OB/ PHARM | Nifedipine

Overview & Mechanism of Action Nifedipine is an oral dihydropyridine calcium-channel blocker primarily used to treat hypertension, variant (Prinzmetal's) angina, and chronic stable angina. It works by inhibiting the influx of extracellular calcium through the membrane pores of myocardial and vascular smooth muscle cells, effectively plugging the channels. Unlike other calcium channel blockers (such as verapamil and diltiazem) that act evenly on the SA/AV nodes and vasculature, nifedipine acts predominantly as a potent peripheral vasodilator with negligible effects on AV nodal conduction. This vasodilation decreases total peripheral resistance, systemic blood pressure, and afterload. For variant angina, it increases myocardial oxygen supply by inducing coronary vasodilation; for chronic stable angina, it decreases myocardial oxygen demand by reducing afterload.

Administration Guidelines Proper administration is critical for nifedipine's safety and efficacy:

• Oral Dosing: Immediate-release capsules and sustained-release tablets must be swallowed whole; they should never be crushed, broken, or chewed. Patients should be advised that empty tablet shells may appear in their stool, which is normal and not clinically significant. While regular doses can be taken with meals to prevent gastric irritation, the specific formulation Adalat CC should be taken on an empty stomach.
• Formulation Interchangeability: Two 30 mg Adalat CC tablets can be safely interchanged with one 60 mg tablet. However, three 30 mg tablets are strictly not interchangeable with a single 90 mg tablet, as this combination results in a substantially higher peak plasma concentration (Cmax).
• Sublingual Route Warning: Sublingual administration of nifedipine liquid to rapidly lower blood pressure (such as in hypertensive urgency) has generated controversy, is generally not recommended, and offers no bioavailability advantage over swallowing the capsules whole.
• Grapefruit Interaction: Patients must avoid grapefruit juice before or after drug administration. Co-administration increases nifedipine's peak plasma concentrations and overall exposure (AUC) by two-fold.

Pharmacokinetics Nifedipine is rapidly and well-absorbed (90%) but undergoes extensive first-pass metabolism in the liver, resulting in an absolute bioavailability of 50% to 70%. It is highly protein-bound (92% to 98%) and undergoes rapid hepatic metabolism into inactive metabolites. Because it is a CYP3A4 substrate, its metabolism can be significantly altered by CYP3A4 inhibitors or inducers. The drug has an elimination half-life of roughly 2 to 5 hours. While immediate-release capsules peak within 30 minutes to 2 hours, sustained-release tablets peak at around 6 hours, allowing for convenient once-daily dosing.

Adverse Reactions Due to its potent vasodilatory effects, the most common mild-to-moderate side effects include peripheral edema (4.0-30.0%), dizziness (4.0-27.0%), flushing (0-25.0%), and headaches (10.0-23.0%). Severe, though less common, adverse reactions can include heart failure, myocardial infarction, arrhythmias, gastrointestinal bleeding, and Stevens-Johnson syndrome.