Methods
Monoclonal Antibody Systemic
Concentration
The mean peak serum concentration of SIL following the first dose of SIL
11 mg kg-1 IV in patients with multicentric Castleman’s disease was
derived from the mean steady-state maximum concentration (Cmax) of 322
μg mL-1 divided by the accumulation ratio (1.7), or 195 μg mL-1
[23]. A dissociation equilibrium binding constant (Kd) for SIL has
been reported as 15 pM [24].
The mean peak serum concentration of TCZ following the first dose of TCZ
8 mg kg-1 in patients with cytokine release syndrome (CRS) during
chimeric antigen receptor T-cell (CAR-T) treatment is 99.5 μg mL-1
[25]. A Kd for TCZ has been reported as 1240 pM [26].
Monoclonal Antibody Concentration in the
Lung
Biodistribution mechanisms and data for therapeutic monoclonal
antibodies were taken from published literature [27]. Specifically,
distribution to the lung has been studied in the context of respiratory
syncytial virus (RSV), where anti-RSV antibodies have been developed.
BALF concentrations have been quantified in studies in cynomolgus
monkeys relative to serum concentrations, and suggest extremely low
BALF:Serum concentration ratios of 0.1-0.2%.
For the purpose of these calculations, an optimistic BALF:Serum
concentration ratio of 0.2% was used. Applying this ratio to peak SIL
and TCZ serum concentrations, peak BALF concentrations are then 2690 pM
and 1370 pM, respectively.
IL-6 and sIL-6R Concentration in the
Lung
Measurements of IL-6 and sIL-6R concentrations in normal subjects,
patients at-risk for ARDS, and subjects experiencing ARDS were obtained
from the literature [5]. In this study, patients with sepsis or
trauma who were at risk for ARDS and patients with ARDS (all cause) were
identified by prospective screening of all patients admitted to the
Medical and Surgical intensive care unit in a three-year period. These
patients would correspond to patients on the World Health Organization
Ordinal Scale for Clinical Improvement of 3 (hospitalized with mild
disease, no oxygen requirement) to 7 (hospitalized with severe disease,
ventilated plus additional organ support) [R&D Blueprint COVID-19
Therapeutic Trial Synopsis Draft Feb 18, 2020]. Vital signs, including
physiologic parameters of hypoxemia (expressed as partial pressure of
oxygen (PaO2) to fraction of expired oxygen (FIO2) ratio, tidal volume,
and static lung compliance), were measured daily but not reported.
Patients were not stratified by mild, moderate or severe ARDS, and the
oxygen requirements of individual patients were not reported.
For the purpose of the present analysis, with respect to IL-6 and sIL-6R
concentrations, normal levels, levels in at-risk subjects on Day 1, and
levels in ARDS subjects on Day 1 were extracted (median, 25th, 75th
percentiles) and analyzed as log-normal distributions. N=300 subjects
were simulated from these lognormal distributions. The degree of
variability reported in Park et.al. exceed 100% for both IL-6 and
sIL-6R in the at-risk and ARDS population, so between-patient
variability is an important consideration. The following table reports
summaries of virtual subject values, as well as the calculated
IL‑6:sIL-6R complex using the formula [IL-6:sIL6R] =
[IL-6]*[sIL-6R] Kd-1 and assuming the reported values in Park
et.al. are free IL-6 and free sIL-6R (Table 1).
Notably the induction level of IL-6 and sIL-6R in ARDS relative to
normal is 370-fold and 4.88-fold, respectively. Given a reported Kd
value for sIL-6R to IL-6 of 5500 pM, binding is not favored and the free
moiety forms predominate over the complex IL-6:sIL-6R (CR). The induced
IL-6:sIL-6R complex level in ARDS relative to normal is 1810-fold. While
out-of-scope for this analysis, affinity for the IL-6:sIL-6R complex for
gp130 is in the range of 10 pM, so the predicted increase in available
IL-6:sIL-6R complex should be understood in that context.
Binding Constant
Selection
A review of the literature provided estimates for binding constants for
SIL:IL-6 complex and TCZ:IL-6R complex. These values represented data
across clinical and nonclinical studies, and modeling from clinical and
nonclinical species. Binding constants for use in our model were set at
the median value encountered, or 15 pM for SIL:IL-6 (Kd_SC in Figure 2)
and 1241 pM for TCZ:sIL-6R (Kd_TR in Figure 2).
Binding Models