Myeloablative Fractionated Busulfan, Fludarabine, Cladribine, Thiotepa and Venetoclax (Cladillac) Conditioning Regimen for Very High-Risk AML/MDS: A Phase 2 Trial

Background: Patients with high-risk AML and MDS have a very poor outcome even with stem cell transplantation (SCT) and need an alternative approach. Busulfan-based conditioning regimens are efficacious in SCT, though relapse remains a major concern. Interestingly, combinations of busulfan and fludarabine with cladribine, venetoclax, and thiotepa have been shown to synergistically kill leukemia cells in preclinical studies. Adding these compounds to busulfan-based regimens may therefore reduce relapse risk post SCT. Here, we intensified a busulfan and fludarabine conditioning regimen by extending the duration of administration from a few days to 3-weeks, while adding cladribine, thiotepa and venetoclax to increase leukemia cell killing and improve SCT outcomes. We investigated this novel regimen in a prospective phase 2 trial enrolling high-risk AML/MDS patients.

Patients and Methods: High-risk AML or MDS was defined by the presence of one of the following criteria; AML: primary refractory or relapsed disease, MRD+, ELN22 adverse risk disease, secondary AML, MDS: IPSS-R > 3.5, poor or very poor risk cytogenetics, bone marrow blasts ≥ 5%, mutated TP53 or RAS pathway genes. Such patients, 18-70 years-old, with a matched or haploidentical related donor, or a 7/8 or 8/8 HLA matched unrelated donors (MUD) received outpatient busulfan (Bu) 100 mg/m² on days -20 and -13, fludarabine 10mg/m², cladribine 10mg/m² and busulfan pharmacokinetically dosed to achieve a total systemic exposure for the course of 20,000±12% µmol.min on days -6 to -3, venetoclax 400 mg daily on days -22 to -3. Graft versus Host (GVHD) prophylaxis was post-transplant cyclophosphamide 50 mg/kg on days +3 and +4, and tacrolimus ± MMF. Primary outcome was progression-free survival (PFS). A total of 50 patients provided more than 80% power to detect an increase in 1-year PFS from historical 30% to 47% with 5% Type I error rate (ClinicalTrials.gov: NCT04708054).

Results: 49 patients, 25 females and 24 males, with a median age of 59 (18-69) years were enrolled between December 2021 to October 2022. Of 25 AML patients, 10 (40%) had primary refractory or relapsed disease, 8 (53%) of 15 patients in remission were MRD+, and 14 (56%) had adverse risk disease (ELN 22). Of 24 MDS patients, 14 (58%) had IPSS-R > 3.5 and 19 (83%) had mod high, high or very high IPSS-M. Eleven of the 49 (22%) patients had mutated TP53. Donor was a matched sibling in 13 (27%), MUD in 27 (55%), mismatched unrelated in 6 (12%), and haploidentical related in 3 (6%). The comorbidity score was ≥ 3 in 23 (47%) patients.

With a median follow up of 23 months, median PFS was not reached (95% CI: 12 months to not reached). 1-year and 2-year PFS was 61% (49-77). 2-year overall survival, non-relapse mortality, and relapse rates were 65% (53-80), 22% (11-34), and 16% (6-27), respectively. Univariate analysis for PFS did not show any association with any of the covariates tested. Univariate analysis for relapse showed a trend towards higher relapse with TP53 positive patients (hazard ratio = 3.693, p = 0.056).

Median time to neutrophil engraftment was 15 (12-32) days and platelet engraftment was 23 (5-221) days. There were no graft failures. At day 30, T cell chimerism was 100% (66-100) donor and myeloid chimerism was 100% (100-100) donor. Grade 2-4 acute GVHD was seen in 37% (23-50), grade 3-4 in 14% (4-24), chronic GVHD in 14% (4-24), and moderate to severe chronic GVHD in 8% (0-16) of patients.

Conclusion: This study met its primary endpoint with encouraging 2-year PFS of 61% in this very high-risk cohort of patients with AML/MDS. Our results warrant further study of this novel regimen.