Tribenzyltin carboxylates complexes, namely tri(4-fluorobenzyl)tin[(N,N-diisopropylcarbamothioyl)sulfanyl]acetate (C1) and tribenzyltin isonicotinate (C9), have been reported to exhibit significant anticancer activity via the death receptor and mitochondrial apoptotic pathway. However, understanding their upstream mechanism remains crucial. This study utilized both in silico and in vitro approaches to elucidate their mechanism of action. Molecular docking analysis indicated that C1 and C9 interacted with the minor groove of DNA. Fluorescence displacement assays confirmed their interaction with DNA through the minor groove. In enzyme inhibition assays, C1 showed significant inhibition of thioredoxin reductase (TrxR) activity, while C9 exhibited a dose-dependent effect, consistent with their docking affinities (- 4.1 and - 4.3 kcal/mol, respectively). Both compounds strongly inhibited human DNA topoisomerase I (TopI), supported by docking energies of - 7.0 and - 6.6 kcal/mol, respectively. The nearly complete inhibition of TopI across all treatment groups highlights it as a key molecular target. Additionally, C1 and C9 bound to thymidylate synthase (TS) with docking energies of - 7.2 and - 7.7 kcal/mol, respectively, leading to a marked reduction in TS levels. Molecular dynamics simulations of the TopI, TS, and TrxR complexes revealed that both C1 and C9 maintained stable binding within their respective active sites throughout the 100 ns trajectories. In summary, C1 and C9 primarily target the DNA minor groove and inhibit key enzymes such as TopI, TrxR, and TS, shedding light on their intricate mechanisms of action. These findings underscore the potential of C1 and C9 as promising multitarget metal-based anticancer agents.
Radiotherapy is an effective treatment modality for breast cancer but, unfortunately, not all patients respond fully with a significant number experiencing local recurrences. Overexpression of thioredoxin and thioredoxin reductase has been reported to cause multidrug and radiation resistance - their inhibition may therefore improve therapeutic efficacy. Novel indolequinone compounds have been shown, in pancreatic cancer models, to inhibit thioredoxin reductase activity and exhibit potent anticancer activity. The present study evaluates, using in vitro breast cancer models, the efficacy of a novel indolequinone compound (IQ9) as a single agent and in combination with ionising radiation using a variety of endpoint assays including cell proliferation, clonogenic survival, enzyme activity, and western blotting. Three triple-negative breast cancer (MDA-MB-231, MDA-MB-468, and MDA-MB-436) and two luminal (MCF-7 and T47D) breast cancer cell lines were used. Results show that treatment with IQ9 significantly inhibited thioredoxin reductase activity, and inhibited cell growth and colony formation of breast cancer cells with IC50 values in the low micromolar ranges. Enhanced radiosensitivity of triple-negative breast cancer cells was observed, with sensitiser enhancement ratios of 1.20-1.43, but with no evident radiosensitisation of luminal breast cancer cell lines. IQ9 upregulated protein expression of thioredoxin reductase in luminal but not in triple-negative breast cancer cells which may explain the observed differential radiosensitisation. This study provides important evidence of the roles of the thioredoxin system as an exploitable radiobiological target in breast cancer cells and highlights the potential therapeutic value of indolequinones as radiosensitisers.***This study was not part of a clinical trial. Clinical trial registration number: N/A.