Phase I Trial of High-Dose Clofarabine and Busulfan as a Myeloablative Regimen Prior to Allogeneic Hematopoietic Stem Cell Transplantation in Patients with Relapsed or Refractory Acute Leukemia.
Authors: Farag Sherif , Wood Lisa L , Schwartz Jennifer E. , Srivastava Shivani , Nelson Robert P. , Robertson Michael , Abonour Rafat , Sayar Hamid , Cripe Larry D. 
Published: October 2019

Abstract
Abstract 198
Fludarabine in combination high-dose busulfan (Bu) is an effective myeloablative preparative regimen for allogeneic stem cell transplantation. At doses used, however, fludarabine has only modest anti-leukemic activity. Clofarabine (Clo) is a second-generation purine nucleoside antimetabolite with significant single agent activity in patients with AML and ALL. The novel combination of Clo with Bu may provide improved disease activity safely. Therefore, we conducted a phase I trial to determine the maximum tolerated dose (MTD) of Clo in combination with Bu in patients with high-risk acute leukemia.
Patients received i.v. Bu (Busulfex) 0.8 mg/kg q 6 hrs on days −6 to −3 and Clo at 30–60 mg/m2/day on days −6 to −2 in successive cohorts. Stem cells were infused day 0. GvHD prophylaxis included sirolimus plus tacrolimus starting day −2 to day 100, tapering to day 180. Patients were eligible if they were 18–60 years, had primary refractory or relapsed and refractory AML or ALL, or were in CR2 or higher, had Karnofsky performance status ≥70%, and adequate organ function. Donors were HLA-matched related (5/6 or 6/6 antigen-matched) or unrelated (10/10 allele-matched). Toxicity was scored using the Common Terminology Criteria for Adverse events, version 3.0. Dose limiting toxicity (DLT) was defined as any grade 3–4 non-hematologic toxicity that did not resolve to grade 2 or less by day 30.
A total of 15 patients were treated at 4 Clo dose levels, 30 (n=3), 40 (n=3), 50 (n=3), and 60 mg/m2 (n=6). Seven males and 8 females of median age 48 (30–58) years, with AML (n=13) or ALL (n=2) were treated. At transplant, leukemia was relapsed and refractory (n=8), primary refractory (n=6), or in CR2 (n=1). Median number of lines of treatment failed before transplant was 2 (1–3). Median marrow blasts at transplant was 12% (3%–83%). Hematopoietic cell transplants were from related (n=9) and unrelated (n=6) donors. All patients engrafted. Median time to neutrophils >0.5×109/l was 16 (12–20) days, and to platelets >20×109/l was 15 (10–42) days. One patient treated at the 30 mg/m2 dose level failed to achieve platelets > 20×109/l. No DLT was observed. Transient Grades 3–4 non-hematological toxicities were evenly distributed across all 4 dose levels, and included vomiting (n=3), mucositis (n=9), hand-foot syndrome (n=1), and elevation of AST/ALT (n=10). Grades 3–4 elevation of AST/ALT occurred in 2 of 3 patients treated at 30 mg/m2, 3 of 3 at 40 mg/m2, 2 of 3 at 50 mg/m2, and 3 of 6 patients at 60 mg/m2 dose levels. AST/ALT peaked at day −1 or 0 and returned to baseline in all patients by day 10, with no long-term sequelae. There was no correlation between Clo dose and peak AST/ALT. One patient developed acute renal failure at the 60 mg/m2 dose on day +12 in association with elevated tacrolimus levels, although the creatinine subsequently normalized. Two patients, both at the 30 mg/m2 dose, developed mild veno-occlusive disease of the liver which was self-limiting. One treatment-related death due to sepsis was observed at day +104 in a patient treated at the 30 mg/m2 dose. Thirteen of 15 patients were in CR by day 30; 2 patients, treated at 40 mg/m2 and 50 mg/m2, respectively, failed to achieve CR. Day 100 mortality was 0. With a median follow-up of 313 days, the 1-year relapse-free survival was 51% ± 15%, and the 1-year overall survival was 61% ± 14%.
Clo at doses as high as 60 mg/m2/day × 5 days in combination with Bu 3.2 mg/kg/day × 4 days is well tolerated and demonstrates promising efficacy in a very-high risk acute leukemia population. The MTD has not been reached. We recommend Phase II testing of Clo 60 mg/m2/day × 5 days in combination with high-dose Bu as a myeloablative regimen for allogeneic stem cell transplantation in patients with acute leukemia.

Disclosures:
No relevant conflicts of interest to declare.