Physiologically-based pharmacokinetic modeling of nafamostat to support dose selection for treatment of pediatric patients with COVID-19

The current coronavirus disease 2019 (COVID-19) pandemic has emerged as a critical global health crisis. This disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As of November 2021, there have been over 250 million cases and greater than 5 million deaths worldwide (https://coronavirus.jhu.edu/map.html). Although the percentage of COVID-19 diagnosis in children is likely to be lower than that of adults, the incidence of COVID-19 in a pediatric population is not well-known. In South Korea, around 15% of the reported COVID-19 cases occurs in individuals less than 19 years of age (http://ncov.mohw.go.kr/). The disease severity generally appears to be milder and the hospitalization rate is also prone to be lower in pediatric patients in comparison with adults [1]. However, the Center for Disease Control and Prevention (CDC) of the United States reported multisystem inflammation syndrome in children (MIS-C) associated with COVID-19 from a minor group of pediatric patients, which arose from the recent report investigating clusters of children manifesting with multi-organ involved severe inflammation and recent SARS-CoV-2 infection [2, 3, 4, 5, 6, 7, 8, 9]. Therefore, a preliminary case definition for MIS-C has been developed by WHO reflecting the clinical and laboratory characteristics and identifying suspected or confirmed cases [10]. Treatment for MIS-C consists of immune modulating drugs such as corticosteroids and immunoglobulin and supportive care for pneumonia, respiratory failure, and sepsis [7, 8, 9, 11].

Medication to treat COVID-19 has not met a demand yet and repurposing drugs with well-established safety are an appealing option. However, several repurposing drugs in particular chloroquine, hydroxychloroquine and lopinavir-ritonavir were investigated for the treatment of COVID-19, and only remdesivir obtained approval from the US Food and Drug Administration for inpatients including children ≥ 12 years old and weighing ≥ 40 kg as well as adults [12, 13, 14, 15, 16, 17, 18, 19].

Nafamostat, a potent inhibitor of various serine proteases, was initially authorized as a short-acting anticoagulant and used for the treatment of pancreatitis in Japan and Korea for more than 20 years with a well-established safety profile [20]. Previous studies have established that serine protease inhibitors targeting TMPRSS2, such as nafamostat, can block SARS-CoV-2 entry and has been demonstrated in vitro and using animal models [21, 22, 23]. It is currently in the clinical trials for the treatment of COVID-19, including a phase 3 clinical trial. In particular, the result of an open-label, randomized phase 2 clinical trial exhibited that in high-risk patients requiring oxygen treatment, nafamostat had a considerably higher recovery and a lower mortality rates in comparison to standard care alone. However, there was no significant discrepancy in time to clinical improvement between nafamostat and standard care [24]. A main route of elimination of nafamostat represents the hydrolytic metabolism to the inactive metabolites such as 6-amino-2-nphthol (AN) and p-guanidinobenzoic acid (pGBA), mainly through arylesterases and carboxylesterase 2, and thus, the unaltered form of nafamostat is less detectable in urine and feces in in vitro or in vivo assessment [25, 26, 27, 28].

Generally, since pediatric clinical researches are difficult to perform and occur with various ethical issues, pharmacokinetic (PK) modeling and simulation are useful tools to define the risk- benefit ratio of a new drug in pediatric drug development and to allow investigators to determine the personalized precision dosing schemes [29]. Furthermore, the models maximize the utilization of available data and simultaneously minimize the need for unnecessary clinical trials by crossing gaps from adults and supporting efficient clinical trial design [30, 31, 32, 33]. Especially as the physiologically-based pharmacokinetic (PBPK) modeling approach in pediatrics takes into account ontogeny of metabolic enzymes and age dependent anatomical and physiological changes, it provides more reliable predictions of plasma drug concentrations for the optimization of the clinical trial design for the recommendation of initial doses in children than traditional allometric scaling [30, 34].

This study aims to establish a whole body intravenous PBPK model of nafamostat in adults and to scale down the adult PBPK model to children and provide pediatric exposures of nafamostat for different age groups and suggest an optimal weight-based nafamostat dose in pediatric populations.

As the COVID-19 pandemic is rapidly outspreading and the number of pediatric patients is gradually growing, severe cases for children such as MIS-C are being reported [2, 3, 4, 5, 6, 7, 8, 9, 45]. However, studies that have investigated pharmacotherapy for the COVID-19 treatment have been performed mostly in adults [46]. Nafamostat is currently being explored for the treatment of COVID-19 in adults and the data that support its exposure in children remain limited. The unmet medical need for providing nafamostat as a therapeutic option in pediatrics demand the use of PBPK modeling and simulation to optimize pharmacotherapies.

In this study, PBPK models of nafamostat for an adult population have been successfully developed. The model provides a consistent representation of dose-exposure relationship following i.v. administration of a dose range and adequately describes plasma concentration-time profiles of nafamostat. More importantly, by scaling the adult PBPK model to pediatrics, we delineate the potential of PBPK modeling approach to more reasonably predict PK in children.

Nafamostat is unstable in plasma and a highly polar drug with few practical methods for its in vivo quantification and only an insufficient number of PK studies with valid nafamostat concentration measurements available [43, 44, 47]. In the same context, the recent randomized controlled study exploring safety and PK/pharmacodynamics reported that the majority of patients exhibited undetectable levels of nafamostat [unpublished data]. We selected a PK study quantified using LCMS rather than a radioisotope-labeling method which often overestimates the parent drug because the metabolites also consist of radioisotope, particularly for unstable compounds. The adult PBPK model was first established using a PK study of i.v. administration of three doses (10, 20, and 40 mg). The kinetic parameters for nafamostat metabolism were input using in vitro experimental data from the literature and several parameters were fitted utilizing population means of the PK study. The resulting adult PBPK model was able to predict nafamostat exposure reliably over a dose range, indicating that the crucial processes driving nafamostat PK were adequately captured. The initial establishment of the model in adults offered a modeling strategy that served as a solid foundation for age extrapolation to improve accuracy of the pediatric model predictions.

Based on the concept of defining absorption, distribution, metabolism and excretion as a function of anatomy, physiology, and biochemical reaction, PBPK modeling and simulation provides the chance of reasonable scaling between adults and pediatrics. Such a strategy is prevalent in the establishment of models in children and is already utilized by other investigators [41, 42, 48]. This study defined six different age groups for predictions of pediatric clearance. Based on the information, including size change and ontogeny of metabolic enzyme of nafamostat, i.e., arylesterases and carboxylesterase 2, the exposures foreach pediatric group was predicted using a virtual population of 100 individuals from that age category. As PK-Sim does not currently equip an ontogeny function for arylesterases and carboxylesterase 2, it was user-provided from the literature and integrated into PK-Sim [38, 39]. In the case of microsomal carboxylesterase 2, its expression increases across the three consecutive age groups, which are children from birth to 3 weeks, between 3 weeks and 6 years, and over 6 years [38]. In the case of arylesterases, its activity continuously increases from birth to 7 years [39]. As age increases, the predicted nafamostat exposures are slightly raised from neonate to infant followed by a steady decrease from infant to child, and then increased from child to adult, as shown in Fig. 3. Whereas the predicted exposures of child subjects were 0.78 fold lower than those of adults, the predicted exposures of all other pediatric groups fell within 0.85 fold when compared to those of adults, indicating the pediatric exposures predicted by PBPK model are not substantially different from those of adults. Based on these finding, we cautiously may recommend a weight based initial dosing regimen for each age group. Assuming 0.2 mg/kg/h currently approved for DIC and acute pancreatitis and being conducted in a clinical trial for COVID-19 is suitable for adults, for neonate, infant, young child, child, and adolescent subjects, the adult dosage regimen is likely to be increased with 0.23 mg/kg/h, 0.22 mg/kg/h, 0.24 mg/kg/h, 0.25 mg/kg/h, and 0.22 mg/kg/h, respectively. For child subjects, the largest dose changes relative to the adult dosage regimen may be considered. Considering nafamostat is primarily eliminated by metabolism, pediatric clearance can be determined by size as well as maturation of the metabolic enzyme, i.e., arylesterases and carboxylesterase 2. In pediatric PBPK models, most efforts have been contributed to the incorporation of age-dependent changes in metabolic clearance. Especially for the enzyme cytochrome P450s (CYP) developmental patterns and clearance of drugs mainly metabolized by these enzymes are relatively well documented [49]. Generally, because of differences in enzyme levels, drugs highly metabolized are administered at a higher mg/kg dose in young child compared with newborns, which was consistent in the case of nafamostat [50]. The hepatic clearance of the drugs can be higher in infants and child as liver blood flow is increased in comparison with adults due to the increased ratio of liver to total body mass in the preceding group [51]. Thus, drugs primarily metabolized in the liver are likely to exhibit a lower exposure rate compared to adults. Similarly, in nafamostat, the predicted exposure of infants, young child, and child were lower than that of adults.

The selection of dose in pediatric populations is generally obtained from adult PK data through two common approaches, PBPK and allometric scaling [52, 53, 54]. In this study, we delineated nafamostat AUC in pediatric patients based on a PBPK model and compared the results from allometric scaling. The comparison analysis indicated that there was a notable separation in predicted pediatric exposure between the methods in neonate, infant, and young child populations, while those of adult, adolescent, and child subjects coincided between them. Generally, PBPK models explain enzyme ontogeny and age dependent alteration in organ development and function. Therefore, they can provide more reliable prediction of plasma drug concentrations [30, 52]. Allometric scaling does not account for drug specific disposition mechanisms, instead extrapolating exposure based on body size and fixed exponent (usually 0.75), which can result in large overestimations of metabolic clearance in very young children due to their immature enzymes, which is in line with our finding [30, 34]. When compared to simple allometry, PBPK models tend to predict higher exposures in younger children. Therefore, PBPK models present more conventional predictions with respect to safety.

This study had several limitations. First, since available PK studies with valid nafamostat concentrations were limited, we utilized only one study using three dosages for establishing the PBPK model and did not validate the model. Second, our recommended pediatric dosing regimen assumes that the exposure-response relationship in pediatric populations will be similar to that of adults and can be rationally extrapolated from adults. Third, owing to the scarcity of available observed pediatric PK data of nafamostat, the predictive performance of the PBPK model could not be verified.

Our study provides reasonable evidence to recommend a nafamostat weight-based dosing regimen with efficacious exposure in pediatric COVID-19 patients. A PBPK model has been established that adequately captures the observed PK profile of nafamostat in adult healthy volunteers. By scaling this model to pediatric populations, the pediatric exposures of nafamostat were predicted and reasonable pediatric doses were cautiously recommended, aiding future investigations of nafamostat PKs in pediatric populations, including the design of clinical trials and precision dosing.