First-generation aromatase inhibitors lacked specificity as well as were associated with substantial toxicity. However, powerful agents that specifically and almost completely inhibit aromatase activity are now in widespread use, and the results of recently published studies suggest that they may be even more effective than tamoxifen.
Additive endocrine therapies, in principle, function by direct action on the hormonally dependent breast cancer recurrence cell. In that regard, estrogen production persists, but its action is prevented at the cellular level by interfering with estrogen and its receptor. Enigmatically, the first such agents to be used successfully were estrogenic compounds (diethylstilbestrol, ethyinylestradiol) administered at pharmacological doses, although androgenic agents were also found to be marginally effective. Subsequently, estrogenic agents were replaced by the equally active but less toxic triphenylethylamine tamoxifen, which was originally considered to be an “antiestrogen.” Subsequent studies have demonstrated that tamoxifen has dualistic agonist and antagonist activity, depending on the tissue of interest.
Consequently, tamoxifen and other cancer rates agents like it have been designated selective estrogen-receptor modifiers (SERMs).
Other hormonal therapies—such as progestational agents (megestrol, medroxyprogesterone acetate) and androgens—have been harder to classify. Both of these types of treatments have been proven to be active against breast cancer, although they have been largely replaced or relegated to third or fourthline therapy by newer, more active and tolerable agents. In theory, they might exert their effects at the cellular stages and would thus be classified with the SERMs as additive agents. On the other hand, neither progesterone nor androgen receptors appear to be fundamentally important for breast cancer behavior, nor neither antiprogestin nor antiandrogen therapy has been particularly successful against breast cancer. Therefore, it has been speculated that the mechanism of progestins action and androgens may be a consequence of suppression of the hypothalamic-pituitary axis via a feedback mechanism. In this case, one might consider these techniques as ablative.
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