Sanofi announced that the US Food and Drug Administration (FDA) has placed the Phase 3 studies of its investigational drug candidate tolebrutinib, on partial clinical hold.

The Phase 3 studies are evaluating tolebrutinib for the treatment of multiple sclerosis (MS) and myasthenia gravis (MG).

Tolebrutinib is an investigational brain-penetrant and bioactive Bruton’s tyrosine kinase (BTK) inhibitor that targets B lymphocytes and microglial cells.

The FDA has paused new enrolment in the US and told the participants who have been enrolled in the last 60 days to refrain from the drug.

However, the US health agency permitted the study participants who have completed at least 60 days to continue their treatment.

The French drugmaker said that the FDA decision was based on a few cases of drug-induced liver injury reported with the use of tolebrutinib in Phase 3 studies.

It was found that the majority of the impacted patients had concurrent complications known historically to predispose them to drug-induced liver injury.

Sanofi, in its statement, said: “Importantly, the elevations of laboratory values used for monitoring liver injury were reversible after drug discontinuation for all cases.

“Following earlier dialogue with FDA about these cases, study protocols were revised in May 2022 to update the monitoring frequency, and enrolment criteria were revised to exclude pre-existing risk factors for hepatic dysfunction.”

Earlier this year, Sanofi unveiled positive preclinical data that showed superior brain penetration and potency for tolebrutinib.

The drug had a superior combination of brain penetration and potency that strengthens its potential to impact neuro-inflammation, said the company.

The French firm said that it is confident that tolebrutinib will be a potentially transformative oral treatment option for people living with MS in the future.

It intends to continue enrolment in the clinical program with the revised study protocols and enhanced safety monitoring in countries outside of the US.

The company said it is working closely with the independent data monitoring committee and investigators worldwide to evaluate the effectiveness of safety measures.