Gastrin I (1-14), human

64Cu-1,4,7-Triazacyclononane-1,4-diacetic acid (NO2A)-6-aminohexanoic acid (6-Ahx)-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2(BBN(7-14)NH2), abbreviated as64Cu-NO2A-(6-Ahx)-BBN(7-14)NH2, is a bombesin (BBN)-based,64Cu-NO2A-conjugated peptide that was synthesized by Lane et al. for use in positron emission tomography (PET) of tumors expressing gastrin-releasing peptide receptor (GRPR) (1, 2).GRPR is a glycosylated G-protein-coupled receptor that is normally expressed in non-neuroendocrine tissues of the breast and pancreas and in neuroendocrine cells of the brain, gastrointestinal tract, lung, and prostate (3, 4). GRPR has been found to be overexpressed in various human tumors, and a large number of BBN analogs have been investigated for GRPR-targeted imaging and therapy (5, 6). These analogs have been synthesized on the basis of either truncated BBN (BBN(6-14) or BBN(7-14)) or full-length BBN(1-14) (7, 8). Chelators and spacers have been used frequently for chelating metals and for improving the kinetics of conjugates (9-11).64Cu is a radiometal with potential applications in diagnostic and therapeutic nuclear medicine. The half-life for64Cu (t1/2= 12.7 h) is long enough for drug preparation, quality control, imaging, and therapy (12, 13). However, use of64Cu is limited by issues ofin vivotranschelation to proteins found in blood and liver (such as superoxide dismutase) (1). A variety of chelators have been investigated for the purpose of stably chelating64Cu (13). In general,64Cu-labeled 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetraacetic acid (64Cu-DOTA) and64Cu-labeled 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (64Cu-TETA) exhibit high uptake and retention in nontarget organs, which limits their application. Cross-bridged (CB) analogs, such as CB-DO2A ((1,4,7,10-tetraazabicyclo[5.5.2]tetradecane-4,10-diyl)diacetic acid), CB-TE2A ((1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-4,11-diyl)diacetic acid), SarAr (1-N-(4-aminobenzyl)-3,6,10,13,16,19-hexa-aza-bicyclo-[6.6.6]eichosane-1,8-diamine), and NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid), demonstrate improved copper containment by enhancing the ligand's rigidity (2, 14).Prasanphanich et al. recently reported that the NOTA-based64Cu-NOTA-8-Aoc-BBN(7-14)NH2conjugate (where 8-Aoc = 8-aminooctanoic acid) exhibited decreased accumulation in hepatic tissue as compared with other chelator-based (DOTA, TETA, and CB-TE2A) conjugates (2, 14). To improve the tumor uptake and maintain the good pharmacokinetic properties of the64Cu-NOTA-8-Aoc-BBN(7-14)NH2conjugate, Lane et al. synthesized a new group of conjugates with the NOTA derivative NO2A and replaced the spacer 8-Aoc with an aliphatic or aromatic linking (1). These conjugates were abbreviated as64Cu-NO2A-(X)-BBN(7-14)NH2, where X denotes the pharmacokinetic modifier, such as AMBA (para-aminobenzoic acid), β-Ala (beta-alanine), 5-Ava (5-aminovaleric acid), 6-Ahx, 8-Aoc, and 9-Anc (9-aminonanoic acid). The β-Ala, 5-Ava, 6-Ahx, and 9-Anc are aliphatic pharmacokinetic modifiers, ranging from three to nine carbons in length, whereas AMBA is an aromatic pharmacokinetic modifier and is more rigid than the aliphatic modifiers. Evidence indicates that a spacing moiety, ranging from three to eight carbons in length, can assist in receptor-mediated uptake (15). Conjugates containing an aromatic linker have significantly higher uptake and retention in PC-3 tumor tissue than those containing hydrocarbon or ether linkers (15, 16). Studies by Lane et al. have shown that the spacer X in the64Cu-NO2A-(X)-BBN(7-14)NH2conjugates has a significant role in clearance, accumulation, and retention of the conjugates in tumor tissue (1). The four conjugates showing the most favorable pharmacokinetic properties and the highest degree of pancreas and tumor accumulation were those in which X = 6-Ahx, 8-Aoc, 9-Anc, or AMBA. PET imaging with these conjugates produced high-contrast images of PC-3 tumor xenografts in severe combined immunodeficient (SCID) mice (1). This chapter describes the data obtained with64Cu-NO2A-(6-Ahx)-BBN(7-14)NH2. Detailed information for other64Cu-NO2A-(X)-BBN(7-14)NH2conjugates is available in MICAD (http://www.ncbi.nlm.nih.gov/books/NBK5330/) (1).

99mTc-Nicotinic acid/tricine/hydrazinonicotinamide-nonsulfated cholecystokinin-8 (99mTc-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).99mTc is a gamma emitter with a physical half-life (t½) 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-NH2. 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-NH2. 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 potentialin vivoimaging of tumors expressing CCK-2Rs. Because of its favorable physical properties and availability,99mTc is still the radionuclide of choice for routine clinical applications (8). Hydrazinonicotinamide (HYNIC) is a bifunctional coupling agent for99mTc 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 that99mTc is bound to the hydrazine group by forming a99mTc(V)=N bond, and other coordination sites are occupied by one or more coligands (1, 12). Although the exact structure of99mTc-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.

Objective and design:C-terminal amidation is an essential processing step towards bioactivation of many peptides including gastrin. This reaction is catalysed by peptidylglycine alpha-amidating mono-oxygenase (PAM, EC 1.14.17.3) which converts the glycine extended precursors on their carboxyl termini to the des-glycine amidated peptide products. In the case of gastrin, most of the amidation is thought to occur in the antrum. However substantial quantities of glycine extended gastrin and PAM are present in plasma. It is unclear whether circulating PAM reflects the secretory activity of the gastrin secreting cell or whether PAM is involved in the postsecretory processing of gastrin. The aim of the present study was to relate the circulating amidation activity to the plasma concentrations of glycine extended and amidated gastrins.Patients and measurements:Plasma PAM, gastrin-amide and gastrin-gly were measured in subjects with different gastrin secretory status: healthy subjects basally and following a meal, members of families with multiple endocrine neoplasia type 1 (MEN-1) with normal and high plasma gastrin, and patients with hypergastrinaemic atrophic gastritis.Results:Patients with MEN-1 and hypergastrinaemia tended to have a higher plasma PAM activity than MEN-1 subjects with normal circulating G-NH2 indicating a cosecretion of hormone and PAM. However in contradistinction to patients with medullary thyroid carcinoma, PAM activity does not appear to be a useful tumour marker of gastrinoma. Hypergastrinaemia from a non-tumour source (hypergastrinaemic non-atrophic gastritis) was associated with a lower plasma PAM activity than in normal subjects and may reflect the secretion of a greater proportion of already amidated gastrin. In general, there was no relationship between plasma PAM activity and the ratio of amidated to non-amidated gastrin suggesting that circulating PAM was not involved in the amidation of gastrin. Feeding increased circulating gastrin but had no effect on plasma PAM activity.Conclusion:The results support the view that gastrin is amidated at the site of its synthesis and that hypergastrinaemia is associated with elevated plasma amidating enzyme activity only when the gastrin originates from tumour sources.