Japanese drugmaker Astellas has received a second approvable letter from US regulators for its immunosuppressant FK506 Modified Release in preventing organ rejection in kidney transplant patients.

The move, which signals a further delay to the drug’s entry onto the all-important US market, pushed the Tokyo-based firm’s shares down nearly 8% on Friday, as investors were hoping that FK506, a once-daily version of Astellas’ immunosuppressant Prograf (tacrolimus), would be launched before the latter’s patent protection runs out in April.

Astellas has been working hard to get its Prograf successor onto the US market since 2005, when it submitted a New Drug Application for the use of FK506 in kidney, heart and liver transplant patients. But the US Food and Drug Administration sent out two approvable letters for kidney and heart recipients and a non-approvable letter for heart transplant patients in early 2007, asking for more information on the drug.

Now, following the receipt of a second approvable letter for FK506 to prevent transplant rejection in kidney patients, Astellas said it is examining its future strategy for the drug, and that it is working with regulators to “better understand the issues raised”.

The company failed to disclose exactly what the issues raised in the approvable letter were, but media reports are claiming that they relate to safety and efficacy data on the drug, and that the delay to the delay to its approval could be up to six months.