Clinical pharmacokinetics and pharmacodynamics of aliskiren.
Aliskiren is the first orally bioavailable direct renin inhibitor approved for the treatment of hypertension. It acts at the point of activation of the renin-angiotensin-aldosterone system, or renin system, inhibiting the conversion of angiotensinogen to angiotensin I by renin and thereby reducing the formation of angiotensin II by angiotensin-converting enzyme (ACE) and ACE-independent pathways. Aliskiren is a highly potent inhibitor of human renin in vitro (concentration of aliskiren that produces 50% inhibition of renin 0.6 nmol/L). Aliskiren is rapidly absorbed following oral administration, with maximum plasma concentrations reached within 1-3 hours. The absolute bioavailability of aliskiren is 2.6%. The binding of aliskiren to plasma proteins is moderate (47-51%) and is independent of the concentration. Once absorbed, aliskiren is eliminated through the hepatobiliary route as unchanged drug and, to a lesser extent, through oxidative metabolism by cytochrome P450 (CYP) 3A4. Unchanged aliskiren accounts for approximately 80% of the drug in the plasma following oral administration, indicating low exposure to metabolites. The two major oxidized metabolites of aliskiren account for less than 5% of the drug in the plasma at the time of the maximum concentration. Aliskiren excretion is almost completely via the biliary/faecal route; 0.6% of the dose is recovered in the urine. Steady-state plasma concentrations of aliskiren are reached after 7-8 days of once-daily dosing, and the accumulation factor for aliskiren is approximately 2. After reaching the peak, the aliskiren plasma concentration declines in a multiphasic fashion. No clinically relevant effects of gender or race on the pharmacokinetics of aliskiren are observed, and no adjustment of the initial aliskiren dose is required for elderly patients or for patients with renal or hepatic impairment. Aliskiren showed no clinically significant increases in exposure during coadministration with a wide range of potential concomitant medications, although increases in exposure were observed with P-glycoprotein inhibitors. Aliskiren does not inhibit or induce CYP isoenzyme or P-glycoprotein activity, although aliskiren is a substrate for P-glycoprotein, which contributes to its relatively low bioavailability. Aliskiren is approved for the treatment of hypertension at once-daily doses of 150 mg and 300 mg. Phase II and III clinical studies involving over 12,000 patients with hypertension have demonstrated that aliskiren provides effective long-term blood pressure (BP) lowering with a good safety and tolerability profile at these doses. Aliskiren inhibits plasma renin activity (PRA) by up to 80% following both single and multiple oral-dose administration. Similar reductions in PRA are observed when aliskiren is administered in combination with agents that alone increase PRA, including diuretics (hydrochlorothiazide, furosemide [frusemide]), ACE inhibitors (ramipril) and angiotensin receptor blockers (valsartan), despite greater increases in the plasma renin concentration. Moreover, PRA inhibition and BP reductions persist for 2-4 weeks after stopping treatment, which is likely to be of benefit in patients with hypertension who occasionally miss a dose of medication. Preliminary data on the antiproteinuric effects of aliskiren in type 2 diabetes mellitus suggest that renoprotective effects beyond BP lowering may be possible. Further studies to evaluate the effects of aliskiren on cardiovascular outcomes and target organ protection are ongoing and will provide important new data on the role of direct renin inhibition in the management of hypertension and other cardiovascular disease.
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