Design and Optimization of Functional Polymer-Modified Liposome Formulations for Enhanced Delivery of Therapeutics for Cancer and Depression

Abstract

Liposomal drug delivery systems have established themselves as a versatile strategy for enhancing the therapeutic efficacy of both anticancer agents and central nervous system (CNS) therapeutics. These systems facilitate improved encapsulation efficiency, regulated drug release kinetics, prolonged circulation time, and surface functionalization for targeted delivery. The work performed in this dissertation encompasses a series of formulation and characterization studies aimed at optimizing liposomal systems for hydrophilic chemotherapeutics—specifically pemetrexed (PMX), 5-fluorouracil (5FU), and doxorubicin (DOX)—as well as the antidepressants paroxetine and venlafaxine. For PMX, a multi-targeted antifolate characterized by rapid systemic clearance and associated dose-limiting toxicities, the effects of incorporating polyethylene glycol (PEG) into the liposomal aqueous core were explored to modulate drug release. Formulations incorporating polyethylene glycol internally in the liposomes displayed a concentration-dependent extension of PMX release, with statistically significant differences observed at late release time points. This finding indicates that the manipulation of the internal liposomal environment can effectively fine-tune drug diffusion release kinetics. Similarly, polyethyleneimine (PEI), a cationic polymer known for its strong electrostatic binding capabilities and pH-buffering properties, was integrated in the aqueous core of liposomes to investigate its effect on encapsulation efficiency and retention of chemotherapeutic agents. This modification significantly improved drug loading and ensured sustained release for PMX, 5FU, and DOX, suggesting the broad applicability of PEI-based systems across a range of hydrophilic pharmacological agents. Moreover, studies examining ionic strength revealed that an increase in NaCl concentration resulted in decreased liposome particle size, likely due to enhanced lipid packing, while maintaining PMX encapsulation efficiency. Nonetheless, elevated ionic strength correlated with accelerated PMX release, possibly due to alterations in bilayer permeability and stability, emphasizing the critical need for precise control over formulation parameters to achieve desired release profiles. When extending these methodologies towards CNS therapeutics, liposomal formulations were developed both with and without PEI for encapsulating paroxetine and venlafaxine, two antidepressants of clinical significance. The inclusion of PEI markedly enhanced encapsulation efficiency and resulted in more sustained release characteristics, thereby addressing challenges related to low bioavailability and limited penetration across the blood-brain barrier. Surface modification studies were conducted on amine-bearing cationic liposomes, specifically those formulated with dimyristoyl phosphatidylethanolamine (DMPE), to explore rapid and scalable functionalization using PEG and the chelating agent dipicolylamine (DPA). The results indicated that PEGylation led to a 44% reduction in surface-accessible amines, while DPA modification resulted in a 28% reduction. These findings underscore the potential for controlled modulation of surface chemistry, making strides toward the creation of stealth nanocarriers with targeted ligand-mediated capabilities. In vitro cytotoxicity evaluations were carried out with PMX. They involved a lactate dehydrogenase (LDH) release assay against 4T1 breast cancer cells to determine the cytotoxic effects of PMX-loaded liposomes in comparison to free drug and blank liposome controls. Statistical analysis using one-way and two-way ANOVA revealed that liposomal PMX exhibited significantly greater and more sustained cytotoxicity over 72 hours, in contrast to the diminishing cytotoxicity of free PMX by that time. These findings suggest that encapsulation within liposomes can extend PMX activity, thereby decreasing dosing frequency while preserving therapeutic efficacy. This work demonstrates the capacity to meticulously engineer both the intraliposomal environment (via PEG or PEI incorporation) and external formulation conditions (ionic strength, surface modification) to optimize liposomal physicochemical properties and drug release characteristics. These strategies have been effectively applied to various drug classes, including antifolate chemotherapeutics, anthracyclines, antimetabolites, and CNS-active antidepressants. The integration of formulation optimization, surface engineering, and in vitro performance evaluation establishes a robust framework for the rational design of nanocarriers, addressing critical challenges in drug delivery to enhance therapeutic outcomes and expand the clinical applicability of liposomal systems in oncology and neuropsychiatric disorders.