Pfizer’s head of research and development has decided to jump ship for AstraZeneca.

Over the past 15 years, Martin Mackay has been with Pfizer through good times (Lipitor, anyone?) and bad (oof, torcetrapib). What would entice him to leave the world’s most successful (by sales) drug maker? Mackay’s not telling, leaving plenty of room for speculation.

Maybe the Scot grew weary of New York and wanted to relocate to AstraZeneca’s headquarters in London (speculation courtesy of Forbes). One analyst noted that AstraZeneca may have some pretty sweet assets, yummy enough to entice Mackay from Pfizer (speculation via WSJ).Or maybe he was tired of sharing his job at Pfizer, as he has done with Mikael Dolsten since last October when Pfizer completed its acquisition of Wyeth. Dolsten was put in charge of biologics; Mackay got the small molecules.

Whatever the reason, Mackay is ultimately leaving a company whose biggest seller (Lipitor) will soon face generic competition for a company whose biggest seller (Nexium) will soon face generic competition. And he will be there to guide AstraZeneca as it restructures its R&D programme, dividing the programme into smaller, more biotechy units – a move that many pharma giants, including Pfizer, have recently embraced.

Dolsten will now head Pfizer’s R&D enterprise on his own.