Imaging Sensitive and Drug-Resistant Bacterial Infection with [¹¹C]-TMP: In Vitro and First-in-Human Evaluation

Abstract

Recently, several molecular imaging strategies have developed to image bacterial infections in humans. Nuclear approaches, specifically positron emission tomography (PET), affords sensitive detection and the ability to non-invasively locate infections deep within the body. Two key radiotracer classes have arisen: metabolic approaches targeting bacterial specific biochemical transformations, and antibiotic-based approaches that have inherent selectivity for bacteria over mammalian cells. A critical question for clinical application of antibiotic radiotracers is whether resistance to the template antibiotic abrogates specific uptake, thus diminishing the predictive value of the diagnostic test. We recently developed small-molecule PET radiotracers based on the antibiotic trimethoprim (TMP), including [11C]-TMP, and have shown their selectivity for imaging bacteria in preclinical models. Here, we measure the in vitro uptake of [11C]-TMP in pathogenic susceptible and drug-resistant bacterial strains. Both resistant and susceptible bacteria showed similar in vitro uptake, which led us to perform whole genome sequencing of these isolates to identify the mechanisms of TMP resistance that permit retained radiotracer binding. By interrogating these isolate genomes and a broad panel of previously sequenced strains, we reveal mechanisms where uptake or binding of TMP radiotracers can potentially be maintained despite the annotation of genes conferring antimicrobial resistance. Finally, we present several examples of patients with both TMP-sensitive and drug-resistant infections in our first-in-human experience with [11C]-TMP. This work underscores the ability of an antibiotic radiotracer to image bacterial infection in patients, which may allow insights into human bacterial pathogenesis, infection diagnosis, and antimicrobial response monitoring.

11C-Para-aminobenzoic acid PET imaging of S. aureus and MRSA infection in preclinical models and humans

“…Given how ubiquitous and conserved this pathway is among different bacteria, the incorporation of PABA into tetrahydrofolate by the bacterial dihydropteroate synthase has been considered as a promising target for pathogen-specific imaging. Inhibitors with nanomolar affinity to key enzymes within the folate synthesis pathway, such as radiolabeled trimethoprim ( 11 C-TMP) and its analog fluoropropyl-trimethoprim ( 18 F-FPTMP), have also been evaluated as bacteria-specific imaging agents ( 18 , 19 ). PABA and its antibacterial structural analog, sulfonamide, enter bacterial cells by passive diffusion across the cell membrane ( 20 ).…”

Antibiotic‐Derived Radiotracers for Positron Emission Tomography: Nuclear or “Unclear” Infection Imaging?

“…Thus, this wildtype DHFR copy in TMP-resistant strains is responsible for the sustained, high levels of intracellular [ 11 C]TMP. [20] The biodistribution of [ 11 C]TMP-PET in healthy human subjects [19] and a proof-of-concept study in patients with known or suspected bacterial infections [20] showed hepatic tracer accumulation and activity in kidneys and bladderrepresenting TMP metabolism and excretion. A patient with biopsy-proven methicillin-sensitive S. aureus (MSSA) discitis, scanned with [ 11 C]TMP, showed asymmetric uptake in the left L4-5 facet that resolved completely on the follow-up scan.…”

Antibiotic‐Derived Radiotracers for Positron Emission Tomography: Nuclear or “Unclear” Infection Imaging?

“…[17] Hence, Sell- 18 F]fluoropropyl-trimethoprim and evaluated their potential as bacterial-specific imaging agents. [18][19][20]…”

“…A dot blot assay with various concentrations of E. coli DHFR incubated with [ 11 C]TMP showed a concentration‐dependent, specific radiotracer binding [21] . To assess bacterial [ 11 C]TMP uptake and determine if antibiotic resistance may affect radiotracer uptake levels, extensive in vitro uptake studies were performed across a panel of wild‐type‐ and TMP‐resistant bacterial strains expressing mutant DHFR enzymes [20] . In all cases, both drug‐susceptible and ‐resistant bacterial strains showed similar and sustained [ 11 C]TMP uptake compared to negative controls (either DHFR knockout strain, heat‐killed, or TMP‐blocked bacteria).…”

“…[ 11 C]TMP has already proven to be selective in imaging of bacterial infections in patients with suspected infections. While antibiotic resistance can directly affect antibiotic‐based tracer uptake, [ 11 C]TMP‐PET imaging has shown promising results in detecting TMP‐resistant and multi‐drug resistant clinical strains [18, 20] . The relatively low tracer uptake ([ 18 F]FP‐TMP‐T/NT<3.0) in the target tissue) could, however, be the limiting factor in its clinical usefulness and is part of an ongoing clinical trial [22]…”