Wolf D M, Langan-Fahey S M, Parker C J, McCague R, Jordan V C
Department of Human Oncology, University of Wisconsin Comprehensive Cancer Center, Madison 53792.
J Natl Cancer Inst. 1993 May 19;85(10):806-12. doi: 10.1093/jnci/85.10.806.
The nonsteroidal anti-estrogen tamoxifen (TAM) is the front-line endocrine treatment for breast cancer, but disease recurrence is common. Treatment failure may occur because tumors become insensitive to TAM. Alternatively, resistance may occur because tumors become stimulated rather than inhibited by TAM. TAM-stimulated growth of MCF-7 human breast tumors has been observed in athymic mice after prolonged treatment with TAM.
Our purpose was to examine the mechanism of treatment failure by determining whether TAM-stimulated tumors acquire the ability to excrete TAM and its anti-estrogenic metabolites or to convert them to estrogenic compounds with weakened antiestrogenic activity.
We used high-pressure liquid chromatography to quantitate TAM and its metabolites in serum and tumors from ovariectomized athymic mice and in MCF-7 cells grown in vitro. We treated tumor-bearing mice with subcutaneous sustained-release preparations of estradiol, TAM, or a nonisomerizable (fixed-ring) analogue and then assessed the activity of these compounds on TAM-inhibited parental MCF-7 tumors and on TAM-stimulated MCF-7 TAM tumors.
We found negligible differences in intratumoral TAM levels between TAM-inhibited parental MCF-7 tumors and TAM-stimulated MCF-7 TAM variants. We did not detect metabolite E (Met E), an estrogenic TAM metabolite, in serum or tumors. Using MCF-7 cells in vitro, we determined that the (Z) isomer of Met E, the form directly produced by TAM metabolism, must be present in the cell at a concentration of over 1000 ng/g to overcome growth inhibition by physiological levels of TAM and antiestrogenic metabolites, but the (E) isomer of Met E was effective at 10 ng/g. We reasoned that conversion of Met E from the (Z) (a weak estrogen) to (E) isomer (a potent estrogen) would be required if formation of Met E were responsible for TAM-stimulated growth. However, fixed-ring TAM, which can only form (Z) Met E, was shown to be as capable as TAM of initiating and maintaining anti-estrogen-stimulated growth of MCF-7 tumors in athymic mice.
Metabolism and isomerization of TAM to estrogenic compounds is not the mechanism of TAM-stimulated growth in our model.
Other potential mechanisms for TAM-stimulated growth, such as estrogen receptor mutation, must be investigated so that effective strategies can be devised to control breast cancer once therapy fails.
非甾体类抗雌激素药物他莫昔芬（TAM）是乳腺癌的一线内分泌治疗药物，但疾病复发很常见。治疗失败可能是因为肿瘤对TAM不敏感。或者，耐药可能是因为肿瘤受到TAM刺激而非抑制。在用TAM长期治疗后，在无胸腺小鼠中观察到TAM刺激MCF-7人乳腺肿瘤生长。
我们的目的是通过确定TAM刺激的肿瘤是否获得排泄TAM及其抗雌激素代谢物或将它们转化为抗雌激素活性减弱的雌激素化合物的能力，来研究治疗失败的机制。
我们使用高压液相色谱法定量来自去卵巢无胸腺小鼠的血清和肿瘤以及体外培养的MCF-7细胞中的TAM及其代谢物。我们用雌二醇、TAM或一种不可异构化（固定环）类似物的皮下缓释制剂处理荷瘤小鼠，然后评估这些化合物对TAM抑制的亲本MCF-7肿瘤和TAM刺激的MCF-7 TAM肿瘤的活性。
我们发现TAM抑制的亲本MCF-7肿瘤和TAM刺激的MCF-7 TAM变体之间瘤内TAM水平的差异可忽略不计。我们在血清或肿瘤中未检测到代谢物E（Met E），一种雌激素性TAM代谢物。使用体外培养的MCF-7细胞，我们确定Met E的（Z）异构体，即TAM代谢直接产生的形式，必须在细胞中的浓度超过1000 ng/g才能克服生理水平的TAM和抗雌激素代谢物的生长抑制，但Met E的（E）异构体在10 ng/g时有效。我们推断，如果Met E的形成导致TAM刺激生长，则需要将Met E从（Z）（一种弱雌激素）转化为（E）异构体（一种强效雌激素）。然而，只能形成（Z）Met E的固定环TAM被证明与TAM一样能够启动和维持无胸腺小鼠中MCF-7肿瘤的抗雌激素刺激生长。
在我们的模型中，TAM代谢和异构化为雌激素化合物不是TAM刺激生长的机制。
必须研究TAM刺激生长的其他潜在机制，如雌激素受体突变，以便在治疗失败后能够设计出有效的策略来控制乳腺癌。
