A Study of Imlunestrant (LY3484356) in Premenopausal Women With Estrogen Receptor-Positive (ER+) Human Epidermal Growth Factor Receptor 2 Negative (HER2-) Early Breast Cancer

This study is specifically looking for premenopausal women (meaning you still have periods) who have been diagnosed with a certain type of early breast cancer. This must be breast cancer that is sensitive to oestrogen (ER-positive) and doesn't have too much of the HER2 protein (HER2-negative). You should generally feel well and be able to swallow tablets or capsules.

For Cohort 1, your breast cancer should be in stages I to III and show a certain level of cell growth (Ki-67 at least 10%). You'd also need to be willing to provide tissue samples from your tumour before and during treatment. For Cohort 2, you should have already had surgery for your ER+, HER2- early breast cancer and received at least 4.5 years of hormone therapy, or at least 2 years with no further ovarian suppression planned.

There are also some reasons why you might not be able to join. For example, if you have inflammatory breast cancer, other serious medical conditions, or have recently had major surgery. You also can't have had certain other cancers unless they've been gone for a long time without treatment. If you could become pregnant, you would need to use effective non-hormonal contraception during the study.

If you decide to take part in this study, what happens next depends on which group (cohort) you would be in. For Cohort 1, the treatment period will be quite short, lasting up to 29 days. For Cohort 2, the treatment might last longer, up to 6 months. During this time, you would take either the new medicine, imlunestrant, or an existing one, tamoxifen, depending on which group you're assigned to. Some participants might also receive a treatment to suppress their ovarian function.

You would have regular appointments and tests so the doctors can monitor how the treatment is affecting you and your cancer. This could include providing tumour samples (for Cohort 1), blood tests, and scans. The team will explain exactly what's involved in your specific case, including how often you'd need to visit the clinic and any follow-up appointments after the main treatment period ends. All the details will be discussed with you before you make any decisions.