Cilazapril is a pyridazine ACE inhibitor. It competes with angiotensin I for binding at the angiotensin-converting enzyme, blocking the conversion of angiotensin I to angiotensin II. As angiotensin II is a vasoconstrictor and a negative feedback mediator for renin activity, lower angiotensin II levels results in a decrease in blood pressure, an increase in renin activity, and stimulation of baroreceptor reflex mechanisms. Kininase II, an enzyme which degrades the vasodilator bradykinin, is identical to ACE and may also be inhibited.
Cilazapril inhibits the production angiotensin II. By doing so, it decreases sodium and water reabsorption (via aldosterone) and it decreases vasoconstriction. The combined effect of this is a decrease in vascular resistance, and therefore, blood pressure.