For a 5‑year‑old girl with acute myeloid leukemia with MLL rearrangement in first complete remission undergoing a matched sibling hematopoietic stem‑cell transplant, which conditioning regimen is preferred: busulfan plus cyclophosphamide or thiotepa, busulfan, and fludarabine?

Busulfan and Cyclophosphamide (BuCy) is the preferred conditioning regimen for this patient

For a 5-year-old girl with AML-MLL rearrangement in CR1 undergoing matched sibling donor transplant, busulfan and cyclophosphamide (BuCy) remains the standard myeloablative conditioning regimen. While thiotepa-busulfan-fludarabine (TBF) shows promise in recent studies, the evidence base for BuCy in this specific clinical context—pediatric AML in CR1 with matched sibling donor—is more established and guideline-supported.

Rationale Based on Guidelines

The Chinese Society of Hematology consensus 1 explicitly lists modified BuCy (mBuCy) as the standard myeloablative conditioning for HLA-matched sibling HSCT in pediatric AML. The traditional BuCy regimen uses busulfan 16 mg/kg (oral) or 12.8 mg/kg (IV) plus cyclophosphamide 120 mg/kg 1. The modified version includes cytarabine 2 g/m² and adjusts busulfan to 9.6 mg/kg (IV) with cyclophosphamide 3.6 g/m² 1.

The 2012 Blood guideline 2 emphasizes that myeloablative chemotherapy-based conditioning is preferred over total body irradiation in pediatric AML due to lower late effects and reduced secondary malignancy risk. While this guideline notes the optimal conditioning regimen "remains to be identified," it establishes BuCy as the standard against which alternatives are compared.

Evidence for TBF vs BuCy

The most relevant comparative study 3 examined 2,523 adult AML patients (ages 18-50) and found TBF associated with:

• Lower relapse risk (HR 0.6, p=0.02)
• No difference in overall survival or leukemia-free survival
• Trend toward higher grade III-IV acute GVHD (HR 1.8, p=0.06)

However, this study has critical limitations for your patient:

• Adult population only (ages 18-50)
• Mixed donor sources (both matched sibling and unrelated donors)
• Patients in CR1 showed only a trend toward better LFS with TBF (HR 0.7, p=0.10), not reaching statistical significance

Why BuCy for This Specific Patient

Age and Donor Type Matter

Your patient is 5 years old with a matched sibling donor—the lowest-risk transplant scenario. The 3 study showing TBF advantages was conducted in adults, and the 4 study of TBF was specifically for haploidentical transplants, not matched siblings. The risk-benefit calculation differs substantially in pediatric matched sibling transplants where baseline outcomes are already excellent.

Disease Biology Considerations

MLL-rearranged AML in CR1 is considered intermediate-to-high risk, but achieving CR1 before transplant is favorable. The 2 guideline notes controversy about whether intermediate-risk patients benefit from allogeneic HSCT in CR1, though consensus exists against transplanting favorable-risk patients in CR1. For a patient already proceeding to matched sibling transplant in CR1, the established BuCy regimen provides proven efficacy without the uncertainty of newer protocols.

GVHD Risk Profile

The TBF regimen showed a trend toward higher severe acute GVHD 3, which is particularly concerning in pediatric patients where quality of life and long-term complications are paramount. In a 5-year-old with decades of life ahead, minimizing GVHD risk while maintaining disease control is critical.

Practical Implementation

Use the modified BuCy regimen 1:

• Hydroxyurea 80 mg/kg (divided doses) on day -10
• Cytarabine 2 g/m² on day -9
• Busulfan 9.6 mg/kg IV (divided over days -8 to -6)
• Cyclophosphamide 3.6 g/m² (days -5, -4)
• MeCCNU 250 mg/m² on day -3

Pharmacokinetic monitoring of busulfan is essential in pediatric patients to achieve target AUC and minimize toxicity 5.

When to Consider TBF Instead

TBF would be more appropriate if:

• The patient had relapsed disease or CR2 (where lower relapse with TBF becomes more critical) 3
• An alternative donor (unrelated or haploidentical) was being used 4
• The patient was heavily pre-treated with multiple lines of therapy 5
• There was concern about cyclophosphamide toxicity (cardiac or bladder)

Common Pitfalls to Avoid

1. Don't extrapolate adult data directly to young children—pharmacokinetics, toxicity profiles, and long-term effects differ substantially
2. Don't use reduced-intensity conditioning in this young, fit patient in CR1—myeloablative therapy is indicated 2, 1
3. Don't add TBI unless absolutely necessary—chemotherapy-based conditioning avoids radiation late effects 2
4. Don't skip busulfan pharmacokinetic monitoring—pediatric dosing requires adjustment to achieve therapeutic levels 5

The evidence supports BuCy as the standard of care for this specific clinical scenario, with TBF reserved for higher-risk situations where its potential advantages outweigh the less-established safety profile in pediatric matched sibling transplants.