Androgen deprivation therapy for advanced prostate cancer: why does it fail and can its effects be prolonged?

Androgen deprivation therapy (ADT) has been the cornerstone of treatment for advanced prostate cancer for over 65 years. Although there can be worrisome side effects, data will be presented that for men with metastatic prostate cancer, immediate ADT can reduce the likelihood of developing the rare but catastrophic sequellae of metastatic disease, although it is unlikely to prolong survival compared with waiting for symptoms before initiating ADT. Additionally, for patients with extremely high risk prostate cancer that is not distantly metastatic (e.g. have a life expectancy from prostate cancer less than 10 years with all other available treatments except immediate ADT) and, whose life expectancy from non-prostate cancer diseases is excellent during this period, early ADT both alone and in conjunction with definitive local treatment prolongs survival. Moreover, ADT seems to be most effective when the cancer volume is low. However, eventually most men receiving ADT experience disease progression. The biological mechanisms explaining how prostate cancer escapes from ADT's control include: 1) Alterations in the androgen receptor (AR) and in the AR co-factors (which modify the responsiveness of the AR to androgens) allow molecules and medications which are not normally AR agonists to act as agonists. 2) The human prostate gland, and particularly prostate cancer, may be able to synthesize androgens from both cholesterol and adrenal androgens. This may occur because prostate cancer tissue has higher concentrations of androgens than does the serum in patients receiving ADT. Thus, castrated men may not be starving their prostate cancers of androgens. 3) The AR in prostatic stroma far more strongly stimulates both malignant and benign prostatic epithelial growth than the epithelial AR does. Indeed, the epithelial AR, particularly in advanced prostate cancer, may have anti-proliferative and anti-tumor progression properties. That is, the AR in the prostatic epithelial cells, particularly malignant ones, may act as a tumor suppressor. Thus, by inhibiting the epithelial AR, its protective effects may be abrogated. The controversial nature of these concepts, as well as the clinical and experimental data which support and question them, will be presented. Additionally, strategies for addressing each of these escape mechanisms, which may be able to prolong responsiveness to ADT, will be discussed.