Evaluation of 99mTc-MAG3-annexin V : influence of the chelate on in vitro and in vivo properties in mice

Abstract We conjugated mercaptoacetyltriglycine (MAG 3 ) to rh-annexin V to permit radiolabeling with 99m Tc in an effort to decrease the high kidney and liver accumulation observed for 99m Tc-labeled Hynic-annexin V. The 36-kDa protein was conjugated at a 5:1 molar ratio with NHS-MAG 3 in HEPES buffer pH 7.8 at room temperature, then quenched with glycine and purified by dialysis. The biopotency of the resulting MAG 3 -annexin was similar to that of Hynic-annexin as determined by a sensitive red blood cell membrane affinity binding assay and a surface plasmon resonance (SPR) assay. The 99m Tc radiolabeling of MAG 3 -annexin resulted in radiochemical yields of 90% under mildly basic pH conditions. Biodistribution data in normal mice clearly showed a significant decrease in kidney and liver uptake at 1 h postinjection for the 99m Tc MAG 3 -annexin compared to the 99m Tc Hynic-annexin (from 24% ID to 4% ID for the liver, and 45% ID to 15% ID for the kidneys, respectively). Autoradiography of the kidneys showed retention of radioactivity in the collecting tubules following administration of both labeled annexins. The 99m Tc MAG 3 -annexin biodistribution was also characterized by a lower retention of radioactivity in the whole body, but with small intestine accumulation over fivefold higher than observed with 99m Tc Hynic-annexin. These findings show a definite improvement in renal and hepatic clearance of the MAG 3 radioligand. However, due to the increased radioactivity uptake in the small intestines, the early in vivo detection of ongoing apoptosis in the lower abdomen might be more difficult with 99m Tc MAG 3 -annexin. Nevertheless, 99m Tc MAG 3 -annexin may be an attractive alternative to 99m Tc Hynic-annexin for the in vivo imaging of phosphatidylserine receptors.