(392b) Improving Selective Targeting to Macrophage Subpopulations through Modifying Liposomes with Arginine Based Materials

Introduction: Two pathways for activating macrophages (MΦ) exist. One of these routes is termed the classically activated M1 pathway is achieved through exposure to lipopolysaccharide (LPS). M1 MΦs are part of the type 1 T helper (Th1) response and are known as pro-inflammatory cells. The other pathway is reached through interleukin-4 (IL-4) and is known as the alternatively activated M2 pathway. M2 MΦ produce pro-angiogenic factors. Tumor-associated MΦ (TAMs) are of the M2 pathway and promote tumor growth through the release of angiogenic molecules. Our goal is to use liposomal drug delivery systems to selectively deliver drugs to TAMs. These polymers will be eventually used to delivery anti-cancer therapeutics to the tumor.

 Materials and Methods: We fabricated liposomes and modified their surfaces with 14 molecules that are chemically similar to arginine in order to investigate the effects of surface modifications on internalization by macrophages. Also, we characterized the size and surface charge of both modified and unmodified liposomes. Finally, the liposomes were loaded with doxorubicin, an anti-cancer drug, and were examined for their loading efficiency and release kinetics at pH 7.4. Changes in the IC₅₀of doxorubicin entrapped in the unmodified and modified liposomes was compared to that of free doxorubicin in M(LPS), M(IL-4), and M(0) macrophages.

Results and Discussion: Liposomes were loaded with doxorubicin and incubated with M(LPS), M(IL-4), and M(0) cells. Dose response curves were fit to a Sigmoidal curve and the half-maximum of inhibitor concentration (IC₅₀) was obtained. The lower IC₅₀values for M(IL-4) cells compared to M(LPS) cells suggests that modifications H and I could be used to improve targeted delivery to TAMs.

PCA was applied to the dataset to determine the relationships between physicochemical properties of the modifiers and the IC₅₀ values. The multidimensional dataset was reduced to a two-dimensional plot to better facilitate analysis of latent relationships. The physicochemical properties were chosen based on previous reports describing attributes of drug molecules such as Lipinskiâ??s rule of five, polar surface area, flexibility, enthalpy, lipophilicity, and charge. Macrophages were observed to have different correlations with these physicochemical properties based on their phenotype. The IC₅₀ values for both M(LPS) and M(0) cells were well aligned on the projection map and were situated between the number of hydrogen bond donors and the number of freely rotating bonds of the modifiers. The M(IL-4) cells were dependent on the zeta potential of the liposomes and the logP (the partition coefficient of the modifier in octanol and water) of the modifiers. Taken together, these insights may elucidate design principles in drug delivery targeted to specific macrophage phenotypes. Further work on larger library is necessary to determine if these relationships between the identified materials properties and IC₅₀values hold for different macrophage polarizations.

Conclusions: Cellular uptake of the liposomes was found to be dependent upon macrophage phenotype and surface modifications. There were also differences in trends between internalization of liposomal FC and the IC₅₀ of liposomal doxorubicin, which were attributed to changes in the ability of doxorubicin to escape the endosome. Two modifications were able to increase the toxicity of encapsulated doxorubicin for M(IL-4) cells over M(LPS) cells, improving targeted deliver to specific macrophage subpopulations.