While early stage and involving a relatively small patient population, the interim phase Ib readout from the combination cohort testing estrogen receptor (ER)-targeting candidate vepdegestrant in combination with CDK4/6 inhibitor Ibrance (palbociclib) in heavily pretreated patients with ER-positive/HER2-negative breast cancer was impressive enough to prompt partners Arvinas Inc. and Pfizer Inc. to expand development work on the program. The results also struck a chord on the Street, with shares of Arvinas (NASDAQ:ARVN) gaining 31% on the day.