Background: The combination of fulvestrant and one of the three approved CDK4/6 inhibitors (i) is the standard of care for patients with ER+/HER2 metastatic breast cancer progressing on first-line endocrine therapy, or relapsing on or within a year of completing adjuvant endocrine therapy. AKT inhibition combined with fulvestrant has recently demonstrated clinically important activity in patients with advanced ER+/HER2- breast cancer progressing on first-line CDK4/6i plus an AI. Monitoring ctDNA dynamics has potential utility as an early response evaluation tool: Lack of suppression of ctDNA at day 15 in patients treated with palbociclib and fulvestrant in the PALOMA-3 trial was predictive for shorter PFS. We designed FAIM to establish whether the addition of the AKT inhibitor, ipatasertib, improves PFS in patients with poor ctDNA suppression during cycle 1 of fulvestrant and CDK4/6i. Methods: FAIM is a phase 2 multi-centre, randomised, open-label superiority trial in patients with ER+/HER2- advanced breast cancer, recruiting at centres in the UK. Patients will undergo ctDNA testing at days 1 and 15 of cycle 1 fulvestrant and CDK4/6 inhibitor. Eligible patients must be aged ≥18, have ER+ (≥1% or Allred score 3/8 or greater) and HER2- advanced breast cancer, measurable disease or assessable bone disease (RECIST 1.1), adequate organ function, fasting glucose ≤150mg/dL and HbA1c ≤7.5% and be eligible for NHS treatment with fulvestrant and CDK4/6i. Patients with diabetes requiring insulin, significant cardiac disease, a history of pneumonitis, or prior exposure to CDK4/6i are excluded. The FoundationOne Liquid CDx assay will be used for ctDNA analysis, a pan-cancer, tumour-agnostic liquid biopsy test that uses error-corrected next-generation sequencing. Patients with poor ctDNA suppression at day 15 will be eligible for the randomised (minimisation) study. Up to 483 patients starting fulvestrant andCDK4/6i will enroll for ctDNA screening to allow 174 patients to enter the randomised study. The primary endpoint of the study is to compare PFS in patients randomised to palbociclib/fulvestrant +/- ipatasertib. Secondary endpoints include safety, overall survival and overall response rate in all randomised patients and PFS in the sub-group of patients with alterations in the PIK3CA/AKT1/PTEN pathway. One hundred patients with ctDNA suppression at day 15, and up to 50 patients with no detectable ctDNA at day 1, will form additional control groups for exploratory outcome comparison to patients with poor ctDNA suppression. Enrolment began in December 2022 and is anticipated to continue for 2 years (NCT04920708). Clinical trial information: NCT04920708.