A HYNIC-conjugated chemotactic
peptide (
fMLFK-HYNIC) was labeled with (99m)Tc using
tricine and TPPTS as coligands. The combination of
fMLFK-HYNIC,
tricine, and TPPTS with (99m)Tc produced a ternary
ligand complex [(99m)Tc(
fMLFK-HYNIC)(
tricine)(TPPTS)] (RP463). RP463 was synthesized either in two steps, in which the binary
ligand complex [(99m)Tc(
fMLFK-HYNIC)(
tricine)(2)] (RP469) was formed first and then reacted with TPPTS, or in one step by direct reduction of [(99m)Tc]
pertechnetate with
stannous chloride in the presence of
fMLFK-HYNIC,
tricine, and TPPTS. The radiolabeling yield for RP463 was usually >/=90% using 10 microg of
fMLFK-HYNIC and 100 mCi of [(99m)Tc]
pertechnetate. Unlike RP469, which decomposed rapidly in the absence of excess
tricine coligand, RP463 was stable in
solution for at least 6 h. [(99)Tc]RP463 was prepared and characterized by HPLC and electrospray mass spectrometry. In an in vitro assay, [(99)Tc]RP463 showed an IC(50) of 2 nM against binding of [(3)H]fMLF to receptors on PMNs. [(99)Tc]RP463 also induces effectively the
superoxide release of polymorphonuclear leukocytes (PMNs) with an EC(50) value of 0.2 +/- 0.2 nM. The localization of RP463 in the
infection foci was assessed in a rabbit
infection model. RP463 was cleared from the blood faster than RP469 and was excreted mainly through the renal system. As a result of rapid blood clearance and increased uptake, the target-to-background ratios continuously increased from 1.5 +/- 0.2 at 15 min postinjection to 7.5 +/- 0.4 at 4 h postinjection. Visualization of the infected area could be as early
as 2 h. A transient decrease in white blood cell count of 35% was observed during the first 30 min after injection of the HPLC-purified RP463 in the infected rabbit. This suggests that future research in this area should focus on developing highly potent antagonists for
chemotactic peptide receptor or other receptors on PMNs and monocytes.