The intracellular delivery of cargos via cell penetrating peptides (CPPs) holds significant promise as a drug delivery vehicle, but a major issue is their lack of cell type specificity, which can lead to detrimental off-target effects. We use an ADEPT-like concept to introduce conditional and selective activation of cellular uptake by using the lysine-rich, cationic, and amphiphilic L17E peptide as a model CPP. By masking the lysine residues of the L17E peptide with enzyme-cleavable acetyl protecting groups, the delivery of the covalently conjugated fluorophore TAMRA to HeLa cells was diminished. Recovery of cellular uptake could be achieved by deacetylation of the masked acetylated L17E peptide using the NAD-dependent sirtuin 2 (SirT2) deacetylase in vitro. Finally, trastuzumab-SirT2 and anti-B7H3-SirT2 antibody-enzyme conjugates were generated for the conditional and selective delivery of a cryptophycin cytotoxin by the L17E peptide. While the masked peptide still demonstrated some cytotoxicity, selective cell killing mediated by the antibody-enzyme conjugates was observed.