If you are planning to study the growth inhibition of HepG2 cells in vitro by using different concentrations of the drug, then MTT assay may be used because the MTT assay is commonly used to assess the cellular cytotoxicity caused by drugs (especially anticancer drugs).
I agree with Malcolm, but make sure you run all the controls. From my experience you have to be careful using the MMT assay. This is because The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay is based on the conversion of MTT into formazan crystals by living cells, this largely determines mitochondrial activity. However, many simple chemical reductants will give an identical signal, as can some media components, enzymes, and components excreted by some cells.