Combining Clofarabine/Fludarabine with Exposure Targeted Busulfan for Pediatric Leukemia: An Effective, Low Toxic TBI-Free Conditioning Regimen

Background: The combination of Clofarabine + Fludarabine + Busulfan (CloFluBu) was found to have synergistic anti-leukemic activity against ALL and AML blasts in vitro(Andersson et al: BBMT 2011). As TBI induces significant late effects in childhood ALL, and AML patients have high relapse rates, we hypothesized that CloFluBu may be an interesting alternative to TBI in ALL and add anti-leukemic activity in AML. Within the “Dutch COG HCT Working Group” we prospectively studied the outcomes of CloFluBu-conditioning regimen for lymphoblastic and myeloid malignancies.

Methods:  Patients from the 2 pediatric HCT programs (LUMC and UMC Utrecht) with a lymphoblastic or myeloid malignancy were included from Aug-2011 to present. Clofarabine 30mg/m2 was given in 1 hour, followed by Fludarabine 10mg/m2 in 1 hour followed by a 3-hour infusion of once daily targeted busulfan (weight-based dosing: with therapeutic drug monitoring). Thymoglobulin was added in unrelated donors (except in AML patients receiving cord blood: CB) and GvHD prophylaxis was according to standard protocols. The cumulative target area under the curve (AUC) for Bu was 90 mg*h/L. Primary endpoint: leukemia free survival (LFS) and overall survival (OS). Other endpoints: acute and chronic graft-versus-host disease (GvHD), VOD, non-infectious lung injury, neutrophil (@day60) and thrombocyte engraftment (@day 180). A predictor analysis was performed using Cox Proportional Hazard Models.

Results:62 patients were included: 28 AML-CR2, 14 ALL-CR1, 3 Infant-ALL, 11 ALL-CR2/3, 6 other (4 MDS, 2 CML, 1CNL). 10/19 ALL-CR1-3 patients with available MRD status prior to HCT were positive: 4 >10e-3 and 6 < 10e-3. Donors used: 25 unrelated CB, 14 (matched) Family donors (FD; 1 Haplo-identical) and 23 Matched Unrelated Donors. Median age at HCT: 10.9 (0.5-17.9) years. Median follow-up 298 (range 18 - 1182) days. The estimated 2-year OS/LFS was 74+/-7% (AML 76%, ALL-CR1 100%, MDS 67%, ALL-CR2-3 59%), with an estimated NRM@1year of  9+/-4% and relapse@1year of 21+/-7%. Other endpoints: Probability of neutrophil engraftment 100% @day60 and thrombocyte engraftment (50x10e9/L) of 84+/-6% @day 180,  only 1 graft failure (1.5%: MUD donor. Succesfully re-grafted with CB) was noted. Toxicity endpoints:  aGvHD 2-4 @ day 180 was 32+/-6% (grade 3-4: 14+-4%), extensive cGvHD@1year 8+-6%, non-infectious lung-injury @ 2 years 12+-4% and no VOD (0%) was noted.

Conclusion: The preliminary results of this TBI-free conditioning regimen (CloFluBu) in myeloid- and lymphoblastic malignancies, showed limited toxicity and encouraging LFS given the high risk group of patients. A longer follow-up and in a larger cohort is needed to draw firm conclusions with regards to the anti-leukemic effect and late effects.