Shares of both Sarepta (SRPT) and Solid Biosciences (SLDB) have been volatile in early trading after Pfizer (PFE) presented “preliminary data drawn from a small number of participants” in an ongoing Phase 1b study of PF-06939926, an investigational gene therapy to potentially treat Duchenne muscular dystrophy, or DMD. Pfizer said preliminary results from open muscle biopsies of the biceps taken 2 months after dosing show detectable mini-dystrophin immunofluorescence signals with a mean of 38% positive fibers taken from participants who received PF-06939926 at 1E14 vg/kg and a mean of 69% positive fibers taken from participants who received PF-06939926 at 3E14 vg/kg. Using an FDA-reviewed LCMS assay, “normal” concentrations of dystrophin were based on pooled skeletal muscle biopsies from 20 pediatric samples, which resulted in mean concentration of just below 3,000 fmol/mg of protein, while levels in the individual samples differed from the mean approximately 50 to 150%. In the ongoing Phase 1b study, mini-dystrophin concentrations 2 months after dosing for all 6 DMD study participants showed a range of 300-1800 fmol/mg of protein, or 10-60% of “normal”. The mean expression level of mini-dystrophin was 23.6% for participants who received PF-06939926 at 1E14 vg/kg and 29.5% for those who received at 3E14 vg/kg. In terms of safety, preliminary results show that the most common adverse events suspected to be related to PF-06939926 are nausea, vomiting, decreased appetite, tiredness and/or fever, which were reported within a few days of dosing by 4 of 6 study participants. Pfizer noted that one of the 6 participants developed a rapid antibody response with activation of the complement system associated with acute kidney injury, hemolysis, and reduced platelet count. “This participant was promptly admitted to a pediatric intensive care unit and received intermittent hemodialysis, as well as 2 intravenous doses of a complement inhibitor. He was discharged from the hospital after 11 days, and his renal function returned to normal within 15 days. None of the other dosed participants had immune-related clinical events. Regardless, in accordance with the original study design, no other participants will be dosed until the specific additional safety monitoring, which has been endorsed by the external data monitoring committee, has obtained all appropriate approvals at the clinical research sites,” the company noted. In early trading after Pfizer’s press release regarding its presentation, Sarepta shares are up 17% to $151.83 while Solid shares are down 5% to $4.80.