GlaxoSmithKline will partner with Shire to co-promote Vyvanse, Shire’s Adderall follow-on ADHD drug. The three-year deal will double the brand’s sales force, with 600 GSK reps targeting primary care physicians and specialists, the companies announced today.

Shire’s sale force of 600 reps will continue to focus primarily on pediatricians and psychiatrists. GSK will begin promoting Vyvanse to physicians in May 2009. The agreement is based on profit-sharing above an agreed-upon, undisclosed, baseline figure, according to a Shire release.

Shire hopes the drug will make inroads with adult patients, according to Michael Boken, senior director of marketing, Vyvanse. “Eighty-five percent of the 8% of kids with ADHD are treated, compared with only 20% of the 2% of adults with ADHD,” said Boken at CBI’s eMarketing forum earlier this month. “We’re most excited about the adult market, and see it as our biggest opportunity.” In order to be diagnosed with ADHD, adults have to experience six months of problems, noted Boken. Vyvanse was approved for adults in April 2008.

GSK’s partnership with Shire could add as much as $500 million to Vyvanse sales five years from now, Bloomberg reported, citing a note Graham Parry, an analyst with Bank of America Securities-Merrill Lynch, wrote to clients. The drug currently garners nearly 12% of the prescription ADHD market, said Michael Cola, president of Shire’s specialty pharmaceuticals business, in the statement.