Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data

Abstract

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A1 adenosine receptor (A1AR). Using the half-life (t1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

[11C]CHDI-626, a PET Tracer Candidate for Imaging Mutant Huntingtin Aggregates with Reduced Binding to AD Pathological Proteins

“…These distinctions were also apparent in the relatively rapid clearance ( t 1/2 ) in liver microsomes. Faster plasma clearance may benefit PET imaging of the parent binding in the brain by minimizing its plasma signals, provided no brain-penetrant radiometabolites are generated. , …”

“…Faster plasma clearance may benefit PET imaging of the parent binding in the brain by minimizing its plasma signals, provided no brain-penetrant radiometabolites are generated. 11,20 In vitro/in vivo MetID studies were conducted to investigate the potential for brain penetration of any human-relevant radiometabolites (Figures S2 and S3 and Tables S1 and S2). In human microsomes, the metabolism of 11 was complex (17 metabolites detected), but most metabolites were of low abundance (Figure S3 and Table S2).…”

Drug-Like Small Molecule HSP27 Functional Inhibitor Sensitizes Lung Cancer Cells to Gefitinib or Cisplatin by Inducing Altered Cross-Linked Hsp27 Dimers

“…The measured values for J2 were 76.2% and 82.7% when treated with 0.5 μg/mL and 5 μg/mL, respectively, and those for NA49 were 146.6% and 101.4%. This indicates that the half-life values of J2 and NA49 can be predicted to be about 3 h and longer than 6 h, respectively ( Table 2 ) [ 38 ]. The differences in plasma stability and predicted half-life between J2 and NA49 can be attributed to the removal of a hydroxyl group at the C-5 position of the chromen-4-one ring in the metabolic pathway of NA49.…”

Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands

“…Although microsomes are considered a less physiologically relevant model than hepatocytes due to the lack of cellular organization, they are still a valuable tool for clearance determination of compounds that are metabolized primarily by phase I enzymes and that do not act as transporter substrates. Results from previous studies showed that xanthine-derived A1AR ligands are metabolized primarily by hepatic P450 enzymes [9] and that scaled microsomal clearance data are in good agreement with measured in vivo clearance [10]. Against this background, hepatic microsomes were preferred over hepatocytes for investigating species differences in A1AR ligand metabolism.…”

“…All compounds listed in Table 1 were synthesized and characterized in our laboratories according to the procedures described in [ 4 , 10 , 39 , 40 , 41 ].…”