Drug Designing: Open Access

Drug Designing: Open Access
Open Access

ISSN: 2169-0138

Review Article - (2016) Volume 5, Issue 3

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Design and Modification of Anticancer Peptides

Cuihua Hu1,3, Xiaolong Chen1,3, Wencai Zhao1,3, Yuxin Chen1,2,3 and Yibing Huang1,2,3*
1Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education, Jilin University, Changchun, China, E-mail: huangyibing@jlu.edu.cn
2National Engineering Laboratory for AIDS Vaccine, Jilin University, Changchun, China, E-mail: huangyibing@jlu.edu.cn
3College of Life Sciences, Jilin University, Changchun, China, E-mail: huangyibing@jlu.edu.cn
*Corresponding Author: Yibing Huang, Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education, College of Life Sciences, Jilin University, 2699 Qianjin Street, Changchun, China, Tel: 86-431-8515-5245 Email:

Abstract

Cancer is a great concern in the public health and development of novel therapeutic agent or strategy become urgently. Anticancer peptides (ACPs) have become promising anticancer drug candidate molecules due to their several extraordinary properties, such as small size, high activity, low immunogenicity, good biocompatibility, diversity of sequence and more modification sites for the functional molecules. However, the low stability and short half-life of peptides are the major barriers for the application. In this review, we focus on the methods of peptide design and modification to improve the stability, half-life time and specificity of ACPs, which including amino acid substitution, cyclization, hybridization, fragmentization, modification of C- and N-terminal of peptide by polymer or targeting molecules, etc.

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Keywords: Anticancer peptide; Design and modification; Tumor homing peptide; Polymer

Introduction

Cancer is a great concern in the public health and development of novel therapeutic agent or strategy become urgently. Compared to other small organic molecules and protein, anticancer peptides (ACPs) have several extraordinary properties, such as small size, high activity, low immunogenicity, good biocompatibility, diversity of sequence and more modification sites for the functional molecules. Thus, ACPs have become promising anticancer drug candidate molecules.

According to the biological effects of ACPs, they can be classified into three groups: inhibitory activity, necrosis activity and apoptosis activity. For the inhibitor activity, most peptides are cellular adhesion molecules such as RGD or YIGSR, etc. and come from the common conservative sequence of basement membrane glypeotein of laminin and fibronectin, etc. [1,2]. They can target on the integrin receptors on the surface of cancer cells and inhibit the migration and metastasis. Thus, they can also be used as targeting molecules for drug designing. For the necrosis activity, most peptides are membrane activity peptides, such as lytic peptide or derived from the antimicrobial peptides (AMPs). In the first, they can quickly bind to the highly negative charged cancer cell membrane by the electrostatic interactions, then destabilize and disrupt the integrity of cell membrane through the hydrophobic interactions leading to necrosis of cancer cells [3-5]. For the apoptosis activity, as we know, most tumor cells resist apoptosis due to a deregulation of pro- and anti-apoptotic proteins, however, partly ACPs can result to the releasing of cytochrome c (Cyt c) and lead to the apoptosis of cancer cells by the permeation and swelling of mitochondria membrane if ACPs internalized inside eukaryotic cells. Briefly, the releasing of Cyt c from damaged mitochondria induces Apaf-1 oligomerization, caspase 9 activation and subsequently the conversion of pro-caspase 3 to caspase 3, which is responsible for many of the hallmarks of apoptotic symptoms [6-8].

The peptide therapeutics is a promising field against different diseases. However, the low stability and short half-life of peptides are the major barriers for the application because most peptides are usually prone to degradation by various proteases in vivo. In this review, we focus on the methods of peptide designing and modification to improve the stability, half-life time and specificity of ACPs, which including amino acid substitution, cyclization, hybridization, fragementization, modification of C- and N-terminal of peptide by polymer or targeting molecules, etc. The schematic diagram of major design and modification strategies of ACPs are shown in Figure 1.

drug-designing-modification-strategies

Figure 1: The schematic diagram of design and modification strategies of ACPs.

Design of ACPs

Up to now, more than 2750 AMPs have been found and 199 peptides exhibited anticancer activities (named ACPs), see the website: http://aps.unmc.edu/AP/main.php. Some ACPs come from the AMPs and exhibit similar properties, such as net positive charge, amphipathic conformation, mechanism, etc. Thus, the method for the design of AMP also can be used for the design of ACP, including amino acid substitution, cyclization, hybridization, fragementization, etc. Based on these methods, the activity, stability of ACP can be improved.

Amino acid substitution

As we know, most anticancer peptides are membrane activity peptides, particularly α -helical cationic anticancer peptides and the net charge, hydrophobicity and helicity usually affect their activities [9,10]. Several researches have been reported that amino acid substitution is a useful tool to improve the activity and specificity of ACPs. Amino acid substitution cannot only change the biophysical parameter of ACP, but also influence the activity and stability of ACP, even alter the mechanism of action of ACP. Temproin-1CEa, a naturally occured α-helical cationic antimicrobial peptide, exhibits a potent anticancer activity and a moderate hemolytic activity [11,12]. Yang et al. designed and synthesized six analogs of temporin-1CEa by reserving the amphipathicity levels and α-helical structural patterns, while changing their cationic property and hydrophobicity. The results indicated that increasing the cationicity while reserving the moderate hydrophobicity may be a strategy to improve the cytotoxicity against tumor cells and decrease the hemolytic activity [13]. Similarly, Alyteserin-2a, is an amphipathic α-helical cationic cell-penetrating peptide, which displays relatively low cytotoxic potency against A549 human non-small cell lung adenocarcinoma cells and low hemolytic activity against human erythrocytes [14]. Several alyteserin-2a analogs were designed, in which amino acids on the hydrophobic face of the helix were replaced by L-tryptophan and amino acids on the hydrophilic face were replaced by one or more L-lysine or D-lysine residues. The Trp-containing peptides display an increasing of cytotoxicity against tumor cells, but the hemolytic activity against human erythrocytes also increase in parallel [15].

A 26-residues amphipathic peptide of V13K can adapt to α-helical conformation in the hydrophobic environment [16]. In order to increase the hydrophobicity of V13K, we designed single Leusubstituted peptides A12L, A20L, double Leu-substituted peptides A12L/A20L and triple ones A12L/A20L/A23L by replacing alanine to leucine at corresponding position. Among of them, peptide of A12L/ A20L showed the strongest activity against HeLa cell line. Our data’s suggested that the hydrophobicity of peptide plays a crucial role in the action against cancer cells due to the necrotic-like membrane disruption mechanism [3]. In addition, helicity was systematically modulated by introducing D-amino acids to replace the original L-amino acids on the non-polar face or the polar face of the helix and the therapeutic index of A12L/A20L against HeLa cells was improved by 9-fold and 22-fold, respectively [17]. These data’s noted us needing to consider the balance of anticancer activity and hemolytic activity by modulation the hydrophobicity and helicity for the designing and modification of ACPs.

Due to the proteolytic degradation of nature peptide, D-amino acids substitution is a useful method to overcome this limitation and enhance peptide stability. NRC-03, acationic anticancer peptide from winter flounder, can cause the lysis of breast cancer and multiple myeloma cells and NRC-03-mediated cell death correlated with peptide binding to anionic molecules of cellular surface [18-20]. However, the effect may be limited because of its susceptibility to proteases. Hilchie et al. [19] designed a analog of NRC-03 by replacing all the original amino acids with the corresponding D-amino acids, named [D]-NRC-03, which exhibited higher anticancer activity than NRC-03 due to the stability against human serum or trypsin [21]. Except D-amino acid, other unnatural amino acids also have been used to modify the anticancer peptide to increase their stability or bioactivity [22-24]. Anticancer peptide of D(KLAKLAK)2 is a well-known pro-apoptotic peptide that locate to mitochondria and disrupt this organelle, also named D(KLA)2 or kla [25]. Horton designed a serious of analogues by exchanging the leucine residue with three different amino acids: phenylalanine (F), cyclohexyl-alanine (FX) or 6-carbon alkyl chain residue (Hex). The data showed that engineered peptides exhibit more toxicity than the parent compound against a wide variety of cancer cell lines, while their cell-type specificity are remained [26-28].

Besides affecting the activity and stability, amino acid substitutions may even alter the mechanism of action of anticancer peptides. Zhou et al. designed a mutant ZXR-2 (FKIGGFIKKLWRSLLA) based on ZXR- 1(FKIGGFIKKLWRSKLA) by replacing Lys to Leu at the 14th position. ZXR-1 is an anticancer peptide derived from a known anticancer peptide mauriporin [29]. It is interesting that ZXR-2 exhibits anticancer activity, but the mechanism is completely different. Briefly, ZXR-1 is a pro-apoptotic peptide, it can translocate into cells and target on the mitochondria, then induce cell apoptosis, while ZXR-2 is a lytic peptide that directly targets on the cell membranes and causes membrane lysis.

Fragmentization

Fragmentization is a strategy to obtain short bioactivity peptide from the bioactivity proteins or the long and complex bioactive peptides. Short peptide has several advantages such as low immunity, easy synthesis, low cost, etc. Here, we divide the fragmentization method into two different types.

The first type is the bioactivity peptide coming from a bioactive protein. For example HPRP-A1 is derived from the N-terminus of ribosomal protein L1 of Helicobacter pylori and exhibited a broad spectrum anticancer activity [30]. AFP is a safe, naturally occurs human protein produced during pregnancy, which has anti-estrogenic and anti-breast cancer activity [31]. The oncostatic activity is localized to an eight amino acid sequence (amino acids 472–479) in domain IIIB. Based on this, a linear peptide of EMTPVNPG was synthesized [32] and then a series of analogues were designed. Among them, the activity of cyclo[EMTOVNOGQ] had been proved [33]. BH3 proteins promote apoptosis through anti-sequestration or direct activation of apoptosis effectors [34,35]. Liu et al. designed an artificial 18-mer BCL- 2 homology 3 peptide (ABH3) via charge tuning and conformation constraining based on original BH3 sequence. The data indicated that it induces cell death in an apoptosis-independent manner through the lytic properties of the peptide that causes disruption of cell membrane [36].

The second type is truncation from the long and complex bioactive peptides [37]. Buforin IIb (RAGLQFPVG[RLLR]3), a histone H2Aderived peptide, has strong cell penetrating ability and anticancer activity against various cancer cell lines but also showed cytotoxicity against normal cells at high concentrations [38]. By stepwise elimination of the C-terminal regular α-helical motif RLLR repeats motif of buforin IIb, Lim et al. designed several peptides consisted of different numbers of motif RLLR and named BR1 (RAGLQFPVGRLLR) and BR2 (RAGLQFPVGRLLRRLLR). The data indicated that BR2 can efficiently internalize into various cancer cell lines without cytotoxicity against normal cells [39]. Cecropin B is an amphipathic polycationic peptide; the signal sequence is located at N-terminus [40]. CB1a was constructed by repeating the N-terminal ten amino acids of CB three times and including a hinge near C-terminus. Compared to CB, CB1a has been demonstrated promising activity against several cancer cells but low toxicity against non-cancer cells and has become a promising anticancer agent [41].

Hybridization

Peptide-peptide hybrid is an important strategy for the modification of ACPs. In the design of AMPs, hybridization is a very common method to improve the activity, reduce the toxicity, etc. [42,43]. In the design of ACPs, most hybrid peptides usually combine cell penetrating peptide or targeting peptide with a lytic peptide or pro-apoptotic peptide, to improve the activity, stability or selectivity of ACP. Here we focus on the modification of cell penetrating peptide, the modification of targeting peptide will be reviewed in next part of targeting modification.

In our previously study, we constructed of a new hybrid peptide, HPRP-A1-TAT(FKKLKKLFSKLWNWKRKKRRQRRR), comprising the cell-permeating peptide TAT(RKKRRQRRR) [44] linked to the C-terminus of HPRP-A1(FKKLKKLFSKLWNWK). Compared to HPRP-A1, HPRP-A1-TAT exhibited stronger anticancer activity and higher therapeutic index [30]. It is interesting that the cellular concentration of HPRP-A1-TAT was higher than that of HPRP-A1 after 24 h incubation with HeLa cells. We suggested that TAT protects HPRP-A1 against degradation, maybe attribute to its high number of positively charged amino acids or the further release of peptides into cancer cells from endocytotic vesicles [30]. Another example of hybrid peptide is r7-kla (D forms). Benedict Law et al. incorporated the mitochondrial membrane disrupting peptide of kla with a cellpenetrating domain of r7. As we know, kla cannot easily penetrate through the cell membrane, r7 as a delivery vector can increase the membrane-crossing ability of kla. Thus, the hybrid peptide of r7-kla showed stronger cellular uptake rate and stability due to the resistant ability to protease digestion and resulted to more cells apoptosis [45].

In addition, the hybrid peptide has also been applied in intravital imaging. Ts (SKKPVPIIY CNRRSGKCQRM) is a mammalian free cell membrane-penetrating peptide [46] and Pc1 (CIRTPKISKPIKFELSG) is a αvβ3-binding peptide [47]. Yan et al. linked Ts to Pc1, and created a hybrid peptide, PTS. Then the hybrid was labeled with an FITC or Cy5.5 as an imaging indicator to evaluate its in vitro and in vivo bioactivity [48].

Except the cell penetrating peptides modification, the hybrid of different AMPs also has been reported. Cecropin A-magainin 2 and cecropin A-melittin hybrid peptides also have been designed and synthesized by Shin and examined the relationships between structure and biological activity [42]. The results suggested that hybridization is a useful method for the design of ACP and the activity of ACP is closed to the structure.

Cyclization

Besides enhancing the biological stability of peptides, cyclization can also stabilize the conformation suitable for better binding to other sites and improved biological activity of ACPs [49,50]. There are two main forms of peptide cyclization, cyclization by the formation of the amide bond between the N-terminal and the C-terminal amino acid residues and cyclizations involving the side chains of individual amino acids [51,52].

Cyclization by the formation of amid bond between the N-terminal and C-terminal amino acids can be also called head-to-tail cyclization [51] or backbone to backbone cyclization [52]. Tørfoss et al. discovered a series of synthetic anticancer heptapeptides (H-KKWβ2,2WKK-NH2) containing a central achiral and lipophilic β2,2-amino acid, which showed high proteolytic stability but low toxicity against normal cells [53]. They further prepared a series of seven to five residue cyclic peptides containing the two most promising β2,2-amino acid derivatives as part of the central lipophilic core and proved that a considerable increase in anticancer potency following head-to-tail peptide cyclization [54].

Conotoxins are disulfide-rich peptides from the venoms of marine cone snail. Some kind of conotoxins can specifically target different subtypes of nicotinic acetylcholine receptors [55] and have very promising anticancer potential [56]. However, they are susceptibility to degradation by proteases. Clark et al. used (Native chemical ligation) NCL to synthesize a range of cyclic conotoxin analogues for evaluation as potential drug leads [57].

TAT is a well-known cell-penetrating peptide. Oh et al. [58] compared the activities and conformations between cyclic TAT and linear TAT. They found that cyclic TAT transduces with higher efficiency than linear TAT and the guanidinium groups are more distant in cyclic TAT meanwhile guanidinium group separation enhances uptake kinetics [59]. Conibear et al. examined three different cyclization approaches using a tumor homing peptide epitope of LyP1 by replacing the disulfide bond with a stable triazole or fluorobenzene ring or grafting it into a h-defensin or cyclotide scaffold. Although these analogs are not as active as expected, but their study highlights the potential of the cyclic cystine ladder and cyclic cystine knotmotifs as stable and versatile peptide scaffolds [60].

Modification Strategy

Targeting modification

Based on the combine peptide library and phage display technology, hundreds of targeting peptides have been identified to specifically distinguish many types of cancer cells and tumor angiogenesis, named Tumor Homing Peptide (THP). These THPs can also be used to delivery chemical agents to the cancer site while with low affinity against the normal cells [61]. Here, we divide the THPs into three different types. The first one, the THPs are tumor specific tumor homing peptides, such as TCP-1, it can specifically target towards the vasculature of orthotropic colorectal cancer [62]. The secondly one, the THPs are tumor independent tumor homing peptides, such as RGD- 4C, NGR, Lyp-1, TMTP1, etc., they can target to the common antigenic markers with broad spectrum [63]. The thirdly one, the THPs not only have homing capability, but also have cell penetrating ability, therefore they can deliver the drug into the depth of tumor tissue, for example, iRGD [64]. As shown above, anticancer peptides (ACPs) are potential candidate drug for cancer treatment, but the drawback of less selectivity toward tumor cells is still the major challenge for clinical use [65]. Based on this problem, many researches have been done by conjugating a THP on the N-terminal or C-terminal of peptide to increase the specificity of ACPs [63], including pro-apoptotic peptide D(KLA)2, Tachyplesin and ATAP, etc. and membrane activity peptide, such as Magainin 2(MG2). In this way, the THP mainly can improve the specificity of ACPs against cancer cell lines and reduce the toxicity of ACPs against normal cells.

RGD

The first isolated THP was RGD (Arg-Gly-Asp) peptide, which was identified by in vivo phage display in tumor bearing mice in 1997. The RGD peptide has high affinity to the alpha v integrin’s when intravenously injected into tumor-bearing mice [66]. Based on the peptide of RGD, a targeting hybrid peptide was synthesized which compose a RGD and an anticancer peptide of Tachyplesin, which is present in leukocytes of the horseshoe crab. The results in vitro showed that RGD-tachyplesin can inhibite the proliferation of both cultured tumor and endothelial cells and reduce the colony formation of TSU prostate cancer cells. Meanwhile, RGD-tachyplesin can induce apoptosis through both mitochondrial and Fas-dependent pathways. The studies in vivo also indicated that the RGD-tachyplesin can inhibit the growth of tumors on the chorioallantoic membranes of chicken embryos and in syngenic mice [67].

NGR

Ellerby et al. [68] also synthesized both CNGRC and ACDCRGDCFC (RGD-4C) conjugated with pro-apoptotic peptide of D(KLA)2. The two homing peptide CNGRC and RGD-4C guided the peptide targeting to cancer cell and internalization. Then the peptide of D(KLA)2, can induce apoptosis by disruption of mitochondrial membranes. Smolarczyk et al. [69] also examined the therapeutic effect of peptide RGD-4C-GGD (KLA)2. The results indicated that the peptide can induce apoptosis in B16 (F10) cell line in vitro and inhibit the tumor growth in vivo by intratumoral administration. It is interesting that the tumor growth is faster and the animals die off after the administration termination.

iRGD

Sugahara et al. identified a novel tumor-penetrating peptide of iRGD (CRGDK/RGPD/EC) [64]. iRGD contains an internalizing RGD sequence, which can bind to αv integrins that are specifically expressed on the endothelium cells of tumor vessels. Then it can be proteolytically cleaved to expose the CRGDK/R sequence on the C-terminal, named C-end Rule (CendR) motif (R/KXXR/K), which has the high affinity to the receptor of neuropilin-1 (NRP-1) and triggers tissue penetration [70]. iRGD has been extensively used as a targeting delivery and penetration tools for nanoparticles [71], peptides [72], monoclonal antibody [73], etc., by chemical conjugated or co-administration [70]. iRGD is one of the most widely used THPs to modify many kind of ACPs, like apoptotic peptide D(KLA)2, ATAP, CDD, Thymopentin, TP5, etc.

ATAP derived from Bfl-1, is an amphipathic α-helix peptide. It can target on mitochondria and induce caspase-dependent apoptosis [74]. De [72] conjugated the iRGD sequence of CRGDKGPDC to the carboxyl-terminal of ATAP and the results indicated that ATAPiRGD can penetrate into cancer cells and distribute the mitochondria network. Meanwhile, the peptide of ATAP-iRGD can also induce apoptosis through release of Cyt c on DU145 cells. In addition, ATAPiRGD- M8 also suppressed tumor growth on DU 145 and PC-3 prostatic cancer xenograft model by intravenous administration.

Wang et al. also modified a variant of D(KLAKLA)2 to D(KLAKLAKKLAKLA)LK, named m(KLA) by iRGD. As we know, the -D(A)K- amino acid bond is a unique substrate of Cathepsin B (CTSB), which is overexpressed in the cytoplasm of tumors and in human tumor-associated cells [76]. The confocal microscope results indicated that iRGD can guide m(KLA) enter into the αvβ3 and NRP1 positive cells, such as MDA-MB-231, SKBR3 and 4T1 after administrated for 2 h and induce apoptosis through both mitochondrial pathway and the death receptor pathway, whereas it cannot enter into the NRP1-negative B16 cells. Furthermore, m(KLA)-iRGD spread extensively within the tumor tissue when it was injected into 4T1 tumor bearing mice. The m(KLA)-iRGD peptide reduction in tumor volume (P<0.05) and the total inhibition of metastasis at the end of the treatment [75].

Thymopoietin pentapeptide (Thymopentin, TP5), is an active fragment of thymopoietin (residues 32-36, Arg-Lys-Asp-Val-Tyr) [77] and has been widely used as immunomodulatory for treating immune deficiency, cancer and infectious diseases. However, poor penetration into tumors limits the clinical use of TP5. In order to overcome this drawback, a homing peptide iRGD was introduced with the C-terminal of TP5 to form TP5-iRGD by Lao et al. [78]. The MTT result showed that TP5-iRGD exhibited high inhibition of cell growth activity against B16F10, MCF-7 and H460 cell line. Meanwhile, the test in vivo on B16 xenograft model also indicated that the TP5-iRGD can significantly inhibit the tumor growth than TP5.

Bit1is a 179-amino acid residues protein. Its C-terminus constitutes the catalytic domain and the N-terminus serves as a mitochondrial localization signal [79,80] and the N-terminal domain has been reported with apoptotic activity and defined cell death domain (CDD). A recombinant protein of iRGD-CDD has been also indicated to trigger the tumor cell death both in cultured tumor cell and the xenograft breast tumor in mice. Repeated treatment with iRGD-CDD strongly inhibited tumor growth, resulting in an average reduction of 77% in tumor volume [81].

T3

Lactaptin, which is a proteolytic fragment of human kappa-casein (residues 57–134) was found in human breast milk. It has been reported that lactaptin has the ability of reducing cell viability and inducing apoptosis in cultured tumor cells [82]. RL2 is the recombinant analogue of lactaptin that has the activity of inducing different of human cancer cells, such as MCF-7, MDA-MB-231, A549, HEP-2 and HA1 [83]. More efforts have been made to obtain the targeting properties of lactaptin. Recently, Nemudraya et al. [84] reported that two targeting peptides of T3(YTYDPWLIFPAN) and iRGD had the specificity for cancer cell and penetration to targeted cancer cell and tumor tissue. They were used to construct the fusion proteins T3-RL2, RL-iRGD-His and RL2-iRGD and evaluate the activity both in vitro and in vivo. The results in vitro showed that the fusion proteins exhibited higher activity than RL2 in MDA-MB-231 and MCF-7 cell lines. The data in vivo also indicated that T3-RL2 protein significantly inhibit tumor growth in a MDA-MB-231 xenograft model compared with RL2.

IL-4Rα ligand

High-affinity interleukin-4 receptor α(IL-4Rα) is highly expressed on the cell surface of various human solid tumors, for example renal cell carcinoma, melanoma, breast carcinoma, ovarian carcinoma, glioblastoma, AIDS-related Kaposi’s sarcoma and head and neck squamous cell carcinoma [85-90]. Recently, a hybrid peptide of IL-4Rα-lytic peptide containing a target peptide targeted to IL-4Rα(KQLIRFLKRLDRNG) and a lytic peptide of KLLLKLLKKLLKLLKKK-OH (bold letters are D-amino acids) has been designed by Yang et al. [91]. The data indicated that the expression levels of IL4Ra in cells were correlated well with IC50 ratio of lytic peptide to IL-4Rα–lytic peptide.

The authors suggested that the increasing of cytotoxic activity can be attributed to the targeting (IL-4Rα) moiety of hybrid peptide. The results in vivo also indicate that the IL-4Rα–lytic hybrid peptide can selectively target to cancer cells and inhibit the tumor growth in the BXPC-3 and MDA-MB-231 xenograft model.

z13

An endometriosis targeting peptide of z13, which displayed the sequence VRRADNRPG, was from T7 phage-based library. z13 can strongly bind with the targeted protein CNGB3 in endometriosis. Sugihara et al. [92] linked z13 to an 18-mer pro-apoptotic peptide, KLAKLAKKLAKLAKKLAK, abbreviated dKLAK (D forms) or a variant peptide of HLAH by replacing lysine with histidine to form hybrid peptide of dKLAK-z13 or HLAH-z13, respectively. The effects of mixture of dKLAK-z13 and HLAH-z13 peptides on baboon endometriosis models in vivo by co-administered were investigated. The results indicated that cells in lesions selectively underwent apoptosis with no effect on neighboring organs and presented a strategy that could be useful to treat peritoneal endometriosis in humans.

EGFR ligand

Oesophageal squamous cell carcinoma (OSCC) is a major histologic type of oesophageal cancer [93] and the key therapy against advanced or metastatic OSCC is chemotherapy [94,95]. Epidermal growth factor receptor (EGFR) is over expressed in OSCC tissues about 71%-88% [96]. Recently, a hybrid peptide as a new agent of EGFRtargeting therapy was designed and synthesized, which contains an EGFR-targeting peptide of YHWYGYTPQNVI and a lytic peptide of KLLLKLLKKLLKLLKKK-OH (bold letters are D-amino acids) [97]. The hybrid peptide could kill EGFR-expressing cells through the combined process of specific binding to EGFR on the cell surface and subsequently disintegrate cell membranes.

TCP-1

TCP-1 (CTPSPFSHC) is a peptide that could target the vasculature of orthotropic colorectal cancer identified by the in vivo phage display technology [62]. The data demonstrated that TCP-1 also recognizes the blood vessels of human colorectal cancer and can be used to deliver fluorescein and the pro-apoptotic peptide. Li et al. [62] conjugated TCP- 1 to the N-terminal of the pro-apoptosis of D(KLA)2 with the double glycine as linker. The results showed that the TCP-1 peptide can guide D(KLA)2 entering into the cancer cell and inducing apoptosis through caspase 3 pathway both by chemical conjugate and uncoupled mixture.

TMTP-1

A 5-amino acid residues peptide of TMTP1 has been reported that it can bind to a series of highly metastatic cancer cell lines both in vitro and in vivo, particularly those from atypical liver micrometasteses that contained small number of neoplastic cells [98]. Ma et al. [99] coupled TMTP1 to the pro-apoptosis peptide D(KLA)2 and named as TMTP-1- DKK. The data in vitro indicated that TMTP1-DKK could trigger rapid apoptosis in human prostate and gastric cancer cells through both the mitochondrial- induced apoptosis pathway and the death receptor pathway. Furthermore, direct injection of TMTP1-DKK into mice with prostate and gastric xenograft cancers also resulted in decreasing of tumor volumes and a significant delay in tumor progression and metastasis in vivo [99].

BRBP1

A linear dodecapeptide peptide of BRBP1 (MYPWTEPSYLSN) was identified through random peptide phage display bio-panning and can against preferentially bind to the human “brain-seeking” breast carcinoma cells (231-BR cells) in a concentration- dependent and energy dependent manner in vitro [100]. Fu et al. [101] designed and synthesized a new hybrid peptide of BRBP1-TAT-KLA, which containing three elements: a brain metastatic breast carcinoma cell (231-BR)-binding peptide BRBP1, a cell penetrating peptide TAT and a pro-apoptotic peptide KLA. In their study, the antitumor activity of BRBP1-TAT-KLA on brain metastatic breast cancer both in vitro and in vivo was evaluated. And the results in vitro showed that the peptide of BRBP1- TAT-KLA efficiently internalized in 231-BR cells and consequently induced the membrane damage of mitochondrial and cellular apoptosis, significantly decreased cell viability and increased apoptotic ability. The results in vivo also demonstrated that BRBP1- TAT-KLA selectively homed to the tumors and induced cellular apoptosis, significantly delayed tumor growth and enhanced antitumor selectivity while without significant toxicity on non-tumor tissues.

Bld-1

A Bld-1 peptide of CSNRDARRC, which binds to bladder tumor cell, has been identified by phage display [102]. Jung et al. combined the tumor targeting peptide Bld-1 with a pro-apoptosis D(KLA)2 and formed a hybrid peptide of Bld-1-kla. The hybrid peptide of Bld-1- kla can selectively bind to HT1376 bladder tumor cells and efficiently internalize into cells but not to other tumors or normal cells, exhibiting higher apoptotic ability than Bld-1 or kla alone. It is interesting that the binding and cytotoxicity of Bld-1-kla was inhibited when pretreated HT1376 cells with Bld-1. The data of confocal microscope showed the peptide localized in the mitochondrial. The test in vivo on tumorbearing mice got the same results that the peptide of Bld-1-kla induced apoptosis of tumor cells and inhibited tumor growth more efficiently than the peptide of kla [103].

LTV, GR and Bombesin

Based on phage peptide libraries, many other cancer-cell targeting peptides have been identified and used for the diagnosis and treatment of tumors. The LTVSPWY peptide (LTV peptide) is a targeting peptide. It can deliver antisense oligonucleotides and small molecules to cancer cells in vitro [104,105]. Furthermore, various fusion proteins carrying the LTV peptide have been designed to target cancer cells in vitro and in vivo [106]. A gastrin-releasing peptide (GNHWAVGHLM, GR peptide) also is a targeting moiety, whose receptor is expressed by most cancer types, including breast, ovarian, prostate and lung cancer [107,108]. Bombesin is a 14-amino acid tumor homing peptide isolated from frog skin [109]. Its receptors are overexpressed in a variety of common human cancers, such as neuroblastoma and small cell lung cancer, as well as cancers of the prostate, kidney, uterus, ovary, breast, pancreas, gastrointestinal tract, head and neck, and esophagus [110]. Liu et al. [111] reported that by attaching magainin 2(MG2) to the N-terminus of Bombesin, the anticancer effect of the hybrid peptide of MG2B increase 10 times over than unconjugated MG2 in vitro. While MG2B also exhibited higher antitumor effects in mice bearing MCF-7 tumor grafts [111].

As shown above, THPs can increase the anticancer activity and selectivity of the ACPs while without change the secondary structure or charges, some of them can help more ACPs enter into the cancer cells and induce cell apoptosis through interrupting mitochondrial membranes. These results suggested that chemically coupled of targeting peptide with the pro-apoptosis peptide or others anticancer drugs is a promising therapeutic strategy for targeted therapy of cancers. We believe that the targeting modification of ACPs will be having a very beautiful future in clinical use. The targeting modifications of ACPs are listed in Table 1.

Homing peptide Anticancer Peptide Cancer Type Reference
  RGD   Tachyplesin TSU prostatic cancer B16 melanoma tumor   [66,67]
RGD D(KLA)2 MDA-MB-435 tumor [68]
RGD-4C D(KLA)2 B16F10 melanoma tumor [69]
iRGD ATAP DU145, PC-3 [72,74]
  iRGD   m(KLA) MDA-MB-231, SKBR3, and 4T1 breast tumor   [75,76]
iRGD TP5 B16F10 melanoma tumor [77,78]
iRGD CDD MCF- 10CA1a, 4T1 tumor [79-81]
T3 RL2 MDA-MB-231, MCF-7 [82-84]
IL-4Rα ligand KQLIRFLKRLDRNG   D(KLA)2   BXPC-3 and MDA-MB-231   [91]
z13 dKLAK baboon endometriosis models [92]
EGFR ligand YHWYGYTPQNVI KLLLKLLKKLLK LLKKK-OH human K-ras mutation negative and positive cancers   [94,95]
TCP-1 D(KLA)2 Orthotopic Colorectal cancer [62]
  TMTP-1   D(KLA)2 PC-3M-1E8 prostate and MKN-45sci gastric   [98,99]
  BRBP-1   D(KLA)2 231-BR brain metastatic breast cancer   [100,101]
Bld-1 D(KLA)2 HT1376 bladder tumor [102,103]
LTV&GR D(KLA)2 MCF-7 and MDA-MB-231 [104-106]
Bombesin magainin II MCF-7 [111]

Table 1: The list of targeting modification of ACPs.

Polymer modification

Polymer is consisting of one or several units, having high molecular weight connected by covalent bond. Polymer generally possesses some special characteristics, such as negligible toxicity, high biocompatibility and strong degradability. Many polymers have been used in drug delivery system to improve the stability and solubility of the drug, for example, PEG, carboxymethyl dextran (CMD), PLLA, etc. In addition, polymer modification also can used to design prodrug system.

Among all the kinds of polymers used for the modification of anticancer peptide, PEG was studied most frequently and thoroughly. It has been approved safety by FDA for food, drug delivery vehicle [112- 115]. PEGylated protein and peptide have many advantages, such as the hydrophilic characteristic of PEG moiety enabling to increase the ability of ACPs to escape from degradation of blood serum protein and making the peptides obtained larger molecular weight. In this part, several strategies for the modification of ACPs have been reviewed as follow:

The PEGylation of ACPs

PEGylated ACPs was first reported by Kawasaki [116]. In this study, a laminin related peptides of YIGSR was conjugated to PEG and formed YIGSRG-[amino-poly(ethylene glycol)] hybrid molecule. It showed a better stability and resistance to blood serum in vivo and exhibited the most potent inhibitory effect on the metastasis of B16 melanoma BL6. Then, Kawasaki [117] developed two novel different methods to conjugate PEG to ACPs which the NH2-PEG and COOH-PEG were successfully synthesized and attached to the amino or carboxyl groups of peptide, respectively. Then, NH2-PEG-COOH was also synthesized by Maeda’s [118-120] and introduced into SPPS, just like an ordinary amino acids. A bifunctional PEG polymer of PDSGR-aaPEG-YIGSR was built combining PDSGR and YIGSR together and exhibited higher inhibiting effect of tumor metastasis.

Similarly, Brinckerhoff et al. [121] also reported the PEGlytion of anticancer peptide MART - 1. It is predicted that immunogenic tumor peptides would have short half life time in vivo due to the degradation of peptidases in plasma. However, when an amino-PEG was attached to the C terminal of MART - 1 to form a MART-1 PEG hybrid, the stability of MART-1 was markedly prolonged; the immunogenicity of these peptides might also be enhanced by creating modifications that enhance stability.

Nisin is an amphiphylic peptide with 34 amino acid residues belonging to the family of lantibiotics and exhibites anticancer activity [122,123]. Guiotto et al. [124] reported that nisin with several shortcomings, including its low solubility in neutral aqueous solutions, instability at physiological pH and rapid breakdown by proteolytic enzymes. The PEG-nisin conjugate could protect nisin from degradation of enzymes and improve its solubility.

It must be noted that high molecular weight PEG moiety leads to a decrease of anticancer activity of peptide-PEG block polymer as the stability increase. For this reason, ACPs modified by short PEG chains was synthesized by Zhang et al. [125]. The studies in vivo and in vitro indicated that PEGylated peptide by short PEG chains exhibited a prolonged circulating life times in plasma without losing of bioactivity. Except the linear PEG, the branched PEG was also employed for the PEGylation of ACPs. Alpha-momorcharin (α-MMC), from a ribosome-inactivating protein (RIP) exhibited excellent cytotoxicity against tumor cells and strong immunogenicity and short half-life time in plasma. Bian et al. [126] synthesized PEGylated-α-MMC using a branched mPEG. The results indicated that the complex preserved moderate anticancer activity and a longer circulation time with 36% acute toxicity and at most 66% immunogenicity decrease.

Although PEGlation of anti-tumor peptide has been widely regarded as an effective method to enhance it stability and solubility, the specific mechanism of PEGlation is still unknown. Towards this goal, Kaneda et al. [127] studied the mechanism of PEGlation; they supposed that the improvement of anticancer activity of peptide can attribute to the prolongation of the half-life time of peptide in the blood.

Imura et al. [128,129] also investigated the biological activity of PEGylated tachayplesin 1, a membrane- acting β-sheet cyclic anticancer peptide. The results demonstrated that the PEGylation of tachayplesin 1 decreased the activity and the cytotoxicity in vitro and increased the specificity of peptide. It is interesting that compared with free peptide; PEGylation did not alter the basic mechanism of membranepermeabilizing. Furthermore, Imura and coworkers got the similarly results when they alter the β-sheet peptide of tachayplesin 1 to α-helical ACPs of magainin 2. These data also provide some useful information for the peptide designing, particularly for the design of prodrug.

Other polymers modification

Except the PEG, other ploymers also were used to modify the ACPs to improve the activity, stability and specificity, for example, Dextran, SMA, PVP, etc. In addition, several polymers have been used in peptide targeting delivery and controlled releasing research. Based on the modification, they can enhance the delivery of peptides to the target site and improve the therapeutic efficacy, while minimizing side effects [130-132]. Due to the enhanced permeability and retention effect (EPR), high molecular weight polymers and nano-sized particles prone accumulate in solid tumors with higher concentrations [133].

Dextran derivative of carboxymethyl dextran (CMD) is frequently used as a drug deliver carrier due to its low glomerular filtration rate and lower hepatic uptake [134,135]. An CMD-peptide conjugates of CMD-s-s-peptide was prepared through the disulfide bond between CMD and EGFRZR-lytic peptide by Gaowa et al. [136,137]. The obtained conjugate could be stimulate-responsive by GSH and release to lytic peptide. Compared to the free peptide, CMD-peptide conjugates were highly accumulated in tumor tissue and the attachment of CMD prolonged the elimination half-life and more effective anticancer activity of peptide after intravenous injection.

Mu et al. [138] modified YIGSR with SMA [poly (styrene comaleic anhydride)] to improve anti-tumor activity of YIGSR and the studies in vivo indicated that bio- conjugate SMA-YIGSR have a prolonged plasma half-time and higher binding affinity than merely YIGSR. Mu et al. [139] further modified YIGSR with Polyvinyl pyrrolidone (PVP). The results showed that the activity and half-life time in plasma of PVPconjugated YIGSR (PVP-YIGSR) was more than 100-fold greater and 15-fold longer than the free YIGSR, respectively.

Gelatin, which is usually obtained from collagen, has been extensively explored for its biocompatibility and biodegradation in the last few decades. Recently, the combination of antitumor hybrid peptide with anionic gelation was developed [137]. The electrospinning fabrication technique is emerging in biomedical application such as cancer therapies and wound healing treatments. Recently, a recombinant silkworm ACP of Bmattacin2 was used to load into PLLA nanofibrous membrane by this technique, PLLA [Poly (L-lactic Acid)] and Bmattacin2 conjugate was successfully prepared, which exhibited significant anti-tumor activity and a good compatibility with the normal cells [140].

Peptide Amphiphiles (PAs) is an amphiphilic molecule composed of alkyl chain and hydrophilic β-sheet forming sequence, which can deliver the AMPs to tumor cells effectively. Standley et al. [141] introduced a novel self-assembled nanoparticle that conjugated KLAK to Pas. It can kill the breast cancer cells not only by disrupting cell membrane, but also through the way of caspase independent and Bax/Bak independent apoptosis pathway. Zha et al. [142] further researched nanoparticle and they reported on the complex of hyaluronic acid and positively charged PA to bear KLAK peptide. Deng et al. [143] conjugated aliphatic acid of various lengths to anticancer peptide of B1, a novel ACPs derived from Cathelicidin-BF15. All results revealed that the modified ACP obtained a higher bioactivity towards tumor cells and indicated that the conjugated aliphatic acid enhanced hydrophobicity and helicity of peptides, which subsequently resulted in higher membrane-lyting capability of ACPs. Thus, this method is suitable for the modification of membrane activity ACPs.

In this part, we focus on the polymer modification of ACPs. From the above data, we think maybe PEGyation is mainly to increase the stability of ACPs, it suitable for the modification of cellular adhesion molecules such as RGD or YIGSR or proapoptotic peptide of D-(KLA)2. While the design of peptide amphiphiles (PAs) maybe a better choice for the modification of membrane activity ACPs because biophysical parameters play an important role for the biological activity of ACP.

Conclusion

In summary, peptide anticancer therapeutics is a promising field against cancer problems. The amino acid substitution, cyclization, hybridization, fragmentization, modification of ACPs are great potential methods and provide many advantages such as increased the half-life time in plasma, enhanced its stability and activity, reduced its toxicity, which could improve the therapeutic efficacy of ACPs. Of course, these methods do not exist lonely; they can be used to modify different ACPs or combined several methods to modify same ACP at the same time. We believe that ACPs as novel anticancer drugs will be play an important role for the clinical practices.

Conflict of Interest

The authors confirm that this article has no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 81373445 to Y.X.C.), the Natural Science Foundation of Jilin Province of China (No. 20150101189JC to Y.X.C., No. 20140101042JC to Y.B.H.).

References

  1. Ruoslahti E (1996) RGD and other recognition sequences for integrins. Annu Rev Cell Dev Biol 12: 697-715.
  2. Höpker VH, Shewan D, Tessier-Lavigne M, Poo M, Holt C (1999) Growth-cone attraction to netrin-1 is converted to repulsion by laminin-1. Nature 401: 69-73.
  3. Huang YB (2011) Studies on mechanism of action of anticancer peptides by modulation of hydrophobicity within a defined structural framework. Mol Cancer Ther 10: 416- 426.
  4. Huang Y, Feng Q, Yan Q, Hao X, Chen Y (2015) Alpha-helical cationic anticancer peptides: A promising candidate for novel anticancer drugs. Mini Rev Med Chem 15: 73-81.
  5. Hoskin DW, Ramamoorthy A (2008) Studies on anticancer activities of antimicrobial peptides. Biochim Biophys Acta 1778: 357-375.
  6. Hoskin DW, Ramamoorthy A (2008) Studies on anticancer activities of antimicrobial peptides. Biochim Biophys Acta 1778: 357-375.
  7. Mai JC, Mi Z, Kim SH, Ng B, Robbins PD (2001) A proapoptotic peptide for the treatment of solid tumors. Cancer Res 61: 7709-7712.
  8. Li H, Kolluri SK, Gu J, Dawson MI, Cao X, et al. (2000) Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3. Science 289: 1159-1164.
  9. Cory S, Adams JM (1998) Matters of life and death: Programmed cell death at Cold Spring Harbor. Biochim Biophys Acta 1377: R25-44.
  10. Huang Y, Yan Q, Hao X, Chen Y (2015) Alpha-helical cationic anticancer peptides promising candidate for novel anticancer drugs. Mini-Reviews in Medicinal Chemistry 15: 73-81.
  11. Papo N, Shai Y (2003) New lytic peptides based on the D,L-amphipathic helix motif preferentially kill tumor cells compared to normal cells. Biochemistry 42: 9346-9354.
  12. Shang D (2009) Molecular cloning of cDNAs encoding antimicrobial peptide precursors from the skin of the Chinese brown frog, Rana chensinensis. Zoolog Sci 26: 220-226.
  13. Wang C, Li HB, Li S, Tian LL, Shang DJ (2012) Antitumor effects and cell selectivity of temporin-1CEa, an antimicrobial peptide from the skin secretions of the Chinese brown frog (Rana chensinensis). Biochimie 94: 434-441.
  14. Yang QZ (2013) Design of potent, non-toxic anticancer peptides based on the structure of the antimicrobial peptide, temporin-1CEa. Archives of Pharmacal Research 36: 1302- 1310.
  15. Conlon JM, Mechkarska M, Arafat K, Attoub S, Sonnevend A (2012) Analogues of the frog skin peptide alyteserin-2a with enhanced antimicrobial activities against Gram-negative bacteria. J Pept Sci 18: 270-275.
  16. Conlon JM, Mechkarska M, Prajeep M, Arafat K, Zaric M, et al. (2013) Transformation of the naturally occurring frog skin peptide, alyteserin-2a into a potent, non-toxic anti-cancer agent. Amino Acids 44: 715-723.
  17. Chen Y (2005) Rational design of alpha-helical antimicrobial peptides with enhanced activities and specificity/therapeutic index. J Biol Chem 280: 12316-12329.
  18. Huang YB (2012) Role of helicity on the anticancer mechanism of action of cationic-helical peptides. International Journal of Molecular Sciences13: 6849-6862.
  19. Hilchie AL (2011) Pleurocidin-family cationic antimicrobial peptides are cytolytic for breast carcinoma cells and prevent growth of tumor xenografts. Breast Cancer Res13: R102.
  20. Hilchie AL (2013) Pleurocidin-family cationic antimicrobial peptides mediate lysis of multiple myeloma cells and impair the growth of multiple myeloma xenografts. Leuk Lymphoma 54: 2255-2262.
  21. Morash MG (2011) The zebrafish embryo as a tool for screening and characterizing pleurocidin host-defense peptides as anti-cancer agents. Disease Models & Mechanisms 4: 622-633.
  22. Hilchie AL(2015) Enhanced killing of breast cancer cells by a D-amino acid analog of the winter flounder-derived pleurocidin NRC-03. Exp Mol Pathol 99: 426-434.
  23. Soudy R (2011) Proteolytically stable cancer targeting peptides with high affinity for breast cancer cells. Journal of Medicinal Chemistry 54: 7523-7534.
  24. Wang S, Noberini R, Stebbins JL, Das S, Zhang Z, et al. (2013) Targeted delivery of paclitaxel to EphA2-expressing cancer cells. Clin Cancer Res 19: 128-137.
  25. Chu HL, Yip BS, Chen KH, Yu HY, Chih YH, et al. (2015) Novel antimicrobial peptides with high anticancer activity and selectivity. PLoS One 10: e0126390.
  26. Javadpour MM, Juban MM, Lo WC, Bishop SM, Alberty JB, et al. (1996) De novo antimicrobial peptides with low mammalian cell toxicity. J Med Chem 39: 3107-3113.
  27. Horton KL, Kelley SO (2009) Engineered apoptosis-inducing peptides with enhanced mitochondrial localization and potency. J Med Chem 52: 3293-3299.
  28. Almaaytah A (2013) Mauriporin, a novel cationic alpha-helical peptide with selective cytotoxic activity against prostate cancer cell lines from the venom of the scorpion Androctonus mauritanicus. International Journal of Peptide Research and Therapeutics 19: 281-293.
  29. Almaaytah A (2014) Antimicrobial and antibiofilm activity of mauriporin, a multifunctional scorpion venom peptide. International Journal of Peptide Research and Therapeutics 20: 397-408.
  30. Zhou XR, Zhang Q, Tian XB, Cao YM, Liu ZQ, et al. (2016) From a pro-apoptotic peptide to a lytic peptide: One single residue mutation. Biochim Biophys Acta 1858: 1914-1925.
  31. Hao X, Yan Q, Zhao J, Wang W, Huang Y, et al. (2015) TAT modification of alpha-helical anticancer peptides to improve specificity and efficacy. PLoS One 10: e0138911.
  32. Bennett JA (1998) Alpha-fetoprotein derived from a human hepatoma prevents growth of estrogen-dependent human breast cancer xenografts. Clin Cancer Res 4: 2877-2884.
  33. Mesfin FB (2000) Alpha-fetoprotein-derived antiestrotrophic octapeptide. Biochim Biophys Acta 1501: 33-43.
  34. Mesfin FB (2001) Development of a synthetic cyclized peptide derived from alpha-fetoprotein that prevents the growth of human breast cancer. J Pept Res 58: 246-56.
  35. Chen L (2005) Differential targeting of prosurvival Bcl-2 proteins by their BH3-only ligands allows complementary apoptotic function. Mol Cell 17: 393-403.
  36. Delbridge AR, Strasser A1 (2015) The BCL-2 protein family, BH3-mimetics and cancer therapy. Cell Death Differ 22: 1071-1080.
  37. Liu Q, Zhao H, Jiang Y, Wu M, Tian Y, et al. (2016) Development of a lytic peptide derived from BH3-only proteins. Cell Death Discov 2: 16008.
  38. Ren SX, Shen J, Cheng AS, Lu L, Chan RL, et al. (2013) FK-16 derived from the anticancer peptide LL-37 induces caspase-independent apoptosis and autophagic cell death in colon cancer cells. PLoS One 8: e63641.
  39. Lee HS, Park CB, Kim JM, Jang SA, Park IY, et al. (2008) Mechanism of anticancer activity of buforin IIb, a histone H2A-derived peptide. Cancer Lett 271: 47-55.
  40. Chen HM, Wang W, Smith D, Chan SC (1997) Effects of the anti-bacterial peptide cecropin B and its analogs, cecropins B-1 and B-2, on liposomes, bacteria, and cancer cells. Biochim Biophys Acta 1336: 171-179.
  41. Lim KJ, Sung BH, Shin JR, Lee YW, Kim DJ, et al. (2013) A cancer specific cell-penetrating peptide, BR2, for the efficient delivery of an scFv into cancer cells. PLoS One 8: e66084.
  42. Shin SY, Kang JH, Hahm KS (1999) Structure-antibacterial, antitumor and hemolytic activity relationships of cecropin A-magainin 2 and cecropin A-melittin hybrid peptides. J Pept Res 53: 82-90.
  43. Lee JK, Seo CH, Luchian T, Park Y (2015) Antimicrobial peptide CMA3 derived from the CA-MA hybrid peptide: Antibacterial and anti-inflammatory activities with low cytotoxicity and mechanism of action in Escherichia coli. Antimicrob Agents Chemother 60: 495-506.
  44. Heitz F, Morris MC, Divita G (2009) Twenty years of cell-penetrating peptides: From molecular mechanisms to therapeutics. Br J Pharmacol 157: 195-206.
  45. Law B (2006) A mitochondrial targeted fusion peptide exhibits remarkable cytotoxicity. Mol Cancer Ther 5: 1944-1949.
  46. Wu JM, Jan PS, Yu HC, Haung HY, Fang HJ, et al. (2009) Structure and function of a custom anticancer peptide, CB1a. Peptides 30: 839-848.
  47. Wu G, Wu H, Li L, Fan X, Ding J, et al. (2010) Membrane aggregation and perturbation induced by antimicrobial peptide of S-thanatin. Biochem Biophys Res Commun 395: 31-35.
  48. Yan X (2016) A hybrid peptide proteins that facilitates transmembrane delivery and its application for the rapid in vivo imaging via near-infrared fluorescence imaging. Frontiers in Pharmacology.
  49. Wu G, Wang X, Deng G, Wu L, Ju S, et al. (2011) Novel peptide targeting integrin αvβ3-rich tumor cells by magnetic resonance imaging. J Magn Reson Imaging 34: 395-402.
  50. Cemazar M, Kwon S, Mahatmanto T, Ravipati AS, Craik DJ (2012) Discovery and applications of disulfide-rich cyclic peptides. Curr Top Med Chem 12: 1534-1545.
  51. Katsara M, Tselios T, Deraos S, Deraos G, Matsoukas MT, et al. (2006) Round and round we go: Cyclic peptides in disease. Curr Med Chem 13: 2221-2232.
  52. Li P, Roller PP (2002) Cyclization strategies in peptide derived drug design. Curr Top Med Chem 2: 325-341.
  53. Roxin Á, Zheng G (2012) Flexible or fixed: A comparative review of linear and cyclic cancer-targeting peptides. Future Med Chem 4: 1601-1618.
  54. Tørfoss V (2012) Improved anticancer potency by head-to-tail cyclization of short cationic anticancer peptides containing a lipophilic ß2,2-amino acid. Journal of Peptide Science 18: 609-619.
  55. Azam L, McIntosh JM (2009) Alpha-conotoxins as pharmacological probes of nicotinic acetylcholine receptors. Acta Pharmacol Sin 30: 771-783.
  56. Dave K, Lahiry A (2012) Conotoxins: Review and docking studies to determine potentials of conotoxin as an anticancer drug molecule. Curr Top Med Chem 12: 845-851.
  57. Tørfoss V, Ausbacher D, Cavalcanti-Jacobsen Cde A, Hansen T, Brandsdal BO, et al. (2012) Synthesis of anticancer heptapeptides containing a unique lipophilic β(2,2)-amino acid building block. J Pept Sci 18: 170-176.
  58. Oh D (2014) Enhanced cellular uptake of short polyarginine peptides through fatty acylation and cyclization. Molecular Pharmaceutics 11: 2845-2854.
  59. Clark RJ, Akcan M, Kaas Q, Daly NL, Craik DJ (2012) Cyclization of conotoxins to improve their biopharmaceutical properties. Toxicon 59: 446-455.
  60. Laakkonen P, Porkka K, Hoffman JA, Ruoslahti E (2002) A tumor-homing peptide with a targeting specificity related to lymphatic vessels. Nat Med 8: 751-755.
  61. Deutscher SL (2010) Phage display in molecular imaging and diagnosis of cancer. Chem Rev 110: 3196-3211.
  62. Conibear AC, Chaousis S, Durek T, Rosengren KJ, et al. (2016) Approaches to the stabilization of bioactive epitopes by grafting and peptide cyclization. Biopolymers 106: 89-100.
  63. Gautam A (2014) Tumor homing peptides as molecular probes for cancer therapeutics, diagnostics and theranostics. Current Medicinal Chemistry 21: 2367-2391.
  64. Sugahara KN, Teesalu T, Karmali PP, Kotamraju VR, Agemy L, et al. (2009) Tissue-penetrating delivery of compounds and nanoparticles into tumors. Cancer Cell 16: 510-520.
  65. Gaspar D, Veiga AS, Castanho MA (2013) From antimicrobial to anticancer peptides. A review. Front Microbiol 4: 294.
  66. Li ZJ, Wu WK, Ng SS, Yu L, Li HT, et al. (2010) A novel peptide specifically targeting the vasculature of orthotopic colorectal cancer for imaging detection and drug delivery. J Control Release 148: 292-302.
  67. Chen Y, Xu X, Hong S, Chen J, Liu N, et al. (2001) RGD-tachyplesin inhibits tumor growth. Cancer Res 61: 2434-2438.
  68. Pasqualini R, Koivunen E, Ruoslahti E (1997) Alpha v integrins as receptors for tumor targeting by circulating ligands. Nat Biotechnol 15: 542-546.
  69. Smolarczyk R, CichoÅ T, Graja K, Hucz J, Sochanik A, et al. (2006) Antitumor effect of RGD-4C-GG-D(KLAKLAK)2 peptide in mouse B16(F10) melanoma model. Acta Biochim Pol 53: 801-805.
  70. Sugahara KN, Karmali TTPP, Kotamraju VR, Agemy L, Greenwald DR, et al. (2010) Co-administration of a tumor-penetrating peptide enhances the efficacy of cancer drugs. Science 328: 1031-1035.
  71. Li X (2016) Tumor vascular-targeted co-delivery of anti-angiogenesis and chemotherapeutic agents by mesoporous silica nanoparticle-based drug delivery system for synergetic therapy of tumor. Int J Nanomedicine11: 93-105.
  72. De G (2014) Amphipathic tail-anchoring peptide is a promising therapeutic agent for prostate cancer treatment. Oncotarget 5: 7734-7747.
  73. Sha H, Zou Z, Xin K, Bian X, Cai X, et al. (2015) Tumor-penetrating peptide fused EGFR single-domain antibody enhances cancer drug penetration into 3D multicellular spheroids and facilitates effective gastric cancer therapy. J Control Release 200: 188-200.
  74. Ko JK, Choi KH, Pan Z, Lin P, Weisleder N, et al. (2007) The tail-anchoring domain of Bfl1 and HCCS1 targets mitochondrial membrane permeability to induce apoptosis. J Cell Sci 120: 2912-2923.
  75. Ellerby HM, Arap W, Ellerby LM, Kain R, Andrusiak R, et al. (1999) Anti-cancer activity of targeted pro-apoptotic peptides. Nat Med 5: 1032-1038.
  76. Chan AT, Baba Y, Shima K, Nosho K, Chung DC, et al. (2010) Cathepsin B expression and survival in colon cancer: implications for molecular detection of neoplasia. Cancer Epidemiol Biomarkers Prev 19: 2777-2785.
  77. Goldstein G (1979) A synthetic pentapeptide with biological activity characteristic of the thymic hormone thymopoietin. Science 204: 1309-1310.
  78. Lao X (2014) Increased antitumor activity of tumor-specific peptide modified thymopentin. Biochimie 107: 277-285.
  79. Jan Y, Matter M, Pai JT, Chen YL, Pilch J, et al. (2004) A mitochondrial protein, Bit1, mediates apoptosis regulated by integrins and Groucho/TLE corepressors. Cell 116: 751-762.
  80. De Pereda JM (2004) Crystal structures of a human peptidyl-tRNA hydrolase reveals a new fold and suggests basis for a bifunctional activity. J Biol Chem 279: 8111-8115.
  81. Chen R, Braun GB, Luo X, Sugahara KN, Teesalu T, et al. (2013) Application of a proapoptotic peptide to intratumorally spreading cancer therapy. Cancer Res 73: 1352-1361.
  82. Nekipelaya VV, Semenov DV, Potapenko MO, Kuligina EV, Kit Yu, et al. (2008) Lactaptin is a human milk protein inducing apoptosis of MCF-7 adenocarcinoma cells. Dokl Biochem Biophys 419: 58-61.
  83. Semenov DV, Fomin AS, Kuligina EV, Koval OA, Matveeva VA, et al. (2010) Recombinant analogs of a novel milk pro-apoptotic peptide, lactaptin and their effect on cultured human cells. Protein J 29: 174-180.
  84. Nemudraya AA, Makartsova AA, Fomin AS, Nushtaeva AA (2016) Tumor-specific peptide, selected from a phage peptide library, enhances antitumor activity of lactaptin. PLoS One 11: e0160980.
  85. Leland P (2000) Human breast carcinoma cells express type II IL-4 receptors and are sensitive to antitumor activity of a chimeric IL-4-Pseudomonas exotoxin fusion protein in vitro and in vivo. Mol Med 6: 165-178.
  86. Qifan W, Fen N, Ying X, Xinwei F, Jun D, et al. (2016) iRGD-targeted delivery of a pro-apoptotic peptide activated by cathepsin B inhibits tumor growth and metastasis in mice. Tumour Biol 37: 10643-10652.
  87. Obiri NI, Hillman GG, Haas GP, Sud S, Puri RK (1993) Expression of high affinity interleukin-4 receptors on human renal cell carcinoma cells and inhibition of tumor cell growth in vitro by interleukin-4. J Clin Invest 91: 88-93.
  88. Puri RK (1991) Expression of high-affinity interleukin 4 receptors on murine sarcoma cells and receptor-mediated cytotoxicity of tumor cells to chimeric protein between interleukin 4 and Pseudomonas exotoxin. Cancer Res 51: 3011-3017.
  89. Husain SR (1997) Interleukin-4 receptor expression on AIDS-associated Kaposi's sarcoma cells and their targeting by a chimeric protein comprised of circularly permuted interleukin-4 and Pseudomonas exotoxin. Mol Med 3: 327-338.
  90. Kawakami K, Leland P, Puri RK (2000) Structure, function, and targeting of interleukin 4 receptors on human head and neck cancer cells. Cancer Res 60: 2981-2987.
  91. Kioi M, Takahashi S, Kawakami M, Kawakami K, Kreitman RJ, et al. (2005) Expression and targeting of interleukin-4 receptor for primary and advanced ovarian cancer therapy. Cancer Res 65: 8388-8396.
  92. Sugihara K, Kobayashi Y, Suzuki A, Tamura N, Motamedchaboki K, et al. (2014) Development of pro-apoptotic peptides as potential therapy for peritoneal endometriosis. Nat Commun 5: 4478.
  93. Kamangar F, Dores GM and Anderson WF (2006) Patterns of cancer incidence, mortality and prevalence across five continents: defining priorities to reduce cancer disparities in different geographic regions of the world. J Clin Oncol 24: 2137-2150.
  94. Ancona E (2001) Only pathologic complete response to neoadjuvant chemotherapy improves significantly the long term survival of patients with resectable esophageal squamous cell carcinoma: Final report of a randomized, controlled trial of preoperative chemotherapy versus surgery alone. Cancer 91: 2165-2174.
  95. Law S (1997) Preoperative chemotherapy versus surgical therapy alone for squamous cell carcinoma of the esophagus: A prospective randomized trial. J Thorac Cardiovasc Surg. 114: 210-217.
  96. Yang L, Horibe T, Kohno M, Haramoto M, Ohara K, et al. (2012) Targeting interleukin-4 receptor α with hybrid peptide for effective cancer therapy. Mol Cancer Ther 11: 235-243.
  97. Laskin JJ, Sandler AB (2004) Epidermal growth factor receptor: A promising target in solid tumours.  Cancer Treat Rev 30: 1-17.
  98. Kohno M, Horibe T, Haramoto M, Yano Y, Ohara K, et al. (2011) A novel hybrid peptide targeting EGFR-expressing cancers. Eur J Cancer 47: 773-783.
  99. Ma X, Xi L, Luo D, Liu R, Li S, et al. (2012) Anti-tumor effects of the peptide TMTP1-GG-D(KLAKLAK)2 on highly metastatic cancers. PLoS One 7: e42685.
  100. Fu B, Zhang Y, Long W, Zhang A, Zhang Y, et al. (2014) Identification and characterization of a novel phage display-derived peptide with affinity for human brain metastatic breast cancer. Biotechnol Lett 36: 2291-2301.
  101. Fu B, Long W, Zhang Y, Zhang A, Miao F, et al. (2015) Enhanced antitumor effects of the BRBP1 compound peptide BRBP1-TAT-KLA on human brain metastatic breast cancer. Sci Rep 5: 8029.
  102. Yang W, Luo D, Wang S, Wang R, Chen R, et al. (2008) TMTP1, a novel tumor-homing peptide specifically targeting metastasis. Clin Cancer Res 14: 5494-5502.
  103. Lee SM, Lee EJ, Hong HY, Kwon MK, Kwon TH, et al. (2007) Targeting bladder tumor cells in vivo and in the urine with a peptide identified by phage display.  Mol Cancer Res 5: 11-19.
  104. Jung HK, Kim S, Park RW, Park JY, Kim IS, et al. (2016) Bladder tumor-targeted delivery of pro-apoptotic peptide for cancer therapy. J Control Release 235: 259-267.
  105. Shadidi M, Sioud M (2003) Identification of novel carrier peptides for the specific delivery of therapeutics into cancer cells. FASEB J 17: 256-258.
  106. Luo H (2011) Tetrameric far-red fluorescent protein as a scaffold to assemble an octavalent peptide nanoprobe for enhanced tumor targeting and intracellular uptake in vivo. FASEB J 25: 1865-1873.
  107. Nagy A, Schally AV (2005) Targeting cytotoxic conjugates of somatostatin, luteinizing hormone-releasing hormone and bombesin to cancers expressing their receptors: A "smarter" chemotherapy. Curr Pharm Des 11: 1167-1180.
  108. Wang XF, Birringer M, Dong LF, Veprek P, Low P, et al. (2007) A peptide conjugate of vitamin E succinate targets breast cancer cells with high ErbB2 expression. Cancer Res 67: 3337-3344.
  109. Anastasi A, Erspamer V, Bucci M (1971) Isolation and structure of bombesin and alytesin, 2 analogous active peptides from the skin of the European amphibians Bombina and Alytes. Experientia 27: 166-167.
  110. Reubi JC, Wenger S, Schmuckli-Maurer J, Schaer JC, Gugger M (2002) Bombesin receptor subtypes in human cancers: detection with the universal radioligand (125)I-[D-TYR(6), beta-ALA(11), PHE(13), NLE(14)] bombesin(6-14). Clin Cancer Res 8: 1139-1146.
  111. Liu S, Yang H, Wan L, Cai HW, Li SF, et al. (2011) Enhancement of cytotoxicity of antimicrobial peptide magainin II in tumor cells by bombesin-targeted delivery. Acta Pharmacol Sin 32: 79-88.
  112. Smith CJ, Volkert WA, Hoffman TJ (2005) Radiolabeled peptide conjugates for targeting of the bombesin receptor superfamily subtypes. Nucl Med Biol 32: 733-740.
  113. Harris JM, Chess RB (2003) Effect of pegylation on pharmaceuticals. Nat Rev Drug Discov 2: 214-221.
  114. Dozier JK, Distefano MD (2015) Site-specific pegylation of therapeutic proteins. Int J Mol Sci 16: 25831-25864.
  115. Hamley IW (2014) PEG-peptide conjugates. Biomacromolecules 15: 1543-1559.
  116. Kawasaki K (1991) Amino acids and peptides. XIV. Laminin related peptides and their inhibitory effect on experimental metastasis formation. Biochem Biophys Res Commun 174: 1159-1162.
  117. Kawasaki K (1995) Amino acids and peptides. XXVI. Laminin-related peptide poly(ethylene glycol) hybrids and their inhibitory effect on experimental metastasis. Biol Pharm Bull 18: 1714-1717.
  118. Maeda M (1998) Amino acids and peptides. XXXIII. A bifunctional poly(ethylene glycol) hybrid of laminin-related peptides. Biochem Biophys Res Commun 248: 485-489.
  119. Maeda M (2001) Amino acids and peptides. XXXVIII. Facile synthesis of laminin-related peptide-poly(ethylene glycol) hybrids by the solid phase method. Chem Pharm Bull 49: 488-491.
  120. Maeda M (1998) Amino acids and peptides. XXXI. Preparation of analogs of the laminin- related peptide YIGSR and their inhibitory effect on experimental metastasis. Chem Pharm Bull 46: 347-350.
  121. Brinckerhoff LH (1999) Terminal modifications inhibit proteolytic degradation of an immunogenic MART-1(27-35) peptide: Implications for peptide vaccines. Int J Cancer 83: 326-334.
  122. Shin JM, Gwak JW, Kamarajan P, Fenno JC (2016) Biomedical applications of nisin. J Appl Microbiol 120: 1449-1465.
  123. Falciani C (2014) Site-specific pegylation of an antimicrobial peptide increases resistance to Pseudomonas aeruginosa elastase. Amino Acids 46: 1403-1407.
  124. Jevsevar S, Kunstelj M, Porekar VG (2010) PEGylation of therapeutic proteins. Biotechnol J 5: 113-128.
  125. Guiotto A, Pozzobon M, Canevari M, Manganelli R, Scarin M, et al. (2003) PEGylation of the antimicrobial peptide nisin A: Problems and perspectives. Farmaco 58: 45-50.
  126. Bian X, Shen F, Chen Y, Wang B, Deng M, et al. (2010) PEGylation of alpha-momorcharin: synthesis and characterization of novel anti-tumor conjugates with therapeutic potential. Biotechnol Lett 32: 883-890.
  127. Kaneda Y, Yamamoto S, Kihira T, Tsutsumi Y, Nakagawa S, et al. (1995) Synthetic cell-adhesive laminin peptide YIGSR conjugated with polyethylene glycol has improved antimetastatic activity due to a longer half-life in blood. Invasion Metastasis 15: 156-162.
  128. Zhang G, Han B, Lin X, Wu X, Yan H (2008) Modification of antimicrobial peptide with low molar mass poly(ethylene glycol). J Biochem 144: 781-788.
  129. Imura Y (2007) Action mechanism of tachyplesin I and effects of pegylation.Biochim Biophys Acta 1768: 1160-1169.
  130. Matsumura Y, Maeda H (1986) A new concept for macromolecular therapeutics in cancer chemotherapy: mechanism of tumoritropic accumulation of proteins and the antitumor agent smancs. Cancer Res 46: 6387-6392.
  131. Dhar S, Kolishetti N, Lippard SJ, Farokhzad OC (2011) Targeted delivery of a cisplatin prodrug for safer and more effective prostate cancer therapy in vivo. Proc Natl Acad Sci U S A 108: 1850-1855.
  132. Imura Y, Nishida M, Matsuzaki K (2007) Action mechanism of PEGylated magainin 2 analogue peptide. Biochim Biophys Acta 1768: 2578-2585.
  133. Kopecek J, Kopecková P, Minko T, Lu ZR, Peterson CM (2001) Water soluble polymers in tumor targeted delivery. J Control Release 74: 147-158.
  134. Rosenblum WI (1968) Effects of dextran-40 on blood viscosity in experimental macroglobulinaemia.  Nature 218: 591-593.
  135. Hattori M, Nagasawa K, Ohgata K, Sone N, Fukuda A, et al. (2000) Reduced immunogenicity of beta-lactoglobulin by conjugation with carboxymethyl dextran. Bioconjug Chem 11: 84-93.
  136. Kim JH, Kim YS, Park K, Lee S, Nam HY, et al. (2008) Antitumor efficacy of cisplatin-loaded glycol chitosan nanoparticles in tumor-bearing mice. J Control Release 127: 41-49.
  137. Gaowa A, Horibe T, Kohno M, Tabata Y, Harada H, et al. (2015) Enhancement of anti-tumor activity of hybrid peptide in conjugation with carboxymethyl dextran via disulfide linkers. Eur J Pharm Biopharm 92: 228-236.
  138. Mu Y, Kamada H, Kaneda Y, Yamamoto Y, Kodaira H, et al. (1999) Bioconjugation of laminin peptide YIGSR with poly(styrene co-maleic acid) increases its antimetastatic effect on lung metastasis of B16-BL6 melanoma cells. Biochem Biophys Res Commun 255: 75-79.
  139. Mu Y (1999) Bioconjugation of laminin-related peptide YIGSR with polyvinyl pyrrolidone increases its antimetastatic effect due to a longer plasma half-life. Biochem Biophys Res Commun 264: 763-767.
  140. Li Z, Liu X, Li Y (2016) Composite membranes of recombinant silkworm antimicrobial peptide and poly (L-lactic Acid) (PLLA) for biomedical application. Sci Rep 6: 31149.
  141. Standley SM, Toft DJ, Cheng H, Soukasene S, Chen J, et al. (2010) Induction of cancer cell death by self-assembling nanostructures incorporating a cytotoxic peptide. Cancer Res 70: 3020-3026.
  142. Zha RH, Sur S, Stupp SI (2013) Self-assembly of cytotoxic peptide amphiphiles into supramolecular membranes for cancer therapy. Adv Healthc Mater 2: 126-133.
  143. Gaowa A, Horibe T, Kohno M, Sato K, Harada H, et al. (2014) Combination of hybrid peptide with biodegradable gelatin hydrogel for controlled release and enhancement of anti-tumor activity in vivo. J Control Release 176: 1-7.
Citation: Hu C, Chen X, Zhao W, Chen Y, Huang Y (2016) Design and Modification of Anticancer Peptides. Drug Des 5:138.

Copyright: © 2016 Hu C, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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