DISCUSSION
Herein, we describe a single-centre, prospective, double-blind,
randomised, placebo-controlled, two-treatment, two-sequence, two-period
crossover study with 400 mg zamicastat/placebo, in healthy male
subjects. The cold stimulus caused a marked sympathetic response
evidenced by a significant increase in BP and HR during CPT, at baseline
(day -1) and after a 10-day treatment with zamicastat or placebo.
CPT was selected to explore the effect of oral zamicastat, at
steady-state conditions, on the overdrive of the sympathetic system and
consequent effects on the cardiovascular system. This test has been used
both clinically and experimentally to evaluate non-baroreflex-mediated
sympathetic neural control in humans, cardiovascular reactivity to
stress in normotensive and hypertensive subjects, and the efficacy of
lifestyle and pharmacological interventions on BP and vascular
reactivity.
Even though our results followed the tendencies presented in other
reports, showing increased BP and HR, the increment in HR was lower than
in other studies, in which the increase could be higher than 20 bpm,
after a 90-second immersion in cold water. However, in other studies CPT
showed to have lower or no effect on HR. On the other hand, the change
in BP was higher than in previous studies, reaching about 30 mm Hg.
These variations may be related to differences in the baseline BP values
of the participants as well as to other demographic and behavioural
patterns such as age, gender, weight, and exercise habits, which have
been shown to influence reactivity to cold-induced stress. Despite the
differences, our results on day 10 illustrate the absence of habituation
or learning from exposure to the cold stimulus. In fact, both BP and HR
kept the same increasing trend with similar magnitude, after 10 days.
This observation combined with the referred influence of demographic and
behavioural patterns on the reactivity to CPT can guide the definition
of future selection criteria that can guarantee the inclusion of
subjects with more robust baseline conditions to promote adaptation and
learning during cold exposure. This would enrich the cohort by reducing
selection bias. However, this endeavour requires a more detailed
analysis of the correlation of the baseline characteristics of our
participants with habituation to the cold stressor.
Women were not eligible for this study to avoid the potential influence
of fluctuation of sex hormones throughout the menstrual cycle with
neurohumoral variations with impact on the cardiovascular system.
Compared to placebo, zamicastat significantly decreased SBP and MAP
response to cold stimulus during CPT, following 10 days of treatment,
evidencing its effect on the overdrive sympathetic response to cold
stimulus. Zamicastat reduced SBP (-4.62 mmHg) and DBP (-1.86 mmHg) when
compared with placebo. The HR slightly increased (0.81 bpm) which may be
related to compensatory hemodynamic effects. The one-sided 95%
confidence interval (-7.77 mmHg) was below the established clinical
margin for SBP (-5.5 mmHg) supported by a statistically significant
difference (p=0.020) between zamicastat and placebo.
The measurement of plasma and urine catecholamines proved to be a valid
tool to evaluate the impact of zamicastat on the SNS. These data showed
that zamicastat 400 mg, given once daily, inhibited DβH plasmatic
activity, increased the 24-h urinary excretion of HVA, and significantly
decreased the 24-h urinary excretion of EPI, NE, and VMA, when compared
to placebo. The decrease of NE in urine 24h explains the reduction of
its metabolite VMA, which reflects a decrease in the total production of
catecholamines caused by zamicastat treatment. On the other hand, the
levels of DA in urine 24h were similar with zamicastat and placebo, but
its metabolite HVA increased, which may imply that a higher amount of DA
was metabolized. Our results confirm that the 24h urinary catecholamines
is a collector of plasmatic levels of catecholamines and, as such, is
more assertive as a potential biomarker of SNS modulation than the
sporadic assessment of catecholamines in plasma, which is more affected
by fluctuations and interferences than an integrated measurement over 24
h.
Overall, the profile of AEs showed that placebo and zamicastat
treatments were similarly well tolerated.
Our results suggest that mechanism of action mediated by zamicastat can
be a promising therapeutic option for morbidities characterized by
excessive activation of the SNS, such as PAH, ischemic disease, among
others.
In conclusion, our study confirmed the effect of zamicastat on the SNS
through the decrease of SBP and MAP response to cold stimulus during CPT
and inhibition of DβH plasmatic activity. In addition, the effect of
zamicastat in 24 h urine catecholamines and their metabolites proved to
be clinically relevant, thereby confirming its potential as SNS
modulator.