Linker Effects on Biological Properties of 111In-Labeled DTPA Conjugates of a Cyclic RGDfK Dimer

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Abstract
In this report, we present in vitro and in vivo evaluation of three 111In-labeled DTPA conjugates of a cyclic RGDfK dimer: DTPA-Bn-SU016 (SU016 = E[c(RGDfK)]2; DTPA-Bn = 2-(p-isothioureidobenzyl)diethylenetriaminepentaacetic acid), DTPA-Bn-E-SU016 (E = glutamic acid) and DTPA-Bn-Cys-SU016 (Cys = cysteic acid). The integrin αvβ3 binding affinities of SU016, DTPA-Bn-SU016, DTPA-Bn-E-SU016, and DTPA-Bn-Cys-SU016 were determined to be 5.0 ± 0.7 nM, 7.9 ± 0.6 nM, 5.8 ± 0.6 nM, and 6.9 ± 0.9 nM, respectively, against 125I-c(RGDyK) in binding to integrin αvβ3, suggesting that E or Cys residue has little effect on the integrin αvβ3 affinity of E[c(RGDfK)]2. It was also found that the 111In-labeling efficiency of DTPA-Bn-SU016 and DTPA-Bn-E-SU016 is 3–5 times better than that of DOTA analogues due to fast chelation kinetics and high-yield 111In-labeling under mild conditions (e.g., room temperature). Biodistribution studies were performed using BALB/c nude mice bearing U87MG human glioma xenografts. 111In-DTPA-Bn-SU016, 111In-DTPA-Bn-E-SU016, and 111In-DTPA-Bn-Cys-SU016 all displayed rapid blood clearance. Their tumor uptake was comparable between 0.5 and 4 h postinjection (p.i.) within experimental error. 111In-DTPA-Bn-E-SU016 had a significantly lower (p < 0.01) kidney uptake than 111In-DTPA-Bn-SU016 and 111In-DTPA-Bn-Cys-SU016. The liver uptake of 111In-DTPA-Bn-SU016 was 1.69 ± 0.18% ID/g at 24 h p.i., while the liver uptake values of 111In-DTPA-Bn-E-SU016 and 111In-DTPA-Bn-Cys-SU016 were 0.55 ± 0.11% ID/g and 0.79 ± 0.15% ID/g at 24 h p.i., respectively. Among the three 111In radiotracers evaluated in this study, 111In-DTPA-Bn-E-SU016 has the lowest liver and kidney uptake and the best tumor/liver and tumor/kidney ratios. Results from metabolism studies indicated that there is little metabolism (111In radiotracers at 1 h p.i. Imaging data showed that tumors can be clearly visualized at 4 h p.i. with good contrast in the tumor-bearing mice administered with 111In-DTPA-Bn-E-SU016. It is concluded that using a glutamic acid linker can significantly improve excretion kinetics of the 111In-labeled E[c(RGDfK)]2 from liver and kidneys.