No 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.

Precision medicine, in which the molecular underpinnings of the disease are assessed for tailored therapies, has greatly impacted cancer care. In parallel, a new pillar of therapeutics has emerged with profound success, including immunotherapies such as checkpoint inhibitors and cell-based therapies. Nonetheless, it remains essential to develop paradigms to predict and monitor for therapeutic response. Molecular imaging has the potential to add substantially to all phases of cancer patient care: predictive, companion diagnostics can illuminate therapeutic target density within a tumor, and pharmacodynamic imaging biomarkers can complement traditional modalities to judge a favorable treatment response. This "Focus on Molecular Imaging" article discusses the current role of molecular imaging in oncology and highlights an additional step in the clinical paradigm termed a therapeutic biomarker, which serves to assess whether next-generation drugs reach their target to elicit a favorable clinical response.

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).

Purpose: Despite the success of chimeric antigen receptor (CAR) T cell therapy against hematological malignancies, successful targeting of solid tumors with CAR T cells has been limited by a lack of durable responses and reports of toxicities. Our understanding of the limited therapeutic efficacy in solid tumors could be improved with quantitative tools that allow characterization of CAR T-targeted antigens in tumors and accurate monitoring of response. Design: We used a radiolabeled fibroblast activation protein (FAP) inhibitor (FAPI) [18F]AlF-FAPI-74 probe to complement ongoing efforts to develop and optimize FAP CAR T cells. The selectivity of the radiotracer for FAP was characterized in vitro and its ability to monitor changes in FAP expression was evaluated using rodent models of lung cancer. Results: [18F]AlF-FAPI-74 showed selective retention in FAP+ cells in vitro, with effective blocking of the uptake in presence of unlabeled FAPI. In vivo, [18F]AlF-FAPI-74 was able to detect FAP expression on both tumor cells as well as FAP+ stromal cells in the tumor microenvironment with a high target-to-background ratio. We further demonstrated the utility of the tracer to monitor changes in FAP expression following FAP CAR T cell therapy, and the PET imaging findings showed a robust correlation with ex vivo analyses. Conclusion: This non-invasive imaging approach to interrogate the tumor microenvironment represents an innovative pairing of a diagnostic PET probe with solid tumor CAR T cell therapy and has the potential to serve as a predictive and pharmacodynamic response biomarker for FAP as well as other stromal cell-targeted therapies.