BEIJING—Foreign pharmaceutical manufacturers in China are facing new challenges as local drug companies step up their efforts to overturn the validity of domestically issued patents.

The industry was rocked in July when China's State Intellectual Property Office (SIPO) revoked patent protection for Pfizer's Viagra (sildenafil citrate). A Chinese-language document on SIPO's Web site says the Patent Reexamination Board (PRB) revoked Pfizer's patent for insufficient disclosure, arguing that Pfizer's application had failed to accurately explain the use of the drug's key ingredients.

Pfizer contended that SIPO retroactively applied a new amendment to the Patent Law when it ruled against its patent. E. Allan Gabor, chairman of Pfizer in China, said the company provided sufficient information in its patent manual to meet Chinese regulations at the time of application, adding that it appeared the PRB "used a higher standard this time around than the first time."

"To take 7 years to evaluate...

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