Advancing 6-bromo-7-[11C]methylpurine to clinical use: improved regioselective radiosynthesis, non-clinical toxicity data and human dosimetry estimates

Background

6-Bromo-7-[¹¹C]methylpurine ([¹¹C]BMP) is a radiotracer for positron emission tomography (PET) to measure multidrug resistance-associated protein 1 (MRP1) transport activity in different tissues. Previously reported radiosyntheses of [¹¹C]BMP afforded a mixture of 7- and 9-[¹¹C]methyl regioisomers. To prepare for clinical use, we here report an improved regioselective radiosynthesis of [¹¹C]BMP, the results of a non-clinical toxicity study as well as human dosimetry estimates based on mouse PET data.

Results

[¹¹C]BMP was synthesised by regioselective N⁷-methylation of 6-bromo-7H-purine (prepared under good manufacturing practice) with [¹¹C]methyl triflate in presence of 2,2,6,6-tetramethylpiperidine magnesium chloride in a TRACERlab™ FX2 C synthesis module. [¹¹C]BMP was obtained within a total synthesis time of approximately 43 min in a decay-corrected radiochemical yield of 20.5 ± 5.2%, based on starting [¹¹C]methyl iodide, with a radiochemical purity > 99% and a molar activity at end of synthesis of 197 ± 130 GBq/μmol (n = 28). An extended single-dose toxicity study conducted in male and female Wistar rats under good laboratory practice after single intravenous (i.v.) administration of unlabelled BMP (2 mg/kg body weight) revealed no test item related adverse effects. Human dosimetry estimates, based on dynamic whole-body PET data in female C57BL/6J mice, suggested that an i.v. injected activity amount of 400 MBq of [¹¹C]BMP will deliver an effective dose in the typical range of ¹¹C-labelled radiotracers.

Conclusions

[¹¹C]BMP can be produced in sufficient amounts and acceptable quality for clinical use. Data from the non-clinical safety evaluation showed no adverse effects and suggested that the administration of [¹¹C]BMP will be safe and well tolerated in humans.