Peptides whose uptake by cells is controllable

A pharmaceutical composition comprising:

• A cyclic molecule of the structure A-X1-B-X2, wherein
• B is a peptide portion of about 5 to about 20 basic amino acid residues, which is suitable for cellular uptake,
• A is a peptide portion of about 2 to about 20 acidic amino acid residues, which when linked with portion B is effective to inhibit or prevent cellular uptake of portion B;
• and
• A and B are linked together and linked via linkers X1 and X2 to form a cyclic compound;
• where
• X1 and X2 are each a cleavable linker of about 2 to about 100 atoms joining A with B to form a cyclic compound where each of X1 and X2 link to both A and B. and
• a pharmaceutically acceptable carrier.

A molecule for transporting a cargo moiety across a cell membrane of the structure A-X-B-C, wherein

• C is a portion comprising a cargo moiety,
• B is a peptide portion of about 5 to about 20 basic amino acid residues, which is suitable for cellular uptake, is covalently linked to portion C, and is effective to enhance transport of cargo portion C across a cell membrane,
• A is a peptide portion of about 2 to about 20 acidic amino acid residues, which when linked with portion B is effective to inhibit or prevent cellular uptake of B-C, and
• X is a cleavable linker of about 2 to about 100 atoms joining A with B-C, which can be cleaved under conditions in or near a target cell.