In mice, the mdr1a and mdr1b genes encode drug-transporting proteins that can cause multidrug resistance in tumor cells by lowering intracellular drug levels. These P-glycoproteins are also found in various normal tissues such as the intestine. Because mdr1b P-glycoprotein is not detectable in the intestine, mice with a homozygously disrupted mdr1a gene [mdr1a(؊͞؊) mice] do not contain functional P-glycoprotein in this organ. We have used these mdr1a(؊͞؊) mice to study the effect of gut P-glycoprotein on the pharmacokinetics of paclitaxel. The area under the plasma concentration-time curves was 2-and 6-fold higher in mdr1a(؊͞؊) mice than in wild-type (wt) mice after i.v. and oral drug administration, respectively. Consequently, the oral bioavailability in mice receiving 10 mg paclitaxel per kg body weight increased from only 11% in wt mice to 35% in mdr1a(؊͞؊) mice. The cumulative fecal excretion (0-96 hr) was markedly reduced from 40% (after i.v. administration) and 87% (after oral administration) of the administered dose in wt mice to below 3% in mdr1a(؊͞؊) mice. Biliary excretion was not significantly different in wt and mdr1a(؊͞؊) mice. Interestingly, after i.v. drug administration of paclitaxel (10 mg͞kg) to mice with a cannulated gall bladder, 11% of the dose was recovered within 90 min in the intestinal contents of wt mice vs. <3% in mdr1a(؊͞؊) mice. We conclude that Pglycoprotein limits the oral uptake of paclitaxel and mediates direct excretion of the drug from the systemic circulation into the intestinal lumen.

We have used mice with a disrupted mdrla P‐glycoprotein gene (mdrla (—/—)mice) to study the role of P‐glycoprotein in the pharmacokinetics of digoxin, a model P‐glycoprotein substrate. [3H]‐digoxin at a dose of 0.2 mg kg−1 was administered as a single i.v. or oral bolus injection. We focussed on intestinal mucosa and brain endothelial cells, two major pharmacological barriers, as the mdrla P‐glycoprotein is the only P‐glycoprotein normally present in these tissues. Predominant faecal excretion of [3H]‐digoxin in wild‐type mice shifted towards predominantly urinary excretion in mdrla (—/—) mice. After interruption of the biliary excretion into the intestine, we found a substantial excretion of [3H]‐digoxin via the gut mucosa in wild‐type mice (16% of administered dose over 90 min). This was only 2% in mdrla (—/—) mice. Biliary excretion of [3H]‐digoxin was not dramatically decreased (24% in wild‐type mice versus 16% in mdrla (—/—) mice). After a single bolus injection, brain levels of [3H]‐digoxin in wild‐type mice remained very low, whereas in mdrla (—/—) mice these levels continuously increased over a period of 3 days, resulting in a ∼200 fold higher concentration than in wild‐type mice. These data demonstrate the in vivo contribution of intestinal P‐glycoprotein to direct elimination of [3H]‐digoxin from the systemic circulation and to the pattern of [3H]‐digoxin disposition, and they underline the importance of P‐glycoprotein for the blood‐brain barrier.

Enhanced drug accumulation in nonmalignant tissues after Pgp blockade should be carefully considered in future clinical trials of Pgp modulation.

P-glycoprotein is a large plasma membrane protein that can cause multidrug resistance in tumour cells by actively extruding substrate drugs out of the cell. These substrates include many anticancer drugs, such as vinca alkaloids, taxanes, epipodophyllotoxins and anthracyclines (reviewed in Endicott and Ling, 1989). The discovery that verapamil was able to reverse multidrug resistance in murine leukaemia cell lines (Tsuruo et al, 1981) initiated the search for reversal agents, which are compounds capable of blocking or inhibiting P-glycoprotein. A major concern for the clinical application of effective reversal agents are the potential consequences of inhibition of endogenous P-glycoprotein. To predict possible adverse effects of reversal agents and to gain more insight into the physiological role of endogenous P-glycoproteins, mice with homozygously disrupted P-glycoprotein genes have been generated at our institute (Schinkel et al, 1994.In humans, only one P-glycoprotein (MDR1) plays a role in multidrug resistance, whereas in mice both mdr1a and mdr1b P-glycoproteins are involved. The tissue distribution of these proteins suggests that the two murine isoforms together perform the same function as the single human MDR1 protein. The mdr1a gene is predominantly expressed in the intestines and in the capillaries of the brain and the testis, mdr1b is mainly expressed in the adrenal gland, pregnant uterus and ovarium. Significant levels of both mdr1a and mdr1b P-glycoprotein are present in liver, kidney, lung, heart and spleen (Cordon-Cardo et al, 1989;Croop et al, 1989). Based on the results of tissue distribution studies, it has been suggested that P-glycoprotein plays a role in the protection of the organism against potentially toxic agents, e.g. by limiting the absorption of orally ingested compounds, by mediating the elimination of substrates from the body and by protecting essential organs such as the brain and the testis against toxic substances in the circulation (Thiebaut et al, 1987;Cordon-Cardo et al, 1989). Recent studies confirmed that P-glycoprotein in the blood-brain barrier protects the brain against the entry of toxic compounds, whereas P-glycoprotein in the intestinal epithelium has been shown to limit the uptake of substrates from the intestinal lumen and to mediate their direct excretion from the bloodstream (Schinkel et al, 1994(Schinkel et al, , 1995Mayer et al, 1996;Sparreboom et al, 1997).To gain a detailed insight into the pharmacokinetic consequences of blocking P-glycoprotein in normal tissues, we previously performed a comprehensive analysis of the plasma pharmacokinetics, tissue distribution and excretion of vinblastine and its metabolites in wild-type and mdr1a(-/-) mice . However, it is of importance to obtain also comparable data on other widely used substrate drugs because it is likely that the impact of endogenous P-glycoprotein on the pharmacokinetics is substrate dependent. Here, we report on the comparative pharmacokinetics of doxorubicin and metabolites in wild-type and mdr1a(-/-...