Long-term adjuvant tamoxifen therapy for breast cancer: the prelude to prevention

Age-related difference in tamoxifen disposition

PURPOSE

To investigate the pharmacokinetic aspects of tamoxifen, such as the pharmacokinetic-pharmacodynamic (toxicity and clinical response) relationship and the influence of hepatic dysfunction, age, treatment duration, and associated chemotherapy on tamoxifen pharmacokinetics.

PATIENTS AND METHODS

Three hundred sixteen patients with breast cancer (247 postmenopausal women) were investigated. Mean age was 58 years (age range, 29 to 85 years). One hundred seventeen patients received tamoxifen as single therapy (adjuvant, 60; neoadjuvant, 17; metastatic, 40); 292 of 316 received 30 mg daily. We obtained 794 blood samples at steady state. Tamoxifen and metabolites, N-desmethyltamoxifen, N-desdimethyltamoxifen, primary alcohol, and 4-hydroxytamoxifen were measured by HPLC.

RESULTS

Serum concentrations of tamoxifen and metabolites showed a wide asymmetrical distribution. Median and extremes were 347 nmol/L (not detectable [ND] to 1677) for tamoxifen, 572 nmol/L (ND to 3132) for N-desmethyltamoxifen, 109 nmol/L (ND to 795) for N-desdimethyltamoxifen, and 59 nmol/L (ND to 390) for primary alcohol. 4-Hydroxytamoxifen was detectable in 9.5% of the samples (ND to 162 nmol/L). Neither the absolute nor the relative concentrations of each compound showed significant variations during treatment. Chemotherapy concomitant with tamoxifen slightly increased the tamoxifen blood concentration. Hepatic dysfunction had no obvious effect on drug concentrations, an exception being a slight reduction in the relative proportion of tamoxifen. The influence of age revealed that concentrations of tamoxifen and metabolites increased significantly with age: women younger than 40 years had a tamoxifen plus metabolite median concentration of 802 nmol/L compared with 2428 nmol/L for women older than 80 years. In the 28 patients in whom tamoxifen-related side effects developed, the proportion of demethylated metabolites was higher than that in patients in whom toxicity did not develop. There was no difference in drug concentrations between responding and nonresponding patients.

CONCLUSION

Despite the tremendous interpatient variability in drug concentrations, the present data show that tamoxifen and metabolite concentrations significantly increase with age.

Clinical uses of antiestrogens

An antiestrogen is a compound that blocks the action of estrogen. Most synthetic antiestrogens have agonistic or antagonistic activity depending on the tissue and the endogenous estrogen mileu. The triphenylethylene derivatives, clomiphene and tamoxifen, are the antiestrogens in greatest clinical use. Their biologic effects, clinical indications, and risks are reviewed. Novel antiestrogens which are beginning to be studied clinically include the benzothiophene derivative, raloxifene and the "pure" antiestrogens such as ICI 182,780. New clinical indications for existing compounds as well as the development of novel antiestrogens may lead to better treatment options for endocrine-dependent conditions.

Effect of tamoxifen on serum cholesterol and lipoproteins during chemohormonal therapy

The effect of tamoxifen on serum cholesterol, high density lipoprotein cholesterol (HDL-cholesterol), low density lipoprotein cholesterol (LDL-cholesterol) and the ratio of LDL-cholesterol to HDL-cholesterol (LDL-C/HDL-C) was investigated in breast cancer patients undergoing therapy for advanced disease. Longitudinal studies in 24 patients treated with tamoxifen (10 mg, twice daily) indicated average decreases in total serum cholesterol (17%) and LDL-cholesterol (27%), whereas the effect of tamoxifen on HDL-cholesterol varied with the individual patient. There was a significant decrease in the LDL-C/HDL-C ratio (33%) consistent with a decreased risk for coronary artery disease. This beneficial influence of tamoxifen on risk factors associated with cardiovascular disease was evident in both premenopausal and postmenopausal patients whether tamoxifen was administered alone or in combination with cytotoxic chemotherapy.

Inhibition of estrone sulfatase and 17 beta-hydroxysteroid dehydrogenase by antiestrogens

Circulating estrone sulfate levels are 10-fold higher than the free estrone and estradiol levels in postmenopausal women and could form a reservoir from which the free estrogens could be synthesized in situ in breast cancer tissues. The enzymes catalyzing conversion of estrone sulfate to free estrone and estradiol are estrone sulfatase and 17 beta-hydroxysteroid dehydrogenase, respectively. Selective blockade of these two enzymes may provide a means of reducing tumor estrogen levels and promoting tumor regression. The present study characterized the kinetics of several potential inhibitors of estrone sulfatase and 17 beta-hydroxysteroid dehydrogenase in vitro in rat breast tumors and compared these effects to those in human tissues. The antiestrogen ICI 164384 as well as tamoxifen and its metabolites inhibit estrone sulfatase via noncompetitive mechanisms at Kis ranging from 11-1130 microM in rat breast tumors. The steroid sulfates (pregnenolone sulfate and dehydroepiandrosterone sulfate) on the other hand, act as competitive inhibitors with Kis ranging from 4 to 6 microM. ICI 164384 and the tamoxifen metabolite 4-hydroxytamoxifen also blocked 17 beta-hydroxysteroid dehydrogenase at concentrations of 470 and 275 microM, respectively. In human breast tumors, 4-hydroxytamoxifen and desmethyltamoxifen blocked estrone sulfatase and 17 beta-hydroxysteroid dehydrogenase but at higher concentrations than in the rat (i.e. IC50s of 1000-2000 microM). The inhibition caused by the antiestrogens requires concentrations at least 100-fold higher than those necessary for antiestrogenic effects. Although blockade of enzyme action is significant in vitro, and could also be in vivo, the effects of antiestrogens on enzyme inhibition are likely to be outweighed by their ability to block estrogen receptor-mediated effects in patients.

Overprolonged adjuvant tamoxifen therapy in breast cancer

While adjuvant tamoxifen therapy given continuously for 2-3 years can lead to a modest improvement in survival rates in early breast cancer, there is no evidence that prolonging tamoxifen administration beyond that time is likely to improve survival rates any further in unselected cases. In the case of advanced disease, an alternating tamoxifen/progestagen regimen has been shown to increase the response rate and also its duration, beyond that to be expected from either agent alone. The next generation of adjuvant trials in breast cancer needs to explore the potential of an alternating tamoxifen/megestrol regimen.

Anti-estrogen induced synthesis of transforming growth factor-beta in breast cancer patients

Estrogen receptor-negative human breast cancer is usually an aggressive, hormone independent tumour. Recent studies show that growth of these tumours could be influenced by anti-estrogens. Anti-estrogens appear to stimulate the production of Transforming Growth Factor-beta (TGF-beta) in hormone dependent as well as independent cell lines suggesting as a result inhibition of cellular proliferation of these cell lines by TGF-beta through an autocrine/paracrine mechanism.