Design of an Acid-Activated Antimicrobial Peptide for Tumor Therapy

Design of pH-responsive antimicrobial peptide melittin analog-camptothecin conjugates for tumor therapy

Melittin, a classical antimicrobial peptide, is a highly potent antitumor agent. However, its significant toxicity seriously hampers its application in tumor therapy. In this study, we developed novel melittin analogs with pH-responsive, cell-penetrating and membrane-lytic activities by replacing arginine and lysine with histidine. After conjugation with camptothecin (CPT), CPT-AAM-1 and CPT-AAM-2 were capable of killing tumor cells by releasing CPT at low concentrations and disrupting cell membranes at high concentrations under acidic conditions. Notably, we found that the C-terminus of the melittin analogs was more suitable for drug conjugation than the N-terminus. CPT-AAM-1 significantly suppressed melanoma growth in vivo with relatively low toxicity. Collectively, the present study demonstrates that the development of antitumor drugs based on pH-responsive antimicrobial peptide-drug conjugates is a promising strategy.

One New Acid-Activated Hybrid Anticancer Peptide by Coupling with a Desirable pH-Sensitive Anionic Partner Peptide

Anticancer peptides (ACPs) are promising antitumor resources, and developing acid-activated ACPs as more effective and selective antitumor drugs would represent new progress in cancer therapy. In this study, we designed a new class of acid-activated hybrid peptides LK-LE by altering the charge shielding position of the anionic binding partner LE based on the cationic ACP LK and investigated their pH response, cytotoxic activity, and serum stability, in hoping to achieve a desirable acid-activatable ACP. As expected, the obtained hybrid peptides could be activated and exhibit a remarkable antitumor activity by rapid membrane disruption at acidic pH, whereas its killing activity could be alleviated at normal pH, showing a significant pH response compared with LK. Importantly, this study found that the peptide LK-LE3 with the charge shielding in the N-terminal of LK displayed notably low cytotoxicity and more stability, demonstrating that the position of charge masking is extremely important for the improvement of peptide toxicity and stability. In short, our work opens a new avenue to design promising acid-activated ACPs as potential targeting agents for cancer treatment.

Constructing New Acid-Activated Anticancer Peptide by Attaching a Desirable Anionic Binding Partner Peptide

Improving the cell selectivity of anticancer peptides (ACPs) is a major hurdle in their clinical utilization. In this study, a new acid-activated ACP was designed by conjugating a cationic ACP LK to its anionic binding partner peptide (LEH) via a disulfide linker to trigger antitumor activity at acidic pH while masking its killing activity at normal pH. Three anionic binding peptides containing different numbers of glutamic acid (Glu) and histidine were engineered to obtain an efficient acid-activated ACP. The conjugates LK-LEH2 and LK-LEH3 exhibited 6.1 and 8.0-fold higher killing activity at pH 6.0 relative to at pH 7.4, respectively, suggesting their excellent pH-dependent antitumor activity; and their cytotoxicity was 10-fold lower than that of LK. However, LK-LEH4 had no pH-responsive killing effect. Interestingly, increasing the number of Glu from 2 to 4 increased the pH-response of the physical mixture of LK and LEH; conversely, they weakly decreased the cytotoxicity of LK, suggesting that the conjugate connection was required to achieve excellent pH dependence while maintaining minimum toxicity. LK-LEH2 and LK-LEH3 were more enzymatically stable than LK, indicating their potential for in vivo application. Our work provided a basis for designing promising ACPs with good selectivity and low toxicity.

pH-Responsive Antimicrobial Peptide with Selective Killing Activity for Bacterial Abscess Therapy

The unusual acidic pH of the abscess milieu is an adverse factor that decreases the therapeutic efficacy of traditional antibiotics. Moreover, avoiding both the undesired killing of commensal bacteria and the development of drug resistance remains difficult during abscess therapy. Hence, we synthesized a series of pH-responsive antimicrobial peptides equipped with efficient bacterial killing activity at pH 6.5 and inactivity at pH 7.4. Among the peptides, F5 exhibited outstanding pH-responsive antimicrobial activity and low toxicity. Fluorescence spectroscopy and electron microscopy illustrated that F5 killed bacteria via a membrane-disruptive mechanism at acidic pH values. Mouse cutaneous abscesses revealed that F5 was equipped with excellent therapeutic ability to reduce the bacterial load and cytokines without causing skin toxicity. In summary, this study reveals a strategy for selectively killing bacteria under the pathologic conditions of abscess sites while avoiding the elimination of commensal bacteria under normal physiological pH levels.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

Novel Peptide-Polymer Conjugate with pH-Responsive Targeting/Disrupting Effects on Biomembranes

Conjugating polymers to peptides has become a new strategy of designing functional antitumor agents for their improved stability and enhanced activity. In this paper, a novel peptide-polymer conjugate PEPc-PMAA with pH responsiveness was designed and synthesized. The isoelectric point of PEPc was studied by dynamic light scattering for the targeting effect. Also, the transmittances of PMAA at different pHs were measured using an ultraviolet-visible spectrophotometer for determining the triggering pH of the disrupting effect. The results showed that PEPc-PMAA was hydrophilic under neutral conditions and changed to be amphiphilic composed of positively charged PEPc and hydrophobic PMAA under acidic conditions. The interactions between PEPc-PMAA and mimic cells were investigated by the measurements of membrane fluidity and cargo leakage from 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine and 1,2-dipalmitoyl-sn-glycerol-3-phospho-(1-rac-glycerol) (DPPG) liposomes. It proved that PEPc-PMAA caused a distinct membrane disturbance of the DPPG liposome at pH 5.5, resulting in more serious cargo leakage. Because of its targeting and disrupting effects on negatively charged biomembranes under acidic conditions, PEPc-PMAA showed its good potential as an antitumor agent.

New designed pH-responsive histidine-rich peptides with antitumor activity

Anticancer peptides have received widespread attention as alternative antitumor therapeutics due to their unique action mode. However, the systemic toxicity hampers their successful utilisation in tumour therapy. Here, the tumour acidic environment was used as a trigger to design a series of histidine-rich peptides by optimising the distribution of histidine and leucine based on the amphiphilic peptide LK, in hoping to achieve desirable acid-activate anticancer peptides. Among all the obtained peptides, L9H5-1 showed enhanced antitumor activity at acidic pH concomitant with low toxicity at normal pH, exhibiting excellent pH-response. At acidic pH, protonated L9H5-1 could rapidly kill tumour cells by efficient membrane disruption as evidenced by in vitro experiments, including increasing intracellular PI uptake and LDH release, dramatic membrane damage and increase of later apoptotic/necrotic cells. Moreover, no cell cycle arrest was observed after treated with L9H5-1. Interestingly, this study found that the new peptides with the same number of histidines and leucines displayed different pH-dependent antitumor activity, indicating that the position of amino acid alteration is extremely important for the design of acid-activated histidine-rich peptides. In short, our work provides a new avenue to develop new acid-activated anticancer peptides as promising antitumor drugs with high efficiency and good selectivity.

A Review for Antimicrobial Peptides with Anticancer Properties: Re-purposing of Potential Anticancer Agents

Abstract In recent years, various research on cancer treatment has achieved significant progress. However, some of these treatments remain disputable because of the emergence and development of drug resistance, and the toxic side effects that were brought about by the lack of selectivity displayed by the treatments. Hence, there is considerable interest in a new class of anticancer molecules that is currently still under investigation termed the cationic antimicrobial peptides (AMPs). AMPs are a group of pervasive components of the innate immunity which can be found throughout all classes of life. The small innate peptides cover a broad spectrum of antibacterial activities due to their electrostatic interactions with the negatively charged bacterial membrane. Compared with normal cells, cancer cells have increased proportions of negatively charged molecules, including phosphatidylserine, glycoproteins, and glycolipids, on the outer plasma membrane. This provides an opportunity for exploiting the interaction between AMPs and negatively charged cell membranes in developing unconventional anticancer strategies. Some AMPs may also be categorized into a group of potential anticancer agents called cationic anticancer peptides (ACPs) due to their relative selectivity in cell membrane penetration and lysis, which is similar to their interaction with bacterial membranes. Several examples of ACPs that are used in tumor therapy for their ability in penetrating or lysing tumor cell membrane will be reviewed in this paper, along with a discussion on the recent advances and challenges in the application of ACPs.

SPA: a peptide antagonist that acts as a cell-penetrating peptide for drug delivery

Although cell-penetrating peptides (CPPs) has been proven to be efficient transporter for drug delivery, ideal peptide vectors for tumor therapy are still being urgently sought. Peptide antagonists have attracted substantial attention as targeting molecules because of their high tumor accumulation and antitumor activity compared with agonists. SPA, a derivative of substance P, is a potent antagonist that exhibits antitumor activity. Based on the amino acid composition of SPA, we speculate that it can translocate across cell membranes as CPPs do. In this study, our results demonstrated that SPA could enter cells similarly to a CPP. As a vector, SPA could efficiently deliver camptothecin and plasmids into cells. In addition, our results showed that SPA exhibited low toxicity to normal cells and high enzymatic stability. Taken together, our results validated the ability of SPA for efficient drug delivery. More importantly, our study opens a new avenue for designing ideal CPPs based on peptide antagonists.

Design of a new pH-activatable cell-penetrating peptide for drug delivery into tumor cells

Cell-penetrating peptides (CPPs) have been considered as potential drug delivery vectors due to their remarkable membrane translocation capacity. However, lack of specificity and extreme systemic toxicity hamper their successful application for drug delivery. Here, we designed a new pH-activatable CPP, LHHLLHHLHHLLHH-NH₂ (LH), by substitution of all lysines and two leucines of LKKLLKLLKKLLKL-NH₂ (LK) with histidines. As expected, histidine-rich LH could be activated and penetrate into cells at pH 6.0, whereas its membrane transduction activity could be shielded at pH 7.4. In contrast, LK showed no obviously different cellular uptake at both pH conditions. Importantly, LH was significantly less cytotoxicity compared with LK at both pH values, suggesting a better safety for further application. In addition, after conjugation of camptothecin (CPT) with LH, this conjugate displayed remarkably pH-dependent antitumor activity than free CPT and LK-CPT. This study provides a new tumor pH-responsive CPP with low toxicity for selective anticancer drug delivery.

pH/Redox-Controlled Interaction between Lipid Membranes and Peptide Derivatives with a "Helmet"

How to reduce the cytotoxicity of antitumor peptides to normal cells remains an ongoing challenge. Here, we designed a pH/redox-responsive supramolecular structure (Pep-V⊂P-PEG) composed of a peptide modified with viologen (Pep-V) and polyethylene glycol bearing pillar[5]arene (P-PEG) as a "helmet". By shielding the hydrophobic moiety of the peptide derivative with pillar[5]arene via host-guest interactions, its disruption on normal cells can be effectively reduced. At acidic pH, the supramolecular structure can selectively adsorb onto negatively charged lipid membranes because of electrostatic interactions. Owing to redox responsiveness of the viologen group, Pep-V could be separated from P-PEG after the addition of reductants and inserted into lipid bilayers, which leads to membrane disruption. Cargo leakage of liposome models was investigated to understand Pep-V⊂P-PEG-induced liposomal membrane disruption under different pH values and redox conditions. Results showed that Pep-V⊂P-PEG caused almost no cargo leakage from (1,2-dipalmitoyl-sn-glycerol-3-phosphocholine) liposomes at pH 7.4 but significant leakage from negatively charged (1,2-dipalmitoyl-sn-glycerol-3-phospho-(1-rac-glyerol)) liposomes at pH 5.0 under a reducing environment. Pep-V⊂P-PEG displayed low destructive effects on mimic normal cells and significant disruption to mimic tumor cells when exposed to a reducing environment that is expected to be a potential antitumor agent.

Disruption of Tumor Cells Using a pH-Activated and Thermosensitive Antitumor Lipopeptide Containing a Leucine Zipper Structure

Antitumor peptides may potentially alleviate the problem of chemoresistance but do not yet target tumor cells and would be cytotoxic to normal cells. Here, we designed a pH-activated and thermosensitive lipopeptide (C6-Pep) containing a leucine zipper and an alkyl chain and assessed the ability of C6-Pep to kill cancer cells. Pep, the same sequence without the N-terminal hexanoic acid moiety, was generated as a less hydrophobic control. First, lipopeptide adsorption into lipid monolayers was studied using Langmuir-Blodgett and polarization modulation infrared reflection adsorption spectroscopy. Under weakly acid conditions, electrostatic interactions between C6-Pep and negatively charged phospholipids increased the adsorption/insertion of C6-Pep (vs Pep) into lipid monolayers. Cargo leakage from liposomes was assayed to model lipopeptide-induced lipid membrane disruption. The ability of C6-Pep to disrupt liposomes depended on the peptide molecular structure/hydrophobicity, solution pH, and temperature-induced uncoiling of the zipper structure; the greatest cargo leakage from the liposome with negative charge was observed for C6-Pep at pH 5.5 under mildly hyperthermic conditions (45 °C). In vitro, C6-Pep was significantly more cytotoxic toward HeLa cells at pH 5.5 under hyperthermic conditions than at pH 7.4 and/or 37 °C. Overall, this study demonstrates that amphipathic C6-Pep can insert into cell membranes in the low-pH tumor microenvironment, whereas the application of heat promotes the uncoiling of the zipper structure, leading to the disruption of tumor cell membranes and cell death. pH-activated and thermosensitive C6-Pep represents a promising tool to kill cancer cells via a strategy that does not invoke chemoresistance and may have low side effects.

A pH-dependent charge reversal peptide for cancer targeting

Naturally occurring cationic antimicrobial peptides exhibit not only antimicrobial activity, but also anticancer activity and are expected to be new weapons in cancer treatment. The selectivity for cancer cells over normal cells is at least partly due to the more negative surface of cancer cells. A lower pH in tumor tissue (pH 6.2-6.9) than that in normal tissues (pH 7.3-7.4) has also been utilized to develop anticancer agents. However, cytotoxicity against normal cells at physiological pH is often an issue. Furthermore, acidic regions can be found in some normal tissues such as the kidneys. Therefore, existing approaches to cancer targeting are not fully satisfactory. In this study, we designed a peptide, HE (GIHHWLHSAHEFGEHFVHHIMNS-amide), with a charge that reverses from -1.5 at pH 7.4 to +6 at pH 5.5 for cancer targeting at low pH based on the antimicrobial peptide magainin 2 by introducing 6 His, an additional Glu, and an amidated terminal. HE interacted with cancer-mimicking negatively charged liposomes in a pH-dependent fashion with a midpoint with a pH of 6.5 just above the membrane surface. The peptide killed human renal adenocarcinoma ACHN cells at pH 6.0, but not at pH 7.4, and was nontoxic against human normal glomerular mesangial cells even at this low pH. Thus, the novel peptide may be a promising lead peptide for cancer therapy, although this derivatization resulted in weakened cytotoxicity.

Multifunctional ATRP based pH responsive polymeric nanoparticles for improved doxorubicin chemotherapy in breast cancer by proton sponge effect/endo-lysosomal escape

Folic acid and trastuzumab functionalized pH responsive polymeric nanoparticles for intracellular doxorubicin delivery in breast cancer.

AS1411 aptamer and folic acid functionalized pH-responsive ATRP fabricated pPEGMA-PCL-pPEGMA polymeric nanoparticles for targeted drug delivery in cancer therapy

Nonspecificity and cardiotoxicity are the primary limitations of current doxorubicin chemotherapy. To minimize side effects and to enhance bioavailability of doxorubicin to cancer cells, a dual-targeted pH-sensitive biocompatible polymeric nanosystem was designed and developed. An ATRP-based biodegradable triblock copolymer, poly(poly(ethylene glycol) methacrylate)-poly(caprolactone)-poly(poly(ethylene glycol) methacrylate) (pPEGMA-PCL-pPEGMA), conjugated with doxorubicin via an acid-labile hydrazone bond was synthesized and characterized. Dual targeting was achieved by attaching folic acid and the AS1411 aptamer through EDC-NHS coupling. Nanoparticles of the functionalized triblock copolymer were prepared using the nanoprecipitation method, resulting in an average particle size of ∼140 nm. The biocompatibility of the nanoparticles was evaluated using MTT cytotoxicity assays, blood compatibility studies, and protein adsorption studies. In vitro drug release studies showed a higher cumulative doxorubicin release at pH 5.0 (∼70%) compared to pH 7.4 (∼25%) owing to the presence of the acid-sensitive hydrazone linkage. Dual targeting with folate and the AS1411 aptamer increased the cancer-targeting efficiency of the nanoparticles, resulting in enhanced cellular uptake (10- and 100-fold increase in uptake compared to single-targeted NPs and non-targeted NPs, respectively) and a higher payload of doxorubicin in epithelial cancer cell lines (MCF-7 and PANC-1), with subsequent higher apoptosis, whereas a normal (noncancerous) cell line (L929) was spared from the adverse effects of doxorubicin. The results indicate that the dual-targeted pH-sensitive biocompatible polymeric nanosystem can act as a potential drug delivery vehicle against various epithelial cancers such as those of the breast, ovary, pancreas, lung, and others.