MIRTAZAPINA farmacocinetica e farmacodinâmica em gatos saudáveis, quimby2010

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Studies on the pharmacokinetics and pharmacodynamics of mirtazapine
in healthy young cats
J. M. QUIMBY*
D. L. GUSTAFSON*
B. J. SAMBER� &
K. F. LUNN*
*Department of Clinical Sciences, College of
Veterinary Medicine and Biomedical
Sciences, Colorado State University, Fort
Collins, CO, USA; �Merial, Analytical
R&D, North Brunswick, NJ, USA
Quimby, J. M., Gustafson, D. L., Samber, B. J., Lunn, K. F. Studies on the
pharmacokinetics and pharmacodynamics of mirtazapine in healthy young
cats. J. vet. Pharmacol. Therap. 34, 388–396.
Mirtazapine pharmacokinetics was studied in 10 healthy cats. Blood was
collected before, and at intervals up to 72 h after, oral dose of 3.75 mg (high
dose: HD) or 1.88 mg (low dose: LD) of mirtazapine. Liquid chromatography
coupled to tandem mass spectrometry was used to measure mirtazapine,
8-hydroxymirtazapine and glucuronide metabolite concentrations. Noncom-
partmental pharmacokinetic modeling was performed. Median half-life was
15.9 h (HD) and 9.2 h (LD). Using Mann–Whitney analysis, a statistically
significant difference between the elimination half-life, clearance, area under
the curve (AUC) per dose, and AUC¥ ⁄ dose of the groups was found. Mirtazapine
does not appear to display linear pharmacokinetics in cats. There was no
significant difference in glucuronidated metabolite concentration between
groups. Pharmacodynamics was studied in 14 healthy cats administered
placebo, LD and HD mirtazapine orally once in a crossover, blinded trial. In
comparison with placebo, cats ingested significantly more food when mirtaza-
pine was administered. No difference in food ingestion was seen between HD
and LD, but significantly more behavior changes were seen with the HD.
Limited serum sampling during the pharmacodynamic study revealed drug
exposure comparable with the pharmacokinetic study, but no correlation
between exposure and food consumed. Mirtazapine (LD) was administered daily
for 6 days with no drug accumulation detected.
(Paper received 23 June 2010; accepted for publication 13 September 2010)
Jessica M Quimby, Department of Clinical Sciences, College of Veterinary Medicine
and Biomedical Sciences, Colorado State University, 300 West Drake Road, Fort
Collins, CO 80523, USA. E-mail: jquimby@colostate.edu
INTRODUCTION
Anorexia is a common clinical sign in cats and is associated with
many systemic diseases. An appropriate diagnostic investigation
is necessary to determine the underlying cause, but in many
cases, anorexia may continue for an extended period of time
during patient evaluation and initial therapy. Nutritional
support is a key component of successful patient management.
While the effects of inadequate nutrition in the diseased state
may be clinically subtle, they involve many organ systems and
are associated with increased morbidity and mortality (Sander-
son, 2000). In the cat specifically, the onset of hepatic lipidosis
following anorexia or hyporexia is well documented (Center,
2005).
In the acutely ill patient, management of anorexia may result
in more rapid recovery. Options for nutritional support include
the use of appetite stimulants, placement of feeding tubes and
administration of total parenteral nutrition. In the chronically ill
patient, management of anorexia likely results in improved
quality of life as well as improvements in survival (Kopple,
1994). For example, appetite stimulants may be helpful in the
long-term management of chronic renal failure, particularly as
ingestion of specially formulated renal diets has been shown to
improve outcome (Elliott et al., 2000; Plantinga et al., 2005;
Ross et al., 2006).
Mirtazapine is a presynaptic a2-adrenergic receptor antago-
nist that increases noradrenergic and serotonergic neurotrans-
mission by blocking presynaptic inhibitory receptors, resulting in
increased norepinephrine release into the synaptic cleft and
therefore increased postsynaptic availability (de Boer, 1996).
The serotonergic effects occur through 5-HT1 receptor-like
activity as well as enhancement of serotonergic transmission
by norepinephrine. In contrast to these effects, mirtazapine is
also a potent postsynaptic 5-hydroxytryptamine (HT)3 and
5-HT2 antagonist. It is thought that the receptor-specific
anti-serotonergic effects help ameliorate side effects of the
J. vet. Pharmacol. Therap. 34, 388–396. doi: 10.1111/j.1365-2885.2010.01244.x.
388 � 2010 Blackwell Publishing Ltd
drug (de Boer, 1996). Although the original mechanism of
action studies was performed in rats and no information is
available for cats, mirtazapine likely exerts its purported effect
through analogous mechanisms in the species of interest.
Mirtazapine was originally introduced to human medicine as
an antidepressant; however, it has recently attracted interest in
veterinary medicine owing to several desirable effects, namely its
significant anti-nausea, anti-emetic and appetite-stimulating
properties. These effects appear to be a result of antagonism of
the 5-HT3 receptor, which is an important receptor in the
physiology of emesis. Noradrenergic effects appear to be
responsible for the antidepressant properties (Timmer et al.,
2000).
An initial uncontrolled clinical trial (Cahill, 2006) and
numerous anecdotal reports have encouraged continued clinical
use of mirtazapine in veterinary medicine. Doses for cats and
dogs have been extrapolated from human doses. A dose of 1.88
or 3.75 mg (1 ⁄ 8 or 1 ⁄ 4 of a 15 mg tablet) every 3 days has been
recommended for cats (Cahill, 2006); however, no pharmaco-
logical studies have been reported to date in support of these
extrapolations. Anecdotal observations by the authors noted that
in most cases, the effect of the drug appears to have dissipated by
the second day after administration, implying a shorter dosing
interval may be more appropriate. Side effects that have been
noted clinically in cats include tremors, muscle twitching and
hyperactivity (Cahill, 2006).
Human pharmacokinetic data have demonstrated that a
number of factors affect the metabolism of mirtazapine. Mirt-
azapine displays linear pharmacokinetics over a wide dose range
in humans, with an elimination half-life (t1 ⁄ 2k) range of 20–40
h. Time to maximum serum concentration (Tmax) ranges from 1
to 2 h, and maximum serum concentration (C max) is reported to
range from 56.1 to 74.5 lg ⁄ L in healthy men with a
concomitant area under the curve to infinity (AUC¥) range of
397–950 lg ⁄ L•h (Timmer et al., 2000). No concentration–effect
relationship has been found in humans treated with mirtaza-
pine, and when used at therapeutic doses, the serum concen-
tration has been reported to range from 5 to 100 lg ⁄ L (Timmer
et al., 2000). Differences in metabolism were found between
sexes and ages, and for patients suffering from hepatic or renal
impairment (Timmer et al., 2000). To provide accurate dosing
recommendations, it would therefore be prudent to investigate
the pharmacokinetics of mirtazapine in veterinary patients. This
is particularly important in feline patients as the drug may be
metabolized more slowly owing to the decreased availability of
some metabolic pathways in this species. For example, cats lack
glucuronyl transferase, which is necessary for glucuronide
conjugation (Lainesse et al., 2006) As mirtazapine is metabo-
lized initially by demethylation and oxidation, followed by
conjugation to glucuronic acid, some differences in drug
metabolism and elimination for felines would be expected
(Timmer et al., 2000). Therefore, this project was designed to
assess the pharmacokinetics and pharmacodynamics of the
commonly prescribed doses of mirtazapine in healthy cats to
provide information to guide the use of mirtazapine as an
appetite stimulant in cats.
MATERIALS AND METHODS
Drug preparation
Commercially available generic 15 mg mirtazapine (Aurobindo
Pharma USA, Inc., Dayton, NJ, USA) tablets were compounded
by the pharmacy at theColorado State University Veterinary
Medical Center according to Professional Compounding Centers
of America� protocol. The method used is guaranteed to produce
accurate compounding within 10% of the target dose. Analysis
of random compounded capsules for mirtazapine content using
liquid chromatography coupled to tandem mass spectrometry
(LC ⁄ MS ⁄ MS) showed accuracies of 94.5 ± 4.6% to the intended
content and a stability of at least 6 months as formulated. The
capsules were compounded within 1 month of utilization and
stored at room temperature. For the pharmacodynamic study,
groups of capsules (lactose placebo, 1.88 and 3.75 mg mirtaza-
pine) were manufactured and randomly coded by the pharmacy
staff who kept the code concealed until completion of the study.
Pharmacokinetic sampling
Seven purpose-bred and three privately owned young adult
healthy cats were enrolled in the pharmacokinetic study after
undergoing a physical examination, complete blood count,
serum biochemistry profile and urinalysis to rule out systemic
disease. Breeds included nine domestic shorthairs and one
domestic longhair. Six cats were spayed females, and four cats
were castrated males. All portions of the project were approved
by the Institutional Animal Care and Use Committee at Colorado
State University, and all owners signed consent forms prior to
participation. Ten cats were randomly assigned to either the
high-dose (HD) group that received 3.75 mg or to the low-dose
(LD) group that received 1.88 mg of mirtazapine. Capsules were
administered orally, followed by 3 mL of water given with a
syringe. The cats were fasted for 12 h before beginning the study.
A jugular catheter was placed under ketamine (20 mg per cat
IV) and butorphanol (0.1 mg ⁄ kg IV) sedation 3 h prior to
mirtazapine administration to allow for ease of sample collection.
Blood samples (1.5 mL) were collected prior to, and at 0.25, 0.5,
1, 4, 8, 24, 48 and 72 h after mirtazapine administration.
Samples were centrifuged within 10 min of collection, and serum
was harvested and stored at )80 �C until analysis.
Mirtazapine analysis
Mirtazapine was measured using liquid chromatography coupled
to tandem mass spectrometry (LC ⁄ MS ⁄ MS) using a method
modified from one previously published (Doherty et al., 2007).
The analysis was carried out in the Pharmacology Core at the
Colorado State University Veterinary Medical Center. An
LC ⁄ MS ⁄ MS-based assay was developed and validated for the
analysis of mirtazapine in cat serum. For sample preparation, a
200 lL aliquot of serum was transferred to a 1.5-mL polypropyl-
ene microcentrifuge tube, and 10 ng of trazodone (10 lL of 1
lg ⁄ mL) was added as an internal standard. Proteins were then
Mirtazapine pharmacokinetics and pharmacodynamics in cats 389
� 2010 Blackwell Publishing Ltd
precipitated by the addition of 500 lL of acetonitrile and vortexing
for 10 min. Samples were centrifuged at 17 900 g for 10 min, and
the resulting supernatant was collected. Standards and quality
assurance ⁄ quality control (QA ⁄ QC) samples were constructed by
spiking 200 lL of blank cat serum with known amounts of
mirtazapine and processing as described above.
Positive-ion electrospray ionization mass spectra were deter-
mined with a 3200 Q-TRAP triple quadrupole mass spectrometer
(Applied Biosystems, Foster City, CA, USA) with a turbo ion-
spray source interfaced to a 1200 Series Binary Pump SL liquid
chromatography system (Agilent Technologies Inc, Santa Clara,
CA, USA) and HTC-PAL autosampler (Leap Technologies,
Carrboro, NC, USA). Chromatography was performed on a
SunFire C18, 2.5-lm, 4.6 · 50 mm column (Waters Corpora-
tion, Milford, MA, USA) with a liquid chromatography gradient
employing 10 mM ammonium acetate (mobile phase A) and
acetonitrile (mobile phase B). Initial chromatographic conditions
were 60:40 (mobile phase A ⁄ mobile phase B) with increasing B
from 40 to 95% from 0 to 2 min, holding at 95% B from 2 to
2.75 min and re-establishing initial conditions from 2.75 to 4
min. The flow rate was 1 mL ⁄ min, injection volume was 4–20
lL, and the total analysis time was 4 min.
Assay performance for each batch was assessed utilizing at
least 10% QA ⁄ QC samples dispersed among unknown samples at
low (1 ng ⁄ mL), mid (10 ng ⁄ mL) and high (100 ng ⁄ mL) ranges
of the standard curve (0.5–500 ng ⁄ mL) with batches failing if
>25% of the QA ⁄ QC samples were outside of the accepted level of
85% accuracy. Accuracy of QA ⁄ QC samples among the batches
analyzed for this study ranged from 94.5 ± 4.6 to 92.2 ± 6.8%.
The lower limit of quantitation (LLOQ) for this assay was based
on the level of detection with >85% accuracy and a CV (%)
<15% and was determined to be 0.5 ng ⁄ mL. Assay performance
was linear to >500 ng ⁄ mL, but 500 ng ⁄ mL was used as the
upper limit to the assay as utilized because of no unknown
sample measurements exceeding this value.
The mass spectrometer settings were as follows: turbo ion-spray
temperature, 550 �C; ion-spray voltage, 1500 V; declustering
potential, 48 V; entrance potential, 4.5 V; collision energy, 37 V;
collision cell exit potential, 2 V; collision cell entrance potential,
11 V; collision gas, N2, medium; curtain gas, N2, 35 units;
nebulizer gas, N2, 40 units; and auxiliary gas, N2, 20 units.
Samples were quantified by the internal standard reference
method in the multiple reaction monitoring mode by monitoring
the transition m ⁄ z 266 fi 195 for the analyte mirtazapine and
m ⁄ z 372 fi 176 for the internal standard trazodone.
8-Hydroxymirtazapine and 8-Hydroxymirtazapine glucuronide
analysis
For the analysis of 8-hydroxymirtazapine and 8-hydroxymirt-
azapine glucuronide, aliquots of cat serum samples were treated
with either b-glucuronidase or saccharic acid. The addition of
saccharic acid to samples was for the inhibition of endogenous
glucuronidase activity. To each 100 lL aliquot of cat serum was
added 1 ng of trazodone (10 lL of 100 ng ⁄ mL) followed by either
2500 units (50 lL of 50 units ⁄ lL) of b-glucuronidase or 50 lL of
300 lM saccharic acid. Samples were vortexed briefly to mix and
incubated at 37 �C for 1 h. Samples were then prepared by
acetonitrile protein precipitation as described for mirtazapine.
Chromatographic conditions used were identical to those used
for mirtazapine, and the mass spectrometer settings used were as
follows: turbo ion-spray temperature, 350 �C; ion-spray voltage,
1500 V; declustering potential, 48 V; entrance potential, 7 V;
collision energy, 37 V; collision cell exit potential, 2 V; collision
cell entrance potential, 11 V; collision gas, N2, low; curtain gas,
N2, 35 units; nebulizer gas, N2, 40 units; and auxiliary gas, N2,
20 units. 8-Hydroxymirtazapine was monitored as a transition
m ⁄ z 282 fi 211. The relative amount of 8-hydroxymirtazapine
and 8-hydroxymirtazapine glucuronide in samples was calcu-
lated based on a standard curve utilizing mirtazapine owing to
the lack of availability of 8-hydroxymirtazapine for use as a
standard. 8-Hydroxymirtazapine values were determined from
the saccharic acid–treated samples, and the amount of
8-hydroxymirtazapine glucuronide was calculated as the amount
in b-glucuronidase minus saccharic acid–treated samples.
Supplies and reagents
b-Glucuronidase (G4511), mirtazapine (M0443), saccharic acid
(S0375) and trazodone (T6154) were purchased from Sigma-
Aldrich Co. (St Louis, MO, USA). All other reagents were of
reagent grade or better.
Pharmacodynamic study
This was a randomized, double-blind, placebo crossover study.
Fifteen privately owned young adult healthy cats were enrolled
in the pharmacodynamic study after undergoing a physical
examination, complete blood count, serum biochemistry profile
and urinalysis to rule out systemic disease. One cat was excluded
owing to the development of an upper respiratory infection.
Breeds included seven domestic shorthairs, four domestic
longhairs, two siamese mixes and one Burmese.Eight cats were
spayed females, and six cats were neutered males. Cats were
fasted for 12 h before beginning the study. Fourteen cats each
received placebo, 1.88 and 3.75 mg of mirtazapine on separate
occasions, 1 week apart in a predetermined random sequence.
Capsules were administered orally, followed by 3 mL of water
administered by syringe. Before capsule administration, a baseline
behavioral assessment was carried out (see Appendix 1; the
behavioral scores from each observation over the 8-h period
were tallied to give the total behavior score for that particular
treatment). One hour after capsule administration, the cats
received a behavioral assessment and one cup of their usual dry
diet, which was weighed on a gram scale (ZL200630016361.1,
China), the accuracy of which was assessed by measuring
National Institute of Standards and Technology traceable
calibration weights. Blood samples were collected for mirtazapine
serum concentration at 2 and 8 h after capsule administration.
At 1-h intervals for an 8-h period, the cats’ remaining food was
weighed and a behavioral assessment was carried out by a single
blinded observer. The cats were released to their owners 9 h after
390 J. M. Quimby et al.
� 2010 Blackwell Publishing Ltd
mirtazapine ⁄ placebo administration. Owners were asked to
complete a home assessment sheet (see Appendix 2) including
perceived changes in appetite, vocalization and activity the
evening of, and the morning after, the administration of
mirtazapine ⁄ placebo.
Estimation of AUC in pharmacodynamic study cats utilizing a limited
sampling approach
Blood samples were collected 2 and 8 h after capsule adminis-
tration for the analysis of mirtazapine serum concentration;
samples were handled as previously described. Estimation of the
AUC of mirtazapine for cats in the pharmacodynamic study was
determined (Panetta et al., 2003). A multiple linear regression
analysis showed that the 8-h time point alone gave the best
correlation to the AUC in the 10 animals from the full
pharmacokinetic study with an r2 = 0.909. Utilizing this model
to predict the AUC0–24 in the training set gave an accuracy and
precision (CV%) of 79.4 ± 21.5 and a median accuracy and
range of 84.8 and 49.6–98.1, respectively.
Assessment of drug accumulation
The accumulation factor at steady-state following multiple doses
was estimated using the following equation: Accumulation
Factor = 1 ⁄ 1-e-kel*T. The terminal elimination rate was used
for estimating the accumulation factor as the applicable kel and
the dosing interval (T) were set at 24 h. In addition to calculation
of accumulation based on the pharmacokinetic data, LD
mirtazapine was administered daily to determine in vivo whether
significant drug accumulation would occur. Four cats (two
female and two male) from the seven purpose-bred cats that
participated in the pharmacokinetic study were utilized for the
daily dosing study. The cats received LD mirtazapine orally once
daily for 6 days. On days 3 and 6, blood samples were collected
immediately before, and 2 h after, capsule administration and
processed as previously described. Subjective behavior assess-
ment was performed during this period (objective assessment
was performed in the pharmacodynamic portion of the project).
Pharmacokinetic analysis
Data were analyzed, and pharmacokinetic parameters were
calculated utilizing noncompartmental methods. Data from the
72-h samples were not included as some cats did not have
detectable levels of drug at this time. Pharmacokinetic param-
eters that were determined included AUC0–t, AUC¥, t1 ⁄ 2k, Tmax
and Cmax. Because mirtazapine was administered via an
extravascular route, absorbed dose is equal to D (dose) ·
bioavailability (F), and thus, the estimation of the parameters
dependent on D [clearance (CL) and volume of distribution (Vd)]
is represented as CL ⁄ F and V ⁄ F (Wagner, 1993).
The data from the initial pharmacokinetic time course study
were utilized to develop a limited sampling approach for
pharmacokinetic analysis during the pharmacodynamic study.
Analysis of the AUC data vs. specific time point values from the 5
HD and 5 LD animals was conducted by multiple linear
regression (MINITAB v15.1.1.0; Minitab Inc., State College, PA,
USA) to determine whether one to three samples could
accurately predict the total AUC value. (Panetta et al., 2003)
The AUC vs. the 8-h time point resulted in the best prediction
with an r2 = 0.909. Therefore, the AUC for the pharmacody-
namic portion of the studies was estimated based on the value of
the 8-h time point utilizing the equation AUC (ng ⁄ mL · h) =
)21.4 + 28.6 · [8 h time point (ng ⁄ mL)].
Statistical analysis
Comparison of the pharmacokinetic parameters between HD and
LD groups was performed with Mann–Whitney statistical analysis.
Comparison of percentage of food ingested and the total behavioral
scores for each day during the pharmacodynamic study was
performed with repeated measures ANOVA and Bonferroni post hoc
analysis. Spearman rank correlation was used to explore the
relationship between drug exposure and pharmacodynamic effect.
PRISM software (GraphPad Software, Inc., La Jolla, CA, USA) was
used for the analysis. Comparison of at-home assessment param-
eters was performed using a mixed-effects logistic regression model
(Littell, 2006) with SAS software (SAS, version 9.2; SAS Institute
Inc., Cary, NC, USA). Cat identity was nested within treatment
sequence (the primary exposure of interest) to account for random
and repeated effects. Treatment sequence was included as a
random effect in each model, regardless of whether a significant
association could be identified when treatment sequence was
analyzed as a fixed effect. Even though incomplete washout was
not expected, this method conservatively accounts for possible
incomplete washout (Littell, 2006). For all analyses, a P-value of
<0.05 was considered to be statistically significant.
RESULTS
Pharmacokinetic study
Ten young adult healthy cats with normal physical examination,
serum biochemistry profile, complete blood count and urinalysis
were entered in the study. Demographics for these cats are
illustrated in Table 1. Comparison of the mg ⁄ kg dose of
mirtazapine between the HD and LD groups, using a Mann–
Whitney test, demonstrated that the doses were significantly
different (P = 0.015). No cats experienced adverse effects from
drug administration although increased activity, social interac-
tion and vocalization were noted subjectively in nine cats
(objective assessment was performed in the pharmacodynamic
portion of the project).
Pharmacokinetic parameters were calculated using a non-
compartmental model and are shown in Table 2. Overall, there
was a statistically significant difference between the elimination
half-life, CL, AUC ⁄ dose and AUC¥ ⁄ dose of the two groups when
they were compared with Mann–Whitney nonparametric statis-
tics (P = 0.03 for all comparisons). Drug concentration curves
for cats in the HD and LD groups are shown in Fig. 1.
Mirtazapine pharmacokinetics and pharmacodynamics in cats 391
� 2010 Blackwell Publishing Ltd
Metabolite analysis revealed that measurable quantities of
8-hydroxymirtazapine, the first product produced by Phase 1,
were detected (Fig. 2). Median AUC was 4.16 ng ⁄ mL•h with a
range of 3.53–4.59 ng ⁄ mL (HD) and 2.63 ng ⁄ mL•h with a range
of 1.94–3.31 ng ⁄ mL (LD). Thus, there was a trend toward
higher values in the HD group, but this did not reach statistical
significance (P = 0.08). The amount of metabolite undergoing
glucuronidation was also measurable, and the median AUC was
11.63 ng ⁄ mL•h with a range of 9.24–11.72 ng ⁄ mL (HD) and
14.32 ng ⁄ mL•h with a range of 12.89–24.69 ng ⁄ mL(LD),
suggesting the same or slightly lower concentration in the HD
group compared to the LD group (Fig. 3). This difference was not
statistically different (P = 0.19).
Pharmacodynamic study
Fourteenyoung adult healthy cats with normal physical
examination, serum biochemistry profile, complete blood count
and urinalysis were entered in the study. Demographics for these
cats are illustrated in Table 1. The effect of mirtazapine
administration on food consumption is illustrated in Fig. 4.
There was a statistically significant difference (P = < 0.0001)
between the amount of food eaten after administration of either
LD or HD mirtazapine in comparison to placebo. Cats ate an
average of 53.2% of their food when receiving the LD and 56.7%
of their food when receiving the HD in comparison to eating 13%
of their food when receiving the placebo. There was no
significant difference in the amount of food consumed between
the two doses of mirtazapine. Nine of 14 cats refused food
entirely when administered placebo, and 8 ⁄ 9 of these cats ate
after mirtazapine was administered at either dose. One cat
refused food entirely regardless of treatment administered.
The effect of mirtazapine administration on interaction,
activity level and vocalization scores is illustrated in Figs 5–7.
There was a statistically significant difference between admin-
istration of either LD or HD mirtazapine in comparison with
Table 1. Summary of demographic data by
study
Study type
Age (years) Weight (kg)
Dose group
Dose (mg ⁄ kg)
Median Range Median Range Median Range
PK single dose
(n = 10 cats)
2.3 1.4–4.1 5.3 3.9–7.8 LD 0.44 0.35–0.48
HD 0.62 0.48–0.73
Daily multiple dosing
(n = 4 cats)
1.4 1.4–1.4 5.1 3.87–6.45 LD 0.37 0.29–0.49
HD N ⁄ A N ⁄ A
PD single dose
(n = 14 cats)
3.3 1.0–4.5 4.2 2.9–6.8 LD 0.45 0.27–0.64
HD 0.89 0.56–1.27
HD, high dose; LD, low dose.
Table 2. Pharmacokinetic parameters of orally dosed mirtazapine in healthy cats
Pharmacokinetic parameter
High dose (HD) (3.75 mg) Low dose (1.88 mg)
Median Range Mean ± SD Median Range Mean ± SD
Cmax
(ng ⁄ mL)
183.9 40.1–272.8 156.5 ± 92.4 45.3 34.1–136.9 73.1 ± 45.5
Cmax ⁄ Dose
(ng ⁄ mL) ⁄ (mg ⁄ kg)
250.6 82.9–399.4 243 ± 132 126.2 77.9–281.9 170 ± 96
Tmax
(h)
1 0.5–4 1.5 ± 1.4 1 1–4 1.6 ± 1.3
Half-life
(h)*
15.9 11.0–21.8 15.3 ± 4.7 9.2 8.7–14.3 10.3 ± 2.3
AUC
(ng ⁄ mL•h)
942.9 443.8–1458.6 990.2 ± 372.4 364.9 289.4–562.0 397 ± 105.6
AUC ⁄ Dose
(ng ⁄ mL•h) ⁄ (mg ⁄ kg)*
1622 917–2135 1567 ± 483 830 771–1157 926 ± 180
AUC¥
(ng ⁄ mL•h)
1047.9 501.8–1498.4 1088.6 ± 400.1 376.1 310.9–570 407.4 ± 102.1
AUC¥ ⁄ Dose
(ng ⁄ mL•h) ⁄ (mg ⁄ kg)*
1670.2 1037.1–2297.3 1725 ± 527 891 816–1173 953 ± 173
CL ⁄ F
(L ⁄ h ⁄ kg)*
0.59 0.43–0.96 0.63 ± 0.22 1.12 0.85–1.27 1.08 ± 0.19
Vdz ⁄ F (L) 13.7 6.9–22.4 14.1 ± 6.2 16 11.3–23.2 16.2 ± 4.9
Values represent the median, range and mean ± SD of pharmacokinetic parameters determined in five separate animals. *Significantly (P < 0.05)
different between the high- and the low-dose group as determined by a Mann–Whitney test. CL, clearance; Vdz, volume of distribution; AUC, area under
the curve.
392 J. M. Quimby et al.
� 2010 Blackwell Publishing Ltd
placebo for all three parameters (interaction P < 0.0001; activity
level P < 0.0001; vocalization P = 0.0028). Bonferroni’s post hoc
analysis determined that there was a statistically significant
difference (P < 0.05) between the LD and HD for interaction and
vocalization scores, but not for activity level (95% confidence
interval: )5.7 to )0.15 for interaction and )5 to )0.7 for
vocalization). Cats were also assessed for the presence of tremors
or twitching, and no events were recorded during the 8-h
observation period. At-home owner assessment of the effect of
mirtazapine on appetite, activity and vocalization is illustrated in
Table 3. Using mixed-effects logistic regression, it was deter-
mined that cats receiving mirtazapine still had a significantly
increased appetite approximately 12 h after drug administration
when compared to cats that received placebo (P = 0.03);
however, there was no significant difference in appetite 24 h
after drug administration (P = 0.18). No statistically significant
Fig. 1. Drug concentration curves for cats in the high-dose and low-dose
groups. Note that the lines deviate, which represents a statistically
significant difference in half-life (P = 0.03) between doses.
Fig. 2. 8-Hydroxymirtazapine metabolite concentrations. 8-Hydroxy-
mirtazapine appears to be present in higher concentrations in the high-
dose group, although the difference was not statistically significant
(P = 0.08).
Fig. 3. Glucuronidated metabolite concentrations. The glucuronidated
metabolite is present in essentially the same quantities in the low-
dose and high-dose groups (P = 0.19).
Fig. 4. Effect of administration of low-dose mirtazapine, high-dose (HD)
mirtazapine or placebo on food consumption. There is a statistically
significant difference in the amount of food ingested between placebo and
either low-dose or HD mirtazapine (P < 0.0001). There is no significant
difference in the amount of food ingested between mirtazapine doses.
Mirtazapine pharmacokinetics and pharmacodynamics in cats 393
� 2010 Blackwell Publishing Ltd
difference was seen for vocalization at either time point (12 h: P
= 0.05, 24 h: P = 0.3) or activity at either time point (12 h: P =
0.15, 24 h: P = 0.33).
Estimation of AUC in pharmacodynamic study cats utilizing a limited
sampling approach
A limited sampling approach was utilized to estimate the AUC of
mirtazapine in the cats for the pharmacodynamic studies
(Panetta et al., 2003). The AUC0–24 estimated in the pharma-
codynamic cats using this limited sampling model was 440.2 ±
137.5 and 1073.3 ± 414.4 ng ⁄ mL · h for the LD and HD,
respectively. These values were not significantly different from
those calculated in the full-course pharmacokinetic study at
these same doses and show essentially the same ratio in exposure
between the HD and LD groups. No correlation between dose and
effect was found when drug exposure, as represented by AUC,
was compared to amount of food eaten (r = 0.53), vocalization
(r = 0.50), activity (r = 0.64) and interaction (r = 0.53).
Assessment of drug accumulation
Calculation of drug accumulation from the pharmacokinetic
data indicated that minimal accumulation (AF = 1.2) was
expected with daily administration of the LD. Demographics of
the four cats that received LD mirtazapine orally daily for 6 days
are presented in Table 1. A median day 3 trough concentration
of 1.4 ng ⁄ mL (range: 0.22–3.36 ng ⁄ mL) and a median 2 h
postadministration drug concentration of 38.6 ± 0.2 ng ⁄ mL
(range: 38.4–38.8 ng ⁄ mL) was observed. On day 6, the median
trough concentration was 1.53 ng ⁄ mL (range: 0–3.27 ng ⁄ mL),
and the median 2 h postadministration drug concentration was
29.5 ng ⁄ mL (range: 24.7–54.8 ng ⁄ mL). These concentrations
were similar to the range measured in the pharmacokinetic
curve at the corresponding time point (range 25.2–80.2 ng ⁄ mL;
median 36.8 ng ⁄ mL). No evidence of drug accumulation was
detected. Subjectively, no cats experienced changes in activity,
social interaction and vocalization during the 6-day time period
(objective assessment was performed in the pharmacodynamic
portion of the project).
DISCUSSION
The efficacy of mirtazapine as an appetite stimulant in cats is
supported by the results of this study, which demonstrated a
significant increase in food consumption after administration of
the drug. Eighty-eight percent of healthy cats that refused to eat
their normal diet while in the hospital subsequently ate when
mirtazapine was administered. No significant difference in the
amount of food consumed was seen between the two doses;
however, a significant increase in behavior such as vocalization
and interaction was noted in the HD group. Limited sampling
during the pharmacodynamic study allowed an estimation of
drug exposure to be calculated. No correlation between drug
exposure and clinical effect of food consumption could be
Fig. 7. Effect of administration of low-dose (LD) mirtazapine,high-dose
(HD) mirtazapine or placebo on vocalization scores. There is a statistically
significant difference in vocalization scores between placebo and either
LD or HD mirtazapine (P < 0.0001). There is also a statistically
significant difference in vocalization scores between administration of LD
or HD (P < 0.05).
Fig. 5. Effect of administration of low-dose (LD) mirtazapine, high-dose
(HD)mirtazapine or placebo on interaction scores. There is a statistically
significant difference in interaction scores between placebo and either LD
or HD mirtazapine (P < 0.0001). There is also a statistically significant
difference in interaction scores between administration of LD or HD
(P < 0.05).
Fig. 6. Effect of administration of low-dose (LD) mirtazapine, high-dose
(HD) mirtazapine or placebo on activity level scores. There is a
statistically significant difference in activity level scores between placebo
and either LD or HD mirtazapine (P < 0.0001). There is no significant
difference in activity level scores between mirtazapine doses.
394 J. M. Quimby et al.
� 2010 Blackwell Publishing Ltd
appreciated, which is consistent with human data (Timmer et al.,
2000). The implication of these findings is that similar efficacy
with less potential for side effects is experienced with the lower
dose, and this may be the most appropriate initial dose for many
cats.
The results of this study also suggest that the dosing interval
in cats is significantly different from that previously postulated
by Cahill: 1 ⁄ 8 to 1 ⁄ 4 of a 15 mg tablet orally every 3 days
(Cahill, 2006). The half-life in humans ranges from 20 to 40 h,
and the drug is administered daily without substantial accumu-
lation (Timmer et al., 2000). In contrast, the half-life of
mirtazapine in cats at the lowest effective dose (1.88 mg) is
approximately 9 h. Based on this half-life, the drug could
theoretically be administered daily in normal cats without
substantial drug accumulation. This is supported by the almost
negligible trough serum concentrations that were measured
during the daily dosing study. Although only a small number of
cats were used and therefore the data were only subjectively
described, 24-h trough concentrations were close to the limits of
detection and no accumulation was seen after daily dosing for 6
days. The authors recognize that serum levels may not
necessarily relate to drug effect in cats and that individual
variation between cats could potentially result in greater drug
accumulation than that measured in this study.
In an effort to evaluate the duration of effect of the drug,
assessments were carried out in the home environment during
the pharmacodynamic study. Observations recorded by owners
in a blinded fashion indicated that the majority of cats did
experience clinical effects of the drug the night after the
medication was administered, and a statistically significant
difference in appetite and behavior was noted for both HD and
LD when compared to placebo. Although a few cats still
experienced effects 24-h postadministration, the majority of cats
had no apparent clinical effect at that time point and no
statistically significant difference in appetite or behavior was
noted for either dose in comparison with placebo. These results
conceivably may have been affected by unknown bias on the
part of the owners or unknown factors affecting the cats in the
24 h after their participation in the pharmacodynamic study.
Dose proportionality is exhibited with mirtazapine in human
pharmacokinetic studies (Timmer et al., 2000), and regardless of
the dose administered, the elimination half-life remains the
same. This does not appear to be true in cats, based on the data
collected in this study, as there was a significant difference
between the elimination half-lives of the HD and LD groups. The
pharmacokinetic study was not a crossover study in design, and
this may have potentially affected the data and interpretation.
However, a significant difference in drug exposure and CL does
appear to exist between the dose groups, while Vd remains
constant. There does not appear to be a significant difference in
absorption between the two groups, so this is unlikely to play a
role. Obesity is one factor that could affect CL, while Vd remains
the same (Dunn et al., 1991). It was noted that one cat in the HD
group was obese; however, the other cats involved in the study
were not considered to be. This could potentially have affected
the study results. Alternatively, the results of the glucuronide
metabolite data may provide some explanation for the apparent
lack of linear pharmacokinetics in this species as there is some
evidence of a trend indicating glucuronide saturation. The
accumulation of higher concentrations of the glucuronide
metabolite might have been predicted in the HD group, as there
were slightly higher concentrations of 8-hydroxymirtazapine
detected in that group. However, there was essentially the same
amount, if not less, of glucuronide metabolite present in the HD
group. Possible explanations for this result include saturation of
the glucuronide enzyme system, which is known to be deficient
in cats (Lainesse et al., 2006), or inhibition of metabolism by the
8-hydroxymirtazapine metabolite. This would be concerning not
only because of a potential for more drug side effects but also
because depletion of glucuronidation ability could affect metab-
olism and serum concentrations of other drugs and supplements.
These findings suggest that it may be more appropriate to
administer the 1.88 mg dose of mirtazapine to avoid these
potential effects on drug metabolism.
In conclusion, administration of mirtazapine to healthy young
cats resulted in a significant increase in appetite. No difference in
the amount of food ingested was seen between the HD and LD,
but significantly more behavior changes were seen with the HD.
A significant difference in half-life was also seen between the HD
and LD, and thus, this drug does not appear to display linear
pharmacokinetics in the cat. The half-life of mirtazapine
administered at the lowest effective dose (1.88 mg) appears to
be shorter than that previously postulated and is potentially
consistent with daily dosing in young healthy cats. Data
collected on clinical effect after a single dose and serum
concentration after repeated dosing support this conclusion. As
it is known that age, renal and hepatic impairment affect the
half-life of mirtazapine in humans, further studies are needed to
Table 3. At-home assessment of appetite, activity and vocalization after administration of mirtazapine. For each treatment type, the number of cats out
of 14 that experienced and increased vs. decreased ⁄ normal appetite, vocalization or activity is noted
Appetite Vocalization Activity
Placebo HD LD Placebo HD LD Placebo HD LD
10–14 h after administration Increased 1 ⁄ 14 6 ⁄ 14 9 ⁄ 14 1 ⁄ 14 7 ⁄ 14 5 ⁄ 14 1 ⁄ 14 6 ⁄ 14 4 ⁄ 14
Normal or decreased 13 ⁄ 14 8 ⁄ 14 5 ⁄ 14 13 ⁄ 14 7 ⁄ 14 9 ⁄ 14 13 ⁄ 14 8 ⁄ 14 10 ⁄ 14
20–24 h after administration Increased 0 ⁄ 14 2 ⁄ 14 4 ⁄ 14 0 ⁄ 14 3 ⁄ 14 2 ⁄ 14 1 ⁄ 14 3 ⁄ 14 4 ⁄ 14
Normal or decreased 14 ⁄ 14 12 ⁄ 14 10 ⁄ 14 14 ⁄ 14 11 ⁄ 14 12 ⁄ 14 13 ⁄ 14 11 ⁄ 14 10 ⁄ 14
HD, high dose; LD, low dose.
Mirtazapine pharmacokinetics and pharmacodynamics in cats 395
� 2010 Blackwell Publishing Ltd
clarify the effects of these factors on the half-life of mirtazapine in
cats. Clinical effects could potentially be different in elderly or ill
animals, and daily dosing may not be appropriate in these
patients.
ACKNOWLEDGMENTS
This study was funded by a grant from the Winn Feline
Foundation. The study sponsors had no involvement in the
planning or execution of the study. Paul S. Morley graciously
provided statistical assistance.
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APPENDIX 1: BEHAVIORAL ASSESSMENT
Interaction
a) Hiding ⁄ sits at back of cage
b) Gets up to greet you when cage is opened
c) Soliciting attention at front of cage
Vocalization (meows per 15 s)
a) 0–10
b) 10–15
c) 15+
Activity
a) Sedentary ⁄ hiding
b) Walking around normally
c) Frantic ⁄ hyperactivity
Tremors
a) None
b) Mild twitch ⁄ bobble (subtle)
c) Noticeable head bobble or twitching
APPENDIX 2: AT-HOME ASSESSMENT
Please observe your cat tonight and tomorrow morning, fill out
this sheet and return to Dr. Quimby.
Cat: __________________________________
Date: ____________________________
The night you bring your cat home
Was your cat’s appetite:
Normal Increased Decreased
Was your cat’s vocalization:
Normal Increased Decreased
Was your cat’s activity level:
Normal Increased Decreased
The morning after
Was your cat’s appetite:
Normal Increased Decreased
Was your cat’s vocalization:
Normal Increased Decreased
Was your cat’s activity level:
Normal Increased Decreased
Additional comments:
396 J. M. Quimby et al.
� 2010 Blackwell Publishing Ltd