Antidiabetic and antimalarial biguanide drugs are metal-interactive antiproteolytic agents

Various biguanide derivatives are used as antihyperglycemic and antimalarial drugs (e.g., 1,1-dimethyl biguanide (metformin), phenylethyl biguanide (phenformin), N-(4-chlorophenyl)-N'-(isopropyl)-imidodicarbonimidic diamide (proguanil)); however, no common mechanism has been suggested in these controversial therapeutic actions. Biguanides bind endogenous metals that inhibit cysteine proteases independently, e.g., Zn(2+), Cu(2+), Fe(3+). Here, various biguanide derivatives are reported to be metal-interactive inhibitors of cathepsin B from mammals and falcipain-2 from Plasmodium falciparum. Structural homologies were identified among the Phe-Arg protease substrate motif and the metal complexes of phenformin and proguanil. Molecular modeling revealed that the position of the scissile amide substrate bond corresponds to the biguanide-complexed inhibitory metal when the phenyl groups are homologously aligned. Binding of the phenformin-metal complex within the active site of human cathepsin B was modeled with computational docking. A major binding mode involved binding of the drug phenyl group at the protease S2 subsite, and the complexed inhibitory metal shared between the drug and the protease Cys29-His199 catalytic pair. Cysteine protease inhibition was assayed with carbobenzyloxy-PHE-ARG-7-aminomethylcoumarin substrate. In the absence of metal ions, phenformin was a weakly competitive protease inhibitor (apparent K(i) several microM); however, metformin was noninhibitory. In contrast, the metal complexes of both metformin and phenformin were protease inhibitors with potency at therapeutic concentrations. Biguanide-metal complexes were more potent cysteine protease inhibitors than either the biguanide or metal ions alone, i.e., synergistic. Similar to chloroquine, therapeutic extracellular concentrations of metformin, phenformin, and proguanil caused metal-interactive inhibition of lysosomal protein degradation as bioassayed in primary tissue using perfused myocardium. The biguanide moiety is identified as a past and future structural scaffold for synthesis of many protease inhibitors. Results are discussed in relation to Zn(2+)-interactive inhibition of insulin degradation in hormone target tissues, and Fe(3+)-interactive inhibition of hemoglobin degradation in parasite food vacuoles. Previous studies on insulin hypercatabolism and insulin resistance are speculatively reviewed in light of present findings.