- Topoisomerase I (topo I) is an essential nuclear enzyme involved in virtually all aspects of gene expression, and is the target of the anti-cancer drugs- camptothecin (CPT) and its derivatives. Improvement of the survival rates of young women with cancer has led to the consideration of the effects of long-term chemotherapy on their fertility. The effect of anticancer drugs on ovarian function was previously investigated; however, no reports are available concerning their effect on the endometrium, whose integrity is an important factor in embryo implantation. Here we used a rat animal model to investigate the expression and activity of topo I in the various physiologic phases of the endometrium and the influence of CPT on its integrity and receptivity. The results show, for the first time, that the endometrial topo I level and activity are influenced by the physiologic phases of the endometrium (estrous cycle) and correlate with the estrogen blood concentration. Treatment with the anti-cancer drug CPT caused histological disruption of the endometrium and deleterious effect on its cyclicity. Moreover, CPT treatment significantly reduced the implantation rate of embryos, suggesting alteration in the receptivity of the endometrium. These results suggest that topo I is important for maintaining the normal physiologic cyclicity and functionality of the endometrium in rats. Anti-cancer agents that target topo I severely impair estrous cycle progression and endometrial integrity and receptivity, emphasizing the importance of addressing the effect of chemotherapy on the endometrial functionality.