Mirtazapine enantiomers in blood and cerebrospinal fluid

Distribution pattern of mirtazapine and normirtazapine in blood and CSF

RATIONALE

The aim of this study was to investigate the distribution pattern of mirtazapine and its metabolite normirtazapine (N-desmethylmirtazapine) in blood and cerebrospinal fluid (CSF).

OBJECTIVES AND METHODS

Concentrations of mirtazapine were measured in blood serum and CSF of 16 patients treated with daily doses of 7.5-60 mg. Daily doses were correlated with serum and CSF concentrations as well as serum levels with those in CSF.

RESULTS

Serum levels of mirtazapine and normirtazapine showed a strong relation to the daily dose of mirtazapine of r = +0.631 and r = +0.732, respectively (p < 0.01). Between the daily doses and the CSF levels of both mirtazapine and normirtazapine, we only found a trend-wise correlation (r = +0.535, p = 0.060). The correlation between mirtazapine and normirtazapine in serum and CSF was highly significant (r = +0.664, p = 0.005 and r = +0.885, p < 0.001, respectively). High discrepancies between (total) mirtazapine levels in serum and CSF indicate a low penetration into CSF with regard to the total serum concentration as the mean of the calculated penetration ratio was 0.16 (SD = 0.11). By correcting the penetration ratio for the plasma protein binding, the mean CSF/serum ratio for the unbound fraction was 1.05 (SD 0.72, range 0.56-3.19) indicating a high passage into CSF.

CONCLUSIONS

Findings indicate a good ability of mirtazapine and normirtazapine to overcome the blood-cerebrospinal fluid barrier and suggest a high ability to enter the brain with sufficient drug levels at the target sites within the brain contributing to clinical efficacy.

Steady-state concentrations of mirtazapine, N-desmethylmirtazapine, 8-hydroxymirtazapine and their enantiomers in relation to cytochrome P450 2D6 genotype, age and smoking behaviour

BACKGROUND AND OBJECTIVE

Mirtazapine is a tetracyclic antidepressant drug available as a racemic mixture of S(+)- and R(-)-mirtazapine. These enantiomers have different pharmacological properties, and both contribute to the clinical and adverse effects of the drug. Cytochrome P450 (CYP) 2D6 has been implicated in the metabolism of S(+)-mirtazapine. However, the effect of CYP2D6 on serum concentrations of the enantiomers of mirtazapine and its metabolites has not been assessed in patients on long-term treatment. The main objective of the study was to evaluate the effect of the CYP2D6 genotype on enantiomeric steady-state trough serum concentrations of mirtazapine and its metabolites N-desmethylmirtazapine and 8-hydroxymirtazapine. The effects of sex, age and smoking behaviour were also assessed.

SUBJECTS AND METHODS

The study included 95 patients who had depression according to the Diagnostic and Statistical Manual of Mental Disorders-4th Edition and were treated for 4 weeks with a daily dose of mirtazapine 30 mg. The serum concentrations of the enantiomers of mirtazapine and its metabolites were analysed by liquid chromatography-mass spectrometry, and the subjects were genotyped for CYP2D6 alleles *3, *4, *5 and *6 and gene duplication.

RESULTS

Three subjects (3%) were classified as ultrarapid metabolizers (UMs), 56 (59%) as homozygous extensive metabolizers (EMs), 30 (32%) as heterozygous EMs and 6 (6%) as poor metabolizers (PMs) of CYP2D6. The median trough serum concentrations of S(+)-mirtazapine were higher in PMs (59 nmol/L, p = 0.016) and in heterozygous EMs (39 nmol/L, p = 0.013) than in homozygous EMs (28 nmol/L). PMs and heterozygous EMs also had higher mirtazapine S(+)/R(-) ratios (0.4) than homozygous EMs (0.3, p = 0.015 and 0.004, respectively). The S(+)-N-desmethylmirtazapine concentration was higher in PMs (16 nmol/L) than in homozygous EMs (7 nmol/L, p = 0.043). There was an association between the CYP2D6 genotype and the ratio between S(+)-8-hydroxymirtazapine and S(+)-mirtazapine, with a significantly higher ratio in homozygous EMs than in heterozygous EMs (0.11 vs 0.05, p = 0.007). The influence of the CYP2D6 genotype on S(+)-mirtazapine, the mirtazapine S(+)/R(-) ratio and S(+)-N-desmethylmirtazapine remained significant after correction for the influence of sex, age and smoking. Smokers had significantly lower concentrations of S(+)-mirtazapine (23 vs 39 nmol/L, p = 0.026) and R(-)-N-desmethylmirtazapine (39 vs 51 nmol/L, p = 0.036) and a significantly lower mirtazapine S(+)/R(-) ratio (0.28 vs. 0.37, p = 0.014) than nonsmokers, and the effect of smoking remained significant after multivariate analysis.

CONCLUSIONS

This study is the first to show the impact of the CYP2D6 genotype on steady-state serum concentrations of the enantiomers of mirtazapine and its metabolites. Our results also support the role of CYP1A2 in the metabolism of mirtazapine, with lower serum concentrations in smokers than in nonsmokers.

Racemic intrathecal mirtazapine but not its enantiomers acts anti-neuropathic after chronic constriction injury in rats

The unique noradrenergic and specific serotonergic antidepressant mirtazapine acts antinociceptive. It is optically active and currently marketed as racemate. In an animal model of acute pain it has been shown that the enantiomers exhibit differential effects: the R(-)-enantiomer showed anti-, the S(+)-enantiomer pronociceptive properties while the racemate acted antinociceptive at low doses and profoundly pronociceptive after high-dose application. Aim of the present study was to evaluate potential enantioselective effects of mirtazapine in neuropathic pain. In a chronic constriction injury model of neuropathic pain, Wistar rats were injected (+/-)-mirtazapine and the enantiomers intrathecally. All substances were dosed between 0.001 and 1mg/kg and compared to vehicle in a randomized and blinded approach. Thermal hyperalgesia and mechanical allodynia were assessed. In contrast to the acute pain results, only racemic mirtazapine exerted significant sustained analgesic effects up to 48 h. Antinociception was observed at all dosages with a maximum in the range of 0.01 mg/kg. Surprisingly, neither enantiomer was pro- nor antinociceptive at any dose or time. Our findings suggest that the synergism of both enantiomers is required to evoke a significant analgesic effect for the treatment of neuropathic pain. Our study gained no evidence for the use of either R(-) nor S(+)-mirtazapine alone. Due to the unique characteristics of (+/-)-mirtazapine and its proven efficacy in acute pain our results suggest that racemic mirtazapine may be a particularly useful antidepressant in the adjunctive treatment of chronic neuropathic pain states and could provide additional benefit to current therapeutic options.