Clinical Impact of Busulfan Pharmacokinetic Monitoring in Pediatric Allogeneic Transplants: Perspectives from A Transplant Center in A Low- and Middle- Income Country.

We evaluated the impact of busulfan pharmacokinetic (Bu‑PK) monitoring on toxicity and outcomes in pediatric patients undergoing busulfan-based myeloablative HSCT for benign (n = 221) and malignant diseases (n = 15) at our center in India (January 2023-December 2025). Bu‑PK using liquid chromatography-tandem mass spectrometry was available to 93 patients whose toxicity and outcomes were compared to 143 patients in whom Bu-PK was not done. Once-daily intravenous busulfan was administered over 3 h. Median busulfan dose administered was 105.3 (IQR 93.2-127.6)% with a wide range of 41.7%-213.1% of the standard weight-based dosing, with no added toxicity in those receiving doses >125% of the weight-based dose. Infections, particularly CMV reactivation, were more frequent in the Bu‑PK group with no increase in mortality. Mixed chimerism was significantly lower in the Bu‑PK group (4.6% vs. 24.1%), with an 84% reduction in odds (OR = 0.16, 95% CI 0.05-0.43,  p < 0.001). The implementation of monitoring was crucial for overcoming pharmacokinetic variability, reducing mixed chimerism, while maintaining an acceptable overall toxicity profile in our pediatric cohort.