Pharmacokinetic studies with PET

Developing Low Molecular Weight PET and SPECT Imaging Agents

Imaging agents for positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) have shown their utility in many situations, answering clinical questions related to drug development and medical considerations. The discovery and development of imaging agents follow a well-understood process, with variations related to available starting points and to the envisaged imaging application. This article describes the general development path leading from the expression of an imaging need and project initiation to a clinically usable imaging agent. The definition of the project rationale, the design and optimization of early leads, and the assessment of the imaging potential of an imaging agent candidate are followed by preclinical and clinical development activities that differ from those required for therapeutic agents. These include radiolabeling with a positron emitter and first-in-human clinical studies, to rapidly evaluate the ability of a new imaging agent to address the questions it was designed to answer.

Determination of pharmacokinetics and tissue distribution of a novel lutetium-labeled PSMA-targeted ligand, 177Lu-DOTA-PSMA-GUL, in rats by using LC–MS/MS

Prostate specific membrane antigen (PSMA) is known to be overexpressed in prostate cancer cells, providing as a diagnostic and therapeutic target for prostate cancer. A lutetium-labeled PSMA targeted ligand, ¹⁷⁷Lu-DOTA-PSMA-GUL is a novel radiopharmaceutical, which has been developed for the treatment of prostate cancer. While the GUL domain of ¹⁷⁷Lu-DOTA-PSMA-GUL binds to the antigen, the beta-emitting radioisotope, ¹⁷⁷Lu-labeled DOTA, interacts with prostate cancer cells. However, the in vivo pharmacokinetics of intact ¹⁷⁷Lu-DOTA-PSMA-GUL has never been characterized. This study aimed to evaluate the pharmacokinetics and tissue distribution of the radiopharmaceutical in rats by using its stable isotope-labeled analog, ¹⁷⁵Lu-DOTA-PSMA-GUL. A sensitive liquid chromatography-tandem mass spectrometry (LC–MS/MS) analysis of ¹⁷⁵Lu-DOTA-PSMA-GUL was developed and validated. Following intravenous injection, the plasma concentration–time profiles of ¹⁷⁵Lu-DOTA-PSMA-GUL showed a multi-exponential decline with the average elimination half-life of 0.30 to 0.33 h. Systemic exposure increased with the dose and renal excretion is the major elimination route. Tissue distribution of ¹⁷⁵Lu-DOTA-PSMA-GUL was most substantial in kidneys, followed by the prostate. The developed LC–MS/MS assay and the in vivo pharmacokinetic data of ¹⁷⁵Lu-DOTA-PSMA-GUL would provide helpful information for further clinical studies to be developed as a novel therapeutic agent for prostate cancer.

An original radio-biomimetic approach to synthesize radiometabolites for PET imaging

To fully exploit the potential of positron emission tomography (PET) imaging to assess drug distribution and pharmacokinetics in the central nervous system, the contribution of radiometabolites to the PET signal has to be determined for correct interpretation of data. However, radiosynthesis and extensive study of radiometabolites are rarely investigated and very challenging for complex drugs. Therefore, an original radio-biomimetic (RBM) approach was developed to rapidly synthesize radiometabolites and non-invasively investigate their kinetics with PET imaging. This method enabled the challenging radiosynthesis of [¹¹C]nor-buprenorphine ([¹¹C]nor-BUP), the main metabolite of buprenorphine (BUP) which has been identified as a substrate of the P-glycoprotein (P-gp) transport function at the blood-brain barrier (BBB). Biomimetic conditions using cytochromes P450 3A4 to convert BUP into nor-BUP were optimized taking into account the short half-life of carbon-11 (t_1/2 = 20.4 min). Those conditions afforded 32% of conversion within 20 min and were applied to the biomimetic radiosynthesis of [¹¹C]nor-BUP from [¹¹C]BUP. Automated radiosynthesis of [¹¹C]BUP according to a procedure described in the literature followed by optimized RBM conditions afforded [¹¹C]nor-BUP in 1.5% decay-corrected radiochemical yield within 90 min and 90 ± 15 GBq/μmol molar activity. HPLC quality control showed chemical and radiochemical purities above 98%. To demonstrate the applicability of the RBM approach to preclinical studies, brain PET images in rats showed a drastic lower uptake of [¹¹C]nor-BUP (0.067 ± 0.023%ID/cm^-3) compared to [¹¹C]BUP (0.436 ± 0.054%ID/cm^-3). P-gp inhibition using Tariquidar increased the brain uptake of [¹¹C]nor-BUP (0.557 ± 0.077%ID/cm^-3).

In Vivo Tissue Pharmacokinetics of Carbon-11-Labeled Clozapine in Healthy Volunteers: A Positron Emission Tomography Study

We investigated clozapine (CLZ) tissue pharmacokinetics in vivo by using carbon-11-labeled CLZ (¹¹C-CLZ) and positron emission tomography (PET). Eight healthy volunteers underwent ¹¹C-CLZ studies wherein computed tomography image acquisition was followed by PET scans (whole-body, four; brain, four). After bolus intravenous ¹¹C-CLZ injection, PET images were acquired at various timepoints for 2–3 hours. Tissue ¹¹C-CLZ signals were plotted over time, and pharmacokinetic parameters were determined. High ¹¹C-CLZ radioactivity was detected in the liver and brain, implying CLZ hepatic metabolism and efficient blood–brain barrier penetration. The urinary and hepatobiliary tracts were involved in ¹¹C-CLZ excretion. Moderate to high radioactivity was observed in the dopaminergic and serotonergic receptor-rich brain regions, indicating CLZ binding to multiple receptor types. To our knowledge, this is the first study to report the determination of ¹¹C-CLZ tissue pharmacokinetics in humans. PET using radiolabeled drugs can provide valuable information that could complement plasma pharmacokinetic data.