DISSOLUTION APPARATUS

The present invention relates to the field of dissolution testing, and in particular to an apparatus and method for intrinsic dissolution testing, a method for preparing a test pellet and its use for dissolution testing.

Dissolution is a dynamic process by which a material is dissolved in a solvent and is characterized by a rate, i.e. by the amount of material dissolved by time. The intrinsic dissolution rate is defined as the rate of dissolution of a pure pharmaceutical active or of a drug product composition, i.e. pharmaceutical active(s) plus excipients, when conditions such as the total exposed surface area of the sample as well as the temperature, agitation- stirring speed, pH, and ionic strength of the dissolution medium are kept constant. The determination of the intrinsic dissolution rate allows for screening of drug candidates and understanding their solution behavior under various bio -physiological and pharmaceutical conditions.

The dissolution of a compound can be described by the Nernst-Brunner equation:

dC/dt = (DS/Vh XCs-C)

wherein dC/dt is the change of drug concentration at time t, D the diffusion coefficient, S the exposed surface area, h the thickness of the diffusion layer, V the volume of the dissolution medium, Cs the saturation solubility of the compound, and C the instantaneous concentration at time t.

Currently, the USP lists the Wood's Intrinsic Dissolution Apparatus from VanKel

Industries, Inc., of Cary, North Carolina, as the official apparatus for determination of intrinsic dissolution rates. See USP 24 - NF 19, Supplement 1, Section 1087, Intrinsic Dissolution (Released 11/01/99).

In a typical intrinsic dissolution experiment 100-500mg of test material is placed in a 0.8 cm diameter (0.5 cm ² ) die cavity, compressed and then placed into the dissolution vessel containing 100-900 ml of dissolution medium. Dissolution is performed under defined mixing conditions and temperature and sample aliquots are removed from the dissolution medium at appropriate time intervals for analysis.

It has now been found that miniaturizing dissolution testing is a helpful tool for easier and faster performing intrinsic dissolution tests.

It is therefore the object of the present invention to provide a device and method for intrinsic dissolution testing which allows for low compound and solvent consumption. It is a further object of present invention to provide a faster screening or testing method for intrinsic dissolution studies. It is a further object of present invention to offer the possibility of parallel intrinsic dissolution studies of many compounds, as well as parallel sample workup. It is a further object of present invention to provide for a device and method for intrinsic dissolution testing which offers flexibility and compatibility with respect to solvent volume, nature of solvent, as well as the nature of test compounds. It is a further object of present invention to provide a device and method for intrinsic dissolution testing with reduced equipment and compound costs.

Hence, the intrinsic dissolution test of present invention is useful for compound characterization in drug development processes, even at an early stage. It is useful for finding the optimal form of an active substance, e.g. crystal habit, crystallinity, amorphism, polymorphism, pseudo-polymorphism, salt or specific particle surface area/size. It is further useful for the identification of appropriate dissolution media, as well as for the identification of in vitro / in vivo correlations.

In detail, present invention relates to a device for intrinsic dissolution testing, comprising

(a) at least one sample holder comprising a tubular section for retaining a test pellet, the tubular section having an inner cross-sectional area from 0.2 mm ² to 13 mm ² ; and

(b) at least one dissolution vessel per sample holder, the dissolution vessel having a dissolution volume from 0.01 ml to 10 ml. Thereby, the term "sample holder" denotes a means suitable for retaining or holding a test sample and putting it into a dissolution vessel so that the test sample is in contact with the solvent.

The sample holder may further comprise means for adjusting the position of the sample holder within the dissolution vessel, controlling its position, or fixing the holder and/or the test sample within the dissolution vessel. It may comprise means for rotating the sample holder/test sample within the dissolution vessel. It may comprise means for facilitating the filling of the test compound. It may comprise means for fixing the sample holder during test pellet formation. It may comprise means for reducing transmission of vibrations to the sample holder during dissolution testing. It may further comprise means for avoiding solvent evaporation.

In particular, the outer shape of the sample holder may be chosen independently from the inner shape suitable for retaining the test sample.

Preferably, the tubular section of the sample holder presents the solvent-exposed surface of the test pellet downwards into the dissolution vessel, i.e. the sample holder is immersed into the solvent from the top of the dissolution vessel.

Further, the device comprises at least one sample holder, which means that in case of more than one sample holder is involved, parallel testing of intrinsic dissolution with only one device is possible. Preferred embodiments will be described below.

The device of present invention is suitable for intrinsic dissolution testing, in particular for dissolution testing of small amounts of test compound. Preferably, the device of present invention is suitable for test material in an amount of 0.1 to 50 mg, more preferred are 1 to 5 mg test compound. Therefore, the sample holder comprises a tubular section for retaining a test pellet wherein the tubular section has an inner cross-sectional area from 0.2 mm ² to 13 mm ² . Tubular section of the sample holder means a section of the sample holder wherein the inside has a tubular shape, the inner surface of the wall(s) running in parallel and with two openings running orthographic to the inner surface.

Preferably, the tubular section of the sample holder has a length from 0.1 to 20 cm. Preferably, the tubular section of the sample holder has a lower length limit of 0.1 cm, 0.5 cm, 1 cm, or 1.5 cm. Preferably, the tubular section of the sample holder has a higher length limit of 20 cm, 15 cm, 12 cm, 10 cm, 8 cm, 6 cm, 5 cm, 4.5 cm, 4 cm, 3.5 cm, 3 cm, 2.5 cm or 2 cm. It is understood that all combinations of lower and higher length limit may be combined.

Inner cross-sectional area means the cross-sectional area orthographic to and delimited by the inner surface of the tubular section, i.e. the orthographic cross-sectional area inside of the tubular section.

Preferred lower limits of the inner cross-sectional area include 0.2 mm ² , 0.38 mm ² , 0.5 mm , 0.7 mm , 1.2 mm , 1.8 mm , 2.2 mm , 2.5 mm , 2.8 mm , or 3.1 mm . Preferred upper limits of the inner cross-sectional area include 12.6 mm ² , 12 mm ² , 9.1 mm ² , 7.6 mm ² , 7.0 mm ² , 6.9 mm ² , 6.6 mm ² , 5.8 mm ² , 5.3 mm ² , 5 mm ² , 4.5 mm ² , 4.2 mm ² , or 3.8 mm ² . It is understood that each lower limit may be combined with each upper limit given in the list. Particularly preferred ranges are from 0.2 mm ² to 9.1 mm ² , from 0.2 mm ² to 7 mm , from 0.2 mm to 6.9 mm , from 0.5 mm to 6.9 mm , from 0.5 mm to 5.8 mm , from 0.7 mm ² to 5 mm ² , from 0.7 mm ² to 3.8 mm ² , and from 0.7 mm ² to 3.2 mm ² .

In a certain embodiment, the device of present invention has a sample holder with a tubular section, wherein the inner cross-sectional area of the tubular section has the shape of a circular area, an elliptic area, or a polygonal area. Hence, the tubular section of the sample holder preferably denotes a section of the sample holder with the inside describing a cylinder or a prism, i.e. a circular cylinder, an elliptic cylinder or a polyhedron.

In a preferred embodiment, the cross-sectional area is a circular area with the dimension having the lower and upper limits as given above. In this embodiment, the cross-sectional area is delimited by the inside diameter of the tubular section. Preferred lower limits of the diameter of the circular cross-sectional area are 0.5 mm, 0.7 mm, 0.8 mm, 1 mm, 1.2 mm, 1.5 mm, 1.7 mm, 1.8 mm, or 1.9 mm. Preferred upper limits of the diameter of the circular cross-sectional area are 4 mm, 3.9 mm, 3.4 mm, 3.1 mm, 3 mm, 2.9 mm, 2.7 mm, 2.6 mm, 2.5 mm, 2.4 mm, 2.3 mm, 2.2 mm, 2.1 mm. A further preferred diameter is 2 mm. It is understood that each lower limit may be combined with each upper limit given in the list. Particularly preferred ranges are from 0.5 mm to 3.4 mm, from 0.5 mm to 2.9 mm, from 0.8 mm to 2.9 mm, from 0.8 mm to 2.7 mm, from 1 mm to 2.5 mm, from 1 mm to 2.2 mm, from mm to mm, or from 1 mm to 2 mm.

The tubular section forms part of the sample holder and is located in the end or tail portion of the sample holder which is in contact with the solvent in the dissolution vessel. Preferably, this is the bottom side of the sample holder. At least a part of the tubular section and the sample holder, respectively, is or will be in contact with the solvent during dissolution testing.

Further, the tubular section of the sample holder is suitable for retaining the test pellet. Thereby, the test pellet takes on the shape of the inner cross- sectional area of the tubular section and has a certain thickness. The bottom or lower side of the test pellet together with the lower edge or bottom line of the tubular section form a substantially plane surface, and the bottom or lower side of the test pellet is allowed to get in contact with the solvent.

Pressed pellet material should have a thickness sufficient to maintain the exposed surface intact during the dissolution experiment, i.e. to prevent the dissolution medium from reaching the rear side of the test pellet or the filling material or plunger.

Hence, the inner cross-sectional area of the tubular section defines the area of the test pellet which will be in contact with the solvent in the dissolution test, i.e. the form and size of the inner cross-sectional area of the tubular section directly relates to the total exposed surface area of the test sample.

"At least one sample holder" means that the intrinsic dissolution test may be performed with a device comprising one sample holder only, but could also comprise more than one sample holder, for instance at least two sample holders. Thereby, the number of sample holders may be freely chosen. As an example, the number of sample holders may be selected from any number from 1 to 1536, additionally including all numbers in between 1 and 1536. Preferably, 1 to 1536, 1 to 384, 1 to 96, 1 to 48, 1 to 32, 1 to 24, 1 to 16, 1 to 12, or 1 to 8 sample holders per device are selected, including all numbers in between.

In a certain embodiment of the invention, the device comprises more than one sample holder, i.e. at least two sample holders. Such an embodiment allows for intrinsic dissolution testing of test samples in parallel. In principle, the number of sample holders maybe freely chosen, for instance from 2 to 1536, additionally including all numbers in between 2 and 1536. Preferably, 2 to 1536, 2 to 384, 2 to 96, 2 to 48, 2 to 32, 2 to 24, 2 to 16, 2 to 12, or 2 to 8 sample holders per device are selected, including all numbers in between.

In present invention, the surface of the test pellet which will be exposed to the solvent is small. Therefore, it is important that the surface exhibits as few and little defects as possible. Even though it is not necessary, it is advantageous to avoid any transfer of the already produced test pellet to the sample holder, thereby ensuring a good quality of the surface.

In a preferred embodiment of the invention, the tubular section of the sample holder forms part of a die for pellet formation. In an embodiment wherein the tubular section of the sample holder forms part of a die, the test pellet is directly formed within the die and tubular section, respectively, by imposing pressure on the test compound within the mould or die. It is advantageous to prepare the test pellet directly in the tubular section of the sample holder. This makes a transfer and insertion of the pellet into the sample holder redundant, thereby avoiding defects in the surface of the pellet resulting from transfer and insertion.

In a certain embodiment of the invention, the bottom part of the tubular section retains the test pellet. Thereby, the bottom or lower side of the test pellet together with the lower edge or bottom line of the tubular section form a substantially plane surface, and the bottom or lower side of the test pellet will be exposed to the solvent. In this embodiment, the test pellet does not fully fill the tubular section, but has a thickness sufficient to maintain the exposed surface intact during the dissolution experiment, i.e. to prevent the dissolution medium from reaching the rear side of the test pellet.

In a certain embodiment of the invention, the bottom part of the tubular section retains the test pellet and the upper part of the tubular section adjacent to the test pellet

retains a filling material. In other words, the tubular section of the sample holder together with the filling material forms a recess for retaining the test pellet, the rear side of the test pellet inside of the tubular section being in direct contact to the filling material. The thickness of the test pellet in this embodiment is in the ranges as given above. The filling material in this embodiment is used to avoid that solvent could dissolve sample material from the rear side, thereby distorting test results.

Preferably, the filling material is an inert material which will not chemically react with the test pellet or with the solvent. It is further preferred, that the filling material is not soluble in the solvent. Preferably, the filling material is an inert plunger or an inert pellet.

Examples for filling material are corn starch, wax or talc of which a pellet may be formed, for instance directly within the tubular section of the sample holder. Examples for an inert plunger material are stainless steel, Teflon ^® (PTFE, polytretrafluorethylene), PEEK (i.e. poly(ether etherketone)), ceramics, Polyethylene or Polypropylene.

In another embodiment of the invention, the tubular section of the sample holder is filled with pressed test material, so that additional filling material is redundant since the pressed test material in this embodiment has a thickness which renders the filling material redundant. Preferably, the term "filled" in this embodiment means that the tubular section is loaded with up to 100%, more preferably with up to 90% of its length with pressed test material.

In a certain embodiment of the invention, the sample holder comprises a section with a funnel shaped inner surface connected to the upper end of the tubular section. Such an embodiment is of advantage in case the tubular section and/or the sample holder forms part of a die. Thereby, the funnel shaped inner surface at the upper end of the tubular section/sample holder will facilitate pouring in the test compound and filling material, respectively.

The device according to the invention comprises at least one dissolution vessel per sample holder, the dissolution vessel having a dissolution volume from 0.01 ml to 10 ml.

In general, the dissolution volume depends on the form and size of the dissolution vessel, the displacement volume of the sample holder immersed into the solvent, and the displacement volume of an optional mixing element. Preferably, the dissolution volume corresponds to the volume of the solvent used per dissolution vessel.

In certain embodiments of the invention, the lower limit of the dissolution volume is 0.01 ml, 0.05 ml, 0.07 ml, 0.1 ml, 0.15 ml or 0.2 ml. In certain embodiments, the upper limit of the dissolution volume is 10 ml, 8 ml, 6 ml, 5 ml, 4 ml, 3 ml, 2 ml, 1.9 ml, 1.8 ml,

1.7 ml, 1.6 ml, 1.5 ml, 1.4 ml, 1.3 ml, 1.2 ml, 1.1 ml, 1 ml, 0.9 ml, 0.8 ml, 0.7 ml, 0.6 ml, 0.5 ml or 0.4 ml. It is understood that each lower limit may be combined with each upper limit given in the list. Particularly preferred ranges are from 0.01 ml to 2 ml, from 0.01 ml to 1.9 ml, from 0.05 ml to 1.9 ml, from 0.1 ml to 1.9 ml, from 0.1 ml to 1.5 ml, from 0.1 ml to 1.3 ml, from 0.1 ml to 1.1 ml, from 0.1 ml to 1 ml, from 0.1 ml to 0.9 ml, from 0.1 ml to 0.8 ml, from 0.1 ml to 0.6 ml, from 0.1 ml to 0.5 ml, or from 0.1 ml to 0.4 ml.

"At least one dissolution vessel per sample holder" means that the intrinsic dissolution test may be performed with one dissolution vessel per sample holder, only. However, in certain embodiments according to the invention, there is more than one dissolution vessel allocated to one sample holder, i.e. at least two dissolution vessels allocated to one sample holder. In such embodiments, the sample holder including the tubular section retaining the test pellet is moved from a first dissolution vessel to a second dissolution vessel and so forth. Such embodiments allow for small solvent volumes per dissolution vessel, or for dissolution vessels with small dissolution volumes, while working under sink conditions. Hence, in a preferred embodiment, at least two separate dissolution vessels are allocated per sample holder.

It may be stated that the dissolution vessels are separated from each other so that there is no substantial solvent exchange from one vessel to the other, except the amount of solvent adhering to the sample holder when being transferred from one vessel to the next. "Separated from each other" hence means without substantial solvent exchange between two vessels, i.e. the solvents of the dissolution vessels are not in contact to each other.

The number of dissolution vessels per sample holder depends on the dissolution volume per dissolution vessel, on the amount of the test compounds and on its solubility. In principle, the number of dissolution vessels per sample holder may be freely chosen as long as it is "at least one" or - in certain embodiments - "more than one", which is taken synonymous to "at least two".

As an example, the following numbers maybe given: from 1 or 2 to 1536, additionally including all numbers in between 1 and 2, respectively, and 1536. Preferably, 1 or 2 to 384, 1 or 2 to 96, 1 or 2 to 48, 1 or 2 to 32, 1 or 2 to 24, 1 or 2 to 16, 1 or 2 to 12, or 1 or 2 to 8 dissolution vessels per sample holder are selected, including all numbers in between.

In certain embodiments of the invention, the inside cross-sectional area of the dissolution vessel is a circular area, an elliptic area or a polygonal area, and in the latter case preferably a rectangular or quadratic area. In case of a polygonal cross-sectional area,

the corners may be angular or rounded, preferably rounded. Preferably, the inside cross- sectional area is a circular area.

The inside bottom of the dissolution vessel may be flat-bottomed, has a V-shape or may be round-bottomed, i.e. has a U-shape. The inside shape of the bottom of the dissolution vessel may be selected to match with the shape of the stirring means. Preferably, the inside bottom of the dissolution vessel may have U-shape.

The inside walls of the dissolution vessel may run parallel or beveled.

In certain embodiments, the dissolution vessel is equipped with means for mixing. The means for mixing are suitable for maintaining a homogeneous solution, i.e. they are appropriate for avoiding phase separations or gradients. Thereby, the solvent may be mixed by stirring, shaking or by ultra-sound, i.e. sonication. A preferred example for mixing by stirring is a magnetic stirrer with stir bars. Alternatively, the sample holder may rotate.

In case a magnetic stirrer and stir bars are used, the stirrer may be a tumble stirrer, a rotating magnet stirrer or a stationary electromagnetic stirrer and the stir bars may be magnetic or magetizable. The stir bars may have any suitable shape, e.g. the shape of a bar, a disk, a cross, an octagonal bar, a bar with pivot ring, or any other, for instance commercially available, shape. The stirring rate may be selected in the range from 1 to 2000 rpm, preferably from 50 to 500 rpm.

In a certain embodiment, the dissolution vessel is equipped with means to prevent evaporation of the solvent. The means to prevent evaporation may comprise a lid or cover of the dissolution vessel. Alternatively, the sample holder itself may be equipped with means to prevent evaporation of the solvent, for instance in the shape of a lid or cover. In case of a microtiter plate representing a set of dissolution vessels, said microtiter plate may be equipped with a cover to avoid solvent evaporation during the test.

In a further embodiment of the invention, the dissolution vessel is equipped with means to freely position the sample holder within the dissolution vessel and the solvent, respectively, thereby controlling the position of the sample holder in the dissolution vessel. Freely positioning the sample holder allows optimizing the test conditions of the intrinsic dissolution test.

In a further embodiment of the invention, the dissolution vessel is equipped with means for temperature control. As described above, the definition of intrinsic dissolution rate also depends on a constant solvent temperature. Therefore, a heating or cooling system for temperature control is advantageous in certain embodiments of the invention

for keeping the solvent temperature constant during dissolution testing. Further, in some embodiments of the invention the test aims to mimic standard test conditions or alternatively physiological conditions. Hence, it is advantageous that the device comprises means for controlling the solvent temperature, preferably at a temperature range from 0 to 50 ⁰ C, for instance at essentially 37°C, essentially 25°C, essentially 20 ⁰ C or essentially at 4°C.

In a further embodiment of the invention, the dissolution vessel is a well of a microtiter plate. Hence, the microtiter plate represents a set of dissolution vessels which are allocated to at least one sample holder. The format of the microtiter plate may be any commercially available format. Preferred formats are 6, 12, 24, 48, 96, 384 or 1536 well plates. Even more preferred is a 96 well format. Further, the device according to the invention may comprise more than one microtiter plate, for instance 2, 3, 4, 5, or 6 microtiter plates.

In principle, all wells of the microtiter plate may be allocated to the same sample holder.

However, in certain embodiments of the invention, an array of dissolution vessels is allocated to one sample holder. "Array" means a part of wells of the microtiter plate, for instance one or more rows of the microtiter plate. In embodiments of the invention wherein one row of dissolution vessels is allocated to one sample holder, preferably 1 to 48, 1 to 32, 1 to 24, 1 to 16, 1 to 12 or 1 to 8 dissolution vessels and wells, respectively, are allocated to one sample holder, depending on the format of the microtiter plate.

Further, in certain embodiments of the invention, the device comprises the same number of sample holders as wells present on the microtiter plate used, i.e. for instance 6, 12, 24, 48, 96, 384 or 1536 sample holders. In such an embodiment, the set of sample holders is preferably moved from a first to a second microtiter plate and so forth.

In a further embodiment of the invention, the sample holder comprises means for adjusting or controlling the position of the sample holder and tubular section retaining the test pellet, respectively, within the dissolution vessel. Thereby, the sample holder may be adjusted in vertical and horizontal direction in order to optimize the position of the test pellet within the solvent of the dissolution vessel. The adjustment of the sample holder hence takes into account the height of the solvent level in the vessel, effects of mixing means in respect of the dissolution behavior of the test pellet, as well as hydrodynamics of the dissolution vessel with the inserted sample holder.

In a further embodiment of the invention, in particular an embodiment comprising a microtiter plate with dissolution vessels, an entire row of sample holders comprises means for adjusting or controlling the position of the sample holders in the dissolution

vessels. The means for adjusting or controlling the position of the sample holders may comprise distance pieces, screws to control vertical or horizontal adjustment, or guiding means, such as guide tracks or guide pins for vertical or horizontal guidance.

In certain embodiments, such distance pieces, screws to control vertical or horizontal adjustment, or guiding means such as guide tracks or guide pins for vertical or horizontal guidance of a row of sample holders may be located on the microtiter plate.

Further, the means for adjusting or controlling the position of the sample holder within the dissolution vessel may be used for guiding the sample holder(s) or row of sample holders from one dissolution vessel to the next dissolution vessel and so forth.

One embodiment of the invention relates to a method for performing intrinsic dissolution testing, comprising the steps of

(a) placing a sample holder retaining a test pellet having a cross sectional area 0.2 mm ² to 13 mm ² into the solvent of a dissolution vessel having a dissolution volume of 0.01 to 10 ml,

(b) optionally mixing the solvent,

(c) optionally removing the sample holder retaining the test pellet from the dissolution vessel and placing it into a subsequent dissolution vessel,

(d) optionally repeating step (c).

As already mentioned above, the inner cross-sectional area of the tubular section of the device defines the cross sectional area and the solvent-exposed surface area of the test pellet, respectively, which will be in contact with the solvent in the dissolution test, i.e. the form and size of the' inner cross-sectional area of the tubular section directly corresponds to the total exposed surface area of the test sample. Hence, all dimensions as given above for the inner cross-sectional area of the tubular section apply to describe the cross- sectional area of the test pellet.

Preferably, the solvent-exposed surface of the test pellet is directed downwards to the bottom of the dissolution vessel.

Further, all dimensions and formats as given above for the dissolution vessel also apply in the embodiment describing the method for performing intrinsic dissolution testing.

The solvent or dissolution medium may be any kind of solvent which is a liquid at the temperature of test performance. Preferred solvents are aqueous media, organic solvents, natural or artificial biological solvents, e.g. plasma, intestinal fluid, or gastric fluids. Solvents may also contain components to improve or to decrease the solubility of the test material, or which improve or increase the dissolution rate of the test material. Particularly preferred are aqueous media, for instance salt solutions or buffers at specific pH values.

Step (b) is optional and not necessary as long as Brownian motion is sufficient for fast and complete mixing. However, in case mixing is performed actively, the means as described above are preferably applied.

Steps (c) and (d) are used preferably when more than one dissolution vessel is allocated per sample holder and in cases where the concentration of the test compound is kept low and/or the dissolution volume is low.

In a preferred embodiment of the invention, the concentration of the test compound in the solvent is kept below 30% of its concentration at saturation. Hereby, "concentration at saturation" or synonymously "saturation concentration" is the point of maximum concentration of the test compound in the solution, i.e. the point at which the solution of test compound can dissolve no more of that test compound and additional amounts of test compound will appear as a precipitate. This point of maximum concentration, the saturation point, depends on the solvent, its temperature as well as the physico-chemical nature of the test substance. The saturation concentration is given in mg per mL of solvent.

The above conditions are referred to as sink conditions. Preferably, the concentration of the test compound in the solvent is kept below 20% of the saturation concentration, even more preferable, the concentration of the test compound in the solvent is kept below 10%. Keeping the concentration of the test compound low may be achieved be choosing a dissolution vessel with a big volume. However, it is preferred in the present invention to apply small solvent volumes per dissolution vessel, i.e. the dissolution volumes as indicated above, and rather use more than one, i.e. at least two, dissolution vessel(s) per sample holder and test pellet, respectively. With such a procedure, the overall amount of solvent used in the dissolution test may be kept small even though sink conditions are maintained.

Hence, in a preferred embodiment of the invention, the concentration of the test compound in the solvent is controlled by moving the sample holder retaining the test pellet through a series of separated dissolution vessels allocated to the same sample holder.

Thereby, number, arrangement and format of the sample holder and dissolution vessels are as described above.

A further embodiment of the invention relates to a method for preparing a test pellet having a cross sectional area of 0.2 mm ² to 13 mm ² , comprising the steps of

(a) closing the bottom part of the tubular section of the sample holder with a plane support,

(b) placing test material into the tubular section of the sample holder, wherein the tubular section has an inner cross sectional area of 0.2 mm ² to 13 mm ² ,

(c) applying pressure onto the test material to form a test pellet in the tubular section of the sample holder,

(d) removing the plane support from the bottom of the sample holder.

Thereby, the plane support has a plane and smooth surface facing the inside of the tubular section when the bottom part of the tubular section of the sample holder is closed. The plane support may be designed as a base plate with a plane and smooth surface.

The test material is preferably in a powdery form for obtaining an even and uniform surface of the test pellet. Further, the test material may essentially consist of active ingredient or drug substance, it may essentially consist of a pharmaceutical excipient, or it may consist of material mixtures, i.e. compositions comprising active ingredient or pharmaceutical excipients, for instance mixtures of active ingredient with pharmaceutical excipients. Mixtures may be obtained by mixing two or more components mechanically, by spray-drying, precipitation and subsequent evaporation or by melting the components.

Further, the test pellet is formed by applying pressure onto the test material. Thereby, the pressure has to be selected in a range that allows compacting test material and form a solid pellet. The pellet has to be sufficiently stable in order to avoid the release of particles or not dissolved material into the dissolution medium. Preferably, the pressure applied is in the range of 50-300 kp/cm ² .

Finally, the plane support is removed from the bottom of the sample holder in order to uncover the surface of the test pellet which will be exposed to the solvent in the dissolution test.

In one embodiment of present invention, the test pellet is captured by the bottom part of the tubular section of the sample holder, such as described above.

In a further embodiment of present invention, a separation layer is placed between the plane support and the test material. The separation layer is placed between the support and the test material to flatten the surface area of the test pellet exposed to the solvent and to facilitate non-destructive removal of the test pellet from the plane support. The separation layer may be a foil, a thin plate, or even a monomolecular layer suitable to facilitate the separation between the test pellet and the plane support. An example for a material suitable as separation layer is polyimide material, for instance in the form of polyimide, Teflon® (PTFE, polytetrafluorethylene) or PEEK (i.e. poly( ether etherketone)) plates or films, or weighting paper.

In a further embodiment of the invention, filling material is placed on the top of the test pellet. The filling material is as described above, for instance it may be an inert plunger. Further, the filling material may be in powdery form which will be pressed into a pellet of filling material being in contact with the rear side of the test pellet.

A further embodiment of the invention relates to the use of a test pellet containing a test material, having a cross-sectional area of 0.2 mm ² to 13 mm ² exposed to solvent in order to perform intrinsic dissolution testing under sink conditions. Preferred cross- sectional areas of the test pellet, test material, solvent and sink conditions are as described above.

In the following, the invention will be described according to the Figures.

Figure 1 shows the sample holder 1, bearing the tubular section 2 retaining the test pellet 3 at the bottom of the tubular section 2 with a pressed filling material 4 on the rear side of the test pellet 3. Further, the sample holder bears a funnel 5 in contact with the tubular section 2. The tubular section 2 retains the test pellet 3 exposed to the solvent , representing the dissolution volume 7 within the dissolution vessel 6. Moreover, the cover 8 prevents solvent evaporation. Stir bar 9, which is either magnetizable or magnetic, is used in combination with a magnetic stirrer (not shown) for mixing the solvent 7.

Figure 2 shows the sample holder 1, bearing the tubular section 2 which forms part of a die for test pellet formation. In this figure, the powdery test material 12 has been placed over the funnel 5 into the tubular section 2. Further, the bottom of the sample holder 1 and tubular section 2, respectively, is closed on the bottom side with a plane support 11 and a separation layer 10. Further, plunger 13 is shown which is used for applying pressure onto

the test material 12. Separation layer 10 is selected to facilitate removal of the tubular section 2 retaining the test pellet, i.e. the test material after being pressurized, from the plane support 11.

Figure 3a shows a set of dissolution vessels 6a, 6b and 6c, which are separated from each other. Sample holder 1 retaining in the bottom part of the tubular section a test pellet 3 is moved from vessel 6a to vessel 6b, then to vessel 6c and so forth. Hence, a set of dissolution vessels 6a to 6c is allocated per sample holder 1.

Figure 3b shows the results of a dissolution experiment performed according to the descripition in Fig. 3a. After exposure of the surface of the test pellet in the sample holder to dissolution medium for a defined time under defined conditions, dissolved material in the dissolution medium is quantified by appropriate analytical methods. The intrinsic dissolution rate of the material is calculated from the measured drug concentration in the dissolution medium (mg/mL), the incubation time per vessel, and the surface area of the test material exposed to the dissolution medium. Under ideal conditions this will result in a constant dissolution rate of the compound with time (mg*min ^"lϊf cm ^'2 ).

Figure 4 shows examples for the inner shape of dissolution vessels. The vessel 6d is flat bottomed, vessel 6e is round-bottomed (U-shaped bottom), and vessel 6f represents a V- shaped bottom.

Figures 5a-c show the test with different dissolution media relating to Example 2.

Explanation of the reference numbers:

1 = sample holder

2 = tubular section

3 = test pellet

4 = filling material 5 = funnel

6 = dissolution vessel

7 = dissolution volume / solvent

8 = cover

9 = stir bar 10 = separation layer

11 = plane support

12 = test material in powder form

13 = plunger

Experimental Part:

1. Influence of the Stirring Speed

Five milligrams of ketoprofen powder (Sigma) were filled into a 2 mm circular sample holder (cross-sectional area of 3.14 mm ² ) and the powder was compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta-HDi at 160kp/cm ² . Afterwards, 10 mg of corn starch were filled into the sample holder as a support and closure on the rear side of the pellet and compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta-HDi at 160kp/cm ² . The sample holder with the test pellet was placed into 0.26 mL of a Titrisol buffer, pH 6.8 (Merck Titrisol buffer pH 7.0 adjusted to pH 6.8 with IN HCl) in a 96-well U-shaped microtiterplate (Nunc). Every 5 min, the sample holder was transferred to the next dissolution vessel (= well) of the microtiterplate for a total of 35 min as described in Figure 3a. Mixing of the dissolution medium in the dissolution vessel was performed using a Multidrive stirrer (VP710, V&P Scientific. Inc., San Diego, CA) at 15% and at 30% of its maximal speed (rotational speed does not increase linearly with % scale) and magnetic tumble stir discs (VP722 F-I, V&P Scientific. Inc., San Diego, CA)(3.43 mm diameter; 0.73 mm thickness). Dissolved Ketoprofen in the dissolution medium was quantified by UPLC analysis (Waters Acquity). Experiments were performed in triplicate at room temperature and values in Table 1 are average values ± Standard deviation. Hence, Table 1 shows an example for the influence of the stirring speed on the intrinsic dissolution rate of ketoprofen.

Table 1

2. Identification of appropriate dissolution media:

Five milligrams of nadoldol (Fig. 5a) or of ketoprofen (Fig. 5b) powder (both Sigma) were filled into 2 mm circular sample holders (cross-sectional area of 3.14 mm ² ) and the powder was compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta- HDi at 160kp/cm ² . Afterwards, 10 mg of corn starch were filled into the sample holders as a support and closure on the rear side of the pellet and compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta-HDi at 160kp/cm ² . The sample holders with the test pellet were either placed into 0.26 mL of a Titrisol buffer, pH 6.8 (Merck Titrisol buffer

pH 7.0 adjusted to pH 6.8 with IN HCl), 20 mM Acetate buffer, pH 4.5 or Titrisol buffer, pH 1.2 (Merck Titrisol buffer pH 1 adjusted to pH 1.2 with IN NaOH) in a 96-well U- shaped micro titerplate (Nunc). The sample holders with the nadolol and ketoprofen test pellets were transferred to the corresponding buffers in the next dissolution vessel (= well) of the microtiterplate every 1 min (Nadolol) and 5 min (ketoprofen) for a total of 7 min (Nadolol) and 35 min (ketoprofen), respectively, as described in Figure 3a. Mixing of the dissolution medium in the dissolution vessel was performed using a Multidrive stirrer (VP710, V&P Scientific. Inc., San Diego, CA) at 15% of its maximal speed and magnetic tumble stir discs (VP722 F-I, V&P Scientific. Inc., San Diego, CA) (3.43 mm diameter; 0.73 mm thickness). Dissolved Nadolol or ketoprofen in the dissolution medium was quantified by UPLC analysis (Waters Acquity). Experiments were performed in triplicate at room temperature and results are plotted as cumulated amount of compound released (in mg) from the sample pellet versus time. Figure 5c shows a direct comparison of the intrinsic dissolution rates of Nadolol and Ketoprofen in 2OmM acetate buffer pH 4.5 after various points in time; for Nadolol and ketoprofen the indicated time points are 1 min and 5 min apart from each other, respectively.

3. Temperature Dependency

Five milligrams of paracetamol powder (Sigma) were filled into 2 mm circular sample holders (cross-sectional area of 3.14 mm ² ) and the powder was compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta-HDi at 160kp/cm ² . Afterwards, 10 mg of corn starch were filled into the sample holders as a support and closure on the rear side of the pellet and compressed directly for 5 min by a Stable Micro System, Texture Analyser, Ta-HDi at 160kp/cm ² . The sample holders with the test pellet were placed into 0.26 mL of water in a 96-well U-shaped microtiterplate (Nunc). The sample holders with the paracetamol test pellets were transferred to the next dissolution vessel (= well) of the microtiterplate every 15 min for 60 min at room temperature or every 5 min for 35min at 37°C as described in Figure 3A. Mixing of the dissolution medium in the dissolution vessel was performed using a Multidrive stirrer (VP710, V&P Scientific. Inc., San Diego, CA) at 15% of its maximal speed and magnetic tumble stir discs (VP722 F- 1, V&P Scientific. Inc., San Diego, CA)(3.43 mm diameter; 0.73 mm thickness). Dissolved paracetamol in the dissolution medium was quantified by UPLC analysis (Waters Acquity). Experiments were performed in triplicate and results are plotted as cumulated amount of compound released (in mg) from the sample pellet versus time.

Table 2: