[99mTc]HYNIC-RGD for imaging integrin αvβ3 expression

There has been increasing interest in peptides containing the Arg-Gly-Asp (RGD) sequence for targeting of alpha(v)beta(3) integrins to image angiogenesis. [F-18]Galacto-RGD has been successfully used for positron emission tomography applications in patients. Here we report on the preclinical characterization of a Tc-99m-labeled derivative for single-photon emission computed tomography. c(RGDyK) was derivatized with HYNIC at the e-amino group of the lysine [c(RGDyK(HYNIC)) or HYNIC-RGD]. Tc-99m labeling was performed using coligands (tricine and EDDA), as well as Tc-99m(CO)(3)(H2O)(3). Radiolabeled peptides were characterized with regard to lipophilicity, protein binding and stability in buffer, serum and tissue homogenates. Integrin receptor activity was determined in internalization assays using alpha(v)beta(3)-receptor-positive M21 and alpha(v)beta(3)-receptor-negative M21L melanoma cells. Biodistribution was evaluated in normal and nude trice bearing M21, M21L and small cell lung tumors. HYNIC-RGD could be labeled at high specific activities using tricine, tricine-trisodium triphenylphosphine 3,3',3"-trisulfonate (TPPTS), tricine-nicotinic acid (NA) or EDDA as coligands. [Tc-99m]EDDA/HYNIC-RGD, [Tc-99m]tricine-TPPTS/HYNIC-RGD and [Tc-99m]tricine-NA/HYNIC-RGD showed protein binding (< 5%) considerably lower than [Tc-99m](CO)(3)-HYNIC-RGD and [Tc-99m]tricine/HYNIC-RGD. [Tc-99m]EDDA/HYNIC-RGD revealed high in vitro stability accompanied by low lipophilicity with a log P value of -3.56, comparable to that of [F-18]Galacto-RGD. In M21 cells for this compound, the highest level of specific and rapid cell uptake (1.25% mg protein(-1)) was determined. In vivo, rapid renal excretion, low blood retention, low liver and muscle uptakes and low intestinal excretion 4 h postinjection were observed. Tumor uptake values were 2.73% ID/g in M21 alpha(v)beta(3)-receptor-positive tumors versus 0.85% ID/g in receptor-negative tumors 1 h postinjection. Small cell lung tumors could be visualized using gamma camera imaging. [Tc-99m]EDDA/HYNIC-RGD shows encouraging properties to target alpha(v)beta(3) receptors in vivo with high stability and favorable pharmacokinetics. Tumor uptake studies showed specific targeting of alpha(v)beta(3)-receptor-positive tumors with tumor-to-organ ratios comparable to those of [F-18]Galacto-RGD. (c) 2006 Elsevier Inc. All rights reserved.