By Deena Beasley and Ankur Banerjee

Regeneron Pharmaceuticals Inc and Sanofi said on Thursday their rheumatoid arthritis drug Kevzara failed to meet the main goals of a U.S. study testing it in the most critically ill COVID-19 patients.

Previous trial results had shown that the drug did not help patients with less severe COVID-19, the disease caused by the novel coronavirus, and shares of Regeneron fell about 3% in after hours trading.

Kevzara belongs to a class of drugs called interleukin-6 inhibitors that are being tried to help regulate a dangerous overreaction to the virus by the body's immune system called "cytokine storm."

Other drugs in the same class, including Roche Holding AG Actemra, are also being studied as treatments for COVID-19.

A separate Sanofi-led trial of Kevzara outside of the United States in hospitalized patients with severe and critical COVID-19 using a different dosing regimen is ongoing, the companies said.

Patients who required mechanical ventilation or high-flow oxygen therapy or treatment in an intensive care unit were considered critically ill. Those who required oxygen without mechanical or high-flow oxygenation were considered severely ill.

More than three dozen U.S. states were seeing a rise in COVID-19 cases, according to a Reuters analysis on Thursday, the latest grim sign that the coronavirus pandemic, once thought to be waning, was again spreading rapidly.

Worldwide, the virus has infected 10.76 million people, resulting in 516,600 deaths.

Shares of Regeneron were down $17.20 at $605.25 after hours. They are up about 67% so far this year.

(Reporting by Ankur Banerjee in Bengaluru; Editing by Aditya Soni and David Gregorio)