EC50 values of dox in cell cultures depends a lot on the incubation time.

Eg. in breast cancer cell HCC38 it changing from 270 nM (after 24 h of incubation) to 12 nM (after 96 h of incubation).

you should better do this experiment in triplicate by yourself and take your calculated value as a reference for future experiments.

As previously reported IC50 values can be influenced by various factors i.e;

Incubation time, assay sensitivity, instrument accuracy, Dox purity.