The orally bioavailable copper salt of D-gluconic acid. In addition to its roles as an enzyme cofactor for cytochrome C oxidase and superoxide dismutase, copper forms complexes with the thiocarbamate disulfiram (DSF) forming DSF-copper complexes, which enhances the DSF-mediated inhibition of the 26S proteasome; proteasome inhibition may result in inhibition of cellular protein degradation, cessation of cell cycle progression, inhibition of cellular proliferation, and the induction of apoptosis in susceptible tumor cell populations.