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\title{Challenges of drug development during the COVID-19 pandemic: key
considerations for clinical trial designs}



\author[1]{Jun Shi}%
\author[2]{Yubo Xiao}%
\author[2]{Yiting Zhang}%
\author[2]{Donghao Geng}%
\author[2]{Danhua Cong}%
\author[2]{Kevin X. Shi}%
\author[2]{Richard J. Knapp}%
\affil[1]{dMed}%
\affil[2]{Affiliation not available}%


\vspace{-1em}



  
  \date{March 14, 2025}


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\selectlanguage{english}
\begin{abstract}
There is an urgent need for targeted and effective COVID-19 treatment. A
number of medications, including hydroxychloroquine, remdesivir,
lopinavir-ritonavir, fapiravir, and tocilizumab, have been identified as
potential treatments for COVID-19. Bringing these repurposed medications
to the public for COVID-19 will require robust and high-quality clinical
trials. This article reviews translational science principles and
strategies for conducting clinical trials in a pandemic and evaluates
recent trials for each drug candidate. We hope that this knowledge will
help focus efforts during this crisis and lead to the expedited
development and approval of COVID-19 therapy.%
\end{abstract}%



\sloppy


\textbf{Challenges of drug development during the COVID-19 pandemic: key
considerations for clinical trial designs~~}

\textbf{Jun Shi,\textsuperscript{1}* Yubo Xiao\textsuperscript{1},
Yiting Zhang\textsuperscript{1}, Donghao Geng\textsuperscript{1}, Danhua
Cong\textsuperscript{1}, Kevin X. Shi\textsuperscript{2}, Richard J.
Knapp\textsuperscript{3}}

\emph{\textsuperscript{1}dMed, Shanghai,
China,\textsuperscript{2}Feinberg School of Medicine, Northwestern
University,\textsuperscript{3}Sanofi (retired), Bridgewater, NJ, USA}

\textsuperscript{\textbf{\#}}\textbf{Correspondence}Jun Shi
(jun.shi@dMedGlobal.com)

\textbf{Running head}Clinical trial design for COVID-19 drug therapies

\textbf{Keywords}SARS-COV-2, COVID-19, drug development, translational
science, clinical trial design, antiviral drugs

\textbf{Scope: Review} article

\textbf{Number of tables: 4}

\textbf{Number of figures: 4}

\textbf{ABSTRACT}There is an urgent need for targeted and effective
COVID-19 treatment. A number of medications, including
hydroxychloroquine, remdesivir, lopinavir-ritonavir, fapiravir, and
tocilizumab, have been identified as potential treatments for COVID-19.
Bringing these repurposed medications to the public for COVID-19 will
require robust and high-quality clinical trials. This article reviews
translational science principles and strategies for conducting clinical
trials in a pandemic and evaluates recent trials for each drug
candidate. We hope that this knowledge will help focus efforts during
this crisis and lead to the expedited development and approval of
COVID-19 therapy.



\textbf{Introduction}

Identification of the 2019 novel coronavirus (SARS-CoV-2) and the
associated coronavirus disease (COVID-19) global pandemic is the
foremost human tragedy affecting our life today. This disease has
reached nearly every country, causing significant morbidity and
mortality as it continues to spread. The development of drugs and
vaccines is critical to win the battle against the SARS-CoV-2 virus. At
present, although different countries have employed different methods of
controlling COVID-19 based on their national conditions and health-care
systems, the general strategy is consistent: 1) flatten the growth curve
of COVID-19 infections by public health measures while reducing their
mortality by supportive care, and 2) accelerate the development of drugs
and vaccines. This article summarizes the challenges of conducting
clinical trials under pandemic conditions; describes potential options
to develop drugs for COVID-19 rapidly; discusses how to improve the
likelihood of trial successes; and reviews currently registered clinical
trials for the treatment and prevention of COVID-19 by analysing
available initial data and assessing the potential success of upcoming
trials.

\textbf{Challenges in conducting clinical trials in a pandemic}

Humans have experienced pandemics before, like the 1918 Spanish
Influenza pandemic, but the present COVID-19 pandemic is perhaps unique
due to modern trends of globalization and changing healthcare practices.
However, we have a limited understanding of the SARS-CoV-2 pathogen and
currently few ways to treat its victims. There is enormous pressure on
our healthcare institutions to preserve the lives of the infected while
finding effective treatments \emph{ab nihilo} . We have long understood
that the safety and efficacy of new medical treatments can only be
evaluated through carefully and systematically designed clinical trials.
How can we preserve this requirement while finding new treatments in the
shortest possible time under conditions of intense crisis?

\textbf{Time pressure} : Conventional drug research and discovery can
easily take a decade from target identification to pivotal phase III
clinical trials. This approach cannot be used for COVID-19. There is
simply not enough time. Instead, we must focus on identifying existing
drugs or drug candidates intended for other indications that may have
efficacy against COVID-19 and put them into accelerated clinical trials.
By leveraging pre-existing drugs with known pharmacological data, we can
reduce the need for dose-finding and toxicologic assessments. Further
dose evaluations can be integrated into an expanded phase III trial
using a combination of clinical, viral load reduction and immune
response as endpoints. This kind of acceleration will place a
substantial burden on regulatory agencies that only the pandemic itself
can justify.

\textbf{Repurposing drugs:} Our current knowledge of the molecular and
biochemical features of the SARS-CoV-2 virus suggest that drugs produced
for related RNA viruses (e.g. Ebola) may also be effective for treating
COVID-19.\textsuperscript{1,2} The clinical development pathway for an
existing drug proposed for a new indication is well understood by
regulatory agencies and is relatively brief. However, because SARS-CoV-2
is new, the repurposed drugs will not have undergone research and early
development optimization for COVID-19.

\textbf{Lack of information:} Once a drug candidate is selected, the
greatest unknowns are the dose, dosing regimen and treatment duration to
be used. Conventionally these are determined in phase II trials with
objectives of proof of concept (PoC) and dose ranging through studying a
drug's pharmacokinetics (PK) and pharmacodynamics (PD). This process may
be compressed or even eliminated through the use of existing therapeutic
dosing guidelines for established indications or acceptance of a near
maximum tolerated dose. Alternatively, two or more dose levels may be
tested in an expanded pivotal safety/efficacy trial. The absence of an
optimal therapeutic dosing regimen for COVID-19 may lead to false
negatives (lack of observed efficacy for an efficacious drug). In
addition, the lack of knowledge about the viral dynamics of SARS-CoV-2
and disease progression means that the therapeutic dosing time window
for treatment is not well defined, potentially leading to misleading
conclusions.

\textbf{Operational challenges} : Epidemiological shifts in disease
burden across the global also complicate clinical trials. COVID-19 was
first observed in the Chinese city of Wuhan. Many clinical trials of
repurposed drugs were performed there.\textsuperscript{3,4} However,
strict public health efforts reduced the number of new cases from
hundreds each day to only a few in a matter of months. As a result,
Wuhan is an unfavourable site for future clinical trials. After the
Wuhan outbreak, COVID-19 cases spiked in Europe, especially Italy and
Spain. At the time of this writing, the nation with the largest disease
burden is the United States, which has the greatest number of total
cases recorded. Clinical trial design needs to take this dynamic into
consideration. It is likely that future clinical trials will have to be
designed as global trials for this reason. Clinical operations can also
be an issue as routine evaluations requiring close patient observations
can't be conducted in settings of home quarantine. There are also
ethical considerations and resource competition between clinical trials
at play that will limit trial conduct or feasibility. Factors include
immense pressure on clinical staff, equipment shortages, and patient
desperation for effective treatment. At present most clinical staff are
almost exclusively focused on the preservation of patient lives.
Diversion of their efforts toward patient selection and treatment
changes necessary for clinical trial conduct could meet serious
resistance. Next, it will be difficult to define appropriate patient
populations for trials. The availability of reverse transcription
polymerase chain reaction (RT-PCR) testing to define SARS-CoV-2
infection is very limited and unequally distributed. A large fraction of
patients with COVID-19 symptoms do not actually know if they are
infected with SARS-CoV-2 virus and the time between symptom onset and
confirmation of infection will vary widely. Furthermore, there is no
well-established standard of care (SOC) on a global level. This will
complicate construction of multinational control groups. Patient
selection also needs to take into account the major differences in
fatality between different age groups and patients with different
co-morbidities. It is also important to consider patient concerns with
respect to their likely enrolment into a clinical trial. In the context
of potentially fatal disease, many patients may withdraw their
participation in trials, especially if they are blinded to their
treatment and they don't know what the treatment they are receiving.
Despite the need for expedited trials, all these issues must be taken
into account for actionable results.

\textbf{Regulation and approval} : Clinical trials need to be reviewed
on scientific and ethical grounds before they can take place. The
requirements for review and approval are well established with
relatively minor national differences. The IND review and approval
process is the standard pathway for drugs entering the clinical phase.
The review process usually takes a few months and involves national
regulatory bodies. Due to time pressure and fast changing disease
incidence, accelerated regulatory review and approval for IND are
required. Some COVID-19 drugs qualify for so called ``investigator-
initiated trials'' which only require local IRB/EC approval. This is a
fast process for initiating clinical trials that will be critical in
this crisis. In investigator-initiated trials, pre-existing drugs are
repurposed for COVID-19 treatment. Because the drugs in these trials
have already undergone review for other indications, less regulatory
oversight is needed for their use in COVID-19.

We present three key ideas for trial design to emphasize high quality
patient selection, study conduct, dose identification, and endpoint
evaluation to produce meaningful results.

\textbf{Key 1 -- Adhere to translational science principles ~}

To design a robust clinical trial for an anti-infection drug, we usually
need to know three sets of information: a) the disease symptomatology
and progression; b) the underlying mechanism of disease periods, i.e.,
dynamics of viral load and immune response; and c) host-virus-drug
interactions. We must have clear objective definitions of disease
progression and improvement. Objectively, this may be reflected by viral
load and host immune response. Viral load can be quantified through
RT-PCR while immune response can be tracked through inflammatory markers
and antibody production. By studying drug PK and PD, maybe indirectly
from previous programs, we can optimize the dosage regimen (dosage,
dosing interval and treatment duration) to maximize the cure rate,
reduce toxicity and avoid drug resistance.~The information obtained is
critical in understanding trial results, as drugs will have different
effects depending on disease progression. For example, a drug may be
falsely dismissed as ineffective if it is given too late in the disease
course. We must go beyond standard clinical evaluations and leverage
basic science measures to enrich trial outcomes.

\textbf{Key 2 -- Leverage Innovative trial designs}

Traditional Phase II studies on COVID-19 are largely missing due to the
time pressure to produce clinically relevant results. Instead, many
teams have progressed directly to exploratory, open-labelled studies,
but these were often underpowered and led to inconclusive results. The
few robust Phase III trials that exist lack COVID-19 applicable Phase II
data and producing them may not be feasible under pandemic conditions.
Instead, simultaneous combination of supportive care and RCTs
(Randomized Clinical Trials) has been recommended as the way to find
effective and safe treatments for COVID-19 and any other future
outbreak.\textsuperscript{5} In addition, there are two types of trial
designs, particularly useful in infectious diseases, that all parties
involved in planning, designing and conducting COVID-19 like trials
should be aware of through educational programs:

Adaptive platform (multi-arm, multi-stage) design.

This approach studies multiple targeted therapies in the context of a
single disease in a continuous manner utilizing a shared control arm.
Individual investigational therapies are allowed to enter or leave the
platform on the basis of a decision algorithm (see Figure
1).\textsuperscript{6} The advantages to this approach include its
flexible design, which allows for multiple drugs to be evaluated,
reduced control population and potential for international deployment.
However, international deployment would require a high degree of
regulatory coordination and normalization of clinical operations across
many different settings.

2) Prophylactic design: These studies focus on a less-studied population
in the COVID-19 pandemic; the close contacts (household or healthcare
workers) of COVID-19 patients. These individuals have a high likelihood
of contracting disease and limiting spread of disease is critical for
managing this crisis. Prospective, cluster-randomized, double-blind,
placebo-controlled studies designed to investigate the efficacy of
anti-viral drugs in prevention of the secondary spread of disease to
close contacts have gained popularity through anti-influenza
trials.\textsuperscript{7} We designed a post exposure prophylaxis (PEP)
trial in Wuhan, China using hydroxychloroquine in mid-February 2020 (see
Figure 2), but it didn't proceed due to the dwindling number of patients
in Wuhan. Scientifically, it would be better if hydroxychloroquine's
antiviral effect toward SARS-CoV-2 was confirmed prior to a PEP trial.
Similarly designed trials and its variations, including pre-exposure
prophylaxis (PrEP) and progression prevention trial in diagnosed
patients using HCQ, are on-going in the US/UK (see Table 4).

\textbf{Key 3 -- Implement 4R standards (right patient, right drug,
right dosage, right timing)}

1. Right patient

The idea of matching drugs to patients is a widely accepted paradigm in
pharmacology. This personalized medicine approach emphasizes the
numerous patient-level factors that can affect therapeutic targets, a
medication's PK, as well as the overall burden of disease.

Patient selection is complicated by heterogeneity in the clinical
manifestations of COVID-19. About 80\% of patients will only have
mild/moderate flu-like or pneumonia symptoms.\textsuperscript{8} The
majority of COVID-19 cases are self-limited and not life-threatening. A
considerable portion of patients, up to 17.9\% of RT-PCR-confirmed cases
in some models,\textsuperscript{9} may be completely asymptomatic for
the entire duration of infection, though some may have objective
subclinical manifestations of disease\textsuperscript{10}. However,
around 20\% of patients are deemed severe cases with significant
dyspnea, hypoxia, or lung imaging findings that require supplemental
oxygen or intensive care and current estimates of COVID-19 case fatality
rates vary from 2-7\%,\textsuperscript{8,11,12} though this figure
varies by location.

A number of factors contribute to the disparate manifestations of
COVID-19; the most well-studied factor is age. It has been reported in
China, Italy, South Korea and United States that elderly patients
comprised approximately 60-83\% of all COVID-19
fatalities.\textsuperscript{13,14,15} A similar trend is seen with
designation of ``severe'' cases as well as rates of hospitalization and
ICU admission, all of which are more prevalent in older
individuals.\textsuperscript{15}

Another factor is cardiovascular disease (CVD). There is a high
prevalence of CVD amongst severe COVID cases and COVID cases overall,
with death rates of individuals with CVD reportedly over four times the
overall mortality rate.\textsuperscript{11} The leading hypothesis for
this is suggested to involve SARS-COV-2's interaction with
angiotensin-converting enzyme 2 (ACE2) and the
renin-angiotensin-aldosterone axis.~Chronic respiratory disease is also
linked with severe and fatal COVID-19
cases,\textsuperscript{15}potentially due to the decreased lung
pulmonary reserve, dysregulated immune system, and disrupted lung
microbiome in these patients.\textsuperscript{16,17} Diabetes is also a
significant risk factor for severe COVID-19 infection, potentially due
to similar mechanisms of immune dysregulation, alterations in ACE2
expression, and increased processing of the SARS-COV-2 spike
protein.\textsuperscript{18} In addition, it has been suggested that
common therapies for these conditions, such as angiotensin converting
enzyme inhibitors, angiotensin receptor blockers, and immunomodulators
like inhaled steroids have an effect on COVID-19
pathophysiology.\textsuperscript{19,20,21}

In designing therapeutic drug trials for COVID-19, we may have to
control for not only age, CVD, pulmonary disease, diabetes, and other
patient-level factors like immunodeficiency, but also control for the
therapies patients have been given.

2. Right Drug

Two approaches for developing scientific treatments for COVID-19 exist:

1) a ``bottom-up'' approach by repurposing already approved drugs or
molecules under clinical development for other indications

2) a ``top-down'' approach targeting at new molecules and vaccines
specifically designed for SARS-COV-2 which can be time-consuming, but
more effective and safer.

COVID-19 is caused by SARS-CoV-2 that is a single-stranded
positive-sense RNA virus.\textsuperscript{22} Since SARS-CoV-2 is a
newly discovered pathogen, no specific drugs are currently available.
Several existing drugs and new drugs that have potential therapeutic
effects are summarized in Table 1. These therapeutics are divided into 4
categories: convalescent plasma or immunoglobulins; direct-acting
antiviral agents (DAA); host cell internalization protein blockers; and
anti-inflammatory drugs.

Convalescent Plasma or immunoglobulin fractions have been used for
treatment of Ebola virus, Middle East respiratory syndrome coronavirus
(MERS-CoV), and SARS-CoV infections.\textsuperscript{23,24,25} A
clinical study of 5 critically ill patients with COVID-19 and acute
respiratory distress syndrome (ARDS) showed that administration of
convalescent plasma containing neutralizing antibodies improved some
patient's clinical status.\textsuperscript{26} Issues associated with
this approach include donner identification, elimination of residual
SARS-CoV-2 risk, off-target immunoglobulin binding, and dose estimation.
Of special concern is the ability to scale this approach to the level
required.

The potential targets of DAA against non-structural proteins include
RNA-dependent RNA polymerase (RdRp), coronavirus main protease (3CLpro),
and papain-like protease (PLpro). A potential target for a DAA against a
viral structural protein was the glycosylated spike (S)
protein.\textsuperscript{27} S protein mediates SARS-CoV-2 entry via
binding to angiotensin-converting enzyme 2 (ACE2) located on the surface
membrane of host cells following which host cell produced transmembrane
protease serine 2 (TMPRSS2) was involved in S protein priming that
facilitates the process of internalization.\textsuperscript{28} Thus,
both host cell membrane proteins ACE2 and TMPRSS2 have become potential
therapeutic targets. In addition, some recent studies also reported that
S protein can also bind to the host cell receptor
CD147\textsuperscript{29} and the glucose regulated protein 78
(GRP78)\textsuperscript{30} that may also mediate internalization.

The protease inhibitor combination lopinavir and ritonavir (Aluvia\selectlanguage{ngerman}®),
and two viral polymerase inhibitors, favipiravir and remdesivir, are
non-structural SARS-CoV-2 protein targets currently in clinical trials.
Galidesivir (BCX4430) is an adenine analogue RdRp inhibitor originally
developed for the treatment of hepatitis C virus. It is currently
undergoing safety testing in early clinical studies and is being
evaluated for its efficacy in treating yellow fever. In preclinical
studies, it exhibits activity against a variety of RNA viruses,
including SARS and MERS.

Both SARS-CoV, responsible for severe acute respiratory syndrome (SARS),
and SARS-CoV-2, responsible for COVID-19, are beta-coronaviruses (CoV)
that share a structurally similar spike glycoprotein (S) complex
surrounding the spherical viral particle that is comprised of a receptor
binding domain (RBD) S1 subunit and a membrane fusion S2
subunit.\textsuperscript{31} The S proteins of SARS-CoV-2 have about
76\% homology to those of SARS-CoV and both recognize and bind to
ACE2.\textsuperscript{31}

While Abidol is mainly used for the prevention and treatment of
influenza virus infections, it may disrupt the binding of S proteins to
ACE2 to prevent viral internalization and is currently in clinical
trials (see Table 1). Soluble recombinant human angiotensin converting
enzyme-2 (srhACE2) was initially proposed as a treatment for general
ARDS but its affinity for the SARS-CoV-2 spike protein could enable a
neutralization with the virus preventing the viral internalization. It
is possible for shrACE2 to treat COVID-19 through a combination of
preventing lung injury by reducing local angiotensin-II levels and
preventing lung epithelial internalization of the virus.

Nafamostat and Camostat are two TMPRSS2 inhibitors with similar
molecular structure that could block the virus internalization process.
Camostat is currently in clinical trials for COVID-19 Infection (see
Table 1).

Hydroxychloroquine (HCQ) / chloroquine (CQ) are anti-malarial drugs used
for treating Lupus and forms of arthritis. HCQ is a derivative of CQ
with therapeutic effects similar to those of CQ, but with reduced toxic
side effects. HCQ/ CQ can inhibit the in vitro replication of several
coronaviruses. Recent publications support the hypothesis that CQ can
improve the clinical outcome of patients infected by SARS-CoV-2. CQ may
interfere with ACE2 receptor glycosylation to inhibit SARS-CoV-2 binding
to target cells. It may also inhibit cleavage of S proteins by
acidifying lysosomes and may inhibit cathepsin activity. HCQ activates
CD8 + T-cells that reduces the production of pro-inflammatory
cytokines\textsuperscript{32,33,34} to limit lung inflammation.

SARS-CoV-2 may cause the rapid release of inflammatory cytokines
resulting in ARDS and multiple organ failure. Anti-inflammatory drugs
can relieve this response. IL-6, CCR5 and JAK kinase are potential
targets for relieving SARS-CoV-2 caused
inflammation.\textsuperscript{35} Tocilizumab
(ACTEMRA\textsuperscript{®}) is a recombinant human monoclonal antibody
that specifically binds to soluble and membrane-bound IL-6 receptors
(sIL-6R and mIL-6R) and inhibits inflammatory IL-6-mediated signal
transduction. CytoDyn's humanized CCR5 antagonist leronlimab (PRO140)
has applied for a phase II clinical trial for adult patients with mild
to moderate respiratory disease after infection with SARS-CoV-2. The JAK
kinase inhibitors baricitinib and ruxolitinib can inhibit JAK-mediated
inflammatory processes and are currently in clinical trials for COVID-19
(see Table 1).

3. Right dosage

\textbf{New drug development paradigm} : Dose is an important issue for
antiviral drug development requiring years of work. The process starts
at the interface between preclinical and Phase I when the first in human
dose is calculated. Under today's model-based drug development paradigm,
this exercise is supported by translational PK-PD modelling using
available data of nonclinical PK, drug metabolism and toxicology. Phase
I dose escalation trials will then be conducted to examine safety,
tolerability and PK in reference of in vitro susceptibility and in vivo
animal data. Phase II studies will be further conducted in target
patients to achieve goals of PoC and dose ranging using biomarkers for
both efficacy such as viral load and safety. Dosage for Phase III trial
will be optimized based on a target PK/PD metric (e.g.,
C\textsubscript{min,ss}/EC90) and population PK of the drug. In
addition, experimental medicine studies will be conducted to study
tissue penetration, and a series of clinical pharmacology studies will
be conducted to examine intrinsic and extrinsic factors including age,
body weight, the stage of the disease, the presence of co-morbidities,
patients' use of other medications for the purposes of adjusting dose in
special populations.

\textbf{Dose estimation for repurposed drugs} : Drug development for
COVID-19, thus far, is dependent on the indication extension of
existing, approved drugs. These anti-viral drugs must have been through
the full pre-clinical discovery and clinical development process with
extensive pharmacological, PK, toxicology and manufacturing work done.
It is essential that drug candidates for repurposing must have shown
efficacy for their original intended indication such that regulatory
approval is achieved or fully expected. Absent these requirements
additional exploratory development work will be required prior to
conduct of a pivotal efficacy trial for the new indication (e.g.
COVID-19).

The only information that should be needed to extend an existing drug
for use to the new indication is the characterization of its target
interaction as nothing else should change. If the new viral target
attacks a different tissue, then additional tissue distribution work may
be required.

The need to characterize an existing drug's interaction at a different
target in the same tissue such as the RdRp for SARS-CoV-2 versus that of
another RNA virus in the lung can be done at the \emph{in vitro} level.
If the EC50 value in this case is different, as it is more likely that
the drug will be less potent, then a proportional dose adjustment is a
useful approximation. In this case, a much greater effort must be made
to estimate the effect of increasing the dose for the new indication. Do
the human PK findings support a simple proportional dose adjustment? Do
the safety findings support a higher exposure level? Small differences
in potency make it likely that the drug can be used for the new
indication but larger ones may well preclude this use. This is
especially true if significant new clinical development efforts must be
made that will substantially increase the time required to get the drug
into patients. Assuming that the outcome supports repurposing the
existing drug for the new indication it still must be tested in an
adequately powered clinical trial for an appropriate patient population.

In COVID-19 drug trials, we found only two trials intended to explore
different dosage regimens, Gilead remdesivir trials examine treatment
durations of 10 days vs 5 days {[}NCT04292730, NCT04292899{]}, and the
PrEP trial by Washington University School of Medicine on HCQ employ
low, mid and high dose regimens {[}NCT04333732{]}. None tried a dosage
higher than its approved level.

Clinical investigators should collaborate closely with the innovator
company to understand the PK, PD, biomarker and safety of drugs
repurposed for the new indication and propose a dosage with highest
possibility of suppressing the virus within the exposure range known to
be safe. In most cases, safety data for levels much higher the than
approved dose may be present in the development data file (i.e., single
or multiple ascending dose studies). However, there can be no
justification for pushing the clinical dose beyond the drug's known
therapeutic window.

The \emph{in vitro-in vivo} translation of antiviral efficacy should be
considered throughout the development process. There are many
considerations when using \emph{in vitro} derived EC50 or EC90 to
predict an efficacious dose \emph{in vivo} . For typical drugs, the
free-drug hypothesis can be used to translate \emph{in vitro} EC values
to \emph{in vivo} efficacious dose based on the theory that unbound drug
in the circulation equilibrates with that at the target site under
steady-state conditions. However, differences in metabolic activity,
drug permeability or transporter expression between the model cell line
and target tissue or organ can result in prediction inaccuracy. In
addition, special attention should be paid for an anti-viral prodrug, as
its active moiety is a metabolite which only stays intracellularly
(e.g., remdesivir).\textsuperscript{36,37} An investigator using a
cultured cell model needs to convince themselves that any such
differences were characterized during the model development and can be
accounted for.

Remdesivir and CQ were found to be relatively more potent (EC50 reported
to be 0.77 and 1.13 \selectlanguage{greek}μ\selectlanguage{english}M, respectively) compared to ribavirin,
penciclovir, nitazoxanide, nafamostat and favipiravir against a clinical
isolate of SARS-CoV-2 in vitro.\textsuperscript{36} This study also
demonstrated that remdesivir functioned at a stage after virus entry
while CQ functioned at both entry, and at post entry stages of the
SARS-CoV-2 infection in Vero E6 cells.\textsuperscript{36} In another
study, the EC50 of HCQ is reported to be 0.72 \selectlanguage{greek}µ\selectlanguage{english}M.\textsuperscript{38}

Remdisivir is an example of a proposed ``repurposed drug'' that appears
not to meet the key standard on clinical efficacy described previously.
An efficacy trial for Ebola where the drug, when combined with the
triple monoclonal antibody ZMapp, failed to show efficacy relative to
ZMapp alone.\textsuperscript{39} Remdisivir is active against SARS-CoV
and MERS-CoV replication in cell models and shows efficacy against
SARS-CoV infection in carboxylesterase 1c knockout
mice\textsuperscript{37} and MERS-CoV infection in rhesus
monkeys\textsuperscript{40}.

CQ and HCQ act at both viral entry and post entry stages so that their
in vitro and in vivo translation can be complicated by major differences
in their lung to plasma concentration ratios. Both CQ and HCQ are highly
distributed to the lung in male CD albino rats\textsuperscript{41} and
HCQ\textsuperscript{42}.

For srhACE2 to occupy viral S protein it must reach the interior of the
lung where the virus is located. To block infection srhACE2 must be
delivered to the site of infection in both the upper and lower
respiratory track areas by inhalation therapy in an adequate amount to
occupy S protein sites on the virus. Intravenous administration is
unlikely to block infection since there is little virus in the blood but
might serve to reduce lung injury though this was not demonstrated in
ARDS patients.

4. Right timing

Timing is important to the administration of therapy during the COVID-19
disease course. While our understanding of the natural history of severe
COVID-19 infection is incomplete, it is thought to progress from
invasion of the respiratory tract and gastrointestinal mucosa, to
dysregulation of the RAAS and immune systems, systemic spread to other
organs, and then finally cytokine storm, sepsis, and acute respiratory
distress syndrome.\textsuperscript{43}

COVID-19 is best treated in its early stages when viral load is low and
host physiology is relatively unperturbed. However, this is complicated
by COVID-19's relatively mild onset, with many patients being
asymptomatic. A study showed that virus shedding was very high during
the first week of symptoms (peak at 7.11 \selectlanguage{ngerman}× 10\textsuperscript{8} RNA
copies per throat swab, day 4).\textsuperscript{44} Furthermore, once
the disease has progressed to its end-stage and organ failure has
occurred, the number of specific therapies available dwindles as drivers
of pathology shift from direct viral effects to secondary systemic
dysregulation of host physiology. Given that most of the trial drugs
currently being studied act on the Sars-Cov-2 viral lifecycle, rather
than the downstream manifestations of COVID-19, these medications may
not have an effect later in illness. This can be seen in the treatment
of influenza, where the anti-viral oseltamivir is only effective if
given in the first 48 hours.\textsuperscript{45} After this period,
there are no specific therapies that have shown benefit, and supportive
care is the standard. Thus, it is important to define patients' disease
progression in clinical trials, as certain drugs may have different
effects when given at different times.

\textbf{Trial analysis ~}

We focused our analysis on the most representative ones including HCQ,
remdesivir, lopinavir/ritonavir, favipiravir~and tocilizumab with the
intent that this will clarify inconclusive results from inadequate
designs and guide potential future strategies.

1) hydroxychloroquine (HCQ)~trials

A search (until April 7th, 2020) was performed on clinical trial
registries of privately and publicly funded clinical trials
(https://clinicaltrials.gov/) focusing on HCQ and COVID-19. A total of
44 registered clinical trials of HCQ were retrieved. Thirty (30) of
these were therapeutic trials for active infection and 21 were
prophylactic trials, with several studies having multi-purpose designs.
The prophylactic trials were separated into 15 pre-exposure (PrEP) and 6
post-exposure prophylaxis (PEP) trials. These trials have exploded in
number over the past month (see Figure. 3), especially in preventative
trials.

CQ was first discovered to have potential activity against COVID-19 in
China\textsuperscript{46} and Chinese experts recommended CQ to patients
diagnosed as mild, moderate and severe cases of COVID-19
pneumonia\textsuperscript{47}. A dose of 500 mg bid for 10 days is
recommended for chloroquine phosphate. HCQ has emerged as a drug of
interest due to its relatively good clinical safety. HCQ has had some
promising preliminary results. Compared to standard care, HCQ treatment
has been associated with increased symptomatic pneumonia improvement
(80.6\% vs. 54.8\%)\textsuperscript{4} and higher levels of viral
clearance (70.0\% vs 12.5\%)\textsuperscript{48} when compared to the
standard of care, though only in relatively small studies. On the other
hand, HCQ has not shown an effect in all studies, with Chen J et al.
reporting no difference in HCQ-treated and control group patients' viral
clearance\textsuperscript{49}. These findings are summarized in Table 3.

HCQ is also being studied in prophylactic trials, both pre- and post-
viral exposure with a much larger sample size in the former (see Table
4). Most of these use a regimen of 800 mg of HCQ sulfate for day 1 and
400 mg daily for the rest of the trial. Median trial duration is 7 days.
These trials are still on-going, but it should be noted that in
pre-exposure studies, the population is mostly healthcare workers who
likely have a disproportionately high amount of viral exposure, versus a
more general lay population in post-exposure trials. Thus, some of these
trials may not be generalizable to a larger population.

Due to the high transmission of SARS-Cov-2, close contacts in household
setting (\textasciitilde{}15\%) and other high-risk close contacts
(\textasciitilde{}10\%) have a high secondary infection
rate.\textsuperscript{51} A preventive strategy using antiviral drugs is
being adopted aimed at reducing transmission and hospitalization. As an
existing drug, HCQ has the advantages of sufficient drug stockpile, good
drug safety and economic cost, which provide the rationale that HCQ is
being widely used.\textsuperscript{52}

In treatment or PEP purpose studies, the most common dosage regimen in
these trials is 800 mg for the first day + 400 mg daily for maintenance
dosing (HCQ sulfate), where the dose interval is mostly twice a day. In
the treatment purpose study, the median treatment period was 7 days
(range: 5-14 days); which is longer than the median treatment period in
the PEP study (median: 5 days, range: 3-11 days). For PrEP study,
designs mainly focused on healthcare workers who are at potential risk
to exposure to the COVID-19 from patients and HCQ is given over a longer
treatment duration by daily regimen or regimens with longer than 24 hour
dosing interval.

Although these large scale prevention trials should only be conducted
after clinical confirmation of anti-viral efficacy by HCQ, their data,
if released, will confirm the efficacy and safety of HCQ against the
COVID-19.

Remdesivir trials

Remdesivir was originally developed by Gilead Science for Ebola virus
infections \textsuperscript{39} and is still under investigation. After
a case report of a severe case of COVID-19 in the US being cured by
remdesivir\textsuperscript{53}, a number of clinical trials on
remdesivir started worldwide.

These trials include the Chinese studies (NCT04252664, NCT04257656)
initiated by Dr. Cao Bin, the global multicenter study initiated by
NIAID in the US (NCT04280705), the solidarity study initiated by WHO
(remdesivir as a group, NCT04330690, NCT04321616) and 2 studies
initiated by Gilead Science (NCT04292730, NCT04292899). More details of
these trials are summarized in Table 2. Since there is no documented
clinical data so far, it is still unknown whether remdesivir is
effective for COVID-19.

Remdesivir is an adenosine analogue that can incorporate into the
nascent viral RNA chains and cause the termination of viral
replication.\textsuperscript{54} In a mouse model of SARS-CoV,
remdesivir significantly reduced lung viral load, improved pulmonary
function, and reduced clinical disease.\textsuperscript{37} According to
previous reports, remdesivir is a potent inhibitor of SARS-CoV-2
replication in infected cells suggesting potential therapeutic activity
against COVID-19.\textsuperscript{36}

The treatment protocol for these trials is 200 mg IV remdesivir on Day
1, followed by 100 mg IV remdesivir as a daily maintenance dose for the
duration of hospitalization, up to 5 or 10 days maximum, depending on
the trial. This is the dose of remdesivir used to treat Ebola virus.
However, this may not be an optimized dose for treatment of COVID-19.
Early studies on remdesivir's action on COVID-19 report a much higher
EC50 and EC90 (0.77 and 1.76 \selectlanguage{greek}μ\selectlanguage{english}M) than that for remdesivir's action
against Ebola (EC50 [?]0.1 \selectlanguage{greek}μ\selectlanguage{english}M). It should also be noted that the Ebola
remdesivir trials from which this dosing regimen was borrowed showed
negative results.\textsuperscript{39}

These trials also use different dose timings, using both symptomatic
disease and RT-PCR to define their therapeutic windows. For example, the
Chinese trials give doses within 8 days of symptom onset for mild and
moderate cases, or 12 days for severe cases while the NIAID trial uses a
window of 3 days after PCR confirmation. The Gilead trials use a window
of 4 days after PCR confirmation. It is unknown what therapeutic timing
window will be most effective.

These trials also study slightly different populations. Only NCT04252664
assesses remdesivir in patients with mild COVID-19. All other studies
are limited to moderate and severe cases.

A recent case series on 53 severe COVID-19 patients who received
remdesivir in the context of compassionate care shows some encouraging
results.\textsuperscript{55} 36 patients (68\%) had an improvement in
oxygen requirement or ventilatory support, 25 patients (47\%) were
stabilized and discharged, and 7 patients (13\%) died. Although this
trial was not conclusive and did not have a control group, historical
comparisons with general care or other clinical trial, such as 22\%
mortality overall in 201 In Wuhan, China\textsuperscript{56} and 22\%
mortality in a recent randomized, controlled trial of
lopinavir--ritonavir\textsuperscript{3}. The compassion-use data suggest
that remdesivir may have clinical benefit in patients with severe
COVID-19. This study is limited by the small size of the cohort, the
relatively short duration of 28 days follow-up, potential missing data,
and the lack of a randomized control group. However, these results are
encouraging and open the door for future randomized, placebo-controlled
trials of remdesivir therapy.

3) Lopinavir-ritonavir

Lopinavir-ritonavir is originally a medication used for HIV but has been
used for other RNA viruses including SARS-CoV. An open-label study
published in 2004 suggested, by comparison with a historical control
group that received only ribavirin, that the addition of
lopinavir--ritonavir (400 mg and 100 mg, respectively) to ribavirin
reduced viral load, incidence of acute respiratory distress syndrome,
and mortality among patients with SARS.\textsuperscript{57} These
results suggest potential activity for SARS-CoV-2 as well.

Compared to HCQ and remdesivir, fewer trials exist for
lopinavir-ritonavir treatment of COVID-19. In the largest trial of
lopinavir-ritonavir in 199 adult patients hospitalized for severe
COVID-19, there was no difference in time to clinical improvement
between patients given 400 mg lopinavir and 100 mg ritonavir twice daily
for 14 days compared to those given standard
care.\textsuperscript{3}Given these weak results, there has been less
interest in lopinavir-ritonavir as a treatment for COVID-19.

However, these discouraging results may partially be explained by
SARS-CoV-2's viral dynamics. The peak of SARS-CoV-2 viral load is around
the time of symptom onset\textsuperscript{58}, but the average time
between symptom onset and randomization in this study was 13 days. At
this time, the application of antiviral drugs may be too late to have
significant effect. Indeed, subgroup analysis showed a minor
acceleration of clinical recovery (16.0 days vs. 17.0 days) and a
reduction in mortality (19.0\% vs. 27.1\%) in those treated within 12
days of symptom onset. In addition, this trial did not assess effect at
different doses and there is no preliminary data to suggest that the
trial dose is effective for COVID-19. Finally, the trial was not blinded
and the authors acknowledge the potential for bias in the subjective
criteria used\textsuperscript{3}. Ultimately despite the negative
results on lopinavir-ritonavir for COVID-19, it is still uncertain
whether or not this drug combination is useful for COVID-19.

4) Favipiravir

Favipiravir is a purine nucleic acid analogue that is ribosylated and
phosphorylated intracellularly into its active metabolite, which may
interfere with viral replication by inhibiting RNA-dependent RNA
polymerase. The EC\textsubscript{50} of fapiravir against SARS-COV-2 in
vitro is 62 \selectlanguage{greek}μ\selectlanguage{english}M (9.7 mg/L)\textsuperscript{36}, which is much higher than
the EC\textsubscript{50} for influenza A and B viruses (0.014~0.55
mg/L). The steady-state peak concentration of fapiravir in human plasma
is 62 mg/L (600 mg, BID).\textsuperscript{59} Its serum protein binding
in the concentration range of 0.3\textasciitilde{}30 mg/L is 54 \% and
radio-tracer studies in monkeys showed that labeled fapiravir in the
lung was approximately 51\% of that in plasma at 0.5 h after dosing.
Assuming that the fapiravir EC\textsubscript{50} concentration in vitro
represents unbound drug and that the free concentration of fapiravir in
the lung is equal to that in plasma (i.e. about 46\%) then the free
concentration of fapiravir in lung is about 14.5 mg/L or about 50\%
greater than the EC\textsubscript{50} value.

An open-label, nonrandomized, controlled trial was conducted at the
Third People's Hospital of Shenzhen to evaluate favipiravir in patients
with COVID-19 pneumonia.\textsuperscript{60} Patients with mild or
moderate pneumonia within 7 days of onset were selected for this study,
and they were more appropriate subjects for antiviral drug trials
compared to the severe patients. A total of 80 patients with mild or
moderate pneumonia treated within 7 days of symptom onset were assigned
to the test group (35) to receive favipiravir (1600 mg twice daily on
Day 1 or 600 mg twice daily on Days 2-14 orally) or to the active
control group (45), to receive lopinavir-ritonavir (400 mg/100 mg twice
daily orally), respectively. In addition, all subjects received
aerosolized interferon-\selectlanguage{greek}α\selectlanguage{english}1b (60\selectlanguage{greek}μ\selectlanguage{english}g twice daily). Treatment continued until
viral clearance was confirmed or for 14 days. The results showed that
the favipiravir arm had shorter viral clearance time (median
(interquartile range, IQR), 4 (2.5 -- 9) d versus 11 (8 -- 13) d, P
\textless{} 0.001) and higher radiographic improvement rate (91.43\%
versus 62.22\%, P = 0.004) compared to the control arm. However, this
trial also has some limitations, such as the small number of enrolments
and like most trials, it is unblinded, which leads to inevitable
selection bias in patient recruitment.

5) Tocilizumab

Cytokine release syndrome (CRS) occurs in many patients with severe
COVID-19, which is also an important cause of death. Interleukin-6
(IL-6) plays an important role in CRS.\textsuperscript{61} As an IL-6
receptor blocker, tocilizumab has been included as an immunotherapeutic
agent for severe and critical patients in the ''Novel Coronavirus
Pneumonia Diagnosis and Treatment Protocol (Trial Seventh Edition)''. At
present, there are several registered clinical trials of tocilizumab in
the treatment of patients with COVID-19. Most trials have targeted
severe patients, and some trials include an increase of inflammatory
factors such as IL-6, CRP or ferritin as one of their inclusion
criteria.

\textbf{Summary}

There is an urgent need for effective anti-viral drugs to combat the
COVID-19 crisis. The identification and development of these drugs
depend on effective clinical trials. However, there are tremendous
challenges for the development of drug therapies under pandemic
conditions, even for drugs repurposed from other indications. Current
barriers that have limited previous trials include lack of
pathophysiologic understanding of disease, unoptimized dose, operational
and regulatory difficulties, poorly defined therapeutic timing windows,
broad non-specific outcome measures, non-randomized and underpowered
trial designs. As a result, though the knowledge previous trials have
provided has been invaluable, conclusive results are lacking. Despite
crisis conditions, we cannot forsake the need for well-designed clinical
trials. We present 3 key ideas, namely, adhering to translational
science principles, leveraging innovative trial design and implementing
4R concept to increase likelihood of trial success and to produce
high-quality data. Furthermore, we think it is particularly important to
establish Global Clinical Trial models (e.g., adaptive platform trials)
that can be implemented rapidly wherever a new pandemic breaks out.
Conducting such trials is certainly a tremendous undertaking that will
need significant resources and commitment. This will require
international cooperation and a shift in the global pharmaceutics
research landscape. Finally, we hope that this unprecedented pandemic
will lead to development of more robust infectious disease research
infrastructure and funding to help mitigate future pandemics.

\textbf{COMPETING INTERESTS}

Authors report no conflicts of interest.

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\end{enumerate}

\textbf{Hosted file}

\verb`20200415COVID19 trial version for submission tables.docx`  available at \url{https://authorea.com/users/312213/articles/442813-challenges-of-drug-development-during-the-covid-19-pandemic-key-considerations-for-clinical-trial-designs}

\textbf{Hosted file}

\verb`20200415COVID19 trial version for submission figures.docx`  available at \url{https://authorea.com/users/312213/articles/442813-challenges-of-drug-development-during-the-covid-19-pandemic-key-considerations-for-clinical-trial-designs}

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