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The antitumor potency of progesterone antagonists is due to their differentiation potential
Abstract:
A new therapy for the progesterone receptor positive mammary carcinoma may be the treatment with progesterone antagonists. This new class of antihormones causes a strong inhibition of tumor growth comparable to the potency of ovariectomy in a panel of experimental mammary carcinomas. The mechanisms of the strong tumor-inhibiting action of progesterone antagonists on experimental mammary carcinomas mainly depends on a progesterone receptor mediated process leading to induction of terminal differentiation and a blockade of the cell cycle. To further characterize the antitumor mechanism of progesterone antagonists we analyzed the effects of Onapristone and ZK 112.993 on DMBA- and MNU-mammary tumors of the rat and MXT-tumors of the mouse after different therapy intervals. These hormone-dependent mammary tumors normally display intraductal growth in papillary, cribiform or solid formation, whereas after treatment periods of 2-6 weeks with progesterone antagonists they displayed dysplastic ductal and acinous formations, usually filled with secretory material. Whereas tumor size, mitotic index, and the grade of tumor malignancy decreased distinctly, the volume fraction of glandular structures in the tumors as well as the appearance of apoptosis increased 3-fold compared to the controls. In addition, the mammary glands of progesterone antagonist treated animals showed the morphological features of differentiation with the appearance of secretory activity. Interestingly, the staining pattern of some of the lectins used, especially UEA 1 binding pattern, fits to the concept of differentiation since recent studies revealed a higher degree of fucosylation only in benign lesions of human breast cancers. Therefore, these data underline the concept of a differentiation potential of progesterone antagonists on progesterone receptor positive mammary carcinomas.
Insights
Progesterone antagonists show potent antitumor effects in progesterone receptor-positive mammary carcinomas. This new therapy induces tumor cell differentiation and inhibits growth, offering a promising treatment strategy.
Area of Science:
- Oncology
- Endocrinology
- Molecular Biology
Background:
- Progesterone receptor (PR) positive mammary carcinoma is a significant health concern.
- Current therapies for PR-positive breast cancer have limitations.
- Progesterone antagonists represent a novel class of antihormones with potential antitumor activity.
Purpose of the Study:
- To investigate the antitumor mechanisms of progesterone antagonists in experimental mammary carcinoma models.
- To characterize the effects of Onapristone and ZK 112.993 on specific mammary tumors.
- To evaluate the potential of progesterone antagonists to induce differentiation in PR-positive mammary carcinomas.
Main Methods:
- Treatment of rat DMBA- and MNU-mammary tumors and mouse MXT-tumors with progesterone antagonists (Onapristone, ZK 112.993).
- Analysis of tumor morphology, growth, mitotic index, malignancy grade, glandular structures, and apoptosis after 2-6 weeks of therapy.
- Assessment of mammary gland differentiation and lectin binding patterns (UEA 1).
Main Results:
- Progesterone antagonists significantly inhibited tumor growth, comparable to ovariectomy.
- Treatment led to dysplastic ductal and acinous formations with secretory material.
- Tumor size, mitotic index, and malignancy grade decreased, while glandular structures and apoptosis increased threefold.
- Mammary glands showed differentiation with secretory activity; UEA 1 binding suggested increased fucosylation.
Conclusions:
- Progesterone antagonists exert strong antitumor effects on PR-positive mammary carcinomas.
- The mechanism involves progesterone receptor-mediated induction of terminal differentiation and cell cycle blockade.
- These findings support the differentiation potential of progesterone antagonists as a therapeutic strategy for PR-positive mammary carcinomas.