Pharmacokinetics of mirtazapine and its main metabolites after single intravenous and oral administrations in rats at two dose rates
© Rouini et al.; licensee BioMed Central Ltd. 2014
Received: 27 August 2013
Accepted: 21 October 2013
Published: 7 January 2014
Mirtazapine (MRZ) is a human antidepressant drug metabolized to 8-OH mirtazapine (8-OH) and dimethylmirtazapine (DMR) metabolites. Recently, this drug has been proposed as a potential analgesic for use in a multidrug analgesic regime in the context of veterinary medicine. The aim of this study was to assess the pharmacokinetics of MRZ and its metabolites DMR and 8-OH in rats.
Eighteen fasted, healthy male rats were randomly divided into 3 groups (n = 6). Animals in these groups were respectively administered MRZ at 2 and 10 mg/kg orally and 2 mg/kg intravenously. Plasma MRZ and metabolite concentrations were evaluated by HPLC-FL detection method. After intravenous administration, MRZ was detected in all subjects, while DMR was only detected in three. 8-OH was not detected. After oral administration, MRZ was detected in 3 out of 6 rats treated with 2 mg/kg, it was detected in 6 out of 6 animals in the 10 mg/kg group. DMR was only detectable in the latter group, while 8-OH was not detected in either group. The oral bioavailability was about 7% in both groups.
The plasma concentration of the MRZ metabolite 8-OH was undetectable, and the oral bioavailability of the parental drug was very low.
KeywordsMirtazapine Metabolites Rats Pharmacokinetics Bioavailability
Mirtazapine (MRZ) is a tetracyclic antidepressant used mainly in patients affected by depression. Less commonly it is also used as a hypnotic, antiemetic, and appetite stimulant, and for the treatment of anxiety, among other indications .
Recently, MRZ use has been extended to veterinary species [2–7]. To the best of the Authors’ knowledge, there is minimal information available on the pharmacokinetics of MRZ in rats . Hence, the aim of this study was to investigate the pharmacokinetics of MRZ and its two metabolites, 8-OH and DMR in rats, after single intravenous (IV) and oral (PO) administrations.
The study protocol was approved by the ethics committee of animal studies at Tehran University of Medical Sciences.
Study one - Twelve Sprague Dawly male rats, aged 8–10 weeks and weighing 250–300 gr, were used. Animals were randomly assigned to two treatment groups (I and II). Each subject belonging to group I received a single oral dose of 2 mg/kg MRZ using the generic drug in the form of a 30 mg/tablet (Sandoz, Italy). After fasting for 12 h overnight, these rats (n = 6) received the treatment by gavage, they remained fasted for 6 h after drug administration. The second group (n = 6) was also treated in the morning following fasting, they were given the same dose of MRZ however this was administered via the intravenous route (achieved by dissolving pure MRZ hydrochloride powder in saline to give a 2 mg/mL solution).
Blood samples for pharmacokinetic analysis (0.5 mL) were collected at intervals of 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 24 h after MRZ administration via a cannula in the animals’ right jugular vein, and placed in collection tubes containing sodium heparin. The blood samples were centrifuged at 1000 g for 10 min within 30 min of collection, and the harvested plasma was stored at -80°C until analysis.
The Shapiro-Wilk test was used to assess the normal distribution of data. The T student test was used to estimate any significant differences between the pharmacokinetics of MRZ and DMR after the two administration routes.
Study one – 2 mg/kg IV and PO administrations
MRZ was quantified in plasma from 0.25 to 24 h or 0.25 to 6 h after IV and PO administrations, respectively. After the IV injection, DMR was quantified in 3 subjects while in the remaining subjects trace levels only were detected (>LOD < LOQ). The AUC ratio of MRZ/DMR was about 26.
DMR was undetectable in the orally administered animals because the concentrations were consistently lower than the LOD. Notably, 8-OH was not detected in any of the plasma samples (regardless of route of administration) either, this is surprising considering the very low LOQ (2 ng/ml) for 8-OH and DMR. Although values were normally distributed according to the Shapiro-Wilk test, wide variations in plasma concentrations were noticed among the rats, especially after PO administration.
Main pharmacokinetic parameters of MRZ after IV (2 mg/kg) and PO (2 and 10 mg/kg) administration of MRZ
IV (2 mg/kg) (n = 6)
PO (2 mg/kg) (n = 3)
PO (10 mg/kg) (n = 6)
0.998 ± 0.002
0.972 ± 0.026
0.967 ± 0.032
AUC 0-∞ (ng h/mL)
1431 ± 659
95 ± 69
1021 ± 240
1763 ± 1074
315.7 ± 112.3
7522 ± 6695
1972 ± 1295
258 ± 91
43210 ± 18018
HL alpha (h)
0.23 ± 0.15
0.10 ± 0.02
2.00 ± 3.16
HL beta (h)
1.7 ± 0.9
2.4 ± 0.5
4.8 ± 1.6
3.60 ± 1.37
2.53 ± 0.91
4.54 ± 1.26
0.63 ± 0.43
0.11 ± 0.04
0.15 ± 0.06
HL abs (h)
0.37 ± 0.22
0.09 ± 0.09
0.14 ± 0.04
T max (h)
/ ± /
0.24 ± 0.23
0.29 ± 0.10
C max (h)
/ ± /
76 ± 39
250 ± 67
/ ± /
6.6 ± 3.1
7.0 ± 4.2
Main pharmacokinetic parameters of DMR after IV (2 mg/kg) and PO (10 mg/kg) administration of MRZ
IV (2 mg/kg) (n = 3)
PO (10 mg/kg) (n = 6)
0.932 ± 0.044
0.991 ± 0.008
0.34 ± 0.30
0.04 ± 0.02
HL λz (h)
3.64 ± 2.55
19.52 ± 7.95
T max (h)
0.83 ± 0.44
1.63 ± 1.60
C max (ng/mL)
14.68 ± 10.17
74.58 ± 30.37
AUC 0 - ∞ (ng h/mL)
54.50 ± 42.98
506.50 ± 274.12
Study two – 10 mg/kg PO administration
Recently, there has been movement to investigate potential applications of MRZ in veterinary medicine. In the past few years, MRZ has been tested in cats [3, 4], dogs  and horses . MRZ caused significant polyphagia in cats . It was hypothesized to be potentially useful for dogs  for treatment of anorexia and anxiety-related diseases ; exploitation of its antiemetic properties (due to antagonism of 5-HT3 receptor) was also considered. In horses, it was suggested as being an analgesic potentially suitable for chronic pain due to its influence on both the noradrenergic and serotonic spinal descending pathways . In rats, MRZ showed significant antinociceptive activity at both the supraspinal and peripheral level .
Pharmacokinetics and metabolism have been reported as being highly variable among these species . Although MRZ has been shown to have a good safety profile, caution should be taken in extrapolating doses from other animal species or humans. The low oral bioavailability value found in the current study is noteworthy. To the best of the Authors’ knowledge, this is the first study to report oral bioavailability of MRZ in rats. The oral bioavailability value in humans is 50% , but no data is present for any animal species. Since MRZ is mainly administered orally, and its bioavailability might vary considerably among animal species, this parameter should be carefully evaluated in each species. However, even though oral bioavailability in rats is low, the effectiveness of the drug (as an analgesic) has been previously demonstrated . This may suggest that the receptors involved are easily activated, even at low drug concentrations. This speculation could also be supported by the earlier observation of its biphasic activity: its effectiveness was reduced when the dose was increased in rats .
Other interesting differences between species have been found in the plasma concentrations of metabolites. 8-OH, which is known to be the predominant metabolite in humans (approximately 40%, ) and dogs , was not detectable in the present study, mirroring what is reported for horses . The metabolism of MRZ in humans is regulated by phase I biotransformation catalyzed by the enzymes CYP1A2 and CYP2D6 (8-hydroxylation), CYP3A4 [9–11] and probably, CYP3A5 (N-demethylation and N-oxidation) . It is unlikely that the variation in plasma metabolite concentrations between previous human studies and the rat samples in the current study are due to species differences in CYP enzymes. In fact, rat and human CYP2D isoforms share a high sequence identity (>70%) . Additionally, recent studies from Matsubara et al.  have identified the new rat CYP3A62 form, and its expression profile is similar to that of human CYP3A4 and rat CYP3A9. The catalytic activities of these enzymes are higher in rats than in humans, but this alone can not account for the large difference in bioavailability. The most plausible explanation is that phase II enzymes account for this difference. It is suspected that the 8-OH metabolite is widely conjugated by glucuronic acid, as several hydroxylated metabolites have shown this metabolic pattern in rats. The rapid elimination of 8-OH as a glucuronide might account for the failure to detect 8-OH in the rat plasma . Further studies are necessary to clarify this issue.
In conclusion, the present study demonstrates that there are large species differences in MRZ pharmacokinetics. Its oral bioavailability is quite low in rats. The in vivo metabolic pattern appears to be different from that in other animal species and humans.
High pressure liquid chromatography
Thanks are due to Organon for providing pure compounds for the HPLC assay. This study was equally supported by both Athenaeum, University of Pisa, Italy (ex 60%) and Drug Design and Development Research Centre, Tehran University of Medical Sciences, Tehran, Iran funds.
- Howland RH: Understanding the clinical profile of a drug on the basis of its pharmacology: mirtazapine as an example. J Psychosoc Nurs Ment Health Serv. 2008, 46: 19-23.Google Scholar
- Giorgi M, Yun HY: Pharmacokinetics of mirtazapine and its main metabolites in beagle dogs: a pilot study. Vet J. 2012, 192: 239-241. 10.1016/j.tvjl.2011.05.010.View ArticlePubMedGoogle Scholar
- Quimby JM, Gustafson DL, Lunn KF: The pharmacokinetics of mirtazapine in cats with chronic kidney disease and in age-matched control cats. J Vet Intern Med. 2011, 25: 985-989. 10.1111/j.1939-1676.2011.00780.x.View ArticlePubMedGoogle Scholar
- Quimby JM, Gustafson DL, Samber BJ, Lunn KF: Studies on the pharmacokinetics and pharmacodynamics of mirtazapine in healthy young cats. J Vet Pharmacol Ther. 2011, 34: 388-396. 10.1111/j.1365-2885.2010.01244.x.View ArticlePubMedGoogle Scholar
- Rouini MR, Lavasani H, Sheikholeslami B, Nikoui V, Bakhtiarian A, Sgorbini M, Giorgi M: Pharmacokinetics of mirtazapine and its main metabolites after single oral administration in fasting/Fed horses. J Equine Vet Sci. 2013, 33: 410-414. 10.1016/j.jevs.2012.07.016.View ArticleGoogle Scholar
- Kilic FS, Dogan AE, Baydemir C, Erol K: The acute effects of mirtazapine on pain related behavior in healthy animals. Neurosciences. 2011, 16: 217-223.PubMedGoogle Scholar
- Giorgi M, Owen H: Mirtazapine in veterinary medicine a pharmacological rationale for its application in chronic pain. Am J Anim Vet Sci. 2012, 7: 42-47. 10.3844/ajavsp.2012.42.47.View ArticleGoogle Scholar
- Ranjan OP, Shavi GV, Nayak UY, Arumugam K, Averineni RK, Meka SR, Sureshwar P: Controlled release chitosan microspheres of mirtazapine: in vitro and in vivo evaluation. Arch Pharm Res. 2011, 34: 1919-1929. 10.1007/s12272-011-1112-1.View ArticlePubMedGoogle Scholar
- Overall KL: Natural animal models of human psychiatric conditions: assessment of mechanism and validity. Prog Neuropsychopharmacol Biol Psychiatry. 2000, 24: 727-776. 10.1016/S0278-5846(00)00104-4.View ArticlePubMedGoogle Scholar
- Bannister K, Bee LA, Dickenson AH: Preclinical and early clinical investigations related to monoaminergic pain modulation. Neurotherapy. 2009, 6: 703-712. 10.1016/j.nurt.2009.07.009.View ArticleGoogle Scholar
- Voortman G, Paanakker JE: Bioavailability of mirtazapine from Remeron® tablets after single and multiple oral dosing. Hum Psychopharmacol. 1995, 10: S83-S96. 10.1002/hup.470100803.View ArticleGoogle Scholar
- Delbressine LP, Moonen ME, Kaspersen FM, Wagenaars GN, Jacobs PL, Timmer CJ, Paanakker JE, van Hal HJ, Voortman G: Pharmacokinetics and biotransformation of mirtazapine in human volunteers. Clin Drug Invest. 1998, 15: 45-55. 10.2165/00044011-199815010-00006.View ArticleGoogle Scholar
- Dahl ML, Voortman G, Alm C, Elwin CE, Delbressine L, Vos R: In vitro and in vivo studies on the disposition of mirtazapine in humans. Clin Drug Invest. 1997, 13: 37-46. 10.2165/00044011-199713010-00005.View ArticleGoogle Scholar
- Venhorst J, Ter Laak AM, Commandeur JN, Funae Y, Hiroi T, Vermeulen NP: Homology modeling of rat and human cytochrome P450 2D (CYP2D) isoforms and computational rationalization of experimental ligand-binding specificities. J Med Chem. 2003, 46: 74-86. 10.1021/jm0209578.View ArticlePubMedGoogle Scholar
- Matsubara T, Kim HJ, Miyata M, Shimada M, Nagata K, Yamazoe Y: Isolation and characterization of a new major intestinal CYP3A form, CYP3A62, in the rat. J Pharmacol Exp Ther. 2004, 309: 1282-1290. 10.1124/jpet.103.061671.View ArticlePubMedGoogle Scholar
- Gibson GG, Skett P: Introduction to Drug Metabolism. 2001, Cheltenham, UK: Nelson Tornesh PublishersGoogle Scholar
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