Japanese drugmaker Eisai has filed a lawsuit to defend its recently launched orally-disintegrating tablet formulation of Alzheimer's disease drug Aricept from generic competition in the USA.

The patent estate for Aricept ODT (donepezil HCl), sold in the USA by Eisai and Pfizer, has been challenged by Mutual Pharmaceutical and United Research Laboratories, although Eisai insists it has protection for the product under a composition-of-matter patent out to November 25, 2010.

Eisai won approval to sell Aricept ODT in October 2004, and sells it in 5mg and 10mg tablets – the same dosage strength as regular Aricept tablets which have been sold in the USA since 1997. The new formulation completed the mutual recognition approval process in Europe last December.

The challenge to the ODT formulation is a blow to Eisai, which has been developing new formulations of Aricept in order to safeguard the franchise when generic competition kicks in. The company recently also signed an agreement with fellow Japanese company Nitto Denko to develop a transdermal patch formulation of the drug, and also won approval for an oral liquid formulation in the USA in October 2004. The company does not break its sales for the drug down by delivery system.

Eisai reported sales 53.7 billion yen ($467mm) for Aricept in the first quarter of fiscal 2007, with $289 million coming from the US market alone, out of total group sales of 154 billion yen. Pfizer reported sales of $88 for the product in the same quarter, ended June 30.

Last December Eisai filed suit against Teva Pharmaceutical Industries after it emerged that the Israeli drugmaker was planning to develop and launch a copycat version of the regular tablet version of Aricept.

Meanwhile, India’s Ranbaxy and Par Pharmaceuticals of the USA have also received tentative approval from the US Food and Drug Administration for 5mg and 10mg donepezil tablets.