MOLECULAR SIMULATION OF POLYACRYLIC ACID PHASE BEHAVIOUR IN PHYSIOLOGICAL CONDITIONS AND ITS PERFORMANCE AS DOXORUBICIN CARRIER

Abstract

Molecular simulations are capable tools for the design of polymeric drug carriers, as it provides a detailed molecular understanding of drug encapsulation and release in such systems. Unlike conventional pharmacokinetic modeling tools that require the experimental results to obtain model parameters, atomistic simulations only require inputs of chemical structures and therefore can minimize the need for in vitro/in vivo experimentation. The detailed molecular insight obtained by these simulations is precious and often beyond the reach of sophisticated experimental facilities. Although initially limited to the prediction of single-molecule behavior (e.g., polymer and drug orientation in a bilayer), gradual advances in computing speed and efficient simulation approaches have made it feasible to employ these methods for phenomena occurring at substantially large length and time scales (e.g., carrier-drug complexation) with modest computational cost and resources. In the present context, the design of polymeric drug carriers for controlled release, pH-responsive drug formulations require a molecular understanding of the changes in physical interactions between the excipient and drug molecules during the dissolution process inside the body. In this study, we study the molecular design of weak pH-responsive polyacrylic acid (PAA) carriers for the delivery of anticancer drug, Doxorubicin (DOX) using four sets of simulations. In the first set of simulations, we performed MD simulations of aqueous PAA solutions. Model oligomers of PAA of different tacticities, molecular weights, degrees of deprotonation, and deprotonation patterns are simulated with water molecules. Deprotonation of PAA chains that occurs with an increase in pH results in an increase in Coulomb repulsion between chain segments on one hand, and a non-monotonic change in the hydrogen bonding between chain segments on the other hand. Therefore, at the single chain level, PAA chains are stretched at higher pH values, where the amount of stretching varies with chain tacticity and salt concentration. While the PAA chains are always more stretched for the fully deprotonated case than compared to the neutral case, radius of gyration (𝑅𝑔) for a given 𝑓 varies with tacticity in the order syndiotactic > atactic > isotactic. One explanation for larger stretching in the syndiotactic case compared to the isotactic case is the difference in the magnitudes of counterion condensation in the two cases. Since the fraction of condensed counterions in the isotactic case is significantly higher than that in the syndiotactic case, it would have relatively lower electrostatic repulsion between chain segments due to the charge screening effect of counterions. ii For the multiple chains simulation, aggregation increases with increase in PAA concentration. Although the counterions have higher mobility than compared to PAA, they mostly reside within the PAA aggregate, thus neutralizing its overall charge. The average 𝑅𝑔 of individual chains increases with increase in PAA concentration, implying an increase in the stretching of individual chains. Three competing changes occur with an increase in solution pH (or equivalently, an increase in 𝑓): (1) electrostatic repulsion between PAA segments increases, (2) 𝐶𝑂𝑂𝐻−𝐶𝑂𝑂𝐻 hydrogen bonding decreases, and (3) 𝐶𝑂𝑂−−𝐶𝑂𝑂𝐻 hydrogen bonding first increases and then decreases. Further insight into the intermolecular interactions can be obtained by a closer inspection of two nearby PAA chains in the concentrated system. The likelihood of counterion bridging increases with increasing either the concentration or the degree of deprotonation of PAA, as the fraction of condensed ions (𝑓𝑐) increases both with increasing concentration and with increase in 𝑓. PAA forms aggregates at higher concentrations, which are relatively denser and contain lesser water (solid-like) at lower pH than compared to higher pH (liquid-like). Apparently, a micelle-like formation (appearance of a dense core of hydrophobic segments) is formed in the end deprotonation case due to the hydrogen bonding between the protonated ends of chains. On the other hand, a network-like formation with no such dense core appears to form in the random deprotonation case, as the protonated groups are randomly distributed along the chains. In the second set of simulations, we simulated multiple oligomers of PAA in model gastric and intestinal fluids, where the degree of deprotonation of PAA oligomers is varied with pH of the medium. Since the gastric fluid has a pH substantially lower than intestinal fluid, PAA oligomers are relatively lesser ionized in gastric fluid in comparison to intestinal fluid and forms aggregates. The effect of pH (or 𝑓) on PAA aggregation behavior is that the compaction of PAA aggregate decreases with increase in pH. In the third set of simulations, multiple PAA oligomers with multiple molecules of cationic anticancer drug, doxorubicin (DOX) are simulated for pH values corresponding to various physiological conditions in aqueous phase. The diffusion coefficient of DOX decreases with an increase in pH due to an increase in the ionic complexation of PAA with DOX, despite a decrease in PAA aggregation. Note that since a purely geometric criteria is used to determine “hydrogen bonding” between 𝐶𝑂𝑂− group of PAA and 𝑁𝐻3+ groups of DOX, the extent of this “ionic complexation” may also be considered as a extent of “hydrogen bonding” between these groups. iii Our observation that the DOX diffusion coefficient decreases with an increase in pH is in close agreement with the experimental studies of similar systems. In the fourth set of simulations, we mimic the dissolution behaviour of PAA-DOX formulations in atomistic MD simulations. Sequential water removal is performed during the MD simulations, followed by re-equilibration, until an amorphous state devoid of water is achieved. This, therefore mimics the reverse of dissolution behavior of amorphous solid dispersions. We study the changes in PAA-DOX interactions and DOX diffusion coefficient as a function of water content, at pH values representative of gastric and intestinal fluids. The results of atomistic simulations can be coupled with a water uptake model to predict the drug release behavior. Diffusion coefficient of DOX decreases with increase in pH of the solution, as the hydrogen bonding between PAA (COOH and COO−) and DOX (NH3+) increases. On removing the water between PAA and DOX, the hydrogen bonding between PAA-DOX increases and PAA-water hydrogen bonding decreases. Thus, both the solubility and diffusivity of DOX reduces. Afterward we study diffusion of DOX by changing the PAA concentration to understand erosion effect on polyacrylic acid (PAA)-doxorubicin (DOX) formulation. Then PAA observe as a fixed carrier by changing DOX concentration with fix PAA concentration with effect on diffusion of DOX. Finally, last consideration chain length variation of PAA with fix PAA and DOX concentration. Our findings are in agreement with recent experimental reports on pH-triggered targeting of tumor cells by PAA-DOX system. Using the results of these four sets of simulations, we devise molecular design principles of pH-responsive polymeric carriers.