Impact of age on in vitro metabolism of clopidogrel: a potential explanation for high on-treatment platelet reactivity in the elderly?

BACKGROUND: High on-treatment platelet reactivity has been reported in 30% of patients on clopidogrel and 50% in elderly patients; however, little is known about the mechanisms of this biological resistance. One hypothesis is an age-related impaired hepatic metabolism of the prodrug clopidogrel, leading to a lower formation of its active metabolite (clopidogrel-AM). OBJECTIVES: To compare the levels of clopidogrel-AM formed in vitro using “old” and “young” human liver microsomes (HLMs) and their consequences on platelet functions. METHODS: We developed an in vitro model using “old” (73.6 ± 2.3 years) and “young” (51.2 ± 8.5 years) HLMs, added to platelet-rich plasma from 21 healthy donors with or without clopidogrel (50 μM) and incubated at 37 °C for 30 (T30) and 45 minutes (T45). Clopidogrel-AM was quantified by liquid chromatography–mass spectrometry/mass spectrometry method. Platelet aggregation was performed by light transmission aggregometry. RESULTS: The generation of clopidogrel-AM increased over time and reached concentrations comparable with those reported in treated patients. At T30, mean clopidogrel-AM concentrations were significantly higher with “young” (8.56 μg/L; 95% CI, 5.87-11.24) than with “old” HLMs (7.64 μg/L; 95% CI, 5.14-10.14; P = .002); and at T45, 11.40 μg/L; 95% CI (7.57-15.22) vs 10.63 μg/L, 95% CI (7.10-14.15), P = .02 (n = 21). Despite a significant inhibition of platelet aggregation, no significant difference was found in light transmission aggregometry (adenosine diphosphate, 10 μM) after clopidogrel metabolism by “old” or “young” HLMs, probably because of low sensitivity of the method to small variations of clopidogrel-AM. CONCLUSION: In this original model combining metabolic and functional approaches, less clopidogrel-AM was produced with HLMs from older patients. This provides support for a decreased CYP450 activity that may contribute to high on-treatment platelet reactivity in elderly patients.