Selective targeting of tumor cells for cancer therapies

After their systematic delivery, several peptides enable the specific targeting of tumor and tumor-associated microenvironments as well as the tumor vasculature. These peptides are known as “tumor-specific internalizing peptides” (TSIPs) or “tumor homing peptides” (THPs).

Similar to anti-cancer peptides, tumor-specific internalizing peptides are usually short peptides in sequence lengths of 3 to 15 amino acids. These peptides specifically recognize and bind to tumor cells or tumor vasculature. Since 1998, in vitro and in vivo phage display technology has been utilized to identify many peptides. Phage display is a molecular biology technique in which proteins or peptides are displayed on the surface of phage as a fusion with one of the phage-coated proteins. Phage display has been used intensively for the screening for protein-protein interactions. This screening method allows the identification of tumor-specific or tumor homing peptides that target specific tumor cells or tumor vasculature.

According to the International Agency for Research on Cancer, an agency of the World Health Organization, cancer is now the world’s biggest killer. The “World Cancer Report” listed 8.2 million deaths from cancer in 2012 and predicts that cancer cases worldwide will rise by 75 % over the next two decades. By then, it is estimated that up to 25 million people may have cancer worldwide. Unfortunately, despite progress in our understanding of the molecular basis of cancer and improvements in treatment options, the mortality rate is still high. This suggests that the availability of new types, more selective drugs that fight cancer would greatly benefit humans.

Tumor-specific internalizing peptides or tumor homing peptides have common sequence motifs like RGD, or NGR, which specifically bind to a surface molecule on tumor cells or tumor vasculature. The best-known examples are the short peptides RGD and NGR. The RGD (Arg-Gly-Asp) peptide is known to bind α integrins, and NGR (Asn-Gly-Arg) binds to a receptor aminopeptidase N present on the surface of tumor endothelial cells, also called tumor angiogenic markers. It is no wonder that tumor-specific internalizing peptides are now used in cancer diagnosis and treatment. So far, many anti-cancer and imaging agents have been targeted to tumor sites in mice models by conjugation them with tumor-specific peptides.

A database called “TumorHoPe” provides comprehensive information about experimentally validated tumor homing peptides and their target cells (http://crdd.osdd.net/raghava/tumorhope/). The database is a manually curated database containing 744 entries of experimentally characterized tumor homing peptides that recognize tumor tissues and tumor-associated microenvironment, including tumor metastasis.

A list of some tumor homing peptide motifs

Models of tumor homing peptides

HN-1 Peptide

Isoforms of peptide ACDCRGDCFCG

The RGD sequence is the primary integrin recognition site in extracellular matrix proteins. Peptide containing the RGD sequence can mimic matrix proteins. A cyclic peptide ACDCRGDCFCG exists in two configurations depending on internal disulfide bonding. The peptide with the 1-4; 2-3 disulfide linkage is the most active.

Un et al. and others investigated the internalization of the HN-1TYR-anti-hRRM2 siRNAR peptide conjugate in human head and neck or breast cancer cells to establish its utility for targeted siRNA delivery into human cancer cells. The researchers investigated the specific internalization of peptides for targeted siRNA delivery to human tumor cells. The scientists used a FITC-HN-1TYR-anti-hRRM2 siRNA construct to image its successful internalization into a human cancer cell line. For the synthesis of the fluorescent siRNA delivery vehicle, FITC-HN-1TYR-anti-hRRM2 siRNAR, a tyrosine, and a FITC was added to the N-terminal end. Next, a synthetic anti-hRRM2 siRNA was synthesized with fluorine, incorporated at its 2’-OH position to avoid degradation by RNases in vivo, and conjugated to the 5’-end of the antisense strand using a hexynyl phosphoramidite linker.

The selected HN1 peptide, a 12mer peptide was isolated by peptide display library screening using a M13 phage library, containing the sequence TSPLNIHNGQKL. It can translocate drugs across the cell membrane into the cytosol, its uptake occurs in a tumor-specific manner, and it can penetrate solid tumors. Ribonucleotide Reductase (RR), composed of the subunits hRRM1 and hRRM2, catalyzes the conversion of ribonucleotides to their corresponding deoxy forms needed for DNA replication. The researchers choose an anti-hRRM2 siRNA to allow for the degradation of hRRM2’s mRNA to suppress tumorigenesis.

In summary, tumor-specific internalizing peptides or tumor homing peptides allow the design of future drug candidates for targeted siRNA delivery into human cancer cells, enabling a more selective treatment of tumors with fewer side effects.

References

Nuria Assa-Munt, Xin Jia, Pirjo Laakkonen, and Erkki Ruoslahti; Solution Structures and Integrin Binding Activities of an RGD Peptide with Two Isomers. Biochemistry 2001, 40, 8, 2373–2378. [ACS]

Bao L, Gorin M.A, Zhang M, Ventura AC, Pomerantz WC, Meraver SD, Teknos TN, Mapp AK, Pan Q.  Preclinical development of a bifunctional cancer cell homing, PKCe-inhibitory peptide for the treatment of head and neck cancer. Cancer Res  2009; 69: 5829-5834. [PMC]

Dudas J, Idler C, Sprinzl G, Bernkop-Schnuerch A, Riechelmann H. Identification of HN-1-Peptide Target in Head and Neck Squamous Cell Carcinoma Cells. ISRN Oncol. 2011;2011:140316. [PMC]

Hong FD, Clayman GL: Isolation of a peptide for targeted drug delivery into human head and neck solid tumors. Cancer Res 60: 6551-6556, 2000. [Cancer Research]

Kapoor P, Singh H, Gautam A, Chaudhary K, Kumar R, et al. (2012); TumorHoPe: A Database of Tumor Homing Peptides. PLoS ONE 7(4): e35187. [PLOS ONE]

Li R, Wang Y, Du J, Wang X, Duan A, Gao R, Liu J, Li B. Graphene oxide loaded with tumor-targeted peptide and anti-cancer drugs for cancer target therapy.   Sci Rep. 2021; 11: 1725. [Nature] 

Potala S, Verma RS.  Targeting head and neck squamous cell carcinoma using a novel fusion toxin-diphtheria toxin/HN-1. Mol Biol Rep  2010; 38: 1369-1397.   [PubMed]

FRANK UN, BINGSEN ZHOU and YUN YEN; The Utility of Tumor-specifically Internalizing Peptides for Targeted siRNA Delivery into Human Solid Tumors. ANTICANCER RESEARCH 32: 4685-4690 (2012).

Wang Y, Wan G, Li Z, Shi S, Chen B, Li C, Zhang L, Wang YS.  PEGylated doxorubicin nanoparticles mediated by HN-1 peptide for targeted treatment of oral squamous cell carcinoma. Int J Pharm.  2017; 525: 21–31 [PubMed]

Wright CL, Pan Q, Knopp MV, Tweedle MF.  Advancing theranostics with tumor-targeting peptides for precision otolaryngology.  World J Otorhinolaryngol Head Neck Surg.   2016; 2: 98-108.  [PMC]

Zheng X. Specificity and feasibility of HN-5 peptide for diagnosis and targeted therapy of head and neck squamous cell carcinomas.  Thesis, The University of Texas Health Science Center at San Antonio., 2007. 

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