PF 4800567 is a drug developed by Pfizer which acts as a selective potent inhibitor of CK1epsilon (IC(50) = 32 nM) with greater than 20-fold selectivity over CK1delta, and has mainly been used in the study of the casein kinase 1 enzymes in the regulation of circadian rhythm, as well as showing potential neuroprotective effects. PF 4800567 was found in animal studies to enhance responses to certain drugs of abuse such as methamphetamine and fentanyl, which suggests a role for CK1-ε in negative regulation of sensitivity to stimulant and opioid drugs.