QUINIDINE IS METABOLISED MAINLY BY THE CYTOCHROME P450 ISOENZYME CYP3A4 & INTERACTIONS MAY OCCUR THROUGH INHIBITION OR ENHANCEMENT OF HEPATIC METABOLISM. CONCOMITANT USE OF DIGOXIN MAY LEAD TO MARKEDLY INCREASED CONCENTRATION OF DIGOXIN. USE WITH OTHER ANTIARRHYTHMIC DRUGS SHOULD BE AVOIDED. RIFAMPICIN, PHENOBARBITONE, PHENYTOIN MAY INCREASE ITS METABOLISM. DRUGS INCREASING URINE PH LIKE SOD BICARBONATE, POT CITRATE, SOME ANTACIDS AND CARBONIC ANHYDRASE INHIBITORS INHIBITS ITS EXCRETION AND HENCE INCREASE ITS PLASMA CONCENTRATION. HIV PROTEASE INHIBITORS NELFINAVIR, RITONAVIR MAY PRODUCE TOXIC CONCENTRATION OF QUINIDINE WHEN GIVEN CONCOMITANTLY. QUINIDINE CAN AFFECT OTHER DRUGS LIKE ORAL ANTICOAGULANTS, ANTIHYPERTENSIVES, ANTIMUSCARINICS, BETABLOCKERS, MUSCLE RELAXANTS & PARASYMPATHOMIMETICS.
ANTIARRHYTHMICS : - QUINIDINE ALSO INHIBITS CYTOCHROME P-450 ISOENZYME CYP2D6 & MAY AFFECT DRUGS METABOLISED BY THIS ROUTE. AMIODARONE INCREASES ITS PLASMA CONC. VERAPAMIL WHEN GIVEN I.V. MAY CAUSE SEVERE HYPOTENSION & MAY INCREASE THE PLASMA CONC. OF QUINIDINE. QUINIDINE MAY INCREASE PLASMA CONC. OF OTHER ANTIARRHYTHMICS LIKE DIGOXIN, AJMALINE, DISOPYRAMIDE, FLECAINIDE & PROPAFENONE .
ANTIBIOTICS :- ERYTHROMYCIN MAY INHIBITS HEPATIC METABOLISM OF QUINIDINE
ANTIFUNGALS :- KETOCONAZOLE, ITRACONAZOLE INHIBIT HEPATIC METABOLISM THUS INCREASING ITS PLASMA CONC.
BETABLOCKERS :- MAY CAUSE SINUS BRADYCARDIA AND/ OR HYPOTENSION WHEN CONCOMITANTLY USED
CALCIUM CHANNEL BLOCKERS :- NIFEDIPINE REDUCES PLASMA CONC.
CIMETIDINE INHIBITS ITS HEPATIC METABOLISM , SO RAISES ITS PLASMA CONC AND PLASMA HALF LIFE.