Purpose: This report presents the synthesis of a cyclic RGD dimer conjugate, MAG(2)-G(3)-E[G(3)-c(RGDfK)](2) (MAG(2)-3G(3)-dimer, G(3) = Gly-Gly-Gly, MAG(2) = S-benzoyl mercaptoacetylglycylglycyl), and evaluation of its (99m)Tc complex, (99m)TcO(MAG(2)-3G(3)-dimer), as a new radiotracer for imaging the tumor integrin alpha(v)beta(3) expression.
Methods: An in vitro displacement assay was used to determine the integrin alpha(v)beta(3) binding affinity of MAG(2)-3G(3)-dimer against (125)I-c(RGDyK) bound to U87MG human glioma cells. The athymic nude mice bearing U87MG glioma xenografts were used for biodistribution and planar imaging studies.
Results: We found that (1) MAG(2) is such a highly effective bifunctional chelator that (99m)TcO(MAG(2)-3G(3)-dimer) can be prepared in high yield (radiochemical purity >95%) and with high specific activity ( approximately 5 Ci/micromol) using a kit formulation; (2) (99m)TcO(MAG(2)-3G(3)-dimer) has very high solution stability in the kit matrix; and (3) (99m)TcO(MAG(2)-3G(3)-dimer) has very fast clearance kinetics from the intestine, liver, and kidneys. Among the (99m)Tc-labeled cyclic RGD peptides evaluated in the xenografted U87MG glioma models, (99m)TcO(MAG(2)-3G(3)-dimer) has the best pharmacokinetics and tumor to background ratios (tumor/liver = 4.29 +/- 1.00 at 30 min postinjection and 8.29 +/- 1.50 at 120 min postinjection; tumor/kidney = 1.16 +/- 0.19 at 30 min postinjection and 2.49 +/- 0.25 at 120 min postinjection). Planar imaging studies showed that tumors in the glioma-bearing mice administered with (99m)TcO(MAG(2)-3G(3)-dimer) can be visualized with excellent contrast as early as 15 min postinjection. (99m)TcO(MAG(2)-3G(3)-dimer) was able to maintain its chemical integrity in kidneys (>80% intact) and liver (>95% intact) over the 2-h period. However, there was significant metabolism (>50% of the injected radioactivity) detected in both urine and feces samples.
Conclusion: (99m)TcO(MAG(2)-3G(3)-dimer) is a very attractive radiotracer for early detection of integrin alpha(v)beta(3)-positive tumors and has significant advantages over the (18)F-labeled RGD peptide radiotracers with respect to the cost, availability, and easiness for routine clinical preparation.