Preferred stereoselective transport of the D-isomer of cis-4-[ ¹⁸F]fluoro-proline at the blood-brain barrier

Generally, L-amino acids are preferably transported into mammalian cells compared with their D-isomers, and only L-amino acids are incorporated into proteins. Former studies, however, indicated that D-[H]proline is accumulated in the brain of mice after injection, while L-[³H]proline is not. We investigated the differential cerebral uptake of the D- and L-isomers of the PET tracer cis-4-[¹⁸F]fluoroproline (D-/L-cis-FPro) and of D-/L-[ ³H]proline (D-/L-Pro) in rats by dual tracer autoradiography and the uptake of D-cis-FPro in two human subjects by PET. The standardized uptake value (SUV) of D-cis-FPro in the cerebral cortex of rats 2 h p.i. was 3.05 ± 1.18 (n = 9) versus 0.06 ± 0.01 (n = 4) for L-cis-FPro (P < 0.001) and 1.29 ± 0.27 (n = 4) for D-Pro versus 0.30 ± 0.14 (n = 9) for L-Pro (P < 0.001). Analysis of the rat brain tissue after injection of D-cis-FPro (n = 3) revealed no radioactivity in the proteins but a relevant part in the form of L-trans-FPro. The PET studies yielded a four- to five-fold higher SUV and influx rate constant in the human cortex for D-cis-FPro than for L-cis-FPro. We conclude that D-cis-FPro and D-Pro are preferably transported at the blood-brain barrier compared with their L-isomers and isomerized to the L-form within the brain. Thus, D-Pro in the plasma might be a source of intracerebral L-proline, which has been shown to act as a modulator of excitatory neurotransmission.