Shire has received a boost with the news that regulators in the USA have confirmed that the attention-deficit hyperactivity disorder drug Vyvanse is entitled to five years of exclusivity.

Following a “thorough” review of “governing statutory and regulatory standards and public comments”, the US Food and Drug Administration has affirmed its earlier decision to grant five-year New Chemical Entity exclusivity to Vyvanse (lisdexamfetamine). The exclusivity period expires on February 23, 2012.

The FDA’s announcement means that Actavis cannot file a generic version of Vyvanse until the above date. The Icelandic firm sued the agency in February in a bid to overturn the exclusivity, claiming that the labelling on Vyvanse states that it is a therapeutically inactive pro-drug that is metabolically converted to dextroamphetamine, eg Shire’s older ADHD treatment Adderall.

The FDA’s validation is therefore great news and analysts Emmanuel Papadakis and James Knight at Collins Stewart said that “more importantly” the decision “reinforces confidence in the patents, which are separate to exclusivity and run to 2023”. Now that Adderall is facing generic competition, Vyvanse is going to be Shire’s major driver of growth.

A deal was signed in March with GlaxoSmithKline to co-promote Vyvanse in the USA and the analysts at Collins Stewart noted that 1,300 reps are now promoting the drug. The co-promotion only got going in May “and will be a 'market build’ story – we expect it to deliver tangible results over the next six months”, they said.

Shire's third-quarter results will be unveiled on Friday and the broker expects group sales of $655 million and underlying earnings per share of $0.16, in-line with consensus. “The main focus of interest should be Vyvanse’s progress and the number of patients taking up velaglucerase”, the company’s investigational Gaucher disease treatment which is available in the USA through an expanded access programme introduced to cope with a potential shortage of the only approved drug for the disease, Genzyme Corp's Cerezyme (imiglucerase).

Collins Stewart noted that last week Genzyme commented that it thinks less than 100 patients have switched from Cerezyme to date.