The US Court of Appeals for the Federal Circuit has reversed the New Jersey District Court's dismissal of Pfizer's patent infringement action against Dr Reddy's Laboratories' generic version of the calcium channel blocker amlodipine besylate (Norvasc; Pfizer). The pharmaceutical industry has been closely monitoring this case, as Dr Reddy's used a novel approach to make its generic drug.

Pfizer's original patent on amlodipine besylate (US Patent 4,572,909) expired in 2003, but was extended until 2007 to compensate for a lengthy review process by the US FDA. At issue in this case was whether the patent extension was limited to Pfizer's marketed product or also applied to other chemical versions of the drug. As with many drugs, Pfizer's original patent protected both the chemical structure of amlodipine besylate and a host of sister compounds, or salts.

Dr Reddy's argued that Pfizer's patent extension did not apply to their version of the drug, amlodipine maleate. The district court agreed and held that the term extension was limited to amlodipine besylate. Although amlodipine maleate is covered by the claims, it is not subject to the extended term. The court reasoned that the 35 USC statute limits the term extension to the first permitted commercial marketing or use of the product and that this was relevant only for amlodipine besylate.

Two of the three Federal Circuit judges disagreed, and concluded that the active ingredient is amlodipine, so it is the same whether administered as the besylate salt or the maleate salt. The Act by its terms extended the term of the patent for the registered uses of the drug product including its salt esters. However, Judge Mayer dissented, arguing that to be eligible for a patent term extension, the product must have been subject to a regulatory review period before its commercial marketing or use. In this case, only amlodipine besylate was subject to regulatory review. The decision allows Pfizer to continue its infringement action in the district court.