Discussion

Cytokine signaling through the IL-6 pathway is complex and depends on multiple factors, including cell type and agonist or antagonist concentration in the tissue environment.
Though IL-6R is necessary for IL-6 signaling, and is a reasonable target for IL-6 signaling inhibition, the increase in IL-6 in ARDS is 30-50 times greater than the increase in IL-6R. Near-complete receptor occupancy of IL-6 by TCZ in the lung could be necessary to achieve a robust decrease in signal transduction. An ability to achieve this depends on three factors: the relative concentrations of IL-6 and sIL-6; the concentrations of antibodies such as SIL and TCZ; and the strength of the binding in each of their respective complexes (SIL:IL-6, TCZ:sIL-6R, and sIL-6R:IL-6).
Observational data on IL-6 and sIL-6R induction in ARDS suggests IL-6 is greatly upregulated, with concentrations reaching 370-fold normal levels, while sIL-6R is only modestly increased, with concentrations reaching 4.88-fold normal levels. Maximal achievable serum siltuxumab concentration exceeds TCZ concentration by approximately two-fold. The dissocation equilibirum concentration for SIL is two to three orders-of-magnitude lower than TCZ, meaning that SIL is better at sequestering IL-6 than TCZ is at sequestering sIL-6. Taking these three factors into account, results of the simulation greatly favor the use of SIL over TCZ to inhibit IL-6 signaling. However, these results are dependent on several key assumptions:
The value for antibody penetration (BALF:serum ratio) of SIL or TCZ has not been reported. The value used in the simulation (0.2%) represented the upper-end of reported values for unrelated monoclonal antibodies. Clearly, having experimentally-obtained penetration values for these compounds would be ideal, but difficult to source for repurposed drugs where the lung has not been a studied as a site of action.
The binding model introduced here is complex, yet only captures a portion of the interactions involved in IL-6 signaling. Specifically, IL-6 binds to both sIL-6R (via the trans pathway in all cells) and membrane-bound IL-6R (mIL-6R) (via the classic pathway on certain cells such as some immune cells). mIL-6R, and therefore classic signaling, are not included in the model. Similarly, soluble gp130 (sgp130) (an important IL-6 modulator in the trans pathway) was not included in the model. Signaling complex formation (IL‑6:sIL‑6R:gp130 and IL-6:mIL-6R:gp130) is not accounted for directly in the model.
Still, suppression of free IL-6 reduces IL-6:mIL-6R and IL-6:sIL-6R, and IL-6:sIL-6R is tracked in the model. If anything, the omission of mIL-6R from the model underscores the relative importance of IL-6 suppression relative to sIL-6R suppression. While signaling complex formation is not tracked in the model (for classic or trans signaling), gp130 is constitutively expressed and is therefore assumed not to be a limiting factor in IL-6 signaling. Similarly, gp130 transmembrane protein binding is not included in the model. However, gp130 affinity for IL-6:sIL-6R is higher than IL-6 for sIL-6R, suggesting that the limiting step is the binding of IL-6 to sIL-6R.
Finally, a review of the literature revealed a range of binding constants reported for SIL:IL-6 and TCZ:sIL-6R (Table 2). The selection of Kd for use in the model was the median reported value (Table 2, Figure 2). The impact of the true Kds being lower or higher is not accounted for in this work.

Considerations for Place in Therapy

A rapid, coordinated innate immune response is the initial line of defense against viral infections. Hyper-inflammatory responses, however, can cause immunopathology. Low pathogenic coronaviruses typically infect the upper airways, while highly pathogenic coronaviruses infect the lower respiratory tract and can cause severe pneumonia, sometimes leading to acute lung injury and ARDS. Disease severity of the highly pathogenic coronaviruses SARS and MERS was influenced by factors such as initial viral titers in the airways, age, and comorbid conditions [28].
The clinical course of SARS progressed in three stages. Robust viral replication dominated the first phase, which lasted a few days. The second phase was associated with high fever, hypoxemia and progression to pneumonia despite a decrease in viral load. The third phase is characterized by strong inflammatory response, in which ~ 20% of patients progressed to ARDS and often death [29]. MERS progresses more rapidly, and has a higher fatality rate than SARS. Common clinical manifestations of MERS resemble those of SARS-CoV-2 and include rapid progression to pneumonia [30, 31]. Like SARS-CoV-2, the majority of MERS patients with shortness of breath progressed to severe pneumonia and required admission to the ICU. Analyses of lungs from patients who died from SARS-CoV showed infection of both the airway and alveolar epithelial cells, vascular endothelial cells, macrophages, monocytes and lymphocytes. Neutrophils and macrophages extensively infiltrated cells [32]. The only tissue samples available for MERS is the analysis of lung tissue from one patient, which were consistent with what was seen in SARS [33].
Virus-induced cytopathic effects and viral evasion of host immune responses are thought to play major roles in disease severity. This argues for antiviral therapy as early as possible in treatment, with adjunctive immunotherapy during the time when patients are at risk for ARDS and in early ARDS. This paradigm would be similar to therapeutic interventions aimed at MERS viral load reduction, which were somewhat beneficial when administered early (but not later in) MERS-CoV infection [30, 34, 35].
IL-6 concentrations skyrocket at the onset of ARDS (i.e. WHO Score ≥ 5 [36]). Immediate treatment with an antibody such as SIL upon hospital admission for patients with low oxygenation needs (i.e. 200 mmHg < PaO2 to FIO2 ≤300 mmHg with positive end-expiratory pressure or continuous positive airway pressure ≥5 cmH2O, delivered invasively or noninvasively; Corresponding to WHO Score of 4 [36]) could drive the immune response out of hyperreactivity. SIL treatment in early ARDS (i.e. Days 1-3) would be crucial to preventing the IL-6 onslaught that leads to lung damage.
Additionally, treatment with an antibody such as TCZ in patients at risk of developing ARDS (i.e. WHO Score of 3 [36]) could mediate the IL-6 signaling response synergistically. Preliminary data suggest IL-6R blockade is most effective in critically ill patients, presumably because ARDS has progressed and IL-6 concentrations have already returned to near-normal. For example, Regeneron and Sanofi are amending their phase 3 trial evaluating sarilumab, an IL-6R antagonist, in hospitalized patients with “severe” or “critical” illness caused by COVID-19 to include only “critical” patients, based on preliminary results that sarilumab provided a clinical benefit to patients requiring mechanical ventilation or high-flow oxygenation or treatment in an intensive care unit, but no notable benefit on clinical outcomes in fpatients who required oxygen supplementation without mechanical or high-flow oxygenation [37].