To evaluate chronic drug toxicity, investigators conduct three major types of long-term animal studies. The first is repeat-dose (chronic) testing to screen for any sort of toxicity that might be detected by in-life observation (e.g., clinical signs, effects on growth and body weight, changes in electrocardiogram or clinical pathology parameters) and postmortem evaluation (e.g., changes visible to the naked eye or by light microscopic examination of tissues, differences in organ weight). Some animals from chronic toxicology studies are retained when the exposure phase is finished to evaluate the reversibility of specific adverse effects. This information enables the potential significance of many positive toxicity findings to be better assessed.

The second type of long-term studies are carcinogenicity studies that specifically evaluate a drug’s ability to cause tumors in animals. These studies use large numbers of animals (400 to 700), last for the natural lifespan of rats and mice (which is approximately two years), and require light microscopic examination of more than 40 tissues from each animal.

The third type of long-term studies are reproductive and developmental toxicity studies that specifically evaluate a drug’s ability to interfere with normal reproduction, pregnancy, fetal development, parturition, the development of offspring to adulthood, and the ability of offspring to reproduce. The studies are normally divided into three subtypes: fertility studies, embryofetal development (teratology), and perinatal/postnatal studies.

There are numerous well-known reasons why high doses of a chemical or drug may produce entirely different effects than those produced by a lower dose. For example, at high doses, the body may be unable to metabolize a drug that is handled in a therapeutically useful and safe way when lower doses are given. This may lead to toxic metabolites being produced at exaggerated dose levels that are not formed at lower, therapeutically useful doses. Alternatively, the body may be unable to eliminate large doses of a drug and the drug may accumulate to levels that cause toxicity in safety tests but would be unlikely to occur in humans taking recommended doses. There may also be marked differences in effects noted in different species with high doses of a drug.

A number of standard or core toxicology studies are usually conducted with new drugs and the study designs often have few if any modifications. For most drugs it is essential to perform:

The information obtained is used as a starting point to plan further studies. A core program of studies contains both short-term and long-term studies.

The specific studies conducted for a drug depend on its eventual route of administration to patients, the duration of patient exposure, and previous clinical and toxicological information on related compounds or drugs, plus regulatory requirements and requests. Depending on these factors, some additional studies are usually added to the core group of studies to be conducted. Beyond this group of studies are a large number of potential studies that are considered when relevant. Some companies are more conservative than others in their approach to toxicology and usually conduct more studies than would be considered to be a minimum by most regulatory agencies.

In addition to long-term toxicology studies and the appropriate battery of short-term studies, a number of specialized studies often must be conducted. The nature of these studies depends on the nature of a drug (i.e., its therapeutic use, its chemical class, and specific clinical and pharmacological characteristics). These studies could involve additional species of particular interest or special strains of animals with certain genetic characteristics.
Also, specific routes of administration, new parameters, or various formulations of the drug could be evaluated. For example, if analyses for impurities in a drug undergoing stability tests demonstrate that 15% of a specific breakdown (i.e., degradation) product is formed, it may be necessary to conduct independent toxicology studies on that breakdown compound. In addition, the biological activity of breakdown product(s) will have to be assessed. If breakdown products are either isomers of the parent drug or biologically similar to the parent drug, then it may be possible to obtain regulatory approval for a shelf life that would permit a certain degree of degradation in the final drug product.

One major “Catch-22” situation often arises in determining the appropriate timing for conducting toxicology studies. This relates to the general desire to wait for the efficacy of a new drug to be established in Phase 2 trials before initiating the long-term toxicology studies (i.e., six- or nine-month repeat-dose studies, reproductive toxicity studies, and lifetime carcinogenicity bioassays, each done in two separate species), coupled with the relatively long time necessary to complete some of these studies (e.g., more than three years for a carcinogenicity study program). Long-term toxicology studies not only are extremely expensive to conduct (approximately two million dollars for each rodent bioassay used to assess carcinogenic potential), but also consume large quantities of investigational drug. The drug may not yet have been synthesized in sufficient amounts to allow such testing to occur at an early stage of development, and there may be various problems associated with obtaining sufficient drug.

Nonetheless, a company must complete several reproductive toxicity studies before enrolling fertile women in Phase 2 clinical trials, so these studies cannot be postponed for a long time. In addition, if the go-no-go point in Phase 2 is used as the time to initiate long-term repeat-dose toxicology studies, or carcinogenicity studies, the Phase 3 pivotal clinical trials on some drugs could be completed and a marketing application ready to submit to a regulatory agency before toxicology reports are completed and ready for submission. Companies will almost always do anything to avoid having toxicology be a rate-limiting step in submitting a New Drug Application or other marketing application.

In practice, one can almost always plan adequate time to complete the toxicology program before all clinical reports are finished. One way of dealing with the Catch-22 situation with long-term studies is to reduce the lag (commonly six months to one year or more), between completion of long-term dosing and availability of the final report. If a problem of timing seems to be likely, then long-term toxicology studies can be initiated earlier in the development program, with the understanding that the money to pay for these studies would possibly be wasted if the drug had to be terminated. The term possibly is used because, whether the drug’s toxicology profile demonstrated the absence or presence of specific findings, the data could provide important information to help plan the future development of the entire chemical series.

Some research and development managers desire to start many toxicology studies on all new investigational compounds as soon as the drug supply is available. This makes sense if there are few drugs undergoing development at the company and the company is in the fortunate position of having abundant resources. Most pharmaceutical companies are clearly not in this position. If a company is prioritizing the development of its investigational drugs, and if it is concerned about not wasting resources, then it is generally wise to delay the start of long-term toxicology studies until one or more well controlled Phase 2 trials are completed. These studies should clearly demonstrate efficacy, confirm that the safety profile and pharmacokinetics are acceptable, and provide an overall indication that a regulatory submission (e.g., Product License Application, New Drug Application) will probably be prepared for the drug.

There is great variation in the quality of contract toxicological laboratories that conduct studies for the pharmaceutical industry. A pharmaceutical company that “bargain shops” looking for the cheapest place to conduct toxicology studies is usually foolish. There is no faster way to kill a promising drug unnecessarily or to delay its progress than by raising toxicological questions or false positive results that must be addressed. Clinical programs are often put “on hold” by the Food and Drug Administration (FDA), by other regulatory authorities, and/or by the company itself when toxicological issues arise.

Factors that are at least as important as price when deciding where to conduct toxicology studies under contract are the ability of the laboratory to start studies quickly, deliver reports on time, respond appropriately to unforeseen events (e.g. unexpected toxicity or technical problems), work effectively with subcontractors, and communicate with the pharmaceutical company sponsor. In addition, the recent regulatory inspection history of the laboratory (by the Occupational Safety and Health Administration, US Department of Agriculture, FDA, etc.), the experience of the staff (especially the study director who will be completely responsible for each study), staff morale, staff turnover rate, and the quality of facilities, equipment, and training programs are all important factors to consider.

High-quality studies are essential and corners should not be cut. In situations where specialized toxicology studies are required, there may only be a few laboratories qualified to do the work. These groups may be unable to schedule a company’s work as soon as desired. This presents a potentially serious dilemma. It is usually better to wait for the best group to conduct the study, provided that the development program is not unacceptably delayed. When delays are unacceptable and the study must be contracted to another group, the contracted study must be monitored intensely.