Bruno Speder is head of clinical regulatory affairs, Clinical Research, SGS Life Science Services, Bruno.Speder@sgs.com.

Bruno Speder

	The novel coronavirus SARS-CoV-2 is the causative agent of COVID-19 respiratory disease, a largely upper respiratory tract disease that can also descend and cause bilateral pneumonia. Its spread has outpaced those of other major infectious outbreaks in the past, and this novel coronavirus epidemic was officially declared a pandemic by the World Health Organization on March 11, 2020. 

	While a first vaccine against this coronavirus could still take some time to develop, mRNA vaccine platforms could offer an early breakthrough, although there are some concerns regarding their efficacy. For antiviral drugs the focus currently lies on repurposing of existing drugs.

	SARS-CoV-2, which thrives in cooler temperate conditions (although this does not preclude it from being prevalent in some warmer climates), is likely to move in waves from one global region to another according to seasonality. The virus shows some similarities to the SARS-CoV-1 virus and COVID-19 disease might resemble influenza disease: on average 5 days incubation, followed by fever, myalgia and a dry cough, which may progress to shortness of breath and approximately 25% of patients who are symptomatic may require hospital treatment at this point-the elderly and those with co-morbidities such as heart/lung disease being particularly susceptible. The disease can then develop into bilateral pneumonia, with patients experiencing chest pains and shortness of breath, which may require supportive therapy i.e. a ventilator.

	Some of the drugs in development for the treatment of COVID-19 disease have existed for a long time, such as nelfinavir and lopinavir, and some of the protease inhibitors from HIV research are being repurposed. Some newer drugs are also of interest: remdesivir, which has been around since the original SARS-CoV and Ebola outbreaks, is being trialed, as is chloroquine. Most potential drug treatments are still in the early phase and work is being undertaken to see how effective these will be.

	In terms of vaccines, there are about 150 in development and some of these are already in Phase 2 development. The nature of clinical advancement, and whether human challenge trials are necessary, will depend on trial sponsors and regulators, although it is expected that a challenge trial may be the only way to prove vaccine efficacy when the pandemic slows, as there will not be sufficient cases to run meaningful Phase 3 trials.

	Challenge trials or controlled human infection modelling (CHIM), brings with it a number of ethical factors which may be-or appear to be-in conflict with the usual approach to medical or clinical intervention. SARS-CoV-2 needs to be grown under a biological safety level (BSL) of 3, much like tuberculosis or cultured HIV, and is a highly infectious organism that produces serious adverse events. Although the challenge model has been an evolutionary process over 200 years since the work of Edward Jenner in 1796, the last 10 years has seen great acceleration in its use. However, it is important to carry out challenges in line with the latest regulations and to test the newest strains or challenge agents.

	Such challenge studies have become increasingly acceptable in light of the practical and ethical issues surrounding animal studies. For SARS-CoV-2 testing, animal testing can be performed using ferrets, Syrian hamster or humanoid transgenic mice, but there might not always be a good correlation with human immune responses and may have poor predictability of therapeutic effects.

	Field studies are also quite difficult in that they can be prolonged because of the slow rate of recruitment of subjects, large required sample size and they can also be complex with very diverse populations with different associated morbidities and mortalities. It may also be problematic to produce a vaccine against a virus that only appears for a few months per year. Human challenge tests (HCTs), or controlled infection models, are potentially more ‘crisp’ because subjects can be infected directly, they can be cared for throughout the test period, and the tests are properly regulated and controlled. In addition, the challenge agent has been properly characterized and subjects are not exposed to a relatively unknown agent in the environment where many other types of co-infections occur.

	Considerations for HCTs include whether to use an attenuated strain of the virus or a homologous strain, and the different attachment pathways also need to be taken into consideration. As well as the ethical issues around using a wild-type strain of the virus, there is also the question of whether to select populations that have low risk factors and screen for the severity of disease. Additionally, the titre to be used in testing needs to be considered, as does the route of inoculation: nasal dosing may be better as oral dosing may give a higher risk of adverse pulmonary effects. These questions, along with many others will need to be discussed with regulators, with the hope that they will be able to give appropriate guidance and advice.

	Development of a SARS-CoV-2 human challenge agent

	The development of a human challenge agent has parallels with that of a drug or vaccine. The agent is first manufactured in compliance with Good Manufacturing Practice (GMP), undergoes non-clinical testing under Good Laboratory Practice (GLP) conditions and then moves into the clinical development phase, in the form a First-in-Human titration study. A human challenge agent is considered by both the EMA and the FDA to be a medicinal product and requires an IND or Clinical Trial Authorization (CTA) application before it can be used in the clinic. One major difference in its development from that of drugs or vaccines is that to date, there is no formal licensing procedure under the form of a marketing authorization application (MAA) or biologics license application (BLA) 

	For the development of a COVID-19 challenge agent, the first point to note is that there are no specific guidelines for developing challenge agents, the main reason being that the development process for a challenge agent is very specific and is highly dependent on the type of agent in question, for example, whether it is a parasite, a bacterium or a virus, all of which need to be developed in different ways.

	With SARS-CoV-2 being a BSL3 organism, this could cause logistical and strategic problems when manufacturing a human challenge agent. The manufacturing process and actual testing needs to be carried out in suitable facilities; therefore, one regulatory discussion would be whether an attenuated challenge agent could be used and considered as a BSL2 organism. This means that the GMP development strategy for the agent is linked to the scientific strategy of the whole program, as the choice of challenge agent could influence the practical execution of the overall program strategy.

	As previously stated, there are no overarching guidelines for the manufacture of human challenge agents but usually vaccine manufacturing guidelines can be used as a basis for their manufacture. A combination of FDA and EMA guidelines should be looked at to ensure the development of a robust manufacturing process that complies with regulations in both the US and Europe. Based on scientific and manufacturing considerations, the challenge agent would be manufactured as a solution for intranasal delivery only and this would need to be discussed with the regulatory authorities to determine what the best approach would be, keeping the safety of the volunteers participating in the study in mind.

	A second important part of the manufacturing strategy for a COVID-19 challenge agent would be the inclusion of an adventitious agent testing program based on existing vaccine guidelines, but here a more risk-based approach could be considered. This would need to be discussed with the authorities for each case but for COVID-19 this could be based on guidance issued by both the FDA and the EMA for the development of therapeutic vaccines for COVID-19 where some of the development principles could be applicable to a human challenge agent.

	Once the agent has been manufactured and a complete testing program completed, in the EU the agent will need to be delivered by a Qualified Person (QP) before use and the manufacturing sites will require an updated GMP manufacturing license. In addition, all the Chemistry, Manufacturing and Controls (CMC) information for the agent will need to be described in the IND CMC section.

	Once any new human challenge agent moved into the manufacturing stage, non-clinical development can begin, which takes place in two parts: an 

cell-based characterization assay to show the 

infectivity properties of the agent; followed by an 

characterization study carried out in an animal model (in the case of SARS-CoV-2 this would be ferret) in compliance with GLP.

	Such a study would be negatively- and positively-controlled and the aim of both the 

studies would be to demonstrate the effectivity and infectivity of such a COVID-19 strain before moving to a FIH titration study. The starting dose for the titration would be based on the data from the animal studies, as would be the case for any other medicinal product, and the non-clinical development data would also form the basis for the design of this FIH study.

	There are also a number of non-clinical development human challenge agent areas that could be more condensed in comparison with traditional drug development, for example pharmacokinetic and product metabolism studies are not usually required for human challenge agents and no formal toxicology studies are expected for this type of agent either, except if the virus is overly virulent or pathogenic, in which case the regulatory authorities may require this type of test to be carried out.

	All the preclinical study results need to be described in the Investigators Brochure (IB) and the IND application dossier. Once this has been completed, there will need to be some regulatory maintenance taking into account the fact that there are no license requirements for human challenge agents but that there is the requirement to maintain an up-to-date Investigational Medicinal Product Dossier CMC file. This should include an ongoing stability results update from a GMP stability results program following a classical 3-month, 6-month, 12-month reporting approach, or with an update each time the agent is used in a challenge study. Updated neutralization assay results will also need to be reported.

	Human challenge unit performance requirements

	Assuming that a BLS2 safety level is required, any human challenge unit undertaking a study into SARS-Cov-2 would need to hold a BSL2 permit and be equipped with BSL2-compliant beds, an airlock/HEPA filtered negative-pressure system and a dedicated BSL2 laboratory. All procedures would need to be discussed with the relevant regulatory authorities and are highly dependent on the nature of the challenge agent.

	Other important regulatory aspects of the study that need to be taken into consideration are the pre-screening of volunteers and the shedding of the virus in the community. Volunteers would need to be pre-screened using an accredited and validated serology screening system to avoid the dangers of shedding of virus in the community so length of stay of volunteers in the human challenge unit needs to be confirmed based on real-life data and data from animal studies. Any subject discharge would be predicated on achieving a negative antigen test result.

	Clinical development of a SARS-CoV-2 human challenge agent

	Due to the still-limited knowledge of the number of subjects infected with COVID-19, the mortality of the infection is difficult to predict but is estimated to be about 1-2%. Symptomatology on its own is insufficient for differentiating SARS-CoV-2 from other pathogens such as influenza. Case isolation and contact tracing are essential in order to slow down the spread of infection and therefore definite diagnosis is required. The lack of a ‘gold standard’ test might lead to delayed diagnosis. The sensitivity of current PCR tests is estimated to be around 65-70% and is dependent on sampling techniques. 

	Chest CT scanning offers higher sensitivity in patients with symptomatic disease for more than 48 hours and makes it possible to diagnose patients that previously had a false negative PCR result, however radiographic signs are aspecific and make it very hard to distinguish between COVID-pneumonia and other viral pulmonary diseases such as influenza pneumonia. 

	For a potential COVID-19 challenge trial, as with every trial, subject safety is essential and starts with the identification of the right population to enter such a trial. During the trial, correct and sensitive markers are required to follow a subject’s health, as are adequate treatment options including the availability of intensive care units and trained staff. Next to subject safety, it is equally important to consider the safety of the clinical trial staff and, by extension, society as a whole. The affected subjects will need to be adequately quarantined and clinical site staff will need to have access to appropriate PPE in order to make it impossible for the virus to infect outside the quarantine area. 

	When evaluating trial populations, whom not to include is a very important question. Age, obesity (BMI) and smoking are known risk factors, while common morbidities associated with bad outcomes include diabetes, cerebrovascular disease and cardiovascular diseases such as hypertension, chronic kidney disease, and COPD. Immuno-compromised subjects should not be included in a trial. Children seem to be less affected, possibly due to receptor differences or immune imprinting from other viruses, while no increased fatality has been seen in pregnant women.

	Stopping rules for these types of trials need to be carefully formulated based on scientific and medical rationale and preferably are unambiguous and based on clinical parameters which are quick and easy to obtain such as pulmonary function, oxygen saturation, temperature, spirometry, physical examination and the need for supportive care. As there are still many unknowns about the disease and its progression, it is essential to be very careful when following up on adverse events and to be very attentive in noting serious adverse reactions. Laboratory findings and CT results can help investigators make decisions on severity and the follow-up of the evolution. There has been much speculation about the use of off-label treatment options, but it is still too early to say whether there is strong evidence for their efficacy. The fact that there is currently no rescue medication available is a significant hurdle when trying to plan a COVID-19 challenge trial. In addition, reports about medications that might harm patients are confusing the issue and no scientific consensus has been reached.

	In conclusion, as this is a droplet infection, adequate containment of the challenge agent is required. Regarding PPE, European FFP2 and FFP3 and US N95 respiratory protective masks can filter out 95% of particles when used correctly. These are the best available at present for use in high-risk procedures that create some type of aerosol, such as intubation or nebulization. However, a large meta-analysis that has been conducted did not find any important benefit over general surgical masks in performing low-risk tasks such as transporting a patient or obtaining a blood pressure.

 is the Head of Clinical Regulatory Affairs & Consultancy, both of

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A first vaccine against this coronavirus could still take some time to develop, but mRNA vaccine platforms could offer an early breakthrough.

Regulatory Considerations Surrounding Human Challenge Studies with the SARS-CoV-2 Virus

	The concept of the human challenge model (HCM) originated from early societal attempts to halt the spread of acute diseases within their own communities. Small pox, for example, was endemic to Eurasia for 600 years with high mortality and morbidity rates and was considered one of the most common barriers to survival into adulthood. It was discovered that by using naturally attenuated material (

 minor) inoculated directly into susceptible or naïve populations (e.g. young children and unexposed individuals living in endemic areas) that death or severe disability, such as blindness or deafness, could drop to as low as 1%-2%, as opposed to 30% where the virus was contracted naturally.

	The Chinese are recorded as using such techniques in 900-1000AD,

 but it is likely that the practice actually spread to the West from India via Mamelukes trading in and around Egypt and the Mediterranean sometime in the 16

 century. Following the successes of Jesty and Jenner’s experiments in the UK, true vaccination utilizing cowpox (

 moved the human challenge model into a different era. Instead of challenge with an attenuated 

, a related virus was used to safely induce immunity. This provided a model for the deliberate inoculation of individuals with live virus for the purpose of healthcare. The practice of human challenge was further developed throughout the 18

 centuries although a series of unethical studies using gonorrhoea, syphilis and a number of other diseases in un-consented subjects (slaves, prostitutes and prisoners) damaged the perception of human challenge as a standard method for gathering data on disease progression and the efficacy of therapeutic interventions.

 century, interest was reviving in the potential of challenge models to offer insight into a number of acute, epidemic diseases. Viruses were an obvious target after the discovery of penicillin, which had cured most of the bacterial infections and DDT that had interrupted the malarial cycle in equatorial regions.

	On the back of this renewed scientific optimism, the UK Medical Research Council (MRC) founded a research facility at Porton Down in 1948 to investigate treatments for the common cold (the Common Cold Research Unit). The unit challenged volunteers with rhinoviruses until the 1980s when it was closed and research moved away, along with the leading scientists, into the private sector. The unit had, however, stimulated other researchers, especially in the US, to emulate the HCM and similar units were set up to challenge volunteers with live agents for malaria, cholera and gastroenteritis (norovirus).

	HCM trials are now being recognized as a route to the accelerated development of novel therapeutic agents. Although originally used in an academic setting, viral challenge studies have progressed since the 1990s to direct application in early phase clinical development studies by both big pharma and small biotechs alike. Phase Ib to Phase IIa viral challenge trials may be used to provide early performance indicators (efficacy), Proof of Concept (PoC), Mode of Action (MoA) and limited safety data and assist in the up / down selection of vaccine and drug candidates. Such relatively low-cost early phase trials provide information that may also be applied in the design of late phase field trials and ultimately inform go/no-go decisions. Viral challenge trials may also be used to provide insights into disease mechanisms, including the up-regulation/expression of host genes; the role of host receptors and viral entry mechanisms; as well as identifying new targets for novel compounds.

	Clinical trials using a live challenge agent have specific infection control requirements. Challenge trial design differs from classic clinical trials in two main aspects: a) the timing and duration of subject isolation (challenge agent dependent) and; b) the timing of interventions (therapeutic dependent). Approaches and methodologies are described in a number of papers and the FDA has also published a guidance document that outlines the principles that underpin challenge studies.

	The HCM has the advantage of measuring Proof of Efficacy (PoE) in human subjects during a defined period of infection with known timepoints for initiation and resolution and scheduled interventional and sampling events. The challenge agent utilized in such trials should be as close as possible to circulating strains so as to emulate the naturally observed pathology and symptomologies associated with wild type infections.

	Data gathered during HCM trials may be dose-defining or dose-timing specific. Therapeutics may fail if an intervention is initiated too early or too late in the course of an infection. Candidate drugs and vaccines may also fail due to adverse events, inferiority, lack of efficacy measures, or hypersensitivity concerns (anti-drug or anti-vaccine antibodies). The HCM may complement traditional early phase studies and act as a bridge to field trials by providing a cost-effective reality check and a window within which to modify candidate therapeutics prior to their application in late phase studies with greater numbers of subjects and the associated increased costs and timelines.

	For HCM studies, the selection of an appropriate challenge agent is essential if data is to be comparable between trials of drug or vaccine candidates. Challenge agents should be capable of inducing disease in healthy adults, have a known route of transmission as well as a well-defined disease progression (timing and severity of signs and symptoms). Although there is no definitive regulation governing the production of challenge agents, it is both accepted and expected that agents are manufactured according to current Good Manufacturing Process guidelines and that, following manufacture, the stock is subject to adventitious testing for purity, studies in animals regarding toxicology and a characterization or titration study in humans prior to authorization for use in a clinical trial. Before selecting a virus for use as a challenge agent, it is recommended to first discuss the approach to selection and manufacture with regulatory authorities via a Scientific and Technical Advice group in Europe or a pre-IND meeting in the US.

	Clinical Trial Authorization (Europe) and Investigational New Drug (US) approvals for use of challenge agents are dependent on the demonstration of efficacy, consistency (quality) and the absence of Serious Adverse Events or long-term sequelae.

	The quality of the challenge agent is only one consideration for the HCM. In a typical influenza challenge trial, subject characteristics are also important, including the immune state of volunteers prior to inoculation (determined during the screening process). Subjects must be naïve to the challenge agent to assure a high attack-rate. To ensure the challenge agent is contained within the Human Challenge Unit (HCU), high levels of infection control are required. Subjects should be quarantined prior to inoculation as well as during the study to not only contain challenge agents but also to reduce the chance of any unplanned transmission of pathogens that may enter the unit. Concomitant or super-infections may confound symptomology and immunity data and invalidate a trial.

	During the course of a HCS, viral titres may be measured to assess viral fitness in the presence or absence of drug (RT-PCR or TCID50). Other measures may include vAUC, seroconversion rates and resistance associated mutations in the viral genome. Subject fitness may be assessed via safety bloods, serum cytokine levels, physical examinations and via the use of a Symptom Scorecard – this would be additional to the objective reporting of adverse events by the study physician. Once volunteers are discharged from the study they may be followed up for a period of time to ensure neither the challenge agent nor the treatment has resulted in persistent physiological damage.

	Influenza challenge models were an early application of the HCM theory. Owing to the pandemic nature of the seasonal influenza outbreaks and the potential for the rapid, global spread of strains with high mortality rates, the establishment of aetiologies, estimation of the relative pathogenicity of circulating strains and the efficacy of candidate vaccines (live and inactivated) needed to be rapidly assessed by authorities. HCMs offer early PK/PD dynamics and proven correlates of protection: measurable markers relating subject fitness to viral fitness.

	The development of such correlates has been essential to the validation of the HCM; investigating concepts related to immunity and protection (blocking, neutralizing and eradication mechanisms).  As the model refines the understanding of humoral and cell-mediated responses, broad symptomologies may assume lesser importance compared to the dynamics of the infection cycle itself (entry and replication rather than down-stream effects).

	Although the HCM has been used primarily as a PoC and dose ranging tool,  it may also be used for more commercial purposes e.g. to fine-tune the primary indication for which it is intended to bring a product onto the market or using early PoC data to raise funding from investors for further development of a product.Some advantages of the HCM over traditional models are shown in Table 1.

Table 1. Advantages of HCM over traditional models.

	Efficiencies (e.g. low cost) of the HCM derive from the certainties inherent in its design. The high attack rate (low subject numbers), well-defined conditions and a uniform cohort reduce noise or complexity compared to a traditional 300+ subject EP field trial.  Within the HCM community there are efforts to further standardize procedures and it may become a requirement that guidelines currently being discussed by opinion leaders, within organizations such as the IABS, FDA and NIH, form an agreed template for future challenge trials. 

	The role of HCM in drug development is gathering momentum in certain therapeutic areas. In the field of respiratory illnesses, especially severe, acute viral disease, it has provided accelerated programs for a number of approaches. The National Institute of Allergies and Infectious Diseases (NIAID), Biomedical Advanced Research and Development Authority (BARDA) and the National Institute for Health (NIH) are showing interest in pooling knowledge with collaborative partners to further promote the HCM. The HCM is also evolving to encompass a wider range of challenge agents and it is possible that bacterial and parasitic agents will be manufactured to cGMP in the near future.

	From discussions with the Regulatory Authorities, it is evident that there is still a little distance to travel until human challenge studies are accepted as the norm rather than an exception in the development of certain therapeutics. However, the HCM may potentially contribute substantially to the Marketing Authorization Application process, including acting as pivotal studies; replacing traditional Phase III efficacy studies. This may occur where the cohort to be challenged very closely resembles the target population in a New Drug Application.

	However to achieve recognition as a late phase study, some of the issues highlighted by the authorities, including the limited size of the safety database, may need to be addressed by increasing the size and diversity of the HCM cohort, applying less stringent inclusion/exclusion criteria to better mimic natural populations, or by potentially combining HCM/ PoC trials with larger safety studies.

	In the field of drug discovery, the HCM has already played a pivotal role in discovery in academia (see: Walter Reed Army Institute of Research and dengue fever).

A wide range of organisms are currently being investigated globally in both resource-rich and resource-poor settings through simple challenge studies using only filtered, non-cGMP agents. It remains unlikely that such studies will progress to late phase through the regulated HCM route if the use of cGMP challenge agents remains a primary requirement, unless governments invest in the manufacture of cGMP organisms for more general access.

Adrian Wildfire is Project Director Infectious Diseases, and Bruno Speder is Head of Clinical Regulatory Affairs, both for SGS Life Science Services.

	1. http://whqlibdoc.who.int/smallate phaseox/9241561106_chp6.pdf

	2.http://www.fda.gov/downloads/BiologicsBloodVaccines/NewsEvents/WorkshopsMeetingsConferences/ucm090496.pdf and

	3. http://www.biopharminternational.com/node/247177?rel=canonical

	4. http://www.atsjournals.org/doi/full/10.1164/rccm.201002-0221OC#.VVXTQ1RBvIU )

	5. Walter Reed Army Institute of Research and its work with dengue -http://jid.oxfordjournals.org/content/209/suppl_2/S49.abstract )

The concept of the human challenge model (HCM) originated from early societal attempts to halt the spread of acute diseases within their own communities

Bruno Speder

Articles

Regulatory Considerations Surrounding Human Challenge Studies with the SARS-CoV-2 Virus

Human Challenge Studies in Early Phase Antiviral Development