Inhibition of intracellular protein-protein interactions (PPIs) involved in cancer pathways is gaining attention as a potential strategy for development of more specific anticancer drugs. In particular, inhibition of MDMX:p53 and MDM2:p53 interactions are of great interest due to their involvement in the development of different type of cancers, including breast and skin cancer. The transcription factor p53 is a tumor suppressor that induces apoptosis of damaged cells. In healthy cells p53 activity is regulated and inhibited by the proteins MDM2 and MDMX, but some cancer cells overexpress MDM2 and/or MDMX and inactivate p53, which facilitate tumor development. A 12-mer dual inhibitor peptide (pDI) was previously reported to be able to target and inhibit MDMX:p53 and MDM2:p53 interactions with nanomolar potency in vitro, but possesses low cell penetrating properties. In the current study we were specifically interested in designing pDI analogues optimised for inhibition of MDMX:p53 and MDM2:p53 interactions, but also with ability to enter into cells so they can reach the intracellular target. Thus, we produced a series of constrained pDI analogues featuring lactam bridges staples that stabilize the bioactive helical conformation and increase the ability to compete for MDM2:p53 and MDMX:p53 interactions and have grafted them into stable scaffolds with cell penetrating properties. We compared all the peptides on their stability, inhibitory potency, toxicity, and ability to enter into cancer cells. Overall, we designed peptides that are stable and are potent inhibitors of MDM2:p53 and MDMX:p53 interactions. In addition, we found that the potency against cancer cells is dependent on the scaffold and the staple strategy, which determine the internalization mechanism and the amount of peptide that reaches the intracellular target. We believe that our results will be useful for the improvement and design of peptide-leads for inhibition of other PPIs involved in cancer pathways.