Efficacy, tolerability, and bone density outcomes of elagolix with add-back therapy for endometriosis-associated pain: twelve months of an ongoing randomized phase 3 trial

Background: Elagolix, an approved oral treatment for endometriosis-associated pain, has been associated with hypoestrogenic effects when used as monotherapy. Hormonal add-back therapy has the potential to mitigate these effects.

Objective: To evaluate efficacy, tolerability, and bone density outcomes of elagolix 200 mg twice daily with 1 mg estradiol/0.5 mg norethindrone acetate (add-back) therapy once daily compared with placebo in premenopausal women with moderate-to-severe endometriosis-associated pain.

Study design: This ongoing, 48-month, phase 3 study consists of a 12-month double-blind period, with randomization 4:1:2 to elagolix 200 mg twice daily with add-back therapy, elagolix 200 mg twice daily monotherapy for 6 months followed by elagolix with add-back therapy, or placebo. The coprimary endpoints were proportion of patients with clinical improvement (termed "responders") in dysmenorrhea and nonmenstrual pelvic pain at month 6. We report 12-month results on efficacy of elagolix with add-back therapy vs placebo in reducing dysmenorrhea, nonmenstrual pelvic pain, dyspareunia, and fatigue. Tolerability assessments include adverse events and change from baseline in bone mineral density.

Results: A total of 679 patients were randomized to elagolix with add-back therapy (n=389), elagolix monotherapy (n=97), or placebo (n=193). Compared with patients randomized to placebo, a significantly greater proportion of patients randomized to elagolix with add-back therapy responded with clinical improvement in dysmenorrhea (62.8% vs 23.7%; P≤.001) and nonmenstrual pelvic pain (51.3% vs 36.8%; P≤.001) at 6 months. Compared with placebo, elagolix with add-back therapy produced significantly greater improvement from baseline in 7 hierarchically ranked secondary endpoints including dysmenorrhea (months 12, 6, 3), nonmenstrual pelvic pain (months 12, 6, 3), and fatigue (months 6) (all P<.01). Overall, the incidence of adverse events was 73.8% with elagolix plus add-back therapy and 66.8% with placebo. The rate of severe and serious adverse events did not meaningfully differ between treatment groups. Study drug discontinuations associated with adverse events were low in patients receiving elagolix with add-back therapy (12.6%) and those receiving placebo (9.8%). Patients randomized to elagolix monotherapy exhibited decreases from baseline in bone mineral density of -2.43% (lumbar spine), -1.54% (total hip), and -1.78% (femoral neck) at month 6. When add-back therapy was added to elagolix at month 6, the change from baseline in bone mineral density remained in a similar range of -1.58% to -1.83% at month 12. However, patients who received elagolix plus add-back therapy from baseline exhibited little change from baseline in bone mineral density (<1% change) at months 6 and 12.

Conclusion: Compared with placebo, elagolix with add-back therapy resulted in significant, clinically meaningful improvement in dysmenorrhea, nonmenstrual pelvic pain, and fatigue at 6 months that continued until month 12 for both dysmenorrhea and nonmenstrual pelvic pain. Elagolix with add-back therapy was generally well tolerated. Loss of bone mineral density at 12 months was greater in patients who received elagolix with add-back therapy than those who received placebo. However, the change in bone mineral density with elagolix plus add-back therapy was <1% and was attenuated compared with bone loss observed with elagolix monotherapy.