In vitro binding affinity vs. in vivo site occupancy: A PET study of four diastereomers of dihydrotetrabenazine (DTBZ) in monkey brain

Abstract

Introduction: In vivo imaging methods such as Positron Emission Tomography (PET) can be used to examine the relationship between in vitro binding affinity and in vivo occupancy of binding sites in the brain for new drug candidates. In this study, PET imaging in monkey brain was used to evaluate that correlation for a set of four diastereomers of the compound dihydrotetrabenazine (DTBZ), the pharmacologically active metabolite of the drug tetrabenazine.

Methods: PET studies of DTBZ diastereomers were completed in a single monkey brain. In vivo occupancies (ED₅₀) were estimated using multiple drug doses and the vesicular monoamine transporter 2 specific radioligand (+)-α-[¹¹C] DTBZ, employing a test-retest sequence of control PET scan, drug administration and a second PET scan completed on a single day.

Results: DTBZ has three chiral carbon centers and eight possible stereoisomers, and in vivo occupancy of the target site VMAT2 was observed only for the four diastereomers of DTBZ having the 11bR absolute configuration. The estimated in vivo occupancies (ED₅₀ values from 0.023 to >3.15 mg/kg) correlated well (R² = 0.95) with the in vitro binding affinities (K_i values of 4 to 600 nM for the VMAT2), and an even better correlation (R² = 0.99) was found for the three isomers with in vitro binding affinities <100 nM.

Conclusions: If the physiochemical (MW, log P, pKa) or physiological (metabolism, transport, protein binding) properties of a set of drug stereoisomers are considered similar, the binding affinities determined from in vitro assays may predict the in vivo occupancies of the target binding site in the monkey brain.

Keywords: Binding affinity; Dihydrotetrabenazine; Occupancy; Stereoisomer.