Evaluation for surrogacy of end points by using data from observational studies: tumor downstaging for evaluating...

In clinical cancer trials for evaluating neoadjuvant chemotherapy, tumor downstaging is frequently used as a surrogate end point for overall survival. We evaluated the surrogacy of tumor downstaging using data from a follow-up observational study in bladder cancer.

A total of 586 patients (from 32 Japanese hospitals) who underwent radical cystectomy for invasive bladder cancer (clinical T2 to T4) between 1990 and 2000 were analyzed. We considered changes over time in clinical stage at diagnosis and pathologic stage at cystectomy as a surrogate end point, and survival time after cystectomy as a true end point. First, we developed a new criterion for tumor downstaging. Second, we statistically evaluated surrogacy for the criterion using Prentice's criteria.

To develop the criterion of end points based on tumor downstaging, we selected the best classification among all possible classifications in an attempt to separate prognosis for patients. The hazard ratios after adjustment for prognostic factors in the intermediate effect patients and the poor effect patients were 1.9 (95% confidence interval, 1.0-3.7) and 5.0 (95% confidence interval, 2.6-9.8), respectively, compared with that in the good effect patients. The conditions for correlation and conditional independency of Prentice's criteria were satisfied approximately. Neoadjuvant chemotherapy has a statistically significant tumor downstaging effect, whereas there was no difference on survival between treatment groups.

The tumor downstaging effect could be an appropriate intermediate end point for screening novel neoadjuvant chemotherapy for invasive bladder cancer. The dataset from follow-up studies were useful for evaluating the surrogacy of end points.