The goal of this clinical trial is to learn if oral probiotics (Clostridium butyricum) work to improve the efficacy of targeted therapy plus immunotherapy in bladder preservation setting for cisplatin-ineligible T2-3N0M0 bladder cancer patients with low serum butyrate.

The main questions it aims to answer are: Do oral probiotics elevate serum butyrate levels and enhance the duation of bladder preservation interval with targeted therapy plus immunotherapy? Researchers will compare oral probiotics combined with targeted therapy plus immunotherapy to standard regimens (targeted therapy plus immunotherapy) to see if oral probiotics can improve its efficacy.

Participants will:

1. Take oral probiotics and/or Disitamab Vedotin (HER2-ADC) and Toripalimab (PD-L1 inhibitor) for one year in total, which is divided into induction treatment period, intensive treatment period and maintenance treatment period.
2. Return to the hospital for evaluation of tumor residual burden according to the follow-up plan, which will include urine cytology, imaging, surgical biopsy, and urine DNA methylation detection.

Inclusion Criteria:

• Patients have histologically confirmed, radiologically staged cT2-3N0M0 urothelial carcinoma of the bladder, in which urothelial carcinoma is the predominant component (>50%).
• Serum butyrate level <46 μg/L as determined by quantitative mass spectrometry.
• HER2 expression is assessed by immunohistochemistry (IHC) on pretreatment tumor specimens, with confirmed IHC ≥1+.
• Patients are deemed ineligible for radical cystectomy based on laboratory evaluation and patient preference.
• Patients considered ineligible for cisplatin therapy and meeting at least one of the following criteria: ECOG performance status >1 or Karnofsky performance status of 60-70%; Creatinine clearance <60 mL/min; Hearing loss ≥ Grade 2 according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) version 5.0; Peripheral neuropathy ≥ Grade 2 (NCI-CTCAE v5.0); New York Heart Association (NYHA) Class III or higher heart failure.
• ECOG performance status of 0-2.
• Adequate cardiac, bone marrow, hepatic, renal, and coagulation functions.

Exclusion Criteria:

• Prior ADCs or PD-1/PD-L1 inhibitor therapy.
• Known hypersensitivity to microbiota-related preparations (microecological products), RC48-ADC or Toripalimab or any of its components.
• Receipt of other approved systemic anticancer therapy or systemic immunomodulatory agents (including but not limited to interferon, interleukin-2, and tumour necrosis factor) within 28 days prior to enrolment.
• Prior radiotherapy for bladder cancer.
• Prior antitumour drug therapy, except for the following: a. For patients who previously received systemic chemotherapy, a treatment-free interval of at least 3 months between the last dose and the start of induction therapy is required; b. Local intravesical chemotherapy or immunotherapy (including BCG) must be completed at least 1 week before initiation of study neoadjuvant treatment.
• Surgery or significant trauma within 28 days prior to enrolment (implantation of vascular access devices and TURBT are not considered).
• Severe chronic or active infection requiring systemic antibacterial, antifungal, or antiviral therapy within 14 days prior to enrolment.
• Receipt of live vaccines within 28 days prior to enrolment.
• Active autoimmune disease requiring systemic treatment and considered by the investigator to potentially interfere with the study treatment.
• Requirement for long-term high-dose corticosteroids or other immunosuppressive agents.
• Clinically significant abnormalities that may affect treatment, including electrolyte disturbances, hypoalbuminaemia, interstitial lung disease, non-infectious pneumonitis, or other uncontrolled systemic diseases. These include uncontrolled diabetes, hypertension, or cardiovascular disease, such as active cardiac conditions within 6 months prior to enrolment.
• Untreated chronic hepatitis B with HBV DNA ≥500 IU/mL (2,500 copies/mL) or known HBV carriers with active disease.
• Active hepatitis C infection.
• History of immunodeficiency, including positive human immunodeficiency virus (HIV) test, other acquired or congenital immunodeficiency disorders, or a history of allogeneic stem cell transplantation or solid organ transplantation.
• Toxicities from prior therapies that have not recovered to baseline or stabilised.
• Presence of other concomitant malignancies.

De-identified individual participant data (IPD) will be made available to qualified researchers, along with the study protocol and statistical analysis plan (SAP), after the publication of primary study results. Requests will be reviewed by the study sponsor and steering committee. Data will be shared via a secure, password-protected research data repository for non-commercial, scientific purposes only, in compliance with applicable regulations and ethical approvals.

IPD will be available starting 6 months after the publication of the primary study results, and will remain accessible for a period of 5 years thereafter.

Access will be granted to qualified independent researchers with a scientifically sound research proposal focused on non-commercial objectives.