I. Comparison of the Regulatory Positioning and Core Concepts of IVRT and IVPT

In vitro release assays (IVRT) and in vitro penetration assays (IVPT) play complementary and crucial roles in drug development, prescription screening, scale-up and variation (SUPAC-SS) processes, and bioequivalence (BE) assessments of generic drugs.

II. Correspondence and Configuration of USP <1724> Standard Equipment Modules in IVRT and IVPT

The configuration differences between the three diffusion devices specified in USP <1724> on IVRT and IVPT:

Model A: Vertical Diffusion Cell (VDC / Franz Cell): Preferred choice for IVRT / Absolutely mainstream for IVPT (manual/automatic)

Model B: Immersion Cell: Suitable for IVRT using the existing USP 2 dissolution apparatus.

Model C: Flow-Through Cell: IVRT/Continuous Flow IVPT for Poorly Soluble APIs

1. Model A: Vertical Diffusion Cell (VDC / commonly known as Franz Cell)

   • In IVRT applications :

     • Construction and Specifications : The standard receptor chamber volume is 5 mL, 7 mL or 12 mL; the pore size diffusion area is usually 0.64 cm2 (9 mm in diameter) or 1.77 cm2 (15 mm in diameter), and the geometric tolerance is strictly limited to ±5.0%.

     • Configuration method : Install an anti-evaporation cover on the donor chamber (to prevent solvent evaporation from changing the API concentration).

   • In IVPT applications :

     • Structure and Specifications : The bottom receiver chamber mostly adopts a jacketed water bath or dry heat constant temperature module; the clamps must be equipped with uniform spring clips or threaded retaining rings to prevent excessive compression and damage to the edge tissue of the skin, and side leakage must not occur.

     • Configuration : The donor chamber is usually kept unoccluded to simulate the evaporation of moisture from the skin surface after clinical wiping, or it is occluded as specified in the prescription.

2. Model B: Immersion Cell

   • In IVRT applications : Compatible with existing USP Apparatus 2 (paddle dissolution apparatus) laboratories. Comes with 150 to 200 mL flat-bottomed glass dissolution cups. Sample depth is precisely controlled via an adjustable-volume PTFE plunger, making it widely applicable for batch release testing.

   • In IVPT applications : Due to the inverted sample well and the excessively large receptor volume (which can easily lead to excessive dilution of trace amounts of API penetrating the skin, resulting in levels below the detection limit), it is rarely used in IVPT .

3. Model C: Flow-Through Cell

   • In IVRT applications : Based on the USP Apparatus 4 principle, a high-precision pulseless piston pump (flow rate typically 4, 8, or 16 mL/min) is used to continuously deliver the medium into the tank. It is suitable for formulations that are extremely difficult to dissolve APIs or require simulation of specific physiological flow rate conditions.

   • In IVPT applications : Microfluidic flow-through transdermal diffusion cells can simulate the clearance effect of continuous blood flow in dermal microvessels, effectively maintaining the leakage conditions of ultra-low concentration APIs.

III. Key Experimental Parameters and Technical Specification Control Matrix

Sample layer: Ointment/cream/patch: Drug delivery surface (IVRT: Unlimited dose / IVPT: 5-15 mg/cm2)

Barrier layer: Synthetic microporous membrane OR ex vivo skin; Temperature control target: 32.0°C ± 1.0°C (physiological surface); Interface (bubble-free); Receptor chamber: Degassed buffer (magnetically stirred); Thermostat setting: 32.0°C ± 0.5°C; Stirring: 100-1000 rpm

1. Barrier medium preparation and quality control :

   • IVRT (Intra-Fluorescent Membrane Evaluation) :

     • Material selection : PES (polyether ether), PVDF, PTFE, Nylon, etc., with a standard pore size of 0.45 μm.

     • Evaluation criteria : The membrane must not physically or chemically adsorb API (adsorption recovery rate must be in the range of 95.0% to 105.0%), and must not show any external damage, deformation or leaching of interfering substances after contact with the medium and substrate.

     • Pre-conditioning : The sample must be completely immersed in the recipient medium for a specified time (e.g., 30 minutes) to thoroughly remove air bubbles from the pores.

   • IVPT (In Vitro Skin Barrier Integrity Testing) :

     • Tissue source : commonly used human abdominal/back surgical slices (skin thickness usually adjusted to approximately 200 to 500 um of dermatomed skin) or frozen-thawed pig ear skin.

     • Barrier Integrity Verification : Transdermal Water Loss (TEWL) testing, transepithelial electrical resistance (TER) measurement, or tritium penetration testing must be performed before testing. Patches with excessively low resistance values (e.g., less than 10 kΩ·cm²) or abnormally high TEWL values must be rejected.

2. Receptor Solution Design Principles :

   • Sink Condition : At the end of the entire test cycle, the concentration of API in the acceptor medium must not exceed 30% of its saturated solubility .

   • Chemical stability and corrosion resistance :

     • Commonly used buffer solutions are phosphate or physiological saline (PBS) with a pH of 5.5 to 7.4.

     • IVPT long-term test (24 to 48 hours) : Additional antimicrobial agents (such as 0.01% to 0.02% sodium azide NaN3 or a mixture of antibiotics) are required to prevent skin tissue decay and autolysis.

     • For poorly soluble drugs with strong lipophilicity, surfactants (such as 0.1% to 2.0% SLS, Tween-20, Brij) or 0.5% to 5.0% fetal bovine serum albumin (BSA) can be added after methodological verification to maintain the trough conditions, but the stratum corneum structure must not be damaged.

3. Temperature control and fluid dynamics :

   • Temperature specifications :

     • Acceptor medium water bath/dry heat system: 32.0°C ± 0.5°C .

     • Membrane surface or stratum corneum surface: The instantaneous temperature must be constant at 32.0°C ± 1.0°C (37.0°C ± 0.5°C for mucosa/deep tissue simulation).

   • Stirring speed and flow field : The speed range is typically 100 to 1000 rpm (200 to 600 rpm is commonly used, with an accuracy of ±2.0 rpm). The magnetic stirrer must be precisely positioned at the center of the bottom of the vertical diffusion tank to avoid vortex or turbulent flow impacting the bottom of the membrane.

   • Bubble-Free Sampling Arm : The sampling arm must be designed with a specific tilt angle and venting structure to prevent bubbles from accumulating under the membrane and obscuring the effective diffusion area.

4. Automated sampling and isothermal and isochoric replenishment :

   • Accuracy requirement : The sampling volume error of the syringe pump must be less than or equal to ±1.0%.

   • Isothermal and isochoric replenishment : Immediately after sampling, replenish the preheated storage tank with fresh medium at the same temperature (32.0°C) in equal volume. It is strictly forbidden to directly replenish with room temperature medium, which may cause local thermal shock.

   • IVPT : Total Reduction and Total Supplementation

   • IVRT : Full removal and full replenishment; Partial removal and partial replenishment.

IV. Data Analysis, Dynamic Modeling, and Statistical Judgment Standards

1. IVRT Data Processing and Judgment (Higuchi Model and SUPAC-SS)

• Higuchi's dynamic equation : Q = R * √(t)

  • Q: Cumulative release per unit membrane area (unit: ug/cm2).

  • R: Release rate (regression slope, unit: ug/cm2 / square hour).

  • t: Experiment time (hours).

• System suitability requirements (SST) :

  • Linearity determination coefficient: R² for linear regression in each pool is greater than or equal to 0.97.

  • Intra-batch variability: The relative standard deviation (%RSD) of the release rates of the six diffusion cells in the same batch must be less than or equal to 10.0%.

• Bioequivalence comparison (90% confidence interval test) :

  • Compare the test sample (T) with the reference sample (R).

  • Take 6 slots each (12 slots in total) or two sets of 12 slots each, and calculate the pairwise matching ratios (36 or 144 ratios in total).

  • The 90% confidence interval of the release rate ratio (T/R) was calculated using the Mann-Whitney/Wilcoxon non-parent number sorting test .

  • Acceptance criteria : The 90% trust range must fall entirely within the range of 75.00% to 133.33% .

2. IVPT Data Processing and Bioequivalence Parameters (Non-Compartment Model Flux Dynamics)

• Dynamic flux (Flux, J) calculation : J = ΔQ / (Δt * A)

  • ΔQ: The amount of drug penetrating within a specific sampling interval (ug).

  • Δt: Sampling time interval (h).

  • A: Effective exposed skin area (cm2).

• Key pharmacokinetic equivalence assessment indicators :

1. Maximum flux (Jmax) : The highest penetration flux value measured during the entire test.

2. Peak time (Tmax) : The time required to reach maximum throughput.

3. Delay Time (Tlag) : The intercept of steady-state flux extrapolated to the time axis, reflecting the time required for a steady-state concentration gradient to be established for the drug to penetrate the stratum corneum.

4. Cumulative Penetration Exposure (AUC 0-t) : The underside of the integral curve of flux over time within a specific period.

• Statistical comparison principle : Log-transformation was used for Jmax and AUC, and the 90% confidence interval was compared to the 80.00% to 125.00% bioequivalence consensus interval.

V. Complete Regulatory Verification System: Compliance with Q1/Q2/Q3 and 21 CFR Part 11

1. Quality Specificity (Q1 / Q2 / Q3) Verification System :

   • Q1 (Qualitative Sameness) : The test preparation and the reference preparation (RLD) contain exactly the same main component and inactive excipient.

   • Q2 (Quantitative Sameness) : The difference between the concentration of each excipient in the test formulation and the RLD must be within ±5.0%.

   • Q3 (Microstructural Sameness) : Confirming that both have the same microstructural arrangement requires comprehensive comparison:

     • Rheological parameters (shear viscosity, yield stress, thixotropy, viscoelastic modulus G' and G'').

     • Microscopic physical properties (crystal morphology, particle size distribution, polymorphism, pH value, specific gravity).

     • IVRT release rate comparison (as the final in vitro confirmatory indicator of physical microscopic release characteristics).

2. Instrument hardware and data integrity regulations require :

   • Hardware and software compliance : Fully compliant with FDA 21 CFR Part 11 and ALCOA+ data integrity standards.

   • Audit Trail : An unalterable timestamp record that fully documents parameter values before and after modification, temperature logs for each well, sampling time, and user account.

   • User access control : It has three or more levels of independent passwords and permissions protection for administrators (Admin), technicians (Tech), and operators (Operator).

   • Validation documentation support : The manufacturer must provide complete DQ (Design Qualification), IQ (Installation Qualification), OQ (Operation Qualification), and PQ (Performance Qualification) standard validation documentation.

VI. Qinwei Technology Perspective

Under current international regulatory trends for transdermal and semi-solid topical preparations, relying solely on endpoint release testing is no longer sufficient to meet the review requirements of TFDA, US FDA, and EMA. IVRT focuses on "micro-process conformity assessment (Q3)," while IVPT directly addresses "in vitro bioequivalence (BE) substitution." The core technological thresholds for a qualified transdermal absorption system lie in high-precision bubble-free venting channels, micro-volume isothermal infusion without pulses, and data integrity in compliance with 21 CFR Part 11. Chinwei Technology is committed to introducing vertical diffusion systems that meet USP <1724> standards, providing complete technical services from initial development of synthetic membrane and skin barrier methods to later IQ/OQ validation and statistical analysis.

VII. References and Sources

• USP <1724> — Semisolid Drug Products — Performance Tests.

• US FDA Guidance for Industry (2022/2023 Draft/Final) — In Vitro Release Test (IVRT) and In Vitro Permeation Test (IVPT) Studies for Topical Drug Products Submitted in ANDAs .

• US FDA SUPAC-SS Guidance for Industry (1997) — Nonsterile Semisolid Dosage Forms: Scale-Up and Post-Approval Changes: Chemistry, Manufacturing, and Controls; In Vitro Release Testing and In Vivo Bioequivalence Documentation .

• EMA Guideline (2018 Draft) — Guideline on Quality and Equivalence of Topical Products (CHMP/QWP/708282/2018) .

• OECD Test Guideline 428 — Skin Absorption: In Vitro Method .

• Taiwan Pharmacopoeia 9th Ed. — General Inspection Methods: Guidelines for Performance Evaluation of Semi-solid Preparations