Re: amyloidosis 에 의한 울혈성 심부전 탐구 2021 ...

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Pathophysiology and Therapeutic Approaches to Cardiac Amyloidosis

Jan M. Griffin, Hannah Rosenblum, and Mathew S. Maurer https://orcid.org/0000-0002-1867-0526 msm10@cumc.columbia.eduAuthor Info & Affiliations

Circulation Research

Volume 128, Number 10

https://doi.org/10.1161/CIRCRESAHA.121.318187

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Abstract

Often considered a rare disease, cardiac amyloidosis is increasingly recognized by practicing clinicians. The increased rate of diagnosis is in part due the aging of the population and increasing incidence and preval‎ence of cardiac amyloidosis with advancing age, as well as the advent of noninvasive methods using nuclear scintigraphy to diagnose transthyretin cardiac amyloidosis due to either variant or wild type transthyretin without a biopsy. Perhaps the most important driver of the increased awareness is the elucidation of the biologic mechanisms underlying the pathogenesis of cardiac amyloidosis which have led to the development of several effective therapies with differing mechanisms of actions. In this review, the mechanisms underlying the pathogenesis of cardiac amyloidosis due to light chain (AL) or transthyretin (ATTR) amyloidosis are delineated as well as the rapidly evolving therapeutic landscape that has emerged from a better pathophysiologic understanding of disease development.

종종 희귀병으로 간주되는 심장 아밀로이드증은 

임상 실무자들 사이에서 점점 더 많이 인식되고 있습니다. 

진단률이 증가한 것은 부분적으로 인구 고령화와 나이가 들면서 심장 아밀로이드증의 발생률과 유병률이 증가했기 때문이며, 생검 없이 변이형 또는 야생형 트랜스티레틴으로 인한 트랜스티레틴 심장 아밀로이드증을 진단하기 위해 핵신티그래피를 사용하는 비침습적 방법의 출현도 한몫을 했습니다. 

아마도 이러한 인식의 증가를 이끈 가장 중요한 동인은 심장 아밀로이드증의 병인에 기초한 생물학적 메커니즘을 밝혀낸 것입니다. 이로 인해 다양한 작용 기전을 가진 여러 가지 효과적인 치료법이 개발되었습니다. 

이 리뷰에서는 

경쇄(AL) 또는 트랜스티레틴(ATTR) 아밀로이드증으로 인한 

심장 아밀로이드증의 병인 기전과 질병 발생에 대한 

병리 생리학적 이해가 향상되면서 

급속하게 발전한 치료 환경에 대해 설명합니다.

light chain (AL) or transthyretin (ATTR) amyloidosis

The elucidation of physiological mechanisms underlying the genesis of misfolded proteins which form amyloid fibrils that deposit in the myocardium and can cause cardiac amyloidosis (CA) has led to development of several effective therapeutic approaches.1 These efforts have led to therapies that have been described as a translational triumph.2 Among the causes of CA, the 2 that account for >95% of cases encountered clinically (Table 1) include (1) immunoglobulin light chain (AL) CA, which is due to a plasma cell dyscrasia with over-production of either κ or λ light chains and (2) TTR (transthyretin) CA, which results from misfolded monomers or oligomers of either wild type (ATTRwt) or variant TTR (ATTRv) CA.3 ATTRv is inherited in an autosomal dominant fashion and is due to one of the >130 mutations in the TTR gene on chromosome No. 18. With the aging of the population, ATTRwt-CA is anticipated to become the most common form of systemic amyloidosis. In this review, we will delineate the mechanisms underlying the pathogenesis of CA and highlight the rapidly evolving therapeutic landscape that has emerged from a better understanding of disease development.

심근에 침착되어 심장 아밀로이드증(CA)을 유발할 수 있는

아밀로이드 섬유소를 형성하는 오접힘 단백질의 발생에 기초한

생리적 메커니즘의 해명이

여러 가지 효과적인 치료 방법의 개발로 이어졌습니다.1

이러한 노력은 번역적 승리로 묘사된 치료법으로 이어졌습니다. 2

CA의 원인 중 임상적으로 발생하는 사례의 95% 이상을 차지하는

두 가지 원인(표 1)은 다음과 같습니다.

(1) 면역글로불린 경쇄(AL) CA는 κ 또는 λ 경쇄의 과잉 생산을 동반한 혈장세포 이상으로 인해 발생하며,

(2) TTR(트랜스티레틴) CA는 이는

     야생형(ATTRwt) 또는 변이형 TTR(ATTRv)의 잘못 접힌 단량체 또는 올리고머로 인해 발생합니다.

CA.3 ATTRv는 상염색체 우성 유전 방식으로 유전되며,

18번 염색체에 있는 TTR 유전자의 130개 이상의 돌연변이 중 하나에 의해 발생합니다.

인구 고령화와 함께

ATTRwt-CA가 전신성 아밀로이드증의 가장 일반적인 형태로 자리잡을 것으로 예상됩니다.

이 리뷰에서는

CA의 병인 기전을 설명하고,

질병의 발달에 대한 이해가 깊어짐에 따라 빠르게 변화하는

치료 환경을 강조하고자 합니다.

Table 1. Major Causes of Cardiac Amyloidosis

FeaturesLight chain cardiac amyloidosis (AL-CA)Transthyretin cardiac amyloidosisWild type (ATTRwt-CA)Variant/hereditary transthyretin (ATTRv-CA)

Age at diagnosis	Fifth to Ninth decade	Seventh to tenth decade	Third to eighth decade
Sex distribution	Roughly equal male:female	Very significant male predominance	Male predominance
Precursor protein	Light-chain	Transthyretin	Transthyretin
Genetic cause	None	None	Autosomal dominant inheritance
Genetic modifier to therapeutic efficacy	t(11,14) presence—poor response to bortezomib but responsive to venetoclax	None	None
Extracardiac involvement	Nerves, kidney, liver, gastrointestinal tract, skin, tongue/soft tissue	Carpal tunnel, lumbar spine, gastrointestinal tract	Nerves
Clinical manifestations	Multi-systemic disease with cardiac and renal involvement (60%–70%); liver (15%) and peripheral/autonomic neuropathy (10%)	Predominant cardiac phenotype with a restrictive cardiomyopathy, atrial and ventricular arrhythmias, and HFpEF	Depends on variant. Val122Ile predominately cardiac, Thr60Ala mixed and Val30Met predominately neuropathic
Prognosis after diagnosis	Depends on stage. Median survival 4–6 mo with advanced heart failure	Depends on stage. Median survival 2–6 y in the absence of treatment	Depends on mutation and stage. Median survival 3–12 y

Expand Table

AL-CA indicates immunoglobulin light-chain cardiac amyloidosis; ATTRv, variant (hereditary, familial) transthyretin amyloidosis; ATTRwt, wild-type transthyretin amyloidosis; CA, cardiac amyloidosis; and HFpEF, heart failure with a preserved ejection fraction.

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Pathophysiology

Despite originating from different precursor proteins, the basic mechanisms underlying amyloid pathogenesis is similar in that the capability of a protein to become amyloidogenic lies in its ability to acquire >1 conformation. Amyloid formation occurs when a protein loses (or fails to acquire) its physiological, functional fold. A number of factors may trigger protein misfolding and aggregation, such as abnormal proteolysis, point mutations, and posttranslational modifications such as phosphorylation, oxidation, and glycation. The misfolded protein or peptide then assembles with similar proteins or peptides to form oligomers, which circulate in the blood and deposit as highly ordered fibrils in the interstitial space of target organs. In CA, the mechanisms of organ dysfunction are likely multifactorial, resulting from a combination of factors including extracellular deposition of amyloid in the parenchymal tissue leading to mechanical disruption of tissue structure, as well as proteotoxicity of the fibrils or prefibrillar proteins leading to inflammation, reactive oxygen species generation, apoptosis, and autophagy, which can be observed even before fibril deposition.4–7 This leads to a restrictive physiology, diastolic dysfunction, and eventually manifests clinically as heart failure.

병태 생리학

서로 다른 전구체 단백질에서 유래되었음에도 불구하고, 아밀로이드 병인의 기본 메커니즘은 유사합니다.

즉,

단백질이 아밀로이드로 변하는 능력은

1개 이상의 형태를 취할 수 있는 능력에 달려 있습니다.

아밀로이드 형성은

단백질이 생리학적, 기능적 폴드를 잃어버리거나(혹은 획득하지 못했을 때) 발생합니다.

비정상적인 단백질 분해,

점 돌연변이,

인산화,

산화,

당화 등의 번역 후 변형과 같은

여러 가지 요인이 단백질 오접힘과 응집을 유발할 수 있습니다.

그런 다음 오접힌 단백질 또는 펩타이드가

유사한 단백질 또는 펩타이드와 결합하여

올리고머를 형성하고,

이 올리고머는 혈액에서 순환하다가

표적 기관의 간질 공간에 고도로 정렬된 섬유소로 침착됩니다.

심장 근육의 기능 장애 메커니즘은 여러 요인에 의해 발생하며,

그 요인은 세포 외부의 아밀로이드가 실질 조직에 침착되어

조직 구조의 기계적 파괴를 초래하는 것,

그리고 섬유소 또는 프리섬유 단백질의 단백질 독성이

염증, 활성 산소 생성, 세포 자멸사,

그리고 아폽토시스를 유발하는 것(섬유소 침착 이전에도 관찰 가능)을

포함합니다.4-7

이로 인해 제한적인 생리학, 이완기 기능 장애가 발생하고,

결국 임상적으로 심부전으로 나타납니다.

Overview of Protein Folding

Protein or peptide folding is a tightly regulated process. In general, proteins require specific 3-dimensional conformations to be soluble and function correctly in the body. The process of protein folding begins after polypeptide chains are synthesized in the endoplasmic reticulum of the cell and a rapid sequence of intracellular folding consisting of conformational modifications is initiated, requiring the use of chaperones and catalysts of folding, to achieve its native structure. In this pathway, conformational intermediates become progressively more organized as they merge, resulting in the most stable native state. In this native structure, there is a minimum of free energy which results from the balance between the internal energy of the protein determined by intramolecular bonds and the level of conformational entropy, determined by the level of randomness of the polypeptide in solution.8

Proteins must remain folded throughout their lifetimes to continue to perform their biological functions and the abundance of each of the thousands of different proteins in a cell must be tightly regulated. This state of a balanced proteome, termed protein homeostasis or proteostasis, requires an extensive network of competing pathways within cells that control the protein synthesis and folding, conformational maintenance, and degradation of proteins present within and outside the cell.9 The proteostasis network serves to ensure that correctly folded proteins are generated at the proper time and cellular location and in amounts allowing stoichiometric assembly in the case of oligomeric protein complexes. Additionally, it prevents proteins from misfolding and aggregating and removes proteins that are misassembled. Normally, misfolded proteins are retained by the endoplasmic reticulum, dislocated to the cytoplasm, and degraded by the proteasome.10 Loss of proteostasis is the underlying cause of diseases associated with protein misfolding, such as amyloidosis.

단백질 폴딩의 개요

단백질 또는 펩타이드 폴딩은

엄격하게 규제되는 과정입니다.

일반적으로 단백질은

체내에서 용해되고 올바르게 기능하기 위해 특정한 3차원 구조를 필요로 합니다.

단백질 폴딩 과정은

세포의 소포체에서 폴리펩티드 사슬이 합성된 후 시작되며,

구조적 변형으로 구성된 세포 내 폴딩의 빠른 순서가 시작되어,

폴딩의 샤페론과 촉매제의 사용을 필요로 하며,

그 결과 고유 구조를 달성하게 됩니다.

이 경로에서,

결합 중간 상태는 결합이 진행됨에 따라

점점 더 조직화되어 가장 안정적인 고유 상태로 변합니다.

이 고유 구조에서는

분자 내 결합에 의해 결정되는

단백질의 내부 에너지와 용액 내 폴리펩티드의 무작위성 수준에 의해

결정되는 결합 엔트로피 수준 사이의 균형으로 인해

최소한의 자유 에너지가 발생합니다.8

단백질은

생물학적 기능을 계속 수행하기 위해 평생 동안 접힌 상태를 유지해야 하며,

세포에 존재하는 수천 가지의 서로 다른 단백질 각각의 양은 엄격하게 조절되어야 합니다.

단백질 항상성 또는 단백질 항상성이라고 불리는 이러한 균형 잡힌 단백질 상태는

세포 내외에 존재하는

단백질의 합성, 폴딩, 형태 유지, 분해를 조절하는

세포 내 경쟁 경로들의 광범위한 네트워크를 필요로 합니다.9

단백질 항상성 네트워크는 올리고머 단백질 복합체의 경우,

정확한 시기와 세포 위치에서 올바르게 폴딩된 단백질이 생성되고,

화학량론적 조립이 가능하도록 하는 역할을 합니다.

또한,

단백질의 잘못된 접힘과 응집을 방지하고,

잘못 조립된 단백질을 제거합니다.

일반적으로,

잘못된 접힘을 일으킨 단백질은 소포체에 의해 유지되고,

세포질로 옮겨져, 프로테아좀에 의해 분해됩니다.10

단백질 항상성(proteostasis)의 상실은

아밀로이드증과 같은 단백질 잘못된 접힘과 관련된 질병의 근본적인 원인입니다.

Mechanisms of Amyloidogenesis

Folded protein structures, in most cases, are only marginally stable, meaning that a substantial proportion of protein species exist in unfolded states. The exposure to various extra-cellular denaturing stimuli causes unfolding of the polypeptide chain, an event which is normally followed by the rapid restoration of the native structure. This extra-cellular unfolding and refolding process causes the exposure of normally hidden hydrophobic residues, and the protein may become the target of ubiquitous endopeptidases. Even minor proteolytic cleavage can destabilize the protein, promote its denaturation, and prevent the restoration of the native structure.

Partial unfolding of the native state of the protein to less thermodynamically stable states is a required step in amyloidogenesis.11 Amyloidogenic and normal protein counterparts are synthesized, but cellular quality control appears to be unable to remove misfolded proteins and they are secreted from the cell.11 Outside the cell, amyloidogenic variants reach a state of equilibrium between fully folded and partially folded forms. Any factor that disrupts the normal 3-dimensional protein structure, such as low pH, oxidation, increased temperature, can shift this equilibrium towards the partially folded state. A misfolded protein must then reach critical local concentration to trigger fibril formation, in conjunction with local factors including glycosaminoglycans and collagen, shear forces, endoproteases, and metals that modulate aggregation and oligomer formation.9,12,13

In both ATTRv and ATTRwt, amyloid aggregation of TTR is preceded by destabilization of the native homotetrameric structure into its constituent monomers and dimers with an exposed hydrophobic surface, followed by misfolding and structural reorganization into amyloid aggregates (Figure 1). Physiological TTR is a homotetramer, mainly synthesized in the liver and the choroid plexus of the brain, circulates in plasma and CSF (cerebrospinal fluid), and serves as a carrier of thyroxine and retinol bound to retinol-binding protein.14 TTR consists of 2 weakly bound dimers, between which lie 2 thyroxine (T4) binding sites. It is at these sites that the dimers of TTR dissociate and the processes of destabilization and unfolding occur. In ATTRv, different mutations lead to a kinetically unstable tetrameric protein with an increased propensity to dissociate into monomers leading to misfolding.15 For example, the Val122Ile (p.Val142Ile) variant destabilizes the TTR tetramer by lowering the kinetic barrier for tetramer dissociation, resulting in a greater extent and faster rate of folded monomer formation that then self assembles into amyloid fibrils in vitro. Despite structural instability, mutant TTR tetramers are secreted with the same efficacy as wild type tetramers if they possess a thermodynamic and kinetic profile the endoplasmic reticulum degradation pathway. In ATTRwt, protein oxidative modifications and failures in the proteostatic machinery and repair mechanisms associated with aging, contribute to native TTR dissociation and aggregation into fibrils.5

아밀로이드 형성의 메커니즘

접힌 단백질 구조는

대부분의 경우 안정성이 매우 낮습니다.

즉, 상당수의 단백질 종이 펼쳐진 상태로 존재한다는 의미입니다.

다양한 세포 외 변성 자극에 노출되면

폴리펩티드 사슬이 펼쳐지는 현상이 발생하는데,

이 현상은 일반적으로 원래의 구조로 빠르게 복원됩니다.

이 세포 외의 펼쳐짐과 다시 접힘 과정은

일반적으로 숨겨져 있는 소수성 잔기(hydrophobic residue)를 노출시키게 되고,

단백질은 어디에나 존재하는 엔도펩티다제의 표적이 될 수 있습니다.

아주 작은 단백질 분해 절단도 단백질을 불안정하게 만들고,

변성을 촉진시키며,

원래의 구조를 회복하는 것을 방해할 수 있습니다.

단백질의 부분적 원상태에서 열역학적으로

덜 안정적인 상태로의 전이는

아밀로이드 형성에 필요한 단계입니다.11

아밀로이드 형성과 정상 단백질은 합성되지만,

세포의 품질 관리는 오접힌 단백질을 제거할 수 없는 것으로 보이며,

오접힌 단백질은 세포 밖으로 분비됩니다.11

세포 밖에서,

아밀로이드 형성의 변이체는

완전히 접힌 형태와 부분적으로 접힌 형태 사이의

평형 상태에 도달합니다.

낮은 pH, 산화, 온도 상승과 같이 정상적인 3차원 단백질 구조를 방해하는 요인은

이러한 평형을 부분적으로 접힌 상태로 바꿀 수 있습니다.

그런 다음,

잘못 접힌 단백질은

글리코사미노글리칸과 콜라겐,

전단력,

엔도프로테아제,

응집과 올리고머 형성을 조절하는 금속을 포함한 국소적 요인과 함께,

피브릴 형성을 촉발하기 위해

임계 국소 농도에 도달해야 합니다.9,12,13

ATTRv와 ATTRwt 모두에서

TTR의 아밀로이드 응집은

원래의 동종사량체 구조가 불안정해져 구성 단량체와 이량체로 분해되고,

그 후 표면이 노출된 소수성 표면을 가진 아밀로이드 응집체로 잘못 접히고

구조가 재구성되는 과정을 거칩니다(그림 1).

생리학적 TTR은 주로 간과 뇌의 맥락막에서 합성되는 동형 이량체이며, 혈장과 뇌척수액(CSF)을 순환하고, 레티놀 결합 단백질에 결합된 티록신과 레티놀의 운반체 역할을 합니다.14 TTR은 2개의 약하게 결합된 이량체로 구성되어 있으며, 그 사이에 2개의 티록신(T4) 결합 부위가 있습니다. 이 사이트에서 TTR의 이합체가 분리되고 불안정화 및 전개 과정이 발생합니다. ATTRv에서, 다른 돌연변이는 운동학적으로 불안정한 4량체 단백질을 생성하고, 이 단백질은 단량체로 분리되는 경향이 증가하여 오접합을 유발합니다.15 예를 들어, Val122Ile(p.Val142Ile) 변이체는 4량체 분리에 대한 운동학적 장벽을 낮춤으로써 TTR 4량체를 불안정하게 만들고, 그 결과 접힌 단량체 형성이 더 광범위하고 더 빠른 속도로 일어나고, 그 단량체는 체외에서 아밀로이드 섬유로 자가 조립됩니다. 구조적 불안정성에도 불구하고, 돌연변이 TTR 테트라머는 열역학 및 운동학 프로파일을 가지고 있는 경우, 원형질막 분해 경로를 통해 야생형 테트라머와 동일한 효능으로 분비됩니다. ATTRwt에서, 단백질의 산화적 변형과 노화와 관련된 단백질 동역학 기계 및 복구 메커니즘의 실패는 원형 TTR의 해리 및 피브릴로의 응집에 기여합니다.5

Figure 1. Pathogenesis of cardiac amyloidosis and therapies. Mechanisms underlying formation of cardiac amyloidosis in transthyretin (TTR) and immunoglobulin light chain (AL). Targets for therapy are enumerated in blue. Effective therapies approved for use are shown in black and experimental therapies are shown in red. ASO indicates antisense oligonucleotide; CPHPC, (R)-1-[6-[(R)-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl] pyrrolidine-2-carboxylic acid; CRISPR, clustered regularly interspaced short palindromic repeat; SAP, serum amyloid P-component; and TUDCA, tauroursodeoxycholic acid. (Illustration credit: Ben Smith).

그림 1. 심장 아밀로이드증의 병인과 치료. 트랜스티레틴(TTR)과 면역글로불린 경쇄(AL)에서 심장 아밀로이드증이 형성되는 기전. 치료 대상은 파란색으로 표시되어 있습니다. 사용이 승인된 효과적인 치료법은 검은색으로, 실험적인 치료법은 빨간색으로 표시되어 있습니다. ASO는 안티센스 올리고뉴클레오티드를, CPHPC는 (R)-1-[6-[(R)-2-카르복시-피롤리딘-1-일]-6-옥소-헥사노일] 피롤리딘-2-카르복실산을, CRISPR은 클러스터링된 규칙적으로 간격을 둔 짧은 회문 반복, SAP는 혈청 아밀로이드 P-성분, TUDCA는 타우루르소데옥시콜산을 나타냅니다.

AL amyloidosis is characterized by a clonal expansion of differentiated plasma cells, that produce misfolding-prone immunoglobulin free light chains (FLC), or a fragment thereof, secreted in excess compared with heavy chains.14,16 The 2 classes of light chains, κ and λ, each consist of an N terminal variable Ig domain attached to a C-terminal constant Ig domain. While excess FLC production is observed in plasma cell disorders including monoclonal gammopathy of undetermined significance, multiple myeloma and Waldenstrom macroglobulinemia, only a fraction of FLCs can form amyloid deposits in vivo. The λ light chains are observed almost twice as commonly as κ in systemic AL amyloidosis.17 DNA sequencing studies have shown presence of germline gene mutations on the variable λ region, Vλ6a and Vλ3r, that reduce the thermodynamic stability of the protein, have a strong association with the development of amyloidosis and account for the propensity of λ light chains to form amyloid deposits.18 Amyloidogenic light chains are kinetically unstable and susceptible to endoproteolysis, which results in the release of amyloidogenic light chain fragments prone to improper aggregation.19

Other less common forms of CA encountered clinically include AA amyloidosis, dialyses associated amyloidosis, and isolated atrial amyloidosis. AA amyloidosis is associated with auto inflammatory condition such as rheumatoid arthritis, inflammatory bowel disease, and hidradenitis suppurativa, particularly when the diagnosis is delayed. SAA (serum amyloid-associated protein) is elevated, and cardiac involvement in AA amyloidosis is always preceded by renal involvement. Effective control of the underlying inflammatory process can halt disease progression and even reverse organ damage.20 β2-microglobulin is the precursor protein for CA associated with long-term dialysis. CA related to β2-microglobulin occurred with low-flow dialysis membranes in patients on dialysis for >9 years in duration. However, newer dialysis technologies reduce serum β2-microglobulin levels in chronic dialysis patients and appear to reduce the risk of developing this form of systemic amyloidosis.21 Isolated atrial amyloidosis, due to deposits of fibrils from atrial natriuretic factor, is extremely common with advancing age and often seen on biopsies of the atrium obtained at the time of cardiac surgery.22 Distinguishing these forms of amyloidosis from those more commonly encountered clinically is essential.

AL 아밀로이드증은 분화된 형질세포의 클론 확장을 특징으로 하며, 이 형질세포는 오접힘이 발생하기 쉬운 면역글로불린 유리 경쇄(FLC) 또는 그 단편을 생산하며, 이는 중쇄에 비해 과다하게 분비됩니다.14,16 경쇄의 두 가지 종류인 κ와 λ는 각각 N 말단 가변 Ig 도메인과 C 말단 상수 Ig 도메인으로 구성되어 있습니다. 불확실한 의미의 단클론 감마병증, 다발성 골수종, 발덴스트롬 거대글로불린혈증을 포함한 형질세포 장애에서 과도한 FLC 생산이 관찰되지만, 생체 내에서 아밀로이드 침착물을 형성할 수 있는 FLC는 극히 일부에 불과합니다. λ 경쇄는 전신성 AL 아밀로이드증에서 κ보다 거의 두 배 더 흔하게 관찰됩니다.17 DNA 염기서열 분석 연구에 따르면 가변 λ 영역인 Vλ6a와 Vλ3r에 있는 생식세포 유전자 돌연변이가 단백질의 열역학적 안정성을 감소시키고, 아밀로이드증의 발생과 밀접한 관련이 있으며, λ 경쇄가 아밀로이드 침전물을 형성하는 경향을 설명할 수 있는 것으로 나타났습니다. 18 아밀로이드 생성 경쇄는 운동학적으로 불안정하고 내단백질 분해에 취약하여, 부적절한 응집이 일어나기 쉬운 아밀로이드 생성 경쇄 단편의 방출을 초래합니다.19

임상적으로 흔하지 않은 다른 형태의 CA로는 AA 아밀로이드증, 투석 관련 아밀로이드증, 그리고 분리된 심방 아밀로이드증이 있습니다.

AA 아밀로이드증은 특히 진단이 지연되는 경우

류마티스 관절염,

염증성 장 질환,

화농성 한선염과 같은 자가 염증 상태와 관련이 있습니다.

SAA(혈청 아밀로이드 관련 단백질)가 상승하고,

AA 아밀로이드증의 심장 침범은

항상 신장 침범이 선행됩니다.

염증 과정을 효과적으로 제어하면

질병의 진행을 멈추고

장기 손상을 되돌릴 수도 있습니다.20

β2-마이크로글로불린은

장기 투석과 관련된 CA의 전구체 단백질입니다.

β2-마이크로글로불린과 관련된 CA는 투석 기간이 9년 이상인 환자에서 저유량 투석막을 사용할 때 발생합니다. 그러나 새로운 투석 기술은 만성 투석 환자의 혈청 β2-마이크로글로불린 수치를 감소시키고 이러한 형태의 전신성 아밀로이드증 발생 위험을 줄이는 것으로 보입니다. 21 심방성 나트륨 이뇨 인자(atrial natriuretic factor)의 섬유소 침착으로 인한 고립성 심방성 아밀로이드증은 나이가 들면서 매우 흔하게 발생하며, 심장 수술 시에 얻은 심방 생검에서 종종 발견됩니다.22 이러한 형태의 아밀로이드증을 임상적으로 더 흔하게 발생하는 아밀로이드증과 구별하는 것이 필수적입니다.

Amyloid Fibril Structure

Despite originating from different precursor proteins, amyloid deposits share several structural properties as observed by electron microscopy. They are composed of rigid, nonbranching fibrils with an average diameter of 7.5 to 10 nm and a cross-ß-sheet secondary structure. Intermolecular hydrogen bonding between the amide and carbonyl groups of the main chain acts as a major stabilizing interaction between protein monomers8,23 that allow formation of the ß-sheet. Both immunoglobulin light chains and TTR protein have extensive ß-structure in the normal folded state, but this region has to be exposed for intermolecular hydrogen bonding between monomers. Contiguous ß-sheet polypeptide chains constitute a protofilament, which are wound around one another to form an amyloid fibril, which frequently have repetitive hydrophobic or polar interactions along the fibril axis.23 This ultrastructure of the fibril allows the intercalation of Congo red dye, conferring the diagnostic property to amyloid of apple-green birefringence under polarized light microscopy.

Beside the fibril core proteins, additional components are known to be part of all amyloid deposits including components of the extracellular matrix, such as lamin, entactin, and collagen, and additional proteins. Serum amyloid P-component (SAP), a glycoprotein that belongs to the pentraxin family, is calcium dependently bound to amyloid fibrils independent of the protein of origin.24 SAP is highly protected against proteolysis,25 making amyloid fibrils highly rigid, resistant to thermal and chemical denaturation and degradation. Proteoglycans or glycosaminoglycans are also common in amyloid deposits and contribute to the carbohydrate composition of amyloid, influencing the conformation of the fibril. Their role is quite complex and seem to contribute to both the genesis and the structural stabilization of amyloid fibrils. Proteoglycans are proposed to represent the initial structural scaffold that facilitates adhesion and orientation of the first nuclei of aggregated amyloid. Despite being a common component of all amyloid deposits, proteoglycans show a degree of chemical and structural heterogeneity and may play a role in the localization of amyloid deposits in tissues.

아밀로이드 섬유 구조

아밀로이드 침착물은

다른 전구체 단백질에서 유래되었음에도 불구하고

전자 현미경으로 관찰했을 때

몇 가지 구조적 특성을 공유합니다.

평균 직경이 7.5~10nm이고,

교차 β-시트 2차 구조를 가진

단단하고 가지가 없는 섬유로 구성되어 있습니다.

주쇄의 아미드기와 카르보닐기 사이의 분자간 수소 결합은

단백질 단량체8,23 사이의 주요 안정화 상호작용으로 작용하여

β-시트의 형성을 가능하게 합니다.

Intermolecular hydrogen bonding between the amide and carbonyl groups of the main chain acts as a major stabilizing interaction between protein monomers8,23 that allow formation of the ß-sheet.

면역글로불린 경쇄와 TTR 단백질 모두

정상적인 접힌 상태에서 광범위한 β-구조를 가지고 있지만,

단량체 사이의 분자간 수소 결합을 위해서는

이 영역이 노출되어야 합니다.

인접한 ß-시트 폴리펩티드 사슬은

원섬유를 구성하고,

이 원섬유는 서로 감겨서 아밀로이드 원섬유를 형성하며,

이 원섬유는 원섬유 축을 따라 반복적인 소수성 또는

극성 상호작용을 갖는 경우가 많습니다.23

이 원섬유의 이러한 초미세 구조는

콩고 레드 염료의 삽입을 가능하게 해,

편광 현미경으로 관찰할 때 사과 녹색 복굴절 현상을 보이는

아밀로이드의 진단적 특성을 부여합니다.

섬유소 단백질 외에도,

라민, 엔타틴, 콜라겐과 같은 세포외 기질 성분과 추가 단백질을 포함한

모든 아밀로이드 침착물의 일부로 알려진 추가 성분이 있습니다.

펜트락신 계열에 속하는 당단백질인

혈청 아밀로이드 P-성분(SAP)은

칼슘에 의존적으로 결합되어 있으며,

기원 단백질과 무관하게 아밀로이드 섬유에 결합되어 있습니다.24

SAP는

단백질 분해로부터 매우 잘 보호되어25

아밀로이드 섬유를 매우 단단하게 만들고,

열적 및 화학적 변성 및 분해에 저항성을 줍니다.

프로테오글리칸 또는

글리코사미노글리칸도 아밀로이드 침착물에서 흔히 발견되며,

아밀로이드의 탄수화물 구성에 기여하여 섬유질의 형태에 영향을 줍니다.

그들의 역할은 매우 복잡하며, 아밀로이드 원섬유의 발생과 구조적 안정화에 기여하는 것으로 보입니다.

프로테오글리칸은

응집된 아밀로이드의 첫 번째 핵의 부착과 배향을 촉진하는

초기 구조적 지지대를 나타내는 것으로 제안되었습니다.

모든 아밀로이드 침착물의 공통적인 구성 요소임에도 불구하고,

프로테오글리칸은 어느 정도의 화학적 및 구조적 이질성을 나타내며,

조직 내 아밀로이드 침착물의 국소화에 중요한 역할을 할 수 있습니다.

Amyloid Deposition

Amyloid deposition in target organs occurs by an extremely complicated aggregation process. The kinetics of fibril formation has an S-shaped growth curve and a discernable lag phase. The duration of each phase of fibril formation is protein-specific and defines the rate at which amyloid deposition occurs.26 First, primary nucleation occurs when soluble oligomers form from monomers. The initial nucleation process is driven by specific adhesive parts of the amyloid proteins and target organ cells may be transiently involved.11 In ATTR, 2 adhesive segments that form the F and H ß-strands in the native TTR structure are the principal drivers of protein aggregation. Upon dissociation into monomers, these strands become exposed and enable stacking into the steric zipper spines of amyloid fibrils.27 Circulating monomers can then add to this existing fibril. Secondary nucleation occurs when the surface of this existing amyloid aggregate catalyzes the formation of new small soluble aggregate. Fragmentation occurs when the existing fibrils break apart, increasing the total number of fibrils. The process of amyloid deposition can be accelerated by the presence of preformed fibrils, or seeds, which can capture and catalyze the conversion of precursors, even at low concentrations, into misfolded, toxic, and aggregation-prone structures.28

Deposition of amyloid in specific organ tissues likely depends on the concurrence of several factors including high local protein concentrations, low pH, and the presence of fibril seeds. Specific interactions with tissue glycosaminoglycans or cell surface receptors may be important.29 In AL amyloidosis, it has been hypothesized that organ tropism may be a function of the variable region gene polymorphisms, leading to interactions between the light chain (or fragment thereof) and tissue constituents such as collagen, lipids, and glycosaminoglycans.19 For example, LV1-44 germ line cells favor deposition in cardiac tissue.18 In ATTRv, the specific site of amino acid substitution determines the propensity for depositing primarily in the peripheral nervous system or cardiac tissue, leading to markedly different phenotypes of disease. Patients with the Val122Ile (p.Val142Ile) TTR variant present predominantly with a cardiomyopathy4 whereas other variants such as Val30Met (p.Val50Met) are associated with neuropathy. While for certain mutations the correlation between the genotype and phenotype is strong, for others clinical features can vary significantly. Furthermore, significant variability in clinical presentation is seen between patients with the same mutation. Cleavage of the TTR monomer into fragmented fibrils may play a role as well in determining the site of deposition as well as the disease penetrance.30 Full-length TTR fibrils are commonly seen in ATTRv caused by the Val30Met (p.Val50Met) variant with early disease and a predominant axonal polyneuropathy with rare cardiac involvement, whereas those with later onset disease often have fibrils composed of a mixture of full-length and truncated TTR. These patients also often have marked cardiac involvement at presentation with concomitant peripheral neuropathy. Similarly, in patients with ATTRwt, amyloid deposits always include both fragmented and full-length TTR fibrils.31 Indeed, environmental and genetic factors that have not been identified must play a role in the pathobiology of TTR amyloidosis.

Mechanisms of Cardiac Dysfunction

The deposition of amyloid fibrils results in cellular injury, tissue damage, and finally organ dysfunction (Figure 2). Although the type of CA cannot be distinguished based on patterns of deposition, it appears there is a predominance of diffuse, peri-cellular, endocardial, and arterial or arteriolar deposits in AL amyloidosis and nodular deposits in TTR amyloidosis.32 In both AL and ATTR-CA large deposits of amyloid in the extracellular space of the myocardium leads to loss of normal tissue architecture and function, progressive biventricular wall thickening and stiffness without compensatory ventricular dilation, leading to a restrictive myopathy, and low cardiac output.11 Early disease is marked by isolated diastolic dysfunction with normal systolic function but as the disease progresses restrictive physiology becomes apparent. Atrial infiltration is present and frequently causes contractile dysfunction. Insights from noninvasive pressure-volume analysis in patients with both wild-type and Val122Ile associated ATTR-CA demonstrate a complex cascade of events occurring overtime marked by decreasing ventricular capacitance and chamber contractility leading to reduced stroke volume, alterations in ventricular-vascular coupling and progressive pump dysfunction not simply due to impairment in diastolic dysfunction but systolic derangements as well33 (Figure 3).

Figure 2. Mechanisms of myocardial dysfunction in transthyretin (TTR) and immunoglobulin light chain (AL) cardiac amyloidosis. In both AL and ATTR-CA, extracellular deposition of amyloid fibrils causes mechanical disruption of normal tissue architecture, leading to impaired relaxation and increased ventricular stiffness. In AL-CA, circulating free light chains cause cytotoxicity by increasing oxidative stress and activation of the p38 MAPK (mitogen-activated protein kinase) signaling pathway. Activation of the p38 MAPK pathway also leads to release of BNP. In ATTR-CA, circulating TTR monomers and oligomers have also been proposed to cause direct cytotoxicity. ROS indicates reactive oxygen species. (Illustration credit: Ben Smith).Open in viewer

Figure 3. Cardiac mechanics in transthyretin (TTR) amyloidosis. Noninvasive pressure-volume loops (top) and isovolumetric pressure-volume area (bottom). Left, Progression of cardiac chamber dysfunction overtime marked by reduced ventricular capacitance and impaired contractility. Collectively, both abnormalities lead to reduced stroke volume and isovolumetric pressure-volume area overtime. Right, Compared with wildtype (WT), patients with V122I associated ATTR-CA have more impaired cardiac function at baseline. Patients with both WT and V122I associated ATTR-CA have reduced ventricular capacitance. However, patients with V122I associated ATTR-CA also have impaired contractility, leading to lower stroke volume and isovolumetric pressure-volume area in V122I associated ATTR-CA compared with WT. PVA indicates pressure-volume area; LVEDP, left ventricular end diastolic pressure.Open in viewer

Mechanical displacement of normal parenchymal tissue by amyloid deposits is insufficient to fully explain organ dysfunction associated with both AL and ATTR-CA (Figure 2). In addition to the mechanical problems imposed by amyloid fibril deposition, small soluble monomers, and oligomers are extremely toxic and believed to play a major role in cell and tissue toxicity. The direct toxic effect of circulating light chains in AL amyloidosis has been postulated to explain discrepancies between myocardial amyloid fibril burden, cardiac dysfunction, and the more aggressive disease trajectory in those with AL compared with ATTR-CA.34 Notably, not only amyloid deposition, but light chain proteotoxicity exhibits specific organ tropism.7 Prefibrillar cardiotropic light chains alter cellular redox state in cardiomyocytes, marked by an increase in intracellular reactive oxygen species, that leads to increased oxidative stress and apoptosis.7 Oxidant stress imposed by the light chains result in direct impairment of cardiomyocyte contractility and relaxation with associated alterations in intracellular calcium handling.4,35 The activation of p38 MAPK (mitogen-activated protein kinase) is one of the molecular mechanisms responsible for cardiotoxicity by increasing oxidative stress and apoptosis. This pathway also mediates brain natriuretic peptide (BNP) transcription, supporting the association between cardiotoxic light chain effects with induced MAPK signaling and elevated BNP (brain natriuretic peptide) levels.36 Indeed, the degree of circulating light chain abnormality is clinically prognostic in patients with AL amyloidosis37 and correlates with cardiac biomarker elevations. Furthermore, reduction in circulating amyloidogenic FLC concentrations by chemotherapy translates into reductions in BNP, despite unaltered amyloid deposition in the myocardium.38

In TTR amyloidosis, an accumulating body of evidence suggests that tissue dysfunction precedes TTR fibril deposition, suggesting as well that circulating prefibrillar proteins cause toxicity. In vitro, TTR monomers and oligomeric intermediates smaller than 100 kDa, but not large aggregates or amyloid fibrils, induce cytotoxicity through interactions with membrane proteins and cholesterol.7 Apoptotic mechanisms are activated through cleavage of caspase 3/7 and superoxide formation.4,6 However, the relevance of these short-term in vitro findings to disease manifestations over months to years remains unclear.6

Presentation and Diagnostic Eval‎uation

Systemic manifestations of TTR deposition, such as carpal tunnel syndrome, lumbar spine stenosis or biceps tendon rupture, may precede cardiac diagnosis of ATTR-CA by several years, survival after which is ≈2 to 5 years without treatment. In contrast, AL-CA is a more rapidly progressing disease, with a median survival of 6 months from the onset of advanced heart failure if untreated. Although a detailed description of the presentation and diagnostic eval‎uation of CA is outside of the scope of this review, Table 2 outlines the cardinal manifestations, approach to diagnostic testing, and characteristic findings for both AL- and TTR-CA.

Table 2. Clinical Manifestations and Diagnostic Eval‎uation of Cardiac Amyloidosis

AL (light chain amyloidosis)TTR (transthyretin amyloidosis)

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AL indicates immunoglobulin light-chain amyloidosis; ATTRv, variant transthyretin amyloidosis; ATTRwt, wild-type transthyretin amyloidosis; ECV, extracellular volume; JVP, jugular venous pressure; and LV, left ventricle.

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Therapeutic Targets

Emerging from the basic molecular mechanisms of the genesis of amyloid fibrils in the myocardium that underlie the development of CA, there are several therapeutic strategies that have either been shown to be effective or are actively being explored in human clinical trials. These approaches broadly include one of 4 strategies including:

1.

Knocking down production of the precursor protein with either gene silencing techniques for TTR that leverage small interfering RNA (siRNA) or antisense oligonucleotides (both of which include approved compounds for ATTRv disease with or without a concomitant cardiomyopathy) and clustered regularly interspaced short palindromic repeat (CRISPR) based approaches; for AL amyloidosis, there is a large and growing armamentarium of anti-plasma cell therapies.

2.

Stabilization of the precursor protein to maintain its normal conformational structure which has led to the development of tafamidis for ATTR-CA39 and the investigation of other TTR stabilizers (eg, AG10), as well the identification of small molecules that kinetically stabilize light chains.40

3.

Degradation/disruption of amyloid fibrils with use of monoclonal antibodies targeted at particular epitopes on misfolded, aggregated proteins that either induce macrophage medicated dissolution or disruption of amyloid formation.41–43

4.

Anti-seeding therapies that involve peptides designed as inhibitors to cap fibril growth.44,45

Effective and Emerging Therapies Based on Biologic Mechanisms

Although ATTR and AL amyloidosis both result in the deposition of amyloid fibrils and damage to the involved organs, treatment regimens are distinct. Therapies are generally more effective if administered before significant cardiac dysfunction has ensued (Figure 4). The following sections will describe:

Figure 4. Model of ATTR-CA progression over time. Myocardial amyloid infiltration occurs before clinically manifested changes in ejection fraction, cardiac biomarkers, and renal function. The ideal emerging therapeutic window for novel therapies is hypothesized to be before significant organ dysfunction has occurred and before rapid and potentially irreversible declines in functional capacity. The relative scale specific to each factor and time course are not proportional. NYHA indicates New York Heart Association. From Circulation. 2019 Jul 2;140(1):27–30.Open in viewer

1.

Reducing the precursor protein or stabilization of TTR amyloid in CA.

2.

Antiplasma cell therapies for the treatment of AL amyloidosis.

3.

Agents targeting the degradation and extraction of TTR or AL amyloid fibrils.

4.

Emerging therapies for ATTR and AL amyloidosis.

Knocking Down or Reducing Precursor Production

There are 2 classes of gene silencer therapy for ATTR amyloidosis currently commercially available or in late-phase clinical trials (Table 3). The first, siRNA molecules and the second, ASOs (antisense oligonucleotides). These molecules knockdown or reduce TTR production, but with slightly different mechanisms. Vitamin A supplementation must be provided in those receiving silencer therapy given that a major function of TTR is to transport retinol, the major circulating form of vitamin A, via retinol-binding protein 4. Thyroxine supplementation is not necessary given the fact that the majority (≈85%) of thyroxine is bound to thyroxine-binding globulin or albumin.

Table 3. Therapies for Transthyretin Cardiac Amyloidosis

DrugPhase of studyIndication by amyloid typeMechanism of actionDoseAdverse effectsConcomitant therapy and monitoringAnnual costPrimary end points

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ASO indicates antisense oligonucleotide; ATTRv, variant transthyretin amyloid; ATTRwt, wild-type ATTR; ATTR-FAP, ATTR familial amyloid polyneuropathy; BMP, basic metabolic panel; CRISPR, clustered regularly interspaced short palindromic repeats; CV, cardiovascular; LFT, liver function test; MMP, matrix metalloproteinase; NSAID, nonsteroidal anti-inflammatory; NYHA, New York Heart Association heart failure class; RISC, RNA-induced silencing complex; TUDCA, tauroursodeoxycholic acid; and UPCR, urine protein-creatinine ratio.

*

Doxycycline alone.

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First Generation Therapies

Inotersen (Tegsedi, Akcea Therapeutics, Inc), an ASO, is a 2′-O-methoxyethyl-modified antisense oligonucleotide inhibitor of TTR production in the liver. It is currently approved in the United States for treatment of stage 1 or 2 polyneuropathy due to ATTRv. ASOs46 are single-stranded, amphipathic DNA sequences which have a high binding affinity for proteins, thus enhancing distribution in the body. ASOs are taken up into the liver by binding with hepatocyte surface proteins, mainly clathrin- or caveolin-mediated uptake,46 and subsequently transported to the nucleus by chaperone proteins. It is here in the nucleus that the ASO binds to the target mRNA protein and via the endonuclease, RNase H2, initiates mRNA degradation. The 2′-O-methoxyethyl modification provides resistance against endogenous degradation of the ASO. The NEURO-TTR trial randomized 172 patients with familial amyloid polyneuropathy (FAP) with or without ATTR-CA to inotersen 300 mg delivered subcutaneously weekly or placebo for 64 weeks. Subjects with New York Heart Association (NYHA) class III or greater were excluded from participation. During the NEURO-TTR trial,47 reduction in serum TTR reached steady-state levels by 13 weeks, with a mean reduction in serum TTR of 74% and median of 79%. Serious adverse events included glomerulonephritis (3%) and thrombocytopenia, with 3 patients having a platelet count <25 000 (3%) and one death from intracranial hemorrhage (platelet count <10 000). These adverse effects have led to Food and Drug Administration (FDA) approval of inotersen with a Risk Eval‎uation and Mitigation Program that requires weekly monitoring of platelet counts and biweekly assessments of eGFR, urinalysis and urine creatinine protein ratio. Inotersen was being investigated in phase II programs for ATTR-CA, however, given the aforementioned side effects, toxicity profile, and the development of longer-acting ASOs, it is no longer being developed for use in ATTR-CA.

Patisiran (Onpattro, Alnylam), a siRNA which has been approved for ATTRv with associated polyneuropathy in the United States, targets the 3′ untranslated region of the TTR mRNA. siRNA are double stranded, hydrophilic molecules containing a sense and an antisense strand and prone to rapid renal excretion.48 As such, patisiran is formulated as a lipid nanoparticle to target hepatocyte uptake. Once in the cytoplasm, the sense strand is removed by Ago2, an intracellular RNA endonuclease, leaving the pharmacologically active antisense strand-Ago2 complex to bind to the target mRNA. This forms the RNA-induced silencing complex and facilitates subsequent degradation of the target mRNA. In a phase II study of patients with ATTRv associated polyneuropathy, serum TTR levels were reduced by over 80% after the second dose of patisiran, when given at a dose of 0.3 mg/kg every 3 weeks.49 The phase 3 APOLLO study, randomized 225 patients with ATTRv with polyneuropathy to patisiran versus placebo, excluding NYHA class III and IV patients.50 In the cardiac subpopulation (n=126) defined by a left ventricular wall thickness ≥13 mm at baseline and no history of hypertension or aortic valve disease, those who were randomized to patisiran demonstrated a reduction in mean LV wall thickness (≈1 mm), global longitudinal strain by −1.4% and NT-proBNP (N terminal pro-BNP) levels reduced by ≈55% after 18 months of therapy. A post hoc analysis showed a reduction of 46% in cardiac hospitalizations and all-cause mortality. The APOLLO-B trial (A Study to Eval‎uate Patisiran in Participants With Transthyretin Amyloidosis With Cardiomyopathy) (URL: https://www.clinicaltrials.gov; Unique identifier: NCT03997383) is a phase III study of patisiran in 300 patients with ATTR-CA, with a 1:1 randomization to placebo. Subjects must be premedicated with antihistamines (H1 and H2), glucocorticoids, and acetaminophen given the proinflammatory nature of the lipid nanoparticle-siRNA complex, which predominately manifests as infusion reactions. The study duration of APOLLO-B involves a 12-month double-bind period followed by an open label extension where all patients will receive treatment with patisiran. The primary end point will be change from baseline at month 12 in the 6-minute walk test.

Second Generation Therapies

Vutrisiran (Alnylam) is an siRNA which is conjugated to a N-acetyl galactosamine (GalNAc), specifically targeting hepatocytes. siRNAs that are conjugated to GalNAc enter cells via interaction with the GalNAc moiety on the ASGPR (asialoglycoprotein receptor) on the hepatocyte. ASGPR is present on the hepatocytes at a high concentration and thus facilitates rapid uptake. Vutrisiran has a greater potency and longer duration of action than other knockdown therapies currently in clinical trial and as such can be administered at a lower dose (25 mg every 3 months), with lower injection volume and longer dosing intervals. Furthermore, as a subcutaneous injection it has greater ease of administration than IV infusions, and no premedications are required given the absence of proinflammatory lipid nanoparticles in the formulation. In a phase 1 study (NCT02797847), a single dose of 25 mg of vutrisiran resulted in a mean maximum TTR reduction of 83% by week 6, which was sustained for 90 days,51 enabling quarterly administration. HELIOS-A (A Study of Vutrisiran in Patients With Hereditary Transthyretin Amyloidosis) was a phase III study which enrolled 164 patients with ATTRv and polyneuropathy with or without cardiac involvment, excluding NYHA class III-IV heart failure (NCT03759379). Patients received either vutrisiran or the reference comparator, patisiran, during the treatment period in a 3:1 randomization, after which all patients will be switched to vutrisiran during the treatment extension period. Vutrisiran met the primary end point, change in modified neurological impairment score+7 from baseline at month 9 as compared with historical placebo data from the APOLLO study in addition to both secondary end points (quality of life assessed by the Norfolk Quality of Life Questionnaire-Diabetic Neuropathy and gait speed assessed by the 10-meter walk test). In addition, vutrisiran showed improvement compared with placebo in the exploratory cardiac end point of change from baseline in NT-proBNP. HELIOS-B is a phase III study of ≈600 ATTR-CA patients, randomized 1:1 to receive 25 mg of vutrisiran every 3 months or placebo (NCT04153149). A subset of patients enrolled in this trial will be permitted to take the TTR stabilizing agent, tafamidis. The trial will run for 30 to 36 months with the primary outcome being a composite of all-cause mortality and recurrent CV hospitalizations.

AKCEA-TTR-LRx (ION 682884, Akcea Therapeutics, Inc) is a ligand (GalNAc linked to the 5′ end) conjugated ASO (ligand conjugated antisense oligonucleotide) in which the ASO portion shares the same base sequence as inotersen. Thus, when the GalNAc is cleaved, ION-682884 has the same mechanism of action as inotersen, however, the GalNAc conjugated drug has a much greater potency (≈51-fold). This allows for lower dose of drug to be administered to achieve a similar therapeutic effect. The ION-682884-CS1 (NCT03728634) study, showed a ≥85% mean reduction in serum TTR levels with a monthly 45 mg dose.52 Furthermore, this dose (and interval of administration) has reportedly occurred without the problematic adverse events seen with inotersen, thought due to the 27-fold lower exposure to active drug seen with ION-682884. Cardio-TTRansform (NCT04136171) is a phase 3 clinical trial which will enroll ≈700 patients with ATTR-CA, randomized 1:1 to receive 45 mg of ION-682884 or placebo subcutaneously once every 4 weeks.53 All patients in this study will be allowed to receive commercial tafamidis concurrently. The treatment period will be 120 weeks with frequent clinical monitoring. The primary end point will be a composite of CV mortality and frequency of CV clinical events comparing the 2 study arms.

CRISPR/Cas9 stands for clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) that is a genome editing approach which is being leveraged to knock down the production of hepatic TTR. Formulations in phase 1 human trials include NTLA-2001 that is composed of human single-guide RNA and a mRNA sequence encoding Cas9 protein encapsulated in a LNP (lipid nanoparticle), that facilitates delivery to hepatocytes. The drug is a highly specific gene editing LNP that disrupts expression‎ of human serum TTR expression‎. The first in human study is designed to investigate the safety and tolerability of NTLA-2001 at doses that are expected to meaningfully decrease the level of the circulating TTR protein. Animal data demonstrate that with a single administration, significant editing of the mouse TTR gene in the liver was enabled with a >97% reduction in serum protein levels that persisted for at least 12 months.54

Stabilizing the TTR Tetramer

Diflunisal is a nonsteroidal anti-inflammatory agent which binds to the T4 binding sites on TTR, though with lower affinity than tafamidis (Table 3). When compared with 3 other kinetic stabilizing agents, diflunisal was found to be the least potent. Dissociation of TTR was shown to be limited to 10% of normal with concentrations of 5.7 µmol/L AG10 (800 mg BID), 10.3 µmol/L tolcapone (3×100 mg over 12 hours), 12.0 µmol/L tafamidis (80 mg daily), and 188 µmol/L diflunisal (250 mg BID).55 Diflunisal is associated with adverse effects such as renal dysfunction, gastrointestinal bleeding, hypertension, and fluid retention, which can exacerbate heart failure in susceptible individuals. However, when used in the treatment of ATTR-CA, diflunisal is prescribed at a dose of 250 mg orally twice daily, which is below the dose recommended for anti-inflammatory activity and appears to be well tolerated.56 Furthermore, in subjects who receive this therapy, prescribers should ensure an eGFR >45 mL/min, <1 mg/kg of furosemide (or equivalent bioavailable loop diuretic) daily without a recent heart failure decompensation, or history of gastrointestinal bleeding. Lohrmann et al57 assessed the effect of diflunisal therapy on cardiac structure and function at 1 year in 81 patients with ATTR-CA. They described a significant increase in serum TTR levels (33 versus 19 mg/dL, P=0.01) and improvement in both left atrial volume index (−1.4 versus +4.6 mL/m2) and cardiac troponin I (−0.01 versus +0.03 ng/mL, P=0.01) in treated compared with untreated groups. In addition, the ATTRwt subgroup were found to have stable global longitudinal strain on echocardiogram (+0.1% versus +1.2%, P=0.03 for treated versus untreated, respectively). As such, diflunisal has been used in an off-label manner in those without significant renal or hematologic comorbidities, and also in allele carriers of mutations who are at high risk of developing disease, with frequent monitoring of renal function and heart failure symptoms.

Tafamidis (Vyndamax/Vyndaqel, Pfizer Inc, New York, NY):

In those of Portuguese descent the most common TTR variant is Val30Met (p.Val50Met), which leads to early-onset FAP, often in the fourth decade of life. Val30Met has a high clinical penetrance, though some carriers develop mild or no manifestations of disease. Coehlo et al58 studied these individuals who were allele carriers of the Val30Met variant but did not develop evidence of disease. They found that these individuals were compound heterozygotes for Val30Met/Thr119Met, which led to the hypothesis that a stabilizing variant (Thr119Met [p.Thr139Met]) can prevent dissociation of the TTR tetramer into monomers even in the presence of a destabilizing variant and highly pathogenic Val30Met mutation. In seminal work, Hammarström et al39 showed that Thr119Met enhances stability of the TTR protein by slowing the rate of dissociation of the dimers. It was based on this discovery that the TTR stabilizer, tafamidis was developed. Tafamidis is a benzoxazole derivative which lacks nonsteroidal anti-inflammatory activity and selectively binds to TTR in the blood, at the T4 binding site. Bulawa et al showed that tafamidis stabilizes wild-type TTR and inhibits amyloidogenesis in a dose-dependent manner, with similar effects on both Val30Met and Val122Ile TTR variants. In a randomized clinical trial of 128 patients with Val30Met FAP (associated polyneuropathy) (Fx-005), tafamidis 20 mg daily delayed neuropathic progression and preserved quality of life compared with placebo in a predefined secondary analysis, after 18 months of treatment.59 It was approved by the European Medicines Agency in 2011 for patients with stage 1 symptomatic polyneuropathy. In the ATTR-ACT trial,60 441 patients with ATTR-CA, NYHA class I-III HF were randomized to receive tafamidis 80 mg, 20 mg, or placebo daily for 30 months. Tafamidis was associated with a lower all-cause mortality versus placebo (29.5% versus 42.9%) and a 32% lower risk of cardiovascular hospitalizations in those with NYHA class I or II HF. However, subjects with NYHA class III symptoms had higher rates of cardiovascular-related hospitalization with tafamidis therapy compared with placebo, highlighting the importance of early diagnosis and treatment. Furthermore, assessment of functional capacity and quality of life parameters showed a lower rate of decline in the distance covered on 6-minute walk test and a lower rate of decline in the Kansas City Cardiomyopathy Questionnaire-Overall Summary score. It was based on this study that tafamidis became the first US FDA-approved TTR stabilizer to treat ATTR-CA in the United States, in May 2019. It is formulated as tafamidis meglumine (20 mg capsules, dose 80 mg daily) and tafamidis free salt (61 mg capsule daily), the latter of which was formulated for patient convenience as a single dose capsule. These formulations are bioequivalent, though are not substitutable on a per mg basis.61

AG10 (Eidos Therapeutics, Inc, San Francisco, CA):

AG10 (Acoramidis) is a small-molecule stabilizer with oral bioavailability which selectively binds to and stabilizes TTR. It mimics the super stabilizing properties of the Thr119Met variant,62 which are thought to be due to the formation of hydrogen bonds between neighboring serine residues at position 117 of each monomer.63 AG10 has been shown to be a potent and selective stabilizer of TTR, exceeding the efficacy of tafamidis in stabilizing WT and variant TTR in serum.64 In the phase 2 AG10-201 study, there was near complete stabilization (>90%) of TTR at peak and trough serum levels, defined by percent occupancy of T4 binding site as measured by Fluorescent Probe Exclusion.65 TTR stabilization was demonstrated by the fact that at 28 days after initiation of therapy, TTR levels rose on average by 51% in those taking 800 mg twice daily of AG10 versus placebo. The Eidos AG10 study (ATTRIBUTE-CM) is a phase 3 trial, which has planned to enroll 510 subjects with ATTR-CA in a 2:1 ratio to AG10 800 mg or placebo twice daily for 30 months (NCT03860935). The co-primary end points are change in distance walked on 6-minute walk test at 12 months (P<0.01) and all-cause mortality and frequency of cardiovascular-related hospitalizations over a 30-month period (P<0.04). The 12-month data are expected towards the end of 2021.

Tolcapone:

Tolcapone is a COMT (catechol-O-methyltransferase) inhibitor typically used for the treatment of Parkinson disease, though also observed to have TTR stabilizing properties. In vitro, it was found that tolcapone has a high affinity for the T4 binding site, displacing radiolabeled T4 from TTR with 4× the efficiency of tafamidis and stabilizing the TTR dimer-dimer interface. Furthermore, tolcapone exhibits stronger TTR aggregation inhibitory activity for both wildtype and Val122Ile than tafamidis66 demonstrating dose-dependent kinetic stabilization of the TTR tetramer. When studied ex vivo, it was shown to bind to and kinetically stabilize tetrameric TTR in human plasma from subjects with wildtype TTR and carriers of Val30Met-TTR. However, tolcapone has a short half-life and a FDA black box warning of potentially fatal, acute liver failure.

With the advent of effective therapies for CA and the potential role of combination therapy, providers may soon be in the enviable position of choosing among therapies. The cost effectiveness of emerging therapies has been questioned,67,68 especially for what may not be a rare disease (eg, ATTRwt). Unfortunately, cost of such therapies could pose a significant obstacle to adoption and increase health disparities.

Antiplasma Cell Therapy

Therapies for AL amyloidosis have evolved considerably over the past decade (Table 4). Autologous stem cell transplant (ASCT) has proven to have the best long-term outcomes, however, only a fraction of patients with cardiac involvement are eligible and with a host of new, novel highly active therapies, the treatment landscape is rapidly evolving. The Mayo group reported that ≈25% of patients are eligible for ASCT69 and only 3.4% (23 of 668 patients) of patients with overt heart failure and AL amyloidosis treated at their center over a 20-year period underwent heart transplant.70 Cardiac damage in AL amyloidosis occurs as a result of light chain toxicity, and so the goal is rapid and complete normalization of the involved light chain. Indeed, the advent of effective antiplasma cell therapies with limited toxicity has led to an evolution in the definition of a deep hematologic response from a complete response to modified, stringent, and absolute involved FLC response, the latter defined as a difference in involved and uninvolved light chain of <10 mg/L71 and an involved free light chain of <20 mg/L.72 In those ineligible for ASCT, medical therapy is pursued with the goal of attaining complete remission (CR), defined as normalization of serum κ and λ free light chains and free light chain (FLC) ratio. Those who do not achieve CR may be classified as having very good partial response, defined as a difference in FLC <40 mg/dL, partial response (decrease in difference in FLC >50%) or no response.73 Cardiac involvement of AL amyloidosis predicts a poor prognosis, which is outlined in the staging system created by the Mayo group. Patients are categorized as stage I to IV based on TnT ≥0.025 ng/mL (high sensitivity TnT ≥40 ng/L), NT-proBNP ≥1800 pg/mL and difference in FLC ≥18 mg/dL, with stage I defined as all below threshold to stage IV being all 3 elevated.37 Typically, patients with a serum NT-proBNP ≥8500 pg/mL or TnT ≥0.05 ng/mL (high sensitivity TnT ≥75 ng/mL) are considered ineligible for ASCT. Although patients may become transplant eligible over time, the benefit of ASCT compared with targeted antiplasma cell therapy is unknown. Cardiac response is defined as a decrease in NT-proBNP by >30% and 300 ng/L (if baseline NT-proBNP >650 ng/L)73 and is associated with a survival benefit in the setting of antiplasma cell therapy.

Table 4. Therapies for Light Chain Cardiac Amyloidosis

DrugMechanism of actionDoseSide effectsConcomitant therapyEfficacy dataSystemic responseOrgan response*

Melphalan76,122 (Alkeran, Evomela)	Alternation of guanine nucleotide, resulting in inter- and intrastrand DNA crosslinks, interfering with DNA replication and transcription	100–200 mg/m2 oral	Fluid retentionDizzinessGastrointestinalMyelosuppressionFever	Dexamethasone (ineligible for ASCT)	HR: 51%–76%† CR: 12%–33%†	18.7% cardiac response with dexamethasone (reduction in IVS of ≥2 mm with resolution of HF)122
Cyclophosphamide (Cytoxan)	Activity via the active metabolite, phosphoramide mustard, at the guanine N-7 position leading to cell apoptosis	100 mg/d oral or 300 mg/m2 oral weekly	AlopeciaHemorrhagic cystitisGastrointestinalMyelosuppressionInfection	Proteasome inhibitorsCD38 monoclonal antibodies	See below under bortezomib	See below under bortezomib
Bendamustine79 (Belrapzo, Bendeka, Treanda)	Induces DNA interstrand crosslinks leading to cytotoxicityInhibits several mitotic checkpoints, promotes inefficient DNA repair	100 mg/m2 IV	MyelosuppressionFatigueNausea/vomitingFever	Dexamethasone	BDex CR 11% VGPR 18% PR 28%	BDex Overall organ response 29% Cardiac response 13%
Lenalidomide83,84 (Revlimid)	IL-6 inhibitors Direct activation of caspase-8 mediated cell death Active JNK to release proapoptotic proteins into the cytosol Increase IL-2 expression‎ and IFN-γ	≈15 mg/d oral	Fluid retentionMyelosuppressionElevated LFTsThromboembolismInfectionIncreases NT-proBNPSkin rash	DexamethasoneMelphalanCyclophosphamide	HR 58%–60% (RMD or RCD)CR 23% (RMD) 11% (RCD)	Overall organ response 50% (RMD) 29% (RCD)Cardiac response 40% (RMD) 23% (RCD)
Pomalidomide86,123 (Pomalyst)	Augment NK cell-dependent cytotoxicity	2–4 mg/d oral	Fluid retentionInfectionThromboembolismSkin rashIncreases NT-proBNP	Dexamethasone	PomDex HR: 48%–68% VGPR: 18%–25%	Overall organ response 15%123 Cardiac response 15%123
Daratumumab100,102 (Darzalex)	Human IgG1κ (immunoglobulin G1 κ) monoclonal antibody against CD38 antigen which leads to cell death via multiple mechanisms including complement mediated and antibody dependent cytotoxicity and apoptosis	1.8 g SC	Infusion related reactions	DexamethasoneBortezomibCyclophosphamide (CyBorD)	HR 76% 92% (Dara-CyBorD)CR 36% 53% (Dara-CyBorD)	Cardiac response 42% with Dara-CyBorD
Corticosteroids	Bind to cytosolic glucocorticoid receptors and translocate to the nucleus where they modulate gene expression‎ resulting in anti-inflammatory and immunosuppressive activity	20–40 mg dexamethasone oral	Fluid retentionThrombosisGastrointestinal hemorrhageInfectionPsychosis	Used as part of a multidrug regimen	Poor hematologic response using high dose dexamethasone as a single agent	Unknown
Bortezomib90 (Velcade)	Proteasome inhibitor—reversible boronic acid inhibitor of the chymotrypsin-like activity to the proteasome	Variable dose and intervals, IV	Peripheral neuropathy	DexamethasoneCyclophosphamideMelphalan	CyBorD90: HR 60% CR 21%	CyBorD90: Overall cardiac response: 17% By cardiac stage‡  Stage II: 29%  Stage IIIa: 17%  Stage IIIb: 4%

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B indicates bortezomib; CR, complete response; CYP, cyclophosphamide; D, dexamethasone; HR, hematologic response rate; IFN-γ, interferon γ; JNK, c-jun terminal kinase; M, melphalan; NT-proBNP, N terminal pro brain natriuretic peptide; PR, partial response; and VGPR, very good partial response.

*

Cardiac response relies heavily on cardiac stage at diagnosis, thus direct comparison between studies is not possible.

†

Response varies with dexamethasone dose.

‡

Cardiac staging is based on Mayo 2004 criteria.91

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Alkylating Agents

Melphalan, cyclophosphamide, and bendamustine belong to a group of chemotherapeutic agents called the nitrogen mustard alkylating agents. Melphalan exerts its effects via alteration of the DNA nucleotide guanine and results in the formation of interstrand and intrastrand DNA crosslinks74 which interfere with DNA replication and transcription. The cytotoxic effects of melphalan are related to cellular concentration and duration of cellular exposure, with some studies linking higher doses of melphalan to excess toxicity in older patients and those with renal impairment. As such, a lower dose of melphalan (≈140 mg/m2) is often used in those over 65 years, and in patients with renal insufficiency, reduced cardiac function or lower performance status without a detrimental effect on long-term outcomes.75 Oral melphalan is typically combined with dexamethasone in the treatment of patients who are ineligible for ASCT or as pre-ASCT conditioning treatment. Palladini et al76 eval‎uated 259 patients with AL amyloidosis treated with oral melphalan and dexamethasone, over 50% of whom had advanced cardiac disease. In those with severe cardiac involvement, the dose of dexamethasone was reduced by 50% given the propensity for fluid retention and exacerbation of heart failure. Hematologic response rate in the full-dose group was 76% versus 51% in the attenuated steroid dose group with a complete response occurring in 31% and 12%, respectively. Median survival was also much lower in the attenuated dose group, 20 months as compared with 7.4 years in the full-dose group. Patients with severe cardiac involvement are at high risk of early death after the initiation of treatment, have a short overall median survival of <18 months and poorer response to therapy.77 Cyclophosphamide acts in a similar manner to melphalan, though exerts its effect via the active metabolite, phosphoramide mustard, at the guanine N-7 position leading to cell apoptosis.78 It is typically used in combination with other therapies such as proteasome inhibitors and CD38 monoclonal antibodies and is associated with side effects including hair loss and hemorrhagic cystitis. Bendamustine was investigated in a phase 2 trial of 31 patients with relapsed AL amyloidosis in combination with dexamethasone, in which 57% achieved a partial response or better (NCT01222260).79 The overall organ response was 29% with a median overall survival of 18.2 months. Bendamustine is considered a third line or salvage therapy.

Steroids

Glucocorticoids (prednisone and dexamethasone) bind to cytosolic glucocorticoid receptors and translocate to the nucleus where they modulate gene expression‎ resulting in anti-inflammatory and immunosuppressive activity.80 Dexamethasone treatment induces upregulation of proapoptotic genes, downregulation of antiapoptotic genes, and activation of intrinsic apoptotic pathways.80 It has a potency 6-fold that of prednisone, however, with greater efficacy comes greater toxicity, particularly in patients with advanced cardiac involvement. In a study by Dhodapkar et al,81 28% of patients experienced volume overload related to treatment with 40 mg daily of dexamethasone (480 mg per cycle). Treatment-related mortality occurred in 6 patients, 4 of whom had advanced CA, and experienced sudden death thought due to a cardiac event. Other severe adverse effects include thrombosis, gastrointestinal hemorrhage, infections, and psychosis especially with high-dose therapy, and as such a low-dose strategy is often employed in older patients and those with heart failure.76 In the recent era, due to their synergistic effect with immunomodulatory agents and proteasome inhibitors, glucocorticoids are used as part of a multidrug regimen to improve response rates and limit toxicity (see below).

Immunomodulatory Agents

Originally marketed as a tranquilizer and antiemetic, thalidomide and its derivatives (lenalidomide and pomalidomide) have antiproliferative, antiangiogenic and immunomodulatory effects. Their antiangiogenic and antiproliferative properties are mediated via inhibition of IL-6 (interleukin 6) expression‎, a growth factor for the proliferation of myeloma cells. They activate apoptotic pathways through direct activation of caspase 8 mediated cell death. Within the mitochondria, they activate c-JNK (jun terminal kinase) which through a series of events result in the release of pro-apoptotic proteins into the cytosol.82 In addition, by activating T cells, they increase expression‎ of IL-2 and IFN-γ (interferon γ) genes, augmenting natural killer (NK) cell-dependent cytotoxicity.82 Lenalidomide has a greater potency than thalidomide, being 50 to 2000× more potent at stimulating T-cell proliferation and 50 to 100× more potent in augmenting IL-2 and IFN-γ production.82 Dexamethasone activates caspase 9, a proapoptotic molecule and is associated with the release of Smac (second mitochondrial-derived activator of caspase) from the mitochondria into the cytosol, a regulator of the activity of molecules that affect apoptosis.82 As such, it augments the antiproliferative effects of lenalidomide. However, notable side-effects include neutropenia, thrombocytopenia, elevations in liver function tests, and thromboembolism and lenalidomide tends to raise NT-proBNP in AL amyloidosis patients.83 With the addition of melphalan to lenalidomide and dexamethasone, overall hematologic response rates of ≈58% are observed,84 with a higher dose of lenalidomide associated with a higher rate of CR, though limited by tolerability at doses >15 mg.84 The addition of cyclophosphamide in place of melphalan shows similar overall hematologic response rates (≈60%), though again has a high burden of toxicity and poor outcomes in those with advanced cardiac disease.85 Overall hematologic response rates with pomalidomide/dexamethasone are similar to those achieved with lenalidomide (40%–60%).85,86 Unfortunately, in those with advanced CA at the time of treatment initiation, outcomes of these treatment combinations remain poor.

Proteasome Inhibitors

The ubiquitin-proteasome system functions as a pathway for intracellular regulation of protein degradation, thus playing a role in maintaining protein homeostasis.87 The ubiquitin-proteasome system involves a series of enzymes which tag the protein with a polyubiquitin chain. The 26S proteasome comprises a 20S core flanked by two 19S caps. The 20S core contains subunits which have proteolytic activity, including caspase-like, trypsin-like, and chymotrypsin-like activity. The proteasome recognizes and binds the tagged protein and subsequently hydrolyzes it into short polypeptides in the 20S core. Proteasome inhibitors bind to the proteasome binding pocket, thus rendering it inactive87 which results in a multitude of downstream events, including accumulation of ubiquitin tagged proteins, inhibition of NF-κB signaling, downregulation of growth factor receptors, suppression of adhesion molecule expression‎, and inhibition of angiogenesis, all of which lead to apoptosis.88 Bortezomib (Velcade) is a first-generation reversible boronic acid inhibitor of the chymotrypsin-like activity to the proteasome. Bortezomib, in conjunction with cyclophosphamide and dexamethasone (CyBorD), has been shown to induce a rapid reduction in light chains in patients with AL amyloidosis and appears to be relatively well tolerated in those with cardiac involvement, however, can cause significant peripheral neuropathic side effects. Furthermore, those with translocation t(11;14) have inferior hematologic response to bortezomib.89 A study of 230 patients from the National Amyloidosis Center (London, England) and the Amyloidosis Research and Treatment Center (Pavia, Italy) of newly diagnosed patients with AL amyloidosis reported an overall hematologic response rate of 60% with a cardiac response of 17%.90 Advanced cardiac stage IIIb patients (Mayo 2004 criteria91; NT-proBNP >8500 ng/L) had a lower overall response of 42% and poorer survival (median survival 7 months), than those with stage II or IIIa disease (overall hematologic response 64% and 69%, respectively), though still showed a survival benefit in those who achieved a hematologic response compared with those who did not (median survival 26 months versus 6 months, respectively).90 In a retrospective study of bortezomib/melphalan/dexamethasone versus melphalan/dexamethasone, the former induced higher hematologic response (69% versus 51%) and CR (42% versus 19%) the difference in response between groups being most notable in those who could not tolerate full-dose dexamethasone.92 The addition of bortezomib in patients with either or both NYHA class III or IV HF and NT-proBNP >8500 ng/L did not improve survival. Second generation proteasome inhibitors include carfilzomib, ixazomib, marizomib, and oprozomib. Carfilzomib (Kyprolis) is an irreversible tetrapeptide epoxyketone which has greater inhibitory activity than bortezomib and has shown activity in patients resistant to bortezomib.93 In a phase 1 dose-escalation study of carfilzomib in patients with previously treated systemic AL amyloidosis, however, cardiac events were common.42 Three of 12 patients had a cardiac event: 1 with cardiac arrest due to ventricular tachycardia, 1 developed a restrictive cardiomyopathy (amyloid negative on biopsy) and 1 had a decline in LV function, all possibly related to carfilzomib therapy and highlighting the need for close cardiac monitoring if this therapy is used. However, in those for whom bortezomib is contraindicated due to peripheral neuropathy, carfilzomib may be effective in appropriately selected candidates without severe cardiac involvement.94 Ixazomib (Ninlaro) is a reversible agent which is hydrolyzed to its active form in aqueous solution and has comparable inhibitory activity to bortezomib.93 It binds to the proteasome β-5 site to inhibit the chymotrypsin-like activity. In 27 patients with relapsed/refractory AL amyloidosis ixazomib showed encouraging hematologic (52%) and organ (56%) response rates with a 45% cardiac response rate (5 of 11 patients).95 It is being investigated in a phase 1/2 study to assess safety and hematologic response rate in combination with CyBorD (NCT03236792). Patients with NYHA class III or IV HF or NT-proBNP >8500 ng/L are excluded. TOURMALINE-AL1 is a phase 3 trial investigating the use of ixazomib with dexamethasone versus physician’s choice (NCT01659658), results of which showed no significant difference in hematologic overall response rate.96 Nevertheless, CR was more frequent with ixazomib (26% versus 18%), and treatment with ixazomib was associated with a significantly longer progression-free survival. Furthermore, the patients who received ixazomib had a higher rate of cardiac and renal responses.

Daratumumab

CD38 is a glycoprotein expressed on a variety of cell types, including normal myeloid and lymphoid cells, but is also highly overexpressed on neoplastic monoclonal plasma cells.97 CD38 is involved in cell signaling and regulation of cytoplasmic calcium flux playing a role in activation, survival, and growth of lymphoid and myeloid cells. Daratumumab (Darzalex) is a human immunoglobulin G1 κ (IgG1κ) monoclonal antibody against CD38 antigen which leads to cell death via multiple mechanisms including complement mediated and antibody dependent cytotoxicity and apoptosis.98 It has been shown to be highly effective in and is approved for the treatment of multiple myeloma,99 which led to investigation of its use in AL amyloidosis. It has been shown to be effective in the treatment of patients with relapse or progression of AL amyloidosis, resulting in a 76% hematologic response rate and 36% CR rate with a median time to response of 1 month.100 The speed with which daratumumab can achieve normalization of light chain levels was highlighted in a report by Hossein et al.101 Two patients with advanced Mayo cardiac stage (stage III and IV) were treated with daratumumab monotherapy and achieved a normal light chain level within one cycle of therapy. ANDROMEDA investigated the safety and efficacy of daratumumab plus CyBorD compared with CyBorD alone in patients with new diagnosed AL amyloidosis (NCT03201965). ANDROMEDA enrolled 388 patients, randomized to CyBorD alone or with daratumumab (1:1). More than one-half of patients assigned to daratumumab achieved a complete hematologic response (53%) compared with only 18% of patients assigned to CyBorD (odds ratio, 5.1; P<0.0001). The 6-month cardiac response rate was 42% for the daratumumab arm compared with 22% for CyBorD alone (P=0.0029).102 In comparison, CR rates in patients receiving CyBorD,90 melphalan with dexamethasone76 or high-dose melphalan with ASCT103 have been reported as 23%, 19%, and up to 48%, respectively. Daratumumab was approved by the FDA in combination with CyBorD for newly diagnosed light chain amyloidosis. Isatuximab acts by inducing internalization of CD38.104 A phase 2 study of isatuximab in patients with relapsed or refractory AL amyloidosis with organ involvement is underway (NCT03499808), however, those with NYHA class IV symptoms or EF <35% are excluded from participation. Elotuzumab is an IgG1κ monoclonal antibody against SLAMF7 (signaling lymphocytic activation molecule F7) which is used in the treatment of multiple myeloma combined with lenalidomide and dexamethasone. The mechanism of action in multiple myeloma cells appears to be antibody dependent, cell-mediated cytotoxicity through recruitment, and activation of NK cells.105 Elotuzumab mediated cell death requires the presence of NK cells since binding to SLAMF7 marks the myeloma cells for recognition by NK cells and also direct binding to SLAMF7 on NK cells themselves causes direct activation and enhanced cytotoxicity.106 Elotuzumab in conjunction with lenalidomide and dexamethasone was successful in inducing hematologic and renal response in a patient with relapsed/refractory multiple myeloma with AL amyloidosis despite 2 stem cell transplants and numerous combinations of chemotherapeutic agents.107 It is currently being studied in a phase 2 trial in conjunction with lenalidomide and dexamethasone with or without cyclophosphamide in patients with relapsed AL amyloidosis (NCT03252600).

Venetoclax

Translocation t(11;14) is the most common cytogenic aberration in AL amyloidosis occurring in up to 60% of patients and confers a poor response to bortezomib.89 Patients with t(11:14) overexpress BCL-2 (B-cell lymphoma 2), a protein involved in programmed cell death regulation.108 BCL-2 mediates suppression of the proapoptotic pathway molecules BAX (Bcl-2-associated X-protein) and BAK (Bcl-2 homologous antagonist/killer).108 Venetoclax, an orally bioavailable agent, is an inhibitor of BCL-2 and thus promotes cell death. It has been shown to be effective in patients with multiple myeloma with translocation t(11;14).109 A phase 1 study (NCT03000660) investigating venetoclax and dexamethasone in relapsed/refractory AL amyloidosis was stopped given the FDA concerns that emerged from the BELLINI clinical trial (NCT02755597, Study M14-031) eval‎uating venetoclax with bortezomib in patients with multiple myeloma in which there was an increased risk of death for patients receiving venetoclax as compared with the control group. However, after subgroup analysis, the BELLINI study has reopened for patients with t(11;14). Similarly, it is expected that the study of venetoclax in AL amyloidosis will reopen for those with t(11;14).

Amyloid Degradation/Extraction

Doxycycline (Table 3) is a tetracycline antibiotic which binds to the bacterial ribosome and inhibits protein synthesis. However, it is the ability to inhibit MMP (matrix metalloproteinase) which is thought to be the basis of its antiamyloidogenic activity.110 Levels of MMP are elevated in AL accompanied with marked diastolic dysfunction when compared with little or no elevation in TTR amyloidosis.110 This led to a phase 2 study of the safety and efficacy of doxycycline in combination with CyBorD for the treatment of AL amyloidosis. D’Souza et al111 reported a low 1-year mortality of 20% and a high stem cell transplant utilization rate of 60%, in this single-arm study.

The use of doxycycline in combination with ursodeoxycholic acid (ursodiol) for the treatment of ATTR-CA was studied in 53 patients.112 Ursodiol is a bile acid sequestrant which has antiamyloid fibril effects as has its taurine conjugate, tauroursodeoxycholic acid. Both have synergistic activity with doxycycline to reduce amyloid fibril burden. Karlstedt et al112 reported an 11% adverse event rate, mainly due to dermatologic and gastrointestinal side effects, and equivocal outcomes with no obvious benefit observed. Overall, it remains unclear as to the place doxycycline and ursodiol have in the treatment of either AL- or ATTR-CA.

NI006 is a recombinant human monoclonal IgG1 antibody that exclusively targets with high affinity the forms of TTR that are disease-associated with amyloid conformation but not physiological forms of TTR. NI006 targets both wild-type TTR as well as TTR variants and induces the clearance of pathological TTR in preclinical models. Currently, NI006 is in phase 1 clinical development in ATTR cardiomyopathy patients (NCT04360434).

CAEL-101 (11-1F4) is an IgG1 monoclonal antibody that binds to κ and λ light chain amyloid fibrils, accumulating in amyloid-laden organs113 leading to elimination of the amyloid protein. In the 1-year follow-up of the phase 1a/1b study, 67% (12 of 18) of renal and cardiac-eval‎uable patients demonstrated organ response.43 Currently, CAEL-101 is being eval‎uated in patients with Mayo stage IIIa (NCT04512235) and IIIb cardiac AL amyloidosis (NCT04504825).

SAP is a glycoprotein which binds avidly to all types of amyloid fibrils. As such, targeting this protein may provide a pathway to extraction of amyloid deposits from affected organs. Bodin et al114 showed that administration of anti-human-SAP antibodies to mice with amyloid deposits containing human SAP activated macrophage mediated phagocytosis of the SAP containing amyloid deposits. In a phase 1 trial, a small-molecule drug, (R)-1-[6-[(R)-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl] pyrrolidine-2-carboxylic acid, was first administered to 15 patients with systemic amyloidosis to reduce circulating SAP, followed by a fully-humanized monoclonal IgG1 anti-SAP antibody.41 Patients with cardiac involvement were excluded from the study. At 42 days following treatment, there was a decrease in amyloid organ burden as indicated by decreased hepatic stiffness and reduction in hepatic amyloid burden on SAP scintigraphy. A phase 2 study failed to show any improvement in cardiac amyloid burden after treatment with anti-SAP antibody (dezamizumab).

NEOD-001 (Prothena) is a humanized form of murine monoclonal antibody 2A4, which binds to an epitope on the misfolded light chain protein though does not bind in the protein’s native conformation. Therefore, it was posited that NEOD-001 could clear AL amyloid deposits in affected organs. Exploratory end points of a phase 1/2 study showed that 8 of 14 patients (57%) with cardiac involvement responded to therapy while the remaining 6 patients showed stable disease.115 A phase 2b study (NCT02632786) in previously treated patients with persistent cardiac dysfunction did not meet its primary or secondary end points, and the phase 3 study in treatment naive patients was discontinued due to futility. However, a signal of efficacy in advanced Mayo Stage IV was found prompting a confirm‎atory trial.

PRX004 (Prothena) is a monoclonal antibody which selectively binds to non-native misfolded TTR but not native TTR. PRX004 is being studied in patients with hereditary ATTR amyloidosis in a phase 1 study (NCT03336580).

Emerging Therapeutics

Anti-seeding (Table 3) refers to inhibition of the aggregation of native TTR onto preformed amyloid fibrils. This may be particularly beneficial in the setting of single organ liver transplantation for ATTRv, after which ongoing cardiac deposition can occur. Preformed amyloid fibrils in the heart can act as a template for further seeding of native TTR. TabFH2 is a peptide inhibitor which binds to the amyloid driving F- and H-stands of fragmented fibrils, thereby impeding self-recognition and seeding.45

The CD38-targeting antibody-drug conjugate STI-6129/CD38-077, comprises a fully human anti-CD38 antibody conjugated to a microtubule inhibitor (duostatin 5.2).116 This antibody-drug conjugate binds avidly to CD38 positive tumor cells, after which it is internalized to exert its cytotoxic effect. STI-6129 is being studied in patient with relapsed or refractory systemic AL amyloidosis in a phase 1 study (NCT04316442).

Advanced Therapies

Heart transplantation may be considered in select patients with end-stage cardiomyopathy secondary to ATTR-CA or AL-CA who have responded to light-chain depleting therapy. Heart transplant candidates should be carefully selected with particular attention being paid to extracardiac involvement such as neuropathic, gastrointestinal, or hepatic manifestations of disease, which could affect posttransplant outcomes.117 Patients with ATTRwt- or ATTRv-CA with the Val122Ile mutation generally require only single organ heart transplant, however, other variants, such as Thr60Ala may need consideration for dual heart/liver transplant. Recent single-center studies have shown that outcomes of appropriately selected patients with either AL- or ATTR-CA are similar to that of nonamyloid cardiomyopathy patients.118,119 Furthermore, the new heart transplantation allocation policy120 provides a pathway for listing these patients with a restrictive cardiomyopathy, who often do not meet the hemodynamic criteria otherwise required for higher priority status. This has led to a decrease in waitlist time/delisting due to clinical deterioration and an increase in the number of patients being transplanted with amyloid cardiomyopathy121 without any change in short-term outcomes. Going forward, it is unclear where heart transplantation will fit in the treatment paradigm, as earlier detection of amyloid cardiomyopathies and novel medical therapies continue to change the landscape of this disease.

Summary and Conclusions

Basic science investigations in the last few decades have led to the elucidation of mechanisms underlying amyloidogenesis and have resulted in the development of effective therapies in multiple classes of compounds. The cardiologist caring for affected patients is in the enviable position of choosing from these therapies that can meaningfully improve the lives of affected patients especially when administered before significant cardiac dysfunction has ensured (Figure 4). The therapeutic landscape in this arena is rapidly evolving and additional breakthroughs are anticipated in the coming years.

Footnote

Nonstandard Abbreviations and Acronyms

ALimmunoglobulin light chain

ASCTautologous stem cell transplant

ASGPRasialoglycoprotein receptor

ASOantisense oligonucleotide

ATTRvvariant transthyretin amyloidosis

ATTRwtwild-type transthyretin amyloidosis

BCL-2B-cell lymphoma 2 receptor

BNPbrain natriuretic peptide

CAcardiac amyloidosis

CRcomplete response

CRISPR/Cas9clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9

CyBorDcyclophosphamide, bortezomib, dexamethasone

FAPfamilial amyloid polyneuropathy

FLCfree light chain

GalNAcN-acetyl galactosamine

IFN-γinterferon γ

IgG1κimmunoglobulin G1 κ

IL-6interleukin 6

MAPKmitogen-activated protein kinase

MMPmatrix metalloproteinase

NKnatural killer

NT-proBNPN terminal pro-BNP

NYHANew York Heart Association

SAPserum amyloid P-component

siRNAsmall interfering RNA

SLAMF7signaling lymphocytic activation molecule F7

Smacsecond mitochondrial-derived activator of caspase

SSASerum amyloid associated protein

TTRtransthyretin

References

1.

Ruberg FL, Grogan M, Hanna M, Kelly JW, Maurer MS. Transthyretin amyloid cardiomyopathy: JACC State-of-the-art review. J Am Coll Cardiol. 2019;73:2872–2891. doi: 10.1016/j.jacc.2019.04.003

Go to Citation

Crossref

PubMed

Google Scholar

2.

Maurer MS, Mann DL. The tafamidis drug development program: a translational triumph. JACC Basic Transl Sci. 2018;3:871–873. doi: 10.1016/j.jacbts.2018.10.001

Go to Citation

Crossref

PubMed

Google Scholar

3.

Maurer MS, Elliott P, Comenzo R, Semigran M, Rapezzi C. Addressing common questions encountered in the diagnosis and management of cardiac amyloidosis. Circulation. 2017;135:1357–1377. doi: 10.1161/CIRCULATIONAHA.116.024438

Go to Citation

Crossref

PubMed

Google Scholar

4.

Manral P, Reixach N. Amyloidogenic and non-amyloidogenic transthyretin variants interact differently with human cardiomyocytes: insights into early events of non-fibrillar tissue damage. Biosci Rep. 2015;35:e00172.