Dendreon, the Seattle-based maker of an immune-boosting therapy for prostate cancer, said today it plans to raise $500 million in a convertible debt financing. The company (NASDAQ: DNDN) said it plans to use the money to seek approval of sipuleucel-T (Provenge) in Europe, build a factory in Europe, continue investing in its U.S. manufacturing plants, and run additional clinical trials. The debt would have to be paid back to investors by January 2016, or the securities could be converted into Dendreon stock holdings, the company said. The offering is being led by JP Morgan Securities, and comes on the heels of the JP Morgan Healthcare Conference, the biggest biotech investment event of the year. The underwriters will have an option to buy another $75 million in debt holdings to cover overallotments, Dendreon said.