Sulforhodamine B (SRB), an aminoxanthene dye, has been used as an assa
y for total cell protein, initially developed as an endpoint assay for
in vitro screening of antitumour agents. In this paper it was investi
gated as a possible endpoint for a cytotoxicity assay using CHO cells.
It is a robust assay with a stable colorimetric endpoint, capable of
semi-automation using microtitre equipment. At optimum concentrations
(0.05-0.1% SRB) the assay is linear with respect to cell number over a
range of 5 x 10(3) to 10(5) cells. In comparative studies with the ne
utral red assay the SRB assay was more sensitive, and in cytotoxicity
assays with test compounds gave comparable dose-response curves. The c
ytotoxicity of five divalent metal chlorides was assessed using the SR
B assay. The order of toxicity was Cd > Hg > Zn > Mn > Mg, that is sim
ilar to the expected in vivo ranking. 16 compounds with reported oral
LD(50) (rat) ranging from 25,800 mg/kg (glucose) to 1 mg/kg (mercuric
chloride) were tested in the assay. The relative toxicities of the com
pounds in the in vitro SRB assay were similar to the relative in vivo
toxicities. The exceptions could be explained by the chemistry of the
compounds and could be attributed to pharmacokinetic properties or mec
hanism of action. This assay can therefore be used to rank chemically
similar compounds but is unsuitable as a precise predictor of in vivo
toxicity.