Homerton Fertility Centre, Homerton University Hospital, London, UK
For the past 60 years clomifene citrate (CC) has been the first line treatment for those with absent or irregular ovulation but who have normal basal levels of endogenous oestradiol (almost all of whom have PCOS). The action of CC is by blocking hypothalamic oestrogen receptors, signaling a lack of circulating oestrogen to the hypothalamus and inducing a change in the pattern of pulsatile release of gonadotrophin releasing hormone (GnRH) and consequently a discharge of FSH. Although CC will restore ovulation in approximately 80 % of patients it will result in pregnancy in only about 35–40 % and 20–25 % will not respond at all and are considered to be ‘clomiphene resistant’. Aromatase inhibitors are non-steroidal compounds that suppress oestrogen biosynthesis by blocking the action of the enzyme aromatase which converts androstendione to oestrogens. Letrozole, the most widely used aromatase inhibitor, is given orally in a dose of 2.5–5 mg/day and is almost free of side effects. In an RCT, 750 women with anovulatory PCOS were randomized to receive either clomifene or letrozole. A 44 % increase in pregnancy rate was achieved by letrozole over clomifene (27.5 % vs 19.5 %). Twinning rate was non-significantly higher in those who received clomifene (7.4 % vs 3.2 %) with no significant difference in the rate of congenital abnormalities. The conclusion would seem to be that letrozole can be regarded as a serious competitor to CC for first-line therapy for induction of ovulation.
Anti-oestrogensClomifene citrateOestradiolHypothalamic-pituitary dysfunctionPCOSFSHOvulationOestrogenGonadotrophin releasing hormoneGnRHFollicular recruitmentFollicular growthAnovulatoryLHClomipheneHuman chorionic gonadotrophinhCGIntra-uterine inseminationIUIOvarian hyperstimulation syndromePregnancyEndometriumEndometrial thicknessOvulation inductionUltrasoundCervical mucusDexamethazoneDehydroepiandrosterone sulphateMetforminAndrogenTestosteroneTheca cellsAromatase inhibitorsLetrozoleMonofollicular ovulation
8.1 Clomifene Citrate
For the past 50 years clomifene citrate (CC) has been the first line of treatment for those with absent or irregular ovulation but who have normal basal levels of endogenous oestradiol (hypothalamic-pituitary dysfunction – WHO Group II). The underlying cause of this type of ovulatory dysfunction is PCOS in a large majority of cases.
8.1.1 Mode of Action
Clomifene citrate is capable of inducing a discharge of FSH from the anterior pituitary and this is often enough to reset the cycle of events leading to ovulation into motion. The release of even small amounts of FSH into the system will often induce ovulation and pregnancy in a large proportion of eu-oestrogenic anovulatory women. This is achieved indirectly, through the action of CC, a non-steroidal compound closely resembling an oestrogen, in blocking hypothalamic oestrogen receptors, signaling a lack of circulating oestrogen to the hypothalamus and inducing a change in the pattern of pulsatile release of gonadotrophin releasing hormone (GnRH).
Clomifene citrate is given orally in a dose of 50–250 mg per day for 5 days from day 2, 3, 4 or 5 of spontaneous or induced bleeding starting with the lowest dose and raising the dose in increments of 50 mg/day per cycle until an ovulatory cycle is achieved. The starting day of treatment, whether on day 2 or through day 5 of the cycle does not influence results . Although 50 mg/day is the recommended dose in the first cycle, less than 50 % will respond to this dose so some practitioners often use a starting dose of 100 mg per day from day 4 or 5, only resorting to 50 mg/day in the case of exquisite sensitivity or persistent cyst formation. There is no apparent advantage of using a daily dose of more than 150 mg which seems neither to significantly increase the ovulation rate nor follicular recruitment . The advantage of the ‘cutting corners’ regimen of starting with a 100 mg daily dose rather than 50 mg, is that it will cut down the number of ‘superfluous’ cycles of treatment until ovulation is achieved and until those resistant to clomifene are identified. A course of six ovulatory cycles is usually sufficient to know whether pregnancy will be achieved using CC before moving on to more complex treatment as approximately 75 % of the pregnancies achieved with CC occur within the first three cycles of treatment . From our own (unpublished) database, we noted that no further pregnancies at all were obtained with CC following seven ovulatory cycles.
Although CC will restore ovulation in approximately 80 % of patients it will result in pregnancy in only about 35–40 % and 20–25 % of anovulatory women with normal FSH concentrations will not respond at all and are considered to be ‘clomiphene resistant’ . Inability of CC to induce ovulation is more likely in patients who are obese, insulin resistant and hyperandrogenic compared to those who do respond . Women with high basal LH levels are also less likely to respond to clomiphene treatment .
As CC blocks the negative feedback mechanism which the eventually rising oestradiol levels would normally invoke, multiple follicle development is relatively common. The risk of multiple gestation is therefore increased and is estimated at about 8–10 % . The vast majority of these are twin pregnancies but the risk may be reduced considerably by ultrasound monitoring and withholding hCG, IUI or intercourse if more than two follicles >14 mm diameter are seen.
The prevalence of congenital abnormalities  and spontaneous abortion  following CC treatment are no different to those seen in spontaneously conceived pregnancies. While mild ovarian enlargement is relatively common, in almost 50 years of practice, I have never seen a full blown ovarian hyperstimulation syndrome as a result of CC treatment.
It is frustrating that the restoration of ovulation does not produce more than a 40 % pregnancy rate. This discrepancy between ovulation and pregnancy rates in patients treated with CC may be partly explained by the peripheral anti-oestrogenic effects of CC at the level of the endometrium and cervical mucus. While the depression of the cervical mucus, occurring in about 15 % of patients, may be overcome by performing intra-uterine insemination (IUI), suppression of endometrial proliferation, unrelated to dose or duration of treatment but apparently idiosyncratic, indicates a poor prognosis for conception if the endometrial thickness on ultrasound scanning does not reach a thickness of 8 mm at ovulation. If endometrial suppression is noted in the first cycle of treatment with CC, it will almost certainly be seen in repeated cycles in the same woman. There is little point in persisting after even one cycle and a step-up to other forms of ovulation induction is recommended.
Monitoring of the clomifene treated cycle by ultrasound evaluation of follicular growth and endometrial thickness on day 12–14 of the cycle is justified by the identification of those who are not responding or have depressed endometrial thickness and is helpful in the timing of natural intercourse or IUI. We found that ultrasound monitoring of CC cycles yielded a cumulative conception rate of 48 % compared with 35 % for those who had unmonitored cycles. Confirmation, or otherwise, of ovulation can be obtained with estimation of the progesterone concentration in the supposed mid-luteal phase. A basal body temperature chart may also be helpful for monitoring but, although cheaper than other methods, is less reliable and may be bothersome for the patients. The added expense of careful monitoring is neutralized by the prevention of protracted periods of possibly ineffective therapy and delay in the inception of more efficient treatment.
8.1.5 Possible Adjuvants to Clomifene Treatment
In order to improve the outcome of treatment with CC, several adjuvants to clomifene treatment have been suggested. A correctly timed ovulation triggering dose of human chorionic gonadotrophin (hCG, 5,000–10,000 IU) is only theoretically warranted when the reason for a non-ovulatory response is that the LH surge is delayed or absent despite the presence of a well developed follicle. Although the routine addition of hCG at mid-cycle seems to add little to the improvement of conception rates  we have found it very useful, given when an ultrasonically demonstrated leading follicle attains a diameter of 19–24 mm, for the timing of intercourse or IUI.
The addition of dexamethazone as an adjunct to clomifene therapy in a dose of 0.5 mg at bedtime, is said to suppress adrenal androgen secretion and induce responsiveness to CC in previous non-responders, mostly hyperandrogenic women with PCOS and elevated concentrations of dehydroepiandrosterone sulphate (DHEAS) . However, glucocorticoid steroid therapy often induces side effects including increased appetite and weight gain and should probably be reserved for women who have congenital adrenal hyperplasia as a cause for their anovulation.
The main action of CC, indirectly stimulating GnRH secretion, not only increases the desired FSH release but also an undesirable increase in LH concentrations. This increase in LH, whose basal level is often already high in women with PCOS, may compromise pregnancy rates in those receiving CC. We have demonstrated that pre-treatment with micronised progesterone is capable of modulating LH pulsatility, reducing LH concentrations and inducing a more favourable environment for ovulation induction with CC . This treatment improved response to CC and yielded consequent pregnancy rates.
Theoretically, metformin seemed to be a promising adjuvant to treatment with CC. The majority of patients who receive CC for anovulatory infertility have PCOS and the majority of these have some degree of insulin resistance. Metformin is said to increase insulin sensitivity and a consequent insulin lowering effect, producing a decrease in testosterone and LH, increase in SHBG and a direct androgen lowering effect on the theca cells. However, the theoretical advantages of a combination treatment of CC and metformin have not been realised in practice. Two large randomised studies showed no significant advantage of the combination treatment compared with clomifene alone [12, 13]. Subsequent large meta-analyses also failed to demonstrate a significant advantage of adjuvant treatment with metformin [14, 15]. Although the results have been disappointing in treatment naive subjects, the addition of metformin to CC may have a place in the treatment of CC resistant women. In an interesting RCT, CC-resistant women with PCOS received either metformin for 6 months and then CC, or hMG alone for ovulation induction . In this small study, as metformin + CC was equally as effective as hMG, less expensive and more convenient, it was suggested as an intermediary step for CC resistant patients, worth trying before resorting to hMG.
A flow chart suggesting possible algorithms for treatment with CC is illustrated in Fig. 8.1.
A flow chart suggesting possible algorithms for the treatment of anovulation with clomiphene citrate
8.1.6 Unexplained Infertilty
Clomifene has also been employed for ovarian stimulation in ovulating women, mainly for idiopathic (unexplained) infertility and often combined with IUI. The rationale is presumably that CC may overcome a subtle defect in ovulatory function and may increase the number of mature follicles so increasing the likelihood of pregnancy . Here the success rate has been, understandably, notably less than in anovulatory women. In a collection of data on the efficacy of treatment for unexplained infertility  the use of CC alone produced disappointing pregnancy rates of 5.6 % per cycle and CC combined with IUI 8.3 % per cycle. While this is significantly superior to timed intercourse alone, it should be remembered that the baseline level from merely expectant treatment in these cases ranges from 1.3 to 4.1 %. Treatment with CC for unexplained infertility cannot be recommended.
As most of the cases for which CC is used for the first line treatment of anovulation associated with PCOS, a suggested stepwise treatment for this cause of infertility is suggested in Fig. 8.2.
Suggested stepwise treatment scheme for infertility associated with PCOS. Details of low-dose FSH and laparoscopic ovarian drilling are given in Chaps. 9 and 11 respectively
8.2 Aromatase Inhibitors
Aromatase inhibitors are non-steroidal compounds that suppress oestrogen biosynthesis by blocking the action of the enzyme aromatase which converts androstendione to oestrogens. Letrozole, the most widely used aromatase inhibitor, has mainly been employed for the treatment of postmenopausal women with advanced breast cancer. It is given orally in a dose of 2.5–5 mg/day and is almost free of side effects.
8.2.1 Mode of Action
It was first hypothesized by Casper and Mitwally , that the efficient oestrogen lowering properties of the aromatase inhibitors could be utilised to temporarily release the hypothalamus from the negative feedback effect of oestrogen. This would allow an increased discharge of gonadotrophins, particularly FSH, from the pituitary. Although the final pathway, the sought after discharge of FSH, is common to both aromatase inhibitors and clomifene citrate (CC), their mechanism of action is obviously very different and this would seem to confer several advantages to aromatase inhibitors for the induction of ovulation.
8.2.2 Possible Advantages of Letrozole
Whereas the main mode of action of CC is an oestrogen receptor blockade and depletion, aromatase inhibitors have no direct effect on these receptors. Aromatase inhibitors should, therefore, not have any deleterious effect on cervical mucus or endometrium, quite frequently a side effect of CC which interferes with the attainment of a pregnancy during ovulation induction therapy. This action of CC, although beneficial at the hypothalamic level, is probably the main reason for the gap between ovulation and pregnancy rates. This can theoretically be avoided when aromatase inhibitors are used for the same purpose.
A further hypothetical advantage of aromatase inhibitors is the fact that when oestrogen production is advanced by the FSH discharge, in contrast with the use of CC, the hypothalamus is able to respond to the oestrogen feedback with a negative feedback mechanism. This will modulate an overzealous discharge of FSH which in turn is more likely to result in a monofollicular ovulation with moderate oestrogen concentrations. This is all the more poignant as aromatase inhibitors have a much shorter half-life (about 2 days) than CC. The prevalence of multiple pregnancies could therefore be expected to be less than that witnessed with the use of CC for ovulation induction.
Following the groundwork to examine the use of the aromatase inhibitor letrozole in reproductive medicine coming from the team of Casper, solid evidence-based data has now been produced to confirm these hypothetical advantages [19–21]. In a series of over 1,100 women who received 2.5–10 mg/day of letrozole, 368 pregnancies were achieved. Only two of these (0.5 %) were twin pregnancies and, very notably, only one (0.2 %) had a fetal anomaly . A massive randomised trial conducted by Legro et al.  should close the discussion regarding the superiority of letrozole over clomifene. This RCT comprised 750 women with anovulatory PCOS who were randomized to receive either clomifene or letrozole. A 44 % increase in pregnancy rate was achieved by letrozole over clomifene (27.5 % vs 19.5 %). Twinning rate was non-significantly higher in those who received clomifene (7.4 % vs 3.2 %) with no significant difference in the rate of congenital abnormalities.
The conclusion would seem to be that letrozole can be regarded as a possible replacement for CC for the first line treatment of anovulatory infertility.
8.2.4 Use of Letrozole in Controlled Ovarian Hyperstimulation
The use of aromatase inhibitors should theoretically result in an accumulation of androgens whose conversion to oestrogens is being blocked. This would, again theoretically, be an unwanted bi-product, especially for women with PCOS who already have an excessive production of androgens. However, paradoxically, this may be a further advantage as androgens may have a stimulatory role in early follicular growth by augmenting follicular FSH receptor expression and therefore amplifying FSH effects . This may explain the relative success of combined letrozole and FSH for ovarian stimulation in improving the response to FSH, reported in two studies. The first , is a report in which a group of poor responders to FSH for IUI were given cotreatment with letrozole, 2.5 mg/day from day 3–7 of the cycle. A lower FSH dose and a significantly higher number of mature follicles was achieved with the combined treatment. These preliminary findings were confirmed in a large series, albeit retrospective and non-randomized, comparing stimulation with FSH alone (145 cycles) or the combined therapy (60 cycles) . The addition of letrozole to gonadotrophin treatment again decreased the dose of gonadotrophins and increased the number of pre-ovulatory follicles. Prospective, randomized trials are needed to verify these interesting findings.
A sub-group of infertile women have been found to express high levels of aromatase P450 in the endometrium and this was associated with poor IVF outcomes . This raises the interesting question of whether letrozole could alleviate this situation and improve results.
8.2.5 Questions Remaining
Many other questions regarding the use of aromatase inhibitors in the treatment of infertility still remain . Trials with aromatase inhibitors have, reasonably, mimicked treatment with CC, being administered on day 3–7 of the cycle. Would treatment beyond day 7 interfere with the E2 rise induced by rising FSH concentrations and have a deleterious effect on the endometrium and oocyte quality? In his commentary, de Ziegler  also questions the timing of aromatase inhibitor administration when the intention is to enhance the sensitivity to FSH receptors by increasing follicular androgen content. Would it not be more logical to prime with aromatase inhibitors before exposure to FSH? Further, although the dose of 2.5 mg of letrozole is standard for the treatment of breast cancer, should the same dose be used for the treatment of infertility? Biljan et al. , for example, found that a daily dose of 5 mg/day produced more mature follicles apparently by further extending the FSH window.
The use of letrozole for induction of ovulation has not yet been sanctioned due to questions of possible teratogenicity raised by one, unpublished, congress presentation of an increased incidence of locomotor and cardiac anomalies in infants whose pregnancy was induced with letrozole compared with a control group of spontaneously conceiving women with an absolutely normal pregnancy. Notwithstanding the scientific mistake in selection of the comparators, the incidence of all malformations was not different between the two groups. A further study quashed these unsubstantiated doubts by reporting a lesser incidence of both minor and major congenital anomalies in a large group of women (n = 911) who conceived using letrozole compared with those who used CC . This valuable, reassuring information is further compounded by that provided in further, very large series of women who received letrozole for ovulation induction with no preponderance of congenital abnormalities [20, 21]. In the light of this data, it remains a mystery to me why letrozole is still contra-indicated for ovulation induction in almost every country in the world.
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