Targeting therapeutic and imaging agents to folate receptor positive tumors.

The membrane-bound folate receptor (FR) is overexpressed on a wide range of human cancers, such as those originating in ovary, lung, breast, endometrium, kidney and brain. The vitamin folic acid is a high affinity ligand of the FR which retains its receptor binding properties when conjugated to other molecules. Consequently, "folate targeting" technology has successfully been applied for the delivery of protein toxins, chemotherapeutic agents, radio-imaging and therapeutic agents, MRI contrast agents, liposomes, gene transfer vectors, antisense oligonucleotides, ribozymes and immunotherapeutic agents to FR-positive cancers. These folate-bearing delivery systems have produced major enhancements in cancer cell specificity and selectivity over their non-targeted formulation counterparts. Hence, it is hopeful that this targeting strategy will lead to improvements in the safety and efficacy of clinically-relevant anti-cancer agents. Therefore, the focus of this review will be to highlight the current status of folate-targeted technology with particular emphasis on the recent advances in this field as well as possible directions for future development.