Conference Coverage

Gene-replacement therapy shows promise in X-linked myotubular myopathy



A gene-replacement therapy called AT132 significantly decreases dependence on a ventilator among children with X-linked myotubular myopathy, according to research presented at the 2020 CNS-ICNA Conjoint Meeting, which was held virtually this year. The treatment also appears to improve patients’ motor function significantly and help them to achieve motor milestones.

Gavel and stethoscope belchonock/Thinkstock

The results come from a phase 1/2 study of two doses of AT132. Three of 17 patients who received the higher dose had fatal liver dysfunction. The researchers are investigating these cases and will communicate their findings.

X-linked myotubular myopathy is a rare and often fatal neuromuscular disease. Mutations in MTM1, which encodes the myotubularin enzyme that is required for the development and function of skeletal muscle, cause the disease, which affects about one in 50,000 to one in 40,000 newborn boys. The disease is associated with profound muscle weakness and impairment of neuromuscular and respiratory function. Patients with X-linked myotubular myopathy achieve motor milestones much later or not at all, and most require a ventilator or a feeding tube. The mortality by age 18 months is approximately 50%.

The ASPIRO trial

Investigators theorized that muscle tissue would be an appropriate therapeutic target because it does not display dystrophic or inflammatory changes in most patients. They identified adeno-associated virus AAV8 as a potential carrier for gene therapy, since it targets skeletal muscle effectively.

Nancy L. Kuntz, MD, an attending physician at Ann and Robert H. Lurie Children’s Hospital of Chicago, and colleagues conducted the ASPIRO trial to examine AT132 as a potential treatment for X-linked myotubular myopathy. Eligible patients were younger than 5 years or had previously enrolled in a natural history study of the disease, required ventilator support at baseline, and had no clinically significant underlying liver disease. Patients were randomly assigned to 1 × 1014 vg/kg of AAT132, 3 × 1014 vg/kg of AT132, or delayed treatment. Participants assigned to delayed treatment served as the study’s control group.

The study’s primary end points were safety and change in hours of daily ventilator support from baseline to week 24 after dosing. The investigators also examined a respiratory endpoint (i.e., maximal inspiratory pressure [MIP]) and neuromuscular endpoints (i.e., motor milestones, CHOP INTEND score, and muscle biopsy).

Treatment improved respiratory function

As of July 28, Dr. Kuntz and colleagues had enrolled 23 patients in the trial. Six participants received the lower dose of therapy, and 17 received the higher dose. Median age was 1.7 years for the low-dose group and 2.6 years for the high-dose group.

Patients assigned to receive the higher dose of therapy received treatment more recently than the low-dose group, and not all of the former have reached 48 weeks since treatment, said Dr. Kuntz. Fewer efficacy data are thus available for the high-dose group.

Each dose of AT132 was associated with a significantly greater decrease from baseline in least squares mean daily hours of ventilator dependence, compared with the control condition. At week 48, the mean reduction was approximately 19 hours/day for patients receiving 1 × 1014 vg/kg of AAT132 and approximately 13 hours per day for patients receiving 3 × 1014 vg/kg of AT132. The investigators did not perform a statistical comparison of the two doses because of differing protocols for ventilator weaning between groups. All six patients who received the lower dose achieved ventilator independence, as did one patient who received the higher dose.

In addition, all treated patients had significantly greater increases from baseline in least squares mean MIP, compared with controls. The mean increase was 45.7 cmH2O for the low-dose group, 46.1 cmH2O for the high-dose group, and −8.0 cmH2O for controls.

Before treatment, most patients had not achieved any of the motor milestones that investigators assessed. After treatment, five of six patients receiving the low dose achieved independent walking, as did one in 10 patients receiving the high dose. No controls achieved this milestone. Treated patients also had significantly greater increases from baseline in least squares mean CHOP INTEND scores, compared with controls. At least at one time point, five of six patients receiving the low dose, six of 10 patients receiving the high dose, and one control patient achieved the mean score observed in healthy infants.

Patients in both treatment arms had improvements in muscle pathology at weeks 24 and 48, including improvements in organelle localization and fiber size. In addition, patients in both treatment arms had continued detectable vector copies and myotubularin protein expression at both time points.

Deaths under investigation

In the low-dose group, one patient had four serious treatment-emergent adverse events, and in the high-dose group, eight patients had 27 serious treatment-emergent adverse events. The three patients in the high-dose group who developed fatal liver dysfunction were among the older, heavier patients in the study and, consequently, received among the highest total doses of treatment. These patients had evidence of likely preexisting intrahepatic cholestasis.

“This clinical trial is on hold pending discussions between regulatory agencies and the study sponsor regarding additional recruitment and the duration of follow-up,” said Dr. Kuntz.

Audentes Therapeutics, which is developing AT132, funded the trial. Dr. Kuntz had no conflicts of interest.

SOURCE: Bönnemann CG et al. CNS-ICNA 2020, Abstract P.62.

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