Incorporation of d-
amino acids into
peptidoglycan is a unique metabolic feature of bacteria. Since d-
amino acids are not metabolic substrates in most mammalian tissues, this difference can be exploited to detect living bacteria in vivo. Given the prevalence of d-
alanine in
peptidoglycan muropeptides, as well as its role in several
antibiotic mechanisms, we targeted this
amino acid for positron emission tomography (PET) radiotracer development. d-[3-11C]
Alanine and the
dipeptide d-[3-11C]
alanyl-d-alanine were synthesized via asymmetric alkylation of
glycine-derived
Schiff-base precursors with [11C]
methyl iodide in the presence of a cinchonidinium phase-transfer catalyst. In cell experiments, both tracers showed accumulation by a wide variety of both Gram-positive and Gram-negative pathogens including Staphylococcus aureus and Pseudomonas aeruginosa. In a mouse model of acute bacterial
myositis, d-[3-11C]
alanine was accumulated by living microorganisms but was not taken up in areas of sterile
inflammation. When compared to existing clinical nuclear imaging tools, specifically 2-deoxy-2-[18F]fluoro-
d-glucose and a
gallium citrate radiotracer, d-
alanine showed more bacteria-specific uptake. Decreased d-[3-11C]
alanine uptake was also observed in
antibiotic-sensitive microbes after antimicrobial
therapy, when compared to that in resistant organisms. Finally, prominent uptake of d-[3-11C]
alanine uptake was seen in rodent models of
discitis-
osteomyelitis and P. aeruginosa
pneumonia. These data provide strong justification for clinical translation of d-[3-11C]
alanine to address a number of important human
infections.