Forbidden fruit: on the analysis of recurrent events in randomized clinical trials

Vulnerable and Stabilized States After Cerebral Ischemic Events: Implications of Kinetic Modeling in the SOCRATES, POINT, and THALES Trials

BACKGROUND AND OBJECTIVES

Trials of acute secondary prevention after minor stroke or transient ischemic attack (TIA), such as SOCRATES, POINT, and THALES, demonstrate a high initial rate of recurrence after ischemic events that drop quickly to a lower rate, suggesting a transient vulnerable clinical state, which may call for different treatments than the subsequent stabilized state. A kinetic model incorporating vulnerable and stabilized states provides estimates of the distinct kinetic rates reflecting the temporal features of underlying stroke mechanisms. We aimed to compare these kinetic rates between treatments and across trials, asking whether these features point to common pathophysiologic processes underlying stroke recurrence, and inform the targeting and timing of enhanced antiplatelet therapy in recurrent stroke prevention.

METHODS

Kaplan-Meier recurrence-free survival curves in the SOCRATES, POINT, and THALES trials were estimated for each treatment group and fitted by nonlinear regression to the 2-state kinetic model, producing estimates of kinetic parameters, with standard errors estimated using the nonparametric bootstrap with repetitive resampling.

RESULTS

For each trial, the 2-state kinetic model fit the survival curves better than did the null (single-state) kinetic model or the Weibull model (p < 0.05). Recurrence rates in the vulnerable state (k 1 ) were 100-fold higher than in the stabilized state (k 2 ). Transition rates from the vulnerable to stabilized state (k 0 ) were still more rapid. Kinetic parameters were consistent across the trials, without significant differences between the trials. Enhanced antiplatelet regimens produced significant reductions in k 1 (aspirin alone: 0.030 ± 0.004 d-1; active treatment: 0.016 ± 0.003 d-1; p < 0.01) but did not affect k 0 or k 2 , suggesting that active treatment only affected risk in the vulnerable state. Modeling based on these kinetic parameters suggests that most of the benefit of active treatment occurred within 3 days.

DISCUSSION

Across multiple trials of acute secondary prevention after minor stroke or TIA, recurrence of stroke is well-described by a 2-state kinetic model postulating vulnerable and stabilized states, with similar kinetic parameters across trials. Enhanced antiplatelet regimens only affected the recurrence rates in the vulnerable state, over a brief period. This analysis suggests that 2 distinct states follow acute cerebral ischemic events, subject to differential impact of immediate or delayed therapies.

Two-State Kinetic Model of Rates of Stroke Recurrence in the POINT Study Population

BACKGROUND AND PURPOSE

Following an acute ischemic stroke or transient ischemic attack, 2 rates of stroke recurrence are suggested by data from trials of acute secondary prevention treatments: a transient rapid rate followed by a persisting slower rate of stroke.

METHODS

A kinetic model was constructed based on underlying vulnerable and stabilized states of patients following acute ischemic events related by fixed transition rates. Its predictions were fitted by nonlinear regression to the observed timing of outcome events in patients in the POINT trial (Platelet-Oriented Inhibition in New TIA and Minor Ischemic Stroke).

RESULTS

The modeled survivor function produced a close fit to the observed data. The model's predicted kinetic rates suggest that, among subjects in the control group, the event rate was 100-fold higher in the vulnerable state than in the stabilized state. Active treatment halved this rapid rate and had little effect on event rates in the stabilized state. If at least one-tenth of the study population began in the vulnerable state, the rate of transition from the vulnerable to the stabilized state was still faster, with a half-life of only 1 to 2 days.

CONCLUSIONS

Examination of kinetics of stroke occurrence, and of the rates associated with modeled state transitions, may provide insights into the underlying pathophysiological events that are targets for acute secondary prevention of stroke.

A kinetic model for the integrated assessment of safety and efficacy in clinical trials

Regulatory agencies, professional societies, and clinical trialists commonly base judgments of treatment benefit on separate assessments of efficacy and safety. When separate assessments were compared with an integrated assessment using a kinetic model of a hypothetical randomized trial of antiplatelet agents in patients with acute coronary syndrome, the former showed treatment A to be superior to treatment B, whereas the latter showed treatment B to be superior to treatment A. In conclusion, comparative judgments regarding the balance between efficacy and safety depend on the model chosen for analysis; kinetic models are particularly suited to the integrated assessment of efficacy and safety relative to regulatory decisions, public policy, guideline development, and clinical care.

Randomized trials, observational registries, and the foundations of evidence-based medicine

Although randomized trials and observational studies are used as the evidentiary basis of clinical practice guidelines, they are not always in agreement. Limitations in the process of randomization in the former and the selective referral of patients for treatment as a consequence of clinical "risk stratification" in the latter are underappreciated causes for these disagreements. As a result, neither is guaranteed to correctly quantify treatment benefit. This essay reviews the operational differences between these alternative evidentiary sources and shows how these differences can affect individual clinical decisions, population-based practice guidelines, and national health policy. In conclusion, the process of evidence-based medicine can be improved by independent agencies charged with the responsibility to identify and resolve these differences.