Synergistic effects of antiprogestins and iNOS or aromatase inhibitors on establishment and maintenance of pregnancy
Section snippets
Introduction and background
The small but potent molecule, nitric oxide, is a common factor in several processes related to implantation. Evidence suggests that cytokines, prostaglandins, histamine, matrix metalloproteinases (MMPs), and hormones all play a role in embryo-maternal interaction and alone or in combination, may alter nitric oxide (NO) levels. NO is a biological free radical that shows chemical activity within numerous cell systems and is responsible for altering vasoactivity, smooth muscle contraction,
NO, antiprogestins and implantation
The nitric oxide system in the endometrium may be a possible target for manipulation by antiprogestins [32], [33]. In mice, both eNOS and iNOS are elevated at the implantation site [12], [33]. In humans both eNOS and iNOS are upregulated by progesterone in both the endometrial vessels and stromal/decidual cells [34], [35]. Interestingly, iNOS continues to increase in and around the implantation site compared to the intersite areas through Day 8 of pregnancy in rodents [33]. Our
iNOS at implantation site and effects of NO inhibition
The importance of NO in the regulation of events in pregnancy has been demonstrated by numerous recent studies [16], [33], [36]. The NO system in early pregnancy may be a possible target for manipulation by antiprogestins [32], [33]. In mice, both eNOS and iNOS are elevated at the implantation site [10], [33]. In humans both eNOS and iNOS are upregulated by progesterone in both the endometrial vessels and stromal/decidual cells [34], [35], [40]. Interestingly, our immunohistochemical studies in
Effects of antiprogestin (onapristone) and NO inhibition with L-NAME or aminoguanidine on implantation in rats
Our studies, in rats and guinea pigs, demonstrate that NOS inhibitors (L-NAME and aminoguanidine) and the antiprogestin, onapristone, act synergistically at the endometrial level [32]. Preimplantation treatment was very effective in preventing the establishment of pregnancy in both species. There were no changes in blood steroids following these treatments. This effect was accompanied by a dramatic vasoconstriction of endometrial blood vessels prior to and during early placentation. In
Effects of antiprogestins and iNOS inhibitors on implantation
Other studies in the rat with a specific iNOS inhibitor and antiprogestins have demonstrated a significant inhibition of development during early pregnancy. When the iNOS inhibitor ZK 810 000 (Berlex Biosciences, Richmond, CA) was given together with the antiprogestin there was a dramatic synergistic effect on implantation (see Fig. 4). The combined dose of 0.3 mg/animal antiprogestin and 7.5 mg iNOS inhibitor blocked implantation for 2 days during the pre-implantation period (Days 4 and 5) of
Effects of NOS inhibition on ability of antiprogestins to initiate preterm delivery in late pregnancy
Treatment of rats with low dose mifepristone (1 mg per day, orally) at Days 17 and 18 of gestation did not influence the progress of pregnancy (Fig. 5). Similarly, treatment with the ZK 810 000 iNOS inhibitor (15 mg per day orally) had little effect by itself on delivery. However, when mifepristone was combined with the iNOS inhibitor >95% of the pups were delivered prematurely (Fig. 5). We conclude that an antiprogestin can act synergistically with an iNOS inhibitor to initiate preterm delivery.
Effects of aromatase inhibitors
The above studies (Fig. 4, Fig. 6) show how effective the ZK 810 000 compound is either in combination with an antiprogestin (mifepristone) in preventing implantation compared with other specific iNOS inhibitors and nonspecific NOS inhibitors when given for 2 days during the preimplantation period (Days 4 and 5) of pregnancy. Since NOS enzymes are similar to P450 enzymes we postulated that perhaps the ZK 810 000 compound is inhibiting P450 enzymes in addition to acting on iNOS. When we treated
Conclusions from our studies
- 1.
NO-progesterone interaction plays a pivotal role during the establishment and maintenance of pregnancy.
- 2.
The synergistic effects of antiprogestins and iNOS inhibitors may be due to the inhibition (dysynchrony) of embryo-maternal dialogue during the peri-implantation period.
- 3.
These results open up new therapeutic perspectives for either inhibiting (antiprogestin combined with iNOS inhibitors for postcoital contraception or induction of labor) or promoting implantation (progesterone and NO
Acknowledgements
The authors would like to acknowledge the work of Tracy Purcell on NO function in the initial phase of this project. The authors would also like to thank Kris Chwalisz and Walter Elger of Jenapharm and John Parkinson and Gary Phillips from Berlex Biosciences (Richmond, CA) for providing us with the study compounds and their advice and assistance. Thanks are also extended to Michael Harper of CONRAD for his helpful advice. This study is supported by funds from the Contraceptive Research and
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