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Glutathione induces cellular resistance against cationic dinuclear platinum anticancer drugs
Bart A J Jansen1, Jaap Brouwer, Jan Reedijk
1Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL-2300 RA, Netherlands.
Abstract:
The sulfur-containing tripeptide glutathione (GSH) is one of the most abundant molecules in cells. Elevated levels of GSH render some types of cancer cells resistant against well-known platinum anti-cancer drugs such as cisplatin and carboplatin. Platinum complexes are often very reactive towards the cysteine residue of GSH, which detoxifies these compounds by a rapid binding mechanism. Clearly, this resistance mechanism poses a severe obstacle to any new platinum drugs designed to overcome cisplatin resistance. In the present study the cytotoxicity of dinuclear platinum compounds of the 1,1/t,t type, as developed by Farrell, is determined in human ovarium A2780 cells and in the cisplatin-resistant cell line A2780cisR, which possesses elevated levels of GSH. Further, the effect of depletion of GSH levels by L-buthionine-S,R-sulfoximine (L-BSO) in A2780cisR was investigated. The experiments show that detoxification by GSH is an effective resistance mechanism against dinuclear platinum compounds. However, the dinuclear complexes are less sensitive towards detoxification compared to cisplatin. This is probably because of the rapid binding of dinuclear cationic complexes to DNA. Compared to cisplatin, the rapid binding to DNA reduces the time during which the drug molecules are exposed to GSH in the cytosol. The reaction of a representative dinuclear compound with glutathione (pH 7, 37 degrees C) was studied in detail by 195Pt NMR. The dinuclear complex BBR3005 ([trans-PtCl(2)(NH(3))(2)(mu-H(2)N(CH(2))(6)NH(2))](2+), abbreviated as 1,1/t,t n=6), follows different pathways in the reaction with GSH, depending on the molar ratio of the reactants. When reacted in stoichiometric amounts (1:1), first a chloride on each platinum is replaced by a sulfur, forming a PtN(3)S product at -2977 ppm. After 2-3 h, this intermediate reacts further to form a sulfur-bridged N(3)Pt-S-PtN(3) species as the main product at -2811 ppm. When BBR3005 is reacted with GSH in a ratio of 1:4, the sulfur-bridged species is not observed. Instead, the final product is trans-Pt(GS)(2)(NH(3))(2) (at -3215 ppm); the same product appears if GSH is reacted with trans-PtCl(2)(NH(3))(2). Apparently, GSH first replaces the chlorides and subsequently degrades the dinuclear compound by replacement of the diaminealkyl linker.
Insights
Dinuclear platinum compounds are less susceptible to glutathione (GSH) detoxification than cisplatin, offering a potential strategy against platinum-resistant cancers. Depleting GSH levels in cancer cells enhances the efficacy of these novel platinum-based drugs.
Area of Science:
- Medicinal Chemistry
- Cancer Biology
- Platinum-based Chemotherapy
Background:
- Elevated glutathione (GSH) levels in cancer cells confer resistance to platinum-based drugs like cisplatin.
- GSH detoxifies platinum compounds through rapid binding to its cysteine residue, hindering drug efficacy.
- This resistance mechanism presents a significant challenge for developing new platinum-based anti-cancer agents.
Purpose of the Study:
- To evaluate the cytotoxicity of dinuclear platinum compounds in cisplatin-sensitive and resistant ovarian cancer cells.
- To investigate the impact of GSH depletion on the efficacy of these dinuclear platinum compounds.
- To elucidate the reaction pathways between a dinuclear platinum complex and GSH.
Main Methods:
- Cytotoxicity assays were performed on A2780 and A2780cisR human ovarian cancer cell lines.
- GSH levels were modulated using L-buthionine-S,R-sulfoximine (L-BSO).
- 195Pt NMR spectroscopy was employed to study the reaction kinetics and products of a dinuclear platinum complex with GSH.
Main Results:
- Dinuclear platinum compounds exhibited cytotoxicity in both sensitive and resistant cell lines.
- GSH depletion using L-BSO enhanced the sensitivity of A2780cisR cells to dinuclear platinum compounds.
- Dinuclear complexes showed reduced sensitivity to GSH detoxification compared to cisplatin, attributed to faster DNA binding.
- 195Pt NMR revealed distinct reaction pathways between the dinuclear complex BBR3005 and GSH, dependent on reactant ratios, forming Pt-S adducts and degradation products.
Conclusions:
- Dinuclear platinum compounds are less susceptible to GSH-mediated detoxification than cisplatin.
- The rapid DNA binding of dinuclear platinum complexes may contribute to their reduced susceptibility to GSH.
- These findings suggest that dinuclear platinum compounds hold promise for overcoming cisplatin resistance in cancers with elevated GSH levels.