Open Journal of Depression
2013. Vol.2, No.4, 54-63
Published Online November 2013 in SciRes (
Open Access
A Review of the Serotonin-Norepinephrine Reuptake Inhibitors:
Pharmacologic Aspects and Clinical Implications for Treatment
of Major Depressive Disorder and Associated Painful
Physical Symptoms*
Levent Alev1#, Alan Lenox-Smith2, Murat Altin3, Héctor Dueñas4
1Eli Lilly-Japan, K.K., Kobe, Japan
2Eli Lilly-UK, Hampshire, UK
3Eli Lilly-Turkey, Istanbul, Turkey
4Eli Lilly-Mexico, Mexico City, Mexico
Received July 16th, 2013; revised August 20th, 2013; accepted September 1st, 2013
Copyright © 2013 Levent Alev et al. This is an open access article distributed under the Creative Commons At-
tribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the
original work is properly cited.
Background: Depression is characterized by low mood, low self-esteem, and loss of interest or pleasure.
Painful physical symptoms (PPS) associated with depression have a negative impact on the probability of
remission. Because both norepinephrine and serotonin are involved with the central regulation of pain, the
serotonin-norepinephrine reuptake inhibitors (SNRIs) may have more success than the selective serotonin
reuptake inhibitors (SSRIs) in impacting PPS as well as the core emotional symptoms of depression.
Methods: Published preclinical and clinical data on the SNRIs (i.e., milnacipran, venlafaxine, and du-
loxetine) have been reviewed, paying special attention to the differentiation of the pharmacological as-
pects of the SNRIs. The efficacy and safety results on depression and associated PPS have also been
summarized. Results: Each of the SNRIs has different profiles regarding the inhibition of binding to hu-
man serotonin and norepinephrine uptake transporters and clinical pharmacokinetics. All SNRIs have data
for alleviating the core symptoms of depression; duloxetine and venlafaxine show efficacy for PPS asso-
ciated with depression. There are also differences in tolerability and adverse events profiles. Conclusions:
Although all SNRIs have the same dual mechanism of action for the treatment of depression, they have
different pharmacologic profiles that may impact clinical outcomes.
Keywords: Duloxetine; Venlafaxine; Milnacipran; Serotonin-Norepinephrine Reuptake Inhibitors; Painful
Physical Symptoms; Major Depressive Disorder
The core symptoms of major depressive disorder (MDD) in-
clude depressed mood or the loss of interest or pleasure in near-
ly all activities, as well as changes in appetite, sleep or psy-
chomotor activity, decreased energy, feelings of worthlessness
or guilt, difficulty concentrating or making decisions, and sui-
cidality. Just as important, however, are the associated symp-
toms (including tearfulness, irritability, brooding, obsessive
rumination, anxiety, phobias, pain, and sexual dysfunction)
(APA, 2000). Given the profound impact of MDD on individu-
als and on society as a whole, the importance of the treatment
of this disorder is difficult to overstate.
After the development of the tricyclic antidepressants (TCAs),
selective serotonin reuptake inhibitors (SSRIs) were an impor-
tant milestone in the pharmacological treatment of MDD. Dur-
ing the SSRIs era, there was renewed interest in the combined
roles of norepinephrine (NE) and serotonin (5 HT) in depres-
sion. Neurotransmitter depletion studies demonstrate that both
5-HT and NE are independently involved in the pathophysiol-
ogy of depression (Delgado et al., 1990; Delgado et al., 1993;
Delgado, Moreno, Potter, & Gelenbert, 1997), and there is evi-
dence that blocking the reuptake of both may result in in-
creased efficacy (Danish University Antidepressant Group,
1986; Danish University Antidepressant Group, 1990; Nelson,
Mazure, Jatlow, Bowers, & Price, 2004). Along with this shift,
there was a new focus on painful physical symptoms (PPS),
which are frequently associated with depression and are an im-
portant factor in treatment outcome (Leuchter et al., 2010).
Serotonin-norepinephrine reuptake inhibitors (SNRIs), with
their selective effects on both 5-HT and NE, may provide both
greater efficacy and increased convenience of a single agent for
the treatment of depression and associated painful symptoms.
Development of these newer antidepressants represents an im-
portant advance in the standard treatment of depression, by
virtue of treating a broader range of symptoms associated with
*Disclosure of interests: Drs. Alev, Lenox-Smith, Altin, and Dueñas are
employees and minor stockholders of Eli Lilly and Company.
#Corresponding author.
Open Access 55
The Link between Painful Physical Symptoms and
Beyond the core diagnostic criteria of depression, there are
symptoms associated with depression that are not unique to
MDD (APA, 2000). In a literature review of 14 studies that
examined the prevalence of pain among patients with depres-
sion (in a mix of primary care and psychiatric settings), the
mean prevalence of pain was 65%, with a range from 15% to
100% (Bair, Robinson, Katon, & Kroenke, 2003). More re-
cently, a United States study (A Randomized Trial Investigat-
ing SSRI Treatment [ARTIST]) conducted in 573 patients with
depression reported that more than two-thirds (69%) of the
patients complained of pain symptoms of mild severity or
above (Bair et al., 2004). In the Sequenced Treatment Alterna-
tives to Relieve Depression (STAR*D) trial, 80% of the par-
ticipants complained of PPS (Leuchter et al., 2010).
Studies have also shown that the presence of PPS makes the
treatment of depressed patients more difficult. Patients with
moderate to severe pain at baseline were 2 to 4 times as likely
to experience poor depression response and health-related qual-
ity of life after 3 months of treatment compared with patients
with no pain (Bair et al., 2004).
Following treatment with an SSRI antidepressant, although
mood symptoms of depression were improved, physical symp-
toms, PPS in particular, were more likely to persist (Greco,
Eckert, & Kroenke, 2004). Also, patients with severe pain at
baseline were less likely to achieve remission than those with
no pain at baseline (odds ratio [OR] = .11, 95% confidence
interval [CI] .05 to .25) and patients whose pain improved early
on were more likely to achieve remission (OR = .90, 95% CI
1.03 to 3.49) (DeVeaugh-Geiss et al., 2010). Finally, Leuchter
et al. (2010) found PPS to be a possible indicator of poorer
treatment outcome for MDD when an initial SSRI was used.
Thus, when painful symptoms and depression are both present,
recovery takes longer and is less likely.
Neurobiology of the Norepinephrine and Serotonin
Pathways and Their Roles in Pain Regulation
Both 5-HT and NE neurons have ascending tracts to the
cerebral cortex and limbic area, but have descending tracts to
the spinal cord as well (Fields, Heinricher, & Mason, 1991).
Serotonergic neurons in the rostral ventromedial medulla act on
various receptor subtypes to exert complex modulatory effects
on nociceptive transmission in the dorsal horn (Table 1). Anti-
nociceptive effects occur when 5 HT1A receptors inhibit excit-
ability of spinothalamic neurons and excitatory interneurons
and presynaptic 5-HT1B/D receptors inhibit neurotransmitter
release from primary afferents. Descending serotonergic path-
ways also have pronociceptive effects. Stimulation of 5-HT2
receptors increases excitability of postsynaptic spinothalamic
neurons. Also, 5-HT3 receptors (cation channels that elicit de-
polarization) act presynaptically to increase neurotransmitter
release from primary nociceptive afferents and postsynaptically
to increase excitability of spinothalamic neurons (Benarroch,
2008). Thus, the descending 5-HT pathways may exert either
inhibitory or excitatory effects on pain perception.
Noradrenergic neurons in the pontine tegmentum also project
to the dorsal horn, where they produce mostly antinociceptive
effects. Norepinephrine primarily inhibits nociceptive transmis-
sion in the dorsal horn via presynaptic α2 receptors in primary
nociceptive terminals. These receptors may also mediate post-
Table 1.
Summary of nociceptive and antinociceptive actions of serotonin and
norepinephrine in the dorsal horn of the spinal cord.
ReceptorAction Target Effect
5-HT1A Postsynaptic
inhibition STNs Pain
5-HT1A Postsynaptic
inhibition Excitatoryinterneurons Pain
5-HT1B/D Inhibit NT
release Primary afferents Pain
5-HT2 Postsynaptic
excitation STNs Pain
5-HT3 Increase NT
release Primary afferents Pain
5-HT3 Postsynaptic
excitation STNs Pain
α2 Inhibit NT
release Primary afferents Pain
α2 Postsynaptic
inhibition STNs Pain
α1 Postsynaptic
excitation Inhibitory Interneuron Pain
Abbreviations: 5-HT = serotonin, NE = norepinephrine, NT = neurotransmitter,
STNs = spinothalamic neurons.
synaptic inhibition of spinothalamic neurons. Activation of
postsynaptic α1 receptors may contribute to antinociception by
increasing the release of γ-aminobutyric acid (GABA) or gly-
cine by local inhibitory neurons (Benarroch, 2008). Thus, 5-HT
and NE function as part of the body’s endogenous analgesic
system but may become dysfunctional in depression and lead to
PPS (Stahl, 2002).
In this review, we report findings of a literature search of the
preclinical and clinical data of the SNRIs, specifically duloxet-
ine, milnacipran, and venlafaxine. Preclinical data focus mainly
on pharmacologic aspects and differentiation between the
SNRIs. Clinical study results are evaluated with regard to effi-
cacy and safety/tolerability in the treatment of MDD. Both
acute and long-term, placebo-controlled or active comparator
studies are considered. Separately, efficacy results are reviewed
with regard to MDD and associated PPS. In addition, we at-
tempt to provide information about the connection between
pharmacologic profile and clinical outcome.
Pharmacological Aspects of the
Serotonin-Norepinephrine Reuptake Inhibitors
Preclinical Data: Reuptake Inhibition of Serotonin
and Norepinephrine
By definition, all SNRIs block the reuptake of both 5-HT and
NE, but the potency and balance of this reuptake inhibition may
help determine their different clinical properties. Table 2 lists
the inhibition constants (Ki, nM) for inhibition of binding to
human NE and 5-HT uptake transporters for the SNRIs (and a
selection of SSRIs and TCAs for comparison). The NE/5-HT
ratio of Ki’s is provided as a measure of the balance of the inhi-
bition of the reuptake of these 2 neurotransmitters. Duloxetine
is a potent (low Ki) and relatively balanced (ratio near 1)
Open Access
Table 2.
Inhibition of binding to human monoamine uptake transporters (Ki,
Drug NE
Ki, nM
Ki, nM
NE/5-HT Ratiob
(1 = balance)
Duloxetinec 7.5 .8 9.4
Venlafaxin ec 2480 82 30
Milnacipr and 200 123 1.6
Fluoxetinee 1021 6.9 148
Paroxetinee 132 .38 330
Sertralined 715 .9 794
Citaloprame >10000 9.5 >1053
Clomipraminef 38 .28 136
Desipramined 3.8 179 .02
aThe lower the number, the larger potency or affinity for transporter inhibition. bA
ratio >1 indicates 5-HT is inhibited to a greater extent than NE. cBymaster et al.
(2001) Neuropsychophar macology , 25, 871-880. dKoch et al. (2003) Neuro-
pharmacol ogy, 45, 935-944. eKoch et al. (2002) Neuropsychopharmacolog y, 27,
949-59. fTatsumi et al. European Journal of Pharmacology, 340, 249-258. Ab-
breviations: 5-HT = serotonin, NE = norepinephrine, SNRI = serotonin-norepi-
nephrine reuptake inhibitors, SSRI = selective serotonin reuptake inhibitors.
5-HT/NE reuptake inhibitor. However, because of the higher
NE/5-HT ratio of venlafaxine, the NE reuptake properties of
this agent are generally not evident until a dose of at least 150
mg/day is achieved (Wong & Bymaster, 2002). Evidence for
this comes from microdialysis experiments in rats showing the
dose required to block NE depletion by 50% (ED50) is 14.9
mg/kg for duloxetine compared with >100 mg/kg for venla-
faxine (Iyengar, Webster, Hemrick-Luecke, Xu, & Simmons,
Davidson et al. (2005) found that venlafaxine (extended re-
lease) at a dose of 225 mg/day produced 56% NE transporter
occupancy using an ex vivo method. Kihara and Ikeda (Kihara
& Ikeda, 1995) also showed using microdialysis that duloxetine
administration in rats results in balanced increases in 5-HT and
NE even at the lowest doses (without having to “push” the
dose). Milnacipran is an even more balanced 5-HT/NE reuptake
inhibitor, but is less potent, as evidenced by its NE and 5-HT
transporter Ki’s.
In addition to NE and 5-HT, duloxetine is a weak inhibitor of
dopamine reuptake. However, it has no significant affinity for
dopaminergic, cholinergic, histaminergic, adrenergic, opioid,
glutamate, or GABA receptors in vitro and does not inhibit
monoamine oxidase (Knadler, Lobo, Chappel, & Bergstrom,
2011). Venlafaxine is also a weak inhibitor of dopamine reup-
take but has no significant affinity for muscarinic cholinergic,
H1-histaminergic, or α1-adrenergic receptors in vitro (Wyeth
Pharmaceuticals, Inc, 2012). Milnacipran has a mild affinity for
N methyl-D-aspartate receptors. However, it has no significant
affinity for serotonergic (5-HT1-7), α- and β-adrenergic, mus-
carinic (M1-5), histamine (H1-4), dopamine (D1-5), opiate, benzo-
diazepine, and GABA receptors in vitro (English, Rey, & Rufin,
Clinical Pharmacokinetics
Clinical pharmacokinetic information for milnacipran, venla-
faxine, and duloxetine is provided in Table 3.
Because milnacipran, venlafaxine, and duloxetine are all at
least partially excreted through the urine, their use should be
carefully considered in patients with renal impairment. For
patients with severe renal impairment, a reduced dose of mil-
nacipran is recommended. Milnacipran is not recommended in
patients with end-stage renal disease. Dose reduction of venla-
faxine is recommended for patients with renal impairment and
patients undergoing dialysis. Duloxetine is not recommended
for patients with severe renal impairment or end-stage renal
In terms of hepatic metabolism, milnacipran is not metabo-
lized by cytochrome P450 enzymes (and no dosage adjustment
is necessary for patients with hepatic impairment) (Forest
Laboratories, Inc, 2012), but venlafaxine (Wyeth Pharmaceuti-
cals, Inc, 2012) and duloxetine (Eli Lilly and Company, 2012;
Nelson et al., 2004) are. Patients with hepatic insufficiency
have decreased duloxetine metabolism and elimination; there-
fore, duloxetine should ordinarily not be used in patients with
this condition (Eli Lilly and Company, 2012; Nelson et al.,
2004). Duloxetine is a cytochrome P450 2D6 (CYP2D6) in-
hibitor, which may result in various drug interactions (Eli Lilly
and Company, 2012; Nelson et al., 2004). Venlafaxine is also
metabolized in the liver, so dosage adjustment is necessary in
hepatically impaired patients (Wyeth Pharmaceuticals, Inc,
2012). Venlafaxine is unique among the SNRIs in that it’s the-
rapeutic properties are thought to be primarily the result of its
metabolite, O-desmethylvenlafaxine. Metabolism of venlafax-
ine into O desmethylvenlafaxine occurs in the liver prince-
pally by CYP2D6 (Lobello et al., 2010)].
Efficacy in the Treatment of Depression and
Associated Painful Physical Symptoms
In the following section, we will review the efficacy of the
SNRIs in their treatment of MDD and associated PPS.
Two clinical trials comparing milnacipran with placebo have
been summarized by Lecrubier et al. (1996). Study 1 was an
8-week, dose-ranging study of outpatients with moderate/severe
depression (Hamilton Depression Rating Scale [HAMD] 17-
item [HAMD-17] total score >22). Both the 50-mg and 100-mg
doses, but not the 25-mg dose, of milnacipran resulted in sig-
nificant improvement on the HAMD-17 and Montgomery-
Åsberg Depression Rating Scale (MADRS) compared with
placebo. Study 2 was carried out in hospitalized patients with
HAMD-17 scores 22. Significant improvement was found on
the HAMD-17 (change from baseline, milnacipran 51%, pla-
cebo 38%; p < .05), but not the MADRS or the Clinical
Global Impressions (CGI) scale. In a placebo-controlled, re-
lapse-prevention study, milnacipran (50 mg twice daily) was
superior to placebo in the prevention of relapse in patients who
had responded to acute treatment and had remained in remis-
sion during a 4-month continuation phase (Rouillon et al.,
2000a; Rouillon, Warner, Pezous, & Bisserbe, 2000b).
A number of clinical trials have compared milnacipran
with SSRIs. In a 6-week, double-blind comparison, fluoxetine
(20 mg/day) exhibited better efficacy than milnacipran
Open Access 57
Table 3.
Pharmacokinetic properties of milnacipran, venlafaxine, and duloxetine.
Milnaciprana Venlafaxine XRb Duloxetinec
Absorption and Distribution
Absolute bioavailability 85% - 90% 45% 43%d
Tmax 2 - 4 hours 5.5 (Ven), 9 (ODV) hours 6 - 10 hours
Plasma protein binding 13% 27% (Ven), 30% (ODV) >90%
Time to steady-state level 36 - 48 hours Within 3 days Within 3 days
Terminal elimination half-life 6 - 8 hours 5 ± 2 (Ven), 11 ± 2 (ODV) hours 8 - 17 hours
Metabolism and Elimination
Active metabolite None ODV None
Main metabolism Glucuronidation Hepatic Hepatic
Excretion in urine 55% (unchanged) 87% 70%
aSavella package insert (Forest Laboratories, Inc, 2012). bEffexor XR package insert (Wyeth Pharmaceuticals, Inc, 2012). cCymbalta package
insert (Eli Lilly and Company, 2012). dLobo, E.D. et al. (2008) Clinical Pharmacokinetics, 47, 191-202. Abbreviations: Tmax = time to peak
concentration, Ven = venlafaxine, ODV = O-desmethylvenlafaxine.
(100 mg/day) on the MADRS, HAMD, CGI-Severity (CGI-S)
(Ansseau et al., 1994). However, the lower efficacy of mil-
nacipran was possibly because milnacipran, which possesses an
elimination half-life of 6 to 8 hours, was given only once daily.
A similar study that used a split dose for milnacipran found no
significant difference in efficacy between milnacipran and fluo-
xetine, though a significant increase in heart rate (HR) was
noted for milnacipran (Guelfi et al., 1998). In a comparison
with fluvoxamine (100 mg twice daily), milnacipran (50 mg
twice daily, 6 weeks) produced a significantly greater reduction
in MADRS scores and a higher response rate (decrease of 50%
or more of the MADRS score from baseline; milnacipran =
78.9%; fluvoxamine = 60.7%) (Clerc, 2001). Comparisons of
milnacipran with paroxetine have shown the 2 antidepressants
have equivalent efficacy in diverse populations (Chang et al.,
2008; Lee et al., 2005; Sechter et al., 2004). A meta-analysis
showed that there were no differences in clinical improvement,
remission, or overall tolerability when comparing milnacipran
with SSRIs (Nakagawa et al., 2008).
Little research has been done on the efficacy of milnacipran
in the treatment of PPS associated with depression. However,
the efficacy of milnacipran in patients with fibromyalgia has
been well-established, and milnacipran has been approved for
this indication by the United States Food and Drug Administra-
tion (FDA) based on 2 pivotal clinical trials (Clauw, Mease,
Palmer, Gendreau, & Wang, 2008; Mease et al., 2009).
The efficacy of venlafaxine has been established in several
placebo-controlled trials, often including an SSRI comparator.
Nemeroff and Thase (2007) found that venlafaxine (75 to 225
mg/day) was superior to placebo (but not to fluoxetine [20 to 60
mg/day]). Shelton et al. (2005) pooled results from 5 active,
placebo-controlled studies and found that the response rate for
venlafaxine was significantly greater than placebo or fluoxet-
ine/paroxetine. However, Schatzberg and Roose (2006) found,
in a placebo-controlled comparison of venlafaxine and fluoxet-
ine, that all 3 groups improved significantly on the HAMD
21-item (HAMD-21) after 8 weeks of treatment, but that there
were no significant differences among the treatment groups on
the change in HAMD-21, MADRS, or CGI scores. Finally, a
relapse-prevention study has shown that relapse rates were
significantly lower for treatment with venlafaxine than with
placebo after 2 years (28.5% vs. 47.3%, p = .005) (Kornstein,
Venlafaxine has been compared with SSRI antidepressants in
several meta-analyses. Meta-analyses (or pooled analyses) are
useful because individual trials are not usually powered to
demonstrate differences between antidepressants. Thase et al.
(2001) found that remission rates for venlafaxine were signifi-
cantly higher than for SSRIs (45% versus 35%, respectively; p
< .001, number needed to treat [NNT] = 10). Smith et al. (2002)
also found an advantage for venlafaxine over the SSRIs (effect
size= .17, 95% CI= .27 to .08). Papakostas et al. (2007)
showed the pooled response rates for venlafaxine to be some-
what greater than rates for the SSRIs (68% versus 61.2%, re-
spectively). Finally, Nemeroff et al. (2008) found the overall
difference in remission rates between venlafaxine and SSRIs
was 5.9% favoring venlafaxine (p < .001, NNT = 17).
Genetic factors may contribute to individual variations in ef-
ficacy with SNRIs. Venlafaxine is metabolized primarily by the
highly polymorphic enzyme cytochrome P450 2D6 to yield the
pharmacologically active metabolite O-desmethylvenlafaxine.
As there is variability in the population with regard to the effi-
ciency of this enzyme, up to 10% of Caucasians will not receive
the full effect of venlafaxine (Lobello et al., 2010; Zanger,
Raimundo, & Eichelbaum, 2004). Lobello et al. (2010) found a
significantly higher percentage of extensive metabolizers achiev-
ed response or remission compared with poor metabolizers, but
found no differences in tolerability. Given duloxetine is mainly
metabolized via cytochrome P450 1A2, it would be expected
that duloxetine metabolism would not be affected, thus efficacy
would also be unaffected. This finding could help explain du-
loxetine-related improvement in depressive symptoms in pa-
tients switching from venlafaxine because of suboptimal re-
sponse (Wohlreich et al., 2005).
Studies with venlafaxine in adults also have established effi-
cacy in the reduction of PPS related to depression, though these
studies were of open-label design. Plesnicar (2010) found a
response rate of 93% (50% reduction of HAMD-17 score from
baseline) and an overall statistically significant improvement on
the Depression and Somatic Symptom Scale (33.6 versus 12.4,
p < .0001) after 8 weeks of treatment with venlafaxine (75 to
Open Access
375 mg/day). Bradley et al. (2003) found, after 12 months of
treatment with venlafaxine (150 mg/day), that HAMD-21
scores and visual analogue scale (VAS) scores for pain had
decreased significantly from baseline. Similar results have been
noted in the elderly. Ibor et al. (2008) found a response rate of
81.6% (as defined by a 50% decrease in HAMD-17 score) and
significant improvement on the VAS for pain (p < .0001) after
24 weeks of treatment with venlafaxine (75 to 225 mg/day) in
patients older than 60 years of age.
There is a question about whether venlafaxine functions
mostly as an SSRI at the lowest dosages. For example, venla-
faxine (75 mg) does not potentiate the NE-mediated venocon-
strictor response, whereas a higher dose (150 mg) does (Ab-
delmawla, Langley, Szabadi, & Bradshaw, 1999). This may
help explain venlafaxine’s increased efficacy at higher doses.
Rudolph et al. (1998) studied 3 dosages (75, 225, and 375
mg/day) and found that the dose-response relationship between
venlafaxine and HAMD-21 total score at Week 6 was statistic-
cally significant (p .01).
The efficacy of duloxetine has been established in several
placebo-controlled, randomized trials (Brannan et al., 2005;
Detke, Lu, Goldstein, McNamara, & Demitrack, 2002; Detke,
Lu, Goldstein, Hayes, & Demitrack, 2002; Goldstein, Mal-
linckrodt, Lu, & Demitrack, 2002; Mallinckrodt et al., 2003;
Nemeroff et al., 2002; Nierenberg et al., 2007; Perahia et al.,
2006; Raskin et al., 2007). Data from 4 randomized, dou-
ble-blind, placebo-controlled trials of duloxetine 60 mg once
daily were pooled for an analysis of the efficacy of duloxetine
versus placebo in MDD treatment (Shelton et al., 2007). Du-
loxetine has also been shown to be effective in the prevention
of relapse of MDD. In a re-analysis of a relapse-prevention
study (Perahia et al., 2009), Kelin et al. (2010) found that, con-
sidering only patients taking 60 mg/day duloxetine, during the
double-blind maintenance phase (up to 52 weeks), 31.7% of
placebo-treated patients experienced a depressive recurrence,
compared with 12.5% of duloxetine-treated patients (p = .004).
Duloxetine has been compared with fluoxetine and paroxet-
ine in 6 clinical trials. In a pooled analysis of those trials, rates
of remission (defined as a HAMD-17 total score 7) for the
active treatments were significantly greater than placebo, but
were not significantly different from each other (duloxetine
40.3%, SSRIs 38.3%, placebo 28.4%). However, a subanalysis
of patients with moderate/severe depression (defined as a
HAMD-17 total score 19) showed duloxetine to be superior to
the 2 SSRIs in this group (35.9% versus 28.6%; p < .046)
(Thase et al., 2007). Another meta-analysis showed the pooled
response rates to duloxetine and SSRIs to be virtually identical
(51.6% versus 51.4%, respectively), though reasons to interpret
this result with caution were cited (Papakostas et al., 2007). In
an open-label study of patients who did not respond to initial
treatment with an SSRI, more than half responded (50% de-
crease in HAMD-17 total score from baseline to endpoint) to
treatment with duloxetine (Perahia, Quail, Desaiah, Corruble, &
Fava, 2008b).
Duloxetine has also been shown to be effective in the treat-
ment of PPS related to depression. Analysis of pooled data
from 2 identical, but independent, placebo-controlled, 9-week
trials showed that patients treated with duloxetine (60 mg/day)
demonstrated significantly greater improvement in overall pain
(as measured on the VAS) compared with those treated with
placebo (p = .016) (Fava, Mallinckrodt, Detke, Watkin, &
Wohlreich, 2004). Brecht et al. (2007) found that, compared
with placebo, duloxetine (60 mg/day) significantly reduced pain
and improved depression, with significant mean changes in
Brief Pain Inventory (BPI)-Short Form average pain scores.
However, Brannan et al. (2005) found that duloxetine-related
improvements in BPI scores at endpoint, compared with pla-
cebo, only approached significance. Mean changes at endpoint
in depression rating scales (HAMD-17 and CGI-S) also did not
differ significantly between duloxetine (60 mg) and placebo
treatment groups. A summary of pooled data from 3 clinical
trials, however, found that, compared with placebo, duloxetine
was associated with significant reduction in pain severity
(Goldstein et al., 2004). Perahia et al. (2008b) found that pa-
tients who did not respond to initial treatment with an SSRI and
were switched to duloxetine (60 mg/day) saw significant im-
provement in VAS overall pain scores.
Comparisons of the Serotonin-Nor epinephrine
Reuptake Inhibitors
Perahia et al. (2008a) found no significant difference be-
tween duloxetine (60 mg/day) and venlafaxine (150 mg/day) as
measured by global benefit risk assessment (efficacy/adverse
events [AEs]; 1.418 versus 1.079, p = .217) and no signifi-
cant differences between the groups on the majority of efficacy
measures. Nausea was the most common treatment-emergent
adverse event (TEAE) for both drugs and was significantly
higher with duloxetine compared with venlafaxine (43.6% ver-
sus 35.0%, p < .05). Significantly more venlafaxine-treated
patients experienced sustained elevations of systolic blood
pressure [BP] (p = .047). More recently, in a re-analysis of the
data reported by Perahia and colleagues, duloxetine has been
shown to be noninferior to venlafaxine (Lenox-Smith, Kelin,
Brnabic, & Bradley, 2011). A meta-analysis comparing 12
antidepressants found an efficacy advantage for venlafaxine
over duloxetine (OR = .77, 95% CI = .60 to .99), but no differ-
ence in efficacy between duloxetine and milnacipran (OR = .97,
95% CI = .69 to 1.38) or milnacipran and venlafaxine (OR
= .79, 95% CI = .58 to 1.08) (Cipriani et al., 2009). However,
weaknesses of this meta-analysis and doubts about its conclu-
sions have been raised. For example, the analysis included data
from treatment arms using subtherapeutic dosages of duloxetine
and most of inferences about duloxetine were drawn from indi-
rect comparisons (Lenox-Smith, D’yachkova, Deberdt, & Ras-
kin, 2010).
Many SNRI-related AEs are predictable from their mode of
action. Thus, one sees the SSRI-type AEs (5-HT effects) such
as nausea, vomiting, dizziness, and sexual dysfunction, but also
the NE events such as raised BP, HR, and sweating. Other
events, such as QT intervals, are drug-specific.
Tolerability and safety of milnacipran have been reviewed by
Puech et al. (1997). Dropouts caused by AEs were 6.1% in the
placebo group, 7.6% with milnacipran, 14.8% with TCAs, and
7.8% with SSRIs. For patients receiving milnacipran, the AEs
noted with a frequency significantly greater than for patients
Open Access 59
receiving placebo were vertigo (5.0%), increased sweating
(4.3%), anxiety (4.1%), hot flushes (3.0%), and dysuria (2.1%).
Treatment with milnacipran resulted in a mild increase in HR
(3.2 beats per minute [bpm]) and a negligible effect on BP (<1
mmHg). In contrast, there was an increased incidence of or-
thostatic hypotension (defined as a decrease of >20 mmHg)
with milnacipran (21%). Milnacipran had minimal effects on
the electrocardiography, including the PR interval, the QRS
duration, and the corrected QT (QTc) space.
Milnacipran has also been reported to improve sexual func-
tion and enjoyment in depressed patients (Baldwin, Moreno, &
Briley, 2008).
Among the 2897 venlafaxine patients in Phase 2 and Phase 3
depression studies reviewed by Rudolph and Derivan (Rudolph
& Derivan, 1996) 18% discontinued treatment due to an AE.
The more common events (occurring in 10% of the venla-
faxine-treated patients and at a rate twice that reported among
placebo-treated patients) were nausea, insomnia, dizziness, som-
nolence, constipation, and sweating. By contrast, 6% of 609
patients treated with placebo during clinical trials withdrew
because of AEs (Rudolph et al., 1996).
Treatment with venlafaxine is associated with sustained in-
creases in BP in some patients. In placebo-controlled studies
with venlafaxine, clinically significant increases in BP (increase
in diastolic BP 15 mmHg and 105 mmHg from baseline)
were observed in 5.5% of patients at doses above 200 mg daily
(Feighner, 1995). The mean increase in BP was 2.93 mmHg
(systolic) and 3.56 mmHg (diastolic) after 8 to 12 weeks of
treatment with doses of >75 mg/day (Wyeth Pharmaceuticals,
Inc, 2012).
The mean change from baseline in QTc for venlafaxine-
treated patients was greater than that for placebo-treated pa-
tients (increase of 4.7 milliseconds [ms] for venlafaxine and
decrease of 1.9 ms for placebo). The mean change from base-
line in HR for venlafaxine-treated patients was significantly
higher than that for placebo (a mean increase of 4 bpm for
venlafaxine and 1 bpm for placebo). In a flexible-dose study,
with venlafaxine doses ranging from 200 to 375 mg/day and
mean dose >300 mg/day, venlafaxine-treated patients had a
mean increase in HR of 8.5 bpm compared with 1.7 bpm in the
placebo group. In addition, a loss of 5% or more of body
weight occurred in 7% of patients treated with venlafaxine
compared to 2% of patients treated with placebo (Wyeth Phar-
maceuticals, Inc, 2012). Finally, sexual dysfunction has been
noted in patients treated with venlafaxine (Lee et al., 2010;
Schweitzer, Maguire, & Ng, 2009).
Abrupt discontinuation has been found to be associated with
the appearance of a variety of new symptoms (Wyeth Pharma-
ceuticals, Inc, 2012).
Safety data for duloxetine have been pooled from the acute
phases of 8 double-blind, placebo-controlled, randomized clini-
cal trials (Hudson et al., 2005). The rates of serious adverse
events (SAEs) for duloxetine- and placebo-treated patients
were .3% and .6%, respectively (p = .282). AEs lead to discon-
tinuation in 9.7% of duloxetine-treated patients, compared to
4.2% of patients receiving placebo (p < .001). TEAEs present
in duloxetine-treated patients with an incidence 5% that were
significantly greater than in patients receiving placebo, were
nausea, dry mouth, constipation, insomnia, dizziness, fatigue,
somnolence, increased sweating, and decreased appetite. Mean
changes in BP and HR were small, and the incidence of in-
creases above normal ranges was low. Duloxetine-treated pa-
tients had a mean decrease in weight of .5 kg, compared with an
increase of .2 kg for patients receiving placebo (p < .001).
Thase et al. (2005) further evaluated the effects of duloxetine
on cardiovascular safety based on data from the 8 placebo-
controlled trials plus active comparator-controlled depression
trials. There was a significant increase for duloxetine compared
with placebo for HR (1.6 versus .6 bpm) and for systolic BP
(1.0 versus 1.2 mmHg); the difference for diastolic BP (1.1
versus .3) was not significant. Also, the effect of duloxetine on
mean changes in supine systolic and diastolic BP was not sig-
nificantly different from that of fluoxetine or paroxetine. Drug-
placebo differences in mean changes in electrocardiograms (eg,
QTc, PR, and QRS intervals) were neither statistically nor
clinically significant, with the exception of duloxetine 120 mg/
day, which significantly decreased PR and QRS intervals com-
pared with placebo.
Delgado et al. (2005) found the incidence of acute treatment-
emergent sexual dysfunction is higher with duloxetine com-
pared with placebo, but is significantly lower when compared
with paroxetine. However, Dueñas et al. (2011) found treat-
ment-emergent sexual dysfunction in duloxetine and SSRI
monotherapy to be comparable (23.4% versus 28.7%, respec-
tively; p = .087).
Duloxetine treatment is also associated with discontinuation
symptoms. In a pooled analysis of 6 short-term treatment trials
in which treatment was stopped abruptly, discontinuation-
emergent adverse events (DEAEs) were reported by 44.3% and
22.9% of duloxetine-treated and placebo-treated patients, re-
spectively (p < .05). Among duloxetine-treated patients report-
ing at least 1 DEAE, the mean number of symptoms was 2.4.
The DEAEs reported significantly more frequently upon abrupt
discontinuation of duloxetine (compared with placebo) were
dizziness (12.4%), nausea (5.9%), headache (5.3%), paresthesia
(2.9%), vomiting (2.4%), irritability (2.4%), and nightmares
(2.0%) (Perahia, Kajdasz, Desaiah, & Haddad, 2005).
Genetic factors may also contribute to individual variations
in adverse reactions to SNRIs. For example, findings from du-
loxetine clinical trials suggest that the incidence of nausea may
be higher in patients from East Asia (37%; 60 mg once daily
[QD] compared with Caucasian patients [14.4%, pooled analy-
sis; 23.4%, pooled analysis; 29.7%, 60 mg QD]) (Lee et al.,
Summary and Conclusions
Remission in major depression is really the goal of treatment:
alleviate a maximum number of depressive symptoms, return to
functional normality, and minimize relapse. One of the difficult
aspects of depression is its multifaceted nature, with many po-
tential associated symptoms. Pain is an important symptom
associated with depression because of its negative impact on
patient functionality and the probability of reaching remission.
Research suggests that chronic pain and depression not only
co-occur but promote the development of each other, such that
chronic pain is a strong predictor of subsequently developing
major depression, and vice versa. It has also been suggested
that pain and depression co-exist symptomatically because they
Open Access
are driven by largely overlapping pathophysiological processes
in the brain and body (Maletic & Raison, 2009). Antidepres-
sants with a dual (5-HT/NE reuptake inhibition) mechanism of
action may be more efficacious in treatment of the PPS associ-
ated with depression (Trivedi, 2004). The biological basis for
this hypothesis lies in the apparently reciprocal relationship
between the descending serotonergic and noradrenergic projec-
tions from the brainstem. Dysfunction in these spinal pain
modulatory pathways, owing to dysregulation of 5-HT and NE
neurotransmission, also believed to play a role in depression,
may lead to a hyperalgesic state in patients with depression
(Stahl, 2002).
Although all SNRIs have the same mechanism of action,
they each also have some unique pharmacologic aspects. Phar-
macodynamics and pharmacokinetics of each of the SNRIs may
help explain the efficacy patterns, posology, and some AEs of
each. Also, to understand the potential for drug interactions and
whether dose adjustments are appropriate for a specific patient
(including hepatic and renal insufficiency), it is important to
consider the intrinsic and extrinsic factors that influence the
pharmacokinetics or pharmacodynamics of SNRIs.
Milnacipran has a relatively high potency for NE reuptake
inhibition than 5-HT compared to other SNRIs. It is generally
given twice daily in MDD treatment due to its short plasma
half-life. It has been shown to be superior to placebo in treating
depression. However, comparisons with SSRIs have yielded
mixed results. Unfortunately, there is little evidence to conclude
whether milnacipran is effective in treating PPS associated with
depression. The AEs for milnacipran (eg, sweating, urinary
hesitancy) tend to be related to NE. Regarding cardiac/BP
changes, milnacipran treatment is associated with orthostatic
hypotension, but has no apparent effect on electrocardiography.
Venlafaxine has differing degrees of inhibition of NE reup-
take depending on dose, whereas 5-HT reuptake inhibition is
moderately potent and present at almost at all doses. It has been
shown to have somewhat better efficacy than the SSRIs in the
treatment of depression. In addition, there is some evidence that
venlafaxine reduces PPS associated with depression. However,
the dose-response effect related to venlafaxine’s ability to block
reuptake of NE requires titration up to higher doses in clinical
practice. Also, venlafaxine has reduced efficacy and/or reduced
tolerability in patients who are poor cytochrome P450 2D6
metabolizers. TEAEs for venlafaxine tend to be more 5-HT-
related. However, venlafaxine treatment is associated with NE-
related increases in BP, HR, and QTc.
Duloxetine is relatively balanced in its potency of 5-HT and
NE reuptake inhibition. It may be administered once daily in
MDD treatment. In addition to alleviating the core symptoms of
depression, duloxetine may have particular efficacy for PPS
associated with depression. There is also some evidence that the
efficacy of duloxetine is superior to that of the SSRIs, at least
among those patients with more severe symptoms. Duloxetine
is the first SNRI approved for treatment of diabetic peripheral
neuropathic pain in most countries. It is also approved for fi-
bromyalgia and chronic musculoskeletal pain (i.e., chronic low
back pain, osteoarthritis). Duloxetine treatment is associated
with a small increase in HR and systolic BP, but not diastolic
BP. No electrocardiographic changes have been noted at nor-
mal doses.
Direct comparison of the benefits and risks of individual
SNRIs can be difficult from the clinical point of view since
there are few head to head studies between SNRIs. More com-
parative clinical and preclinical studies may be of benefit.
In conclusion, although all SNRIs have a similar mechanism
of action for MDD treatment, they have different pharma-co-
logic profiles that may impact clinical outcomes. Careful con-
sideration of the pharmacologic differentiations of SNRIs can
help to achieve better efficacy and safety results in the clinical
practice of MDD treatment.
We thank Dr. Rodney Moore (inVentiv Health Clinical,
funded by Eli Lilly and Company) for his assistance in drafting
the manuscript.
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