Defining Resistance and Sensitivity Thresholds for IC50
IC50 has an exact definition, that is half-maximal inhibitory concentration. To get this you need to make a series of dilutions of each of your drugs and test your cell lines in each concentration and assess how they grow (eg measure Optical density, colony size, cell count or whatever applies to your cells and setup). Then you can set up a inhibition curve that is a function of the concentration. This most likely looks like a sigmoid curve and the inflection point gives you the IC50 value.

An example image form the wikipedia page of IC50 (recommended to read):


So in your case, you already have IC50 values (for wild types I assume), and you need to assess your (probably mutant ?) cell lines to see if they are sensitive or resistant. Standard lab practice recommends using wild-type controls and at least 3 parallel controls. You can compare the average growth of WTs to your cell line at the IC50 concentration you already got in your dataset, and calculate standard deviation, and 95% confidence interval. With some simplification if the 95%CI of your cell line is greater than the wt 95%CI it is most likely that your cell line is resistant to that drug, on the other hand if its lower then that cell line is sensitive to the drug in question.