Total number of words: 5706
Abstract
Early studies suggesting that chloroquine (CQ) and hydroxychloroquine
(HCQ) could benefit coronavirus patients brought these old medicines
back to the spotlight. This led to an increase in demand and price,
turning their counterfeiting a pharmacovigilance issue worldwide.
Meanwhile, lack of evidence on effectiveness and safety concerns have
reduced their clinical trials in severe COVID-19 cases. Despite the
knowledge that CQ and HCQ toxic effects are stereo specific rather than
their therapeutic effects, these drugs are available only as racemates.
In this context, this work brings a discussion about chiral switching to
their eutomers so that CQ and HCQ distomers would become impurities,
what may be a viable alternative to test new dose-response curves. Even
if it is proven that the use of pure CQ and HCQ enantiomers are useless
against COVID-19, chiral switching would certainly improve safety and
efficacy in the treatment of many autoimmune inflammatory diseases,
benefiting chronic users of these drugs.
Keywords: chiral switch, enantiomers, chloroquine,
hydroxychloroquine, counterfeit, COVID-19, autoimmune inflammatory
diseases.
1. Introduction: Why is this fuss all about?
In late 2019, a group of patients with a severe acute respiratory
syndrome (SARS) of unknown cause appeared in Wuhan, Hubei province,
China . Since then, outbreaks of this syndrome had quickly spread across
Chinese territory, resulting in thousands of confirmed cases. The
Chinese Center for Disease Control and Prevention organized an intensive
outbreak investigation program and attributed the etiology of this
disease to a new virus belonging to the coronavirus (CoV) family.
Initially, the new virus was called 2019-nCoV, and on February
11th of 2020 the World Health Organization (WHO)
announced that the disease caused by the new CoV should be called
”COVID-19”, which stands for ”coronavirus disease 2019”. Subsequently,
experts from the International Committee on Taxonomy of Viruses (ICTV)
renamed the newly discovered CoV to SARS-CoV-2 due to its similarity to
the coronavirus responsible for the first epidemic of SARS (SARS-CoV) in
2002 . Because of its alarming speed of transmission COVID-19 advanced
worldwide and reached all continents, causing the WHO to declare a
pandemic state on March 11th of the same year .
By the beginning of September, there were more than 26 million confirmed
cases, and more than 870 thousand deaths throughout 216 affected
countries . Although SARS-CoV-2 is not the most lethal among
coronaviruses, its speed of propagation makes it one of the deadliest
viruses known. There is still no vaccine against it and no medication
has proven effective in containing the pandemic, although several
clinical trials are in progress. Meanwhile, treatment for COVID-19
remains symptomatic and supportive. In a race against time, scientists
are trying to repurpose the use of old medicines. It has been reported
that most of COVID-19 patients received antiviral therapy in China.
Current treatment options are mainly based on previous experience that
have shown clinical benefits in treating influenza, Ebola, the Middle
East Respiratory Syndrome (MERS), SARS, and other viral infections.
Ribavirin, lopinavir, ritonavir, remdesevir, nelfinavir, arbidol have
been tested but none of which proved to be effective against COVID-19.In vitro data have shown that some anthelmintic, such as
nitazoxanide, ivermectin and niclosamide, inhibit replication of a
variety of viruses, leading them to be tested against COVID-19. However,in vivo results are poor and inconclusive .
Huge inflammatory responses known as cytokine storms are one of the
features of severe cases of COVID-19 and are often associated with
increased mortality, so the use of systemic anti-inflammatory drugs is a
potential therapeutic strategy. However, timely and appropriate use of
corticosteroids, such as methylprednisolone, are recommended only to
patients that are presenting ongoing deterioration in the oxygenation
index, or rapid progression of radiological findings, or excessive
activation of immune responses. Together with immunosuppressive agents,
broad-spectrum antibiotics, such as carrimycin and azithromycin, have
also been prescribed as adjunct therapies to prevent bacterial secondary
infection .
Some antimalarial drugs have shown to have
immunomodulatory/anti-inflammatory properties and antiviral activity as
well. Since the late 1960s it is known that chloroquine (CQ) can inhibit
the growth and spread of different viruses, (including the SARS-CoV)in vitro , but subsequent in vivo experiments were
controversial . Early studies in China and France claiming the use of CQ
and its derivative hydroxychloroquine (HCQ) could be beneficial to
coronavirus patients brought these drugs back to the spotlight. The
spread of these news and the adoption of CQ and HCQ in public policies
to combat COVID-19 by some of the largest world economies, such as the
United States and Brazil, have turned these drugs by far the most
popular proposed for treatment and prophylaxis, appearing in several
clinical studies registered on the United States National Institutes of
Health (NIH) website .
These apparently good news led to an increase in demand and
self-medication with CQ and HCQ, raising concerns about possible
retinopathy and serious cardiac impairment by these drugs. These
concerns were confirmed in some of the clinical trials performed. In
addition to an increased risk of cardiac complications, CQ and HCQ do
not resulted in the reduction of mortality of hospitalized COVID-19
patients, when compared with standard of care, what has prompted WHO to
discontinue clinical trials using these drugs in critically ill
patients. However, clinical trials to evaluate the efficacy of CQ and
HCQ in pre or post-exposure prophylaxis are still ongoing. Although
there is already evidence that HCQ has no effect in altering the course
of the disease in individuals without comorbidities, it is possible that
this drug is more beneficial for patients who are at higher risk groups.
More clinical trials need to be conducted in this regard .
Furthermore, due to its widespread use as an antimalarial, CQ has been
one of the most counterfeit medicines for many years. CQ and HCQ are
also widely used in the treatment of autoimmune inflammatory diseases
like lupus erythematous and rheumatoid arthritis due to their off-label
immunomodulatory activity. The increase in demand due to its
popularization in the treatment of COVID-19 and the lockdown imposed on
the two largest producers of active pharmaceutical ingredients, China
and India, caused a shortage in the market and the prices to skyrocket.
This, in addition to putting at risk the continuity of treatment of the
patients who make chronic use of these drugs, ended up triggering an
increase in CQ counterfeiting around the world, turning it into a major
pharmacovigilance issue .
However, CQ and HCQ are currently marketed as racemates, a mixture of
equal quantities of two enantiomers. It is known that the enantiomers in
a drug available as racemic mixture may have distinct pharmacological
and/or toxicological properties. Chiral switch is the substitution in
the marketplace of a racemic drug for its single-enantiomer version .
Chiral switching CQ and HCQ to their eutomers, the enantiomers that
cause the desired therapeutic responses, may be an alternative to reduce
their toxic adverse effects, allowing a safer administration of higher
doses and a better evaluation of the effectiveness against COVID-19 over
a wider dose-response range. In addition, the marketing of the pure
eutomer could improve the quality of life for those who continuously
take these drugs, and also turn the enantiomeric purity assessment an
useful indicator for counterfeit and substandard medicines
identification . Therefore, these molecules still have properties that
should be properly exploited to really exhaust all possibilities against
diseases.
2. SARS-CoVs mechanism of infection
With a diameter of approximately 60 to 140 nm, round or elliptical
shaped and often pleomorphic, coronaviruses (CoVs) are RNA viruses which
strand of genetic material is the longest known. According to their
genetic characteristics, coronaviruses are classified into four genera:
α-CoV, β-Cov, γ-CoV and δ-Cov. Genetic sequencing of the coronavirus
isolated from patients who contracted COVID-19 revealed that SARS-CoV-2
belongs to the β-CoV genus. . CoVs are given this name because of their
appearance as a crown (coronam, crown in Latin) when viewed under the
electron microscope due to the presence of spicules of glycoproteins
around their envelope that serve as a docking point at the host cell.
The SARS-CoVs glycoprotein spikes are comprised of two subunits, S1 and
S2. During the infection of the human cell, the S1 subunit binds to an
angiotensin-converting enzyme type 2 (ACE-2) like receptor located in
the membrane of the host cell. The virus can then enter the cell either
through endocytosis or can fuse its membrane to the cell’s membrane and
release its genome and protein content directly into the cytoplasm
(Figure 1).
Figure 1
Either way the ACE-2 bound spike must be cleaved by specific proteases
to initiate the virus replication process. At the cell surface the spike
is cleaved mainly by the transmembrane protease serine-2 (TMPRSS2),
ensuring virus and host-cell membrane fusion. In the endocytosis pathway
the spike is cleaved by cathepsin L within the endosome, provoking the
fusion between the viral membrane and the endocytic membrane. These two
mechanisms ultimately result in cell infection .
3. COVID-19: who are the players?
3.1 The role of the angiotensin converting enzyme
The angiotensin converting enzyme (ACE) is a transmembrane enzyme widely
distributed in the body’s tissues and is an essential component of the
renin-angiotensin-aldosterone system (RAAS), mediating a number of
systemic and local effects in blood pressure control, tissue perfusion
and extracellular volume (Figure 2).
Figure 2
ACE is a dicarboxy-peptidase enzyme that acts removing the
carboxy-terminal dipeptide from the decapeptide angiotensin I (Ang I)
resulting in the octapeptide angiotensin II (Ang II; Ang I-8). By
binding to the angiotensin II (Ang II) type 1 receptors
(AT1) in the cardiovascular system, Ang II plays an
important role in blood pressure homeostasis, acting on multiple organs
through intense vasoconstriction, aldosterone release from the adrenal
glands, pituitary secretion of vasopressin and reabsorption of sodium
and water at the kidneys .
The ACE homologous enzyme, ACE-2,
plays a fundamental physiological role in maintaining local homeostasis.
Despite a more than 42% homology in the amino acid sequence of their
catalytic sites, ACE and ACE-2 are biochemically and pharmacologically
distinct. ACE-2 is a monocarboxypeptidase and, unlike ACE, it is only
capable of hydrolyzing one amino acid at a time from the
carboxy-terminal portion of its substrate. Thus ACE-2 converts the
decapeptide Ang I into the nonapeptide Ang I-9 and converts the
octapeptide Ang II to the heptapeptide Ang I-7. Although the function of
Ang I-9 is not fully understood, it is suggested that it acts as an
endogenous competitive inhibitor of ACE, since Ang I-9 is also a
substrate for ACE. Ang I-7, in turn, produces a cardiovascular and renal
vasodilator effect when it binds to the Mas oncogene receptor,
which negatively regulates the effects of Ang II mediated by the
AT1 receptor (Figure 2). This indicates the important
protective physiological role played by ACE-2 in counter to the effects
of Ang II. .
ACE-2, is present in various organs and tissues, such as the bronchi,
bronchioles and lungs, cardiovascular system (including the coronary
heart vessels), renal tubular epithelium, esophagus, intestines, testes,
endocrine pancreas, bile ducts, retina and brain. However, its
distribution is not uniform meaning that some organs might be more prone
to infection than others. This can explain the spread of the virus
beyond the respiratory tract and the damage to certain organs found in
the severe form of the disease .
3.2 The role of angiotensin II in the inflammatory process
Ang II is also involved in the development of pro-inflammatory stimuli
in different organs and tissues. Overexposure of cells to Ang II
increases the expression of AT1 and AT2receptors. The activation of AT1 receptors by Ang II, in
addition to promoting the production of reactive oxygen species (ROS)
that lead the cell to oxidative stress, increases the production of the
lectin-type oxidized low-density lipoprotein receptor 1 (LOX-1). LOX-1
increases the expression of messenger RNA for the formation of more
AT1 receptors and the intracellular production of
pro-inflammatory signaling molecules such as, interleukin-6 (IL-6),
interleukin-10 (IL-10), tumor necrosis factor α (TNF-α) and
intercellular adhesion molecules, favoring the action of monocytes of
the immune system. The activation of AT2 receptors, in
turn, increases the intracellular production of pro-apoptotic molecules.
Thus, the expression/activation of AT1 and
AT2 receptors in response to Ang II and the inflammatory
environment is a feedback process that amplifies the inflammatory
response and cell death in a chained reaction . Cellular infection by
SARS-CoVs decreases the availability and expression of ACE-2,
unbalancing the local renin-angiotensin system (RAS) and favoring
over-stimulation by Ang II, which further intensifies the inflammatory
response .
3.3 Other immunologic features
In addition, the rapid viral replication in the host cell causes massive
pyroptosis at the pulmonary epithelium and endothelium, resulting in
acute injuries and an intense inflammatory process in the lung tissue,
leading to an abrupt decrease in respiratory capacity, typical in severe
SARS-CoVs infections . Unlike apoptosis (where cell death occurs in a
programmed and organized manner, shrinking gradually until it is finally
phagocyted), pyroptosis is a process of rapid and highly inflammatory
cell death (Figure 3). The cascade of biochemical reactions triggered by
pyroptosis ends up in the formation of an inflammasome and rupture of
the cell membrane in a desperate attempt by the host cell to prevent the
pathogen from using the cellular machinery for its own replication . The
rupture of infected cells produces viremia (the release of new viral
particles in the bloodstream) and spreads the infection to other organs
and tissues where ACE-2 receptors are available. The pyroptosis process
also provokes the extravasation of immunomodulatory molecules in the
bloodstream, triggering inflammatory responses and mobilizing immune
system cells such as, macrophages and T helper lymphocytes and the
consequent release of interferon γ (IFN-γ), which leads to the
recruitment of more inflammatory cells, initiating an inflammatory chain
reaction known as “cytokine storm”. This exaggerated and
out-of-control response of the innate immune system causes tissue damage
and is considered to be the main cause of acute respiratory syndrome and
multiple organ failure seen in severe cases of COVID-19 . The risk of
developing a cytokine storm is particularly high in elderly patients,
who are more likely to have pre-existing chronic inflammation and an
enabling inflammatory environment. The suppression of the cytokine storm
with immunomodulatory drugs is an important way to prevent further
aggravation of COVID-19 patients. However, this approach must be started
at the right time, because if administered too early the pharmacological
suppression of the immune system will make it difficult to eliminate the
virus in the body .
Infection of immune cells also contributes to the spread of the virus
from the lungs to other organs via the lymphatic circulation, competing
with viremia for the rapid spread of the disease in the body. A probable
mechanism for the spread of the virus via the immune system, which may
be related to the reduction of defense cells and evolution to the severe
form of the disease, is a phenomenon known as antibody-dependent
enhancement (ADE). Antibodies produced after infection by a first viral
agent can cross-react with a second viral agent (which does not need to
belong to the same species) and produce clusters of antigens with
antibodies or with components of the complement system that will be
internalized by phagocytic cells through its FcRg type receptors. Once
internalized through this mechanism, the virus is not destroyed. On the
contrary, the viral particle replicates and leaves the host cell to
infect others, and by leaving the cell it still induces it to secrete
pro-inflammatory cytokines. This phenomenon has been described in other
infections by SARS, MERS and Dengue virus and may explain the
geographical variability of severe cases of COVID-19. Particularly
worrying in the case of COVID-19 is that this infection mechanism is
independent of ACE-2 receptors, and that there are other types of
non-immune cells that also have FcRg receptors. Interestingly, the lower
the antibody titer remaining from the primary infection, the greater the
potentiation of the secondary infection. Other types of coronaviruses
can infect humans, causing mild flu or even no noticeable symptoms.
Although these infections are not sufficient to cause seroconversion and
produce a lasting immune response, during the post-convalescence period,
when the antibody titer produced is low, is when there is the greatest
risk for potentiation of the secondary infection dependent on antibodies
produced in the primary one. This feature of the immune system in
response to some viral infections imposes an additional challenge to
produce a vaccine against SARS-CoV-2, where artificial immunization
could actually increase the risk of developing the severe form of the
disease .
It is already known that the serum conversion caused by SARS-CoV-2 is
not long-lasting and that IgG titers decline considerably after a few
weeks after the period of convalescence. The ADE phenomenon has already
been seen in other coronavirus infections. The occurrence and
contribution of ADE to the evolution of severe cases of COVID-19 cannot
be ruled out . Therefore, immunomodulatory drugs could be useful in the
treatment of artificially induced ADE and in preventing its natural
occurrence as well.
4. CQ and HCQ back to the spotlight
4.1 Immunomodulatory Effects
CQ and HCQ were originally developed as anti-malarial drugs, but their
accumulation in lymphocytes and macrophages results in anti-inflammatory
properties that has led them to an off label use as part of the standard
strategy for the treatment of patients suffering from autoimmune
diseases such as, rheumatoid arthritis and systemic lupus erythematosus.
Although the mechanisms behind the immunomodulatory properties of CQ and
HCQ are not fully understood it is believed that they exert direct
molecular effects on lysosomal activity, autophagy and immunologic
signaling pathways .
CQ and HCQ are both weak bases, permitting their accumulation in acidic
intracellular compartments, such as the lysosomes. Lysosomes are
digestive structures and together with other vesicles like
autophagosomes, recycles intracellular material and debris (autophagy)
and helps processing phagocyted external antigens to be presented via
MHC class II receptors to T CD4+ lymphocytes.
Autophagy and autoantigen presentation by antigen-presenting cells to
CD4+ T cells is a probable mechanism behind the onset
of some autoimmune diseases. CQ can destabilize lysosomal membranes and
thus provoke the leakage of lysosomal enzymes into the cytoplasm.
Furthermore, the lysosomal pH is optimal for the hydrolytic enzymes’
activity. By increasing the pH inside the lysosomes, CQ and HCQ might
impair the maturation of the vesicle and inhibit antigen presentation
and subsequent immune activation (Figure 1) . It is also proposed that
CQ and HCQ inhibits autophagy mainly by provoking a severe
disorganization of the Golgi and endo-lysosomal systems and therefore
impairing autophagosome fusion with lysosomes .
CQ and HCQ also interfere with the toll-like receptor (TLR) signaling
pathway. TLR are membrane spanning receptors that recognize structurally
conserved molecules derived from pathogens (such as viral nucleic acids)
that are usually expressed on sentinel cells like macrophages and
dendritic cells. TLR can also be found in the endoplasmic reticulum of
these cells. Upon ligand binding or phagocytosis, TLR can either be
internalized or transported from the endoplasmic reticulum to
endolysosomes where they are cleaved by lysosomal enzymes and thus
becoming active. Once activated TLR triggers a signaling cascade that
leads to the expression of pro-inflammatory cytokines such as, IL-1,
IL-6, IFN-γ and TNF-α. CQ and HCQ, change the endosomal pH, interfering
in enzymatic TLR activation. Both drugs can also directly bind to the
pathogen nucleic acid inside the endosome, and hence steric blocking the
linkage of TLR to its ligand molecule .
4.2 Antiviral Effects
As weak bases, CQ and HCQ are only capable of crossing membranes
whenever in the non-protonated form. When entering acidic organelles
such as the endosome, Golgi vesicles, and lysosomes these molecules
become protonated and thus imprisoned and accumulate. Some viruses, such
as coronaviruses, enter their host cells by endocytosis, targeting the
virus to lysosomal vesicles where the low pH and hydrolases will disrupt
the viral particle, hence liberating its nucleic acid. Accumulated CQ
and HCQ in the lysosomes raise the local pH impairing this pH-dependent
step for viral entry. CQ can also interfere with the post-translational
modification of viral proteins which usually involve proteases and
glycosyltransferases within the endoplasmic reticulum or the trans-Golgi
network vesicles, where a low pH is required . CQ also seems to
interfere with the terminal glycosylation of cellular ACE-2 receptors,
the receptor that SARS-CoVs target for cell entry, a probable mechanism
for preventing the virus infection and spread .
5. What is being overlooked?
5.1 CQ and HCQ Stereopharmacology
The parent molecule for CQ (C18H26ClN3) is quinine, and HCQ
(C18H26ClN3O) is CQ’s hydroxyl derivative. Both are alkylated
4-aminoquinolines and amphiphilic weak bases based on two fused aromatic
rings having conjugated double bonds, the 4-aminoquinoline nucleus. Both
drugs are water soluble and cross cell membranes well but because HCQ is
more polar and hence less lipophilic, it has more difficulty diffusing
across cell membranes. CQ and HCQ have two basic groups corresponding to
the quinoline-ring nitrogen and the diethylamino side-chain nitrogen
that are thought to contribute to the accumulation of these drugs in
intracellular compartments, especially lysosomal compartments, which
seems to be paramount for their activity and potential interaction with
nucleic acids. At a physiologic pH of 7.4, CQ is monoprotonated but
still soluble in lipid and able to traverse cell membranes. However,
biprotonated CQ will occur in acidic organelles, such as the lysosome
(pH of 4–5), and be sequestered and prevented from traversing back out
to the cytoplasm .
Both CQ and HCQ have a chiral carbon linked to the secondary amine,
therefore they occur as enantiomers (R and S isomers) (Figure 4).
Figure 4
As chiral drugs, both enantiomers exhibit distinct pharmacokinetics,
efficacy and toxicity profiles that can be translated into performance
variances that could suggest which enantiomer can be a viable, less
toxic, therapeutic agent. Eutomer is the name given to the
pharmacologically active enantiomer, that is, it is the one that fits
perfectly in the pharmacological receptor, causing the desired
therapeutic responses. Distomer is the name given to the enantiomer that
does not fit or does not fit perfectly with the pharmacological
receptor. In general, the dystomer is responsible for toxic and
undesirable side effects . Although the interconversion between
enantiomers might occur in vivo for some drugs, CQ and HCQ are
stereochemically stable and the interconversion between the two
enantiomers has not been observed .
Regarding pharmacokinetics, CQ and HCQ are almost completely absorbed
orally within 2 to 4 hours after administration, with a plasma peak in
about 3 to 12 hours. Due to their ability to be sequestered inside
tissues, CQ and HCQ have a high volume of distribution and long
half-lives estimated at 40 - 60 days. About 33 to 70% of the
bioavailable dose of CQ binds to plasma proteins, being S-CQ more highly
bound to plasma proteins than R-CQ. CQ and HCQ are metabolized by the
liver undergoing dealkylation. Its main metabolites are the desethyl and
bisdesethyl forms, all pharmacologically active. The main route of
elimination is renal, with 40 to 60% being excreted unchanged or
metabolized. The S enantiomer for both CQ and HCQ is excreted by the
kidneys preferentially compared to the R enantiomer , meaning that R-CQ
is more bioavailable than its S counterpart.
Regarding efficacy and toxic features, CQ and HCQ binds strongly to
melanin-containing tissues like the skin and the eyes, which might
explain certain tissue-specific mechanisms, such as retinopathy, or the
efficacy in the treatment of skin manifestations. Since sequestration of
the R-CQ enantiomer in ocular tissues is greater than that of the S-CQ
enantiomer, racemates are more prone to retinopathy. Furthermore, CQ has
proarrhythmic activity. It prolongs the QT interval of the
electrocardiogram and causes the potentially lethal long QT syndrome.
This is due to its capacity to inhibit the cardiac inward rectifier
potassium current, which can trigger lethal ventricular arrhythmias.
Studies revealed that CQ selectively blocks cardiac potassium ion
channels in the cytoplasmic portion through a three-point interaction,
indicating that the three-dimensional fit may be stereo specific.
Therefore, it is possible that one of CQ enantiomers might display
weaker interactions with respect to its optically active counterpart,
thus resulting in a lesser detrimental effect on cardiac function .
It has been demonstrated that S-CQ has a higher antimalarial activity
than R-CQ . Results from in vitro antiviral activity against SARS-CoV-2
in Vero E6 cells showed that both CQ and HCQ enantiomers exhibited
antiviral effect in a concentration-dependent manner. The antiviral
activity of S-CQ was relatively similar to the racemic CQ, whereas the
efficacy of the S-HCQ was much better than its racemates . There is
still no data on the role of stereochemistry in the anti-inflammatory
effects of CQ and HCQ. However, the available data suggest that S-HCQ
act as the eutomer and therefore it may be a more effective and less
toxic alternative for the treatment of malaria, autoimmune inflammatory
diseases and even COVID-19.
5.2 Switch or not switch: what are the advantages?
About half of the drugs currently available on the market are chiral
drugs, that is, they have at least one chiral center and, therefore, are
presented in at least two stereoisomeric forms as a pair of enantiomers.
Among chiral drugs, approximately 90% are racemates .
The relevance of chirality in the pharmaceutical industry became evident
from the famous case of thalidomide in the 1960s. Thalidomide was sold
as a racemate and used to be prescribed to ameliorate the nausea
commonly felt during the first trimester of pregnancy. Unfortunately, it
was discovered later that the S-enantiomer of thalidomide is
teratogenic, causing the malformation of thousands of born babies. From
this discovery, the Food and Drug Administration (FDA), started to
demand a more rigorous evaluation of chiral drugs . CQ was familiar to
the Germans as early as 1934 but was industrially developed because of
intense antimalarial drug development efforts in the USA during World
War II. The relative safety, efficacy, and low cost brought CQ to the
front lines to treat malaria, and it was extensively used for almost two
decades after its first introduction in 1944–45 . According to today’s
regulations, CQ probably would not had been approved as a racemate. This
ended up triggering the study and development of new models for safer,
more effective and with a better therapeutic profile chiral drug.
Therefore, one strategy adopted by the pharmaceutical industry for safer
drugs development is chiral switching, which is the replacement of a
racemate approved on the market by one of its pure enantiomers.
Chiral switching is not new to the pharmaceutical industry. Since the
mid-1990s, this practice has become common as an alternative to the
extension of patent protection against the production of generic
racemate drugs. Successful examples are the drugs Lipitor
(atorvastatin), Plavix (clopidogrel bisulfate) and Nexium
(esomeprazole), which were the globally best-selling drugs in 2008 and
together generated 30 billion dollars for their manufacturing
laboratories. The active pharmaceutical ingredients of these medicines
are the pure enantiomers that have been registered and protected under
patents that derived from the patents of their corresponding racemates .
From a clinical point of view, the main advantage of chiral switching is
the removal of the distomer in the formulation. As a result, there are
improvements in the therapeutic index, lower adverse effects and the
possibility of higher dose levels and/or longer periods of
administration, besides facilitating safety and efficacy studies .
The chiral switch of HCQ was first accomplished in the early 1990s
claiming the S enantiomer for treatments of malaria, rheumatoid
arthritis, and lupus erythematosus. Later studies on HCQ and CQ
enantiomers have been conducted but none have led to a single-enantiomer
drug approval for any indication. Nevertheless, several syntheses of HCQ
and CQ enantiomers have been reported, including a simple method for
large-scale production of the CQ enantiomers. In the current pandemic
context, emergency drug approvals should be considered. The toxicity
profiles of CQ and HCQ have been well known for many years. This could
speed up the approval process and a successful chiral switch of HCQ or
CQ could become highly advantageous .
Furthermore, considering that in the formulation of a drug the substance
of therapeutic interest is the eutomer, it can be concluded that in a
racemate there are 50% of impurities that, in addition to not bringing
any benefit to the patient, may still offer risk. Therefore, high
enantiomeric purity values are mandatory for single enantiomers drugs.
In this case, the distomers are impurities and must be controlled and
monitored in the finished products. To do so, fast and convenient
methods using high performance liquid chromatography and chiral
stationary phases for assessing enantiomeric purity values of CQ
enantiomers are now available .
Finally, the commercialization of the pure CQ and HCQ enantiomers can be
a way to prevent counterfeiting of the drug, and the evaluation of the
enantiomeric purity can be a quick way to identify and rule out false or
substandard drugs allegedly marketed as pure enantiomers.
6. Discussion and conclusions
Although new vaccines are being developed and put to the test in record
time, the immune dynamics of SARS-CoV-2 imposes major challenges to be
overcome. The rapid decline of IgG in exposed patients and the risk of
antibody-dependent enhancement (ADE) are challenges that can compromise
the safety of new vaccines under development. Without a lasting immune
response, the risk of reinfection exists, and it is possible that
COVID-19 will reappear in recurring cycles. Safe and efficient drugs
that can be used for prophylaxis and prevention of the severe form of
this disease are highly desirable. Although recent studies have also
demonstrated the ineffectiveness of QC and HCQ in the treatment of mild
hospitalized cases of COVID-19 , studies to evaluate the efficacy of
these drugs in outpatients and in the prophylaxis of individuals
frequently exposed, such as health professionals, continue. CQ and HCQ
are drugs with great prophylactic and treatment potential, but their
toxic side effects must be considered to prevent them from being
indiscriminately used for this purpose, as a single dose of 30 mg/kg can
be fatal . CQ and HCQ have already shown to be effective in vitroin reducing the infection of other viruses, such as the dengue, zika,
influenza and SARS viruses. Apparently, the antiviral action of CQ and
HCQ occurs by inhibiting the endosomal viral replication pathway that is
common to many enveloped viruses. This mechanism of action seems to be
related only to the physicochemical properties of the molecules.
Although the work of Li et al (2020) has reported that the S-HCQ
is more effective against SARS-CoV-2 than the racemate or its R
counterpart, there is no further explanation about the mechanism behind
this stereoselectivity. Nevertheless, while scientists are in a hurry
trying to find a safe and effective dose for the prophylactic use of HCQ
, Li’s work prospects that the use of S-HCQ enantiomerically pure might
be a way to eliminate the adverse toxic effects already attributed to
the R form, probably allowing to double the therapeutic index of HCQ and
a wider dose-response range for evaluation of the effectiveness against
COVID-19.
Since CQ and HCQ are currently marketed as racemates, chiral switching
these drugs might also benefit rheumatoid arthritis and lupus
erythematosus patients to whom CQ and HCQ are well established as part
of the treatment protocol. The adoption of the eutomer alone could allow
the reduction of the administered dose with the maintenance of the same
therapeutic effect, and reduction in toxicity and metabolic burden, all
of which are highly desirable features for the chronic use of drugs.
In addition to potential clinical benefits, the adoption of CQ and HCQ
eutomers as the only marketed molecule could be commercially
advantageous and an useful asset against counterfeiting. CQ has a long
history of counterfeiting and substandard commercialization specially in
regions where malaria still is endemic . The increase in demand and
shortage of CQ and HCQ in the market due to COVID-19, besides raising
the prices imposes a risk to treatment continuity of autoimmune disease
patients. This has brought a golden opportunity for counterfeiting and
commercialization of substandard CQ and HCQ medicines, which has raised
worldwide. Simple methods for large-scale production of CQ enantiomers
are promptly available. Chiral switching these drugs would discourage
counterfeiting since CQ and HCQ distomers would become impurities.
Convenient and quick evaluation methods, such as the determination of
enantiomeric purity, could then become a useful tool in the
identification of counterfeited and substandard formulations.