CARRIER MECHANISMS INVOLVED IN THE TRANSEPITHELIAL TRANSPORT OF BIS(POM)-PMEA AND ITS METABOLITES ACROSS CACO-2 MONOLAYERS

Purpose, To investigate the role of carrier mechanisms in: [1] the pol arized transport of the bis(pivaloyloxymethyl)- [bis(POM)-] ester prod rug of the antiviral agent 9-(2-phosphonylmethoxyethyl)adenine [PMEA] and [2] the directional secretion of its metabolites. Methods. Caco-2 monolayers were used to study the modulating effect of carriers on the transport of bis(POM)-PMEA and the efflux of intracellularly formed m etabolites mono(POM)-PMEA and PMEA from the cells. The interaction of bis(POM)-PMEA and its metabolites with;the efflux mechanisms present i n Caco-2 monolayers was investigated by testing the effect of various concentrations of verapamil (30, 100, 300 mu M) Or indomethacin (10-50 0 mu M) On transport and efflux.Results. Polarity in transport of bis( POM)-PMEA (50 mu M) across Caco-2 monolayers was noted: transport of t otal PMEA [=bis(POM)PMEA, mono(POM)-PMEA and PMEA] was significantly h igher in basolateral (BL) to apical (AP) direction (14.5 +/- 0.4%) tha n transport in the opposite (AP to BL) direction (1.7 +/- 0.2%). This difference was reduced in a concentration dependent way when verapamil (0-100 mu M) was included in both AP and BL incubation media. After l oading the cells with bis(POM)-PMEA (100 mu M) for I hr, studies on ef flux of PMEA and mono(POM)-PMEA from the Caco-2 monolayers over a 3 hr period, revealed that both metabolites were preferentially secreted t owards the AP compartment. Efflux of PMEA towards AP and BL compartmen ts amounted to 14.6 +/- 1.1% and 5.3 +/- 0.4%, respectively, of the in itial intracellular amount of total PMEA, while efflux of mono(POM)-PM EA towards AP and BL compartments was limited to 2.3 +/- 0.1% and 0.5 +/- 0.1%, respectively. When 10 mu M indomethacin was included in the AP incubation medium, efflux of PMEA was decreased to 7.8 +/- 0.3% and 3.3 +/-:0.3% towards the AP and BL compartments, respectively. The de crease in efflux by indomethacin was concentration-dependent up to 100 mu M. Transepithelial transport of total PMEA was also reduced in the presence of 30 mu M indomethacin, as reflected in smaller concentrati ons of PMEA and mono(POM)PMEA in the acceptor compartment, irrespectiv e of the transport direction. Conclusions. The data obtained in this s tudy suggest that bis(POM)-PMEA is substrate for a P-glycoprotein-like carrier mechanism in Caco-2 monolayers, while its metabolites mono(PO M)-PMEA and PMEA are transported by a non-P-glycoprotein efflux protei n.