Received for publication April 27, 2006.
Revised August 25, 2006.
Accepted for publication August 28, 2006.

Identification of bombesin (Bn) receptor subtype-specific ligands: effect of N-methyl scanning, truncation, substitution and evaluation of putative reported selective ligands

Abstract

Mammalian bombesin (Bn) receptors include the gastrin-releasing peptide receptor (GRPR), neuromedin B receptor (NMBR) and bombesin receptor subtype 3 (BRS-3). These receptors are involved in a variety of physiological/pathologic processes including thermoregulation, secretion, motility, chemotaxis and mitogenic effects on both normal and malignant cells. Tumors frequently overexpress these receptors and their presence is now used for imaging and receptor-mediated cytotoxicity. For these reasons there is an increased need to develop synthetic, selective receptor subtype specific ligands, especially agonists for these receptors. In this study we used a number of strategies to identify useful receptor subtype selective ligands including synthesizing new analogs (N-methyl-substituted constrained analogs, truncations, substitutions) in DTyr⁶,Ala¹¹,Phe¹³,Nle¹⁴]Bn(6-14), which has high affinity for all Bn receptors and is metabolically stable, as well as completely pharmacologically characterized analogs recently reported to be selective for these receptors in [Ca²⁺]_i assays. Affinities and potencies of each analog were determined for each human Bn receptor subtype. N-methyl substitutions in position 14,12,11,10,9,8 did not result in selective analogs and except in position 11 which markedly decreased affinity/potency. N terminal truncations or position 12 substitutions did not increase selectivity as previously reported by others. Of the four shortened analogs of [DPhe⁶,Ala¹¹,Phe¹³,Nle¹⁴]Bn(6-14) reported to be potent selective BRS-3 ligands on [Ca²⁺]_i assays, only Ac-Phe,Trp,Ala,His(tBzl),Nip,Gly,Arg-NH₂ had moderate selectivity for hBRS-3, however it was less selective than previously reported Apa¹¹ analogs, demonstrating these are still the most selective BRS-3 analogs available. However, both of these analogs should be useful templates to develop more selective BRS-3 ligands.

Key words: BRS-3, bombesin, bombesin receptors, gastrin-releasing peptide, neuromedin B, peptide structure-function

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