Some of you have been asking me about the difference between clomiphene and ulipristal

Guys, some of you have been asking me about the difference between clomiphene and ulipristal as to why both are partial agonist, but ulipristal has anovulatory action.
Its a very complicated mechanism and hence i dont go in to detail in the lecture. However as many of you have asked, i have tried to simplify it in this diagram.
• In follicular phase FSH is released which facilitates maturation of follicles as well as moderate increase in estrogen, which has inhibitory effect on LH release (as we dont want ovulation before full maturation of follicles).
• However after maturation of follicles there is a significant increase in estrogen, that stimulates de-novo synthesis of progesterone in hypothalamus. This progesterone stimulates progesterone receptors in gonadotrope cells and increase release of LH (LH surge). This results in ovulation and synthesis of progesterone.
• Progesterone antagonises the effect of estrogen on hypothalamus and kills the LH surge.
• Now when i give ulipristal, which is a partial agonist at progesterone receptors. When progesterone level is low i.e. in the early phase, it acts as agonist and just like progesterone or drug levonogestrel block ovulation.
• But progesterone or levonorgestrel cant achieve this effect if given immediately before ovulation as denovo synth of progesterone is complete in hypothalamus.
• So in this case the second mechanism is where in presence of high progesterone released by hypothalamus, it behaves as an antagonist and decreases LH surge and inhibits ovulation.
• Thus ulipristal is considered to be better than levonorgestrel as it is effective even if taken just before ovulation.
So guys by now you would have understood, why i dont cover this complicated mechanism in lecture.