USP 2 biorelevant dissolution of a BCS Class 2 lipophilic base (salt)

PURPOSE

To establish the dissolution behaviour of an immediate release formulation of lipophilic basic salt in three different biorelevant media using a standardized USP Apparatus 2 method.

To examine how the results from simple in vitro dissolution tests can be used to gain an understanding of the drugs in vivo release. This can be of huge benefit to developers because it enables them to optimize the release profile of their drug product. Furthermore, the prognostic ability of the media helps in the development of superior drug products for patients because insights into potential food effects can be seen prospectively by developers.

DRUG BACKGROUND

The molecule's structure is a lipophilic, benzofuran based basic salt. The maximum dose strength immediate release tablet (200 mg) was tested in the in vitro dissolution tests. In patients, the drug is known to have a longer Tmax (x2) in vivo in the fasted state and has a significant positive food effect as reflected by increased Cmax and AUC (both ×3) when taken with a high-fat meal. The administration directions state the drug can be administered either with or without food.

METHOD

Media preparation

Three dissolution media simulating the fasted stomach, fasted intestine and fed stomach were prepared using FaSSIF/FeSSIF/FaSSGF Powder and the corresponding buffer concentrates with instructions generated from the Media Preparation Tool.

Preliminary assessment

Preliminary assessment studies were carried out to ensure the HPLC-UV analysis (to check the media did not interfere with drug product analysis) and dissolution set up (including filter adsorption) were satisfactory. A summary of the method used is provided in Table 1.

Table 1 HPLC method

Parameter

Value

Mobile phase

60% Acetonitrile 40% H2O 2.65mL/L triethylamine, 1.5mL/L orthophosphoric acid

pH

3

Column

C18HS-3 75mm x 4.6mm x 3µm

Wavelength (nm)

240

Retention time (min)

3.5

Temperature (°C)

30

Analysis time (min)

3.5

Flow rate (mL/min)

2

Filter adsorption

Filter adsorption was checked by passing each medium containing the drug substance through the filter (n = 3) and establishing the volume that required to presaturate the filter.

Dissolution conditions

The conditions provided in Table 2 were taken from the existing QC method as the basis for the biorelevant dissolution. These parameters were used for all three media tested.

Table 2 Dissolution conditions

Parameter (unit)

Value

USP Dissolution apparatus

2

Volume (mL)

900

Rotation speed (RPM)

75

Temperature (°C)

37

Vessels for each media

6

The only deviation from the QC method was the sampling points were extended to up to 2 hours and included 10, 20, 30, 45, 60, 90 and 120 minute sampling points. The tablets were tested in three different biorelevant media (FaSSIF, FeSSIF and FaSSGF) for 2 hours with n=6 vessels according to a standardized biorelevant dissolution method based around the dissolution procedure and methodology as described in USP Chapter <1092>. The average % drug release at each time point was determined by HPLC and plotted against the time when the sample was removed from the dissolution vessel.

RESULTS

Preliminary assessment

HPLC analysis

HPLC samples were diluted x10 fold prior to injection.

None of the media interfered with drug analysis. No significant modifications to the method were necessary.

Filter adsorption

To presaturate the filter it was found that at least 6 mL of the dissolution media containing drug should be passed through the filter prior to the withdrawal of the first sample from each vessel at the 10 minute time point.

Figure 1 Dissolution in simulated fasted state gastric media

This immediate release drug has a low and slow dissolution rate in simulated fasted gastric medium (FaSSGF); less than 10% of the dose is dissolved after 2 hours. This reflects the base's surpisingly low water solubility even in acidic conditions and indicates that drug dissolution of this base in fasted stomach fluid will be low.

Figure 2 Dissolution in simulated fasted state intestinal fluid

The tablets exhibited 72% drug release after 2 hours and the dissolution profile still did not reach a plateau. This suggests this poorly soluble basic drug has a moderate to high solubility in the simulated fasted intestinal fluids but a slow dissolution rate as demonstrated by only about a third of the drug being released after 20 minutes.

Figure 3 Dissolution in simulated fed state intestinal fluid

In the simulated fed intestine fluid the drug demonstrated a much quicker dissolution rate compared to the fasted state. There was almost complete release between 10 and 20 minutes compared to a third of the dose released in the fasted state intestinal medium. This is largely because of the presence of the physiological surfactants in the fed state fluids that increase the solubility of this water insoluble lipophilic drug quite dramatically.

DISCUSSION

For this molecule, the results provide a logical basis for establishing a link between the drug's biopharmaceutical properties, the drug product’s dissolution and the drug’s subsequent absorption.

In vitro dissolution

Despite being a basic salt, the in vitro dissolution results in fasted stomach fluid showed low and slow release. This reflects the low solubility of the basic molecule even in an acidic environment.

In contrast, dissolution was markedly quicker in the simulated intestinal fluids. The fed media showed a very fast dissolution in comparison to the fasted intestinal media. This is due to the lipophilic nature of the drug and the increased solubility with the physiological surfactants present in the fed intestinal fluid.

Stable

In all chromatograms, no new impurity peaks appeared in any of the media even after two hours of dissolution. The molecule was chemically stable in all three media under the conditions tested.

Understanding the impact of the in vitro dissolution results

It is clear to see the benefit of how in vitro biorelevant results can be used to understand the qualitative in vivo performance; the slower Tmax and lower exposure in the fasted state in vivo is reflected by the low solubility and slow dissolution in FaSSGF and the slow in vitro dissolution in FaSSIF.

In sharp contrast, the faster Tmax and higher exposure in patients after a meal is reflected by the very fast in vitro dissolution in FeSSIF.

For a quantitative assessment and correlation of the in vitro dissolution results to in vivo pharmacokinetics, the physiological parameters of the gut as well as the post absorptive processes of the drug would need to be considered.

Despite this significant difference in the in vivo exposure in the fasted and fed state which is reflected by the in vitro dissolution, the drug very surprisingly does not have a food effect restriction even though it has a relatively narrow therapeutic window. This is probably because the drug was developed and registered before regulatory authorities made it mandatory to assess the effects of food on bioavailability of a new drug.

CONCLUSION

Biorelevant dissolution testing is important for gaining an understanding of the relationship between the in vitro drug release in dissolution media (FaSSIF, FeSSIF and FaSSGF) simulating the key fluids of the gastrointestinal tract and its likely in vivo dissolution without interference from post absorptive processes.  

The surprisingly low solubility of this lipophilic basic salt in simulated in vitro fasted gastric media coupled with the very slow dissolution in simulated in vitro fasted gastric media help explain the long Tmax of the drug and lower Cmax in vivo. In contrast, the very fast dissolution in the simulated fed state intestinal media helps reveal why the drug has a higher Cmax and shorter Tmax after a meal. These in vitro results can enable developers to identify if their drug product has any in vivo dissolution difficulties when administered to patients either in the fasted or fed state. Furthermore, the results can help support the administration regimen of the drug product for the patient.

Importantly, these tests can be carried out using simple in vitro USP dissolution apparatus and HPLC analysis.