Imexon (NSC-714597) is an
aziridine-containing imminopyrolidone in Phase I clinical trials. The current studies compared
biological indices of cytotoxicity in 7 human
multiple myeloma (MM) cell lines to develop a correlative model for
imexon sensitivity. In the MM cell lines there was a wide range of sensitivity to
imexon measured by standard cytotoxicity assays (MTT) and by viability/apoptosis/
necrosis (
Annexin-V-FITC/PI) measurements. The following sensitivity pattern was observed in order of decreasing sensitivity: IM-9 > 8226/S > MM.1S, ARH-77, H929 > 8226/I > U266. The same descending rank order was seen for loss of mitochondrial membrane potential (
MMP), generation of
reactive oxygen species (ROS) and, at high
drug concentrations,
thiol depletion. Cell cycle analysis showed
imexon sensitive cells accumulate at the G2/M interphase. Although there was a positive correlation between increasing CuZnSOD levels and
imexon resistance, no relationship was found for
catalase, Bcl-2, mitochondrial
thioredoxin or MnSOD levels. These findings suggest consistent phenotypes for
imexon sensitivity and resistance in human MM cell lines exposed to
drug for 48 h, with a combination of apoptosis and
necrosis. Resistance is correlated with CuZnSOD expression, reduced
drug accumulation, lack of ROS generation and maintenance of
MMP. Oxidation of cellular
thiols occurs only at high (supra-cytotoxic)
drug levels and is, therefore, weakly correlated with cytotoxicity. This unique mechanism involving oxidation and the previously reported absence of myelosuppression suggests that
imexon may be rationally combined with existing
cytotoxic agents to improve therapeutic activity in MM.