With the development of phage display technology, hundreds of tumor homing peptides (THPs) and their derivatives have been discovered. Studies have confirmed that these tumor homing peptides have the potential to find tumor cells in animals and can specifically deliver antitumor drugs to tumor sites. Among them, some tumor homing peptides also have transmembrane properties, it allowing drugs to enter tumor cells more effectively to play a therapeutic role. At present, research on tumor-homing peptides for tumor diagnosis and treatment has already entered different clinical trials.

The source of tumor homing peptide (THPs)?

In 1985, Smith reported for the first time the display of peptides on the surface of phage. In 1990, Scott et al. developed and constructed a phage display peptide library based on phage display technology. Phage peptide library is a very important branch of phage display technology. The peptide library is a recombinant phage library composed of a large number of single phage with different peptides, that is, the target receptor is used to screen the phage peptides that interact with it, and after elution and amplification, the specific recombinant phage is enriched and then can analyze the structure and sequence of the selected peptide.

Since then, the phage display peptide library has become an effective screening tool to identify biological targeting peptide ligands (such as pure proteins, antibodies, antigens, and whole cells). Phage display technology includes three screening methods: in vitro phage display , in vivo phage display, and in vivo/in vitro phage display technology. Researchers use these methods to screen and identify a large number of tumor homing peptides.

Types of tumor homing peptides?

Many tumor homing peptides screened by phage display technology are non-specific, while other tumor homing peptides have relatively specific affinity for specific tumor types:

Non-specific

1. Homing peptides targeting tumor blood vessels

Compared with normal blood vessels, tumor blood vessels have the characteristics of distortion and leakage in morphology. The important components of tumor angiogenesis (such as vascular endothelial cells, smooth muscle cells and extracellular matrix) and integrins, cell surface polysaccharides and endothelial growth ,the expression of factor receptors, etc. are different. Based on the difference between tumor blood vessels and normal blood vessels, phage technology was used to screen and identify many tumor vascular homing peptides. These peptides can bind to molecular markers in tumors, and most of these molecular markers play an important role in tumor angiogenesis.

RGD (Arg-Gly-Asp) and NGR (Asp-Gly-Arg) are the first tumor homing peptides discovered in the late 1990s and are also classic representatives of tumor vascular homing peptides. The receptors of RGD are integrins p3 and p5, and NGR has affinity for peptidase and aminopeptidase N (APN). At present, a large number of research reports have shown that RGD and NGR and their derivatives (such as RGD-4C) can be used in the diagnosis and anti-tumor angiogenesis treatment of various tumors. Subsequent studies found that the tumor vascular homing peptides were iRGD, IF7 and TMTP1.

2. Homing peptide targeting tumor lymphatic vessels

Many tumor homing peptides that target tumor lymphatic vessels have been discovered in the research. Tumor lymphatic vessel homing peptide can specifically target tumor lymphatic vessels, which indicates that tumor lymphatic vessels and tumor blood vessels have their own unique tumor-specific markers.

Ly-1 (CGNKRTRGC) is the first tumor lymphatic vessel homing peptide to be discovered. Its receptor is cell surface mitochondrial protein P32, which can specifically target breast cancer and osteosarcoma and other tumors. LyP-2 (CNRRTKAGC) is a similar sequence to LyP-1 and is also a tumor lymphatic vessel homing peptide. In addition, LyP-1 and LyP-2 do not bind to normal skin tissues and cervical tissues. Although LyP-1 and LyP-2 have similar sequences, the types of tumors they target and the receptors that target tumor lymphatic vessels are different.

In addition to the above-mentioned two tumor lymphatic vessel homing peptides, there are research reports that REA (CREAGRKAC), AGR (CAGRRSAYC) and LSD (CLSDGKRKC) three peptides can specifically bind to tumor lymphatic vessels. The above research results show that people are expected to conduct tumor diagnosis and targeted therapy from the direction of tumor lymphatic vessels.

3. Dual-targeted tumor homing peptide

Studies have found that certain peptides not only target tumor blood vessels or tumor lymph vessels alone, but also home to tumor cells. Dual-targeting homing peptide refers to the same tumor, which can simultaneously target tumor blood vessels and tumor cells, or simultaneously target tumor lymphatic vessels and tumor cells. However, for the same tumor, these peptides generally do not target tumor blood vessels and tumor lymphatic vessels at the same time, because tumor lymphatic vessels and tumor blood vessels each have their own unique tumor-specific markers.

F3 (KDEPQRRSARLSAKPAPPKPEPKPKKAPAKK) is a 31 amino acid long peptide derived from a fragment of high mobility group protein 2. F3 can simultaneously home to vascular endothelial cells and tumor cell nuclei in transplanted tumor animal models such as leukemia HL-60 cells and breast cancer MDA-MB-435 cells. Using phage display technology, it was also found that in addition to F3, KRK and pentapeptide CDTRL are also dual-targeting homing peptides for tumor blood vessels and tumor cells.

Some non-specific tumor homing peptides

Some tumor homing peptides can bind to specific tumors. Compared with non-specific tumor homing peptides, they further achieve a single tumor target. Koivistoinen et al. identified a heptapeptide (Thx, ARRPKLD) by in vitro phage display technology; Thx peptides showed significant binding to a variety of lung cancer cell lines, but did not bind to control cell lines. Zhang et al. used the in vitro screening of phage display peptide library in fresh isolated gastric cancer tissue samples and found an AAD peptide (AADNAB KSFPV) that specifically binds to gastric cancer. AAD peptide can specifically bind to gastric cancer BCG823 cells and gastric cancer tissues, but does not bind to normal gastric mucosa and other tumor tissues (such as breast cancer and colon cancer, etc.).

 Some tumor-specific homing peptides：

Tumor homing peptide that can penetrate cell membrane

Tumor homing peptide can effectively gather more drugs to tumor tissue. In order to have a therapeutic effect, the drug also needs to enter tumor cells. Most chemical drugs can only penetrate a distance of 3 to 5 cell diameters and enter the extravascular tumor tissue. Most tumor homing peptides have tumor homing ability but no cell penetration ability, so the bound drugs cannot be loaded into tumor tissue.

Researchers have discovered a new type of tumor homing peptide. These peptides can further penetrate tumor blood vessels and interstitium based on the targeting of tumor homing, so that the drugs contained in them can effectively enter tumor cells to exert therapeutic effects. . These peptides are called tumor homing penetrating peptides. They often contain 3 independent modules: homing area, tissue penetration area and protease recognition site. The coordination of these three modules can ensure a multi-step, highly specific tumor homing and infiltration process.

The mechanism of tumor-homing penetrating peptides includes the following three steps: (1) After intravenous injection, the tumor-homing sequence allows the tumor-homing peptides to reach the target tumor blood vessels, and then bind to their respective receptors; (2) After the receptor is bound, the tumor homing peptide is processed by endogenous proteases to expose the CendR sequence at the C-terminal; (3) The active CendR sequence binds to the extravasated NRP-1 receptor, penetrates the tissue and associates with the remaining tumor then peptide enters the cell.

Compared with cell-penetrating peptides, tumor-homing peptides have increased tumor targeting properties and are more penetrating than ordinary tumor-homing peptides. These two characteristics make them useful in tumor-targeted drug delivery systems. more effective.

 Tumor homing peptides with cell penetrating properties

Application of tumor homing peptide

1. Tumor diagnosis

In recent years, a large number of studies have reported the combination of cyclic RGD peptide, NGR peptide and F3 peptide and other tumor homing peptides with various contrast agents (such as radionuclides, nanoparticles and near-infrared fluorescent dyes) to achieve tumor specificity Imaging diagnosis

The radionuclide-labeled tumor homing peptide consists of three parts: tumor homing peptide, radionuclide and chelating agent, and is measured by positron emission tomography (PET)/single photon emissioncomputed tomography technology ( SPECT) realizes tumor imaging. At present, 18F-Galacto-RGD and 99mTc-NC100692 have been successfully used in clinical practice. However, related studies have found that their half-life is very short and will be cleared from the blood soon.

Some studies further combine tumor homing peptides with multivalent compounds or nanoparticles to have many advantages. Nanoparticles increase the binding sites of tumor homing peptides, which can achieve multiple functions such as targeting, imaging and treatment, and also enable tumor homing peptides to be combined with a variety of contrast agents to achieve multimodal imaging. For example, some nanoparticles (including perfluorocarbons, microbubbles, single-walled carbon nanotubes, liposomes, super paramagnetic iron oxide nanoparticles (SPIONPs), gold nanoparticles, and paramagnetic quantum dots (pQDs) can be combined with tumor homing peptides for tumor imaging.

Among them, magnetic nanoparticles (such as SPIO NPs and pQDs) are combined with RGD peptides to achieve in vivo tumor imaging through magnetic resonance imaging (MRI) technology.

2. Tumor treatment

In order to achieve precise tumor targeting, increase tumor target accumulation, and improve tumor treatment levels, tumor-targeted drug delivery systems have become a hot spot in current research. The tumor-homing properties of tumor-homing peptides enable it to be used as a targeting carrier in tumor-targeted drug delivery systems, combining cytotoxic drugs, liposomal drugs, therapeutic proteins, pro-apoptotic peptides, nanoparticles, nucleic acids and radioactivity. Specific delivery of nuclides and other substances to the tumor site to achieve more effective tumor targeted therapy

Reports have shown that by linking the cyclic RGD peptide with the fifth-generation polyamide-amine dendrimer and labeling it with fluorescein isothiocyanate, this composite tumor homing peptide can carry doxorubicin to treat integrins A variety of tumors overexpressing ave3. Since this cyclic RGD peptide-targeted polymer is water-soluble and can release doxorubicin stably, slowly and continuously, it can exert effective therapeutic effects while targeting tumors.

In addition, studies have shown that pH-sensitive nanoparticles modified with RGD peptide can enhance the therapeutic effect of paclitaxel contained therein, and their cytotoxicity, cell uptake capacity and tumor aggregation ability are stronger than those of the paclitaxel group without RGD peptide modification.

Compared with antibodies, peptides are smaller, can penetrate into tissues more effectively, and generally do not produce immune responses; compared with small molecules, peptides are more specific; in addition, the cost of preparing peptides is also lower. These advantages make tumor homing peptide an ideal targeting carrier in tumor targeted drug delivery systems, which can specifically deliver various contrast agents, drugs and nanoparticles to tumor blood vessels or lymphatic vessels, thereby achieving tumor diagnosis and such as monitoring to brought new hope for early tumor diagnosis and targeted therapy.

References

1.Zhang WJ，SuiYX，Budha A，eta1． Affinity peptide developed by phage Display selection for targeting gastric cancer[J]．WorldJGastroenterol，2012， 18f17)：2053-2060．

2.PasqualiniR，Koivunen E，Ruoslaht iE． Alpha vintegrinsas receptors for tumor

Targeting by circulating ligands[J]．Nat Biotechno.1997，15(6)：542-546．

3.Laakkonen P，Porkka K，HofmanJA， efa1．A tumor-homing peptide with

a targeting specificity related to lymphatic vessels[J]．NatMed，2002， 8(7)：751-755

4.Hu Q，Gao X，GuG，efa，．Glioma therapy using tumor homing and penetrating peptide-functionalized PEG-PLA nanoparticles loaded with paclitaxelLI．Biomaterials，2013,34(22)：5640—5650.