Effects of Lasofoxifene Versus Fulvestrant on Vaginal and Vulvar Symptoms in Patients With ESR1-Mutated, ER+/HER2-, Metastatic Breast Cancer From the ELAINE 1 Study

Background: Lasofoxifene, a novel endocrine therapy (ET), showed antitumor activity versus fulvestrant in women with ESR1-mutated, metastatic breast cancer (mBC) that progressed on prior ET (phase 2, ELAINE 1 study). We investigated changes in genitourinary syndrome of menopause (GSM) vulvar-vaginal symptoms with lasofoxifene and how patient/disease characteristics affect baseline vulvar-vaginal symptoms in ELAINE 1.

Methods: Women were randomized to oral lasofoxifene 5 mg/day or IM fulvestrant 500 mg (days 1, 15, and 29, then every 28 days) until disease progression/severe toxicity. Changes in mean vaginal (VAS) and vulvar (VuAS) assessment scales, and their composite (average of all symptom scores/patient), from baseline to week 16, and mean baseline VAS/VuAS scores by patient/disease characteristics, were descriptively summarized.

Results: Of 103 enrolled patients, 72 (70%) completed the VAS/VuAS (mean age 61.5 years). Vaginal (40%)/vulvar (25%) dryness and vaginal pain (22%) were the most frequently reported symptoms; 26% reported ≥1 moderate/severe symptom. Lasofoxifene decreased the mean composite VAS/VuAS, VAS, and VuAS from baseline to week 16 by 74%, 74%, and 79%, respectively; fulvestrant increased them by 36%, 15%, and 63%, respectively. Baseline vaginal/vulvar symptoms were more severe if patients were under age 40, had no visceral disease, used adjuvant tamoxifen previously, or had longer AI duration in the adjuvant/metastatic settings.

Conclusions: Oral lasofoxifene (5 mg/day), but not fulvestrant, appears to improve GSM vaginal symptoms in women with mBC. These preliminary findings suggest further study is needed; such will be explored in the phase 3, registrational, ELAINE 3 trial in patients with ESR1-mutated, ER+/HER2- mBC.