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

Abstract

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).

Keywords: Biomimetic; Carbon-11; Nor-buprenorphine; PET imaging.