Teva Pharmaceutical Industries’ just-acquired IVAX business unit has become the latest generics company to mount a challenge to Astrazeneca’s best-selling gastrointestinal drug Nexium in the USA, filing for approval to market a delayed-release capsule version of the product.

IVAX joins Ranbaxy Laboratories in filing for a generic version of Nexium (esomeprazole), which is one of AstraZeneca’s most important growth products with sales up 18% to $4.63 billion in 2005, with $3.13 billion of that total coming from the USA.

In filing its Abbreviated New Drug Application in the USA, IVAX has asserted that AstraZeneca’s patents on Nexium are invalid, and the originator company now has 45 days to start a lawsuit that would block entry of the generic for 30 months. If IVAX won the suit, of course, it could get the all-clear to launch sooner.

While now accepted as a fact of life for research-based pharmaceutical companies, this type of patent challenge has a particularly unwelcome resonance for AstraZeneca at present, coming right after AstraZeneca lost a lawsuit aimed at defending its cardiovascular product Toprol XL (metoprolol) from generic competition in the USA.