In a bid to slow down the effects of the US patent expiry of Fosamax, Merck & Co is to launch an authorised generic version of the blockbuster osteoporosis drug.

The New Jersey-based giant confirmed that it has signed an agreement with an unnamed firm to launch its own generic form of the 70mg formulation of Fosamax (alendronate) on February 6, just as the branded treatment’s patent expires. Fosamax, a bisphosphonate, is a major product for Merck and third quarter sales reached $725 million.

Full-year revenue from the drug is likely to be in the region of $3 million, but this figure will fall to $1.85 billion, according to analysts at Cowen & Co and fall over 50% again in 2009. However the launch of an authorised generic will take some of the wind out of the sails of Barr Laboratories and Teva Pharmaceutical Industries.

The two generics specialists are planning to launch their copycat versions of Fosamax on February 6 as well and they were looking to share 180-day exclusivity for the product. However Merck’s move threatens that windfall though both Barr and Teva noted that the news comes as no surprise.

The fact that Merck will sell generic alendronate is also likely to hurt makers of other bisphosphonates, notably Procter & Gamble and Sanofi-Aventis’ Actonel (risedronate) as doctors plump for the cheaper version. The whole bisphosphonate class is currently under review by the US Food and Drug Administration after two different studies in older women with osteoporosis of Fosamax and Novartis’ Reclast/Zometa (zoledronic acid) could increase the risk of atrial fibrillation.