Coronavirus entry starts with the S protein binding to a target receptor on the cell surface, where after fusion is mediated at the cell membrane, delivering the viral nucleocapsid inside the cell for subsequent replication ¹⁴. The S protein is famous for causing syncytial formation between infected cells and other receptor-bearing cells around them, emphasizing that the S protein does not function in just the virion state alone.

A neutralizing antibody targeting the S protein on the surface of 2019-nCoV is likely the first therapy contemplated by biomedical researchers in academia and industry, providing passive immunity to disease ¹⁵. The recently published genome sequence of 2019-nCoV (GenBank: MN908947.3) allows researchers to perform gene synthesis in the lab and consider expressing the S protein as an immunogen. Traditional methods of screening mice or rabbits for neutralizing antibodies may be too slow for this outbreak, but faster methods such as using phage or yeast display libraries that express antibody fragments could be used quickly to identify lead candidates for viral neutralization ^{16, 17}. The challenge is that any antibody candidate would need to be rigorously tested in cell culture and animal models to confirm that it can neutralize 2019-nCoV and prevent infection. Furthermore, several isolates would need to be tested that are circulating in the population to try to assess if sufficient breadth of coverage is obtained with the neutralizing antibody. Information from other coronaviruses species like SARS would be helpful as to where to target the best epitope in order to produce neutralizing antibodies (the receptor-binding domain in the S protein is a key target) ¹⁸, but again this is a slow and challenging process, which may not yield significant gains for several months. Moreover, ultimately a cocktail of antibodies may be required to ensure full protection for patients, which would add additional complexity for formulation and manufacturing. Like some of the therapeutic options discussed below, the ability to express any lead candidates in lower organisms for protein expression (bacteria, yeast, insect cells) would facilitate faster production of therapy for patients ¹⁹.

An alternative strategy of generating neutralizing antibodies against 2019-nCoV S protein would be to immunize large animals (sheep, goat, cow) with the 2019-nCoV S protein, and then purifying polyclonal antibodies from the animals ²⁰. This strategy may serve an expedited service in the setting of an outbreak and has many advantages such as simplifying production and manufacturing, but has limited guarantees that each animal would produce neutralizing antisera, or what the antibody titer would be in each animal ²¹. Moreover, there is also the human immune response against foreign immunoglobulins to other species, which would potentially complicate any treatment scenarios ²². In a truly desperate scenario, this strategy may be viable for a short-term, but would not easily scale in the 2019-nCoV outbreak, which is already rapidly multiplying.

Beyond targeting the surface proteins of 2019-nCoV, one could also target the RNA genome itself for degradation. This RNA genome sequence of 2019-nCoV was recently published (GenBank: MN908947.3), and one strategy that could be considered then, is the use of small interfering RNA (siRNA) or antisense oligonucleotides (ASO) to combat the virus by targeting its RNA genome ²³. The challenge with this strategy is multi-fold. First, the conserved RNA sequence domains of CoV-2019 are not known. Identifying conserved sequences is essential in order to optimize siRNA targeting and avoid viral escape of the oligonucleotide strategy. One could look at genome homology of 2019-nCoV to the SARS virus for comparison of conserved sequences, but this would still be guesswork. A second challenge is how the oligonucleotides would be delivered into the lungs. Advances have been made into delivery vehicles such as lipid nanoparticles that can mediate some delivery into the lungs ²⁴. It is unknown, however, if enough siRNA’s or ASO’s would be effectively delivered within the lungs to stop the infection or make a difference in its clinical course. For example, if 25% of alveolar epithelial cells in the lung had siRNA or ASO in them, that efficiency might be a great success for traditional gene therapy, but would hardly make any difference in a viral infection. Such an explanation is also likely why siRNA candidates against Ebola failed in trials ²⁵, despite significant success in preclinical animal models ^{26, 27}. Lastly, even if one assumed that siRNA was effective clinically, there is a limited ability to scale up manufacturing of siRNA drugs to a large infected population. Current siRNA and ASO therapies are manufactured for rare diseases, and there are no available resources existing to manufacture the medications quickly.

Ideal agents to fight 2019-nCoV would be approved small molecule drugs that could inhibit different aspects of the viral life cycle, ultimately inhibiting replication. Two classes of potential targets are viral polymerases ²⁸ and protease inhibitors ²⁹, both of which are components of human immunodeficiency virus (HIV) and hepatitis C virus (HCV) antiviral regimens. Pilot clinical studies are already ensuing by desperate clinicians with various repurposed antiviral medicines. This has been done in every viral outbreak previously with limited success, outside of case reports ³⁰. Indeed, during the Ebola outbreak, none of the repurposed small molecule drugs were definitively shown to improve the clinical course across all patients ³¹. The 2019-nCoV could be different, and there are initial positive reports that lopinavir and ritonavir, which are HIV protease inhibitors, have some clinical efficacy against 2019-nCoV, similar to prior studies using them against SARS ³². Research should continue to be undertaken to screen other clinically available antivirals in cell culture models of 2019-nCoV, in hopes that a drug candidate would emerge useful against the virus that could be rapidly implemented in the clinic. One promising example could be remdesivir, which interferes with the viral polymerase and has shown efficacy against MERS in mouse models ³³. For further information, reviews of previous drug repurposing efforts for coronaviruses are provided ^{34, 35}. Though these repurposed medications may hold promise, it is still reasonable to pursue novel, 2019-nCoV specific therapies to complement potential repurposed drug candidates.

A simple but potentially very effective tool that can be used in infectious outbreaks is to use the serum of patients who have recovered from the virus to treat patients who contract the virus in the future ³⁶. Patients with resolved viral infection will develop a polyclonal antibody immune response to different viral antigens of 2019-nCoV. Some of these polyclonal antibodies will likely neutralize the virus and prevent new rounds of infection, and the patients with resolved infection should produce 2019-nCoV antibodies in high titer.

Patients with resolved cases of 2019-nCoV can simply donate plasma, and then this plasma can be transfused into infected patients ³⁷. Given that plasma donation is well established, and the transfusion of plasma is also routine medical care, this proposal does not need any new science or medical approvals in order to be put into place. Indeed, the same rationale was used in the treatment of several Ebola patients with convalescent serum during the outbreak in 2014–2015, including two American healthcare workers who became infected ³⁸.

As the outbreak continues, more patients who survived infection will become available to serve as donors to make antisera for 2019-nCoV, and a sizeable stock of antisera could be developed to serve as a treatment for the sickest patients. Unfortunately, the exponential growth of the outbreak would work against this strategy, since the growing number of cases would likely outstrip the ability of previous patients to provide donor plasma as treatment. Moreover, convalescent patient sera would have significant variability in the potency of antiviral effect, making it less ideal ³⁷. While transfusion medicine services should certainly pursue convalescent patient sera as an option right now for patient treatment, it is ultimately limited in its effective scope of controlling the outbreak.