Possible Interaction: Quinidine and Verapamil

“Since an increasing number of patients is likely to be treated concurrently with digoxin, quinidine and a calcium channel blocker, a study was done to show whether coadministered quinidine and verapamil would cooperate to elevate the SDC.”

“and the combination verapamil 80 mg plus quinidine base 160 mg t.i.d.),”

“For both models, quinidine administration decreased population digoxin oral clearance by approximately 45% and verapamil therapy decreased population digoxin oral clearance by approximately 30%.”

“ Quinidine has been the first line drug for many years and has been proven to be highly effective especially when combined with Verapamil.”

“Pretreatment with verapamil , 80 mg and 120 mg (every 8 hours for 3 days), reduced the oral clearance of quinidine from 17.0 L/hr to 11.6 and 11.3 L/hr, respectively (P < 0.01).”

“The half‐life of quinidine was also significantly prolonged by verapamil.”

“These results indicate that verapamil impairs the metabolism of quinidine to 3‐hydroxyquinidine and reduces the oral clearance of quinidine to a degree that could be of clinical significance given the narrow therapeutic index of this drug.”

“With MDCKII-WT cells, Papp of quinidine dropped from 2 to 1, in the presence of verapamil.”

“The presence of 2 μM verapamil had no effect on mefloquine resistance; however, the presence of verapamil modulated the activities of QN and QD in a manner parallel to that observed for CQ.”

“ Quinidine (2 x 10(-5) M) blocked and verapamil (10(-5) M) reduced hyperpolarization responses, suggesting involvement of Ca(2+)-dependent K+ channels.”

“In healthy subjects, VE increases quinidine t1/2 and markedly decreases its metabolic clearance.”

“Recent clinical work has questioned the safety of a combined therapy of oral quinidine and intravenous verapamil , because some patients were reported to react with severe hypotension probably due to drug interactions with vascular alpha-adrenergic receptors.”

“Thus, recent work has questioned the safety of combined therapy with β-blockers and verapamil,5 , 6 and occasional reports have described catastrophic results.7 , 8 We describe here three patients who became hypotensive after receiving oral quinidine and intravenous verapamil.”