COVID-19 has created unprecedented challenges for the healthcare system, and until an effective vaccine is developed and made widely available, treatment options are limited. A challenge to the development of optimal treatment strategies is the extreme heterogeneity of presentation. Infection with SARS-CoV-2 results in a syndrome that ranges in severity from asymptomatic to multi-organ failure and death. In addition to local complications in the lung, the virus can cause systemic inflammation and disseminated micro thrombosis, which can cause stroke, myocardial infarction or pulmonary emboli3-6. Risk factors for poor COVID-19 outcome include advanced age, obesity, diabetes and hypertension 1,7-13.
Computational analyses can provide insights into the transmission, control, progression and underlying mechanisms of infectious diseases. Indeed, epidemiological and statistical modeling has been used for COVID-19, providing powerful insights into co-morbidities, transmission dynamics and control of the disease 14-17. However, to date, these analyses have been population dynamics models of SARS-CoV-2 infection and transmission or correlative analyses of COVID- 19 comorbidities and treatment response. Simple viral dynamics models have been also developed and used to predict the SARS-CoV-2 response to anti-viral drugs18,19. All these models, however, do not explicitly consider the biological or physiological mechanisms underlying disease progression or the time-course of response to various therapeutic interventions.
Several therapies targeting various aspects of COVID-19 pathogenesis have been proposed and have either completed – or are currently being tested in – clinical trials 20. Despite strong biologic rationale, these treatments have generally produced conflicting results in the clinic. For example, the initial trials of the anti-viral remdesivir have been mixed: the original trial from China failed21, while the recent trial in the US is more encouraging and has led to the approval of remdesivir in the US and other countries 22. Other anti-viral drugs alone or in combination are also showing promise 23.
Other potential treatments include anti-inflammatory drugs and anti-thrombotic agents. Because of the systemic inflammation seen in many patients, anti-inflammatory drugs have been tested, including anti-IL6/IL6R therapy (e.g., tocilizumab, siltuximab) and anti-JAK1/2 drugs (e.g. barcitinib). It is not clear if these drugs will be effective as stand-alone treatments, particularly after the recent failure of tocilizumab in a phase III trial 3,24-26. In addition, given that a common complication of COVID-19 is the development of coagulopathies with microvascular thrombi potentially leading to the dysfunction of multiple organ systems4,5, anti-thrombotic drugs (e.g., low molecular weight heparin) are being tested. Recognizing the interactions of COVID-19 with the immune system 27, the corticosteroid dexamethasone have been tested, showing some promising results. Given the large range of patient comorbidities, disease severities, and variety of complications such as thrombosis, it is likely that patients will have heterogeneous responses to any given therapy, and such heterogeneity will continue to be a challenge for clinical trials of unselected COVID-19 patients28.
Here, we developed a systems biology-based mathematical model to address this urgent need. Our model incorporates the known mechanisms of SARS CoV-2 pathogenesis and the potential mechanisms of action of various therapeutic interventions that have been tested in COVID-19 patients. In previous work, we have exploited angiotensin receptor blockers (ARBs) and angiotensin converting enzyme inhibitors (ACEis) for the improvement of cancer therapies and developed mathematical models of the renin-angiotensin system in the context of cancer desmoplasia 29-32 . Using a similar approach, we developed a detailed model that includes lung infection by the SARS-CoV-2 virus and a pharmacokinetic/pharmacodynamic (PK/PD) model of infection and thrombosis to simulate events that take place throughout the body during COVID- 19 progression (Figure 1 and Supplementary Figure 1). The model is first validated against clinical data of healthy people and COVID-19 patients and then used to simulate disease progression in patients with specific co-morbidities. Subsequently, we present model predictions for various therapies currently employed for treatment of COVID-19 alone or in combination, and we identify protocols for optimal clinical management for each of the clinically- observed COVID-19 phenotypes.
Model Description
The model includes SARS-CoV-2 infection, the renin angiotensin system (RAS), inflammatory and anti-inflammatory cytokines, innate and adaptive immune cells, and factors involved in the coagulation cascade (Fig. 1). SARS-CoV-2 enters the cell by docking to ACE2, a key component of the RAS. ACE2 can be membrane-bound or soluble, and it regulates inflammation by converting Ang II to Ang 1-7 and Ang I to Ang 1-9; as opposed to Ang I and Ang II, which lead to inflammation. Ang 1-7 and Ang 1-9 have anti-inflammatory effects. Intracellular virus initiates inflammatory pathways through toll-like receptors and NFkB, which produces interferons and other inflammatory cytokines. The viral antigens, along with inflammatory cytokines, cause activation of naïve T cells, creating virus-specific T effector cells. T cell activation is controlled by viral antigen strength and the presence of PD-L1 / PD-1 inhibition. We combine inflammatory cytokines into a single variable, but explicitly account for IL6 production via the trans pathway in epithelial and endothelial cells and the canonical pathway in immune cells. In the presence of inflammatory cytokines and virus, neutrophils can produce neutrophil extracellular traps (NETs).
Because the virus can infect endothelial cells, we also consider viral dissemination via the blood stream, and the possibility of systemic infection and thrombosis. We include the major organs in a PK/PD model, with physiological blood flow patterns explicitly modeled. Infection of endothelial cells, combined with high levels of inflammatory cytokines in the plasma, can result in thrombosis. Damage to virally-infected endothelial cells and the production of NETs can exacerbate the thrombosis, and microthrombi can enter the blood stream to accumulate in other organs, including the brain, heart and lung. We use a simplified model of the coagulation pathways, assuming that formation of microthrombi is proportional to the number of infected endothelial cells, the presence of neutrophil NETs, and the level of inflammatory cytokines. Transport of oxygen from the alveolar space to the blood vessels in the lung is calculated using a modified diffusion model, which accounts for damage-induce thickening of the alveolar membrane 33.