Biorelevant Dissolution Methods

Unlike dissolution methods used for QC in which their design is primarily based upon drug substance physicochemical properties and formulation principles, biorelevant dissolution methods are designed to closely simulate physiological conditions in the GI tract. However, it should be noted that the physico-chemical properties of the drug substance (e.g., solubility) and its formulation (e.g., immediate- or extended-release dosage forms) play a key role in selecting an appropriate type of biorelevant dissolution medium (e.g., gastric or intestinal medium), apparatus (e.g., a single vessel or multiple vessels), and test conditions (e.g., agitation speed and duration of a dissolution test), since these drug substance and formulation characteristics impact the location where the drug dissolution takes place in the GI tract. For instance, weak acids that are not soluble in the stomach (low pH) are usually very soluble in the small intestine (high pH).

In addition, for drugs that are unstable in an acidic method, a delayed-release formulation can be employed to ensure that the drug release occurs only in the high pH GI region (the small intestine).

3.7.3.1 Biorelevant Dissolution Media for Gastric Conditions

For BCS Class I drugs that are formulated in immediate-release dosage forms or any products that dissolve rapidly and completely in an acidic medium, it is logical to use a dissolution medium that reflects the gastric conditions. The minimum physiological parameters that need to be considered here include pH, surfactants, and enzymes. Food effects may also be considered if significant food effects are observed in vivo. It should be noted that biorelevant dissolution media or methods are designed primarily to mimic GI conditions in healthy subjects under the fasted and fed state, since in vivo bioequivalence studies are generally performed using these healthy subjects.

The pH in the stomach has a significant influence on the dissolution rate due to its effect on the solubility of a drug substance. In the fasted state of young healthy subjects, values of gastric pH are generally between 1.4 and 2.1 (Dressman et al., 1990). However, the fasted state gastric pH values are found to be higher in subjects who are either over 65 years old or receiving gastric acid blocker therapy (Russell et al., 1993; Christiansen, 1968). The gastric pH values also increase immediately following meal ingestion (pH 3-7) (Dressman et al., 1998). The gastric pH resumes the fasted state values in approximately 2-3 h depending on the size and content of the meal (Dressman et al., 1998).

The surface tension of gastric fluid is lower than that of water, and it was measured in the 35-50 mNm-1 range (Finholt and Solvang, 1968;Finholt etal., 1978; Efentakis and Dressman, 1998). Although the decrease in surface tension suggests the presence of surfactants in the stomach, substances that lower the surface tension in vivo have not been identified unequivocally. The enzyme, pepsin, is also found to be in gastric fluid. The presence of this enzyme in the stomach causes a major problem for protein and polypeptide stability in addition to the acidity of the gastric environment.

Based upon the physiological factors described above, to simulate gastric conditions in the fasted state, the pH values of a gastric dissolution medium should be in the pH range of 1.5-2.5. In addition, surfactants, such as SLS, should be added into the medium to lower its surface tension close to the in vivo values. As mentioned earlier, for some capsules, an enzyme (pepsin) can be added to the medium to ensure timely dissolution of the shell by preventing pellicle formation. A sample composition for SGF in the fasted state is shown in Table 3.2 (Dressman, 2000). Due to its simplicity, this medium can also be used for QC dissolution testing.

To simulate the fed state in the stomach, the use of milk (Macheras et al., 1987) and Ensure1® (Ashby et al., 1989) may be appropriate, since these media offer appropriate ratios of fat to protein and fat to carbohydrate. However, these two media are not suitable for routine quality assurance testing due to the difficulties in filtering and separating the drug substance from the medium for analysis.

Table 3.2. Sample composition for simulating gastric conditions in the fasted state (Klein 2005)_

SGF composition

Sodium chloride Hydrochloric acid Triton x 100

Deionized water qs ad 300 mL

3.7.3.2 Apparatus and Test Conditions for Simulating the Stomach

The basket and paddle methods are frequently used, in conjunction with biorelevant media for gastric conditions, to simulate the drug release in the stomach under fasted and fed conditions. Since these two devices consist of a single vessel for each dosage form and are operated with a fixed volume of a single medium, they are best suited for drug products in which the majority of drug release occurs in the same section of the GI tract. Although the relationship between in vivo hydrodynamics or motility and rotational speed is still not well understood, the range of 50-100 rpm, which is established empirically, appears to give data that can be used to establish IVIVC.

In comparison to the basket and paddle methods, the reciprocating cylinder (USP Apparatus 3) and the flow-through cell (USP Apparatus 4) may offer some advantages regarding simulating gastric conditions. Apparatus 3, which originates from the official disintegration tester (Borst et al., 1997), can be used to improve the study of food effects in the stomach by simulating changes in the composition and motility with time due to gastric secretion and digestion using a series of different media and agitation rates in the vessels. Similarly, Apparatus 4 also provides the possibility of changing the composition of a medium and flow rate during the test.

Regarding the volume of these gastric media, it depends on the volume of administered fluids and endogenous secretion. For instance, in the fasted state, gastric juice secretion is usually low. Therefore, by considering the quantity of fluid that is ingested with the dosage form, the medium volume should be in the order of 200-300 mL. The duration of a dissolution test should reflect the time available for dissolution in the stomach that is a function of the emptying pattern, which can vary considerably depending on the size of the solid particles as well as the size and the content of the meal (Meyer et al., 1988; Moore et al., 1981). If a drug formulated in an immediate-release dosage form is administered in the fasted state and is well absorbed from the upper small intestine, it is appropriate to run the dissolution test with SGF for 15-30 min. Nevertheless, there are still discrepancies in various pharmacopoeia regarding the duration of a dissolution test for immediate-release drug products (e.g., 30-120 min).

3.7.3.3 Biorelevant Dissolution Media for Intestinal Conditions

For poorly soluble drugs (e.g., BCS Class II drugs that are neutral or weak acids), it may be more appropriate to use a dissolution media that mimics the intestinal conditions. The dissolution of drug products in the small intestine is influenced by physiological factors including but not limited to pH, endogenous secretions from the pancreas and gall bladder (e.g., bile salts, lecithin, and digestion enzymes), and food effects. The physiological aspects related to drug absorption in the colon will not be addressed here and can be found elsewhere (Dressman et al., 1997).

The pH values of intestinal conditions are considerably higher than those of gastric conditions, and were measured to be in the range of 5.5-6.0 for the duodenum, 6.5 in the jejunum, 7 in the proximal, and 7.5 in the distal ileum (Dressman et al., 1998). The high pH values in the small intestine are attributed to the neutralization effect of bicarbonate ion secreted by the pancreas. It should be noted that pH values gradually increase from the duodenum to the ileum, resulting in a pH gradient in the small intestine. In the fed state, the pH values in the duodenum (4.2-6.1), jejunum (5.2-6.2), and ileum (6.8-7.8) are generally lower than those in the fasted state (Dressman et al., 1990; Ovesen et al., 1986; Fordtran and Locklear, 1966).

In the small intestine, secretion of bile from the gallbladder in the duodenum leads to a high concentration of bile salts and phospholipids (lecithin), resulting in the formation of mixed micelles even in the fasted state. These bile salts and lecithin may have a significant enhancing effect upon the dissolution rate of poorly soluble drugs by improving the wettability of solids and by increasing the solubility of a drug substance into mixed micelles (Mithani et al., 1996). As for gastric secretion, the rate of bile secretion also depends strongly on the prandial state, in which the concentration of bile salts and lecithin further increases in the presence of food. Since the majority of bile salts (>90%) is reabsorbed by the active transport mechanism (Davenport, 1982), a decreasing gradient of bile salts is observed along the small intestine. The digestion enzymes, such as lipases, pep-tidases, amylases, and proteases, are also secreted by the pancreas in the small intestine in response to food ingestion. Lipases and peptidases present a stability problem for some drugs and hence may influence the dissolution process.

A commonly used medium for simulating fasting conditions in the proximal small intestine is fasted state simulated intestinal fluid (FaSSIF). As evidenced in the above discussion, one apparent difference between the SGF and FaSSIF is that this simulated intestinal medium contains bile salts and lecithin. Thus, the dissolution rate of poorly soluble, lipophilic drugs may be improved greatly in this medium in comparison to the dissolution rate observed in simple aqueous solutions. The composition of this medium is given in Table 3.3 (Dressman, 2000) and it was based on experimental data in dogs and humans for the concentration of bile components, pH value, buffer capacity, and osmolality (Greenwood, 1994). The pH value was chosen to be 6.5, which closely resembles the values measured from the midduodenum to the proximal ileum. Sodium taurocholate was often used as a representative bile salt since cholic acid is one of the more common bile salts in human bile (Carey and Small, 1972). In addition, because the pKa of

Table 3.3. Sample composition for simulating the fasted state conditions in the small intestine (note that the recommended volume for dissolution studies is 1 L) (Klein 2005)_

FaSSIF composition

Table 3.3. Sample composition for simulating the fasted state conditions in the small intestine (note that the recommended volume for dissolution studies is 1 L) (Klein 2005)_

FaSSIF composition

Sodium taurocholate

3mM

Lecithin

0.75 mM

NaH2PO4

3.9 g

KCl

7.7 g

NaOH

qs ad pH 6.5

Deionized water

qs ad 1L

Table 3.4. Sample composition for simu-

lating the fed state conditions in the small

intestine (note that the recommended vol-

ume for dissolution studies

is 1 L) (Klein

2005)

FeSSIF composition

Sodium taurocholate

15 mM

Lecithin

3.75 mM

Acetic acid

8.65 g

KCl

15.2g

NaOH

qs ad pH 5.0

Deionized water

qs ad 1L

taurine conjugate is very low, precipitation and an alteration in the micellar size with small variations in pH values are unlikely to occur within the pH range in the proximal small intestine (pH 4.2-7). The ratio of phospholipids to bile salts employed in these media is approximately 1:3, which reflects the in vivo ratio that is generally found to be between 1:2 and 1:5 (Dressman et al., 1998).

In comparison to the fasted state, a dissolution medium simulating intestinal conditions in the fed state should assume a lower pH value, higher buffer capacity, and osmolarity (Greenwood, 1994). In addition, as described earlier, lipids in food further simulate the release of bile salts and phospholipids, which certainly have major effects on the dissolution rate of the drug. Most of these factors should be taken into consideration during the development of a dissolution medium for simulating the proximal small intestinal conditions in the fed state. The sample composition of the fed state simulating intestinal fluid (FeSSIF) is given in Table 3.4 (Dressman, 2000). It should be noted in Table 3.4 that an acetic buffer is used here instead of the phosphate buffer to achieve the higher capacity and osmolarity while maintaining the lower pH value, and that taurocholate and lecithin are present in considerably higher concentrations than those in the fasted state medium.

3.7.3.4 Apparatus and Test Conditions for Simulating Small Intestine

Using biorelevant media that mimic intestinal conditions (e.g., FaSSIF and FeS-SIF), the basket and paddle methods can also be employed to study the drug release in the small intestine. The advantages and disadvantages of these two apparatus used for simulating intestinal conditions are similar to those used for simulating gastric conditions. Since the relationship between in vivo hydrodynamics (or motility) and rotational speed is not known, the agitation rate (50-100 rpm) is once again determined empirically to give data that provide the best IVIVC.

With the possibility of varying the composition of media and the agitation rate (or the flow rate), both the reciprocating cylinder and flow-through cell systems can be used to simulate the pH and composition changes from the duodenum to the ileum. Furthermore, the flow-through cell system can be operated as an open system, allowing removal of dissolved drugs and hence providing sink conditions for poorly soluble drugs to mimic conditions in the small intestine. However, the open system mode requires a large volume of media. Therefore, its practical use is severely limited to product development, especially when biorelevant media are used.

With regard to the volume of the fasted state simulated intestinal medium, phar-macokinetic studies in the fasted state show that by ingesting 200-250 mL of water with the dosage form, a total volume of 300-500 mL will become available in the proximal small intestine. Based upon this evidence, a volume of 500 mL is recommended for the FaSSIF. The total volume of the fed state simulated intestinal medium should take into consideration the volume of coadministered fluid, the volume of fluid ingested meal, and the secretions of the stomach, pancreas, and bile (Fordtran and Locklear, 1966). As a result, in comparison to FeSSIF, a larger volume (up to 1L) is generally required for dissolution testing using FeSSIF.

3.7.3.5 Biorelevant Methods for Extended-Release Dosage Forms

For extended-release drug products, the dissolution method must capture, at minimum, the changes in composition, pH, and residence times along the GI tract, since absorption of these dosage forms takes place throughout the entire intestine. Thus, the reciprocating cylinder and flow-through cell systems can be used, in conjunction with different biorelevant dissolution media, to assess the in vivo release behavior of extended-release dosage forms.

3.7.3.6 Remaining Challenges

Currently, similar to the dissolution test used for QC, the biorelevant dissolution method is generally drug product specific. In other words, no universal biorelevant methods have been devised. If the same drug is formulated differently, even a subtle difference in the formulation may require the development of different in vitro dissolution methodology in order to obtain an IVIVC. Although some progress has been made in understanding the GI tract environment (e.g., pH, composition, and volume), the establishment of IVIVC is still primarily based upon a trial and error approach (Zhang and Yu, 2004). Furthermore, none of the dissolution media, apparatus, and test conditions described previously for gastric and intestinal conditions reflects all physiological parameters that are important for determining the effects of composition, food, motility patterns, and transit times on drug release in the stomach and small intestine. In addition, transient changes in composition, motility, and volume in both the fasted and fed states are not fully captured by the current biorelevant dissolution methods.

Therefore, developing biorelevant methods that truly capture the drug release behavior under in vivo conditions remains extremely challenging, since the physiological environment of the GI tract is still not fully understood. For instance, despite the fact that the hydrodynamics in the GI tract are known to play an important role in dissolution, they have not been studied in detail. Thus, to devise such dissolution methods, we must seek a complete understanding of how all the key factors such as composition, hydrodynamics, volume, and transit times affect the dissolution of drugs in the GI tract. We can then utilize this knowledge in the design of biorelevant dissolution testing.

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  • Katrin
    Why vessel volume will be 500 ml in fed state?
    7 years ago

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