Metformin is a widely prescribed medication for the treatment and management of type 2 diabetes. It belongs to a class of biguanides, which are characterized by a wide range of diverse biological properties, including anticancer, antimicrobial, antimalarial, cardioprotective and other activities. It is known that biguanides serve as excellent N-donor bidentate ligands and readily form complexes with virtually all transition metals. Recent evidence suggests that the mechanism of action of metformin and its analogues is linked to their metal-binding properties. These findings prompted us to summarize the existing data on the synthetic strategies and biological properties of various metal complexes with metformin and its analogues. We demonstrated that coordination of biologically active biguanides to various metal centers often resulted in an improved pharmacological profile, including reduced drug resistance as well as a wider spectrum of activity. In addition, coordination to the redox-active metal centers, such as Au(III), allowed for various activatable strategies, leading to the selective activation of the prodrugs and reduced off-target toxicity.

Recent studies have suggested that the commensal microbiome positively affects cancer prognosis and treatment outcomes. However, only a few strategies for regulating the microbiome composition have been reported. In this study, we identified gold(I) complexes that selectively inhibited nonbeneficial bacterial strains in vitro without affecting commensal Lactobacillus strains. In contrast, clinically used drugs demonstrated comparable effects on both commensal and noncommensal strains. Consistent with the in vitro results, the selected gold(I) complex induced favorable changes in the intratumoral and gastrointestinal microbiomes in vivo. Furthermore, its anticancer efficacy was found to be dependent on the composition of microbiome and correlated with the production of short-chain fatty acid bacterial metabolites. Structure–activity relationship studies have dissected the contribution of each structural component in both the in vivo efficacy and microbiome-modulating properties. Transcriptomic analysis of tumors revealed microbiome-associated gene signaling pathways. These findings provide valuable insights for future research on microbiome-modulating anticancer drugs, presenting potential avenues to optimize cancer treatment outcomes and mitigate side effects such as gastrointestinal dysbiosis. Furthermore, our study provides insights into the involvement of microbiome in the mechanism of action of metal-based chemotherapeutics.