Systematic review of time to subsequent therapy as a candidate surrogate endpoint in advanced solid tumors.

Aim: Time to subsequent therapy (TST) is an end point that may complement progression-free survival (PFS) and overall survival (OS) in determining the treatment effect of anticancer drugs and may be a potential surrogate for PFS and OS. We systematically reviewed the correlation between TST and both PFS and OS in published phase 2/3 studies in advanced solid tumors. Materials & methods: Trial-level correlational analyses were performed for TST versus PFS (by investigator and/or central review) and TST versus OS. Results: Of 21 included studies, nine (43%) used 'time to first subsequent therapy or death' (TFST) as the TST end point; 11 (57%) used different definitions ('other TST end points'). There was a strong correlation between TFST and PFS by investigator (medians: R 2  = 0.88; hazard ratio [HR]: R 2  = 0.91) and TFST versus PFS by central review (medians: R 2  = 0.86; HRs: R 2  = 0.84). For TFST versus OS there was medium/poor correlation for medians (R 2  = 0.64) and HRs (R 2  = 0.02). Conclusion: TFST strongly correlates with PFS, but not with OS.