Dose Optimization of rhIL‐7‐hyFc for Patients With Lymphopenia Using a Neonatal Fc Receptor‐Mediated Recycling‐Based and Target‐Mediated Drug Disposition Pharmacokinetic Model

ABSTRACT

Recombinant human interleukin‐7 hybrid Fc (rhIL‐7‐hyFc) is a homodimer of rhIL‐7 fused to a hyFc. Exogenous IL‐7 promotes T cell proliferation and increases lymphocyte count, making it a potential treatment option for lymphopenia and cancer. To improve therapeutic efficacy, rhIL‐7‐hyFc was developed as a long‐acting IL‐7. This study aimed to create a pharmacokinetic model for rhIL‐7‐hyFc by incorporating neonatal Fc receptor (FcRn)‐mediated recycling and target‐mediated drug disposition (TMDD) of the IL‐7 receptor. Data were collected from a randomized, double‐blind, placebo‐controlled phase 1 trial involving 30 healthy volunteers who received single doses of rhIL‐7‐hyFc. Volunteers received 20 or 60 mg/kg subcutaneously, 60 mg/kg intramuscularly (IM), or a placebo. Clinical data were provided by Genexine Inc. (Seoul, Republic of Korea). A TMDD‐FcRn–mediated recycling pharmacokinetic model was developed using NONMEM 7.5 software, assisted by PsN 5.3.1 software. A quasi‐steady‐state approximation was used to describe drug‐receptor and drug‐FcRn interactions. The model evaluation included goodness of fit, visual predictive checks, and bootstrap analysis. Based on the pharmacokinetic parameters of the final model, a simulation was conducted to select the dosage regimen, ensuring a probability of at least 0.8 for meeting both safety and efficacy criteria. The model successfully described the pharmacokinetic profiles of 24 patients administered rhIL‐7‐hyFc. Based on the simulation results, 670–800 μg/kg every 3 weeks, 1010–1530 μg/kg every 6 weeks, and 1510–2190 μg/kg every 9 weeks IM were proposed. These results may help further understand rhIL‐7‐hyFc characteristics and, moreover, provide guidance for selecting the appropriate dosing regimen in future clinical trials.

Summary.

• What is the current knowledge on the topic?

  • ○
    IL‐7 is a promising therapeutic agent for lymphopenia because of its ability to stimulate lymphocyte proliferation. To improve its short half‐life, rhIL‐7‐hyFc was developed. This fusion protein, combining IL‐7 with the hyFc domain, considerably extends the half‐life to 26.8–63.3 h, offering a substantial improvement over rhIL‐7.
• What question did this study address?

  • ○
    In the present study, a TMDD‐FcRn–mediated recycling pharmacokinetic model of rhIL‐7‐hyFc was developed. The model identified a dosing regimen that met both safety and efficacy criteria simultaneously. This study addressed that clinical dosing regimens could be reliably extrapolated from microdose data using a semi‐mechanistic PK model.
• What does this study add to our knowledge?

  • ○
    The study proposed the following dosing regimens for rhIL‐7‐hyFc: 670–800 μg/kg Q3W, 1010–1530 μg/kg Q6W, and 1510–2190 μg/kg Q9W IM.
• How might this change clinical pharmacology or translational science?

  • ○
    The developed model can be used to describe the pharmacokinetic profiles of other IgG fusion drugs administered via the IM and SC routes. This study can serve as a mechanistically informed framework for future studies aiming to bridge microdosed data to higher clinical doses.

1. Introduction

Interleukin‐7 (IL‐7) plays a crucial role in T cell function, including T cell development and the maintenance of mature T cells. The IL‐7 receptor is composed of a common γ chain (CD132) and two α chains (CD127). The γ chain exists in most hematopoietic cells, whereas IL‐7 receptor alpha (IL‐7Rα) is mainly found in lymphocytes. After binding to its receptor, IL‐7 promotes lymphocyte survival and differentiation through the Janus kinase/signal transducers and activators of transcription and phosphatidylinositol 3‐kinase and protein kinase B pathways [1]. IL‐7 can significantly increase T‐cell numbers, and it has a lower risk of side effects, such as cytokine storms, compared to other interleukins [2]. Therefore, IL‐7 treatment is being introduced and developed in clinical trials for its potential to improve the immune response in patients with limited naïve T‐cell or T‐cell depletion [1]. In a study that administered recombinant human IL‐7 (rhIL‐7) to patients with refractory malignancy, exogenously administered rhIL‐7, particularly the non‐glycosylated form, had a relatively short half‐life, ranging from 6.46 to 9.80 h, similar to other cytokines [3].

To overcome the short half‐life of rhIL‐7, a long‐acting IL‐7, rhIL‐7 hybrid Fc (rhIL‐7‐hyFc), is being developed [4]. rhIL‐7‐hyFc is a novel recombinant protein that is structured by hybridizing the immunoglobulin D (IgD) and immunoglobulin G4 (IgG4) domains with two IL‐7s molecules. In the neonatal Fc receptor (FcRn) region, IgD provides hinge flexibility, whereas IgG4 extends the half‐life. The design aims to increase the drug's half‐life through FcRn‐mediated recycling. FcRn, a heterodimeric major histocompatibility complex Class 1 receptor, interacts with the Fc region of IgG to prevent intracellular catabolism, thereby extending the half‐life of both IgG and IgG‐fused drugs.

The results of the phase 2a clinical trial for relapsed/refractory checkpoint inhibitor‐negative microsatellite‐stable colorectal and pancreatic cancers were announced. In this clinical trial, rhIL‐7‐hyFc was administered at 1200 μg/kg intramuscularly (IM) every 6 weeks (Q6W) in combination with pembrolizumab 200 mg intravenously (IV) every 3 weeks (Q3W), resulting in an overall response rate (ORR) of 15.4% per RECIST v1.1 and 30.8% per iRECIST v1.1 (iORR) [5]. Moreover, the results of a phase 1b/2 clinical trial for patients with recurrent/metastatic triple‐negative breast cancer were announced. In this clinical trial, rhIL‐7‐hyFc was administered at 1200 μg/kg IM every 9 weeks (Q9W) in combination therapy, resulting in an ORR of 21.2% [6]. To improve the efficacy of drug treatment, a pharmacokinetic model was designed to explore an optimal dosing regimen.

In the present study, we described the pharmacokinetic profile of microdosed rhIL‐7‐hyFc in healthy adults through the target‐mediated drug disposition (TMDD)‐FcRn‐mediated recycling pharmacokinetic model to optimize the dose for patients with lymphopenia. Microdosing refers to the early evaluation of the pharmacokinetics of new drug candidates with minimal drug exposure. Since the exposure is limited, it is considered safe, leading to an increase in microdosing studies conducted as phase 0 clinical trials before phase I trials. The TMDD phenomenon is often seen as a factor that complicates the extrapolation of drug exposures from a microdose to therapeutic doses. However, microdosing studies can provide valuable insights into the role of target binding in humans, especially for drugs with potential TMDD effects [7]. This study employs semi‐mechanistic population pharmacokinetic modeling to predict the safety and efficacy of therapeutic doses based on microdosing data, considering drug‐receptor interactions. Additionally, it includes the interaction between the drug and the FcRn, which is known to bind to IgG and influence its pharmacokinetic profile [8]. Through this, the study offers a more robust explanation of nonlinearity, providing a solid rationale for extrapolation. This approach enables safer dose optimization and potentially serves as a valuable foundation for future research.

2. Methods

2.1. Pharmacokinetic Data Collection and Model Development of rhIL‐7‐hyFc

Pharmacokinetic data were collected from a phase 1 clinical trial (NCT02860715) approved by the Institutional Review Board of Seoul National University Hospital. Either rhIL‐7‐hyFc or a matching placebo was randomly administered to 30 subjects in an 8:2 ratio, with single doses of 20 and 60 μg/kg given subcutaneously or 60 μg/kg given IM. Detailed information is provided in a previous study by Lee et al. [4]

The dataset consisted of 410 time versus concentration observations obtained from 24 healthy individuals. Blood samples were collected for analysis of serum concentration of total IL‐7, which were measured using the Quantikine HS ELISA Human IL‐7 immunoassay kit (R&D Systems, Minneapolis, MN). The assay type was a solid phase sandwich ELISA. The measured concentrations were assumed to represent the unbound IL‐7. The lower limit of quantification was set at 0.031 ng/mL. The molecular weight of rhIL‐7‐hyFc was 104 kDa, and the administered dose was converted to pmol. IL‐7 concentration was calculated in pM based on a value of 20.5 kDa per mol of the drug. Additionally, based on the structure of rhIL‐7‐hyFc (a homodimeric IL‐7 fused to the hyFc domain), it was assumed that 1 mol of rhIL‐7‐hyFc binds to 2 mol of IL‐7R in the human body. Baseline IL‐7 levels for each patient were calculated as the average values from screening, dosing, and immediately before drug administration. The IL‐7 concentrations measured were then adjusted by subtracting the baseline value to calculate the concentration of exogenous IL‐7.

The 2‐compartment models, with or without TMDD alone or TMDD‐FcRn–mediated recycling, were evaluated for the structural model. The model development process is presented in Figure S1. Some parameters used to construct the model were assumed to be the same as those from previously reported literature or in‐house experimental values by Genexine to avoid over‐parameterization. The rate of drug uptake from the interstitial space to the distribution space (k _uptake) was set at 0.00952 1/h, calculated based on the IgG loss rate in patients with familial hypercatabolic hypoproteinemia [9]. These patients are deficient in FcRn‐mediated recycling, leading to a rapid loss of IgG. Therefore, the rate of IgG loss in these patients can be used to calculate the uptake rate of IgG‐fused drugs in the general population. The association rate constant (k _on) and dissociation rate constant (k _off) for the binding of the drug to the IL‐7R and FcRn receptors were provided by Genexine and were measured using biolayer interferometry. The recycled fraction of the FcRn‐IgG complex (FR) was set to 0.715, a value from previous publications [10, 11]. FcRn concentration was obtained from the minimum and maximum endosomal FcRn concentrations calculated from tissue FcRn [12]. The concentration of IL‐7 receptor was extracted from Figure 3 in the study by Monti et al. [13] A detailed table including the parameters, their values, and specific rationales is provided in Table S1.

FIGURE 3.

The probabilities of meeting the safety and efficacy conditions for each regimen are shown. The red line indicates the probability of meeting the efficacy criterion, and the blue line indicates the probability of meeting the safety criterion. The dark blue shade indicates the dose range where the probability of satisfying both conditions exceeds 0.8.

Stepwise covariate modeling (SCM) was performed using PsN, applying predefined p value thresholds (p < 0.05 for forward inclusion and p < 0.01 for backward elimination) based on a likelihood ratio test. Demographic covariates, such as age, sex, weight, height, and body mass index were included in the analysis. Covariates were retained in the final model only if their inclusion significantly reduced the objective function value.

2.2. Application of Quasi‐Steady‐State (QSS) Approximation for Drug‐Target Interaction

TMDD and QSS approximations were employed to explain drug‐receptor interactions. The binding of rhIL‐7‐hyFc to the IL‐7 receptor is a pure TMDD process. Although FcRn is not a pharmacological target of rhIL‐7‐hyFc, its interaction with FcRn can be explained by the TMDD process, considering the internalization rate constant as the degradation rate constant of the drug‐target complex [14].

A QSS model was applied to the TMDD of the drug‐receptor and drug‐FcRn interactions. In the QSS approximation, the free drug, the target, and the complex are assumed to be in a quasi‐steadystate, where the binding rate is balanced by the sum of the dissociation and internalization rates [15]. These are described in Equations (1) and (2):

$$K_{\mathrm{SS}1}=\frac{k_{\text{recycle}}+k_{\mathrm{off}1}}{k_{\mathrm{on}1}}$$ (1)

$$\begin{array}{c}{K}_{\mathrm{SS}2}=\frac{k_{\mathrm{int}}+k_{\mathrm{off}2}}{k_{\mathrm{on}2}}\end{array}$$ (2)

K _SS1, K _SS2, k _off1, k _off2, k _on1, and k _on2 represent the equilibrium, binding, and dissociation constants for drug–FcRn and drug–CD127 interactions, respectively. k _int refers to the internalization rate of the drug–receptor complex, whereas k _recycle refers to the internalization rate of the drug–FcRn complex. Specific details of the model were referenced from Dua et al. [16] and Ngo et al. [14]

Parameter optimization for the models was performed using the first‐order conditional estimation with interaction method in NONMEM 7.5 and PsN 5.3.1 software [17]. The model with the smallest objective function value was selected. Goodness of fit (GOF), bootstrap analysis (n = 1000), and visual predictive check (VPC, n = 500) methods were used for model evaluation.

2.3. Simulation Study for Optimal Dosing of rhIL‐7‐hyFc for Clinical Trial Design

Simulation‐based dose optimization for rhIL‐7‐hyFc was performed using the final developed model with 1000 simulations for each dose scenario. Simulations were conducted starting from a minimum dose of 30 μg/kg, with maximum doses set at 1200 μg/kg for Q1W, 3600 μg/kg for Q3W, 7200 μg/kg for Q9W, and 10,800 μg/kg for Q12W. The pharmacokinetic data was evaluated for at least 3 weeks, which corresponds to a period exceeding five times the maximum half‐life. Therefore, it was assumed that no accumulation would occur even under multiple dosing conditions. Body weight and baseline values were randomly generated to calculate the mean and standard deviation from the observed data.

First, the previously reported results of a phase 1 clinical trial (NCT03478995) conducted in patients with locally advanced or metastatic solid tumors were referenced to establish safety criteria. In this clinical trial, 21 patients were administered doses of 60, 120, 240, 480, 720, 960, 1200, and 1700 μg/kg IM at Q3W intervals [18]. The 1200 μg/kg Q3W regimen was determined to be the maximum tolerated dose (MTD), and the corresponding exposure was regarded as the maximum tolerated exposure (MTE). In the present study, we conducted simulations with varying doses and dosing intervals based on the developed model. The area under the curve from the time of dosing to the last measurable concentration (AUC_last) was calculated for each regimen and compared with the MTE, i.e., AUC_last at 1200 μg/kg Q3W (AUC_{last_1200 Q3W}). The safety criterion was established as AUC_last/AUC_{last_1200 Q3W}, where a value of 1 or less was considered safe.

Second, the results of a cell proliferation assay for human IL‐7 in phytohemagglutinin‐activated peripheral blood lymphocytes were referenced to determine the efficacy of the drug on lymphopenia, revealing a half maximal effective concentration (EC₅₀) of 18.1907 pM [19]. Because lymphopenia is directly associated with the absolute lymphocyte count, this value was set as the lower limit for efficacious levels. The trough concentration (C _trough) was defined as the sum of the baseline value and the minimum concentration of exogenous IL‐7 after administration. The efficacy criterion was established as C _trough/EC₅₀, where a value of 1 or greater was considered indicative of effectiveness.

For each dosing regimen, the number of simulated individuals out of 1000 who met the efficacy criterion was calculated and set as the probability of satisfying the efficacy criterion. Similarly, the ratio of individuals who met the safety criteria was calculated to determine the probability of satisfying the safety criteria. Dosing regimens were selected if the probabilities of both efficacy and safety criteria exceeded 0.8.

3. Results

3.1. TMDD‐FcRn–Mediated Recycling Pharmacokinetic Model and Evaluation

The final model structure developed to explain the pharmacokinetic data after drug administration is shown in Figure 1. In this model, the drug was administered SC or IM and distributed in the interstitial space, referred to as V _I. The drug in the interstitial space then moves to both the central compartment, where IL‐7R is located, and the distribution space, referred to as V _D, where the FcRn receptor is located. In this model, the process of IL‐7R binding and subsequent internalization in the central compartment is described as TMDD, whereas the process of drug binding to FcRn and subsequent recycling in V _D is explained as FcRn‐mediated recycling. The parameter estimates, inter‐individual variabilities, and relative standard errors for rhIL‐7‐hyFc are listed in Table 1. The pharmacokinetic parameters generally exhibited low relative standard error, except for the apparent volume of distribution (V _D) of the drug at the FcRn‐mediated recycling space, which showed a shrinkage of 66.2%. The ƞ‐shrinkage was very small for the apparent clearance at 5% and the apparent inter‐compartment clearance at 3%.

FIGURE 1.

Structure of the final model with target‐mediated drug disposition and neonatal Fc receptor‐mediated recycling. CL, Apparent clearance of drug at central compartment; FR, Recycling fraction of FcRn bound drug; IM, Intramuscular injection; K _int, Internalization rate constant of drug target complex in central compartment; K _recycle, Recycling rate constant from distribution space to central compartment due to its binding with FcRn; K _SS1, Quasi‐steady‐state equilibrium contant of drug‐FcRn interaction; K _SS2, Quasi‐steady‐state equilibrium contant of drug‐CD127 interaction; Q, Apparent inter‐compartment clearance of drug between distribution space and central compartment; SC, Subcutaneous injection; V _C, Apparent volume of distribution of drug at central compartment; V _D, Apparent volume of distribution 2 space of drug at the FcRn‐mediated recycling space.

TABLE 1.

The final parameter estimates of rhIL‐7‐hyFc population pharmacokinetic model and bootstrap results.

Parameter (unit)	Observed value		Shrinkage (%)	Bootstrap replicates
	Median	RSE (%)		Median	2.5th	97.5th
CL (L/h)	8.91	18.7		7.93	2.42	12.29
V C (L)	39	20.2		43.08	21.65	100.99
Q (L/h)	5.46	34.4		6.87	2.11	18.61
V D (L)	0.138	66.2		0.066	0.004	0.149
K rec (1/h)	0.00107	21.6		0.00111	0.00065	0.00163
K int (1/h)	0.375	43.2		0.30	0.005	3.50
V I_IM (L)	442	29		539	212	923
V I_SC (L)	1680	17.5		1786	971	3075
Individual variability
CL (%)	64.4	18	5	73.3	13.1	555
Q (%)	96.6	17	3	56.8	28.6	165.9
Residual variability
Proportional error (%)	33.5	6.2		33	27	41
Additive error (pmol/L)	1.05	11.2		1.09	0.48	1.76

Abbreviations: CL, Apparent clearance of drug at central compartment; K _int, Internalization rate constant of drug target complex in central compartment; K _rec, Recycling rate constant from distribution space to central compartment due to its binding with FcRn; Q, Apparent inter‐compartment clearance of drug between distribution space and central compartment; RSE, Relative standard error; V _C, Apparent volume of distribution of drug at central compartment; V _D, Apparent volume of distribution 2 space of drug at the FcRn‐mediated recycling space; V _{I_IM}, Apparent volume of distribution of drug at distribution 1 space of intermuscular injection; V _{I_SC}, Apparent volume of distribution of drug at distribution 1 space of subcutaneous injection.

According to the basic GOF plot for the final model (Figure S2), both the population and individual predictions were similar to the observed values. The individual weighted residuals were close to zero and evenly distributed without showing any specific pattern. Conditional weighted residuals fell within the ±2 range, with trends centered around zero. Therefore, we concluded that the final parameters were adequately estimated.

A VPC plot is shown in Figure 2, where the model‐predicted confidence intervals are appropriate. The observed median values were well contained within these confidence intervals, and the prediction intervals were sufficiently narrow. The covariate analysis identified no covariates meeting the significance thresholds for inclusion. This was considered to be due to the weight‐based dosing regimen employed in this study. Table 1 presents the results of the bootstrap analysis, where most parameter estimates were closely aligned with the bootstrap median estimates and fell within the 2.5th–97.5th percentile range, indicating that the model's parameters were unbiased.

FIGURE 2.

Visual predictive check results: The red line indicates the 50th percentile of observations, and the lower and upper blue lines indicate the 5th and 95th percentiles, respectively. The gray points are observations. The blue shades indicate model‐predicted confidence intervals of the 5th and 95th percentile, and the red shade indicates model‐predicted confidence intervals of the 50th percentile.

3.2. Dosage Regimen of rhIL‐7‐hyFc for Clinical Trial Design

Based on the simulation results for 1000 patients receiving doses of 300–1200 μg/kg Q1W IM, 300–3600 μg/kg Q3W IM, 300–7200 μg/kg Q6W IM, or 300–10,800 μg/kg Q9W IM, the probabilities of meeting the safety and efficacy conditions were calculated, as shown in Figure 3. For the Q1W regimen, the efficacy criterion was met across all dose levels; however, the safety criterion was not met. For the Q3W regimen, both efficacy and safety were above 80% for doses ranging from 670 to 800 μg/kg. For the Q6W regimen, both efficacy and safety exceeded 80% at 1010–1530 μg/kg. For the Q9W regimen, both efficacy and safety were above 80% at doses of 1510–2190 μg/kg.

4. Discussion

A semi‐mechanistic pharmacokinetic model applying TMDD‐FcRn–mediated recycling of rhIL‐7‐hyFc was developed. This model is based on the interaction of the Fc region of rhIL‐7‐hyFc with both FcRn and IL‐7 receptors and includes administration via both IM and SC injections.

Monoclonal antibodies injected IM or SC are distributed into the interstitial space, circulate through the lymphatic system, and then reach the systemic circulation. FcRn is primarily expressed in vascular endothelial cells of various tissues. It protects IgG from catabolism by forming an FcRn‐IgG complex, allowing it to be recirculated into the bloodstream. IgG molecules that do not bind to FcRn are degraded via the lysosomal pathway. Consequently, drugs conjugated to IgG exhibit an extended half‐life due to the FcRn‐mediated recycling process. FcRn binds to IgG, preventing its degradation and facilitating its recycling. Therefore, targeting FcRn in drug design is becoming increasingly important, and IgG‐fused drugs can provide sustained therapeutic effects.

Owing to the excessive number of parameters in the TMDD model, simpler models based on approximations have been developed. In the present study, several approximations were used to make fitting the model to the data more feasible, necessitating some assumptions. First, the interaction between the drug and target was modeled using a QSS approximation, which assumes that the binding rate is balanced by the sum of the dissociation and internalization rates [k _on*L_C* R = ((k _off + k _int)*P]. The same assumptions are made in this study. Second, it was assumed that the active concentrations of the drug binding IL‐7 receptor and FcRn remained constant before and after drug administration. According to Ngo et al., when the drug concentration was considerably lower than the endogenous IgG concentration of 74,700 nM, the drug had a negligible effect on the receptor concentration [14]. In the present study, the average blood concentration of the drug was as low as 0.012 nM, suggesting that the receptor concentration did not change substantially before and after drug administration, and that competition with endogenous IgG could be ignored.

In this study, the V _I was set differently for each administration route, as IM administration led to faster absorption than SC administration due to the differences in the size and number of blood vessels between muscle and subcutaneous tissue.

The parameters related to drug internalization of the drug‐receptor complex (k _int) were estimated to be 0.375 1/h, which was similar to 0.231 1/h, an in vitro elimination rate of the IL‐7‐receptor complex calculated from the half‐life of 3 h [20].

The model was developed using serum IL‐7 data measured after the administration of rhIL‐7‐hyFc to patients in a clinical trial. The final model was evaluated using bootstrapping, basic GOF, and VPC methods, which confirmed its accuracy. Using the final model, simulations for 300–1200 μg/kg Q1W, 300–3600 μg/kg Q3W, 300–7200 μg/kg Q3W, and 300–10,800 μg/kg Q9W were conducted. The minimum dose was set such that the probability of C _trough exceeding the EC50 in the cell proliferation assay was at least 0.8, and the maximum dose was set such that the probability of AUC_last being lower than or equal to AUC_{last_1200 Q3W} was at least 0.8. Based on these criteria, dosing regimens of 670–800 μg/kg Q3W, 1010–1530 μg/kg Q6W, and 1510–2190 μg/kg Q9W IM was proposed, and for the Q1W regimen, no dose range met the safety criterion. Considering that 1200 μg/kg Q3W was the MTD, the maximum tolerated exposure (MTE) can be considered compatible at 2400 μg/kg Q6W and 3600 μg/kg Q9W. The proposed regimens, which remain below these exposure levels, can thus be considered safe.

The present study has a few limitations. First, since the drug is still undergoing clinical trials, there is no established dosage, and the effective dose in patients is unknown. However, by determining the EC₅₀ in human peripheral blood lymphocytes using a cell proliferation assay, it is possible to make an indirect prediction about the effects in patients with lymphopenia. Second, the proposed dosing regimens were not further tested in the clinic. This is because the modeling analysis was conducted after the drug had progressed to phase 2 trials, and it relied on toxicity criteria derived from later‐phase clinical data. Lastly, the dataset used for model construction included doses up to 60 μg/kg, whereas simulations were performed for higher doses ranging from 300 to 10,800 μg/kg. Given that this drug is biologics which is likely to exhibit a TMDD pattern, such extrapolation may raise concerns regarding the validity of model predictions at higher doses.

However, this study has the advantage of adopting the TMDD model compared to traditional compartment models. We compared the simulation results of the TMDD‐FcRn–mediated recycling pharmacokinetic model and traditional compartment models with results from external studies that reported individual C _max and AUC values [21, 22, 23], and as a result, our model showed better predictive performance than compartment models (Figure S3).

As our model incorporates TMDD with FcRn‐mediated recycling, it explains dose nonlinearity through drug‐receptor interactions, providing a solid rationale for extrapolation beyond the observed dose range. Such an approach offers a more robust and scientifically grounded explanation of nonlinearity and serves as a mechanistically informed framework for future studies aiming to bridge microdosed data to higher clinical doses.

Author Contributions

H.S.J., S.W.L., W.J., H‐y.Y., S.L., A.K., J‐w.C., and H.L. wrote the manuscript; H.S.J., S.W.L., W.J., D.C., M.‐S.B., H‐y.Y., S.L., A.K., J‐w.C., and H.L. designed the research; H.S.J., S.W.L., W.J., H.J., H‐y.Y., S.L., A.K., J‐w.C., and H.L. performed the research; H.S.J., S.W.L., W.J., H.J., H‐y.Y., S.L., A.K., J‐w.C., and H.L. analyzed the data; D.C., M.‐S.B., and H.L. contributed new reagents/analytical tools.

Conflicts of Interest

D.C. is an employee of NeoImmuneTech Inc. M.‐S.B. is an employee of Genexine Inc. All other authors declared no competing interests for this work.

Supporting information

Data S1.

Data S2.

Data S3.

Data S4.

Data S5.