Purpose: Peptidomimetic thrombin inhibitors derived from Nalpha-(2-naphthylsulfonyl)-3-amidino-phenylalanine with different basic and acidic substituents were investigated with respect to their intestinal transport behavior.
Methods: Intestinal permeability coefficients were studied using Caco-2 monolayers and a reversed-phase HPLC method for quantitation.
Results: Apparent permeability coefficients Papp of compounds with a free amidino group were in general low (<10 x 10(-8) cm/s) and independent of the structure of the amide part (C-terminus). Polarized efflux, however, was strongly affected by substituents in the amide moiety yielding the following efflux ratios (ER): methylpiperidide (1) (ER 45) > piperidine carboxylic acid methylester (ER 6-11) > piperidine carboxylic acids (ER 1.9-2.9) > piperazide (ER -0.17). Efflux of (1) was temperature-dependent, but independent of the enantiomeric configuration, accompanied by an increase in transepithelial electrical resistance (TEER), and could be reduced by P-gp inhibitors (PSC 833, Cremophor EL) but not by indomethacin. Replacement of the amidine group of (1) by aminomethyl-, amino-, and oxamidine- moieties drastically increased absorptive permeability (46-68 fold) with ER < 3.4. In contrast, the oxamidine with a C-terminal nipecotic acid residue (8) displayed also a temperature dependent efflux- without altering TEER (ER 22). This efflux was sensitive to PSC833/Cremophor EL and indomethacin.
Conclusions: Basic and acidic residues of amidino-phenylalanine-derived thrombin inhibitors mediate affinity to intestinal efflux pumps. presumably P-gp and MRP. P-gp mediated efflux was related to a net positive charge and accompanied by an increased TEER. Among the methylpiperide (1) promoieties studied the oxamidino group seems to be very promising in overcoming both transport and efflux problems frequently encountered with peptidomimetics containing amidines.