Pharmacokinetics, Pharmacodynamics and Safety of Ticagrelor in Children with Sickle Cell Disease: Results of a Two-Part Phase 2 Study

INTRODUCTION: Activated platelets promote the adhesion of sickle cells to endothelial cells and the formation of platelet-leukocyte aggregates, contributing to vaso-occlusion (VOC). It has been suggested that inhibition of platelet reactivity may reduce the rate of VOCs in sickle cell disease (SCD). A study with ticlopidine reported a reduced rate of VOCs versus placebo. A recent, placebo-controlled, pediatric study with prasugrel showed a non-significant reduction in VOCs events, which was likely due to the low level of platelet inhibition. Ticagrelor is a reversible P2Y₁₂ receptor antagonist shown to inhibit ADP-induced platelet aggregation and is approved for use in adults with coronary artery disease (CAD). A phase 2 study was conducted to characterize the pharmacokinetic (PK), pharmacodynamic (PD) and safety profiles of ticagrelor in children aged 2-17 years with SCD (HbSS or HbS/β0; NCT02214121).

METHODS: This study was conducted in 2 parts: Part A was an open-label, randomized, dose-ranging study in which patients received two single doses of ticagrelor 1 week apart, followed by twice daily (bid) ticagrelor for 1 week. Initially, single doses of 0.125-0.563 mg/kg were given, followed by repeated dosing with 0.125 mg/kg bid. Individual platelet response was used to inform further dosing. After an interim analysis including the first 14 patients in Part A, single doses were increased to a maximum of 2.25 mg/kg and repeated dosing up to 0.75 mg/kg bid. Part B (optional) immediately followed the repeated dosing in Part A: patients were randomized 2:1 to continue ticagrelor with the same dose used during Part A or matching placebo as a double-blind treatment for 4 weeks. Safety was assessed during both parts, pain and analgesic use were recorded daily in an eDiary, and painful crises were reported at each visit. Exposure to ticagrelor and the active metabolite (AR-C124910XX), as well as the subsequent platelet inhibition, were determined after single and repeated doses in Parts A and B.

RESULTS: A total of 45 children with SCD, mean age 11.2 years (range: 3-17) received study drug in Part A and 23 (15 ticagrelor, 8 placebo) children, mean age 10.0 years (range: 3-17) in Part B. Most patients were of African descent (35/45 in Part A and 22/23 in Part B). Ticagrelor exposure increased approximately dose-linearly, with maximum plasma concentration values ranging from 15.2 to 566.6 ng/mL (with 0.125 mg/kg to 2.25 mg/kg) for ticagrelor and 3.1-156.0 ng/mL for AR-C124910XX. The parent:metabolite ratio in children with SCD was similar to that previously observed in adults. Mean P2Y₁₂ reaction units (PRU) after single doses of ticagrelor decreased from pre-dose to 2 hours post-dose (change from baseline: -23.4 to -207.8 PRU for 0.125 mg/kg to 2.25 mg/kg), and returned towards baseline up to the last timepoint (6 or 8 hours' post-dose). In Part B, no difference was seen between the placebo and ticagrelor groups in pain ratings and analgesic use, although the study was not statistically powered to detect differences in these outcomes. Ticagrelor was well tolerated, with no bleeding during treatment and no patient discontinued treatment due to an adverse event.

CONCLUSIONS: The PK/PD profile of ticagrelor in children with SCD was suitably characterized and the exposure-response relationship appeared similar to that previously observed in adults with CAD. Ticagrelor was well tolerated and no safety concerns were raised. The potential to impact SCD-related pain crises needs further evaluation, as the number of patients, low doses for some patients and short study duration limit the efficacy conclusions from this study.