Guanidine Ligands as Versatile Scaffolds for the Development of Antitumor Ruthenium–Based Metallodrugs

The straightforward catalytic synthesis of guanidine ligands has been evaluated as a strategy for rapidly developing ruthenium metallodrugs as antitumor agents. A series of structurally guanidine ligands, obtained via guanylation reactions, were used as scaffolds for generating RAPTA‐C derivatives. Consequently, a novel family of guanidine–ruthenium derivatives was synthesized, fully characterized, and assessed as potential alternatives to platinum‐based chemotherapy. Their antitumor activity was evaluated in vitro across various cancer cell lines, using RAPTA‐C and cisplatin as metallodrug controls. Two ruthenium–guanidine derivatives were identified as the most promising candidates, exhibiting IC 50 values between 3 and 5 μM across multiple cancer cell lines. These compounds were selected for further investigation. Notably, their significant cytotoxicity in triple‐negative breast cancer (TNBC) cells suggests a mechanism of action independent of receptor status. In vitro experiments, such as cell cycle assessment, apoptosis detection, Matrigel invasion tests, and measurement of ROS production, corroborated this hypothesis. Compared to cisplatin and RAPTA‐C, the most promising ruthenium complexes, RuG3 and RuG8, demonstrated enhanced cytotoxicity across various cancer cell lines. As a proof of concept, fluorescence lifetime imaging microscopy (FLIM) was used to visualize the intracellular distribution of a luminescent derivative in MCF7 breast cancer cells. The results showed homogeneous cytoplasmic distribution, while the guanidine ligand contributed to increased stability. Furthermore, photophysical studies showed that in aqueous solution, the protonation of the guanidine group triggers the isomerization of the luminescent complex, yielding the active and more symmetrical species. Finally, in vivo safety studies in mice highlighted the potential of the selected compound as a promising candidate for further preclinical development in MIC. Overall, these findings provide valuable insights into the use of guanidines as ancillary ligands for designing new ruthenium‐based antitumor agents as an alternative to platinum‐based chemotherapy.