Sept 10 (Reuters) - U.S. health regulators approved the use of Medivation Inc's advanced prostate cancer drug Xtandi in men who have not yet received chemotherapy, the company said on Wednesday, significantly expanding the potential patient population for the oral medicine.

The expanded Food and Drug Administration approval, that will also enable the drug to better compete with Johnson & Johnson's Zytiga, triggers $90 million in milestone payments to Medivation by Japanese drugmaker Astellas Pharma Inc under a collaboration agreement.

Xtandi, known chemically as enzalutamide, originally gained U.S. approval in 2012 for use in patients with castration-resistant prostate cancer that has spread beyond the prostate only after they had first received chemotherapy treatment.

"The average duration of treatment should double and the addressable patient population triple in the pre-chemo setting," Sanford Bernstein analyst Geoffrey Porges said in a research note earlier this week.

Astellas reported worldwide Xtandi sales of $227 million in the second quarter.

The additional approval was based on results of a Phase III trial in which Xtandi demonstrated an overall survival benefit, reducing the risk of death by 29 percent compared with a placebo. It also significantly reduced the risk of disease progression and the time for need for chemotherapy or skeletal problems in cancer that had spread to the bones.

Medivation shares rose to $98.35 in after hours trading from a Nasdaq close at $96.89.

(Reporting by Bill Berkrot; Editing by James Dalgleish)