The interaction of phenylbutazone and diazepam with smectite were studied in in-vivo and in-vitro. The kinetics of both drugs were investigated in healthy subjects after oral administration as monotherapy or in association with smectite. Smectite did not substantially alter the kinetics of phenylbutazone, whereas the peak plasma concentration of diazepam was reduced to 91%, and the time of peak concentration was prolonged by 153% of the control values. The in-vitro investigations were conducted at pH 5.5 and 8 and showed that there was no interaction between phenylbutazone and smectite, but that it adsorbed diazepam. The findings suggest that smectite delays the absorption of basic drugs and does not alter the absorption kinetics of acidic drugs.

Surfactants are classically used to improve the solubilization of lipophilic drugs such as digoxin. Polysorbate 80 and Cremophor EL (polyoxyl 35 castor oil) are such surfactants but they may also modulate the action of P-glycoprotein, an energy-dependent "counter-transport" system implicated in the phenomenon of multidrug resistance in cancer cells. P-glycoprotein is also present in the intestine on the apical membrane of mature enterocytes and can potentially reduce the absorption of a wide range of drugs. In this study, using the improved everted gut sac method, the effects of Polysorbate 80, Cremophor EL and cyclosporin on the absorption of digoxin were studied. An increase in the uptake of digoxin in the presence of these three products could be shown with our in vitro model. Cremophor EL and Polysorbate 80 had no toxic effects at the concentrations used. These results suggest that surfactants such as Cremophor EL and Polysorbate 80 should not only support solubilization but can also modulate the P-glycoprotein system to improve the bioavailability of poorly absorbed drugs.

1 The pharmacokinetics of atenolol, after 200 mg orally, were studied in 18 patients with terminal renal insufficiency (creatinine clearance less than 5 ml/min), of whom twelve were being treated by chronic dialysis. 2 The peak plasma level, 1.59 +/‐ 0.43 mg/l, was reached in 4.7 +/‐ 2.1 h. 3 Without dialysis treatment, the apparent plasma half‐ life of atenolol was greatly increased (73.4 +/‐ 28.8 /). During dialysis, it dropped to 7.5 +/‐ 3.7 h but returned to 51.2 +/‐ 17.3 h after dialysis. The plasma atenolol plot was a rising slope for a few hours after the end of dialysis. 4 Renal clearance of atenolol was very low (4.6 +/‐ 1.5 ml/min). 5 Plasma clearance during dialysis was 42.6 +/‐ 21.3 ml/min for a mean blood flow‐rate of 236 +/‐ 25 ml/min through a cuprophane membrane dialyser. 6 These results suggest that dosage should be modified for these patients.

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