Abstract
Treatment strategies that target host entry factors have proven an effective means of impeding
viral entry in HIV and may be more robust to viral evolution than drugs targeting viral proteins
directly. High-throughput functional screens provide an unbiased means of identifying genes
that influence the infection of host cells, while retrospective cohort analysis can measure the
real-world, clinical potential of repurposing existing therapeutics as antiviral treatments. Here,
we combine these two powerful methods to identify drugs that alter the clinical course of
COVID-19 by targeting host entry factors. We demonstrate that integrative analysis of genome-
wide CRISPR screening datasets enables network-based prioritization of drugs modulating viral
entry, and we identify three common medications (spironolactone, quetiapine, and carvedilol)
based on their network proximity to putative host factors. To understand the drugs’ real-world
impact, we perform a propensity-score-matched, retrospective cohort study of 64,349 COVID-19
patients and show that spironolactone use is associated with improved clinical prognosis,
measured by both ICU admission and mechanical ventilation rates. Finally, we show that
spironolactone exerts a dose-dependent inhibitory effect on viral entry in a human lung epithelial
cell line. Our results suggest that spironolactone may improve clinical outcomes in COVID-19
through tissue-dependent inhibition of viral entry. Our work further provides a potential approach
to integrate functional genomics with real-world evidence for drug repurposing.
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2
Introduction
Host cell entry represents a critical stage of the SARS-CoV-2 replication cycle that determines
the tropism and virulence of emerging variants (1). SARS-CoV-2 entry relies canonically on
binding between viral Spike protein and host ACE2, followed by processing of the Spike protein
by endogenous proteases, most notably TMPRSS2 and furin (2, 3). However, the complete
entry process relies both directly and indirectly on a network of hundreds of host genes that
remains poorly understood (4, 5).
Heterogeneity in expression patterns of accessory entry factors, which facilitate viral adhesion,
cleavage events, and membrane fusion, is a chief determinant of viral susceptibility in terms of
both tissue types and patient subgroups (6, 7). Consequently, drug interactions with such host
factors can promote or inhibit viral entry in vitro and have in some cases demonstrated clinical
efficacy in large-scale studies (8–10). With the emergence of SARS-CoV-2 variants with
significant genome-wide mutational load, such as the BA.2 and XE strains, therapeutics and
vaccines targeting viral proteins of early strains have shown reduced efficacy in recent
outbreaks (11–13). Further, structural and functional study of the Omicron mutational landscape
revealed a range of adaptive mutations that facilitate antibody neutralization, vaccine protection,
and high-affinity viral interactions with host ACE2 receptor (12, 14, 15). There consequently
exists a pressing need to identify therapies that modulate host entry factors, which may be both
more robust to future variants and complementary to existing direct-acting antivirals such as
nirmatrelvir and molnupiravir (16, 17).
High-throughput forward genetic screens, primarily performed with CRISPR-Cas9 systems in
knock-out (CRISPR-KO) and activation (CRISPRa) formats, provide a powerful tool to identify
host genes that facilitate viral entry (18–25). Such methods enable causal inference of single-
gene effects that may be confounded in gene expression assays by both epistatic patterns and
immune mechanisms distinct from viral entry. CRISPR screens can also quantify gene effects in
distinct cell types and different perturbation schemas, which provides specific mechanistic
insights but can limit the generalizability of findings from any individual experiment (26).
We hypothesized that integrated analysis of multiple viral-entry functional screens would reveal
a shared network of host entry genes, with more generalizable implications for drug repurposing
than would be possible using individual datasets. We performed drug-target network analysis
using all publicly available, genome-wide CRISPR screens of SARS-CoV-2 viral entry, which
identified three common drugs, spironolactone, carvedilol, and quetiapine, as potential
modulators of viral entry. Furthermore, we conducted a retrospective clinical outcome analysis
of these drugs using medical records from 64,349 COVID-19 patients, which supported a
significant protective role for spironolactone. Finally, we demonstrated that spironolactone
exerts a time-dependent inhibitory effect on SARS-CoV-2 viral entry in human lung cells,
suggesting that spironolactone may mediate a milder disease course by suppressing viral entry.
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3
Methods
CRISPR screening data
We identified all published, genome-wide CRISPR-KO screens of SARS-CoV-2 entry in human
cells available as of May 2022, with five screens meeting criteria. Three screens were
performed in HuH-7 liver cells, and two were performed in A549 lung epithelial cells.
Multiplicities of infection (MOI) for each screen ranged from 0.21 to 0.4. We also identified all
genome-wide CRISPRa screens in human cells available as of May 2022, with three screens
meeting criteria. These included two screens in Calu-3 lung epithelial cells and one in HEK293T
embryonic kidney cells, with MOI ranging from 0.05 to 0.5. Raw data for each screen was
obtained from the respective source publication (Figure 1B). For each screen, fold-change
values were converted to Z-scores, with positive scores indicating genes whose expression
promoted viral entry.
Functional enrichment analysis
Enrichment analyses were performed using the prerank gene set enrichment analysis (GSEA)
algorithm. KEGG pathway gene sets were obtained from the KEGG Pathway Database, and
gene ontology molecular function (GOMF) gene sets were downloaded from the MSigDB
Signatures Database (27, 28). In all cases, sets with a minimum size of 3 and maximum set size
of 500 were considered. We defined significance as a false discovery rate (FDR) less than 0.05.
Network-based drug rankings
Drug-gene interactions for FDA-approved drugs were obtained from DrugBank and used to
construct a bipartite graph for each screen containing two node classes, compounds and genes,
and undirected edges representing known interactions between protein-coding genes and FDA-
approved drugs (29). The complete network contained 7,233 nodes (4,927 compounds and
2,306 genes) and 14,863 edges. For each screen, a unique subgraph was defined
encompassing gene hits, measured as the top 5% of genes by Z-score, and their corresponding
drug interactions.
In each subgraph, compounds were ranked by normalized degree centrality (NDC), defined as
node degree normalized to the total number of possible neighbors, reflecting each compound’s
proximity to host entry factors identified in the screen. Alternative measures of centrality,
including betweenness centrality and eigenvector centrality, were also evaluated. We defined
drug hits as the top 1% of drugs by NDC rank for each screen and selected for follow-up all
drugs identified as hits in at least three screens.
Cohort construction
De-identified patient medical records were obtained from the Northwestern Medicine Electronic
Data Warehouse. Records were filtered to include only patients who had a positive COVID-19
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4
test by means of reverse transcription-polymerase chain reaction (RT-PCR) testing. Patients
who had more than one positive test were sorted by date, and the first was selected to create a
unique set of patient identifiers. A 6-month time window preceding the first positive COVID-19
test was created for use as a filter for medication treatment status.
Patient characteristics that served as covariates considered in the analysis included race,
gender, age, postal code, and medical comorbidities such as hypertension, diabetes, metastatic
cancer, rheumatological disease, autoimmune disorders, and kidney failure, which are reported
according to Charlson Comorbidities (30). The full list of comorbidities is provided as
supplementary material (Supplementary Table S1). Records in which both race and postal
code were missing (n = 8,201) were removed, as data was not missing at random. The final
database included 64,349 unique patients with a positive COVID-19 test and complete
comorbidity labels.
For each of the medications under investigation, treatment groups were identified by string
matching to generic/brand names for the medication of interest. We defined users as those with
medication orders occurring within 6 months preceding their first recorded COVID-19-positive
specimen, such that treatment status connotes drug exposure prior to COVID-19 diagnosis.
Propensity score matching
Propensity score matching was performed using the psmpy package in Python 3.7 (31). In our
use case, logistic regression was executed where the treatment state (for the medication of
interest) was regressed on the set of covariates defined previously. Due to the size imbalance
between treatment and control group sizes, logistic regression was run repeatedly on a
balanced sample to generate a probability integer for each observation by folding iteratively over
the larger class and averaging over repeated patient indexes. The logit of the logistic regression
prediction was calculated for control and treatment observations, as it more closely
approximates a normal distribution (32). Following this, a unidimensional k-nearest neighbors
(k-NN) algorithm was fitted to the logit scores of the control group. The treatment group was
then fitted to the model, calculating Euclidean distance as the similarity metric. Am exclusionary
caliper size of 0.25 of the standard deviation of the distance was implemented to reduce distant
matches. To prevent samples from sharing the closest first match, matching was performed
without replacement. In this way, treatment-control patient pairs were identified in which each
subject had an approximate equal likelihood of receiving the drug of interest.
To verify adequate matching, a Cohen’s D statistic (standardized mean difference) was
calculated before and after matching, ensuring that the matched cohort had smaller covariate
effect sizes. Additionally, the two cohorts’ logit scores and the number of patients in each
category were compared to confirm similarity.
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5
Statistical analysis
Clinical outcomes under investigation included admission to the intensive care unit (ICU) and
initiation of mechanical ventilation. Odds ratios for each clinical endpoint, with corresponding
confidence intervals and p-values, were calculated using both McNemar’s test for matched
treatment-control pairs and a chi-square test for the same cohorts without pairing.
Ethics statement
The clinical study was approved by the Northwestern University Institutional Review Board.
Software
All computational analyses were performed in a Python 3.7 environment.
Replicating vesicular stomatitis virus (VSV) pseudovirus generation
Recombinant VSV expressing eGFP in the 1st position (VSVdG-GFP-CoV2-S) was generated
using plasmid-based methods. The plasmid to rescue this virus was generated by inserting a
codon optimized SARS-CoV2-S based on the Wuhan-Hu-1 isolate (Genbank:MN908947.3),
which was mutated to remove a putative ER retention domain (K1269A and H1271A) into a
VSV-eGFP-dG vector (Addgene, Plasmid #31842) in frame with the deleted VSV-G. The control
virus VSVdG-RABV-G SAD-B19 was also generated by inserting Rabies virus G in the same
vector. Both viruses were rescued in 293FT/VeroE6 cell co-culture and amplified in VeroE6 cells
and titrated in VeroE6 cells over-expressing TMPRSS2. Sequencing of the amplified virus
revealed an early C-terminal Stop signal (1274STOP) and a partial mutation at A372T (~50%) in
the ectodomain.
Pseudovirus infection assay using Replicating VSV pseudovirus
HEK293FT cells were plated in clear 96-well plates at 2x104 cells per well approximately 24
hours prior to infection in 100 uL of media containing 10% FBS. Cells were infected with
VSVdG-CoV2-S or VSVdG-RABV-G at an MOI of 0.1. Infection was performed by diluting virus
in media without FBS and adding 150 uL of diluted virus per well. After addition of the virus, the
plate was spun at 900 x g for 60 minutes at 30°C. Infection was tracked over time using an
Incucyte system (Sartorius) in a 37°C and 5% CO2 incubator using 4x magnification and
detecting GFP. GFP+ cells were counted using Incucyte Analysis software and data were
reported as GFP positive foci per well after normalization to confluence.
In vitro viral entry inhibition experiments
All inhibitor assays use 96-well plates coated with Poly-D-Lysine (Thermo Fisher, A3890401) at
a concentration of 50 ug/mL for 2 hours at room temperature. The plates were then washed with
PBS three times, and 1 x 104 cells were plated in a final volume of 100 uL of culture media. The
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6
next day, 20 uL of media was removed from each well and replaced with a 5X concentration of
the inhibitor in culture media at the indicated dilution. The cells were then returned to 37oC. Two
hours later, diluted SARS-CoV-2 Spike pseudotyped lentiviruses (for an ~MOI of 0.05-0.15)
were added to each well. Plates were spinfected and assayed as described above. Drugs
(Sigma-Aldrich) were diluted in PBS via vigorously vortexing to a concentration of 100 mM prior
to dilution in culture media.
Results
To identify host subnetworks that facilitate SARS-CoV-2 viral entry, we obtained all genome-
wide CRISPR screens measuring the impact of individual genes’ expression on viral infection in
human cells (Figure 1A). The screens accounted for a variety of cellular contexts, including
both lung and non-lung cell types, and functional perturbations, namely loss-of-function (i.e.,
CRISPR-KO) and gain-of-function (i.e. CRISPRa) screens (Figure 1B). The final dataset
collection included five CRISPR-KO and three CRISPRa screens, enabling resolution of
context-dependent entry mechanisms that could not be identified by only the KO datasets.
The eight screens exhibited variable levels of correlation at the single-gene level, consistent
with their heterogeneous cellular contexts (Figure 2A). 88% (7/8) of screens were significantly
correlated with at least one other screen, while 26% (7/28) of all pairwise comparisons revealed
significant positive correlation after correcting for multiple testing (Figure 2B). Gene-level
agreement was higher within each class of screen, with significant positive correlation among
60% (6/10) of CRISPR-KO and 33% (1/3) of CRISPRa screen pairs. Both pairs of screens
performed in lung-derived tissue were significantly positively correlated at the single gene level,
but cellular context was not significantly associated with gene-level correlation overall (p =
0.0825).
We next quantified functional pathway enrichment among each screen using GSEA. 20 KEGG
pathways were significantly enriched in at least two screens, including several pathways with
known involvement in SARS-CoV-2 entry (Figure 2C). Pathways involved in glycosaminoglycan
and phosphoglycerides were most strongly enriched, consistent with their essential role in viral
attachment (33). We also observed significant de-enrichment of pathways involved in
neurodegenerative disease, including Alzheimer’s, Huntington’s, and Parkinson’s diseases, as
well as synaptic signaling broadly. Correlations among normalized pathway enrichment scores
for each screen were generally higher than correlations for individual genes, although screens
with higher gene-level correlations tended to have higher pathway correlations as well (Figure
2D).
We next constructed unweighted networks representing known interactions between FDA-
approved drugs and entry genes identified in individual screens (Figure 3A). Each network
contained an average of 116 (standard deviation 8.00) genes, 605 (208) drugs, and 758 (328)
edges each, corresponding to an average density of 1.06% (0.112%). The mean degree of each
graph was 6.50 (2.86) for gene nodes, 1.23 (0.104) for drug nodes, and 2.03 (0.256) overall,
and 3.81% (1.45%) genes per screen, on average, lacked known interactions with any drugs.
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7
We prioritized drugs for downstream analysis based on centrality within each hit network. As
both local (NDC) and global (betweenness) measures of centrality yielded similar rankings (⍴ =
0.99, p < 0.001), we used NDC for subsequent analyses due to its interpretability. No drug met
significance in all screens, while 248 drugs were significant in at least one dataset. 33 drugs met
significance in at least three datasets (Figure 3B). Drug hits encompassed a range of functional
categories, with a predominance of psychoactive compounds. Tricyclic antidepressants were
the most common category, comprising 18.2% (6/33) of hits, followed by atypical antipsychotics,
which comprised 9.1% (3/33).
We next performed a propensity-score-matched retrospective clinical analysis to evaluate
whether use of candidate drugs was associated with COVID-19 disease severity (Figure 4A).
We obtained electronic medical records from a large academic hospital system, which yielded
64,349 patient records with a positive COVID-19 test. Of the drugs meeting centrality
significance, only three medications had a sufficient treatment cohort size for PSM analysis:
carvedilol, quetiapine, and spironolactone (Figure 4B). Additionally, we included metformin as a
positive control, as it has demonstrated a significant negative association with COVID-19
severity in clinical studies (9, 34–36).
We observed a significant negative association between spironolactone use and progression to
ICU admission (OR 0.42; CI95 0.22-0.78; p = 0.004). The association between spironolactone
use and progression to mechanical ventilator status was also strongly negative, although not
statistically significant (OR 0.31; CI95 0.073-1.00; p = 0.052). The positive control, metformin,
also exhibited a significant negative association with progression to ICU admission (OR 0.44;
CI95 0.32-0.61; p < 0.001) and a nominally significant association with progression to
mechanical ventilator status (OR 0.46; CI95 0.25-0.82; p = 0.007). We did not observe a
significant association with progression to either ICU admission or mechanical ventilator status
for either carvedilol or quetiapine.
Given that spironolactone use was associated with a significant reduction in risk of severe
COVID-19 in our cohort analysis, we evaluated whether its mechanism could be mediated by
inhibition of viral entry. We performed a SARS-CoV-2 pseudoviral entry assay in a human lung
epithelial cell line at varying doses of spironolactone, observing a time- and dose-dependent
drug effect on viral entry (Figure 5A). We observed an initial peak of viral entry at 4-8 hours
after infection, at which time there was a small but significant increase in infection at the highest
dose (Figure 5B). Following the first peak, there was a larger and sustained decrease in viral
infection at higher spironolactone doses, consistent with an overall inhibitory effect on viral entry
(Figure 5C).
Discussion
Our analysis demonstrates that genome-wide CRISPR screens provide a basis for systematic
prioritization of drug candidates in COVID-19, many of which are not evident in methods reliant
on gene expression studies or association hits alone. We identify three common medications,
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spironolactone, quetiapine, and carvedilol, as potential modulators of SARS-CoV-2 infection
based on their interactions with host entry factors. We perform a propensity-score-matched,
retrospective cohort study of clinical outcomes for COVID-19 patients on these medications,
showing that spironolactone use is associated with reduced likelihood of ICU admission. We
further show that spironolactone inhibits SARS-CoV-2 pseudoviral entry in a human lung
epithelial cell line in a dose-dependent manner, providing a potential mechanism for the
observed therapeutic effect.
Functional genomic screens enable measurement of the effect of individual host genes on viral
entry, but their noise levels and context dependence limit direct clinical applicability. We
addressed this barrier by combining data from multiple genome-wide CRISPR screens across
several experimental and cellular contexts, inferring drug activity based on abundance of known
drug targets among hits, and selecting drugs with high representation among screens for follow-
up analysis. Such a prioritization method benefits from direct biological interpretability, whereby
top candidate drugs modulate large numbers of entry factors, while enabling higher sensitivity
for detecting potential effects.
Pharmacologic antagonists of the renin-angiotensin-aldosterone system (RAAS), such as
spironolactone, have been proposed as potential inhibitors of SARS-CoV-2 infection due to their
role as indirect modulators of ACE2 expression (37, 38). For instance, spironolactone has been
observed to decrease expression of soluble ACE2, which normally promotes viral adhesion,
while simultaneously increasing expression of the membrane-associated form (39, 40). Its anti-
androgenic action may also decrease expression of TMPRSS2, leading to impaired Spike
protein proteolysis and activation (41). Finally, given that COVID-19 is strongly associated with
both hypokalemia and hypocalcemia, RAAS antagonists that maintain cation homeostasis, such
as spironolactone, may mitigate both ion-channel-mediated viral entry and the clinical sequelae
of associated electrolyte imbalances (42, 43). We note that, excluding the cytochrome P450
family, all spironolactone-targeted entry factors in our analysis classify as either androgen
signaling proteins (PGR, NR3C1, SRD5A2, and SHBG) or voltage-gated cation channels
(CACNB3, CACNA2D1, CACNA1B, CACNA2D3, and CACNA1I), supporting a possible
combination of mechanisms. Further studies in relevant cell or tissue contexts could help to
elucidate the physiological role of these putative host factors/pathways during viral infection.
Despite a variety of mechanistic hypotheses, few investigations of either clinical efficacy or in
vitro effect have been performed for spironolactone in COVID-19. A recent interventional study
of spironolactone-sitagliptin combination therapy showed statistically non-significant
improvements in clinical outcomes, including mortality, ICU admission, intubation rate, and end-
organ damage, for patients on spironolactone (44). Another interventional trial showed no
significant clinical improvements for COVID-19 patients on potassium canrenoate, a
mineralocorticoid receptor antagonist similar to spironolactone (45). However, sample size was
limited in both studies, and our retrospective study design enables analysis of a substantially
larger treatment cohort.
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Our study has several limitations, including the use of a single healthcare system (across 11
hospitals) for clinical analysis, and the use of pseudotyped virus for in vitro validation. Further
investigations, including well-powered randomized controlled trials, will be necessary to
determine the therapeutic role for spironolactone in COVID-19.
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FIGURES
Figure 1. Integrated analysis of SARS-CoV-2 entry networks for drug repositioning
A. Genome-wide gene essentiality scores for viral entry were obtained from five CRISPR-KO
screens and three CRISPRa screens. These were combined with high-confidence drug-target
interactions from DrugBank to generate drug-gene subnetworks for hit genes from each screen.
Drugs are prioritized based on normalized degree centrality within each subnetwork, and top
drugs are validated through propensity-score matched hazard analysis of retrospective
electronic medical record data.
B. Descriptions of each functional screen considered in the analysis.
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Figure 2. Correlations and functional enrichment patterns among screens
A. Cluster map of individual gene ranks for all screens. Red bars indicate knockdown screens,
and blue bars indicate activation screens. Hierarchical clustering groups knockdown and
activations screens together.
B. Pairwise correlation measurements among screens. Annotations indicate significance at the
nominal level (*), after correcting for multiple testing among screens (**), and after correcting for
multiple testing among all pairs (***). A majority of knockdown screens demonstrate correlation
at a nominal or higher significance. Correlations between knockdown and activation screens
exist at a lower frequency.
C. Pathways with significant enrichment in multiple screens involve cellular adhesion and
synaptic transport. Annotations indicate significance at FDR values of 0.4 (*), 0.05 (**), and
0.001 (***).
D. Inter-screen correlations for KEGG pathway enrichment scores are stronger than for
individual gene scores. Annotations are the same as in (B).
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Figure 3. Network-based prioritization of candidate drugs
A. Summary network showing overlap among the individual screen networks. Drug and gene
nodes are labeled by average centrality and by the frequency at which they were labeled a hit in
individual screens. Psychotropic drugs, particularly tricyclic antidepressants and atypical
antipsychotics, show high aggregate centrality, while calcium and potassium channels represent
highly targeted entry genes.
B. Aggregate ranking of candidate drugs. Bars indicate centrality across all screens. Heat map
shows percentile rank in each screen. Blue boxes indicate screens in which the given drug was
labeled as significant, and bold text indicates drugs with sufficient cohort numbers for
retrospective clinical investigation.
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Figure 4. Retrospective clinical validation of candidate drugs
A. Study design. Propensity score matched treatment and control groups were defined for each
drug of interest in patients with a COVID-19 diagnosis. Clinical endpoints were ICU admission
and mechanical ventilation.
B. Cohort sizes, odds ratios, and significance levels for individual drugs and endpoints. Bold text
indicates drug-endpoint combinations that were significant after Bonferroni correction (ɑ = 0.05;
m = 8). Statistically significant negative associations with ICU admission were observed for both
spironolactone and metformin, which was included as a positive control.
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Figure 5. Spironolactone inhibits SARS-CoV-2 pseudoviral entry in human lung cells
A. Infected cell density over time at increasing doses of spironolactone. n = 3 replicates for all
time points measured.
B. During the initial transitory entry spike at 4 hours, spironolactone was associated with a mild,
dose-dependent increase in viral entry, which was significant at the highest dose.
C. At 24 hours post infection (steady-state phase), spironolactone conferred a dose-dependent
inhibitory effect on viral entry. Brackets show all significant sequential differences.
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