Prostate cancer (PCa) is the most common cancer in men in the UK. Initially PCa is androgen sensitive; it then progresses to hormone resistant PCa (HRPC), which eventually becomes resistant to chemotherapy as well. Doxazosin, an α1A adrenergic antagonist used in the management of benign prostatic hyperplasia, decreases the incidence of PCa (Harris et al, 2007). Furthermore, in vitro studies have shown that doxazosin reverses drug resistance to chemotherapy (Takara et al, 2009), with an additive anti-neoplastic effect when combined with chemotherapy (Cal et al, 2000) or radiotherapy (Ceullar et al, 2002). These cytotoxic actions of doxazosin against PCa are by an adrenergic-independent mechanism (Kyprianou et al, 2000). However, the molecular target and type of cell death induced by doxazosin is unclear. Our objective was to identify, using chemical inhibitors of autophagy and transmission electron microscopy (TEM), whether autophagy is the mode of cell death induced by doxazosin. PC-3 (p53 null, HRPC) cells were grown in F-12 HAM media and skin fibroblasts were grown in DMEM, respectively, at 37 C in humidified 5% CO2. Both media were supplemented with 1 % antibiotic solution containing penicillin/streptomycin and 8% foetal bovine serum. (a) PC-3 cells were seeded (5000 cell/well) in 96-well plates. After 24 h incubation they were pre-treated for 4 h with/without autophagy inhibitors, 3-methyl adenine (2 mM) and dansylcadaverine (75 µM), and then exposed to doxazosin (37 µM dissolved in H2O) for a further 48 h. Subsequently, cell viability was assessed by CellTiter 96® assay. Results are expressed as means ± S.E.M. and statistical differences are determined using Student’s two-tail paired t test. (b) Each cell line was treated for 48 h with doxazosin (37 µM) or vehicle (control) and morphology assessed by TEM. Pre-treatment with 3-methyl adenine or dansylcadaverine significantly (p=0.02 and p<0.0001, respectively) reduced cytotoxic action of doxazosin. TEM appearances of untreated PC-3 cells (Fig.1) remarkably differed from those treated with doxazosin (Fig 2). The latter showed distinct morphological characteristics of autophagy with autophagosomes containing mitochondria (mitophagy) accompanied by extensive lipofuscin-like accumulation. Skin fibroblasts also demonstrated identical findings on TEM following exposure to doxazosin. Thus, p53-null status of PC-3 cells could not be accounted for autophagy as the skin fibroblasts have p53 gene. Also, notable features of apoptosis and necrosis such as karyorrhexis and pyknosis were absent in both treated groups. We have shown, for the first time doxazosin-induced PCa cell death by autophagy (mitophagy) that results in lipofuscin like accumulation. This novel finding may be exploited as a potential therapeutic target for the treatment of HRPC.