Results Of a Phase I Study Of Clofarabine and 2 Gy Total Body Irradiation (TBI) As a Non-Myeloablative Preparative Regimen For Hematopoietic Stem Cell Transplantation (HSCT) In High Risk Children With Hematologic Malignancies: A Multi-Center TACL Study

Reduced intensity conditioning (RIC) regimens for HSCT in hematologic malignancies are successful in reducing transplant related mortality (TRM) and making HSCT available for a select group of high risk patients. Despite HSCT and its associated graft-versus-leukemia (GVL) effect, the risk of relapse remains high with RIC regimens. Clofarabine is a second generation purine analogue with significant anti-leukemic activity in pediatric patients. We postulated that a non-myeloablative (NMA) preparative regimen with clofarabine and 2 Gy TBI may lead to sustained engraftment with low TRM in high-risk children with hematologic malignancies undergoing related and unrelated HSCT.

The primary aim of the Phase I study was to establish the maximum feasible dose (MFD) of clofarabine in conjunction with 2 Gy TBI that leads to durable engraftment with acceptable toxicity in a high-risk pediatric population. Patients between 1-21 years of age with leukemia and undergoing HSCT with significant co-morbidities or any condition that will make them at high risk of TRM with a myeloablative regimen were eligible. Patients had to be in CR or CRp (no morphologic disease, ANC >0.75 x 10⁹/L but without platelet recovery) status prior to HSCT with a PS score of >50 with no active infection, a creatinine clearance of >70 ml/min/1.73m² and liver enzymes <5X ULN. Patients were stratified according to donor source- related donor (RD) or unrelated donor (UD; 10/10 or 9/10 HLA match) arms. Peripheral blood stem cells (PBSC) were the preferred graft source (BM was allowed for RD only). Cyclosporine and mycophenolate mofetil were used as GVHD prophylaxis. A phase I modified 3+3 design was used with engraftment, severe toxicity and TRM as study end-points. Starting dose of clofarabine was 40 mg/m² x 5 days with escalation to 52 mg/m² in case of non-engraftment and acceptable toxicity in the first cohort or de-escalation to 30 mg/m²for dose limiting toxicity (DLT) or TRM. Patients completed the dose evaluation at 100 days post-HSCT, but were followed for 1 year post- transplant and were censored at relapse or death.

Seventeen pediatric patients with a median age of 9 years (range 1-18) enrolled on the study between 7/2010 and 5/2013 and 16 have completed 100-day post-HSCT evaluations. Ten patients had ALL (CR2 =5; CR3 =4 and CR4=1) while 7 had AML (PIF=1; CR1= 1; CR2=3 and secondary AML=3); 9 received UD and 8 received RD grafts. All patients were heavily pre-treated and the indication for the NMA approach was: prior HSCT (n=6), prior liver transplant (n=2), trisomy 21 (n=2) or significant comorbidities (n=7).

There was no observed TRM or DLT in the first cohort (clofarabine 40 mg/m²/dose x 5 days) in both the UD and RD strata. The dose was escalated to 52 mg/m²/dose in the next cohorts. All patients tolerated the maximum dose well and no DLT was reported. Significant non-hematologic toxicities (CTC ≥ grade 3) observed were- transient transaminitis (ALT increase in 30% and AST increase in 17%); anorexia (35%) and skin reactions in 16%.

PBSC was the stem cell source for all patients, except 3 who had received BM from a RD. Median cell dose was 6.8 x 10⁶ CD34+/kg (range: 2.2-11.5 x 10⁶/kg). All patients achieved neutrophil and platelet engraftment at a median of 19 days (range: 9-25) and 11 days (range: 3-29), respectively. CD3 chimerism analysis at day+30 post-transplant showed donor engraftment in all patients; 15 were full donor (>95%) and 2 showed mixed chimerism (53% and 81% donor).

Ten (58%) patients developed acute GVHD, 8 (47%) had ≤ Grade II and 2 (11%) had Grade III acute GVHD. Seven (41%) patients relapsed (4/8 RD and 3/9 in UD arms) at a median time of +106 days (range: 88-190) post-HSCT and all of them died due to disease. Ten patients (58%) are alive and disease free; six have completed 1 year evaluations and 4 are still in follow-up.

The MFD of clofarabine in combination with 2 Gy TBI was determined to be 52 mg/m². This regimen is well tolerated and leads to brisk and sustained donor engraftment in a pediatric population that is at high risk of TRM.

Thompson:Sanofi Aventis: Speakers Bureau.