A pharmaceutical composition comprising a PD-1 inhibitor and a Hsp90 inhibitor, or a tautomer, or a pharmaceutically acceptable salt thereof, wherein the Hsp90 inhibitor is 3-(2,4-dihydroxy-5-isopropyl-phenyl)-4-(1-methyl-indol-5-yl)-5-hydroxy-[1,2,4] triazole or 5-hydroxy-4-(5-hydroxy-4-(1-methyl-1H-indol-5-yl)-4H-1,2,4-triazol-3-yl)-2-isopropylphenyl dihydrogen phosphate. Also provided is a method for treating cancer in a subject in need thereof, using the pharmaceutical composition described herein.