A comparison in monkeys of (99m)Tc labeled to a peptide by 4 methods

Although a number of different strategies for labeling peptides with (99m)Tc have been developed, only a few studies have compared the in vivo properties of (99m)Tc when attached to different chelators. Furthermore, these comparisons are usually in mice, whereas results obtained in nonhuman primates may be expected to be more relevant to the clinical situation.

We evaluated the influence of 4 common chelators on the biodistribution in monkeys of (99m)Tc-labeled HNE-2, a 6.7-kDa peptide being investigated as an inflammation/infection imaging agent. The peptide was conjugated with the N-hydroxysuccinimide ester of mercaptoacetyltriglycine (MAG3), mercaptoacetyltriserine (MAS3), hydrazinonicotinamide (HYNIC), and the cyclic anhydride of diethylenetriaminepentaacetic acid (DTPA). After radiolabeling, each peptide was administered intravenously to rhesus monkeys with a Staphylococcus aureus-induced focal inflammation/infection.

Quantification of radioactivity accumulation by regions of interest over 3 h after administration in monkeys showed important differences among labeling methods: For example, at 3 h, kidney accumulation varied in percentage injected dose per organ (%ID per organ) from 31 %ID per organ (HYNIC) to 18 %ID per organ (MAG3), whereas liver varied from 7.8 %ID per organ (MAG3) to 2.8 %ID per organ (MAS3). Radioactivity accumulation in the lesion was independent of labeling method. These organ accumulations were compared with that obtained earlier in mice by sacrifice and dissection also at 3 h and at the same administered dosage. In the rodent, kidney levels varied from 45 %ID per organ (HYNIC) to 12 %ID per organ (MAS3) and liver levels varied from 6.5 %ID per organ (DTPA) to 2.0 %ID per organ (MAS3).

In agreement with previous work from this laboratory and elsewhere, the method of radiolabeling had an important effect on the biodistribution of (99m)Tc. Furthermore, although biodistribution results in mice should be used with caution to predict biodistributions in primates, in major organs, these results in mice and monkeys were similar.