Defining the Most Appropriate Primary End Point in Phase 2 Trials of Immune Checkpoint Inhibitors for Advanced Solid Cancers: A Systematic Review and Meta-analysis

Abstract

Importance: Checkpoint inhibitors have a unique mechanism of action that differs from chemotherapy or targeted therapies. The validity of objective response rate (ORR) as a surrogate for progression-free survival (PFS) and overall survival (OS) in checkpoint-inhibitor trials is uncertain.

Objective: To determine the types of primary end points used in phase 2 checkpoint-inhibitor trials, and to assess the strength of associations for ORR with PFS and OS.

Data sources: Trials listed in electronic databases from 2000 to 2017 (PREMEDLINE, MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials).

Study selection: Advanced solid cancers in phase 2 and phase 3 trials.

Data extraction and synthesis: Correlations between ORR odds ratios and hazard ratios (HRs) for PFS and OS were examined for randomized comparisons. Within checkpoint-inhibitor treatment arms, correlations for ORR with 6-month PFS and 12-month OS rates were examined. All analyses were weighted by trial size. Multivariable models to predict 6-month PFS and 12-month OS rates from ORR were developed and their performance validated in an independent sample of trials.

Main outcomes and measures: Correlation coefficient (r) of ORR with PFS and OS.

Results: Of 87 phase 2 trials identified, ORR was the most common (52 [60%]) primary end point. Twenty randomized clinical trials with 25 treatment comparisons were identified. Checkpoint-inhibitor therapy was associated with pooled ORR of 24% (95% CI, 18%-31%). For randomized comparisons, r between ORR odds ratio and PFS HR was 0.63 (95% CI, 0.35-0.89), ORR odds ratio and OS HR was 0.57 (95% CI, 0.23-0.89), and between PFS HR and OS HR was 0.42 (95% CI, 0.04-0.81). Within the checkpoint-inhibitor arms, r correlation coefficients between ORR with 6-month PFS, ORR with 12-month OS, and 6-month PFS with 12-month OS were 0.37 (95% CI, -0.06 to 0.95), 0.08 (95% CI, -0.17 to 0.70), and 0.74 (95% CI, 0.57-0.92), respectively. In validation, when 6-month PFS was used to predict 12-month OS, there was a good calibration between actual and predicted 12-month OS. When ORR was used to predict 6-month PFS and 12-month OS rates, respectively, the actual vs predicted rates calibrated poorly.

Conclusions and relevance: In checkpoint-inhibitor trials, ORR correlated poorly with OS. For future phase 2 studies, 6-month PFS rate is recommended as an end point.

Figure 1.. Flow Diagram of Study Inclusions and Exclusions

Figure 2.. Correlations in Relative Treatment Effect

A, Ratio of objective tumor response and hazard ratio (HR) for overall survival (OS). B, Hazard ratio for progression-free survival (PFS) and HR for OS. Each circle represents a trial or treatment comparison. The circle size is proportional to the number of patients.

Figure 3.. Correlations in Absolute Treatment Effect Within the Checkpoint-Inhibitor Arms

A, Objective tumor response and 12-month overall survival (OS) rate. B, Six-month progression-free survival (PFS) rate and 12-month OS rate. Each circle represents a trial or treatment comparison. The circle size is proportional to the number of patients.

Figure 4.. Observed vs Predicted 12-Month Overall Survival (OS) Rate

A, Observed 12-month OS rate and 12-month OS rate predicted by 6-month progression-free survival (PFS). B, Observed 12-month OS rate and 12-month OS rate predicted by objective response rate (ORR). Each data point represents a phase 2 trial from the validation cohort of phase 2 studies.