In-vitro and in-vivo metabolism of different aspirin formulations studied by a validated liquid chromatography tandem mass spectrometry method

Low-dose aspirin (ASA) is used to prevent cardiovascular events. The most commonly used formulation is enteric-coated ASA (EC-ASA) that may be absorbed more slowly and less efficiently in some patients. To uncover these non-responders patients, the availability of proper analytical methods is pivotal in order to study the pharmacodynamics, the pharmacokinetics and the metabolic fate of ASA. We validated a high-throughput, isocratic reversed-phase, negative MRM, LC-MS/MS method useful for measuring circulating ASA and salicylic acid (SA) in blood and plasma. ASA-d4 and SA-d4 were used as internal standards. The method was applied to evaluate: (a) the in vitro ASA degradation by esterases in whole blood and plasma, as a function of time and concentration; (b) the in vivo kinetics of ASA and SA after 7 days of oral administration of EC-ASA or plain-ASA (100 mg) in healthy volunteers (three men and three women, 37-63 years). Parameters of esterases activity were V-max 6.5 +/- 1.9 and K-m 147.5 +/- 64.4 in plasma, and V-max 108.1 +/- 20.8 and K-m 803.2 +/- 170.7 in whole blood. After oral administration of the two formulations, t(max) varied between 3 and 6 h for EC-ASA and between 0.5 and 1.0 h for plain-ASA. Higher between-subjects variability was seen after EC-ASA, and one subject had a delayed absorption over eight hours. Plasma AUC was 725.5 (89.8-1222) for EC-ASA, and 823.1(624-1196) ng h/mL (median, 25-75% CI) for plain ASA. After the weekly treatment, serum levels of TxB(2) were very low (<10 ng/mL at 24 h from the drug intake) in all the studied subjects, regardless of the formulation or the t(max). This method proved to be suitable for studies on aspirin responsiveness.

Automated summary

A high-throughput LC-MS/MS method was validated for measuring aspirin and salicylic acid in blood and plasma. The study found differences in absorption rate and plasma AUC between oral administration of enteric-coated aspirin and plain aspirin, with higher between-subjects variability seen after enteric-coated aspirin.