Tamoxifen-DNA adduct formation in monkey and human reproductive organs

Abstract

The estrogen analog tamoxifen (TAM), used for adjuvant therapy of breast cancer, induces endometrial and uterine tumors in breast cancer patients. Proliferation stimulus of the uterine endometrium is likely involved in tumor induction, but genotoxicity may also play a role. Formation of TAM-DNA adducts in human tissues has been reported but remains controversial. To address this issue, we examined TAM-DNA adducts in uteri from two species of monkeys, Erythrocebus patas (patas) and Macaca fascicularis (macaque), and in human endometrium and myometrium. Monkeys were given 3-4 months of chronic TAM dosing scaled to be equivalent to the daily human dose. In the uteri, livers and brains from the patas (n = 3), and endometrium from the macaques (n = 4), TAM-DNA adducts were measurable by TAM-DNA chemiluminescence immunoassay. Average TAM-DNA adduct values for the patas uteri (23 adducts/10(8) nucleotides) were similar to those found in endometrium of the macaques (19 adducts/10(8) nucleotides). Endometrium of macaques exposed to both TAM and low-dose estradiol (n = 5) averaged 34 adducts/10(8) nucleotides. To examine TAM-DNA persistence in the patas, females (n = 3) were exposed to TAM for 3 months and to no drug for an additional month, resulting in low or non-detectable TAM-DNA in livers and uteri. Human endometrial and myometrial samples from women receiving (n = 8) and not receiving (n = 8) TAM therapy were also evaluated. Women receiving TAM therapy averaged 10.3 TAM-DNA adducts/10(8) nucleotides, whereas unexposed women showed no detectable TAM-DNA. The data indicate that genotoxicity, in addition to estrogen agonist effects, may contribute to TAM-induced human endometrial cancer.

Fig. 1.

TAM–DNA adduct formation in (A) organs of three female patas monkeys given a daily oral TAM dose, for 3 months, that was 25–40% higher than the human daily dose; (B) organs of three female patas monkeys given TAM daily for 3 months [as in (A)] followed by a 1-month drug-free recovery period. Values are TAM–DNA adducts/10⁸ nucleotides (mean ± SE, n = 2 assays/DNA sample) and were determined by TAM–DNA CIA (lower limit of detection 1.26±0.16 adducts/10⁸ nucleotides). TAM–DNA adducts were undetectable in ovary, kidney and esophagus (data not shown).

Fig. 2.

TAM–DNA adduct formation in endometrium of female macaques exposed for 4 months to daily oral TAM at a human equivalent dose (n = 4, closed square) and TAM plus low-dose estradiol (E₂) (n = 5, open square). TAM–DNA adducts were determined by TAM–DNA CIA (lower limit of detection 1.26±0.16 adducts/10⁸ nucleotides) and values are expressed as TAM–DNA adducts/10⁸ nucleotides (mean ± SE, n = 2 assays/DNA sample). Values for monkeys exposed to TAM alone are significantly different from values for monkeys exposed to TAM plus E₂ (*P = 0.027).