Cholecystokinin-B/Gastrin Receptor Binding Peptides: Preclinical Development and Evaluation of Their Diagnostic and Therapeutic Potential

The high sensitivity of pentagastrin stimulation in detecting primary or metastatic medullary thyroid cancer (MTC) suggests widespread expression of the corresponding receptor type on human MTC. Indeed, autoradiographic studies demonstrated cholecystokinin (CCK)-B/gastrin receptors not only in >90% of MTCs but in a high percentage of small cell lung cancers and potentially a variety of gastrointestinal adenocarcinomas. In a pilot study, we have demonstrated the feasibility of radiolabeled gastrin-I to target CCK-B receptor-expressing tissues in vivo in animals and patients (T. M. Behr et al. , Eur. J. Nucl. Med., 25: 424–430, 1998). The aim of the present study was to systematically optimize, in a preclinical model, suitable radioligands for targeting CCK-B receptors in vivo . For this purpose, a variety of CCK/gastrin-related peptides, all having in common the COOH-terminal CCK-receptor binding tetrapeptide sequence Trp-Met-Asp-PheNH 2 or derivatives thereof, were studied. They were radioiodinated by the Iodogen or Bolton-Hunter procedures. The peptides tested were members of the gastrin- or cholecystokinin families or possessed characteristics of both, which differ by the intramolecular position of a tyrosyl moiety (occurring in native or sulfated form). Their stability and affinity were studied in vitro and in vivo ; their biodistribution and therapeutic efficacy were tested in nude mice bearing s.c. human MTC xenografts. Diethylene-triamine-pentaacetate derivatives of suitable peptides were synthesized, evaluated, and labeled with 111 In. All members of the CCK or gastrin family were stable in serum (with t ½ s of several hours at 37° C); nevertheless, the stability of those peptides was highest that bore the NH 2 -terminal pGlu residues ( e.g. , big gastrin, gastrin-I, caerulein, and others) or d-amino acids. In accordance to their comparably low affinity, nonsulfated members of the CCK family showed fairly low uptake in the tumor and other CCK-B receptor-expressing tissues ( e.g. , the stomach). Sulfated CCK derivatives performed significantly better but additionally displayed a high uptake in normal, CCK-A receptor-expressing tissues (such as the liver/gallbladder, pancreas, and bowel). Best tumor uptake and tumor:nontumor ratios were obtained with members of the gastrin family, probably because of their selectivity and affinity for the CCK-B receptor subtype. Pilot therapy experiments in MTC bearing animals showed significant antitumor efficacy as compared with untreated controls. 111 In-Labeled diethylene-triamine-pentaacetate derivatives of minigastrin showed excellent targeting of CCK-B receptor-expressing tissues in animals and a normal human volunteer. These data suggest that CCK/gastrin analogues may be a useful new class of receptor binding peptides for diagnosis and therapy of CCK-B receptor-expressing tumors, such as MTC or small cell lung cancer. Nonsulfated gastrin derivatives may be preferable because of their CCK-B receptor selectivity, and hence, lower accretion in normal CCK-A receptor-expressing organs. Further preclinical as well as clinical studies are ongoing.