Quisinostat is a brain-penetrant radiosensitizer in glioblastoma

In recent years, histone deacetylase inhibitors (HDACi) have garnered considerable interest for the treatment of adult and pediatric malignant brain tumors. However, owing to their broad-spectrum nature and inability to effectively penetrate the blood-brain barrier, HDACi have failed to provide significant clinical benefit to glioblastoma (GBM) patients to date. Moreover, global inhibition of HDACs results in widespread toxicity, highlighting the need for selective isoform targeting. While no isoform-specific HDACi are currently available, the second-generation hydroxamic acid-based HDACi quisinostat possesses sub-nanomolar specificity for class I HDAC isoforms, particularly HDAC1 and 2. Recently, we demonstrated that HDAC1 is the essential HDAC in GBM. Here, we provide the first report on the neuro-pharmacokinetic, pharmacodynamic and radiation-sensitizing properties of quisinostat in preclinical models of GBM. We demonstrate that quisinostat is a well-tolerated and brain-penetrant molecule that significantly extends survival when administered in combination with radiation in vivo. The pharmacokinetic-pharmacodynamic-efficacy relationship was established by correlating free drug concentrations and evidence of target modulation in the brain with survival benefit. Together, these data provide a strong rationale for clinical development of quisinostat as a radiosensitizer for the treatment of GBM.