Here is the design of a clinical trial:

Official Title: A Single-arm, Multicenter Phase II Clinical Study to Evaluate the Efficacy and Safety of Tyrosine Kinase Inhibitor (TKI) in Combination With Anti-PD-1 Antibody in TKI-responded Microsatellite Stability/Proficient Mismatch Repair (MSS/pMMR) Metastatic Colorectal Adenocarcinoma.

Included conditions:
- MSS
- pMMR
- Metastatic Colorectal Adenocarcinoma

Study Armgroups:
- {'label': 'TKI ± anti-PD-1 antibody', 'type': 'EXPERIMENTAL', 'description': 'According to response to TKI, the combination of anti-PD-1 treatment would be determined.', 'interventionNames': ['Drug: TKI ± anti-PD-1 antibody']}

Interventions:
- {'type': 'DRUG', 'name': 'TKI ± anti-PD-1 antibody', 'description': 'After one cycle of TKI, evaluations would be performed according to RECIST v 1.1. (1) obvious response (A): CR, PR or shrunken SD, or cavitation in metastatic lung lesions, or decrease in the density of liver metastatic targets ≥15%; (2) general response (B): enlarged SD; (3) poor response (C): PD. TKI ＋ anti-PD-1 antibody will be administered in group A. The subjects in group C will exit the study. The subjects in group B will continue TKI for another one cycle, the obvious response subjects will entered group A, the general response subjects will keep in arm B and continue the TKI monotherapy, and the poor response subjects will exit the study.\n\n1. Fruquintinib 5mg or regorafenib 120mg, qd po, 3 weeks, Q4w. (The investigators decide to choose fruquintinib or regorafenib)\n2. Toripalimab injection 240mg, Q3w, until the disease progresses or lasts for two years.', 'armGroupLabels': ['TKI ± anti-PD-1 antibody']}

Primary Outcomes:
- {'measure': '9-month progression-free survival (PFS) rate (in subjects receiving TKI followed by TKI in combination with anti-PD-1 antibody)', 'description': 'From the date of first dose of treatment to the first of either disease progression, relapse or death from any cause, evaluate patient PFS rate at 9 months', 'timeFrame': '9 months'}

Please estimate the sample size based on the assumption: 
Type 1 error rate is 0.05, power is 0.90, and a 10% dropout rate is considered.
Sample size calculation
        This is a single-arm phase II clinical trial, with 9-month PFS rate as the primary endpoint. A Simonâs two-staged design is used to calculate the sample size.
        The null hypothesis is that the true 9-month PFS rate is 27.1%, which was reported by the FRESCO trial. The 9-month PFS rate of the TKI+anti-PD-1 antibody (arm A) group is supposed to be 55%, yielding a type 1 error rate of 0.05 and power of 0.90. The simon2stage package of STATA software (version 15.0) is used to obtain the sample size. The sample size of arm A is 25 cases. Sixteen evaluable patients are recruited in the first stage. If more than four patients can reach 9âmonths according to iRECIST 1.1 criteria, nine additional patients will be added to the second stage (for a total of 25 patients). The null hypothesis will be rejected if more than 10 patients achieve 9-month PFS in 25 patients.
        According to the FRESCO study, the proportion of patients with CR+PR+shrunken SD in the fruquintinib group was 52.9% after 8-week treatment. Hence, to ensure that there are 25 patients in arm A, 48 patients should be selected at the initial stage. Considering a 10% dropout rate, we need to recruit 53 patients in this trial.