Neutral Red versus MTT assay of cell viability in the presence of copper compounds

•Copper compounds (≥10 μM) interfere in MTT but not in Neutral Red assay of cell viability.•The interference seems related to a copper (II) interaction with formazan.•The Neutral Red assay is more sensitive than the MTT assay with P19 neurons.

Copper is essential for numerous physiological functions, and copper compounds may display therapeutic as well as cytotoxic effects. The MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide) assay is a standard test largely used in cytotoxicity studies. This report shows that low micromolar levels of copper compounds such as Cu(II)Urea2, Cu(II)Ser2 and CuCl2 can interfere with the MTT assay making improper the detection of formazan product of MTT reduction. Comparatively, the Neutral Red assay appears to be sensitive and showing no interference with these compounds. The lactate dehydrogenase alternative assay cannot be used because of inhibitory effect of these copper compounds on the enzyme activity.