Cialis, the second-largest erectile dysfunction drug after Pfizer's Viagra, has patents in over 30 countries and the drug earned it $971 million in worldwide sales in 2006.
Earlier this year, the patent office turned down an application for Novartis' cancer drug Gleevec. However, Lilly has secured a process patent in India on Cialis, which means it can produce the same drug but through a different process.
The drug is available in the market but in the absence of a product patent, it competes against other cheap versions of the drug.
Lilly's attempt to secure both product and process patents for Cialis in the country was opposed by domestic drug maker Ajanta Pharma, which held that the product was not a new invention.
Ajanta Pharma said Indian scientists from the Central Drug Research Institute (CDRI), Lucknow, had synthesised the basic components of the drug in 1970.
Ajanta also pointed out that the scientists had secured a US patent (no: 3,917,599) for their invention five years later.
The patent office, in its verdict early this year, found merits in the opponent's claim thereby rejecting a product patent for Cialis.
"Mere discovery of any new property of known substances are not patentable under the Indian Patent Act," said Nitya Nand, former director of CDRI and a co-author of the Indian patent.
In other words, he explained, Cialis was an obvious extension of the knowledge CDRI had patented. In fact, Ajanta has appealed to the Intellectual Property Appellate Board (IPAB) against the grant of the process patent to Lilly.
Meanwhile, Lilly has moved a revision petition against the rejection of its patent application, "We strongly believe that the rejection should be reconsidered and overturned, and we have requested the patent office to do so," Lilly said in an e-mail response.
"We are confident that the government is committed to providing meaningful protection and we would await their decision," the company spokesperson said.
Lilly sources said that tadalafil is not a derivative of a known substance but a new chemical entity that 72 other countries have deemed to be novel and non-obvious from all known compounds.
"Only India has rejected this compound, and indeed, only in India has anyone even attempted to oppose the patent, despite the important commercial value of the compound," company sources said.
The patent offices that have granted the patent include those from developed countries like the US, Europe and Japan, as well as from developing countries like China, the sources said.
Incidentally, Cialis was one of the early products that received exclusive marketing rights (EMR) before the full- fledged amendment to the patent law came in place.
The EMR for its application [85/DEL/1995] was challenged by Ajanta in the Calcutta High Court and a stay order followed.
Patent experts said the ruling may impact any pending patent decisions in other countries.
"The implications are not yet clear but the fact that the molecule was known way back in 1975 can have an adverse impact in other jurisdictions," said Gopakumar Nair, a Mumbai-based patent expert.