Reacting to this, the drug major's scrip ended the day 5.06 per cent higher at Rs 748.95 on the BSE. During the day, the stock surged 11.52 per cent to Rs 795.
At the NSE, Natco's scrip was up 4.56 per cent at Rs 747.
Hyderabad-based Natco markets Copaxone (Glatiramer Acetate) -- used in the treatment of relapsing-remitting multiple sclerosis -- in the US market.
The drug's patent to Teva is set to expire in May.
It added: "The final rejection issued today finds the first claim of the re-issue application, which is identical to the only claim in the '808 Patent, to be indefinite and both claims of the reissue application to be unpatentable based on obvious-type double patenting. In its ruling, the PTO refused to extend patent protection on Copaxone(R) past the patents that will expire in May 2014.