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Although stapled α-helical peptides can address challenging targets, their advancement is impeded by poor understandings for making them cell permeable while avoiding off-target toxicities. By synthesizing >350 molecules, we present workflows for identifying stapled peptides against Mdm2(X) with in vivo activity and no off-target effects. Key insights include a clear correlation between lipophilicity and permeability, removal of positive charge to avoid off-target toxicities, judicious anionic residue placement to enhance solubility/behavior, optimization of C-terminal length/helicity to enhance potency, and optimization of staple type/number to avoid polypharmacology. Workflow application gives peptides with >292x improved cell proliferation potencies and no off-target cell proliferation effects (> 3800x on-target index). Application of these 'design rules' to a distinct Mdm2(X) peptide series improves (> 150x) cellular potencies and removes off-target toxicities. The outlined workflow should facilitate therapeutic impacts, especially for those targets such as Mdm2(X) that have hydrophobic interfaces and are targetable with a helical motif. Stapled α-helical peptides are promising for targeting challenging targets such as transcription factors, but achieving sufficient cell permeability while avoiding off-target cleavage is difficult. Here, the authors present workflows for identifying stapled peptides against Mdm2(X) with in vivo activity and no off-target effects based on comprehensive investigations of their properties. [ABSTRACT FROM AUTHOR]