The Importance of Formulation Design in Oral GLP Toxicology Studies

• Catalent Pharma Solutions

Introduction

Toxicology studies that are carried out in compliance with the principles of Good Laboratory Practice (GLP) are somewhat unique when compared with most other preclinical in vivo studies. The need to achieve high exposures, either by administering large doses of the drug molecule or by using enabling formulations, can make the identification of a suitable vehicle challenging. For short investigative toxicology studies, a relatively wide range of vehicles may be used as excipient toxicity is mitigated by the limited study duration. However, from the standpoint of formulation development, toxicology studies conducted in early clinical development are primarily designed to identify acceptable vehicles for use in longer duration first-in-human-enabling studies. As a result, greater consideration must be given to not only the safety profile of the excipients used in these formulations, but also the complexity of manufacturing and formulation stability that will be required over the longer dosing interval.

This article will focus solely on the oral route of administration and covers focus areas in the early drug development process that are critical for a successful toxicology study, including:

• Key decision points in designing formulations for a GLP toxicology study;
• How formulation selection relates to the scientific and regulatory requirements of a toxicology study;
• Why formulation development plays a crucial role in GLP toxicology studies;
• Formulation approaches and limitations for various animal species;
• Optimizing Return on Investment (RoI).

Precursors to Efficient Formulation Development

Even during preclinical studies, having a scientifically-sound potencyand stability-indicating analytical method is crucial to the success of a formulation development program. An ideal analytical method needs to be sensitive, linear, precise, and accurate for the quantitation of the drug molecule. At the same time, the method needs to demonstrate specificity through forced degradation studies to establish the stabilityindicating nature of the method. The ability of the method to quantitate the amount of drug in test articles is important to ensure the correct dose of drug is administered to the animals. Related substance tracking and quantitation in test articles are critical to drug impurity qualification for setting release and stability specifications for the clinical product. This means that the method should be able to capture changes to the molecule due to environmental changes, including the impact of excipients, storage conditions, and manufacturing processing. The analytical method plays such a key role in enabling efficient formulation development and therefore should not be a second priority.

Additionally, comprehensive preformulation data is essential during formulation development. At a minimum, there should be an understanding of the drug molecule’s physical characteristics, solidstate stability, solubility and solution stability across the biological pH range, and permeability. Keep in mind that the preformulation data should be established using the most stable polymorph and the selected salt form. Salt forms should be considered if they improve the molecule’s solubility, stability, or manufacturability. However, they can have implications when developing enabled formulations as discussed later in this article. Changing polymorphs or salt forms mid-way through formulation development could significantly impact the time and cost of a program, and should be avoided.

GLP Toxicology Study and Formulation Development Goals

The purpose of GLP toxicology studies is to elucidate the safety profile of a drug. To reach toxic drug levels, the required dose is likely much higher than the eventual clinical dose. Achieving dose proportional increase in exposure simplifies development and ensures a predictable response. However, drugs, especially those with solubility and permeability issues, may display a non-linear, non-dose proportional exposure (Figure 1). Their total exposure likely plateaus before reaching a toxicity level and in worse cases, the exposure fails to meet the minimum effective level. These drugs display unpredictable and highly variable exposure, which may lead to unforeseen safety issues as more subjects are dosed.

Figure 1. Dose-Concentration Curves of a Non-Linear, Non-Dose Proportional and Linear, Dose Proportional Drugs

The first step to designing a GLP toxicology formulation is to identify the formulation platform and dosage presentation needed to meet this challenge. The following questions should be considered:

• What is the toxicology dose(s) I need to achieve my exposure goals?
• What are the selected animal species? Are the results translatable to humans and are there any peculiarities in dosing these animal species?
• Will dosing crystalline API provide sufficient exposure based on the efficacy and toxicity established in the dose range finding, dose escalation, or exploratory toxicology studies?
• If the API has low inherent solubility in biorelevant media, how can I increase exposure?
• What role do permeability, transporters, efflux, and metabolism play in exposure?

Formulation Strategies and Dosing Considerations

For the best chance of a successful study outcome, the formulation approach should be based on a sound understanding of the drug molecule’s properties. A good starting point for any development program is to establish where the drug falls in the developability classification system (DCS) as shown in Figure 2. This system is based on the biopharmaceutics classification system (BCS) but it is designed to be used as a formulation selection tool rather than a regulatory tool to obtain biowaivers. Molecules that fall in the DCS I category can usually be dosed as simple solutions or suspensions to animals. Molecules falling into the DCS III/IV categories have low permeability that may complicate their practical use as a drug. Molecules in the DCS II category face poor solubility issues and are prime candidates for enhanced formulations to increase solubility and therefore exposure.

Figure 2. Developability Classification System¹

If the native form of drug molecule (salt or free form) shows dose proportionality in early pharmacokinetic studies and is expected to provide toxicity level exposure, it would be easiest to dose the molecule in the form of a simple solution or suspension for GLP studies. This approach is suitable for both rodent and non-rodent species, as is required for a GLP study. This dosing approach is typical for DCS I molecules and provides a wide range of dose flexibility. Simple aqueous solutions are an excellent approach which eliminates dosing uniformity concerns of some other approaches. When developing a high concentration solution formulation for a well-tolerated drug, pH manipulation and surfactant/cosolvent/complexing agent incorporation are possible ways to maximize and maintain solubility at the higher dose levels. When a solution formulation is not possible, a suspension can be developed to fulfil the need to achieve the required high drug concentration. The suspension media may consist of surfactants such as sodium lauryl sulfate (SLS) and/or suspending agents such as methylcellulose. A suitable suspension will be one that is easy to administer at the intended dose concentration, homogeneous upon preparation, easily resuspended to obtain homogeneity after storage, and it should be able to maintain physical and chemical stability over a practical dosing period to allow flexibility of preparation at the toxicology site. Alternatively, a powder-in-capsule (PIC), a capsule filled with a simple blend, or a tablet, can also be developed. These solid dosage presentations are acceptable for non-rodent species, but not suitable for rodents due to dosage size. For DCS I molecules, a final clinical formulation is likely a traditional solid oral dosage form.

For DCS II molecules displaying good permeability but poor solubility, a simple solution formulation or a suspension may not provide dose proportionality or sufficient exposure to achieve toxicity level. Therefore, bioavailability enhancement technologies will be needed. An enhanced formulation can improve kinetic solubility of the molecule and potentially prevent or retard precipitation, but it generally will not improve permeability of the molecule (DCS III/IV). For molecules where absorption is limited by their dissolution rate (DCS IIa), micronization or co-micronization with a surfactant is a straightforward technology to reduce particle size, increase surface area, and/or improve wettability. On the other hand, when a molecule is truly solubility limited (DCS IIb), more complex formulation approaches may be necessary.

DCS IIb molecules that are water insoluble but lipophilic (often called “grease balls”) can be formulated in lipid-based and self-emulsifying drug-delivery systems. This approach can not only enhance solubility of the molecule but potentially influence lymphatic transport, thus allowing for bioavailability enhancement. Lipid-based formulations are versatile, relatively easy to develop, cost efficient to manufacture, and can be easily adapted as the clinical formulation.

For DCS IIb molecules that experience solubility limitations due to their stable crystalline structure (often called “brick dust”), amorphous dispersions could be the answer. Technologies such as spray drying and hot melt extrusion have gained popularity over the past few decades. These technologies can greatly improve solubility by rendering the molecule amorphous in the presence of a carrier excipient such as a polymer, making transient supersaturation possible to facilitate absorption. But amorphous dispersions can face physical and chemical stability challenges and are more complex and expensive to manufacture than traditional dosage forms. In addition, the free form of the drug molecule is best suited to these technologies as the salt can work to destabilize the dispersion and reduce the overall drug loading. Overall, these more sophisticated formulations should be considered only when other options are exhausted.

As mentioned, molecules facing permeability issues (DCS III/IV), high first pass metabolism, transporter uptake, and/or efflux add an additional layer of challenge to a program. The use of certain excipients may enhance alternative uptake pathways such as via the lymphatic system or inhibit active efflux pumps. However, the path to success for molecules with these limitations is less clear than those with solubility limitations.

Main Takeaway

• Solutions and suspensions are best for GLP toxicology studies for DCS I molecules in both rodents and non-rodents and they are the more consistent and dose flexible forms compared to traditional solid oral dosage forms such as capsules or tablets
• For solubility-limited molecules (DCS II), an enhanced formulation can provide improved aqueous solubility which could translate to bioavailability enhancement
• Within the enhanced technologies, (co)micronization and lipidbased formulations are more straightforward and cost efficient than spray drying and hot melt extrusion.

Excipient Considerations

Excipients are part of every toxicology formulation and therefore decisions surrounding them can have a significant impact on the program, including its costs, regulatory pathway, and timeline considerations. The safety and tolerability of excipients used in combination should be carefully considered. When using novel excipients with poorly soluble molecules to achieve adequate solubility and maximize exposure, it should be considered that their continued use in future clinical formulations requires extensive safety and toxicology data adding time, cost, and risk to a program. Some innovative excipients, such as cyclodextrin complexing agents, are still more expensive than standard excipients, even though generic versions are now available. In addition, it should be noted that certain excipients are known to be problematic for certain animal species, and thus should be avoided. Polysorbate 80 is known to cause anaphylactic reactions in dogs² and should not be used in GLP toxicology studies conducted using dogs.

From Animal to Human

Generally speaking, the GLP toxicology formulation should closely resemble the formulation intended for clinical use. Yet often, the GLP toxicology and the clinical formulations are not the same due to the nature of a toxicology formulation requiring a significantly higher dose strength. Although the dosage presentations do not have to be the same, the formulation platform should be consistent. If a suspension of crystalline material is used in the GLP toxicology study, a powderin-bottle or a powder-in-capsule presentation is likely adequate for the clinical studies. If an amorphous dispersion is used in the GLP toxicology study, an amorphous platform should be considered for the clinical studies. However, if the predicted clinical dose is orders of magnitude lower than the GLP doses, it is possible that an amorphous form will not be required for the clinical studies. Calculating the solubility limited absorbable dose (SLAD) proposed as part of the DCS, may help to determine whether it is reasonable to attempt to use a crystalline presentation of the drug candidate in early clinical trials.

Closing Thoughts

It usually takes two-to-four months to complete the analytical and formulation development activities to support a GLP toxicology study. This timeline can be extended if analytical methods are challenging to develop, and/or if an enhanced formulation approach is needed. Making a sound investment in a GLP formulation early can save developers time and money in the long run. Developers do not want to discover during their GLP studies that reformulation is required to achieve exposure, head off solubility issues, or to overcome in-use stability problems during dosing. Timely and successful completion of GLP studies is an essential step to getting new drugs to patients, which is the ultimate goal of any drug development program.

References

1. Bulter, J. & Dressman, J. (2010). The Developability Classification System: Application of Biopharmaceutics Concepts to Formulation Development. Journal of pharmaceutical sciences, 99(12), 4940-4954.
2. Qiu, S. et. al., (2013). Complement activation associated with polysorbate 80 in beagle dogs. International Immunopharmacology, 15(1), 144-149.