The twists and turns in Shire Pharmaceuticals’ bid to defend its top-selling drug Adderall XR (mixed amphetamine salts) from generic competition continue and, on Friday, Impax Laboratories filed a countersuit claiming that one of Shire’s patents in the USA is invalid.

Last month, Shire initiated legal proceedings against Impax and another US generics company, Barr Laboratories, alleging that their attempts to market copycat versions of the attention deficit hyperactivity disorder treatment infringed its patent (No 6,913,768) [[21/10/05e]]. The patent was issued to Shire on July 5, 2005.

Shire is already suing the two companies for infringing two other US patents in cases due to come to trials in January and February 2006, respectively [[10/01/03b]] [[05/01/04d]].

Adderall XR is one of Shire's top-selling drugs, bringing in third quarter sales of some $166 million dollars.