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.