The successful implementation of gene therapy approaches in the clinic still awaits the development of optimized processes for the manufacturing of gene transfer vectors. To date, a majority of clinical trials have been using vectors derived from Murine Leukemia Viruses (MLV) (Andreadts et al. 1999, hltp://wvvw.wHev.gp.uk/genrned). Despite of their limitations (i.e. relatively low titer and activity, use limited to dividing cells), MLV vectors lead to permanent gene transfer and expression and the first clinical success of gene therapy has been obtained using this technology (Cavazzana-Calvo et al. 2000). From the technological point of view, the main problem associated with the use of retroviral vectors is the relatively low vector liter generallyobtained in non-optimized production systems. The titers seldom rise above 10~6 infectious particles per ml. The use of reactor systems in a perfusion mode has been shown to improve this situation (Merten et al. 2001A). Medium optimization would be a cost effective way to optimize the production systems, but only rudimentary results are available. One possibility of medium optimization consists in the reduction of the formation of toxic by-products, mainly lactate and ammonium, which are synthesized andexcreted by the cells due to an unbalanced metabolism of glucose and gfutamine. For instance, we could show that lactate at a concentration beyond 5 mmol/1 was growth inhibitory for TeFLY GA18 cells leading thus to a reduced vector titer (Merten et al.2001B). The reduction of the production and of the finally toxic concentration of these by-products can be achieved by either replacing glucose and glutamine by other nutrients which are differently metabolized, such as fructose (Cristofalo & Kritclievsky 1965, Imamura et al. 1982, Wolfrom et al. 1983, Zielke et al. 1984, Barngrover et al. 1985, Nahapetian et al. 1986) or glutamate (Hassell et al. 1987), or by reducing the glucose (Graff et al. 1965, Hu et al. 1987, Kurokawa et al. 1984, Konstantinov et al. 1995) or glutamine (Glacken et al. 1986, 1988) concentration in the medium, respectively.
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