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), [¹¹C]-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 [¹¹C]-TMP in patients infected with both TMP-sensitive and TMP-resistant organisms to demonstrate the clinical feasibility of the tool. RESULTS. We observed robust [¹¹C]-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 [¹¹C]-TMP, and that despite the AMR, these strains should be “imageable.” Clinical imaging of patients with [¹¹C]-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).

Although the precise molecular mechanisms responsible for neuronal death and motor dysfunction in late-onset Parkinson’s disease (PD) are unknown, evidence suggests that mitochondrial dysfunction and neuroinflammation occur early, leading to a collective increase in reactive oxygen species (ROS) production and oxidative stress. However, the lack of methods for tracking oxidative stress in the living brain has precluded its use as a potential biomarker. The goal of the current study is to address this need through the evaluation of the first superoxide (O₂^•−)-sensitive radioactive tracer, [¹⁸F]ROStrace, in a model of late-onset PD. To achieve this goal, MitoPark mice with a dopaminergic (DA) neuron-specific deletion of transcription factor A mitochondrial (Tfam) were imaged with [¹⁸F]ROStrace from the prodromal phase to the end-stage of PD-like disease. Our data demonstrate [¹⁸F]ROStrace was sensitive to increased oxidative stress during the early stages of PD-like pathology in MitoPark mice, which persisted throughout the disease course. Similarly to PD patients, MitoPark males had the most severe parkinsonian symptoms and metabolic impairment. [¹⁸F]ROStrace retention was also highest in MitoPark males, suggesting oxidative stress as a potential mechanism underlying the male sex bias of PD. Furthermore, [¹⁸F]ROStrace may provide a method to identify patients at risk of Parkinson’s before irreparable neurodegeneration occurs and enhance clinical trial design by identifying patients most likely to benefit from antioxidant therapies.

Neuroinflammation is implicated as a key pathologic mechanism in many neurodegenerative diseases and is thought to be mediated in large part by microglia, native phagocytic immune cells of the CNS. Abnormal aggregation of the protein α-synuclein after phagocytosis by microglia is one possible neuropathophysiological mechanism driving Parkinson’s disease (PD). We conducted a human pilot study to evaluate the feasibility of targeting the inducible isoform of nitric oxide synthase using the [¹⁸F]NOS radiotracer to measure neuroinflammation in idiopathic PD. Ten adults consisting of 6 PD patients and 4 healthy controls (HC) underwent one hour of dynamic [¹⁸F]NOS positron emission tomography (PET) brain imaging with arterial blood sampling. We observed increased [¹⁸F]NOS whole brain distribution volume (V_T) in PD patients compared to age-matched healthy controls (p < 0.008) via a 1-tissue compartment (TC) model. The rate constant K1 for transport from blood into tissue did not differ between groups (p = 0.72). These findings suggest elevated oxidative stress, a surrogate marker of inflammation, is present in early-stage idiopathic PD and indicate that [¹⁸F]NOS PET imaging is a promising, non-invasive method to measure neuroinflammation.

An ultrahigh throughput in silico screen was conducted on 42 M compounds in the Mcule database and led to the identification of a new lead compound for radiotracer development for use with the imaging technique, positron emission tomography.