| The folate receptor (FR) is a highly selective tumor marker overexpressed in greater than 90% of ovarian carcinomas. Two general strategies have been developed for the targeted delivery of drugs to FR-positive tumor cells: (1) coupling to a monoclonal antibody against the receptor and, (2) coupling to a high affinity ligand, folic acid. Folic acid, a high affinity ligand of the FR, retains its receptor binding properties when derivatized via its gamma-carboxyl group. Folate conjugation has been successfully applied in vitro for the receptor-specific delivery of protein toxins, anti-T-cell receptor antibodies, interleukin-2, chemotherapy agents, gamma-emitting radiopharmaceuticals, magnetic resonance imaging contrast agents, liposomal drug carriers, and gene transfer vectors. This dissertation presents novel approaches to using folate receptor-mediated delivery. First, FR-targeted liposomes were evaluated in FR(+) tumor cells as carriers for a lipophilic boron compound for neutron capture therapy (Chapter 2). These carriers were capable of specific receptor binding and FR-mediated endocytosis in vitro. Second, novel pH-sensitive liposome formulations based on incorporation of oleyl alcohol were evaluated (Chapter 3). FR-targeting was used in combination with these pH-sensitive liposomes for the purpose of drug delivery and were evaluated in vitro. These FR-targeted, pH-sensitive liposomes, entrapping the membrane-impermeable chemotherapeutic agent, cytosine-beta-D-arabinofuranoside (araC), showed ∼17-times greater FR-dependent cytotoxicity in a FR(+) cell line compared with araC delivered via FR-targeted non-pH-sensitive liposomes. The third approach involved a novel lipophilic conjugate of folate, folate-polyethylene glycol (PEG)-cholesterol, which was synthesized and evaluated for FR-mediated delivery of liposomes to tumor cells (Chapter 4). As an alternative to phospholipids, cholesterol was found to be an effective anchor for incorporation of a targeting ligand into liposomes. All three approaches demonstrated that the small size, convenient availability, simple conjugation chemistry, and lack of immunogenicity of folic acid make it an ideal ligand for targeted delivery of drug carriers to tumors. |
