Peptidoglycan-Targeted [F]3,3,3-Trifluoro-d-alanine Tracer for Imaging Bacterial Infection.

Imaging is increasingly used to detect and monitor bacterial infection. Both anatomic (X-rays, computed tomography, ultrasound, and MRI) and nuclear medicine ([In]-WBC SPECT, [F]FDG PET) techniques are used in clinical practice but lack specificity for the causative microorganisms themselves. To meet this challenge, many groups have developed imaging methods that target pathogen-specific metabolism, including PET tracers integrated into the bacterial cell wall. We have previously reported the d-amino acid derived PET radiotracers d-methyl-[C]-methionine, d-[3-C]-alanine, and d-[3-C]-alanine-d-alanine, which showed robust bacterial accumulation and . Given the clinical importance of radionuclide half-life, in the current study, we developed [F]3,3,3-trifluoro-d-alanine (d-[F]-CF-ala), a fluorine-18 labeled tracer. We tested the hypothesis that d-[F]-CF-ala would be incorporated into bacterial peptidoglycan given its structural similarity to d-alanine itself. NMR analysis showed that the fluorine-19 parent amino acid d-[F]-CF-ala was stable in human and mouse serum. d-[F]-CF-ala was also a poor substrate for d-amino acid oxidase, the enzyme largely responsible for mammalian d-amino acid metabolism and a likely contributor to background signals using d-amino acid derived PET tracers. In addition, d-[F]-CF-ala showed robust incorporation into peptidoglycan, as detected by HPLC/mass spectrometry. Based on these promising results, we developed a radiosynthesis of d-[F]-CF-ala via displacement of a bromo-precursor with [F]fluoride followed by chiral stationary phase HPLC. Unexpectedly, the accumulation of d-[F]-CF-ala by bacteria was highest for Gram-negative pathogens in particular . In a murine model of acute bacterial infection, d-[F]-CF-ala could distinguish live from heat-killed , with low background signals. These results indicate the viability of [F]-modified d-amino acids for infection imaging and indicate that improved specificity for bacterial metabolism can improve tracer performance.