Citation: Teixeira-Guedes, C.; Brás, A.R.; Teixeira, R.G.; Valente, A.; Preto, A. Ruthenium(II)–Cyclopentadienyl- Derived Complexes as New Emerging Anti-Colorectal Cancer Drugs. Pharmaceutics 2022, 14, 1293. https://doi.org/10.3390/ pharmaceutics14061293 Academic Editor: Carlos Alonso- Moreno Received: 18 May 2022 Accepted: 15 June 2022 Published: 17 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). pharmaceutics Article Ruthenium(II)–Cyclopentadienyl-Derived Complexes as New Emerging Anti-Colorectal Cancer Drugs Catarina Teixeira-Guedes 1,2,3 , Ana Rita Brás 1,2 , Ricardo G. Teixeira 4 , Andreia Valente 4, * ,† and Ana Preto 1,2, * ,† 1 Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal; cigteixeira@gmail.com (C.T.-G.); pg31015@alunos.uminho.pt (A.R.B.) 2 Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 3 Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Trás dos Montes and Alto Douro, Quinta de Prados, 5000-801 Vila Real, Portugal 4 Centro de Química Estrutural, Institute of Molecular Sciences and Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal; rjteixeira@fc.ul.pt * Correspondence: amvalente@fc.ul.pt(A.V.); apreto@bio.uminho.pt (A.P.) † These authors contributed equally to this work. Abstract: Colorectal cancer (CRC) is one of the most common malignancies and one of the leading causes of cancer-related death worldwide, urging the need for new and more efficient therapeutic approaches. Ruthenium complexes have emerged as attractive alternatives to traditional platinum- based compounds in the treatment of CRC. This work aims to evaluate anti-CRC properties, as well as to identify the mechanisms of action of ruthenium complexes with the general formula [Ru(η 5 -C 5 H 4 R)(PPh 3 )(4,4 ′ -R ′ -2,2 ′ -bipyridine)][CF 3 SO 3 ], where R = CH 3 , CHO or CH 2 OH and R ′ = H, CH 3 , CH 2 OH, or dibiotin ester. The complexes (Ru 1–7) displayed high bioactivity, as shown by low IC 50 concentrations against CRC cells, namely, RKO and SW480. Four of the most promising ruthenium complexes (Ru 2, 5–7) were phenotypically characterized and were shown to inhibit cell viability by decreasing cell proliferation, inducing cell cycle arrest, and increasing apoptosis. These findings were in accordance with the inhibition of MEK/ERK and PI3K/AKT signaling pathways. Ruthenium complexes also led to a decrease in cellular clonogenic ability and cell migration, which was associated with the disruption of F-actin cytoskeleton integrity. Here, we demonstrated that ruthenium complexes, especially Ru7, have a high anticancer effect against CRC cells and are promising drugs to be used as a new therapeutical strategy for CRC treatment. Keywords: ruthenium complexes; anti-colorectal cancer drugs; apoptosis; cell cycle arrest; cytoskeleton 1. Introduction Cancer is one of the leading causes of death worldwide and an important barrier to increasing average life expectancy. According to World Health Organization (WHO), it is responsible for approximately one in every six deaths, accounting for nearly 10 million deaths in 2020 all over the world [1,2]. In particular, colorectal cancer (CRC) ranks as the third most commonly diagnosed cancer in both men and women and the second in terms of higher mortality [2,3]. The current CRC treatment is based on surgery and chemotherapy, that is, 5-fluorouracil (5-FU), the most used chemotherapeutic agent [4,5]. 5-FU is an analog drug of uracil that, at the intracellular level, is converted to several active metabolites, disrupting RNA and DNA synthesis [6]. However, due to its low efficacy, it is often used in combination with platinum-based drugs [7]. Since the discovery of cisplatin in 1965 [8], the use of metal-based agents has in- creasingly gained interest in clinical practice for cancer treatment. Platinum(II) drugs, Pharmaceutics 2022, 14, 1293. https://doi.org/10.3390/pharmaceutics14061293 https://www.mdpi.com/journal/pharmaceutics