inducerea ovulatiei

1. A new era in ovulation induction
Hananel Holzer, M.D.,a
Robert Casper, M.D.,b
and Togas Tulandi, M.D., M.H.C.M.a
a
Department of Obstetrics and Gynecology, McGill University, Montreal, Quebec; and b
University of Toronto, Toronto,
Ontario, Canada
Objective: To evaluate the efficacy of aromatase inhibitors in ovulation induction, superovulation, and IVF.
Design: A literature search was conducted with the key words “aromatase inhibitor,” “letrozole,” “anastrazole,”
“ovulation induction,” “ovulation,” and “superovulation” in MEDLINE, EMBASE, and the Cochrane Database
of systematic reviews.
Result(s): Ovulation induction with letrozole is associated with an ovulation rate of 70%–84% and a pregnancy
rate of 20%–27% per cycle. In one study, ovulation and pregnancy rates with letrozole seemed to be higher than
those of anastrazole. In superovulation, letrozole is associated with few developing follicles and thick endome-
trium. The use of letrozole for superovulation is associated with a pregnancy rate higher than with the use of
clomiphene citrate (CC) (16.7% vs. 5.6%). The addition of letrozole to FSH treatment leads to a decreased FSH
requirement. The pregnancy rate for treatment with letrozole and FSH was similar to that for FSH alone.
Conclusion(s): Aromatase inhibitors are as effective as or superior to CC in ovulation induction and in
superovulation. Unlike CC, they do not carry an antiestrogenic effect on the endometrium. Given the advantages
of aromatase inhibitors, they can be used to replace CC as ovulation-inducing drugs. Their role in IVF remains
to be determined. (Fertil Steril௡ 2006;85:277–84. ©2006 by American Society for Reproductive Medicine.)
Key Words: Infertility, anovulation, ovulation induction, superovulation, controlled ovarian hyperstimulation,
aromatase inhibitors, letrozole, anastrazole
The most widely used treatments for infertility are ovulation
induction and superovulation. In anovulatory women, the
purpose of treatment is to induce the development of at least
one follicle, whereas in other causes of infertility ovarian
stimulation is used to increase the number of follicles—an
approach known as superovulation or controlled ovarian
hyperstimulation (1, 2). Ovarian stimulation can be achieved
by administration of exogenous gonadotropins or by aug-
menting endogenous FSH with clomiphene citrate (CC) treat-
ment. These treatments are associated with a risk of ovarian
hyperstimulation syndrome and multiple gestations (3).
For more than 4 decades, CC has been the first line of
treatment for ovulatory disorders (4–7). Clomiphene citrate
is easy to use and results in ovulation in most patients
(60%–90%), but the pregnancy rates are disappointing
(10%–40%). This has been attributed to its peripheral an-
tiestrogenic effects, mainly on the endometrium and the
cervical mucus (8–13). The multiple pregnancy rate associ-
ated with CC is between 10% and 20% (14).
Gonadotropins are more effective than CC but are expen-
sive and associated with higher risk for ovarian hyperstimu-
lation syndrome and multiple gestations. Unlike CC, gonad-
otropins do not exert a peripheral antiestrogenic effect.
In view of the disappointing results of CC treatment and
the cost and possible complications of gonadotropins, fertil-
ity care providers have been looking for a new, easy to use,
less expensive, and more effective drug. It seems that a new
group of ovulation-inducing drugs has finally arrived.
AROMATASE INHIBITORS
Aromatase is a member of the cytochrome P450 hemoprotein-
containing enzyme complex super family. It catalyzes the
rate-limiting final step in estrogen (E) production, the hy-
droxylation of androstenedione to estrone and of T to E2
(Fig. 1). Its activity can be demonstrated in the ovaries,
adipose tissue, placenta, brain, muscle, fibroblasts, osteo-
blasts, liver, and breast (15–28).
For many years, aromatase inhibitors have been used as an
adjunct treatment for breast cancer. They could be steroidal
or nonsteroidal inhibitors. Steroidal inhibitors are derivatives
of androstenedione that act as false substrates, binding irre-
versibly to the androgen-binding site (29–31). The first-
generation aromatase inhibitor is aminoglutethimide, a nonspe-
Received February 15, 2005; revised and accepted May 12, 2005.
Reprint requests: Togas Tulandi, M.D., M.H.C.M., McGill University, De-
partment of Obstetrics and Gynecology, 687 Pine Avenue West, Mon-
treal, Quebec H3A 1A1, Canada (FAX: 514-843-1448; E-mail: togas.
tulandi@mcgill.ca).
MODERN TRENDS
Edward E. Wallach, M.D.
Associate Editor
2770015-0282/06/$32.00 Fertility and Sterilityா Vol. 85, No. 2, February 2006
doi:10.1016/j.fertnstert.2005.05.078 Copyright ©2006 American Society for Reproductive Medicine, Published by Elsevier Inc.

2. cific steroidal inhibitor that causes medical adrenalectomy.
The side effects of this substance are lethargy, nausea, skin
rash, and fever (32). Formestane, a second-generation aro-
matase inhibitor, is more selective and has fewer side effects;
however, it must be administered intramuscularly, and many
patients experience local reactions (33).
The third-generation aromatase inhibitors include two
nonsteroidal inhibitors, anastrozole and letrozole, and a ste-
roidal agent, exemestane. Anastrozole and letrozole are se-
lective aromatase inhibitors. They are reversible and highly
potent. Their intrinsic potency is considerably greater than
that of aminoglutethimide, and at doses of 1–5 mg decreases
E levels by 97%–Ͼ99%.
Letrozole and anastrazole are completely absorbed after
oral administration, with a mean half-life of approximately
45 hours (range, 30–60 hours). Clearance from the systemic
circulation is mostly by the liver. Their adverse effects are
gastrointestinal disturbances, asthenia, hot flushes, headache,
and back pain (34–41).
AROMATASE INHIBITORS AND OVULATION INDUCTION
Estrogen exerts a negative feedback on the hypothalamic–
pituitary axis and decreases the release of FSH from the
pituitary gland (42–46). Mitwally and Casper (47) first pos-
tulated that blocking E production by inhibiting aromatiza-
tion would release the hypothalamic–pituitary axis from
estrogenic negative feedback. As a result, FSH secretion
increases, stimulating the development of ovarian follicles
(Fig. 1). Because aromatase inhibitors block high levels of E
from androgen conversion, the effects in women with poly-
cystic ovary syndrome (PCOS) are more prominent (47). In
addition, androgens that normally converted to estrogens
accumulate in the ovary, and these androgens increase fol-
licular sensitivity to FSH (47). Unlike CC, aromatase inhib-
itor does not deplete E receptors or produce a negative effect
on the endometrium. Clomiphene citrate, on the other hand,
has a longer half-life (2 weeks) that results in prolonged
central E receptor depletion (48).
The use of aromatase inhibitors for ovarian stimulation
and controlled ovarian hyperstimulation (superovulation) is
indeed promising.
AROMATASE INHIBITORS AS A SINGLE DRUG
FOR OVULATION INDUCTION
Ovulation Induction
In the first reported series (49), 10 patients with PCOS
resistant to CC or with endometrial thickness of Ͻ5 mm
were treated with letrozole. Seven patients ovulated; the
mean number of follicles was two. One patient conceived,
and another had a biochemical pregnancy. The endometrial
thickness just before ovulation was 7–9 mm.
In a small study (50), 12 PCOS patients with inadequate
response to CC were treated with letrozole. Of 12 patients, 9
(75%) ovulated, and 3 conceived; the mean endometrial
thickness was 8.1 mm (6.2 mm with CC). The same inves-
tigators (51) subsequently evaluated the effects of a single
dose of letrozole (20 mg on day 3 of the cycle) in 7 patients
(nine cycles). The number of follicles, serum E2 level, and
endometrial thickness in women treated with a single dose
were similar to those in women treated with a 5-day letrozole
regime.
FIGURE 1
(A) The pituitary–ovarian axis in the follicular
phase. Estradiol is produced by the ovarian
granulosa cells and exerts a negative feedback
effect on FSH release from the pituitary gland.
(B) Effects of aromatase inhibitor. Aromatization of
androgens to estrogens is inhibited, the
hypothalamic–pituitary axis is released from the
negative feedback, and FSH secretion is
increased. The androgens accumulated in the
ovary increase the ovarian sensitivity to FSH. The
overall effect is stimulation of development of
ovarian follicles. IGF I ϭ insulin-like growth factor I.
Testosterone Estradiol
Androstendione Estrone
Aromatase
Pituitary
gland
Ovary
EstradiolFSH
A
Testosterone Estradiol
Androstendione Estrone
Aromatase
Pituitary
gland
Ovary
EstradiolFSH
Aromatase
Inhibitor
Ovary:
Androgens ?
FSH receptors ?
IGF I ?
Sensitivity to FSH ???
B
Holzer. A new era in ovulation induction. Fertil Steril 2006.
278 Holzer et al. A new era in ovulation induction Vol. 85, No. 2, February 2006

3. Al-Omari et al. (52) treated women with CC-resistant
PCOS with 2.5 mg of letrozole (22 patients) or 1 mg of
anastrazole daily from day 3 to day 7 of the cycle (18
patients). Endometrial thickness was greater in the letrozole
group (8.2 mm) than in the anastrazole group (6.5 mm). The
ovulation rate in the letrozole group was 84.4% and in the
anastrazole group was 60%. The pregnancy rates were also
statistically higher in the letrozole than in the anastrazole
group (18.8% vs. 9.7% per cycle, respectively). The mean
number of leading follicles was 1.7 in both groups. There
were no cases of multiple gestations. Although the ideal dose
of anastrazole is still unknown, it seems that the dose of 1 mg
daily is much too low.
Studies evaluating the use of aromatase inhibitors in-
volved mostly women who had failed CC treatment, because
of either CC resistance or thin endometrium. In all studies,
patients responded well to aromatase inhibitors. Their use is
associated with ovulation rates of 70%–88%, endometrial
thickness of 7–9 mm, and pregnancy rates of 20%–27% per
cycle (Table 1) (48–50). Clearly, treatment with aromatase
inhibitors in this group of patients is associated with a good
ovulation rate, thick endometrium, and a considerable num-
ber of pregnancies. With this treatment, multiple developing
follicles appear on cycle day 7, but at mid-cycle only a single
dominant follicle is found.
Superovulation (Controlled Ovarian Hyperstimulation)
In a randomized, double-blind study, 19 normal healthy
ovulatory volunteers not desiring pregnancy were random-
ized to receive either letrozole 2.5 mg daily (9 patients) or
CC 50 mg daily (10 patients) on days 5–9. The results were
compared with untreated cycles of the same patients. A
single dominant follicle was found in the untreated cycles:
2.2 in the CC cycles and 1.7 in the letrozole cycles. The
endometrial thickness was comparable between the un-
treated and treated cycles. Compared with natural cycles, CC
resulted in higher E2 levels, whereas the levels in the letro-
zole group were lower (53).
In another randomized trial of 49 patients with unex-
plained infertility (54), 24 patients were treated with CC and
25 with letrozole. Compared with CC, letrozole treatment
was associated with a lower E2 level, fewer follicles (1 vs.
2), thicker endometrium (8.6 mm vs. 6.9 mm), and more
stromal blood flow. The investigators concluded that letro-
zole provides a better uterine environment. Moreover, there
is a trend for a higher pregnancy rate in the letrozole group
than in the CC group (16.7% vs. 5.6%).
In a small trial of eight patients (55), letrozole (2.5 mg
daily for 5 days) produced significantly lower E2 concentra-
tions and fewer follicles than CC (100 mg daily). Ovulatory
women with inadequate response to CC (thin endometrium)
seem to do better when treated with aromatase inhibitors.
These studies suggest that the use of letrozole is associ-
ated with thick endometrium and improved stromal blood
TABLE1
Aromataseinhibitorsforovulationinduction.
Authors(reference)TreatmentNo.ofpatientsanddiagnosis
Meanendometrial
thickness(mm)
Ovulationrate
(%)Conception
MitwallyandCasper(49)Letrozole10,PCOSresistanttoCCor
endometriumϽ5mm
7–9702(1biochemical
pregnancy)
MitwallyandCasper(50)Letrozole12,PCOS,inadequate
responsetoCC
8.1753of12
MitwallyandCasper(51)Letrozole,single
dose
3PCOS,4unexplained
(9cycles)
988.91pregnancy
Al-Omarietal.(52)Letrozolevs.
anastrazole
40PCOSresistanttoCC8.2(letrozole)84.4letrozole,
60anastrazole
27%(letrozole)
6.5(anastrazole)16.6%(anastrazole)
Holzer.Anewerainovulationinduction.FertilSteril2006.
279Fertility and Sterilityா

4. flow, conditions that are favorable for implantation. Com-
pared with those of CC, pregnancy rates with letrozole
treatment seem to be higher (Table 2).
AROMATASE INHIBITORS AS AN ADJUNCT
TREATMENT WITH GONADOTROPINS
Withholding E2-negative feedback before gonadotropin ther-
apy might increase the number of preovulatory follicles and
reduce the gonadotropin requirement. Clomiphene citrate
has been used for this purpose but with discouraging results,
perhaps because of its peripheral antiestrogenic effect on the
endometrium and cervix (56–60). Aromatase inhibitors, on
the other hand, might serve the same purpose without the
unwanted antiestrogenic effect (61–63).
In a prospective, nonrandomized study involving women
with unexplained infertility or mild male factor (61), 36
women were treated with letrozole and FSH, 18 women with
CC and FSH, and 56 women with FSH only. Follicle-
stimulating hormone was administered on days 3–7, until the
day of hCG administration. Compared with the FSH-only
group, the required amount of FSH was significantly lower
in the letrozole and FSH and in the CC and FSH groups.
There was no difference in the number of follicles of Ͼ18
mm among the three groups. The pregnancy rate per cycle in
the CC and FSH group (10.5%) was significantly lower than
in the letrozole and FSH group (19.1%) and the FSH-only
group (18.7%). Healy et al. (62) evaluated women who were
superovulated with gonadotropin or a combined regimen of
gonadotropin and letrozole (5 mg daily on days 3–7). Go-
nadotropin was started on day 3 of the cycle as well (“over-
lapping approach”). There were 145 cycles in the gonadotropin-
only group and 60 cycles in the letrozole and gonadotropin
group. Patients treated with a combined regimen required
less gonadotropin and developed more follicles Ͼ14 mm but
had thinner endometrium. However, there was no significant
difference in the pregnancy rates between the two groups
(gonadotropin 20.9%, gonadotropin and letrozole 21.6%).
In another nonrandomized study (63), women with PCOS
and those with other causes of infertility (unexplained infer-
tility, male factor infertility, and endometriosis) were treated
with either FSH alone or a combination of letrozole and
FSH. In women with PCOS (n ϭ 26), the pregnancy rate in
the letrozole and FSH group was 26.5% per cycle and in the
FSH-only group (n ϭ 46) was 18.5% per cycle. In women
without PCOS, there was no significant difference in preg-
nancy rates between the letrozole and FSH (n ϭ 63) and
FSH-alone (n ϭ 308) groups. The overall pregnancy rate
was 11% per cycle. However, the addition of letrozole to
FSH treatment led to a lower FSH requirement without a
significant difference in number of follicles Ͼ16 mm.
These studies suggest that the addition of letrozole to
gonadotropins decreases gonadotropin requirements, in-
creases the number of preovulatory follicles, and decreases
endometrial thickness without a negative effect on preg-
nancy rates (Table 3).
TABLE2
Useofletrozoleforsuperovulation.
Authors(reference)Treatment
No.ofpatients
anddiagnosis
Meanno.of
dominantfollicles
Meanendometrial
thickness(mm)Pregnancy
Fisheretal.(53)Letrozolevs.CC19,normal
volunteers
2.2CCvs.1.7LNodifferencefrom
naturalcycles
Volunteersnotdesiringpregnancy
Sammouretal.(54)Letrozolevs.CC49,unexplained
infertility
2CCvs.1L8.6mmLvs.6.9
mmCC
Pregnancyratepercycle:16.7%
Lvs.5.6%CC
Fatemi(55)Letrozolevs.CC15,unexplained
infertility
2CCvs.1L8.0mmLvs8.3
mmCC
Totalpregnancies:37.5%CCvs.
28.6%L
Note:Lϭletrozole.
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280 Holzer et al. A new era in ovulation induction Vol. 85, No. 2, February 2006

5. COMBINATION TREATMENT FOR POOR RESPONDERS
Aromatase inhibitors decrease negative feedback on FSH
secretion and increase ovarian sensitivity to FSH (64–67).
The use of aromatase inhibitors in “poor FSH responders” is
therefore appropriate. In one study (68), investigators eval-
uated 12 patients with unexplained infertility who had re-
sponded poorly to FSH alone. Patients were treated with
letrozole 2.5 mg daily on days 3–7 and FSH (50–225 IU/
day) from day 7 of the cycle (“sequential approach”). When
two leading follicles were 20 mm, hCG was administered,
followed by IUI. The pregnancy rate with combined treat-
ment was 21% in three cycles. Compared with FSH alone,
combined letrozole and FSH is associated with a signifi-
cantly lower FSH requirement and more mature follicles.
This preliminary report suggests a potential benefit of aro-
matase inhibitors in improving ovarian response in poor FSH
responders (68).
To date, studies evaluating the effect of adjuvant treatment
with letrozole to gonadotropins are heterogeneous. However,
compared with gonadotropin alone, the addition of letrozole
is associated with less gonadotropin requirement and an
equal number (or more) of leading follicles (61–63). In one
study (62), endometrium was significantly thinner in the
group treated with combined letrozole and FSH, but the
pregnancy rate did not differ. In this study, the investigators
used an “overlapping” approach (62).
AROMATASE INHIBITORS IN ASSISTED
REPRODUCTIVE TECHNOLOGIES
Despite encouraging experience with aromatase inhibitors as
an adjunct treatment in ovulation induction and superovula-
tion, there is a paucity of information regarding their use in
IVF programs. Two studies evaluated the effects of aro-
matase inhibitors in poor responders.
One study (69) was a randomized trial of 38 women with
a history of poor ovarian response to gonadotropins. Thirteen
women received 2.5 mg letrozole daily from days 3 to 7 and
75 IU of recombinant FSH daily from days 3 to 8, whereas
25 others underwent a long GnRH agonist (GnRH-a) proto-
col and were stimulated with FSH. The combined letrozole
and FSH group required a significantly lower total dose of
FSH as compared with the GnRH-a and FSH group. Except
for the E2 levels in late follicular phase, which were signif-
icantly higher in the GnRH-a and FSH group, the treatment
outcomes in all respects, including the pregnancy rates, were
statistically comparable. This study confirms that the addition of
letrozole to FSH is an effective way of lowering the FSH
requirement and the cost of IVF in poor FSH responders (69).
In another study (70), 27 poor responders who had failed
to respond to a microdose flare-up protocol were treated with
a GnRH antagonist combined with letrozole and FSH. The
control group included 258 patients receiving the flare-up
protocol with no history of poor response to FSH. Although
a greater number of oocytes were observed in the control
TABLE3
LetrozolecombinedwithFSHtreatment.
Authors(reference)No.ofpatients,diagnosis,andtreatmentOutcomes
MitwallyandCasper(61)Letrozole/FSH(36pts),CC/FSH(18pts),
FSHonly(56pts).Unexplainedinfertility,
mildmalefactor
CombinedgroupsreceivedlessFSH;nodifferenceinnumberof
follicles,endometriumthinnerinCC/FSH;pregnancyrates:
19.1%(L/FSH),10.5%(CC/FSH),18.7%(FSHonly)
Healeyetal.(62)FSHalone(165cycles)vs.letrozole/FSH
(60cycles)forsuperovulation
Additionofletrozoleledtodecreasedgonadotropin
requirement,morefollicles,andthinnerendometrium;similar
pregnancyrates
MitwallyandCasper(63)PCOS:letrozole/FSH(26pts),FSHalone
(46pts).Ovulatoryinfertility:letrozole/
FSH(63pts),FSHalone(308pts)
Additionofletrozoleisassociatedwithlessgonadotropin
requirement,similarnumberoffollicles,andhigherpregnancy
rateinpatientswithPCOS
Note:ptsϭpatients.
Holzer.Anewerainovulationinduction.FertilSteril2006.
281Fertility and Sterilityா

6. group, the implantation and on-going pregnancy rates were
similar (70).
The ability of aromatase inhibitors to decrease the dose of
FSH for ovarian stimulation has led to their use in women
with breast cancer who wish to preserve their fertility before
chemotherapy. Oktay et al. (71) compared the use of tamox-
ifen alone or letrozole combined with low-dose FSH in
women with breast cancer who desired embryo cryopreser-
vation. The combined protocol was associated with lower
peak E2 levels and a higher number of embryos.
The role of aromatase inhibitors in assisted reproductive
technology remains unclear. In theory, the low E2 level in
combined letrozole and FSH stimulation could result in a
reduced incidence of ovarian hyperstimulation syndrome
and premature luteinization, a favorable endometrium, and a
high implantation rate. In addition, it is associated with a
reduced gonadotropin requirement. Poor FSH responders
would benefit from this combined treatment (Table 4).
TREATMENT PROTOCOL AND PREGNANCY
Preliminary studies of letrozole used a daily dose of 2.5–7.5
mg for 5 days or a single dose of 20 mg orally. The dose of
2.5 mg was adopted from a daily dose of letrozole as an
adjunct hormonal treatment for breast cancer (72). The 5-day
treatment was taken from the 5-day CC regimen. Adminis-
tration on days 3–7 of the cycle allows sufficient time for
letrozole to be cleared from the body, leaving only negligible
levels close to the time of ovulation (72).
It seems that, compared with a 2.5-mg daily dose, a 5-mg
dose yields more follicles (73). We recently compared these
two doses and found that 5 mg of letrozole daily was
associated with a higher number of follicles and a higher
pregnancy rate (74). In another randomized trial of 238
cycles of superovulation in women with idiopathic infertility
(75), we compared 7.5 mg of letrozole and 100 mg of CC
daily for 5 days. The pregnancy rates in both groups were
similar, but the miscarriage rate in the CC-treated cycles was
higher.
Information about a single-dose treatment is still limited.
Supposedly a single oral dose of letrozole provides a benefit
of maximal E suppression early in the cycle and of early
clearance. In a small study (51), nine patients were treated
with a single dose of 20 mg letrozole on day 3 of the cycle.
Eight patients ovulated, and one conceived. These results seem
to be equivalent to those achieved with a 5-day regimen.
Recently, Mitwally et al. (76) reported pregnancy rates
and pregnancy outcomes for the first 2 years of treatment
with letrozole, either as a single agent or when combined with
FSH. The results were compared with those of treatment-
induced pregnancy with CC, FSH, or CC and FSH, and
spontaneous pregnancies. The pregnancy rates were compa-
rable among the FSH, CC and FSH, letrozole and FSH, and
the letrozole-only groups, but lower in the CC-only group.
The miscarriage rates were similar among the groups,
whereas the rates of multiple pregnancies were higher when
CC was used (Fig. 1). Because of the heterogeneity of patient
groups and the variable protocols, firm conclusions cannot
be made.
CONCLUSIONS
Aromatase inhibitors are a new group of drugs to join the
arsenal of fertility treatments. They are orally administered,
easy to use, and relatively inexpensive, with minor side
effects. Anastrazole and letrozole are third-generation aro-
matase inhibitors that have been used for ovulatory disorders
and for superovulation. To date, letrozole has been studied
much more extensively than anastrazole. The data on letro-
zole suggest that it can be used to replace CC as the first-line
treatment for women with ovulatory disorders. Compared
with CC, its use is associated with thicker endometrium. For
superovulation, there is a trend for higher pregnancy rates
with letrozole than with CC. When letrozole is added to
gonadotropin regimens, it leads to less gonadotropin require-
ment and a pregnancy rate that is comparable to that with
gonadotropin-only treatment. The role of aromatase inhibi-
tors in assisted reproductive technologies remains to be seen.
The ideal dose of letrozole is unknown; however, it seems
TABLE 4
Letrozole use in assisted reproductive technologies.
Authors (reference) Treatment No. of patients Outcome
Goswami et al. (69) 2.5 mg letrozole day 3–7 ϩ
75 IU rFSH day 3–8 vs.
long GnRH agonist
protocol ϩ FSH
38 poor responders Addition of letrozole led to
less FSH requirement;
comparable outcomes
Schoolcraft et al. (70) Letrozole/FSH vs. flare-up
protocol
27 poor responders
vs. 258 controls
More oocytes in controls;
similar pregnancy and
implantation rates
Note: rFSH ϭ recombinant FSH.
Holzer. A new era in ovulation induction. Fertil Steril 2006.
282 Holzer et al. A new era in ovulation induction Vol. 85, No. 2, February 2006

7. that the dose of 5 mg daily for 5 days is the most effective.
Aromatase inhibitors are promising new drugs for the induction
of ovulation and superovulation. After 4 decades of CC treat-
ment, a new era of ovulation induction has finally arrived.
REFERENCES
1. Lunenfeld B, Van Steirteghem A, Bertarelli Foundation. Infertility in
the third millennium: implications for the individual, family and soci-
ety: condensed meeting report from the Bertarelli Foundation’s second
global conference Hum Reprod Update 2004;10:317–26.
2. Fisch P, Casper RF, Brown SE, Wrixon W, Collins JA, Reid RL, et al.
Unexplained infertility: evaluation of treatment with clomiphene citrate
and human chorionic gonadotropin. Fertil Steril 1989;51:828–33.
3. Fluker MR, Urman B, Mackinnon M, Barrow SR, Pride SM, Yuen BH.
Exogenous gonadotropin therapy in World Health Organization groups
I and II ovulatory disorders. Obstet Gynecol 1994;83:189–96.
4. Franks S, Adams J, Mason H, Polson D. Ovulatory disorders in women
with polycystic ovary syndrome. Clin Obstet Gynaecol 1985;12:
605–32.
5. Hull MGR. The causes of infertility and relative effectiveness of
treatment. In: Tempelton AA, Drife JO, eds. Infertility. London:
Springer, 1992:33–62.
6. Kistner RW. Induction of ovulation with clomiphene citrate (clomid).
Obstet Gynecol Surv 1965;20:873–900.
7. Adashi EY. Clomiphene citrate: mechanism(s) and site(s) of action.
Fertil Steril 1984;42:331–44.
8. Gonen Y, Casper RF. Sonographic determination of a possible adverse
effect of clomiphene citrate on endometrial growth. Hum Reprod 1990;
5:670–4.
9. Eden JA, Place J, Carter GD, Jones J, Alaghband-Zadeh J, Pawson ME.
The effect of clomiphene citrate on follicular phase increase in endo-
metrial thickness and uterine volume. Obstet Gynecol 1989;73:187–90.
10. Nakamura Y, Ono M, Yoshida Y, Sugino N, Ueda K, Kato H. Effects
of clomiphene citrate on the endometrial thickness and echogenic
pattern of the endometrium. Fertil Steril 1997;67:256–60.
11. Yagel S, Ben-Chetrit A, Anteby E, Zacut D, Hochner-Celnikier D, Ron
M. The effect of ethinyl estradiol on endometrial thickness and uterine
volume during ovulation induction by clomiphene citrate. Fertil Steril
1992;57:33–6.
12. Homburg R. Management of infertility and prevention of ovarian
hyperstimulation in women with polycystic ovary syndrome. Clin Ob-
stet Gynecol 2004;18:773–8.
13. Randall JM, Templeton A. Cervical mucus score and in vitro sperm
mucus interaction in spontaneous and clomiphene citrate cycles. Fertil
Steril 1991;56:465–8.
14. Mitwally MF, Biljan MM, Casper RF. Pregnancy outcome after the use
of an aromatase inhibitor for ovarian stimulation. Am J Obstet Gynecol
2005;192:381–6.
15. Cole PA, Robinson CH. Mechanism and inhibition of cytochrome
P-450 aromatase. J Med Chem 1990;33:2933–44.
16. Akhtar M, Njar VCO, Wright JN. Mechanistic studies on aromatase and
related C-C bond cleaving P-450 enzymes. J Steroid Biochem Mol Biol
1993;44:375–87.
17. Santen RJ, Manni A, Harvey H, Redmond C. Endocrine treatment of
breast cancer in women. Endocr Rev 1990;11:221–65.
18. Lonning PE. Aromatase inhibition for breast cancer treatment. Acta
Oncol 1996;35(Suppl 5):38–43.
19. Steinkampf MP, Mendelson CR, Simpson ER. Regulation by follicle-
stimulating hormone of the synthesis of aromatase cytochrome P-450 in
human granulosa cells. Mol Endocrinol 1987;1:465–71.
20. Tsai-Morris CH, Aquilana DR, Dufau ML. Cellular localization of rat
testicular aromatase activity during development. Endocrinology 1985;
116:38–46.
21. Bulun SE, Simpson ER. Competitive reverse transcription polymerase
chain reaction analysis indicates that levels of aromatase cytochrome
P450 transcripts in adipose tissue of buttocks, thighs, and abdomen of
women increase with advancing age. J Clin Endocrinol Metab
1994;78:428–32.
22. Means GD, Kilgore MW, Mahendroo MS, Mendelson CR, Simpson
ER. Tissue-specific promoters regulate aromatase cytochrome P450
gene expression in human ovary and fetal tissues. Mol Endocrinol
1991;5:2005–13.
23. Roselli CE, Horton LE, Resko JA. Distribution and regulation of
aromatase activity in the rat hypothalamus and limbic system. Endo-
crinology 1985;117:2471–4.
24. Matsumine H, Hirato K, Yanahara T, Tamada T, Yoshida M. Aroma-
tization by skeletal muscle. J Clin Endocrinol Metab 1986;63:717–20.
25. Berkovitz GD, Brown TR, Fujimoto M. Aromatase activity in human
skin fibroblasts grown in cell culture. Steroids 1987;50:281–95.
26. Shozu M, Simpson ER. Aromatase expression of human osteoblast-like
cells. Mol Cell Endocrinol 1998;139:117–29.
27. Shozu M, Sebastian S, Takayama K, Hsu WT, Schultz RA, Neely K, et
al. Estrogen excess associated with novel gain-of-function mutations
affecting the aromatase gene. N Engl J Med 2003;348:1855–65.
28. Nelson LR, Bulun SE. Estrogen production and action. J Am Acad
Dermatol 2001;45:S116–24.
29. Bhatnagar AS, Hausler A, Schieweck K, Lang M, Bowman R. Highly
selective inhibition of estrogen biosynthesis by CGS 20267, a new
non-steroidal aromatase inhibitor. J Steroid Biochem Mol Biol 1990;
37:1021–7.
30. Coombes RC, Goss P, Dowsett M, Gazet JC, Brodie A. 4Hydroxyan-
drostenedione treatment of postmenopausal patients with advanced
breast cancer. Lancet 1984;2:1237–9.
31. Broodie AM, Njar VJ. Aromatase inhibitors and breast cancer. Semin
Oncol 1996;23:10–20.
32. Lipton A, Santen RJ. Medical adrenalectomy using aminoglutethimide
and dexamethasone in advanced breast cancer. Cancer 1974;33:503–12.
33. Goss PE, Powles TJ, Dowsett M, Hutchison G, Brodie AM, Gazet JC,
et al. Treatment of advanced postmenopausal breast cancer with the
aromatase inhibitor, 4-hydroxyandrostenedione: phase II report. Cancer
Res 1986;46:4823–6.
34. Winer EP, Hudis C, Burstein HJ, Chlebowski RT, Ingle JN, Edge SB,
et al. American Society of Clinical Oncology technology assessment on
the use of aromatase inhibitors as adjuvant therapy for women with
hormone receptor-positive breast cancer: status report 2002. J Clin
Oncol 2002;20:3317–27.
35. Buzdar A, Jonat W, Howell A, Jones SE, Blomqvist C, Vogel CL, et al.
Anastrozole, a potent and selective aromatase inhibitor, versus meges-
trol acetate in postmenopausal women with advanced breast cancer:
results of overview analysis of two phase III trials. J Clin Oncol
1996;14:2000–11.
36. Marty M, Gershanovich M, Campos B, Romieu G, Lurie H, Bonaven-
tura T, et al. Letrozole, a new potent, selective aromatase inhibitor (AI)
superior to aminoglutethimide (AG) in postmenopausal women with
advanced breast cancer (ABC) previously treated with anti-estrogens
[abstract]. Proc Am Soc Clin Oncol 1997;16:156.
37. Bergh J, Bonneterre J, Illiger HJ, Murray R, Nortier J, Paridaens R, et
al. Vorozole (Rivizor) versus aminoglutethimide (AG) in the treatment
of postmenopausal breast cancer relapsing after tamoxifen. Proc Am
Soc Clin Oncol 1997;16:156.
38. Dowsett M, Lonning PE. Anastrozole—a new generation in aromatase
inhibition: clinical pharmacology. Oncology 1997;54(Suppl 2):11–4.
39. Dranitsaris G, Leung P, Mather J, Oza A. Cost-utility analysis of
second-line hormonal therapy in advanced breast cancer: a comparison
of two aromatase inhibitors to megestrol acetate. Anticancer Drugs
2000;11:591–601.
40. Sioufi A, Gauducheau N, Pineau V, Marfil F, Jaouen A, Cardot JM, et
al. Absolute bioavailability of letrozole in healthy post-menopausal
women. Biopharm Drug Dispos 1997;18:779–89.
41. Sioufi A, Sandrenan N, Godbillon J, Trunet P, Czendlik C, Howald H,
et al. Comparative bioavailability of letrozole under fed and fasting
conditions in 12 healthy subjects after a 25 mg single oral administra-
tion. Biopharm Drug Dispos 1997;18:489–97.
283Fertility and Sterilityா

8. 42. McNeilly AS. The control of FSH secretion. Acta Endocrinol Suppl
(Copenh) 1988;288:31–40.
43. Kamat A, Hinshelwood MM, Murry BA, Mendelson CR. Mechanisms
in tissue-specific regulation of estrogen biosynthesis in humans. Trends
Endocrinol Metab 2002;13:122–8.
44. Naftolin F, MacLusky NJ. Aromatization hypothesis revisited. In: Serio
M, ed. Differentiation: basic and clinical aspects. New York: Raven
Press, 1984:79–91.
45. Naftolin F, MacLusky NJ, Leranth CZ, Sakamoto HS, Garcia-Segura
LM. The cellular effects of estrogens on neuroendocrine tissues. J
Steroid Biochem 1988;30:195–207.
46. Naftolin F. Brain aromatization of androgens. J Reprod Med 1994;39:
257–61.
47. Mitwally M. Casper R. Aromatase inhibitors in ovulation induction.
Semin Reprod Med 2004;22:61–78.
48. Young SL, Opsahl MS, Fritz MA. Serum concentrations of enclomi-
phene and zuclomiphene across consecutive cycles of clomiphene
citrate therapy in anovulatory infertile women. Fertil Steril 1999;
71:639–44.
49. Mitwally MF, Casper RF. Aromatase inhibition: a novel method of
ovulation induction in women with polycystic ovarian syndrome. Re-
prod Technol 2000;10:244–7.
50. Mitwally MF, Casper RF. Use of an aromatase inhibitor for induction
of ovulation in patients with an inadequate response to clomiphene
citrate. Fertil Steril 2001;75:305–9.
51. Mitwally MF, Casper RF. Single-dose administration of an aromatase
inhibitor for ovarian stimulation. Fertil Steril 2005;83:229–31.
52. Al-Omari WR, Sulman WR, Al-Hadithi N. Comparison of two aro-
matase inhibitors in women with clomiphene-resistant polycystic ovary
syndrome. Int J Gynaecol Obstet 2004;85:289–91.
53. Fisher SA, Reid RL, Van Vugt DA, Casper RF. A randomized double-
blind comparison of the effects of clomiphene citrate and the aromatase
inhibitor letrozole on ovulatory function in normal women. Fertil Steril
2002;78:280–5.
54. Sammour A, Biljan MM, Tan SL, Tulandi T. Prospective randomized
trial comparing the effects of letrozole (LE) and clomiphene citrate
(CC) on follicular development, endometrial thickness and pregnancy
rate in patients undergoing super-ovulation prior to intrauterine insem-
ination (IUI) [abstract]. Fertil Steril 2001;76(Suppl 1):S110.
55. Fatemi HM. Clomiphen citrate versus letrozole for ovarian stimulation:
a pilot study. Reprod Biomed Online 2003;7:543–6.
56. Kemmann E, Jones JR. Sequential clomiphene citrate-menotropin ther-
apy for induction or enhancement of ovulation. Fertil Steril 1983;39:
772–9.
57. Jarrell J, McInnes R, Cooke R, Arronet G. Observations on the com-
bination of clomiphene citrate-human menopausal gonadotropin-human
chorionic gonadotropin in the management of anovulation. Fertil Steril
1981;35:634–7.
58. Lu PY, Chen AL, Atkinson EJ, Lee SH, Erickson LD, Ory SJ. Minimal
stimulation achieves pregnancy rates comparable to human menopausal
gonadotropins in the treatment of infertility. Fertil Steril 1996;65:
583–7.
59. Dickey RP, Olar TT, Taylor SN, Curole DN, Rye PH. Sequential
clomiphene citrate and human menopausal gonadotrophin for ovulation
induction: comparison to clomiphene citrate alone and human meno-
pausal gonadotrophin alone. Hum Reprod 1993;8:56–9.
60. Rose BI. A conservative, low-cost superovulation regimen. Int J Fertil
1992;37:339–42.
61. Mitwally MF, Casper RF. Aromatase inhibition reduces gonadotrophin
dose required for controlled ovarian stimulation in women with unex-
plained infertility. Hum Reprod 2003;18:1588–97.
62. Healey S, Tan SL, Tulandi T, Biljan M. Effects of letrozole on super-
ovulation with gonadotropins in women undergoing intrauterine insem-
ination. Fertil Steril 2004;80:1325–9.
63. Mitwally M, Casper R. Aromatase inhibition reduces the dose of
gonadotropin required for controlled ovarian hyperstimulation. J Soc
Gynecol Investig 2004;11:406–15.
64. Weil SJ, Vendola K, Zhou J, Adesanya OO, Wang J, Okafor J, et al.
Androgen receptor gene expression in the primate ovary: cellular lo-
calization, regulation, and functional correlations. J Clin Endocrinol
Metab 1998;83:2479–85.
65. Weil S, Vendola K, Zhou J, Bondy CA. Androgen and follicle-stimulating
hormone interactions in primate ovarian follicle development. J Clin
Endocrinol Metab 1999;84:2951–6.
66. Vendola K, Zhou J, Wang J, Famuyiwa OA, Bievre M, Bondy CA.
Androgens promote oocyte insulin-like growth factor I expression and
initiation of follicle development in the primate ovary. Biol Reprod
1999;61:353–7.
67. Vendola KA, Zhou J, Adesanya OO, Weil SJ, Bondy CA. Androgens
stimulate early stages of follicular growth in the primate ovary. J Clin
Invest 1998;101:2622–9.
68. Mitwally M, Casper R. Aromatase inhibition improves ovarian re-
sponse to follicle-stimulating hormone in poor responders. Fertil Steril
2002;77:776–80.
69. Goswami SK, Das T, Chattopadhyay R, Sawhney J, Kumar J,
Chaudhury K, et al. A randomized single-blind controlled trial of
letrozole as a low-cost IVF protocol in women with poor ovarian
response: a preliminary report. Hum Reprod 2004;19:2031–5.
70. Schoolcraft W, Surrey E, Minjarez D, Gardner DK. Antagonist/letro-
zole protocol for patients failing microdose agonist flare stimulation.
Fertil Steril 2002;78(Suppl 1):S234.
71. Oktay K, Buyuk E, Akar M, Rosenwaks Z, Libertella N. Fertility
preservation in breast cancer patients: a prospective controlled compar-
ison of ovarian stimulation with tamoxifen and letrozole for embryo
cryopreservation [abstract]. Fertil Steril 2004;82(Suppl 2):S1.
72. Mitwally M, Casper R. Aromatase inhibition for ovarian stimulation:
future avenues for infertility management. Curr Opin Obstet Gynecol
2002;14:255–63.
73. Biljan M, Tan SL, Tulandi T. Prospective randomized trial comparing
the effects of 2.5 and 5.0 mg of letrozole (LE) on follicular develop-
ment, endometrial thickness and pregnancy rate in patients undergoing
super-ovulation [abstract]. Fertil Steril 2002;78:S55.
74. Al-Fadhli R, Sylvestre C, Buckett W, Tulandi T. A randomized trial of
superovulation with letrozole 2.5 mg and 5 mg. Fertil Steril (In Press).
75. Al Fozan H, Al-Khaduri M, Tan SL, Tulandi T. A randomized trial of
letrozole versus clomiphene citrate in women undergoing superovula-
tion. Fertil Steril 2004;82;1561–3.
76. Mitwally MF, Biljan MM, Casper RF. Pregnancy outcome after the use
of an aromatase inhibitor for ovarian stimulation. Am J Obstet Gynecol
2005;192:381–6.
284 Holzer et al. A new era in ovulation induction Vol. 85, No. 2, February 2006