Introduction
The pharmaceutical industry is under tremendous pressure to increase productivity and lower costs while at the same time discover and develop novel therapies. Current estimates of the cost developing a drug range from $1-3 billion dollars, and takes 12-15 years for a drug to reach the market [1]. Increased concerns related to safety and late-stage drug failure are key contributors of driving up the cost of clinical trials. Between 1991 and 2005, the cost of clinical trials has increased over 300% [2]. Therefore, one mechanism of improving pharmaceutical productivity is to make sure that those targets and compounds that advance into clinical development have the greatest chance of success in the intended patient population.
Despite the fact that drug safety is a key concern for FDA, the public and the pharmaceutical industry, the way toxicology is applied within the drug development process has not changed significantly in over three decades. The average expenditure for preclinical drug safety is only six percent of the entire R&D expense [2] with the bulk of this expense on regulatory toxicology studies. Thus, limited resources have been applied to reducing safety issues at an early stage despite elevated concerns related to safety and the high attrition rate due in part to safety [3]. Revolutionizing the way toxicology is applied within the biopharmaceutical industry is one approach to dealing with the productivity gap within the industry. Selecting the therapeutic target, and ultimately, the lead compound, are two of the most critical decisions a pharmaceutical company makes, but these decisions often lack a holistic preclinical safety assessment and strategy to enable future success. This paper will focus on revolutionizing the way toxicology is applied in the R&D process; moving from a hazard identification and risk assessment paradigm to one that reduces or eliminates risk prior to major expenditures.
Figure 1- Outline of a Discovery Toxicology Paradigm
Discovery Toxicology Strategy
Discovery toxicology is essentially the safety assessment that occurs prior to regulatory toxicology and is used to select the target and lead compound. The goal of any discovery toxicology strategy should be a holistic view of identifying potential safety issues related to the target, compound, use in the intended patient population, and then establish a screening strategy to minimize or eliminate these risks. This can be broken down into three major categories; intended therapeutic use, target selection, and compound selection. Although details and specific assays or study types can not be addressed in this paper, a high level overview of each category is provided with an emphasis on new chemical entities (NCE) as outlined in Figure 1.
Therapeutic Use
A thorough understanding of how a NCE will be used therapeutically is a critical and often overlooked process in the discovery space. How a new NCE will be used in the market place is typically addressed after proof-of-concept is achieved where Phase III trial design and market authorization are discussed. With the focus on the pharmacology of the drug, very little attention is paid to potential safety attributes in the market place. However, this can be detrimental for the compound, a company and even patients. As early as target selection, understanding the patient population and pathophysiology of the disease, possible combination strategies, and common over-the-counter medications the patients may take are key elements to capture. Understanding these areas will help drive the selection of targets, and the type of screening strategy needed toselect a NCE that will have a greater chance of success. Another advantage to this approach is that the non-clinical and clinical development strategies required for registration are discussed at a very early stage of a program. Examples of disease area considerations related to safety are provided below:
- Diabetes – Given that many of these patients have underlying cardiovascular complications, cardiovascular safety is critical to address for new NCEs in this area. Combination therapy is also common so the discovery toxicology strategy should address the potential safety risks in combining the new NCE with standard of care. For instance, combining PPAR agonists with statins can lead to severe muscle toxicity which has been attributed to exaggerated mitochondrial toxicity [4]. Therefore, targets or compounds that diminish mitochondrial function or elevate blood pressure in this therapeutic area should be avoided or de-risked early.
- HIV – Highly active anti-retriviral therapy (HAART) in HIV patients combines a host of antiviral compounds. Therefore, the ADME and safety attributes of NCEs to treat HIV patients needs to be well understood. For instance, reverse transcriptase inhibitors are known mitochondrial toxins [5]; therefore, any new NCE that interferes with mitochondrial function may exacerbate the toxicities associated with reverse transcriptase inhibitors. In addition, elevated lipids and cholesterol are also seen with HAART therapy [6]. NCEs that elevate lipids, cholesterol or have a cardiovascular risk may lead to late-stage or post-market attrition.
- Elderly Population – developing drugs for use in the elderly population can be complex due to concomitant medications, multiple disease states, diminished organ function and the metabolic changes commonly observed with aging. Decreases in antioxidant defenses, alterations in transporters and drug metabolizing enzymes, as well as increases in mitochondrial DNA mutations have all been observed in aging subjects [7-9]. Therefore, it is important to eliminate the risk of inducing adverse drug reactions with any NCE intended for use in the elderly by devising a screening strategy that takes into consideration these challenges.
Target Selection
Selecting a target is one of the most important decisions a company can make. The potential safety issues of inhibiting a novel target is another important and often overlooked area that can be a costly mistake for any company. For instance, selecting a target that has the potential of producing teratogenic effects for an indication requiring treatment in women of child bearing potential can significantly decrease the value of that target, not to mention complicate clinical development. At the point of considering which target to advance for a given indication, in addition to addressing do-ability from a biology and chemistry standpoint, the potential safety concerns need to be weighed with each target. Ranking the targets based on biology, chemistry and safety will allow the organization to apply resources on those programs that have a greater chance of success.
Safety assessment of a target is primarily literature based. Reviewing the biology of a target, a toxicologist can speculate on potential safety issues that could arise when the target is inhibited. This may include information from competitors, transgenic animals and other available publications. Once the potential safety concerns are identified, the risks should be evaluated in the context of the proposed therapeutic indication and a risk mitigation strategy established. In addition to the mitigation strategy, understanding where the target is distributed and how it functions across species is important to evaluate. Knowledge of other targets that might be inhibited due to homology are also important elements to incorporate into a mitigation strategy.
To mitigate the potential risk of inhibiting a target, several approaches should be considered. Transgenic animals are powerful tools to understand the in vivo effects of completely knocking out a target. However, target knockouts can produce embryonic lethality and compensatory pathways during fetal development can induce phenotypic changes. Therefore, safety derived from transgenic animals need to be viewed with caution. Other potential methods to assess safety of inhibiting a target may include conditional knockouts, use of antisense or siRNA, known chemical inhibitors, or monoclonal antibodies. In addition to in vivo approaches, in vitro assays using siRNA to look at functional changes in cells can be a powerful technique to address specific safety-related questions.
Understanding where a target distributes in humans as well as in the pharmacology and toxicology species is important information to obtain early within a program. This allows for the appropriate evaluation of tissues in early in vivo efficacy and safety studies. If the distribution of a target is not available from literature sources, immunohistochemistry can be performed using tissue micro-arrays from human and animal tissues to map the distribution of a given target.
In addition to tissue distribution across species, understanding any differences in the physiological function of the target across species is a key element of understanding target safety. Major differences in function across species is important to understand, not only to help establish an appropriate animal model of disease, but also to ensure any safety signals identified in animals are placed into context. This information may also be important for the appropriate selection of the toxicology species used to support clinical trials.
Compound Selection
Selecting the NCE to advance into regulatory toxicology studies and clinical development is another critically important decision a company makes. The NCE should exhibit the appropriate pharmacological, physiochemical, ADME and toxicological properties to be a successful drug for the desired indication. Over the last five years, greater emphasis on safety as a result of attrition has begun to take place within the industry [10]. However, the movement to reduce safety risks early has been slow to evolve with limited resources specifically dedicated to this endeavor.
The NCE selection process starts with the discovery toxicology strategy; understanding what safety attributes are important to screen out of a molecule for the desired indication. Compound screening initially starts at the series selection phase. At this phase, hundreds of compounds per week might be evaluated for safety attributes using in-silico predictive tools and high-throughput in vitro screens to help with chemistry design. To select a series to advance to further testing, safety endpoints that might be used include genotoxocity assays, binding assays that predict QT prolongation, selectivity profiling, and mitochondrial screens.
Once a series is selected the next step is to select pre-leads. Safety assays at this stage build upon what are used for series selection, which may include broad ligand binding against a host of other receptors, chemical reactivity screens and in vitro cytotoxicity profiling. Lead selection may also include the conduct of efficacy studies in animals. Capturing safety endpoints such as clinical chemistry and histopathology from these efficacy studies can be an efficient way of identifying in vivo safety signals at a very early stage. This is also an animal sparing paradigm that captures as much information as possible from animal work conducted. Additional in vivo studies may include a short-term rodent study with selected molecules to assess early in vivo safety in healthy animals. Cardiovascular studies and other endpoints might be incorporated based on what is known about the compound, the patient population and probable concomitant medications.
Narrowing down the pre-leads to the final selection of a NCE essentially builds on the previous screens to include a rodent and non-rodent 14-day repeat dose toxicity study. These non-GLP studies often serve as future dose-range finding studies, but also provide a more robust confirmation of the safety of a select few compounds, as these studies do include full clinical pathology examination along with microscopic examination of multiple tissues. Novel biomarkers and toxicogenomicendpoints might be incorporated into these studies to help differentiate the pre-leads. Additional in vivo cardiovascular studies using telemeterized animals should also be conducted for most programs to select the lead.
Benefits
The benefits of establishing a discovery toxicology strategy that enables selection of targets and NCEs with reduced risk of late-stage failure will not only reduce drug development costs but also increase toleration in patients and potentially avoid a highly publicized post-market failure. The cost to do this early safety work is significantly offset by the benefits to an organization. For instance, the average cost of taking a single compound from candidate selection through the regulatory toxicology studies and IND filing is ~$7 million dollars. Therefore, preventing attrition in the initial regulatory toxicology space can save the organization $7 million dollars. A two-week in vivo toxicology screening study at a cost of ~$15 thousand dollars can often identify toxicities that would occur in the regulatory toxicology space. The return on investment for this single example is in the millions of dollars. Adding all the potential savings by removing those compounds from further development at such an early stage would have significant return on investment with the potential of savings in the hundreds of millions of dollars.
Challenges
The challenges to fully implement this strategy are both scientific and behavioral. On the scientific front, understanding mechanisms of drug-induced toxicities and developing high-throughput assays predictive of these mechanisms are key to establishing new testing paradigms. Historically, in vitro toxicology assays were developed in an effort to predict target organ toxicity in humans. However, these in vitro systems have failed to deliver high-throughput assays predictive of target organ effects in vivo. Rather than developing in vitro assays predictive of target organ effects, assays designed to look at specific mechanisms of drug-induced toxicities at the intracellular level are the types of assays that will enable the development of predictive high-throughput safety screens. These assays may include measures of mitochondrial toxicity, apoptosis, oxidative stress, ER stress, DNA damage and cell cycle disruption. Selecting NCEs that do not inhibit these intracellular pathways will reduce the risk of potential target organ effects in vivo as well as reduce the risk of synergistic or additive toxicities with other medications that may occur later in development or post-market.
The behavioral challenges relates to the way toxicologist, pharmacologists and chemists apply early safety strategies and screens in the selection of targets and the design of NCEs. A common fear in applying new strategies and screens used to select NCEs is that projects will become paralyzed with ambiguous safety data and potentially good compounds will be killed due to the poor predictive nature of an in vitro assay. At the project level, it is often easier to ignore the possible early safety attributes and wait to see what happens in vivo later in the discovery and development process However, it is this frame of mind that prevents the early implementation of these novel safety strategies and delays the productivity gains that this approach provides.
Keys to Success
Key to a successful discovery toxicology strategy is to address potential safety concerns that could terminate a target or compound where it makes the most sense for the project and has the greatest impact. Most programs should establish a multi-staged, tiered approach with specific fit-for-purpose assays that builds confidence in safety of the target and enables selection of a compound for further development. The specific strategy and actual assays employed are dictated by the target and intended use of the NCE. Therefore, the discovery toxicology strategy needs to accommodate the unique attributes of the target, the compound and therapeutic application, along with assays that are “fit-for-purpose” for that specific project. Flexibility in approach and a strong collaborative interdisciplinary team consisting of biology, pharmacology, chemistry, drug metabolism, drug safety and clinical are also keys to success.
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Greg Stevens, Ph.D., is currently Executive Director, Drug Safety R&D at Pfizer, Inc. Greg started with Agouron Pharmaceuticals in 1996 which later became Pfizer. His combined tenure with Agouron and Pfizer encompasses over 13 years of drug discovery and development experience with increasing levels of responsibilities. From day one, Greg has sought to revolutionize the way toxicological sciences are applied to drug discovery and development. His pioneering efforts to move drug safety much earlier in the drug discovery process has helped Pfizer transform the way targets and compounds are selected. His current responsibilities are to manage the Drug Safety Research activities for Pfizer’s La Jolla facility.
This article was printed in the November/December 2009 issue of Pharmaceutical Outsourcing, Volume 10, Issue 7. Copyright rests with the publisher. For more information about Pharmaceutical Outsourcing and to read similar articles, visit www.pharmoutsourcing.com and subscribe for free.