title: Enzyme-Cleavable Polymeric Micelles for the Intracellular Delivery of Proapoptotic Peptides.
abstract: Peptides derived from the third Bcl-2 homology domain (BH3) renormalize apoptotic signaling by antagonizing prosurvival Bcl-2 family members. These potential peptide drugs exhibit therapeutic activities but are limited by barriers including short circulation half-lives and poor penetration into cells. A diblock polymeric micelle carrier for the BIM BH3 peptide was recently described that demonstrated antitumor activity in a B-cell lymphoma xenograft model [Berguig et al., Mol. Ther. 2015, 23, 907-917]. However, the disulfide linkage used to conjugate the BIM peptide was shown to have nonoptimal blood stability. Here we describe a peptide macromonomer composed of BIM capped with a four amino acid cathepsin B substrate (FKFL) that possesses high blood stability and is cleaved to release the drug inside of target cells. Employing RAFT polymerization, the peptide macromonomer was directly integrated into a multifunctional diblock copolymer tailored for peptide delivery. The first polymer block was made as a macro-chain transfer agent (CTA) and composed of a pH-responsive endosomolytic formulation of N,N-diethylaminoethyl methacrylate (DEAEMA) and butyl methacrylate (BMA). The second polymer block was a copolymer of the peptide and polyethylene glycol methacrylate (PEGMA). PEGMA monomers of two sizes were investigated (300 Da and 950 Da). Protein gel analysis, high performance liquid chromatography, and coupled mass spectrometry (MS) showed that incubation with cathepsin B specifically cleaved the FKFL linker and released active BIM peptide with PEGMA300but not with PEGMA950. MALDI-TOF MS showed that incubation of the peptide monomers alone in human serum resulted in partial cleavage at the FKFL linker after 12 h. However, formulation of the peptides into polymers protected against serum-mediated peptide degradation. Dynamic light scattering (DLS) demonstrated pH-dependent micelle disassembly (25 nm polymer micelles at pH 7.4 versus 6 nm unimers at pH 6.6), and a red blood cell lysis assay showed a corresponding increase in membrane destabilizing activity (<1% lysis at pH 7.4 versus 95% lysis at pH 6.6). The full carrier-drug system successfully induced apoptosis in SKOV3 ovarian cancer cells in a dose-dependent manner, in comparison to a control polymer containing a scrambled BIM peptide sequence. Mechanistic analysis verified target-dependent activation of caspase 3/7 activity (8.1-fold increase), and positive annexin V staining (72% increase). The increased blood stability of this enzyme-cleavable peptide polymer design, together with the direct polymerization approach that eliminated postsynthetic conjugation steps, suggests that this new carrier design could provide important benefits for intracellular peptide drug delivery.
authors.affiliation: Department of Bioengineering and Molecular Engineering and Sciences Institute, University of Washington , Seattle, Washington 98195, United States.
authors.forename: Hanna B
authors.initials: HB
authors.lastname: Kern
authors.affiliation: Department of Bioengineering and Molecular Engineering and Sciences Institute, University of Washington , Seattle, Washington 98195, United States.
authors.forename: Selvi
authors.initials: S
authors.lastname: Srinivasan
authors.affiliation: Department of Bioengineering and Molecular Engineering and Sciences Institute, University of Washington , Seattle, Washington 98195, United States.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Convertine
authors.affiliation: Fred Hutchinson Cancer Research Center , Seattle, Washington 98108, United States.
authors.forename: David
authors.initials: D
authors.lastname: Hockenbery
authors.affiliation: Fred Hutchinson Cancer Research Center , Seattle, Washington 98108, United States.
authors.forename: Oliver W
authors.initials: OW
authors.lastname: Press
authors.affiliation: Department of Bioengineering and Molecular Engineering and Sciences Institute, University of Washington , Seattle, Washington 98195, United States.
authors.forename: Patrick S
authors.identifier: 0000-0001-6939-6371
authors.initials: PS
authors.lastname: Stayton
mesh_terms: D002401:Cathepsin B / Q000737:chemistry; D045744:Cell Line, Tumor; D004337:Drug Carriers / Q000737:chemistry; D016503:Drug Delivery Systems; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008689:Methacrylates / Q000737:chemistry; D008823:Micelles; D010455:Peptides / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry; D011108:Polymers / Q000737:chemistry*
chemical_list: D004337:Drug Carriers; D008689:Methacrylates; D008823:Micelles; D010455:Peptides; D011108:Polymers; C524499:polyethylene glycol methacrylate; D011092:Polyethylene Glycols; D002401:Cathepsin B; C519537:diethylaminoethyl methacrylate; C016284:butyl methacrylate
keywords: RAFT polymerization; apoptosis; cathepsin B; intracellular drug delivery; pH-responsive polymer; therapeutic peptide