Abstract

The choroid plexus uptake of [³H]cefadroxil was studied in peptide transporter 2 (PEPT2) wild-type and null mice as a function of temperature, transport inhibitors, pH, and saturability. At normal pH (7.4) and temperature (37°C), the uptake of 1 μM cefadroxil was reduced by 83% in PEPT2^–/– mice as compared with PEPT2^+/+ mice (p < 0.001). A further reduction was achieved in null animals by reducing the temperature to 4°C, or by adding saturating concentrations of unlabeled cefadroxil or p-aminohippurate (p < 0.05). Glycylsarcosine coadministration could inhibit the uptake of cefadroxil in PEPT2^+/+ mice (p < 0.01) but not PEPT2^–/– mice. Although a proton-stimulated uptake of cefadroxil was demonstrated in PEPT2^+/+ mice (pH 6.5 versus pH 7.4; p < 0.01), no pH dependence was observed in PEPT2^–/– mice. Kinetic parameters for cefadroxil (without p-aminohippurate) in wild-type mice were: V_max = 5.4 pmol/mg/min, K_m = 34 μM, and K_d = 0.0069 μl/mg/min; in the presence of p-aminohippurate, the parameters were: V_max = 4.1 pmol/mg/min, K_m = 27 μM, and K_d = 0.0064 μl/mg/min. In null animals, the kinetic parameters of cefadroxil (without p-aminohippurate) were: V_max = 2.7 pmol/mg/min, K_m = 110 μM, and K_d = 0.0084 μl/mg/min; in the presence of p-aminohippurate, only a K_d = 0.010 μl/mg/min was observed. Based on kinetic and inhibitor analyses, it was determined that (under linear conditions), 80 to 85% of cefadroxil's uptake in choroid plexus is mediated by PEPT2, 10 to 15% by organic anion transporter(s), and 5% by nonspecific mechanisms. These findings demonstrate that PEPT2 is the primary transporter responsible for cefadroxil uptake in the choroid plexus. Moreover, the data suggest a role for PEPT2 in the clearance of peptidomimetics from cerebrospinal fluid.