12. STATISTICAL CONSIDERATIONS 12.1 Overview The proposed study is an open-label, single-arm phase II study of GVAX in combination with pembrolizumab in patients with metastatic colorectal cancer who have progressed at least two lines of therap ies. Subjects with microsatellite instability Phase 2 Study of GVAX (with CY) and pembrolizum ab in MMR-p advanced colorectal cancer J16154/Version 5/ January 17, 2018 55(MSI) would not be eligible for this study. The primary objective of the trial is to determine whether the combination of GVAX with pembrolizumab administered in this patient population yields a clinically compelling ant itumor activity measured as objective response rate (ORR, assessed by RECIST 1.1). Secondary endpoints include safety, duration of response (DOR), progression-f ree survival (PFS), overall survival (OS), and immunologic correlates. The study is planned with 25 evaluable subjects based on a two-stage design that allows early termination for lack of efficacy. A safety run-in phase will be in cluded to ensure that the combination of GVAX and pembrolizumab is safe. The study will be continuously monitored for adverse events after the safety run-in phase. At the beginning of the study, GVAX plus pembrolizumab will be tested in the first 6 patients in the safety run-in portion of the study. These subjects will be monitored for unacceptable toxicities (defined in Section 4.7) through Cycle 1 of treatment before continuation with further accrual. The study will continue without pause to complete the accrual goal for the first stage if no more than 1 of the first 6 subjects experiences unacceptable toxicities (as defined as dose –limiting toxicities in cycle one in section 4.7) in the run-in. If 2 of the 6 subjects experience unacceptable toxicities, the study will be paused for a review. If 3 or more of the first 6 subjects experience unacceptable toxicities with the combination therapy during the run-in, the study will be discontinued. The probability of observi ng 2 or more patients with DLT among the first 6 subjects is greater than 64% if th e true incidence rate is greater than 33%. 12.2 Sample Size and Accrual Approximately 25 patients with metastatic colorectal cancer who have progressed at least two lines of therapies will be enrolled according to this single-arm, Phase 2 trial, with ORR as the primary endpoint. Enrollment will be carried out in 2 stages so that the study can terminate early if GVAX in combination with pembrolizumab is not sufficiently effective. A recently published phase III study showed that TAS-102 resulted in an ORR of 1.6% (95% confid ence interval 0.7-3%) and the agent had an overall survival benefit vs. placebo in a similar patient population [42]. Simon’s minimax two-stage design will be employed to test the null hypothesis that the true ORR is 3% or less (not cons idered clinically compelling for this combination). In the first stage, 15 subjects will be accrued. If none of these 15 subjects respond, the study will be terminated. Otherwise, 10 additional subjects will be accrue d to target a total of 25 treated and response evaluable subjects. If 3 or more responses are observed in these 25 subjects, we will conclude the combination of pembrolizumab with GVAX is promising and warrant further investig ation. The study could also be terminated early as soon as 3 responses with the combination treatment are confirmed. The probability of stopping the trial early for futility is 63% if the true ORR is 3% or less. This design yields 89% power at a one-sided type I error rate of 5% when the true response rate is 20%. Subjects who are wit hdrawn from the study prior to receiving one cycle of the combination treatme nt of cyclophosphamide, GVAX, and Phase 2 Study of GVAX (with CY) and pembrolizum ab in MMR-p advanced colorectal cancer J16154/Version 5/ January 17, 2018 56pembrolizumab for reasons other than disease progression will be replaced. The study may enroll 28-30 subjects to ensure 25 woul d be evaluable for the primary efficacy endpoint. More than 300 colorectal cancer patients seek primary oncology care at JHH and an additional 200 patients or more come to JHH for consultation each year. The majority of these patients have MMR-p metastatic di sease. We estimate that approximately a third of the colorectal cancer patients seen at JHH would be poten tially eligible for enrollment for the proposed study. We expect to enroll 2-3 patients per month. We estimate that we could reach the accrual goal within 1 year. 12.3 Statistical analysis 12.3.1 Analysis of Primary Endpoint The objective response rate defined as the proportion of response evaluable subjects who have a complete response (CR) or partial response (PR) using RECIST 1.1 criteria at any time during the study. The evaluable population includes all subjects who have completed at least two dose of combination therapy and have received at least one follow-up scan. It is known that this crude proportion underestimates the true ORR due to the adaptive nature of Simon’s two stage design. Thus we will also report the bias-reduced estimator for ORR due to Whitehead [43] and the adjusted 95% confidence intervals that account for the adaptiveness of the study design [44]. 12.3.2 Analysis of Secondary Efficacy Endpoints Overall survival is defined as the duration of time from start of study treatment to time of death. For subjects lost to follow- up or whose vital stat us is unknown, every effort will be made to determine the date such subjects were last known to be alive. Such efforts may include phone calls, cer tified mail, and the checking of public records. Subjects who are alive or lost to fo llow-up as of the data analysis cutoff date will be right-censored.