N α-Boc-l-Asp (OBn)-l-Lys (Z)-O t Bu (reversin 121, 1), an inhibitor of the P-gp ABC transporter, was used to conceive compounds inhibiting the drug efflux occurring through the Hoechst 33342 and daunorubicin transport sites of P-gp, respectively H and R sites. Replacement of the aspartyl residue by trans-4-hydroxy-l-proline (4 (R) Hyp) gave compounds 11 and 15 characterized by half-maximal inhibitory concentrations (IC50) of ...
