Sangamo Therapeutics and Pfizer have reported additional findings from the Phase I/II Alta clinical trial of investigational gene therapy SB-525 to treat patients with severe haemophilia A.

SB-525 is made up of a recombinant adeno-associated virus serotype 6 vector (AAV6) that encodes the complementary deoxyribonucleic acid for B domain deleted human Factor VIII (FVIII).

During the study, SB-525 was generally well-tolerated and led to a dose-dependent increase in FVIII activity levels.

The initial two patients who received 3e13 vg/kg dose were observed to quickly research normal FVIII activity levels without any reported bleeding events. The response was found to be sustained for 24 weeks, the extent of follow-up.

Data showed that the two patients who were recently administered with the dose are showing FVIII activity kinetics consistent with the first patients at similar early time points.

The companies further announced results from ten patients across 9e11 vg/kg, 2e12 vg/kg, 1e13 vg/kg, and 3e13 vg/kg dose cohorts.

A dose-dependent rise in FVIII levels and a dose-dependent decrease in the FVIII replacement therapy use were observed in subjects across these cohorts.

The two patients in the 1e13 vg/kg cohort demonstrated durable FVIII activity levels at weeks 52 and 32, while the four patients on the 3e13 vg/kg dose had FVIII activity data at 24, 19, six and four weeks of follow-up, respectively.

In the 3e13 vg/kg cohort, the first two patients retained normal range at 24 and 19 weeks of follow-up, respectively.

The next two subjects exhibited rapid FVIII activity kinetics at six and four weeks of follow-up and appeared to be consistent with the first two patients at similar early time points.

One treatment-related serious adverse event (SAE) was reported, with the patient experiencing hypotension and fever six hours following SB-525 infusion.

Sangamo Therapeutics Research and Development executive vice-president Adrian Woolfson said: “The results show that SB-525 is well tolerated, that Factor VIII levels in the first two patients in the 3e13 vg/kg cohort reached normal, sustained levels as measured using a chromogenic assay, and that variability of Factor VIII activity is low, both within each patient and within each dose cohort.”

The companies intend to further assess gene therapy in a registrational study. Pfizer will be responsible for SB-525’s late-stage development and manufacturing.