Designing Sub-20 nm Nanocarriers for Small Molecule Delivery: Interplay among Structural Geometry, Assembly Energetics, and Cargo Release Kinetics

Biological constraints in diseased tissues have motivated the need for small nanocarriers (10-30 nm) to achieve sufficient vascular extravasation and pervasive tumor penetration. This particle size limit is only an order of magnitude larger than small molecules, such that cargo loading is better described by co-assembly processes rather than simple encapsulation. Understanding the structural, kinetic, and energetic contributions of carrier-cargo co-assembly is thus critical to achieve molecular-level control and predictable in vivo behavior. These interconnected set of properties were systematically examined using sub-20 nm self-assembled nanocarriers known as three-helix micelles (3HM). Both hydrophobicity and the "geometric packing parameter" dictate small molecule compatibility with 3HMs alkyl tail core. Planar obelisk-like apomorphine and doxorubicin (DOX) molecules intercalated well within the 3HM core and near the core-shell interface, forming an integral component to the co-assembly, as corroborated by small angle X-ray and neutron-scattering structural studies. DOX promoted crystalline alkyl tail ordering, which significantly increased (+63%) the activation energy of 3HM subunit exchange. Subsequently, 3HM-DOX displayed slow-release kinetics (t1/2=40 h) at physiological temperatures, with ~50x greater cargo preference for the micelle core as described by two drug partitioning coefficients (micellar core/shell Kp1 ~24, and shell/bulk solvent Kp2 ~2). The geometric and energetic insights between nanocarrier and their small molecule cargos developed here will aid in broader efforts to deconvolute the interconnected properties of carrier-drug co-assemblies, and to understand nanomedicine behavior throughout all the physical and in vivo processes they are intended to encounter.