Targeting cancer stem cell OXPHOS with tailored ruthenium complexes as a new anti-cancer strategy

Background Previous studies by our group have shown that oxidative phosphorylation (OXPHOS) is the main pathway by which pancreatic cancer stem cells (CSCs) meet their energetic requirements; therefore, OXPHOS represents an Achille’s heel of these highly tumorigenic cells. Unfortunately, therapies that target OXPHOS in CSCs are lacking. Methods The safety and anti-CSC activity of a ruthenium complex featuring bipyridine and terpyridine ligands and one coordination labile position (Ru1) were evaluated across primary pancreatic cancer cultures and in vivo, using 8 patient-derived xenografts (PDXs). RNAseq analysis followed by mitochondria-specific molecular assays were used to determine the mechanism of action. Results We show that Ru1 is capable of inhibiting CSC OXPHOS function in vitro, and more importantly, it presents excellent anti-cancer activity, with low toxicity, across a large panel of human pancreatic PDXs, as well as in colorectal cancer and osteosarcoma PDXs. Mechanistic studies suggest that this activity stems from Ru1 binding to the D-loop region of the mitochondrial DNA of CSCs, inhibiting OXPHOS complex-associated transcription, leading to reduced mitochondrial oxygen consumption, membrane potential, and ATP production, all of which are necessary for CSCs, which heavily depend on mitochondrial respiration. Conclusions Overall, the coordination complex Ru1 represents not only an exciting new anti-cancer agent, but also a molecular tool to dissect the role of OXPHOS in CSCs. Results indicating that the compound is safe, non-toxic and highly effective in vivo are extremely exciting, and have allowed us to uncover unprecedented mechanistic possibilities to fight different cancer types based on targeting CSC OXPHOS.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This study was supported by a Fero Foundation Grant (B.S.,Jr.); Rámon y Cajal Merit Award (RYC-2012-12104) from the Ministerio de Economía y Competitividad, Spain (B.S.,Jr.); funding from the Beca Carmen Delgado/Miguel Pérez-Mateo from AESPANC-ACANPAN Spain (B.S.,Jr.); a Conquer Cancer Now Grant from the Concern Foundation (Los Angeles, CA, USA) (B.S.,Jr.); a Coordinated grant (GC16173694BARB; B.S.,Jr.) and SEED grant (IDEAS222917FERN; M.A.F.-M.) from the Fundación Asociación Española Contra el Cáncer (AECC); FIS grants PI18/00757 and PI21/01110 (B.S.,Jr.) and PT20/00045 (A.M.T.R and B.S.,Jr.), (co-fnanced through Fondo Europeo de Desarrollo Regional (FEDER) “Una manera de hacer Europa”); an IGNICIA proof of concept grant (IN855A-2018/16) “RuCSC - targeting cancer stem cells using ruthenium compounds”, an initiative of the Agencia Gallega de Innovación (GAIN) to facilitate the access of I+D+i projects to the market (J.L.M and B.S.,Jr.), a La Caixa Research Consolidate grant (CC21-20122; J.L.M and B.S.,Jr.); Spanish national grants PID2019-110320RB-I00 (M.A.F.-M.) from the Ministerio de Ciencia e Innovación (MCIN), IJC2019-040358-I funded by MCIN/AEI/10.13039/501100011033 (J.R.), PID2019-108624RB-I00, PDC2021121508-I00, PID2022-137318OB-I00 and ORFEO-CINQA network RED2018102387-T (J.L.M.), the Consellería de Cultura, Educación e Ordenación Univer‑ sitaria 2015-CP082, IN855A 2018/16, ED431C-2021/25 and Centro Singular de Investigación de Galicia accreditation 2019-2022: ED431G 2019/03 (J.L.M.), the European Union (European Regional Development Fund-ERDF correspond‑ ing to the multiannual fnancial framework 2014-2020), and the European Research Council Advanced Grant No. 340055, Proof-of-concept Grant No. 899334 (J.L.M.). a Max Eder Fellowship of the German Cancer Aid (111746) (P.C.H.); the German Research Foundation (DFG, CRC 1279 “Exploiting the human peptidome for Novel Antimicrobial and Anticancer Agents”) (P.C.H.).S