Tc-Nicotinic acid/tricine/hydrazinonicotinamide-nonsulfated cholecystokinin-8

Tc-Nicotinic acid/tricine/hydrazinonicotinamide-nonsulfated cholecystokinin-8 (Tc-NA/tricine/HYNIC-nsCCK-8) is a radiolabeled peptide developed for single-photon emission computed tomography (SPECT) imaging of tumors that express the gastrin/cholecystokinin-2 (CCK-2) receptor (1). Tc is a gamma emitter with a physical half-life () of 6.01 h. The gastrointestinal peptides gastrin and CCK have various regulatory functions in the brain and gastrointestinal tract (2). Gastrin and CCK have the same COOH-terminal pentapeptide amide sequence, which is the biologically active site (3). Human gastrin is a peptide of 34 amino acids that also exists in several C-terminal truncated forms (4), which include the minigastrin, a 13-residue peptide with the sequence of LEEEEEAYGWMDF-NH. CCK exists in a variety of biologically active molecular forms that are derived from a precursor molecule of 115 amino acids (5). They range from 4 to 58 amino acids in length and include sulfated (Tyr residue) and unsulfated CCK-8, which has the structure DYMGWMDF-NH. They bind to and act through transmembrane G-protein–coupled receptors (6). Two different CCK receptor subtypes have been identified in normal tissue. CCK-1 (CCK-A, alimentary) receptors have low affinity for gastrin, and CCK-2 (CCK-B, brain) receptors have high affinity for gastrin (5). They also differ in terms of molecular structure, distribution, and affinity for CCK. These receptors have been found to be expressed or overexpressed on a multitude of tumor types (6). CCK-2 receptors (CCK-2Rs) have been found most frequently in medullary thyroid carcinomas, small cell lung cancers, astrocytomas, and stromal ovarian cancers (2). CCK-1 receptors (CCK-1Rs) have been identified in gastroenteropancreatic tumors, meningiomas, and neuroblastomas. Reubi et al. (7) designed a series of radiolabeled CCK-8 (CCK fragment 26-33) peptides that showed high specificity for potential imaging of tumors expressing CCK-2Rs. Because of its favorable physical properties and availability, Tc is still the radionuclide of choice for routine clinical applications (8). Hydrazinonicotinamide (HYNIC) is a bifunctional coupling agent for Tc labeling of peptides and proteins that can achieve high specific activities without interfering with the amino acid sequence responsible for receptor binding (9-11). In this approach, it is suggested that Tc is bound to the hydrazine group by forming a Tc(V)=N bond, and other coordination sites are occupied by one or more coligands (1, 12). Although the exact structure of Tc-HYNIC-ligands has not been established, it is believed that the choice of coligand can influence the stability and hydrophilicity of the radiolabeled peptide (9, 13). Using the HYNIC labeling strategy and nicotinic acid (NA)/tris(hydroxymethyl)-methylglycine (tricine) as the coligands, Laverman et al. (1) successfully labeled both sulfated CCK-8 (sCCK-8) and nonsulfated CCK-8 (nsCCK-8) for CCK receptor imaging in mice bearing tumors that express CCK. It has been shown that the sCCK-8 peptide displays high affinity for both the CCK-1Rs and CCK-2Rs (1, 14) The study showed that nsCCK-8 had affinity for the CCK-2Rs but little affinity for the CCK-1Rs.