We evaluated the potential usefulness of a new radiolabeled substance P (SP) analog, [111In-DTPA-Arg1]SP, as a radiopharmaceutical for the in vivo detection of SP receptor-positive (SPR+) immunologic disorders (i.e., inflammatory bowel disease and arthritis) and tumors (i.e., carcinoid).
Methods: Substance P, [DTPA-Arg1]SP and [3-(p-hydroxyphenyl)propionyl-Arg1]SP (Bolton-Hunter-SP, [BH-SP]) were tested as competitors for 125I-BH-SP to SPR in rat brain cortex membranes. An autoradiographic displacement study of the submandibular gland of the rat with the 125I-BH-SP as radioligand and [DTPA-Arg1]SP as competitor was performed. Tissue distribution and ex vivo autoradiography were studied in rats, with and without pretreatment with the selective nonpeptide antagonist CP96,345 to quantify specific binding. In vivo metabolism of [111In-DTPA-Arg1]SP was performed in control rats. Gamma-camera scintigraphic studies were carried out with control rats to visualize the SPR+ salivary glands in rats bearing the SPR+ transplantable pancreatic tumor CA20948 and in rats with SPR+ adjuvant arthritic joints, which was induced after injection of a homogenate of Mycobacterium tuberculosis.
Results: Substance P, [DTPA-Arg1]SP and BH-SP dose-dependently inhibited binding of 125I-BH-SP to SPR in rat brain cortex membranes with IC50 values of 0.2, 4 and 2 nM, respectively. In an autoradiographic displacement study of the submandibular gland with 125I-BH-SP as radioligand, an IC50 of 2.7 nM was found for [DTPA-Arg1]SP. In vivo metabolism of the radiopharmaceutical in the rat revealed a renal clearance rate of 50% of the injected radioactive dose in 30 min and a rapid enzymatic degradation of the radiopharmaceutical, resulting in an effective half-life of the intact radiopharmaceutical in blood of approximately 3 min. Tissue distribution and ex vivo autoradiographic studies in rats showed uptake and specific binding of radioactivity in isolated tumors and submandibular and parotid glands. Optimum SPR+ target-to-background ratios were found 24 hr after injection of [111In-DTPA-Arg1]SP. Visualization of normal SPR+ tissues, such as the salivary glands by gamma camera scintigraphy, after administration of [111In-DTPA-Arg1]SP was demonstrated in untreated rats. Pathological SPR+ processes were visualized both in rats bearing the transplantable pancreatic tumor CA20948 and in those with adjuvant mycobacteria tuberculosis-induced arthritic joints.
Conclusion: [Indium-111-DTPA-Arg1]SP can be used successfully to visualize SPR+ processes in vivo by gamma camera scintigraphy.