Formulating a plan for toxicology studies

GLP toxicology studies are designed to provide an insight into a drug’s safety profile, however, formulations need to be carefully developed before these studies can be effectively performed. In this article, Stephie Lee, scientist, Oral Drug Delivery at Catalent Pharma Solutions, goes through the important considerations for a successful GLP toxicology study outcome.

GLP toxicology studies are designed to give an insight into a drug’s safety profile. As the aim is to predict at what level a drug becomes toxic, the doses administered are likely to be substantially greater than those that will ultimately be given to patients. If exposure increases with dose in a linear fashion, development is likely to be simpler and responses become more predictable.

However, this is frequently not the case, particularly if the drug is poorly soluble or permeable, where its exposure may plateau before toxicity is achieved, or may not even reach the minimum effective level. Because the exposure is neither linear nor proportional for these drugs, the lack of predictability may lead to unforeseen safety concerns as increasing numbers of subjects are dosed.

It is therefore important to design studies very carefully, with the first step to identify a formulation that can provide the exposure needs. Several factors are to be considered: the toxicology dose needed to achieve exposure goals; which animal species are being used, and what the expected translatable results to humans will be; whether crystalline form of the drug will give sufficient exposure, based on the efficacy and toxicity observed in dose range-finding and escalation or exploratory toxicology studies; how exposure can be increased if the drug has low solubility; and what role permeability, metabolism, transporters and efflux have on exposure.

Formulation choice

A successful study outcome is more probable if the approach to formulation is based on a sound knowledge of a molecule’s properties. Therefore, a good starting point is to determine where in the developability classification system (DCS) it falls. (Figure 1).

Class 1 molecules, with good solubility and permeability, are the easiest to work with, as it is normally appropriate to dose them to animals as a solution or suspension. For molecules in Class 2, poor solubility must be overcome, and some form of enhanced formulation should be used to increase solubility and, therefore, exposure. Those in Class 3 and Class 4 have low permeability, which can make them tricky to use as drugs.

Formulation development

When the native form of a molecule exhibits dose proportionality in early pharmacokinetic studies (often DSC Class 1 molecules) and is anticipated to give sufficient exposure, a solution or suspension should suffice. It allows for great dose flexibility and is suitable for both non-rodent species and rodents. Solubility can often be maximised and maintained by optimising the pH, or adding surfactants, co-solvents, or complexing agents to the mix. If a solution is not possible, then a suspension may be developed instead. Typical suspension media include surfactants like sodium lauryl sulphate and suspending agents such as methylcellulose. Appropriate suspensions should be easy to administer at the desired concentration, homogeneous upon preparation, and easy to re-homogenise after storage.

If a solution or suspension is not practical, potential alternatives include tablets, or capsules filled with the neat drug substance or a simple blend. While these can be given to non-rodent species, the dosage size may be too large for rodents. The final clinical formulation is often a capsule or tablet for a DCS Class 1 molecule, whether the GLP toxicology studies are done with a solution, a suspension, or a solid dosage form.

Improving solubility

DCS 2 molecules have good permeability but poor solubility, and therefore a simple solution or suspension is unlikely to provide dose proportionality or sufficient exposure to achieve toxicity. Bioavailability enhancement will be required to improve the molecule’s kinetic solubility, and possibly also retard or prevent precipitation. However, development strategy is not usually as straightforward as for molecules in DCS 3 and 4, where permeability is poor.

For those DCS 2 molecules where absorption is limited by dissolution rate, micronisation, or co-micronisation with a surfactant, may suffice. For those that are solubility-limited but lipophilic, the answer may lie in using a lipid-based delivery system. These formulations are usually straightforward to develop, cost-efficient to make and can be adapted to give a clinical formulation.

For those where solubility is a result of its stable crystalline structure, an amorphous dispersion may be necessary, using spray drying or hot melt extrusion technologies. However, physical and chemical stability challenges are common, and they are more expensive and difficult to develop than the traditional dosage forms. Therefore, an amorphous dispersion should only be considered as a last resort.

Addressing permeability

Any molecule that falls into DCS Class 3 or 4 has permeability challenges, which makes formulation for a GLP toxicology study more complicated. It may be possible to promote alternative uptake pathways, such as via the lymphatic system or inhibit active efflux pumps, by using certain excipients.

Excipient considerations

Careful consideration of excipients is also important: they can affect costs, regulatory pathway and timeline. If a novel excipient is employed to increase exposure, it must always be remembered that extensive supporting safety data will be required. Others, while safe for humans, may have adverse effects in the animal species used to conduct the study.

The formulation used in the GLP toxicology study should, ideally, be related to the formulation that will be used in the clinic, albeit at a higher dose than will be given to human subjects. It does not have to be exactly the same formulation or dosage form, but if the toxicology formulation is, say, crystalline or amorphous, then the clinical formulation should be too. The exception would be, if the predicted human dose is much lower, then it could be that an amorphous form will not be necessary.

It takes time to develop a formulation for GLP toxicology studies, and to carry out the necessary analytical work to support development and testing of the formulation. Typically, this will require two to four months. It can, however, save both time and money in the long run, as some of the critical questions around solubility, permeability and exposure for the drug will already have been answered.