Myeloid cell leukemia-1 (Mcl-1) plays an important role in survival, chemo- and radioresistance of head and neck squamous cell carcinoma (HNSCC). Cyclin-dependent kinase 9/cyclin T (CDK9) promotes excessive production of multiple pro-survival proteins including Mcl-1, leading to impaired apoptosis of cancer cells. As such, CDK9 is an emerging therapeutic target in cancer therapy. We herein report the first study of targeting CDK9 as a treatment strategy for hypopharyngeal squamous cell carcinoma (HSCC), an aggressive malignancy associated with one of the worst prognoses within HNSCC. We showed that mRNA levels of Mcl-1 were significantly higher in HSCC tumor tissues than in the adjacent non-tumor mucosae. In addition, the levels of Mcl-1 mRNA correlated with the tumor size and clinical stage of HSCC patients. CDKI-73, a potent CDK9 inhibitor, was capable of downregulating the expression of Mcl-1 in the HSCC cells by suppression of the CDK9 mediated phosphorylation of RNA polymerase II. CDKI-73 effectively induced apoptosis as a single agent and synergized anti-tumor activity of cisplatin in HSCC cells. Taken together, our study presents compelling evidence for developing CDK9 inhibitors, such as CDKI-73, as new therapeutic strategy for HSCC. (C)2016 Elsevier Inc. All rights reserved.