Preclinical development and first-in-human study of KA2507, a selective and potent inhibitor of histone deacetylase 6, for patients with refractory solid tumors

Purpose: Inhibition of histone deacetylase 6 (HDAC6) is predicted to deliver both direct antitumor activity and modulation of the antitumor immune response. This study describes the development of a novel HDAC6 inhibitor. Patients and Methods: KA2507 was characterized in HDAC biochemical and cellular target engagement assays and in preclinical efficacy models of melanoma and colorectal cancer. In a phase I study, KA2507 was administered orally using a 3+3 dose-escalation design (NCT03008018). Results: KA2507 is a potent and selective inhibitor of HDAC6 (biochemical IC₅₀ = 2.5 nmol/L). Preclinical models demonstrated antitumor efficacy in syngeneic tumor-bearing mice, with translational studies highlighting modulation of the antitumor immune response. Twenty patients were treated in a phase I study. KA2507 was well tolerated; dose-limiting toxicity was not observed up to the maximum dose administered. Pharmacokinetic profiling supported twice-daily oral dosing. Pharmacodynamic analysis demonstrated selective HDAC6 target engagement in peripheral blood cells, free from off-target class I HDAC activity. Stable disease was the best clinical response (7 patients). Three of these patients (adenoid cystic carcinoma, n = 2; rectal adenocarcinoma, n = 1) had prolonged disease stabilization that lasted for 16.4, 12.6, and 9.0 months, respectively. Conclusions: KA2507 is a potent and selective inhibitor of HDAC6 showing antitumor efficacy and immune modulatory effects in preclinical models. In a phase I study, KA2507 showed selective target engagement, no significant toxicities, and prolonged disease stabilization in a subset of patients. Further clinical studies of KA2507 are warranted, as a single agent or, preferably, combined with other immuno-oncology drugs.