P–325 The impact of letrozole on endometrial thickness in IVF cycles

Abstract Study question Does daily administration of letrozole during IVF stimulation affect endometrial thickness ? Summary answer Patients treated with letrozole during fresh IVF cycles had a thinner endometrium on the day of trigger compared to patients who did not receive letrozole. What is known already Letrozole supplementation is commonly used during fertility preservation for breast cancer patients to reduce peak estrogen levels with no adverse effects on embryo outcomes. Studies in poor responders have found that letrozole use resulted in a shorter duration of stimulation and a lower total dose of gonadotropin, with no detrimental effect on IVF outcomes. In normal responders, studies have shown an increase in blastocysts obtained, but have not yet shown an increase in clinical pregnancy rates. There is concern that when a fresh embryo transfer is planned letrozole use may negatively affect endometrial thickness and subsequently diminish pregnancy rates. Study design, size, duration In a retrospective cohort study between January 2009 and June 2019 at a single academic fertility center, we compared the endometrial thickness in 97 cancer patients who underwent IVF-fertility preservation with daily letrozole use to 158 cancer patients who underwent IVF-fertility preservation without letrozole. Participants/materials, setting, methods All women diagnosed with cancer were referred for fertility preservation prior to gonadotoxic treatment exposure and were less than 40 years old at the time of oocyte retrieval. All patients who received letrozole started on day one of stimulation and continued until the day of oocyte retrieval. The primary outcome was endometrial thickness on the day of trigger. The secondary outcomes were number of oocytes retrieved, number of MII retrieved, and maximal estradiol level. Main results and the role of chance During the study period, 336 cancer patients underwent fertility preservation. Eighty-one patients were excluded; 50 because they had an intrauterine device or were on long term oral contraceptives and 31 because endometrial thickness was not documented. Of the remaining 255 patients, 86 had breast cancer, 95 had a hematological cancer and 74 had various other cancers. Ninety-seven cancer patients treated with letrozole were compared to 158 cancer patients who did not receive letrozole. Patients who received letrozole were significantly older (34 vs 28yrs, P < 0.0001). There were no significant differences in baseline characteristics such as BMI, AFC nor in the total duration for stimulation. Endometrial thickness on the day of trigger was significantly less in letrozole treated patients (8 vs 9mm, P < 0.003). There were no significant differences in total number of oocytes retrieved (12.5 vs 11, P = 0.126) nor in the number of mature oocytes (8 vs 8, P = 0.312). Patients in the letrozole group received a higher total gonadotropin dose (2680IU vs 1980IU, P = 0.016). The maximum estradiol level was significantly lower in patients treated with letrozole (1068 vs 3838ml/dl, P = <0.0001). A regression analysis showed that using letrozole during stimulation decreased the endometrial thickness by 0.81mm (CI –1.37 to –0.253, P = 0.005). Limitations, reasons for caution The retrospective nature of this study could have introduced selection and misinformation bias. We report on cancer patients where all oocytes or embryos were vitrified. Without fresh embryo transfer data, it is unclear if a thinner endometrium due to letrozole will effect the implantation or pregnancy rate. Wider implications of the findings: As the use of letrozole expands beyond cancer patients and poor responders, it is important to understand the impact on the endometrium. This study shows that letrozole reduces endometrial thickness. However, the effect on endometrial function remains unknown. Further study is needed before letrozole can be used with fresh transfers. Trial registration number 2020–6370