The modern patient is increasingly susceptible to bacterial infections including those due to multi-drug resistant organisms (MDROs). Noninvasive whole-body analysis with pathogen-specific imaging technologies can significantly improve patient outcomes by rapidly identifying a source of infection and monitoring the response to treatment, but no such technology exists clinically. Methods We systematically screened 961 random, radiolabeled molecules in silico as substrates for essential metabolic pathways in bacteria, followed by in vitro uptake in representative bacteria – Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and mycobacteria. Fluorine-labeled analogs, that could be developed as positron emission tomography (PET)-based imaging tracers, were evaluated in a murine myositis model. Results We identified three novel, non-toxic molecules demonstrating selective bacterial uptake: para-aminobenzoic acid (PABA), with uptake in all representative bacteria including Mycobacterium tuberculosis; mannitol, with selectively uptake in S. aureus and E. coli; and sorbitol, accumulating only in E. coli. None accumulated in mammalian cells or heat-killed bacteria, suggesting metabolism-derived specificity. In addition to an extended bacterial panel of laboratory strains, all three molecules rapidly accumulated in respective clinical isolates of interest including MDROs such as methicillin resistant S. aureus (MRSA), extended-spectrum beta-lactamase (ESBL)-producing, and carbapenem-resistant Enterobacteriaceae. In a murine myositis model, fluorine-labeled analogs of all three molecules could rapidly detect and differentiate infection sites from sterile inflammation in mice (P=0.03). Finally, 2-deoxy-2-[F-18]fluoro-D-sorbitol (18F-FDS) can be easily synthesized from 2-deoxy-2-[F-18]fluoro-D-glucose (18F-FDG). PET, utilizing 18F-FDS synthesized using current good manufacturing practice, could rapidly differentiate true infection from sterile inflammation to selectively localize E. coli infection in mice. Conclusion We have developed a systematic approach that exploits unique biochemical pathways in bacteria to develop novel pathogen-specific imaging tracers. These tracers have significant potential for clinical translation to specifically detect and localize a broad range of bacteria, including MDROs.

Imaging studies are frequently used to support the clinical diagnosis of infection. These techniques include computed tomography (CT) and magnetic resonance imaging (MRI) for structural information, and single photon emission computed tomography (SPECT) or positron emission tomography (PET) for metabolic data. However, frequently there is significant overlap in the imaging appearance of infectious and non-infectious entities using these tools. To address this concern, recent approaches have targeted bacteria-specific metabolic pathways. For example, radiolabelled sugars derived from sorbitol and maltose have been investigated as PET radiotracers, since these are efficiently incorporated into bacteria but are poor substrates for mammalian cells. We have previously shown that para-aminobenzoic acid (PABA) is an excellent candidate for development as a bacteria-specific imaging tracer as it is rapidly accumulated by a wide range of pathogenic bacteria, including metabolically quiescent bacteria and clinical strains, but not by mammalian cells. Therefore, in this study we developed an efficient radiosynthesis for [11C]PABA, investigated its accumulation into Escherichia coli and Staphylococcus aureus laboratory strains in vitro, and shown that it can distinguish between infection and sterile inflammation in a murine model of acute bacterial infection.

5-[18F]F-pyrazinamide (5-[18F]F-PZA), a radiotracer analog of the first-line tuberculosis drug pyrazinamide (PZA), was employed to determine the biodistribution of PZA using PET imaging and ex vivo analysis. 5-[18F]F-PZA was synthesized in 60 min using a halide exchange reaction. The overall decay-corrected yield of the reaction was 25% and average specific activity was 2.6 × 106 kBq (70 mCi)/μmol. The biodistribution of 5-[18F]F-PZA was examined in a pulmonary Mycobacterium tuberculosis mouse model, where rapid distribution of the tracer to the lung, heart, liver, kidney, muscle, and brain was observed. The concentration of 5-[18F]F-PZA was not significantly different between infected and uninfected lung tissue. Biochemical and microbiological studies revealed substantial differences between 5-F-PZA and PZA. 5-F-PZA was not a substrate for pyrazinamidase, the bacterial enzyme that activates PZA, and the minimum inhibitory concentration for 5-F-PZA against M. tuberculosis was more than 100-fold higher than that for PZA.