
==== Front
Neurol Ther
Neurol Ther
Neurology and Therapy
2193-8253
2193-6536
Springer Healthcare Cheshire

39126603
643
10.1007/s40120-024-00643-4
Original Research
Population Pharmacokinetic Modeling and Simulation for Dose Optimization of GB-5001, a Long-Acting Intramuscular Injection of Donepezil, in Healthy Participants
Khwarg Juyoung 1
Lee Heeyong 2
Yu Kyung-Sang 1
Seol Eunyoung eunyoung.seol@g2gbio.com

2
http://orcid.org/0000-0003-4188-2786
Chung Jae-Yong jychung@snubh.org

3
1 https://ror.org/04h9pn542 grid.31501.36 0000 0004 0470 5905 Department of Clinical Pharmacology and Therapeutics, Seoul National University College of Medicine and Hospital, Seoul, Republic of Korea
2 R&D Center, G2GBIO, Inc., Cheongju, Republic of Korea
3 https://ror.org/04h9pn542 grid.31501.36 0000 0004 0470 5905 Department of Clinical Pharmacology and Therapeutics, Seoul National University College of Medicine and Bundang Hospital, Seongnam, Republic of Korea
10 8 2024
10 8 2024
10 2024
13 5 14531466
21 5 2024
1 7 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, which permits any non-commercial use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc/4.0/.
Introduction

GB-5001 is an intramuscular (IM) formulation of donepezil under development for the treatment of Alzheimer’s disease. The objective of this study was to develop a population pharmacokinetic (PK) model for donepezil in both IM and oral formulations, and to optimize the IM dosage of GB-5001 using bioequivalence (BE) simulation.

Methods

A population PK model of donepezil was developed using NONMEM. It was based on plasma concentration data from a Phase 1 dose escalation study, which involved a single administration of donepezil IM formulation at doses of 70, 140, and 280 mg, and the oral formulation at 10 mg. The model was evaluated based on goodness-of-fit plots, conditional weighted residuals, visual predictive checks, and bootstrapping. BE simulations were conducted using a parallel design between various doses of the IM formulation and the 10-mg dose of oral formulation.

Results

The PKs of donepezil were best described by a two-compartment model, which incorporated distinct absorption compartments for the IM (dual first-order absorption and simultaneous zero-order absorption with lag time) and oral (first-order absorption with lag time) formulations. Based on the simulation results, an IM dosage range of 210–215 mg in a sample size of over 92 was estimated to achieve a success rate of approximately 80% for BE.

Conclusion

The population PK model well explained the PKs of donepezil following administration of both the IM and oral formulations. This model could be applied for the design and dose selection of future BE trials.

Trial Registration

ClinicalTrials.gov identifier, NCT05525780.

Keywords

Alzheimer’s disease
Donepezil
Intramuscular
Long-acting formulation
G2GBIOissue-copyright-statement© Springer Healthcare Ltd., part of Springer Nature 2024
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pmcKey Summary Points

Why carry out this study?	
GB-5001 is an intramuscular (IM) formulation of donepezil, currently under development for the treatment of Alzheimer’s disease, designed to address the unmet needs of an increased administration interval and enhanced patient compliance.	
This study aimed to develop a population pharmacokinetic (PK) model for donepezil in two formulations, IM and oral, and to optimize the dose of the IM formulation using bioequivalence (BE) simulations.	
What was learned from the study?	
A population PK model incorporating a two-compartment structural model with distinct absorption compartments effectively described the PK of donepezil in both IM and oral formulations.	
The population PK model was utilized for BE simulations across various sample sizes and doses of the IM formulation compared to a 10-mg dose of the oral formulation. The simulations suggested an IM dose range of 210–215 mg to achieve an approximately 80% success rate in BE.	

Introduction

Alzheimer’s disease (AD) is a neurodegenerative disorder marked by a progressive decline in cognitive or behavioral impairment [1]. Although the true pathogenesis of AD is still controversial, research has shown that AD is associated with the presence of amyloid-beta protein deposition, neurofibrillary tangles, neuronal dystrophy, and decreased synaptic acetylcholine levels [2–4]. Cholinergic neurotransmission has been one of the major treatment targets for the symptomatic improvement of cholinergic dysfunctions of AD [5]. Acetylcholinesterase (AChE) inhibitors, such as donepezil, rivastigmine, and galantamine, have been widely used for the symptomatic treatment of AD.

Donepezil, characterized by a non-competitive and reversible inhibitor of AChE, alleviates symptoms by inhibiting the hydrolysis of AChE, which enhances cholinergic neurotransmission. The U.S. Food and Drug Administration (FDA) has approved the tablet formulation of donepezil for once-daily administration in mild to severe AD at doses ranging from 5 to 23 mg. However, maintaining daily adherence to oral dosing presents a significant challenge in managing AD. Recent studies showed a oral donepezil discontinuation rate of approximately 20–40% [6, 7]. The most common reason for discontinuation is the occurrence of adverse events (AEs), but other reasons, such as poor treatment awareness and inconvenience of administration, were also noted [7]. Moreover, a 3-year patient follow-up study suggested that non-adherence to donepezil may further increase the risk of discontinuation [8]. To address this issue, alternative formulations with extended dosing intervals, such as sustained-release tablets, a transdermal patch, or intramuscular (IM) injection have been proposed to enhance patient compliance. Several studies have demonstrated the 1- to 6-month IM formulations are more effective in terms of patient adherence and compliance than daily oral tablets in human immunodeficiency virus pre-exposure prophylaxis [9], prostate cancer [10], and schizophrenia [11].

GB-5001 is a novel once-monthly IM injectable developed as a sustained-release formulation of donepezil for the treatment of AD using G2GBIO’s patent technology, InnoLAMP®. Donepezil is encapsulated into a FDA-approved biodegradable polymeric matrix (PLGA) and sustainedly released through the degradation of the polymer and the diffusion of donepezil. The biodegradation rate of PLGA copolymers depends on the molar ratio of lactic and glycolic acids in the polymer chains and the molecular weight of the polymer [12, 13]. A spherical shape and monodisperse GB-5001 donepezil microspheres (Dv50: 47 µm) were manufactured by membrane emulsification.

As a long-acting formulation with a monthly treatment interval, GB-5001 could significantly improve adherence to donepezil treatment in AD, which is a major concern among the elderly patient population affected by AD. Furthermore, by bypassing the gastrointestinal tract, GB-5001 is expected to enhance the bioavailability and reduce the AEs compared to the conventional oral formulation.

The objective of this study was to develop a population pharmacokinetic (PK) model for donepezil in both IM and oral formulations, and to optimize the IM dosage of GB-5001 by comparing it with the oral formulation using bioequivalence (BE) simulation.

Methods

This clinical study received approval from the institutional review board at Advarra, an independent review committee based in Columbia, MD, USA. The study was registered in the clinical trials registry (NCT05525780), and conducted in compliance with International Council for Harmonisation Good Clinical Practice and the Declaration of Helsinki. Written informed consent was obtained from all participants.

Study Participants

Healthy, non-smoking male participants aged between 18 and 55 years with a body mass index of 18.5–30.0 kg/m2 were eligible for participation in this study. Enrolled participants had no clinically significant conditions, as determined by their medical history, electrocardiogram, vital signs, laboratory results, and physical examination. Participants with a history of rhabdomyolysis, or at risk of rhabdomyolysis due to conditions of muscular disorders, hypothyroidism, liver or kidney disease, were excluded.

Clinical Study Design

A randomized, double-blind, placebo-controlled, dose-escalation study was conducted to evaluate the PKs of donepezil from single dose of the IM (GB-5001 70 mg, 140 mg, and 280 mg) and the oral formulation (Aricept tablet 10 mg). A total of 48 participants were assigned to four cohorts. The first three cohorts, each consisting of 12 participants, were allocated to one of the three dosage levels of the IM formulation. Within these cohorts, participants were randomized in a 3:1 ratio to receive a single IM dose of either GB-5001 or a matching placebo, under fasting conditions. IM injection was administered in the gluteal muscle. Additionally, a fourth cohort consisting of 12 participants were assigned to receive a single dose of the oral formulation under fasting conditions.

Following administration of the IM formulation, PK blood samples for donepezil were collected at 0 (pre-dose) and at 0.5, 1, 2, 4, 6, 8, 12, 24, 48, 72, 96, 120, 168, 240, 312, 360, 432, 504, 552, 600, 648, 720, 768, 840, 1008, 1176, and 1512 h post-dose. For the oral formulation, PK blood samples were collected at 0 (pre-dose) and at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 6, 8, 12, 24, 48, 96, 120, 144, and 240 h post-dose. The blood samples were centrifuged at 2000 ± 20 g for at least 10 min at 4 °C, and then stored at − 20 °C until bioanalysis.

Bioanalysis

The plasma concentration of donepezil was measured by a validated high-performance liquid chromatography–tandem mass spectrometry (Sciex API 4000) in positive ion mode equipped with a Turbo Ionspray® interface. Donepezil-d4 was used as an internal standard for the validation of the analysis method. The between-run precision ranged from 2.06 to 3.55%, and the within-run precision ranged from 1.23 to 4.02%. The sensitivity for between-run measurements was 2.57%. The lowest limit of quantification was < 100.00 pg/mL with a calibration curve range of 100.00–50,000.00 pg/mL.

Pharmacokinetic Analysis

PK analysis was performed using Phoenix WinNonlin® (version 8.3.4; Certara, NJ, USA). The maximum plasma concentration (Cmax) and time to reach Cmax (Tmax) were directly observed from the concentration data. The area under the concentration–time curve (AUC) from time zero until the last observed concentration (AUCt) was calculated using the linear-up–log-down trapezoidal method. The AUC from time zero to infinity (AUCinf) was calculated using the formula AUCt + Clast/λz, where Clast represents the last measurable concentration and λz represents the terminal elimination rate constant. λz was estimated through linear regression analysis of the terminal portion of the log-concentration versus time plot. Apparent total body clearance (CL/F) was calculated as dose/AUCinf, and the apparent volume of distribution (Vd/F) was calculated as dose/(λz·AUCinf). The terminal elimination half-life (T½) was calculated as ln(2)/λz. The assessment of dose proportionality of AUCinf, AUCt and Cmax for the IM formulation was conducted using a power model approach, where the natural logarithms of the PK parameters, AUCinf, AUCt and Cmax , were considered as dependent variables, and the natural logarithm of the dose involved as the independent variable. If the two-sided 90% confidence interval (CI) for the slope lies entirely within the interval [1 + ln(0.5)/ln(ρ), 1 + ln(2)/ln(ρ)], where ρ is defined as the ratio of the highest to the lowest dose, then dose proportionality is suggested across the explored dose range.

Pharmacokinetic Model Development

The nonlinear mixed effect modeling (NONMEM) software version 7.5.1. (ICON Development Solutions, Ellicott City, MD, USA) was used for the development of the population PK model of the donepezil. Parameter estimation was performed using the first-order conditional estimation with interaction method. R software version 4.1.3 and Perl-speaks-NONMEM version 5.3.0 were used for data processing and graphical representation of the NONMEM analysis results. The model was evaluated based on goodness-of-fit (GOF) plots, conditional weighted residuals (CWRES), and a visual predictive check (VPC) of the observed and predicted concentrations. Furthermore, using the final PK model, bootstrapping with 1000 replications was conducted for the validation of the parameter estimates. The median value and the 95% CIs of the parameters from the bootstrap results were obtained, and compared with the final parameter estimates.

Simulation

BE trials were simulated using the final PK model of donepezil. The simulation employed a parallel study design between oral administration of donepezil 10 mg and IM administration of donepezil at doses ranging from 185 to 225 mg in increments of 5 mg. Dosing scenarios for oral administration consisted of a once-daily dosing regimen for 28 days, while the dosing scenario for IM administration included three administrations at 4-week intervals. The total sample size for BE simulation ranged from 50 to 104 in increments of 6, with 500 simulations performed for each combination of dose of the IM formulation and sample size. The simulation results were analyzed using the noncompartmental method with NonCompart R package (version 4.2.1) to calculate Cmax and AUC at steady state. The 90% CIs of simulated Cmax and AUC at steady state were compared with the conventional BE criteria of 0.8–1.25. The percentage of trials that met the criteria was calculated for each combination of dose of the IM formulation and the number of participants.

Safety Assessments

Safety was evaluated using monitoring of AEs, clinical laboratory tests, vital signs measurements, electrocardiograms, and physical examinations.

Results

Demographics

A total of 48 healthy male participants were enrolled and completed the study. The 36 participants enrolled in the cohort for the IM formulation (GB-5001 or placebo) were primarily of white race (94.4%), and the mean ± standard deviation (SD) for age, height, weight, and BMI were 39.9 ± 9.8 years, 176.6 ± 5.3 cm, 78.7 ± 7.3 kg, and 25.3 ± 2.6 kg/m2, respectively. All 12 participants enrolled in the cohort for the oral formulation were of white race (100%), and the mean ± SD for age, height, weight, and BMI were 39.5 ± 10.2 years, 174.3 ± 4.4 cm, 82.4 ± 9.1 kg, 27.1 ± 2.1 kg/m2, respectively. There were no significant differences in demographic characteristics among the cohorts.

Pharmacokinetic Results

The IM formulation was slowly absorbed, exhibiting a double-peak profile (Fig. 1). The median Tmax ranged approximately from 433 to 434 h, and the mean terminal elimination half-life was around 150–190 h, both being similar across dose groups (Table 1). For the dose proportionality analysis of the IM formulation, the point estimate and the 90% CI for the slope of AUCinf, AUCt and Cmax were 1.043 (0.842–1.245), 1.031 (0.842–1.220), and 1.012 (0.833, 1.190), respectively. Compared with the dose-proportionality interval criterion (0.500–1.500), all the PK exposures (AUCinf, AUCt and Cmax) were dose-proportional across the IM dose range of 70–280 mg.Fig. 1 Mean plasma time–concentration profile of donepezil after administration of A IM formulation 70 mg, 140 mg, and 280 mg, and B oral formulation 10 mg in linear scale

Table 1 PK parameters of donepezil for the intramuscular and oral formulation

Parameter	Intramuscular (GB-5001)	Oral (Aricept)	
70 mg
(N = 9)	140 mg
(N = 9)	280 mg
(N = 9)	10 mg
(N = 12)	
Cmax (μg/L)	11.98 ± 4.05	24.43 ± 7.88	47.63 ± 11.72	18.02 ± 3.80	
Tmax (h)	432 [361–504]	434 [361–1010]	433 [431–839]	2.25 [1.50–4.45]	
AUCt (h·μg/L)	6800 ± 2200	15,000 ± 4400	29,000 ± 10,000	670 ± 120	
AUCinf (h·μg/L)	6900 ± 2200	15,000 ± 4800	30,000 ± 13,000	770 ± 210	
T½ (h)	170 ± 22	150 ± 13	190 ± 83	88 ± 30	
CL/F (L/h)	11 ± 3.2	11 ± 4.2	11 ± 3.7	14 ± 3.0	
Vd/F (L)	2700 ± 1100	2300 ± 930	2700 ± 730	1600 ± 300	
Data are presented as the arithmetic mean ± standard deviation except for Tmax, for which the median [minimum–maximum] is presented

AUCt area under the concentration–time curve (AUC) from time zero until the last observed concentration, AUCinf AUC from time zero to infinity, Cmax maximum plasma concentration, CL/F apparent total body clearance, T½ terminal elimination half-life, Tmax time to reach Cmax, Vd/F apparent volume of distribution

Population Pharmacokinetic Analysis

The PKs of donepezil were best described by a two-compartment model following oral and IM administrations (Fig. 2). Different absorption compartments were defined for the oral and IM formulations. For the PK model of the oral formulation, a first-order absorption with a lag time was incorporated. The oral bioavailability was fixed to 1, considering the high oral bioavailability of Aricept [14]. The absorption of the IM formulation was modeled in three phases: dual first-order absorption with lag time and a simultaneous zero-order absorption. Considering the different distribution and elimination profiles between the oral and IM formulations, the volume of distribution (Vd), clearance (CL), inter-compartmental clearance (Q), and residual errors of donepezil concentration were separately modeled for each formulation. Inter-individual variability (IIV) was described using an exponential model. IIV was included in CL and central Vd for both oral and IM formulation. IIV for peripheral Vd and oral bioavailability was included for the oral formulation. A combined additive and proportional residual error model was employed to describe the random error of plasma concentrations. The estimated parameters for the oral and IM formulations are shown in Table 2. The GOF plot demonstrated that the PK model effectively described the observed concentrations (Fig. 3). Furthermore, the CWRES predominantly lay within the range of − 2 to 2, indicating symmetric distribution centered around 0 (Fig. 4). The median parameters obtained from the bootstrap analysis were similar to the final parameter estimates of the population PK model. Moreover, these final parameter estimates were well contained within the 95% CIs derived from the bootstrap analysis.Fig. 2 Structural model for oral and intramuscular (IM) formulations of donepezil. ALAG1 absorption lag time (depot 1); ALAG4 absorption lag time (depot 4); ALAG5 absorption lag time (depot 5); CL clearance; F1 oral bioavailability; F4 fraction of dose (depot 4); F5 fraction of dose (depot 5); KA absorption rate constant (depot 1); KA4 absorption rate constant (depot 4); KA5 absorption rate constant (depot 5); Q intercompartmental clearance

Table 2 Parameter estimates of the population pharmacokinetic model of donepezil for oral and intramuscular formulation

Formulation	Parameter		Final model	Bootstrapa	
Estimate	RSE	Shrinkage	Median	95% CI	
Oral	Oral bioavailability	F1	1 (fixed)	–	–	–	–	
Absorption rate constant (depot 1)	KA	0.203 h−1	11.40%		0.21	(0.165–0.257)	
Clearance	CL	14.3 L/h	6.50%		14.35	(12.602–16.282)	
Volume of distribution (central)	V2	39.5 L	36.50%		40.737	(19.526–73.488)	
Intercompartmental clearance	Q	84.9 L/h	9.60%		86.514	(70.362–103.327)	
Volume of distribution (peripheral)	V3	1080 L	5.90%		1073.55	(957.33–1202.795)	
Absorption lag time (depot 1)	ALAG1	0.931 h	1.70%		0.93	(0.887–0.956)	
IIV on clearance		0.0475 (22.1%)b	39.20%	48.30%	0.045	(0.009–0.08)	
IIV on volume of distribution (central)		1.59 (197.6%)b	31.10%	45.60%	1.498	(0.636–2.525)	
Covariance clearance vs. volume of distribution (central)		− 0.144 (− 52.0%)b	68.60%		− 0.131	(− 0.311–0.024)	
IIV volume of distribution (peripheral)		0.0153 (12.4%)b	44.10%	61%	0.015	(0.002–0.031)	
IIV oral bioavailability		0.0162 (12.8%)b	28.60%	50.70%	0.013	(0.003–0.023)	
Proportional error		0.137	11.50%		0.132	(0.106–0.162)	
Additive error		138 pg/mL	13%		137.786	(85.692–202.99)	
Intramuscular	Fraction of dose (depot 4)	F4	0.748	1.90%		0.748	(0.719–0.778)	
Fraction of dose (depot 5)	F5	0.145	11.20%		0.144	(0.106–0.172)	
Absorption rate constant (depot 4)	KA4	0.00402 h−1	6.70%		0.004	(0.004–0.005)	
Absorption rate constant (depot 5)	KA5	0.0134 h−1	17.20%		0.015	(0.011—0.034)	
Clearance	CL	10.3 L/h	6.10%		10.295	(9.112–11.595)	
Volume of distribution (central)	V2	503 L	29%		511.553	(259.082–834.504)	
Intercompartmental clearance	Q	185 L/h	18.10%		183.88	(128.741–250.004)	
Volume of distribution (peripheral)	V3	1160 L	8.30%		1176.22	(1026.616–1357.204)	
Duration of zero order absorption	D2	648 h	0.20%		647.833	(574.904–820.454)	
Absorption lag time (depot 4)	ALAG4	235 h	0.50%		234.934	(231.951–236.813)	
Absorption lag time (depot 5)	ALAG5	645 h	0.20%		644.194	(637.819–709.858)	
IIV on clearance		0.0936 (31.3%)b	24%	18.10%	0.088	(0.053–0.137)	
IIV on volume of distribution (central)		1.13 (144.8%)b	39.70%	20.70%	1.06	(0.496–2.37)	
Covariance clearance vs volume of distribution (central)		0.113	55.20%		0.118	(− 0.02–0.239)	
Proportional error		0.223	5%		0.219	(0.196–0.241)	
Additive error		23.5 pg/mL	14.40%		23.353	(15.668–30.726)	
CI confidence interval, RSE relative standard error, IIV inter-individual variability

a605 successful bootstrap runs were included; 307 runs with minimization terminated were skipped when calculating the bootstrap results, and 88 runs with estimates near a boundary were skipped when calculating the bootstrap results

bCoefficient of variation calculated as e2-1×100

Fig. 3 Diagnostic plots of the final population pharmacokinetic model. CWRES conditional weighted residuals

Fig. 4 Visual predictive plot of the population pharmacokinetic model for donepezil A intramuscular and B oral formulation. Dots are observed concentrations. The blue solid lines are the 5% and 95% percentile of the observed concentrations, and the blue-shaded areas are the 95% confidence intervals of the simulated 5% and 95% percentile. The red solid line is the 50% percentile of the observed concentrations, and the red-shaded area is the 95% confidence interval of the simulated 50% percentile

Bioequivalence Simulation

The proportion of trials meeting the BE criteria of 0.8–1.25 were calculated for each set of simulation conditions, which included different doses of the IM formulation GB-5001 and various sample sizes (Fig. 5). For the Cmax, the simulation results showed that approximately 80% of the simulated trials met the BE criteria for doses over 210 mg in participant numbers of over 92. For AUClast, doses ranging from 185 to 215 mg in participant number of more than 98 demonstrated a BE success rate around 80%. Considering the aim of achieving a success rate of approximately 80% for both parameters, Cmax and AUClast, doses of IM formulation between 210 and 225 mg would be suitable for studies involving over 92 participants. When assuming a success rate of around 60%, doses of the IM formulation between 205 and 220 mg would be appropriate for studies involving over 98 participants.Fig. 5 Simulated percentage of successful bioequivalence for A Cmax and B AUC between oral and intramuscular formulation. The dosing scenario for simulation included the once-daily administration of the oral formulation for 28 days and the administration of the intramuscular formulation 3 times every 4 weeks. AUC area under the concentration–time curve at steady state, Cmax maximum plasma concentration at steady state

Safety Results

Among 36 participants who received the IM formulation (GB-5001 or placebo), a total of 28 treatment emergent adverse events (TEAEs) were reported by 17 participants, 20 of which were considered drug-related. Among 12 participants who received the oral formulation (Aricept®), a total of 22 TEAEs were reported by 7 participants, 21 of which were considered drug-related. The ratio of reported TEAEs was higher for the oral formulation (Table 3). For the IM GB-5001 or placebo, the most commonly reported TEAEs were in the SOC of gastrointestinal disorders (5.6%), general disorders, and administration site conditions (22.2%), investigations (13.9%), and nervous system disorders, (13.9%). TEAEs classified under the system–organ class “General disorders and administration site conditions” were all injection site pain, and there was no correlation between the IM dose and the injection site pain. For the oral Aricept®, the most commonly reported TEAEs were in the SOC of Gastrointestinal disorders (50.0%), General disorders and administration site conditions (8.3%), Nervous system disorders, (41.7%), and Skin and subcutaneous tissue disorders (25.0%). All TEAEs were mild or moderate. No deaths or serious AEs were reported in this study, and no participants were discontinued due to TEAEs. There were no TEAEs related to laboratory abnormalities, vital signs, ECGs, and physical assessment results during the study. Table 3 Summary of treatment emergent adverse events following a single administration of intramuscular and oral formulation of donepezil

SOC		Treatment		
Term	Placebo
(N = 9)	Intramuscular (GB-5001)	Oral (Aricept)	
70 mg
(N = 9)	140 mg
(N = 9)	280 mg
(N = 9)	Overall
(N = 36)	10 mg
(N = 12)	
Gastrointestinal disorders, n (%) E	0	0	1 (11.1) 1	1 (11.1) 1	2 (5.6) 2	6 (50.0) 8	
General disorders and administration site conditions, n (%) E	2 (22.2) 2	5 (55.6) 5	1 (11.1) 1	0	8 (22.2) 8	1 (8.3) 1	
Investigations, n (%) E	2 (22.2) 3	2 (22.2) 2	1 (11.1) 1	0	5 (13.9) 6	0	
Nervous system disorders n (%) E	2 (22.2) 3	1 (11.1) 1	0	2 (22.2) 2	5 (13.9) 6	5 (41.7) 6	
Skin and subcutaneous tissue disorders n (%) E	0	0	0	0	0	3 (25.0) 3	
Each subject contributes once to each of the incidence rates, regardless of the number of occurrences. The Overall column includes subjects from all treatment groups of GB-5001, including placebo. The gastrointestinal disorders include abdominal pain, constipation, diarrhoea, nausea, and vomiting. The general disorders and administration site conditions include administration pain, fatigue, injection site hypersensitivity, and injection site pain. The investigations include increased alanine aminotransferase, increased blood creatine phosphokinase, and increased blood pressure

TEAE treatment emergent adverse event, SOC system organ class, n (%) number and percent of subjects with TEAEs, E event

Discussion

A population PK analysis of donepezil for both oral and IM formulation was conducted. A two-compartment model was developed considering different disposition characteristics of the two formulations. The model diagnostics including GOF and VPC plots, and the bootstrap results from 1000 replicates indicated a robust fit of the model to the data.

The estimated parameter for central Vd showed a significant difference between the oral and IM formulations, with values of 39.5 L for the oral formulation and 503 L for the IM formulation. This can be linked to the nature of the modified release products, where the absorption rate is intentionally slowed by changes in the formulation. The elimination half-life, as determined through noncompartmental analysis, was about twofold longer for the IM formulation compared to the oral formulation. This could lead to flip–flop pharmacokinetics, a situation where absorption is much slower than elimination, influencing the elimination process. For this reason, PK parameters including Vd and CL may have been affected [15].

The absorption of the IM formulation was described in three phases, which were the dual first-order absorption processes with lag time and a simultaneous zero-order absorption. This approach well described the PK profile of the IM formulation, which exhibited a minimal but consistent increase after administration, and then subsequently showed a distinct double peak. Furthermore, the three-phase absorption model was justified, taking into account the varying release rates of the modified-release particles contained in the IM formulation.

Through the PK modeling and simulation, the optimal dose of IM formulation and appropriate sample size to achieve BE with the donepezil 10-mg oral formulation was estimated. For both primary endpoints, Cmax and AUC at steady state, the proportion of successful BE trials increased with a larger sample size. This proportion increased with increased dose for Cmax, but decreased for doses exceeding 195 mg for AUC. Considering both Cmax and AUC, doses of IM formulation between 210 and 215 mg were suggested to attain a success rate of approximately 80% in sample sizes over 92 participants. To ensure safety and to select doses where the systemic exposure of the IM formulation was similar to the oral formulation, a profile with decreased Cmax and similar AUC would be more appropriate. Taking this into account, a dose range of 190–205 mg is also considered plausible.

The IM formulation of donepezil offers a beneficial therapeutic option for patients suffering from AD, owing to its decreased administration frequency. The number of TEAEs reported was higher for the oral formulation (Aricept®) in comparison to each group of subjects taking the IM formulation (placebo or any dose level of GB-5001) during the Phase 1 study. Recently, the FDA approved the first transdermal patch formulation for once-weekly administration [16]. Although transdermal patches are advantageous for being non-invasive and easy to apply and remove, the IM formulation prolongs administration intervals to months, thereby presenting a substantial benefit compared to the patch formulation. Also, the IM formulation can overcome skin-related AEs of the patch formulation.

There are a few limitations in this study. Firstly, the study was conducted with a small sample size, with 12 participants allocated to each cohort. However, considering the exploratory nature of this phase 1 study, the sample size was appropriate for evaluating the PK and safety profiles and for developing the population PK model. Secondly, all participants in this study were male. Since donepezil has not shown gender differences in PK, safety, or efficacy, the study results are not expected to be significantly affected.

Conclusion

A two-compartment model with multiple absorption compartments best described the population PKs of donepezil for the IM and oral formulations. An IM dosage range of 210–215 mg in a sample size of over 92 would be appropriate for achieving BE in a success rate of approximately 80%. This study will inform the decision-making process in the design and dose selection of future clinical trials. A pivotal BE study is planned to be conducted on a selected dose based on this modeling study to demonstrate the BE of the PKs and safety profiles of the IM formulation compared to the oral formulation in patients with AD.

Acknowledgements

We thank the participants of the study.

Author Contributions

Eunyoung Seol and Heeyong Lee contributed to the study conception and design. Juyoung Khwarg and Jae-Yong Chung analyzed the data and wrote the manuscript. Eunyoung Seol, Heeyong Lee, Kyung-Sang Yu provided critical feedback on the manuscript. All authors read and approved the manuscript for publication.

Funding

This study and the journal’s Rapid Service Fee was funded by G2GBIO, Inc., Cheongju, Republic of Korea.

Data Availability

The data supporting the published results of this study may be shared upon a reasonable request made to the corresponding author or sponsor.

Declarations

Conflict of Interest

Eunyoung Seol and Heeyong Lee are employees of G2GBIO, Inc. Juyoung Khwarg received a scholarship from the BK21FOUR education program. Kyung-sang Yu and Jae-Yong Chung do not have any conflict of interest for this study.

Ethical Approval

This clinical study received approval from the institutional review board at Advarra, an independent review committee based in Columbia, MD, USA. This study was registered in the clinical trials registry (NCT05525780), and conducted in compliance with International Council for Harmonisation Good Clinical Practice and the Declaration of Helsinki. Written informed consent was obtained from all participants.
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