Roles of Nonpolar and Polar Intermolecular Interactions in the Improvement of the Drug Loading Capacity of PEO-<i>b</i>-PCL with Increasing PCL Content for Two Hydrophobic Cucurbitacin Drugs

Molecular dynamics (MD) simulation was used to study the roles of nonpolar and polar intermolecular interactions in the improvement of the drug loading capacity of poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) with increasing PCL content for two water insoluble anticancer drugs: Cucurbitacin B (CuB) and Cucurbitacin I (CuI). In particular, random binary mixture models containing 10-12 wt % drug and remaining PEO-b-PCL with three different PCL/PEO (w/w) ratios (0.5, 1, and 2) were used to calculate their Flory-Huggins interaction parameters (chi). The MD simulation results show that, for both CuB and CuI, the computed chi decreases (i.e., affinity increases) with increasing PCL/PEO ratio. Such results are consistent with our experimental observation that increasing the PCL/PEO (w/w) ratio from 1 to 4.8 significantly increases the drug loading capacity of micelles formed by PEO-b-PCL for both drugs. Analysis of the energy data shows that increasing affinity (loading) at higher PCL/PEO ratio is attributed to the increase in favorable polar interactions and to the formation of additional hydrogen bonds (H-bonds) between the drugs and the PCL block rather than to the increase in the hydrophobic characteristics of the diblock copolymer as one would normally expect. In fact, the nonpolar intermolecular interactions became more unfavorable at higher PCL/PEO ratio. Analysis of the radial distribution functions of the model mixtures indicates that at high PCL/PEO ratio, multiple H-bond sites on the PCL block interacted with single H-bond sites on the drug molecules. However, at low PCL/PEO ratio, only single H-bonds formed between various H-bond sites on the drug molecules and those of the PCL and PEO blocks. It seems that formation of H-bonds between multiple H-bond sites on the PCL block and single H-bond sites on the drug molecules is responsible for inducing drug/PEO-b-PCL affinity. The finding also explains the experimental observation that release rates of both drugs decrease with increasing PCL/PEO ratio and that the decrease in the release rate of CuB is more pronounced than that of CuI. Our simulation results show that the number of H-bonds formed between CuB and the PCL block is much higher than that of CuI at high PCL/PEO ratio.

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