Treosulfan/fludarabine/thiotepa conditioning treatment before allogeneic hematopoietic stem cell transplantation for children with hematological malignancies

Pediatric patients with hematological malignancies are commonly treated with busulfan- or fractionated total body irradiation (TBI)-based myeloablative conditioning regimens prior to allogenic hematopoietic stem cell transplantation (allo-HSCT). However, both these treatment options are associated with an increased risk of adverse events (AEs). Treosulfan-based conditioning prior to allo-HSCT has been shown to have myeloablative, immunosuppressive, and antineoplastic effects that are associated with a reduced risk of non-relapse mortality (NRM) in adult patients with AML and MDS. Therefore, Krzysztof Kalwak et al. published results of a phase II, non-randomized, prospective study that evaluated the safety and efficacy of treosulfan/fludarabine conditioning in children in Bone Marrow Transplantation.¹

Study design

Clinical endpoint

• Engraftment and complete donor-type chimerism (defined as ≥ 95% donor cells)
• NRM
• Disease relapse/progression (RI)
• Relapse/progression-free survival (RFS/PFS)
• Acute/chronic graft-versus-host disease (GvHD)
• GvHD-free and relapse/progression-free survival (GRFS)
• cGvHD-free and relapse/progression-free survival (CRFS)
• Overall survival (OS)

Treatment and dosing

• Individualized body surface area (BSA)-adapted intravenous (IV) treosulfan dose was given to all patients on Days −6 to −4, as follows: BSA of ≤ 0.5 m² was given 10 g/m²/day; BSA > 0.5–1.0 m² was given 12 g/m²/day; or BSA > 1.0 m² was given 14 g/m²/day (Table 1)
• A total dose of 150 mg/m² fludarabine (IV) was given: 30 mg/m²/day fludarabine was administered from Day −7 to −3
• The option of administrating an intensified regimen by additional thiotepa infusion was given at the investigator’s discretion to 65 patients: thiotepa (IV) 2 × 5 mg/kg/day was given on Day −2; total dose was 10 mg/kg
• Allogeneic hematopoietic stem cells were given at Day 0

Patient characteristics

• Between November 2014 and July 2015, 70 pediatric patients (aged 28 days to 17 years) with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), or juvenile myelomonocytic leukemia (JMML) were enrolled in this study at 18 transplantation sites in five European countries (Table 1). 

Table 1. Patient characteristics and transplant data by hematological malignancy¹

	Disease
	ALL (N = 23)	AML (N = 29)	MDS (N = 10)	JMML (N = 3)	Overall (N = 65)
Age [years]
Median	12.0	8.0	14.0	2.0	11.0
ICH age group [n (%)]
28 days to 23 months	2 (8.7%)	4 (13.8%)	1 (10.0%)	1 (33.3%)	8 (12.3%)
2–11 years	7 (30.4%)	14 (48.3%)	2 (20.0%)	2 (66.7%)	25 (38.5%)
12–17 years	14 (60.9%)	11 (37.9%)	7 (70.0%)	0 (0.0%)	32 (49.2%)
Number of HSCT [n (%)]
First	22 (95.7%)	28 (96.6%)	8 (80.0%)	2 (66.7%)	60 (92.3%)
Second	1 (4.3%)	1 (3.4%)	2 (20.0%)	1 (33.3%)	5 (7.7%)
Treosulfan dose [n (%)]
10 g/m2/day −6, −5, −4	1 (4.3%)	3 (10.3%)	0 (0.0%)	1 (33.3%)	5 (7.7%)
12 g/m2/day −6, −5, −4	5 (21.7%)	13 (44.8%)	3 (30.0%)	2 (66.7%)	23 (35.4%)
14 g/m2/day −6, −5, −4	17 (73.9%)	13 (44.8%)	7 (70.0%)	0 (0.0%)	37 (56.9%)

ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; ICH, International Council of Harmonization; JMML, juvenile myelomonocytic leukemias; MDS, myelodysplastic syndrome; N/A, not applicable

^aFor ALL and AML subjects only.

Results

• The median follow-up was 41.8 months (range for surviving patients, 24.2–57.5 months)
• The most common donor type was matched-unrelated in patients with acute lymphoblastic leukaemia. 60.9% of these patients received from a bone marrow source and 39.1% from a peripheral blood source  

Engraftment and chimerism

• 64 (98.5%) patients achieved reconstitution of granulopoiesis
• There was one reported death (1.4%) prior to engraftment, 15 days after allo-HSCT
• Maximum conditional cumulative incidence of engraftment, 43 days after allo-HSCT: 100% (90% CI, 97.7–100)
• There were no reported primary graft failures. One patient with ALL had reduced neutrophil and leukocyte count, but had 100% donor chimerism, and subsequently received a stem cell boost 113 days after transplant
• Incidence of complete donor-type chimerism:
  • Day +28: 98.4% (90% CI, 92.8–9)
  • Day +100: 92.2% (90% CI, 84.3–9)
  • 12 months: 92.6% (90% CI, 83.8–4)
• The incidence of complete donor-type chimerism was not significantly influenced by treosulfan dose, donor type, or patient’s age group. However, diagnosis of JMML and recipients of second allo-HSCT showed significantly reduced donor chimerism.

NRM and RI incidence

• Cumulative incidence of NRM at 36 months: 3.1% (90% CI, 0.0–6)
• Cumulative incidence of RI at 36 months: 21.7% (90% CI, 13.2­–1)
  • Subgroup analyses: treosulfan dose, donor type, or patient’s age group had no statistically significant effect, but underlying disease did (p < 0.0001)
  • RI of ALL was 26.1%; AML, 17.2%; MDS, 0.0%; and JMML, 100%. The incidence of RI at second allo-HSCT increased from 18.5 to 60.0% (p = 0.0140)

RFS/PFS

• Kaplan–Meier estimate of RFS/PFS at 36 months: 73.6% (90% CI, 63.3–5)
• Exploratory subgroup analyses showed second HSCT (first HSCT [76.4%]; second HSCT [40%]; p = 0.0234) and underlying disease (ALL, 69.6%; AML, 79.3%; MDS, 88.9%; JMML, 0%; p = 0.0001) had a statistically significant impact

GvHD

• Cumulative incidence of Grades I–IV acute GvHD at Day 100: 45.3% (90% CI, 35.1–5), Grades II–IV: 26.6% (90% CI, 17.5–35.6), and Grades III–IV: 7.8% (90% CI, 2.3–13.3)
• Cumulative incidence of chronic GvHD at 36 months: 25.8% (90% CI, 16.7–9), and for moderate/severe chronic GvHD: 19.4% (90% CI, 11.1–27.7)
• The three different treosulfan dose groups had no difference in GvHD incidence

GRFS and CRFS

• Kaplan–Meier estimate of GRFS at 36 months: 56.7% (90% CI, 45.9–1)
  • GRFS at 36 months: ALL, 56.5%; AML, 69.0%; and MDS, 36.0% (JMML, not applicable)
• Kaplan–Meier estimate of CRFS at 36 months: 58.2% (90% CI, 47.4–5)
  • CRFS for ALL, 60.9%; AML, 69.0%; MDS, 36%; and JMML, 0.0%; p = 0.0017

OS

• Kaplan–Meier estimate of OS after HSCT at 36 months: 83.0% (90% CI, 73.7–3)
  • OS for ALL: 78.3%, AML: 86.2%, MDS: 90%, (JMML, not applicable)
• Exploratory analysis: patients aged 28 days to 23 months had a higher OS (n = 8; 100%; 90% CI, 100–100) compared to patients aged 12–17 years (n = 32; 74.9%; 90% CI, 59.5–1)
• Individual BSA-related dose calculation showed the 10 g/m² treosulfan group achieved an OS of 100% and the 14 g/m² dose group had an OS of 75.6% (90% CI, 61.5–1)

Safety

• 11 patients had died by the time of data cutoff, eight due to relapse/progression and three patients (4.6%) due to transplantation-related complications
• Median time from transplantation to death: 9.72 months (range, 0.5–8)
• Treatment-emergent AEs were reported in 96.9% of patients (63/65)
• The most prominent severe Common Terminology Criteria for Adverse Events (CTCAEs) were Grade III oral mucositis (43.1%), infections and infestations (30.8%), nausea and vomiting (16.9% for both), and diarrhea (15.4%)
• Skin and subcutaneous tissue symptoms of at least CTCAE Grade III were considered to be within the acceptable margin (12.3%), regardless of thiotepa addition

Pharmacokinetics

• 290 pharmacokinetic (PK) samples from 54 patients were analysed
• The mean AUC_last (± SD) was comparable for the three dose groups
  • 10 g/m² dose group: 1,686 ± 345 µg h/mL
  • 12 g/m² dose group: 1,599 ± 33 µg h/mL
  • 14 g/m² dose group: 1,848 ± 283 µg h/mL
• The median C_max was comparable in the different dose/BSA groups
  • 10 g/m² dose group: 700 ± 218 µg/mL
  • 12 g/m² dose group: 634 ± 192 µg/mL
  • 14 g/m² dose group: 650 ± 98 µg/mL

Conclusion

The treosulfan/fludarabine/thiotepa conditioning regimen with BSA-adapted dosing was deemed tolerable and efficacious in children with hematological malignancies. The cumulative incidences of OS and NRM compared well to other conditioning regimens. However, a limitation of this study is the small sample size of patients treated, particularly in the MDS and JMML subgroups. Therefore, the poor outcome shown for patients with JMML should be interpreted with caution. Based on recent clinical data, the European Commission has approved treosulfan in combination with fludarabine as part of conditioning treatment prior to allo-SCT in children older than 1 month with malignant diseases. This study provides further confirmation of the myeloablative potential of the treosulfan/fludarabine/thiotepa conditioning regimen, which induced a good rate of complete donor-type chimerism.