Synergistic combination of microtubule targeting anticancer fludelone with cytoprotective panaxytriol derived from panax ginseng against MX-1 cells in vitro: experimental design and data analysis using the combination index method.

This brief article focuses on two aims: i) To investigate the in vitro pharmaco-dynamic interactions of combining synthetic potent microtubule targeting anticancer agent, Fludelone (FD) with cyto-protective agent, Panaxytriol (PXT) derived from Panax ginseng, and ii) To illustrate step-by-step operation for conducting two-drug combination in vitro using the combination index method, in terms of experimental design, data acquisition, computerized simulation and data interpretation. The Chou-Talalay method for drug combination is based on the median-effect equation, which provides the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI<1, =1, and >1 indicates synergism, additive effect and antagonism, respectively. Based on these algorithms, computer software, CompySyn, is used for determining synergism and antagonism at all doses or effect levels simulated automatically. The use of Chou-Talalay's CI method in quantifying synergism or antagonism is increasing steadily during the past two decades, however, confusing questions and pitfalls were still frequently raised by insufficient understanding of the theory, especially reflected when researchers trying to use the computerized software to design and conduct experiments. In order to specifically address the confusions and to illustrate the practical features of this method, in this paper, a selected example is given based on our unpublished data regarding the combinational pharmacologic interactions of FD and PXT against the growth of breast cancer cell line MX-1. The step-by-step operation from experimental design to the real data analysis is illustrated. The results indicated that FD and PXT combination in vitro exerted synergistic effect when cell growth inhibition was greater than 45%, with CI ranged 0.836-0.609 for the fractional inhibition of Fa=0.50~0.90, as shown by the Fa-CI plot and by the isobologram. Thus, quantitative conclusion of synergism is obtained using the Chou-Talalay CI method, under the well-defined simple conditions for the FD and PXT combinations in vitro.