A pharmaceutical composition including a compound represented by the formula (I) or a pharmacologically acceptable salt thereof wherein two R moieties each independently are a C1-3 alkyl group or are groups bonded to each other to form a C2-5 alkylene group; A is an optionally substituted C6-10 aryl group or an optionally substituted heteroaryl group; and R1, R2 and R3 each independently are an optionally substituted linear or branched C1-4 alkyl group, wherein the pharmaceutical composition is administered in combination with an immunotherapeutic agent.