PD-1 blockade combined with antiangiogenic therapy has synergistic effects and has initially shown therapeutic effects. involve administering one cycle of TKIs (fruquintinib or regorafenib). Patients will be divided into three armsarm A (obvious response to TKIs), arm B (general response to TKIs) and arm C (poor response to TKIs)according to their response to TKIs, as determined by significant changes in imaging findings. Patients in arm A will then receive TKIs in combination with anti-PD-1 antibody, patients in arm C will withdraw from the study, and those in arm B will continue to take TKIs for another one further cycle. Next, patients with obvious response to TKIs will be reallocated to arm A, those with general response to TKIs will stay in arm B and will continue to take TKIs, and patients with poor response to TKIs will withdraw from the study. Administration of arm A or arm B will last until disease progression or intolerable toxicity. Anti-PD-1 antibody can be administered for up to 2 years. This trial will provide necessary data to improve the prognosis of patients with MSS/pMMR mCRC. Trial registration number NCT04483219; Pre-results. Keywords: gastrointestinal tumours, Ranolazine protocols & guidelines, immunology, gastroenterology Strengths and limitations of this study This is the first study to try to evaluate the efficacy and safety of tyrosine kinase inhibitors (TKIs) in combination with antiprogrammed cell death protein 1 (anti-PD-1) antibody in TKI-responsive patients with microsatellite stable/proficient mismatch repair (MSS/pMMR) metastatic colorectal adenocarcinoma (mCRC). The results will provide useful data for precision immunotherapy in patients with MSS/pMMR mCRC. The novelty of the present study lies in the screening phase of the study design, which could help screen patients who could benefit more from PD-1 blockade combined with antiangiogenic therapy and could also provide a safe dose for the following combined therapy. The initial screening of the study is based on response to TKIs, which will be determined based on imaging data (eg, Ranolazine tumour shrinkage, appearance of cavity or reduction in density). The study setting, criteria, inclusion, interventions and outcomes are based on a pragmatic approach to ensure external validity. Study limitations include its single-arm design, which has no comparator, making it hard to assess internal validity. Introduction Metastatic colorectal cancer (mCRC) is one of the main causes of cancer-related deaths worldwide.1 Approximately 40% of patients with mCRC are diagnosed at an advanced stage, which could only receive palliative treatment.2 3 Chemotherapy combined with targeted therapy has significantly improved the prognosis of patients with mCRC.4C10 MULK However, the prognosis of patients with mCRC after multiple-line therapeutic strategies remains poor due to the high Ranolazine tumour load, high level of malignancy and strong drug resistance. At present, irinotecan combined with cetuximab, regorafenib, fruquintinib or trifluridine, and tipiracil hydrochloride tablets (TAS-102) are the regimens for palliative therapy after second-line treatment.11C15 However, overall therapeutic efficacy remains unsatisfactory (the progression-free survival (PFS) is about 4 months and the overall survival (OS) fluctuates between 6 and 9 months).11C15 Thus, exploring palliative therapies after second-line treatment is urgently required. Over the recent years, immunotherapy has provided a new opportunity to treat solid tumours.16 17 To date, studies have mainly concentrated on assessing the efficacy of combined therapies and the superiority of screening of population. It has been discovered that approximately 90% of patients with mCRC have microsatellite stable/proficient mismatch repair (MSS/pMMR) tumours. Yet the results of early trials on programmed cell death protein 1 (PD-1) blockade for patients with MSS/pMMR mCRC remain unsatisfactory, highlighting an urgent need for strategies to enhance the immune response to cancer therapy.18C21 PD-1 blockade combined with antiangiogenic therapy has shown to be effective against some types of cancer (eg, hepatocellular carcinoma).22 23 Antiangiogenic drugs can have an important role in transforming the microenvironment. They can promote the normalisation of tumour blood vessels, which can enhance tissue perfusion and infiltration of immune cells to the tumour, thereby enhancing the effects of immunotherapy. Moreover, the activation/reprogramming of immune cells can influence tumour blood vessels.20 Tumour vascular normalisation and immune reprogramming can improve the tumour microenvironment via a benign cycle of mutual Ranolazine enhancement.24 Therefore, combining immunotherapy with antiangiogenesis is supposed to have a synergistic effect in converting the tumour immune microenvironment from an immunosuppressive status to a state of immune promotion. Fruquintinib and regorafenib are multitargeted tyrosine kinase inhibitors (TKIs) mainly applied for antiangiogenesis. They can induce tumour necrosis, release several new antigens, improve the immunosuppressive Ranolazine microenvironment and induce tumour vascular normalisation. In addition to killing tumour cells, fruquintinib and regorafenib can also convert the immune-suppressive properties of the tumour microenvironment, which can sensitise PD-1 blockade, ultimately improving the prognosis of patients with MSS/pMMR mCRC.25 26 A phase Ib trial from Japan (REGONIVO) assessed the safety and efficacy of regorafenib combined with nivolumab in the treatment of mCRC,.