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

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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.


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.


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.

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