This is a randomized, double blind, multi-institutional phase II therapeutic study of Indoximod or placebo after the completion of standard of care sipuleucel-T (Provenge®) in men with asymptomatic or minimally symptomatic metastatic prostate cancer that is castration resistant (hormone refractory). Patients are randomized to receive either twice daily oral Indoximod or placebo for 6 months beginning the day after the third and final sipuleucel-T infusion.