Abstract: Mesothelioma growth inhibitory effect of standard cisplatin-pemetrexed concomitant therapy is potentiated by combination with an anti-CD26 antibody. A high therapeutic effect and excellent safety is obtained by the concomitant use of an anti-CD26 antibody and gemcitabine. By conjugating an anti-CD26 antibody (YS110) with triptolide via a divalent cross-linking agent for concomitant therapy using the anti-CD26 antibody, a novel antibody-drug conjugate (Y-TR1) very highly effective for CD26-positive malignant mesothelioma cells has been successfully obtained.