Scholarly article on topic 'Are GnRH antagonists comparable to agonists for use in IVF?'

Are GnRH antagonists comparable to agonists for use in IVF? Academic research paper on "Basic medicine"

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Academic research paper on topic "Are GnRH antagonists comparable to agonists for use in IVF?"

Human Reproduction Vol.22, No.11 pp. 2805-2813, 2007

Advance Access publication on September 14, 2007

Are GnRH antagonists comparable to agonists for use in IVF?

J.A. Huirne, R. Homburg and C.B. Lambalk1

Division of Reproductive Medicine Department of Obstetrics and Gynaecology Vrije Universiteit medical centre (VUmc), PO Box 7075, 1007 MB Amsterdam, the Netherlands

Correspondence address. Tel: +0031-204440070; Fax: +0031-204440045; E-mail: cb.Lambalk@vumc.nl

We believe that appropriate comparison of optimal GnRH agonist and antagonist regimens has not been performed yet.

Currently available meta-analyses included all comparative studies between GnRH agonists and antagonists performed o

so far, including less than optimal GnRH antagonist regimens. After critical appraisal of the various studied GnRH §

antagonist regimens in terms of follicular development and IVF outcome, we postulate that early suppression of endogen- §

ous FSH results in optimal follicular development. Additionally, stable and early suppression of LH and progesterone f

levels during the entire period of stimulation may be an advantage for implantation and pregnancy outcome. In this §

respect, single dose and particularly flexible protocols seem to be less advantageous. Early FSH and LH suppression pp

can be achieved by early GnRH antagonist administration (stimulation day 1) or by oral contraceptive (OC) pretreat- h

ment. More studies comparing long GnRH agonist protocols with 'long' GnRH antagonist protocols, with enough |

power to identify differences in pregnancy rates, are required before appropriate comparison can be made. o

Keywords: GnRH agonists; GnRH antagonists; IVF; stimulation protocols; oral contraceptive pretreatment j

l r / t U i

the follicular phase (Albano et al., 1997; Ganirelix dose-finding Si study group, 1998; Olivennes et al., 1998; Huirne et al., y 2004b). Initially two general approaches emerged; (i) the 8, single dose protocol, in which one injection is administered « late in the follicular phase around stimulation day 7 or 8, and o (ii) the multiple dose regimens in which the antagonist is admi- g nistered daily from stimulation day 6 onwards. Soon, compara- ° tive studies with long GnRH agonist protocols were initiated, g without certainty about the possible optimal GnRH antagonist 3 administration strategy (Albano et al., 2000; Borm and 2 Mannaerts, 2000; Olivennes et al., 2000; European Middle 55 East Orgalutran study group, 2001; Fluker et al., 2001). Several drawbacks of the GnRH antagonist regimens emerged with these first phase III studies. (i) The numbers of oocytes retrieved were in favour of the long GnRH agonist arms compared with the fixed multiple dose day 6 and fixed single dose GnRH antagonist regimen (Albano et al., 2000; Borm and Mannaerts, 2000; Olivennes et al., 2000; European Middle East Orgalutran study group, 2001; Fluker et al., 2001; Roulier et al., 2003). (ii) The initiation of FSH administration in a GnRH antagonist regimen is cycle dependent. It is mostly started on day 2 or 3 of the natural cycle which made treatment planning and scheduling more difficult (Huirne and Lambalk, 2001; Huirne et al., 2005). (iii) Although the pregnancy rates were not different in various individual studies, a

The first in vitro fertilization (IVF) therapies were performed in natural unstimulated IVF cycles. Nowadays, gonadotrophins are given to induce multiple follicular development and GnRH analogues for the prevention of premature LH surges in IVF. Without the use of GnRH analogues, LH surges occur in ~20% of stimulated IVF patients (Edwards et al., 1996; Janssens et al., 2000). Preventing LH surges using GnRH analogues improves oocyte yield with more embryos, allowing better selection, leading to an increase in pregnancy rates (Templeton et al., 1998). It took 15 years of experience with the GnRH agonists in IVF to identify the optimal protocol (the long protocol starting in the midluteal phase of the preceding cycle) with regard to the best IVF results in a general population (Daya, 2000; Huirne et al., 2004a). GnRH agonist administration causes gonadotrophin suppression via pituitary desensitization, after an initial short period of gonadotrophin hypersecretion. In contrast, GnRH antagonists cause immediate and rapid gonadotrophin suppression, by competitive occupancy of the GnRH receptor and therefore intuitively a more logical choice to use in IVF for the prevention premature LH surges. Theoretically, GnRH antagonists could be administered at any time during the early or mid-follicular phase of a treatment cycle to prevent a premature LH surge. With this in mind, around a decade ago, the first dose-finding studies in IVF started GnRH antagonist medication on a fixed day late in

© The Author 2007. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

meta-analysis including the first five comparative studies of fixed GnRH antagonist protocols compared with long agonist protocols, indicated 5% less clinical pregnancies in the antagonist groups (Al-Inany and Aboulghar, 2002). The initial reported results of these comparative studies, together with the results of national large database evaluations (Devaux et al., 2004; Griesinger et al., 2005), which were not in favour of the GnRH antagonist, made the GnRH antagonist for many clinicians a second choice. Is this justified? Analysis of the national IVF registry in Germany from 2000 to 2003 demonstrated that GnRH antagonists were often utilized in cycles with an unfavourable a priori prognosis, in patients with advanced age and with a higher number of previously unfavourable cycles (Griesinger et al., 2005). Sub-analysis of patients with equal demographic and clinical features resulted in similar pregnancy rates independent of whether GnRH agonists or antagonists were used (Engel et al., 2006).

Furthermore, the meta-analysis of Al-Inany and Aboulghar (2002), included one study, with different starting doses of FSH in the comparative arms, with a possible risk of confounding (Olivennes et al., 2000). Differences no longer reach statistical significance if this study is left out of the analyses: OR 0.8 (0.63-1.01) (Thesis Al-Inany 2006), although this may also be due to insufficient power once this study is omitted.

Now that over 200 clinical trials involving GnRH antagonists in IVF have been published, it is a good time to compare results with those achieved with the long GnRH agonist protocol, the most commonly adopted protocol for assisted reproductive treatment cycles worldwide. Tables 1 and 2 provide an overview of all currently available comparative RCT's (published as full papers), including their design and regimen used, with or without oral contraceptive (OC) pretreatment.

Very recently, two meta-analyses have been published with conflicting interpretations (Al-Inany et al., 2006; Kolibianakis

Table 1: Characteristics of current published randomized controlled trials comparing GnRH agonist and antagonists in IVF without OC pretreatment

Authors

Number of

patients

Population

Type of

agonist

Agonist protocol

Type of

antagonist

Antagonist protocol

Starting dose FSH (IU)

Long agonist versus fixed MD Albano et al. (2000) European study (2000) European Middle East Orgalutran Study Group (2001)

Fluker et al. (2001) Lee et al. (2005)

Simon et al. (2005)

95/198 General

244/486 General 119/236 General

105/208 General 41/20 General

(20/20/21)

14/14/14 Oocyte donors

Long agonist versus fixed SD

Olivennes et al. (2000)a 43/126

General

Long agonist versus flexible antagonist (MD)

Hohmann et al. (2003) 58/111 General

(45/48/49)

30/30 General

58/58 General

Check et al. (2004) Loutradis et al. (2004)b

Buserelin Long MD Buserelin Long MD Triptorelin Long MD

Leuprorelin Long MD Buserelin Long MD

Buserelin Long MD

Triptorelin Long SD

Triptorelin Long MD

Leuprolide Long MD Triptorelin Long MD hCG (18 mm)

Badrawy et al. (2005) 50/50 General

Xavier et al. (2005) 65/66 General

Marci et al. (2005) 30/30 Poor resp

Rombauts et al. (2006) 111/111 General

Short agonist versus fixed antagonist multiple dose (MD) Martinez et al. (2003) 23/21 Poor resp

Short agonist versus flexible antagonist multiple dose (MD) Akman et al. (2001) 24/24 Poor resp

Malmusi et al. (2005) 30/30 Poor resp

Schmidt et al. (2005) 24/24 Poor resp

De placido et al. (2006) 67/66 Poor resp

Short agonist versus flexible antagonist single dose (SD) Roulier et al. (2003)c 364/307 General

Buserelin Buserelin

Long MD Long MD

Leuprorelin Long MD Nafarelin Long MD

Triptorelin Short MD

Leuprorelin

Triptorelin

Leuprorelin

Triptorelin

Cetrorelix Ganirelix Ganirelix

Ganirelix Cetrorelix

Ganirelix

OC/short MD (CD2)

Short MD (CD1) OC/short MD Flare up MD (S1)

Fixed S6 MD Fixed S6 MD Fixed S6 MD

Fixed S6 MD Fixed S5 MD or fixed S7 SD Fixed S6 MD 0.25 mg or fixed S6 MD 2 mg

Cetrorelix Fixed SD (S7)

Cetrorelix Flex MD ( > 14 mm)

Ganirelix Cetrorelix

Ganirelix Cetrorelix

Cetrorelix Ganirelix

Flex (>14 mm) Flex MD (>14 mm) hCG (20 mm)

Flex MD (>14 mm) Flex MD (>14 mm or E2 conc) Flex MD (>14 mm) Flex MD (>14 mm)

Cetrorelix Fixed S7 MD

150 or 225 hMG 150 rFSH 150 rFSH

225 rFSH 225 hMG

150 rFSH

225 versus 150 hMG

150 rFSH, CD2 or CD5 with cetrorelix 300 rFHS or hMG 225 rFSH (+75IU rFSH commenc with antag.) 225 Menogon 150-450 rFSH

375 rFSH 200 rFSH

150 rFSH + 150 HMG

Cetrorelix Flex MD ( > 14 mm) 300 rFSH + 300 hMG

Ganirelix Ganirelix Cetrorelix

Flex MD (>14 mm) Flex MD (>12 mm) Flex MD (>14 mm), 2 days 0.125 mg/day than 0.25 mg/day

450 rFSH

300 rFSH + 150 hMG 300 rFSH (+150 LH, commenc. with antag.)

Triptorelin Short SD (CD1) Cetrorelix Flex SD (> 14 mm) 150-225 rFSH

aStarting dose was 225 IU in the Cetrorelix and 150 IU hMG in the Triptorelin. bDifferent criteria for hCG administration in both groups, 18 versus 20 mm. cInadequate randomization.

MD, multiple dose; SD, single dose; flex, flexible protocol; OC, oral contraceptives; E2, estradiol; S6, stimulation day 6 etc.; CD2, cycle day 2; hMG, human menopausal gonadotrophine; rFSH, recombinant follicle stimulating hormone; LH, luteinizing hormone; hCG, human chorion gonadotrophin.

Table 2: Characteristics of current published randomized controlled trials comparing GnRH agonist and antagonists in IVF including OC pretreatment

Authors

Number of Population Type of Agonist patients agonist protocol

Type of Antagonist antagonist protocol used

Start FSH: Starting dose

number of days FSH after last OCP

OC/long agonist versus OC/fixed antagonist protocol

Hwang et al. (2004)a 29/27 PCOS Buserelin Long MD ( +OC) Cetrorelix

Sauer et al. (2004)b

Cheung et al. (2005)

Huirne et al. (2006a) Prapas et al. (2005)c

25/24 General Leuprorelin OC/long SD

(25/24/25)

OC/MD fixed (0.125mg/day OCP+4 to OCP+9) (0.25 mg/day OCP+10 to hCGd) Cetrorelix OC/fixed SD S7

33/33 Poor Buserelin

responders 91/91 General Buserelin

75/73 Oocyte Triptorelin

donors

OC/long MD Cetrorelix

Long MD OC/long MD

Cetrorelix Ganirelix

OC/fixed SD S7+LH OC/fixed MD S6 2 or 3

OC/fixed MD S6 5 OC/fixed MD S8 2

150 hMG

225 rFSH

300 rFSH

150-225 rFSH 300 rFSH

OC/long agonist versus OC/flexible antagonist protocol

Bahceci et al. (2005) 75/73 PCOS Leuprolin OC/long MD Cetrorelix OC/flex MD 3 150-300

(>14 mm) uFSH/hMG

Barmat et al. (2005) 40/40 General Leuprolin OC/long MD Ganirelix OC/flex MD 5 300 rFSH

(>12 mm)

Rombauts et al. (2006) 111/111 General Nafarelin long MD Ganirelix OC/flex MD 2 200 rFSH

Koichi et al. (2006)d (>14 mm)

66/63/63 General Buserelin OC/long MD Cetrorelix OC/flex MD 5 225-300uFSH

(>14 mm)

Vlaisavljevic et al. (2003)e 236/226 General Goserelin OC/long SD Cetrorelix OC/flex MD 5 150-225r/uFSH

(>12-14 mm)

hort agonist versus OC/flexible antagonist

Schmidt et al. (2005) 24/24 Poor Leuprolin Flare up MD Ganirelix OC/flex MD 2 300 rFSH

responders (>12 mm) + 150 hMG

aDose of antagonist was increased from 0.125 to 0.25 mg/day on day 10 after last OCP. bThree arm study; Leurolin (n — 25) versus Cetrorelix (n — 24) versus Cetrorelix + LH (n — 25).

cSome women were used twice as donord 3 arm study; Buserelin (n — 66) versus Cetrorelix, startdose 225 IU FSH, FSH increase to 300 if follicle > 14 mm (n — 63) versus Cetrorelix, startdose 225 IU FSH, FSH decreased to 75IU + 200 hCG/day if follicle > 14 mm (n — 63). MD, multiple dose; SD, single dose; flex, flexible protocol; OC, oral contraceptives; OCP, oral contraceptive pill; E2, estradiol; S6, stimulation day6 etc.; CD2, cycle day 2; hMG, human menopausal gonadotrophin; rFSH, recombinant follicle stimulating hormone; LH, luteinizing hormone; hCG, human chorion gonadotrophin.

Inadequate randomization.

et al., 2006a). Both studies reported highly significantly shorter duration of stimulation (—1.5 and —1.1 days) and less oocytes (—1.6 and — 1.1) retrieved using GnRH antagonists compared with agonists (Al-Inany et al., 2006; Kolibianakis et al., 2006a). The first meta-analysis including 27 relevant published papers, abstracts and proceedings, showed significant differences with respect to clinical (OR = 0.84, 95% CI 0.72-0.97) and ongoing pregnancy/live birth rate (OR = 0.82, 95% CI: 0.69-0.98) in favour of the agonist regimen (Al-Inany et al., 2006). However, the quality, method of design and analysis could not be assessed for all of the individual papers used. The other meta-analysis, with 22 RCT's published as full papers in peer reviewed journals, could not identify significant differences with respect to the probability of live birth independent of population studied, gonado-trophin type used for stimulation, type of agonist protocol or whether a fixed or flexible GnRH antagonist regimen was used (Kolibianakis et al., 2006a). In the latter meta-analysis, an expected live birth was calculated on the basis of clinical and ongoing pregnancies for all studies not reporting live birth rate, which remains an estimate. OC pretreatment (OCP) was only sub-analysed (in a subgroup 'additional intervention group'), if OCP was given in only one of the treatment arms, but not when it was applied in both arms. We think that this is not entirely correct as OCP may have a bigger effect in a GnRH antagonist regimen with

■A ?

mostly high fluctuation in hormonal levels during the treatment compared with the long GnRH agonist regimen in which hormonal levels are more or less stably suppressed. It may be an effect modifier and should, in our opinion, be taken into account in the analyses. Thus the question is still unanswered; what is the current place of the GnRH antagonist in IVF/ICSI?

After critical appraisal of currently available studies using GnRH antagonists, we think that the differences in reported outcome measurements could be the consequence of the large variation of employed GnRH antagonist regimens (Fig. 1). In this respect, it is likely that two phenomena play an important role to facilitate optimal IVF results when GnRH analogues are used:

(i) Stable and low LH and progesterone levels throughout the stimulation phase to achieve optimal conditions for implantation and

(ii) Sustained low levels of endogenous FSH before stimulation is started to allow optimal synchronization of the follicular cohort.

What are the facts that may help to determine the optimal GnRH analogue regimen?

Long versus short or ultra-short GnRH agonist regimen

Many treatment schedules with the use of GnRH agonists in IVF therapy have been designed and studied (for review see

Figure 1: Variations in duration and initiation time of the GnRH antagonist in different GnRH antagonist protocols

MD, multiple dose; SD, single dose; CD1, cycle day 1; S1, stimulation dayl; S6, stimulation day 6; S7, stimulation day 7; hCG, day of human chorionic gonadotrophin administration

Huirne et al., 2004a). Several investigators tried to shorten the duration of GnRH agonist administration by later administration or early cessation. However, the long protocol (starting in the midluteal phase of the preceding cycle) gave the best IVF results with regard to oocyte yield and pregnancy rates (Daya, 2000). This protocol induces profound suppression of endogenous release of gonadotrophins during the early follicular phase, allowing the early antral follicles to grow co-ordinately in response to exogenous gonadotrophins to accomplish simultaneous maturation (Fig. 2a). This leads to an extended widening of the FSH window, increased FSH requirement and in the end more mature follicles and retrieved oocytes (Daya, 2000).

Fixed versus flexible and short versus long GnRH antagonist regimens

The initially developed GnRH antagonist regimens started relatively late in the follicular phase on a fixed day, mostly stimulation day 6. Under these circumstances, the luteo-follicular transitory rise of endogenous FSH starts the stimulation of a cohort of follicles that vary in stage of development. Subsequently exogenous FSH allows further development of a few leading large follicles and several smaller follicles (Albano et al., 2000; Borm and Mannaerts, 2000; European Middle East Orgalutran study group, 2001; Fluker et al., 2001; Huirne et al., 2004a; Huirne et al., 2005). As the criteria to administer hCG are based on the size of the leading largest follicles, consequently a number of follicles will still be immature at that time (Fig. 2b). Logically, the stimulation period will be shorter with less FSH required but also the number of oocytes will be reduced compared with the long GnRH agonist protocol (Albano et al., 2000; Borm and Mannaerts, 2000; Olivennes et., 2000; European Middle East Orgalutran study group, 2001; Fluker et al., 2001). We think that the

relatively higher levels of FSH during the early follicular phase in various initially developed GnRH antagonist regimens, results in less synchronization of the follicular cohort with less oocytes retrieved (Fig. 2b). More oocytes may result in increased pregnancy rates due to increased number of embryos available for improved embryo selection (Temple-ton and Morris, 1998) and cryopreservation. However, differences in number of oocytes are in the range of 1 or 2 and it is debatable if this slightly smaller number of oocytes retrieved can be held responsible for possible differences in pregnancies.

Yet another feature of the originally employed GnRH antagonist protocols may have contributed negatively. It is likely that the higher LH, estradiol and progesterone levels during the early follicular phase in most of these GnRH antagonist regimens compared with the long agonist regimen (Fig. 3a and b) may play a role. Significantly lower ongoing pregnancy rates are seen in patients with elevated progesterone at initiation of stimulation of fixed day 6 GnRH antagonist cycles (Kolibianakis et al., 2004b). The level of LH suppression 2 days after commencement of GnRH antagonist therapy in a fixed day 6 protocol is possibly associated with ongoing pregnancy, the higher the LH levels the lower the probability of achieving an ongoing pregnancy (Kolibianakis et al., 2004a). Possibly early closure of the implantation window occurs (Develioglu et al., 1999) through earlier expression of progesterone receptors in the follicular phase and down-regulation of estrogen receptors by the exposure to supraphysiological steroid hormone levels (Kolibianakis et al., 2002; Papanikolaou et al., 2005). These findings support the proposed facilitating/activating mode of hormonal control of endometrial receptivity (de Ziegler, 1995). According to this theory, once endometrium is primed by estradiol,

Figure 2: (a) Synchronized follicular development after FSH administration in a long GnRH agonist regimen and (b) Follicular development in a fixed day 6 GnRH antagonist regimen without OC pretreatment

The long GnRH agonist protocol suppresses endogenous FSH levels, leading to a follicular cohort of all small follicles at the initiation of FSH administration without leading larger follicles. After exogenous FSH administration, FSH levels remain above the threshold, resulting in a synchronized follicular development. As soon as one or two follicles meet the hCG administration criteria, most follicles will be of more or less similar size and sensitive for hCG. In the fixed GnRH antagonist protocol, endogenous FSH levels are not suppressed during the early follicular phase. The luteo-follicular transition induces FSH levels above the threshold for a short period until hormonal feedback occurs, leading to the initiation of follicular growth of a few leading follicles. After exogenous FSH administration, FSH levels arise above threshold again and will initiate several additional follicles to grow. As soon as the leading follicles meet the hCG criteria, several other follicles will be of smaller sizes and may not be sensitive for hCG yet. Such an asynchronized cohort may therefore result in less oocytes retrieved, compared to the long agonist protocol

the duration of progesterone exposure is the crucial point leading to a receptive endometrium. Other studies could not find an effect of the absolute LH concentrations on stimulation day 8 or the day of hCG administration during a fixed GnRH antagonist regimen on ovarian response and IVF outcome (Bosch et al., 2003; Penarrubia et al., 2003; Merviel et al., 2004). Differences between the various studies, with regard to the level of LH suppression, study populations and type of GnRH antagonist regimen used, may play a role in the conflicting results. Furthermore, one study indicates that the stability of LH levels rather than absolute LH values are associated with clinical pregnancy as no pregnancies occurred if the LH and progesterone levels changed too markedly (either increase or decrease) during GnRH antagonist administration (Huirne et al., 2005).

In order to reduce the number of antagonist injections and the duration of stimulation, flexible protocols were developed. Instead of starting with the GnRH antagonist on a fixed day, administration was made dependent on the follicular size. GnRH antagonist injections were started as soon as the follicles reach a size of > 14, 15 or 16mm after 5 days of stimulation (Ludwig et al., 2002; Hohmann et al., 2003; Kolibianakis et al., 2003b; Escudero et al., 2004; Klipstein et al., 2004; Mochtar et al., 2004). Overall, this implies that almost 50% of the patients will start with a GnRH antagonist beyond day 6 of FSH stimulation (Kolibianakis et al., 2003b; Escudero et al., 2004; Mochtar et al., 2004). Such protocols in particular allow higher LH, estradiol and progesterone levels, especially when antagonist treatment is started beyond day 6 and are associated with lower pregnancy rates (Kolibianakis et al.,

Figure 3: Schematic overview of expected FSH and LH concentrations in various GnRH analogue regimens

a and b are regimens without oral contraceptive pill (OCP) pretreatment: (a) long GnRH agonist protocol and (b) fixed day 6 GnRH antagonist protocol. (c) FSH is started 2 days after the last OCP and (d) FSH is started 5 days after the last OCP in a fixed day 6 GnRH antagonist regimen

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2003b), whereas an earlier start (cycle day 4 or 5) of GnRH antagonists in these protocols is associated with improved pregnancy rates (Lainas et al., 2005). In this respect, starting with a GnRH antagonist on day 1 compared with day 6 will even further decrease the exposure to LH and estradiol during the early follicular phase (Kolibianakis et al., 2003a). However, the rather high-pregnancy rates (52% per embryo transfer) were not different in this small study. Additionally, this regimen will increase the cost due to the extended period of GnRH antagonist administration.

The long GnRH agonist protocol is favourable to flexible start antagonist protocols with respect to the number of follicles on the day of hCG and number of oocytes retrieved (Hohmann et al., 2003; Weghofer et al., 2004; Ragni et al., 2005). Again asynchronous follicle development through absent suppression of early endogenous FSH secretion could explain this (Figs 2 and 3 a and b). Overall, it seems that the low gonadotrophin levels prior to stimulation created by the long agonist protocol are of particularly favourable with regard to IVF/ICSI yield and outcome.

OCP or estrogen pretreatment versus GnRH antagonist only protocols

GnRH antagonist regimen with estrogen or OC pretreatment offers a simple alternative to achieve gonadotrophin suppression during the early follicular phase (De Ziegler et al., 1998, Van Heusden et al., 1999). This mechanism can be used to overcome the cycle dependency of GnRH antagonist regimens by inducing a withdrawal bleeding so that starting time of

hormonal stimulation can be planned. OC or estrogen pretreat-ment has been evaluated over the past several years. In some studies, gonadotrophin administration was started 2 or 3 days (Cedrin-Durnerin et al., 2004; Cheung et al., 2005; Bahceci et al., 2005; Huirne et al., 2006b; Huirne et al., 2006c; Rom-bauts et al., 2006) and others, 4 or 5 days after OC withdrawal (Obruca A et al., 2000; Vlaisavljevic et al., 2003; Hwang et al., 2004; Sauer et al., 2004; Barmat et al., 2005; Huirne et al., 2006a; Koichi et al., 2006; Kolibianakis et al., 2006b) in either flexible or fixed GnRH antagonist protocols. OC pre-treatment using GnRH antagonists with subsequent starting of FSH 2 or 3 days after the last OC intake is associated with deep suppression of LH and FSH levels and improved synchronization of the follicular cohort development compared with GnRH antagonist only protocols (Huirne et al., 2006b; Rom-bauts et al., 2006). Similarly, improvement of the synchronization of the follicular cohort was also observed if stimulation was started 3 days after estradiol pretreatment in GnRH antagonist protocols in a general population (Franchin et al., 2003) and in poor responders with promising pregnancy rates (Dragi-sic et al., 2005). Whereas this effect is not seen when FSH stimulation was started on day 5 after the last OCP (Obruca A et al., 2000; Kolibianakis et al., 2006b). Apparently, timing the start of exogenous gonadotrophin administration after OCP-pretreatment affects follicular development (Cedrin-Durnerin et al., 2007). Fig. 3c and d show a schematic presentation of the expected patterns of LH and FSH levels during the follicular phase if stimulation is started on day 2 or day 5 after the last OC intake. Straightforward comparison of starting with

gonadotrophin administration on day 2 versus day 5 after the last OC intake indeed showed stronger gonadotrophin suppression and less large follicles in the early stimulation period, if stimulation was started earlier after OC withdrawal (Huirne et al., 2007). The drawbacks of OC pretreatment are that the stimulation period is increased and more gonadotrophins are needed. Several RCT's comparing OC pretreated GnRH antagonist with long agonist protocols could not observe significant differences with respect to the number of oocytes retrieved and pregnancy rates (Hwang et al., 2004; Sauer et al., 2004; Barmat et al., 2005; Cheung et al., 2005; Bahceci et al., 2005; Huirne et al., 2006a; Rombauts et al., 2006). Although some studies indicate lower implantation rates after OC pretreatment (Huirne et al., 2006b; Rombauts et al., 2006) or increased pregnancy loss compared with GnRH antagonist only regimens (Kolibianakis et al., 2006b), similar luteal endometrial development was found in OC pretreated flexible GnRH antagonist protocol in comparison to a long GnRH agonist protocol (Saadat et al., 2004) or a short GnRH agonist protocol (Schmidt et al., 2005) and in addition when a fixed day 6 antagonist was compared to a long agonist protocol (Simon et al., 2005).

GnRH agonists and antagonists: have we compared them in the optimal way?

Taken all together, the optimal GnRH analogue regimens seem to be regimens ensuring stable FSH and LH suppression during the entire stimulation period. In this respect, the long agonist or long fixed antagonist regimen seemed to be preferred (i.e. long OC pretreated fixed GnRH antagonist protocol or a long GnRH antagonist protocol from stimulation day 1 onwards). We stress that an optimal comparison of GnRH agonists versus antagonist requires the comparison of the optimal regimens of both compounds. So far, only a few individual studies compared the long GnRH agonist protocol with OC pretreated fixed GnRH antagonist protocols (Table 2). Most individual RCT's comparing OC pretreated GnRH antagonist (fixed or flexible) with a long GnRH agonist protocol, could not identify significant differences in number of oocytes retrieved and pregnancy rates in a general IVF population (Vlaisavljevic et al., 2003; Hwang et al., 2004; Sauer et al., 2004; Barmat et al., 2005; Huirne et al., 2006a; Rombauts et al., 2006), PCO patients (Bahceci et al., 2005) or poor responders (Cheung et al., 2005). Most studies comparing OC pretreated flexible GnRH antagonist protocols with long GnRH agonist protocols in a general IVF population, are in favour of the agonist with respect to number of oocytes retrieved and pregnancy rates. OC pretreated fixed GnRH antagonist protocols may be more favourable than OC pretreated flexible GnRH antagonist protocol, although this has so far not been studied in a direct way. On the basis of the lower number of side effects and lower number of required (GnRH analogue) injections with similar ability to schedule ovum pickup, OC pretreated GnRH antagonist regimen may be an attractive alternative for the commonly used long GnRH agonist protocol (Huirne et al., 2006a).

To further explore our idea that the OC pretreated fixed GnRH antagonist regimen is comparable to the long GnRH

agonist protocol, more large RCT's comparing these regimens are required. Only thereafter can a fair comparison be made by a meta-analysis of sufficient power to identify significant differences in pregnancy rates. To identify a difference in clinical pregnancy rate of 5% (with pregnancy rates in the region of 25%), using ß of 0.2 and a of 0.05 with a two-tailed hypothesis test, over 1252 patients would be required in each treatment arm.

Conclusion

After critical appraisal of the current GnRH antagonist studies, we believe that stable and early suppression of endogenous gonadotrophins may be advantageous to achieve follicular synchronization and the highest clinical pregnancy rates. This may be achieved by either a long GnRH agonist protocol or a 'long'

GnRH antagonist protocol (i.e. OC pretreated fixed GnRH o

antagonist protocol). In this respect, short or flexible regimens o

seem to be far from optimal. Appropriate comparison of GnRH e

agonist and antagonist regimens requires the inclusion of the ^

optimal regimens of both compounds. Most meta-analyses g

that have been performed so far included all GnRH antagonist p

regimens performed, including the less then optimal regimens. h

More (larger) randomized controlled trials of sufficient 3

power to identify significant differences in pregnancy rates o

comparing OC pretreated fixed GnRH antagonist regimen or 0

a long (starting stimulation day 1) fixed GnRH antagonist 0

regimen with long GnRH agonist regimens are required to |

allow optimal comparison between GnRH agonists and antag- .

onists for their use in IVF or ICSI therapy. /

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Submitted on October 13, 2006; resubmitted on April 18, 2007; accepted on

July 10, 2007