The in vitro anti-cancer activity of gold, palladium and platinum-based metallodrugs

Abstract

Background: Cancer is a group of diseases characterized by cells that proliferate uncontrollably and are likely to invade distant sites within the body. Cancer development occurs from a series of molecular events that alter the normal function and properties of cells. These include events, such as the evasion of apoptosis and limitless replicative potential. Cervical cancer is the most prevalent cancer among women in Sub-Saharan Africa. In South Africa, 21% of women in the general population harbour the human papillomavirus infection, which is the precursor for developing cervical cancer. The current treatment regimen has improved cancer mortality, but some complications still limit the treatment, such as toxicity, chemoresistance, and lack of selectivity. Therefore, this has stimulated research into developing anticancer agents that are less toxic and more selective. Metallodrugs have been used for centuries to treat several diseases, and currently, there is an increase in using metallodrugs as anticancer agents. An example of a commonly used metallodrug in cancer therapy is cisplatin – a platinum-based drug used to treat a variety of cancers. The discovery of cisplatin increased investigations to identify other metals for their potential use in cancer treatment, such as gold and palladium.

Methods: Six metal-based complexes, diphenylphosphino-2-pyridylgold(I) chloride (AE20), 2-(2-diphenylphosphino) ethyl) pyridyl-gold(I) chloride (AE76), 2-(diphenylphosphino)-2’(N,N-dimethylamino)biphenyl gold(I) chloride (AE125), dichlorobis[2-(di-Cyphosphino)ethyl]-aminepalladium (II) (AE190), dichlorobis[2-(di-Cyphosphino)ethyl]-amineplatinum (II) (AE193) and palladium(tetrahydrothiophene) [(diphenylphosphino-2-pyridyl)] AuCl chloride (AE177), were evaluated in vitro for their effects on cell proliferation and death in cancer cells. These complexes are hypothesized to have in vitro anticancer activity. The two cell lines used in this study were cervical adenocarcinoma cells (HeLa) and African green monkey kidney cells (Vero). The 3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay was conducted to assess the cell viability and to determine the 50% cytotoxic concentration (CC50) of the complexes in the cell lines, and these CC50 values obtained were used to determine the selectivity index of each complex. The effects of the complexes on cell proliferation, cell cycle progression and the induction of apoptosis were evaluated using flow cytometry and real-time cell electronic sensing. The apoptotic pathway induced by the complexes was determined by measuring caspase activity. A clonogenic survival assay was conducted to evaluate whether the cells undergo unlimited cell division and gain their reproductive integrity after being treated with the complexes for an extended period.

Results: Five of the six complexes were found to be cytotoxic at low concentrations (CC50<10 µg/ml), with one complex (AE190) having a high CC50 value (> 10 µg/ml). Four of the six complexes were found to be selective to HeLa cells (SI>1); with AE190 (a palladium-based complex) and AE193 (a platinum-based complex) showing a lack of selectivity, SI=0.21 (CC50= 12.19 µg/ml) and SI= 0.85 (CC50=7.31 µg/ml), respectively. However, this was similar to the control cisplatin (SI=0.46). These complexes induced early apoptosis and cell cycle arrest in HeLa cells (p<0.05). Complexes, AE20 and AE76 induced G2/M arrest, whereas AE125, AE190, AE193, AE177, and the positive control roscovitine led to S-phase arrest. The clonogenic survival assay showed the effects of the complexes on the reproductive integrity of HeLa cells after long-term exposure to the complexes. The plating efficiency of HeLa cells decreased significantly when cells were treated with AE125 and AE193 at CC50 compared to untreated cells. At 2×CC50, cells treated with AE125, AE190 and AE193 lost their reproductive integrity, which was evident by the lack of colony formation. The decrease in the plating efficiency indicates that the complexes affected the ability of individual cells to form colonies and can, therefore, act as anticancer agents. This finding corroborates the apoptosis results, where there was a statistically significant increase in cells undergoing apoptosis compared to untreated cells. RT-CES analysis indicated that treatment with the complexes induced a concentration-dependent cellular response (cytotoxic and cytostatic).

Conclusion: Overall, all the complexes had antiproliferative effects in HeLa cells, which was evident by the induction of early apoptosis and cell cycle arrest in ≥20% of the treated cells. Treatment with AE125, AE190, AE193 and cisplatin proved to affect the colony formation ability of cells following a decrease in plating efficiency. These complexes can, therefore, be regarded as promising anticancer agents warranting further investigation.