The redox balance of cells is key to normal cell physiology. It is maintained by 3 systems: GSH/GSSG, NADPH/NADP; Thioredoxin (red)/Thioredoxin (oxd). Of these 3 systems, GSH/GSSG is the most widely studied for its implication in diseased states and for the development of rational therapeutic approaches (Townsend, A. J., Leone-Kabler, S., Haynes, R. L., Wu, Y., Szweda, L., and Bunting, K. D. (2001). Selective protection by stably transfected human ALDH3A1 (but not human ALDH1A1) against toxicity of aliphatic aldehydes in V79 cells. 130-132, 261-273). The diseased states associated with an imbalance in GSH/GSSG include major pathologies like cancers (Estrela, J. M., Ortega, A., and Obrador, E. (2006). Glutathione in cancer biology and therapy. Crit. Rev. Clin. Lab. Sci. 43, 143-181; O'Brien, M. L., and Tew, K. D. (1996). Glutathione and related enzymes in multidrug resistance. Eur. J. Cancer Oxf. Engl. 1990 32A, 967-978). Their one common aetiology is oxidative stress brought about by ROS and/or Reactive Nitrogen Species (RNS) that first cause a decrease in GSH due to the direct detoxification of ROS and RNS. This initial decrease in GSH is followed subsequently by a compensatory increase in GSH synthesis that cells bring into play in order to continue the detoxification of ROS/RNS and of newly formed electrophilic products such as 4-hydroxynonenal (HNE) and malondialdehyde (MDA)) produced by ROS attack on cellular lipids (Esterbauer, H., Schaur, R. J., and Zollner, H. (1991). Chemistry and biochemistry of 4-hydroxynonenal, malonaldehyde and related aldehydes. 11, 81-128).
The GSH paradox in cancer cells is that instead of the deficit in intracellular GSH that would have been expected, it is precisely the opposite that was found experimentally in many different cancer cells (Estrela, J. M., Ortega, A., and Obrador, E. (2006). Glutathione in cancer biology and therapy. Crit. Rev. Clin. Lab. Sci. 43, 143-181). But this increase in GSH has negative therapeutic repercussions as it protects cancer cells from chemo and radio therapies (Carretero, J., Obrador, E., Esteve, J. M., Ortega, A., Pellicer, J. A., Sempere, F. V., and Estrela, J. M. (2001). Tumoricidal activity of endothelial cells. Inhibition of endothelial nitric oxide production abrogates tumor cytotoxicity induced by hepatic sinusoidal endothelium in response to B16 melanoma adhesion in vitro. J. Biol. Chem. 276, 25775-25782).
In addition, if low levels of GSH must be obtained in cancer cells for chemotherapy to be effective, this is not the case for normal cells for not inducing collateral damage thereto.
The therapeutic approaches that are presently being used to lower cellular GSH in order to combat the chemoresistance of cancer cells, target GSH itself or the enzymes involved in GSH synthesis, GSH degradation and GSH efflux. There are already 10 GSH-lowering compounds that are in phases I, II and III of clinical trials as anticancer agents (Tew, K. and Townsend D (2011) Redox platforms in cancer drug discovery and development. Curr. Opin. Chem. Biol. 15, 156-161). They all have to be administered in combination with standard anti-cancer drugs, e.g. cyclophosphamide, taxol, vincristine, melphalan, etc.
Furthermore, the enzymes targeted by these GSH-lowering drugs are those involved in GSH synthesis (gamma glutamyl cysteine ligase), GSH degradation (gamma-glutamyl transpeptidase) and GSH efflux (GSH-S-transferase). These same enzymes are however essential for protecting normal cells from ROS attack. Hence, there is a strong possibility of collateral damage to normal cells as the drugs cannot be delivered selectively to cancer cells and to cancer cells only.
In view of this, there is a need to find other therapeutic solutions which specifically and selectively target GSH in cancer cells.
The inventors of the present invention have unexpectedly found that a combination comprising an aminothiolester compound or a pharmaceutically acceptable salt thereof, in particular the S-methyl 4-(dimethylamino)-4-methylpent-2-ynethioate or a pharmaceutically acceptable salt thereof, and more particularly the 4-(Dimethylamino)-4-methyl-2-pentynethioic acid S-methyl ester fumarate, and a compound able to increase the H2O2 level in cancer cells of a subject, is useful as a medicament and able to treat cancer in a subject, wherein cancer cells of said subject do not overproduce H2O2. In particular, they found that a combination comprising an aminothiolester compound or a pharmaceutically acceptable salt thereof, in particular the S-methyl 4-(dimethylamino)-4-methylpent-2-ynethioate or a pharmaceutically acceptable salt thereof, and more particularly the 4-(Dimethylamino)-4-methyl-2-pentynethioic acid S-methyl ester fumarate, and a compound able to increase the H2O2 level in cancer cells of a subject, is useful as a medicament and able to treat cancer in a subject, wherein cancer cells of said subject do not overproduce H2O2 and have a level of GSH below 0.5 nmol for 25 000 cells.
Without being bound by any theory, when the compound able to increase the H2O2 level in cancer cells of a subject would have induced an increase in H2O2 level in the cancer cells, then the aminothiolester compound or a pharmaceutically acceptable salt thereof, would increase the levels of intra cellular metabolites that are produced by H2O2 attack, and, at the same time, GSH would thus be consumed in the detoxification of these electrophilic metabolites. As a result, insufficient GSH would be available in cancer cells to act as a scavenger of H2O2. Hence, levels of H2O2 should increase and should trigger-off the H2O2-dependent mechanisms in the mitochondrial (intrinsic) pathway of apoptosis.
In normal cells that have not undergone initially an attack by H2O2, intracellular GSH levels are already high (due to the absence of H2O2) so that the levels of any H2O2-induced electrophiles are below those in their cancer counterparts. However, H2O2 levels in normal cells can concomitantly rise when a compound able to increase the H2O2 level in cancer cells of a subject is used, if this compound is able to increase the level of H2O2 in both normal and cancer cells. In this latter case, H2O2 levels in normal cells will however still be lower than those in cancer cells and will thus remain below their apoptotic threshold upon treatment with the aminothiolester compound according to the invention or a pharmaceutically acceptable salt thereof.