Targeting Gene Therapy—A Reality?
Authors: Glenn Dranoff
Journal: Molecular Therapy (2000)

Two dozen investigators interested in cancer gene therapy convened in Hilton Head Island, South Carolina last November to assess the state of the enterprise. The William Guy Forbeck Foundation, an organization devoted to promoting cancer research through small, highly interactive meetings, was the host for this most illuminating session. Spirited exchange of viewpoints and critical analysis of experimental and clinical data, superbly orchestrated by cochairmen John Minna (University of Texas, Southwestern) and Inder Verma (Salk Institute), provided all the participants with a deeper understanding of the limitations and prospects for cancer gene therapy. Overall, this highly informative conference underscored the critical need for the field to nurture effective collaborations between scientists exploring basic mechanisms and clinicians evaluating antitumor activities. In broad terms, cancer gene therapies are designed to elicit tumor destruction either by improving the delivery of toxic gene products to cancer cells or by enhancing host antitumor immune defenses. Because efficient gene transfer and expression underlie both of these approaches, considerable discussion surrounded the issues of vector design and development. Verma provided an overview of the strengths and weaknesses of existing gene transfer systems, underscoring the principle that investigators must match particular vectors with particular applications. For example, therapeutic strategies that require persistent gene expression, such as the transfer of drug resistance genes into hematopoietic stem cells (Bertino and O'Connor, Memorial Sloan-Kettering and Antman, Columbia) demand integrating vectors, while strategies in which short-term gene expression is sufficient, such as tumor vaccination schemes, can employ nonintegrating systems. Several key variables that thwart stable gene expression in vivo were highlighted, including viral promoter inactivation, loss or functional compromise of transfected cell populations, and immune recognition of expressed viral proteins (Verma). Notwithstanding these obstacles, increasing evidence suggests that vectors derived from adeno-associated virus (AAV) are likely to yield sustained transgene expression in vivo. In rhesus monkeys, AAV vectors have effectuated relatively stable circulating levels of erythropoietin over the course of 1 year (Wilson, University of Pennsylvania). Moreover, regulatory elements have been successfully incorporated into these vectors, allowing for controlled in vivo gene expression as a function of exogenously administered pharmacologic agents. The ability to target specific gene products to cancer cells is a prerequisite for those therapeutic schemes exploiting the activities of toxic gene products. A promising approach to this problem involves the intratumoral injection of replication conditional viruses; these agents preferentially traverse the lytic phase of the viral life cycle in mitotic cancer cells, thereby evoking tumor cell destruction. Preliminary evidence that this strategy may accomplish tumor selectivity in humans is emerging from studies with the defective herpes virus G207, as initial clinical testing of this vector in glioma patients has failed to reveal any neurotoxicity (Martuza, Georgetown). Given the large transgene capacity of engineered herpes viruses, it may be possible to augment tumor cell killing through the inclusion of toxic gene products or immunostimulatory molecules. A second scheme for achieving cancer cell selectivity entails the introduction of a wild-type p53 gene into malignant cells (Roth, M. D. Anderson). Some evidence suggests that this manipulation can elicit tumor cell apoptosis, but leave similarly transfected normal cells seemingly unaffected. Additional studies of the mechanisms underlying these intriguing effects are clearly warranted. A third strategy for consummating tumor cell selectivity consists of the incorporation of tumor-specific promoters into vector expression cassettes. These sequences may restrict transgene expression to cancer cells; the L-plastin promoter may be an attractive candidate for this purpose (Deisseroth, Yale). In contrast to these approaches, cancer cell-specific transgene expression might be achieved in vivo by preferentially targeting gene transfer to malignant cells. As adenoviruses are being injected systemically in a variety of therapeutic efforts, there is considerable interest in improving their selectivity. The engineering of adenoviral vectors to accomplish CAR-independent gene transfer constitutes a promising line of study (Curiel, University of Alabama–Birmingham). Although CAR is one receptor which adenoviruses naturally exploit for target cell entry, genetic modification of the viral capsid proteins can result in altered viral tropism. This genetic strategy can be used in concert with bispecific targeting complexes to enhance further virus–target cell interactions. Encouraging results have also been obtained with retroviral vectors in which the viral envelope protein has been coupled to a singlechain antibody directed against a target cell surface moiety (Verma). The dovetailing of this technology with improvements in lentiviral vector design may ultimately allow application of this system to in vivo gene delivery, especially to the central nervous system. A third strategy for achieving target cell-selective transduction involves the use of phage display libraries to identify peptides that mediate efficient uptake by specific cells. This powerful technology has been effectively employed to characterize peptides that stimulate entry into dendritic cells, a result with significant implications for vaccine development (Johnston, University of Texas Southwestern). This targeting scheme may also facilitate the development of engineered bacterophages as novel gene transfer systems. With accumulating evidence that cancer patients frequently develop both humoral and cellular immune responses to tumor-associated antigens, immunotherapy has become a major focus of cancer gene therapy research. Dendritic cells have received considerable attention in this regard because of their striking abilities to stimulate potent T and B cell responses. A very elegant scheme based on this insight involves the transfection of bone marrow-derived dendritic cells with whole tumor cell RNA (Gilboa, Duke University). This strategy permits the manufacture of patient-specific vaccines, obviating the requirement for identifying candidate target antigens. A second approach to constructing patient-specific vaccines involves engineering autologous tumor cells to secrete granulocyte–macrophage colony-stimulating factor (GM-CSF). Evidence that this strategy can enhance cellular and humoral antitumor immunity in patients with metastatic melanoma has been marshaled (Dranoff, Dana-Farber Cancer Institute). A variation of this theme entails vaccination with irradiated, allogeneic tumor cell lines engineered to secrete GM-CSF (Pardoll, Johns Hopkins). Underlying this approach is the rationale that shared tumor antigens may also serve as effective vaccine targets. Preliminary data in support of this idea are emerging from initial clinical trials in patients with pancreatic and prostate adenocarcinoma. Another promising strategy exploits gene transfer to augment the expansion and persistence of antigen-specific T cells following adoptive transfer (Greenberg, University of Washington). In this scheme, CD8-positive T cells are engineered to express chimeric cytokine receptors which empower the transduced cells to respond to autocrine cytokine production, thus reducing their dependence on helper T cell function. Although it is clear that several cancer gene therapies have demonstrated intriguing biologic activity in experimental systems and early clinical testing, a focal point of the conference was a vigorous discussion of the challenges confronting the next stages of clinical testing. Most patients treated to date have been in Phase I studies, where the primary objectives are the delineation of safety, toxicity, and feasibility. Participants in these trials are in the most advanced stages of illness, harboring substantial burdens of multiresistant disease; hence, it is extremely difficult to appraise clinical end points within these small, nonrandomized studies. Investigators must thus prioritize specific approaches and expeditiously advance them into definitive clinical efficacy testing within the setting of minimal residual disease. The criteria for such selection remain to be fully clarified, but should include validating the scientific principle underlying the therapy in cancer patients. A limited understanding of the therapeutic mechanism need not preclude definitive evaluation, however, since the establishment of clinical efficacy would provide the most favorable opportunity for elucidating relevant effector mechanisms. Given the substantial cost and logistical difficulties of conducting large, randomized studies, more attention should be devoted to innovative clinical trial designs that may afford a provisional assessment of clinical activity. At the dawn of a new millenium, the Forbeck symposium helped clarify the promise and problems of cancer gene therapy. Whether this emerging field will transform the insights of molecular genetics into tangible cancer treatments remains uncertain; nevertheless, the concerted efforts of scientists and clinicians working in this area are likely to change our understanding of the host–tumor relationship.