Frank Baldino Jr., chief executive and founder of biotechnology company Cephalon, died Thursday night at the age of 57, the company said. The cause of death was not disclosed.

Cephalon announced in August that Baldino would take a temporary medical leave and said the company, which makes the sleep disorder drugs Provigil and Nuvigil, would be run in his absence by Kevin Buchi, its chief operating officer.

On Dec. 6 the company said Baldino would be on medical leave indefinitely and that Buchi would continue to perform his duties.

Announcing his death, Fritz Bittenbender, vice president of public affairs, said that "in deference to Frank and his leadership, we are not going to make any announcements about the transition in the near future."

Baldino, who received a Ph.D. in pharmacology from Temple University, founded Cephalon roughly 20 years ago and built it on the back of Provigil, a drug for a rare sleep disorder called narcolepsy. Provigil was subsequently approved in other indications.

The company now focuses on central nervous system disorders such as pain, and cancer. Cephalon has a market value of $4.7 billion.

From 1981 to 1987, Baldino served as senior research biologist in the medical products department at DuPont, where he was responsible for developing research strategies for identifying novel neuropharmaceutical agents.

Among many other positions, Baldino chaired the Executive Council of the Harvard Division of Sleep Medicine and was a member of the boards of Temple University, the University of the Sciences in Philadelphia and the Franklin Institute.