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Lactose VitE Gélu+

Figure 1 Absolute bioavailability in dogs of an experimental Merck API from three formulations: a lactose/API mixture packed into capsules, a vitamin E TPGS-based formulation and a Gelucire-based formulation. Source: From Ref. 9.

Lactose VitE Gélu+

Figure 1 Absolute bioavailability in dogs of an experimental Merck API from three formulations: a lactose/API mixture packed into capsules, a vitamin E TPGS-based formulation and a Gelucire-based formulation. Source: From Ref. 9.

slight modifications to them, may be able to adequately address both QC and IVIVC needs, especially when they are used as the basis for formulating APIs with medium lipophilicity. More work is needed to determine how wide the field of application is in terms of both API and formulation type.

Biorelevant Dissolution in Compendial Apparatus

The biorelevant media were conceived to provide better forecasts of oral bioavailability for poorly soluble APIs. The idea was to account for the better wetting and solubilisation of the APIs in presence of bile components. These effects can improve API solubility and dissolution rate by more than an order of magnitude. A prerequisite for the improvement in solubility is that the API has lipophilicity sufficient for interaction with the mixed micelles; typically the logP value should be greater than two to facilitate the partitioning and as the logP increases, so will the extent of solubilization. A more complete discussion of the prediction of absorption of lipophilic APIs with biorelevant dissolution tests can be found in the review article by Dressman and Reppas (10).

Intestinal Biorelevant Media

For many lipid-based formulations, dispersion in the aqueous environment will be aided and abetted by the bile components in the biorelevant media and, if a single phase is formed, characterizing release into the biorelevant media may well be sufficient to forecast the release properties in the intestine. The current composition of the intestinal media is provided in Tables 2 and 3.

Gastric Biorelevant Media

In many cases, dispersion and release will commence in the stomach and it may be worthwhile to study these phenomena under gastric as well as intestinal conditions, especially during the development phase. A suitable, physiologically relevant release-testing medium composition for the fasted state is given in Table 1. Simulation of the fed state in the stomach presents a significantly greater challenge—due to the presence of fats, carbohydrates, and proteins one has to deal with a multiphase medium and the associated problems with separating the phases and analysing each for the API. Various approaches to simulating the fed state conditions in the stomach have been addressed by Klein et al. (11). These authors also proposed viscosified Ensure Plus (a total nutrition drink) as a release medium which would simulate most of the physical properties of the gastric contents in the fed state. In reality, this medium is difficult to work with and efforts are continuing to design a medium which adequately addresses the gastric content composition and yet lends itself to analysis of the API without too much investment of time and effort.

Current research efforts seek to identify a release test medium consisting of a suitable dilution of heat-treated milk with a buffer simulating the gastric secretions to create a kind of "snapshot" composition of the gastric contents corresponding to that observed about one hour after meal ingestion. Although use of such a medium still requires centrifugation and separation of the proteins via precipitation, and is therefore obviously unsuitable for routine QC purposes, it is able to simulate the key processes that can occur in the stomach—partitioning into the lipid phase, incorporation into milk protein (e.g., casein) micelles and dispersion into the aqueous phase. Thus the approach is well suited to studying a wide variety of release mechanisms in a medium that can provide contributions of the different mechanisms in roughly the same proportions as would be experienced by the dosage form when ingested with a usual Western meal.

To summarize, the use of biorelevant dissolution media in a standard compendial dissolution apparatus has the advantages of using well-known media and equipment, and accounting for a range of release mechanisms, but brings with it more work in terms of media preparation and sample analysis. Therefore, this approach is best suited to formulation screening work and evaluating mech-anism(s) of release. It is also somewhat limited in range by the fact that not all lipid-based formulations will disperse well in these media.

Adaptation of Biorelevant Dissolution Tests for Screening of Lipid-Based Formulations

An important criterion for selecting a lipid-based formulation to improve API bioavailability is that the solubility of the API is much better in the lipid-based formulation than in aqueous media. The API solubility can be evaluated in a wide variety of lipid-based formulations to determine which is the most favourable to solubility. However, in addition to solubilizing capacity, the chosen formulation must also be able to release the API under physiological conditions. At present, a good screening method for this step in the process appears to be lacking.

To overcome this deficit, a project was initiated between F. Hoffmann-La Roche (Basel) and the Johann Wolfgang University in Frankfurt (12). The approach was to create a medium throughput screening tool based on a 96 well filter plate system (MACA CO2 S5, Millipore). The system consisted of a 96 well receiver plate, with a 96 well filter plate polycarbonate filter (PCF, 0.4 ^m) placed on top. Small holes between the wells in the filter plate enabled the removal of samples from the receiver side without having to remove the filter plate from the set-up. Figure 2 shows the configuration of the system, with a donor and a receiver compartment separated by the PCA filter, thus allowing the lipid formulation phase to remain separated from the aqueous phase during the course of the experiment. The concentration in the aqueous receiver compartment can thus be determined without any further phase separation work. Since the filter is hydrophilic, it allows the aqueous phase to equilibrate with the filter and doesn't impede passage of micellar solutions (of the API and/or lipid phase) from the lipid phase into the aqueous receiver compartment. Some key experimental considerations are:

1. To ensure the wettability of the filter membrane and to avoid generation of air bubbles in the system during the experiment, the filter is

wetted with 25 ^L of receiver medium prior to addition of the lipid-based formulation into the donor compartment.

2. A typical volume for the lipid-based formulation in the donor compartment is 100 ^L, and a typical volume of 300 ^L is used for the receiver compartment.

3. Working with such small volumes, it is prudent to run the experiments at room temperature to reduce evaporation of the media.

4. The receiver compartment is stirred with a miniature magnetic stirrer (300 rpm).

5. Transfer is studied as a function of time, by removing volumes from the receiver compartment at suitable times and replacing with fresh medium.

6. Additionally, 5 ^L samples can be removed and examined microscopically to determine whether the receiver phase remains homogeneous or if phase separation occurs. Phase separation may occur if the surfactants in the formulation interact unfavourably with those in the receiver medium (typically, bile components in media such as FaSSIF and FeSSIF). Figure 3 shows examples of lipid-based formulations that form homogenous systems with SGF, FaSSIF, and FeSSIF under these conditions and some which phase separate.

The methodology was applied to an experimental Roche compound. With a clogP value of 5.35, this API is highly lipophilic. Nevertheless, its solubility in triglycerides was very low. Solubility in mono-/diglyceride mixtures was also insufficient to completely solubilize the API dose (anticipated to be 5-20 mg) in

Figure 3 Comparison of biorelevant media and formulation of Roche compound diluted in biorelevant media (magnification X 100).

a single-unit dosage form volume of approximately 1 mL. However, the excipient, Capmul, provided somewhat better solubilization (21 mg/mL). Finally, the API solubility in surfactants such as Labrasol and Tween 80 was even higher, with additive effects observed in lipid/surfactant combinations.

Release tests were conducted for several formulations using biorelevant media in the screening method. A typical set of results for the release test are shown in Table 5. Concentrations of the API of about 0.1 mg/mL were achieved within 100 minutes in SGF, FaSSIF, and FeSSIF from the Capmul formulation; with addition of Tween 80 at a ratio of 9:1 the concentration after 100 minutes rose to almost 0.2 mg/mL.

The formulations were subsequently administered to monkeys to determine whether they would perform according to the predictions from the in vitro screening method. The results, shown in Figure 4, confirm the utility of the screening method: the plasma levels of the Roche API were substantially higher with the Tween/Capmul formulation than with Capmul alone. Even more interestingly, a

Table 5 Release of an Experimental Roche Compound from Several Formulations in the 96 Well Plate Screening Model (mg/mL)

Tween 80-Capmul Formulations (starting concentration in the formulation was 20 mg/mL)

Table 5 Release of an Experimental Roche Compound from Several Formulations in the 96 Well Plate Screening Model (mg/mL)

Tween 80-Capmul Formulations (starting concentration in the formulation was 20 mg/mL)

Formulation

Medium

10min

30min

50min

70min

100min

Capmul

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