Co-phenotrope slightly reduced rate, but not extent, of absorption of a single dose of quinidine.
Clinical evidence,mechanism, importance and management
In one study,8 healthy subjects were given a single 300mg dose of quinidine sulfate alone and after taking two tablets of co-phenotrope (atropine sulfate 25 micrograms, diphenoxylate 2.5 mg; Lomotil) at midnight on evening before and another two tablets next morning an hour before quinidine (See reference number 1). It was found that maximum plasma quinidine levels were reduced by 21 % from 2.1 to 1.65 micrograms/mL by co-phenotrope, time to maximum level was prolonged from 0.89 to 1.21 hours,and there was a slight increase in elimination half-life from 5.7 to
6.8 hours. While these results were statistically significant, changes
were relatively small and it seems doubtful if they are clinically relevant, particularly as extent of absorption was unchanged. However it needs to be emphasised that because quinidine formulation used was an immediate-release preparation, these results may not necessarily apply to sustained-release preparations, and also may not apply if multiple doses of quinidine are used.
1. Ponzillo JJ,Scavone JM, Paone RP, Lewis GP, Rayment CM, Fitzsimmons WE. Effect ofdiphenoxylate with atropine sulfate on the bioavailability of quinidine sulfate in healthy subjects. Clin Pharm (1988) 7, 139–42.