2021-09-15
Development and Application of Flow-Through Cell Method in Early Formulation Development
This paper mainly refers to the research papers of Jiang B. Fang et al: Development and Application of a Biorelevant Dissolution Method Using USP Apparatus 4 in Early Phase Formulation Development, summarizes the suggestions on the optimization of the flow cell method and the research cases of the flow cell method in the early formulation development.
1. Introduction to Flow-Through Cell Method
In the 1950s, the flow-through cell method was applied to the drug dissolution test of oral preparations. In the 1990s, the flow-through cell method was officially incorporated into the United States Pharmacopoeia and became USP Apparatus 4Compared with the traditional QC dissolution method (basket method / paddle method), the flow-through cell method has the following advantages:(1) During the whole dissolution experiment of insoluble drugs, good sink conditions can be maintained.
(2) It can easily change the dissolution medium and change the flow rate to simulate the conditions in vivo.
(3) It can more effectively simulate the hydrodynamics in the gastrointestinal tract.
(4) It is suitable for in vitro release test of different formulation, including tablets, capsules, suppositories, powders, etc.
(5) The in vivo and in vitro correlation of dissolution test results is better.
Many literatures point out that the application of traditional QC dissolution method has certain limitations in guiding the development of early drug formulation. The dissolution test by flow-through cell method is carried out in an environment closely similar to the physiological conditions in vivo, including pH environment, hydrodynamics and duration. Therefore, the test results of flow-through cell method have potential ability to predict the bioavailability of drugs in vivo.
2. Optimization of Flow-Through Cell Method
（1）Flow RateThe flow rate of dissolution medium is generally recommended to be in the range of 4 ~ 16ml / min, although higher flow rate can and has been adopted. In this study, the flow rate is 8ml / min (22.6mm diameter cell), and its average flow velocity is about 0.00033m/s, which is considered to be close to the flow velocity of fluid in the body (for example, the velocity of intestinal fluid is generally 0.0002 ~ 0.0008 m/s).
In addition, the research literature does not recommend the use of lower flow rate. When the flow rate is lower than 6 ml/min, the dissolution curve becomes more unstable. A similar phenomenon was observed when a small flow-through cell with a diameter of 12 mm was used. This may be due to the decrease of hydrodynamic flow uniformity due to low flow rate.
（2）Dissolution MediumFour standard biologically relevant dissolution media were used: SGF, SIF, FeSSIF and FaSSIF, which represent similar pH and components under gastric, intestinal, fasting or eating conditions, respectively. Generally, SGF is used first, and then SIF is switched. FeSSIF and FaSSIF are used instead of SIF when food impact assessment is required.
（3）Other Method ParametersThe bottom of the flow-through cell is filled with glass beads with a diameter of 1mm; A 0.7 µm filter is installed on the top of the flow-through cell; Use glass wool when necessary to reduce back pressure, etc.
3. Research Cases
The research literature lists four research cases using flow-through cell:
Research Case 1: Batch to batch variability
The API a of research case 1 is BCS Class II compound, weakly basic, and the pKa value is 1.5. It was prepared into 10mg immediate release (IR) tablets for early clinical research. The dissolution results of the original clinical supply batch (Lot 1) tested by theflow-through cell method are very similar to the in vivo plasma concentration curve of the clinical study of this batch.
However, when the QC dissolution method (paddle method, 900 ml, 0.1 N HCl, 50 rpm) was used, it was found that the in vitro dissolution rate of remanufactured product Lot 2 of the same formula was slightly slower than that of original Lot 1. Since the quantitative standard of in vitro release has not been established in the early stage of clinical development, it is difficult to determine whether Lot 2 is suitable to continue as a clinical supply batch.
The results of flow-through cell method showed that the in vitro release of Lot 2 was significantly different from that of Lot 1. Compared with the original batch (Lot 1), the cumulative dissolution rate of compound A was only 60%. The preclinical crossover study of these two batches was conducted with beagle dogs. The results showed that the C max of Lot 2 decreased by about 70% and the AUG decreased by about 65%, which further confirmed the test results of the flow-through cell method.
Research Case 2: Effect of pH Regulator
The main drug component B of research case 2 is BCS Class II compound, methanesulfonate, with a pKa value of 5.1.
Both formulations (Lot 2 and Lot 3) used fumaric acid as pH regulator, in which Lot 2 contained 15% fumaric acid (intragranular) and Lot 3 contained 20% fumaric acid (intragranular 15% and extragranular 5%). Two other formulations without pH regulators were used as the control group (Lot 1 and Lot 4).
When SGF was used first and then the dissolution medium was changed to SIF, the dissolution results of flow-through cell method showed that all four formulations had similar concentration and time distribution curves and similar cumulative dissolution rates. Several pharmacokinetic studies (male beagle dogs) also showed that there was no significant difference in AUC of all tested formulas, which was in good agreement with the in vitro data.
However, when SIF is used as dissolution medium alone, there is a significant difference in the dissolution results of flow-through cell method between the formulations with and without pH regulator, and Lot 2 and Lot 3 are significantly higher.
The results show that if the drug will disintegrate and release in the environment with low pH (strong acidity), such as in the stomach, the weak acid may not play the expected role of improving drug bioavailability as a pH regulator. If the drug is released at a higher pH (neutral or alkaline), the use of weak acids as pH regulators can play the expected role.
Research Case 3: Influence of Formulation Design and Excipients
The main drug component C of research case 3 is BCS Class II compound, weak acid, with pKa values of 4.0 and 7.9.
The excipients of the two formulations are similar but slightly different: Lot 1 formulation uses microcrystalline cellulose (pH 101), lactose monohydrate (impalpable 313) and HPMC; Lot 2 formulation used microcrystalline cellulose (pH 102), lactose monohydrate (fastflo 316), and no HPMC.
The results of the two formulations using the traditional QC dissolution method are very similar. However, the test results of flow-through cell method show that the in vitro dissolution result of Lot 1 is much better than that of Lot 2, so it is predicted that the in vivo performance of Lot 1 will be better than that of Lot 2. This in vitro prediction result was later confirmed by non clinical in vivo pharmacokinetic data: Although the T max values of Lot 1 and lot 2 are similar, Lot 1 has about 3 times larger C max value and about 4 times larger Aug value than Lot 2.
Research Case 4: Assessment and Prediction of Food Impact
The effect of food on the absorption and bioavailability of small molecule drugs is one of the key factors to be considered in the process of drug development. In the early stages of development, it is necessary to understand and predict the impact of food to maximize drug bioavailability and help design the most effective animal and human clinical studies.
By comparing the test results of FeSSIF and FaSSIF as dissolution medium, the flow-through cell method can be used to eva1uate the effect of food in vitro.
The research literature shows the results of in vitro food impact assessment of lansoprazole rapid disintegration tablets and danazol capsules.
Lansoprazole has better bioavailability in fasting state, which is consistent with the test results of flow-through cell method. At the same time, it can be found that the in vitro dissolution curve of flow-through cell method under fasting state is very similar to the in vivo blood drug concentration curve (as shown in the figure below).
For danazol, it is reported that its bioavailability after eating is at least three times higher than that during fasting. The in vitro dissolution test results of flow-through cell method (as shown in the figure below) are consistent with this conclusion.
Summary
The authors use the in vivo bioavailability results of commercially available drugs to guide the development of flow-through cell dissolution method, so as to ensure that the selected method parameters, such as flow rate, are optimized in hydrodynamics. This method is a biologically related in vitro dissolution method with standard parameters and has high universality. Since there is no need for a large number of product specific development, this not only saves the development time of the method, but also directly tests the powder or particles without using the dosage form in the early stage of compound development. The flow-through cell method can effectively study the drug release in vitro in biological related environment, and predict the in vivo performance of different types of drug compounds and solid oral dosage forms.
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