4617 Background: Androgen ablation is the mainstay of systemic therapy for prostate cancer, with cytotoxic chemotherapy reserved for hormone refractory disease. Earlier use of chemotherapy at the time of androgen dependence when there are potentially fewer multi-resistant clones, may yield greater benefit. This study addresses the ideal timing of systemic treatments in the androgen-dependent Shionogi tumour model. The hypothesis that stress-induced changes in gene expression after chemotherapy can induce a hormone independent phenotype was further explored in the LNCaP setting. Methods: Three groups of ten mice bearing Shionogi xenografts were treated with: 1) initial castration and delayed paclitaxel, 2) initial paclitaxel and delayed castration, or 3) combined therapy. Endpoints were time to tumour progression and time to sacrifice. The additive effects of combined therapy were also assessed through in vitro proliferation studies on LNCaP cells. Spotted microarray and RT-PCR analyses were carried out on mRNA from LNCaP cells exposed to paclitaxel to identify gene expression changes over time. Results: Mice receiving combined therapy demonstrated a significant improvement in time to progression (median 65 vs. 38 days, p=0.004) and time to sacrifice (median 83 vs. 66 days, p<0.014) versus sequential therapy. A marked lack of response to castration in the mice treated with paclitaxel first was observed. Similar effects were seen in vitro. Microarray studies identified increases in several genes known to play a role in androgen independence in response to paclitaxel exposure. Quantitative RT-PCR demonstrated an increase of expression of clusterin, bcl-2, bcl-xL and Hsp27 of between 30% and 60% after 48 hours of paclitaxel exposure. Conclusions: In laboratory models of prostate cancer, the combination of androgen deprivation and paclitaxel is more effective than sequential treatment. Molecular studies confirm that chemotherapy can induce alterations in gene expression associated with androgen independent proliferation. These findings provide pre-clinical proof-of-principle for ongoing clinical trials addressing the role and timing of systemic therapies in prostate cancer. No significant financial relationships to disclose.