Infection detection in mice using 99mTc-labeled HYNIC and N2S2 chelate conjugated to the antimicrobial peptide UBI 29-41

Earlier we reported that UBI 29-41, a synthetic peptide corresponding to residues 29-41 of human ubiquicidin, directly labeled with technetium-99m ((99m)Tc-UBI 29-41) distinguishes bacterial and fungal infections from sterile inflammations in animals. This study was undertaken to evaluate the radiochemical and biological characteristics of (99m)Tc-UBI 29-41 labeled through the intermediacy of a HYNIC or N(2)S(2) moiety, which were introduced at the N-terminus of UBI 29-41 during solid phase synthesis, with (99m)Tc-UBI 29-41. Methods were as follows: UBI 29-41 and HYNIC- or N(2)S(2)-conjugated peptide were labeled with technetium-99m. Preparations of these radiolabeled UBI 29-41 were purified by HPLC and Sep-Pak. Next, the stability of these tracers in human serum was challenged for 24 hours and their in vitro binding to bacteria assessed. Using scintigraphy up to 2 hours after injection of the tracer and ex vivo countings at the last interval we evaluated the ability of the three tracers to detect bacterial infections in mice inoculated with 2 x 10(7) viable Staphylococcus aureus or Klebsiella pneumoniae as well as their biodistribution. We observed the following results: HPLC analysis of purified (99m)Tc-HYNIC-UBI 29-41, (99m)Tc-UBI 29-41 and (99m)Tc-N(2)S(2)-UBI 29-41 revealed that within 60 minutes >90% of the radioactivity was associated with the peptide. In addition, the stability of these radiolabeled UBI 29-41 peptides in human serum was excellent. All three tracers bound equally well to bacteria in vitro. After intravenous injection into mice with an experimental bacterial infection (99m)Tc-HYNIC-UBI 29-41 and (99m)Tc-UBI 29-41 were rapidly removed from the circulation mainly by renal clearance (at t = 120 minutes approximately 60% of the injected dose/gram tissue; % ID/g). In contrast, (99m)Tc-N(2)S(2)-UBI 29-41 was removed mainly by the liver (t = 120 minutes; 52% ID/g) showing deposits in the intestines (t = 120 minutes; 31% ID/g) and to a lesser extent by renal clearance (19% ID/g). All three tracers rapidly detected bacterial infections in mice and highest accumulation (target-to-nontarget ratios between 3.2 and 3.6 and between 2.9 and 4.4 for infections with S. aureus and K. pneumoniae, respectively) was found at 2 hours after injection of the tracer. In conclusion, purified (99m)Tc-HYNIC-UBI 29-41 and (99m)Tc-N(2)S(2)-UBI 29-41 were as effective as (99m)Tc-UBI 29-41 in detecting infections in mice injected intramuscularly with bacteria. However, (99m)Tc-N(2)S(2)-UBI 29-41 should not be advised for the imaging of abdominal infections as this tracer, in contrast to the other tracers, is cleared via the liver and intestines.