Drug resistance and pseudoresistance: an unintended consequence of enteric coating aspirin

Abstract

Background: Low dose aspirin reduces the secondary incidence of myocardial infarction and stroke. Drug resistance to aspirin might result in treatment failure. Despite this concern, no clear definition of aspirin resistance has emerged, and estimates of its incidence have varied remarkably. We aimed to determine the commonality of a mechanistically consistent, stable, and specific phenotype of true pharmacological resistance to aspirin-such as might be explained by genetic causes.

Methods and results: Healthy volunteers (n=400) were screened for their response to a single oral dose of 325-mg immediate release or enteric coated aspirin. Response parameters reflected the activity of the molecular target of aspirin, cyclooxygenase-1. Individuals who appeared aspirin resistant on 1 occasion underwent repeat testing, and if still resistant were exposed to low-dose enteric coated aspirin (81 mg) and clopidogrel (75 mg) for 1 week each. Variable absorption caused a high frequency of apparent resistance to a single dose of 325-mg enteric coated aspirin (up to 49%) but not to immediate release aspirin (0%). All individuals responded to aspirin on repeated exposure, extension of the postdosing interval, or addition of aspirin to their platelets ex vivo.

Conclusions: Pharmacological resistance to aspirin is rare; this study failed to identify a single case of true drug resistance. Pseudoresistance, reflecting delayed and reduced drug absorption, complicates enteric coated but not immediate release aspirin administration.

Clinical trial registration: URL: http://www.clinicaltrials.gov. Unique identifier: NCT00948987.

Figure 1

Study design (A) Phase 1 and 2; (B) Phase 3.

Figure 2

Response to 7 days of low dose aspirin (81mg, enteric coated) in Phase 3. Assessment of platelet aggregation using arachidonic acid as the stimulus in the absence and presence of aspirin (100 μmol/L) added to isolated platelets ex vivo. (A) Individuals who qualified as “non-responders” in Phase 1 AND Phase 2 (n=27). One subject showed uninhibited platelet aggregation following one week of aspirin treatment, but responded to aspirin addition ex vivo. (B) Individuals who were classified as responders on two occasions of single dose aspirin administration in Phase 1 and Phase 2 (n= 25). Two subjects showed uninhibited platelet aggregation but responded to aspirin addition ex vivo.

Figure 3

Relative cumulative frequency distributions of the arachidonic acid (325 mg) induced platelet aggregation response (A–C), serum thromboxane formation (D–F) and urinary thromboxane metabolite excretion (G–I) stratified by drug formulation and assessment time. The density of the data points is visualized by color coding – green high density/frequency, red – low density /frequency. Classification thresholds (“responders” vs. “non-responders”) as determined iteratively using random forest classifiers are shown as blue arrows, which also depict the fraction of responders in the study groups. Urinary TxM excretion failed to predict individual response status although pre- and postdose distributions segregated in each group (p<0.05).

Figure 4

Consistency of the response to distinct stimuli of platelet aggregation. Four stimuli were used to assess the functional impact of aspirin treatment on platelet aggregation: arachidonic acid (A), collagen (B), adenosine diphosphate (ADP, C) and protease activated (thrombin) receptor 1 activating peptide (PAR-1 AP, D). Data are displayed as scattergrams of slope vs. maximum aggregation. Data points are color coded by the response status as determined by 95% inhibition of serum TxB₂ concentration. Thus, “responders” are plotted in green and “non-responders” in red. Shades of gray represent densities of the two-dimensional distribution. Classification thresholds as determined iteratively using random forest classifiers and 95% confidence intervals are shown in blue. (A) The test sensitivity of arachidonic acid induced platelet aggregation was >90% with an error rate of 10% when volunteers with a residual platelet function following aspirin administration of 16% maximal aggregation (95% confidence interval 12% to 62%) or 11%/min slope (95% confidence interval 8%/min to 17%/min) were classified as “non-responders”. (B, C) Inducing platelet aggregation with collagen or ADP resulted in sensitivities of 80% and 75% and error rates of 25% and 36% at thresholds of 41%/min slope (95% confidence interval 39%/min to 42%/min) and 75% maximal aggregation (95% confidence interval 72% to 76%), respectively. Maximal aggregation was not a predictor of response status in collagen stimulated platelets; slope was not a predictor of response status in ADP stimulated platelets. (D) As expected, the PAR-1AP response did not associate with inhibition of serum Tx formation by aspirin.