Kevin Denny, MBA, DABT- Executive Technical Director, WuXi AppTec
According to the U.S. FDA, a drug's maximum tolerated dose (MTD) is the "highest dose that does not produce unacceptable toxicity." But the toxicity level considered "unacceptable" can be a relative term. Some level of toxicity would likely be acceptable—and even expected—in drugs used to treat rare diseases or those used in life-threatening medical situations. That is why defining a dose exploration protocol early in the preclinical phase is crucial to successful and safe drug development.
Single-dose MTD testing represents a moment in time during preclinical dose exploration, but it is nonetheless essential for early drug development. The intent of the MTD exploration is finding the range in which a dose exhibits signs of toxicity to a dose considered the highest dose tolerable with extended dosing. Estimating MTD in early drug development can be challenging, when the pharmacologically active dose may be close to the dose that produces overt toxicity. The actual value of these early-stage studies is to inform the starting clinical dose regimen submitted in IND applications.
MTD Testing and Safety Pharmacology for IND Applications
MTD testing establishes a therapeutic baseline for a specific drug. Failing to identify it during early development can lead to sub[1]optimal or skewed dose levels in first-in-human (FIH) clinical trials. For this reason, MTD studies should be conducted on most new drug candidates. Exceptions would include endogenous hormones and proteins (e.g., recombinant clotting factors) used at physiologic concentrations or prophylactic vaccines. The data derived from these studies establish a baseline and informs where the rest of the toxicology program goes. It is critical for scientists to know where toxicology exists—and where it does not—as they prepare investigational new drug (IND) applications.
MTD studies also provide crucial information to guide the safety pharmacology studies that are needed for IND applications. The U.S. FDA defines safety pharmacology studies as those that "investigate the potential undesirable pharmacodynamic (PD) effects of a substance on physiological functions in relation to exposure in the therapeutic range and above." Safety pharmacology studies have three primary objectives:
- To identify undesirable pharmacodynamic properties of a substance that may have relevance to its human safety.
- To evaluate adverse pharmacodynamic and/or pathophysiological effects of a substance observed in toxicology and/or clinical studies.
- To investigate the mechanism of the adverse pharmacodynamic effects observed and/or suspected.
Every IND application should identify these core objectives and explain how they will be achieved during the study. The cardiovascular, respiratory and central nervous systems comprise the vital organs in which most safety pharmacology testing occurs. Specific endpoints scientists look at in these organ systems include:
- Central Nervous System: Motor activity, behavioral changes, coordination, sensory/motor reflex responses and body temperature.
- Cardiovascular System: Blood pressure and heart rate.
- Respiratory System: Respiratory rate and other measures of respiratory function. Also note that clinical observation of animals is generally inadequate to assess respiratory function.
In vivo, safety pharmacology studies should investigate the dose-response relationship of any adverse effects. The onset and duration of the adverse effect should also be investigated and compared to a dose that elicits the proposed or expected pharmacodynamic impact. If no adverse effects are identified, investigators should submit the highest tested dose that produced a moderate adverse effect in a related study with a similar administration route and exposure duration.
Conversely, in vitro, studies should establish a concentration-effect relationship. A range of drug concentrations should be used to increase the likelihood of seeing an effect on the test species. In the absence of an effect, the range of concentrations selected will need to be justified in the IND application.
MTD Study Design
Acute toxicity studies, short-duration dose-escalation studies and dose-ranging studies can all be used to derive a potential new drug's MTD. The MTD is often determined by acute toxicity studies including short duration dose escalation studies or dose ranging studies. These studies use a minimum number of test species and include toxicological endpoints such as clinical observations, body weight changes and clinical pathology to determine potential acute toxicities.
This maximum tolerated dose can be used to support doses to be evaluated in longer-term safety assessments. These studies are typically conducted before the GLP in vivo pharmacokinetic (PK) and ADME studies. Maximum tolerated dose (MTD), maximum feasible dose (MFD), limit dose (1000mg/kg/day), exposure saturation and dose providing a 50-fold margin of exposure are all ways to derive a drug's dosage, but MTD is the most common.
As mentioned, MTD testing is typically done early in development and starts with a single dose followed by an observation period. Typically, the observation period is 1-3 days between a dose escalation to allow for delayed toxicity observation. This represents a single moment in time (i.e., acute toxicity) and is used to set the parameters for the 14- day and 28-day (i.e., subacute) studies needed for investigational new drug (IND) applications. However, these tests are conducted while scientists are still learning about their drug. These studies are the first opportunity for scientists to learn about potential dose related toxicities on an acute basis.
Sometimes toxicity remains elusive even when increasing exposure to 1000 milligrams per kilogram dose (i.e., the limit dose). This is more common in large-molecule compounds because of the drug's ADME characteristics and how they impact test species. When that happens, scientists need to increase the dosage to reveal some level of toxicity—this helps generate valuable dosage data and appease regulators looking for potential safety concerns. In some cases, the MTD cannot be determined (i.e., large molecules) and in other cases, one of the other methods such as MFD, limit dose, exposure saturation or 50X margin of exposure would be more appropriate. All of these options for determining the high dose for toxicology studies are described in ICH M3(R2).
The U.S. FDA explains dose escalation as such: “In situations where a dose of 1000 mg/kg/day does not result in a mean exposure margin of 10-fold to the clinical exposure and the clinical dose exceeds one gram (g) per day, doses in toxicity studies can be increased to 2000 mg/kg/day or the MFD, whichever is lower. In rare situations where the dose of 2000 mg/kg/day results in an exposure that is less than the clinical exposure, a higher dose up to the MFD can be considered.”
MTD Testing Considerations
With any high-dose toxicology studies, ethical questions can arise. Lethal Dose 50 (LD50) studies were one such cause for concern. The Animal Research Act of 1985 defines this test as one in which any “material or substance is administered to animals to determine the concentration or dose that will achieve a predetermined death rate.” In most cases, the predetermined death rate was 50%. Thankfully, traditional LD50 studies have been indefinitely suspended as the industry has agreed that death is not an appropriate endpoint in a toxicological study. Hence, the reason for the creation of the MTD study. It gives investigators an idea of the top dose while measuring less severe endpoints and preserving test species.
Despite the potential for high dosages, MTD studies are typically designed with animal welfare in mind. MTD, MFD, limit dose, exposure saturation and 50-fold margin of exposure are all high-dose toxicological studies, but they all seek to prevent dosing animals when it would not add value to predicting clinical safety. Determining the MTD before conducting PK or ADME studies will adhere closer to the 3Rs—replacement, reduction and refinement of animals used in research—and ultimately minimize animal morbidity.
Clinical observations, body weight changes and clinical pathology— i.e., blood tests and liver function—are often the most common toxicological endpoints. Body weight loss is often a sensitive indication that a dose level will not be tolerated with multiple doses. MTD studies conducted these days are not designed to cause mortality in any percentage of the test species. In fact, seizures, lethargy and weight loss greater than 10% are all less-severe signs of toxicity and demonstrate that the MTD has been reached or exceeded.
It is rare, but finding MTD levels with some compounds can take weeks. This means investigators escalate dose levels every day or every few days while monitoring for adverse effects. Obviously, this requires prolonging the discomfort in test species until adverse effects are identified. Some investigators design studies to test several test articles in the same day at varying doses to try to avoid this scenario. However, that can require an exorbitant number of test species all at once, without providing clear PK data until several days later. Most investigators prefer to use the same group of animals over a period of time and just wait the predetermined number of days in between dosages.
It is also worth noting that regulators want to see clear evidence of when compounds are toxic and when they are not. Studies that fail to demonstrate those dose levels or that generate inconclusive data raise questions. The outcome of such scenarios can be redoing MTD testing, IND applications placed on clinical hold or creating the need to do additional studies which increase the time before clinical testing can start. None of these options are preferable at critical junctures in the drug development timeline.
A Final Word on MTD Testing
One of the first steps in any drug development program should be MTD testing. It provides a toxicological launching pad from which all other testing initiates. Whether your goal is to satisfy regulators, set parameters for follow-up testing or preserve as many test species as possible, it is critical to establish safe dose levels.
The difference between a successful and an unsuccessful IND application often comes down to data quality. Clear dose levels, seamless administration routes, thoughtful approaches to animal welfare and easy-to-follow protocols all contribute to data quality. If drug developers or sponsors do not have the in-house resources or experience to achieve this level of data quality, they might consider working with a proven, experienced lab testing partner who does.
Kevin Denny is a board-certified toxicologist with extensive regulatory and nonclinical toxicology experience managing multiple IND-enabling programs from clinical development to NDA submission. A talented Study Director and Nonclinical Project Team Lead, he has a successful track record of planning, designing, and implementing drug development strategies through all regulatory submissions including IND, CTA, NDA, MAA, BLA and responses to CRLs. He has managed and developed new GLP laboratories in both the United States and France. With his proven capabilities to collaborate and manage nonclinical development across departments, outside investigators and Contract Research Organizations (CROs), he has moved projects into and through clinical development with successful approvals. Mr. Denny holds master's degrees in Regulatory Affairs and Quality Assurance and Toxicology as well as an MBA and has authored or co-authored multiple approved NDAs, a BLA approval and multiple IND submissions. Currently, Mr. Denny is using his expertise as a consultant focusing on CNS, neurodegenerative disease, Stroke and Traumatic Brain Injury.
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