99mTc-labeled [N40,Pro1,Tyr4]bombesin

Excerpt

^99mTc-labeled [N₄⁰,Pro¹,Tyr⁴]bombesin ([^99mTc]Demobesin 3) is a bombesin (BN) analog developed by Nock et al. for molecular imaging of tumors expressing gastrin-releasing peptide (GRP) receptor (GRPR) (1).

BN is an amphibian neuropeptide consisting of 14 amino acids (pGlu-Gln-Arg-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH₂) (2, 3), and it was first isolated from frog skin in 1970 (4). The search for its mammalian counterpart led to the discovery of GRP, which consists of 27 amino acids. GRP and BN share an identical C-terminal region (-Trp-Ala-Val-Gly-His-Leu-Met-NH₂), which is necessary for receptor binding and signal transduction (5, 6). In addition to the release of gastrin, GRP- and BN-like peptides also produce a wide range of other biological responses in diverse tissues, such as secretion of endocrine and exocrine glands, maintenance of circadian rhythm, regulation of satiety, and contraction of smooth muscles (7). They also act as potential growth factors for both normal and cancerous cells (3, 5, 6). There are four members of the BN receptor family, including three mammalian receptors: GRPR (BB₂ or BRS2; 384 amino acids), neuromedin B receptor (NMBR, BB₁, or BRS1; 390 amino acids), and BN-like receptor 3 (BB₃, BRS3, or orphan; 399 amino acids) (5, 7, 8); the fourth receptor (BB₄) has only been found in amphibians. GRPR is the only well characterized receptor of this family. GRPR is a glycosylated, 7-transmembrane, G-protein–coupled receptor that, upon binding with its ligands, gives rise to a complex cascade of intracellular reactions. It is normally found in non-neuroendocrine tissues of the breast and pancreas, and in neuroendocrine cells of the brain, gastrointestinal tract, lung, and prostate (9). Interestingly, GRPR is overexpressed in prostate cancer as well as in tumors of the breast, lung, pancreas, ovary, kidney, and gastrointestinal tract. It has been reported that GRPR is expressed at a high density in the intraepithelial neoplasia and primary carcinoma of the prostate, whereas normal prostate tissue and, in most cases, benign prostate hyperplasia are predominantly negative for GRPR (10-13).

GRPR has attracted significant interest as a target for tumor detection, tumor staging, and evaluation of tumor response to therapy (5, 6, 8, 11). A large number of BN derivatives have been developed, and they have been labeled with ^99mTc, ¹⁷⁷Lu, ⁶⁷Ga, and ¹¹¹In for single-photon emission computed tomography (SPECT) and with ⁶⁴Cu, ⁶⁸Ga, and ¹⁸F for positron emission tomography. The published BN derivatives can be generally classified as truncated BN (6–14 or 7–14) or full-length BN (1-14) analogs (10, 12-18). The truncated BN analogs seem to be favorable for in vivo applications because they are usually more stable than the full-length tetradecapeptides and still bind to the GRPR adequately. However, the full-length peptides offer different labeling methods by attachment of functional groups to the amino acids 1–6. In many cases, the amino acids on positions 13 (Leu) and 14 (Met) have been replaced by unnatural amino acids (cyclohexylalanine (CyHAla) and norleucine (Nle)), and Lys has been placed on position 3 for attachment of radiolabels with reactive esters. Spacers, chelators, or radiometals have also been widely used for conjugation and for favorable kinetics (19). The BN derivatives can also be divided into agonists and antagonists (1, 20, 21). By far, most BN derivatives are agonists. Agonists are internalized into and accumulate within cells, and it has been assumed that they exhibit higher uptake by cancer cells than antagonists. However, some reports have shown that uptake of antagonists in vivo into tumor xenografts is higher than that of agonists (20, 21). Antagonists may have stronger binding for the GRPR than agonists (20, 21), and it has been suggested that antagonists may bind to all of the receptors, whereas agonists may only bind to the high affinity sites of the receptors.

An optimal BN-like radiotracer needs to meet several requirements: high affinity for GRPR, with rapid and specific tumor uptake; high hydrophilicity, with preferred renal excretion and low hepatobiliary excretion; and high stability but relatively rapid clearance from blood (6). Despite a large number of published derivatives, a valid comparison among them for the feasibility of tumor imaging is difficult because standardization between studies is lacking. Generally speaking, the majority of the radiotracers have relatively high renal and hepatic uptake, resulting in low tumor/liver and tumor/kidney ratios. Nock et al. synthesized four BN analogs, Demobesins 3–6, and labeled them with ^99mTc for imaging of GRPR-expressing tumors (1). The [^99mTc]Demobesins 3–4 were synthesized on the basis of the parent BN tetradecapeptide sequence with minor modifications, while the [^99mTc]Demobesins 5–6 were truncated peptides on the basis of the essential residues (the BN(7–14) motif) needed for receptor interaction. Different from [^99mTc]Demobesins 3 and 5, the oxidation-sensitive Met¹⁴ was replaced by Nle¹⁴ in [^99mTc]Demobesins 4 and 6. Their studies showed that all four BN analogs had high affinity for the human GRPR, efficiently internalized in the GRPR-expressing PC-3 cells in a time- and dose-dependent manner. The [^99mTc]Demobesins 3 and 4 presented high and rapid localization in the tumors with rapid clearance via the kidneys, while the [^99mTc]Demobesins 5 and 6 showed a lower tumor uptake and excessive hepatobiliary excretion.