Validation of 6-minute walk distance as a surrogate end point in pulmonary arterial hypertension trials

Abstract

Background: Nearly all available treatments for pulmonary arterial hypertension have been approved based on change in 6-minute walk distance (Δ6MWD) as a clinically important end point, but its validity as a surrogate end point has never been shown. We aimed to validate the difference in Δ6MWD against the probability of a clinical event in pulmonary arterial hypertension trials.

Methods and results: First, to determine whether Δ6MWD between baseline and 12 weeks mediated the relationship between treatment assignment and development of clinical events, we conducted a pooled analysis of patient-level data from the 10 randomized placebo-controlled trials previously submitted to the US Food and Drug Administration (n=2404 patients). Second, to identify a threshold effect for the Δ6MWD that indicated a statistically significant reduction in clinical events, we conducted a meta-regression among 21 drug/dose-level combinations. Δ6MWD accounted for 22.1% (95% confidence interval, 12.1%- 31.1%) of the treatment effect (P<0.001). The meta-analysis showed an average difference in Δ6MWD of 22.4 m (95% confidence interval, 17.4-27.5 m), favoring active treatment over placebo. Active treatment decreased the probability of a clinical event (summary odds ratio, 0.44; 95% confidence interval, 0.33-0.57). The meta-regression revealed a significant threshold effect of 41.8 m.

Conclusions: Our results suggest that Δ6MWD does not explain a large proportion of the treatment effect, has only modest validity as a surrogate end point for clinical events, and may not be a sufficient surrogate end point. Further research is necessary to determine whether the threshold value of 41.8 m is valid for long-term outcomes or whether it differs among trials using background therapy or lacking placebo controls entirely.

Figure

Results of the meta-regression analysis showing the relationship between changes in 6-minute walk distance (6MWD) between baseline and 12-week follow-up at the drug/ dose level by study on the odds of a clinical event at 12 weeks. The circles each represent a drug/dose combination, with sizes proportionate to study weights (detailed in Table 4 and based on inverse variance weighting). The shaded gray area corresponds to the bounds of the 95% prediction intervals. The threshold value is indicated on the horizontal axis at 41.8 m.