Clinical Pharmacology of MS-275, A Histone Deacetylase Inhibitor

The goal of this escalating single-dose phase I research study was to determine the safety, tolerability, pharmacokinetics, pharmacodynamics as well as in vitro metabolism and plasma protein binding of MS-275, a novel histone deacetylase inhibitor, in patients with solid tumors and lymphomas. A validated LC/MS assay was developed to quantitate MS-275 in plasma, human liver microsomes and urine. The pharmacokinetic (PK) evaluation was done using a non-compartmental approach. In-vitro plasma protein binding profile of MS-275 was characterized by a validated micro-equilibrium dialysis method. In vitro phase I and phase II hepatic metabolism of MS-275 were evaluated using human liver microsomes. A correlative covariate analysis was performed in an effort to explain the wide inter-individual variability among patients.Results from the study demonstrate that the validated LC-MS assay is specific, accurate, precise and sensitive. MS-275 demonstrates a substantial inter-individual PK variability in systemic exposure and clearance; exposures increase in near-proportion, while peak concentrations increase more than-proportionally with an increase in dose. Mean apparent oral clearance (CL/F) is independent of dose and exhibits apparent dose-independent PK behavior over the studied dose range. Oral absorption is highly variable. MS-275 has a 50-fold longer half-life in humans compared to pre-clinical species. PK/PD analysis showed significant correlation between occurrence of DLT and higher systemic exposures. Although there was an increase in the acetylation of histone H3 and H4 over time, preliminary analysis showed no significant correlation between PK parameters and change in % histone acetylation after 24 hours. MS-275 is moderately bound to plasma proteins. Hepatic phase I and II metabolic pathways are only minor routes of elimination, and MS-275 is neither a substrate for liver-specific organic anion transporting proteins, OATP1B1 and OATP1B3, nor a substrate for gastrointestinal efflux transporters ABCB1 (P-gp) or ABCG2. No significant correlation was found between CL/F and demographic, body measures and other clinical covariates, and inter-patient variability in CL/F remained similar in magnitude even after correcting dose for body surface area (BSA) or other body measures. BSA is not a significant predictor of MS-275 PK, and flat-fixed dosing can be used in the future