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The Selective Estrogen Receptor Modulator Trioxifene (LY133314) Inhibits Metastasis and Extends Survival in the PAIII Rat Prostatic Carcinoma Model

Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, Indiana 46285

Trioxifene (LY133314) is a selective estrogen receptor modulator (SERM) with competitive binding activity against estradiol for estrogen receptor (ER) and antagonistic activity against ER-mediated gene expression. The PAIII rat prostatic adenocarcinoma (PCa) is an androgen receptor-negative, ER- and ERß-positive, spontaneously metastatic rodent tumor cell line. After s.c. implantation of 10⁶ PAIII cells in the tail, s.c. administration of trioxifene (2.0, 4.0, 20.0, or 40.0 mg/kg-day) for 30 days produced significant (P < 0.05) inhibition of PAIII metastasis from the primary tumor in the tail to the gluteal and iliac lymph nodes (maximum nodal weight decreases, 86% and 88% from control values, respectively). PAIII metastasis to the lungs was significantly inhibited by trioxifene treatment. Numbers of pulmonary foci in PAIII-bearing rats were significantly (P < 0.05) reduced by trioxifene administration in a dose-related manner (maximal reduction, 98% from control values). Continual administration of the compound significantly (P < 0.05) extended survival of PAIII-bearing rats. Trioxifene inhibited the proliferation of PAIII cells at micromolar levels in vitro but did not slow growth of the primary tumor growth in the tail. Trioxifene administration also produced regression of male accessory sex organs. In PAIII-tumor-bearing animals, trioxifene administration produced a maximal regression of 76% for ventral prostate and 64% for seminal vesicle (P < 0.05 for both). SERMs may be preferable to estrogens given their efficacy in experimental PCa models and relative lack of side effects observed in clinical trials. Our data support the contention that trioxifene represents a SERM with potential antimetastatic efficacy for the treatment of androgen-independent, metastatic PCa.

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