In vivo monitoring of viable bacteria by SPECT using 99mTc-HYNIC(GH)2-UBI 29-41 and 99mTc-HYNIC(Tricine)2-UBI 29-41

Abstract Background: The number of bacterial infections that—for various reasons—are challenging to cure continues to increase. One such reason is persister cell infection. To investigate persister formation and persister infections, viable bacteria must be evaluated in the same animal over time. In this study, the feasibility of monitoring viable bacteria by SPECT using two labeled peptides was evaluated. Results: Two types of ubiquicidin (UBI) 29-41 labeled with technetium-99m, 99mTc-HYNIC(GH)2-UBI 29-41 and 99mTc-HYNIC(Tricine)2-UBI 29-41, were synthesized. The in vitro binding of these labeled peptides to Staphylococcus aureus was measured. For the in vivo study, each labeled peptide was injected into S. aureus infected mouse thigh after treatment with various doses of ciprofloxacin (CPFX). Two hours after injection, the accumulation of each labeled peptide at the infection site was assessed by SPECT, and then the number of viable bacteria was determined from the accumulation detected. The peptide labeling was successful, and the radiochemical purity was 91±9% (GH, n=8) and 100% (Tricine, n=8). The in vitro binding of the labeled peptides to S. aureus (5×108 cfu) without serum was 78.9% (GH) and 85.5% (Tricine) of the total 99mTc activity. With serum, the binding rate was 67.5% (GH) and 13.3% (Tricine). The accumulation of labeled peptide was calculated from the SPECT images, and that in the bacterial infection site (left thigh) was higher than that in the non-infection site (right thigh) for both peptides. Good correlation was found between the target-to-non-target (T/NT) ratios of each labeled peptide and the viable bacterial count at the infection site, and 99mTc-HYNIC(Tricine)2-UBI 29-41 had a wider range than 99mTc-HYNIC(GH)2-UBI 29-41.Conclusion: Using the SPECT/labeled peptide method, it was possible to monitor viable bacterial count in the range 103–108 cfu, which is appropriate for tracking viable bacterial counts in the same animal over time.