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Mitochondrial toxicity assessment

Impairment of mitochondrial function is increasingly implicated in the etiology of drug-induced toxicity. For example, mitochondrial dysfunction was found to play a role in the toxicity of troglitazone and cerivastatin which were withdrawn from the US market in 2000 and 2001 respectively.¹

Mitochondria produce >90% of the cellular energy requirements in the form of adenosine triphosphate (ATP) via oxidative phosphorylation.

Many cell lines developed for use in vitro are metabolically adapted for growth under hypoxic and anaerobic conditions using high glucose media and derive most of their energy from glycolysis rather than mitochondrial oxidative phosphorylation (a process termed the Crabtree effect). This reduces the cells susceptibility to mitochondrial toxicants.²

Circumventing the Crabtree effect by replacing glucose with galactose in the cell media increases the reliance of the cells on mitochondrial oxidative phosphorylation to obtain ATP. By comparing the toxic effects of different drugs in the glucose and galactose media, it is possible to detect mitochondrial impairment and identify if this is a primary effect or secondary to other cytotoxic mechanisms.²

Cyprotex evaluates mitochondrial toxicity using HepG2 cells, U-87 MG cells or other cell lines (available on request).

‘Clinical signs of drug induced mitochondrial impairment range from obvious and severe to more subtle reflections of modest loss of mitochondrial capacity such as exercise intolerance, malaise, and mild lactic acidosis.’

¹Dykens JA and Will Y (2007) Drug Discovery Today 12; 777-785

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Protocol

Mitochondrial toxicity assessment protocol

Media Assessed	Supplemented DMEM containing 25 mM glucose Supplemented DMEM containing 10 mM galactose
Cells Types Available	HepG2, U-87 MG, other custom cell lines
Test Compound Concentration	0.03, 0.1, 0.3, 1, 3, 10, 30 and 100µM (different concentrations available)
Final DMSO Concentration	0.5 %
Number of Replicates	3 replicates per concentration
Quality Controls	0.5% DMSO (vehicle control) Papaverine (positive control) Chlorpromazine (negative control)
Compound Requirements	100µL of 20mM solution
Analysis Method	MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide], determined by absorbance.
Data Delivery	IC₅₀ determination in the presence of glucose and galactose media Minimum effective concentration (MEC) determination in the presence of glucose and galactose media Fold change in Glu/Gal IC₅₀

Drug induced mitochondrial toxicity is shown by members of important drug classes, including the thiazolidinediones, statins, fibrates, antivirals, antibiotics, and anticancer agents.²

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Data

Data for mitochondrial toxicity assessment

The Cyprotex mitochondrial toxicity assay has been validated using a number of different mitochondrial toxicants and non-mitochondrial toxicant compounds.

A mitochondrial toxicant is indicated by a greater than five-fold change in IC₅₀ value observed in the galactose media compared to the glucose media. Figure 1 illustrates the data for the mitochondrial toxicant, papaverine (A), and the non mitochondrial toxicant, tamoxifen (B). A 7.91 fold increase in IC₅₀ value is observed for papaverine in galactose media compared with glucose media (table 1). No fold change was observed with the non-mitochondrial toxicant (tamoxifen).

Figure 1

Effect of papaverine (A) and tamoxifen (B) on HepG2 cell loss when cells are grown in glucose or galactose media.

Compound	Media	Minimum Effective Concentration( µM)	IC₅₀ (µM)	Fold Change in IC₅₀
Papaverine	Glucose	4	15.5	7.91
Papaverine	Galactose	1	1.96	7.91
Tamoxifen	Glucose	40	14.3	0.85
Tamoxifen	Galactose	40	16.9	0.85

Table 1

IC₅₀ fold change when HepG2 cells are exposed to papaverine or tamoxifen in galactose media compared with glucose media.

References

¹ Dykens JA and Will Y (2007) Drug Discovery Today 12; 777-785

² Marroquin LD et al. (2007) Toxicol Sci 97(2); 539-547

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