Model-based translation from piglets to healthy volunteers: Prediction of TNF-α and IL-6 time-courses in human endotoxin challenge studies
Anders Thorsted (1), Elisabet I. Nielsen (1), Peter Matzneller (2), Markus Zeitlinger (2), Sven Benson (3), Matthijs Kox (4), Peter Pickkers (4), Lena E. Friberg (1)
(1) Department of Pharmaceutical Biosciences, Uppsala University, Uppsala, Sweden, (2) Department of Clinical Pharmacology, Medical University of Vienna, Vienna, Austria, (3) Institute of Medical Psychology & Behavioural Immunobiology, University Hospital Essen, University of Duisburg-Essen, Germany, (4) Department of Intensive Care Medicine, Radboud University Medical Center, Nijmegen, The Netherlands.
Objectives: Endotoxins (ETX), also known as lipopolysaccharides (LPS), are components in the plasma membrane of Gram-negative bacteria that lead to immune activation when administered to mammals. Consequently, ETX is widely utilized in studies into innate immune responses, both in vitro and in animals as well as in vivo in healthy volunteers. The purpose of the current work was to assess if a previously developed quantitative model based on studies in piglets could be used to predict the cytokine time-courses observed in ETX challenge studies in healthy volunteers.
Methods: A non-linear mixed effects model previously established the relation between infused ETX (E. coli O111:B4) in rates from 0.063 to 16.0 μg/kg/h, and the production of the cytokines tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6) in a population of piglets (n=116, median body weight of 27 kg) [1]. The model described ETX kinetics with one compartment disposition and non-linear elimination (Vmax, Km, Vc), with the time-course stimulating the production of TNF-α, parameterized as a turn-over model (mean transit time MTTTNF and baseline S0TNF), through a sigmoidal Emax relationship (Emax, EC50, γ) and incorporated tolerance. The time-courses of ETX and TNF-α both stimulated the production of IL-6, parameterized as a turn-over model with transit compartments (MTTIL6, S0IL6), through linear effect models (E1,TNF, E2,ETX).
Cytokine data were obtained from healthy volunteers (n=230, median body weight of 79 kg [2-8]) challenged with ETX as a bolus (0.4, 0.8, 1.0 and 2.0 ng/kg) or continuous infusion (1.0 ng/kg bolus followed by 1.0 ng/kg/h for three hours). Allometric scaling by body weight was applied to four parameters in the preclinical model (Vmax, Vc, MTTTNF, MTTIL6) with fixed exponents of -0.25, 1.0, 0.25 and 0.25, respectively, while the typical cytokine baselines were allowed to differ between the species (S0TNF, S0IL6). In a second step, refinements were explored. For example, differences in the exposure-response relationships between ETX, TNF-α and IL-6 were evaluated, as doses differed substantially between piglets and healthy volunteers because humans are much more sensitive to ETX. Parameters were estimated with SAEM in NONMEM 7.4.3 and M3 was used to incorporate any samples below the limit of quantification. The focus of goodness of fit diagnostics was VPCs with acceptable overlap between the median observation and median simulation for each dose level.
Results: When refitting the cytokine baselines only, the observed human time-courses of TNF-α and IL-6 were both underpredicted. To refine the model, the potency parameter (EC50) in the ETX-TNF-α exposure-response relationship was re-estimated, resulting in a human estimate of 31.5 ETX units/L (11% of the estimate for piglets) with an improvement in model fit (ΔOFV=-1777). This resulted in acceptable prediction of the median time-courses of TNF-α across all five dose groups (including combined bolus and infusion administration). For IL-6, additional modifications were required, as the relationship established for piglets resulted in underprediction of the effect in lower dose groups, and a prolonged stimulation in higher dose groups. The model for IL-6 was updated by re-estimating the contribution of TNF-α on IL-6 production (E1,SLP) resulting in a slope of 11.5 (12 times the estimate for piglets, ΔOFV=-231), and tolerance was implemented in E1,SLP, resulting in lower stimulation of IL-6 by TNF-α with time (half-life of 40 min, ΔOFV=-421).
Conclusions: Although species differences were apparent for some parameters, the preclinical model structure worked well to predict the median time-courses of TNF-α and IL-6 observed in healthy volunteers. Aside from allometric scaling and cytokine baselines, quantification of species differences in three parameters related to the ETX-TNF-α and TNF-α-IL-6 exposure-response relationships were required to fit the data well. This highlights the potential variations in how immune components affect each other at different ETX doses and between species, and the complexity of the innate immune response. Successful translational models can be of value by informing future study designs and for translating preclinical results to the clinic.
References:
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