Liposome Click Membrane Permeability Assay for Identifying Permeable Peptides.

PURPOSE: To develop a novel, target agnostic liposome click membrane permeability assay (LCMPA) using liposome encapsulating copper free click reagent dibenzo cyclooctyne biotin (DBCO-Biotin) to conjugate azido modified peptides that may effectively translocate from extravesicular space into the liposome lumen. METHOD: DBCO-Biotin liposomes were prepared with egg phosphatidylcholine and cholesterol by lipid film rehydration, freeze/thaw followed by extrusion. Size of DBCO-Biotin liposomes were characterized with dynamic light scattering. RESULTS: The permeable peptides representing energy independent mechanism of permeability showed higher biotinylation in LCMPA. Individual peptide permeability results from LCMPA correlated well with shifts in potency in cellular versus biochemical assays (i.e., cellular/ biochemical ratio) demonstrating quantitative correlation to intracellular barrier in intact cells. CONCLUSION: The study provides a novel membrane permeability assay that has potential to evaluate energy independent transport of diverse peptides.

A comprehensive lipid binding and activity validation of a cancer-specific peptide-peptoid hybrid PPS1.

We recently identified a peptide-peptoid hybrid, PPS1, which recognizes lipids that have an overall negative charge, such as phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidic acid (PA), and phosphatidylinositol (PI), but that does not bind to neutral lipids, such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), and sphingomyelin (SM). The simple dimeric version of PPS1, PPS1D1, displayed strong cytotoxicity to cancer cells over normal cells in vitro and tumor burden in vivo. In this study, we comprehensively characterized the direct binding and activity of PPS1 on PS, PG, and PA using liposome-based assays and lung cancer cell lines that express these negatively charged lipids. First, the fluorescence polarization (FP) binding studies of fluoresceinated-PPS1 (PPS1-FITC) to PS-, PG-, and PA-containing PC-liposomes showed that the binding of PPS1 to PC-liposomes increased as concentrations of these lipids increased. In terms of activity, PPS1D1 induced the release of calcein from large, unilamellar PC-liposomes containing 15-30% PS, PG, and PA. PPS1D1 had no activity when the liposomes were composed of 100% PC. This effect was higher at 30% lipids than 15%, and the EC50 for PG and PA were higher than that of PS, indicating that PPS1D1 is more specific towards PS. PPS1D1 binds to and induces significant cytotoxicity in lung cancer cell lines H1693, HCC95, and H1395, which express negatively charged lipids, but had no effect on normal HBEC30KT cells, which has mostly PC in the outer layer. In addition, a series of previously developed PPS1D1 derivatives, which retain or lose activity, were tested with these liposome-based assays, and the data were equivalent to previous observations. This study provides comprehensive binding and activity validations of a unique peptide-peptoid hybrid, PPS1, on negatively charged lipids PS, PA, and PG that are elevated on cancer cell surfaces relative to normal human cell surfaces.

Identification of the minimum pharmacophore of lipid-phosphatidylserine (PS) binding peptide-peptoid hybrid PPS1D1.

We previously reported a unique peptide-peptoid hybrid, PPS1 that specifically recognizes lipid-phosphatidylserine (PS) and a few other negatively charged phospholipids, but not neutral phospholipids, on the cell membrane. The dimeric version of PPS1, i.e., PPS1D1 triggers strong cancer cell cytotoxicity and has been validated in lung cancer models both in vitro and in vivo. Given that PS and other negatively charged phospholipids are abundant in almost all tumor microenvironments, PPS1D1 is an attractive drug lead that can be developed into a globally applicable anti-cancer agent. Therefore, it is extremely important to identify the minimum pharmacophore of PPS1D1. In this study, we have synthesized alanine/sarcosine derivatives as well as truncated derivatives of PPS1D1. We performed ELISA-like competitive binding assay to evaluate the PS-recognition potential and standard MTS cell viability assay on HCC4017 lung cancer cells to validate the cell cytotoxicity effects of these derivatives. Our studies indicate that positively charged residues at the second and third positions, as well as four hydrophobic residues at the fifth through eighth positions, are imperative for the binding and activity of PPS1D1. Methionine at the first position was not essential, whereas the positively charged Nlys at the fourth position was minimally needed, as two derivatives that were synthesized replacing this residue were almost as active as PPS1D1.

Identification of lipid-phosphatidylserine (PS) as the target of unbiasedly selected cancer specific peptide-peptoid hybrid PPS1.

Phosphatidylserine (PS) is an anionic phospholipid maintained on the inner-leaflet of the cell membrane and is externalized in malignant cells. We previously launched a careful unbiased selection targeting biomolecules (e.g. protein, lipid or carbohydrate) distinct to cancer cells by exploiting HCC4017 lung cancer and HBEC30KT normal epithelial cells derived from the same patient, identifying HCC4017 specific peptide-peptoid hybrid PPS1. In this current study, we identified PS as the target of PPS1. We validated direct PPS1 binding to PS using ELISA-like assays, lipid dot blot and liposome based binding assays. In addition, PPS1 recognized other negatively charged and cancer specific lipids such as phosphatidic acid, phosphatidylinositol and phosphatidylglycerol. PPS1 did not bind to neutral lipids such as phosphatidylethanolamine found in cancer and phosphatidylcholine and sphingomyelin found in normal cells. Further we found that the dimeric version of PPS1 (PPS1D1) displayed strong cytotoxicity towards lung cancer cell lines that externalize PS, but not normal cells. PPS1D1 showed potent single agent anti-tumor activity and enhanced the efficacy of docetaxel in mice bearing H460 lung cancer xenografts. Since PS and anionic phospholipid externalization is common across many cancer types, PPS1 may be an alternative to overcome limitations of protein targeted agents.