Receptor-Mediated Targeting by TTPs for Facilitating Delivery

Receptor-mediated targeting by TTPs leverages the overexpression of specific receptors on the tumor surface. These peptides are specifically designed to bind these receptors, facilitating the targeted delivery of therapeutic agents, imaging compounds, and diagnostic markers directly to the tumor site17. On binding to their target receptor, these peptides can facilitate the internalization of the peptide–receptor complex, allowing for intracellular delivery of therapeutic agents (Figure 2 )18. TTPs are engineered to bind with high affinity and specificity to the receptors that are overexpressed on tumor cells19. This selective binding ensures that the peptide is delivered to tumor cells, sparing healthy tissues. Upon receptor binding, the peptide–receptor complex is internalized by the cancer cell through endocytosis. This internalization allows the payload to be delivered directly into cancer cells, thereby enhancing therapeutic efficiency (Figure 2 )20. Once inside the cell, the therapeutic agent (, drug, toxin, or gene therapy vector) is released, where it can exert its intended effect21.

Most notably, clathrin-mediated endocytosis is the predominant route for internalization of receptor–ligand complexes in most mammalian cells. In CME, receptor complexes accumulate in clathrin-coated pits (~100–150 nm in diameter), where adaptor proteins (., AP-2) recruit clathrin triskelia to form a coated vesicle. Dynamin then pinches off the vesicle, which uncoats and fuses with early endosomes22. Vesicles of this size (~100 nm) are well-suited for the bulk uptake of peptide–drug conjugates. Typically, acidification within late endosomes and lysosomes promotes cargo release, but also risks enzymatic degradation; thus, TTP designs often incorporate endosomal escape motifs to ensure payload release into the cytosol before lysosomal degradation23. Moreover, caveolin-mediated endocytosis occurs via flask-shaped caveolae (~60–80 nm in diameter) enriched in caveolin-1 and Cavin proteins. Ligand–receptor binding induces caveolar budding in a dynamin-dependent manner, forming caveolar carriers that bypass early endosomes and lysosomes, often trafficking to caveosomes or the Golgi and endoplasmic reticulum24. The smaller vesicle size and nonacidic routing protect sensitive cargo (., peptides, proteins, nucleic acids) from degradation, but may slow release kinetics, necessitating specialized release triggers in TTP designs25.

Implications for TTP Design and Drug Release

For CME-internalized cargos, engineering pH-sensitive or membrane-disruptive elements (., histidine-rich sequences) can accelerate endosomal escape, thus maximizing cytosolic delivery before lysosomal degradation26. CvME avoids lysosomes, thereby protecting delicate agents like siRNA and proteins from degradation. However, because it operates more slowly, special linkers responsive to specific signals (., redox-sensitive disulfides) may be required to release cargos at the optimal time27. Targeted trafficking and differential routing can be leveraged to direct payloads to specific intracellular organelles; for example, CvME-mediated trafficking to the ER favors the delivery of unfolded protein therapeutics28.

This process ensures that the cytotoxic effect remains confined to cancer cells, thereby reducing systemic side effects29. Some common receptors targeted by tumor-targeting peptides are integrins, which are involved in tumor angiogenesis and metastasis, thus making them highly effective targets for TTPs. For example, RGD peptides (arginine-glycine-aspartic acid) specifically target these integrins to deliver therapeutic agents and imaging compounds30. EGFR, which is overexpressed in various cancer types, is targeted by peptides to inhibit growth signals and deliver cytotoxic agents. Peptides that bind EGFR can deliver chemotherapeutic drugs specifically to EGFR-expressing tumor cells31. Similarly, HER2 is commonly overexpressed in breast cancer and other tumor types, facilitating the effective delivery of therapeutic agents by HER2-targeting peptides32. Folate receptors are overexpressed in certain cancers, making folate-conjugated peptides useful for targeted drug delivery. Moreover, folate-linked peptides facilitate the delivery of chemotherapy drugs to folate receptor-positive tumors33. Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancer cells, making it an ideal target for peptide-based delivery systems34. Peptides targeting PSMA can deliver radiolabeled compounds for imaging or therapeutic agents, enabling targeted treatment35. Carbonic anhydrase IX (CA-IX) is overexpressed in hypoxic tumors and can be targeted by peptides to deliver therapeutic agents or imaging probes36.

Targeting the Tumor Microenvironment

The tumor microenvironment (TME) is a highly complex, adaptive system comprising malignant cells, immune cells, stromal elements, blood vessels, and extracellular matrix (ECM) components. It not only drives tumor growth but also significantly contributes to therapeutic resistance, immune evasion, and metastasis37. Acknowledging the TME's active role in tumor biology has led to the development of therapeutic strategies aiming to disrupt its supportive functions, including vascular normalization, immune response reprogramming, and ECM remodeling, ultimately enhancing the efficacy of conventional therapies38. A key factor underlying the complexity of the TME is the genetic and phenotypic heterogeneity within tumor cell populations. This diversity enables cancer cells to interact with surrounding stromal components via distinct paracrine signaling pathways, which shape their behavior and further promote treatment resistance. The influence of this heterogeneity extends to various stromal cells, including cancer-associated fibroblasts (CAFs), which respond to tumor-derived signals and contribute to ECM remodeling, immune modulation, and therapy resistance39. Although CAFs represent a substantial stromal population, they are part of a broader cellular network that includes endothelial cells, pericytes, and immune infiltrates. Endothelial cells form blood vessels that sustain tumor growth and enable metastatic dissemination. Working in concert, TME components ensure that the tumor remains protected and fully functional40. A more detailed discussion of CAF biology and its therapeutic implications appears in Section 3.3. Here, the focus remains on emphasizing the TME as a whole, underscoring the need for integrated therapeutic approaches that target both tumor cells and their supportive ecosystem to overcome resistance and improve clinical outcomes41.

Inhibiting Angiogenesis

Inhibiting angiogenesis is a crucial strategy in cancer therapy that aims to starve the tumor of the blood supply essential for its growth and metastasis42. Tumor-targeting peptides can be designed to specifically bind to angiogenic markers on endothelial cells, thereby delivering therapeutic agents that inhibit the formation of new blood vessels43. This targeted approach ensures that anti-angiogenic treatments are delivered precisely where they are needed, helping reduce systemic toxicity and optimizing therapeutic efficiency44. Vascular endothelial growth factor (VEGF) and its receptor (VEGFR) are key regulators of angiogenesis45. TTPs can be designed to bind to VEGF or VEGFR, blocking their interaction and inhibiting the angiogenic signaling pathway. For example, Bevacizumab (Avastin) is a monoclonal antibody against VEGF46.

Modulating Immune Response

An immunosuppressive environment persists in most tumors, leading to a diminished immune response and allowing cancer cells to remain undetected due to myeloid-derived suppressor cells, regulatory T cells, tumor-associated macrophages, and inhibitory cytokines47. Patients experiencing these immunosuppressive effects can be treated with immune checkpoint inhibitors, which remove inhibitory signals on T cells and allow them to fight the tumor again. When PD-L1 or PD-L2 bind to the PD-1 receptor on activated T cells, the cells become exhausted, and robust immune responses are halted. Pembrolizumab (Keytruda) is a humanized IgG4 monoclonal antibody that binds the programmed cell death-1 (PD-1) receptor on activated T cells and prevents its interaction with PD-L1 and PD-L2, restoring T-cell proliferation and cytotoxicity against tumor cells48. Nivolumab likewise targets PD-1 to release the PD-1–mediated brake on T cells, and has demonstrated clinical efficacy across multiple advanced malignancies by enhancing T-cell–mediated tumor cell killing49.

Therapeutic cancer vaccines represent another modality to stimulate antitumor immunity by presenting tumor antigens to a patient’s antigen-presenting cells. Sipuleucel-T (Provenge) is an FDA-approved autologous cellular vaccine for metastatic prostate cancer in which a patient’s dendritic cells are harvested, incubated with a fusion protein of prostatic acid phosphatase and granulocyte–macrophage colony-stimulating factor, and then reinfused to elicit a sustained, antigen-specific T-cell response and prolong overall survival50. Finally, reprogramming of tumor-associated macrophages from a pro-tumorigenic M2-like state to an antitumorigenic M1-like phenotype can be achieved by targeting the colony-stimulating factor-1 receptor (CSF-1R). Small-molecule inhibitors or peptides against CSF-1R deplete or re-educate M2 macrophages, enhancing antigen presentation and fostering a pro-inflammatory microenvironment conducive to tumor rejection51.

Targeting Cancer-Associated Fibroblasts (CAFs)

Beyond directly targeting cancer cells, tumor-targeting peptides are designed to disrupt the tumor-supportive functions of cancer-associated fibroblasts (CAFs), which are essential to tumor progression. Cancer-associated fibroblasts (CAFs) are a major stromal component of the tumor microenvironment (TME) and play a critical role in supporting tumor progression, invasion, angiogenesis, and immune evasion52. With growth factors, cytokines, chemokines, and enzymes, CAFs modify cancer cells’ responses by remodeling components of the ECM. This remodeling helps tumor cells migrate more easily to other parts of the body and can also force some of the drug’s dose to remain in the intestines, thus reducing its effectiveness. In addition to shaping the stroma, CAFs secrete TGF-β, VEGF, and FGFs, thereby promoting faster growth of cancer cells and fostering new blood vessel development. Moreover, cells in the CAF system release cytokines and chemokines that inhibit the immune response against cancer within the body53.

Recent research finds that CAF populations have many different functions. Distinct subtypes such as myofibroblastic CAFs (myCAFs) and inflammatory CAFs (iCAFs) differ in their phenotypic markers and roles. While myCAFs contribute to ECM stiffening through expression of α-smooth muscle actin (α-SMA) and collagen crosslinking enzymes, iCAFs are characterized by the secretion of pro-inflammatory cytokines like IL-6 and CXCL12, which enhance immune evasion and drive tumor growth54. Targeting CAFs therapeutically has become an area of intense investigation. One promising approach involves the use of tumor-targeting peptides (TTPs) that bind selectively to fibroblast activation protein (FAP), a surface protein highly expressed on CAFs. These peptides can serve as carriers for cytotoxic agents or imaging probes, enabling precise delivery to the CAF-rich regions of tumors55. Peptides designed to inhibit key signaling pathways, such as Hedgehog signaling, have also shown potential in reducing CAF activation and tumor-supportive functions. Additionally, efforts are underway to develop peptide-based inhibitors against matrix metalloproteinases (MMPs) and other ECM-modifying enzymes secreted by CAFs, aiming to limit their remodeling activity and improve drug penetration56.

By specifically disrupting CAF functions, these strategies aim to break down the protective stromal barrier that surrounds tumors, reduce resistance to chemotherapy, and enhance overall treatment efficacy. As understanding of CAF heterogeneity continues to evolve, tailored interventions may offer more precise and effective ways to neutralize their tumor-promoting roles57.

Disrupting Extracellular Matrix

Disrupting the extracellular matrix (ECM) within the tumor microenvironment (TME) is a critical strategy in cancer therapy58. Using tumor-targeting peptides (TTPs) against the extracellular matrix can reduce tumor growth and make other treatment methods more effective59. Targeting the ECM with TTPs can inhibit these processes and enhance the effectiveness of other therapies60. Attaching peptides to specific ECM components, such as fibronectin or collagen, can modify tumor development61. Peptides can inhibit matrix metalloproteinases (MMPs), which degrade ECM components and facilitate tumor invasion. For example, peptides mimicking MMP inhibitors, such as marimastat, can help prevent ECM degradation62. Peptides designed to bind specific ECM components can alter a tumor’s structural integrity. For example, peptides targeting fibronectin or collagen in the ECM are notable examples63. Disruption of the extracellular matrix with tumor-targeting peptides offers a multifaceted approach to cancer therapy by interfering with the structural and signaling functions of the ECM that support tumor growth and invasion64.

Exploiting Hypoxia and Acidity

Tumors often develop regions with low oxygen levels due to abnormal blood vessel formation and rapid tumor growth that consumes oxygen more quickly than it can be adequately supplied. Hypoxia triggers the expression of hypoxia-inducible factors (HIFs), which help tumors survive, promote new blood vessel formation, and spread. To capitalize on this feature, TTPs and prodrugs can be tailored to activate specifically in hypoxic areas while remaining inactive under normal oxygen levels, thus reducing damage to healthy tissues. For example, hypoxia-responsive linkers such as 2-nitroimidazole are commonly used. Under low oxygen, nitro-reductase enzymes convert the nitro group into an amino group, triggering drug release65. Azobenzene-based linkers have also been incorporated into antibody drug conjugates (ADCs) for selective drug delivery in hypoxic tumor tissue66.

Prodrugs are inactive compounds that only become active in hypoxic conditions, targeting low-oxygen tumor cells specifically. Hypoxia-responsive peptides are engineered to release their drugs when exposed to low oxygen levels. These peptides are often combined with drugs that are triggered by HIFs or enzymes overexpressed in hypoxia, like nitroimidazole derivatives67. To further extend TTP applications beyond simple ligand–receptor binding, recent designs incorporate stimuli‑responsive linkers and motifs that react specifically to TME cues, most prominently pH and hypoxia. Among pH‑sensitive linkers, hydrazone bonds are the most widely used. They remain stable in blood (pH 7.2–7.4) but hydrolyze rapidly in the mildly acidic TME (pH 6.5–6.9) or endosomal compartments (pH ≤ 5.5)68. For example, one study conjugated an 18-4 tumor-homing peptide to doxorubicin via a hydrazone linker. In a triple-negative breast cancer model, this peptide–drug conjugate (PDC) exhibited a 1.4-fold increase in intratumoral doxorubicin accumulation and a 1.3–2.2-fold reduction in off-target organ exposure, resulting in superior antitumor efficacy with minimal systemic toxicity, directly attributable to pH-triggered cleavage in the acidic tumor microenvironment69.

Acetal linkers offer an alternative pH‑sensitive strategy with tunable hydrolysis kinetics; one study examined multiple acetal‑based linkers and showed that each unit decrease in pH increased the acetal hydrolysis rate by an order of magnitude. At pH 5.0, half‑lives ranged from seconds to days, whereas stability at pH 7.4 was maintained70. In addition, hypoxia‑responsive motifs (., 2‑Nitroimidazole) are among the most common hypoxia‑sensing groups. Under low‑oxygen conditions (pO < 10 mmHg), intracellular nitro-reductases reduce the nitro group to an aminoimidazole, converting a hydrophobic motif to a hydrophilic one. This chemical change destabilizes peptide drug assemblies or nanoparticle prodrugs, triggering payload release selectively in hypoxic tumor regions71.

Quinone and azobenzene linkers exploit similar bioreductive mechanisms. Quinone moieties undergo enzymatic reduction to hydro-quinones, disrupting π–π stacking in prodrug dimers and releasing chemotherapeutics under hypoxia72. Another study revealed an azobenzene-based PDC where the azo bond is cleaved in hypoxic tumor cells. This cleavage not only liberates the drug but also alters its subcellular localization, enhancing cytotoxicity specifically in oxygen‑deprived regions73.

Drug Delivery

Peptides are conjugated with cytotoxic drugs to form peptide-drug conjugates, ensuring selective delivery to tumor cells while minimizing systemic toxicity74. Peptides targeting integrins or other specific receptors overexpressed on tumor cells, such as the RGD peptide for αvβ3 integrin, belong to this category75, 76. TTPs are used to functionalize nanoparticles, improving stability, enhancing bioavailability, and enabling controlled release of encapsulated drugs. Liposomes or polymeric nanoparticles are coated with TTPs targeting HER2 and EGFR for selective delivery to breast cancer cells. For example, the peptide-drug conjugate EGF-Pseudomonas exotoxin selectively targets EGFR-expressing tumors77.

Imaging and Diagnostics

TTPs play a crucial role in advancing imaging and diagnostic applications in cancer management. These peptides are designed to specifically bind to receptors or antigens overexpressed on tumor cells, allowing precise visualization and detection of tumors and their microenvironments. TTPs are conjugated with fluorescent dyes, allowing for the visualization of tumors using fluorescence microscopy or imaging systems78. Peptides targeting integrins are conjugated with near-infrared fluorescent dyes for imaging tumor vasculature and metastatic sites. TTPs are labeled with positron-emitting radionuclides (such as F or Cu), enabling the detection of tumors through PET scans79. Peptides targeting somatostatin receptors, which are overexpressed in neuroendocrine tumors, are labeled with Ga for PET imaging80. TTPs are labeled with gamma-emitting radionuclides, such as mTc, allowing for SPECT imaging of tumors81. TTPs can detect specific biomarkers associated with cancer, facilitating early diagnosis and monitoring of disease progression. Peptides targeting EGFR are used in assays to detect elevated levels of EGFR in blood samples of patients with certain cancers82. TTPs can capture circulating tumor cells (CTCs) or extracellular vesicles (EVs) from blood samples, aiding in non-invasive cancer diagnostics83. Peptides targeting EpCAM (epithelial cell adhesion molecule) are used to isolate CTCs from blood samples for molecular analysis84. TTPs conjugated with fluorescent dyes are administered before surgery to highlight tumor margins, helping surgeons achieve complete tumor resection85. Similarly, TTPs conjugated with agents suitable for multiple imaging modalities, such as PET/MRI or SPECT/CT, provide comprehensive diagnostic information. Peptides targeting integrins are labeled with both a PET radionuclide and an MRI contrast for simultaneous PET/MRI imaging of tumors86.

Therapeutic Vaccines

Tumor-associated antigens (TAAs) or neoantigens (mutated antigens unique to tumor cells) are identified and used to develop peptide-based vaccines. These peptides derived from these antigens are presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs), such as dendritic cells. This presentation leads to activation of T cells, particularly cytotoxic T lymphocytes (CTLs), which can recognize and kill tumor cells expressing these antigens87. Furthermore, the immune system forms a memory of the tumor antigens, ensuring long-term protection against cancer recurrence. Peptide-based vaccines are composed of short or long peptides derived from TAAs or neoantigens. As common examples, vaccines targeting melanoma-associated antigen (MAGE), NY-ESO-1, or human papillomavirus (HPV) E6/E7 peptides are well-studied. Dendritic cell (DC) vaccines are also increasingly recognized; they are loaded with tumor antigens and then reintroduced into the patient to stimulate a robust immune response88. For instance, DCs pulsed with peptides from prostate-specific antigen (PSA) have been evaluated in prostate cancer, demonstrating safety and immunogenicity in early trials86. Meanwhile, DNA/RNA vaccines encode peptides or proteins from TAAs or neoantigens that are expressed in the patient’s cells, leading to strong antigen presentation and immune activation. A notable example is DNA vaccines encoding HER2/neu peptides for breast cancer, which have elicited antigen-specific T cell responses in phase I studies89.

Photodynamic Therapy

Photodynamic therapy (PDT) is a minimally invasive treatment modality that uses light-activated compounds known as photosensitizers (PSs) to induce cytotoxic effects in targeted cells. Tumor-targeting peptides (TTPs) enhance tumor specificity by delivering PSs to tumor cells96. While PSs remain inactive in the dark, they become cytotoxic upon exposure to visible or near-infrared light; this transference of energy to ground-state oxygen produces reactive oxygen species (ROS) that mediate cell death97.

Immunotherapy and Radiotherapy

TTPs can deliver radionuclides to tumor cells for targeted radiotherapy, minimizing radiation exposure to healthy tissues. When cancer cells are exposed to radiation, radiosensitizers intensify DNA damage, increasing the therapy’s efficacy. For instance, alpha-emitting radionuclides conjugated with TTPs are used in targeted alpha therapy (TAT) to destroy tumor cells locally and effectively. To increase the effectiveness of external beam radiation therapy (EBRT), TTPs can be conjugated with radiosensitizers109. Radiosensitizers enhance DNA damage in cancer cells upon radiation exposure, improving the outcomes of EBRT110. Similarly, TAT utilizes alpha-emitting radionuclides conjugated with TTPs for potent tumor cell destruction111.

Methodological Advancements and Peptide Engineering

Phage display and computational modeling have expanded peptide libraries, while advanced computational tools have facilitated the identification of high-affinity peptides. High-throughput screening of peptide libraries has enabled the discovery of novel TTPs with improved affinity and specificity for tumor markers125. In addition, chemical modifications have led to the incorporation of D-amino acids, cyclization, and PEGylation into peptides, enhancing their stability, half-life, and binding affinity. Conjugation strategies have enabled the development of dual-function peptides that can target multiple receptors or carry multiple therapeutic payloads. Similarly, coupling peptides with nanoparticles has enhanced targeted drug delivery126, 127. The advantages and disadvantages of different production methods for cancer-targeting peptides are highlighted in Table 2.

Multifunctional Peptides

Peptides are engineered to target multiple receptors or pathways simultaneously, which enhances their efficacy and reduces the likelihood of resistance. Peptides that combine both therapeutic and diagnostic functions pave the way for theragnostic (simultaneous therapy and diagnostics)131, 132, 133, 134.

Delivery Systems

Incorporation of TTPs into nanoparticles, liposomes, or micelles significantly improves their stability, bioavailability, and targeted delivery. Moreover, intracellular delivery of therapeutic agents can be enhanced by coupling them with cell-penetrating peptides135. Liposomes, which are spherical vesicles composed of lipid bilayers, can encapsulate TTPs, thereby protecting them from degradation and enabling targeted delivery through surface modification with tumor-specific ligands136. Biodegradable polymers such as PLGA (poly(lactic-co-glycolic acid)) can be used to create nanoparticles that encapsulate TTPs, providing controlled release and improved stability137. Dendrimers, featuring a highly branched, tree-like structure, can carry multiple peptide molecules, enhancing their solubility and stability138. Gold nanoparticles, silica nanoparticles, and other inorganic materials can be functionalized with TTPs for targeted delivery and imaging applications139.

Translational Status of Nanoparticle-Based Tumor-Targeting Peptide Strategies

Recent advancements in nanoparticle-based TTP strategies have demonstrated promising results in both preclinical and clinical settings. Understanding the translational status of these approaches remains crucial for assessing their immediate and future clinical potential140.

Personalized Medicine

patient-specific development of TTPs builds on the unique molecular profile of a patient's tumor, maximizing treatment efficacy. This approach leverages the unique molecular and genetic profiles of each patient's tumor to design highly specific and effective therapeutic agents. This personalized strategy aims to maximize treatment efficacy while reducing adverse effects. Tumor profiling, target identification, and peptide synthesis are crucial elements in the development of patient-specific TTPs145, 146, 147.

Advanced Computational Peptide Design

The integration of AI and ML has substantially advanced the design of tumor-targeting peptides. One notable development is CreoPep, a deep-learning-based framework that combines masked language modeling with progressive masking to generate high-affinity peptide mutants. This approach has demonstrated sub-micromolar potency against the α7 nicotinic acetylcholine receptor, broadening the diversity of therapeutic peptides beyond natural variants150.

Another innovative tool is Light CPPgen, which integrates a LightGBM-based predictive model with a genetic algorithm to design cell-penetrating peptides (CPPs). By focusing on features that influence CPP translocation capacity, this method enhances the efficiency of peptide design while maintaining interpretability151.

Synergy with CRISPR/Cas-Based Screening

CRISPR/Cas-based genetic alteration screens have emerged as a powerful tool for identifying novel targets in cancer immunotherapy. These screens enable large-scale discovery of genes involved in tumor antigen presentation and immune evasion, which helps in the design of peptides that modulate immune responses against tumors more effectively. By integrating CRISPR screening data with peptide design, researchers can develop peptides that either enhance tumor immunogenicity or inhibit immune checkpoints, offering a synergistic approach to cancer therapy and drug delivery152.

Advanced Biomaterials for Tumor Microenvironment (TME) Responsive Drug Release

Because the TME is highly diverse and heterogeneous, delivering medications to tumors remains challenging. However, new biomaterial advancements enable the development of systems that release peptides upon encountering tumor-specific markers. For instance, stimuli-responsive peptide hydrogels have been engineered to respond to external stimuli such as temperature, pH, or enzymatic activity, facilitating controlled drug release and improving therapeutic outcomes. Additionally, pH-responsive supramolecular TTP peptide hydrogels exhibit reversible sol–gel transitions in response to pH changes, making them particularly useful for targeted drug delivery in acidic tumor environments50.