Apotex has launched the first generic version of GlaxoSmithKline blockbuster antidepressant Paxil (paroxetine) in the US, several months earlier than expected.

After legal wrangling dating back to 1998, Paxil exclusivity was dealt a fatal blow last March when a US court ruled that Apotex version did not infringe on GSK's patent.

GSK has launched is spoiling tactic of putting its own generic paroxetine onto the market via a royalties-based licensing agreement with another generic firm, Par Pharmaceuticals.

Paxil was GSK biggest selling product in 2002 with 69% of its global sales of £2.1 billion earned in the US market.

GSK has been able to protect itself from a huge shock to its revenues by launching last year the longer-lasting formulation Paxil CR, which has captured approximately 40% of new Paxil prescriptions in the US.

But market analysts Decision Resources predict generic paroxetine will become the "Drug of choice in the management of anxiety disorders" for US doctors, undermining sales of all the leading branded depression treatments.

Decision Resources say generic Paxil will have an even greater impact on prescribing behaviour than the launch of generic Prozac in 2001 and, along with prospective generic competition for Lundbeck Cipramil, will prompt prescribers to switch from the original products and also from other antidepressants, such as Pfizer Zoloft and Wyeth Effexor XR.

Apotex Chief Executive Dr Barry Sherman said: "We consider it outrageous that GSK has kept an affordable alternative from consumers for years by asserting patents that they knew, or ought to have known, were either invalid or that our product did not infringe."

Apotex says it also has the capacity to launch its generic Paxil onto the European market, where the drug is known as Seroxat.

Apotex and GSK remains locked in patent litigation over the product with a court case expected in 2004. If Apotex were found to have infringed GSK's patent it would be liable to treble damages because of having already launched the product.

GSK's second-best selling antidepressant Wellbutrin recently gained FDA approval for an extended release version. GSK is treating Wellbutrin XL as a new product launch in its own right to persuade prescribers away from generic versions of the original.

Generic competition for the original product means total Wellbutrin sales are expected to fall from the £840 million forecast for 2003 to £350 million in 2004.

pharmafocus@pharmafile.co.uk