Polymeric micellar paclitaxel phosphorylates Bcl-2 and induces apoptotic regression of androgen-independent LNCaP prostate tumors

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Abstract
BACKGROUND Paclitaxel has been difficult to evaluate in preclinical tumor model systems because its poor solubility requires a Cremophor EL formulation, which results in lethal anaphylaxis. We tested the effectiveness of a novel polymeric micellar paclitaxel on androgen‐independent tumor growth in the LNCaP tumor model. METHODS Athymic male mice bearing LNCaP tumors were castrated and allowed to grow until their PSA levels increased to three times above precastration levels. The animals were then treated with 0.5 mg intravenous polymeric micellar paclitaxel once daily for the first 5 days of a 3‐week cycle. In total, three cycles were given. Tumor volume and serum PSA levels were measured weekly to monitor tumor progression. RESULTS In vitro mitogenic assays demonstrated that polymeric micellar paclitaxel was effective in inhibiting LNCaP cell growth with an IC50 of 5 nM. Paclitaxel precipitated apoptosis in vitro at a concentration of 1 nm and higher, confirmed by DNA laddering. Western blotting demonstrated that paclitaxel treatment phosphorylated and inactivated Bcl‐2. In mice bearing LNCaP tumors treated with micellar paclitaxel, tumors regressed rapidly with the commencement of micellar paclitaxel treatment. Tumor size decreased 91% and PSA level decreased 96% after three cycles of treatment. TUNEL immunostaining of the tumor treated with micellar paclitaxel showed marked apoptosis when compared with the control. No significant side effects or mortality was observed in the micellar paclitaxel group (n = 7). In contrast, all (n = 7) mice treated with conventional Cremophor EL paclitaxel died within 1 day of injection. CONCLUSIONS The polymeric micellar paclitaxel formulation is water‐soluble and capable of inducing complete response in mice bearing androgen‐independent LNCaP tumors. The lack of toxicity of polymeric micellar paclitaxel permits in vivo preclinical testing of paclitaxel‐based combination regimens. Prostate 44:156–163, 2000.

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