Relapsed or refractory acute leukemia (R/R AL) is a life-threatening blood cancer with poor outcomes and limited treatment options. PID23 Injection is an innovative in vivo CAR-T therapy that delivers a viral vector encoding three targets (CD19, BCMA, and CD70) directly into patients, enabling their own T cells to generate functional CAR-T cells against leukemia cells. This single-center, single-arm, open-label, dose-escalation study (3 dose levels: 0.8×10⁹, 2×10⁹, and 4×10⁹ TU) plans to enroll 3-18 patients with R/R AL aged 3-75 years, ECOG 0-2, and positive for at least one target. The primary objective is to evaluate safety, tolerability, and determine the recommended dose. Secondary objectives include preliminary efficacy (remission, survival), pharmacokinetics (CAR-T expansion), pharmacodynamics (cytokine changes), and exploratory viral clearance. After a single intravenous infusion, patients are hospitalized for ≥3 weeks, followed by monthly visits for 3 months, then every 3 months for up to 2 years. Enrollment is from May 2026 to May 2027, with follow-up through May 2029. The study is conducted at Zhujiang Hospital of Southern Medical University (PI: Prof. Li Yuhua).

Inclusion Criteria:

1. Patient or legal guardian voluntarily signs the informed consent form (ICF), demonstrating understanding of the study purpose and procedures, and willingness to participate.
2. Age 3 to 75 years, inclusive, male or female.
3. Diagnosis of relapsed or refractory acute leukemia per guideline criteria:

Relapsed: reappearance of leukemic cells in peripheral blood or bone marrow blasts ≥5% after achieving complete remission (CR); Refractory: failure to achieve CR after 2 courses of standard induction chemotherapy; relapse within 12 months after consolidation/intensification therapy; relapse after 12 months with no response to conventional chemotherapy; 2 or more relapses; extramedullary leukemia relapse or persistence; relapse after allogeneic hematopoietic stem cell transplantation.

4.Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2. 5.Life expectancy ≥12 weeks. 6.Bone marrow morphology showing ≥5% primitive/immature lymphocytes (blasts). 7.Tumor cells positive for CD19, BCMA, or CD70 expression by flow cytometry. 8.Adequate major organ function, defined as:

1. Cardiac: left ventricular ejection fraction (LVEF) ≥40% by echocardiogram;
2. Renal: serum creatinine ≤2.0× upper limit of normal (ULN), or creatinine clearance ≥50 mL/min (Cockcroft-Gault formula);
3. Hepatic: ALT and AST ≤3.0×ULN (≤5.0×ULN if with liver involvement); total bilirubin ≤2.0×ULN (≤3.0×ULN for Gilbert's syndrome);
4. Pulmonary: oxygen saturation ≥92% on room air;
5. Hematologic: absolute neutrophil count (ANC) ≥1.0×10⁹/L, platelets ≥50×10⁹/L, hemoglobin ≥80 g/L (with bone marrow involvement, ANC ≥0.5×10⁹/L, platelets ≥20×10⁹/L permitted) - assessments allowed after transfusion or hematopoietic growth factor support.

9. For women of childbearing potential, negative serum pregnancy test; all participants agree to use reliable (non-rhythm) contraceptive methods from ICF signing through 1 year post-PID23 infusion.

Exclusion Criteria:

1. Prior treatment with CAR-T or other genetically modified cell therapies, unless the investigator determines that safety risks have been adequately excluded.
2. Received the following anti-tumor therapies prior to PID23 infusion: Chemotherapy or molecular targeted therapy within 14 days or 5 half-lives (whichever is longer) (excluding conditioning chemotherapy and intrathecal chemotherapy; intrathecal therapy must be stopped ≥1 week prior to PID23 infusion); Radiotherapy to non-hematopoietic sites within 7 days; Radiotherapy to hematopoietic sites within 14 days.
3. Any of the following cardiac conditions:

(1) New York Heart Association (NYHA) Class III or IV congestive heart failure; (2) Myocardial infarction or coronary artery bypass grafting (CABG) within 6 months prior to enrollment; (3) Clinically significant ventricular arrhythmia, or unexplained syncope (excluding vasovagal or dehydration-related); (3) History of severe non-ischemic cardiomyopathy. 4.Active or uncontrolled infection requiring systemic therapy within 1 week prior to screening.

5.Grade 2-4 acute graft-versus-host disease (GVHD) or moderate-to-severe chronic GVHD within 4 weeks prior to screening.

6.Cerebrovascular accident or seizure within 6 months prior to screening. 7.Deep vein or arterial thrombosis event within 6 months prior to screening. 8.Active malignancy other than acute leukemia (excluding: inactive disease with treatment completed >2 years; adequately treated cervical carcinoma in situ, basal/squamous cell skin carcinoma, localized prostate cancer post-curative surgery, ductal carcinoma in situ post-curative surgery).

9.Received (attenuated) live vaccine within 4 weeks prior to screening. 10.Any other condition that, in the investigator's judgment, makes the patient unsuitable for participation in this study.

IPD will not be shared due to: (1) ongoing study with long-term follow-up through 2 years post-infusion; (2) small sample size (3-18) with high participant re-identification risk; (3) sensitive genetic information (CAR-T viral vector integration data) contained in the IPD; and (4) lack of infrastructure and funding for IPD sharing at this single-center investigator-initiated study. Comprehensive study findings will be published in peer-reviewed journals. Requests for access to raw IPD for legitimate research purposes may be directed to the corresponding author via email.