A double-blind, randomized, placebo-controlled, proof of concept study to investigate the safety and efficacy of the combined administration of 0.5 mg sublingual testosterone and 10 mg tadalafil in women with hypoactive sexual desire disorder

Female sexual dysfunction (FSD) refers to various disturbances or impairments
of sexual function, including a lack of interest in sexual activity, repeated
failure to attain or maintain sexual excitement, and inability to attain an
orgasm following sufficient arousal. A recent study estimated that 43% of
women suffer from sexual dysfunction in the United States (US).1 Low sexual
desire (22% prevalence) and sexual arousal problems (14% prevalence) belong to
the most common categories of sexual dysfunction of women. These categories
are convenient in providing working definitions and an accepted lexicon for
researchers and therapists. However, it may be incorrect to assume that these
disorders are fully independent of each other. Both case studies and
epidemiological studies demonstrate that these disorders can overlap and may be
interdependent. In some cases, it may be possible to identify the primary
disorder that led to the others, but in many cases, this may be impossible. In
view of the recognition that sexual desire and arousal problems often coexist,
it has recently been recommended that a new combined diagnosis of Sexual
Interest/Arousal Disorder replace the separate diagnoses of hypoactive sexual
desire disorder (HSDD) and female sexual arousal disorder (FSAD) in the fifth
edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM),
scheduled for publication in 2013.
The 3 (transitional and overlapping) phases of the human sexual response can
each be disrupted, leading to low sexual desire, sexual arousal problems, and
hampered orgasm. The phases are regulated by relatively independent
neurotransmitter functions, and dysfunctions are candidates for
psychopharmacological treatment. Traditionally, motivated behaviors have been
divided into appetitive and consummatory components. Activities aimed at
obtaining reward and satisfactions belong to the appetitive component. The
fundamental appetitive motivational process is an intrinsic brain function and
is especially related to the predictive value of stimuli for reward.
Processing of motivationally relevant information (ie, stimuli predicting
reward) causes an increase in activity of the meso accumbens dopaminergic
system (ie, dopamine neurons of the ventral tegmental area [VTA] innervating
the nucleus accumbens). The activity of this system is increased during
flexible approach behavior when anticipating reward related to copulation.2
Increasing activity in these dopaminergic pathways facilitates sexual
motivation, in particular anticipatory sexual behavior.3
Anticipating sexual reward will produce arousal of the genitals, in which at
least 2 key neurotransmitters are involved: acetylcholine and nitric oxide
(NO). Acetylcholine and NO both promote erections in men and lubrication and
swelling in women. Orgasm, the consummatory phase of human sexual response, is
facilitated by descending spinal noradrenergic fibers and innervation of the
genitals, and inhibited by descending spinal serotonergic fibers.
In many mammalian species, female sex steroids are necessary for the expression
of female sexual behavior. As a result, the capability for copulation in these
animals is limited to the period of ovulation.4,5 Humans (as well as higher
primates) show sexual intercourse outside the periovulatory period. In humans,
testosterone is clearly involved in female sexual behavior.6 The disappearance
of testosterone following ovariectomy and adrenalectomy is accompanied by a
complete loss of libido,7,8 while substitution of this steroid maintains sexual
desire and fantasies after surgical menopause.9
An important aspect of sexual motivation is physiological sexual responding.
Measured as an increase in vaginal vasocongestion elicited by sexual stimuli,
this responding is considered to be preparatory for copulatory behavior.10 In
hypogonadotropic, hypogonadal females, substitution with testosterone
undecanoate 40 mg orally per day during an 8 week period enhanced vaginal
responsiveness.10 This effect was not found in another group of
hypogonadotropic, hypogonadal patients (unpublished data). In both studies,
subjects received testosterone each morning, but subjects in the first study
were tested in the afternoon, and subjects in the second study were tested in
the morning. The different outcomes on physiological responding between these
experiments may be caused by a time dependent effect of testosterone on vaginal
arousal. A third study was conducted to determine the effect of a single dose
of testosterone sublingually, as compared with placebo, on vasocongestion
during presentation of visual erotic stimuli.11 On treatment days, 8 sexually
functional women were exposed, at intervals of 1.5 hours, to 6 erotic films
depicting intercourse. The intake of testosterone caused a sharp increase in
plasma levels of testosterone of short duration. About 3 to 4.5 hours after
this testosterone peak, a striking increase in vaginal responsiveness was
observed when the subjects were exposed to the visual sexual stimuli. These
findings demonstrate a time lag in the effect of sublingually administered
testosterone on genital arousal in sexually functional women. This study was
replicated 2 years later, with the same results.12
The results of the above mentioned studies demonstrate that testosterone is
involved in female sexual motivation in a time dependent fashion. The
influence of sex steroids on sexual behavior might be explained by the
existence of a steroid responsive neural network, a highly interconnected group
of sex hormone receptor-containing neurons in the brain.13 This network is not
a closed circuit, but serves reproductive aims by functioning as an integrating
and activating center between external sensory cues, hormonal processes, and
reproductive behavior. This is partly accomplished by selective filtering of
sensory input and amplification of signals that may facilitate sexual
behavior. It is assumed that an increase in vaginal vasocongestion induced by
sexual stimuli is preparatory for copulatory behavior. Visual exposure to
sexual intercourse between members of the species of the onlooker is a potent
releasing stimulus for such a preparatory motivational response. Both men and
women have a marked capacity to respond to erotic films with a genital
response.14 The increased motivational sensitivity for sexual cues induced by
testosterone presumably results from an altered brain state in which
dopaminergic, serotonergic, and noradrenergic pathways are involved. Indeed,
the VTA,15 the dorsal raphe nucleus (serotonergic),16 and the nucleus coeruleus
(noradrenergic)17 are all modulated by androgen activity in rats.
In several studies, it has been shown that selective type 5 phosphodiesterase
(PDE 5) inhibitors improve erectile function in men with erectile dysfunction,
on average close to normal function.18 In the penis, NO released from nerves
and endothelium, induces production of cyclic guanosine monophosphate (cGMP);
cGMP plays a key role in relaxing smooth muscle, necessary for the induction of
an erection. This nucleotide is hydrolyzed by phosphodiesterases, of which PDE
5 exerts the main activity in the corpora cavernosa. Therefore, PDE 5
inhibitors will, during sexual stimulation, enhance the action of NO/cGMP on
erectile function.19 The genitals of both sexes have common embryological
origins. Recently, it has been shown that the clitoris consists of an erectile
tissue complex, which embeds the anterior vaginal wall. Clitoral erection and
the anterior wall of the vagina are highly involved in female sexual arousal
and response. Sildenafil, a PDE 5 inhibitor, has been shown to improve sexual
performance in sexually functional women.14 Two recent studies showed
increased genital sensation in subsets of postmenopausal women with arousal
disorder.20,21 Data on tadalafil is lacking. Only one study has been described
investigating effects of tadalafil in women.22In this study, 3 women using
serotonin enhancing agents reported improved sexual functioning following 20 mg
tadalafil use. These studies suggest that a PDE 5 inhibitor might be beneficial
for women with sexual dysfunction. However, most studies show that the use of a
PDE5 inhibitor in female sexual dysfunction (mostly HSDD or FSAD) is not
efficacious, possibly because central mechanisms of sexual arousal are not
targeted. 23,24
Sexual functions are the result of an interaction between central and
peripheral processes. Combining medications in such a way that both central
and peripheral processes are enhanced - for example by combining testosterone
and a PDE 5 inhibitor - is theorized to have a synergistic effect, because
enhancement of central processes alone should enhance peripheral processes and
vice versa. Enhanced central and peripheral mechanisms then receive enhanced
feedback stimulation from each other, increasing each process in a cumulative
manner. Indeed, in previous studies,23,24 the combination treatment of
sublingual testosterone (0.5 mg) and vardenafil (10 mg) induced a very strong
and significant increase in vaginal pulse amplitude, coupled with subjective
reports of more intense genital sensations and sexual lust in women suffering
from FSD, whereas neither substance alone had an effect. In an experiment on
the efficacy of on demand use of sublingual testosterone (0.5 mg) combined with
sildenafil (50 mg), physiological and subjective sexual responses were
evaluated in an ambulatory psychophysiology laboratory at home for 1 week.
Subsequently, sexual improvement and satisfaction during use of the this
combination was measured (in the bedroom) for 3 weeks. The combination
significantly improved sexual functioning in both phases (manuscript submitted).
The findings that sublingual testosterone combined with sildenafil or combined
with vardenafil can increase measures of sexual arousal in women with HSDD,
suggests that combining 0.5 mg sublingual testosterone with tadalafil may also
have such an effect. Sublingual testosterone has a delay in effect of
approximately 4 hours.11,12 A single dose can thus induce increased sexual
responses in women without FSD,11,12 but not with HSDD.23,24 To allow the
pharmacodynamic effect of a single dose of sublingual testosterone to optimally
overlap with the pharmacodynamic effect of sildenafil or vardenafil, these two
PDE5 inhibitors have to be administered approximately 2 hours after the
testosterone administration. Tadalafil has maximum concentration 2 hours after
ingestion, and a terminal half life of approximately 18 hours. Thus, when
sublingual testosterone and tadalafil are combined, both compounds can be taken
simultaneously. Simultaneous administration of two drugs is easier and could
improve protocol adherence. The present small exploratory study is directed at
investigating the possible efficacy of the combination of a single dose of 0.5
mg sublingual testosterone with 10 mg oral tadalafil, administered
simultaneously.

Primary Objective
To investigate the possible efficacy of combined administration of 0.5 mg
sublingual testosterone and 10 mg tadalafil in increasing sexual satisfaction
during sexual activity in the domestic setting in healthy female subjects with
hypoactive sexual desire disorder (HSDD).

Secondary Objectives
The secondary objectives are as follows:

To investigate the possible efficacy of combined administration of 0.5 mg
sublingual testosterone and 10 mg tadalafil in increasing vaginal pulse
amplitude (VPA), clitoral blood volume (CBV) and subjective ratings of sexual
desire and arousal in the laboratory, in healthy female subjects with
hypoactive sexual desire disorder (HSDD).

To evaluate the safety of combined administration of 0.5 mg sublingual
testosterone and 10 mg tadalafil

This is a double blind, randomized, placebo controlled proof of concept (small
exploratory) study, with 2 psychophysiological laboratory measurement sessions
(placebo & active) separated by at least 4 days, and a 1 week at home period
where subjects have 2 coital events, separated by at least 4 days.
Subjects will visit the study site a total of 4 times: 1 screening visit, 2
visits for psychophysiological measurement and 1 follow up visit. During the
psychophysiological measurement visits, VPA, CBV and subjective desire and
arousal will be measured, and the subject*s health will also be monitored. At
the end of the last psychophysiological measurement visit, study medication for
the 1-week at home period will be dispensed. After a coital event at home,
sexual satisfaction will be measured using a sexual satisfaction questionnaire.
At follow up (whilst still blinded), an in depth psychological interview is
given in order to further investigate possible drug dependent differences
between sexual events.

administration of sublingual testosterone (0.5 mg) and an oral tablet
containing tadalafil (10 mg)

Use of T+Tad is expected to improve the level of sexual satisfaction for a
single sexual event.
Dosed separately, testosterone and tadalafil administration at the doses and
frequency proposed for this study are not known to result in any serious health
risks.
Testosterone and sildenafil(a shorter acting PDE5 inhibitor) have been dosed
together in several studies ( A total of 172 subjects have been dosed with
testosterone and sildenafil). No significant adverse events (AEs) have been
observed when testosterone and sildenafil was administered together. Because
the combination of T+Tad is roughly equivalent, we also expect this combination
to be safe.
The amount of testosterone (0.5 mg) used roughly corresponds to what women
produce endogenously per day.25 After sublingual administration of 0.5 mg T,
circulating testosterone increases to supraphysiological levels for only a very
short period of time and quickly returns to baseline levels within 3 hours. In
contrast to this, continuous androgen administration chronically raises
testosterone levels to supraphysiological levels, which can result in
masculinization effects such as hirsutism; it has also been linked to an
increased risk of developing specific types of cancer and adverse
cardiovascular effects. Sublingual T (0.5 mg) however, only contains a very
modest amount of testosterone, and in this study, the maximum amount of
testosterone administered will be twice 0.5 mg; this level of testosterone
exposure is significantly less than levels that have been associated with the
above mentioned side effects. Hence, the suggested dose and dosing regimen for
testosterone, as a component of T+Tad, is considered to be safe.
Tadalafil has been approved for use in men by the FDA and the EMA (amongst
others) for erectile dysfunction and pulmonary hypertension. In men it is well
tolerated, the most frequent AEs (>10%) being headache and dyspepsia. In the
here used dosis (10 mg) it is considered to be safe.
Data on the effect of tadalafil and testosterone on oral contraceptives are
lacking. For this reason, participants on oral contraceptives will be
instructed to use a second contraceptive method (double barrier). All
participants will be instructed not to become pregnant during the study.