Japanese drug maker Astellas Pharma Inc said on Wednesday it would begin mid-stage trials of a new drug candidate to help prevent organ rejection in kidney transplants.

Astellas said its immunosuppressant ASP0485 had a different mechanism from its top-selling transplant drug Prograf and is expected to be used together with Prograf.

The drug maker is under pressure to develop new transplant drugs as Prograf's U.S. patent expires next April.

Astellas will start Phase II trials of ASP0485 in North America and Europe and plans to develop the product globally, including Japan.

The drug candidate is already sold on the market under the name Amevive to treat psoriasis, a chronic skin disorder.

Astellas has also developed a once-daily version of Prograf, which is currently given twice a day, and which has been approved in Europe.

In the United States, it has submitted additional data for the once-daily version as requested by the Food and Drug Administration for liver transplants and is set to submit more data on kidney transplants later this year.

Around 16,000 kidney transplants are performed in the United States annually and Prograf is used in around 80 percent of those transplants.

Immunosuppressants are, however, different from other drugs in that the risk of organ rejection means doctors are more cautious about using generics.