**1. Introduction**

Today, humans consume food and vegetables rich in phenolic compounds and are continuously exposed to increased volume of xenobiotics. These chemicals such as phenolic compounds as well as pharmaceutical agents induce cellular toxicity and/or genomic instability most especially in the liver [1]. These chemicals are absorbed and biotransformed by the liver to their metabolites that may be less or more toxic to cells. Consequently, the evaluation of hepatocytotoxicity and erythropoietic cytotoxicity are key components of safety assessment during drug development [2]. Cytotoxicity is the ability of chemicals to destroy the functioning of living cells [3, 4]. Cytotoxic

chemicals cause damages to the components of DNA and thus induce mutations that may increase the risk of cancer development. Cytotoxicity and cell viability assessments are based on targeting of functions of different cells, including the liver and bone marrow [5, 6].

Cytotoxicity assays are experimental methods used in pharmacology and *in vitro* toxicology studies to measure loss of cellular functions [5–7]. These assays generally screen xenobiotics and predict human toxicity. These assays predict the risk of these xenobiotics towards human health by relating the cytotoxic effects between the *in vitro* and *in vivo* systems. There are different *in vitro* cytotoxic assays that have been employed in this stead with different results [5–7]. In these *in vitro* systems commonly used, cells are treated or exposed to a chemical or test compound and incubated for some period. Afterwards, a marker is measured to reflect the number of viable cells present compared to the positive (toxin) and negative (vehicle) control treatments. In addition, dead cells are also evaluated to distinguish between cytotoxicity and growth arrest. Knowledge of the number of live and dead cells present during or after the end of the experiment enhances the statistical robustness of these assays [5–8]. These assays commonly estimate dead cells via two common ways: (a) penetration of an otherwise non-permeable dye into the cells due to loss of cell membrane integrity; (b) the ability of indicator molecules to partition into a compartment not achievable if the cell membrane is intact. Therefore, these cytotoxicity assays directly or indirectly assess cytotoxicity by providing information on cell membrane integrity, cell metabolism, cell machinery, cell mortality and cell proliferation [5–8]. Some of these sensitive and reproducible assays are used to obtain colorimetric, luminometric and fluorometric measurements hence entails the use of different dyes such as trypan blue exclusion dye, neutral red, acridine orange and propidium iodide [1, 9–12]. These dyes offer different advantages and disadvantages hence a combination of two or more dyes is usually employed in *in vitro* studies to avoid overestimation or underestimation of the toxicity of a chemical substance thereby increasing the reliability of the results obtained.
