Quinidine administration increases steady state serum digoxin concentration in horses.

The research investigates if the administration of quinidine increases the concentration of digoxin in the blood of horses, and the findings confirm it does, largely due to a reduction in the renal clearance of digoxin.

Objective and Procedure

• The aim of the study was to assess the influence of quinidine administration on serum digoxin concentration in horses, a question that arises from the recognized interaction between these two drugs in other species.
• To achieve this aim, digoxin was administered orally to six horses for seven consecutive days. This allowed for steady-state conditions for digoxin to be established. Steady-state status was confirmed once peak and trough serum digoxin concentrations remained statistically consistent over three days (Days 4, 5, and 6).
• On Day 6, serum digoxin concentration was measured at different time intervals following digoxin administration. On Day 7, the horses were administered quinidine orally according to a specific schedule and the serum digoxin concentration was again measured at the same time intervals as the day before.
• The study also monitored and measured the clearance rates of both creatinine and digoxin from the kidneys on both days (6 and 7).

Findings

• The results pointed towards a notable increase in the serum concentration of digoxin in the horses after the administration of quinidine. The mean concentration of digoxin approximately doubled 15 and 30 minutes after the administration of quinidine was initiated and this elevated concentration persisted for the next 12 hours.
• Concurrently, the rate of renal clearance (the process through which the kidneys remove substances) for both digoxin and endogenous creatinine decreased. However, the decrease in digoxin clearance was greater.
• Among all the horses, serum quinidine concentrations reached the therapeutic range in five out of the six horses.

Conclusion

• The findings support the hypothesis that the administration of quinidine increases steady-state serum digoxin concentration in horses. Furthermore, the study sheds light on the mechanism behind this change, suggesting it is correlated to a decrease in renal clearance of digoxin.
• Interestingly, this study found that some of the reduction in renal clearance is due to decreased glomerular filtration, a result that is not consistent with the findings in other species.