US regulators are refusing to approve Sanofi and Regeneron's sarilumab to treat patients with rheumatoid arthritis until certain manufacturing issues are addressed.

The US Food and Drug Administration has issued a Complete Response Letter highlighting certain deficiencies identified during a routine good manufacturing practice inspection of the Sanofi Le Trait facility where sarilumab is filled and finished, one of the last steps in the manufacturing process.

These deficiencies must be rectified before the drug can be approved; Sanofi says it has already begun implementing corrective actions and is working closely with the FDA towards a timely resolution that addresses these concerns.

Sarilumab is an investigational, fully human monoclonal antibody directed against the IL-6 receptor that has been developed for the treatment of patients with active, moderate-to-severe forms of the condition.

The drug's application contains data from around 2,500 patients who had an inadequate response to previous treatment regimens, including seven studies from the global SARIL-RA Phase III program.

Recently released data from the SARIL-RA-TARGET study showed "clinically relevant and statistically significant improvements" versus the placebo arm on co-primary endpoints of improving symptoms of the disease and physical function.

Improvement in signs and symptoms of RA at 24 weeks, as measured by the American College of Rheumatology score of 20 percent improvement (ACR20), were: 61 percent in the sarilumab 200mg group; 56 percent in the sarilumab 150mg group; and 34 percent in the placebo group, all in combination with disease-modifying anti-rheumatic drugs.

The drug is also currently under review Europe.