Liposomal targeting of glucocorticoids to inhibit tumor angiogenesis

Abstract

Glucocorticoids (GC) have inhibitory actions on solid tumor growth due to suppressive effects on tumor angiogenesis and inflammation. When evaluating the preclinical studies on solid tumor growth inhibition, it appears that GC-induced antitumor effects are achieved by using substantially higher doses than the minimal doses needed to achieve inhibition of inflammatory disease processes. The high doses of GC used in these antitumor studies resulted in death of some of the animals due to opportunistic infections, indicating that severe systemic side effects can occur. As GC are also rapidly cleared from the circulation, and localize at tumor sites only to a very limited extent, targeted delivery of GC to tumor tissue is an attractive strategy to increase intratumoral drug concentrations. A strategy investigated for GC targeting to tumors is the incorporation of GC into long-circulating liposomes (LCL). Our previous studies showed that prednisolone phosphate (PLP) encapsulated in LCL (LCL-PLP) can inhibit tumor growth after intravenous administration (i.v.). These antitumor effects of LCL-PLP are the result of the tumor-targeting property of the liposome formulation. The mechanism by which LCL-PLP inhibits tumor growth is unclear. Therefore, the primary aim of this thesis was to investigate the mechanisms of action of LCL-encapsulated GC that could explain their inhibitory effects on murine tumor models. We investigated the effects of liposome-encapsulated PLP versus free PLP on angiogenic protein production in tumor tissue in vivo and on viability and proliferation of tumor and endothelial cells in vitro. Taken together, our studies point to a strong inhibitory effect of liposomal PLP on tumor angiogenesis by reduction of the intratumoral production of the majority of pro-angiogenic factors produced by tumor-associated macrophages (TAM). To evaluate whether this finding can be generalized to other types of GC, we encapsulated GC other than PLP in LCL for targeting to tumor tissue. The antitumor activity, adverse effects, and mechanisms of action of LCL containing synthetic GC other than prednisolone were investigated. In addition to prednisolone, the following GC as phosphate derivatives were tested: budesonide disodium phosphate, dexamethasone disodium phosphate, and methylprednisolone disodium phosphate. They were selected due to their difference in the ranking order of their potency in terms of activation of the human glucocorticoid receptor. All liposomal glucocorticoids exerted strong antitumor effects which are enabled by the tumor-targeting property of the long-circulating liposome formulation that increases intratumoral drug concentration and prolongs the inhibitory effects. LCL-encapsulated GC showed different potencies on different pathways involved in tumor angiogenesis and inflammation. We also tested the antitumor effects of LCL containing therapeutic agents other than GC. Interestingly, statins with a primary activity not related to angiogenesis (e.g. cholesterol lowering in cardiovascular diseases) also possessed anti-angiogenic activity on tumor growth when they are tumor-targeted delivered. Finally our studies provide a comparison between the antitumor effects of DoxilTM or CaelyxTM (in Europe) (doxorubicin entrapped in LCL) and LCL-PLP. These results suggest that Doxil does not act primarly through TAM. The antitumor activity of Doxil seems to be primarily the result of cytotoxic effects on tumor cells.