Supramolecular assemblies of amphiphilic block copolymers having polypeptide segments offer significant advantages for tailoring spatial arrangement based on secondary structures in their optically active backbones. Here, we demonstrated the critical effect of α-helix bundles in cisplatin-conjugated poly(L- (or D-)glutamate) [P(L(or D)Glu)-CDDP] segment on the packaging of poly(ethylene glycol) (PEG)-P(L(or D)Glu)-CDDP block copolymers in the core of polymeric micelles (CDDP/m) and enhanced micelle tolerability to harsh in vivo conditions for accomplishing appreciable antitumor efficacy against intractable pancreatic tumor by systemic injection. CDDP/m prepared from optically inactive PEG-poly(D,L-glutamate) (P(D,LGlu)), gradually disintegrated in the bloodstream, resulting in increased accumulation in liver and spleen and reduced antitumor efficacy. Alternatively, CDDP/m from optically active PEG-P(L(or D)Glu) maintained micelle structure during circulation, and eventually attained selective tumor accumulation while reducing nonspecific distribution to liver and spleen. Circular dichroism and small-angle X-ray scattering measurements indicated regular bundled assembly of α-helices in the core of CDDP/m from PEG-P(L(or D)Glu), which is suggested to stabilize the micelle structure against dilution in physiological condition. CDDP/m suffered corrosion by chlorides in medium, yet the optically active micelles with α-helix bundles kept the micelle structure for prolonged time, with slowly releasing unimers and dimers from the surface of the bundled core in an erosion-like process, as verified by ultracentrifugation analysis. This is in sharp contrast with the abrupt disintegration of CDDP/m from PEG-P(D,LGlu) without secondary structures. The tailored assembly in the core of the polymeric micelles through regular arrangement of constituting segments is key to overcome their undesirable disintegration in bloodstream, thereby achieving efficient delivery of loaded drugs into target tissues.

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Spontaneous formation of polymeric metallosomes with uniform size (~100 nm) was found to occur in aqueous medium through the reaction of an anticancer agent, (1,2-diaminocyclohexane)platinum(II) (DACHPt), with a Y-shaped block copolymer of ω-cholesteroyl-poly(L-glutamic acid) and two-armed poly(ethylene glycol) (PEGasus-PLGA-Chole). Circular dichroism spectrum measurements revealed that the PLGA segment forms an α-helix structure within the metallosomes, suggesting that secondary-structure formation of metallocomplexed PLGA segment may drive the self-assembly of the system into vesicular structure. These metallosomes can encapsulate water-soluble fluorescent macromolecules into their inner aqueous phase and eventually deliver them selectively into tumor tissues in mice, owing to the prolonged blood circulation. Accordingly, fluorescent imaging of the tumor was successfully demonstrated along with an appreciable antitumor activity by DACHPt moieties retained in the vesicular wall of the metallosomes, indicating the potential of metallosomes as multifunctional drug carriers.

No abstract

The ubiquitin-proteasome system is central in the regulation of cellular proteins controlling cell cycle progression and apoptosis, drawing much interest for developing effective targeted cancer therapies. Herein, we developed a novel pH-responsive polymeric-micelle-based carrier system to effectively deliver the proteasome inhibitor MG132 into cancer cells. MG132 is covalently bound to the block copolymer composed of polyethylene glycol (PEG) and polyaspartate through an acid-labile hydrazone bond. This bond is stable at physiological condition, but hydrolytically degradable in acidic compartments in the cell, such as late-endosomes and lysosomes, and thus, it was used for controlled release of MG132 after EPR-mediated preferential accumulation of the micelles into the tumor. MG132-loaded micelles have monodispersed size distribution with an average diameter of 45nm, and critical micelle concentration is well below 10(-7)M. In vitro studies against several cancer cell lines confirmed that MG132-loaded micelles retained the cytotoxic effect, and this activity was indeed due to the inhibition of proteasome by released MG132 from the micelles. Real-time in vitro confocal-microscopy experiments clearly indicated that MG132-conjugated micelles disintegrated only inside the target cells. By intravital confocal micro-videography, we also confirmed the prolonged circulation of MG132 loaded micelles in the bloodstream, which lead to tumor specific accumulation of micelles, as confirmed by in vivo imaging 24h after injection. These micelles showed significantly lower in vivo toxicity than free MG132, while achieving remarkable antitumor effect against a subcutaneous HeLa-luc tumor model. Our findings create a paradigm for future development of polymeric-micelle-based carrier system for other peptide aldehyde type proteasome inhibitors to make them effective cohort of the existing cancer therapeutic regiments.