Brain tumors are the leading cause of cancer deaths in persons younger than 35 years. The incidence of central nervous system tumors is more than twice that of Hodgkin's disease, more than half that of melanoma and, in women, the frequency of mortality caused by tumors of the central nervous system is almost equivalent to that caused by ovarian cancer. In children, brain tumors are the most common solid tumor and are second only to leukemia as an overall cause of childhood cancer. (Dale, D. C. and Federman, D. D., 1995, Scientific American Medicine, Scientific American, Inc., New York, Chapter 7.) Most brain tumors are inoperable; and even for those brain tumors that are operable, the surgery is extremely difficult and frequently leads to neurological disorders.
The in vivo application of retroviral vector-mediated gene therapy has been applied to the treatment of brain tumors (Oldfield et al., 1993, Hum. Gene Ther.; 4:39-69; Culver et al., 1992 Science 256:1550-2). Perhaps, the most widely studied application of gene therapy utilizes retroviruses genetically engineered to express proteins that activate a relatively nontoxic pro drug to form a highly toxic agent. For example, retroviral producer cells expressing susceptibility factors have been transplanted into the brain tissue of patients in order to kill the tumor cells (Barba, D. et al., WO 93/04167). One particular application of the system utilizes the thymidine kinase gene of the Herpes simplex virus which confers sensitivity to anti-viral drugs such as ganciclovir and acyclovir (Barba et al., WO 93/04167; Moolten, F. L. et al., 1986, Cancer Research 46:5276-5281). The HSV-TK gene product catalyzes the phosphorylation of a number of nucleoside analogues which are poor substrates for the TK of mammalian cells. For example, the antiherpes drug acyclovir exhibits minimal toxicity to cells lacking HSV-TK activity, but is activated in cells expressing HSV-TK to a toxic form capable of inhibiting DNA synthesis and which has been shown to exhibit selective cytoxicity to cells expressing the HSV-TK gene.
One concern associated with the use of retroviral vector-mediated gene therapy is that the implanted producer cells might not continue to survive and/or express the therapeutic genes for the time periods required to achieve the maximum therapeutic benefit. It is generally known that cells directly implanted into the brain die within about a two to four week period (see, for example, Itukura, T. et al., 1988, J. Neurosurg. 68:955-959). In some instances, the adherence of cells to microcarriers, prior to implantation in vivo, has been shown to enhance the long-term viability of transplanted cells (Cherskey et al.; WO 9206702) but to date this method has not been successfully applied to retroviral producer cell lines.