DNA Adduct Formation in the Livers of Female Sprague−Dawley Rats Treated with Toremifene or α-Hydroxytoremifene

Abstract

Toremifene, an analogue of tamoxifen in which the ethyl side chain has been replaced with a 2-chloroethyl substituent, is used as a chemotherapeutic agent in postmenopausal women with advanced breast cancer. Toremifene is metabolized in a manner similar to that of tamoxifen, with alpha-hydroxytoremifene being a predominant metabolite in incubations in vitro. DNA adducts have been detected previously in liver DNA upon the administration of toremifene to rats; however, the identity of these adducts is unknown. In the present study, we have characterized the DNA adducts produced by alpha-hydroxytoremifene and have compared the extent of hepatic DNA adduct formation in rats administered toremifene, alpha-hydroxytoremifene, or tamoxifen. alpha-Hydroxytoremifene was synthesized, further activated by sulfation, and then reacted with salmon testis DNA. After enzymatic hydrolysis to deoxynucleosides, HPLC analysis indicated the formation of two major DNA adducts, which were characterized as (E)- and (Z)-alpha-(deoxyguanosin-N2-yl)toremifene on the basis of 1H NMR and mass spectral analyses. To assess the formation of toremifene DNA adducts in vivo, female Sprague-Dawley rats were treated intraperitoneally with toremifene, alpha-hydroxytoremifene, or tamoxifen. 32P-Postlabeling analyses of hepatic DNA from the tamoxifen-treated rats indicated three DNA adducts at a total level of 2,200 +/- 270 adducts/108 nucleotides. DNA adducts were not detected (<5 adducts/108 nucleotides) in the livers of rats treated with toremifene. Two DNA adducts, of which the major one coeluted with the 3',5'-bis-phosphate of (E)-alpha-(deoxyguanosin-N2-yl)toremifene, were present at a level of 57 +/- 12 adducts/108 nucleotides in hepatic DNA from rats administered alpha-hydroxytoremifene. The low level of hepatic DNA adduct formation observed with both toremifene and alpha-hydroxytoremifene, as compared to that with tamoxifen, may be due to the limited esterification of alpha-hydroxytoremifene and/or the poor reactivity of alpha-sulfoxytoremifene.

Toremifene in the treatment of breast cancer

“…Nevertheless, endometrial cancer rates are increased in women taking tamoxifen [13] . The chlorine substitution in the structure of toremifene alters its metabolism such that DNA adducts are much less likely to form [13,[63][64][65][66] and a case-control study based on records of 38000 Finnish breast cancer patients appears to suggest that toremifene is considerably less frequently associated with endometrial cancer than is tamoxifen [OR 2.9; 95%CI: 0.3-3.9 vs 0.9; 95%CI: 0.3-3.9) [17] . However, a recent meta-analysis of studies involving a total of 7242 patients with early or advanced breast cancer found no difference in the number of endometrial cancers between patients treated with toremifene or tamoxifen, although the follow-up was relatively short in the majority of studies.…”

Potential of Selective Estrogen Receptor Modulators as Treatments and Preventives of Breast Cancer

“…Toremifene is as effective as tamoxifen in the treatment of ER-positive breast cancer but with the potential of fewer genotoxic effects, since it does not produce DNA adducts in rat liver and human endometrium [113]. The mechanism for the reduced genotoxicity of toremifene can be explained as follows: Tamoxifen-DNA adducts are primarily formed via sulfonation of the α-hydroxylated tamoxifen metabolites, but the α-hydroxy metabolites of toremifene is poorly esterified or sulfonated, and even sulfonated α-hydroxy toremifene, α-sulfoxytoremifene, reacts poorly with DNA [114, 115]. However, there are some reports to show toremifene induces DNA damages and hepatocarcinogenesis in rats [116, 117].…”

Mass spectrometry for the assessment of the occurrence and biological consequences of DNA adducts

“…DNA adducts can be induced by a variety of endogenous or exogenous sources of chemical species, such as byproducts of lipid peroxidation, 1,2 chemotherapeutic drugs, 3–5 tobacco carcinogens, 6–8 and environmental contaminants. 9,10 If not repaired correctly or timely, these adducts may lead to myriads of alterations within the cell, including, but are not limited to, cell death, mutations in the genome, and aberrant cell cycle control.…”