The (Tc(N)(PNP))2+ metal fragment labeled cholecystokinin-8 (CCK8) peptide for CCK-2 receptors imaging: In vitro and in vivo studies

The radiolabeling of the natural octapeptide CCK8, derivatized with a cysteine residue (Cys-Gly-CCK8). by using the metal fragment [^99mTc(N)(pNp3)]²⁺ (PNP3 = N,N-bis(dimethoxypropylphosphinoethyl)methoxyethylaniine) is reported. The [^99mTc(N)(NS-Cys-Gly-CCK8)(PNP3)]⁺ complex was obtained according to two methods (one-step or two-step procedure) that gave the desired compound in high yield. The complex is stable In aqueous solution and in phosphate buffer. In vitro challenge experiments with an excess of cysteine and glutathione indicate that no transchelation reactions occur, confirming the high thermodynamic stability and kinetic inertness of this compound. Stability studies carried out in human and mouse serum, as well as in mouse liver homogenates, show that the radiolabeled compound remains intact for prolonged incubation at 37°C. Binding properties give K_d (19.0 ± 4.6 nmol/1) and B_max (∼10⁶ sites/cell) values in A431 cells overexpressing the CCK2-R. In vivo evaluation of the compound shows rapid and specific targeting to CCK2-R, a fourfold higher accumulation compared to nonreceptor expressing tumors.