Development of novel polymeric micellar drug conjugates and nano-containers with hydrolyzable core structure for doxorubicin delivery

Novel micelle-forming poly(ethylene oxide)-block-poly(ε-caprolactone) (PEO-b-PCL) block copolymers bearing doxorubicin (DOX) side groups (PEO-b-P(CL-DOX)) on the PCL block were synthesized. Prepared block copolymers were characterized, assembled to polymeric micellar drug conjugates and assessed for the level of DOX release at pH 7.4 and pH 5.0 using a dialysis membrane to separate released and conjugated drug. The possibility for the degradation of PCL backbone for PEO-b-P(CL-DOX) micelles was investigated using gel permeation chromatography. Micelle-forming DOX conjugate did not show any signs of DOX release at 37 °C within 72 h of incubation at both pHs, but revealed signs of poly(ester) core degradation at pH 5.0. In further studies, PEO-b-PCL micelles bearing benzyl, carboxyl or DOX groups in the core were also used as micellar nano-containers for the physical encapsulation of DOX, where maximum level of drug-loading and control over the rate of DOX release was achieved by polymeric micelles containing benzyl groups in their core, i.e., PEO-b-poly(α-benzylcarboxylate-ε-caprolactone) (PEO-b-PBCL) micelles. The in vitro cytotoxicity of chemically conjugated DOX as part of PEO-b-P(CL-DOX) and physically encapsulated DOX in PEO-b-PBCL against B16F10 murine melanoma cells was assessed and compared to that of free DOX. Consistent with the results of in vitro release study, cytotoxicity of micellar PEO-b-P(CL-DOX) conjugate (IC50 of 3.65 μg/mL) was lower than that of free and physically encapsulated DOX in PEO-b-PBCL (IC50 of 0.09 and 3.07 μg/mL, respectively) after 24 h of incubation. After 48 h of incubation, the cytotoxicity of conjugated DOX (IC50 of 0.50 μg/mL) was still lower than the cytotoxicity of free DOX (IC50 of 0.03 μg/mL), but surpassed that of physically encapsulated DOX in PEO-b-PBCL (IC50 of 1.54 μg/mL). The results point to a potential for PEO-b-P(CL-DOX) and PEO-b-PBCL as novel polymeric micellar drug conjugates and nano-containers bearing hydrolyzable cores for DOX delivery.