Sex-dependent metabolism of ketamine and (2R,6R)-hydroxynorketamine in mice and humans

Abstract

Background: Ketamine is rapidly metabolized to norketamine and hydroxynorketamine (HNK) metabolites. In female mice, when compared to males, higher levels of (2R,6R;2S,6S)-HNK have been observed following ketamine treatment, and higher levels of (2R,6R)-HNK following the direct administration of (2R,6R)-HNK.

Aim: The objective of this study was to evaluate the impact of sex in humans and mice, and gonadal hormones in mice on the metabolism of ketamine to form norketamine and HNKs and in the metabolism/elimination of (2R,6R)-HNK.

Methods: In CD-1 mice, we utilized gonadectomy to evaluate the role of circulating gonadal hormones in mediating sex-dependent differences in ketamine and (2R,6R)-HNK metabolism. In humans (34 with treatment-resistant depression and 23 healthy controls) receiving an antidepressant dose of ketamine (0.5 mg/kg i.v. infusion over 40 min), we evaluated plasma levels of ketamine, norketamine, and HNKs.

Results: In humans, plasma levels of ketamine and norketamine were higher in males than females, while (2R,6R;2S,6S)-HNK levels were not different. Following ketamine administration to mice (10 mg/kg i.p.), C_max and total plasma concentrations of ketamine and norketamine were higher, and those of (2R,6R;2S,6S)-HNK were lower, in intact males compared to females. Direct (2R,6R)-HNK administration (10 mg/kg i.p.) resulted in higher levels of (2R,6R)-HNK in female mice. Ovariectomy did not alter ketamine metabolism in female mice, whereas orchidectomy recapitulated female pharmacokinetic differences in male mice, which was reversed with testosterone replacement.

Conclusion: Sex is an important biological variable that influences the metabolism of ketamine and the HNKs, which may contribute to sex differences in therapeutic antidepressant efficacy or side effects.

Keywords: Ketamine; antidepressant; depression; humans; hydroxynorketamine; mice.

Figure 1.. Simplified metabolism of ketamine to form the hydroxynorketamines.

(R,S)-ketamine (KET) undergoes N-demethylation to form (R,S)-norketamine (norKET) which is further metabolized to form (R,S)-dehydronorketamine (DHNK). (R,S)-norKET can be hydroxylated at the carbon 4, 5, and 6 positions, to form the (2,4)-, (2,5)-, and (2,6)-hydroxynorketamines (HNKs), respectively. Through a minor metabolic pathway, (R,S)-KET can also be directly hydroxylated to form the (2,6)-hydroxyketamines (HKs) which are subsequently N-demethylated to form the (2,6)-HNKs.

Figure 2.. Plasma concentrations of ketamine and metabolites following ketamine administration to male and female mice.

Plasma concentrations of (A) ketamine (KET), (B) norketamine (norKET), and (C) (2R,6R;2S,6S)-hydroxynorketamine (HNK) following a single intraperitoneal injection of ketamine (10 mg/kg) to intact male (M) and female (F) mice measured at 10, 30, 60 and 120 minutes. Data are the mean ± SEM. n=4 per sex and time point. *Indicates male vs. female comparison; *p<0.05, **p<0.01, ***p<0.001. Inset area under the concentration vs. time curve (AUC).

Figure 3.. Gonadectomy alters plasma levels of ketamine and hydroxynorketamine following ketamine treatment in male, but not female, mice.

(A-C) Plasma concentrations of (A) ketamine (KET), (B) norketamine (norKET), and (C) (2R,6R;2S,6S)-hydroxynorketamine (HNK), following a single intraperitoneal injection of ketamine (10 mg/kg) to female mice 10 days after sham surgery or gonadectomy (GDX) measured at 10, 30, 60 and 120 minutes. (D-F) Plasma concentrations of (D) KET, (E) norKET, and (F) (2R,6R;2S,6S)-HNK following a single intraperitoneal injection of ketamine (10 mg/kg) to male mice 10 days after sham surgery or gonadectomy (GDX). Data are the mean ± SEM. n=3–4 per surgical condition and time point. *Indicates sham vs. GDX comparison; *p<0.05, **p<0.01, ***p<0.001. Inset area under the concentration vs. time curve (AUC).

Figure 4.. Plasma concentrations of (2R,6R)-hydroxynorketamine following direct administration to intact male and female, and gonadectomized male, mice.

Plasma concentrations of (2R,6R)-hydroxynorketamine (HNK) following a single intraperitoneal injection (10 mg/kg) to (A) intact male (M) and female (F) mice, and (B) to gonadectomized or sham-operated male mice measured at 5, 10, 30, 60 and 120 minutes. Data are the mean ± SEM. n=3–5 per sex or surgical condition and time point. *Indicates sham vs. GDX comparison; *p<0.05, **p<0.01, ***p<0.001. Inset: values of the area-under-the-concentration vs. time curve (AUC) for male and female mice.

Figure 5.. Testosterone modulates the metabolism of ketamine in male mice.

Plasma concentrations of (A) ketamine (KET), (B) norketamine (norKET), and (C) (2R,6R;2S6S)-hydroxynorketamine (HNK) following a single intraperitoneal injection of ketamine (10 mg/kg) to sham-operated male mice with control implants (SHAM), gonadectomized male mice with control implants (GDX), and gonadectomized male mice with testosterone-releasing implants (GDX+T) measured at 10, 30, 60 and 120 minutes. Inset: area under the curve (AUC). Data points and error bars represent mean and SEM, respectively. n=4–5 per surgical condition and time point. *Indicates SHAM vs. GDX comparison; **p<0.01, ***p<0.001. ^#Indicates GDX vs. GDX+T, ^##p<0.01, ^###p<0.001. Inset: values of area under the concentration vs. time curve (AUC).

Figure 6.. Plasma concentrations of ketamine and metabolites following intravenous ketamine infusion to humans.

Plasma concentrations of (A) ketamine (KET), (B) norketamine (norKET), (C) (2R,6R;2S,6S)-hydroxynorketamine (HNK), (D) (2R,4R;2S,6S) and (2S,6R;2R,6S)-HNK, and (E) (2R,4S;2S,4R) and (2S,5S;2R,5R)-HNK following an intravenous infusion of ketamine (0.5 mg/kg over 40 min) to male (M; n=19) and female (F; n=38) human subjects measured at 40, 80, 120, 230 minutes, and one day post-infusion. Data are the estimated marginal mean ± SEM. *p<0.05, **p<0.01, ***p<0.001. Inset area under the concentration vs. time curve (AUC).