Interaction of lovastatin with mitoxantrone in human colon cancer cell line in vitro
Authors: George Geromichalos, Dimitrios Paikos, Eleni Stergiou, Ioannis Stergiou, Athanasios Papageorgiou, Pelagia Stravoravdi, Ioannis Moshos, Soultana Voyatzi, Evangelos Briasoulis
Journal: Cancer Research (2007)

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA

4407 

Purpose: Lovastatin (LOV), has profound effects upon several cellular functions intervening with signal transduction pathways of cholesterol metabolism and isoprenylation. LOV is a specific inhibitor of hydroxymethylglutaryl-CoA (HMG-CoA) reductase, the primarytarget in the current clinical treatment of hypercholesterolemias. LOV has been reported to decrease the production of isoprenoids and thereby inhibits the isoprenylation of several proteins such as Ras, Rap and G. Manipulating cholesterol metabolism may open several possibilities of interfering with the growth of malignant cells. Mitoxantrone (MXT), a clinically important antitumor drug, is an effective drug inhibiting DNA topoisomerase-II (topo-II), acting also as a DNA binder. Since LOV and MXT attack different enzymes in signal transduction, we tested the hypothesis that these drugs might act synergistically. In order to investigate to what extent the inhibition of cholesterol biosynthesis pathway is involved in the signal transduction of apoptosis after MXT-induced DNA damage, or the role of MXT in the mechanism of HMG-CoA reductase inhibition by LOV, molecular modeling studies incorporating docking approaches, have also been adopted. This approach could be informative in pointing out the mechanism of the interaction in a molecular level.

Methods: HT29 human colon cancer cells were grown in adherence in 96-well microplates and exposed to both agents for 48 hrs. Drug cytotoxicity was estimated using the SRB colorimetric assay. The combined drug interaction was assessed with the median-effect analysis and the Combination Index (CI). In our calculations the X-ray crystallographic structures of both target enzymes were used. 

Results: The CI values illustrated mainly synergistic and only in some cases additive and antagonistic effects according to concentration ratio applied and the level of cell-kill. The order of synergy (CI  100:1 > 200:1. Moreover, from docking studies of MXT and LOV on the structure of HMG-CoA reductase, it is concluded that both molecules adopt almost the same binding position inside the ligand binding cavity of the active site of the enzyme. Comparison with the structures of LOV and MXT bound to both human enzymes (HMG-CoA reductase and topo-II) revealed a similar mode of binding but marked differences in specific interactions that account for their differences in affinity. Binding of MXT at the same time with LOV may enhance LOV binding and subsequent enzyme inhibition. This could explain the synergistic interaction between the two drugs.

Conclusions: The in vitro results suggest that LOV in combination with MXT may be more effective than MXT alone in treating human colon cancer and might allow the use of lower, less toxic doses of MXT. The present data, provide a basis for the rational design of animal tumor model protocols.