ABSTRACT
Objectives: In this study, we developed angiotensin-converting enzyme 2 (ACE2)-specific, peptide-derived 68Ga- and 18F-labeled radiotracers, motivated by the hypotheses that ACE2 is an important determinant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) susceptibility and that modulation of ACE2 in coronavirus disease 2019 (COVID-19) drives severe organ injury. Our current efforts are focusing on broader dissemination of ACE2-targeted PET radiotracers based on chelation of [18F]AlF enabling advanced murine and potentially human studies. Method(s): A series of NOTA-conjugated peptides derived from the known ACE2 inhibitor DX600 were synthesized, with variable linker identity. Since DX600 bears 2 cystine residues, both linear and cyclic peptides were studied. An ACE2 inhibition assay was used to identify lead compounds, which were labeled with 68Ga and 18F-AlF to generate the corresponding peptide radiotracers (68Ga-NOTA-PEP). The most potent 68Ga and 18F-AlF DX600 derivatives were subsequently studied in a humanized ACE2 (hACE2) transgenic model. Result(s): Cyclic DX-600-derived peptides had markedly lower half-maximal inhibitory concentrations than their linear counterparts. The 3 cyclic peptides with triglycine, aminocaproate, and polyethylene glycol linkers had calculated half-maximal inhibitory concentrations similar to or lower than the parent DX600 molecule. Peptides were readily labeled with 68Ga and 18F-AlF, and the biodistribution of both tracers was determined in an hACE2 transgenic murine cohort. Pharmacologic concentrations of coadministered NOTA-PEP (blocking) showed a significant reduction of 68Ga-NOTA-PEP4 signals in the heart, liver, lungs, and small intestine. Ex vivo hACE2 activity in these organs was confirmed as a correlate to in vivo results. The biodistribution of both tracers was similar, with apparent blocking observed in the lungs using the 18F-AlF peptide that needs to be verified via additional experiments. Conclusion(s): NOTA-conjugated cyclic peptides derived from the known ACE2 inhibitor DX600 retain their activity when N-conjugated for 68Ga or 18F-AlF chelation. In vivo studies in a transgenic hACE2 murine model using the lead tracer, 68Ga-NOTA-PEP4, showed specific binding in the heart, liver, lungs and intestine-organs known to be affected in SARS-CoV-2 infection. Blocking studies using the 18F-AlF labeled correlate showed modulation of PET signals in the normal lungs. These results suggest that 68Ga-NOTA-PEP4 or the 18F-AlF correlate could be used to detect organ-specific suppression of ACE2 in SARS-CoV-2-infected murine models and COVID-19 patients.Copyright © 2023 Southern Society for Clinical Investigation.