We synthesized and evaluated four
hydrazino nicotinamide (HYNIC) derivatized chemotactic
peptide analogs: For-NleLFK-HYNIC (HP1), For-MLFK-HYNIC (HP2), For-MLFNH(CH2)6NH-HYNIC (HP3), and For-MLF-(D)-K-HYNIC (HP4), for in vitro bioactivity and receptor binding. The
peptides were radiolabeled with 99mTc via a
glucoheptonate co-
ligand and their biodistribution determined in rats (n = 6/time point) at 5, 30, 60 and 120 min after injection. Localization of the
peptides at sites of deep thigh
Escherichia coli infection was determined by radioactivity measurements on excised tissues in rats (n = 6/time point) and rabbits as well as
scintillation camera imaging in rabbits (n = 6). All
peptides maintained
biological activity (EC50s for O2 production by human PMNs: 12-500 nM) and the ability to bind to the
oligopeptide chemoattractant receptor on human PMNs (EC50s for binding: 0.12-40 nM). After incubation with 99mTc-glucoheptonate, radiolabeled
peptides were isolated by HPLC at specific activities of > 10,000 mCi/microM. Technetium-99m-labeled
peptides retained receptor binding with EC50s < 10 nM. Blood clearance of all four
peptides was rapid. Biodistributions of the individual
peptides were similar, with low levels of accumulation in most normal tissues. In rats, all of the
peptides concentrated at the
infection sites (T/B ratio: 2.5-3:1) within 1 hr of injection. In rabbits, outstanding images of the
infection sites were obtained, with T/B ratios of > 20:1 at 15 hr after injection. This study demonstrates that 99mTc-labeled chemotactic
peptide analogs are effective agents for the external imaging of focal sites of
infection.