Hum. Reprod. Advance Access originally published online on February 14, 2006
Human Reproduction 2006 21(6):1432-1435; doi:10.1093/humrep/del020
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Efficacy of combined metformin–letrozole in comparison with metformin–clomiphene citrate in clomiphene-resistant infertile women with polycystic ovarian disease
Department of Infertility & IVF, Vali-e-Asr Hospital, Tehran University of Medical Sciences, Tehran, Iran
1 To whom correspondence should be addressed at: Department of Infertility & IVF, Vali-e-Asr Reproductive Health Research Center, Vali-e-Asr Hospital, Tehran University of Medical Sciences, Keshavarz Blvd., PO Box:14194, Tehran, Iran. E-mail: fsohrabvand{at}yahoo.com
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Abstract |
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BACKGROUND: Adding metformin to clomiphene citrate in clomiphene-resistant polycystic ovary syndrome (PCOS) patients increases ovulatory response. However, because of anti-estrogenic effects of clomiphene it may be associated with lower pregnancy rate, offsetting the ovulation rate benefit. Letrozole is an aromatase inhibitor which induces ovulation without anti-estrogenic effects. METHODS: Infertile women with PCOS were randomly divided into metformin–letrozole (29 patients) and metformin–clomiphene groups (30 patients). After an initial 6–8 weeks of metformin, they received either letrozole (2.5 mg) or clomiphene (100 mg) from day 3–7 of their menstrual cycle. Estradiol (E2) levels, number of follicles, pregnancy rates and endometrial thickness were measured on the day of HCG administration. RESULTS: Mean total E2 and E2 per mature follicle were significantly higher in clomiphene group without a difference in mean number of mature follicles >18 mm and ovulation rate. Endometrial thickness was significantly higher in letrozole group. The pregnancy rate in letrozole group (10 patients, 34.50%) as compared with clomiphene group (5 patients, 16.67%) did not show significant difference, whereas full-term pregnancies were higher in letrozole group [10 patients (34.50%) versus 3 patients (10%)]. CONCLUSION: In clomiphene-resistant PCOS patients, the combination of letrozole and metformin leads to higher full-term pregnancies.
Key words: clomiphene citrate/letrozole/metformin/PCOS/pregnancy rate
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Introduction |
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Infertility has been attributed to various factors, amongst which anovulation is the cause of about 40% of all female infertilities. Polycystic ovary disease (PCOD) is the major cause of anovulation (Fleming et al., 2002
), the incidence of which has been reported to reach 6% in infertile females (Heard et al., 2002). Anti-estrogenic drugs, such as clomiphene citrate, are the first-line therapy for ovulation induction in these patients and are capable of producing ovulation in 70–75% of cases. Satisfactory response is seen in 70% of patients when clomiphene citrate is given at a dosage of 50–100 mg/day. If patients fail to respond to a dosage of 150 mg/day, they are considered as clomiphene resistant (Fleming et al., 2002). Animal studies show that a high dose of clomiphene citrate has adverse effects on fertility and the initial stages of fetal growth and development, though not proven in human studies (Mitwally and Casper, 2002a). In addition, studies have shown a significant difference between rate of ovulation and pregnancy and a higher abortion rate in patients undergoing clomiphene citrate therapy. Thus, the use of a simpler oral drug, as a safe alternative to clomiphene citrate, can produce a new horizon in ovulation induction. Letrozole is a newly designed selective aromatase inhibitor, with the above characteristics, which can be used to induce ovulation in infertile women with polycystic ovary syndrome (PCOS) (Mitwally and Casper, 2002a). Previous studies have shown the beneficial effects of metformin in PCOS patients by improving pregnancy rate and the metabolic situation and by decreasing the complications of pregnancy such as gestational diabetes (Batukan and Baysal, 2001; Vandermolen et al., 2001). In addition, the beneficial effects of combined metformin–clomiphene therapy have also been reported in clomiphene-resistant PCOS patients (Vandermolen et al., 2001). Studies have also been performed concerning the beneficial role of letrozole in clomiphene-resistant patients (Mitwally and Casper, 2002b). However, no studies have yet compared the effects of combined metformin–letrozole therapy with those of metformin–clomiphene citrate. The aim of this study was to compare and determine the efficacy of combined metformin–letrozole administration to that of metformin–clomiphene citrate in clomiphene-resistant infertile women with PCOS.
Materials and methods
This study was begun after obtaining consent from the Deputy of Research and the Medical Ethics Committee of Tehran University of Medical Sciences. In this single-blind randomized clinical trial, 120 ovarian cycles were studied in 60 clomiphene-resistant patients with PCOS, who were chosen among 115 PCOS patients attending the infertility clinic of Vali-e-Asr Hospital (Tehran, Iran) during the years 2003–2004. The major criteria for diagnosis of PCOS were oligo- and/or anovulation, clinical or biochemical, signs of hyperandrogenism and polycystic ovaries which is in accord with the revised 2003 Rotterdam criteria of PCOS. Thyroid function, prolactin level, hysterosalpingography and husband’s sperm analysis were checked for normal values. Inclusion criteria were consisting of PCOS patients who had failed to become pregnant after three courses of 150 mg clomiphene citrate (considered as clomiphene resistant), whereas the values of the above mentioned tests were normal. Exclusion criteria included patients with a history of liver and kidney failure, cardiovascular disease, diabetes (based on criteria set by the American Diabetic Association) or patients who consumed metformin or drugs effecting insulin secretion or clomiphene citrate in the previous 2 months. The patients were visited and examined by two gynaecologists. A series of blind envelopes numbered from 1 to 60 had been prepared. Each patient was invited to pull out an envelope and was placed by the clinic secretary in either the metformin–letrozole group (Group A: envelopes number 1–30) or metformin–clomiphene citrate group (Group B: envelopes number 31–60) (Netter, 1970). All patients of both groups received 1500 mg metformin (Glucophage, Merck, West Drayton, UK) a day (500 mg three times a day) for 6–8 weeks. If pregnancy occurred, the patient was excluded from the study. However, in case of failure of pregnancy after the end of this period, the patients in the metformin–clomiphene group (Group B) were given 100 mg clomiphene citrate for 5 days starting from day 3 of their menstrual cycle, and those in the metformin–letrozole group (Group A) received 2.5 mg letrozole (Femara, Novartis, Quebec, Canada) for 5 days from day 3 of their menstrual cycle. The condition of the ovaries was determined by transvaginal sonography every other day from day 12 of the cycle by a single sonographist. A total of 10 000 IU of HCG was administered to those in whom at least one ovarian follicle was 
18 mm in size. Endometrial thickness, number of mature follicles, estradiol (E2) level and the ratio of E2 to number of mature follicle were determined on the day of HCG administration. The patients were advised to have intercourse every other day for one week, starting 24–36 h after receiving HCG. In order to confirm ovulation, transvaginal sonography was performed. In case of delayed menstruation in a patient who had ovulated, 
-HCG was measured, and pregnancy was observed by transvaginal sonography. In case of pregnancy and observation of the fetal heart rate, metformin was discontinued, and in case of therapeutic failure, the patients were advised to continue metformin and to participate in the three courses of therapy. The statisticians, sonographist and gynaecologists involved in this trial were blind to the kind of treatment, whereas patients were not blind to the kind of letrozole and clomiphene tablets, because of their known different shapes. SPSS version 12.0 software was used for statistical analysis and the t-test and chi-square tests were used as appropriate. P-values less than 0.05 were considered as statistically significant.
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Results |
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On the whole, 120 ovarian cycles were studied in 59 patients [53 cycles in 29 patients in the letrozole group (Group A) and 67 cycles in 30 patients in the clomiphene citrate group (Group B)]. After metformin administration, one of the patients in the letrozole group became pregnant and was excluded from the study. A significant statistical difference was not observed between Group A and B with respect to mean demographic variables, including age, BMI, duration of infertility, regular menses after metformin and mean number of mature follicles (diameter >18 mm) (Tables I and II). Mean total E2 and mean E2 level per mature follicle on the day of HCG administration was significantly higher among patients in the clomiphene group as compared with those in the Group A (1664 ± 1349 and 981.35 ± 648.44 pM/l versus 783.38 ± 251.50 and 447.60 ± 133.36 pM/l) (Table I). Mean endometrial thickness was also significantly lower in the Group B (0.55 ± 0.28 cm versus 0.82 ± 0.13 cm) (Table I). The pregnancy rate between the two groups did not show significant difference: 10 patients (34.50%) as compared with 5 patients (16.67%) (P = 0.2). Two miscarriages occurred in the clomiphene citrate group in the first trimester, whereas none were seen in the letrozole group. Gestational age in the time of delivery in both groups was between 37 and 39 weeks, with no preterm labour. Mean birth weight was 2860 g in Group A and 2900 g in Group B, which were both lighter than Iran’s mean birth weight as it is 3100 g for girls and 3300 g for boys. There were no abnormalities in the two groups. Full-term pregnancies were significantly higher in the letrozole group than the clomiphene group: 10 patients (34.50%) as compared with 3 patients (10%) (P = 0.045). The rate of pregnancy in the cycles was 7% (5 of 67 cycles) in the clomiphene group and 19% (10 of 53 cycles) in the letrozole group, which was not statistically significant (P = 0.06) (Tables I and II).
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Discussion |
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The results of this study indicate that clomiphene-resistant women with PCOS experience higher pregnancy rates and fewer abortions when they receive combined metformin–letrozole in comparison with metformin–clomiphene. No significant relationship between age, BMI or duration of infertility in the clomiphene citrate or the letrozole groups was observed. According to the results of this study, mean endometrial thickness on the day of HCG administration was significantly less in subjects taking clomiphene citrate than those who received letrozole (0.55 ± 0.28 versus 0.82 ± 0.13 cm), which is similar to the results achieved by Mitwally et al. (2005)
. However, in the study performed by Al-Fozan et al. (2004), a significant relationship was not found between these two groups. It is probable that the cause of endometrial thickening in patients receiving letrozole is because of improved vascularization as compared with clomiphene citrate (Fisher et al., 2002). Other studies also show that clomiphene citrate can cause inadequate endometrial thickness in 15–50% of patients (Fisher et al., 2002) and have negative effects on the quality or quantity of the cervical and endometrial mucosa (Mitwally and Casper, 2001). These complications may be attributed to the anti-estrogenic effect and the relatively longer half-life of clomiphene citrate, thus decreasing endometrial thickness by its long-term effect in decreasing the number of estrogen receptors (Mitwally and Casper, 2001). A significant statistical relationship did not exist between the frequency of ovulation in either group; neither was there a significant relationship between the mean number of mature ovarian follicles (diameter >18 mm). In the study performed by Al-Fozan et al. (2004), a significant relationship did not exist between the number of follicles measuring more than 14 and 18 mm among patients who were studied for ovulation induction and intrauterine insemination (IUI) in the two groups. However, in the study performed by Mitwally et al. (2005), the mean number of mature follicles was significantly higher in the letrozole group versus clomiphene citrate group. According to the findings of this study, mean total E2 and E2 level per mature follicle were significantly higher in the clomiphene citrate group than the letrozole group on the day of HCG administration (1664.63 versus 981.38 pM/l and 783.38 versus 447.60 pM/l). High supraphysiological levels of estrogen attained during ovarian stimulation with clomiphene citrate may explain some of the adverse effects of clomiphene on the outcome of infertility treatment, although reducing estrogen synthesis by aromatase inhibitor may ameliorate such deleterious effects. In the current study, there was no difference between the clomiphene citrate group and letrozole group with respect to the adverse effects of metformin. Also, a significant relationship did not exist between the letrozole and clomiphene citrate groups in regard to pregnancy rate, although a non-significant increase in pregnancy rate was observed in patients who received letrozole (34.50 versus 16.67%); this almost two-fold increase in pregnancy rate could have been significant if a larger group of patients was included. Because two abortions occurred in the clomiphene citrate group, and none in the letrozole group, full-term pregnancies were significantly higher in the latter as compared with the former group. All newborns in both groups were healthy without any abnormality. The study performed by Mitwally and Casper (2001), which assessed the effect of letrozole administration in 10 women with PCOS, showed that pregnancy occurred in 20% of cases. Another study (Sammour et al., 2001), which also studied the efficacy of letrozole and clomiphene citrate in 49 women with idiopathic infertility, also showed pregnancy rate to be higher in patients receiving letrozole than those receiving clomiphene citrate (16.7 versus 5.6%). As seen in both studies, pregnancy rate is higher in patients receiving letrozole than those receiving clomiphene citrate but is lower when compared with the current study. Thus, we may speculate that the combination of metformin–letrozole is better than letrozole alone, particularly in overweight women who have more intense anovulatory state with higher androgen levels producing a more resistant hypothalamic pituitary ovarian axis. Further studies are required for this hypothesis to be confirmed.
In the study performed by Al-Fozan et al. (2004), per-cycle pregnancy was 11.5% in the letrozole and 8.9% in the clomiphene citrate groups, which was not statistically significant. According to results derived from the current study as well as previous ones, it seems that the risk of miscarriage is higher than expected in the clomiphene group, which may be because of changes in peripheral estrogen level in the cervical and endometrial mucosa. Clear evidence exists concerning the hazardous effects of clomiphene citrate accumulation during pregnancy and the initial stages of development in mouse and rabbit but has not been proven in other studies (Mitwally and Casper, 2002a). Hypotheses have been stated concerning the direct adverse effects of clomiphene citrate on oocytes, but views are variable. One of the probable causes of the low success rate of clomiphene citrate is inadequate uterine blood flow during the early luteal phase and the stage of implantation (Mitwally and Casper, 2002a). Finally, there are difficulties in ovulation in women with oligomenorrhea and PCOS, which may be because of insulin resistance and its related factors (Fleming et al., 2002). It is strongly believed that high serum insulin level is related to PCOS pathogenesis. Metformin is an effective drug in diabetes and can increase tissue sensitivity to insulin as well as decrease plasma insulin level and hepatic glucose production. In PCOS patients, metformin can decrease the level of L.H. and ovarian androgen level as well as correct hyperinsulinemia (Heard et al., 2002). The effect of metformin on the activity of ovaries has been shown in clinical trials (Fleming et al., 2002) and has been shown to correct irregular menses by producing ovulation (Mitwally et al., 2005). In addition, Nestler et al. (1998) showed that metformin increases ovarian response to clomiphene citrate in obese women with PCOS. According to these findings, the current study was novel in that we used metformin simultaneously with clomiphene and letrozole. Aromatase is the enzyme necessary for converting androstenedione to estrone and finally to E2 in peripheral tissues (Bast et al., 2000). Aromatase inhibitors can prevent peripheral estrogen production in patients in whom peripheral estrogen secretion is increased (Bast et al., 2000). These drugs have high potency, and estrogen level can be controlled by 97–99% at a dosage of 1–5 mg/day. With the same mechanism, selective aromatase inhibitors such as letrozole are used to induce ovulation, especially in infertile women with PCOS (Mitwally and Casper, 2002a). The use of aromatase inhibitors in the initial follicular phase has a negative feedback effect on the hypothalamus and pituitary glands thereby causing GnRH, LH and FSH secretion with resultant ovarian follicular growth stimulation. They may also have direct action on the ovaries and increase follicular sensitivity to FSH. Women with PCOS may also have relatively low levels of ovarian aromatase. High androgen levels result in the formation of multiple small ovarian follicles. In addition, androgens increase the number of FSH receptors in the ovaries, which results in increasing FSH sensitivity. High exogenous FSH or low estrogen production because of aromatase inhibitors will lead to growth of one or more ovarian follicles (Bast et al., 2000). Regarding previous studies and the results of the present study, we can assume that letrozole is a suitable alternative to clomiphene citrate, especially in cases not responding to this drug, or it can be a first-line drug in ovarian stimulation and treatment of anovulation. It seems that letrozole and its drug group are safe, reliable and cheap drugs with therapeutic value (Mitwally et al., 2005). On the other hand, regarding the positive effect of letrozole in producing FSH sensitivity and satisfactory E2 elevation (at normal physiological levels), it can have better therapeutic effects in infertile females. In addition, because serum clearance of letrozole is faster than clomiphene citrate (50 h versus 4 weeks) and does not lead to a decrease in the estrogen receptors, it is probable that letrozole does not produce deleterious effects similar to that found with clomiphene citrate on the endometrium, although it can lead to pregnancy at similar or even higher rates (Fisher et al., 2002). However, the optimum dose of letrozole remains unknown and further studies are necessary in this field (Mitwally and Casper, 2005). The results of the current study comparing metformin–clomiphene citrate with metformin–letrozole in clomiphene-resistant PCOS patients show that combined metformin–letrozole therapy leads to higher pregnancy rates with the maintenance of pregnancy until full term. In addition, regarding that in the population under study, all the PCOS patients were overweight (BMI, 29–30 kg/cm2) by complete chance, it seems that based on the findings of this study and especially in these patients, combined metformin–letrozole is probably the best choice of therapy. Although this study was the first to be performed on clomiphene-resistant overweight PCOS patients receiving metformin–letrozole, it is hoped that further trials will be conducted on a larger scale.
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Submitted on July 27, 2005; resubmitted on December 3, 2005; accepted on January 13, 2006.
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