Our results indicate that in an animal model, 1) one dose of mifepristone at first-trimester human equivalent (~ 4–6 weeks, 53,56,57), induces a complete abortion and 2) the administration of progesterone (at a human equivalent to approximately 6–9 hours 64) following mifepristone administration, in a pregnant rat, reverses the effects of the mifepristone, resulting in living offspring at the end of gestation (D21) in the majority (81.3%) of rats in the reversal group (i.e., received mifepristone + progesterone).
The survival of the offspring following the administration of mifepristone appears to directly correlate with the progesterone administration given the clear and evident initiation of the abortion process based on the presence of the same characteristic physical responses of abortion initiation in the reversal group as the abortion group (weight loss and uterine bleeding). In fact, there was no significant difference in regard to uterine bleeding between the two groups. However, in regard to body weight, while never reaching the same percentage body weight gain as that of the normal pregnancy group after drug/vehicle administration, potentially due to some fetal demise in some cases, the successful reversal group displayed weight gain at a rate that paralleled the normal pregnancy group until D21 following the abortion-initiation-associated-weight loss. These results, in addition to the approximate fetal weight, as calculated from the uterine weight per fetus, and the average uterine diameters, appear to support the normal development of the surviving fetuses in the reversal group.
This is in contrast to the rats in which progesterone failed to induce a reversal (18.8%), whose weights paralleled those of the abortion group with no significant differences across days between the two groups through D21. Moreover, the unsuccessful reversal group also displayed an absence of any gestational sacs and living fetuses as evident through ultrasonography, including the absence of any cardiac activity, and confirmed in the weights of the collected uteri, which were not significantly different from those of the abortion group.
In relation to the reversal group relative to the normal pregnancy group, percentage weight gain progression (rate of weight gain), uterine weight, and number of fetuses were not significantly different from each other (although the number of fetuses indicated a tendency toward significance), again appearing to support the normal development of the surviving fetuses in the reversal group.
Ultrasound scans and heart rate measurement, in our study, also indicated no difference between the normal pregnancy group and the reversal group. While further investigation is warranted and necessary, this may potentially reflect the absence of detrimental consequences following the administration of progesterone after abortion initiation using mifepristone, at least at the level of cardiac activity. In fact, a case report of an unsuccessful abortion, using mifepristone, in a twin pregnancy, indicated no postnatal abnormalities 65. Additionally, previous literature has reported no evidence of a significant increase in major malformations in the continuation of pregnancy following mifepristone exposure 66,67. Moreover, additional literature has indicated that progesterone/17 alpha-hydroxyprogesterone caproate administration during gestation does not appear to negatively impact the health of the offspring resulting from that pregnancy 39,46,68,69.
The administration and actions of the natural agonist, progesterone, in the presence of the antagonist, mifepristone, appears to be in concordance with the literature and our understanding of the pharmacological functioning of reversible competitive antagonism 70–75, where sufficient levels of the agonist can override a given concentration of an antagonist. The higher doses of progesterone necessary can, at least in part, be explained by the necessity to overcome the higher affinity of mifepristone to the progesterone receptor 76. Another factor that may contribute to the mechanisms at play in this process is the metabolic clearance rate which, in rats, following high acute progesterone levels, has been shown to not only lead to an increase in the progesterone levels, but to also reduce its clearance 77.
Limitations and Future Direction
As with every scientific study, there are limitations that need to be considered. Thus, a limitation of this study is that the actual individual fetal weights were not directly measured but were addressed based on the indirect measurement obtained from the uterine weights and the number of observed living fetuses. However, despite this limitation, the average uterine weight per fetus appears to indicate approximate fetal weights that are well in line with those previously documented generally for rats 78–80.
Another aspect requiring consideration in animal models is the potential differences and similarities in gestation between the animal and human pregnancy. In the case of a rat model, similarities to human pregnancy include hemochorial placentation 81,see 82 for review, while differences include a divergence in the primary source of progesterone synthesis that takes place in humans during the luteo-placental shift 34,83. This shift occurs around the 7th week in a human pregnancy 34, but does not occur in the rat, where the corpus luteum remains the primary source of progesterone 84. In this regard, while the capacity to accurately indicate the exact human-equivalent timing is a potential limitation, based on previous literature 53,56,57, our study was conducted prior to the time equivalent to the luteo-placental shift in humans, given that the mifepristone and progesterone administration took place at the equivalent of ~ 4–6 human weeks of pregnancy. Additionally, in relation to the timing of progesterone administration relative to mifepristone, as indicated above, and based on previous literature 64, it appears that the timing used in this study (approximately 10–15 minutes after mifepristone administration) is equivalent to between approximately 6 and 9 human hours post-mifepristone administration. While times less than 10 minutes were not tested in this study, it is anticipated that a shorter gap between the administration of the two drugs would result in better outcomes. The fundamental reasoning behind the timing was to avoid co-administration and the necessity to ensure that we observed the physical symptoms (as reported) of the initiation of the abortion process. This is fundamental to ensure no ambiguity in the interpretation of the results of progesterone’s capacity to reverse the mifepristone-initiated abortion.
While the findings from our study, as per any pre-clinical study conducted in animal models, cannot be extrapolated directly to the clinical/human level, they provide the possibility of objectively and ethically investigating progesterone-mediated reversal of mifepristone-induced abortion. Such a limitation needs to be considered in the interpretation of the data. However, despite this limitation, this study provides a novel and more accurate model for progesterone-mediated reversal of mifepristone-induced pregnancy termination than previous research 52, with a clear initiation of abortion, followed by a recovery that leads to full-term gestation. This study does not address the physiological and behavioral aspects following birth in the case of reversal, in either the mother or the offspring, but provides the foundation necessary for additional research, including at the physiological and behavioral levels. Additionally, the study does not address the critical time period in which mifepristone-induced abortion can be reversed by the administration of progesterone.
Based on the findings of this current work indicated above, in addition to our previous research pertaining to behavioral and physiological consequences resulting from mifepristone-induced pregnancy termination 54, future research will seek to integrate these findings into addressing, in greater detail, the impact of mifepristone-induced termination and the progesterone-mediated reversal at the behavioral, physiological and ultimately neurological levels.