When I tested the toxicity of my inhibitor on MDCK, all the cells die at 100 uM. However interestingly at 200 uM or 300 uM the MTT values go up back to half the negative control! Any idea what's up here?
The MTT assay is a measure of absorbance at certain wavelength - say around 550 nm. If your drugs have absorbance at this range, then, the higher the drug concentration the higher the absorbance - or the loss of MTT absorbance by your drugs is compensated by the direct absorbance of the drugs in the wavelength used. That is why we use both a blank for the MTT and another blank for all concentrations of the drugs in the absence of the MTT. If you subtract the absorbance you get from the drug-blank, it would come close to the blank that you said you did.
Do you know that some drugs directly affect the absorbance of MTT? HABTEMARIAM, S. (1995). Catechols and quercetin reduce MTT through iron ions: a possible artefact in cell viability assay. Phytotherapy Res. 9(8), 603–605
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