A series of potent and selective D3 receptor (D3R) analogues with diazaspiro alkane cores were synthesized. Radioligand binding of compounds 11, 14, 15a, and 15c revealed favorable D3R affinity (Ki = 122–25.6 nM) and were highly selective for D3R vs D3R (ranging from 264–905-fold). Variation of these novel ligand architectures can be achieved using our previously reported 10–20 minute benchtop C–N cross-coupling methodology, affording a broad range of arylated diazaspiro pre-cursors.

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.

BACKGROUND Several molecular imaging strategies can identify bacterial infections in humans. PET affords the potential for sensitive infection detection deep within the body. Among PET-based approaches, antibiotic-based radiotracers, which often target key bacterial-specific enzymes, have considerable promise. One question for antibiotic radiotracers is whether antimicrobial resistance (AMR) reduces specific accumulation within bacteria, diminishing the predictive value of the diagnostic test. METHODS Using a PET radiotracer based on the antibiotic trimethoprim (TMP), [ 11 C]-TMP, we performed in vitro uptake studies in susceptible and drug-resistant bacterial strains and whole-genome sequencing (WGS) in selected strains to identify TMP resistance mechanisms. Next, we queried the NCBI database of annotated bacterial genomes for WT and resistant dihydrofolate reductase (DHFR) genes. Finally, we initiated a first-in-human protocol of [ 11 C]-TMP in patients infected with both TMP-sensitive and TMP-resistant organisms to demonstrate the clinical feasibility of the tool. RESULTS We observed robust [ 11 C]-TMP uptake in our panel of TMP-sensitive and -resistant bacteria, noting relatively variable and decreased uptake in a few strains of P . aeruginosa and E . coli . WGS showed that the vast majority of clinically relevant bacteria harbor a WT copy of DHFR, targetable by [ 11 C]-TMP, and that despite the AMR, these strains should be “imageable.” Clinical imaging of patients with [ 11 C]-TMP demonstrated focal radiotracer uptake in areas of infectious lesions. CONCLUSION This work highlights an approach to imaging bacterial infection in patients, which could affect our understanding of bacterial pathogenesis as well as our ability to better diagnose infections and monitor response to therapy. TRIAL REGISTRATION ClinicalTrials.gov NCT03424525. FUNDING Institute for Translational Medicine and Therapeutics, Burroughs Wellcome Fund, NIH Office of the Director Early Independence Award (DP5-OD26386), and University of Pennsylvania NIH T32 Radiology Research Training Grant (5T32EB004311-12).

Previously, we reported a 3-(2-methoxyphenyl)-9-(3-((4-methyl-5-phenyl-4H-1,2,4-triazol-3-yl)thio)propyl)-3,9-diazaspiro[5.5]undecane (1) compound with excellent dopamine D 3 receptor (D 3 R) affinity (D 3 R K i = 12.0 nM) and selectivity (D 2 R/D 3 R ratio = 905). Herein, we present derivatives of 1 with comparable D 3 R affinity (32, D 3 R K i = 3.2 nM, D 2 R/D 3 R ratio = 60) and selectivity (30, D 3 R K i = 21.0 nM, D 2 R/D 3 R ratio = 934).Fragmentation of 1 revealed orthosteric fragment 5a to express an unusually low D 3 R affinity (K i = 2.7 μM). Compared to piperazine congener 31, which retains a high-affinity orthosteric fragment (5d, D 3 R K i = 23.9 nM), 1 was found to be more selective for the D 3 R among D 1 -and D 2 -like receptors and exhibited negligible off-target interactions at serotoninergic and adrenergic G-protein-coupled receptors (GPCRs), common off-target sites for piperazinecontaining D 3 R scaffolds. This study provides a unique rationale for implementing weakly potent orthosteric fragments into D 3 R ligand systems to minimize drug promiscuity at other aminergic GPCR sites.