Contextualizing the results of HELIOS-B in the broader landscape of clinical trials for the treatment of transthyretin cardiac amyloidosis

Abstract

This focused review will highlight the results of HELIOS-B, the first randomized outcomes trial evaluating a gene silencing treatment for transthyretin cardiac amyloidosis (ATTR-CM). In HELIOS-B, vutrisiran was tested against placebo and demonstrated a 28% reduction in the composite of all-cause mortality and recurrent cardiovascular events. Additionally, there were clinically significant benefits on the 6-min walk test, Kansas City Cardiomyopathy Questionnaire, and NYHA class. Discontinuation rates and adverse events were similar between treatment and control arms, suggesting that vutrisiran is well tolerated. In this review, these promising results are explored and compared with other treatment trials in ATTR-CM.

Transthyretin cardiac amyloidosis (ATTR-CM) is a rare disease characterized by progressive myocardial tissue infiltration with amyloid aggregates composed of misfolded TTR, ultimately resulting in end organ damage and dysfunction [13]. TTR is a homotetrameric polypeptide normally synthesized within the liver and secreted into the plasma as a carrier protein for thyroxine and retinol binding protein/vitamin A. In ATTR amyloidosis, a cascade consisting of tetramer dissociation, misfolding, and aggregation into amyloid fibrils forms the mechanistic basis for amyloid deposition in myocardial tissue. This cascade can occur in the setting of a point mutation in the TTR gene rendering it more unstable, termed variant ATTR amyloidosis, or by an age-related process with an unclear etiology and a non-mutated TTR genotype, termed wild-type ATTR amyloidosis [9].

If amyloidosis is like a flooded basement, therapies targeting the amyloidogenic cascade can be thought of with two main goals: (1) turn off the running water and (2) clean up the mess that is left over. So far, treatments studied in phase 3 trials that have led to approvals fall within the first category and have the potential to prevent progression of ATTR cardiomyopathy (ATTR-CM) and ATTR polyneuropathy (ATTR-PN), thereby mitigating functional impairments, reductions in quality of life, and increased mortality. These therapies include TTR protein stabilizers which bind to the native functional TTR protein and prevent tetramer dissociation, halting the amyloidogenic cascade. They also include TTR silencers which knockdown protein production. Therapies which “clean up the mess” are currently in clinical trials. This mini-review article will focus on “turning off the running water” by examining the results of HELIOS-B [15], the first randomized outcomes trial evaluating a TTR silencing treatment for ATTR-CM. Findings from HELIOS-B will subsequently be contextualized in the landscape of other clinical trials in ATTR-CM.

RNA interference in ATTR-CM.

TTR silencing can be accomplished by taking advantage of RNA interference (RNAi) [12]. RNAi is a cellular housekeeping function normally employed by eukaryotic cells to modulate post-transcriptional gene expression. In vivo, noncoding RNA known as microRNA is produced within the nucleus and transported to the cytoplasm where binding occurs with mRNA, inducing post-transcriptional gene silencing. The synthetic counterpart to microRNA, termed small interfering RNA (siRNA) [4], was first shown by Elbashir and colleagues [3] in 2001 to interfere with mRNA translation in mammalian cells, setting the stage for the development of RNAi therapeutics for use in human subjects. Currently available RNAi therapeutics fall into two classes: siRNA and antisense oligonucleotides (ASOs). In the case of siRNA therapeutics, double-stranded RNA molecules are incorporated into cells, followed by the removal of the sense strand and binding of the antisense strand to the RNA-induced silencing complex (RISC), leading to cleavage of the mRNA target. Patisiran and vutrisiran are categorized as siRNA therapies. In contrast, ASOs are single-stranded RNA molecules which are incorporated into hepatic cells and bind directly to the target TTR mRNA, with inotersen and eplontersen as notable examples.

The clinical benefits of RNAi therapy were first realized with APOLLO, a phase 3, multicenter trial investigating the efficacy and safety of the RNAi therapeutic agent patisiran in patients with hereditary ATTR-PN [1], leading to US FDA approval in 2018 [16]. The favorable effects of patisiran in ATTR-PN were subsequently extended to ATTR-CM in APOLLO-B, an international, multicenter, double-blind, randomized, placebo-controlled trial of patisiran in patients with hereditary or wild-type ATTR-CM [11]. This was a short 12-month trial demonstrating a reduction in the primary endpoint of change in 6-min walk distance (14.69 m; 95% confidence interval [CI], 0.69 to 28.69; p = 0.02) and secondary endpoint of change in Kansas City Cardiomyopathy Questionnaire Overall summary (KCCQ-OS) score (3.7 points; 95% CI, 0.2 to 7.2; p = 0.04). Despite positive trial findings, the US FDA rejected a supplemental new drug application to expand patisiran treatment to hereditary or wild-type ATTR-CM, citing small benefits in 6-min walk distance and KCCQ-OS, without an observed benefit in patients on background therapy with tafamidis.

Vutrisiran is a next-generation, long-acting RNAi therapy which also silences TTR mRNA translation. Compared with patisiran, vutrisiran results in a similar degree of TTR silencing (median TTR reduction trough for vutrisiran vs patisiran: 86.2% vs 78.2%) with a smaller variability in peak to trough TTR concentrations [2]. Whereas patisiran requires frequent intravenous administration every 3 weeks and weight-based dosing, vutrisiran is given by a health-care provider intramuscularly at a fixed dose of 25 mg every 3 months, reducing the frequency and duration of visits. While higher one-time doses of vutrisiran led to a greater magnitude of TTR reduction, steady-state reductions were similar in the 25 mg quarterly dosing cohort compared with higher doses [8]. Vutisiran was first evaluated clinically in HELIOS-A, an 18-month multinational open label phase 3 trial for the treatment of ATTR-PN [2]. Like patisiran in APOLLO, vutrisiran demonstrated significant improvements in neuropathy endpoints, leading to its approval by the US FDA in 2022 for the treatment of ATTR-PN.

Results of HELIOS-B.

A secondary analysis of HELIOS-A demonstrated beneficial changes in both cardiac structure and biomarkers in a subgroup of ATTR-PN patients with cardiac involvement [5], setting the stage for HELIOS-B for ATTR-CM. This phase 3 trial sponsored by Alnylam Pharmaceuticals randomized 655 patients from 2019 to 2021 to vutrisiran 25 mg subcutaneously every 3 months or placebo for up to 36 months [15]. Following this, patients were able to roll over into the open label extension where all patients received vutrisiran. It is important to note that approximately 40% of participants were being treated with tafamidis at the time of randomization and another 13% were started on tafamidis during the trial period.

Participants enrolled in HELIOS-B were reflective of patients routinely encountered in clinical practice with ATTR amyloidosis during this time period: the average age was 77 years, 92% were male, and 89% were diagnosed with wild-type ATTR amyloidosis [15]. Baseline N Terminal prohormone of brain natriuretic peptide (NT-proBNP) was 2021 pg/mL in the vutrisiran and 1801 pg/mL in the placebo arms, and National Amyloidosis Center stage was 1 in 64% in the vutrisiran and 70% in the placebo arms.

The primary endpoint was a composite of all-cause mortality and recurrent cardiovascular (CV) events and was met in the overall cohort (HR 0.72, p-value 0.01; n = 654) and those receiving vutrisiran monotherapy without tafamidis (HR 0.67, p-value 0.02; n = 395). Among secondary endpoints, all-cause mortality at 42 months of treatment was reduced by 35% in the entire study population (p = 0.01), and 34% in the monotherapy population (p = 0.045). Finally, favorable changes to health status and functional capacity were also observed at month 30; the least-squares mean change from baseline in KCCQ-OS was 5.8 points (p < 0.001) and for 6-min walk test distance was 26.5 m (p < 0.001). With respect to the primary endpoint, inspection of the Kaplan–Meier curve for the overall study population revealed a treatment benefit beginning at month 6 which was sustained for the remainder of the trial duration. In prespecified subgroups, similar effects were noted on the primary endpoint and on all-cause mortality, though younger age (< 75 years), lower NT-proBNP (≤ 2000 pg/mL), wild-type disease, and lack of concomitant tafamidis use seemed to derive the greatest benefit from therapy. The incidence of overall and serious adverse events was similar between treatment arms, with adverse events leading to discontinuation of vutrisiran in 10 patients (3%) and placebo in 13 patients (4%). Taken together, these results suggest that vutrisiran produces clinically apparent benefits in function, quality of life, and CV outcomes with an acceptable safety profile.

Comparison to other approved therapies.

The results of HELIOS-B will likely result in 3 FDA-approved ATTR-CM therapies by 2025: tafamidis (studied in ATTR-ACT[10]), acoramidis (studied in ATTRIBUTE-CM[7]), and vutrisiran. While the design of these 3 trials will be compared in the following section, a cross-trial comparison of endpoints is not possible due to many factors. All 3 trials met their primary endpoint, though these endpoints were defined using different criteria and statistical plans. The win ratio was used as an endpoint in ATTR-ACT and ATTRIBUTE-CM, with death and CV hospitalization being the first two hierarchical events for both trials, and change in NT-proBNP and 6-min walk distance added as the third and fourth event in ATTRIBUTE-CM [7, 10]. The primary endpoint in HELIOS-B was a time to event analysis of death and recurrent CV events (defined as CV hospitalization or urgent heart failure visit). All 3 studies used statistical methods that gave more weight to mortality relative to other events for each endpoint. The duration of each trial was also different: ATTR-ACT and ATTRIBUTE-CM lasted 30 months while HELIOS-B followed patients to 36 months. Additionally, in order to capture more events, HELIOS-B had the unconventional change in statistical plan to include patients in the open label extension (where all patients were receiving vutrisiran) to accrue more all-cause mortality events through month 42.

Furthermore, the timing, inclusion criteria, and baseline characteristics of enrolled participants for each trial were different (Table 1). For example, ATTR-ACT enrolled patients from 2013 to 2015, ATTRIBUTE-CM from 2019 to 2020, and HELIOS-B from 2019 to 2021. Patients were required to have endomyocardial biopsy evidence of ATTR-CM in ATTR-ACT, leading to a very high bar to be enrolled in the trial, while ATTRIBUTE-CM and HELIOS-B included patients predominantly with an imaging-based diagnosis. Baseline characteristics also revealed that disease stage was lower in HELIOS-B compared with ATTRIBUTE-CM and ATTR-ACT, based upon NT-proBNP levels, NYHA Class, and National Amyloidosis Center stage. Finally, tafamidis drop-in was allowed after 12 months in ATTRIBUTE-CM (14.5% in acoramidis-treated patients and 21.8% in the placebo arm), while 53% of patients in HELIOS-B were treated with tafamidis at some point in the study.

Table 1.

Comparison of treatments for ATTR-CM

	ATTR-ACT [10, 14, 17] Tafamidis ( n = 264)	ATTRIBUTE-CM [7] Acoramidis ( n = 421)	HELIOS-B [15] Vutrisiran ( n = 326)
Age, years	75 (range 46–88)	77.4 ± 6.5	77.0 (range 45–85)
Sex
Males	91.3%	91.2%	91.7%
Females	8.7%	8.8%	8.3%
Race
White	79.9%	87.4%	85.0%
Black	14.0%	4.8%	7.1%
Genotype
Wild type	76.1%	90.3%	88.7%
Variant	23.9%	9.7%	11.3%
NYHA class
I	9.1%	12.1%	15.0%
II	61.4%	69.6%	76.7%
III	29.5%	18.3%	8.3%
NT-proBNP, pg/mL	2995.9 (1752–4862)	2326 (1332–4019)	2021 (1138–3312)
NAC stage
1	45.1%	57.2%	63.8%
2	36.0%	31.8%	30.7%
3	18.9%	10.9%	5.5%
Tafamidis use
Baseline	N/A	0%	40%
Drop-in	N/A	14.5%	13.5%

NYHA, New York Heart Association; NT-proBNP, N terminal prohormone of brain natriuretic peptide; NAC, National Amyloidosis Center.

In all 3 studies, there is a signal that “less sick” patients (i.e., based upon younger age, lower NTproBNP, lower NYHA class) had better outcomes. A cross-trial comparison highlights the fact that enrolling less sick patients at earlier stage disease in treatment trials from ATTR-ACT to ATTRIBUTE-CM to HELIOS-B has led all-cause mortality to approach the population expected 3-year rate (Fig. 1).

Fig. 1.

Time to all-cause mortality compared among trials in ATTR-CM. Survival curves for the treatment and placebo arms of ATTR-ACT, ATTRIBUTE-CM, and HELIOS-B were superimposed in the same graph. Population age-expected mortality is also depicted and based upon the Social Security actuarial life Table (4.1% mortality per year based upon 2019 life table for median age of 77 years with 91% males). Relative to ATTR-ACT, an improvement in overall mortality can be seen in the treatment arms of ATTRIBUTE-CM and HELIOS-B which enrolled patients at an earlier disease stage, some of whom received combination therapy. The overall mortality of the treatment arm of HELIOS-B approached the age-expected mortality for the average patient in this trial. Direct comparison among treatment options should not be performed due to differences in eras and baseline characteristics across trials

Future directions.

With respect to other RNAi protein knockdown therapies, the second-generation antisense oligonucleotide eplontersen is currently being studied in the randomized CARDIO-TTRansform trial for patients with ATTR-CM (NCT04136171) with topline results expected in late 2025, following positive results from NEURO-TTRansform which demonstrated significant therapeutic benefits in terms of function and quality of life for patients with ATTR-PN. A small interfering RNA from Alnylam named ALN-TTRsc04 can be dosed every 6–12 months (compared with vutrisiran dosed every 3 months) and is also undergoing early investigation. Finally, gene editing with the CRISPR-Cas9 treatment NTLA-2001 is enrolling in the phase 3 CV outcomes trial MAGNITUDE (NCT06128629). Phase 1 data suggests 90–95% TTR protein reduction with a single drug infusion [6].

In summary, the current and future therapeutic options for ATTR amyloidosis are expanding rapidly, and, using the flooded basement analogy, can be grouped into treatments that turn off the running water and treatments that clean up the mess that is left over. RNAi therapy has emerged as a cornerstone for both ATTR-CM and variant ATTR-PN to turn off the running water and slow or stop further TTR protein deposition. HELIOS-B demonstrated significant benefits in ATTR-CM, positively affecting quality of life, functional capacity, and survival for patients presenting with this clinical phenotype. While both TTR stabilizing and RNAi therapy delay or halt ATTR progression, additional studies and subanalyses of HELIOS-B are needed to assess the effects of combination therapy with vutrisiran and tafamidis. Additionally, future studies will address whether “cleaning up the mess” of amyloid fibrils leftover with fibril depleter therapies is of added benefit.

Data availability

No datasets were generated or analysed during the current study.