The combination of immune checkpoint inhibitors (ICIs) and tyrosine kinase inhibitors (TKIs) has significantly improved clinical outcomes in patients with advanced renal cell carcinoma (RCC). However, 40%-60% of patients still develop primary or acquired resistance. For those with resistant disease, current immune rechallenge strategies have yet to demonstrate clear clinical benefit. Emerging evidence indicates that the gut microbiota plays a critical role in modulating responses to immunotherapy. Microbiota-modulating approaches, including fecal microbiota transplantation (FMT) and live bacterial formulations such as CBM588, have shown preliminary potential to enhance sensitivity to immunotherapy and improve patient outcomes. Nevertheless, whether gut microbiota modulation can resensitize advanced RCC to immunotherapy remains to be investigated.

The REMEDY-RCC trial is an investigator-initiated, prospective, open-label, phase II, parallel two-cohort exploratory study primarily designed to investigate whether gut microbiota modulation can restore sensitivity to immunotherapy in patients with advanced RCC. Given the substantial differences in the tumor immune microenvironment and response to ICIs between clear cell RCC (ccRCC) and non-clear cell RCC (nccRCC), this study employs a parallel two-cohort design to stratify these two patient populations: a primary cohort consisting of patients with metastatic ccRCC, and an exploratory cohort consisting of patients with metastatic nccRCC. The primary and secondary endpoints will be analyzed in the primary cohort, whereas the exploratory cohort will be used solely for exploratory and descriptive analyses. The study plans to enroll approximately 33 adult patients with advanced or metastatic RCC, including 27 with metastatic ccRCC and 6 with metastatic nccRCC. Eligible patients will receive the following sequential interventions: bowel decontamination with amoxicillin-clavulanate potassium followed by polyethylene glycol (PEG) solution, high-dose gut colonization and maintenance supplementation with live Clostridium butyricum powder combined with soluble dietary fiber, and immune rechallenge therapy consisting of a PD-1 immune checkpoint inhibitor (ICI) combined with four cycles of a CTLA-4 ICI, followed by maintenance therapy with the PD-1 ICI alone. Treatment will continue until disease progression, unacceptable toxicity, completion of the planned treatment cycles, or withdrawal of consent.

The REMEDY-RCC trial requires specific follow-up for enrolled patients. Radiological response is assessed longitudinally using computed tomography (CT). Tissue and body fluid samples collected from patients will be used for biomarker and multi-omic analyses.

The primary endpoint of the trial is the objective response rate (ORR) in the primary cohort, as assessed by the Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Sample size for the primary cohort was determined using Simon's optimal two-stage design, with a one-sided α of 0.10 and power (1-β) of 0.80. The unacceptable response rate (P0) was set at 17.4%, based on the ORR observed in the FRACTION-RCC study for nivolumab plus ipilimumab rechallenge in patients with metastatic ccRCC who had progressed after prior immunotherapy. The target response rate (P1) was established considering that FMT has been shown to increase ORR by approximately 20% in immunotherapy-resistant melanoma. Given that metastatic RCC and melanoma are both considered immunologically responsive tumor types, a comparable therapeutic benefit from gut microbiota remodeling is anticipated. Accordingly, P1 was set at 37.4%. This effect size is clinically meaningful and would be non-inferior to the clinical efficacy of cabozantinib in metastatic RCC that has progressed following immunotherapy.

The primary cohort will initially enroll six patients for safety assessment. If the treatment is deemed safe, further accrual will follow Simon's two-stage design, and enrollment of the exploratory cohort will be considered. After these six patients, an additional six patients will be enrolled, bringing the total to 12, at which point an interim analysis will be performed. If the number of objective responses at the interim analysis is ≤ 2, the trial will be terminated early. If ≥3 objective responses are observed, an additional 12 patients will be enrolled, resulting in a total sample size of 24 patients. Accounting for a potential 10% dropout rate, the study plans to enroll 27 patients in total.

The exploratory cohort is planned to enroll no more than 6 patients (not exceeding 20% of the total enrollment), with a fixed-sample descriptive design. The sample size calculation is not based on statistical hypothesis testing, and all efficacy endpoints will be reported using descriptive statistics. The total planned sample size is 33 patients, comprising 27 patients in the primary cohort and 6 patients in the exploratory cohort.

Inclusion Criteria:

• Pathological Diagnosis and Subtype Specification Histologically confirmed metastatic renal cell carcinoma (mRCC) of either clear cell type or non-clear cell type, with non-clear cell histologies restricted to papillary, chromophobe, or unclassified subtypes. Sarcomatoid features are allowed. Other non-clear cell subtypes, including but not limited to collecting duct carcinoma and renal medullary carcinoma, are excluded.

Based on the pathological subtype, enrolled patients will be assigned to two cohorts: the primary cohort consists of patients with histologically confirmed metastatic clear cell RCC, and the exploratory cohort consists of those with histologically confirmed metastatic non-clear cell RCC (subtypes limited to papillary, chromophobe, or unclassified).

• Immunotherapy Resistance Patients must have received at least two cycles of PD-1/PD-L1 ICI therapy and demonstrated disease progression either during treatment or within six months after the last dose. Progressive disease must be confirmed by the investigator according to RECIST 1.1 criteria, with additional radiographic confirmation performed within four weeks after the initial documented progression.
• Prior Treatment Lines and Sequence Restriction Patients must have received no more than two prior lines of systemic therapy, with no more than one line consisting of a PD-1/PD-L1 ICI. This PD-1/PD-L1 ICI based therapy may have been administered as first-line or second-line treatment for advanced or metastatic RCC, or as neoadjuvant or adjuvant therapy. Furthermore, this line of therapy must be the most recent prior treatment line, meaning that no other anticancer therapy has been administered following its completion.
• Lesion Requirement Patients must have at least one measurable lesion per RECIST v1.1, as assessed by the investigator or by local radiologic evaluation.
• Pre-treatment Tumor Biopsy Quality The pre-treatment tumor biopsy specimen must contain viable tumor tissue accounting for at least 75% of the total examined area.
• Prior Radiotherapy Washout Participants who have received palliative radiotherapy for non-central nervous system lesions must complete such therapy at least 2 weeks prior to the first dose of study treatment. Participants for whom the only measurable lesions reside within a previously irradiated field are eligible for enrollment, provided that such lesions have shown clear progression and remain accurately measurable.
• Recovery from Prior Treatment Toxicity All toxicities from prior anticancer therapies must have recovered to baseline or to Grade ≤1 according to the CTCAE version 5.0, with the exception of controllable AEs such as hypothyroidism that are clinically insignificant or remain stable under supportive care.
• General Baseline Status Patients must be ≥18 years of age, have an Eastern Cooperative Oncology Group (ECOG) performance status <2, an American Society of Anesthesiologists (ASA) score ≤2, and a life expectancy ≥6 months.
• Adequate Liver Function Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels ≤3 × upper limit of normal (ULN), or ≤5 × ULN in patients with liver metastases. Total bilirubin ≤1.5 × ULN, except for patients with Gilbert's syndrome, in whom bilirubin ≤3.0 mg/dL is allowed.
• Adequate Renal Function Creatinine clearance ≥30 mL/min (Cockcroft-Gault formula) or serum creatinine ≤1.5 × ULN, and urine protein-to-creatinine ratio ≤1 mg/mg.
• Adequate Bone Marrow Function White blood cell count >2.0 × 10⁹/L, absolute neutrophil count >1.5 × 10⁹/L, platelet count >100 × 10⁹/L, lymphocyte count ≥0.3 × 10⁹/L, and hemoglobin ≥90 g/L, with no blood transfusion within 14 days prior to assessment.
• Viral Serology Patients must be negative for human immunodeficiency virus (HIV), negative for hepatitis B surface antigen (HBsAg), or positive for hepatitis B core antibody (HBcAb) with negative HBV DNA, and negative for hepatitis C antibody (HCV Ab), or positive with negative HCV RNA.
• Informed Consent Patients must provide written informed consent.
• Compliance Patients must be willing and able to adhere to the study protocol.

Exclusion Criteria:

- Recent Microbiota Interference Patients are excluded if they have received systemic antibiotics within 4 weeks prior to screening, or have received any live bacterial preparations or commercial prebiotics within 2 weeks prior to screening, or are currently using or plan to use probiotics, yogurt, or foods supplemented with bacteria during the treatment period.

- Immune-Related Severe Toxicity or Autoimmune Status

Patients are excluded if they meet any of the following criteria:

• Have a history of permanent discontinuation of immunotherapy due to immune-mediated adverse events;

  • Have active or known autoimmune disease, except for type 1 diabetes mellitus, hypothyroidism requiring only hormone replacement therapy, and vitiligo;

    • Have an immunosuppressive status, defined as confirmed immunodeficiency, receipt of systemic corticosteroids at a dose equivalent to >10 mg of prednisone per day or other immunosuppressive agents (excluding inhaled or topical corticosteroids), or a history of active autoimmune disease requiring systemic therapy within the past 2 years.

      - Prior Specific Immunotherapy or Excessive Therapy Patients are excluded if they have received prior treatment with a CTLA-4 ICI or more than two prior systemic therapy regimens.

      - Gastrointestinal Risk Factors Patients are excluded if they have a history of inflammatory bowel disease (including Crohn's disease or ulcerative colitis); chronic severe diarrhea; or grade ≥3 immune-related colitis; or present with gastrointestinal metastatic lesions; or have undergone major abdominal surgery that may impair intestinal absorption or peristaltic function.

      - Oral Administration Barrier Patients are excluded if they have dysphagia or are unable to take the required oral study medication.

      - Rapid Tumor Progression Patients are excluded if they have rapidly progressive disease such that, in the opinion of the investigator, they cannot safely tolerate a 4-week washout period and a 2-week reconditioning period.

      - Active Interstitial Lung Disease or Pneumonitis Patients with active interstitial lung disease or pneumonitis, or a history of either condition requiring systemic steroid therapy, are excluded.

      - Uncontrolled Brain Metastases or Central Nervous System Disease Patients with known brain metastases, cranial epidural disease, or significant vasogenic edema are excluded unless they have received adequate radiotherapy or surgery and have been stable for at least 4 weeks prior to the first dose of study treatment.

      - Severe Cardiac Disease Patients are excluded if they have a history of myocarditis or congestive heart failure (New York Heart Association [NYHA] class III-IV), unstable angina, uncontrolled severe arrhythmia, or myocardial infarction within 6 months prior to enrollment.

      - Recent Other Antineoplastic Treatment Patients are excluded if they have received, prior to the first dose of study treatment:any small molecule kinase inhibitor within 2 weeks; cytotoxic agents, biologics, or other systemic anticancer therapies within 4 weeks; radiotherapy for bone metastases within 2 weeks, or any other radiotherapy within 4 weeks.

      • Planned Other Antineoplastic Treatment Patients are excluded if they require any other systemic or local antineoplastic therapy during the study period, with the exception of bisphosphonates or denosumab for the management of bone metastases.
      • Active Second Malignancy Patients are excluded if they have a second active malignancy diagnosed within the preceding 2 years, with the exception of indolent or early-stage tumors, including non-melanoma skin cancer and carcinoma in situ.
      • Laboratory Abnormalities

Patients are excluded if they meet any of the following criteria:

• White blood cell count <2.0 × 10⁹/L, absolute neutrophil count <1.5 × 10⁹/L, or platelet count <100 × 10⁹/L;

  • AST/ALT >3 × ULN, or >5 × ULN in patients with liver metastases, or total bilirubin >1.5 × ULN (except for Gilbert's syndrome); ③Creatinine clearance <30 mL/min or serum creatinine >1.5 × ULN.

    • Other Uncontrolled Comorbidities

Patients are excluded if they have any of the following:

• Uncontrolled tumor-related pain, with painful lesions requiring palliative radiotherapy to be managed prior to enrollment;

  • Uncontrolled pleural effusion, pericardial effusion, or ascites requiring drainage more frequently than once per month, the presence of an indwelling catheter such as a PleurX® device is permitted; ③Moderate to severe hepatic impairment, defined as Child-Pugh class B or C; ④Uncontrolled or symptomatic hypercalcemia.

    • Other General Exclusions

Patients are excluded if they meet any of the following criteria:

• Have received prior allogeneic stem cell transplantation or solid organ transplantation;

  • Have active hepatitis B (HBV DNA positive), active hepatitis C (HCV RNA positive), or HIV infection;

    • Have undergone major surgery within 4 weeks prior to study treatment;

      • Have a history of allergy to food or antibiotics; ⑤Are pregnant, breastfeeding, or plan to conceive or father children during the study period (from pre-screening or screening visit until 120 days after the last dose); ⑥Require dialysis, have malabsorption syndrome, uncontrolled electrolyte disturbances, or require specific anticoagulants such as warfarin; ⑦Have any medical or psychological condition that may compromise safety or compliance.