Analytics of dissolution testing of products containing nanosized drugs with a view to predicting plasma profiles

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The oral bioavailability of a drug substance is strongly related to its aqueous solubility.
Only complete dissolution during the GI-passage can maintain an optimal
bioavailability. Poor aqueous drug solubility results, according to the Nernst-Brunner
equation into a slow dissolution rate, sometimes too slow for complete dissolution in
the GI tract. The dissolution rate increases with decreasing particle size and therefore
increasing surface area of the drug particles. In consequence,, micronization of the
drug is applied to increase oral bioavailability, but often meets with modest success.
Recently developed techniques were applied to decrease the particle size into the
nanometer range. For some substances, pharmacokinetic parameters could be
influenced decisively, e.g. the obviation of a food effect for the drugs aprepitant and
fenofibrate.
The assessment of a dosage form is investigated by dissolution testing. For a
reasonable assessment of such tests, a separation of solid and liquids has to be
ensured within an appropriate time frame. For particle sizes of about 150 nm it
appears questionable whether such separation can be succeeded by classical
techniques, e.g. the use of syringe filters with a pore size of 0.45 μm.
The aims of this thesis were to investigate the suitability of various analytical
techniques in analysis of dissolution tests containing nanosized drug substance.
Furthermore, a suitable analytical tool is applied to establish an in vitro – in vivo
correlation of the nanosized drug fenofibrate.
At first, several techniques were investigated in theory to assess their ability to ensure
a rapid and complete separation of solids and liquids. The classical dialysis, turbidity
measurement and UV-measurement via fiber optics were excluded from further
investigation due to various reasons, e.g. the speed of separation for dyialisis. The
asymmetrical flow field-flow fractionation appeared to be a promising tool, but lack of
equipment precluded further investigation.
The ultrasonic resonance technology (ResoScan), the microdialysis and the use of
centrifugal filter devices have shown to be inappropriate for the analytics of nanosized
drugs in dissolution test. The use of syringe filters with various pore sizes and the ionselective
electrode (ISE) was promising, so these techniques were examined more
intensively.
The syringe filters with various filter pore sizes were investigated for their ability to
hold back colloidal drug. Fenofibrate was chosen as model drug, since this is
commercially available both as micronized and nanosized formulation (Lipidil TerR and
Lipidil 145 ONER), enabling direct comparison.
The experiments with micronized fenofibrate which contains little or no colloidal
fenofibrate yielded similar dissolution profiles, irrespective of filter pore size; f2 was
always greater than 65, indicating less than 5% difference between the dissolution
profiles in any medium.
Using a pore size of 0.1 μm or less, the maximum concentration of drug achieved in
solution from the nanosized formulation was commensurate with the saturation
solubility of fenofibrate in all tested media. Filtration with a pore size of 0.2 μm or 0.45
μm generated concentrations exceeding the saturation solubility. These results, in
combination with higher standard deviations of the analytical results, indicate that the
apparent “supersaturation” is caused by colloidal fenofibrate, which is too fine to be
held back by these filters. The f2-value of less than 50 when comparing the profiles
obtained from 0.1 μm and 0.2 μm filter pore size indicates that the choice of filter pore
size is crucial to the interpretation of the dissolution profiles. To separate nanosized
drug from molecularly dissolved fenofibrate in Lipidil 145 ONER, a filter pore size of 0.1
μm or less appears to be appropriate.
It was observed that the experimental increase of dissolution rate is not congruent
with common hypothesis regarding the boundary layer h for decreasing particle sizes
and subsequent application of the Nernst-Brunner equation.
The initial dissolution rates of both formulations were investigated by using a filter
pore size of 0.1 μm. The results were utilized in an in silico model (STELLAc) to
correlate the in vitro results with in vivo data (Model A). In the preprandial state a good
in correlation was established for the micronized fenofibrate, while for the nanosized
fenofibrate the plasma levels were overpredicted.
The model was expanded to investigate the impact of an absorption step at the
intestinal membrane on the in vitro – in vivo correlation. It was found that even a
minor deceleration of absorption results in varied plasma profiles caused by a lagged
appearance of drug in the blood.
For both formulations the rate determining step was identified: When changing from
the micronized to the nanosized formulation, the rate-determining step for absorption
may change from completely dissolution-controlled to at least partly permeationcontrolled
in the fasted state.
In the fed state, gastric emptying appears to be rate-determining for absorption of
fenofibrate from both the micronized and the nanosized formulation.
Another technique appears to be suitable for analysis of nanosized drugs in dissolution
testing. The Ion-selective electrode (ISE) is a recently developed analytical system
measuring the changes of the electrochemical potential in solutions. A transformation
via the Nikolski – Eisenmann equation results into the concentration of the respective
drug in solution. Since only dissolved drug is detected, obviating the need for
separation of dissolved from undissolved drug, this system appears to be very
promising in the analytics of nanocrystalline drugs.
Diphenhydramine_HCl was chosen as model substance for the ISE studies. It was the
goal of investigation to test compatibility of the ISE with complex media, e.g. all
biorelevant dissolution media. This is done in advance of application of the ISE in these
media for nanocrystalline drug substance. The results were compared to manual
sampling, filtration and subsequent HPLC-UV analysis.
The results demonstrate that the ion-selective electrode is suitable for measurements
of diphenhydramine HCl in fasted state biorelevant media (FaSSGF, FaSSIF, FaSSIF-V2)
as both a stand-alone system (Method A) and in conjunction with a single point
conventional assay (Method B). The results acquired are similar to those obtained by
manual sampling and subsequent HPLC-UV analysis. The ISE also delivers satisfactory
results in a milk-based medium (FeSSGF), in which it has distinct advantages over
manual sampling with HPLC-UV analysis by obviating the need for sample preparation.
The application of the ISE in FeSSIF type media will need further study.
Finally, as an on-line technology, ISE offers more efficient generation of dissolution
profiles than conventional sample-based methods.