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Polyphenolic Promiscuity, Inflammation-coupled Specificity: Whether PAINs Filters Mask an Antiviral Asset

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The Covid-19 pandemic has elicited much laboratory and clinical research attention on vaccines, mAbs, and certain small-molecule antivirals against SARS-CoV-2 infection. By contrast, there has been comparatively little attention on plant-derived compounds, especially those that are understood to be safely ingested at common doses and are often commonly consumed in the diet. With broader scope for assays and trials against a diverse array of viral infections, we review elucidations of the pharmacokinetic mechanisms of polyphenolic compounds over the past two decades and survey their putative frequent-hitter behavior. Many polyphenols are indeed promiscuous binders, suggesting a candidate mechanism of non-specific inhibition combined with inflammation-targeting specificity. Since such a specificity mechanism combined with promiscuity poses a possible pathway of inhibiting viral replication uniquely in infected tissue, we highlight pre-clinical studies of polyphenol aglycones that reduce virion replication. It is hoped that greater awareness of the potential specificity of polyphenolic activation to sites of pathogenic infection will spur renewed research and clinical attention on assaying and trialing against several infectious viral diseases.
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Polyphenolic Promiscuity, Inflammation-coupled Specificity:
Whether PAINs Filters Mask an Antiviral Asset
Rick Sheridan
a
* , Kevin Spelman, PhD
b,c
a
EMSKE Phytochem; b
Massachusetts College of Pharmacy and Health Sciences; c
Health, Education & Research
Abstract
The Covid-19 pandemic has elicited much laboratory and clinical research attention on vaccines, mAbs, and
certain small-molecule antivirals against SARS-CoV-2 infection. By contrast, there has been comparatively
little attention on plant-derived compounds, especially those that are understood to be safely ingested at
common doses and are often commonly consumed in the diet. With broader scope for assays and trials
against a diverse array of viral infections, we review elucidations of the pharmacokinetic mechanisms of
polyphenolic compounds over the past two decades and survey their putative frequent-hitter behavior. Many
polyphenols are indeed promiscuous binders, suggesting a candidate mechanism of non-specific inhibition
combined with inflammation-targeting specificity. Since such a specificity mechanism combined with
promiscuity poses a possible pathway of inhibiting viral replication uniquely in infected tissue, we highlight
pre-clinical studies of polyphenol aglycones that reduce virion replication. It is hoped that greater awareness of
the potential specificity of polyphenolic activation to sites of pathogenic infection will spur renewed research
and clinical attention on assaying and trialing against several infectious viral diseases.
1. Introduction
Currently approved therapies for infection by SARS-CoV-2, the etiological agent of the COVID-19 pandemic,
include monoclonal antibodies and certain repurposed drugs such as dexamethasone. Monoclonal antibody
therapy suffers from difficult logistics to administer, and the approved repurposed drugs have only modest
effect on disease outcome. Some novel therapeutics developed for coronaviruses
1,2 as well as further
repurposed drugs
3 are under regulatory review as antivirals for SARS-CoV-2. Each has its own strengths but
also drawbacks such as perceived concerns about mutagenicity, time to production ramp-up, or side-effect
profile.
Persistently low worldwide vaccination rates, the potential for breakthrough infections, and the ability for
vaccinated individuals to achieve viral loads sufficient to infect others
4
, suggest that there remains ample scope
for a replication-inhibiting antiviral in the panoply of pandemic-alleviating healthcare tools.
Natural products may present a potentially untapped source of antiviral activity. Plants must resist viruses
whose constituent peptides are restricted to the same twenty proteogenic amino acids. Plant virus proteins
share similar fundamental constraints on protein secondary and tertiary structure as viruses with mammalian
hosts. Plants’ secondary metabolites are known particularly for plant-protection. Prevalent among the
secondary metabolites are polyphenols. One of the three primary polphyenol classes are flavonoids
5
.
Flavonoids are a family of over eight thousand unique compounds that provide several advantages to plants.
6–8
These compounds are responsible for some pigment and aroma of flowers and fruits, thereby attracting
pollinators.
9–11 Various flavonoids also protect plants from both biotic and abiotic stressors,
9,12,13 providing
* Author to whom correspondence should be addressed: rick@emske-phytochem.com
antimicrobial defenses,
9,11,14 acting as UV filters,
9,11,15 and serving as signaling molecules.
9,11,16 Further, despite
sparse literature on the topic, several flavonoids are also demonstrated to inhibit several plant viruses.
17–21
Recent research has demonstrated antiviral modes of activity for flavonoids by targeting neuraminidase
22,23
,
proteases
23–25 and DNA/RNA polymerases.
24
A frequent target of coronavirus antivirals is the SARS-CoV-2 main protease, owing especially to the
successful history of protease inhibitors on reducing HIV replication. Several polyphenols showed potent
antiviral activity to SARS-CoV’s main protease.
26–30 Among these, the polyphenolic flavonoid hesperetin ( 1 )
was unique in potently inhibiting the action of the main protease in cell-based assay.
26 Hesperetin
dose-dependently inhibited cleavage activity of the 3CLpro in expressed in Vero E6 cells with an IC50 of 8.3
μM.
26
However, polyphenols like hesperetin are disfavored by industrial medicinal chemists for proceeding through
the hit-to-lead (H2L) stage of the drug discovery pipeline.
31,32 Polyphenols are categorized among the
Pan-Assay INterference compounds (PAINs)
33 (other terms are “frequent-hitters”, “promiscuous inhibitors”,
“privileged structures/scaffolds”, and “invalid metabolic panaceas”
34
), and are suggested to obscure the results
of various assays. They also bind broadly to assays’ protein targets themselves.
Fig. 1 : Four flavonoid aglycones referenced in the present work: hesperetin ( 1 ); diosmetin ( 2 ); quercetin ( 3 );
Due to the ongoing pressing need for further COVID treatment strategies, we review the pharmacokinetic and
putative frequent-hitting behavior of polyphenols’ as a class with an eye toward ascertaining 1) their potential
as an antiviral 2) whether or not polyphenols simultaneously should pose risks to ordinary healthy cellular
processes.
2. What defines a polyphenol?
While IUPAC has defined the term “phenols”
35
, a definition of polyphenols remains yet to be formally accepted.
Quideau (2011) explored definitions of polyphenols extensively, providing an applicable description:
The term “polyphenol” should be used to define plant secondary metabolites
derived exclusively from the shikimate-derived phenylpropanoid and/or the
polyketide pathway(s), featuring more than one phenolic ring and being devoid of
any nitrogen-based functional group in their most basic structural expression .”
5
In describing polyphenols in part based on their provenance provides excellent exclusivity. However, one
could question whether it is helpful to exclude phenols such as acacetin which have only one phenolic ring.
For large-scale cheminformatic purposes, which challenge the application of biosynthesis pathway criteria, an
applicable definition may be to treat polyphenols as any molecule with more than one phenolic ring but lacking
elements other than C, H, and O.
3. Poor polyphenol PK perception
The therapeutic efficacy of any antiviral whose purpose is to reduce viral replication requires maintaining a
efficacious concentration of the ligand at its putative target for an extended period of time. Conservatively, this
period should ideally be of long duration relative to a virus’s replication time to reach peak viral load. An
interval typically measured in days in the case of SARS-CoV-2 infection in humans.
36
However, polyphenolic compounds’ potential for efficacy for any particular pathology is criticized due to a prima
facie poor pharmacokinetic ADMET profile. Consider, for example, diosmetin ( 2 ). A primary intermediate
metabolite of the pharmaceutical formulation known as Daflon (comprised of 90% diosmin, and 10% other
flavonoids expressed as hesperidin, diosmetin, linarin, and isorhoifolin), it and similarly proportioned
formulations are prescribed in many countries around the world for chronic venous insufficiency (CVI).
Ingested diosmin becomes diosmetin through Phase I metabolism through the intestinal wall, and then is either
glucuronidated (primarily) to glucuronides ( 4 and 5 ), sulfated, or methylated through Phase II metabolism in the
liver.
37–47 Serum analysis on healthy individuals demonstrates negligible presence of the aglycone in plasma,
and low sustained levels of the diosmetin conjugates (primarily glucuronides) in plasma, with t
max of 2.3 hrs and
t
1/2 ranging from 8-70 hrs. 38–42,44–47 Stachulski and Meng (2013) and Tranoy et al. (2014) note that most
glucuronides are rapidly eliminated by the kidneys, posing an apparent limitation to their efficacy.
48,49
Glucuronidation further reduces bioavailability to the intracellular compartment as the glucuronide moiety
imparts a hydrophilicity that prevents cellular uptake.
49
Fig. 2 : Common glucuronide metabolites of the referenced aglycones: diosmetin-3’-O-glucuronide ( 4 ); diosmetin-3’-7-O-glucuronide
( 5) ; hesperetin-3’-O-glucuronide ( 6 ); hesperetin-7–O-glucuronide ( 7 ); quercetin-3-O-glucuronide ( 8 );
Note that a similar metabolic pathway can be described for other flavonoid aglycones. In the case of
hesperidin, it is hydrolyzed to hesperetin, ultimately primarily becoming glucuronides ( 6 and 7). Or quercetin,
primarily to glucuronide ( 8). Russo et al. (2018)
39 provide a prototypical example of flavonoid plasma
pharmacokinetics as demonstrated by diosmetin, which is reproduced and linearized in Figure 3a and b,
respectively.
A)
B)
Fig. 3 a) Diosmetin plasma concentration (as ascertained following deglucuronidation), after administration of a 50mg/kg micronized
diosmin formulation to rats. Image licensed under CC by 4.0 from Russo et al. (2018)
39
b) same plot with linearized axes.
Yet to our knowledge 1) no quantitative bioavailability assays of diosmetin have taken place in non-plasma
compartments such as extracellular fluid and tissue in humans; 2) tissue distribution studies of flavonoids in
animal models are few. More in vivo distribution data to support therapeutic insights into polyphenols would be
valuable.
4. Drawbacks of polyphenol PK analysis
On broader review of the polyphenol pharmacokinetic literature, five insights about pharmacological assays
emerge:
1. The most commonly obtained pharmacological assay for concentration of polyphenols or their
metabolites is blood plasma analysis, rather than interstitial fluid or intracellular fluid.
37–47,50–59
2. A polyphenol’s plasma concentration profile alone provides no data on tissue distribution or
biotransformation.
50,59,60
3. It is very difficult to sample intracellular fluid for drug/metabolite concentration profiling to the exclusion
of extracellular and serum fluid.
61,62
4. Even radiolabeled assaying of all possible elimination routes fails to provide a complete accounting of
polyphenol dosage intake.
59
5. Plasma samples of polyphenols are more frequently obtained from healthy individuals, rather than
those suffering from a particular pathology.
37–47,51–59
Therefore if any particular pharmaceutical candidate’s PK profile achieves significant distribution in organs
other than those associated with either the GI tract or renal tract, it would be unascertainable from serum
analysis alone. Further, if any particular pathology has an effect on a compound’s tissue distribution (whether
by causing sequestering in sanctuary sites, or adduct formation with the target in tissue both of which
represent an increase in the volume of distribution), then plasma analysis alone remains poorly positioned to
provide the relevant readout. Rather, tissue analysis in sacrificed animal models, or comprehensive
radiolabeled elimination quantitation in humans, would be required.
Walle et al. (2001) demonstrated such a radiolabeled analysis
59
. Notably, they found that carbon dioxide was a
major metabolite of quercetin ( 3 ) in humans,
59 suggesting a rarer elimination pathway than typically
encountered by pharmacological analysis. Even with this exotic elimination route taken into account, the full
dose of quercetin was not always accounted for. One can speculate that sequestration of quercetin products in
tissue compartments was maintained past the 72-hr study period.
Moreover, while DeBoer et al. (2005)
63 and Bieger et al. (2008)
64 demonstrated that quercetin reaches certain
tissues other than those associated with GI and renal tracts in healthy animal models, even these studies still
fall short of addressing any putative bioavailability of flavonoids uniquely to tissue affected by diseased
circumstances.
5. The flavonoid paradox
To begin addressing these pharmacokinetic challenges, we look at a subtle but critically important aspect of the
pharmacokinetic profile of polyphenols such as flavonoids as ascertained from the literature.
Menendez et al. (2012) and Perez-Vicaino et al. (2013) posed the problem of the “flavonoid paradox”.
65,66 The
paradox is summarized by the observation that several flavonoid polyphenols have been shown to
demonstrate therapeutic effects for various pathologies in vivo, and yet their pharmacological profiles suggest
poor bioavailability with rapid plasma clearance.
The paradox is resolved by the following deconjugation mechanism: During inflammation (as happens during
infection of several etiologies), phagocytes arrive at the extracellular fluid surrounding the sites of inflammation.
The phagocytes express β-glucuronidase which accomplishes deglucuronidation (also known as
deconjugation) of the flavonoid glucuronide into its aglycone form. The deconjugated flavonoid aglycone
subsequently diffuses through the cell membrane where they can reach their target. The mechanism is
summarized in Table 1. For purposes of this review only, the mechanism steps are labeled stages B, C, D, E,
and F for clarity.
Table 1 - Deglucuronidation-through-inflammation mechanism steps
Stage B - Flavonoid aglycones are glucuronidated prior to arrival in the bloodstream
Stage C - Neutrophils and macrophages are attracted to site(s) of inflammation
Stage D - Beta-glucuronidase is expressed by neutrophils and macrophages
Stage E - Serum flavonoid glucuronides are deglucuronidated (‘deconjugated’) by
β-glucuronidase at site of inflammation
Stage F - Flavonoid aglycones diffuse through cell membrane
Pathology-selective activation of glucuronide drugs by beta-glucuronidase has been widely understood and
exploited in the context of anti-tumor agents.
49 However, it is the “deconjugation in inflammation hypothesis”
that was developed and verified progressively over the period 2001-2019 across several polyphenols in vitro,
in animal models, and in humans under inflammation conditions.
65,67–78 Steps of the pathway were verified
across the polyphenols luteolin, quercetin, daidzein, and kaempferol, as well as the ellagic acid metabolites
urolithin A, iso-urolithin A, and the single-phenol urolithin B. We propose standardizing the mechanism’s
naming to the technical term ‘deglucuronidation-through-inflammation’ or DTI. The ‘Shimoi pathway’ may also
serve as a convenient shorthand that recognizes the lead researcher to first propose and study this
mechanism with specific attention to inflammation with polyphenols (by way of luteolin).
6. Validation of deglucuronidation-through-inflammation
Demonstration of the evidence generated through the deglucuronidation-through-inflammation body of
work
65,67–78
, is provided against the model’s labeled stages C-F in Figure 4.
A)
B)
Fig. 4 Deconjugation-through-inflammation literature basis A) in vitro support B) in vivo support
While the deglucuronidation-through-inflammation hypothesis has been extensively reviewed by others
66,79–84
,
to our knowledge, this review is the first to unify the body of work into one cohesive, accessible evidentiary
framework.
7. The promiscuous inhibition of polyphenols
Promiscuous inhibition poses two primary implications for medicinal chemistry assaying. The first is the
non-specific binding of protein / enzyme targets themselves. The second is the disruption of assay integrity by
inhibiting non-target enzymes used for assay readout. As it can be difficult to distinguish between these two,
orthogonal assays are sometimes performed to verify a target binding interaction.
Promiscuity could take any of several forms. A promiscuous ligand could simply be highly conforming to a
protein surface’s geometry, with a high number of hydrogen donors & acceptors to more likely “stick”
nonspecifically to any given protein site’s own set of h-donors and h-acceptors. Another mechanism sees
promiscuous inhibition take the form of colloidal aggregations.
85 In this mechanism, upon reaching a certain
concentration, the ligand forms tightly-packed spherical aggregates with itself, even inside the cell.
86,87
Proteins and enzymes non-specifically bind to the surface of the aggregation and are inhibited in the process.
88
Often seen as a nuisance originally, it is now also seen as a source of opportunity in drug discovery as
well.
87,89,90 Deliberate study of aggregation in cell-based assays is a nascent sub-field
91 so cataloguing of
non-specific aggregation among polyphenols in cells merits further investigation.
Fig. 5 Non-specific aggregation inhibition model. Figure reproduced from Auld et al. (2017)
88 under CC BY-NC-SA 3.0
Quercetin has earned a reputation as a promiscuous inhibitor
31,92–94 as well as having served as one of the first
aggregators identified.
85,95,96 Luteolin
93
, curcumin
93
, myricetin
93,94,97–99 and tannic acid
93,94 are also promiscuous
inhibitors, where myricetin and tannic acid have been further identified as aggregators.
94
Of 123,844 assay records hosted by Pubchem and compiled by Gilberg et al. (2016)
100
, their isolation of the
most promiscuous 466 of them (99.6% percentile) contains 13 polyphenols based on our
cheminformatic-oriented definition.
The catechol functional group, while not the exclusive province of polyphenols (and nor do polyphenols all
contain catechol), certainly correlates with polyphenols. Bael and Holloway (2010), highlights catechol as a
prominent PAINS functional group
101 even as Capuzzi et al. (2017) cautions against blind application of PAINS
filters
102
. And yet Jasial et al. (2017) demonstrates that the catechol functional group is in the top ten percentile
(9.5) of primary activity assays in Pubchem, and in the top seven percentile (6.9) of functional groups in
Pubchem confirmatory assay activity.
103
8. Therapeutic role of a promiscuous binder?
The final step of a putative polyphenol deglucuronidation-based antiviral mechanism requires that a
promiscuous-binding compound once inside a virus-infected human cell will arrest viable virion production.
The complete proposed mechanism is presented in Table
Table 2 - Proposed deglucuronidation-based antiviral mechanism
Stage A - Infection by any of several virus species induces inflammation.
Stage B - Flavonoid aglycones are glucuronidated prior to arrival in the bloodstream
Stage C - Neutrophils and macrophages are attracted to site(s) of inflammation
Stage D - Beta-glucuronidase is expressed by neutrophils and macrophages
Stage E - Serum flavonoid glucuronides are deglucuronidated (‘deconjugated’) by β-glucuronidase
at site of inflammation
Stage F - Flavonoid aglycones diffuse through cell membrane
Stage G - Flavonoid aglycones cause non-specific inhibition within the cell - interfering with
both ordinary cellular processes and the etiological source of inflammation (such as viral
replication)
Table 2 is illustrated graphically in Figure 6 - by way of one of the most studied flavonoids in the
pharmacokinetic literature, quercetin. Inhibitory mechanisms of viral replication could be due to direct inhibition
of viral proteins and enzymes, or by slowing ordinary cellular metabolic mechanisms such as respiration,
translation, transcription as co-opted by the infecting virus. In one case, that of fisetin applied to Dengue
fever,
104 fisetin showed no direct activity against DENV virions outside the cell yet effectively inhibited
replication in-cell. The study’s authors suggest it could be due to forming complexes with RNA or inhibition of
RNA polymerases. While inhibition of the dengue RdRp would represent a virus-specific inhibition, it remains
intriguing to consider that the replication inhibition could also be due to non-specific inhibition in a weakened
cell.
Fig. 6 : Proposed model of Shimoi mechanism for entry into intracellular compartment during viral infection, with quercetin serving in
the role of the aglycone, and quercetin-3-O-glucuronide as the glucuronide (adapted from Perez-Vizcaino et al 2013
66
).
9. Application of DTI to antiviral assaying and clinical trialing
Given that many forms of viral infections are known to induce inflammation, it would be a logical extension to
study whether consumption of certain flavonoids of sufficient quantity and in bioavailable forms could serve to
reduce the rate of viral replication in the early stages of viral infection. The mechanism of action could be by
inhibition of viral entry to cells, direct inhibitory action on viral enzymes in-cell, or non-specific promiscuous
disruption of the co-opted metabolism of infected cells.
The literature provides early in vitro evidence of achievable inhibition by phenolic flavonoids spanning across
Dengue virus, Influenza-A virus (IAV), Chikungunya virus, Foot-and-mouth disease virus (FMDV), Japaneses
Encephalitus Virus (JEV) and SARS-CoV-2.
Table 3 - in vitro evidence of in-cell viral inhibition (reported IC50) by phenols and polyphenols
DENV
FMDV
Influenza-A
JEV
CHIKV
SARS-CoV-2
Luteolin
9.7-10.0μM
105
15.9μM
106
Isoginkgetin
1.9-2.0μM
105
Quercetin
95.6-118μM
107
8.9-25.8μM
108
18.2μM
110
Baicalin*
97-235μM
22
0.8-3.2μM
111
7.0μM
112
2.9 μM
113
Curcumin
14.0μM
114
0.5-3.8μM
115,116
< 30μM
116
3.9μM
117
0.4-38 μM
110,118,119
Fisetin
150μM
104
29.5μM
112
Quercetagetin
43.5μM
112
Hesperetin
8.5μM
120
Naringenin
18-180μM
104
6.8 μM
120
< 35μM
121
* one phenol only
Much care must be applied in interpreting in vitro viral replication inhibition results. Where an IC50 value is
defined against a measure of viral RNA copies/mL, then qRT-PCR will show a difference of a single unit of Ct.
For comparison, SARS-CoV-2 infection typically presents a Ct range between 10 and 40 for acute infection vs
non-detectable viral load, respectively.
36 However, in vitro and in vivo Ct values are not directly comparable, as
in vitro reduction of viral replication may exhibit nonlinear effects at the in vivo scale, especially when the
effects of the innate and specific immune system are considered. Where due analysis of toxicity allows, a
higher IC value can be targeted, such as IC90 or even IC99.
122
Viral inhibitory assays typically report the Selectivity Index (SI), defined as the ratio of the cytotoxicity (CC50) to
the inhibitory concentration (IC50). A SI < 1 means that the ligand’s cytotoxicity to cells occurs at a lower
concentration than its inhibition of the target. Selectivity indices of 5 or greater are preferred. Ligand
candidates suffering from lower selectivity indices may be excluded from further investigation. However, the
deglucuronidation-through-inflammation mechanism would suggest that dismissing polyphenol ligands with a
low selectivity index could be overly conservative. Given that most polyphenols circulate in plasma as
glucuronides, and are only deglucuronidated to their aglycone form locally to the site of inflammation, a low
selectivity index may be acceptable and even preferable. This of course will depend on how efficiently the
deglucuronidation process discriminates between the localities of healthy and infected cells that induce the
inflammatory process.
Given the specificity that deglucuronidation-through-inflammation affords, and the putative validity of
aggregation-based nonspecific binding mechanisms,
86 the standard practice of applying aggregate-dissociating
detergents such as Triton X-100 is called into question for antiviral assays of phenols that are known or
expected to act through the DTI mechanism. A revisiting of relevant results of in vitro assays in the literature
where such a detergent was applied would be appropriate. However, such a modification to laboratory practice
should be considered carefully as the tendency for an aggregation to bind assay-specific enzymes could still
benefit from detergent application.
Non-specific inhibition can be a double-edged sword. Dong (2014) demonstrates that the aglycone kaempferol
increased IAV viral titers by log-2 compared with untreated mice, hastening their loss.
123 This was attributed to
attenuation of antiviral host-defense factor expression such as IFNα, IFNβ, IFNγ. By contrast, hesperidin was
protective of the mice. Further laboratory and clinical investigation of demonstrably promiscuous-binding
polyphenols in in vitro viral infection culture and in vivo will continue to be valuable. Results of recent clinical
trials of a polyphenol, quercetin, even while preliminary, do encourage further investigation.
124,125 Attention
would be particularly appropriate against those viruses that are known to induce inflammatory responses such
as influenza A (IFV-A), dengue (DENV), chikungunya (CHIKV), and coronavirus (SARS-CoV-2).
10. Additional observations on antivirals trialing of polyphenols
While it is important to maintain ligand concentration at the target for a period sufficient to exert the relevant
mechanism of action, it is worth noting that this period can be extremely short. Although not a polyphenol,
artemisinin enjoys enormous efficacy against the malaria parasite P. falciparum with a T
max at less than 2 hours
and a half-life of 2-5 hours.
126,127 Also, the dosage is of utmost importance. Following due analysis of toxicity,
protease inhibitors can target a C
min dose (minimum concentration between consecutive doses) of many
multiples of the IC50 value
128 to achieve faster viral clearance. Indirect antiviral effects of certain polyphenols
may also be possible, such as non-specific upregulation of immunosurveillance, as well as modulation of
specific immune cells.
129–131
Also, as promiscuous binders, due attention should be applied to inhibition of liver enzymes for drug-drug
interactions
44
, especially of drugs that study subjects might concomitantly consume for the same or unrelated
conditions. For example, among the polyphenols studied are those known to bind to CYP1A2
44
, CYP3A4
44 and
OATP1A2, the latter giving rise to the famous “grapefruit effect”.
132 Conversely, this P450 or other liver enzyme
inhibition may be advantageous to increase serum concentrations of verified pharmaceuticals, such as
fluvoxamine, in context of combination therapy (J. Duke, personal communication, 28 Nov 2009) for superior
joint bioavailability.
Finally, as a given polyphenol can demonstrate differing bioavailabilities between different dosage forms,
57
consideration should also be given to oral delivery type, such as aqueous, softgel, dry tablet form, and degree
of micronization. Further, owing to strong bioavailability and/or release rate, delivery in the form of original
plant matter while controlling for phytochemical content should also be considered.
72,133,134
11. Evolutionary Role
A BLAST search demonstrates that the gene coding for β-glucuronidase (GUSB, and uidA in bacteria
135
) is
extensively common across the animal kingdom (data not shown). Its homolog β-galactosidase (40% identity)
is also commonly expressed in bacteria (data not shown). β-glucuronidase has several documented
purposes
135
. It targets glucuronic acid in the gut,
136,137 and is associated with the degradation of
glucuronate-containing glycosaminoglycan.
138 But its extensive expression on, and release from, neutrophils
attracted in response to inflammatory signals is a mechanism whose genetic etiology and species prevalence
will require further work to elucidate.
Given the long history of herbivory in animals (and associated polyphenolic compound ingestion), and the high
prevalence frequency of the β-glucuronidase-coding gene GUSB across vertebrates, this mechanism could be
a long-ago evolved broad response under selection pressure of viral pathologies in ancestral species. It would
be worthwhile for future investigators to probe the genetic basis for neutrophilic β-glucuronidase expression
and its orthology across vertebrate species in order to better localize how this response evolved.
12. Conclusion
In this paper, we add to the body of evidence in the literature that polyphenols are a frequent-binding class of
chemicals produced by plants. We show that the pharmacology of polyphenols may allow for viral
infection-fighting potential due to the human body’s inflammatory response and provide conjecture as to the
evolutionary basis for a putative inflammation-induced antiviral function. Future work could include quantifying
the effect of in vitro antiviral studies under inflammation with neutrophils present for such viral targets as
SARS-CoV-2, CHIKV, DENV, and IAV/IBV.
Acknowledgements
The authors would like to thank Stéphane Quideau PhD, Jurgen Bajorath PhD, Pamela Weathers PhD, Brian
Shoichet PhD, Pierre Laurin PhD, and Scott Ferguson PhD for helpful tips in the course of the preparation of
this manuscript; and Yu Wai Chen PhD, Francisco Perez-Vizcaino PhD, and Erik de Clercq Phd for
encouragement, and would especially like to recognize and appreciate Stephen Molnar PhD for insights and
reviewing drafts of this manuscript. They are indebted to Fabiola de Marchi for expert chemical structure
graphic drafting. Finally, they would like to thank Priyanshu Jain for unflagging support and encouragement in
the course of preparation.
The figures were generated using the ChemSketch software package.
139
Preprint versions of this manuscript are posted to Zenodo
140 and ChemRxiv .
141
Competing Interests statement
RS leads research & development for EMSKE Phytochem, focusing on efficacy verification of phytochemical
constituents. He also leads a venture in the agricultural advisory industry.
KS is an adjunct professor at MCPHS and CSO of Health, Education & Research and an advisor and
consultant for the natural products industry.
The authors declare no competing interest.
--------
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