Synthesis and microPET evaluation of [¹¹C] 7-iodo-2-[4-methoxy-3-(2-(4-methylpiperidin-1-yl)ethoxy)phenyl]isoindolin-1-one as a candidate 5-HT_2C receptor imaging agent in the nonhuman primate brain

Objectives: The 5-HT_2C receptor (5-HT_2CR) is one of 14 5-HT receptor subtypes that binds the endogenous neurotransmitter serotonin. Substantial preclinical and clinical findings on 5-HT_2CR agonists and antagonists demonstrated 5-HT_2CR as a potential therapeutic target for the treatment of schizophrenia, anxiety, obesity, depression, drug abuse, and Parkinson’s disease. Although PET imaging of 5-HTRs has been progressing for almost three decades, attempts for the development of 5-HT_2CR PET tracers were made just recently and with limited success. We developed a novel ¹¹C-labeled radioligand, [¹¹C] 7-iodo-2-[4-methoxy-3-(2-(4-methylpiperidin-1-yl)ethoxy)phenyl]isoindolin-1-one (1) with high brain penetrability, selectivity, and binding outside the choroid plexus . We report here the synthesis, radiosynthesis and microPET evaluation of [¹¹C]1 in nonhuman primate brain as a promising PET imaging antagonist for 5-HT_2CR.

Methods: (1) and its corresponding normethyl precursor (2) were synthesized via multi-step synthetic approaches. In vitro competition binding assays of (1) were conducted by the NIMH Psychoactive Drug Screening Program (PDSP). [¹¹C]1 was prepared via O-methylation of (2) with [¹¹C]CH₃I in the presence of 0.1 M Bu₄NOH in DMF followed by HPLC purification. Log P of 1 was measured between 1-octanol and phosphate buffer at PH 7.4. [¹¹C]1 was intravenously administered to anesthetized adult rhesus monkey for dynamic microPET imaging to assess in vivo regional brain uptake using a Siemens MicroPET Focus 220 scanner. A baseline study was initially performed to determine the extent of brain uptake in vivo. To test for specific binding with [¹¹C]1, a second imaging study was conducted in which [¹¹C]1 was administered 30 min after a dose of 0.3 mg/kg SB-242084, an antagonist commonly used for 5-HT_2C blockade.

Results: (1) displayed a high affinity for 5-HT_2CR (Ki = 1.1 nM) and a high selectivity over 5-HT_2AR (Ki > 1000 nM) and 5-HT_2BR (Ki = 126 nM). [¹¹C]1 was obtained in an average 36% ± 10% decay-corrected radiochemical yield (n = 8) with a radiochemical purity of >97% and a specific activity of 0.5-1.2 Ci/µmol. (1) displays moderate lipophilicity with a log P7.4 of 2.89. In the baseline study, [¹¹C]1 exhibited excellent brain blood barrier penetration and showed high uptake in the choroid plexus (the region with the highest density of 5-HT_2CR) and hippocampus, with low uptake in the cerebellum, a region known to be deficient in 5-HT_2CR. The corresponding time-activity curves (TACs) of [¹¹C]1 show that ratios of uptake in choroid plexus and hippocampus to that in cerebellum peaked at 3.7 and 2.7, respectively. Administration of a dose of SB-242084 resulted in a reduction of radioactivity at the choroid plexus and hippocampus supporting that brain regional uptake of [¹¹C]1 reflected specific 5-HT_2CR binding.

Conclusions: We have developed a novel ¹¹C-labeled radioligand [¹¹C]1 for 5-HT_2CR. [¹¹C]1 exhibited high brain uptake with excellent contrast in nonhuman primate brain. Therefore, [¹¹C]1 is a suitable candidate for in vivo 5-HT_2CR PET imaging, which will be further studied. Acknowledgement: This work was funded by a grant from the National Institute of Health (1R21MH108928).

• Download figure
• Open in new tab
• Download powerpoint