Transthyretin Kinetic Stabilizers for ATTR Amyloidosis: A Narrative Review of Mechanisms and Therapeutic Benefits

Abstract

Transthyretin amyloidosis (ATTR amyloidosis) is a systemic disease affecting multiple organ systems, particularly the heart and peripheral nervous system. Decades of research suggest the disease is caused by the dissociation, misfolding, and aggregation of transthyretin (TTR), resulting in extracellular deposition of amyloid fibrils in tissue and organs. If untreated, ATTR amyloidosis leads to substantial functional impairment, quality-of-life burden, and mortality. Because dissociation of the TTR tetramer is rate-limiting for aggregation and amyloid fibril formation, small molecules that bind to and stabilize the natively folded tetramer of TTR have been developed. Subunit exchange experiments demonstrated that tafamidis and acoramidis effectively slow TTR tetramer dissociation and aggregation in plasma at concentrations achieved with approved oral doses in patients with ATTR amyloidosis. In randomized controlled clinical trials, these TTR kinetic stabilizers have significantly reduced cardiomyopathy progression and improved quality of life in patients with variant or wild-type disease (tafamidis is also approved to slow polyneuropathy progression). Current availability of two kinetic stabilizers has increased interest in their pharmacological properties and clinical effects, including potential similarities and disparities. In this review, the mechanisms involved in TTR kinetic stabilization are summarized with preclinical and clinical study findings on the use of the kinetic stabilizers tafamidis and acoramidis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40119-025-00423-7.

Key Summary Points

Transthyretin amyloidosis is a systemic, progressive disease caused by the dissociation, misfolding, and aggregation of transthyretin (TTR), resulting in extracellular deposition of amyloid fibrils in tissue and organs.

The TTR kinetic stabilizers tafamidis and acoramidis are small molecules that bind to the unoccupied thyroid hormone binding sites of native tetrameric TTR, slowing dissociation and preventing subsequent aggregation.

In randomized controlled clinical trials, tafamidis and acoramidis have significantly reduced cardiomyopathy progression and improved quality of life in patients with transthyretin amyloid cardiomyopathy.

Tafamidis has accumulated long-term clinical trial and real-world evidence supporting its safety, tolerability, and effectiveness; long-term data on acoramidis are still emerging from ongoing clinical trials and real-world use.

Studies employing a subunit exchange assay have not found significant differences in biochemical binding and potency between tafamidis and acoramidis at the plasma concentrations achieved by their standard dosages, despite claims that acoramidis is a more potent TTR kinetic stabilizer.

Introduction

Transthyretin amyloidosis (ATTR amyloidosis) is a systemic, progressive disorder caused by the dissociation, misfolding, and aggregation of the transthyretin (TTR) protein, resulting in the formation of circulating soluble aggregates and the deposition of amyloid fibrils in the extracellular matrix of tissue and organs [1, 2]. The process of amyloid fibril formation seems to be dependent on the slow dissociation of the TTR tetramer into metastable dimers that dissociate into monomers, which can rapidly misfold and self-assemble into amyloid fibrils [3–7]. In ATTR amyloidosis, the formation of soluble aggregates and fibrils together appear to cause significant damage to organs (especially the heart) and the peripheral, autonomic, and central nervous systems [5, 6].

TTR aggregation leading to amyloidosis occurs due to either single point mutations in one allele encoding TTR or aging [8]. Variant ATTR amyloidosis (ATTRv amyloidosis) is typically inherited in an autosomal dominant manner [9]. Thus, almost all patients with ATTRv amyloidosis are heterozygotes, and disease pathogenesis is the result of mutant TTR subunits being incorporated into tetramers otherwise composed of wild-type TTR subunits, hastening dissociation and increasing the risk of TTR aggregation. An estimated 5,000 to 40,000 people globally have ATTRv amyloidosis [10]. The aggregation of different TTR sequences can produce distinct disease phenotypes and clinical manifestations [8]. ATTRv amyloidosis can present as a principal axonal polyneuropathy, often with accompanying autonomic nervous system dysfunction and commonly with deterioration of other organ systems, such as the heart and gastrointestinal tract, upon progression. Patients with ATTRv amyloidosis can also present with principal cardiomyopathy with dysautonomia [11, 12]. Regardless of the mutation, multisystem involvement is common in patients with ATTRv amyloidosis [13].

Aging-associated wild-type ATTR amyloidosis (ATTRwt amyloidosis) is the most common type of ATTR amyloidosis and is caused by sporadic wild-type TTR aggregation (no TTR mutations in ATTRwt amyloidosis) leading to cardiomyopathy, perhaps owing to an aging-associated decline in catabolism [8, 14, 15]. Amyloid fibrils are deposited in the interstitial space of the myocardium in individuals with ATTRwt cardiomyopathy (ATTRwt-CM), often leading to increased wall thickness, diastolic dysfunction, cardiac arrhythmias, and heart failure [16]. ATTRwt amyloidosis most often presents as cardiomyopathy and heart failure, but signs and symptoms suggesting a mixed phenotype, such as sensorimotor and/or autonomic neuropathy and musculoskeletal dysfunction, may accompany cardiovascular manifestations [12, 13, 17–19]. Findings from the Transthyretin Amyloidosis Outcomes Survey (THAOS) showed that approximately one-third of patients with symptomatic ATTRv cardiomyopathy (ATTRv-CM) or ATTRwt-CM had a mixed phenotype of both cardiac and neurologic symptoms, reinforcing the phenotypic heterogeneity of this disease [11].

Disease progression in patients with ATTR amyloidosis inevitably leads to substantial loss of function, impaired quality of life (QOL), and death [13, 20–22]. Median overall survival rates range from 2.5 to 6 years in untreated patients with transthyretin amyloid cardiomyopathy (ATTR-CM) and depend on the disease type and particular TTR variant [20, 21]. More recently, real-world data from patients with ATTR-CM from THAOS revealed survival rates of 70.0% and 59.3% in untreated patients at 30 and 42 months, respectively [23].

Kinetic stabilizers are small molecules that bind to the unoccupied thyroid hormone binding sites of native tetrameric TTR in a way that slows down dissociation, preventing subsequent aggregation, including amyloid fibril formation [24]. The TTR kinetic stabilizers tafamidis and acoramidis have been examined in randomized controlled clinical trials, with favorable outcomes, strongly supporting the hypothesis that TTR aggregation drives the pathology of ATTR amyloidosis [25, 26]. Diflunisal is a nonsteroidal anti-inflammatory drug (NSAID) and repurposed TTR kinetic stabilizer that has also been explored in clinical trials for the treatment of ATTR amyloidosis [27]. Although diflunisal has not received regulatory approval for this disease, limiting it to off-label use, it will be covered briefly in this review.

The similarities and disparities observed among TTR kinetic stabilizers have become an important topic of discussion, as our understanding of the amyloid cascade and their mechanisms of action and clinical effects has grown. In this review, the basic biology of the TTR protein is examined, along with the critical steps involved in TTR kinetic stabilization. Additionally, evidence from preclinical and clinical investigations of the safety and efficacy of TTR kinetic stabilizers is summarized. Although the efficacy and safety of these agents have been established in clinical trials, it is important to note that methodological differences in these trials preclude meaningful comparisons between outcomes. Furthermore, as described in detail below, studies quantifying the extent of subunit exchange employing a subunit exchange assay, the gold standard for evaluating the efficacy of inhibiting TTR dissociation, have not found significant differences in biochemical binding and potency between tafamidis and acoramidis at the plasma concentrations achieved by their standard dosages [28], despite claims that acoramidis is a more potent TTR kinetic stabilizer [26].

Although this review focuses on TTR kinetic stabilizers, a distinct category of agents has also received regulatory agency approval for the treatment of ATTRv amyloidosis manifesting in polyneuropathy [29]. The TTR messenger RNA (mRNA) degraders patisiran [30], inotersen [31], vutrisiran (also approved for ATTR-CM) [32], and eplontersen [33] decrease plasma TTR concentrations by > 75%, slowing concentration-dependent TTR aggregation. The clinical benefits of TTR mRNA degraders in ATTR amyloidosis further support the hypothesis that aggregation of newly biosynthesized TTR is the main driver of postmitotic tissue degeneration in ATTR amyloidosis. Enhanced efficacy via combined treatment with a TTR kinetic stabilizer and a TTR mRNA degrader is being explored in clinical trials.

This article is based on previously conducted studies and does not contain any new studies with human participants performed by any of the authors.

TTR Biology

Native TTR is a 55-kDa tetramer, made up of four 127-residue β-sheet-rich subunits that circulate in the bloodstream at a concentration of approximately 3–5 µM (Fig. 1) [24, 34–39]. The liver secretes more than 90% of TTR into the bloodstream [40], whereas the choroid plexus and retinal pigment epithelial cells secrete most of the TTR into the cerebrospinal fluid and vitreous fluid of the eye, respectively [41–43]. Most individuals have two wild-type alleles, which means they have wild-type TTR homotetramers circulating in their blood, cerebrospinal fluid, and vitreous fluid. As noted above, nearly all patients with ATTRv amyloidosis are heterozygotes. Thus, they have three TTR heterotetramers composed of mutant and wild-type subunits with distinct stoichiometries circulating and two homotetramers composed entirely of mutant subunits or entirely wild-type subunits in their circulation (Fig. 1) [6, 44].

Fig. 1.

Structure of TTR. A Native TTR is a 55-kDa tetramer, made up of four 127-residue β-sheet-rich subunits, B Patients with ATTRv amyloidosis have three TTR heterotetramers composed of mutant and wild-type subunits with distinct stoichiometries and two homotetramers composed entirely of mutant subunits or entirely wild-type subunits. ATTRv amyloidosis, variant transthyretin amyloidosis; TTR transthyretin.

Reproduced from Powers and Kelly (2021) with permission from Elsevier Science & Technology Journals

The primary function of TTR is to transport holo-retinol-binding protein, but it also serves as a back-up carrier of thyroid hormone in the blood [37, 45–47]. Because albumin and thyroid binding globulin transport nearly all the thyroid hormone in the blood, > 99% of the thyroxine (T₄) binding sites in TTR are unoccupied, thus allowing kinetic stabilizers such as tafamidis and acoramidis to bind [38, 48, 49].

Mechanism of Kinetic Stabilization

The discovery of TTR tetramer dissociation as the rate-limiting step for TTR aggregation supported the development of a class of therapies known as kinetic stabilizers for the treatment of ATTR amyloidosis [4–7, 44, 50–60]. Tafamidis and acoramidis bind selectively and with high affinity to the native tetrameric state of TTR to stabilize it and prevent the tetramer dissociation that leads to formation of aggregates, including amyloid fibrils, that cause ATTR amyloidosis (Fig. 2) [51, 61]. Although TTR has two small-molecule binding sites, only one site must be occupied to effectively prevent tetramer dissociation since dissociation becomes sufficiently slow relative to TTR turnover, principally by the liver [53, 62]. Therefore, small-molecule binding to the native state increases the kinetic stability of TTR and thereby reduces populations of non-native or misfolded states [54, 63].

Fig. 2.

Summary of the pathobiology of the TTR aggregation cascade and impact of kinetic stabilizer therapy. ATTR transthyretin amyloidosis, TTR transthyretin

The degree of pharmacological kinetic stabilization attained depends on the fraction of TTR tetramers that have at least one of their binding sites occupied by a kinetic stabilizer (binding to the second site, as noted above, provides no further benefit). This fraction in turn depends on the affinity of the kinetic stabilizer for TTR; the affinity of the kinetic stabilizer for albumin (TTR’s main competitor for ligand binding); and the plasma concentrations of kinetic stabilizer, TTR, and albumin [50, 64]. It does not depend on the relative contributions of enthalpy and entropy to the affinities, nor does it depend on the precise mode of binding, as long as the binding can only occur in the native tetramer (and not to dissociated monomers or dimers). In other words, for kinetic stabilization, binding is binding. It does not matter if the binding is enthalpically driven or entropically driven, if it “mimics” the structural effects of known TTR stabilizing mutations, or if it stabilizes TTR in ways that cannot be duplicated by mutations.

The interrelationships among the factors that dictate TTR binding site occupancy, as well as other pharmacological properties, are essential determinants of the stabilization potency of individual kinetic stabilizers. Notably, the benefit of a kinetic stabilizer’s high affinity for TTR may be offset by its low plasma concentration or other pharmacological properties. For example, the relatively short half-life of acoramidis (effective half-life, ~6 h [65]; terminal half-life, 25 h [28]) as well as its modest oral bioavailability and substantial metabolism may diminish the impact of the agent’s kinetic stabilizing potency. Increases or decreases in TTR plasma concentration, achieved by fluctuations in the oral dose, have a direct impact on the level of kinetic stabilization observed [28].

TTR Kinetic Stabilization Testing Methods

The assays used to determine the extent to which small-molecule binding stabilizes proteins have often measured the resistance to denaturing perturbations as the primary output (Supplementary Material Table 1). For example, the increases in TTR resistance to acid- or urea-mediated denaturation in the presence of a kinetic stabilizer have been widely used to evaluate kinetic stabilizers [7, 51, 55]. The disadvantage of these methods is that the perturbation changes ligand affinity as well as TTR stability and thus may not reflect the extent of stabilization under physiological conditions [66]. A method that avoids this problem is fluorescence probe exclusion (FPE), in which the rate of binding of a probe that becomes fluorescent in the thyroxine-binding site of TTR is measured in the presence and absence of a kinetic stabilizer. Decreases in the probe’s rate of binding indicate that the probe is occluded from the binding pocket by the kinetic stabilizer. This assay is performed under physiological conditions. However, TTR has two binding sites, and the binding to the second site is usually weaker than binding to the first site. Thus, it is the second site from which the kinetic stabilizer is displaced in this assay and, as noted above, binding to this second site is essentially irrelevant to kinetic stabilizer activity [61].

In contrast to the above-mentioned methods, TTR subunit exchange directly measures binding-induced tetramer stabilization by kinetic stabilizers [50, 53]. Using this method, TTR homotetramers with and without tags are mixed in the presence and absence of a kinetic stabilizer. The rate at which the subunits exchange between the two homotetramers to generate heterotetramers is essentially the same as the tetramer dissociation rate. Because the subunit exchange assay can be performed in TTR’s native environment (blood plasma) and tetramer dissociation is the rate-limiting step for TTR amyloidogenesis, this assay is the gold standard for evaluating the efficacy of a TTR kinetic stabilizer in inhibiting TTR dissociation.

Finally, it has been widely observed that TTR plasma concentrations increase upon initiation of kinetic stabilizer therapy, and it has been suggested that the magnitude of this increase can be used as a surrogate measure for kinetic stabilizer efficacy. This increase is likely due to a combination of improved secretion efficiency of TTR from hepatocytes in the presence of stabilizers (although this effect may be small for wild-type TTR and variants that are only mildly destabilizing [67]), decreases in the rate of TTR turnover by lysosomal degradation, and decreases in the extent of TTR misfolding in the blood plasma [68, 69]. The last of these processes is likely the most relevant to TTR disease pathogenesis, whereas the other two are of unclear significance. Thus, the value of the TTR plasma concentration as a biomarker for therapeutic efficacy is uncertain and may even differ for different stabilizers and different TTR genotypes. Indeed, no association was found between increases in TTR plasma concentration and response to kinetic stabilizer therapy in a cohort of Portuguese patients with transthyretin amyloid polyneuropathy (ATTR-PN) [69]. Moreover, higher TTR concentrations may increase the difficulty of achieving complete binding, especially if the stabilizer is not in substantial excess relative to TTR. An “ideal” kinetic stabilizer would stabilize TTR to misfolding in blood plasma without changing its turnover rate and thereby increasing the TTR concentration.

Preclinical TTR Kinetic Stabilizer Investigations

The TTR kinetic stabilizing properties of tafamidis and acoramidis have been demonstrated using various assays. With the “gold standard” subunit exchange assay, investigators observed that tafamidis and acoramidis performed comparably at drug plasma concentrations achieved at therapeutic doses [28].

Tafamidis (2-(3,5-dichlorophenyl)-1,3-benzoxazole-6-carboxylic acid) is a kinetic TTR stabilizer with a proven safety profile established over its 14-year commercial history. Tafamidis is highly orally bioavailable [70], such that a lower drug dose is needed to achieve an effective drug concentration, with less risk of unanticipated toxicity [71], and exhibits a long terminal half-life (~49 h [72]). The potency of tafamidis in preventing TTR subunit exchange—a direct measure of its ability to prevent tetramer dissociation—has been demonstrated in human plasma [28, 64, 70]. The latter research showed that at the plasma concentrations typically achieved by the 80-mg, once-daily dose of tafamidis (~20–30 μM), its binding to TTR is sufficiently strong and selective to reduce the rate of tetramer dissociation by about 96%. Most recently, Tess et al. demonstrated through in vitro and in vivo studies that tafamidis achieves near-maximal TTR stabilization through single-site binding at its therapeutic dose, indicating a high affinity for TTR and effective inhibition of tetramer dissociation, reducing unbound TTR by approximately 92%, and resulting in a similar reduction in markers of disease progression [73]. Additionally, experiments quantifying plasma oligomer concentrations using peptide probes also showed that tafamidis meglumine 20 mg once daily slowed tetramer dissociation and TTR aggregation in plasma in patients with ATTRv polyneuropathy (ATTRv-PN) [74]. The combination of these findings underscores that tafamidis, at its therapeutic dose, achieves a high degree of TTR stabilization.

Acoramidis (AG10) is a TTR kinetic stabilizer originally synthesized as an analog of a compound found to be one of the best hits from a high-throughput screen for TTR ligands [61, 63]. In preclinical testing, acoramidis demonstrated effective kinetic stabilization of native tetrameric TTR in vitro, selective binding to wild-type TTR in human serum, and effective stabilization of variant and wild-type TTR in human serum [61]. Subunit exchange experiments conducted in human plasma demonstrated that the typical plasma concentrations achieved with oral dosing of acoramidis hydrochloride (HCl) of 800 mg twice daily (~10–15 μM) lowered the TTR tetramer dissociation rate to approximately 4% of normal [28]. In a phase 1 study of acoramidis in healthy volunteers, stabilization in wild-type and variant TTR > 90% was observed with this kinetic stabilizer at steady state of the highest administered dose (acoramidis HCl 800 mg twice daily) using an FPE assay that indirectly measures TTR binding site occupancy [75]. In a phase 2 study in patients with ATTR-CM receiving acoramidis HCl 400 mg or 800 mg twice daily or placebo, in vitro TTR stabilization again measured by FPE assay, was > 90% at peak and trough concentrations, with a > 50% increase in serum TTR levels after 28 days of treatment with the 800-mg twice-daily dose compared with baseline [76] (Supplementary Material Table 2). In an open-label extension study wherein patients could continue on acoramidis HCl 800 mg twice daily, mean TTR stabilization at month 45 was 99% as measured by FPE assay [77]. However, interpreting findings from studies using the FPE assay is challenging given its limitations. Importantly, tafamidis and acoramidis exhibited similar potency and binding affinity when measured using subunit exchange methodology—findings that dispel potential claims of superiority for either stabilizer.

Diflunisal is an NSAID with moderate binding affinity and selectivity for TTR. However, because diflunisal is present at high plasma concentrations after oral dosing, it is effective at kinetically stabilizing TTR in vivo [78, 79]. In subunit exchange experiments, diflunisal at plasma concentrations of approximately 100 to 200 μM, achieved by a daily oral dose of 500 mg, was associated with excellent kinetic stabilization despite moderate potency due to its very good oral bioavailability (i.e., a lower amount of administered drug is required to achieve the desired plasma concentration and therapeutic effect [71]) and modest albumin binding [28]. In preclinical studies, diflunisal was also shown to provide effective kinetic stabilization of TTR tetramers in human plasma as a result of its high oral bioavailability [79, 80]. As a result of these findings, this NSAID has been repurposed to treat patients with ATTR amyloidosis. However, diflunisal has not received regulatory approval for this disease, limiting it to off-label use.

Clinical Investigations of TTR Kinetic Stabilizers

Evidence from clinical trials conducted in patients with ATTR amyloidosis supports the efficacy, tolerability, and safety of tafamidis and acoramidis (clinical trial data summarized in Supplementary Material Table 2). Although a meaningful comparison of trial results for these TTR kinetic stabilizers is not possible due to substantial differences in trial methodology, a summary of their clinical safety and efficacy profiles is provided below.

Tafamidis is the first disease-modifying drug to receive regulatory approval for the treatment of ATTR amyloidosis, first receiving approval in the European Union for ATTR-PN in 2011, and subsequently in more than 40 other countries worldwide [81]. Further, tafamidis became the first disease-modifying drug approved to treat ATTR-CM (variant and wild-type), a milestone reached in the United States and Japan in 2019, in the European Union in 2020, and later in more than 50 other countries. In a pivotal double-blind, placebo-controlled trial, in the efficacy-evaluable population, a significantly higher proportion of patients with early-stage V30M (p.V50M) disease who received tafamidis meglumine 20 mg satisfied neurologic response criteria (< 2 point worsening in the Neuropathy Impairment Score-Lower Limbs [NIS-LL]) compared with patients who received placebo after 18 months of treatment (60.0% vs. 38.1%, respectively; p < 0.05) [82]. Tafamidis-treated patients also had a better preserved QOL (Norfolk QOL-Diabetic Neuropathy total score [TQOL], 0.1 vs. 8.9; p < 0.05). Because statistically significant effects on the coprimary neurologic and QOL outcomes were not observed in the intent-to-treat population, tafamidis did not receive regulatory approval for ATTR-PN in the United States. Similar incidences of serious adverse events and adverse events leading to discontinuation were seen in the tafamidis and placebo groups. In an open-label extension of this pivotal study, tafamidis was shown to maintain neurologic function and QOL over 30 months of follow-up [83]. Patients who switched from placebo to tafamidis had significant reductions in the monthly rate of change in neurologic function (NIS-LL, from 0.34 to 0.16/month; p = 0.01) and QOL deterioration (TQOL, from 0.61 to −0.16/month; p < 0.001).

In an open-label study, tafamidis meglumine 20 mg once daily effectively stabilized TTR and preserved QOL in patients with non-V30M ATTRv-PN for 12 months [84]. In subsequent post hoc analysis of findings from an 18-month, double-blind, placebo-controlled randomized trial in patients with V30Met ATTRv-PN [85] and the aforementioned 12-month open-label study of patients with non-V30M disease [84], tafamidis was shown to comparably delay neurologic progression in patients with V30M and non-V30M genotypes in ATTRv-PN regardless of the severity of neurologic impairment at baseline [86].

Results from additional open-label extension studies, providing data from up to 9 years of treatment, support the sustained tolerability and effectiveness of tafamidis in slowing disease progression in ATTR polyneuropathy [87–89]. Barroso et al. reported less polyneuropathy progression in patients who received tafamidis at the start of the randomized study compared with patients who switched to tafamidis after 18 months of placebo after up to 6 years of follow-up [87]. In addition, significant slowing of polyneuropathy progression and QOL deterioration was observed with long-term tafamidis treatment in the patients who switched from placebo to tafamidis versus those with prior placebo treatment. Real-world evidence from the THAOS registry also supports the effectiveness profile of tafamidis beyond conventional clinical trials. Based on THAOS data, tafamidis-treated patients with ATTR-PN had significantly less deterioration in neurologic progression (NIS-LL, p < 0.001) and QOL (TQOL score, p < 0.001) than untreated patients with ATTR-PN over a 2-year follow-up period [90]. In 2019, a comprehensive, integrated analysis was conducted using safety data from interventional, observational, and real-world (THAOS) studies and post-marketing surveillance reports of tafamidis in patients with ATTR-PN [91]. This analysis did not uncover any new safety signals and supported the known safety profile of tafamidis in ATTR-PN.

Evidence from the tafamidis clinical trial program also demonstrated its safety and efficacy in ATTRv-CM and ATTRwt-CM. In a phase 2, open-label trial conducted in patients with early-stage ATTRwt-CM (of predominantly cardiac phenotype) who received tafamidis meglumine 20 mg daily, TTR stabilization and preserved New York Heart Association (NYHA) functional status were observed in 89.3% and 71.4% of patients, respectively, after 12 months of treatment [25]. Tafamidis was well tolerated in most patients. In the subsequent pivotal, randomized, placebo-controlled registration study, Tafamidis in Transthyretin Cardiomyopathy Clinical Trial (ATTR-ACT), conducted in patients with wild-type and variant disease, tafamidis meglumine (20 mg and 80 mg pooled) was significantly more effective than placebo in reducing the combination of all-cause mortality and cardiovascular-related hospitalization, the two-component hierarchical primary outcome analyzed using the Finkelstein–Schoenfeld method over 30 months [92]. The win ratio, defined as the number of pairs of treated patient wins divided by the number of pairs of placebo patient wins, was 1.7 (95% confidence interval [CI], 1.3, 2.3) for the two-component hierarchical analysis. Reductions in the risk of death from any cause and cardiovascular-related hospitalization of 30% and 32%, respectively, were achieved with tafamidis versus placebo. Consistent benefits were generally seen across the prespecified subgroups analyzed; namely, ATTRv versus ATTRwt subgroups, 80-mg versus 20-mg dose subgroups, and NYHA class I or II versus class III subgroups. However, cardiovascular-related hospitalizations were more common with tafamidis than placebo in patients with NYHA class III, a finding attributed to the longer survival of tafamidis-treated patients during a period of more severe disease. After 30 months of treatment, tafamidis versus placebo also reduced the decline from baseline in functional capacity (distance walked for the 6-min test, 75.7 m [standard error (SE) ± 9.2; p < 0.001]) and QOL (Kansas City Cardiomyopathy Questionnaire – Overall Summary [KCCQ-OS] score, 13.7 [SE ± 2.1; p < 0.001) and was associated with a smaller increase in N-terminal pro-B-type natriuretic peptide (NT-proBNP) levels (least squares [LS] mean difference, −2180.54 [95% CI −3326.14, −1034.95]). The adverse event profile of tafamidis was similar to that of placebo in the study. Discontinuations due to adverse events were less common in the tafamidis versus the placebo group, as were adverse events such as diarrhea, urinary tract infections, acute kidney injury, dyspnea, and pleural effusion. In a post hoc analysis of ATTR-ACT, after 30 months of follow-up, patients receiving tafamidis had a significantly smaller reduction in the estimated glomerular filtration rate than those receiving placebo (LS mean difference, 3.99 mL/min/1.73 m² [95% CI 1.31, 6.68]) [93]. Additionally, among patients who completed the study, a significantly greater proportion of patients receiving tafamidis versus placebo had improvement in chronic kidney disease staging (17.7% vs. 7.2%; odds ratio, 2.75 [95% CI 1.10, 6.90]). Another post hoc analysis from ATTR-ACT found less pronounced worsening in four echocardiographic measures in patients receiving tafamidis 80 mg versus placebo (LS mean difference: left ventricular [LV] stroke volume, 7.02 mL [95% CI 2.55, 11.49]; LV global longitudinal strain, −1.02% [95% CI −1.73, −0.31]; septal E/e′, −3.11 [95% CI −5.50, −0.72]; lateral E/e′, −2.35 [95% CI −4.01, −0.69]) [94]. In addition, an analysis of atrial fibrillation/flutter from ATTR-ACT found that these arrhythmias were prognostic for all-cause mortality, but that baseline or historical atrial fibrillation/flutter did not impact tafamidis treatment outcomes, with similar efficacy observed regardless of arrhythmia history [95].

Subsequent analyses of data from ATTR-ACT [92] and the open-label, long-term extension of ATTR-ACT showed that the 80 mg dose of tafamidis meglumine provided a significantly lower risk of all-cause mortality versus the 20 mg dose (hazard ratio [HR], 0.70 [95% CI 0.50, 0.98]) after a median follow-up of 51 months [96]. In addition, Elliott et al. reported that all-cause mortality was lower (HR 0.64 [95% CI 0.41, 0.99]) among patients with NYHA class III symptoms at baseline who received continuous tafamidis treatment (i.e., tafamidis in both ATTR-ACT and the long-term extension) compared with patients who received delayed tafamidis treatment (i.e., placebo in ATTR-ACT and tafamidis in the long-term extension) over a median follow-up of almost 5 years, demonstrating the importance of early treatment in patients with ATTR-CM regardless of the severity of their heart failure symptoms [97]. Over a similar follow-up period, a 47% reduction in mortality risk was observed with continuous tafamidis treatment versus delayed tafamidis treatment in patients with LV ejection fractions < 50% and ≥ 50% at baseline (HR 0.53 [95% CI 0.37, 0.76] and 0.53 [95% CI 0.34, 0.82], respectively) [98]. Findings from post hoc analyses of ATTR-ACT and the long-term extension also support the efficacy of tafamidis in patients with ATTR-CM who were aged < 80 and ≥ 80 years [99]. In addition to clinical trial data, recent analyses from THAOS provide real-world evidence of improved survival with tafamidis in ATTR-CM. Patients with ATTR-CM and a predominantly cardiac phenotype who received tafamidis in clinical practice had higher survival rates at 30 and 42 months (84.4% [95% CI 80.5, 87.7] and 76.8% [95% CI 70.9, 81.7], respectively) compared with patients who did not receive tafamidis (70.0% [95% CI 66.4, 73.2] and 59.3% [95% CI 55.2, 63.0], respectively) [23]. Similar findings were observed in a contemporary cohort of patients with ATTR-CM and a mixed phenotype [100]; survival rates at 30 months were 81.5% (95% CI 66.7, 90.2) in tafamidis-treated patients and 75.1% (95% CI 66.1, 82.0) in tafamidis-untreated patients. No new safety signals were identified in these analyses of real-world data.

The tafamidis free acid 61-mg capsule formulation, developed for patient convenience, with demonstrated bioequivalence to tafamidis meglumine 80 mg [101], is currently widely approved for the treatment of ATTR-CM. In the above-mentioned open-label, long-term extension study of ATTR-ACT, all patients were successfully switched from tafamidis meglumine 80 mg or 20 mg to tafamidis free acid 61 mg formulation after a median 39-month treatment period [96].

Acoramidis was approved in the United States in November 2024, in the European Union in February 2025, and in Japan in March 2025 for the treatment of ATTR-CM in adults based on the results of the pivotal ATTRibute-CM phase 3 study [26]. Its approval is currently under review in other countries. The recommended oral dose is 712 mg (two 356-mg tablets) twice daily (1,424 mg daily in total) of a newer formulation of acoramidis that is bioequivalent to acoramidis HCl 800 mg twice daily used in clinical trials. Investigators for the ATTRibute-CM study evaluated the effects of acoramidis HCl 800 mg twice daily versus placebo in patients with ATTR-CM over a 30-month period [26]. A significant benefit favoring acoramidis over placebo was observed with respect to the hierarchical primary outcome (p < 0.001), which included four components—death from any cause, cardiovascular-related hospitalization, NT-proBNP, and 6-min walk distance—analyzed using the Finkelstein–Schoenfeld method. The investigators reported a win ratio of 1.8 (95% CI 1.4, 2.2) for the primary four-component hierarchical analysis and a win ratio of 1.5 (95% CI 1.1, 2.0) for the secondary two-component hierarchical analysis of death from any cause and cardiovascular-related hospitalization (the analysis used for tafamidis approval). Acoramidis reduced the occurrence of the composite of all-cause mortality or first CV-related hospitalization versus placebo (35.9% vs. 50.5%; HR 0.64 [95% CI 0.50, 0.83]; p = 0.0008) [102]. Acoramidis also reduced the decline from baseline in functional capacity versus placebo (LS mean difference in distance walked for the 6-min test, 39.6 m [95% CI 21.1, 58.2]; p < 0.001) and preserved QOL (LS mean difference in KCCQ-OS score, 9.9 points [95% CI 6.0, 13.9]; p < 0.001) [26]. The frequency of adverse events was similar in the acoramidis and placebo groups, with a lower frequency of serious adverse events seen in the acoramidis group. Subsequent analyses of ATTRibute-CM and its open-label extension study (wherein all patients received acoramidis HCl 800 mg BD) showed reduced mortality (HR 0.64 [95% CI 0.47, 0.88]; p = 0.006) and CV-related hospitalizations (HR 0.53 [95% CI 0.41, 0.69]; p < 0.0001) at month 42 in patients who received continuous acoramidis versus those who received placebo then acoramidis [103]. To date, no data are available for acoramidis in patients with ATTR-PN. A phase 3 efficacy and safety study of acoramidis in patients with symptomatic ATTR-PN ended early prior to recruitment completion (NCT04882735). An ongoing trial is evaluating acoramidis for the prevention of transthyretin amyloidosis in asymptomatic carriers of a known pathogenic TTR variant (NCT06563895).

Diflunisal has also been investigated in several clinical trials. In a 2-year, investigator-initiated, multicenter placebo-controlled randomized clinical trial conducted in patients with ATTRv-PN, diflunisal 250 mg twice daily significantly reduced the rate of polyneuropathy disease progression and preserved QOL [27]. Findings from another open-label clinical study of patients with ATTRv amyloidosis who were not candidates for liver transplantation showed that the effects of diflunisal on neurological and cardiac functions were sustained after 2 years of treatment in patients and that the drug was well tolerated [104]. In a systematic review, which included six studies of diflunisal administered in 400 patients with ATTR-CM, diflunisal use was associated with reduced mortality and orthotopic heart transplants [105]. However, like other NSAIDs, diflunisal is associated with a risk of very serious adverse effects such as gastrointestinal bleeding, renal dysfunction, and worsening heart failure, which calls for careful monitoring with chronic use [106]. The tolerability and safety of this kinetic stabilizer likely depend on dose as well as patient age and disease severity, but serious adverse events, including deterioration of renal function and thrombocytopenia, have been reported in diflunisal-treated patients with ATTRwt and ATTRv amyloidosis [104, 107, 108].

Summary and Conclusions

Research conducted over the past several decades has dramatically advanced our understanding of the amyloid cascade and interventions that may be useful in modulating amyloidogenic processes. This knowledge has allowed the development of TTR kinetic stabilizers, designed to reduce the concentration of dissociated, non-native TTR in order to slow aggregation and formation of amyloid fibrils. Tafamidis, acoramidis, and diflunisal are small molecules that selectively bind to and kinetically stabilize TTR, interrupting the amyloidogenic cascade triggered by tetramer dissociation. Although diflunisal is not approved for the treatment of ATTR amyloidosis and is limited to off-label use, tafamidis first received regulatory approval for ATTR-PN in 2011 and for ATTR-CM in 2019, and acoramidis first received approval for ATTR-CM in 2024. Using the gold-standard TTR subunit exchange methodology, investigators have shown that tafamidis and acoramidis exhibit similar potency in preventing tetramer dissociation at plasma concentrations achieved via approved oral doses, contradicting claims of superiority (e.g., the proposed “super-stabilizer” properties of acoramidis). In pivotal clinical trials, both tafamidis and acoramidis have been shown to significantly reduce mortality and improve function and QOL at a once-daily 80 mg dose for tafamidis and a twice-daily 800 mg dose for acoramidis HCl, with favorable safety and tolerability profiles, although a meaningful comparison of the safety and efficacy of these kinetic stabilizers in patients with ATTR-CM is not possible in the absence of head-to-head clinical trials. Furthermore, tafamidis has a robust body of long-term clinical trial and real-world evidence demonstrating its safety, tolerability, and efficacy in ATTR amyloidosis, whereas long-term data on acoramidis are still emerging.

Medical Writing/Editorial Assistance

Medical writing support was provided by Donna McGuire and Emily Balevich of Envision Pharma Group and was funded by Pfizer.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contributions

Evan Powers, Jeffery Kelly, Leslie Amass, Lori Baylor, Isabel Fernández-Arias, and Steve Riley contributed to the concept of this review article, drafted and/or critically revised the article, and approved this version for publication.

Funding

This work was sponsored by Pfizer. Pfizer funded the rapid service fee for this journal. Jeffery W. Kelly acknowledges support by NIH DK46335 to develop tafamidis and an understanding of its function.

Data Availability

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current review.

Declarations

Conflicts of Interest

Evan Powers and Jeffery Kelly discovered tafamidis, and receive royalties and sales milestone payments. Leslie Amass, Lori Baylor, Isabel Fernández-Arias, and Steve Riley are employees of Pfizer and hold Pfizer stock/stock options.

Ethical Approval

This article is based on previously conducted studies and does not contain any new studies with human participants performed by any of the authors.