Early this year, Johnson & Johnson prostate cancer blockbuster Zytiga took an unexpected hit when a U.S. court stripped away a key patent, opening the prospect of generic launches later this year—a prospect J&J is working hard to avoid. But as of Wednesday, the $1.2 billion drug has copycat competition from another quarter.

India's Sun Pharma won an FDA nod for Yonsa, a drug containing the same active ingredient as Zytiga—abiraterone acetate—in a formulation that doesn't step on J&J's patents. It's approved in combo with methylprednisolone to treat patients with metastatic castration-resistant prostate cancer.

Sun Pharma will launch Yonsa in the U.S. under a licensing deal with Churchill Pharmaceuticals, its original developer. Churchill is eligible for upfront and sales-based milestones and royalties. 

Zytiga is approved for use in tandem with a different steroid, prednisone, to treat patients with mCRPC and high-risk castration-sensitive prostate cancer. The drug generated $1.23 billion in the U.S. last year.  

Johnson & Johnson is obviously hoping to cling to as much of that revenue as it can. Even after Zytiga's adverse patent ruling in January, J&J executives said on a conference all in April that the drugmaker doesn't expect generic competition this year.

Credit Suisse analyst Vamil Divan sees things differently. After the patent ruling, Divan wrote that his team didn’t "expect the final patent remaining on Zytiga to hold up to challenge, and we have been anticipating a generic entry following the expiration of a 30-month stay in October 2018."  An official patent challenge from a generics maker triggers that automatic delay under the Hatch-Waxman Act. 

Still, J&J's Zytiga sales are somewhat insulated from potential competition because the drug generated about half of its revenue outside of the U.S. last year. 

After the patent ruling, Johnson & Johnson said it will ask for a rehearing at the Patent Trial and Appeal Board or will appeal to "vigorously" defend its patent. 

Zytiga's case is one example of a high-profile patent loss for pharma as part of an inter partes review at the U.S. Patent and Trademark Office. The industry has pushed for change, claiming it's unfair that drugmakers have to defend patents at the administrative level and in federal courts, too. Last month, the Supreme Court ruled the process is constitutional. 

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