Mumbai, March 20: AstraZeneca, the $25.7 billion pharmaceutical giant, has red-flagged a threat to its business from generic drug makers in emerging markets who have started to sniff an opportunity to make cheaper versions of its two best-selling drugs — Nexium and Pulmicort — before its patents on these drugs expire.

The patent for Nexium — a popular drug used to treat acid reflux and heartburn symptoms in patients — expires in 2015 in the US and this year in the EU and Canada.

The patent for Pulmicort — an anti-asthma drug that is sold in India as well — expires in the US in 2019 for repsules and 2018 for the flexhaler formulation.

“We are currently facing challenges in the US from numerous generic drug manufacturers regarding our patents for Nexium and Pulmicort, two of our key products,” AstraZeneca said in its annual report filed with the US market regulator.

AstraZeneca did not name the generic players that were looking to tap the opportunities thrown up by the looming patent expiries. But it is known that Ranbaxy has been readying plans to launch Nexium in the US during a six-month exclusivity period for the first drug maker that steps up to the plate with an application to make a generic version before the patent on a blockbuster drug expires.

Ranbaxy is understood to be in talks with companies for sourcing ingredients to make a generic version of the Nexium. The decision to seek out suppliers came after the US Food and Drugs Administration barred the Indian drug maker from supplying active pharmaceutical ingredients made from its Toansa facility in Punjab. It isn’t known whether any Indian company is exploring opportunities to make a cheaper version of the Pulmicort.

Nexium is AstraZeneca’s second biggest revenue spinner in the US after Crestor. In 2013, Nexium raised revenues of $2.12 billion, down from the $2.27 billion in 2012 and $2.4 billion in 2011. Crestor is the top selling drug in AstraZeneca’s business with revenues of $2.9 billion in 2013. Nexium had combined revenues of $699 million in the EU, Canada and Japan. Pulmicort reported revenues of $224 million in the US and a combined revenue of $265 million in the EU, Canada and Japan.