ACP-5862 is a major active, circulating, pyrrolidine ring-opened metabolite of Acalabrutinib with an IC50 of 5.0 nM for Bruton tyrosine kinase (BTK). ACP‐5862 is a weak time‐dependent inactivator of CYP3A4 and CYP2C8. Acalabrutinib is an orally active, irreversible, and highly selective BTK inhibitor, with an IC50 of 3 nM and EC50 of 8 nM.