Concomitant diuretic therapy
As with other ACE inhibitors, patients on diuretics, especially those on recently instituted diuretic therapy, may occasionally experience an excessive reduction of blood pressure after initiation of therapy with quinapril hydrochloride tablets. The possibility of hypotensive effects with quinapril hydrochloride tablets may be minimized by either discontinuing the diuretic or cautiously increasing salt intake prior to initiation of treatment with quinapril hydrochloride tablets. If it is not possible to discontinue the diuretic, the starting dose of quinapril should be reduced (see DOSAGE AND ADMINISTRATION).
Agents increasing serum potassium
Quinapril can attenuate potassium loss caused by thiazide diuretics and increase serum potassium when used alone. If concomitant therapy of quinapril hydrochloride tablets with potassium-sparing diuretics (eg, spironolactone, triamterene, or amiloride), potassium supplements, or potassium-containing salt substitutes is indicated, they should be used with caution along with appropriate monitoring of serum potassium (see PRECAUTIONS).
Tetracycline and other drugs that interact with magnesium
Simultaneous administration of tetracycline with quinapril hydrochloride tablets reduced the absorption of tetracycline by approximately 28% to 37%, possibly due to the high magnesium content in quinapril hydrochloride tablets. This interaction should be considered if coprescribing quinapril hydrochloride and tetracycline or other drugs that interact with magnesium.
Increased serum lithium levels and symptoms of lithium toxicity have been reported in patients receiving concomitant lithium and ACE inhibitor therapy. These drugs should be coadministered with caution and frequent monitoring of serum lithium levels is recommended. If a diuretic is also used, it may increase the risk of lithium toxicity.
Drug interaction studies of quinapril hydrochloride with other agents showed:
- Multiple dose therapy with propranolol or cimetidine has no effect on the pharmacokinetics of single doses of quinapril hydrochloride tablets.
- The anticoagulant effect of a single dose of warfarin (measured by prothrombin time) was not significantly changed by quinapril coadministration twice-daily.
- Quinapril hydrochloride tablet treatment did not affect the pharmacokinetics of digoxin.
- No pharmacokinetic interaction was observed when single doses of quinapril hydrochloride and hydrochlorothiazide were administered concomitantly.
- Co-administration of multiple 10 mg doses of atorvastatin with 80 mg of quinapril hydrochloride resulted in no significant change in the steady-state pharmacokinetic parameters of atorvastatin.