Book Description

For the treatment of cancer, peptides hold great potential as both targeting and therapeutic agents. One particularly promising anti-cancer strategy is peptides derived from the third Bcl-2 homology domain (BH3), which antagonize pro-survival Bcl-2 proteins and induce apoptosis. Unfortunately, before the clinical potential of peptides can be realized, a number of drug delivery barriers must be overcome. Namely, peptides have short circulation half-lives, are susceptible to degradation by extracellular proteases, and are unable to cross cell membranes and access intracellular targets. An antibody-targeted, pH-responsive polymeric system was recently developed and implemented for the intracellular delivery of the pro-apoptotic BH3 peptide BIM1. Unfortunately, the delivery properties of this system were limited by the poor stability of the disulfide-linkage used for conjugating BIM to the polymeric carrier. It was the objective of this thesis to develop highly stable polymer-peptide conjugates for the targeted and intracellular delivery cancer drugs. Initially, steric hindrance was investigated for enhancing the stability and delivery properties of disulfide-linked polymer-BIM conjugates. Two methyl groups were introduced onto the peptide’s disulfide-adjacent carbon by substituting BIM’s C-terminal cysteine with pencillamine and conjugating the peptide to the polymeric carrier via disulfide exchange. In a murine xenograft model of B-cell lymphoma, steric hindrance significantly enhanced conjugate stability, peptide half-life and peptide deposition into tumors. However, benefits were relatively minor with much left to be desired. Next an enzyme-labile peptide linker was developed that is highly stable in human serum and efficiently cleaved in cancer cells to release active BIM peptide. A methacrylamido-peptide macromonomer containing BIM capped with a four amino acid (FKFL) cathepsin B substrate was synthesized and directly integrated into the polymeric delivery vehicle via RAFT polymerization. The resulting cathepsin-B cleavable BIM prodrug system demonstrated potent apoptotic activity in ovarian cell cultures and is currently being investigated for apoptotic activity and therapeutic efficacy in intraperitoneal ovarian cancer xenograft model. Lastly, peptide monomer technology was alternatively implemented for tumor-specific targeting. A peptide monomer containing the EGFR-targeting sequence GE112 was polymerized into a hydrophilic polymeric drug delivery system in combination with an ester-linked camptothecin prodrug monomer. GE11 was shown to enhance targeting and activity of the polymeric prodrug in ovarian cancer cell cultures. [1] Berguig GY, Convertine AJ, Frayo S, Kern HB, Procko E, Roy D, Srinivasan S, Margineantu DH, Booth G, Palanca-Wessels MC, Baker D, Hockenbery D, Press OW, Stayton PS. Intracellular delivery system for antibody-Peptide drug conjugates. Mol Ther. 2015 May;23(5):907-17. [2] Li Z, Zhao R, Wu X, Sun Y, Yao M, Li J, Xu Y, Gu J. Identification and characterization of a novel peptide ligand of epidermal growth factor receptor for targeted delivery of therapeutics. FASEB J. 2005 Dec;19(14):1978-85.