[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 αvβ3 integrins to image angiogenesis. [18F]Galacto-RGD has been successfully used for positron emission tomography applications in patients. Here we report on the preclinical characterization of a 99mTc-labeled derivative for single-photon emission computed tomography.c(RGDyK) was derivatized with HYNIC at the amino group of the lysine [c(RGDyK(HYNIC)) or HYNIC-RGD]. 99mTc labeling was performed using coligands (tricine and EDDA), as well as 99mTc(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 αvβ3-receptor-positive M21 and αvβ3-receptor-negative M21L melanoma cells. Biodistribution was evaluated in normal and nude mice 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. [99mTc]EDDA/HYNIC-RGD, [99mTc]tricine–TPPTS/HYNIC-RGD and [99mTc]tricine–NA/HYNIC-RGD showed protein binding (<5%) considerably lower than [99mTc](CO)3/HYNIC-RGD and [99mTc]tricine/HYNIC-RGD. [99mTc]EDDA/HYNIC-RGD revealed high in vitro stability accompanied by low lipophilicity with a log P value of −3.56, comparable to that of [18F]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 αvβ3-receptor-positive tumors versus 0.85% ID/g in receptor-negative tumors 1 h postinjection. Small cell lung tumors could be visualized using γ camera imaging.[99mTc]EDDA/HYNIC-RGD shows encouraging properties to target αvβ3 receptors in vivo with high stability and favorable pharmacokinetics. Tumor uptake studies showed specific targeting of αvβ3-receptor-positive tumors with tumor-to-organ ratios comparable to those of [18F]Galacto-RGD.