First-in-human immunoPET imaging of HIV-1 infection using 89Zr-labeled VRC01 broadly neutralizing antibody.

A major obstacle to achieving long-term antiretroviral (ART) free remission or functional cure of HIV infection is the presence of persistently infected cells that establish a long-lived viral reservoir. HIV largely resides in anatomical regions that are inaccessible to routine sampling, however, and non-invasive methods to understand the longitudinal tissue-wide burden of HIV persistence are urgently needed. Positron emission tomography (PET) imaging is a promising strategy to identify and characterize the tissue-wide burden of HIV. Here, we assess the efficacy of using immunoPET imaging to characterize HIV reservoirs and identify anatomical foci of persistent viral transcriptional activity using a radiolabeled HIV Env-specific broadly neutralizing antibody, 89Zr-VRC01, in HIV-infected individuals with detectable viremia and on suppressive ART compared to uninfected controls (NCT03729752). We also assess the relationship between PET tracer uptake in tissues and timing of ART initiation and direct HIV protein expression in CD4 T cells obtained from lymph node biopsies. We observe significant increases in 89Zr-VRC01 uptake in various tissues (including lymph nodes and gut) in HIV-infected individuals with detectable viremia (N = 5) and on suppressive ART (N = 5) compared to uninfected controls (N = 5). Importantly, PET tracer uptake in inguinal lymph nodes in viremic and ART-suppressed participants significantly and positively correlates with HIV protein expression measured directly in tissue. Our strategy may allow non-invasive longitudinal characterization of residual HIV infection and lays the framework for the development of immunoPET imaging in a variety of other infectious diseases.

18F-AraG PET for CD8 Profiling of Tumors and Assessment of Immunomodulation by Chemotherapy.

Most clinical trials exploring various combinations of chemo- and immunotherapy rely on serial biopsy to provide information on immune response. The aim of this study was to assess the value of 18F-arabinosyl guanine (18F-AraG) as a noninvasive tool that profiles tumors on the basis of the key player in adaptive antitumor response, CD8+ cells, and evaluates the immunomodulatory effects of chemotherapy. Methods: To evaluate the ability of 18F-AraG to report on the presence of CD8+ cells within the tumor microenvironment, we imaged a panel of syngeneic tumor models (MC38, CT26, LLC, A9F1, 4T1, and B16F10) and correlated the signal intensity with the number of lymphocytes found in the tumors. The capacity of 18F-AraG to detect immunomodulatory effects of chemotherapy was determined by longitudinal imaging of tumor-bearing mice (MC38 and A9F1) undergoing 2 types of chemotherapy: oxaliplatin/cyclophosphamide, shown to induce immunogenic cell death, and paclitaxel/carboplatin, reported to cause immunogenically silent tumor cell death. Results: In the tumor panel, 18F-AraG revealed strikingly different uptake patterns resembling cancer-immune phenotypes observed in the clinic. A statistically significant correlation was found between the 18F-AraG signal and the number of PD-1-positive CD8+ cells isolated from the tumors (r2 = 0.528, P < 0.0001). In the MC38 model, paclitaxel/carboplatin did not result in an appreciable change in signal after therapy (1.69 ± 0.25 vs. 1.50 ± 0.33 percentage injected dose per gram), but oxaliplatin/cyclophosphamide treatment led to close to a 2.4-fold higher 18F-AraG signal (1.20 ± 0.31 vs. 2.84 ± 0.93 percentage injected dose per gram). The statistically significant increase in signal after oxaliplatin/cyclophosphamide was also observed in the A9F1 model (0.95 ± 0.36 vs. 1.99 ± 0.54 percentage injected dose per gram). Conclusion: The ability of 18F-AraG PET to assess the location and function of CD8+ cells, as well immune activity within tumors after immune priming therapy, warrants further investigation into its utility for patient selection, evaluation of optimal time to deliver immunotherapies, and assessment of combinatorial therapies.

Sensing Living Bacteria in Vivo Using d-Alanine-Derived 11C Radiotracers.

Incorporation of d-amino acids into peptidoglycan is a unique metabolic feature of bacteria. Since d-amino acids are not metabolic substrates in most mammalian tissues, this difference can be exploited to detect living bacteria in vivo. Given the prevalence of d-alanine in peptidoglycan muropeptides, as well as its role in several antibiotic mechanisms, we targeted this amino acid for positron emission tomography (PET) radiotracer development. d-[3-11C]Alanine and the dipeptide d-[3-11C]alanyl-d-alanine were synthesized via asymmetric alkylation of glycine-derived Schiff-base precursors with [11C]methyl iodide in the presence of a cinchonidinium phase-transfer catalyst. In cell experiments, both tracers showed accumulation by a wide variety of both Gram-positive and Gram-negative pathogens including Staphylococcus aureus and Pseudomonas aeruginosa. In a mouse model of acute bacterial myositis, d-[3-11C]alanine was accumulated by living microorganisms but was not taken up in areas of sterile inflammation. When compared to existing clinical nuclear imaging tools, specifically 2-deoxy-2-[18F]fluoro-d-glucose and a gallium citrate radiotracer, d-alanine showed more bacteria-specific uptake. Decreased d-[3-11C]alanine uptake was also observed in antibiotic-sensitive microbes after antimicrobial therapy, when compared to that in resistant organisms. Finally, prominent uptake of d-[3-11C]alanine uptake was seen in rodent models of discitis-osteomyelitis and P. aeruginosa pneumonia. These data provide strong justification for clinical translation of d-[3-11C]alanine to address a number of important human infections.

High Enantiomeric Excess In-Loop Synthesis of d-[methyl-11C]Methionine for Use as a Diagnostic Positron Emission Tomography Radiotracer in Bacterial Infection.

Currently, there exists no accurate, noninvasive clinical imaging method to detect living bacteria in vivo. Our goal is to provide a positron emission tomography (PET) method to image infection by targeting bacteria-specific metabolism. Standard of care methodologies detect morphologic changes, image immunologic response to infection, or employ invasive tissue sampling with associated patient morbidity. These strategies, however, are not specific for living bacteria and are often inadequate to detect bacterial infection during fever workup. As such, there is an unmet clinical need to identify and validate new imaging tools suitable for noninvasive, in vivo (PET) imaging of living bacteria. We have shown that d-[methyl-11C]methionine (d-[11C]Met) can distinguish active bacterial infection from sterile inflammation in a murine infection model and is sensitive to both Gram-positive and Gram-negative bacteria. Here, we report an automated and >99% enantiomeric excess (ee) synthesis of d-[11C]Met from a linear d-homocysteine precursor, a significant improvement over the previously reported synthesis utilizing a d-homocysteine thiolactone hydrochloride precursor with approximately 75-85% ee. Furthermore, we took additional steps toward applying d-[11C]Met to infected patients. d-[11C]Met was subject to a panel of clinically relevant bacterial strains and demonstrated promising sensitivity to these pathogens. Finally, we performed radiation dosimetry in a normal murine cohort to set the stage for translation to humans in the near future.