DESCRIPTION
Nifedipine extended-release tablets are an extended release tablet dosage form of the calcium channel blocker nifedipine. Nifedipine is dimethyl 1,4-dihydro-2,6-dimethyl-4-(o-nitrophenyl)-3,5-pyridinedicarboxylate, C17H18N2O6, and has the structural formula:
 

 

Nifedipine, USP is a yellow powder, practically insoluble in water but soluble in ethanol. It has a molecular weight of 346.33. Nifedipine extended-release tablets consist of an external coat and an internal core. Both contain nifedipine, the coat as a slow release formulation and the core as a fast release formulation. Nifedipine extended-release tablets, USP contain either 30 mg, 60 mg or 90 mg of nifedipine for once-a-day oral administration.
Inert ingredients in the formulation are: colloidal silicon dioxide, hydroxypropyl cellulose, hypromellose, lactose monohydrate, magnesium stearate, polydextrose, polyethylene glycol, titanium dioxide and triacetin. The 60 mg tablets also contain FD&C Yellow No. 6 Aluminum Lake and the 90 mg tablets also contain D&C Red No. 27, FD&C Blue No. 2 and FD&C Red No. 40.
USP Dissolution Test Pending.
CLINICAL PHARMACOLOGY
Nifedipine is a calcium ion influx inhibitor (slow-channel blocker or calcium ion antagonist) which inhibits the transmembrane influx of calcium ions into vascular smooth muscle and cardiac muscle. The contractile processes of vascular smooth muscle and cardiac muscle are dependent upon the movement of extracellular calcium ions into these cells through specific ion channels. Nifedipine selectively inhibits calcium ion influx across the cell membrane of vascular smooth muscle and cardiac muscle without altering serum calcium concentrations.
Mechanism of Action
The mechanism by which nifedipine reduces arterial blood pressure involves peripheral arterial vasodilatation and, consequently, a reduction in peripheral vascular resistance. The increased peripheral vascular resistance, an underlying cause of hypertension, results from an increase in active tension in the vascular smooth muscle. Studies have demonstrated that the increase in active tension reflects an increase in cytosolic free calcium.
Nifedipine is a peripheral arterial vasodilator which acts directly on vascular smooth muscle. The binding of nifedipine to voltage-dependent and possibly receptor-operated channels in vascular smooth muscle results in an inhibition of calcium influx through these channels. Stores of intracellular calcium in vascular smooth muscle are limited and thus dependent upon the influx of extracellular calcium for contraction to occur. The reduction in calcium influx by nifedipine causes arterial vasodilation and decreased peripheral vascular resistance which results in reduced arterial blood pressure.
Pharmacokinetics and Metabolism
Nifedipine is completely absorbed after oral administration. The bioavailability of nifedipine as nifedipine extended-release tablets relative to immediate release nifedipine is in the range of 84% to 89%. After ingestion of nifedipine extended-release tablets under fasting conditions, plasma concentrations peak at about 2.5 to 5 hours with a second small peak or shoulder evident at approximately 6 to 12 hours post dose. The elimination half-life of nifedipine administered as nifedipine extended-release tablets is approximately 7 hours in contrast to the known 2 hour elimination half-life of nifedipine a