The research was presented online as part of the 2020 AAN Science Highlights.
SMA results from a mutation in SMN1, which encodes the SMN protein necessary for motor function. Deficiency of this protein causes motor neurons to die, resulting in severe muscle weakness. At 2 years of age, untreated patients with SMA type 1 generally die or require permanent ventilation.
The Food and Drug Administration approved onasemnogene abeparvovec-xioi under the brand name Zolgensma in May 2019. The gene-replacement therapy, which is administered once intravenously, delivers a fully functional copy of human SMN1 into the target motor neuron cells. It is indicated as treatment for SMA in infants younger than 2 years of age.
Preliminary STR1VE data
Preliminary data from the phase 3 STR1VE study were scheduled to be presented at the meeting. The open-label, single-arm, single-dose study enrolled symptomatic patients with SMA type 1 (SMA1) at multiple US sites. Enrollment was completed in May 2019.
The study included 10 male patients and 12 female patients. Participants’ mean age at dosing was 3.7 months. Of 19 patients who could have reached age 13.6 months at data cutoff, 17 (89.5%) were surviving without permanent ventilation, compared with a 25% survival rate among untreated patients. One of the 19 patients died, and the event was judged to be unrelated to treatment. Another of the 19 reached a respiratory endpoint or withdrew consent.
The population’s mean baseline Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) score was 32. This score increased by 6.9, 11.7, and 14.3 points at months 1, 3, and 5, respectively. Half of the 22 infants sat independently for 30 or more seconds, and this milestone was achieved at a mean of 8.2 months after treatment. Five of six (83%) patients age 18 months or older sat independently for 30 or more seconds, which was one of the study’s primary endpoints. As of March 8, 2019, treatment-emergent adverse events of special interest were transient and not associated with any sequelae.
The STR1VE study was sponsored by AveXis, the maker of onasemnogene abeparvovec-xioi. Several of the investigators are employees of AveXis, and others received funding from the company.
Long-term follow-up in START
Long-term follow-up data for participants in the phase 1/2a START study also were scheduled to be presented. Patients who completed START were eligible to participate, and the trial’s primary aim was to evaluate the long-term safety of onasemnogene abeparvovec-xioi. Patients are intended to have five annual visits, followed by 10 annual phone calls, and the investigators request local physicians or neurologists to transfer patient records. Safety assessments include medical history and record review, physical examination, clinical laboratory evaluation, and pulmonary assessments. Efficacy assessments include evaluation of the maintenance of developmental milestones.
As of May 31, 2019, 13 patients in two cohorts had been enrolled and had had a baseline visit. For patients in Cohort 2, the mean age and time since dosing were 4.2 years and 3.9 years, respectively. All patients in Cohort 2 were alive and did not require permanent ventilation. Participants did not lose any developmental milestones that they had achieved at the end of START. Two patients were able to walk, and two could stand with assistance during long-term follow-up. This result suggests the durability of the treatment’s effect. No new treatment-related serious adverse events or adverse events of special interest had occurred as of March 8, 2019.
Dr. Jerry R. Mendell
“We know from accumulating experience that treating infants by gene therapy is safe,” said Jerry R. Mendell, MD, the principal investigator and an attending neurologist at Nationwide Children’s Hospital in Columbus, Ohio. “Of the 15 patients we had in our first trial, only four adverse events related to the gene delivery were encountered, and only two of these were considered serious adverse events [i.e., liver enzymes that were 10 times greater than normal laboratory levels]. These laboratory tests occurred without accompanying clinical symptoms or signs. All were suppressed by corticosteroids and related to the liver inflammation. This pattern of safety has been seen in our very large gene therapy experience. No long-term surprises were encountered.”
The START study was sponsored by AveXis. Several of the investigators are employees of AveXis, and others received funding from the company.