Shares in Astellas have fallen on the news that a Novartis unit has launched a generic version of the Japanese drugmaker’s blockbuster transplant drug Prograf in the USA.

The Swiss major’s Sandoz division has introduced its generic equivalent of Prograf (tacrolimus), the immunosuppressive treatment used to help prevent rejection of a kidney or liver transplant. It is the first generic tacrolimus product to be launched in the USA despite losing patent protection in April 2008.

Sandoz chief executive Jeff George noted that the firm and parent Novartis have “deep experience in the transplant field and we are deeply committed to providing more cost-saving treatment options for this community in the future". Their products include the cyclosporine drugs Sandimmune and Neoral, used for the prevention of organ rejection following kidney, liver or heart transplantation, as well as Myfortic (mycophenolic acid) delayed-release tablets, indicated for kidney transplants, and Simulect (basiliximab), for the prevention of acute organ rejection.

The arrival of generic competition in the USA is a major blow for Astellas, seeing as how, according to IMS Health, Prograf had sales there of $929 million for the year ended April 2009.

In response, the US unit of the Tokyo-based group is preparing to challenge the US Food and Drug Administration in a Washington DC court, after the agency dismissed Astellas request to demand certain clinical trials before approving generic versions of Prograf.

Astellas noted that the FDA had “substantially denied” its request that the agency take additional measures to protect transplant recipients, “a unique and vulnerable patient population, from substitute critical dose immunosuppressant drugs that have not demonstrated bioequivalence in rigorous clinical trials”. As a result it is heading to the courts, saying that under the FDA's decision, “bioequivalence testing is required only in healthy volunteers, not the patient population that will be treated with the new product”.

The FDA has also denied Astellas' request for labelling changes that require doctors to be notified whenever a “substituted oral formulation” is about to be provided to a transplant patient so that the physician can decide whether “additional drug blood concentration testing should be done to ensure the health and safety of the patient”. Astellas is also challenging this decision in the court.

William Fitzsimmons, Astellas’ senior vice president of development, noted that “this is a specialised category of medicine and transplant patients are vulnerable to small differences in drug concentrations, which can lead to significant differences in their treatment outcome. We look forward to open discussion about this important issue."