 Part A: This is a phase Ib trial combining the CDK46 inhibitor palbociclib with the PI3K inhibitors taselisib, or pictilisib. There are two treatment arms during the dose escalation phase where patients will receive either taselisib OR pictilisib in combination with palbociclib. Palbociclib, taselisib and pictilisib can all be given orally once daily with food, in a 21-days-on and 7-days-off schedule. Once the MTD is reached, the combination with the optimum safety and PKPD profile will be taken forward to the dose expansion phase Part B. 