Nov 3 (Reuters) - Sarepta SRPT.O said on Monday that its late-stage study testing two gene-targeted therapies for Duchenne muscular dystrophy did not meet the main goal, sending its shares plummeting over 36% in overnight trading.

The study took nine years to complete and enrolled 225 patients aged 6 to 13 with Duchenne, a rare genetic disorder that causes progressive muscle weakness and mostly affects boys.

The trial evaluated the therapies, casimersen and golodirsen, which belong to a class of drugs called phosphorodiamidate morpholino oligomers, or PMOs, that help patients produce a functional dystrophin protein.

While patients showed numerical improvement in climbing four steps after 96 weeks, the difference did not reach statistical significance.

Sarepta reported an observed difference of 0.05 steps per second on the primary endpoint.

It also noted that the COVID-19 pandemic disrupted trial participation and data collection, adding complexity to the study and potentially affecting the results.

Despite missing statistical significance, Sarepta said the therapies slowed disease progression by 30% when excluding COVID-affected patients, with long-term data showing a nearly three-year delay in the need for a wheelchair.

J.P. Morgan analyst Anupam Rama said, "we do see a fairly clear rationale for the miss (COVID)" and supportive trends when excluding these patients.

Despite the setback, Sarepta said it plans to meet with the U.S. Food and Drug Administration to discuss converting the drugs' current accelerated approvals into full approvals.

On a conference call, executives said they "really do not believe that there's a risk of losing marketing authorization" given the "extraordinarily beneficial safety profile."

Rama added: "We do see strong rationale for potential full approval," but cautioned that "regulatory processes can be a wild card in the current environment."

Duchenne muscular dystrophy usually begins in early childhood and worsens over time, making walking, breathing and other daily activities increasingly difficult.

Sarepta has lost about 80% of its market value this year, as mounting regulatory scrutiny and three patient deaths linked to its separate gene therapy, Elevidys, have triggered clinical trial holds and raised concerns about the company's safety oversight.

The company said it expects to finalize safety labeling talks with the FDA soon, likely resulting in a boxed warning and removal of the non-ambulatory use from Elevidys' label.