With competition heating up in the spinal muscular atrophy (SMA) space, Biogen has been hoping FDA approval of a new dosing regimen for Spinraza (nusinersen) could help refresh interest in the antisense oligonucleotide (ASO) — but its market plans have been put on pause after the US regulator declined to greenlight the supplemental application. Biogen said Tuesday that it received a complete response letter (CRL) for a high-dose Spinraza regimen that would lower the number of loading doses to two, rather than four, followed by higher maintenance doses every four months.The letter did not flag any problems with Spinraza's data package, Biogen said, which was based on results from the Phase II/III DEVOTE study, shared at last year's World Muscle Society (WMS) meeting. The trial met its primary endpoint in treatment-naïve, symptomatic infants, who achieved significantly greater improvements in motor function than untreated matched controls.Instead, the FDA is requesting a technical information update be included in the chemistry, manufacturing and controls (CMC) portion of the drug's application. Issues related to CMC are the most frequent reason for an FDA rejection, a recent FirstWord analysis found. The agency in July posted a batch of CRLs associated with more than 200 new drug applications, and deficiencies at manufacturing facilities or subpar quality of the manufactured drug product led to a combined 60% of all rejections (see – Vital Signs: What we learned from the FDA's CRL data dump).Luckily for Biogen, the fix appears to be easy — it said the FDA has shared options to resolve the application's issue, with a resubmission planned "promptly.""While this outcome was unexpected, we remain committed to bringing the high dose regimen to people living with SMA," said Priya Singhal, head of development at Biogen. The company noted that high-dose Spinraza was recently approved in Japan, and it's currently under review in the EU. High-dose Spinraza was one of two new SMA options expected to win FDA approval this week — and Scholar Rock's entrant, apitegromab, met a similar fate to Biogen's ASO (see – Spotlight On: How will newcomers reshape the SMA market?).Separately on Tuesday, the US regulator issued a CRL to Scholar Rock's application for apitegromab, citing CMC issues. The letter identified problems with Catalent's Indiana fill-finish facility, and were not specific to the antibody. Once the issues at the facility have been addressed, Scholar Rock said it will resubmit its application for apitegromab with the hope that the FDA "will be able to act expeditiously" on the re-filing.If Biogen and Scholar Rock are successful with their resubmissions, their products will compete with Roche's oral small molecule Evrysdi (risdiplam), which has charted a steep growth trajectory since its launch in mid-2020. Last year, the more conveniently-dosed oral drug posted higher revenue figures than both the standard dosing regimen of Spinraza and Novartis' gene therapy Zolgensma (onasemnogene abeparvovec) (see – Vital Signs: Roche outmanoeuvres another gene therapy).Biogen and partner Ionis Pharmaceuticals also have another SMA shot-on-goal in the works. Registrational studies are being prepped for salanersen after Phase I data for the ASO — administered intrathecally once per year — demonstrated that it can slow neurodegeneration in patients previously treated with Zolgensma.