On 8 August 2024, as part of its second quarter (Q2) financial results and business progress reporting, Scholar Rock disclosed new positive data from its Phase II TOPAZ trial (NCT03921528). Results show continued and sustained motor function over four years in non-ambulatory patients with spinal muscular atrophy (SMA) treated with apitegromab and nusinersen. The positive data lays the foundation for apitegromab’s potential as an adjunctive therapy for SMA.

Apitegromab is one of three myostatin inhibitors currently in late-stage development as adjunctive therapies to approved disease-modifying treatments to improve motor function in SMA patients.  Key opinion leaders (KOLs) previously interviewed by GlobalData highlighted that despite the availability of disease-modifying therapies – Biogen’s Spinraza (nusinersen), Roche’s Evrysdi (risdiplam), and Novartis’s gene transfer therapy Zolgensma (onasemnogene abeparvovec) – there remains a significant disease burden for SMA patients who continue to have chronic motor impairments and functional deficits. As such, the prospect of an adjunctive therapy that could improve muscle strength and mobility will be welcomed.

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Apitegromab is a monoclonal antibody believed to improve muscle strength by inhibiting myostatin activation through the selective binding of myostatin in skeletal muscle. In the TOPAZ trial, type II and type III SMA patients received an intravenous infusion of apitegromab every four weeks in addition to the patients’ background treatment with an SMN [survival motor neuron protein]-targeted therapy (nusinersen or risdiplam).

Sustained motor function improvement at 48 months

Scholar Rock reported that at 48 months, more than 90% of the non-ambulatory patients enrolled in the study remained on apitegromab treatment and there was a sustained motor function improvement from baseline when analysed annually, from one year to four years of treatment.  At 48 months, a mean change of 5.3 points from baseline (95% CI: 1.5, 9.2; n=23) on the Hammersmith Functional Motor Scale (HFMSE) was observed in non-ambulatory patients aged two to 21 years who received treatment with nusinersen combined with apitegromab. The mean change in HFMSE was greater for patients between two and 21 at 6.4 points (95% CI: 1.8, 11.0, n=19). In addition, the mean change in Revised Upper Limb Module (RULM) for patients ages 2–21 was 3.6 points (95% CI: 2.0, 5.3; N=22) and 6.4 points (95% CI: 2.7, 6.3; n=18) for patients aged two to 12 years.

If positive results are achieved in Scholar Rock’s ongoing Phase III trials, SAPPHIRE (NCT05156320) and the open-label extension ONYX study (NCT05626855), apitegromab could be the first myostatin inhibitor to be approved as an adjunctive therapy for SMA. However, KOLs noted that intravenous infusions are typically administered by professionals in a healthcare institution. As such, apitegromab’s route of administration could impair accessibility and compliance, particularly in non-ambulant patients.

Myostatin inhibitors to contribute $259.4 million in sales

Apitegromab will compete with two other myostatin inhibitors also in Phase III development: Roche’s RG-6237 (MANATEE [NCT05115110]) and Biohaven’s taldefgrobep alfa (RESILIENT [NCT05337553]). RG-6237 and taldefgrobep alfa are being developed as subcutaneous injections that could offer greater convenience than intravenous infusions if they can be administered at home by patients or their caregivers.

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According to GlobalData’s recently published report Spinal Muscular Atrophy: Opportunity Assessment and Forecast, the anticipated launches of the three myostatin inhibitors during the forecast period of 2022 to 2032 will be a key driver of growth for the SMA market.  Such therapies, if proven to be well tolerated and effective at improving muscle mobility and function, are expected to play a significant role in improving the quality of life of SMA patients. Together, the myostatin inhibitors are forecast to contribute $259.4 million in sales to the SMA market across the seven major markets, France, Germany, Italy, Japan, Spain, the UK and the US, by 2033.