Aspirin 25 mg, Dipyridamole 200 mg (ER) Hard-gelatin capsules

Aspirin inhibits cyclooxygenase and reduces prostaglandin production. The cited product information links this pathway to analgesic, antipyretic, anti-inflammatory and platelet effects.

For the cited dispersible-tablet reference, plasma peaks occur about 20–30 minutes after dissolution and overall bioavailability is approximately 70%. Aspirin is converted to salicylic acid, then to further metabolites that are eliminated through the kidneys. Reported half-lives are about 15–20 minutes for aspirin and 2–3 hours for salicylic acid. These estimates do not describe enteric-coated absorption or salicylate overdose.

The antithrombotic action of aspirin and extended-release dipyridamole capsules is the result of the additive antiplatelet effects of dipyridamole and aspirin.

Dipyridamole — Dipyridamole inhibits the uptake of adenosine into platelets, endothelial cells and erythrocytes in vitro and in vivo; the inhibition occurs in a dose-dependent manner at therapeutic concentrations (0.5 to 1.9 mcg/mL). This inhibition results in an increase in local concentrations of adenosine which acts on the platelet A2-receptor thereby stimulating platelet adenylate cyclase and increasing platelet cyclic-3',5'-adenosine monophosphate (cAMP) levels. Via this mechanism, platelet aggregation is inhibited in response to various stimuli such as platelet activating factor (PAF), collagen and adenosine diphosphate (ADP).

Dipyridamole inhibits phosphodiesterase (PDE) in various tissues. While the inhibition of cAMP-PDE is weak, therapeutic levels of dipyridamole inhibit cyclic-3',5'guanosine monophosphate-PDE (cGMP-PDE), thereby augmenting the increase in cGMP produced by EDRF (endothelium-derived relaxing factor, now identified as nitric oxide).

Aspirin — Aspirin inhibits platelet aggregation by irreversible inhibition of platelet cyclooxygenase and thus inhibits the generation of thromboxane A2, a powerful inducer of platelet aggregation and vasoconstriction.

There are no significant interactions between aspirin and dipyridamole. The kinetics of the components are unchanged by their co-administration as aspirin and extended-release dipyridamole capsules.

Dipyridamole — Peak plasma levels of dipyridamole are achieved 2 hours (range 1 to 6 hours) after administration of a daily dose of 400 mg aspirin and extended-release dipyridamole capsules (given as 200 mg BID). The peak plasma concentration at steady-state is 1.98 mcg/mL (1.01 to 3.99 mcg/mL) and the steady-state trough concentration is 0.53 mcg/mL (0.18 to 1.01 mcg/mL).

Aspirin — Peak plasma levels of aspirin are achieved 0.63 hours (0.5 to 1 hour) after administration of a 50 mg aspirin daily dose from aspirin and extended-release dipyridamole capsules (given as 25 mg BID). The peak plasma concentration at steady-state is 319 ng/mL (175 to 463 ng/mL). Aspirin undergoes moderate hydrolysis to salicylic acid in the liver and the gastrointestinal wall, with 50% to 75% of an administered dose reaching the systemic circulation as intact aspirin.

Dipyridamole — When aspirin and dipyridamole extended-release capsules were taken with a high fat meal, dipyridamole peak plasma levels (Cmax) and total absorption (AUC) were decreased at steady-state by 20 to 30% compared to fasting. Due to the similar degree of inhibition of adenosine uptake at these plasma concentrations, this food effect is not considered clinically relevant.

Aspirin — When aspirin and extended-release dipyridamole capsules were taken with a high fat meal, there was no difference for aspirin in AUC at steady-state, and the approximately 50% decrease in Cmax was not considered clinically relevant based on a similar degree of cyclooxygenase inhibition comparing the fed and fasted state.

Dipyridamole — Dipyridamole is highly lipophilic (log P=3.71, pH=7); however, it has been shown that the drug does not cross the blood-brain barrier to any significant extent in animals. The steady-state volume of distribution of dipyridamole is about 92 L. Approximately 99% of dipyridamole is bound to plasma proteins, predominantly to alpha 1-acid glycoprotein and albumin.

Aspirin — Aspirin is poorly bound to plasma proteins and its apparent volume of distribution is low (10 L). Its metabolite, salicylic acid, is highly bound to plasma proteins, but its binding is concentration-dependent (nonlinear). At low concentrations (<100 mcg/mL), approximately 90% of salicylic acid is bound to albumin. Salicylic acid is widely distributed to all tissues and fluids in the body, including the central nervous system, breast milk, and fetal tissues. Early signs of salicylate overdose (salicylism), including tinnitus (ringing in the ears), occur at plasma concentrations approximating 200 mcg/mL [see Overdosage (10)].

Dipyridamole — Dipyridamole is metabolized in the liver, primarily by conjugation with glucuronic acid, of which monoglucuronide which has low pharmacodynamic activity is the primary metabolite. In plasma, about 80% of the total amount is present as parent compound and 20% as monoglucuronide. Most of the glucuronide metabolite (about 95%) is excreted via bile into the feces, with some evidence of enterohepatic circulation. Renal excretion of parent compound is negligible and urinary excretion of the glucuronide metabolite is low (about 5%). With intravenous (i.v.) treatment of dipyridamole, a triphasic profile is obtained: a rapid alpha phase, with a half-life of about 3.4 minutes, a beta phase, with a half-life of about 39 minutes, (which, together with the alpha phase accounts for about 70% of the total area under the curve, AUC) and a prolonged elimination phase λz with a half-life of about 15.5 hours. Because of the extended absorption phase of the dipyridamole component, only the terminal phase is apparent from oral treatment with aspirin and extended-release dipyridamole capsules which was 13.6 hours.