The focus of the research work for the doctoral thesis was to evaluate two apparatuses which were developed in collaboration with the company Lek Pharmaceuticals d.d., a Sandoz company, and the Faculty of Mechanical Engineering Ljubljana. The experimental work was divided into two sections to evaluate and optimize both, the Advanced Gastric Simulator (AGS) and the Intestine Model for Simulating the Peristaltic Action (IMSPA). The principle of action is similar for both apparatuses. They consist of a flexible silicone container, which represents the lumen of the organ, and the constriction mechanisms, which generate physical pressure on the container's wall, therefore influencing the shape and movement of the container's interior. The IMSPA additionally consists of a platform, with a two-axis actuator, to enable the movement of the fluid inside the container. The specific hydrodynamic conditions, closely resembling the peristaltic movement in the human stomach and the small intestine, are created by the action of the platform and the sequentially-acting constriction mechanisms. The settings of the parameters, such as the amplitude of the mechanisms' aperture, the speed and the delay between the adjacent mechanisms, can be selected with the computer software. Considering the close physical similarity of the apparatuses with the anatomy of the human stomach and intestine, the purpose of this doctoral thesis was to research and evaluate the actual applicability of the new models for the prediction of drug release kinetics from the pharmaceutical dosage forms.
Several dosage forms with different active pharmaceutical ingredients were tested. The results were compared with the classical in vitro dissolution tests (USP II). The AGS was used for testing ordinary tablets, mini-tablets and capsules; and the IMSPA was used for evaluating the matrix tablets of two different polymer types – polyethylene oxide (PEO) and hydroxypropylmethyl cellulose (HPMC). The comparison showed, that the new models for the drug dissolution from solid oral pharmaceutical formulations using the simulation of the peristalsis in the human stomach and intestine, successfully and explicitly separated the tested (different) samples based on their composition or the selected test conditions.
An important part of our research work was the optimization of the apparatuses that led to larger repeatability (RSD values decreased from initial average values of 15-20% to values under 10%). Preparation of the precise protocol of the test execution with experimentally set parameters (defining the most appropriate position for the application of the dosage form into the apparatus and the manner of replacing the medium) significantly decreased the variability between the measurements and assured the repeatable test execution, which is a necessary factor for research and routine work. The experiments for decreasing the variability comprised not only the test procedure optimization, but also the mechanical optimization. The final shape and dimensions of the silicone tube and the suitable material selection were defined in the IMSPA. The model for industrial manufacturing of the silicone tube was designed in collaboration with a company, specializing in polymer materials, assuring the absence of major differences between the silicone tubes that were present when made by hand. This is recommended also for the silicone container in the AGS, which is still in the development phase.
During the research work we evaluated the influence of the test conditions. In the AGS, we compared the closed and open system (the flow of fluid through the apparatus) and tested the influence of various flow rates and programs on the drug release. Furthermore, we tested the programs with different speeds and intensities in both apparatuses to evaluate their influence on the drug release kinetics. The amplitude and the speed of contractions (and the amplitude and the speed of the platform movement) affecting the fluid movement inside the silicone containers were continually modified. Results showed that by modification of the amplitude and the period of movement, we can change the hydrodynamic conditions in the silicone interior and consequently influence the drug release from different dosage forms. The custom software enables us to reach in vivo values of the peristaltic contraction frequency.
The apparatuses enable the selection of certain other parameters to approach in vivo conditions. The volume of the medium used for the dissolution test can be selected to approach more realistic in vivo conditions (≤ 250 mL) compared to the USP II, where medium volume is normally 500-900 mL. A special valve was mounted in the AGS for simulating the pylorus and enabling the collection of the samples and the detection of the dissolved drug at the outlet of the container. In this manner, not only can the drug dissolution rate be determined but also, how fast the dissolved drug is emptied from the stomach, which is very likely the most important parameter for the estimation of the drug levels available for absorption further along the small intestine. Optional values of the flow rate in and out of the container can be selected, although in vivo relevant flows were chosen most frequently during our experiments. The pyloric valve was upgraded with a mechanical optimization in a way to enable time-dependent continuous flow rate adjustment during the test in order to simulate the in vivo pattern of the gastric emptying of fluids. This method offers the possibility of in vitro simulation of gastric emptying in the fasted state after drinking one glass of water (240 mL), which is a prescribed protocol for in vivo pharmacokinetic studies according to the U.S. agency FDA (Food and Drug Administration). The statement that with the apparatuses some of the in vivo parameters can be simulated, was supported by additional measurements with the endoscopic capsule SmartPill®, which enabled experiments with pressure measurements inside both apparatuses and direct comparison with the available literature in vivo data. In contrast with the conventional apparatuses, the AGS and the IMSPA enabled the design of programs to create direct mechanical influence in the silicone containers due to the physical contact of the silicone wall and the capsule created by the contractions, reaching in vivo relevant pressure values. We also showed that there is a dependency between the contraction amplitude and the measured pressure. The higher the constriction mechanism was set towards narrowing, the higher the measured pressure of the contraction became. The importance of establishing the relationship between the intensity of the different programs and the generated pressure lies in the possibility to develop various motility patterns for simulating the different phases of the specific gastric movement.
Based on the experiments and successfully determined in vitro-in vivo correlations, it can be concluded that the AGS and the IMSPA can be used for in vitro dissolution testing for several dosage forms. The apparatuses distinguish well between the tested samples in different dissolution media with the adequate repeatability. In addition, the models enable controlled test execution due to the possibility of controlling the parameters that influence the drug release kinetics. In both models, appropriate media volume in the container, sampling volume, insertion position of the dosage form, the manner of replacing the medium and various motion patterns can be selected. In the AGS, a controlled flow rate through the silicone container can be monitored and altered. The advantage of the novel apparatuses compared to the classical in vitro tests is in the possibility of controlling all the mentioned parameters, either isolated or combined. Solely the influence of one selected parameter on the drug release from the tested dosage form independently of other conditions can be studied as to whether it may play a role in the drug release process. Furthermore, the experiment combining all the parameters simultaneously is especially important when trying to simulate in vivo conditions with the goal to enable a good prediction of in vivo behavior of the dosage form.
To evaluate the in vivo relevance of the new models, the dissolution profiles were compared to in vivo data. The newly developed in vitro methods established a close simulation of the in vivo drug release in several examples. The results from the pharmacokinetic studies for immediate-release mini-tablets and both types of matrix tablets were available.
When comparing the two samples of the mini-tablets, various flow rates and motion programs with different intensities were tested. The predicting power of the model was evaluated with calculated determination coefficients (R2). It was shown that the AGS provided a better correlation with the in vivo data (R2=0.99) compared to the USP II (R2=0.87).
In the IMSPA the method for testing the PEO matrix tablets was developed, which showed better correlation with observed in vivo ratios compared to the USP II. Various media, media volume and motion patterns were tested. The similarity factor (f2) was calculated for the USP II method as well as for the IMSPA method. According to the FDA guidelines, the dissolution profiles are not considered to be equal, if f2<50, which was achieved when using the IMSPA method only. The research work was continued with the testing of the two HPMC matrix tablets, where the highest level of in vitro - in vivo correlation (Level A) was achieved, which confirmed the further applicability of the model for testing the matrix tablets. Successful prediction of the plasma concentration profiles for HPMC matrix tablets was established. The results were achieved by using the same software program as in the case of the PEO matrix tablets, which additionally presents the benefit of this particular program to further research possibilities for determining influences on the drug release kinetics for other mechanically susceptible dosage forms.
In this doctoral thesis, we presented the innovative methodology for drug dissolution testing using the two new models with the advantage of the close resemblance to the in vivo conditions. The influence of mechanical and hydrodynamic stress, as well as some other in vivo relevant conditions on the drug release kinetics, can be researched. Based on the obtained results, it can be assumed that the apparatuses simulate well, some of the parameters influencing the drug release kinetics due to its shape and the specific movement. The new apparatuses were successfully implemented in the regular research work and the main parameters influencing the test conditions were evaluated. Fully controlled and repeatable experimental conditions, including the control of the hydrodynamic influences, as well as the direct physical contact of the silicone wall and the dosage form and the proper media selection, can lead to a design of a dissolution method showing discriminatory results, especially for mechanically susceptible dosage forms. The results of the current thesis additionally outline the prospects for future research, where the goal is to achieve an even better predictive power by closer replication of the true (measured) in vivo conditions inside the gastrointestinal tract.
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