In what circumstance, if any, would you consider fulvestrant + capivasertib over CDK4/6 inhibitors in a patient who has HR+, PIK3CA mutant metastatic breast cancer?  

A PIK3CA mutation is prognostic but not predictive for response to a CDK4/6i. The only situation I would consider skipping over CDK4/6i therapy and doing fulvestrant+capi in the 1st line metastatic setting is if the patient had recurrence on adjuvant abema/ribo +AI and NGS showed a PIK3CA mutation. ...

Only in second-line with appropriate mutations as per NCCN guidelines.

If a patient had a PIK3CA mutation and was progressing on front-line endocrine therapy + a CDK4/6 inhibitor, I would treat with fulvestrant and capivasertib rather than fulvestrant and another CDK4/6 inhibitor in the second-line setting.