Canada's top court on Thursday stripped Pfizer of its patent for Viagra, saying the pharmaceutical giant failed in the patent to identify the active compound in the drug.
"Patent 2,163,446 is void," the Supreme Court said in its ruling.
The court explained that under Canadian patent rules an inventor must "properly disclose the invention and how it works" in order to be granted "exclusive monopoly rights" for it.
It concluded that Pfizer knew and did not properly disclose that sildenafil is the only compound in Viagra, out of several listed compounds, that induces penile erection in impotent males.
"The patent system is based on a 'bargain,' or quid pro quo: the inventor is granted exclusive rights in a new and useful invention for a limited period in exchange for disclosure of the invention so that society can benefit from this knowledge," the court said.
"If there is no proper disclosure, then there can be no exclusive monopoly rights."
Generic pharmaceutical company Teva brought the suit against Pfizer hoping to produce a generic version of Viagra before Pfizer's patent was due to expire in 2014.
The high court noted Pfizer had conducted tests proving sildenafil was the key to treating erectile dysfunction, and so concluded that "the invention was the use of sildenafil for the treatment of ED."
But Pfizer's patent only states that "one of the compounds (in Viagra) induces penile erection in impotent males, without specifying which is the effective compound."
Teva said in a statement it would soon start producing a generic version of the drug that will be "priced significantly lower than Viagra," which will result in "millions in savings to consumers" and "make this medication accessible to people who might otherwise not have been able to afford it."
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