The invention provides a method of treating cancer in a subject in need thereof, comprising administering to the subject:(a) a compound of Formula I:or a pharmaceutically acceptable salt thereof, wherein R is hydrogen or an acyl group； and (b) a PD-1 signaling inhibitor； wherein the compound of Formula I or pharmaceutically acceptable salt thereof and the PD-1 signaling inhibitor are administered in amounts which in combination are therapeutically effective. The invention further provides a pharmaceutical composition comprising a compound of Formula I or a pharmaceutically acceptable salt thereof, a PD-1 signaling inhibitor and a pharmaceutically acceptable carrier or excipient.
