A portion of gastrointestinal tumours are sensitive to chemotherapy. However, cross-resistance can be formed between different chemotherapy drugs. The choice of treatment after chemotherapy resistance becomes a major issue. Changing the way in which chemotherapeutic drugs are administered, such as different doses and cycles of chemotherapeutic drugs, affects tumour resistance.

Targeted therapy and immunotherapy provide new hope for chemotherapy resistance. Except for a few gastrointestinal tumours that have been approved for targeting, there are fewer targeted drugs for the digestive system. In consideration of the resistance to chemotherapy in gastrointestinal tumours driven by multiple genes, whole exon sequencing, RNA sequencing, and drug screening based on the PDX model can increase the effectiveness of targeted drugs through prediction and screening, and provide information on genetic changes and related mechanisms. Immune checkpoint inhibitors are a new systemic treatment method after chemotherapy and targeted therapy and a treatment option after chemotherapy resistance. Effective markers of immune checkpoint inhibitors, such as TMB, MSI, CPS, etc. have become hot spots for treatment. Different immunotherapy strategies are being developed for different tumour immune states. Effective biomarkers and mechanisms are still needed. Chemotherapy and targeted therapy can directly kill tumour cells, leading to the exposure of tumour antigens and increasing the efficacy of immunotherapy. Chemotherapy can destroy immune cells and may reduce the efficacy of immunotherapy. Choosing different combinations of different systematic treatment schemes and different administration sequences will likely achieve better therapeutic effects.

This Special Issue invites researchers, scientists, and physicians to submit research articles, reviews, and clinical studies on this topic, to address the urgent issues and insights into therapy for gastrointestinal tumours.

Potential topics include but are not limited to the following:

• Novel chemotherapeutic strategies to reduce chemotherapy resistance and mechanisms
• Genetic changes and mechanisms of chemotherapy resistance in gastrointestinal tumours
• Combinations of chemotherapy, targeted drugs, immunotherapy, and synergistic mechanisms
• Effective markers used for the management of chemotherapy resistance and immune checkpoint inhibitors