Biological Evaluation of d-[F]Fluoroalanine and d-[F]Fluoroalanine- as Positron Emission Tomography Imaging Tracers for Bacterial Infection.

d-Amino acids such as d-alanine are substrates for bacterial peptidoglycan biosynthesis and are selectively taken up by bacteria and not mammalian cells. Consequently, d-amino acid metabolism is an attractive target for antibiotic discovery and the development of bacteria-specific imaging agents. d-Fluoroalanine and the deuterium-labeled analogue fludalanine (MK641) were originally explored as antibiotics by Merck but failed in clinical trials due to unaccepted toxicity. Herein, we synthesized a fluorine-18 labeled d-fluoroalanine, d-3-[F]fluoroalanine (d-[F]FAla), and its deuterated analogue, d-3-[F]fluoroalanine- (d-[F]FAla-), and evaluated their capability to image bacterial infection. Both d-[F]FAla and d-[F]FAla- can accumulate up to 0.64-0.78% ID/cc in the infectious area at 15 min postinjection. Despite the reduction of defluorination not being observed for deuterated F-labeled d-fluoroalanine, these radiolabeled d-alanine analogues were able to differentiate bacterial infection from sterile inflammation in a soft-tissue model of infection.