Peptides in Oncology

Overview

Peptides have emerged as versatile tools in oncology, serving roles that span diagnostics, targeted therapy, drug delivery, and immunotherapy. Their high target specificity, favorable tissue penetration (relative to antibodies), rapid blood clearance, and ease of chemical modification make them well-suited for cancer applications where precision targeting is paramount.

The oncology peptide landscape includes natural hormone analogs that have been in clinical use for decades (somatostatin analogs, GnRH agonists), newer peptide-drug conjugates that deliver cytotoxic payloads selectively to tumors, radiolabeled peptides for both imaging and therapy, and peptide vaccines that harness the immune system against tumor cells.

Tumor-Targeting Peptides

Receptor-Targeted Peptides

Many tumors overexpress specific receptors on their cell surfaces, providing molecular addresses that peptide ligands can exploit:

• Somatostatin receptors (SSTRs) — Overexpressed in neuroendocrine tumors, small cell lung cancer, and some breast cancers. Octreotide and its analogs are the prototypical receptor-targeted peptides in oncology.
• Integrin receptors — The RGD (arginine-glycine-aspartate) motif binds alpha-v-beta-3 integrins overexpressed on tumor vasculature and many tumor cell types. RGD-based peptides are used for both imaging and targeted drug delivery.
• Bombesin/gastrin-releasing peptide receptors — Overexpressed in prostate, breast, and pancreatic cancers
• Cholecystokinin-2 (CCK2) receptors — Overexpressed in medullary thyroid carcinoma and small cell lung cancer
• GnRH receptors — Exploited by luteinizing hormone-releasing hormone (LHRH) analogs in prostate and breast cancer
• Prostate-specific membrane antigen (PSMA) — Targeted by peptide-based ligands for prostate cancer imaging and therapy

Cell-Penetrating Peptides in Oncology

Cell-penetrating peptides (CPPs) can transport therapeutic cargoes across cell membranes, enabling delivery of molecules that cannot enter cells on their own. In oncology, CPPs are being investigated to deliver:

• Proapoptotic peptides that activate programmed cell death pathways
• Peptide inhibitors of intracellular protein-protein interactions
• Nucleic acid therapeutics (siRNA, antisense oligonucleotides) for gene silencing
• Stapled peptides targeting transcription factors and other intracellular oncoproteins

Peptide-Drug Conjugates (PDCs)

Peptide-drug conjugates link tumor-homing peptides to cytotoxic payloads through cleavable or non-cleavable linkers. PDCs offer potential advantages over antibody-drug conjugates (ADCs): smaller size for better tumor penetration, lower immunogenicity, faster clearance from non-target tissues, and simpler manufacturing.

PDCs in oncology development target a variety of receptors, with PSMA-targeting and somatostatin receptor-targeting conjugates among the most advanced. The cytotoxic payloads include traditional chemotherapeutic agents, toxins, and photosensitizers for photodynamic therapy.

Radiopeptide Therapy

Peptide Receptor Radionuclide Therapy (PRRT)

PRRT represents one of the most clinically mature applications of peptides in oncology. In this approach, tumor-targeting peptides are conjugated to chelating agents that bind therapeutic radionuclides:

• Lutetium-177 dotatate (Lutathera) — Approved for somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors. This somatostatin analog delivers beta-emitting lutetium-177 directly to SSTR-expressing tumor cells, causing localized DNA damage and cell death.
• Lutetium-177 PSMA-617 (Pluvicto) — Approved for PSMA-positive metastatic castration-resistant prostate cancer. This small-molecule PSMA ligand (with peptide-like properties) delivers lutetium-177 to prostate cancer cells.

Theranostic Pairs

A distinctive feature of radiopeptide oncology is the theranostic paradigm — using the same targeting peptide with different radioisotopes for imaging (diagnosis) and therapy:

• Gallium-68 dotatate for PET imaging paired with lutetium-177 dotatate for therapy
• Gallium-68 PSMA-11 for imaging paired with lutetium-177 PSMA-617 for therapy

This approach enables patient selection (identifying those whose tumors express the target), treatment planning, and response monitoring using the same molecular targeting platform.

Emerging Radiopeptides

The next generation of radiopeptide therapeutics includes alpha-emitting isotopes (actinium-225, bismuth-213) paired with tumor-targeting peptides, which deposit more energy over shorter distances and may be more effective against micrometastases.

Peptide-Based Immunotherapy

Peptide Cancer Vaccines

Peptide vaccines for cancer immunotherapy use tumor-derived epitopes to activate cytotoxic T cells against cancer cells. Personalized neoantigen vaccines — designed from an individual patient's tumor mutations — represent the cutting edge of this approach and are being evaluated in combination with immune checkpoint inhibitors.

Immune Checkpoint Peptide Inhibitors

While most checkpoint inhibitors are monoclonal antibodies, peptide-based alternatives are in development. Small peptides and cyclic peptides targeting the PD-1/PD-L1 interaction offer potential advantages in terms of manufacturing cost, tissue penetration, and reduced immunogenicity.

Peptide-Based Immune Modulators

Several peptides modulate the tumor immune microenvironment:

• Thymosin alpha-1, a naturally occurring thymic peptide, enhances T-cell and dendritic cell function
• Antimicrobial peptides such as LL-37 exhibit direct antitumor and immunomodulatory activity
• Peptide inhibitors of indoleamine 2,3-dioxygenase (IDO) aim to reverse tumor-mediated immune suppression

Peptides in Cancer Diagnostics

Molecular Imaging

Radiolabeled peptides are used extensively in nuclear medicine for cancer detection:

• Somatostatin receptor scintigraphy (OctreoScan) and PET/CT (gallium-68 dotatate) for neuroendocrine tumors
• Gallium-68 PSMA PET/CT for prostate cancer staging
• RGD-based PET tracers for angiogenesis imaging

Fluorescence-Guided Surgery

Tumor-targeting peptides conjugated to near-infrared fluorescent dyes enable real-time visualization of tumor margins during surgery, improving the completeness of surgical resection while preserving normal tissue.

Clinical Landscape

The oncology peptide pipeline is substantial, spanning all clinical trial phases:

• Established therapies — Somatostatin analogs (octreotide, lanreotide), GnRH agonists/antagonists (leuprolide, degarelix), PRRT (lutathera, pluvicto)
• Late-stage development — Next-generation PRRT agents, PDCs, combination PRRT with checkpoint inhibitors
• Early-stage — AI-designed tumor-targeting peptides, alpha-emitting radiopeptides, multi-specific peptides, stapled peptide PPI inhibitors

Challenges

Peptide oncology faces several ongoing challenges:

• Renal accumulation — Many peptides are cleared through the kidneys, causing dose-limiting nephrotoxicity, particularly for radiopeptides
• Tumor heterogeneity — Not all cells within a tumor express the target receptor uniformly
• Resistance — Tumors may downregulate target receptors in response to therapy
• Short circulation time — Rapid blood clearance limits tumor exposure; half-life extension strategies from peptide stability research are being applied to oncology peptides

The convergence of advanced screening technologies, computational design, novel conjugation chemistries, and combination strategies with immunotherapy continues to expand the role of peptides across the oncology spectrum.