Determination of IC50

The IC₅₀ of each drug was assessed as a single agent over a period of 72 h. The inhibitory form of the Hill function was used to fit the concentration-response curves.

with R, the number of per mL cells (2 mL per well); R0_cells, the baseline number of cells per mL at zero drug concentration (control); I_max, the maximum inhibition; IC₅₀, the concentration that inhibits 50% of the cell growth, γ, the Hill coefficient, and C, the concentration of the drug. Because of different seeding numbers due to replicate experiments, the baselines were fixed to experimental values for TFP only.

The equation fittings were performed using the software ADAPT 5 (15) with the maximum likelihood method. The variance model used was as follows:

with Y(1) the response, PV(1) the intercept fixed to 0.001, and PV(2) the slope.

The IC₂₀, IC₄₀, and IC₆₀ values were calculated from the following:

with IC_X the concentration inhibiting X% of the maximal effect.