Phosphomycin (phos), a large spectrum antibiotic with preferential localization into the kidneys, can be labeled with technetium-99m at various pHs. However, the final pH in the labeling of phos by [ 99m Tc]pertechnetate in the presence of SnCl 2 seems to play a significant role in determining the biological properties of the final labeled product. Radiopharmacological characteristics of various forms of 99m Tc-phos obtained at pHs 6.8, 4.5 and 2.5 were studied in mice, rats and rabbits. Significant differences in these products were shown by biodistribution studies in Balb/C mice and Wistar rats and confirmed by whole-body autoradiographies in mice and γ-camera studies in rats and rabbits. 99m Tc-phos, pH 6.8, has a high uptake in kidneys from 2 min after injection (10.25 ± 1.11 %ID) to 3 h after injection (8.26 ± 0.42 %ID) and a rapid urinary excretion (19.9 ± 7.35 %ID at 2 min after injection and 75.11 ± 3.48 %ID at 3 h). Intrarenal pharmacokinetics studied by autoradiography showed a high concentration in the inner medulla and pelvis at 1 min after injection followed by an increased localization in kidney cortex, like 99m Tc-DMSA. Preferential uptake in bones was shown by 99m Tc-phos, pH 2.5, from 5 min to 3 h after injection (15.05 ± 2.67 %ID at 3 h compared to 1.79 ± 0.19 %ID of 99m Tc-phos, pH 6.8, at the same time). A high urinary excretion of 99m Tc-phos, pH 2.5, is shown as in the case of other bone agents (76.5 ± 3.46 %ID at 3 h post-injection) but without any kidney retention (1.80 ± 0.26 %ID in kidneys 3 h post-injection). 99m Tc-phos, pH 4.5, was diffusely distributed in liver, kidneys, bones without any specific organ affinity and followed intestinal and urinary accumulation (19.98 ± 5.63 and 53.98 ± 4.24 %ID at 3 h post-injection respectively). The results show that the final pH in phos labeling by 99m Tc produces radiopharmaceuticals with different biological properties: 99m Tc-phos, pH 6.8, is a kidney agent with cortex localization, whereas 99m Tc-phos pH 2.5, is a good bone seeker.
