Folate targeting

Folate targeting is a method utilized in biotechnology for drug delivery purposes. This Trojan Horse process, which was created by Drs. Christopher P. Leamon and Philip S. Low, involves the attachment of the vitamin, folate (folic acid), to a molecule/drug to form a 'folate conjugate'. Based on the natural high affinity of folate for the folate receptor protein (FR), which is commonly expressed on the surface of many human cancers, folate-drug conjugates also bind tightly to the FR and trigger cellular uptake via endocytosis. Molecules as diverse as small radiodiagnostic imaging agents to large DNA plasmid formulations have successfully been delivered inside FR-positive cells and tissues. Folate targeting is a method utilized in biotechnology for drug delivery purposes. This Trojan Horse process, which was created by Drs. Christopher P. Leamon and Philip S. Low, involves the attachment of the vitamin, folate (folic acid), to a molecule/drug to form a 'folate conjugate'. Based on the natural high affinity of folate for the folate receptor protein (FR), which is commonly expressed on the surface of many human cancers, folate-drug conjugates also bind tightly to the FR and trigger cellular uptake via endocytosis. Molecules as diverse as small radiodiagnostic imaging agents to large DNA plasmid formulations have successfully been delivered inside FR-positive cells and tissues. Folic acid (FA, folate or vitamin B9), is a vital nutrient required by all living cells for nucleotide biosynthesis and for the proper metabolic maintenance of 1-carbon pathways. Aside from its cofactor role for intracellular enzymes, FA also displays high affinity for the folate receptor (FR), a glycosylphosphatidyinositol-linked protein that captures its ligands from the extracellular milieu and transports them inside the cell via a non-destructive, recycling endosomal pathway. The FR is also a recognized tumor antigen/biomarker. Because of this, diagnostic and therapeutic methods which exploit the FR’s function are being developed for cancer. The FR is an emerging therapeutic target for diagnosis and treatment of cancer and chronic inflammatory diseases. Expression of the FR is selectively upregulated on certain malignant cells and activated macrophages. Overexpression of the FR on these types of cells is clinically significant because they designate areas where the physiological symptoms of disease are most extensive. The malignant cells indicate the presence of tumors associated with ovarian, lung, breast, kidney, brain, endometrial, and colon cancer. Macrophages become activated in chronic diseases such as rheumatoid arthritis, Crohn’s disease, ulcerative colitis, psoriasis, atherosclerosis, diabetes, and most other inflammatory diseases. From a mechanistic perspective, the FR functions to concentrate exogenous ligands (e.g. folates and folate-drug conjugates) into the cell cytosol by endocytosis. The term endocytosis refers to the process whereby the plasma membrane invaginates and eventually forms a distinct intracellular compartment. The endocytic vesicles (endosomes) rapidly become acidified to allow the FR to release its ligand. Afterwards, the empty FR returns to the cell surface where it can participate in another round of ligand-mediated endocytosis. The discovery of vitamin-mediated drug targeting in plants led to the hypothesis that folate-targeted therapies could be of clinical use. After proteins covalently bonded to biotin were successfully transported into plant cells through receptor-mediated endocytosis, a similar technique was attempted with folate and animal cells. Targeted drug therapy is advantageous because it deposits the drug at the specific location where it can be most useful in treating the disease. Similarly, folate-targeted imaging therapy helps visualize areas where the FR is expressed at higher levels. With greater control over where exogenous agents are delivered, diagnostic and treatment therapies are more effective and cause fewer side effects. Specificity of folate conjugates for the FR has been shown by competition tests with free folate. When this ligand, known to bind the FR, is added in excess of the folate conjugate, it outcompetes the conjugate, indicating that the folate conjugate specifically binds the FR, and not other receptors, in the process of receptor-mediated endocytosis. Addition of an enzyme that frees the folate receptor from the cell membrane and addition of antibodies to the FR also reverse the internalization of folate conjugates, providing further evidence that folate conjugates bind the FR with specificity. While some drugs and radioimaging agents are delivered to cells as folate conjugates in a one-to-one folate-to-conjugate ratio, folate-targeted liposomes allow for the delivery of larger amounts of chemotherapeutic agents. In this technique, drug particles are enveloped in a plasma membrane-bound vesicle. Folate is attached to polyethylene glycol bound to the phosphate heads of membrane phospholipids, thus directing the liposomes to FRs of tumor cells, by which they are engulfed. Elevated expression of the FR occurs in many human malignancies, especially when associated with aggressively growing cancers. Recently, it was proposed that this relationship may possibly be used for prognostic purposes. Non-mucinous ovarian cancer (the majority of ovarian cancers) was the first tumor type to be associated with FR 'over-expression', and it was later shown that this antigen was identical to that found on KB tumor cells and in placental tissue. Several studies confirmed that ~80-90% of ovarian tumors over-express the FR. Other gynecological cancers also over-express the receptor as well as pediatric ependymal brain tumors, mesothelioma, and breast, colon, renal and lung tumors. The FR may also be found associated with cancer, particularly when related to myeloid leukemia and perhaps head and neck carcinomas. Taken together, the total number of tumors that express the FR is very large; therefore, FR-targeted strategies could have significant impact on cancer treatment for patients diagnosed with FR-positive disease. The FR is expressed on many different types of malignant tissues and in large quantities. But, not all human cancers within a particular indication will express the FR. Because novel FR-targeted therapies are now being tested clinically, having the ability to screen patients for FR-positive disease could certainly increase the efficiency of and decrease the time for clinical investigations of these novel agents.