Abstract
154 Background: Androgen deprivation and direct androgen receptor (AR) antagonism are standard treatments for castration resistant prostate cancer (CRPC). The advent of new drugs such as abiraterone acetate (AA) and enzalutamide rendered highly efficient implementations of both anti-androgen strategies. However, reports of therapy resistance or cross resistance soon accompanied reports of successful clinical trials of these new drugs. Moreover, case reports of an AA-withdrawal phenomenon surfaced, an outcome normally associated with AR antagonism e.g. by bicalutamide with high AR-affinity and the potential to switch from antagonism to AR-activation. Methods: CRPC cell model VCaP and VCaP resistant to permanent treatment of 5 µmol/L AA (Janssen-Cilag, Germany) were analyzed under AA and for AA withdrawal. Expression of AR, AR-V7, androgen regulated genes, CYP17A1 and AKR1C3 was quantitated by real time RT-PCR. Further analyses were performed by western blots, prostate-specific antigen (PSA) ELISA kits and cell proliferation BrdU tests. Results: VCaP cells resistant to 5 µmol/L AA after 7 months revealed excessive AR and CYP17A1 expression and a marked decline of tumor cell proliferation in response to AA withdrawal or to a similar extent to added 1 nmol/L testosterone. Proliferation dropped to less than 50% when both procedures were combined. Elimination of tumor cells was accompanied by a massive release of PSA which was most pronounced in the combined treatment. Conclusions: Overexpression of AR turning into a liability as a therapeutic target for a testosterone surge has been reported by several research groups. AA withdrawal from AA resistant cells complies with these necessities by overexpression of AR and CYP17A1 formerly arrested by AA. In patients a PSA decline upon AA withdrawal may be explained by an excessive loss of PSA secreting tumor cells. Therefore, AA qualifies uniquely for effective intermittent androgen deprivation of CRPC.
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