Background/Objectives: Cancer remains one of the major challenges of our century. Organometallic ruthenium complexes are gaining recognition as a highly promising group of compounds in the development of cancer treatments. Methods: Building on the auspicious results obtained for [Ru(η⁵-C₅H₅)(PPh₃)(bipy)][CF₃SO₃] (TM34), our focus has shifted to examining the effects of incorporating bioactive ligands into the TM34 framework, particularly within the cyclopentadienyl ring. Results: In this study, we report the synthesis and characterization of two new ruthenium(II) complexes with the general formula [Ru(η⁵-C₅H₄CCH₃=R)(PPh₃)(bipy)][CF₃SO₃], where R represents a nicotinic acid derivative (NNHCO(py-3-yl)) (1) or an isoniazid derivative (NNHCO(py-4-yl)) (2). The complexes were fully characterized using a combination of spectroscopic techniques and computational analysis, revealing the presence of E/Z-hydrazone isomerism. Stability studies confirmed the robustness of both complexes in biological media, with compound 1 maintaining good stability in buffer solutions mimicking physiological (pH 7.4) and tumor-like (pH 6.8) environments. The cytotoxicity of the complexes was evaluated in vitro in several human cancer cell lines, namely melanoma (A375), alveolar adenocarcinoma (A549), epidermoid carcinoma (A431), and breast cancer (MDA-MB 231). Conclusions: Both compounds exhibited moderate to high cytotoxic activity, with complex 1 showing a greater propensity to induce cell death, particularly in the A431 and MDA-MB 231 cell lines.

Dysregulation of Fibroblast Growth Factor Receptors (FGFRs) signaling has been associated with breast cancer, yet employing FGFR-targeted delivery systems to improve cytotoxic agents is still sparsely exploited. Herein, we report four new bi-functional ruthenium-peptide conjugates (RuPCs) with FGFR-targeting and pH-dependent releasing abilities, envisioning the selective delivery of cytotoxic Ru complexes to FGFR(+)-breast cancer cells, upon specific accumulation and controlled activation at the acidic tumoral microenvironment. The antiproliferative potential of the RuPCs and active Ru complexes was evaluated in four breast cancer cell lines with different FGFR expression levels (SK-BR-3, MDA-MB-134-VI, MCF-7, and MDA-MB-231) and in the normal human dermal fibroblasts (HDF) cell line, at pH 6.8 and 7.4 that mimics the tumor microenvironment and normal tissues/bloodstream, respectively. The RuPCs showed higher cytotoxicity in cells with higher level of FGFR expression at acidic pH. Additionally, RuPCs showed up to 6-fold higher activity in the FGFR(+) breast cancer lines compared to the normal line. The release profile of Ru complexes from RuPCs corroborates the antiproliferative effects observed. Remarkably, the cytotoxicity and releasing ability of RuPCs were shown to be strongly dependent on the peptide conjugation position in the Ru complex. Complementary molecular dynamic simulations and computational calculations were performed to help interpret these findings at the molecular level. In summary, we identified a lead bi-functional RuPC that holds strong potential as a FGFR-targeted chemotherapeutic agent.