Which of the following drugs is not an inhibitor of P - glycoprotein?

Ans. is 'd' i.e., Phenobarbitone P-glycoprotein* P-glycoprotein (P gp) is a product of multidrug resistance 1 gene (ABCB1).* P-glycoprotein plays an important role in pharmacokinetics of drugs.* P-glycoprotein is an ATP-binding cassette (ABC) transporter and is an important factor to limit membrane permeability in several tissues and/or elimination pathways into urine (renal tubles) and bile (liver).* P-glycoprotein has following effects on drugs which are substate for P-gp-i) Decreasing the absorption of drugs and toxins in intenstine by efflux transport from enterocyts, i.e., exsorption of drug.ii) Decreasing the distribution of drug in brain by efflux transport at BBBiii) Excretion of drug by kidney (renal tubular secretion) and liver (secretion into bile)iv) Multiple drug resistance of a cancer by export (explusion) of anticancer drugs from the cells.* It is worth noting that in the intestine (enterocytes) both P-glycoprotein and CYP3A4 (A drug metabolizing enzyme also present in liver) are found. Both of these have same effect, i.e., decreasing the absorption of drug.i) P-glycoprotein - by efflux of drug from enterocytes into the lumen.ii) CYP3A4 - by metabolizing the drug into the enterocytes, so that less drug is available for absortion into the circulation.* Some drugs are substrate for both CYP3A4 and P-gp. Examples :i) CCBs: Verapamil, diltiazamii) Anticancer drugs: Etoposide, daunorubicin, doxorubicin, paclitaxel, vincrisitineiii) Antimicrobials: HIV protoease inhibitors (indinavir, ritonavir), erythromycin, ketoconazoleiv) Immunosuppressants: Cyclosporine, tacrolimus, sirolimus.v) Other: Digoxin, fexofenadine, loperamideInibitors of P - glycoprotein* Verapamil* Quinidine* ErythromycinInducers of P - glycoprotein* Phenobarbitone* Rifampicin