Profiling tofersen as a treatment of superoxide dismutase 1 amyotrophic lateral sclerosis

ABSTRACT

Introduction

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive motor neuron disorder with a fatal outcome 3–5 years after disease onset due to respiratory complications. Superoxide dismutase 1 (SOD1) mutations are found in about 2% of all patients. Tofersen is a novel oligonucleotide antisense drug specifically developed to treat SOD1-ALS patients.

Areas covered

Our review covers and discusses tofersen pharmacological properties and its phase I/II and III clinical trials results. Other available drugs and their limitations are also addressed.

Expert opinion

VALOR study failed to meet the primary endpoint (change in the revised Amyotrophic Lateral Sclerosis Functional Rating Scale score from baseline to week 28, tofersen arm vs. placebo), but a significant reduction in plasma neurofilament light chain (NfL) levels was observed in tofersen arm (60% vs. 20%). PrefALS study has proposed plasma NfL has a potential biomarker for presymptomatic treatment, since it increases 6–12 months before phenoconversion. There is probably a delay between plasma NfL reduction and the clinical benefit. ATLAS study will allow more insights regarding tofersen clinical efficacy in disease progression rate, survival, and even disease onset delay in presymptomatic SOD1 carriers.

KEYWORDS:

• Amyotrophic lateral sclerosis
• antisense oligonucleotide
• genetics
• neurofilament light chain
• superoxide dismutase 1
• tofersen

1. Introduction

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive and fatal motor neuron disorder characterized by upper and lower motor neuron degeneration [1]. In general, death occurs 3–5 years after disease onset mainly due to respiratory failure and its complications [1,2]. The overall worldwide ALS prevalence and incidence are 4.42 (95% CI 3.92–4.96) per 100.000 population and 1.59 (95% CI 1.39–1.81) per 100.000 person-years, respectively [3].

ALS is presently classified as either sporadic (sALS) or familial (fALS). fALS, which represents 10 to 15% of cases, means that there is more than one occurrence of ALS and related disorders (like frontotemporal dementia) in a family [4,5]. So far at least 40 ALS-related genes have been associated with ALS [5,6]. C9orf72, SOD1, TARDPB, and FUS are the most common mutated genes in ALS being related to 47.7% of fALS and 5.2% of sALS cases [6,7].

Superoxide dismutase 1 (SOD1) was the first ALS-related gene identified in 1993 (SOD1-ALS) [8–10]. In the European population, SOD1 variants occur in 14.8% of fALS and 1.2% of sALS, while in Asian population in 30% and 1.5%, respectively [6]. More than 200 SOD1 mutations have been described [4,11]. SOD1 gene encodes for the Cu/Zn SOD1 protein, a powerful antioxidant enzyme protecting cells from superoxide radicals’ toxicity [12,13]. While the pathophysiological mechanisms associated with motor neuron degeneration in SOD1-ALS remain to be fully elucidated, pathogenic variants are responsible for a SOD1 protein toxic gain of function [12,14,15]. This toxic protein allows hydroxyl radicals accumulation, leading to both nuclear and mitochondrial DNA damage and protein misfolding within motor neurons and astrocytes, and ultimately to cellular dysfunction and degeneration [13].

Although SOD1 variants share a common pattern of inheritance (autosomal dominant), their frequency, penetrance, and phenotypic expression (onset age and disease progression rate) is highly heterogeneous [11,16–18]. The most common clinical presentation for patients with this mutation is leg weakness with predominant lower motor neuron signs [11]. It should be considered that there are geographical differences regarding SOD1 genotypes distribution (p.A4V with a higher prevalence in North America [17], p.D90A in Europe [19] and p.H46R in Japan [20]).

Pre-fALS study has found increased serum and CSF levels of neurofilament light chain (NfL) in individuals with SOD1 variants associated with faster disease progression, 6–12 months before phenoconversion, indicating NfL as a potential biomarker for predicting ALS onset in this specific population [21–23].

Up to now, there were no effective treatments targeting SOD1-ALS patients. However, the identification of ALS-related genes has advanced our understanding of disease pathophysiology and contributed to the development of therapeutics directly challenging the underlying genetic process. Tofersen, a novel antisense oligonucleotide (ASO) drug, was specifically designed for SOD1-ALS patients’ treatment. This SOD1 targeting ASO induces RNase H-mediated degradation of SOD1 mRNA, thus reducing the SOD1 protein synthesis and their toxic intracellular accumulation [24].

2. Overview of the market

Despite decades of research, there is still a significant unmet need of effective treatments in ALS field to delay disease progression and improve survival.

Riluzole and edaravone are the U.S. Food and Drug Administration (FDA)-approved drugs for ALS treatment. However, the European Medicines Agency (EMA) has only approved riluzole (administered orally 50 mg twice daily), in 1996.

Riluzole inhibits the release of glutamic acid from neurons, partly due to inactivation of voltage-dependent sodium channels on glutamatergic nerve terminals, and blocks some of the postsynaptic effects of glutamate by noncompetitive blockade of N-methyl-D-aspartate (NMDA) receptors [25]. Its phase III, randomized, double-blind, placebo-controlled trials showed a modest survival increase [26], which was supported by latter population studies [27]. Indeed, most population studies comparing riluzole vs. riluzole-free ALS patients confirmed a more significant impact on survival than the one reported in the trials, particularly in early treated patients [28–30]. Riluzole is very well-tolerated, the most common adverse events are asthenia and mild elevation in aminotransferase levels [26].

Edaravone is a free radical scavenger (antioxidant) given intravenously or orally. Although the first randomized, double-blind, placebo-controlled phase III study did not show a significant difference in the revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R) score over 24 weeks [31], a post-hoc analysis suggested a potential effect of edaravone in a particular ALS subgroup who meet specific criteria, in particular short disease duration [32]. An additional trial, selecting a very specific population of ALS patients with short-disease duration, showed a slight but significant positive effect on the rate of functional decay over 6 months, without a positive impact on the respiratory function [33]. This latter trial supported edaravone intravenous treatment approval by U.S. FDA in 2017. Injection site pain, gait disturbances, headache, and allergic reactions side effects have been described [31]. U.S. FDA approved the oral formulation for treating ALS in 2022, with the same dosing regimen as the intravenous formulation, an initial treatment cycle of daily dosing for 14 days, followed by a 14-day drug-free period and subsequent treatment cycles consisting of daily dosing for 10 out of 14-day periods, followed by 14-day drug-free periods. Recently, the ADORE phase III trial, comparing continuous daily treatment of oral edaravone + riluzole vs. placebo + riluzole, during a 48-week period, showed no benefit from edaravone (https://www.ferrer.com/en/results-study-ADORE).

PB-TURSO is a potentially neuroprotective oral drug due to its ability to inhibit motor neuron apoptosis through reduction in the production of oxygen free radicals, as well as a decrease in endoplasmic reticulum stress and caspase activation. The phase II randomized, double-blind, multicentre, placebo-controlled CENTAUR trial evaluated the efficacy and safety of PB-TURSO (a sachet containing a fixed co-formulation of 3 g of PB and 1 g of TURSO given twice daily) in a total of 137 ALS patients. A slower function decline than placebo as measured by the ALSFRS-R score over 24 weeks was observed, leading to its conditional approval in Canada in June 2022 and full approval in U.S. in September 2022. Adverse events with the PB-TURSO were mainly gastrointestinal [34]. A long-term survival analysis of participants in CENTAUR suggested that PB-TURSO treatment at baseline resulted in a 6.5 month longer median survival as compared to patients initiating medication after 6 months [35]. Nevertheless, the PHOENIX phase III trial has been completed and was recently found that it did not reach its primary or secondary endpoints (https://www.amylyxalstrial.com). That led its developer Amylyx Pharmaceuticals to intend removing PB-TURSO from the market in U.S. and Canada April 2024.

3. Introduction to the drug

Tofersen is a 20-base residue ASO with a mixed backbone structure [RNA-DNA-RNA (5-10-5) gapmer]. There are 19 inter-nucleotide linkages, with 15 of them being 3′-O to 5′-O phosphorothioate diesters and the remaining 4 being 3′-O to 5′-O phosphate diesters. Ten out of 20 sugars are 2-deoxy-D-ribose, while the other 10 consist of 2′-O-(2-methoxyethyl)-D-ribose (MOE). The arrangement of residues involves five MOE nucleosides located at both the 5′ and 3′ ends of the molecule, surrounding a central gap containing 10 2′-deoxynucleosides. Both cytosine and uridine bases are methylated at the 5′-position [36].

This ASO is administered intrathecally into the CSF. Within motor neurons it binds directly to SOD1 mRNA creating an RNA-DNA hybrid, which promotes RNase H-dependent enzyme activation. This specific enzyme cleavages SOD1 mRNA, which leads to its reduction and the respective SOD1 protein in the motor neurons [24,36].

The recommended dosage for adults is 100 mg (per 15 mL) per administration. The first three doses (loading phase) are given once every 2 weeks, followed by one monthly dose (maintenance phase). While the maximum tofersen concentration in the CSF is achieved at the ending of the loading phase, during the maintenance phase there is minimal to no accumulation. Its peak plasma concentration occurs 2 to 6 hours after intrathecal injection, and it does not accumulate following monthly doses [36].

Tofersen is not an inducer or inhibitor neither metabolized by cytochrome P450 enzymes. Exonuclease (3’ and 5’) is the main enzyme involved in its metabolism though hydrolysis. The excretion of tofersen has not been studied. However, the estimated effective half-life within central nervous system is about 4 weeks [36].

4. Clinical efficacy

In a phase I/II ascending-dose trial, tofersen with different doses (20, 40, 60 and 100 mg) and placebo were investigated in a total of 50 adults with SOD1-ALS. Five doses of tofersen and placebo were given intrathecally over 12 weeks (day 1, 15, 29, 57 and 85). Primary endpoints included safety and pharmacokinetics, while change in CSF SOD1 protein concentration between baseline and day 85 represented the secondary outcome. Tofersen reduced consistently CSF SOD1 protein concentration over 12 weeks, being the highest reduction (36%) observed with the highest administrated dose (100 mg) [24].

The phase III, randomized, double-blind, placebo-controlled VALOR trial evaluated the efficacy and safety of tofersen in a total of 108 adults with SOD1-ALS. Seventy-two patients received eight doses of tofersen 100 mg and 36 received a placebo over 24 weeks, followed by an open-label extension (OLE) for up to 236 weeks. Change in ALSFRS-R score from baseline to week 28 among patients with a faster disease progression rate was considered the primary endpoint. Prognostic criteria for faster disease progression were based on SOD1 genotype and the estimated slope of the ALSFRS-R score from onset symptoms to trial screening. Secondary endpoints considered changes in biomarkers (CSF SOD1 protein concentration and plasma NfL levels), slow vital capacity and handheld dynamometry in 16 muscles. Tofersen group showed reduction in CSF SOD1 protein concentration (29% vs. 16%) and plasma NfL levels (60% vs. 20%) in comparison to placebo over 28 weeks, but no improvement in clinical outcomes [37].

At an interim analysis at 52 weeks of patients who had completed VALOR and enrolled in an OLE study, striking reductions in plasma NfL level were seen in patients previously receiving placebo and who initiated tofersen in the OLE. Earlier initiation of tofersen leads to a notable deceleration in the decline of clinical and respiratory function, strength, and overall quality of life in patients with SOD1-ALS [37].

Recently, a 12-month German multicentre cohort study from the tofersen early access program including a total of 24 SO1-ALS patients corroborates VALOR study and its OLE, showing a reduction of NfL serum levels, and phosphorylated neurofilament heavy chain CSF levels [38].

5. Post-marketing surveillance

The most common side events were mainly related to lumbar puncture for tofersen intrathecal administration. Asymptomatic pleocytosis was remarkably observed in tofersen-treated SOD1-ALS cases in contrast to placebo group (42% vs. 8%) [24,37]. Neurological serious adverse events occurred in 7% of SO1-ALS patients under tofersen treatment, including myelitis, radiculitis, aseptic meningitis, and papilledema and increased intracranial pressure [37].

Two drug-related serious adverse events (myeloradiculitis) were reported in the ‘real-world’ study of tofersen in Germany [38].

There is no information regarding the use of tofersen during pregnancy and breastfeeding. Due to its molecular weight of 7127 Da, the excretion in human breast milk should be residual. In addition, the infant gastrointestinal tract should be able, at least partially, to degrade tofersen, and its systemic absorption should be minimal [39].

6. Regulatory affairs

On 25 April 2023, FDA granted accelerated approval of tofersen (trade name Qalsody®) for the treatment of SOD1-ALS. On 23 February 2024, EMA has recommended granting market authorization of tofersen. In March 2024, Health Canada has agreed to review applications seeking the approval of tofersen. This drug is available in some other countries via expanded access programmes outside U.S. Although phase III VALOR study failed to meet the primary clinical endpoint (change in ALSFRS-R score from baseline to week 28 in tofersen group vs. placebo), its approval was based on biomarker response (reduced plasma NfL levels) [37] given the strong association between ALS progression rate and survival with NfL levels [40]. However, confirmatory studies regarding tofersen clinical benefit are still mandatory.

7. Conclusion

Tofersen is the first successfully developed gene therapy in ALS field, specifically indicated for SOD1-ALS cases. In addition, it is the first time that a treatment was approved based on its biomarker response. PrefALS study have suggested plasma NfL as a potential biomarker in ALS population, given its gradual increasing over 6–12 months before phenoconversion (presymptomatic phase). In the light of these findings, a decrease in plasma NfL and therefore axonal degeneration may favor a therapeutic effect. The delay between biomarker response and the clinical benefit is a possible explanation for the VALOR study failure regarding the primary outcome, as the trial duration was relatively short (28 weeks).

The ongoing ATLAS study is designed to evaluate the impact of initiating tofersen at the time of neurofilament levels increase in SOD1 presymptomatic carriers, with mutations associated with high or complete penetrance and faster disease progression. Results from this trial will surely be relevant not only to confirm tofersen efficacy in slowing SOD1-ALS progression rate and increasing survival, but also delaying disease onset in these high-risk presymptomatic carriers [41].

Although treating only about 2% of all cases, tofersen represents a new hope for the ALS community and their families and caregivers, as it opens other future possibilities for treating other ALS-related genes.

8. Expert opinion

The identification of ALS-related genes and the concomitant design of gene therapies have allowed to explore new possible drugs challenging the underlying genetic process.

Tofersen decrease neuronal loss as determined by the marked reduction of neurofilament levels, and slow disease progression, in particular of fast progressors. In the future other solutions will be tested for treating SOD1-ALS patients, in particular using viral vectors with small interfering RNA.

This drug is probably the first of many that will successfully treat genetic-related ALS patients, in the future. Currently, a phase III randomized, placebo-controlled FUSION trial (NCT04768972) of ION363 (Jacifusen) for FUS-ALS is running (initiated in 2021). The primary outcome includes ALSFRS-R change, discontinuation due to deterioration, time to rescue, and ventilator assistance-free survival. Other phase I and phase I/II trials are being explored using ASO, for example ataxin-2 ASO (ALSpire, NCT04494256) for reducing ataxin-2, a RNA-binding protein involved in RNA metabolism.

Tofersen revolutionized ALS treatment, particularly those with SOD1 pathogenic mutations. The disparity between biomarker response and absence of clinical benefit in VALOR trial suggests that for future trials design a longer period for monitoring disease progression is needed. In addition, machine leaning approaches could be used for ALS patient stratification given the considerable clinical heterogeneity [42]. This profiling would allow fine-tune the trial design and identify potential patient strata benefitting from treatment.

Asymptomatic SOD1 carriers’ identification is mandatory not only for genetic counseling, but also for regular follow-up, particularly those with genotypes associated with a faster disease progression. However, there are still many unanswered questions regarding tofersen use, including its chronic impact on disease progression, survival, onset delay in asymptomatic SOD1 carriers, and even the effect of silencing both SOD1 alleles in humans. Moreover, it will be critical to define criteria for identifying the SOD1 mutations that will benefit from therapy. ATLAS study and real-world data evidence regarding tofersen clinical efficacy and safety are more than welcome.

Many different drugs acting in different disease mechanisms have been tested for treating ALS in well-designed clinical trials including large number of patients [43], with negative results. This indicates that we need a new approach. The future will demonstrate that treating genetic mutations associated with ALS is the right track for having therapeutic success. For this target, more intense genetic research is necessary, and systematic genetic profiling of all ALS patients is recommended.

Article highlights

• ALS is fast progressive neurodegenerative disease with no treatment able to halt disease progression.

• Superoxide dismutase 1 (SOD1) was the first ALS-related gene identified and is associated with 2% of the total ALS cases in the Western-population.

• Riluzole and tofersen are approved by U.S. FDA and EMA, but intravenous edaravone is only approved by U.S. FDA.

• Riluzole has a modest effect in increasing survival, but both phase III phenylbutyrate-taurursodiol and oral edaravone trials were negative.

• Tofersen is a novel antisense oligonucleotide (ASO) drug, specifically designed for SOD1-ALS patients’ treatment.

• The phase III trial (VALOR), evaluating the efficacy and safety of tofersen, showed a striking decrease in CSF SOD1 protein concentration and plasma NfL levels, in comparison to placebo over 28 weeks.

• Earlier initiation of tofersen leads to a notable deceleration in the decline of clinical and respiratory function.

• The ongoing ATLAS study is designed to evaluate the impact of initiating tofersen in SOD1 presymptomatic carriers, and further assessing the utility of employing serum NfL as a reliable disease-related biomarker.

• Tofersen is a precision treatment tool for ALS, targeting patients with SOD1 pathogenic mutations.

Declaration of interest

Both authors are investigators of the ADORE and Phoenix trials which investigated oral Edaravone and AMX-0035 respectively. Oral Edaravone and AMX-0035 are both therapeutic options for ALS. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was not funded.