Current Gene Therapy - Volume 1, Issue 4, 2001

A better understanding of the molecular events responsible for the development of drug resistance in cancer cells has emerged in recent years. It is now established that tumor cells can acquire drug resistance by alterations of pathways involved in the regulation of apoptosis and that failure to activate this pathway in cancer cells may confer resistance to chemotherapy. This resistance to drug-induced apoptosis is likely to play an important role in tumors that are refractory to chemotherapy. The identification of points in the apoptotic pathway at which dysregulation occurs opens up new therapeutic opportunities in situations where conventional cytotoxic chemotherapy approaches fail. Although these gene therapy-based strategies are still in their infancy they will likely lead to more effective treatments for human cancers. This review will focus on gene therapy strategies developed to specifically target the apoptic pathway and how these strategies can affect the sensitivity of tumor cells to chemotherapy

Peripheral T lymphocytes are a target of choice for many gene therapeutic strategies. Retrovirus-mediated transduction allows genomic integration and long-term expression of transgenes in target cells. Over many years, low transduction efficiency into primary T lymphocytes has limited clinical application of existing protocols. Recently, gene transfer rates >50 percent have been achieved facilitating clinical studies. More attention is thus being focused on the ability of gene-modified cells to carry out innate as well as conferred functions in vivo and the influence of culture conditions, retroviral vector and host response thereon.

Herpes simplex virus type 1 (HSV1) has a number of properties which could potentially be exploited in the development of vectors for the delivery of genes to the nervous system. These include a natural tropism for neurons, a large viral genome allowing the insertion of multiple exogenous genes, and the ability to establish asymptomatic life-long latent infections. Despite these inherent advantages, the development of HSV vectors successfully exploiting all these properties has been problematical. This has mainly been due to either vector toxicity or an inability to maintain transgene expression in the long term. Recent progress towards overcoming these problems and several applications of the technology are discussed.

Utilization of chemotherapy for treatment of tumors is mainly limited by its hematological toxicity. Because of the low level expression of drug resistance genes, transduction of hematopoietic progenitors with multidrug resistance 1 (MDR1) or multidrug resistance-associated protein 1 (MRP1) genes should provide protection from chemotherapy toxicity. Successful transfer of drug resistance genes into hematopoietic cells might allow the administration of higher doses of chemotherapy and, therefore, increase regression of chemosensitive tumors. In addition, this approach can be used to select in vivo transduced cells by their enrichment after administration of cytotoxic drugs.Our group has studied the potential value of MRP1 to protect hematopoietic cells. The interest in the use of MRP1 as an alternative to MDR1 gene transfer for bone marrow protection lies in its different modulation. Indeed, classical P-gp reversal agents, tested in clinic to decrease MDR1 tumor resistance, have little or no effect on MRP1 function. This would allow, in the same patient, the use of reversal agents to decrease P-gp tumor resistance without reversing bone marrow protection of the transduced hematopoietic cells provided by MRP1. We constructed two different MRP1-containing vectors with either the Harvey retroviral long terminal repeat (LTR) or phosphoglycerate kinase (PGK) as promoters and generated ecotropic producer cells. MRP1 transduced fibroblasts were more resistant to doxorubicin, vincristine, and etoposide and their chemoprotection was increased after selection with chemotherapeutic agents in the presence of glutathione, a co-factor for MRP1 function. Lethally irradiated mice were engrafted with bone marrow (BM) cells transduced with MRP1 vectors (PGK promoter). We demonstrated that high expression of MRP1 in murine hematopoietic cells reduces doxorubicin-induced leukopenia and mortality. In addition, in vivo selection of MRP1-transduced BM cells was achieved following doxorubicin administration and allowed a better chemoprotection after the second chemotherapy cycle.This article reviews the data of chemoprotection and selection with MRP1 gene transfer.

Efficient virus and non-virus vector systems for gene transfer to tumor cells have been developed and tested in cell culture and in animal experiments. With some of the earliest and most comprehensively evaluated vectors, such as retroviruses, advanced clinical trials were performed in tumor patients. Malignant primary brain tumors (gliomas) have been chosen for the first clinical studies on novel gene therapy approaches because these tumors are non-metastatic and develop on the largely postmitotic background of normal glial and neuronal tissue. However, the human cancer gene therapy studies performed so far were not as successful as preclinical animal experiments. Furthermore, the clinical studies did not address major limiting factors for in vivo gene therapy, such as insufficient gene transfer rates to the tumor with the used local delivery modalities, and the resulting inability of a particular transgene-prodrug system to confer permanently eradicating cytotoxicity to the whole neoplasm. Critical evaluation of gene transfer and therapy studies has led to the conclusion that, even using identical vectors, the anatomical route of vector administration can dramatically affect both the efficiency of tumor transduction and its spatial distribution, as well as the extent of intratumoral and intracerebral transgene expression. This review concentrates on different physical methods for vector delivery to malignant primary brain tumors in experimental or clinical settings: stereotactic or direct intratumoral injection or convection-enhanced bulk-flow interstitial delivery intrathecal and intraventricular injection and intravascular infusion with or without modification of the blood-tumor-barrier. The advantages and drawbacks of the different modes and delivery routes of in vivo vector application, and the possibilities for tumor targeting by modifications of the native tropism of virus vectors or by using tissue-specific or inducible transgene expression are summarized.

Attempts to improve the efficacy of adoptive T-cell therapies have led to the development of innovative strategies that combine the high specificity of antibody molecules with the efficient trafficking properties and effector functions of immune cells. These antigen-selective cell therapies are designed to convert therapeutically important native antigens expressed on the cell surface (tumor associated antigens, viral envelope proteins) into recruitment points of effector functions, and address the goal of major histocompatibility complex- and exogenous cytokine-independent activation of mature effector T-cells. The most promising and best characterized antigen-selective strategy is based on the genetic manipulation of the recognition specificity of T-cells by grafting the recognition specificity of a monoclonal antibody onto a lymphocyte triggering molecule (TCR-associated polypeptides, Fc epsilon RI-gamma chain). Upon encountering specific antigen, cells harboring chimeric immune receptors (CIRs) are able to undergo specific stimulation and kill antigen bearing cells in both in vitro and in vivo model systems. Initial studies have focused on terminally differentiated effector cell-based protocols. However, recent data indicate that progenitor cell-based therapies allow the permanent generation of stable populations of CIR-expressing cells of multiple lineages, leading to long-term persistent systemic immunity. Emerging gene therapy strategies are based on the use of biespecic antibody fragments. The advantages of these biespecic antibody-mediated immune recruitment (BIR) approaches (trans-recruitment and multieffector activation) could complement conventional CIR-based immunotherapies. Although further scientific progress is required regarding the selection of the ideal effector cell / s and the definition of the optimal targeting and recruitment systems, clinical trials recently initiated in patients with advanced cancer and human immunodeficiency virus infection should help us to determine the real efficacy of these approaches. The relevance of these and other emerging concepts to cell-mediated immunotherapy is discussed.