An appeals court in the USA has said that generics companies Teva and Ranbaxy can continue to share the 18-day marketing exclusivity period for copycat versions of Merck & Co’s cholesterol-lowering brand Zocor, barring other companies from entering the market until December 20.

The court upheld an earlier ruling in May which granted Teva and Ranbaxy joint exclusivity in the face of a legal challenge by rival generics company Sandoz, which also wanted a piece of the action.

The 180-day exclusivity period, a reward for companies who are the first to file for approval of a new generic drug, provides them with a financial incentive for investing in the development of copycat drugs and can be enormously lucrative.

Zocor (simvastatin) lost its patent protection on June 23, opening up a $3.14 billion US market to generic rivals. The drug, a star performer in the Merck's portfolio and the second biggest-selling cholesterol drug in the world, raked in sales of $4.4 billion last year, and the group has previously forecast a decline to $2.3-$2.6 billion for 2006 because of generic erosion.

Ranbaxy is claiming around 60% share of the market for the 80mg strength of simvastatin tablets, the only dose for which it has exclusivity, which accounted for sales of around $500 million a year in 2005.

Teva has exclusivity for the 5mg, 10mg, 20mg and 40mg doses, and while it has not disclosed its market share it said generic simvastatin was among the key drivers of the spectacular 127% sales growth it reported in the third quarter of 2006.