UNSULFATED DTPA-CCK AND DOTA-CCK ANALOGS AS SPECIFIC HIGH-AFFINITY LIGANDS FOR CCK-B RECEPTOR-EXPRESSING HUMAN AND RAT-TISSUES IN-VITRO ANDIN-VIVO

Receptors for regulatory peptides such as somatostatin or vasoactive i ntestinal polypeptide are expressed by a number of human neoplasms and can be visualized in vivo with peptide receptor scintigraphy. Recentl y, the CCK-B receptor, which binds both gastrin and cholecystokinin wi th high affinity, was shown using in vitro methods to be overexpressed in a number of human tumor tissues, including medullary thyroid carci nomas, small cell lung cancers, astrocytomas, gastrointestinal tumors, and stromal ovarian cancers. In the present study, we have designed n ovel, unsulfated CCK octapeptide analogs linked to the metal chelating DTPA and DOTA, and have tested them for their binding affinity to CCK -B receptor-positive tissue from human tumors: The most potent compoun ds assayed were DTPA-[Nle(28,31)]-CCK(26-33) (MP2286) and DTPA-[D-Asp( 26),Nle(28,31)]-CCK(26-33) (MP2288) with an IC50 of 1.5 nM, For compar ison, analogs with C-terminal DTPA, such as [Nle(28,31),Aphe(33)(p-NH- DTPA)]-CCK(26-33) and CCK-(26-33)-NH(CH2)(2) NH-DTPA, had an IC50 of > 100 nM. DOTA-[D-Asp(26),Nle(28,31)]-CCK(26-33) had an IC50 Of 3.9 nM. The compounds were selective for CCK-B receptors as they did not bind with high affinity to CCK-A receptors expressed in human tumors (meni ngiomas or gastroenteropancreatic tumors). In vivo rat biodistribution studies with indium-111 labeled MP2286 and MP2288 showed that the pri mary mode of clearance was renal, and the primary sites of uptake (% I D/g 24 h p.i.) were kidneys (0.270 and 0.262, respectively) and the ga strointestinal tract. The CCK-B receptor-expressing gastric mucosa sho wed specific in vivo accumulation of In-111-labeled MP2288 which could be blocked in the presence of excess unlabeled MP2288. In-111-labeled MP2286 and MP2288 were also found to be stable in human plasma wherea s both compounds were degraded in urine (>40% after 3 h at 37 degrees C). The affinity, specificity, biodistribution, and stability of these two DTPA-CCK analogs :indicate that these compounds have substantial promise for use in the in vivo visualization of CCK-B receptor-express ing tumors.