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PurposeTo examine the potential of stratum corneum (SC) sampling via tape-stripping in humans to assess bioequivalence of topical acyclovir drug products, and to explore the potential value of alternative metrics of local skin bioavailability calculable from SC sampling experiments.MethodsThree acyclovir creams were considered in two separate studies in which drug amounts in the SC after uptake and clearance periods were measured and used to assess bioequivalence. In each study, a “reference” formulation (evaluated twice) was compared to the “test” in 10 subjects. Each application site was replicated to achieve greater statistical power with fewer volunteers.ResultsSC sampling revealed similarities and differences between products consistent with results from other surrogate bioequivalence measures, including dermal open-flow microperfusion experiments. Further analysis of the tape-stripping data permitted acyclovir flux into the viable skin to be deduced and drug concentration in that ‘compartment’ to be estimated.ConclusionsAcyclovir quantities determined in the SC, following a single-time point uptake and clearance protocol, can be judiciously used both to objectively compare product performance in vivo and to assess delivery of the active into skin tissue below the barrier, thereby permitting local concentrations at or near to the site of action to be determined.

Study ObjectivesWe compared the bioavailability of racemic amphetamine (d-amphetamine and l-amphetamine) from a manipulation-resistant immediate-release (IR) amphetamine sulfate capsule (AR19) versus amphetamine sulfate IR tablets (reference). In this open-label, randomized, two-period, two-treatment, two-sequence, crossover study, 36 healthy volunteers aged 18-45 received a single dose (20-mg capsule) of AR19 in one period and a single dose (2 x 10-mg tablets) of reference in another period, after a 10-hour overnight fast. Each drug administration was separated by a washout period of at least 6days. Bioequivalence for d- and l-amphetamine was assessed using time to peak concentration (Tmax), peak concentration in plasma (Cmax), and area under the plasma concentration-time curve from time-zero to the time of the last quantifiable concentration (AUClast) and extrapolated to infinity (AUCinf). All 36 volunteers completed both treatment sequences. Mean (standard deviation; SD) Tmax for d- and l-amphetamine was similar for AR19 (2.84[1.05]; 3.05[1.22], respectively) and reference (2.52[0.75]; 2.75[1.00], respectively). The geometric least-squares mean ratios and 90% confidence intervals were within the boundary of 80%-125% for bioequivalence for Cmax (d-amphetamine, 98.35% [96.12-100.64]; l-amphetamine, 98.82% [96.42-101.28]), AUClast (d-amphetamine, 99.45% [96.92-102.05]; l-amphetamine, 99.29% [96.55-102.10]), and AUCinf (d-amphetamine, 99.50%[96.77-102.30]; l-amphetamine, 99.23% [96.06-102.50]). A total of 13 mild adverse events were reported by 7 volunteers (AEs; AR19, n=5; reference, n=8). No serious AEs were reported. AR19 was well tolerated and was bioequivalent to reference when administered as a 20-mg dose in healthy volunteers.Funding Acknowledgements: This study was funded by Arbor Pharmaceuticals, LLC.

This study evaluated the bioequivalence of a newly developed liposomal amphotericin B (LAmB) formulation (DKF-5122, Amphosom™) relative to AmBisomeⓇ and mechanistically characterized the pharmacokinetic property. The two-period crossover trial included a patient cohort receiving once-daily infusions for 5 days per period and a healthy-adult cohort receiving a single dose per period. Both cohorts received 3 mg/kg intravenously, and plasma concentrations of LAmB and free drug (fAmB) were analyzed. Bioequivalence in healthy adults was evaluated using conventional criteria. A joint population model was developed based on healthy-adult data, linking LAmB and fAmB through first-order liposomal release and linear disposition. Model adequacy was assessed by goodness-of-fit diagnostics, prediction-corrected visual predictive checks, and bootstrap analysis. Thirty-one participants contributed to the dataset (six patients and twenty-five healthy adults). In healthy adults, Test-to-Reference geometric mean ratios (90% confidence intervals) were 1.08 (1.04–1.12) for the maximum concentration of LAmB and 1.01 (0.94–1.08) for the area under the curve; for fAmB, the corresponding values were 1.00 (0.91–1.10) and 1.01 (0.95–1.07), meeting conventional bioequivalence criteria. Both LAmB and fAmB were well described by 3-compartment models, and the only formulation-related difference was a statistically significant reduction in the central compartment volume of LAmB for the Test formulation. However, this difference was not of a magnitude that would meaningfully affect the BE outcome. Covariate effects were not clinically relevant. Amphosom™ achieved pharmacokinetic bioequivalence to AmBisomeⓇ, and the joint model explained the observed similarity by quantifying liposomal release and systemic disposition of LAmB and fAmB. ClinicalTrials.gov identifier: NCT05749380.

The most common method for establishing bioequivalence (BE) is to demonstrate similarity of concentration–time profiles in the systemic circulation, as a surrogate to the site of action. However, similarity of profiles from two formulations in the systemic circulation does not imply similarity in the gastrointestinal tract (GIT) nor local BE. We have explored the concordance of BE conclusions for a set of hypothetical formulations based on budesonide concentration profiles in various segments of gut vs. those in systemic circulation using virtual trials powered by physiologically based pharmacokinetic (PBPK) models. The impact of Crohn’s disease on the BE conclusions was explored by changing physiological and biological GIT attributes. Substantial ‘discordance’ between local and systemic outcomes of VBE was observed. Upper GIT segments were much more sensitive to formulation changes than systemic circulation, where the latter led to false conclusions for BE. The ileum and colon showed a lower frequency of discordance. In the case of Crohn’s disease, a product-specific similarity factor might be needed for products such as Entocort® EC to ensure local BE. Our results are specific to budesonide, but we demonstrate potential discordances between the local gut vs. systemic BE for the first time.

For successful oral drug development, defining a bioequivalence (BE) safe space is critical for the identification of newer bioequivalent formulations or for setting of clinically relevant in vitro specifications to ensure drug product quality. By definition, the safe space delineates the dissolution profile boundaries or other drug product quality attributes, within which the drug product variants are anticipated to be bioequivalent. Defining a BE safe space with physiologically based biopharmaceutics model (PBBM) allows the establishment of mechanistic in vitro and in vivo relationships (IVIVR) to better understand absorption mechanism and critical bioavailability attributes (CBA). Detailed case studies on how to use PBBM to establish a BE safe space for both innovator and generic drugs are described. New case studies and literature examples demonstrate BE safe space applications such as how to set in vitro dissolution/particle size distribution (PSD) specifications, widen dissolution specification to supersede f2 tests, or application toward a scale-up and post-approval changes (SUPAC) biowaiver. A workflow for detailed PBBM set-up and common clinical study data requirements to establish the safe space and knowledge space are discussed. Approaches to model in vitro dissolution profiles i.e. the diffusion layer model (DLM), Takano and Johnson models or the fitted PSD and Weibull function are described with a decision tree. The conduct of parameter sensitivity analyses on kinetic dissolution parameters for safe space and virtual bioequivalence (VBE) modeling for innovator and generic drugs are shared. The necessity for biopredictive dissolution method development and challenges with PBBM development and acceptance criteria are described.

Objective: This study aimed to assess the bioequivalence of two oral methocarbamol formulations, as follows: the test (T) methocarbamol 1500 mg tablets and the reference (R) Robaxin® 500 mg tablets (3 tablets, total dose: 1500 mg) under fasting conditions, and compare their pharmacokinetic performance. Methods: This was a single-center, phase I, randomized, open-label (blinded for analytical determination), two-sequence, two-period, crossover, bioequivalence study. A total of 32 healthy volunteers were randomly assigned to receive the T-R or R-T administration sequence. Each volunteer received a single dose of each methocarbamol formulation (T or R) separated by a washout period of 7 days. To evaluate the pharmacokinetic profile, blood samples were collected at nineteen time points after dosing. Results: The arithmetic mean Cmax was 31.72 µg/mL for R and 32.39 µg/mL for T, and the arithmetic mean AUC0−t was 90.25 h × µg/mL and 89.72 h × µg/mL, respectively. All adverse events reported were mild for both formulations. The 90% confidence intervals for the corresponding logarithmically transformed geometric mean ratios of Cmax and AUC0−t fell within the acceptance interval of 80.00–125.00%, as their values were 91.67–112.47% for ln(Cmax) and 92.34–103.47% for ln(AUC0−t). Conclusion: Therefore, one tablet of methocarbamol 1500 mg was found to be bioequivalent to the Robaxin® 500 mg tablets (3 tablets), with comparable tolerability and safety profiles.