Simultaneous population pharmacokinetic modelling of ketamine and three major metabolites in patients with treatment‐resistant bipolar depression

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT • (R,S)‐ketamine is a phencyclidine derivative that was initially developed as an anaesthetic agent and which is currently being studied in the treatment of pain and depression. After administration, the drug is extensively N‐demethylated to (R,S)‐norketamine. The pharmacokinetics of ketamine and norketamine have been extensively studied in volunteers and patients after the administration of anaesthetic and sub‐anaesthetic doses. However, ketamine and norketamine are extensively transformed into a series of diastereomeric hydroxyketamines and hydroxynorketamines and (R,S)‐dehydronorketamine metabolites. The plasma kinetics of these metabolites have not been elucidated. WHAT THIS STUDY ADDS • The current study expands the characterization of the disposition kinetics of (R,S)‐ketamine and (R,S)‐norketamine and presents a population pharmacokinetic analysis of (R)‐ketamine, (S)‐ketamine, (R)‐norketamine, (S)‐norketamine, (R)‐dehydronorketamine, (S)‐ dehydronorketamine and (2S,6S;2R,6R)‐hydroxynorketamine and the serum concentration–time profiles of multiple ketamine metabolites observed in the plasma of patients after a single 40 min infusion of a sub‐anaesthetic dose of the drug. The data demonstrate that while norketamine is an initial metabolite, it is not the major circulating metabolite and suggest that the determination of the downstream metabolites of ketamine may play a role in the pharmacological effects of the drug. AIM To construct a population pharmacokinetic (popPK) model for ketamine (Ket), norketamine (norKet), dehydronorketamine (DHNK), hydroxynorketamine (2S,6S;2R,6R)‐HNK) and hydroxyketamine (HK) in patients with treatment‐resistant bipolar depression. METHOD Plasma samples were collected at 40, 80, 110, 230 min on day 1, 2 and 3 in nine patients following a 40 min infusion of (R,S)‐Ket (0.5 mg kg−1) and analyzed for Ket, norKet and DHNK enantiomers and (2S,6S;2R,6R)‐HNK, (2S,6S;2R,6R)‐HK and (2S,6R;2R,6S)‐HK. A compartmental popPK model was constructed that included all quantified analytes, and unknown parameters were estimated with an iterative two‐stage algorithm in ADAPT5. RESULTS Ket, norKet, DHNK and (2S,6S;2R,6R)‐HNK were present during the first 230 min post infusion and significant concentrations (>5 ng ml−1) were observed on day 1. Plasma concentrations of (2S,6S;2R,6R)‐HK and (2S,6R;2R,6S)‐HK were below the limit of quantification. The average (S) : (R) plasma concentrations for Ket and DHN