Cancer Targeting Peptides for Imaging and Therapeutics

Cancer-targeting peptides have short amino acid sequences designed to specifically bind to cancer cells or their microenvironment. These peptides allow targeted drug delivery, imaging, and therapy. These peptides recognize and bind to molecules overexpressed on cancer cells or tumors, such as receptors, integrins, or specific antigens.

Cancer is a complex disease with significant challenges for the scientific and medical communities. Cancer leaves a substantial number of patients helpless, and the efficacy of conventional chemotherapies is often poor, and many patients suffer from off-target effects.

To develop better cancer treatments, cancer targeting, and anti-cancer peptides may offer new avenues for therapeutics. Cancer-targeting peptides can be coupled to imaging agents, chemotherapies, and/or nanocarriers to increase their on-site delivery, allow better tumor mass contouring in imaging, and increase the efficacy of chemotherapies to reduce adverse effects.

Types of Cancer-Targeting Peptides

Tumor-Homing Peptides

Tumor-homing peptides selectively bind to cancer cells or the tumor microenvironment. They improve the specificity of drug delivery, imaging, and therapeutic applications by targeting receptors or proteins overexpressed in tumors. These peptides recognize surface markers unique to cancer cells, such as RGD peptides that bind to integrins, such as αvβ3.

Mechanisms

Receptor-Mediated Targeting: Tumors often overexpress certain receptors that these peptides can recognize and bind to. RGD Peptides target αvβ3 and αvβ5 integrins. These integrins are overexpressed in angiogenic, rapidly growing tumor blood vessels.

Tumor Microenvironment Targeting: Some tumor-homing peptides recognize proteins in the tumor stroma, such as fibronectin or collagen. For example, iRGD Peptides (internalizing RGD) bind integrins and penetrate tumors by interacting with neuropilin-1 (NRP-1), enhancing drug delivery.

pH- or Enzyme-Responsive Activation: Some peptides remain inactive until they reach the acidic or enzyme-rich tumor microenvironment. C-end Rule (CendR) peptides, for example, bind to neuropilin receptors after being cleaved by proteases in tumors. Tumors and their vasculature overexpress the neuropilin receptor. CendR peptides improve the transport of therapeutic agents deep into tumors, increasing drug accumulation and effectiveness. CendR peptides contain a C-terminal motif (R/KXXR), where X represents any amino acid. After binding integrins, CendR peptides undergo proteolytic cleavage, exposing the R/KXXR motif. This motif binds NRP-1 and triggers endocytosis and transvascular transport. This amino acid sequence binds to αvβ3/β5 integrins on tumor cells or blood vessels. The activation of NRP-1 induces bulk transport of drugs, nanoparticles, or antibodies deep into tumor tissues.

Table1: Examples of Tumor Homing Peptides

Applications of Tumor Homing Peptides

Targeted Drug Delivery: Peptides conjugated to chemotherapy drugs, nanoparticles, or siRNA allow precise targeting.

Tumor Imaging: Radiolabeled tumor homing peptides improve PET and MRI scans.

Peptide-Based Therapies: Some tumor homing peptides disrupt tumor growth by interfering with signaling pathways.

Cell-Penetrating Peptides (CPPs)

Cell-penetrating peptides (CPPs) are short peptides, typically 5 to 30 amino acids in length, facilitating the delivery of molecules, such as drugs, nucleic acids, or nanoparticles, across the cell membrane. These peptides are widely used in cancer therapy to improve the intracellular uptake of therapeutic agents. Some CPPs, like TAT peptides, help transport drugs or nanoparticles into cancer cells.

CPPs penetrate cell membranes through two primary pathways:

Direct Penetration (Energy-Independent): CPPs interact with the lipid bilayer, leading to temporary membrane destabilization and direct diffusion into the cytoplasm.

Endocytosis-Mediated Uptake (Energy-Dependent): CPPs are internalized via receptor-mediated endocytosis, macropinocytosis, or clathrin/caveolin-mediated endocytosis. CPPs can be designed with escape mechanisms to avoid degradation in endosomes.

Types of Cell-Penetrating Peptides

Cationic CPPs (Electrostatic Interaction): Cationic peptides are rich in positively charged amino acids such as arginine, or lysine that interact with negatively charged cell membranes. For example, the TAT (Trans-Activator of Transcription) Peptide, derived from HIV-1, is commonly used for drug delivery.

Amphipathic CPPs (Hydrophobic & Hydrophilic Domains): These peptides contain both hydrophobic and hydrophilic regions, aiding in membrane penetration. For example MAP (Model Amphipathic Peptide), increasing the efficiency of intracellular transport.

Hydrophobic CPPs (Membrane Fusion): Hydrophobic CPPs utilize hydrophobic residues to integrate into membranes and deliver cargo, for example, Crotamine, a peptide derived from snake venom, that is effective in cancer targeting.

Table 2: Examples of Cell-Penetrating Peptides

Applications in Cancer Therapy

Targeted Drug Delivery: CPPs conjugated with chemotherapy drugs such as doxorubicin or paclitaxel enhance intracellular delivery.

Gene Therapy: CPPs aid in transporting siRNA, mRNA, or CRISPR components into cancer cells.

Nanoparticle Transport: CPP-modified nanoparticles improve the efficiency of cancer treatment.

Imaging & Diagnostics: CPPs conjugated with fluorescent or radiolabeled markers are useful for tumor imaging.

Enzyme-Activated Peptides (EAPs)

Enzyme-activated peptides are designed to remain inactive until they encounter specific enzymes overexpressed in the tumor microenvironment. These peptides enhance targeted drug delivery, reducing off-target effects and improve therapeutic outcomes in cancer treatment. Peptides that are cleaved by tumor-specific enzymes, such as MMPs, activate selective drug release in the tumor site.

Mechanism of Action

Prodrug Activation: For prodrug activation, peptides are linked to drugs or imaging agents and remain inactive until cleaved by tumor-associated enzymes.

Tumor-Specific Enzyme Recognition: These peptides contain cleavage sites recognized by overexpressed enzymes in tumors, ensuring selective activation at the cancer site.

Table 3: Key Enzymes Targeted in Cancer

Examples of Enzyme-Activated Peptides

MMP-Responsive Peptides: For example, the PLGLAG peptide sequence is cleaved by MMP-2/9 to release drugs selectively at tumor sites.

Cathepsin B-Cleavable Peptides: Peptide-drug conjugates with Cathepsin B-sensitive linkers release cytotoxic agents in tumor cells.

NGR Peptides: This peptide is recognized by CD13 (aminopeptidase N) in tumor vasculature, enhancing selective drug delivery.

Thrombin-Responsive Peptides: These peptides are used in prodrug activation and tumor-targeted clotting therapy.

Applications in Cancer Therapy

Targeted Drug Delivery: Peptide-drug conjugates activated by tumor enzymes improve chemotherapy precision.

Tumor Imaging: Enzyme-cleavable peptides linked to fluorophores or radiotracers enable real-time tumor visualization.

Prodrug Strategies: Enzyme-sensitive peptide carriers prevent premature drug activation in healthy tissues.

Immunogenic Peptides

Immunogenic peptides are short peptides capable of triggering an immune response. In cancer therapy, they are used in cancer vaccines and immunotherapies to stimulate the immune system to recognize and destroy tumor cells. These peptides are designed to mimic tumor antigens and activate T cells, enhancing the body’s ability to fight cancer. Since immunogenic peptides stimulate the immune system to recognize and destroy cancer cells, they are often used in cancer vaccines.

Mechanism of Action

Antigen Presentation: These peptides are processed by antigen-presenting cells (APCs), such as dendritic cells and are then displayed on major histocompatibility complex (MHC) molecules.

T Cell Activation: Cytotoxic T cells (CD8+ T cells) recognize peptides presented by MHC class I and kill cancer cells. Helper T cells (CD4+ T cells) recognize peptides on MHC class II, boosting immune response.

Anti-Cancer Peptides

Many cancer targeting peptides can also act as anti-cancer peptides. These anti-cancer peptides (ACPs) have the ability to target and kill cancer cells while minimizing harm to normal cells and recently have gained attention as potential therapeutic agents due to their specificity, lower toxicity, and ability to overcome drug resistance.

Types of Anti-Cancer Peptides

Cell-Penetrating Peptides (CPPs) facilitate the delivery of drugs or other peptides into cancer cells.

Pro-apoptotic Peptides trigger programmed cell death in cancer cells.

Anti-angiogenic Peptides inhibit the formation of blood vessels that supply tumors.

Immunomodulatory Peptides enhance the immune system’s ability to target cancer cells.

Membrane-Disrupting Peptides selectively interact with and disrupt cancer cell membranes, leading to cell death.

Membrane-Disrupting Peptides target the negatively charged cancer cell membrane to cause cell lysis. ACPs can activate or induce the apoptosis pathways in cancer cells by targeting mitochondria or caspases. Many ACPs like Angiostatin inhibit angiogenesis by blocking tumor blood vessel formation. Other such as certain defensins and cathelicidins stimulate immune responses against tumors.

Examples are LL-37, a human cathelicidin peptide with anti-tumor properties; Melittin, a peptide from bee venom that disrupts cancer cell membranes; TK90 & TK91, synthetic peptides with high selectivity for cancer cells; the Bax-BH3 Peptide, induces apoptosis in cancer cells by targeting mitochondria.

However, the use of some of these peptides is still challenging because of their low stability and the potential for degradation. Peptides are prone to enzymatic degradation in the body. Luckily, chemical modifications such as cyclization and PEGylation can enhance stability. To enhance delivery to targets in cells or tissues, nanoparticles, liposomes, and conjugation with cell-penetrating peptides can improve peptide delivery, for example, to tumors. Depending on the sequence, peptide synthesis and modifications can be expensive, but advancements in biotechnology may reduce costs.

Table 4: Immunogenic Peptides in Cancer Therapy

Examples of Immunogenic Peptides

NY-ESO-1 Peptide are found in various cancers; used in peptide-based vaccines.

HER2 Peptide is targeted in breast cancer immunotherapy.

HPV-E7 Peptide is used in vaccines for HPV-related cervical cancer.

Neoantigen Peptides are specific to patient-specific mutations used in personalized cancer vaccines.

Applications in Cancer Immunotherapy

Peptide-Based Cancer Vaccines – Stimulate immune response against tumors (e.g., Sipuleucel-T for prostate cancer).

T Cell Therapy Enhancement – Peptides prime T cells for stronger anti-cancer activity.

Combination Immunotherapy – Used with immune checkpoint inhibitors (e.g., anti-PD-1) for better efficacy.

Applications of Cancer-Targeting Peptides

Targeted Drug Delivery: Peptide-drug conjugates (PDCs) improve specificity, reducing side effects.

Imaging & Diagnostics: Radiolabeled peptides enhance PET and MRI imaging for tumor detection.

Therapeutics: Peptide-based cancer vaccines and peptide inhibitors disrupt key cancer pathways.

A Selection of References

Anti-cancer peptides: Peptides-with-Anti-Cancer-Properties

Argenziano, M., Berlier, G., Brusa, P., Cavalli, R., Chirio, D., Dosio, F., Gallarate, M., Peira, E., Stella, B., & Ugazio, E. (2021). Developing Actively Targeted Nanoparticles to Fight Cancer: Focus on Italian Research. Pharmaceutics.  Pharmaceutics

Cancer Research Peptides: Catalog-peptides/cancer-research-peptides

CPPs for siRNA delivery: Cell-penetrating-peptides-for-the-delivery-of-siRNA-into-cells

Gupta R, Ambasta RK, Pravir Kumar. Autophagy and apoptosis cascade: which is more prominent in neuronal death? Cell Mol Life Sci. 2021 Dec;78(24):8001-8047. [PMC, Springer]

FDA approved for tumor imaging: FDAd-approves-first-radiopharmaceutical-peptide-drug-conjugate

Le Joncour V, Laakkonen P. Seek & Destroy, use of targeting peptides for cancer detection and drug delivery. Bioorg Med Chem. 2018 Jun 1;26(10):2797-2806. Science

Peptide receptors for cancer therapies:  Peptide-receptors-are-important-for-cancer-therapies

Peptides with anti-cancer properties: Peptides-with-Anti-Cancer-Properties

Therapeutic peptides for cancer therapies: Therapeutic-Peptides-for-Treating-Cancer

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