Merck & Co has signed a deal for an authorized generic form of its blockbuster Fosamax osteoporosis drug

Merck & Co Inc has signed a deal for an authorized generic form of its blockbuster Fosamax osteoporosis drug, to become available after the U.S. patent on the world's top-selling osteoporosis drug lapses on February 6, 2008.

Fosamax, whose chemical name is alendronate, has global annual sales of about $3 billion. It was the first member of the widely used family of treatments known as bisphosphonates that inhibit cells called osteoclasts that break down bone tissue.

An authorized generic is a copycat form of a company's branded medicine that is sold through a licensing agreement, usually with a generic-drug manufacturer. Such deals allow the original seller of a branded drug to hold on to a greater revenue stream from the medicine once it loses patent protection and becomes prey to competition from generics. An authorized generic can significantly hurt sales of rival generics waiting to be launched once a branded drug's patent lapses.

Analysts have said cheaper generics will not only batter sales of Merck's branded Fosamax, but could hurt rival once- weekly bisphosphonate Actonel — which is sold by Procter & Gamble Co and French drugmaker Sanofi-Aventis. Boniva, a newer bisphosphonate sold by Roche Holding AG and GlaxoSmithKline Plc, is only taken once a month.

Fosamax is also sold in a formulation that is combined with either 2800 units or 5600 units of Vitamin D, a nutrient that supports bone and other tissues.