Exploration of the treating the host concept, using repurposed
agents acting on the renin angiotensin system – ATC
C09.
From the preceding discussion, a framework for treating the host and
repurposing can be employed to explore the eligibility of drugs that
influence the renin angiotensin system as candidates for clinical
trials. In particular to identify
what is known about these agents in regard to:
1) Pathogenesis 2) Target homology of COVID-19 3) Prevention/ Treatment
and 4) Dose-Response curve in relation to safety.
1. Pathogenesis. To treat the host requires an
understanding of the physiology of this response to that condition (in
this case COVID). In addition, knowledge is needed of chemical,
structural and immunological homology with the novel action of the
repurposed drug and the molecular events that drive the consequences of
viral pathogenesis in COVID-19 disease. As the main causes of death in
COVID are pulmonary vasoconstriction, pulmonary oedema, thrombosis, and
fibrosis, a focus on RAS inhibition, a key driver of (a) Pulmonary
Complications – pro-thrombotic and inflammation in the lung (b) Innate
Immune Response and (c) Vascular Response - vasoconstriction,
inflammation and coagulation is key.
1a. Pulmonary Complications – pro-thrombotic and inflammatory
lung disease. During early 2020, an enormous amount was written on the
pathogenesis of COVID-19, including comprehensive summary review
articles with strong emphasis on pulmonary pathophysiology in this
condition (11-13, inter alia ). The relationship between the
dysregulation of the pulmonary renin angiotensin system (RAS) and the
progression to severe acute respiratory syndrome (SARS) including the
intermediate stages of infections and pneumonia has been discussed at
length. By way of summary the fundamental initiation is the role of ACE2
as the receptor for SARS coronavirus infection (14). It is the impact of
this initiation that drives a pulmonary dysregulation of the finely
balanced RAS. This balance involves the generation of angiotensin II by
ACE and the inactivation of that peptide by ACE2. The functional role of
ACE2, its catalytic product angiotensin1-7 and the MAS receptor are the
counterbalance moieties for angiotensin II and its AT1R receptor (11).
The interaction of angiotensin 1-7 with MAS receptors promotes an
anti-inflammatory response, vasodilation an anti-fibrotic response and
is seen as protective. Down regulation of ACE2 in acute lung injury
models with SARSCoV infection supports a protective role for RAS
blockade in this form of pulmonary infection (11). The core hypothesis
is that a dysregulated RAS is core to the pathophysiology of COVID-19
driven lung damage and acute respiratory distress syndrome (15). This
builds upon much earlier experimentation showing the angiotensin II
receptor blocker (ARB) losartan could effectively reduce acute lung
injury induced by SARS-CoV (16).The summary statement is most apt:“Therefore, in acute lung injury, ACE, Ang II, and AT1R function
as lung-injury-promoting factors, while ACE2 protects from lung
injury” (17).
It follows that from a repurposing
standpoint that there exists a strong target homology with
agents that promote RAS inhibition and the pathogenesis of COVID-19
disease in the lung in terms of pathophysiology and pharmacology.
1b. Innate Immune Response. Angiotensin II, via
activation of the AT1R receptor, is pivotal in modulating immune
responses. As highlighted by Veerappan et al 2008 (18) mast cells are
found in the submucosal tissue in human bronchi in the upper and lower
respiratory tree. Mast cells rapidly recognise pathogens and mount an
immune response that includes plasma extravasation, immunological and
cytokine modulation, leukocyte recruitment and ultimately cellular
inflammation (19). What is critical is the role of mast cells in the
bronchi in the release of renin and the initiation of the production of
the angiotensin II that acts through AT1R receptors (18). Moreover, in
an experimental model of hypersensitivity, anaphylactic mast cell
degranulation has been shown to result in angiotensin
bronchoconstriction (18). Of significance is the observation that mast
cell chymase can generate angiotensin II independently
of ACE activity (20), the salient point being that ACE is membrane bound
and chymase upon release moves within interstitial spaces. ACE also
modulates macrophage and neutrophil function in its role in the innate
and adaptive response (21).
Angiotensin II mediates its pro-inflammatory responses by signalling
through AT1R receptor.
These responses include, leukocyte recruitment, increased production of
reactive oxygen species by receptor (AT1R) activation of NADPH oxidase,
contributing to inflammation and the stimulation of the innate immune
responses (summarised in 21). As mentioned above angiotensin 1-7 serves
as a counter to the pro-inflammatory influence of angiotensin II.
Angiotensin 1-7 achieves this by acting through the MAS receptors
expressed at the surface of the bronchial smooth muscle and alveolar
epithelium (12). With the viral infection in COVID-19 the cytokine storm
is a key characteristic. The release of large amounts of
pro-inflammatory cytokines and chemokines (IFNα, IFNγ, IL-1β, IL-6,
IL-12, IL-18, IL-33, TNFα, TGFβ, CXCL10, CXCL8, CXCL9, CCL2, CCL3,CCL5)
by immune effector cells drives the aberrant inflammatory response (22).
Many cells of the innate immune system have been shown to have an
important role in the development of human COVID lung. Pulmonary RAS has
a key role in bronchoconstriction consistent with the release of renin
and subsequent angiotensin II formation with mast cell degranulation
(18). Moreover, in an animal model of hypersensitivity mast cell
degranulation was associated with angiotensin II mediated constriction,
inhibited with an AT1R blocker or an inhibitor of renin
(18). From a repurposing
standpoint that there exists a strong target homology with
agents that promote RAS inhibition and the modulation of innate immune
responses in the lung.
1c. Vascular; vasoconstriction, inflammation and
coagulation.
There is growing evidence that there exists a confluence of vascular and
diffuse alveolar damage with SARS and possibly also COVID-19 disease. In
some of the data with COVID-19
pneumonia, McGonagle et al (23)
have drawn attention to the similarities of blood vessel wall oedema,
modest vessel wall immune cell infiltration, hyaline thrombosis and
haemorrhagic change described earlier for patients with SARS, consistent
with the observation that disseminated intravascular coagulation may
develop late in COVID-19 disease. Consistent with this view Teuwen et al
2020 (24) suggested that pulmonary endothelial cells (ECs) have been
largely overlooked as a target in COVID-19, as the attending ECs altered
vessel barrier function promotes a pro-coagulative vascular inflammatory
state and an inflammatory cell infiltration. They also highlight the
fact that SARS-CoV-2 binds to the ACE2 receptor impairing the enzyme
that is the counterbalance for the vasoactive angiotensin II. The lung
pathology COVID-19 disease displays microvascular thrombosis and
haemorrhage with extensive alveolar and interstitial inflammation termed
lung-restricted vascular immunopathology (23). In the early stage is
distinct from diffuse pulmonary intravascular coagulopathy. Cao and Li
(25) also similarly highlighted the two hallmarks of critical patients
with COVID-19 disease namely progressive inflammation and the trend to
hypercoagulation. Furthermore, they draw attention not only to the
inflammatory state as a trigger for coagulation, but endothelial cell
disseminated intravascular coagulation, characterising the COVID-19
hypercoagulable state as displaying elevated D-dimer (a fibrin
degradation product) and fibrinogen concentrations and prolonged
prothrombin time (25).
Previous studies have shown that in pulmonary hypertension there is a
reduced ACE2 activity and that augmentation of ACE2 reduced oxidant and
inflammatory markers as well as improving pulmonary hemodynamics (26).
Thirty years ago, our team established that in hypertensive blood
vessels the endothelial cell mediated relaxation was impaired and this
could be reversed with impairment of prostanoid system or treatment with
the ACEI captopril (27). Collectively demonstrating a key role of the
vascular endothelium in coagulation, inflammation and vasoconstriction
and the capacity for abnormal function driven by the RAS. The RAS
influence on the vasculature also extends to trophic factors. The
neurotrophic peptide NGF is believed to be a pro inflammatory mediator
in airways and promotes bronchial hyperresponsiveness, Freund-Michel et
al. (28). Of significance was our demonstration that the AT1R blocking
agent losartan reversed the abnormal concentrations of NGF in blood
vessels in hypertension (29).
As we indicated earlier there is growing evidence that there exists a
confluence of vascular and diffuse alveolar damage with SARS and
possibly COVID-19. Gonzalez‑Jaramillo et al 2020 in a commentary on the
double burden of COVID-19, also implicates the RAS in chronic pulmonary
disease such as pulmonary fibrosis and pulmonary arterial hypertension
(30). Similarly, PAH is characterized by reduced ACE2 activity and the
production of Angiotensin-(1–7) by ACE2 activating Mas receptors
activation improves experimental models of PAH (26).
As pointed out by Cao and Li 2020 (25), SARS-CoV-2 can initially
replicate in the upper respiratory track and later in the disease course
replicate in the lower respiratory tract generating a secondary viremia
with a subsequent attack on organ targets expressing ACE2. A dynamic
consistent with the possibility outlined by Kreutz et al 2020 (11) and
other groups that ACEIs and ARBs may be associated with lower incidence
and/or improved outcome in patients with lower respiratory tract
infections. From a repurposing standpoint that there exists a strong target homology with agents that promote RAS inhibition that involves blood vessel and vascular endothelial cell damage.
2. Target homology of COVID-19 and therapeutic
agents .
SARS-CoV-2 is an RNA virus and they have high mutation rates up to a
million times higher than their hosts, and it is this high rate that
enables the viruses to avoid host immunity and to achieve drug
resistance (Pachetti et al 2020- (31). From the standpoint of drug
repurposing the selected drug should target, not the virus, but that
aspect of the pathophysiology of COVID-19 that is tightly linked to the
conserved mutational characteristics of the virus and its class.
2a. Conserved target for SARS-CoV-2. It is now generally
accepted that ACE2 is the receptor for SARS-CoV and SARS-CoV-2 to enable
entry to the host (32). Moreover, South et al 2020 (33) suggested that
angiotensin may promote both lung injury and ARDS in SARS-CoV,
SARS-CoV-2/COVID-19 and possibly in MERS.
Using crystal structures for NL63 coronavirus (NL63-CoV) and SARS
coronavirus (SARS-CoV) receptor binding domains Wu et al 2011
demonstrated virus binding hot spots on their common receptor, the human
angiotensin-converting enzyme 2 (ACE2). From that experimentation Wu et
al 2011 (34) suggested that the hot spot features were amongst the
driving force for the convergent evolution of these two viruses. It
follows that this would infer multiple binding opportunities for
differing or mutated viruses but restricted to a common protein target.
The targeted physiological process for driving pulmonary inflammation
will be hard for a virus to abandon without significant penalty and can
be viewed as conserved. From a repurposing standpoint that there exists
a strong target homology with agents that promote RAS
inhibition and the common conserved site of the pathogenesis for
COVID-19 and related viruses.
3. Prevention to
Treatment. An understanding of how the repurposed drug provides benefit
in treating the host across the cascade from prevention to the
life-threatening consequences of other viral pathogenesis in COVID-19
disease. Cao and Li 2020 importantly summarise the clinical course of
SARS-CoV-2 infection by dividing it into three phases: viremia phase,
acute phase (e.g. with pneumonia) and severe or recovery phase (25).
From the standpoint of drug repurposing in COVID we have adopted a
similar approach, with particular focus on the severe phase, where the
host system has switched on innate immune system, affecting
inflammation. oedema, and vasconstriction. Our treatment focus in this
phase is on RAS inhibition and (a)
predisposition and age (b)
predisposition with hypertension and co-morbidities (c) treatment and
secondary infection and (d) intubation.
3a. Predisposition and age . Increasing age has been
shown to be a strong correlative factor with morbidity and mortality
with COVID. As summarized by Shahid et al. studies have highlighted the
relatively higher mortality rates in older adults (35). While a variety
of reasons may underpin this predisposition, it would appear that the
RAS has a strong input to aging by way of mitochondrial function,
specifically a strong relationship between free radicals, the
mitochondria and aging, with excessive free radical production linked to
dysregulation of RAS (36). Key in this dysregulation is the angiotensin
receptor AT1R and its linkage to NADPH and increased production of
reactive oxygen species (ROS), and a role of RAS in decreasing ROS
scavenging enzymes. As highlighted by Wilson et al 2016 (37) in
mitochondria the receptors for angiotensin (AT1R and AT2R) are linked to
respiration and nitric oxide production. Furthermore the presence of
angiotensin 1-7 in purified mitochondria and the conversion of
angiotensin I to angiotensin 1-7 by endopeptidases suggests a
intramitochondrial pathway for formation of angiotensin 1-7 (37).
Recently Wang et al (38) have shown that overexpression of ACE2 through
mitochondrial function enhances the protective effects of endothelial
progenitor cells (EPC) on endothelial cell injury. It remains to be
determined whether the dysregulation of RAS described in COVID-19 also
similarly has a deleterious effect on mitochondrial function, as there
appears to be an emerging possibility that a complex intersection
between aging, mitochondrial function, apoptosis, the RAS and
coronaviruses exist. Lai et al conducted a proteome analysis on human
promonocyte HL-CZ cells expressing SARS CoV3CL., finding that 36% of
the up regulated proteins were located in the mitochondria, including
the apoptosis-inducing factor ATP synthase beta chain and cytochrome c
oxidase. They suggested that SARS CoV3CLpro could induce
mitochondrial-mediated apoptosis (39). Additionally, it is possible that
mitochondrial dysregulated RAS and free radical production in aging acts
in concert with viral RAS dysregulation in COVID-19 disease.
From the standpoint of repurposing
what is important is the observation that inhibition of RAS with
enalapril and losartan preserve mitochondrial function from the effects
of aging (40).