formulation design
HOW TO ADMINISTER AMORPHOUS SOLID DISPERSIONS TO SMALL LABORATORY ANIMALS ?
Amorphous solid dispersions (ASDs) are frequently used to enhance solubility and biovailability of active pharmaceutical ingredients (APIs) in discovery and development programs. However, ASDs present unique challenges when it comes to administration to small laboratory animals, like mice or rats.
Indeed, ASDs are solid products (unlike many other solubility-enhancing formulations which are liquid in their native form, like cyclodextrin solutions, lipid-based formulations or nanosuspensions). While specific capsules for administration of solid dosage forms to small laboratory animals exist (for example size 9 capsules with a filling volume of 0.025 mL), the mg-quantities of formulation that can be administered from such capsules are usually well below the target dosing ranges in early pharmacology and toxicology studies (100-1000 mg/kg).
Administration of high API doses to small laboratory animals is much more readily achieved from liquid formulations, which can be administered at dosing levels up to 10 mL per kg body weight (which corresponds to 2.5 mL of formulation for a 250 g rat or 0.25 mL to a 25 g mouse). Prior dispersion of ASD powder in a liquid vehicle and subsequent administration as a suspension therefore opens up the possibility of giving much higher doses than those attainable with a capsule.
However, dispersion of an ASD powder in a liquid vehicle creates a new set of problems. Practitioners generally prefer mild aqueous buffers as gavage vehicles for reasons of tolerability, but exposure to aqueous media can detract greatly from the in vivo performance of ASDs. First, water may migrate into the ASD powder, thereby plasticizing the amorphous matrix and inducing API crystallization. Second, API may be released into the aqueous gavage vehicle, where it is susceptible to precipitation into a less soluble form. Both effects may lead to dramatic API crystallization in the gavage vehicle prior to administration, which can negate to large extent the ASD's potential to improve oral absorption.
Another practical problem that arises when dispersing ASDs at high concentrations in aqueous media is the increase in viscosity due to polymer hydration. For some polymers (like HPMC E5 which is often used as an ASD carrier polymer), exposure to water for a couple of minutes may enhance viscosity to levels that are incompatible with administration through a 18-20 G gavage tube.
Two main tactics can be employed to reduce the risk of API crystallization during dispersion in a gavage vehicle, notably the use of polymers with pH-dependent solubility and the use of non-aqueous gavage vehicles. These techniques also help in keeping viscosity under control such that syringeability through narrow gavages tubes remains practicable.
Designing ASDs based on polymers with pH-dependent solubility
A wide variety of pharmaceutical-grade polymers with pH-dependent solubility are commercially available. Nearly all of these materials have originally been designed to serve as enteric coating polymers, but many of them have also shown great utility as carriers for ASDs (for example the cellulose derivative hydroxypropylmethyl cellulose acetate succinate, HPMCAS). These “enteric” polymers do not dissolve at the pH values typically encountered in the stomach (pH 1-4), but show rapid dissolution at intestinal pH (pH 5-8). ASDs based on enteric polymers can usually be successfully administered from gavage vehicles with acidic pH. Given the insolubility of the carrier polymer at these low pH values, the risk of water penetration and subsequent API crystallization is greatly reduced. For the same reasons, the extent of API release from the ASD powder during dispersion in the gavage vehicle is also diminished.
From a practical point of view, a good starting point is usually to install a pH in the gavage vehicle that is 1 pH-unit below the dissolution pH of the polymer, and to keep buffer capacity as low as possible to minimize impact on the pH in the duodenum - where one wants rapid polymer dissolution to occur.
An additional benefit of this approach is that API release is targeted to the the small intestine - the site of absorption. This form of targeting reduces the potential performance loss due to API supersaturation and precipitation in the stomach (where API absorption capacity is negligible relative to the small intestine).
To verify whether an ASD powder is resistant to gavage vehicle-induced API crystallization, one can disperse the ASD powder in the vehicle of interest, take samples as a function of time and assess these for potential crystallinity with a technique like X-ray diffraction or polarised light microscopy.
Using non-aqueous excipients as gavage vehicles
In case the usage of enteric polymers is not an option (for instance because early formulation design work has shown that none of the enteric polymers show adequate ability to stabilize the amorphous form of the API), one can evaluate the utility of non-aqueous gavage vehicles. The key here is to select vehicles with good tolerability, as generally relatively high volumes of vehicle will be co-administered to deliver the target ASD dose. Frequently used excipients for this purpose include pharmaceutical-grade oils that are rich in triglycerides, like corn oil or soybean oil. Dispersion of ASD powders in such vehicles carries a much lower risk of ASD matrix plasticization and premature API crystallization. However, the verification of the absence of crystallization on dispersion in the vehicle is still warranted.
When interpreting the pharmacokinetic data, one should bear in mind that the vehicle may also have assisted (sometimes dramatically) in promoting intestinal solubilization and API absorption (as lipids and lipophilic excipients have the potential to alter the gastrointestinal milieu, which we broadly discussed in one of our earlier articles). While this may be a desirable outcome in discovery and preclinical studies, the "vehicle effect" may be lost later on in development when ASDs are administered without gavage vehicle (e.g. in dog studies or early clinical studies).
Other attention points when administering ASDs via oral gavage
The typical way in which practitioners go about gavage administration is to prepare multiple doses in bulk via dispersion of an accurately weighed amount of ASD powder in an accurately measured volume of gavage medium. This volume of bulk dispersed formulation is then stirred while individual doses are collected from the container with a syringe. As is the case with administration of any dispersed formulation, ensuring homogeneity of the dispersed phase (ASD powder) in the continuous phase (the gavage vehicle) is crucial to obtaining dose uniformity. Care should therefore be taken when preparting the gavage dispersion to avoid agglomeration and inhibit potential sedimentation.
Wettability of ASD powders based on enteric polymers in acidic gavage vehicles also tends to be poor, and agglomeration is likely to occur. This risk may be mitigated via addition of the powder under vigorous stirring/ultrasonication, and/or the addition of very low amounts of surfactants to the gavage vehicle (concentrations should be minimized here as surfactants can plasticize the ASD matrix and induce API crystallization). Dispersion of ASD powders, whether or not they are based on enteric polymers, in lipophilic vehicles (e.g. corn oil) generally also requires vigorous mixing to ensure adequate deagglomeration.
Problems of ASD powder sedimentation can usually be mitigated via adequate stirring throughout the administration period, but in case that unacceptable sedimentation is observed, a viscosity-increasing agent may be added to the vehicle.
If you would like to learn more about our approach to amorphous solid dispersion design, you can find more information here or reach out to one of our experts directly via the button below.
Indeed, ASDs are solid products (unlike many other solubility-enhancing formulations which are liquid in their native form, like cyclodextrin solutions, lipid-based formulations or nanosuspensions). While specific capsules for administration of solid dosage forms to small laboratory animals exist (for example size 9 capsules with a filling volume of 0.025 mL), the mg-quantities of formulation that can be administered from such capsules are usually well below the target dosing ranges in early pharmacology and toxicology studies (100-1000 mg/kg).
Administration of high API doses to small laboratory animals is much more readily achieved from liquid formulations, which can be administered at dosing levels up to 10 mL per kg body weight (which corresponds to 2.5 mL of formulation for a 250 g rat or 0.25 mL to a 25 g mouse). Prior dispersion of ASD powder in a liquid vehicle and subsequent administration as a suspension therefore opens up the possibility of giving much higher doses than those attainable with a capsule.
However, dispersion of an ASD powder in a liquid vehicle creates a new set of problems. Practitioners generally prefer mild aqueous buffers as gavage vehicles for reasons of tolerability, but exposure to aqueous media can detract greatly from the in vivo performance of ASDs. First, water may migrate into the ASD powder, thereby plasticizing the amorphous matrix and inducing API crystallization. Second, API may be released into the aqueous gavage vehicle, where it is susceptible to precipitation into a less soluble form. Both effects may lead to dramatic API crystallization in the gavage vehicle prior to administration, which can negate to large extent the ASD's potential to improve oral absorption.
Another practical problem that arises when dispersing ASDs at high concentrations in aqueous media is the increase in viscosity due to polymer hydration. For some polymers (like HPMC E5 which is often used as an ASD carrier polymer), exposure to water for a couple of minutes may enhance viscosity to levels that are incompatible with administration through a 18-20 G gavage tube.
Two main tactics can be employed to reduce the risk of API crystallization during dispersion in a gavage vehicle, notably the use of polymers with pH-dependent solubility and the use of non-aqueous gavage vehicles. These techniques also help in keeping viscosity under control such that syringeability through narrow gavages tubes remains practicable.
Designing ASDs based on polymers with pH-dependent solubility
A wide variety of pharmaceutical-grade polymers with pH-dependent solubility are commercially available. Nearly all of these materials have originally been designed to serve as enteric coating polymers, but many of them have also shown great utility as carriers for ASDs (for example the cellulose derivative hydroxypropylmethyl cellulose acetate succinate, HPMCAS). These “enteric” polymers do not dissolve at the pH values typically encountered in the stomach (pH 1-4), but show rapid dissolution at intestinal pH (pH 5-8). ASDs based on enteric polymers can usually be successfully administered from gavage vehicles with acidic pH. Given the insolubility of the carrier polymer at these low pH values, the risk of water penetration and subsequent API crystallization is greatly reduced. For the same reasons, the extent of API release from the ASD powder during dispersion in the gavage vehicle is also diminished.
From a practical point of view, a good starting point is usually to install a pH in the gavage vehicle that is 1 pH-unit below the dissolution pH of the polymer, and to keep buffer capacity as low as possible to minimize impact on the pH in the duodenum - where one wants rapid polymer dissolution to occur.
An additional benefit of this approach is that API release is targeted to the the small intestine - the site of absorption. This form of targeting reduces the potential performance loss due to API supersaturation and precipitation in the stomach (where API absorption capacity is negligible relative to the small intestine).
To verify whether an ASD powder is resistant to gavage vehicle-induced API crystallization, one can disperse the ASD powder in the vehicle of interest, take samples as a function of time and assess these for potential crystallinity with a technique like X-ray diffraction or polarised light microscopy.
Using non-aqueous excipients as gavage vehicles
In case the usage of enteric polymers is not an option (for instance because early formulation design work has shown that none of the enteric polymers show adequate ability to stabilize the amorphous form of the API), one can evaluate the utility of non-aqueous gavage vehicles. The key here is to select vehicles with good tolerability, as generally relatively high volumes of vehicle will be co-administered to deliver the target ASD dose. Frequently used excipients for this purpose include pharmaceutical-grade oils that are rich in triglycerides, like corn oil or soybean oil. Dispersion of ASD powders in such vehicles carries a much lower risk of ASD matrix plasticization and premature API crystallization. However, the verification of the absence of crystallization on dispersion in the vehicle is still warranted.
When interpreting the pharmacokinetic data, one should bear in mind that the vehicle may also have assisted (sometimes dramatically) in promoting intestinal solubilization and API absorption (as lipids and lipophilic excipients have the potential to alter the gastrointestinal milieu, which we broadly discussed in one of our earlier articles). While this may be a desirable outcome in discovery and preclinical studies, the "vehicle effect" may be lost later on in development when ASDs are administered without gavage vehicle (e.g. in dog studies or early clinical studies).
Other attention points when administering ASDs via oral gavage
The typical way in which practitioners go about gavage administration is to prepare multiple doses in bulk via dispersion of an accurately weighed amount of ASD powder in an accurately measured volume of gavage medium. This volume of bulk dispersed formulation is then stirred while individual doses are collected from the container with a syringe. As is the case with administration of any dispersed formulation, ensuring homogeneity of the dispersed phase (ASD powder) in the continuous phase (the gavage vehicle) is crucial to obtaining dose uniformity. Care should therefore be taken when preparting the gavage dispersion to avoid agglomeration and inhibit potential sedimentation.
Wettability of ASD powders based on enteric polymers in acidic gavage vehicles also tends to be poor, and agglomeration is likely to occur. This risk may be mitigated via addition of the powder under vigorous stirring/ultrasonication, and/or the addition of very low amounts of surfactants to the gavage vehicle (concentrations should be minimized here as surfactants can plasticize the ASD matrix and induce API crystallization). Dispersion of ASD powders, whether or not they are based on enteric polymers, in lipophilic vehicles (e.g. corn oil) generally also requires vigorous mixing to ensure adequate deagglomeration.
Problems of ASD powder sedimentation can usually be mitigated via adequate stirring throughout the administration period, but in case that unacceptable sedimentation is observed, a viscosity-increasing agent may be added to the vehicle.
If you would like to learn more about our approach to amorphous solid dispersion design, you can find more information here or reach out to one of our experts directly via the button below.
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