INTRODUCTION Since the pioneering work of Avery et al. in 1944 (1) the transmission of genetic material in procaryotes has been routinely accomplished by soluble DNA (transformation) (2), by bacteriophage (transduction) (3), or by direct contact between bacteria (conjugation) (4). Therefore, the introduction of foreign DNA performed later on eucaryotic cells was called transfection only for historical and semantic reasons. In the field of human gene therapy, various viral and synthetic vectors have been designed to introduce genes into cells. However, the relative ineffi- ciency of these different in situ gene transfer methodolo- gies is currently the major drawback of this therapeutic approach. One of the major concerns of gene therapy is the treatment of malignant diseases (5). Among the different strategies used in experimental or clinical studies, gene-directed enzyme prodrug therapy is an attempt to kill tumor cells by targeting in these cells a ‘suicide’ gene encoding for an enzyme that modifies a non-toxic pro- drug into a toxic molecule. Then, the specific conversion of the non-toxic prodrug to highly cytotoxic metabolites by the gene-modified malignant cells leads to their de- struction (6). Several enzyme/prodrug systems have been proposed for cancer therapy. These include for instance the Escherichia coli cytosine deaminase/5-fluorocytosine (7), the E. coli purine nucleoside phosphorylase/6-methyl- purine-2′deoxyriboside (8) or the herpes virus thymidine kinase/ganciclovir systems (9). In this later case, ganci- clovir, which is not a substrate for human kinases, is converted to ganciclovir-monophosphate by the herpes thymidine kinase-transduced gene, and then to the anti- metabolite triphosphate form by intracellular kinases. This gene therapy approach, which has shown promising results in experimental brain tumors in rodents, was recently applied to the treatment of human brain tumors (10). In this clinical protocol, the intratumoral implan- tation of murine fibroblast cells producing retroviral vectors was performed in patients with progressive growth of recurrent malignant brain tumors. Unfortunately, in spite of the detection of a local antitumor activity, this gene therapy approach failed to modify the clinical course of the treated patients. Therefore, the enhancement of the therapeutic efficacy of any suicide gene therapy re- quires at least to increase the bystander effect, to enhance the levels of transduction of the prodrug-converting gene and to extend the ability of the vector to reach infiltrated tumor cells. This later point could be achieved with the use of synthetic vectors, which are expected to spread Medical Hypotheses (1999) 52(6), 605–607 © 1999 Harcourt Brace & Co. Ltd Article No. mehy.1998.0816 Mycoplasma: a new potential vector for gene therapy? F. Berger, 1 C. Canova, 2 J.-M. Vicat, 1 A.-L. Benabid, 1 D. Wion 1 1 INSERM U318, Neurobiologie Préclinique, CHU Michallon, BP217, Grenoble, France 2 INSERM XR298, CHU Angers, Angers, France Summary The introduction of foreign genetic material in living cells is the basis of the current protocols of gene therapy. The concept that the de novo synthesis of a foreign therapeutic protein requires the entrance of the corresponding gene into target cells via virus or synthetic vectors is directly inherited from experiments on bacterial transduction or transformation. However, the difficulties inherent in the penetration and the expression of foreign DNA into eucaryotic cells are probably responsible for the low efficiency of this therapeutic approach. In this paper, we explore the possibility of avoiding these limiting critical steps by expressing the foreign gene on the surface rather than inside the target cells by the use of mycoplasma, the smallest reported living cell. The absence of transfer of genetic information between this vector and eucaryotic target cells, the sensibility of mycoplasmas to antibiotics and their cytadherance are among the interesting features of this potential vector. The interest of this new vector in the case, e.g. of the gene-directed enzyme prodrug therapy of solid tumors, is discussed. Received 22 June 1998 Accepted 25 August 1998 Correspondence to: Didier Wion MD, INSERM U318, Neurobiologie Préclinique, CHU Michallon, BP217, 38043 Grenoble, Cedex 09, France. E-mail: Didier.Wion@ujf-grenoble.fr 605