Two First-Ever Rare Disease Approvals Landed the Same Day. Both Were Measured by Proxy.
- The FDA approved Regeneron‘s Pasatru (garetosmab-grts) on 19 August 2026, the first treatment for fibrodysplasia ossificans progressiva, an ultra-rare disorder affecting roughly 900 people worldwide. In the Phase 3 OPTIMA trial (n=63), new heterotopic ossification lesions fell 90% at 10 mg/kg and 94% at 3 mg/kg against placebo at week 56.
- The same day, Ultragenyx received accelerated approval for GENGLYCOS (pariglasgene brecaparvovec-opnr), the first therapy targeting the cause of glycogen storage disease type Ia. The Phase 3 GlucoGene study treated 46 participants aged eight and over, showing reduced cornstarch requirement (p<0.001). GSDIa affects 1,500 to 2,500 patients in the US.
- Both approvals rest on measures a step removed from patient experience. Pasatru’s flare-up benefit is clinician-assessed: patient-reported flare-ups showed no significant difference from placebo. GENGLYCOS is indicated to reduce daily cornstarch intake, with continued approval contingent on verifying clinical benefit.
- Ultragenyx received a Priority Review Voucher and will supply two years of data on 50 commercially treated patients against 20 controls, drawn from patients ineligible because of anti-AAV8 antibodies. Regeneron has EU review underway, Japan planned, and a paediatric trial, OPTIMA 2, due to start this year.
Clinical read
Both are real firsts for diseases that had nothing, and neither should be talked out of existing. The question is narrower. What did each trial measure, and how far is that from what a patient would notice.
- Pasatru’s flare benefit exists only in the clinician’s assessment. Clinician-assessed flare-ups fell 88% at 10 mg/kg. Patient-reported flare-ups showed no significant difference from placebo in any group. Regeneron discloses this plainly. The indication wording carries it too, and most coverage will skip it.
- The OPTIMA dose response runs in opposite directions on the two endpoints. The 3 mg/kg arm beat 10 mg/kg on lesions, 94% against 90%, then delivered a 15% flare reduction against 88%. The approved starting dose is 10 mg/kg. Ask which endpoint drove that choice.
- GENGLYCOS is indicated to reduce cornstarch, and the confirmatory arm is not randomised. The 20 controls are patients who sought treatment but carry anti-AAV8 antibodies. Seropositivity is not random. Treat the confirmatory package as observational when modelling the risk of withdrawal.
Read the original source (Regeneron) → | Read the original source (Ultragenyx) →
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