In vitro and in vivo assessment of 99mTc-UBI specificity for bacteria

Abstract

Technetium-99m labeled ubiquicidin peptide 29-41 ( 99mTc-UBI) is a cationic human antimicrobial peptide fragment that has been shown to bind bacteria in vitro and accumulates at sites of infection in experimental animals. To help determine if 99mTc-UBI is bound to the bacterial cell envelope by a simple nonspecific electrostatic interaction, a comparative study of the in vitro binding of 99mTc-UBI and two different 99mTc labeled cationic peptides ( 99mTc-Tat-1-Scr and 99mTc-Tat-2-Scr) to bacteria and to two tumor cell line (LS174T and ACHN) was performed. The in vivo specificity of 99mTc-UBI for infection in mice was also evaluated using dual labels in the same animal and comparing the target/non-target ratio for 67Ga-citrate and 99mTc-UBI at sites of induced infection and sterile inflammation. Under conditions of this study, the in vitro binding of 99mTc-UBI, 99mTc-Tat-1-Scr and 99mTc-Tat-2-Scr to S. aureus was 35, 78 and 87% respectively. While the binding of 99mTc-Tat-1-Scr and 99mTc-Tat-2-Scr was 37 and 33% to colon tumor cells (LS174T) and 39 and 41% to renal tumor cells (ACHN) respectively, the binding of 99mTc-UBI to both cell types was much lower at less than 4%. In vivo studies revealed that there is a significant difference (p < 0.05) in the radioactive accumulation of 99mTc-UBI between the sites of infection and inflammation compared to 67Ga-citrate. Thus, 99mTc-UBI showed an average infection/inflammation ratio of 2.08 ± 0.49 compared to 1.14 ± 0.45 for 67Ga-citrate. In conclusion, the in vitro and in vivo results provide evidence that a specific mechanism is responsible of the 99mTc-UBI bacterial intracellular accumulation.