Wyeth Ltd filed a Para IV infringement suit against the Ahmedabad based Zydus Cadila and its US subsidiary, Zydus Inc, for the latter's move to market generic equivalent of its major anti depressant drug, Effexor XR, an extended release of venlafaxine hydrochloride in the US.

The law suit filed in the US District Court for the District of Delaware alleges that Zydus has filed an Abbreviated New Drug Application (ANDA) with the US FDA for venlafaxine hydrochloride extended release formulation even as three patents of the same product from Wyeth, US Patent Nos. 6,274,171 (issued on August 14, 2001), 6,403,120 (issued on June 11, 2002), and 6,419,958 (issued on July 16, 2002), are still valid.

According to the petition filed by Wyeth, Zydus Cadila has developed the generic copies of Effexor XR capsules with strengths of venlafaxine hydrochloride 37.5, 75 and 150 mg extended release and has filed ANDA 90-174 with the US drug regulator seeking approval to market the product in US market. The three patents of Wyeth pertaining to the product is valid up to 2017, it is learnt.

Effexor, a leading product in the antidepressant market, accounted for 16.9 per cent (USD 3.8 billion) of the company's annual sales in 2007. The product belongs to a different class of antidepressant (SNRIs) than older drugs (SSRIs), and is often effective for patients not responding to SSRIs. The patent on Effexor's active ingredient (venlafaxine) ran out in mid 2008, although Wyeth still holds patent on the capsule form until 2010, as per reports.

Cadila Healthcare in June 2008 has announced launch of venlafaxine hydrochloride tablets in the US market. The global market for CNS-related pharmaceuticals which was recorded at around USD 56 billion in 2005 is dominated by the anti-depressant drug class with a market share of more than 30 per cent, according to a market study published in 2007.

Studies have also forecasted a splurge of generic copies of Effexor in the US market from 2009, as the patent of the base ingredient has expired on 2008. Further, the market is expected to increase widely in the coming years, as the World Health Organization (WHO) predicts that by 2020 depression will be the second largest cause of the global health burden.

Apart from Zydus, Lupin Pharmaceuticals, Sun Pharmaceuticals and Torrent Pharmaceuticals has already moved to tap the Effexor market. While Lupin and Torrent are facing patent litigation, Wyeth has not sued Sun Pharmaceuticals as the ANDA filing was on tablet form.