Binding and interstitial penetration of liposomes within avascular tumor spheroids

The liposomal delivery of cancer therapeutics, including gene therapy vectors, is an area of intense study. Poor penetration of liposomes into interstitial tumor spaces remains a problem, however. In this work, the penetration of different liposomal formulations into prostate carcinoma spheroids was examined. Spheroid penetration was assessed by confocal microscopy of fluorescently labeled liposomes. The impact of liposomal surface charge, mean diameter, lipid bilayer fluidity and fusogenicity on spheroid penetration was examined. A variety of different liposome systems relevant to clinical or preclinical protocols have been studied, including classical zwitterionic (DMPC:chol) and sterically stabilized liposomes (DMPC:chol:DOPE-PEG2000), both used clinically, and cationic liposomes (DMPC:DOPE:DC-chol and DOTAP), forming the basis of the vast majority of nonviral gene transfer vectors tested in various cancer trials. Surface interactions between strongly cationic vesicles and the tumor cells led to an electrostatically derived binding-site barrier effect, inhibiting further association of the delivery systems with the tumor spheroids (DMPC:DC-chol). However, inclusion of the fusogenic lipid DOPE and use of a cationic lipid of lower surface charge density (DOTAP instead of DC-chol) led to improvements in the observed intratumoral distribution characteristics. Sterically stabilized liposomes did not interact with the tumor spheroids, whereas small unilamellar classical liposomes exhibit extensive distribution deeper into the tumor volume. Engineering liposomal delivery systems with a relatively low charge molar ratio and enhanced fusogenicity, or electrostatically neutral liposomes with fluid bilayers, offered enhanced intratumoral penetration. This study shows that a delicate balance exists between the strong affinity of delivery systems for the tumor cells and the efficient penetration and distribution within the tumor mass, similar to previous work studying targeted delivery by ligand-receptor interactions of monoclonal antibodies. Structure-function relationships from the interaction of different liposome systems with 3-dimensional tumor spheroids can lead to construction of delivery systems able to target efficiently and penetrate deeper within the tumor interstitium and act as a screening tool for a variety of therapeutics against cancer.