A Phase II Study of Low Dose Melphalan with Thiotepa and Fludarabine As Preparative Regimen for Alternative Donor Transplantation

Introduction

Alkylating agents such as thiotepa have shown to improve progression-free survival when added to reduced intensity conditioning (RIC) for patients with hematological malignancies undergoing allogeneic hematopoietic stem cell transplant (alloHCT). However, when combined with other conditioning agents such as high dose melphalan, it is seen to increase non-relapse mortality. In a retrospective analysis of double umbilical cord blood transplant (dUCBT) for AML, thiotepa 10mg/kg with high dose melphalan at 140mg/m2 and fludarabine 160mg/m2 showed 32% grade III gastrointestinal toxicity, 23% relapse, 28% 1-year treatment related mortality and 59%1-year overall survival¹. Another retrospective study in haplo-identical transplants for AML using same doses of thiotepa, melphalan and fludarabine showed 32% non-relapse mortality and 44% relapse rate by day 72 post-transplant².

We hypothesized that reducing the dose of melphalan when combining with thiotepa and fludarabine can be a safe and effective reduced intensity conditioning regimen for alternative donor transplant in this patient population.

Methods

We designed an open label, single arm, non-randomized study with the primary objective of measuring leukemia-free survival (LFS). Between 2018 to 2023, we enrolled patients with high-risk hematological malignancies who did not have matched related donors. Conditioning regimen consisted of Melphalan 100mg/m2 on day -8, thiotepa 10mg/kg on day -7 (dose adjusted to 5mg/kg for age over 60 years), fludarabine 160mg/m2 in divided doses on days -6, -5, -4 and -3. Post-transplant cyclophosphamide was added as 50mg/kg given on each of days +3 and +4 for haplo-identical transplant. Graft versus host disease (GVHD) prophylaxis of tacrolimus and mycophenolate mofetil (MMF) was started on day -5 for dUCBT and T +5 for haplo-identical transplant. Patients were followed for 12 months post-transplant or until treatment failure, defined as relapse, graft failure or death.

Results

Forty patients were enrolled, two did not proceed to transplant. Median follow-up of transplanted patients was 11.9 months (0.5-31.8). Median age was 52.5 years (15-71 years) with 26% (10/38) older than 60 years. Eleven patients (28%) were female. Nineteen patients (50%) had AML, others had ALL, MDS, CMML, CML or T-cell lymphomas. Twenty one patients (55%) were classified as high risk disease, based on molecular/cytogenetics or clinical data. Thirty-one (81.5%) were in complete remission (CR) based on disease evaluation, however twenty three patients (60.5%) had high disease risk index prior to transplant. Median co-morbidity index (CMI) score was 3 (0-5) and five patients (13%) had CMI score of 5. Twenty-eight (73.6%) underwent haplo-identical transplant, and ten underwent dUCBT. Day +100 bone marrow performed in 32/38 patients demonstrated morphologic CR in 100% patients. Overall survival (OS) at 12 months was 76.3% (95% CI: 59.4-86.9%). LFS at 12 months was 68.4% (95%CI: 51.1-80.7%). CMI score had significant impact on PFS (p=0.0315) and OS (p=0.0075). There was no difference in LFS between patients receiving dUCBT and haplo-identical transplants. Cumulative relapse rate at 1-year was 11% (95%CI: 0.033-0.227), cumulative non-relapse mortality at 1-year was 21% (95%CI: 0.098-0.352). Ten patients died during the study, from sepsis (4/10), COVID (1/10), relapsed disease (3/10) and GVHD (2/10). Graft rejection was seen in 2.6% (1/38) and graft failure was seen in 13.1% (5/38). Median time to neutrophil engraftment (ANC>500) was at 20 days (13-38 days) and median time to platelet engraftment was at 29 days (16-165 days).

Conclusion

Using a RIC regimen with lower dose melphalan, thiotepa, and fludarabine, we noted a 1-year LFS of 68.4% in patients with hematological malignancies undergoing alternative donor transplant. Relapse rates and NRM at 1-year was much lower than in previously reported studies^1,2. These promising data suggest that this novel RIC regimen warrants further investigation in larger clinical trials of alternative donor transplantation.