Efficacy and Safety of UCBT With TMI -Based Conditioning Regimen for Adults With Refractory/Relapsed Aplastic Anemia

Refractory/relapsed aplastic anemia (AA) in adults remains a clinical challenge that is frequently encountered and urgently needs to be resolved. Refractory AA is ineffective to the standard first-line treatment regimen, which is based on antithymocyte globulin (ATG) combined with cyclosporine A (CsA) as the standard immunosuppressive therapy (IST), accounting for approximately 30%-40% of all AA patients. It has been reported that the 5-year mortality rate of refractory severe aplastic anemia (r-SAA) exceeds 40% after diagnosis. Brosdky et al. reported that the 10-year event-free survival (EFS) of SAA treated with high-dose cyclophosphamide-based IST is only 28%. The efficacy is not satisfactory. Thrombopoietin receptor agonists such as eltrombopag, as an important treatment for r-SAA, have a hematologic response rate of only 41.8% with monotherapy for 52 weeks, with an overall low response rate and unsatisfactory quality of response. The 24-week complete blood count response is only 16.4%, and the 52-week response is only 21.8%. Clonal hematopoiesis is also a long-term concern for IST. Multiple centers have reported that the early efficacy of IST combined with thrombopoietin receptor agonists for treating SAA has improved, but the long-term overall efficacy has not substantially exceeded that of IST alone. The bone marrow failure of non-severe aplastic anemia (NSAA) is less severe than that of SAA, and the main treatment methods are IST ± thrombopoietin receptor agonists or combined with androgens, similar to SAA, with similar long-term efficacy challenges. Therefore, the development of new and effective treatment methods is an urgent clinical need.

Refractory AA is ineffective to IST, possibly mainly due to the insufficient number or functional abnormalities of residual hematopoietic stem/progenitor cells in patients. Therefore, salvage treatment should prioritize hematopoietic stem cell transplantation (HSCT) to supplement normal hematopoietic stem cells and rebuild hematopoiesis. According to domestic and international guidelines for the diagnosis and treatment of adult AA, unrelated donor allogeneic hematopoietic stem cell transplantation or haploidentical hematopoietic stem cell transplantation are the main salvage treatment methods for refractory SAA, with hematopoietic stem cells mainly derived from peripheral blood or bone marrow. However, the availability of suitable donors is one of the main difficulties for unrelated donor allogeneic hematopoietic stem cell transplantation or haploidentical hematopoietic stem cell transplantation. Many AA patients cannot receive early treatment due to the lack of suitable hematopoietic stem cell donors and have to wait for a long time. Unrelated cord blood is an ideal source of hematopoietic stem cells due to its easy availability, low immunogenicity, and low incidence of chronic graft-versus-host disease (cGVHD) after transplantation. However, Delatour et al. reported that the 3-year overall survival (OS) of patients with refractory AA treated with cord blood transplantation is only 38% ± 6%, with unsatisfactory efficacy. Nevertheless, with the optimization of conditioning regimens, the popularization of high-resolution HLA typing, and the continuous improvement of supportive care, the efficacy of unrelated cord blood transplantation (UCBT) is expected to improve.

The optimization of the conditioning regimen for UCBT is a crucial factor in determining patient outcomes. Hiramoto et al. reported that patients received conditioning regimens of total body irradiation (TBI) (≤4Gy) combined with fludarabine/melphalan (Flu/Mel) or fludarabine/cyclophosphamide (Flu/Cy). The results showed that a radiation dose of 4 Gy is an independent influencing factor for neutrophil engraftment. Compared with the Flu/Cy group, a higher proportion of patients in the Flu/Mel group received 4 Gy (96% vs. 68%, p<0.01), but there was no difference in engraftment success rates (77.8% vs. 72%). Compared with TBI, TMI has many advantages: (1) TMI clears residual T cells in the recipient's body, optimizes spatial competition, and better prepares the bone marrow niche for cord blood stem cells; (2) It can more accurately target the bone marrow region; (3) It reduces toxicity to other organs and can better protect the patient's organ function; (4) It reduces the risk of long-term tumors; (5) It lowers the radiation dose: the TMI dose is 4-6 Gy, while the TBI dose reaches 12 Gy; (6) TMI is applicable to a wider range of people and is more tolerable for elderly/weak patients. Therefore, TMI has potential advantages in the conditioning regimen for refractory/relapsed severe aplastic anemia.

Therefore, we have designed and proposed for the first time the Phase II clinical study titled "A Phase II Study on the Efficacy and Safety of Umbilical Cord Blood Transplantation (UCBT) with a Total Marrow Irradiation (TMI)-Based Conditioning Regimen for the Treatment of Refractory/Relapsed Aplastic Anemia (AA) in Adults." This study employs a TMI (4 Gy)/Flu/Mel conditioning regimen and single-unit cord blood stem cell transplantation to treat adult refractory/relapsed AA, aiming to evaluate the efficacy and safety of this regimen.

Inclusion criteria

• Subjects suitable for enrollment in this study must meet all of the following criteria:

• Age ≥18 years and <50 years, regardless of gender.
• Diagnosis of AA according to the guidelines of the British Society for Haematology. The bone marrow shows hypocellularity, without infiltration of abnormal cells or marrow fibrosis. The peripheral blood must meet at least two of the following three criteria: absolute neutrophil count (ANC) <1.5×10⁹/L; platelet count <50×10⁹/L; hemoglobin <100 g/L. Based on the severity of the disease, AA is classified into severe aplastic anemia (SAA) and non-severe aplastic anemia (NSAA).

The diagnostic criteria for severe aplastic anemia (SAA) are as follows:

Peripheral blood criteria

At least two of the following three criteria must be met:

Absolute neutrophil count (ANC) <0.5×10⁹/L; Reticulocyte count <20×10⁹/L; Platelet count <20×10⁹/L; Bone marrow criteria The degree of bone marrow cellularity is <25% of normal; if ≥25% but <50% of normal, then the proportion of residual hematopoietic cells should be <30%.

Bone marrow aspiration results show hypocellular or severely hypocellular marrow, with a marked decrease in hematopoietic cells and an increase in non-hematopoietic cells such as lymphocytes and reticular cells.

Other：Other diseases causing pancytopenia, such as myelodysplastic syndrome (MDS), paroxysmal nocturnal hemoglobinuria (PNH), and congenital marrow failure syndromes, should be excluded.

If ANC <0.2×10⁹/L, the diagnosis is very severe aplastic anemia (VSAA); Those who do not meet the criteria for SAA are classified as non-severe aplastic anemia (NSAA);

• Meet the criteria for refractory AA: ①SAA: no response to first-line ATG + CSA treatment for more than 6 months; ②NSAA: ①persistent transfusion dependence of two or more blood cell lines, ②meet any of the following criteria: no response to immunosuppressive therapy with cyclosporine for more than 12 months, or no response to androgen and/or TPO receptor agonist therapy for more than 6 months, ④or progression to SAA; Meet the criteria for relapsed AA: initially effective to first-line immunosuppressive therapy, but pancytopenia recurs after reduction or discontinuation of immunosuppressive therapy, meeting the criteria for SAA.
• Karnofsky score ≥60, ECOG score ≤2, HCT-CI index ≤2.
• No HLA-identical sibling donor or no HLA-identical unrelated donor available.
• Understanding of the study procedures and voluntary written informed consent.

Exclusion criteria

• Subjects with any of the following conditions are not eligible for enrollment in this study:

• Pancytopenia and hypoplastic marrow diseases caused by other reasons (including PNH, etc.).
• Use of intermediate or high doses of cyclophosphamide (≥20mg/kg/d) for immunosuppressive therapy within 3 months before enrollment.
• History of hematopoietic stem cell transplantation.
• Known or suspected contraindications or allergies to fludarabine, melphalan, or other drugs.
• Patients with uncontrolled bleeding and/or infection after standard treatment before screening.
• Active viral hepatitis (hepatitis B, hepatitis C, etc.), HIV infection, or syphilis at baseline or screening.
• Creatinine clearance rate <60 ml/min before treatment, or serum creatinine >140 μmol/L.
• Pregnant or breastfeeding women.
• Participation in another clinical trial within 3 months before enrollment.
• Any other condition that the investigator deems may prevent the subject from completing the study or pose significant risks to the subject.