Sigalit Meker

Synthesis, Characterization, Cytotoxicity, and Hydrolytic Behavior of C2- and C1-Symmetrical TiIV Complexes of Tetradentate Diamine Bis(Phenolato) Ligands: A New Class of Antitumor Agents

We recently introduced a new class of bis(isopropoxo)-Ti(IV) complexes with diamine bis(phenolato) ligands that possess antitumor activity against colon HT-29 and ovarian OVCAR-1 cells that is higher than that of the known Ti(IV) compounds titanocene dichloride and budotitane as well as that of cisplatin. Herein, we elaborate on this family of compounds; we discuss the effect of structural parameters on the cytotoxic activity and hydrolytic behavior of these complexes, seeking a relationship between the two. Whereas complexes with small steric groups around the metal center possess high activity and lead mostly to formation of O-bridged polynuclear complexes with bound bis(phenolato) ligand upon water addition, bulky complexes hydrolyze to release all free ligands and are inactive. Slightly increasing the size of the N-donor substituents probably weakens the ligand binding in solution, and, thus, rapid hydrolysis is observed, leading to a lack of cytotoxicity, supporting the requirement for ligand inertness. Replacing the two isopropoxo ligands with a single catecholato unit gives a complex with a different geometry that exhibits slower hydrolysis and reduced cytotoxicity, suggesting some participation of labile ligand hydrolysis in the cytotoxicity mechanism. A crystallographically characterized O-bridged polynuclear species obtained from a biologically active bis(isopropoxo) complex upon water addition is inactive, which rules out its participation as the active species, yet suggests some role of the particular steric and electronic requirements allowing its formation in the activity mechanism. Additional measurements support rapid formation of the active species in the presence of cells prior to O-bridged Ti(IV) cluster formation.

High Antitumor Activity of Highly Resistant Salan–Titanium(IV) Complexes in Nanoparticles: An Identified Active Species†

A nanoformulated trinuclear hydrolysis product of a bis(alkoxo) salan-Ti(IV) complex shows high antitumor activity, which identifies it as an active species in cells. Additional highly stable mononuclear derivatives also show high activity, when formulated into nanoparticles, thus evincing that biologically friendly Ti(IV) can provide high cytotoxicity with controlled biological function.

Anti‐proliferative Activity of Nano‐Formulated Phenolato Titanium(IV) Complexes Against Cancer Cells

Nanoparticles of titanium(IV) complexes of phenolato ligands were formed and evaluated for cytotoxicity toward human HT‐29 colon cancer, murine T‐25 lymphoma, and murine HU‐2 multidrug‐resistant (MDR) cells. The nano‐formulation, besides increasing the complexes′ shelf lives, is particularly efficient in overcoming limitations in solubility and cell‐penetration, thus enhancing biological accessibility; large complexes that were inactive when measured in a non‐formulated form showed marked activity when nano‐formulated. For active and accessible small complexes, the effect of the formulation was negligible. Most complexes showed similar activity toward MDR cells and their drug‐sensitive analogues, further increasing their therapeutic potential. An exception is a particularly hydrophobic complex, which is presumably more accessible to interaction with the membrane ABCB1 (MDR1) transporter active in the multidrug resistance of HU‐2 cells. The most efficient compound is a mononuclear complex of a single hexadentate ligand, combining particularly high activity and hydrolytic stability with accessibility aided by the nano‐formulation.

Specific Design of Titanium(IV) Phenolato Chelates Yields Stable and Accessible, Effective and Selective Anticancer Agents

Octahedral titanium(IV) complexes of phenolato hexadentate ligands were developed and showed very high stability for days in water solutions. In vitro cytotoxicity studies showed that, whereas tetrakis(phenolato) systems are generally of low activity presumably due to inaccessibility, smaller bis(phenolato)bis(alkoxo) complexes feature high anticancer activity and accessibility even without formulations, also toward a cisplatin-resistant cell line. An all-aliphatic control complex was unstable and inactive. A leading phenolato complex also revealed: 1) high durability in fully aqueous solutions; accordingly, negligible loss of activity after preincubation for three days in medium or in serum; 2) maximal cellular accumulation and induction of apoptosis following 24-48 h of administration; 3) reduced impact on noncancerous fibroblast cells; 4) in vivo efficacy toward lymphoma cells in murine model; 5) high activity in NCI-60 panel, with average GI50 of 4.6±2 μm. This newly developed family of Ti(IV) complexes is thus of great potential for anticancer therapy.

Anticancer Metal Complexes: Synthesis and Cytotoxicity Evaluation by the MTT Assay

Titanium (IV) and vanadium (V) complexes are highly potent anticancer agents. A challenge in their synthesis refers to their hydrolytic instability; therefore their preparation should be conducted under an inert atmosphere. Evaluation of the anticancer activity of these complexes can be achieved by the MTT assay. The MTT assay is a colorimetric viability assay based on enzymatic reduction of the MTT molecule to formazan when it is exposed to viable cells. The outcome of the reduction is a color change of the MTT molecule. Absorbance measurements relative to a control determine the percentage of remaining viable cancer cells following their treatment with varying concentrations of a tested compound, which is translated to the compound anticancer activity and its IC50 values. The MTT assay is widely common in cytotoxicity studies due to its accuracy, rapidity, and relative simplicity. Herein we present a detailed protocol for the synthesis of air sensitive metal based drugs and cell viability measurements, including preparation of the cell plates, incubation of the compounds with the cells, viability measurements using the MTT assay, and determination of IC50 values.

Highly Stable Tetra-Phenolato Titanium(IV) Agent Formulated into Nanoparticles Demonstrates Anti-Tumoral Activity and Selectivity

Titanium(IV) complexes exhibit high potential as anti-tumor agents, particularly due to their low intrinsic toxicity and cytotoxicity toward cisplatin resistant cells. Nevertheless, Ti(IV) complexes generally undergo rapid hydrolysis that previously hampered their utilization as anticancer drugs. We recently overcame this difficulty by developing a highly stable Ti(IV) complex that is based on tetra-phenolato, hexadentate ligand, formulated into organic nanoparticles. Herein we investigated the activity of this complex in vitro and in vivo. Although inactive when tested directly due to poor solubility, when formulated, this complex displayed (a) high cytotoxicity toward cisplatin resistant human ovarian cells, A2780-cp, with resistance factor of 1.1; (b) additive behavior in combination with cisplatin toward ovarian and colon cancer cells; (c) selectivity toward cancer cells as implied by its mild activity toward non-cancerous, fibroblast lung cells, MRC-5; (d) high stability and durability as manifested by the ability to maintain cytotoxicity, even following one week of incubation in 100% aquatic medium solution; and (e) in vivo efficacy toward solid tumors of human colon cancer cells, HT-29, in nude mice without any clinical signs of toxicity. These features support the formulated phenolato Ti(IV) complex being an effective and selective anti-tumoral agent.

Abstract 5496: ‘Salan’ titanium(IV) complexes: A new class of anti-cancer agents.

Early attempts at using titanium compounds (derivatives of titanocene dichloride and budotitane) as anti-cancer treatment were essentially unsuccessful. The main reason was their rapid hydrolysis in biological media, leading to a variety of unidentified products that hampered the elucidation of mechanistic aspects. Lately, we have introduced a new class of Ti(IV) complexes of salan type diamino bis(phenolato) ligands. These compounds demonstrate substantially enhanced hydrolytic stability, as well as cytotoxic activity towards a variety of cancer cell lines in vitro. The IC50 of different derivatives of these compounds towards murine mammary carcinoma, lymphoma, multi-drug resistant lymphoma, as well as human leukemia, melanoma and pancreatic cancer cell lines, is in the range of 0.6 - 5.7μM, as compared to 70-100μM of the older titanium compounds and to 50-60μM of cisplatin (Manna C.M. et al. ChemMedChem, 7, 703 (2012)). Exposure of murine T-69 and human OVCAR-1 and HT-29 cell lines to one of the salan Ti(IV) compounds resulted in increased levels of p53 and growth arrest in G-1 of the cell cycle implying activation of the apoptotic pathway in response to Ti(IV) complexes.The well-identified and highly stable hydrolysis products of the titanium compounds, which had previously been formulated into nanoparticles to enhance solubility and assist cellular penetration (Meker, S. et al. Angew. Chem. Int. Ed. 51,10515 (2012)), were active both in vitro (IC50 -0.45μM) in murine lymphoma and in vivo (50% tumor growth inhibition) of the same lymphoma cells following their introduction into syngeneic mice. These findings suggest the potential use of solubilized active titanium compound derivatives as a new class of anti-cancer agents. Citation Format: Ori Braitbard, Sigalit Meker, Maya Stolarov, Jacob Hochman, Edit Tshuva. ‘Salan’ titanium(IV) complexes: A new class of anti-cancer agents. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5496. doi:10.1158/1538-7445.AM2013-5496

Abstract 1762: Highly potent anti-cancer ‘salan’ Titanium(IV) complexes: Structure - function relationship

Cisplatin is a widely used platinum-based metallo-chemotherapeutic drug that is considered an efficient treatment mainly for testicular and ovarian cancers. However, its narrow activity range and severe side effects trigger studies of other potent transition metal complexes. Two titanium(IV) anti-cancer agents that have been previously studied extensively as cisplatin alternatives are titanocene dichloride and budotitane. Despite their high activity, these complexes failed clinical trials mainly due to their poor water stability and formation of unidentified aggregates in water solutions. Our research group focuses on development of new, better-suited families of anti-tumor Ti(IV) complexes lacking Cp or diketonato ligands, and their investigation as anti-tumor agents. We recently reported the synthesis, characterization, and cytotoxicity of “salan” type Ti(IV) complexes [1]. The well defined hydrolytic behaviour, high stability, and high cytotoxicity of these compounds are strongly correlated to their particular structure. This enables fine tuning of complex properties by structural modifications. Herein we elaborate on the investigation of these salan titanium(IV) compounds. The correlation between the complex properties: structure, hydrolytic stability and cytotoxicity will be discussed, revealing insights on the biological mechanism of their cytotoxicity, suggesting an apoptotic cell death pathway.Additionally, further biological evaluation is presented, evincing that these compounds are markedly more active than cisplatin towards human and murine cancer-derived cell lines, including colon, ovarian, lung, cervix, pancreas, leukaemia, skin and breast, with activity also against cisplatin-resistant and multi-drug-resistant cells, and with minor effect on primary murine cells. In vivo experiments are now under way to determine both safety, as well as the anti-cancer activity of these complexes. [1] D. Peri, S. Meker, C. M. Manna, E. Y. Tshuva, Inorg. Chem., 2011, 50, 1030. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1762. doi:1538-7445.AM2012-1762