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Metallothionein expression and transient resistance to electrophilic antineoplastic drugs.

Intrinsic and acquired resistance to antineoplastic agents remains an important impediment to cancer therapy. Intracellular metallothioneins appear to be one factor in determining the responsiveness of malignant and normal cells to electrophilic anticancer agents. Metallothioneins are not only constitutively expressed but their expression can be transcriptionally activated by a host of endogenous and exogenous substances; this is a reversible phenomenon. This inducibility affords the possibility that cells can respond quickly to toxic substances and that under certain conditions transient drug resistance may occur. We review the evidence implicating metallothioneins in anticancer drug resistance and discuss this in the context of the possibility of transient resistance.