Adverse drug reactions (ADRs) are the fourth leading cause of death worldwide. In the U.S. alone, 2.2 million serious ADRs are reported annually in the hospitalized population, with over 100,000 deaths.
Models that better predict a drug’s efficacy and a drug’s toxicity cost-effectively are the need of the hour.
“A major challenge of drug safety testing is that preclinical studies with laboratory animals are not always predictive of human safety,” said Albert P. Li, Ph.D., president and CEO of In Vitro ADMET Laboratories. Dr. Li cited the frequent clinical trial failures due to safety concerns with drug candidates that have been chosen based on preclinical animal safety data.
Dr. Li’s work suggests that “there may be species-specific differences in toxicity, resulting in discordance of results between laboratory results, animal, and human findings.” Liver metabolism differs greatly between other animal species and humans. For instance, as a drug is metabolized to a more toxic (metabolic activation) or less toxic (metabolic detoxification) compound, preclinical results with laboratory animals may not always translate into applications for humans.
The involvement of metabolism in toxicity may also lead to individual differences in toxicity, which may explain the phenomenon of idiosyncratic toxicity, according to Dr. Li.
Idiosyncratic drug toxicity is human-specific, occurring as a rare event (<1/5,000) and therefore, impossible to study in clinical trials or experimental animals. In fact, current trends suggest that regulatory agencies actively discourage animal testing in research. Europe has banned the use of animal testing for all cosmetics. Indeed, many drugs have been withdrawn from the market due to safety concerns.
Instead, human cells are being used (e.g., induced pluripotent stem cells [iPS]) to yield cardiomyocytes or Parkinson’s neurons. As well, tissue engineering efforts have created three-dimensional tissue equivalents designed for transplantation into patients, and screened for toxicity. In vitro and in silico modeling of tissues and diseases using lab-on-a-chip technologies are available. As agencies such as the NIH, FDA, and the DARPA seek new ideas and proposals, researchers have come up with cutting-edge solutions.
At the “International Conference on Predictive Human Toxicity and ADME/Tox Studies”, experts from the biotech and pharma industry discussed recent advances toward accurately predicting human toxicities and screening early in the drug discovery process.
Cryopreserved Human Hepatocytes
“We now have optimized the isolation, culturing, and cryopreservation of human hepatocytes to retain high viability and, most importantly, retention of drug metabolism capability similar to the human liver in vivo,” said Dr. Li. “These cryopreserved human hepatocytes now represent the gold standard for human drug metabolism research,” he added.
Working closely with the FDA, Dr. Li and colleagues have developed an in vitro assay for the identification of drugs that cause liver failure. “Using the ratio of reactive oxygen species:cellular ATP content as the endpoint, we were able to identify drugs that are known to cause severe liver failure (drug induced liver injury, or DILI) with a specificity and sensitivity of near 90%,” he said.
According to Dr. Li, “this novel assay complements the assays that we have previously developed with human hepatocytes such as the Metabolism Comparative Cytotoxicity Assay (MCCA) or the Cytotoxic Metabolic Pathway Identification Assay (CMPIA).”
MCCA and CMPIA are designed to evaluate the key role of hepatic metabolism in drug toxicity. Indeed, hepatic toxicity is a major manifestation of drug toxicity since liver is exposed to a high bolus concentration of the drug after oral administration (first-pass effect). Also, the liver metabolizes a relatively nontoxic compound to form highly reactive and toxic metabolites, Dr. Li explained.
“We have developed an additional assay with the IdMOC (Integrated Discrete Multiple Organ Co-culture) system with which we can detect toxicity to nonhepatic cell types by toxic metabolites formed by hepatocytes.
“We believe that our battery of assays would improve our ability to identify drugs and drug candidates with human-specific toxicity that cannot be detected using the classical toxicological approaches with laboratory animals.”
Dr. Li proposes that the in vitro human hepatocyte assays be used early in drug discovery to eliminate compounds that have potential human toxicity. Further testing in classical toxicological assays of the compounds that are nontoxic to human cells in vitro can eliminate compounds with complicated toxicity profiles that may not be detectable with cells in vitro.
The “combined human in vitro and animal in vivo” approach emphasizes the human-specific properties obtained in vitro, combined with in vivo parameters obtained with laboratory animals. It should provide data that are predictive of human toxicity in vivo, according to Dr. Li.
Integrating Human and Animal Approaches
“We can now put forward an insightful approach evaluating DILI using an integrated human in vitro-animal in vivo approach,” said David Kwok, Ph.D., CEO, BRI Biopharmaceutical Research. “You still need to have an in vivo animal model such as the one involving animal hepatocytes, given that safety assessment in animals is still a regulatory requirement,” he explained.
“At BRI, we advocate the use of the high-content hepatocyte imaging assay in animal hepatocytes (rat, dog, and other relevant species) in support of in vitro/in vivo correlation and justification for a relevant animal species during subsequent IND-enabling preclinical safety evaluation,” said Dr. Kwok.