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Modified release drug delivery systems are an important tool in the clinical management of many chronic diseases: clinicians appreciate the flexibility afforded by these systems while patients benefit from reduced incidences of side-effects and more convenient dosing regimens (Takada and Yoshikawa, 1999). The therapeutic, convenience and overall cost advantages of MR systems have rendered them widely used, especially for the treatment of chronic illness like asthma and hypertension (Vyse and Cochrane, 1989; Keating, 2006; Hiremath and Saha, 2008).
MR systems for oral administration of drug substances can be classified into three main groups; diffusion-controlled systems in which the drug diffuses through a polymer membrane or matrix; chemically-controlled systems, which release drug substances via polymer degradation or cleavage of drug from the polymer chain, and solvent-activated systems, which can either be osmotically driven or polymer-swelling controlled (Takada and Yoshikawa, 1999). One way of achieving controlled release of the drug is application of an insoluble polymer coating, such as ethylcellulose to a dosage form. The ethylcellulose coating controls the release of drug by diffusion through the intact polymeric membrane at constant rate; therefore, the plasma concentration is at steady- state (Kallai et al., 2010).
Regardless of the MR system, the process of oral drug delivery is problematic and hampered by physiological barriers associated with variable gastrointestinal tract (GI) transit times, presence of food, pH, ionic
composition, surface tension, disease state and gut metabolism. This shows that optimizing the uptake of drugs with unfavourable physicochemical characteristics can be a formulation challenge (Singh and Kim, 2007; McConnell et al., 2008; Mudie et al., 2010).
Dissolution testing is one of the most important tools during formulation development and quality control of solid dosage forms (Kramer et al., 2005; Aulton, 2007). Dissolution testing allows manufacturers of pharmaceutical products to assess the rate of drug release, consistency of batches, and can even provide information on in-vivo behaviour of a drug product (Dressman et al., 2007). However, unlike the conventional oral drug delivery systems, MR systems are not usually amenable to standard dissolution testing methodologies, typically involving the use of simple compendia buffers or standard pharmacopeia methods (Fadda and Basit, 2005). Thus, in recent years, there have been attempts by several research groups to develop the most appropriate dissolution testing methodologies that are able to reflect physiological conditions (Dressman et al., 2007), especially with respect to in vitro - in vivo correlation studies.
Gastrointestinal tract physiology and drug disposition
1.2.1 Anatomical and Physiological Considerations
The gastrointestinal (GI) tract is a muscular tube that consists of four major anatomical areas; the oesophagus, the stomach, the small intestine and the colon as shown in figure 1. The wall of the GI tract is structured by four layers; the serosa (the outer layer), the muscularis externa (the smooth muscle tissue), the submucosa (the connective tissue layer) and the mucosa (the inner layer of the lumen). The surface area of the lumen is
large and therefore is ideally suited for the disintegration, dissolution and absorption of orally administered dosage forms (Walker and Whittlesea, 2007).
The effective absorption of many drug substances is reduced during the digestive process (figure 1). This is because there are various physiological factors that affect the rate of drug dissolution. Firstly, absorption of the drug depends highly on the GI transit time, which in turn, influences the localization of the dosage form in areas of the GI tract where absorption takes place. Secondly, the formulation is affected by a number of factors such as mechanical forces, nature of mucosa, the surface area of the lumen, the pH and the presence of the bacteria and enzymes in the gut (Dressman and Kramer 2009).
Figure 1; Represents the physiology of the GI tract (Encyclopedia, 1999)
The stomach is not the major site of absorption but the orally administered drug first comes in contact with the stomach, in order to release the drug that is affected by pH variation in the GI tract (Dressman et al., 2007; McConnell et al., 2008). Therefore, the MR preparations are developed by considering the factors that affects the drug release (Aulton, 2007). Bearing this, MR products are more complicated and difficult to attain a standard dissolution test irrespective of the physiological factors under consideration (Dressman and Kramer 2009).
1.2.2 Solubility, dissolution and absorption of drugs from the GI tract
Limited absorption and bioavailability is universally recognized as a major challenge in oral administration of drug substances (Stagemann et al., 2007). In order for a drug to exert a pharmacological action, its molecules must be soluble in physiological fluids of the GI tract. Therefore, aqueous solubility is one of the indicators of bioavailability. Dissolution, on the other hand, refers to the process of transfer of molecules from the solid state into solution and is mathematically described by the Noyes-Whitney Equation (Aulton, 2007). The dissolution rate is a rate limiting step in the absorption process kof oral drugs, especially those that are poorly soluble (Amidon et al., 1995). Indeed, recent developments in biopharmaceutics have shown that bioavailability of many drugs could be predicted by a well designed dissolution testing protocol (Shinkuma et al., 1984; Dressman and Reppas, 2000; Garbacz et al., 2009). This is the basis for the biopharmaceutics classification system, proposed by Gordon Amidon in 1995 (Amidon et al., 1995).
Research interest in the use of the so-called biorelevant dissolution media over the last decade has led to the testing of dissolution of drug substances under conditions that mimic the physiological environment of the GI tract. However, questions do remain regarding the completeness of the compositions in fully representing the GI tract conditions. Accordingly, some workers have advocated for the use of media whose ionic composition is matched to in-vivo conditions (Fadda and Basit, 2005; McConnell et al., 2008) rather than the biorelevant approach of Dressman and co-workers above. The systems advocated for are physiological Hank's and Kreb's bicarbonate buffers, which are thought to simulate the ionic composition and buffer capacity of intestinal fluids better than phosphate buffer systems used in biorelevant media (Fadda and Basit, 2005). However, to date, the use of physiological bicarbonate buffers has been problematic due to their inherent instabilities (Dressman et al., 2007; Fadda and Basit, 2005), limiting their wide-adoption in the pharmaceutics.
1.2.3 Biopharmaceutics of modified release dosage forms
The processes leading to the absorption and subsequent bioavailability of a therapeutic agent after oral administration are complex and unpredictable. Whereas the absorption of nutrients from the GI tract is a natural phenomenon, for which the GI tract has evolved over millions of years to do, absorption of drugs is an unnatural and subject to significant variability (Dressman et al., 2007). The physicochemical properties of the drug and its dosage form are key determinants to successful delivery as are the physiological factors existing in the GI tract. Clearly, successful delivery of a drug from the GI involves achieving the right balance between these two "forces". In recent years, there has been growing appreciation of
biopharmaceutics with respect to inter and intra-subject variability of orally administered drugs, the causes of which being mainly physiological.
Taking the pH, for instance, it is well-known that the pH fluctuates throughout the GI tract and varies with meal volume and content or volume of secretions (Mudie et al., 2010). Abrahamsson et al., (2004). A key finding is that food delays disintegration of dosage forms which can lead to delayed dissolution rate of the drug (Abrahamsson et al., 2004). The small intestine is generally accepted as the main site of drug absorption in humans; the stomach and the colon contributing much less to uptake of drugs administered orally (Masaoka et al., 2006). Variability in gastric emptying can, therefore, influence whether an orally administered dose is able to access to the site of optimum drug absorption. Moreover, gastric emptying is itself influenced by several factors, including the size and type of meal as well as physiological factors. For MR formulations, which by nature, are non-disintegrating, this can mean failure to function as designed. On the other hand, the small intestine transit time may determine whether absorption is complete or not as it determines how long the dosage form remains in the region that is conducive to drug absorption (Mudie et al., 2010). With respect to physicochemical factors of the drug or the dosage form, Muschert et al., (2009) demonstrated that the drug release could be affected by the level of the ethylcellulose coating, which, over the long term, drug release patterns could significantly altered. Suffice to say, knowledge of the physiological factors that affect the release of the drug can facilitate the development of dosage forms and accompanying tests that are more representative to human condition (Mudie et al., 2010).
With respect to in-vitro dissolution testing of MR systems, different types of apparatus are currently used. The most commonly used systems are the USP Apparatus 1 and 2 (Fadda et al., 2009). These systems are relatively cheap and easy to use. However, data obtained may not be as accurate as the data obtained from USP Apparatus 3 or 4 whose designs take account of behaviour to that of GI tract and might yield better in-vitro in-vivo correlations (Garbacz et al., 2009).
The buffer media is also one of the major factors that are being re-examined, with some authors advocating for more physiologically relevant dissolution media systems (Muddi et al., 2010; Fadda and Basit, 2005). In particular, the role of bicarbonate -based buffers in yielding better in-vivo disintegration times compared to phosphate buffers has been recently reported (Fadda et al.,2009). One of the greatest challenges with respect to adopting bicarbonate buffers, however, is the continuous rise in pH as the carbon dioxide gas escapes, which renders the media unstable and less suited to compendia dissolution testing. Clearly, there is a need for further work in this area, and so far as it is apparent, no work has been reported with respect to dissolution profiles of controlled release reservoir systems.
Diltiazem hydrochloride is a calcium channel blocker that prevents the influx of calcium ions in cardiac muscle. Therefore, it is effective in the management of chronic heart disease, including angina pectoris, myocardial ischemia and hypertension. The structural formula of diltiazem is shown below (Fig 2):
Figure 2: Structural formula of diltiazem HCl (Dailymed, 2011)
Diltiazem is available as immediate release tablets or capsules and MR preparations. The commonest brand names in the UK included in British National Formulary are CARDIZEM CDÂ®, CARDIZEM LAÂ®, and DILACOR XRÂ® (BNF 2008). In clinical use, the dose timing of diltiazem is critical as pH is known to affect the solubility of the drug (Sood and Panchagnula.,1998). Therefore, MR diltiazem was considered a suitable model for the study in order to determine if the release mechanism of the drug from MR system was influenced by media composition.
Aims and objectives of the study
The main aim of the study was to compare the rate of dissolution of modified release diltiazem HCl drug in both the Hank's bicarbonate buffer and the phosphate buffer at pH 7.4. The specific objectives of the study were as follows:
To determine the total drug content of the MR diltiazem HCl in each capsule and to undertake various other quality assurance test such as disintegration, drug solubility and weight uniformity. This would ensure that the ADIZEM-SR capsules are ideal to get tested.
To undertake dissolution test of MR diltiazem HCl pellets in both the Hank's and phosphate buffer in USP 2 paddle apparatus. To examine the rate of dissolution in USP 4 apparatus as this would enable to obtain data that is accurate. Therefore, the USP 4 data could also confirm the results gained from USP 2 paddle apparatus.
To investigate the release of diltiazem HCl drug from the ethylcelullose coated pellets, few Scanning Electron Microscope (SEM) samples of original pellets were taken. Intended to take few SEM samples of pellets when removed from both the physiological buffers. This would enable to elicit any pore formation or cracks formed on the surface of the pellets.
Materials and Methods
Table 1: List of reagents and materials used in the study
Controlled-release Diltiazem HCl (120mg) capsules (AdizemÂ®, Napp Pharmaceuticals Limited, Cambridge, England) were purchased from a local retail outlet. The formulation consisted of pellets coated with Ethylcellulose. Other materials and reagents used for the research project are tabulated below (Table 1) and were used as received.
VWR International Ltd, Leicestershire, England
Fisher Scientific, Loughborough, England
Fisher Scientific, Loughborough, England
BDH Laboratory Supplies, Poole, Dorset, England
Fisher Scientific, Loughborough, England
Alfa Aesar, Heysham, Lancashire, England
Fisher Scientific, Loughborough, England
Fisher Scientific, Loughborough, England
Materials and Method
2.2.1 Preparation of the dissolution media:
Media sufficient to undertake the appropriate dissolution test were prepared according to the formulas shown in the tables below (Table 2 and Table 3).
Hank's Buffer (pH 7.4)
Quantities for 5 L
Table 2: Formula used to prepare Hank's buffer (pH 7.4)
Briefly, the weight of each compound required to prepare the required volume of media was calculated and amount of each compound was separately weighed.
Materials and Methods
The quantity of water required was degassed by using a degasser (Caleva, Model: SW39) and set aside in a 10 l tank. All the ingredients, with the exception of NaHCO3 and CaCl2, were added stepwise to a proportion of the degassed water in a 1000 ml beaker and stirred well before they were transferred into a carboy. The pH of the prepared media was checked using a pH meter (Corning pH meter 240, Model: SW39) and where necessary, was adjusted by adding 1M HCl or NaOH solution.
Phosphate Buffer (pH 7.4)
Quantities for 5L
Table 3: Formula used to prepare phosphate buffer (pH 7.4)
After the weight of each compound required to prepare the appropriate volume of media was established, the amount of water required was collected and degassed as described above. A proportion of the degassed water was poured into a beaker in order to dissolve the weighed compounds. The solution was then transferred in to the 10 l carboy and thoroughly mixed. The pH of the media was checked and if required, adjusted with 1M HCl or NaOH.
Materials and Methods
2.2.2 Preparation of standard calibration curves
Standard solutions (5-100 mg/l) containing pure diltiazem HCl powder were prepared by accurately weighing out 0.5 g of the drug using a sensitive analytical balance (Kerns ABS/ABJ, Model: ABJ 120-4M). After the drug was dissolved in the appropriate media solution (stock solution) in a 1000 ml volumetric flask, the five different standards were prepared as shown in table 4 below:
Media [Hank's/ Phosphate]
Table 4: Diltiazem HCl calibration standards
Once the five different standards were prepared, the absorbance was determined using a UV-Vis spectrometer (Agilent Technologies, Model: 8453) at a wavelength of 236nm.
Materials and Methods
2.2.3 Dissolution testing using USP Apparatus 2 (Paddle) bath
The extents of diltiazem HCl released from the coated pellets at different time intervals was done with the aid of a USP Apparatus 2 (Paddle) (Erweka, Model; DT 600 HH). The apparatus was set for a total run time of 12 hours, a paddle speed of 100 rpm and the temperature of 37â°C. The capsules (n=6) were placed into six different flasks containing 900ml of the dissolution media. At specified times (i.e., 0.5h, 1.0h, 2.0h, 4.0h, 6.0h and 8.0h), samples (4 ml) were withdrawn from the vessels with the aid of a syringe and filtered with a syringe filter (0.4 Âµm, Millex, Millipore). Immediately, fresh media of equivalent volume was added to the vessel. The amount of drug released in each sample was determined with a UV-Vis spectrometer at a wavelength of 236nm. The test was done in triplicate.
2.2.4 Dissolution testing using USP Apparatus 4 (Flow-through) bath
Dissolution testing in the USP Apparatus 4 was undertaken in a CE7 Smart flow-through apparatus (Sotax AG), equipped with six, 22.6 mm diameter test cells, a 6-cylinder piston pump, a media splitter and an on-line UV-Vis spectrophotometer. The open-loop configuration was utilized. A 5 mm-sized glass bead was placed in the tip of each cell followed with a total of 1.7 g of 1 mm-sized glass beads. A glass microfiber filter (MNGF1, 0.7Â Î¼m pore size, 25Â mm diameter, Whatman, Germany) was placed on the top of the cell. During the experiment, the capsule was mounted on top of the beads. Experiments were performed in triplicate at 37Â â°C in the appropriate dissolution media at a flow rate of 8 ml/min. All drug analyses were performed automatically by the on-line spectrophotometer at programmed
Materials and Methods
time intervals. The results are presented as the mean value of minimum six tablets in concentration (mg/ml) and as cumulative % drug release.
2.2.5 Scanning electron microscope (SEM)
Scanning electron microscope was performed to analyse the characteristics of the ethylcellulose coated pellets. The ADIZEM-SR capsules (n= 3) that contained pellets were emptied. The pellet was carefully placed on to the metal stub that has a conductive carbon sticker. A new carbon sticker that conducts was replaced each time when different pellet was placed. The metal stub that has the pellet was carefully placed in the SEM. The SEM was operated at 12.5 to 20 kV and the samples of pellets were obtained at Ã-140 magnification.
2.2.6 Total drug content in Adizem SR capsules
Capsule (n=5) contain diltiazem that were randomly selected and their contents were carefully emptied into a glass mortar. Using a glass pestle, the pellets were gently ground into a fine powder. To the powdered mass, 200ml of a 50:50 mixture of methanol and acetone was added. Using the pestle, the mixture was gently mixed to ensure all the drug had dissolved. Thereafter, 10 ml of the mixture was carefully withdrawn with a pipette and transferred into a separate volumetric flask (100 ml) and made to the mark with the 50:50 mixtures of methanol and acetone. After this was thoroughly mixed, a portion was removed, filtered and the drug content analysed with the aid of the UV-Vis spectrometer at a wavelength of 236nm. The drug content was determined using a standard, prepared by weighing out 0.1000g of pure diltiazem HCl dissolved in 100ml of the 50:50 mixtures of methanol and acetone. Generally, three absorbance readings were taken for each of the samples prepared.
Physical characteristics of the ADIZEM SR 120mg capsule
The result for the quality control test regarding the physical characterisation, the characteristics of the pellets, the drug solubility and the total drug content were as follows:-
3.1.2 Weight uniformity
The results for weight variation are shown in table 5 below:-
0.125 g + 0.05
0.216 g + 0.05
0.215 g + 0.05
Table 5: The variation of weight for three different ADIZEM-SR capsules of 120mg.
The result shows that the weight variation was below 5 % RSD. Therefore, the weight variations of the capsule meet the BP requirements for weight, as the percentage of RSD was below.
The disintegration time for the modified release diltiazem capsule was less than 30 seconds, when in contact with the media at 37â°C. According to the British Pharmacopeia (BP) 1993, the capsule should generally disintegrate within 30 minutes. Hence, the disintegration time for this specific modified release diltiazem capsule falls within specification of the BP.
3.1.4 Scanning Electron Microscope (SEM)
Figure 3: SEM pictures of 'A' and 'B' shows diltiazem pellets that are coated with ethylcellulose (x140).
The samples of pellets taken by SEM as shown in figure 3A and figure 3B clearly illustrates that, there are no pores or cracks present on the outer surface of the pellets that could lead the diltiazem drug to leak from the matrix when in contact with the media. However, the SEM samples proof that, the complete release of diltiazem drug within four hours was not due to this reason.
Intended to obtain some interesting samples of pellets after placing it in both the physiological buffers but was unable to do due to time constrain.
3.1.5 Drug Solubility.
Diltiazem HCl is in Biopharmaceutical Classification Scheme (BCS) I. Therefore, the drug has high solubility and high permeability. Due to this reason, the drug was rapidly absorbed across the intestinal tract and thus shows good bioavailability. The rate of dissolution could be affected by high solubility of the drug.
3.1.6 Total drug content in Adizem SR capsules
According to the BP 2009, the monograph for diltiazem hydrochloride show that, the total content of diltiazem drug in each capsule should be between 118.20mg to 121.20mg. The total drug content in each ADIZEM-SR diltiazem capsule was 120.34 mg + 0.007, which therefore falls within the specification of the BP 2009.
3.2 Standard calibration curve
The tabulated data of absorbance of each standard from Hank's and phosphate buffer was transferred in to a graphical form (figure 4) as shown below;
Figure 4: The graph 'A' and 'B' shows the standard calibration curve for diltiazem HCl in Hank's and phosphate buffer.
The graph was used to calculate the drug concentration for the unknown samples collected from the USP 2 paddle apparatus dissolution tester.
3.3 The in-vitro drug release of diltiazem HCl in Hank's and phosphate buffer in USP 2 paddle apparatus
The dissolution profile of diltiazem capsule in USP 2 paddle apparatus shows a rapid dissolution rate, when examined at 100 rpm and 37Â°C + 1. Interestingly, the data obtained from USP 2 apparatus (figure 5) illustrates a complete dissolution that occurred within 4 hours from the coated pellets in both the bicarbonate and phosphate buffer.
Figure 5: Mean dissolution rate of controlled release diltiazem capsule in Hank's and phosphate buffer (pH 7.4). Data obtained from USP 2 paddle apparatus at a speed of 100 rpm and temperature of 37â°C+1.
The statistical T-test for USP 2 paddle apparatus.
USP 2 Paddle Apparatus
Avg % of drug released at 1hr
Avg % of drug released at 4hr
Avg % of drug released at 8hr
46.762 + 19.094
75.791 + 24.857
83.401 + 21.110
29.300 + 5.077
73.736 + 21.901
84.075 + 15.708
Table 6: P-value for the percentage of drug released from USP 2 paddle apparatus for both bicarbonate and phosphate buffer at pH 7.4.
Initially within the first one hour (figure 5), the percentage of drug released in Hank's buffer was 23% compared to phosphate buffer which was 15%. Generally, this shows that Hank's buffer released the drug immediately compared to the phosphate buffer. However, the statistical analysis of T-test shows that, the P-value for the first one hour was P>0.05 ( table 6). This value suggests that, there is no significant difference between Hank's and phosphate buffer.
Although, between 2 to 4 hours (figure 5), the rate of drug released from Hank's and phosphate buffer was similar. The P>0.05 at 4 hour (table 6) represents that; there is no significant difference in the release rate in both the physiological buffers.
The maximum rate of release of diltiazem drug was achieved at 4 hours (figure 5). This is because at 2 hours the rate of drug release was only 30%, whereas at 4 hours the rate of drug release increased 10 % i.e. 40% in both Hank's and phosphate buffer. This shows that the release rate of drug from the coated pellets was rapid. Within the time interval of 4 to 6 hours, the complete dissolution has taken place in both the physiological buffers. The maximum drug released was 45% at 6 hours in both the buffers. Between 6 hours to 8 hours, there was no difference between the percentages of drug release in both Hank's and phosphate buffer as P>0.05 (table 6). This shows that, the diltiazem drug has fully released from the matrix of the coated pellets.
3.3.1 The in-vitro drug release of diltiazem HCl in Hank's and phosphate buffer in USP 4 apparatus
The result obtained from the USP 4 dissolution tester (figure 6 and figure 7) generates similar values that support the data from USP 2 paddle apparatus.
Figure 6: The drug release profile of controlled release diltiazem capsules in Hank's buffer at pH 7.4.
The figure 6 demonstrates that, the percentage of drug released at 1.5 hours (i.e. 90 minutes) in Hank's buffer was 40%. The rate of drug release was same in all six flasks in USP apparatus 4. At 3 hours (i.e. 180 minutes), all the six flasks has a varied rate of release of the drug from the coated pellets. The percentage of drug released varied from 83% to 98%. At 4.5 hours (i.e. 270 minutes), all the six flasks continued with the same rate of drug release. Between 3.5 hours to 4.5 hours (i.e. 210 minutes to 240 minutes) complete dissolution has taken place because the percentage of drug released was constant.
Figure 7: The drug release profile of controlled release diltiazem capsules in phosphate buffer at pH 7.4.
However in figure 7, the percentage of drug released in phosphate buffer at 1.5 hours was 40%. The release rate was similar in all the six flasks of the USP apparatus 4. At 2 hours (i.e. 120 minutes), all the six flasks had different rate of release of the drug. This varied rate of drug release was rapid in phosphate buffer compared to Hank's buffer in USP apparatus 4.
At 3 hours (i.e.180 minutes) the rate of drug released from the coated pellets ranged from 65% to 100% from six different flasks. From the time interval of 3.5 hours (i.e. 210 minutes) to 4 hours (i.e. 240 minutes), the complete dissolution has taken place as the percentage of drug released was stabilised in all six different flasks. This was same in Hank's buffer.
3.3.2 Comparison between the in-vitro drug release of diltiazem HCl in both the physiological buffers in USP 2 paddle apparatus and USP 4 apparatus
Figure 8: Mean dissolution rate of controlled release diltiazem hydrochloride capsule in Hank's and phosphate buffer (pH 7.4). Data attained from USP 4 apparatus at a flow rate of 8ml/min and temperature of 37â°C+1.
Statistical T-test for USP 4 apparatus
USP 4 Apparatus
Avg % of drug released at 1.5hr
Avg % of drug released at 2.5hr
Avg % of drug released at 4.5hr
53.460 + 23.576
74.724 + 33.129
80.313 + 35.805
50.435 + 22.298
71.588 + 32.294
76.785 + 35.017
Table 7: P-value for the percentage of drug released from USP 4 apparatus for both bicarbonate and phosphate buffer at pH 7.4.
Compared to the average percentage of drug released in USP 2 paddle apparatus, the data obtained from the USP 4 apparatus demonstrates the exact same trend. Initially in the first one hour (i.e. 60 minutes), the rate of drug release was 32% in both the Hank's and phosphate buffer. However, the P-value (P<0.05) (table 7 ) suggest that there is significant difference on the release rate of the drug between both the physiological buffer. Therefore, this does not support the data obtained for the first one hour from USP 2 paddle apparatus, as the graph (fig 5) and P-value (table 6) suggest that there is no significant difference on the percentage of drug released from both the buffers.
Interestingly, the rate of release was increased between 1 to 2 hour because the percentage increase was from 30% to 70% i.e. 40 % increased. The release rate of the drug was not significantly different in both the physiological buffers, as the P-value was P>0.05 (table 7). Whereas in USP 2 paddle apparatus, the rate of release was quite prolonged, as the increase was between 1 hour to 4 hours in both the Hank's and phosphate buffer.
The percentage of drug released was stabilised between 3.5 hours to 4 hours in both the physiological buffers, as the P-value was P>0.05 (table 7). This shows that complete dissolution has taken place from the coated pellets. The USP 2 apparatus also had the similar trend because the drug release was constant within the period 6 hours. This shows that, the capsules do not acquire the characteristics of controlled release preparations.
Discussion and Conclusion
The importance of modified release diltiazem HCl is that it enables to sustain a constant therapeutic steady state concentration of drug at the site of action. The constant effect of the drug is achieved through the controlled release of drug from the matrix of the pellets. The drug release rate from the modified release preparation is controlled by various mechanisms such as dissolution, diffusion, osmosis and ion-exchange controlled (Wen and Park, 2010).
The dissolved drug diffuses out from the matrix
Diltiazem HCl drug
The media enters the matrix through the polymer membrane by osmosis.
When in contact with media
Figure 9: Mechanism of drug release from a controlled reservoir pellet system (Wen and Park, 2010)
The in-vitro findings (figure 5 and figure 8) from this work clearly show that the release rate of the drug from the matrix of the ethylcelullose coated pellets was rapid. As described above, the diffusion of the drug molecules out from the pellets is controlled by the semi-permeable polymer membrane, ethylcellulose. The main rate limiting step in this type of mechanism was the diffusion of the media through the polymer membrane which enabled the diltiazem in the matrix to be dissolved and subsequently diffuse out through the membrane driven by the concentration gradient. Thus, the rate of drug release was mainly controlled by the thickness of the polymer membrane, and is well described by Fick's law, which relates the rate of dissolution and particle size or the surface area, the concentration gradient and the thickness of the coating (Wen and Park, 2010).
As the diltiazem drug is highly soluble, the drug release would be expected to be fairly rapid. This is in accordance with what was found in the present work. Increasing the ethylcelullose coating on pellets would be expected to decrease the rate of drug release (Sadeghi et al., 2002). However, as discussed by Asa, 1998 the characteristics of the pellet also contribute to the dissolution of the drug. The SEM (figure 3) illustrated that the pellets were hard, non-porous and had a smooth surface. As the pellets were small in size (average size), thus, the applied ethylcelullose coating level was low compared to large sized pellets. As the pellets were small in size, the media was able to penetrate easily through the polymer membrane and caused increased rate of drug dissolution (Bernard et al., 1999).The equal distribution of polymer and thickness of the coating on the surface of the pellets is vital as this accomplish the constant release of the drug (Heng, 2005).
Another factor that could affect was the pH of the dissolution media that had an influence on the ethylcelullose polymer membrane of pellets. The pH in the GI tract varies with the content, volume of secretion and meal volume (Mudie et al., 2010). The pH of the dissolution media ranged from 7.40 to 7.50 but the pH variation was more likely in bicarbonate buffer. As the pH was above 6, the carboxyl group in ethylcelullose polymer was dissociated and led to increased permeability of the membranes (Bernhard et al., 1999). This therefore, enhanced the uptake of the media, increasing the rate of dissolution of diltiazem (Bernhard et al.,1999). As the standard physiological buffers represent the intestinal fluid, the rapid rate of dissolution of modified release preparations in-vivo could lead to dose dumping and exerts toxic effects on the body.
Analysing the two standard buffers (i.e. Hank's physiological buffer and phosphate buffer), the osmolality of the buffers were different and primarily affected the mechanism and the release rate of the diltiazem HCl drug from the coated pellets. The media with the lower osmolaity had enhanced penetration into the matrix of the coated pellets enabling it to be more available for drug dissolution. In addition, ahigh hydrostatic pressure was created inside the matrix that acted against the ethylcelullose coating causing the polymer membrane to crack. The crack formation on the polymer is caused when the mechanical stability of ethylcelullose polymer exceeds in the built up of the hydrostatic pressure (Muschert et al., 2009). This resulted in increased rate of diltiazem drug being released from the matrix of the coated pellets. Unfortunately, no confirmatory tests were done to visualize the cracks on the polymer membranes as the SEM samples of
pellets removed from the dissolution media could not be conveniently manipulated and due to time limitations, no further work was done to optimize their handling. However, osmolality is not considered as a major factor that affects the dissolution of the diltiazem HCl in-vivo since the change in the rate of drug release caused due to the change in osmolality is unlikely to raise in-vivo (Muschert et al., 2009).
With respect to hydrodynamics caused by different agitation mechanisms, the paddle speed set in USP 2 paddle apparatus represents the in-vivo patterns of the GI motility. This was set to 100 rpm as it has been demonstrated that the speeds between 75 and 125 rpm produce good pharmacokinetic data (Scholz et al., 2003). Agitation is necessary because it allows the dissolved drug, after diffusing out of the matrix of the coated pellets, to be easily dispersed into the media. This subsequently created a concentration gradient between the media and the pellets and allowed the entry of media in to the matrix that led to increased dissolution of diltiazem drug. Eventually this led the dissolution rate to stabilise because the concentration of drug in the pellets were low compared to the concentration of drug in the dissolution media.
However, the dissolution rate of modified release capsules in USP 4 apparatus was dependent upon factors such as tablet orientation in each flow through cells and the surface area of the pellets. As the capsule was placed in the horizontal position, a high velocity of the media was caused around the capsule which led to a high cross- sectional area. Thereby, this caused an increased rate of dissolution of the diltiazem hydrochloride drug. The surface area of the pellets that contained diltiazem hydrochloride was also one of the important factors that enhanced the increased rate of
dissolution. This was because the thickness and the size of the pellets affected the rate of entry of the media (i.e. physiological Hank's buffer or phosphate buffer) and thereafter affected the rate of dissolution of diltiazem HCl drug (Stephen, 2000).
However, despite the influence of the above factors, the study's findings show that the percentage of the drug released in both buffers (i.e. Hank's and phosphate buffer) was similar (figure 5 and figure 8). This was further confirmed by the findings obtained from the USP 4 apparatus (figure 6 and figure 7). This shows the ionic composition of both the standard physiological buffers have no effect on the dissolution rate of modified release diltiazem, and therefore, on the account of these current findings, the earlier findings of Fadda and Basit (2005) and Fadda et al.,(2009) suggesting that physiological bicarbonate buffers are more discriminative of drug release probably do not apply with modified release systems. It would be of interest to determine if this is generally applicable to neutral or acidic drugs formulated in modified release reservoir systems as well.
4.2 Conclusions and Suggested Future work
This study investigated drug release profiles of diltiazem HCl in bicarbonate and phosphate buffer from modified release pellets. It was established that drug release was fairly rapid (maximum release in 4 hours, complete release between 4 to 4.5 hours) in both buffer systems. Drug release in physiological bicarbonate buffer compared to phosphate buffer at pH 7.4 was not significantly different, had no significant difference in the rate of drug released, and thus, the ionic composition of the buffers had minimal effect on the dissolution rate of the model drug.
To widen on this new knowledge, further research could be undertaken, for instance, to investigate the effect of MR diltiazem HCl drug on fed and fasted state, to consider the variation in the viscosity of the media and further examine the effects of adding surfactants to the media.
Furthermore, an in-vitro in-vivo study could be undertaken to establish the relationship of the in-vitro dissolution result to actual behaviour in patients. Currently, achieving data that resembles the in-vivo conditions is challenging but vital as it enable to design a product that is effective and able to positively impact or lead to quality of life in patients. Another aspect of the work could consider working with in-house prepared capsules (with better control on ingredients) and investigate differences with commercial samples.