Gastrin-Releasing Peptide (GRP) and Bombesin: The GRPR Family in Cancer Theranostics, Appetite Research, and Neuropeptide Biology (2026)

# Gastrin-Releasing Peptide (GRP) and Bombesin: The GRPR Family in Cancer Theranostics, Appetite Research, and Neuropeptide Biology (2026)

Gastrin-releasing peptide (GRP) and its amphibian ortholog bombesin form one of the most versatile neuropeptide systems in biomedical research. Originally discovered as a gastric acid secretagogue, GRP/bombesin has since been characterized as a critical regulator of appetite and satiety, a growth factor for multiple cancer types, and — most recently — a high-value molecular target for cancer imaging and theranostics through its receptor, GRPR.

Bombesin was first isolated from the skin of the European frog Bombina bombina in 1971. When its mammalian counterpart, gastrin-releasing peptide, was subsequently identified in the human gastrointestinal tract, the conserved N-terminal sequence revealed that bombesin was effectively the amphibian homolog of GRP — a pattern of evolutionary conservation that established GRP/bombesin as one of the classic examples of amphibian skin peptides illuminating mammalian biology. Today, the GRP/bombesin system encompasses multiple receptor subtypes (GRPR, NMBR, BRS-3) and three endogenous peptide families, making it a rich subject for comparative receptor pharmacology, appetite neuroscience, and oncology research.

> Research Use Only: All GRP and bombesin research compounds are sold strictly for laboratory investigation. This profile is intended for researchers and does not constitute medical advice, clinical guidance, or endorsement for human or animal use.

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The GRP/Bombesin Peptide Family

Bombesin (Amphibian)

Bombesin is a 14-amino-acid peptide (pGlu-Gln-Arg-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH₂) isolated from frog skin. Its C-terminal sequence (-Trp-Ala-Val-Gly-His-Leu-Met-NH₂) is highly conserved across the entire bombesin superfamily and is essential for receptor binding and activation. The C-terminal amidation is required for full biological activity — a feature shared with many gut neuropeptides.

Gastrin-Releasing Peptide (GRP)

GRP is the principal mammalian bombesin-like peptide, encoded by the GRP gene and processed to multiple bioactive forms:

• •GRP-27: The full-length 27-amino-acid form, produced primarily in GI enteric neurons and brain
• •GRP-10 (neuromedin C): The C-terminal decapeptide of GRP; shares the conserved C-terminal receptor-binding sequence with bombesin
• •GRP-18: An intermediate processing product

GRP is produced predominantly in:

• •Enteric neurons of the stomach and intestine (primary peripheral source)
• •Pulmonary neuroendocrine cells and small cell lung cancer cells (historically, GRP was first characterized as a "bombesin-like peptide" in human lung)
• •Brain — hypothalamic and limbic nuclei relevant to appetite and social behavior
• •Prostate and other glandular tissues

Neuromedin B (NMB)

Neuromedin B is the second mammalian bombesin-like peptide, acting preferentially at NMB receptors (NMBR) rather than GRPR. NMB is predominantly expressed in the brain and pituitary, contributing to different aspects of the bombesin system's CNS actions.

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The Bombesin Receptor Family: GRPR, NMBR, and BRS-3

The bombesin receptor family in mammals consists of three structurally related class A GPCRs:

GRPR Structure (2023 Cryo-EM)

A landmark 2023 study resolved the cryo-EM structures of human GRPR in complex with its Gq protein, bound to both GRP agonist and a bombesin-derived antagonist — providing the first atomic-resolution view of GRPR activation and blockade (PubMed 36724251). Key structural insights include:

• •The conserved C-terminal "pharmacophore" of GRP/bombesin (Trp-Ala-Val-Gly-His-Leu-Met-NH₂) occupies the orthosteric binding pocket
• •Antagonist binding induces a distinct receptor conformation that prevents Gq engagement
• •Structural determinants of GRPR vs. NMBR selectivity reside in TM4 and TM5 and the second extracellular loop

Signaling Pathways

GRPR activation drives:

1. Gαq/PLCβ/IP₃ → intracellular Ca²⁺ release → PKC activation → the primary pathway for smooth muscle contraction, gastric acid secretion, and mitogenic signaling in cancer cells

2. MAPK/ERK → proliferative and survival signaling; central to GRPR's role in cancer cell growth

3. PI3K/Akt → anti-apoptotic survival signaling; relevant to cancer biology

4. β-arrestin recruitment → receptor internalization and desensitization; relevant to agonist vs. antagonist pharmacology in imaging research

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Cancer Biology: GRPR as a Theranostic Target

GRPR has emerged as one of the most actively investigated peptide receptor targets in cancer research over the past two decades. Its value as a theranostic (simultaneous diagnostic and therapeutic) target derives from dramatic overexpression across multiple common cancer types, combined with minimal expression in most normal adult tissues.

GRPR Overexpression Across Cancer Types

Prostate Cancer Theranostics Research

GRPR research in prostate cancer has accelerated dramatically because it complements the widely studied PSMA (prostate-specific membrane antigen) targeting system. Key research findings:

• •GRPR is highly expressed in early-stage, low-grade prostate cancer where PSMA expression may be lower, making GRPR-targeted imaging potentially complementary for comprehensive tumor detection
• •GRPR expression is inversely correlated with Gleason grade in some datasets — higher in well-differentiated tumors — suggesting utility at different disease stages than PSMA
• •Multiple radiolabeled bombesin antagonist compounds have been developed for PET/CT and SPECT imaging, with RM26 (¹⁷⁷Lu/⁶⁸Ga-labeled antagonists) among the most studied in preclinical models
• •A 2024 comprehensive review established GRPR as a novel biomarker and therapeutic target in prostate cancer theranostics (PubMed 38442298)
• •An earlier review of prostate cancer theranostics targeting GRPRs provides the foundational framework (PubMed 29256046)

Breast Cancer Theranostics Research

GRPR-targeted imaging is particularly relevant for breast cancer research because GRPR expression is high in estrogen receptor-positive tumors — a major breast cancer subtype. A 2024 systematic scoping review examined GRPR as a theranostic target across the breast cancer research landscape (PubMed 38342656). Key findings:

• •Radiolabeled GRPR antagonists outperform agonists for imaging due to superior pharmacokinetics, reduced receptor-mediated internalization, and cleaner background signal
• •The shift from GRPR agonists to antagonists represents a major paradigm change in the field — agonist-driven receptor internalization, while valuable for therapeutic radioligand delivery, causes problematic redistribution in pure imaging contexts
• •GRPR targeting may complement HER2 and hormone receptor imaging to provide comprehensive molecular profiling

Peptide-Drug Conjugates (PDCs)

Bombesin/GRP analogs have been extensively investigated as homing peptides for targeted delivery of cytotoxic payloads — the GRP receptor analog of antibody-drug conjugates (ADCs):

• •Bombesin is used as the GRPR-targeting vector, conjugated via linkers to cytotoxic warheads including daunorubicin, doxorubicin, and camptothecin analogs
• •The 2023 review of bombesin-based peptide-drug conjugates systematically evaluated the structural requirements for optimal GRPR targeting and payload delivery (PubMed 36834815)
• •A parallel approach uses bombesin as a tumor-targeting ligand on the surface of nanoparticles and liposomes, leveraging GRPR-mediated endocytosis for enhanced intracellular drug delivery (PubMed 31921534)

Agonist vs. Antagonist Research Paradigm

A key conceptual development in GRPR cancer research is the recognition that antagonists — not agonists — are generally preferred for imaging applications:

• •Agonists drive GRPR internalization, which delivers more radionuclide payload intracellularly (relevant for therapy) but reduces cell-surface accumulation (suboptimal for imaging)
• •Antagonists block GRPR without triggering internalization, leading to higher surface accumulation, better tumor-to-background ratios, and superior imaging contrast
• •This agonist-versus-antagonist distinction is now a fundamental principle in GRPR theranostic compound design

The current landscape of GRPR-targeted imaging and therapy using radiolabeled compounds was comprehensively reviewed in PubMed 35668669.

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GI Research: Gastric Acid and Motility

GRP was originally characterized as a gastric acid secretagogue — the "bombesin-like peptide" that stimulates gastrin release from G cells of the gastric antrum, driving acid secretion. Key GI research findings:

• •GRP released from enteric neurons activates GRPR on gastric G cells → gastrin release → parietal cell HCl secretion
• •GRP stimulates pancreatic enzyme secretion, synergizing with CCK (see CCK Research Profile)
• •GRP accelerates intestinal transit — a prokinetic action that complements its gastric secretagogue role
• •GRP also stimulates gallbladder contraction, contributing to postprandial bile flow

The GRP/GRPR system in the GI tract exemplifies the "neuroendocrine" integration of neural (enteric neuron GRP release) and hormonal (gastrin) signals in regulating digestive function.

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Appetite and Satiety Research

One of the most extensively studied behavioral actions of bombesin/GRP is the suppression of food intake. This satiety-inducing action was among the first behavioral effects identified for an amphibian skin peptide and has been conserved across vertebrate evolution.

Central and Peripheral Satiety Mechanisms

• •Peripheral administration of bombesin/GRP suppresses food intake in multiple species, indicating that gut-derived GRP signals the brain to reduce feeding
• •Central (ICV) bombesin administration suppresses feeding at doses lower than peripheral administration, suggesting central GRPR/NMBR activation contributes to satiety independent of peripheral signals
• •Brain sites mediating bombesin-induced satiety include the paraventricular nucleus (PVN), dorsomedial hypothalamic nucleus (DMH), ventromedial hypothalamic nucleus (VMH), and periaqueductal gray — overlapping with satiety circuits engaged by CCK, leptin, and GLP-1 signaling
• •Blockade of brain bombesin/GRP receptors with antagonists increases food intake in satiated rats, demonstrating endogenous tone of this system in satiety maintenance (PubMed 8388660)

GRPR Knockout Research

GRPR-deficient mice exhibit disrupted feeding and body weight regulation:

• •Increased meal size without compensatory reduction in meal frequency
• •Altered satiety signaling — GRPR KO mice continue eating beyond normal satiety endpoints
• •Modest obesity phenotype under some dietary conditions
• •These phenotypes confirm GRPR's role in meal termination and satiety signaling (PubMed 12176666)

For comparison with other satiety peptides, see profiles on Peptide YY, CCK, Amylin, and Nesfatin-1.

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CNS Research: Beyond Appetite

Stress and Anxiety Research

Bombesin/GRP receptors in the amygdala and limbic system contribute to stress-related behavior:

• •GRPR is expressed in the basolateral amygdala and contributes to fear conditioning
• •GRP released in the amygdala during emotionally significant experiences modulates fear memory consolidation
• •Bombesin/GRP has been characterized as having a "dual role" in feeding and stress — stress activates the GRP system, which may suppress appetite while modulating the emotional response

Itch Research

An unexpected discovery in GRPR biology is its role in itch (pruritus) signaling in the spinal cord:

• •GRPR is expressed on spinal cord interneurons that relay itch signals
• •GRP and bombesin administered spinally induce scratching behavior
• •The 2023 cryo-EM structural study of GRPR specifically highlighted itch research as one of the key application areas alongside cancer (PubMed 36724251)
• •GRPR antagonists are under research investigation as potential anti-itch tools

Social Memory Research

Recent research has implicated GRPR in social behavior and social memory:

• •GRPR is expressed in hippocampal CA2 — a region specifically implicated in social memory
• •GRP/GRPR signaling in CA2 modulates social memory consolidation
• •This hippocampal GRPR function connects GRP biology to social neuroscience research, an area of growing interest

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Bombesin as a Research Tool in Nuclear Medicine Models

Bombesin's utility as a research scaffold extends beyond its biology. The peptide's reliable GRPR-targeting properties and established radiolabeling chemistry make it one of the most extensively used peptide scaffolds in nuclear medicine research:

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Key Research Findings Summary

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Research Limitations and Outstanding Questions

1. Agonist vs. antagonist theranostics: The optimal paradigm for different cancer stages and treatment goals (pure imaging vs. targeted radionuclide therapy) requires further clarification — agonist-driven internalization may be preferable for therapeutic payloads

2. GRPR heterobivalent targeting: Combining GRPR and PSMA targeting in single constructs is an active research area, but the optimal linker chemistry and pharmacokinetics remain to be optimized

3. BRS-3 biology: The orphan bombesin receptor BRS-3 lacks a well-characterized endogenous ligand, making its physiological role speculative; it represents a major gap in understanding the complete bombesin receptor system

4. GRP in social memory: The emerging role of GRPR in CA2 hippocampal social memory is mechanistically exciting but requires deeper characterization, particularly regarding the specific GRP source and circuit logic

5. Species-specific pharmacology: Different bombesin receptor subtypes show variable expression patterns across species, complicating translation of rodent data to human biology

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Summary

GRP and bombesin anchor one of the most pharmacologically productive peptide systems in biomedical research. Discovered in frog skin and subsequently identified in human lung and gut, bombesin/GRP acts through a family of three class A GPCRs — GRPR, NMBR, and the orphan BRS-3 — with distinct tissue distributions and physiological roles.

In cancer research, GRPR has become a major theranostic target, with bombesin analogs serving as tumor-homing vectors for radioligand therapy, PET/CT imaging, and peptide-drug conjugate delivery across prostate, breast, and lung cancers. The 2023 cryo-EM structure of the human GRPR provides the molecular foundation for next-generation compound design. In neuroscience, the GRP/bombesin system regulates appetite and satiety through hypothalamic circuits, modulates fear memory in the amygdala, controls itch signaling in the spinal cord, and contributes to social memory in hippocampal CA2.

Few peptide systems match GRP/bombesin in the breadth of research domains they illuminate — from structural pharmacology to nuclear medicine to appetite neuroscience.

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All GRP and bombesin research compounds described herein are intended exclusively for laboratory investigation under Research Use Only (RUO) conditions. This content does not constitute medical advice, clinical guidance, or endorsement for human or animal use.