Recent evidence suggests that the quinazoline-based α1-adrenoceptor antagonists, doxazosin and terazosin, exhibit a potent apoptotic effect against prostate tumor epithelial cells, whereas tamsulosin, a sulfonamide-based α1-adrenoceptor antagonist, was ineffective in inducing a similar apoptotic effect against prostate cells (Cancer Res., 60: 4550–4555, 2000). In this study, to identify the precise molecular mechanism underlying this apoptosis induction, we examined whether doxazosin and terazosin (both piperazinyl quinazolines) affect prostate growth via an α1-adrenoceptor-independent action. Transfection-mediated overexpression of α1-adrenoceptor in human prostate cancer cells, DU-145 (that lack α1-adrenoceptor), did not alter the ability of prostate cancer cells to undergo apoptosis in response to quinazolines. Significantly enough, there was no modification of the apoptotic threshold of the androgen-sensitive prostate cancer cells, LNCaP, to either quinazoline-based α1-agonist by androgens. Furthermore, human normal prostate epithelial cells exhibited a very low sensitivity to the apoptotic effects of doxazosin compared with that observed for the malignant prostate cells. These findings provide the first evidence that the apoptotic activity of the quinazoline-based α1-adrenoceptor antagonists (doxazosin and terazosin) against prostate cancer cells is independent of: (a) their capacity to antagonize α1-adrenoceptors; and (b) the hormone sensitivity status of the cells. This may have potential therapeutic significance in the use of quinazoline-based α1-adrenoceptor antagonists (already in clinical use for the treatment of hypertension and benign prostate hyperplasia) for the treatment of androgen-independent human prostate cancer.