HDAC9 and head and neck squamous cell carcinoma: To investigate whether selective class IIa HDAC inhibition can be used to sensitize HNSCC cells to cisplatin, CHDI0039 was tested in combination with cisplatin in the cisplatin-resistant Cal27CisR clones Cal27CisR_VC, Cal27CisR_HDAC4, and Cals27CisR_HDAC5 by MTT assays.