IN-VITRO AND IN-VIVO STUDIES OF SUBSTANCE-P RECEPTOR EXPRESSION IN RATS WITH THE NEW ANALOG [INDIUM-111-DTPA-ARG(1)]SUBSTANCE-P

We evaluated the potential usefulness of a new radiolabeled substance P (SP) analog, [In-111-DTPA-Arg(1)]SP, as a radiopharmaceutical for th e in vivo detection of SP receptor-positive (SPR+) immunologic disorde rs (i.e., inflammatory bower disease and arthritis) and tumors (i.e., carcinoid). Methods: Substance P, [DTPA-Arg(1)]SP and [3-(p-hydroxyphe nyl)propionyl-Arg(1)]SP (Bolton-Hunter-SP, [BH-SP]) were tested as com petitors for I-125-BH SP to SPR in rat brain cortex membranes. An auto radiographic displacement study of the submandibular gland of the rat with the I-125-BH-SP as radioligand and [DTPA-Arg(1)]SP as competitor was performed. Tissue distribution and ex vivo autoradiography were st udied in rats, with and without pretreatment with the selective nonpep tide antagonist CP96,345 to quantify specific binding. In vivo metabol ism of [In-111-DTPA-Arg(1)]SP was performed in control rats. Gamma-cam era scintigraphic studies were carried out with control rats to visual ize the SPR+ salivary glands in rats bearing the SPR+ transplantable p ancreatic tumor CA20948 and in rats with SPR+ adjuvant arthritic joint s, which was induced after injection of a homogenate of Mycobacterium tuberculosis. Results: Substance P, [DTPA-Arg(1)]SP and BH-SP dose-dep endently inhibited binding of I-125-BH-SP to SPR in rat brain cortex m embranes with IC50 values of 0.2, 4 and 2 nM, respectively. In an auto radiographic displacement study of the submandibular gland with I-125- BH-Sp as radioligand, an IC50 of 2.7 nM was found for [DTPA-Arg(1)]SP. In vivo metabolism of the radiopharmaceutical in the rat revealed a r enal clearance rate of 50% of the injected radioactive dose in 30 min and a rapid enzymatic degradation of the radiopharmaceutical, resultin g in an effective half-life of the intact radiopharmaceutical in blood of approximately 3 min. Tissue distribution and ex vivo autoradiograp hic studies in rats showed uptake and specific binding of radioactivit y in isolated tumors and submandibular and parotid glands. Optimum SPR + target-to-background ratios were found 24 hr after injection of [In- 111-DTPA-Arg(1)]SP. Visualization of normal SPR+ tissues, such as the salivary glands by gamma camera scintigraphy, after administration of [In-111-DTPA-Arg(1)]SP was demonstrated in untreated rats. Pathologica l SPR+ processes were visualized both in rats bearing the transplantab le pancreatic tumor CA20948 and in those with adjuvant mycobacteria tu berculosis-induced arthritic joints. Conclusion: [Indium-111-DTPA-Arg( 1)]SP can be used successfully to visualize SPR+ processes in vive by gamma camera scintigraphy.