Oxidation of Raloxifene to Quinoids:  Potential Toxic Pathways via a Diquinone Methide and o-Quinones

Abstract

Raloxifene was approved in 1997 by the FDA for the treatment of osteoporosis in postmenopausal women, and it is currently in clinical trials for the chemoprevention of breast cancer. Before widespread use as a chemopreventive agent in healthy women, the potential cytotoxic mechanisms of raloxifene should be investigated. In the current study, raloxifene was incubated with GSH and either rat or human liver microsomes, and the metabolites and GSH conjugates were characterized using liquid chromatography-tandem mass spectrometry. Raloxifene was converted to raloxifene diquinone methide GSH conjugates, raloxifene o-quinone GSH conjugates, and raloxifene catechols. For comparison, three raloxifene catechols were synthesized and characterized. In particular, 7-hydroxyraloxifene was found to oxidize to the 6,7-o-quinone. As compared with raloxifene diquinone methide, which has a half-life of less than 1 s in phosphate buffer, the half-life of raloxifene 6,7-o-quinone was much longer at t 1/2 ) 69 ( 2.5 min. The stability offered by raloxifene 6,7-o-quinone implies that it may be more toxic than raloxifene diquinone methide. Cytotoxicity studies in the human breast cancer cell lines S30 and MDA-MB-231 showed that 7-hydroxyraloxifene was more toxic than raloxifene in both cell lines. These results suggest that raloxifene could be metabolized to electrophilic and redox active quinoids, which have the potential to cause toxicity in vivo.

Quinone Methides and their Prodrugs: A Subtle Equilibrium Between Cancer Promotion, Prevention, and Cure

“…Raloxifene is metabolized by CYP3A4 in several hydroxylated compounds, including 3-hydroxy-RA and 7-hydroxy-RA [17,18]. Adducts of RA with reduced glutathione (GSH) have also been detected [19].…”

“…Adducts of RA with reduced glutathione (GSH) have also been detected [19]. Furthermore, a study of the precise mechanism of oxidation using 18 O labeled reagents has led to the conclusion that a DQM (16) was the intermediate species for the production of 7-hydroxy-RA and the glutathione adduct. Due to the very short half-life (< 1 second) of RA DQM, it has been postulated that this molecule could not react with nucleophiles just after its formation inside CYP3A4 [19].…”

“…One can expect that, owing to the electrophilic character of QM, mutagenic adducts with DNA could be formed and cause the development of cancers of the genital system, as it is the case with TAMO and RA for instance. Albeit adducts of their respective QM and DNA have been observed, the involvement of the former ones in carcinogenic effects of the drugs has not been established but only suspected [18,21].…”

Structural modulation of reactivity/activity in design of improved benzothiophene selective estrogen receptor modulators: induction of chemopreventive mechanisms

“…1B; refs. 4,5). These quinoids are able to act as electrophiles and oxidants leading to modification of cellular biomolecules and thus potentially to cytotoxicity and genotoxicity (3).…”

Kollicoat® IR: Minimizing the Risks for Oxidative Degradation of Drugs

“…A further study suggests that the formation of N-oxide was not limited to formulation only but was also observed in the synthesis of raloxifene [17]. Oxidative degradation of raloxifene may lead to adverse reactions [18]. Thus, careful consideration should be given during the synthesis and formulation development of peroxide sensitive drugs like raloxifene and others to avoid the mechanical stress and risks for API's degradation.…”