Ruxolitinib Pharmacokinetics and Pharmacodynamics in Children with Acute and Chronic Graft-versus-Host Disease

We evaluated the pharmacokinetics (PK) of oral ruxolitinib in children with steroid-refractory acute graft-versus-host disease (aGVHD) (age <12 years) and chronic GVHD (cGVHD) (age ≤18 years) using our published pediatric dosing. PK sampling was performed before and 2 hours after ruxolitinib administration in patients with established cGVHD. More extensive PK analyses were performed in patients with newly diagnosed aGVHD or cGVHD before and .5, 1, 2, 4, and 6 hours after ruxolitinib administration in patients weighing >10 kg and before, 3+, and 6+ hours in children weighing <10 kg. pSTAT1, pSTAT3, and pSTAT5 expression levels were measured on CD4⁺ and CD8⁺ T cells before and 2 hours after ruxolitinib administration as a pharmacodynamic marker of JAK/STAT inhibition. Thirteen patients were prospectively enrolled, including 8 with existing cGVHD (age 0 to ≤18 years), 4 with new-onset steroid-refractory aGVHD (age 0 to <12 years) and 1 with newly diagnosed steroid-refractory cGVHD. Great variability in PK was seen. Mean oral clearance (CL/F) was 7.76 ± 4.09 L/h (range, 3.1 to 15.3 L/h). The average elimination half-life was 2.32 ± 1.0 hours. Mean ruxolitinib clearance was higher in children age <2 years versus those age >2 years (12.1 ± 3.0 L/h versus 5.7 ± 2.8 L/h; P = .005) and was reduced with concurrent treatment with azoles and azithromycin. We saw a variable reduction in pSTAT1/3/5 expression on T cells at time of peak ruxolitinib absorption (2 hours after dosing). Children <10 kg had lower ruxolitinib exposure, possibly due to inherent increased drug clearance or variability in dosing methods, leading to decreased drug absorption.