Enhanced antitumor activity of low-dose continuous administration schedules of topotecan in prostate cancer

Purpose: The objective of this study was to determine the antitumor effects of alternate dosing schedules of topotecan in prostate cancer. Results: A concentration-dependent increase in cytotoxicity was observed in PC-3 and LNCaP cells after topotecan treatment using conventional and metronomic protocols. A significant increase in potency (2.4-18 fold, after 72 h) was observed following metronomic dosing compared with conventional dosing administration in both cell lines. Metronomic dosing also increased the percentage of PC-3 cells in the G(2)/M, compared with control, but did not alter LNCaP cell cycle distribution. Metronomic dosing increased p21 protein expression in LNCaP and PC-3 cells compared with conventional dosing. The observed in vitro activity was confirmed using an in vivo model of human prostate cancer. Metronomic dosing and continuous infusion decreased tumor volume significantly (p <= 0.05) compared with control and conventional topotecan treatment, but had no effect on tumor vascular staining. Methods: The cytotoxicity of topotecan after conventional or metronomic dosing was determined by examining cellular morphology, mitochondrial enzymatic activity (MTT), total cellular protein (SRB), annexin V and propidium iodine (PI) staining, cell cycle and protein gel blot analysis in human prostate cancer cell lines (PC-3 and LNCaP) and the effects metronomic or continuous infusion on tumor growth in an in vivo tumor xenograft model. Conclusions: These data support the hypothesis that low-dose continuous administration of topotecan increases potency compared with conventional dosing in prostate cancer. These data also suggest the novel finding that the enhanced antitumor activity of topotecan following low-dose exposure correlates to alterations in cell cycle and increased p21 expression.