CAMBRIDGE, Mass.--Millennium Pharmaceuticals announced that three of its partners have extended their strategic alliances with the company. Millennium's four-year collaboration with Pfizer, which was due to expire in December 1998, was expanded and extended for an additional two years, while a partnership with Astra will be extended for four more years. In the field of oncology, Millennium's partnership with Eli Lilly will continue an additional two years. No financial terms were disclosed.