Shares at Japanese drugmaker Astellas have fallen over 5% on the news that the FDA has issued a non-approvable letter over a new once-daily formulation of its immunosuppressant drug Prograf for heart transplantation.

The company also received a setback from the agency which has asked for additional information on the new treatment, FK506MR (tacrolimus) in two other indications, in the case of organ rejection in kidney and liver transplant patients. Astellas gave few details about the FDA’s communication and simply stated that it is “internally reviewing the comments outlined in the action letters” and is working with the agency to address any issues they have raised.

The US patent on Astellas' twice-daily version of Prograf runs out in 2008, and the firm had hoped to extend the lifecycle of the drug by getting the green light for this new formulation. Astellas estimates Prograf sales to hit 170 billion yen ($1.4 billion) for the fiscal year which ends in March, and the drug currently contributes around a fifth of the firm’s revenues. In the USA, it is currently used in 80% of kidney transplants, more than 90% of liver transplants and 60% of heart transplants.

Still, not all hope is lost for FK506MR and Hiroshi Tanaka, a senior analyst at Mizuho Securities, told Reuters that "as long as they get approval for kidney and liver transplants, even if they don't get approval for heart transplants, then it shouldn't be that much of a problem."

Antibody deal with Kirin

Meantime, Astellas also announced that it has signed an agreement with fellow Japanese firm Kirin to develop and market an immunosuppressive fully human monoclonal antibody.

The firms said that the anti-CD40 antibody had shown strong immunosuppressive activity in preclinical studies and is also expected to be used for certain autoimmune diseases. Astellas will pay Kirin up to 5.8 billion yen if all milestones are reached.