Structure–activity studies of Ti(IV) complexes: aqueous stability and cytotoxic properties in colon cancer HT-29 cells

Abstract

As part of our research efforts in the area of titanium-based antitumor agents, we have investigated the cytotoxic activity of [Ti(4)(maltolato)(8)(mu-O)(4)], (Cp-R)(2)TiCl(2) and (Cp-R)CpTiCl(2) (R = CO(2)CH(3) and CO(2)CH(2)CH(3)), and three water-soluble titanocene-amino acid complexes-[Cp(2)Ti(aa)(2)]Cl(2) (aa = L: -cysteine, L: -methionine, and D: -penicillamine)-on the human colon adenocarcinoma cell line, HT-29. The capacity of [Ti(4)(maltolato)(8)(mu-O)(4)] to donate Ti(IV) to human apo-transferrin and its hydrolytic stability have been investigated and compared to the previously reported data on modified titanocenes with either hydrophilic ancillary ligands or the functionalized cyclopentadienyl ligands. Notably, the titanium-maltolato complex does not transfer Ti(VI) to human apo-transferrin at any time within the first seven days of its interaction, demonstrating the inert character of this species. Stability studies on these complexes have shown that titanocene complexes decompose at physiological pH while the [Ti(4)(maltolato)(8)(mu-O)(4)] complex is stable at this pH without any notable decomposition for a period of ten days. The antitumor activity of these complexes against colon cancer HT-29 cells was determined using an MTT cell viability assay at 72 and 96 h. The titanocene-amino acid and the (Cp-R)(2)TiCl(2)/(Cp-R)CpTiCl(2) (R = CO(2)CH(3)) complexes were not biologically active when human transferrin was absent; they also were inactive when human transferrin was present at dose-equivalent concentrations. (Cp-R)(2)TiCl(2) and (Cp-R)CpTiCl(2) (R = CO(2)CH(2)CH(3)) showed cytotoxic activity in HT-29 cells comparable to that which is displayed by titanocene dichloride. The titanium-maltolato complex had higher levels of cytotoxic activity than any other titanocene complex investigated. Transferrin may be important in protecting the titanium center from hydrolysis, but this may be achieved by selecting ligands that could result in hydrolytically stable, yet active, complexes.

Cytotoxicity of a Ti(IV) compound is independent of serum proteins

“…In one study, structurally similar titanocenes that deliver Ti(IV) to Tf at comparably fast rates and degrees of saturation were found not only to poorly induce cytotoxicity in HT-29 colon cancer cells but also to induce at different half-maximal inhibitory concentrations (IC 50 ) (15). In more conclusive studies, the addition of Tf to cell-viability assays did not improve the cytotoxicity of different Ti(IV) compounds in several cell lines, with the exception of titanocene dichloride in some instances (16)(17)(18)(19).…”

Cytotoxic Titanium(IV) Complexes: Renaissance

“…[19,21,34,94,95] Moreover, steric and electronic effects were found to have a major influence on both the rate of hydrolysis and also on the general hydrolytic behavior and nature of products. Where steric crowding near the metal site allows [L 1,2,4,6 Ti(OiPr) 2 ], the isopropoxo groups indeed dissociate first to give a new polynuclear O-bridged species with bound phenolato ligand which is itself stable for days as observed for complex 17 (Scheme 6) [24,54] and complex 20 (Scheme 7) [74] However, where steric crowding around the metal center is too large, despite strong ligand binding leading to relatively slow hydrolysis [L 3 Ti-(OiPr) 2 ], [96] or where the electronic features weaken ligandmetal binding leading to rapid hydrolysis [L 5,8 Ti(OiPr) 2 ], the cluster is not obtained and simple ligand release as bisphenol is observed instead (Figure 3). As an example, UV/ Vis studies with L 1 Ti(OiPr) 2 are presented in Figure 4; formation of a new species with bound ligand following isopropoxo release is detected by the LMCT band shift as well as by the decay in absorbance to a value greater than zero, which is only observed within several hours, while no change is observed in the first seconds based on stoppedflow measurements.…”

“…In addition, the water-stable tetranuclear titanium complex 20 based on the diketonato analogue maltol ligand 19 (Scheme 7) does not donate Ti IV to transferrin, yet is active against HT-29 cell lines in-vitro with values comparable to that of 1, unlike other titanocene derivatives studied in comparison. [74] Thus, it would appear that further work is necessary in establishing transferrin as the exclusive agent of active transport for cytotoxic titanium complexes. Scheme 7. A recent paper proposes an alternate theory for titanium transport to cells through the action of serum albumin (HSA).…”

“…These results are also in agreement with the observation that the maltol tetranuclear complex (20, Scheme 7) does not deliver Ti IV to transferrin. [74] It is thus obvious that a carefully designed inert ligand is required to stabilize an active species and enable cytotoxic activity, and support such role for the Cp and diketonato ligands in 1 and 2. [56,74] …”

“…We thus see that this ligand family enables stabilization of the active species, and appears to be involved in the interaction with the biological target. [74] Steric effects on the N-donors and on the aromatic rings, both at positions near the metal site as well as distant from it, were explored. In general, steric bulk has a negative effect on cytotoxicity as reported for titanocene derivatives (3, 4, Scheme 2), [13] with no particularly larger influence of the near-metal site bulk (Figure 8).…”