Synthesis and biological evaluation of a novel 99m Tc -Nitrido Labeled Substance P Derivative

Aim. Substance P (SP) is an 11-amino acid neuropeptide of the tachykinin family. SP has the greatest affinity for the neurokinin-1 (NK1) receptor presented in a variety of normal tissue such as brain, lymphatic tissue, thymus and salivary glands. High affinity binding sites for SP have been found in various neoplasms with increased expression of NK-1 receptor. SP also plays an important role in the pathogenesis of inflammatory disease. Recently, an In-111 labeled SP analog has been developped for diagnostic imaging. This study investigated the preparation, characterization, and first biologic evaluation in vivo of a novel substance P analog with 99mTc(V), which has more ideal imaging characteristics and availability. The complex is characterized by the presence of a terminal [Tc=N]2+ multiple bond. The complex is represented by the general formula [99mTc(N)(SNS-SP)(X)], where SNS-SP is the tridentate Substance P derivarive [SP = Cys-Cys-Arg-Pro-Lts-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met] and X is a monodentate ligand; specifically, we used the monophosphine tris(2-cyanoethyl)phosphine (PCN) for modeling this “3+1” complex. Materials and Methods. Reagents required for the preparation of the Tc-99m complex were provided through a two-vial freeze-dried kit formulation (vial A and B). The preparation involves two steps: (1) formation of the [188Tc≡N]2+ core (Vial A), (2) reaction of this core with the SNS-SP derivative and the PCN to afford the “3+1” complex nitrido technetium(V) [99mTcN(SNS-SP)(PCN)] (vial B). Purification of the complex from excess of reagents and free ligand was carried out by passing the activity through a reversed-phase SepPak cartridge. The radiochemical yield was determined by HPLC chromatography. SPECT studies and X-ray data acquisition have been performed using the integrated digital X-ray imaging system of YAP-(S)PET scanner. Results. The radiochemical yield for the complex 99mTcN(SNS-SP)(PCN) was found to be in the range 94.5 ± 3.0. Evaluation of stability of 99mTc complexe in serum and towards transchelation with GSH and cysteine revealed no significant change in radiochemical purity (RCP) after 24 h of incubation at 37°C. Imaging data indicates high activity concentration of the complex in kidneys and urinary bladder; a lower concentration was observed in liver and activity traces in digestive apparatus. Some spots are comparable with thymus and salivary glands. Conclusion. The new ”3+1” complex have been prepared in high yeld. 99mTcN(SNS-SP)(PCN) can be interested for imaging SP receptor positive tissue (inflammatory diseases and neoplasms with increased expression of SP receptor) and may also be an attractive alternatives to In-111 labeled molecules for SP receptor.