Download Effects of different doses of venlafaxine on serotonin and

International Journal of Neuropsychopharmacology (2007), 10, 41–50. Copyright f 2006 CINP
doi:10.1017/S1461145705006395
Effects of different doses of venlafaxine on
serotonin and norepinephrine reuptake in
healthy volunteers
ARTICLE
CINP
Pierre Blier1,3, Élise Saint-André1, Chantal Hébert1,3, Claude de Montigny1,
Normand Lavoie2 and Guy Debonnel1
1
Department of Psychiatry, McGill University, Montréal, QC, Canada
Department of Urology, McGill University, Montréal, QC, Canada
3
University of Ottawa Institute of Mental Health Research, Ottawa, ON, Canada
2
Abstract
Venlafaxine is generally considered to be a dual 5-HT and NE reuptake inhibitor when it is used at doses
above 75 mg/d in humans. While its 5-HT reuptake-inhibiting property has been demonstrated, some
controversy still exists regarding the doses of venlafaxine required to inhibit NE reuptake. Healthy male
volunteers received, on a double-blind basis, paroxetine (20 mg/d), desipramine (100 mg/d), nefazodone
(300 mg/d), or venlafaxine (150 or 300 mg/d) in the last 5 d of a 7-d period of administration. Inhibition of
5-HT reuptake was estimated by determining the degree of depletion of whole-blood 5-HT, while that
of NE was assessed by measuring the attenuation of the systolic blood pressure increases produced
by intravenous injections of tyramine. Paroxetine, both regimens of venlafaxine, and to a lesser extent
desipramine significantly decreased whole-blood 5-HT content. Nefazodone failed to produce any
significant change. Desipramine abolished the tyramine pressor response, whereas all other drug
regimens left this parameter unaltered. Venlafaxine and paroxetine acted as potent 5-HT reuptake
inhibitors in the present study. In contrast, neither the moderate nor the high dose of venlafaxine
displayed any significant inhibiting activity in this model assessing NE reuptake in peripheral NE
terminals. The validity of the model was confirmed by the potent inhibitory action of desipramine on NE
reuptake. While the reasons for this unexpected lack of action remain unclear, venlafaxine appeared to be
an effective NE reuptake agent in depressed patients using the same approach.
Received 13 September 2005 ; Reviewed 30 October 2005 ; Revised 14 November 2005 ; Accepted 22 November 2005 ;
First published online 11 May 2006
Key words : Antidepressant, nefazodone, paroxetine, reuptake inhibition, tyramine.
Introduction
Almost all tricyclic antidepressant drugs are potent
norepinephrine (NE) reuptake inhibitors. In contrast,
their potency to inhibit the reuptake of serotonin
(5-hydroxytryptamine ; 5-HT) is more variable, ranging from extremely potent for clomipramine to
ineffective for trimipramine (Hyttel, 1982). In contrast,
it is acknowledged that the non-tricyclic agent
venlafaxine is an effective 5-HT reuptake inhibitor,
but there remains uncertainty regarding the doses
necessary to inhibit the NE transporter in humans.
In synaptosomal preparations of rodent brains,
venlafaxine displays only moderate affinity for 5-HT
Address for correspondence : Professor P. Blier, University of Ottawa
Institute of Mental Health Research, Royal Ottawa Hospital, Lady
Grey Building, Suite 2043, Ottawa, Ontario, Canada K1Z 7K4.
Tel. : 613-722-6521 (ext. 6908) Fax : 613-761-3610
E-mail : [email protected]
reuptake sites, and at NE reuptake sites, its affinity has
generally been reported to be in the micromolar range,
which may be considered of questionable physiological significance (Béı̈que et al., 1998). Specifically,
its activity on NE reuptake is about 10 times weaker
than that of sertraline, which is considered a selective
5-HT reuptake inhibitor (SSRI ; Bolden-Watson and
Richelson, 1993). Similar in-vitro data have been
reported in human cell lines (Owens et al., 1997 ;
Tatsumi et al., 1997). These observations using different preparations and assays show considerable
variability in the potency and selectivity of venlafaxine, thereby raising concerns about the mechanism
of action of venlafaxine in the treatment of mood and
anxiety disorders.
The in-vivo assessment of the potency of venlafaxine to inhibit 5-HT and NE reuptake in the rat
brain have, however, yielded very different results.
For instance, the dose of venlafaxine necessary to
42
P. Blier et al.
inhibit the firing rate of 5-HT neurons in anaesthetized
rats is the same as for the SSRI paroxetine (Béı̈que
et al., 1999). Such a suppression of firing activity is
due to the increased activation of cell body 5-HT1A
autoreceptors by enhanced amounts of synaptic 5-HT,
resulting from 5-HT reuptake inhibition. This dose
equivalency between these two drugs is, therefore,
quite different from the 100-fold greater in-vitro
potency of paroxetine when compared to that of
venlafaxine (Béı̈que et al., 1998 ; Tatsumi et al., 1997).
In a similar model indirectly assessing NE reuptake
activity, venlafaxine was only three times less active
than the potent tricyclic NE reuptake inhibitor
desipramine in suppressing the firing rate of NE
neurons in the locus coeruleus (Béı̈que et al., 1999).
Among the possible explanations for this discrepancy
between in-vitro and in-vivo results is the low
plasma protein binding of venlafaxine when compared to those of paroxetine and desipramine, being
y30 % for venlafaxine vs. 95 % for the latter two drugs
(Ereshefsky and Dugan, 2000 ; Puozzo and Leonard,
1996). Presumably, the greater free fraction of venlafaxine may allow greater brain penetration.
Despite the fact that the 5-HT and NE systems in
the rodent and the human brain are similar, the effectiveness of various antidepressant drugs to inhibit
5-HT and NE reuptake needs to be evaluated in
healthy volunteers and in patient populations. Studies
examining such actions of venlafaxine in both groups
of subjects are still lacking. In the present study,
the capacity of venlafaxine and of other reference
antidepressant agents to inhibit 5-HT reuptake was
evaluated from their ability to deplete whole-blood
content of 5-HT. Since blood platelets can only store
5-HT through a 5-HT reuptake transporter that is
nearly identical to that present on 5-HT neurons in the
brain, and that y90 % of the total blood 5-HT is in
platelets, the sustained inhibition of 5-HT reuptake
depletes whole-blood 5-HT (Artigas et al., 1985 ;
Demet et al., 1978). The activity of the NE transporter
was evaluated by measuring the increase in the systolic blood pressure (BP) produced by the intravenous
injection of tyramine. Tyramine penetrates peripheral
NE terminals through their NE transporters and then
releases NE which transiently increases BP (Hoffman
and Lefkowitz, 1990). NE reuptake inhibition therefore
attenuates this response (Ghose, 1984).
Subjects and methods
Study subjects
Volunteers were recruited among students, outside
the faculty of medicine, on the McGill University
campus. Prior to their inclusion in the study, 40
volunteers underwent a structured clinical interview
(SCID for non-patients), a physical examination, an
electrocardiogram and laboratory tests consisting of a
complete blood count, electrolytes, liver enzymes,
urea and creatinine, hepatitis B and HIV testing, and
urine drug screening. Participants met the following
inclusion criteria : (1) male, between the ages of 18
and 40 years old ; females were not included to
avoid having to control for menstrual cycle or
possible metabolic interference with oral contraceptives ; (2) body weight in the normal range as
indicated by body mass index ; (3) no significant
medical illness, including migraines, high blood
pressure (BP>140/90), hepatitis B or HIV ; (4) no
evidence suggestive of a past or present history of
mental disorders. Subjects were excluded from the
study for any of the following reasons : (1) use of
an investigational drug within 30 d of study entry ;
(2) orthostatic hypotension (a fall in systolic BP>10
mmHg) ; (3) use of any psychotropic medication or
any illicit drug ; (4) regular use of any medication other
than acetaminophen or vitamins and mineral supplements ; (5) history of difficult phlebotomy. None of the
subjects smoked tobacco. Any significant abnormality
upon physical examination or laboratory tests was
also a reason for exclusion.
A total of 37 healthy male volunteers entered
the study. There was one dropout in the high-dose
venlafaxine group due to side-effects. This subject was
replaced to have six persons in each group.
Drug regimens
Volunteers were randomly assigned to one of six
groups : placebo, paroxetine (Paxil1) 20 mg/d, nefazodone (Serzone1) 300 mg/d, desipramine 100 mg/d
(Norpramine1), venlafaxine immediate release (IR)
150 mg/d (Effexor IR1, intermediate dose), or venlafaxine IR 300 mg/d (Effexor IR1, high dose). The
regimens were chosen on the basis of their clinical
efficacy in depression. The dose of paroxetine was
the minimal effective dose for depression and was
not meant to test its potential noradrenergic action
reported to occur with high doses (Gilmor et al., 2002).
The two venlafaxine regimens on the basis of their
antidepressant action were superior to SSRIs, at least
in some studies (Kelsey, 1996 ; Thase et al., 2001).
Following randomization, investigators and subjects
remained blind to the drug administration groups.
The capsules containing placebo or one of the five
different medications were identical. For the first 2 d,
all subjects took one capsule in the morning and one
5-HT/NE reuptake inhibition in healthy volunteers
capsule at bedtime. They were started at bedtime on
the day the baseline data were collected. The paroxetine group received 10 mg in the morning, the nefazodone group received 75 mg b.i.d., the desipramine
group 25 mg b.i.d., the low-dose venlafaxine group
37.5 mg b.i.d., and the high-dose venlafaxine group
75 mg b.i.d. For the next 5 d, all subjects took one
capsule in the morning and two capsules at bedtime.
The paroxetine group received 10 mg b.i.d., the nefazodone group received 150 mg b.i.d., the desipramine
group 50 mg b.i.d., the low-dose venlafaxine group
75 mg b.i.d., and the high-dose venlafaxine group
150 mg b.i.d.. The assessment of NE and 5-HT
reuptake processes was performed at baseline (day 0)
and on day 7 with the last dose being taken about
2–3 h before testing. Taking into account these gradual
titrations, the half-life of the drugs and that of their
metabolites, it was deemed that 5 d at the same dose
would be sufficient to achieve steady-state plasma
levels (Devane, 1998). The plasma levels of venlafaxine
and desmethyl venlafaxine were carried out by Dr
Gilles Caillé at the University of Montreal using a
method approved by Wyeth and summarized in
Debonnel et al. (2006).
Assessment of 5-HT reuptake
Platelet 5-HT content has become widely used
for studying the activity of the 5-HT transporter on
serotonergic neurons (Arora et al., 1984). Blood platelets can take up and store 5-HT in a fashion very
similar to that of 5-HT neurons, but lack the enzymatic
system necessary to synthesize 5-HT and most of the
5-HT receptors subtypes (Demet et al., 1978 ; Pletscher,
1986). Thus, any 5-HT present in the platelets has been
taken up by the transport system, and concentration
constitutes an index of the efficacy of 5-HT uptake sites
on 5-HT neurons. Following blockade of the 5-HT
reuptake process, platelet 5-HT content gradually
decreases as older platelets that have previously taken
up 5-HT are removed from the circulation. They are
replaced by new ones that are rendered incapable of
effective 5-HT uptake by the reuptake- inhibiting
drugs. Moreover, as a very large proportion (90–95 %)
of blood 5-HT is sequestrated in the platelets, wholeblood 5-HT content largely reflects platelet 5-HT content (Artigas et al., 1985 ; Demet et al., 1978 ; Pletscher,
1986). Blood 5-HT content measured on day 7 was
compared to the baseline level and its reduction
was used as an index of the degree of 5-HT reuptake
inhibition.
An adapted HPLC procedure using an electrochemical detector for assessing 5-HT levels in
43
samples from total blood, from approaches previously
described was utilized (Kilts et al., 1981 ; Koch and
Kissinger, 1980 ; Korpi, 1984 ; Sasa et al., 1978). The
procedure involves deproteination of blood samples
by addition of perchloric acid (4 M) in presence of
antioxidants and application of the supernatant on an
amberlite CG-50 cation exchange resin column from
which 5-HT is eluted with an acetate buffer (pH 5.1).
We used an internal standard (2 nmol/ml N-methyl5-hydroxytryptamine oxalate salt) and its per cent
recovery was used to correct for methodological losses
during the extraction and chromatographic procedures and ensure correction for losses during quantification by HPLC. One-millilitre whole-blood aliquots
were thawed and 0.5 ml of the internal standard
working solution was added as well as acetate buffer
to make up the volume to 2 ml. A total of 0.2 ml perchloric acid was used to precipitate serum proteins,
the mix was vortexed for 1 min and centrifuged at
4000 rpm for 10 min with a Brinkmann Eppendorf
centrifuge. The supernatant was transferred to glass
tubes containing 2 ml of sodium acetate buffer. The
pH was adjusted to 5.0¡0.1 with 6 mol/l ammonium
hydroxide.
The HPLC equipment consisted of a Waters HPLC
pump (510 model ; Oregon City, OR, USA), a standard
analytical Hypersil column (Chromatographic
Sciences Company, Montreal, Quebec), an electrochemical detector (model 400, Princeton Applied Research, Oakridge, TN, USA) with an Ag/AgCl
reference electrode set at +0.5 V potential to eliminate
interference caused by catecholamines, metanephrines
and other metabolites of catecholamines, and a gold
working electrode. An autosampler (Hitachi model
7200 L, Chromabec, Montreal, Quebec) was used for
automatic injection of samples and a Waters 740 data
module integrator was used to record chromatograms,
calculate the area under the curve and obtain ratios of
5-HT and internal standard. The mobile phase containing 100 mg disodium EDTA, 25.3 ml concentrated
ammonium hydroxide, 32.3 ml glacial acetic acid, and
162.5 ml methanol with a pH adjusted to 5.1 was filtered through 0.45 picometres membrane filters and
degassed for 15 min before use.
Assessment of NE reuptake
Tyramine acts indirectly as a sympathomimetic agent,
i.e. it is taken up in NE neurons by the NE transporter
and then it produces a release of NE from its
intraneuronal stores (Hoffman and Lefkowitz, 1990).
NE reuptake inhibition therefore attenuates this
response (Ghose, 1984). In turn, NE acts on the
44
P. Blier et al.
vascular post-synaptic a1-adrenoceptors, which causes
a vasoconstriction and a secondary elevation of
systolic BP (Ghose, 1984). Pretreatment with drugs
that block the high-affinity NE reuptake site, prevent
the increase in BP that usually follows tyramine
administration by preventing tyramine entry into
NE neurons (Hoffman and Lefkowitz, 1990). The
tyramine test consists of measuring the increase in
systolic BP of patients receiving a fixed dose of
tyramine, or measuring the dose of tyramine necessary to cause a predetermined increase in systolic BP
(Ghose, 1984 ; Ghose and Turner, 1975). Either a decrease in the pressor response to the same doses of
tyramine, or an increase in the dose of tyramine
necessary to cause a given increase in systolic BP, is
considered to be a reliable index of NE reuptake
blockade. In the present study, the effects of two doses
of intravenous tyramine (4 mg and 6 mg) on systolic
BP were assessed.
The following procedure was followed at each visit :
the subject was connected to a Criticare 508 apparatus,
which automatically registered the heart rate (HR) and
BP, and a catheter was installed in an antecubital vein
in one arm. Following a resting period of 30 min in
the supine position, during which HR and BP were
recorded every 10 min, blood was drawn for 5-HT
level measurements.
A test dose of 0.5 mg tyramine was then administered intravenously in order to rule out the presence
of a pheochromocytoma, which would not have
been detected by the physical examination and
laboratory tests. After 5 min, and if there was no significant elevation in BP after the test dose, a dose
of 4 mg tyramine was administered. The BP and HR
were then measured every 2 min for 10 min. This time
period allowed us to detect the peak increase in BP,
which usually occurs 2 min after a bolus injection of
tyramine, as well as sufficient time to let BP return
to its baseline level. If the increase in BP did not
reach 30 mmHg after the 4-mg tyramine dose, a
second dose of 6 mg was given, and the BP and HR
were measured as previously described. The difference between the systolic BP measured just prior to
the tyramine load and that measured 2 min after was
considered the index for the action of tyramine.
Blockade of NE reuptake manifests itself by a decrease
in the elevation of BP shown prior to beginning the
drug regimen (effect of tyramine measured on day 1).
Evaluation and statistical analysis
Serotonin reuptake data are expressed in whole-blood
5-HT concentrations in pmol/l compared to baseline
levels. These results, as well as the systolic, diastolic
BPs and the pulses were analysed using the two-tailed
paired Student’s t test. The systolic BP response to
tyramine was considered to be the difference between
the systolic BP measured just prior to the tyramine
load and that measured 2 min post-administration.
ANOVA for repeated measures for doses of tyramine
and treatments were conducted on the data to assess
the effects of the different drug regimens on the
pressor response to loads of 4 mg and 6 mg tyramine.
Post-hoc analyses were conducted where appropriate
with Bonferroni’s method. All values are expressed as
means¡S.E.M. The level of significance was set at
p<0.05.
Ethics
This study was carried out in accordance with the
Declaration of Helsinki and was approved by the
Research Ethics Board of the Royal Victoria Hospital,
McGill University. All subjects gave informed written
consent. Subjects were given $150 (CDN) as compensation for lost time related to their participation.
Results
Plasma levels of the antidepressant drugs
In the six subjects who received 150 mg/d venlafaxine,
the mean level of the parent compound was 67 ng/ml
(range 43–97) and that of its metabolite, desmethyl
venlafaxine, was 338 ng/ml (range 235–415). The
total level of venlafaxine plus its metabolite was
404¡31 ng/ml (range 319–512). In the six subjects
who received 300 mg/d venlafaxine, the mean level of
the parent compound was 170 ng/ml (range 75–265)
and that of the metabolite was 540 ng/ml (range
369–699). The total level of venlafaxine plus its metabolite in this second group was 710¡67 ng/ml (range
516–930). These levels are consistent with those
obtained in depressed patients (Debonnel et al., 2006).
The mean ratio of the metabolite/parent compound
was 4.7 (range 1.5–7.4). Despite this apparent wide
range of metabolic activity, it is important to mention
that the total concentration of venlafaxine active
moieties varied by less than 2-fold in both groups of
subjects. Furthermore, the two subjects who had the
lowest and the highest metabolite/parent compound
ratio still had a total level of active agents within
15 % of the mean of their respective group.
The mean level of paroxetine was 53¡19 ng/ml
(range 21–110, n=6), values that are fully consistent
with those reported using 20 mg/d (Gilmor et al.,
2002). The mean desipramine level was 119¡39 ng/
ml (range 71–295, n=6). The levels of nefazodone
in the six subjects varied from 66 to 2990 ng/ml
(mean¡S.E.M. 925¡480), those of the hydroxymetabolite varied from 35 to 499 ng/ml (mean¡S.E.M.
263¡72), and mCPP levels were low as expected
(mean¡S.E.M. 29¡12 ng/ml, range <3–79 ng/ml).
The mean concentration of des-ethyl-hydroxynefazodone, a metabolite with an uncharacterized
biological activity, was of 1315¡250 ng/ml (range
553–2051).
Modifications of cardiovascular parameters by
the antidepressants
Despite the small number of subjects tested in each
group, and the numerous factors that can alter such
parameters, it was decided to assess BP and pulse at
baseline and after the 1-wk duration of the study,
when the subjects were lying down immediately
before starting the tyramine infusions. In the placebo
group, there was a mean significant drop of 13 beats/
min (range +2 to x27 ; d.f.=5, t=2.92, p=0.03), with
five of the six subjects presenting a decrease at day 7.
In the desipramine group, there was a mean significant increase of 14 beats/min (range x4 to +32 ;
d.f.=5, t=2.63, p=0.046), with five of the six subjects
presenting an increase during the second session. The
other medications did not produce any significant
change in pulse (p>0.5, data not shown). With regards
to systolic and diastolic BPs, the only significant
difference noted among all the treatment groups, was
the increased mean diastolic pressure by 5 mmHg
in the paroxetine group (range +2 to +9 ; d.f.=5,
t=3.51, p=0.017).
Depletion of whole-blood 5-HT levels by the
antidepressant drugs
The mean concentration of 5-HT was 2446¡376
pmol/l at baseline in the placebo group and 2344¡411
7 d later, with five of the six subjects having a f10 %
variation within that 1-wk period. This mean difference was not statistically significant (d.f.=5, t=1.05,
p=0.34 ; see Figure 1). Similarly in the nefazodone
group, there was a non-significant mean 10 pmol/l
decrease after 7 d administration (d.f.=5, t=0.15,
p=0.884), suggesting a lack of sustained effect of
nefazodone on 5-HT reuptake (Figure 1). In the desipramine group, however, there was a small but significant mean 19 % decrease in whole-blood 5-HT
(d.f.=5, t=3.19, p=0.02), all six subjects showing
a decrease in this parameter after 7 d administration
(range 8–33 % decreases, Figure 1). In the paroxetine
% change in whole-blood 5-HT
5-HT/NE reuptake inhibition in healthy volunteers
20
45
Desipramine Nefazodone
300 mg/d Venlafaxine
100 mg/d
0
−20
Placebo
*
−40
−60
−80
−100
*
Paroxetine
20 mg/d
*
*
150 mg/d
300 mg/d
Figure 1. Percentage changes of whole-blood 5-HT levels
after 7 d administration of placebo or the various
antidepressant agents. There were six subjects in each
group. They received half of the indicated doses for the
first 2 d of administration and the full doses for the next
5 d on a twice-daily regimen. The last dose was given 2–3 h
prior to drawing blood. The baseline levels, expressed in
pmol/l, were 2446¡377 in the placebo group, 2585¡344
in the desipramine group, 1901¡430 in the paroxetine
group, 1344¡131 in the nefazodone group, 2326¡407 in
the 150 mg/d venlafaxine group and 2083¡254 in the
300 mg/d venlafaxine group. * Indicates p<0.05 using the
paired two-tailed Student’s t test.
group, all subjects had a robust decrease in wholeblood 5-HT (range 45–96 % ; d.f.=5, t=3.02, p=0.03 ;
Figure 1). In the 150 mg/d venlafaxine group, all subjects also had a marked decrease in their whole-blood
5-HT level (range 52–85 % ; d.f.=5, t=4.45, p=0.007 ;
Figure 1). Finally, in the high-dose venlafaxine group,
the decrease was profound and more homogeneous
than in the last two groups (range 73–82 % ; d.f.=5,
t=8.02, p<0.001 ; Figure 1).
Effects of the antidepressant medications on the
tyramine pressor response
The modifications in systolic BP were compared using
a two-way ANOVA, after ensuring that the normality
and equal variance tests were passed (p=0.23 and
0.70 respectively). It revealed a significant source of
variance of dose of tyramine (F=61.43, d.f.=1,
p<0.001), of treatment (F=6.54, d.f.=11, p<0.001),
but not of treatmentrdose (F=1.44, d.f.=11, p=0.16).
All the subsequent pairwise comparisons were carried
out using Bonferroni’s method considering a p value
smaller than 0.05 as significant. The 6-mg dose of
tyramine produced a numerically greater increase in
systolic BP than the 4-mg dose in all the groups at
baseline, although it reached statistical significance
only in the nefazodone group (t=4.37). The mean
46
P. Blier et al.
Increase in systolic blood pressure
by tyramine (mmHg ± S.E.M.)
30
Placebo
Desipramine
100 mg/d
Paroxetine
20 mg/d
Nefazodone
300 mg/d
25
20
15
10
5
0
6
4 mg
6
6 mg
*
*
6
4 mg
6
6 mg
6
4 mg
6
6 mg
6
4 mg
6
6 mg
Figure 2. Alterations of systolic blood pressures within 2 min of the intravenous injection of tyramine in a bolus manner.
The number of subjects studied in each group is indicated at the bottom of the histograms. They received half of the
indicated doses for the first 2 d of administration and the full doses for the next 5 d on a twice-daily regimen. The last dose
was given 2–3 hours prior to the tyramine challenge. * Indicates p<0.05 using the Bonferroni method as post-hoc tests
following an ANOVA for repeated measures. The difference between the effects of the 4-mg and 6-mg doses of tyramine
at baseline was significant only in the nefazodone group. %, Prior to administration ; &, after 7 d administration.
40
Increase in systolic pressure produced
by tyramine (mmHg ± S.E.M.)
increase produced by the 4-mg dose was 12 mmHg
and a statistically significant further increase of
9 mmHg was produced by the 6-mg dose.
Placebo administration did not significantly alter
the tyramine response (t=1.99), thus indicating test–
retest reliability of this paradigm over a 1-wk period.
The only medication that significantly altered the
tyramine pressor response was desipramine (t=5.96).
This drug nearly abolished the pressor response
to both doses of tyramine (t values of 5.06 and
5.45 at 4 mg and 6 mg respectively). The paroxetine,
nefazodone and the low- and high-dose venlafaxine
regimens yielded non-significant t values of 0.15, 1.90,
1.28 and 0.64 respectively (Figures 2, 3).
35
30
The results of the present study indicate that paroxetine and venlafaxine are potent inhibitors of 5-HT
reuptake. Desipramine exerted a small, albeit significant, inhibition of 5-HT reuptake, whereas nefazodone
was devoid of sustained inhibitory action on this
process (Figure 1). Among these four antidepressant
drugs, only desipramine exerted a significant attenuation of the tyramine pressor response (Figures 2
and 3). These data indicate that venlafaxine, even
at a dose that is four times higher than its minimal
effective dose in depression (i.e. 75 mg/d), did
300 mg/d
25
20
15
10
5
0
Discussion
Venlafaxine
150 mg/d
6
4 mg
6
6 mg
6
4 mg
6
6 mg
Figure 3. Alterations of systolic blood pressures within
2 min of the intravenous injection of tyramine in a
bolus manner. The number of subjects studied in each
group is indicated at the bottom of the histograms.
They received half of the indicated doses for the first
2 d of administration and the full doses for the next
5 d on a twice-daily regimen. The last dose was given
2–3 h prior to the tyramine challenge. The treatments
did not produce significant alterations of the tyramine
pressor response, using the Bonferroni method as
post-hoc tests following an ANOVA for repeated
measures. %, Prior to administration ; &, after 7 d
administration.
5-HT/NE reuptake inhibition in healthy volunteers
not appear to inhibit the NE reuptake process in the
present study in healthy volunteers.
The assessment of whole-blood 5-HT diminutions
by reuptake inhibitors yields an underestimation of
the actual degree of reuptake inhibition. This is
because, even if a reuptake inhibitor would rapidly
and completely inhibit the 5-HT reuptake pump of
platelets, their content needs an extended delay to be
depleted as a result of 5-HT being released and not
being refilled through reuptake. Indeed, when the
treatment with a dose of 75 mg/d venlafaxine is
extended from 1 to 4 wk, the degree of depletion
increases over time despite venlafaxine achieving its
steady-state level within a few days (Debonnel et al.,
2006). Nevertheless, the advantage of this approach is
that it readily permits detection if various substances
exert a sustained action on the 5-HT reuptake transport system. In the case of nefazodone, for instance, it
was reported that 200 mg nefazodone administered
twice daily inhibited by 34 % the ex-vivo platelet 5-HT
reuptake in healthy male volunteers 2–4 h after its oral
administration, but not 12 h after the last dose. In the
same study, nefazodone was devoid of effects on the
whole-blood levels of 5-HT. In contrast, a daily dose
of 20 mg fluoxetine produced a sustained inhibition
of platelet 5-HT reuptake and a whole-blood 5-HT
depletion of up to 80 % (Salazar et al., 1994).
It may appear surprising that desipramine at a
relatively low dose actually produced a significant
reduction of whole-blood 5-HT. Nevertheless, prior
reports have documented the lack of absolute specificity of this drug for the NE reuptake process. For
instance, Javors et al. (2000) estimated that after a
prolonged administration of desipramine in patients,
the IC50 required to inhibit 5-HT platelet depletion was
195 ng/ml. Moreover, Gilmor et al. (2002) showed that
the reuptake of 5-HT was also significantly inhibited
(by 18 %) when using the plasma of the patients containing 100 ng/ml desipramine in a human cell line in
vitro. The present results are thus strikingly similar to
those previously published. Although the three sets of
data indicate that desipramine produces a consistent
inhibition of 5-HT reuptake transporters in humans,
they do not imply that it would necessarily alter 5-HT
levels in the brain. This is presumably because there is
a reserve in the number of 5-HT transporters (5-HTT)
in order to maintain the effectiveness of the process. In
humans, it is estimated that y80 % of the 5-HTT have
to be occupied to produce an antidepressant response
using drugs that inhibit 5-HT reuptake (Meyer et al.,
2004). Indeed, that study has clearly shown that
the minimal effective doses of various 5-HT reuptake
inhibitors, including venlafaxine, produced an 80 %
47
occupancy of 5-HTT sites in the human brain, as
assessed using a labelled 5-HTT ligand and positron
emission tomography. Perhaps the strongest clinical
evidence that tricyclic antidepressants do not exert a
potent inhibitory action on the 5-HT reuptake process,
with the exception of clomipramine, is that they are
not effective in the treatment of obsessive–compulsive
disorder, unlike the SSRIs. In contrast, clomipramine
is a potent 5-HT reuptake inhibitor and it is the
only tricyclic drug with an anti-obsessional action
(Goodman et al., 1997).
The robust inhibitory action of desipramine on
the tyramine pressor response was expected based
on prior reports from ours, and several other laboratories. Indeed, amitriptyline and imipramine (which
are metabolized to nortriptyline and desipramine
respectively) potently inhibit this response, as
does nortriptyline itself, maproptiline, clomipramine
(which is metabolized to the potent NE reuptake
inhibitor desmethyl-clomipramine), and the nontricyclic NE reuptake inhibitors tomoxetine (now
denoted atomoxetine) and reboxetine (Gobbi et al.,
2003 ; Harvey et al., 2000 ; Hassan et al., 1985, 1989 ;
Seppala et al., 1981 ; Slater et al., 2000 ; Turcotte et al.,
2001 ; Zerbe et al., 1985). In contrast, the SSRI paroxetine at its minimal effective dose in depression of
20 mg/d was devoid of effect in this test (Hassan et al.,
1985, 1989 ; Figure 2). This is consistent with the results
of Gilmor et al. (2002) showing a 27 % inhibition of
NE reuptake ex vivo using the plasma of patients
containing 100 ng/ml paroxetine. It remains to be
determined, however, if higher regimens of paroxetine
(50 mg/d or more), consistently producing levels
of y200 ng/ml paroxetine, would attenuate the tyramine pressor response, given that the ex-vivo inhibitory action of such plasma levels of NE reuptake is
40 % (Gilmor et al., 2002). Nefazodone did not alter
the tyramine pressor response consistently with its
relative absence of effect at the human NE transporter
(Ki=618 nmol/l ; Owens et al, 1997). However, it
is conceivable, although improbable, that its high
affinity for a1-adrenoceptors may have prevented a
rise in systolic BP. Indeed, nefazodone did not even
inhibit 5-HT reuptake despite having a higher affinity
for 5-HT than for NE reuptake sites (Ki=459 nmol/l ;
Owens et al., 1997).
The lack of inhibitory action of venlafaxine
on the tyramine pressor response was unexpected,
especially at the 300 mg/d regimen. It is noteworthy
that Harvey et al. (2000) reported an attenuation of this
parameter using a 375 mg/d regimen of venlafaxine.
The attenuation was, however, only of y20 % with
respect to the baseline value with the standard error
48
P. Blier et al.
for that mean value nearly reaching the 1.0 ratio level
(i.e. no change prior to and following treatment). It
should be noted that this value was not significantly
different from those of the low-dose venlafaxine regimen and of the SSRI sertraline due to the relatively
high standard deviations in all three conditions. The
authors proposed that 5-HT reuptake inhibition could
somehow enhance the tyramine pressor response, but
prior evidence and the present report indicate that
this not the case (Hassan et al., 1985, 1989 ; Figure 2).
Their conclusion was solely based on the observation
that the 375 mg/d venlafaxine regimen failed to
separate from the maproptiline group. Consequently,
it appears that these experiments did not produce any
evidence for a significant attenuation of the tyramine
pressor response using 375 mg/d, as did the present
ones using 300 mg/d. In contrast, in a companion
paper, a significant attenuation was demonstrated
at regimens of 225 and 375 mg/d venlafaxine in
depressed patients (Debonnel et al., 2006). Possible
bases for this healthy volunteer/depressed patient
difference are discussed in that paper. Nevertheless,
it is important to emphasize here that this healthy
volunteer/depressed patient difference is not present
with amitriptyline, with the drug being effective in
both groups of subjects (Ghose and Coppen, 1977 ;
Hassan et al., 1989 ; Wilkins et al., 1985).
Aside from the tyramine pressor test on BP, other
peripheral human models have been utilized to
demonstrate a NE reuptake action of venlafaxine.
For instance, tyramine-induced pupillary response,
reduced salivary flow, BP and HR alterations, and
cardiac NE reuptake have all reported significant
effects for 75 mg/d venlafaxine (Bitsios et al., 1999 ;
Melichar et al., 2001 ; Watkins et al., 2003). These
approaches, although they demonstrate a significant
degree of NE reuptake inhibition in the periphery,
probably did not correspond to a threshold of NE
reuptake inhibition sufficient to alter the central
NE reuptake process necessary to produce an antidepressant response potentially superior to that of
SSRIs (Kelsey, 2000 ; Thase et al., 2001). It is interesting
to note, however, that in the NE-induced constriction
of the dorsal vein of the hand, 75 mg/d venlafaxine
and 20 mg/d paroxetine were ineffective, but 100 mg/d
desipramine as well as 150 mg/d venlafaxine produced significant inhibitory effects (Abdelmawla et al.,
1999). The latter model would thus correspond to a
clinical action of venlafaxine superior to that of SSRIs
at doses greater than 75 mg/d, presumably being
mediated by an additional NE reuptake inhibition.
All these physiological models represent attempts at
estimating NE reuptake inhibition in the brain. Upon
the availability of a specific ligand of NE reuptake sites
for use in positron emission tomography studies, the
degree of occupancy of NE transporters by various
regimens of venlafaxine will be directly measurable.
Nevertheless, such results will have to be correlated
with clinical data, as done by Meyer et al. (2004) for the
5-HT reuptake process, in order to determine at what
degree of occupancy of NE transporters is the NE
reuptake process significantly inhibited.
In summary, 5-HT reuptake inhibition was clearly
documented with 20 mg/d paroxetine and with
150 and 300 mg/d venlafaxine. The adequacy of the
tyramine pressor response to document the presence
of NE reuptake inhibition was indicated by the
inhibitory action of the NE reuptake inhibitor
desipramine and the lack of effect of paroxetine and
nefazodone. In contrast, the two regimens of venlafaxine did not result in NE reuptake inhibition in
healthy volunteers.
Acknowledgements
This work was supported by Wyeth Canada. We
would like to thank Dr Gilles Caillé and his team for
carrying out, under contract, the venlafaxine plasma
level determinations. Details of this procedure are
provided in Debonnel et al. (2006). We would like to
thank GlaxoSmithKline and Bristol–Myers Squibb for
the plasma level assays of paroxetine and nefazodone
respectively. Desipramine levels were determined in
the main laboratory of the Royal Victoria Hospital in
Montreal. P. Blier is an Adjunct Professor in the
Department of Psychiatry at McGill University.
Statement of Interest
P. Blier, G. Debonnel and C. de Montigny have
paid consultants, remunerated speakers and
received other research grants from Wyeth. P.
has been contracted to produce an animated
gramme for educational purposes for Wyeth.
been
have
Blier
pro-
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