Effect of stereochemistry on the transport of Aca-linked β-turn peptidomimetics across a human intestinal cell line Original Research Article

Transcellular transport is one of the most important barriers facing the development of new therapeutic agents. However, little is known about the specific effects of structure and particularly stereochemistry on cell permeability. An attractive in vitro model has been developed for the direct assessment of cell transport, using the immortalized human epithelial cell line, Caco-2. The present study assesses the effects of stereochemistry on transport in a commonly used β-turn model system. Thus, l,l- and l,d-Ala-Ala were cyclized with aminocaproic acid, resulting in macrocycles in which the dipeptides correspond to the i + 1 and i + 2 positions of a β-turn. The transport of these dipeptides across a Caco-2 cell monolayer was determined, along with corresponding acyclic models (l,l- and l,d-CH3CH2C(O)-Ala-Ala-n-Pr). The transport studies were carried out in the presence and absence of verapamil, a known inhibitor of the apically polarized efflux system present in Caco-2 cells. Both apical→basolateral and basolateral→apical transport were measured. Measurements made in the presence of verapamil showed that the cyclic peptides experienced a ca. 4–5-fold difference in intrinsic flux depending on stereochemistry, with the l,d isomer being transported at a higher rate. These differences disappeared in the acyclic cases examined (permeability coefficient ratios of the L, D/L, L isomers were 1.04–1.13). These observations are discussed in terms of the conformations and hydrogen-bonding characteristics of the compounds as determined by NMR spectroscopy.