Cellular Influx and Cytotoxicity of Oxaliplatin Analogues

Abstract

This thesis describes the first systematic investigation of the relationship between lipophilicity and reactivity on the one hand and cellular accumulation and cytotoxic activity on the other hand of a new class of oxaliplatin derivatives with different substituents at position 4 of the cyclohexane ring and different leaving ligands. Furthermore, the contribution of hCTR1 and hOCT1-3 on the influx of oxaliplatin was investigated. Information regarding the influence of oxaliplatin on the expression of these transporters, as well as on their localization inside cancer cells is still limited. In contrast to the studies conducted in the past, in this study no transfected cell lines were used.<br /> Lipophilicity and reactivity are important determinants of the platinum influx. In contrast to the early influx phase, in which lipophilicity predominantly governs the influx rate, the late influx phase is mainly controlled by the reactivity of a platinum compound. Passive diffusion is suggested as the preponderant influx mechanism in the early influx phase and protein-mediated transport as the major influx mechanism in the late influx phase.<br /> Lipophilicity and reactivity are also determinants of the cytotoxic potency of platinum compounds and of tumor resistance. Although lipophilicity enhances the influx of platinum complexes, especially in the early influx phase, an increased lipophilicity leads to a reduction of cytotoxic potency of the investigated oxaliplatin analogues. This might be due to alterations in the reactivity of platinum complexes with bulky substituents in the intracellular environment or increased intracellular sequestration of lipophilic complexes. On the other hand, an increased lipophilicity is associated with the advantage of overcoming resistance. This does not seem to be related to increased influx. Changes in the reactions occuring before binding to the DNA, as well as different recognition and processing of the formed DNA adducts due to the variation of the carrier ligand might be another explanation for this phenomenon which, however, requires clarification in future investigations. Contrary to lipophilicity, an increase in reactivity enhances the cytotoxic potency of a platinum compound. Furthermore, resistance against platinum complexes of low reactivity appears to be reduced, possibly because these compounds are less affected by cytosolic inactivation. Therefore, a well-balanced relationship between lipophilicity and reactivity may be a starting point for optimization of the oxaliplatin structure. A log P value of approximately 0 and a low reactivity should support (a) passive diffusion as well as (b) the possibility of overcoming resistance.<br /> In any event, passive diffusion represents an important mechanism of oxaliplatin influx. Furthermore, it is supposed that oxaliplatin is transported by hCTR1 and that this transport is reduced in resistant cells. It is questionable whether the thereby decreased influx contributes to resistance. Influx of oxaliplatin via hOCT1 occurs in sensitive cells, however is unlikely in resistant cells. This fact might partly account for the reduced influx of oxaliplatin in resistant cells and contribute to resistance of HCT-8ox cells. hOCT2 is assumed to be involved in the influx of oxaliplatin in both cell lines to a similar extent suggesting no major relevance of this transporter for resistance. It could be demonstrated that cimetidine, routinely used as inhibitor of hOCT2 in influx studies, interacts with oxaliplatin under physiological conditions. The data strongly suggest that the results of studies, in which a platinum complex is coincubated with a transporter inhibitor, should be reinterpreted. hOCT3 may participate in cellular processing of oxaliplatin. However, its involvement in the influx of the platinum drug remains questionable. The relevance of this transporter for oxaliplatin resistance cannot be ruled out, but requires more detailed investigation.<br /> The results of this thesis contribute to the elucidation of influx of platinum compounds and of resistance provoked by reduced influx. Based on these results platinum complexes with improved activity against oxaliplatin-resistant tumor cells can be developed. Additionally, the results may support the efforts to refine individualized chemotherapy by consideration of the expression profile of the investigated transport proteins.