Intestinal absorption studies on peptide mimetic α-methyldopa prodrugs

Two dipeptide mimetic prodrugs, 1 and 2, and two tripeptide mimetic prodrugs, 3 and 4, of L-α-methyldopa were evaluated for intestinal absorption by in-situ single-pass rat jejunal perfusion studies and by in-vitro uptake experiments in brush-border membrane vesicles (BBMVs) prepared from rat intestine. In the perfusion studies, compound 1 demonstrated a 3.5-fold increase in permeability (P(m)(*) = 2.27) as compared with that of α-methyldopa (P(m)(*) = 0.65), indicating that this prodrug was better absorbed in the intestine than its parent drug. Other prodrugs showed no significant improvement in intestinal permeability. The results correlated with the results of BBMV uptake studies. In the presence of an inward proton gradient, compound 1 showed Michaelis-Menton saturable kinetics of BBMV uptake with a low value of Km (0.06 ± 0.13 mM) and a high value of V(max)/K(m) (36.38 nmol (mg protein)^-1/30 s mM^-1) at a low concentration range and a linear uptake at high concentrations with K(d) 0.14 ± 0.02 mM. Compounds 2 and 3 were mainly taken up in BBMVs via passive diffusion. Compound 4 was taken up in BBMVs basically via the carrier-mediated transport system, while the rate of uptake was much lower than that of compound 1. The uptake of compounds 1 and 4 was significantly inhibited by dipeptides L-Gly-L-Pro and L-Gly-L-Phe, and cephradine, a β-lactam known to be transported via the dipeptide carrier system, indicating that both compounds were taken up in BBMVs via the H⁺-coupled dipeptide-mediated transport system. In contrast to the complicated uptake profile of α-methyldopa, the higher rate of BBMV uptake with less variation demonstrated on compound 1 suggested that the attached nonessential amino acid moiety, D-phenylglycine, is a feasible delivery tool in carrying the parent drug through the intestine.