Metabolism of DPA-714, a new peripheral benzodiazepine receptor PET ligand

Objectives The translocator protein, formerly known as the peripheral benzodiazepine receptor expressed by microglial cells is highly upregulated during neuroinflammatory processes. During the development of [¹⁸F]-N, N-Diethyl-2-(2-[4-(2-Fluoroethoxy)-Phenyl]-5,7-Dimethyl-Pyrazolo[1,5-a]Pyrimidin-3-yl)-Acetamide ([¹⁸F]-DPA-714) as a new probe for PET studies, high amounts of radiometabolites were detected in the plasma of baboons. Therefore in order to understand the metabolic pathways of the ligand, the oxidative metabolism of DPA-714 was investigated in different species.

Methods The in vitro metabolism of DPA-714 was carried out using rat, baboon and human liver microsomes as well as human recombinant CYPs. Microsomal incubations were analyzed by LC/MS and the structure of the metabolites were characterized by LC/MS-MS.

Results Seven metabolites were produced by rat liver microsomes by the following metabolic pathways: N-dealkylation (m/z=371), methyl and/or phenyl hydroxylations (m/z=415) or multiple oxidation reactions (m/z=335,337,387,369). In rats, N-dealkylation was mainly catalysed by P4503A, whereas phenyl hydroxylation by CYP4501A. In baboon, the main metabolites were identified as hydroxylated, N-dealkylated and O-dealkylated products resulting in the loss of the fluorine atom (m/z=353). The same metabolites were produced by human liver microsomes and recombinant CYP3A4.

Conclusions The major routes of metabolism were identified as methyl and/or phenyl hydroxylations, N-dealkylation and O-dealkylation. Species differences were observed in the oxidative metabolism of DPA-714. Studies are in progress to identifie the in vivo metabolites of the tracer in baboon prior to human studies.