RGD peptide in cancer targeting: Benefits, challenges, solutions, and possible integrin–RGD interactions

1Three amino acids compose the structure of the RGD peptide: arginine (Arg), glycine (Gly), and aspartic acid (Asp). The RGD motif is formed by a linear arrangement of these amino acids., This peptide can be found in cell adhesion and signaling proteins, such as fibronectin, vitronectin, and fibrinogen.,, RGD peptides' principal role is to facilitate cell adhesion by binding to the integrin receptors on the cell's surface. Integrins are proteins located in the transmembrane that are essential for cell signaling, motility, and survival. They participate in a variety of biological processes, such as immune response, wound healing, and angiogenesis.,,, In the timeline of RGD peptide, the RGD sequence was first identified as a crucial motif for cell adhesion in the extracellular matrix in 1984. In the 1990s, the RGD peptide began to be modified by scientists to enhance its binding affinity and selectivity to specific integrin receptors. By 1997, biomaterials were incorporated with RGD peptide for tissue engineering, promoting cell adhesion and tissue regeneration. In the early 2000s, the use of RGD peptide as a targeting ligand for drug delivery systems was explored by researchers, enabling specific delivery to cells expressing integrin receptors. In 2010, RGD peptide‐conjugated nanoparticles were developed for targeted delivery of anticancer drugs to tumor cells, showing promising results in preclinical studies. Since 2015, RGD peptide has also been utilized in the development of scaffolds and hydrogels for tissue engineering applications, facilitating cell adhesion and promoting tissue regeneration. In 2020, RGD peptide was incorporated into bioinks used in 3D bioprinting, enabling precise deposition of cells and promoting their attachment to the printed structures. They found that these integrins bind to their native ligands that contain the RGD sequence. This discovery opened up new possibilities for utilizing RGD peptides as targeting motifs in cancer treatment for both therapeutic and diagnostic purposes. The absence of intellectual property protection further fueled the development of various RGD‐based agents, including fluorescence markers, radiopharmaceuticals, drug conjugates, nanoparticles, and micelles. While RGD applications related to angiogenesis and αvβ3‐integrin have been prominent in the field of radiopharmacy, it is important to note that RGD is not limited to these specific targets. Researchers have also explored the affinity and selectivity of RGD peptides for other integrins, such as αvβ6‐integrin. The development of radiolabeled ligands targeting αvβ3‐integrin has been a significant breakthrough in radiotheranostics for cancer therapy. The future of RGD in targeted cancer therapy holds great promise. Ongoing research is focused on the continuous development of novel RGD‐based ligands and exploring the potential of integrins in therapy. RGD peptides may find applications beyond cancer therapy, including in the field of cancer immunotherapy. The complexity of integrin functions suggests that future research on RGD‐related molecules will encompass diverse areas of molecular medicine and life science.,,,,,, RGD peptides can be modified to enhance its anticancer properties. A few Examples of RGD analogues that were used for the targeting cancer were depicted in Figure 1.

2Specific integrins identify the RGD motif within proteins and bind to this peptide, allowing cell adhesion and communication. RGD peptides have been widely explored for diverse uses in medicine and biotechnology due to their ability to improve cell adhesion and target particular integrin receptors.,,,, To increase cell adhesion and growth, RGD peptides can be introduced into biomaterials such as hydrogels and scaffolds. This may enhance these materials' biocompatibility and efficacy in the regeneration and repair of tissues.,,,

3To target particular integrin receptors that are overexpressed in illnesses such as cancer, RGD peptides can be coupled to therapeutic molecules or nanoparticles.,,,, Integrins, notably αvβ3 and αvβ5, are elevated in malignant cells and vasculature. The medicine may be administered selectively to the tumor location by connecting RGD peptides to drug carriers, limiting the impact on normal cells and tissues. RGD peptides can help drug carriers get into cells by engaging with integrin receptors, which are found on the cell's surface. This receptor‐dependent endocytosis can boost the absorption of drugs in the cell, resulting in greater therapeutic effectiveness (Figure 2).,,, This is especially effective for delivering drugs with low cell permeability or that breakdown quickly in the extracellular environment. RGD‐functionalized drug carriers can be engineered to carry numerous therapeutic agents to the target area, such as chemotherapeutic medicines and genes. This can aid in the treatment of drug resistance and improve the overall therapeutic impact.

4RGD‐functionalized drug carriers can be programmed to release their payload in response to external stimuli like as pH, temperature, or the presence of certain enzymes.,, This can guarantee that the medicine is only delivered at the intended spot, reducing adverse effects, and boosting therapeutic efficacy. To observe integrin expression in animal models, RGD peptides can be tagged with imaging agents like fluorescent dyes or radiotracers.,, This can aid in identifying and monitoring illnesses caused by aberrant integrin expressions, such as cancer and cardiovascular disease. Furthermore, RGD peptides can be combined with imaging agents to allow for real‐time monitoring of drug distribution and treatment response. This can aid in the optimization of treatment procedures and give crucial information on the efficacy of the therapy.,,,,

5By inhibiting the interaction between integrins and their ligands, RGD peptides can be utilized alone to limit tumor development and angiogenesis. This has the potential to alter signaling pathways involved in cancer cell survival, migration, and invasion. Angiogenesis inhibition is a viable treatment option for cancer and other disorders characterized by excessive blood vessel formation. RGD peptide‐conjugated medicines can preferentially target angiogenesis‐related integrins, resulting in more effective anti‐angiogenic therapy.,,,

6RGD peptides, RGD peptide‐conjugated medicines, and RGD peptide‐conjugated nanoparticles/nanocarriers have considerable potential for targeted drug delivery and imaging. Their capacity to selectively target integrin‐expressing cells may result in better treatment results, fewer side effects, and enhanced disease identification and monitoring. New medications targeting the RGD peptide have the potential to enhance the lives of millions of individuals suffering from illnesses such as cancer, cardiovascular disease, and inflammatory ailments. A thorough review can aid in the consolidation of existing information regarding RGD peptides, their interactions with integrins, and their significance in many biological processes. This can serve as a good basis for researchers to find new therapeutic targets. A review of RGD peptides can have a substantial influence on future drug development by improving our understanding of the peptide, revealing knowledge gaps, promoting innovation, and aiding drug design.

7A class of cell surface receptors known as integrins is essential for cell adhesion, migration, and signaling. They are heterodimeric proteins composed of two subunits, α and β, which come together to create different integrin pairs. Certain integrins are particularly appealing targets for RGD peptide‐based therapeutics because they are overexpressed on the surfaces of cancer cells and the tumor vasculature in malignancies.,,, Integrins that recognize the RGD motif particularly interact with the RGD peptide. Several cancer cell types, including melanoma, glioblastoma, and breast, prostate, and ovarian malignancies, overexpress the αvβ3 integrin.,,,, Additionally, it supports angiogenesis by being abundantly expressed in the tumor vasculature.,, RGD peptides that target the αvβ3 integrin can aid in preventing tumor angiogenesis, invasion, and proliferation. Similar to αvβ3, αvβ5 integrin is overexpressed in a variety of cancer types and contributes to tumor angiogenesis.,,, To stop the creation of new blood vessels and restrain tumor growth, RGD peptides can interfere with this integrin's function. α5β1 integrin, which is overexpressed in several malignancies including breast, lung, and colon cancers, is involved in cell adhesion and migration.,,, RGD peptides can help stop cancer cells from migrating and invading by targeting the α5β1 integrin. Numerous malignancies, such as pancreatic, lung, and colon tumors, have elevated levels of the integrin αvβ6.,,, Cancer cell invasion and metastasis include the αvβ6 integrin. It is possible to stop the spread of cancer by targeting this integrin using RGD peptides. The surface of platelets expresses the αIIbβ3 integrin, which is essential for platelet aggregation. Even while platelet aggregation is not directly connected to tumor cells, tumor cells can use it to encourage metastasis. Inhibiting platelet aggregation and lowering the risk of metastasis can be accomplished by targeting αIIbβ3 integrin with RGD peptides.,,, RGD peptides can be utilized to specifically target cancer cells and the tumor vasculature by engaging with these integrins, improving drug delivery efficiency, and minimizing adverse effects on healthy tissues. RGD‐functionalized drug carriers are a viable option for cancer therapy since this focused approach has demonstrated promise in studies.

8The potential applications of the RGD peptide in cancer targeting have garnered significant attention within the field of cancer research. The discovery of the RGD peptide's cancer targeting properties has opened up novel avenues for the precise delivery of drugs and imaging agents in cancer therapy. By linking anticancer drugs or imaging agents to the RGD peptide, cancer cells can be specifically targeted while minimizing harm to healthy cells. This targeted approach holds promise for improving the effectiveness of cancer treatments and reducing undesirable side effects. Additionally, the RGD peptide can be employed in the development of imaging agents that are specific to cancer. By attaching a radioactive or fluorescent label to the RGD peptide, it becomes feasible to visualize and detect tumors using diverse imaging techniques such as positron emission tomography (PET) or fluorescence imaging.,,,,,,,

9RGD peptides are derived from natural proteins, which may trigger an immune response in some patients (Table 1, Figure 3). This could lead to the production of antibodies against the RGD peptide, potentially reducing the effectiveness of the therapy or causing adverse reactions.,,, Peptides, including RGD peptides, can be susceptible to degradation by proteases in the body. This may limit their stability and reduce their effectiveness in drug delivery.,, To overcome this issue, researchers often use modified or cyclic RGD peptides, which exhibit increased stability and resistance to protease degradation. The synthesis of RGD‐functionalized drug carriers can be complex and costly, particularly when multiple components, such as drugs, imaging agents, and stimuli‐responsive materials, are involved.,,, This may limit the widespread adoption of RGD peptide‐based drug delivery systems. Integrin expression can vary between different types of cancer and even between individual tumors of the same type., This heterogeneity may affect the effectiveness of RGD peptide‐based drug delivery, as the therapy may be more effective in some patients than in others.

10To overcome the potential immunogenicity of RGD peptides, altering the peptide structure can help reduce immunogenicity. For example, enlarging the peptide ring of c(RGDyK) by introducing an amino sequence serine‐glycine‐serine (SGS) has been shown to reduce the incidence of anaphylaxis after repeated intravenous c(RGDyKSGS)‐liposome stimulation. This modification was effective in reducing the incidence of anaphylaxis post the repeated intravenous c(RGDyKSGS)‐liposome stimulation. Therefore, the introduction of the SGS sequence into c(RGDyK)‐liposomes serves as a strategy to mitigate the immunogenicity‐associated issue and enhance the safety profile of these drug delivery systems. Incorporating RGD peptides into stealth lipids, such as polyethylene glycol (PEG)‐lipids, can help reduce immunogenicity by shielding the peptide from the immune system.,, This approach can also prolong the circulation time of the RGD peptide in the bloodstream, allowing for more effective delivery to the target site. A promising strategy for attenuating the immune response is presented when RGD peptide‐based lipids are combined with immunosuppressive agents. This approach was exemplified by the study mentioned in the provided document, where RGD‐modified lipid nanoparticles (LNPs), specifically RGD‐PEG‐lipid modified through the post‐insertion method, were utilized. This modification allowed the nanocarrier system to be preferentially targeted toward integrin‐expressing tumor endothelial cells (TECs). In a recent study, an immunosuppressive agent, aPD‐1 monoclonal antibody (mAb), was employed. The programmed cell death protein 1 (PD‐1) receptor, a key regulator of T‐cell responses, was inhibited by this antibody, thereby enhancing antitumor immune responses. By combining RGD‐modified LNPs with aPD‐1 mAb and siVegfr2 (small interfering RNA against Vegfr2), a reduction in tumor‐infiltrating lymphocytes (TILs), indicative of immune response mitigation, was achieved. Furthermore, vascular normalization was induced, and tumor growth was suppressed through this combination approach. It is worth noting that RGD‐PEG‐lipid was the lipid utilized in the study, and aPD‐1 mAb was the specific immunosuppressive agent employed. This strategy, which involves the utilization of RGD peptide‐based lipids and immunosuppressive agents, aligns with the broader goal of improving the efficacy of gene therapy while mitigating potential adverse immune reactions. It thus represents a promising avenue for further exploration in cancer treatment., This approach can be particularly useful when the RGD peptide is used in combination with other therapeutic agents, such as small molecules or antibodies.,,, The choice of delivery system can have a significant impact on the immunogenicity of RGD peptides, which may include liposomes, nanoparticles, and micelles, etc. It is needed to find the most suitable option for reducing immunogenicity while maintaining therapeutic efficacy.,,, Considering the diversity of major histocompatibility complex (MHC) alleles in different populations and races, designing personalized peptides based on an individual's MHC profile can help reduce the risk of immunogenicity. It is essential to carefully evaluate each approach's benefits and drawbacks to determine the most suitable method for a specific application.

11The RGD peptide's poor solution stability can be overcome by cyclizing the linear RGD peptide., It can improve its stability greatly by lowering conformational flexibility and making it less sensitive to proteolysis. A covalent link is formed between two amino acids in the linear sequence to accomplish cyclization. Multiple copies of RGD sequences incorporated into a single molecule can boost their affinity for integrin receptors, compensating for lower individual binding affinities. Multimeric structures incorporating surface‐bound RGD peptides, such as dendrimers or nanoparticles, have shown increased biological activity. The addition of polyethylene glycol (PEG) chains to the RGD peptide improves its solubility, decreases immunogenicity, and extends its circulation duration in vivo.,,, PEGylation protects peptides from enzymatic breakdown while retaining bioactivity. By embedding or conjugating RGD peptides onto biomaterial surfaces such as scaffolds, controlled release over time can be achieved which could enhance local concentration at local sites.,, Co‐administration or conjugation with serpins may protect the peptide against early hydrolysis by inhibiting the breakdown of enzymes., Serpins, also known as serine protease inhibitors, are a superfamily of proteins that play a crucial role in regulating protease activity. They are characterized by a conserved structure and mechanism of action. Serpins inhibit serine proteases by forming a covalent complex with the protease, leading to its inactivation. This interaction involves a reactive center loop (RCL) within the serpin molecule that acts as a bait for the protease. Serpins are involved in various physiological processes, including blood clotting, immune response, and inflammation. They also have implications in diseases such as cancer, thrombosis, and neurodegenerative disorders. Chemical changes such as N‐methylation or D‐amino acid substitution to particular amino acids within an RGD motif may assist increase resistance to enzymatic cleavage without reducing receptor affinity. Non‐peptide analogs or peptidomimetics that replicate the structure and function of the RGD peptide can improve stability, bioavailability, and potency. When compared to endogenous peptides, these molecules are generally less vulnerable to proteolysis.,, It is feasible to overcome the restricted stability of RGD peptides while retaining their biological activity for diverse therapeutic purposes by using these tactics singly or in combination.

12Due to the necessity for specialized modifications or multimeric structures, the synthesis of RGD peptides can be complicated, time‐consuming, and expensive. To address these obstacles, new RGD motifs with simpler structures or more accessible synthetic methods may provide equal biological activity while lowering synthesis complexity and expense.,,,

13Solid‐phase peptide synthesis (SPPS) provides for the effective step‐by‐step assembly of amino acids on a solid platform, eliminating purification stages and enhancing total yield. Automated SPPS synthesizers reduce manual effort and human error even further. Adjusting reaction parameters such as temperature, solvent choice, coupling reagents, or protecting groups may increase peptide synthesis efficiency and prevent side reactions that cause contaminants. Continuous improvement in production procedures through optimization studies will increase process efficiency, resulting in less waste, higher product quality, and lower cost per unit. Scaling up production processes by investing in larger reactors/fermenters will result in lower cost per unit product as fixed expenses are dispersed over a larger number of units produced.,,,

14For longer or more complex RGD‐containing peptides or proteins, chemical ligation techniques like native chemical ligation (NCL) enable the convergent assembly of multiple smaller peptide fragments with greater synthetic accessibility.,,, Producing RGD peptides using recombinant DNA technology in bacterial (e.g., E.coli), yeast (e.g., Pichia pastoris), insect cells (e.g., baculovirus system), or mammalian cell cultures can provide higher yields compared to traditional chemical synthesis methods at lower costs per batch.,,,, Efficient bioconjugation strategies such as click chemistry allow for site‐specific attachment of functional moieties like PEG chains without requiring additional protection/deprotection steps during peptide synthesis.,,,

15By adopting these strategies individually or in combination, it is possible to overcome the complexity and cost challenges associated with the synthesis of RGD peptides without compromising their therapeutic potential.

16Heterogeneous expression of integrins on different cell types and tissues can impact the targeting specificity and therapeutic efficacy of RGD peptides. To overcome this challenge, design RGD peptide derivatives that selectively target specific integrin subtypes overexpressed in pathological conditions such as cancer or inflammation. This can be achieved by modifying amino acid sequences, incorporating additional functional groups, or using peptidomimetics.

17The application of targeted extracellular matrix (ECM)‐derived peptides for promoting neovascularization in a rodent model of myocardial infarction is the focus of the document. One specific approach mentioned in the document is the conjugation of targeting moieties, such as antibodies or aptamers, to RGD peptide constructs. The interaction of the RGD peptide sequence with integrin receptors on the surface of cells is well‐known. By conjugating targeting moieties, such as antibodies or aptamers, to RGD peptide constructs, the specificity of the peptides toward cells with co‐expression patterns related to disease conditions is increased. This results in an increased likelihood of the peptides binding to and interacting with cells specifically involved in the disease process, such as cells in the infarcted area of the heart. The reduction of off‐target effects on normal cells is achieved by increasing the specificity of the peptides. This is important as it minimizes any potential negative impact on healthy cells and tissues. The conjugation of targeting moieties to RGD peptide constructs enables a more precise and targeted delivery of the peptides to the desired cells, thereby enhancing their therapeutic potential. Hence, a promising strategy for improving the specificity and effectiveness of ECM‐derived peptides in promoting neovascularization in the context of myocardial infarction is offered by the approach of conjugating targeting moieties to RGD peptide constructs. Conjugating other targeting moieties (e.g., antibodies, aptamers) to RGD peptide construct increases specificity toward cells with co‐expression patterns related to disease conditions while reducing off‐target effects on normal cells. Developing stimuli‐responsive drug delivery systems is also helpful because they can release their cargo only under certain physiological/pathological conditions like acidic tumor microenvironments or enzymatic cleavage by proteases specifically upregulated in diseased tissue.,,, Nanoparticles and nanocarriers could utilize engineered surface‐bound RGD motifs for enhanced cellular uptake via receptor‐mediated endocytosis while carrying a therapeutic payload within the core/shell matrix., Designing prodrugs that are activated upon binding to a specific integrin subtype through enzymatic cleavage or conformational changes, etc. resulting from receptor‐ligand interaction can release an active RGD molecule at targeted sites.,, A combination of RGD peptide‐based therapies with other treatments like chemotherapy agents or immune checkpoint inhibitors could enhance treatment efficacy while potentially mitigating resistance mechanisms associated with single‐agent therapy.,,,

18Identifying biomarkers indicative of high integrin expression levels corresponding to likely responders allows for better patient stratification and personalized treatment approaches. Determining the optimal dosing, frequency, and routes of administration would maximize therapeutic efficacy while minimizing off‐target effects on normal tissues with lower integrin expression levels.,,, Utilizing imaging techniques like PET or MRI in conjunction with RGD‐based probes for visualizing changes in integrin expression during therapy allows early identification of responders and facilitates timely adjustments to the treatment plan if necessary.,,, By employing these strategies individually or in combination, it is possible to overcome challenges posed by heterogeneous integrin expression when using RGD peptides for targeted therapies while maximizing their clinical potential.

19RGD peptides are short peptide fragments derived from the amino acid sequence of several extracellular matrix proteins, such as fibrinogen, fibronectin, vitronectin, collagen, and laminin. They are widely present in fibronectin, laminin, fibrinogen, osteopontin, and vitronectin.

20RGD can be divided into two types: RGD: This is a tripeptide sequence of RGD.RGD polypeptide: This is a functional peptide containing RGD.

21RGD: This is a tripeptide sequence of RGD.

22RGD polypeptide: This is a functional peptide containing RGD.

23Depending on the application and the integrin targeted, RGD can be chemically modified or replaced by a similar peptide which promotes cell adhesion. For example, RGD peptides can be cyclized, or made into a cyclic compound, via disulfide, thioether, or rigid aromatic ring linkers. This leads to an increase in binding affinity and selectivity for integrin αVβ3 relative to αIIBβ3.

24Another type of RGD‐derived peptide is the “internalizing RGD” or “iRGD.” This peptide has the ability to recognize the integrin receptor on the cancer cell surface like its ancestor with an additional outstanding feature to penetrate to extravascular space of tumor and ability to penetrate to cancer cells unlike the original peptide.,,,,,

25Cilengitide is a cyclic RGD peptide that targets αvβ3 and αvβ5 integrins (Figure 4). It has been investigated as a potential treatment for glioblastoma, a type of brain cancer. Cilengitide has shown promising results in preclinical studies (Table 2), but its efficacy in clinical trials has been limited. One disadvantage of cilengitide is its potential immunogenicity, which may reduce its effectiveness in some patients.,,, Eptifibatide is also a cyclic RGD peptide that targets αIIbβ3 integrin, which is involved in platelet aggregation. Eptifibatide is used as an antiplatelet agent to prevent blood clots in patients with acute coronary syndrome or undergoing percutaneous coronary intervention. One advantage of eptifibatide is its rapid onset of action, which can help prevent thrombotic events.,,, Abegrin is a cyclic RGD peptide that αvβ3 integrin, which is overexpressed in several types of cancer. Abegrin has been investigated as a potential treatment for various cancers, including breast cancer and pancreatic cancer. The associated clinical trials can be found in Table 3. One advantage of Abegrin is its high specificity for αvβ3 integrin, which can improve the efficacy of the therapy.,,,

26The clinical studies focus on various types of cancer, including ovarian cancer, lung cancer, head and neck cancer, glioblastoma, non‐small cell lung cancer, cervical cancer, and others. Each study aims to evaluate the specific application of the drugs in the context of these cancer types. The studies utilize different imaging techniques, such as PET/CT (positron emission tomography/computed tomography) and PET/MRI (positron emission tomography/magnetic resonance imaging), to assess tumor characteristics, angiogenesis, and response to treatment. These imaging techniques provide valuable information about the distribution and activity of the drugs within the body. Some studies have secondary objectives, such as evaluating immunologic responses, determining potential toxicities, assessing the predictive value of imaging techniques, and correlating imaging parameters with clinical treatment response (Table 4).

27RGD‐based radiopharmaceuticals are designed to specifically target integrin αvβ3 receptors. These receptors are proteins found on the surface of cells, including cancer cells and activated endothelial cells involved in angiogenesis. The RGD sequence has a specific structure that allows it to bind to these receptors. To enable imaging of tumor angiogenesis, the RGD‐based radiopharmaceuticals are labeled with short half‐life isotopes like 68Ga and 99mTc (Table 4). These isotopes emit positrons (in the case of 68Ga) or gamma rays (in the case of 99mTc). PET/CT or SPECT/CT imaging techniques are used to detect and capture these emissions. By administering the radiopharmaceuticals to patients and performing PET/CT or SPECT/CT scans, it becomes possible to visualize and quantify the extent and characteristics of tumor angiogenesis. This imaging approach provides valuable information for various aspects of cancer management. First, it aids in cancer diagnosis by providing detailed images that can help identify the presence and location of tumors. It can also assist in staging, which involves determining the extent and spread of the disease. Furthermore, this imaging technique is useful in treatment planning. By visualizing tumor angiogenesis, healthcare professionals can better understand the blood supply to the tumor and make informed decisions regarding treatment strategies. It can help determine the most appropriate treatment approach, such as the use of chemotherapy, radiation therapy, or targeted therapies. Additionally, molecular imaging of tumor angiogenesis using RGD‐based radiopharmaceuticals allows for the monitoring of therapeutic response. By comparing images taken before and after treatment, healthcare professionals can assess the effectiveness of the chosen treatment and make adjustments if necessary. Hence, RGD‐based radiopharmaceuticals, labeled with short half‐life isotopes like 68Ga and 99mTc, target integrin αvβ3 receptors. This enables molecular imaging of tumor angiogenesis using PET/CT or SPECT/CT imaging techniques. By visualizing and quantifying the extent and characteristics of tumor angiogenesis, this imaging approach provides valuable information for cancer diagnosis, staging, treatment planning, and monitoring of therapeutic response. It offers insights into the biology and behavior of tumors, aiding in cancer management and treatment decision‐making.

28Additionally, conditionally replicative adenoviruses (CRAds) are a type of adenovirus that have been genetically modified to selectively replicate in cancer cells while sparing normal cells. This targeted replication within cancer cells can lead to their destruction and potentially provide a therapeutic benefit. One specific type of CRAd mentioned in Table 4 is Ad5‐Delta 24RGD. This CRAd is designed to infect and replicate within cancer cells that overexpress integrin αvβ3 receptors, which are commonly found on the surface of tumor cells. By targeting these receptors, Ad5‐Delta 24RGD can selectively infect and replicate within cancer cells, leading to their destruction. Another CRAd mentioned is DNX‐2401. This CRAd is being evaluated in clinical trials for the treatment of glioblastoma, a type of aggressive brain cancer. DNX‐2401 is delivered directly into the tumor, either through intratumoral injection or into the resected tumor cavity. Once inside the tumor, DNX‐2401 can replicate and spread, targeting and destroying cancer cells. In addition to glioblastoma, CRAds are also being studied in various types of cancer, including ovarian cancer. Clinical trials are being conducted to evaluate the safety, tolerability, and efficacy of CRAds in these cancer types. These trials aim to determine the appropriate dosing, administration routes, and potential side effects of CRAds in order to assess their therapeutic potential. By selectively targeting cancer cells and replicating within them, CRAds have the potential to provide a targeted and effective treatment approach for various types of cancer. However, it is important to note that clinical trials are still ongoing, and further research is needed to fully understand the safety and efficacy of CRAds in different cancer types.

29Apatinib is an antiangiogenic drug that selectively inhibits vascular endothelial growth factor receptor 2 (VEGFR‐2). VEGFR‐2 is a protein that plays a crucial role in angiogenesis, the process by which new blood vessels are formed to supply nutrients and oxygen to tumors.

30By specifically targeting and blocking VEGFR‐2, apatinib disrupts the signaling pathway that promotes angiogenesis. This inhibition of angiogenesis can help to starve tumors of their blood supply, potentially slowing down their growth and progression. Clinical studies are currently being conducted to evaluate the efficacy and safety of apatinib in patients with different malignancies. These studies aim to assess the drug's ability to inhibit angiogenesis and its impact on tumor growth and patient outcomes. In the context of molecular imaging, 18F‐ALF‐NOTA‐PRGD2 PET/CT scans are being used in these clinical studies to predict the response to apatinib treatment. PRGD2 is a peptide that specifically binds to integrin αvβ3 receptors, which are overexpressed in tumor‐associated blood vessels. By labeling PRGD2 with the radioactive isotope 18F and performing PET/CT scans, researchers can visualize and quantify the expression of integrin αvβ3 receptors in tumors. These PET/CT scans provide valuable information about the extent and characteristics of tumor angiogenesis, which can help predict the response to apatinib treatment. By assessing the level of integrin αvβ3 receptor expression, researchers can gain insights into the tumor's angiogenic activity and its potential sensitivity to apatinib therapy (Table 4).

31The RGD peptide is one of the most commonly used peptides for targeted cancer therapy. The RGD peptide has a high affinity for integrin receptors, which are overexpressed on the surface of many cells. Other peptides can compete with RGD for integrin receptor binding and have been investigated for targeted cancer therapy. examples of peptides that have been investigated for targeted cancer therapy include the iRGD peptide, which can penetrate tumor tissue and enhance drug delivery,,, and the CRGDK homing peptide, which has a high affinity for αvβ3 integrin.,, However, the RGD peptide remains one of the most widely studied and utilized peptides for targeted cancer therapy due to its high affinity for integrin receptors and its ability to selectively target tumor cells (Table 5).

32The iRGD (internalizing RGD) peptide is a modified version of the traditional RGD peptide with an amino acid sequence: CRGDKGPDC. It has been developed to improve tumor targeting and penetration abilities.,, The RGD motif within the iRGD peptide binds to αvβ3 and αvβ5 integrins overexpressed on tumor endothelial cells, facilitating initial accumulation at the tumor site.,,, Following proteolytic cleavage of the iRGD peptide after binding to integrins, a CendR motif (C‐terminal arginine or lysine residue) is exposed which interacts with neuropilin‐1 (NRP‐1) receptors present on tumor cells and vasculature, leading to enhanced tissue penetration and cellular uptake via receptor‐mediated endocytosis.,,, Potential applications of iRGD peptides in cancer treatment include conjugating chemotherapy agents or cytotoxic drugs for targeted delivery, coupling with nanoparticles or nanocarriers for efficient drug transport, enhancing gene therapy approaches by conjugating viral vectors or gene‐editing tools and boosting immunotherapy efficacy through targeted delivery of immune checkpoint inhibitors.

33The CRGDK peptide is a pentapeptide containing an RGD sequence flanked by cysteine residues at both ends, resulting in improved stability due to cyclization via disulfide bond formation. Its applications are similar to those associated with other RGD‐based peptides but may offer better resistance against enzymatic degradation due to its cyclic structure.,, Some potential applications include tumor targeting via selective binding to αvβ3/αvβ5 integrins overexpressed on cancerous tissues, conjugation with chemotherapeutic or cytotoxic agents for targeted treatments, development of nanoparticles or nanocarriers to improve drug delivery and reduce off‐target effects.,,

34Both iRGD and CRGDK peptides are derived from the RGD peptide family but possess unique properties that can be exploited for various treatment purposes. The iRGD peptide demonstrates improved tumor targeting and penetration abilities via dual receptor binding (integrins and NRP‐1), while the cyclic structure of CRGDK offers enhanced stability against proteolytic degradation. Each peptide has potential applications in targeted chemotherapy, nanoparticle‐based drug delivery systems, gene therapy, and immunotherapy approaches for cancer treatment.

35Integrins are heterodimeric proteins composed of α and β subunits. In humans, there are 18 α subunits and 8 β subunits, which can combine to form different integrin receptors. The extracellular domain of integrins contains a ligand‐binding head region, a single‐pass transmembrane domain, and a cytoplasmic tail that interacts with intracellular signaling molecules. The ligand‐binding head region consists of a β‐propeller domain and a βI‐like domain, which together form a binding pocket for the RGD motif. The RGD motif is a short peptide sequence found in various ECM proteins, such as fibronectin, vitronectin, and collagen. It plays a crucial role in cell adhesion and migration by interacting with integrin receptors. The arginine residue in the RGD motif forms a salt bridge with a conserved glutamic acid residue in the integrin receptor, while the glycine and aspartic acid residues contribute to the overall conformation and stability of the motif.,,,,

36The interaction between the RGD motif and integrin receptors is a multistep process. Initially, the RGD motif binds to the ligand‐binding head region of the integrin receptor, primarily through interactions with the α subunit. This binding induces conformational changes in the integrin, leading to the exposure of a high‐affinity state. This conformational change allows the integrin to bind to other ECM proteins, leading to the formation of focal adhesions and initiation of downstream signaling events. The interaction between the RGD motif and integrin receptors triggers various intracellular signaling pathways. These pathways regulate cell survival, proliferation, migration, and differentiation. One of the well‐studied signaling pathways is the focal adhesion kinase (FAK) pathway, which activates downstream signaling molecules, including mitogen‐activated protein kinases (MAPKs) and phosphatidylinositol 3‐kinase (PI3K). These signaling events ultimately regulate cellular processes such as cytoskeletal rearrangement, gene expression, and cell cycle progression. The RGD motif‐integrin interaction is essential for numerous physiological processes, including embryonic development, tissue homeostasis, and immune response. It is involved in cell adhesion and migration during wound healing, angiogenesis, and tissue regeneration. Dysregulation of the RGD motif‐integrin interaction has been implicated in various pathological conditions, including cancer metastasis, fibrosis, and autoimmune diseases. Targeting this interaction has emerged as a potential therapeutic strategy for these diseases.,,,,,,,

37The interaction between the RGD motif and integrin receptors is a complex and highly regulated process that plays a crucial role in cell adhesion, migration, and signaling. Understanding the detailed information about this interaction provides insights into the mechanisms underlying physiological and pathological processes. Further research in this field may lead to the development of novel therapeutic approaches targeting the RGD motif‐integrin interaction for the treatment of various diseases.

38Breast cancer is a complex illness with several subtypes that have diverse genomic profiles, clinical characteristics, and therapy responses. Integrins, a kind of cell surface receptor, are important in cell adhesion, migration, and signaling and are frequently dysregulated in breast cancer. The RGD peptide is known to interact with numerous integrins, making it an attractive option for breast cancer‐targeted therapy and diagnostics.,, Understanding the particular integrin expression patterns in various breast cancer subtypes can aid in the development of RGD peptide‐based techniques for more effective and tailored breast cancer therapy.,,

39TNBC is distinguished by the lack of expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). It is a dangerous subtype with few therapeutic choices and a dismal prognosis. Integrins αvβ3, αvβ5, and α5β1 are widely overexpressed in TNBC,, and their interaction with RGD peptides has been studied for targeted drug delivery, imaging, and treatment.,,, MDA‐MB‐231 is a breast cancer cell line generated from a patient with triple‐negative/basal‐like breast cancer, which is distinguished by the lack of ER, PR, and HER2 expression and has a poor prognosis. Another breast cancer cell line produced by a patient with triple‐negative/basal‐like breast cancer is MDA‐MB‐468. It has been found that MDA‐MB‐231 and MDA‐MB‐468 cells have integrins αvβ3, αvβ5, and α5β1, which can bind with RGD peptides (Table 6).,,,,

40Luminal breast cancer is distinguished by the presence of hormone receptors (ER and/or PR) and has a better prognosis than TNBC and HER2‐positive breast cancer. Integrins αvβ3, αvβ5, and α5β1 can also be overexpressed in luminal breast cancer, and RGD peptide‐based techniques for targeted therapeutics and diagnostics have been investigated.,,, MCF‐7 is a frequently used breast cancer cell line obtained from a patient with luminal A breast cancer, which is distinguished by the presence of estrogen receptor (ER) and/or progesterone receptor (PR) expression and low levels of human epidermal growth factor receptor 2 (HER2). T47D is another breast cancer cell line generated from a luminal A breast cancer patient. MCF‐7 and T47D cells have been shown to express integrins αvβ5, and α5β1, which can bind RGD peptides.,, MCF‐7 cells are typically employed as a model for low integrin αvβ3 expression, and similarly, T47D cells exhibit a low level of integrin expression.,,,

41Lung cancer is a diverse collection of cancers that may be divided into two types: non‐small cell lung cancer (NSCLC) and small cell lung cancer (SCLC). NSCLC is further classified into three major subtypes: adenocarcinoma, squamous cell carcinoma, and giant cell carcinoma. SCLC is distinguished by its fast development and early metastasis., RGD peptides have been demonstrated in multiple lung cancer cell lines to bind with integrins αvβ3 and αvβ5.,, These interactions could be used in targeted drug delivery and imaging applications, highlighting the promise of RGD peptides for lung cancer targeted treatment. HCC827 (NSCLC, adenocarcinoma), PC‐9 (NSCLC, adenocarcinoma), H1975 (NSCLC, adenocarcinoma), and H69 (SCLC) are the most often utilized lung cancer cell lines.,,, These cells have been shown to express integrins αvβ3 and αvβ5, which can bind to RGD peptides.

42Cervical cancer is the fourth most frequent cancer in women globally, with HPV infection being the leading risk factor. Squamous cell carcinoma (SCC) and adenocarcinoma (AC) are the two primary histological subtypes of cervical cancer., Several integrins have been shown to interact with RGD peptides, adding to the complexity and heterogeneity of cervical cancer. In cervical cancer, integrins that engage with RGD peptides typically bind to fibronectin (α5β1, αvβ3, and αvβ6) and vitronectin (αvβ3 and αvβ5).,,, Integrin α5β1 overexpression has been found in several cervical cancer cell lines (e.g., HeLa), where it contributes to cell adhesion and migration via interactions with its natural ligand‐fibronectin with an RGD sequence.,,, Due to its different binding specificity from the other mentioned integrins, Integrin α2β1 does not directly interact with classical linear/cyclic‐RGD peptides, but studies suggest that it may be involved indirectly through crosstalk/signaling pathways related to cervical cancer progression mediated by other RGD‐binding integrins such as α5β1. These integrins' expression and function differ depending on the stage and severity of cervical cancer.,, The use of RGD‐based inhibitors to target these integrins provides a promising treatment option for cervical cancer, with the potential to enhance patient outcomes and lessen the burden of this deadly illness. RGD peptides, on the other hand, have been demonstrated to interact with integrins αvβ3 and αvβ5, particularly binding to fibronectin in cervical cancer cell lines such as HeLa (SCC) and SiHa (SCC).,

43Colorectal cancer (CRC) is a complex disease with several molecular subgroups that can interact with RGD peptides.,, The interaction of integrins with RGD peptides is important in the development, metastasis, and angiogenesis of colorectal cancer and colon cancer. Several integrins, including αvβ3, αvβ5, αvβ6, α5β1, and α2β1, can interact with RGD peptides in CRC.,,, Accordingly, targeting these integrins and their interactions with RGD peptides may constitute a viable therapeutic method for the treatment of CRC. Integrin αvβ3 is significantly expressed in CRC and has been linked to tumor growth, angiogenesis, and metastasis. The interaction between αvβ3 and RGD peptides increases cell adhesion, migration, and invasion, which contributes to CRC cell aggression. Inhibiting the αvβ3‐RGD connection has been demonstrated in preclinical models to inhibit tumor development and angiogenesis, indicating that targeting this integrin may be a potential treatment approach for CRC. Another RGD‐binding integrin that is increased in CRC is integrin αvβ5. It has been associated with cell adhesion, migration, and invasion regulation, and with the activation of growth factor signaling pathways such as the VEGF pathway. Targeting the αvβ5‐RGD connection might aid in the inhibition of tumor development and angiogenesis in CRC. Integrin expression and interactions with RGD peptides may change between cell lines and subtypes. HCT116, HT‐29, SW620, Caco‐2, LoVo, LS174T, DLD‐1, SW480, and Colo205 colorectal cancer cell lines contain integrin αvβ3, which can interact with RGD peptides., The RKO cell line has been demonstrated to interact with RGD peptides, which can attach to integrin αvβ5.,

44Hepatocellular carcinoma (HCC), intrahepatic cholangiocarcinoma (ICC), and combination hepatocellular‐cholangiocarcinoma (cHCC‐CC) are all subtypes of liver cancer. The interaction between integrins and RGD peptides is critical in the development, metastasis, and angiogenesis of liver cancer. Several integrins, including αvβ3, αvβ5, α5β1, and α6β1, have been found to interact with RGD peptides in liver cancer.,,,,

45These integrins have been shown to interact with RGD peptides in several liver cancer cell lines. These interactions contribute to tumor growth, metastasis, angiogenesis, and therapy resistance, making them attractive therapeutic targets for the treatment of liver cancer. Integrins αvβ3, αvβ5, and α5β1 are expressed by HepG2, and Huh7 (well‐established HCC cell lines).,,

46Pancreatic cancer is a formidable adversary in the world of oncology, characterized by its aggressive nature, limited treatment options, and poor prognosis. Pancreatic cancer is a very aggressive and diverse disease, with pancreatic ductal adenocarcinoma (PDAC) being the most frequent subtype. The search for innovative approaches to combat this deadly disease has led researchers to explore the application of RGD peptides, which have shown promise in targeting integrins—a group of cell surface receptors that play pivotal roles in tumor growth, invasion, angiogenesis, metastasis, and therapy resistance. In pancreatic cancer, the dysregulation of integrin expression on cancer cells has been extensively studied. Specifically, integrins αvβ3, αvβ5, and αvβ6 have been implicated in pancreatic cancer progression, making them attractive therapeutic targets. Several other integrins, including α5β1, and α6β1, may also interact with RGD peptides in pancreatic cancer.,,, Integrins αvβ3, αvβ5, and α5β1 are expressed by PANC‐1, MIA PaCa‐2, BxPC‐3, and AsPC‐1 (all well‐established PDAC cell lines).,,,,,

47One promising approach involves utilizing RGD peptides with high binding affinity for integrins, particularly αvβ3, to enhance drug delivery specificity. RGD peptides have been incorporated into liposomal drug carriers, allowing the targeted delivery of anticancer therapeutics directly to pancreatic cancer cells. This modification enhances the tumor specificity of vesicles, increasing the precision of treatment. RGD‐conjugated albumin nanoparticles have shown promise as delivery vehicles for therapeutic agents in pancreatic cancer. These nanoparticles, by targeting integrin avβ3 receptors on pancreatic cancer cells, enhance drug penetration, improve antitumor efficacy, and inhibit tumor growth and metastasis. RGD peptides have also been employed in conjunction with gemcitabine, a standard drug for pancreatic cancer treatment, to enhance drug penetration into tumors. The coadministration of GEM and RGD peptides has demonstrated effectiveness in reducing tumor size, particularly in cell line‐based xenografts, emphasizing the potential clinical utility of this approach In the realm of diagnostic imaging, RGD peptides have been utilized to improve the labeling and uptake of superparamagnetic iron oxide (SPIO) nanoparticles in pancreatic cancer cells. This approach enhances the sensitivity of pancreatic cancer imaging, aiding in early detection and precise evaluation. Another innovative application of RGD in pancreatic cancer involves using a quantum dots‐RGD probe as a photosensitizer in photodynamic therapy (PDT). This approach inhibits cell proliferation and induces apoptosis in pancreatic carcinoma cells, offering a promising avenue for clinical treatment.

48The application of RGD peptide in pancreatic cancer has emerged as a multifaceted approach encompassing targeted drug delivery, tumor‐selective immunotherapies, photodynamic therapy, diagnostic imaging, and more. By targeting integrins, RGD peptides offer a promising avenue to enhance the specificity and efficacy of pancreatic cancer diagnosis and therapy. However, continued research is imperative to fully realize the potential of RGD‐based approaches in combating this challenging disease.

49Kidney cancer, commonly known as renal cell carcinoma (RCC), is a diverse group of malignancies that develop from renal tubular epithelial cells. RCC is classified into four subtypes: clear cell RCC (ccRCC), papillary RCC (pRCC), chromophobe RCC (chRCC), and collecting duct carcinoma (CDC). Each subtype has different histological characteristics, molecular changes, and clinical consequences.,

50The most prevalent subtype of RCC is ccRCC. It is distinguished by transparent cytoplasm and a well‐defined cell membrane. The deletion of the von Hippel–Lindau (VHL) tumor suppressor gene, which leads to the stability of hypoxia‐inducible factors (HIFs) and the stimulation of angiogenesis, is the most prevalent genetic change in ccRCC. Integrins such as αvβ3, αvβ5, and α5β1 may interact with RGD peptides in ccRCC.,, RGD peptides may reduce cell adhesion, migration, and invasion in ccRCC cell lines such as 786‐O and A498, by targeting these integrins. The second most prevalent kind of RCC is pRCC. The presence of papillary structures bordered by cuboidal or columnar cells marks it. pRCC is classified into two subtypes: Type 1, which has a better prognosis, and Type 2, which is more aggressive and has a worse prognosis., The prognosis for CDC is poor, with a high prevalence of metastasis and resistance to traditional therapy., There is little known about the roles of integrins and RGD peptides in chRCC, and CDC. Integrins such as αvβ3 and αvβ5 may, however, have a role in both pRCC, chRCC and CDC, as they are implicated in cell adhesion, migration, and invasion in other RCC subtypes.,

51Integrins are a kind of cell adhesion molecule that is required for the growth and spread of cancer. By targeting these molecules with RGD peptides, researchers were able to selectively deliver drugs to cancer cells and the tumor vasculature while preventing injury to healthy organs. This technique has recently shown a lot of promise, with RGD‐functionalized drug carriers demonstrating increased efficacy and lower toxicity when compared to traditional chemotherapy. RGD‐functionalized drug carriers have also been shown to improve therapeutic efficacy by increasing medicine absorption and retention inside the tumor microenvironment. Because different cancer types may require different treatments or dosages, the precision of this method allows for the development of individualized treatment strategies. RGD peptides and their conjugates can help detect and track the progression of cancer by serving as imaging agents and improving therapy administration. RGD‐conjugates are expected to become a more important weapon in the fight against cancer as research in this field advances.

52Hossein Javid: Conceptualization (equal); data curation (equal); formal analysis (equal); methodology (equal); project administration (equal); writing – original draft (equal). Mahsa Akbari Oryani: Data curation (equal); formal analysis (equal); validation (equal); writing – original draft (equal). Nastaran Rezagholinejad: Conceptualization (equal); resources (equal); software (equal). Ali Esparham: Conceptualization (equal); formal analysis (equal); writing – original draft (equal). Mahboubeh Tajaldini: Formal analysis (equal); methodology (equal); software (equal). Mehdi Karimi‐Shahri: Project administration (equal); supervision (equal); validation (equal); visualization (equal); writing – review and editing (equal).

53Funding information is not applicable/no funding was received.

54The authors declare that they have no conflict of interest.

55Ethics approval for this type of article (A review) is not applicable.

56Not applicable.