Merck & Co has suffered a blow after a US court declared that the patent on Temodar is unenforceable, a decision that could soon see Teva Pharmaceutical Industries marketing its generic version of the big-selling brain cancer drug.

Judge Sue Robinson of the US District Court in Delaware has issued a decision in a lawsuit brought against Teva over its Abbreviated New Drug Application to sell a copycat version of Temodar (temozolomide), saying the patent is “not enforceable due to prosecution laches”, which refers to an "unreasonable and unexplained delay” in prosecution “and/or inequitable conduct”. Merck acquired the drug, a chemotherapy for the treatment of adults with newly-diagnosed glioblastoma multiforme and refractory anaplastic astrocytoma (two forms of brain cancer), through its recent takeover of Schering-Plough.

Temodar is a billion dollar blockbuster and has annual US sales of $369 million, according to Teva, whose ANDA already has tentative marketing approval from the Food and Drug Administration. Once the official thumbs-up is given, which could be imminent following the Delaware decision, the Israeli drugmaker is eligible to receive 180 days of market exclusivity.

Merck believes that the patent on Temodar is valid until February 2014. The company’s general counsel Bruce Kuhlik said “we are very disappointed with the court's ruling, and we continue to believe the patent for Temodar in the USA is valid and enforceable”. He added that the judge’s verdict “reflects a step in the lengthy patent litigation process and we plan to appeal this decision”.