Indian pharmaceutical company Sun Pharma has announced that the U.S. Food and Drug Administration (FDA) has tentatively approved the company’s generic version of the successful anti-diabetes drug Januvia, manufactured by American giant Merck Sharp & Dohme (MSD).

Januvia has been one of the best-selling drugs on the U.S. market, with total sales of nearly $2.5 billion in 2012. The news of the FDA approval of a generic version has caused many experts to believe that Sun Pharma may be challenging Merck’s patent. The announcement was considered surprising by specialists because Sun Pharma sided with Merck in the legal proceedings it started against Glenmark over patent infringements on Januvia, the Economic Times noted. A spokesperson for Sun Pharma confirmed the tentative approval from the FDA but declined to comment on the nature of the filing.

According to the news source, the generic version of Januvia can be launched in 2022 at the earliest. However, experts commented that if an application is filed between 12 and 14 years prior to the expiration of a drug patent, it is very likely that the application is a Para IV type — either challenging the validity of the patent or a claim that the generic version would not infringe Merck’s patent. In fact, 2022 is when the patent for the basic component of Januvia expires, but the patents related to the salt of the monophosphate version that is marketed are set to expire four years later.

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An industry analyst, who wished to remain anonymous, explained to the Economic Times that the date on which Sun Pharma filed its abbreviated new drug application for the salt Januvia — Oct. 18 2010 — is two days after the NCE-1 date, a cut-off date for filing a Para IV application, suggesting a very likely challenge of patent validity. If this is true, Merck will be able to legally challenge Sun Pharma before the patent expires. Unless this happens, there could be a generic version of the drug in the U.S. market at least four years before Merck’s patent on the salt (Januvia) expires, the expert commented.

In 2011, Sun Pharma signed an agreement with Merck on marketing, promoting and distributing MSD’s diabetes products, including Januvia, in India. A spokesperson for Merck told the Economic Times that the company was not in litigation with Sun Pharma in the United States regarding sitagliptin, which is the generic name for Januvia. Receiving tentative approval from the U.S. FDA does not give the company the right to produce a generic product before the patent expiry date, the spokesperson added. Merck denied having taken legal action against Sun Pharma or any other generic drug maker and noted that the company has “taken a position of respect” for the sitagliptin patent.

Meanwhile, two other generic drug makers have filed applications at roughly the same time. These were from U.S. generic drug maker Mylan and Sandoz, the generic subsidiary of Swiss giant Novartis, the news website said.