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Gene-Replacement Therapy for SMA1 May Necessitate New Rating Measures to Capture Patients’ Motor Function Gains

CHOP-INTEND scores plateaued in a phase I trial of AVXS-101 even as patients reached new motor milestones.


CHICAGO—The Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) was designed to track motor function in patients with spinal muscular atrophy type 1 (SMA1), but the test may not be sensitive to advances in motor function made by patients who receive an investigational gene-replacement therapy, researchers said at the 47th Annual Meeting of the Child Neurology Society.

AVXS-101 is an adeno-associated virus serotype 9 (AAV9)-based gene-replacement therapy that contains a copy of the SMN1 gene. AVXS-101 crosses the blood–brain barrier and is meant to treat the deletion or loss of function of the SMN1 gene in patients with SMA. The treatment may result in sustained SMN protein expression with a one-time dose.

Researchers conducted a phase I, open-label, dose-escalation study to study the treatment. Fifteen patients received AVXS-101 intravenously, and 12 participants received the proposed therapeutic dose.

“Patients treated with AVXS-101 achieved and maintained motor milestones unprecedented in the natural history of SMA1,” said Linda P. Lowes, PT, PhD, Principal Investigator for the Center for Gene Therapy at Nationwide Children’s Hospital in Columbus, Ohio, and Associate Professor of Pediatrics at Ohio State University, and colleagues. “CHOP-INTEND is a reliable tool for detecting early treatment impact, but appears insensitive to further motor function gains among AVXS-101–treated children as they age and develop.”

Linda P. Lowes, PT, PhD

Linda P. Lowes, PT, PhD

For example, the scale did not capture skills such as independent rolling and sitting that had not been seen in patients with SMA1, the researchers said.

CHOP-INTEND uses a 0–64-point scale where higher scores indicate better motor function. By six months, essentially no untreated patients with SMA1 achieve a score of 40 or greater points on the CHOP-INTEND, and scores may decrease between age 6 and 12 months. In the phase I trial of AVXS-101, CHOP-INTEND score increased from baseline by a mean of 9.8 points at one month postdose, 15.4 points at three months postdose, and 25.4 points at 24 months (n = 12 at all time points). Eleven of the 12 patients maintained a CHOP-INTEND score greater than 40 points for a mean of 19.5 months. Two children were able to crawl, pull to a stand, and stand and walk independently.

In the study, patients who achieved additional sitting milestones did not have accompanying increases on their CHOP-INTEND scores. For example, the 11 patients who sat for at least 5 seconds had a mean CHOP-INTEND plateau score of 56.9, while the 10 patients who sat for at least 10 seconds had a mean CHOP-INTEND plateau score of 57.0 and the nine patients who sat for at least 30 seconds had a mean CHOP-INTEND plateau score of 57.7.

The investigators suggested that the addition of scoring items such as supported sitting, head lifting, and walking could “mitigate the artificial ceiling effect” with the current scoring system. Other modifications to CHOP-INTEND, such as assessing one side of a patient instead of both sides, could reduce test administration time without affecting scores.

Furthermore, several CHOP-INTEND items may not perform well in assessing patients treated with AVXS-101 because they test primitive reflexes that fade as children live longer (eg, spinal incurvation) or children may become too heavy for the tests (eg, ventral suspension).

The study was sponsored by AveXis.

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