Pfizer was celebrating yesterday after a US judge upheld its patents for blood pressure drug Norvasc (amlodipine besylate) and blocked generic competition to the multibillion-dollar product until September 2007.

The federal court in Chicago ruled that the patent covering the active ingredient in Norvasc was valid and still enforceable, preventing Canadian generics firm Apotex from launching its copycat version.

Apotex has yet to respond publicly to the news, but is expected to appeal the decision. Meanwhile, other generics companies, including Mylan Laboratories and Dr Reddy’s Laboratories, have taken aim at the Norvasc franchise in the USA, and two cases are due to reach the courts later in 2006.

Norvasc is Pfizer’s second-largest product, bringing in $3.5 billion to the firm’s coffers in the first nine months of 2005, $1.7 billion of which was from the USA. So the extra period of exclusivity is likely worth a great deal to the world’s biggest drugmaker, given that big-selling products can lose upwards of 70% of their sales in the first year of generic competition. That said, the product has already lost patent protection in much of Europe, but ex-US sales still managed to climb fractionally in the same nine-month period.

Norvasc is just one of a number of Pfizer’s products that are exposed to generic competition, and the loss of exclusivity for drugs such as antibiotic Zithromax (azithromycin) and antifungal Diflucan (fluconazole) is the main reason the drugmaker has trimmed its earnings outlook for 2005. Pfizer is scheduled to report its fourth-quarter results later today.

But lately Pfizer has been successful in preventing early competition from generic companies intent on busting its patents. The Norvasc victory follows an even more significant case involving Pfizer’s top-selling cholesterol-reducer Lipitor (atorvastatin), in December. In this lawsuit, Pfizer successfully defended the $12 billion brand from a challenge by India’s Ranbaxy Laboratories, although the latter company is appealing that verdict.