GlaxoSmithKline's best-selling asthma drug Advair is proving tough to copy by generic rivals, the Wall Street Journal reports.

That challenge could allow GSK, the British drug maker with its North American headquarters in Research Triangle Park, to protect Advair even after its loses patent protection next year.

Typically, losing such protection opens the door for cheaper copycat versions and hurts sales. But Advair combines two drugs in a fine powder that's inhaled through an intricate device called a Diskus.

Generic-drug makers Teva Pharmececeutical and Sandoz, a subsidiary of Novartis, have each recruited GSK to help develop a generic rival, the newspaper reports. The difficulty represents a challenge for generics companies, trying to replicate increasingly complex drugs.

Oriel Therapeutics, a Durham company developing treatments for asthma and other respiratory ailments, has been acquired by Sandoz, the generic-drug division of pharmaceutical giant Novartis.

Financial terms of the deal weren't disclosed. In 2007, Oriel raised $26.5 million in venture-capital financing.

The company was founded in 2001 by Anthony Hickey and Timothy Crowder, based on inhaled drug-delivery technology developed at the UNC School of Pharmacy.

Oriel's products include experimental, generic treatments for asthma and chronic obstructive pulmonary disease, or COPD. The company also developed a new type of disposable dry powder inhaler to improve delivery of respiratory drugs.