The work aims to determine the drug permeation driving force through the Paracellular pathway reported by the authors from sorbitan monostearate microparticles encapsulated the drug7. In that work, it was proved the molecular dispersion of the drug led to improving its Paracellular pathway permeation. To be assured from complete dispersion of the drug molecules and sorbitan monostearate, the solid dispersion is prepared by melting the physical mixture of the required amounts of the drug and the carrier. In addition, it could be expected there is no loss in both components of the solid dispersion as a result of the absence of a third component, which can dissolve either of the components or both. This led to assurance from nearly remaining the ratios of the components.
The effect of drug dispersion in the wax matrix on the drug processing could be noticed from the Table 1. The flow property of all drug-matrices ratios was markedly improved from fair to passable flow in the case of the pure drug to very free-flowing except that prepared with 75% TDC, which is free-flowing. That may be due to the low concentration of the wax matrix compared to other ratios. The closest of the angle of repose values on using different concentrations of the wax matrix may be due to the method of preparation, which is based on melting the drug and the matrix to form a clear melted solution before solid mass formation after cooling. Therefore, the homogeneity of the dispersion of the drug in the dispersed substance could be expected. These results indicate the advantage of using sorbitan monostearate as a dispersed matrix since it improves the flowability of the dispersed drug which is sometimes considered as a limiting step in the pharmaceutical manufacture processes. In addition, to assure the homogeneity of using different concentrations of the dispersed media, it will be preferring to use the melting method to get a clear melt solution. This may be a disadvantage in using the melting technique for thermos-labile substances but it may be not considered because the melting point of sorbitan monostearate is around 56 °C. The compressibility index of the solid dispersed drug in sorbitan monostearate by using melting method is completely improved from poor compressible substance in case of the pure drug to excellent compressibility substance for all solid dispersed drug-sorbitan monostearate ratios36.
Since metformin-sorbitan monostearate solid dispersions were prepared by using melting technique, this led to supposes that there is no loss for both drug and matrix and the actual drug content should be equal to theoretical drug content. On the determination of the actual drug content, from a Table 2, it can be noticed that the actual drug content (ADC) is nearly equal to the theoretical drug content (TDC). The deviation of ACD from TDC is nearly constant and is maximum on using 25% TDC. That is maybe due to the insolubility of sorbitan monostearate in the extraction solution. ADC increased nearly parallel to increasing the TDC.
Partition coefficient
The partition coefficient of the pure drug and all drug solid dispersion products with different ratios were studied. Figure 1 showed that the partition coefficient of 25% drug dispersed in sorbitan monostearate as solid dispersion increased markedly than that of the pure drug. On using 33% drug in the solid dispersion form, the value of Po/w is again increased and remains constant on increasing the drug percentage in the solid dispersion products. Since the drug-sorbitan monostearate solid dispersion is prepared by the melting method to produce a melt clear solution and the partition coefficient value depends on the lipophilicity of the drug, it can be concluded that the melting of the drug led to its dispersion in the polar part of the carrier. In this case, the polar part of the carrier could be only the polar part of an image of the surfactant which is, maybe, the surfactant micelle. This conclusion was also previously reported by the author7.
Effect of the theoretical drug content on its partition coefficient.
Dissolution profile
The drug release profile from different matrices prepared with different metformin-span ratios was studied. From Fig. 2, it can be noticed that there is rapid initial and incomplete drug release and both depend on the drug-matrix ratio. These two features are well-known and characterized for each drug dispersed in the molecular state in an insoluble matrix which is, maybe, also our case. Increasing the wax ratio led to decreasing the burst effect and total drug release and the reverse could be noticed concerning the drug. That is maybe due to the insolubility of sorbitan monostearate in the dissolution media. In addition, the high solubility of the drug in the dissolution media may enhance the burst effect.
Drug release profile from different prepared drug-span matrices.
Permeation profile
The intestinal sacs for assaying the permeability are a quick and sensitive technique for determining the overall intestinal integrity or comparative transport of a specific molecule, with the added benefit of intestinal site-specificity. The apparent permeability [Papp] or permeation coefficient of a molecule through the intestinal barrier could be calculated37,38. The benefits of the application of the intestinal sac would be increased after solving the critical points facing its application by the author. Mady et al.7 succeed to solve the critical points of the technique by suspension the sac to the dissolution shaft in the dissolution media of the dissolution apparatus. This modification led to creating instead of drug release profile to drug permeation profile on using standard dissolution apparatus. The author discussed the critical points about the suggested solution, which may increase the value of the application of the technique as a modified non-everted sac.
Figure 3 showed that the addition of tween 80 to the pure drug enhances its permeability concerning the initial, rate and total drug permeated in the experimental time. That is due to different reported mechanisms5,7,9,10 These effects would be more pronounced from the different solid dispersion products of the drug in sorbitan monostearate. The drug permeation profiles from the different drug-sorbitan monostearate solid dispersion products showed an overlap profile style concerning the initial, rate, and the total amount of drug released. This may be due to the drug dispersion in the carrier matrix and the use of an amount of the products containing the same actual drug concentration. Mady et al.7 discussed the essential of metformin HCL encapsulation in the molecular state in the carrier to improve its permeability. The conclusion of the author about the essential molecular dispersion of the drug in sorbitan monostearate as a carrier represents the basic philosophy to maximize its entrapment in the matrix. Increasing the initial drug permeated and total drug permeated leads to expecting improving the onset of action and decreasing the drug reported dose. Although metformin is reported to be safe from producing a hypoglycemic effect, decreasing the dose leads to decreasing the GIT side effect especially in older patients.
The permeability profiles of the pure drug, drug-tween, and different molecular solid dispersions of the drug in sorbitan monostearate matrices.
The permeability parameters of metformin HCl, metformin-tween, and different dispersion of metformin-sorbitan monostearate products across the non-everted sac were calculated and summarized in Table 3. The values of r2, in each case, are high enough to consider a good fitting of the calculated permeation data. In every case, there is no lag time. In the case of metformin HCl, the absences of lag time may be due to its high-water solubility which may be responsible for the presence of intercept values with y abscissa in concentration. The presence of an intercept value represents, in this case, the rapid saturation of the Paracellular pathway tissues of the intestinal wall with the drug before drug transport7. This finding is reinforced by the fact that about 90% of metformin HCl is absorbed via the Paracellular pathway5,6, and the absence of lag time. The addition of tween 80 to the drug led to increasing of all permeation parameters by different reported mechanisms5,7,9,10. The permeation parameters of the drug from its different molecular dispersion products in sorbitan monostearate are markedly increased than that from drug-tween although the intercept values are nearly the same. In addition, the permeation parameters of the drug from its solid dispersion products prepared with different ratios are nearly equal which is reflected in the drug absorption enhancement (DAE %). The drug absorption enhancement percent was calculated according to Eq. (9)7
$$mathrm{% DAE}=frac{mathrm{The, cumulative ,amount ,of ,drug ,penetrated ,from ,the ,dosage ,form}}{mathrm{The ,cumulative ,amount ,of ,pure ,drug ,penetrated}} times 100$$
(9)
From Table 3, it can be noticed that the drug absorption enhancing percent from different molecular drug solid dispersion percent are nearly equal.
Metformin HCl is a class III drug (highly water-soluble). Entrapment of the drug in sorbitan monostearate led to a huge increasing its partition coefficient indicating the entrapment of the drug occurred in the polar part of the surfactant. This led to the change of the drug from hydrophilic to the lipophilic entrapped image in sorbitan monostearate. The formed image led to the previously reported increasing the partition coefficient and decreasing the drug release. Emulsification of the image by tween 80, led to a marked increasing the apparent permeability and consequently the total permeability of the drug. Emulsification of the image by tween 80 may be changed the image surface from lipophilic to hydrophilic and that may explain the absence of the lag time and the presence of the intercept with y abscissa in concentration. Changing the formed image surface to hydrophilic by adding tween 80 may be led to increasing the Paracellular pathway of the image-encapsulated drug. The similarity of the intercept values to the drug-tween indicating the same permeation mechanism of the image to the drug itself (paracellular pathway). These results could be also confirmed from the histogram representation of the total drug permeation Fig. 4.
Histogram representation of the apparent permeability of metformin (M-to-S) from the pure drug, drug-tween, and drug-sorbitan monostearate solid dispersion products.
The similarity of the drug permeation profiles, the permeability coefficient, total permeation percent, and drug absorption enhancement percent from its different solid dispersion products prepared with different ratios suggesting that the role of sorbitan monostearate is only entrapment of the drug in its polar part as an image. This image entrapment process represents the drug permeation driving force, which is responsible for the paracellular permeation enhancement effect and this effect does not affect all drug-matrix ratios.
Pharmacodynamics evolution of the metformin HCl products
Evaluation of the in-vivo effect of metformin hydrochloride is monitored by using its pharmacodynamics marker parameter (lowering the blood glucose level after oral administration). The blood glucose level was measured before the drug administration, which represents the glucose level at zero time. After the drug administration, the plasma glucose level is monitored as a function of time. The blood glucose level would be expressed as a percent and the profile of drug glucose concentration is plotted as a function of time Fig. 5A,B.
(A) Profiles of the change in glucose levels percent versus time. (B) Profiles of the change in glucose levels percent versus time (SD error bar).
A drop in the blood glucose level could be noticed from the first hour after oral drug administration. The effect of the high-water solubility of the drug and its paracellular pathway could be also reported as a result of the beginning of dropping of the blood glucose level after 15 min. The maximum and rate of blood glucose level from drug-tween are higher than that from drug alone which confirms the results of the drug permeation through the modified permeation non everted sac test. Expected results could be noticed from the glucose dropping level as a result of administration of drug dispersed in sorbitan monostearate matrix. A clear significant difference between the pharmacodynamic effect of the pure drug and the dropping of glucose level after administration of lower, middle, or higher drug entrapped in sorbitan monostearate at p ˂ 0.05 on applying ANOVA test was found.
From Fig. 5A, it can be noticed that the dropping of glucose level profile after administration of lower (25%), middle (50%), and higher (75%) drug entrapped in sorbitan monostearate are intersecting at more than one point to the degree of congruence. Applying the ANOA test concluded there is no significant difference between the pharmacodynamic effect (dropping of glucose level) of the three products at p ˂ 0.05, which confirms again the drug permeation results from the modified non-everted sac technique. It should be reported that each point represents the mean of 6 blood glucose level measurements at that time with standard deviation as shown in Fig. 5B.
The area above the curves (AAC) was calculated from Fig. 5. The results are tabulated in Table 4. Drug pharmacodynamic enhancement percent was calculated according to Eq. (10).
$$mathrm{Drug, pharmacodynamic ,enhancement ,% }=frac{(mathrm{AAC ,of ,treated ,drug}-mathrm{AAC ,of ,pure ,drug})}{(mathrm{AAC ,of ,pure ,drug})}times 100$$
(10)
From Table 4, it can be noticed that the drug pharmacodynamic enhancement % increased according to the following order: 75% ˃ 50% ˃ 25% ˃ drug-tween. The mean of pharmacodynamic enhancement percent is 64%, which is equal to drug absorption enhancement effect % as a result of drug molecular dispersion in sorbitan monostearate Table 3.
Drug intestinal permeation—pharmacodynamic correlation
FDA defined in-vitro-in-vivo correlation (IVIVC) as “a predictive mathematical model describing the relationship between an in vitro property of a dosage form and a relevant in vivo response”. In general, the in vitro property is the rate or extent of drug dissolution or release while the in vivo response is the plasma drug concentration or amount of drug absorbed39,40,41,42. FDA guidance described 4 levels for IVIVC which are A, B, C, and multiple C40. In this study, level A was selected because of its highest category of correlation since it correlates a point-to-point relationship41,42.
The pharmacodynamic marker of metformin is its dropping effect on the blood glucose level. Therefore, it was tried to correlate (point-to-point correlation) the percentage of drug permeated and its dropping of blood glucose level percentage. From Fig. 6A–E, it can be noticed that the drug permeation percent curve is the opposite superimposed to the blood glucose dropping percent curve. That is maybe due to the glucose dropping effect is dependent on the blood drug concentration. The area between the two curves would be decreased by increasing the percent of drug molecular dispersed in the span matrix until intercepted at the special point.
Point to point (level A) correlation of the drug permeation profiles and its pharmacodynamics effect: (A) metformin HCL; (B) metformin-tween 80; (C) 25% drug in span-tween. (D) 50% drug in span-tween; (E) 75% drug in span-tween.
Since Level A correlation is a linear relationship between two variables, it was tried to create a mathematic line correlation between the drug permeation enhancing percent and its pharmacodynamics enhancing percent (dropping of glucose level) as a point-to-point correlation. Table 5 shows the value of the correlation coefficient in every case is high enough to conclude a linear correlation between the drug permeation enhancing percent and its pharmacodynamic enhancing percent. Decreasing the slope values of the products than from both drug alone and drug plus tween confirm the drug penetration enhancements, which is consequently lead to more dropping of blood glucose level. The nearly similar slope values of the low, middle, and high drug entrapment products are in agreement with the insignificant difference in the drug permeability and pharmacodynamic effect between the three products. In addition, the results supported the conclusion about the drug permeation driving force is its molecular dispersion in the sorbitan monostearate suggesting image, which is not dependent on the percentage of drug entrapped. Decreasing the intercept values of the products from the pure drug may be, indicates the unstatutable paracellular pathway of the drug absorption from the suggested image, which facing the pure drug absorption.
From the above study, it was found the following: 1. the drug permeation profiles from all drug molecular dispersed products in sorbitan monostearate (low, middle and high) are found to be overlapped to each other. 2. The drug pharmacodynamic effect of the prepared products is intersecting at more than one point to the degree of congruence. 3. Applying the ANOA test showed there is no significant difference between the pharmacodynamic effect (dropping of glucose level) of the three-drug molecular dispersed products at p ˂ 0.05. 4. The mean drug absorption (permeation) percent is equal to the mean pharmacodynamic percent. Accordingly, it can be reported that the paracellular enhancement of sorbitan monostearate to metformin is based on its dispersion in the matrix, which is confirmed by the author before7, and the enhancement does not depend on the drug–matrix ratio. This conclusion is supported by the results of the ANOVA test, which shows no significant difference between the drug products’ pharmacodynamic effects. Accordingly, the mean pharmacodynamic effect of the low, middle and high drug molecular dispersed in the matrix was calculated and is represented in Fig. 7 with bar standard deviation.
Profiles of the change in glucose levels percent versus time (SD error bar).
