Introduction

It is estimated that one in four deaths in the United States is due to malignant disease, corresponding to over 1,500 deaths per day.(1) Although effective treatments for many types of cancers exist, options such as chemotherapy have deleterious effects on vital non-tumor bodily tissues. Their detrimental effects limit the viable dosage given to patients and consequently, their resulting effectiveness. A predominant goal in oncology is therefore the development of tumor specific treatment options, and numerous strategies have been explored to achieve this goal.

Most of these strategies take advantage of increased tumor vessel leakage due to the enhanced permeation and retention effect or utilize tumor ligand targeting.(2) The former has most recently involved the usage of liposomal and nanoparticle carriers for delivering pharmacologic agents and enzymes to sites of disease and is the delivery mechanism for such drugs as Doxil, Caelyx, and Abraxane.(3) The latter involves taking advantage of engineered humanized or fully humanized antibody fragments for greater therapeutic specificity in over 100 ligand-targeted therapeutics in clinical trials.(4)

However, these strategies are now being accompanied by the usage of stem cells which have been shown to selectively accumulate in malignant tissue. Stem cells can be transduced with numerous viral vectors to express select transgenes. The ability of stem cells to exhibit tropism to sites of pathology and express factors which selectively affect tumors makes them a novel and powerful mechanism for cancer treatment. To date, stem cells expressing therapeutic genes have been tested in animal models of intracranial glioma, medulloblastoma, melanoma brain metastasis, disseminated neuroblastoma and breast cancer lung metastasis. Every one of these trials demonstrated significant antitumor response to treatment.(5)