Radiosynthesis of PET tracers [11C]BBAC and [11C]BBPC for brain orexin receptors

Objectives Orexin receptors (OX1R and OX2R) play a major role in several brain disorders. In order to better study these receptors, we developed the radiosynthesis methods for PET radioligands that would allow in vivo imaging of the orexin receptors.

Methods [11C]CH3OTf was transported by a stream of argon (20~30mL/min) at room temperature through a mixture of the precursor (S)-1-(2-(1H-benzo[d]imidazol-2-ylthio)acetyl)-N-(biphenyl-2-yl)pyrrolidine-2-carboxamide or (S)-1-(3-(1H-benzo[d]imidazol-2-yl)propanoyl)-N-(biphenyl-2-yl)pyrrolidine-2-carboxamide (0.6 mg), NaOH (5 µL, 5N) and acetone (0.4 mL), respectively. Following the end of trapping, the reaction mixture was diluted with water (1 mL) and directly injected into a semi-preparation RP-HPLC (Phenomenex C18,10×250 mm,10µm) and eluted with 50% acetonitrle in 0.1M ammonium formate at a flow rate of 10mL/min. The radioactive products [11C]BBAC and [11C]BBPC were collected based on retention time of 6~7min and 4~5min based on γ-detector, and diluted with 100mL of deionized water, and passed through a C-18 Sep-Pak cartridge. Reconstitution of the products was in 1mL of absolute ethanol, and followed by dilution with 9mL of saline and passage through a sterile filter that afforded [11C]BBAC and [11C]BBPC, respectively. Following the development of the radiochemistry, PET scans were performed. 2.03 mCi of [11C]BBAC and 1.94mCi of [11C]BBPC were administered as a bolus into an anesthetized (isoflurane) female rhesus monkey while in a PET scanner, and emission data were collected for 122min.

Results [11C]BBAC and [11C]BBPC were synthesized successfully. Although radioactivity was observed in the area of the pituitary, no brain region inside the blood brain barrier had significant detectable binding consistent with specific receptor binding to the radiotracers.

Conclusions These are the first reported in vivo PET tracers for orexin receptors. However, the lack of quantifiable brain uptake means that further work is required to develop a radiotracer for this particular brain target.

Research Support NIDA, NIM