A disease burden that makes the participant ineligible for endocrine therapy per the investigator's best judgment (e.g., symptomatic visceral disease that is potentially life-threatening in the short-term)

Malignancies other than breast cancer within five years prior to study treatment initiation (except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma or Stage I endometrioid uterine cancer).

With the exception of alopecia, any unresolved toxicities from prior therapy greater than CTCAE grade 1 at the time of starting study treatment.

Known abnormalities in coagulation such as bleeding diathesis, or treatment anticoagulants precluding intramuscular injections of fulvestrant or LHRH, if applicable.

Prior exposure to any chemotherapy or anti-cancer agents other than those specified in the protocol (e.g. hormonal therapy such as LHRH agonists) without appropriate washout period before randomization/enrollment, for example, randomization within 3 half-lives of a small molecule anti-cancer agent, or within 4 weeks for any antibody-based anticancer agents.

Concurrent use of herbal or natural products intended as treatment or prophylaxis for any type of cancer.

Radiotherapy within 2 weeks prior to the first dose of study intervention

Major surgical procedure (excluding placement of vascular access) or significant traumatic injury within 4 weeks of the first dose of study intervention or an anticipated need for major surgery during the study.

Strong inhibitors of CYP3A4 or strong/moderate inducers of CYP3A4 within 2 weeks prior to the first dose of capivasertib (3 weeks for St John's wort), Note that adequate washout or dose reduction may be required for some CYP3A substrates with a narrow therapeutic window prior to initiating capivasertib dosing.

Any concomitant medication that may interfere with fulvestrant, cetirizine, and loperamide safety and efficacy based on the prescribing information of fulvestrant, cetirizine, and loperamide and local clinical guidelines.

Clinically significant abnormalities of glucose metabolism as defined by any of the following:

1. Participants with diabetes mellitus type 1 or diabetes mellitus type 2 requiring insulin treatment.
2. HbA1c ≥8.0% (63.9 mmol/mol).

Spinal cord compression or brain metastases unless asymptomatic, treated and stable and not requiring steroids within four weeks prior to study treatment initiation.

Leptomeningeal metastases.

Inadequate bone marrow reserve or organ function as demonstrated by any of the following laboratory values:

1. Absolute neutrophil count <1.5 × 109 /L.
2. Platelet count <100 × 109 /L.
3. Hemoglobin <9 g/dL (<5.59 mmol/L). NOTE: any blood transfusion must be >14 days prior to the determination of a haemoglobin ≥9 g/dL (≥5.59 mmol/L).
4. Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) >2.5 times upper limit of normal (ULN) if no demonstrable liver metastases or >5 × ULN in the presence of liver metastases.
5. Total bilirubin >1.5 × ULN (participants with confirmed Gilbert's syndrome may be included in the study).
6. Creatinine >1.5 × ULN concurrent with creatinine clearance <50 mL/min (measured or calculated by Cockcroft and Gault equation); confirmation of creatinine clearance is only required when creatinine is >1.5 × ULN.

As judged by the investigator, any evidence of severe or uncontrolled systemic diseases, including uncontrolled hypertension, or active infection including hepatitis B, hepatitis C and human immunodeficiency virus (HIV). Participants with HIV that is controlled (not detectable viral load) with highly active antiretroviral therapy (HAART) are eligible to participate. Screening for chronic conditions is not required.

Prior treatment with AKT or PI3K inhibitors.

History of skin or gastrointestinal disorders that, in the investigator's opinion, gives reasonable suspicion of a disease or condition that contraindicates the use of capivasertib.

History of hypersensitivity to active or inactive excipients of capivasertib, fulvestrant and LHRH agonists (if applicable, i.e., concomitant LHRH agonist required in this study) or drugs with a similar chemical structure or class to capivasertib, fulvestrant or LHRH agonists (if applicable, i.e., concomitant LHRH agonist required in this study).

History of intolerability to cetirizine or loperamide.

Refractory nausea and vomiting, malabsorption syndrome, chronic gastrointestinal diseases, inability to swallow the formulated product or previous significant bowel resection, or other condition that would preclude adequate absorption of capivasertib.

Past medical history of interstitial lung disease, drug-induced interstitial lung disease, radiation pneumonitis which required steroid treatment, or any evidence of clinically active interstitial lung disease.

Evidence of dementia altered mental status or any psychiatric condition that would prohibit understanding or rendering of informed consent.