The present invention is directed to an amphipathic peptide and methods of using the amphipathic peptide for delivering small molecule agents to a cell. Ideally, the amphipathic cell penetrating peptide comprises less than approximately 50 amino acid residues with at least 6 arginine residues, at least 12 Alanine Residues, at least 6 leucine resiues, optionally at least one cysteine residue, and at least two but no greater than three glutamic acids wherein the arginine residues are evenly distributed along the length of the peptide； and the peptide has a defined ratio of arginine to negatively charged amino acid residues and a defined ratio of hydrophilic amino acid residues to hydrophobic amino acid residues. The present invention is also directed to a nanoparticle and cell delivery system comprising the amphipathic cell penetrating peptide of the invention. The peptide, nanoparticle or cell delivery system of the invention may be used in therapy. For example, the peptide may be used as a therapeutic agent delivery system, in which the therapeutic agent may include nucleic acids or other small molecules.
