Induction and Inhibition of Intestinal P-glycoprotein and Effects on Etoposide Absorption

Abstract

P-Glycoprotein (Pgp) actively pumps a number of antineoplastic drugs, such as etoposide, out of cancer cells and causes multidrug resistance. Pgp also expresses at the brush border membrane of the small intestine under the normal physiologic condition. We hypothesized that induction or inhibition of intestinal Pgp might increase or decrease the etoposide efflux from blood into the intestinal lumen and, therefore, decrease or increase the absorption of etoposide, respectively. Etoposide absorption was studied using an everted gut sac preparation of rat jejunum and ileum. In this study, we found that treating rats with sodium arsenite (10 μg/kg), sodium butyrate (0.5 mg/kg), or 3-methylcholanthrene (3-MC, 170 μg/kg) for 5 days decreased the intestinal absorption of etoposide in both jejunum and ileum. Western blotting with C219, a monoclonal antibody of Pgp, confirmed the increase of the intestinal Pgp in both ileum and jejunum. With reversed-transcription/polymerase chain reaction of rat mdrlb gene, we found that arsenite and butyrate induction increased mdrlb mRNA in the jejunum, whereas the inducing effect of arsenite, butyrate, or 3-MC in the ileum was not significant. Addition of C219 at 0.1 U/ml or 5'-adenylylimidodiphosphate at 0.2 N, a non-hydrolyzable ATP analog, increased the absorption of etoposide. Quinidine, an antiarrythmics, has been demonstrated to circumvent the multidrug resistance in cell lines, possibly by interfering with Pgp function. Adding quinidine at 1 mg/ml to the everted gut sac preparation also substantially increased the absorption of etoposide. In situ absorption of etoposide was also studied by intraluminal perfusion of the drug in anesthetized rats. Intravenous infusion of quinidine at either 1 or 2 mg/hr both increased serum level of etoposide. Such an increase was shown to be dose-dependent. Intravenous infusion of etoposide at 0.2 mg/hr resulted in luminal exsorption of the drug in small intestine. The intestinal clearance rate of etoposide was 41.7±7.2 ml/hr/kg. When etoposide was administered at the same infusion rate with 1 mg/hr of quinidine, the intestinal clearance of etoposide decreased to 18.4±3.9 ml/hr/kg. These in situ data confirmed that the intestinal Pgp mediated the efflux of etoposide and the use of Pgp-inhibiting agent such as quinidine may increase the intestinal absorption of etoposide. In addition to etoposide, the efflux of rhodamine 123, which is a common substrate used to measure Pgp function, in the everted gut sacs was also studied. Despite of the Pgp level increase in both ileum and jejunun shown in Western blotting, rhodamine 123 efflux increased only in the jejunum, but not in the ileum. The results suggest that there are multiple members of Pgp family in the rat small intestine. Rhodamine 123 was probably transported by a Pgp encoded by mdrlb, and etoposide was transported by another Pgp member.