The National Institutes of Health Microphysiological Systems Program focuses on a critical challenge in the drug discovery pipeline

The National Institutes of Health has partnered with the US Food and Drug Administration and the Defense Advanced Research Projects Agency to accelerate the development of human microphysiological systems (MPS) that address challenges faced in predictive toxicity assessment and efficacy analysis of new molecular entities during the preclinical phase of drug development. Use of human MPS could provide better models for predicting the efficacy of new molecular entities in clinical trials. It is also anticipated that improvements in predicting drug toxicities early in the drug development process through the use of MPS or human organs-on-a-chip will decrease the need to withdraw new therapies from the market and minimize or eliminate deaths due to unidentified drug toxicities.

Although advances in basic and preclinical science continue to fuel the drug discovery pipeline, only a small fraction of compounds in the pipeline meet criteria for approval by the US Food and Drug Administration (FDA) [1]. Current success rates for compounds that reach phase II and phase III clinical trials have dropped to 18% and 50%, respectively [2,3], with failures in the clinic and removal prior to FDA review being attributed to a lack of effi cacy, unidentifi ed toxicities in the preclinical phase, and an ever-changing business strategy for some of the drug developers [2,3]. Since the drug discovery pipeline is both labor and resource intensive, alternative approaches that would enable early indications and potentially more reliable readouts of toxicity or effi cacy could provide signifi cant improvements in the success rate and associated cost benefi ts for this process. To address this challenge in drug development and regu la tory science, the National Center for Advancing Trans lational Sciences (NCATS), through the Cures Acceleration Network and in conjunction with the National Institutes of Health (NIH) Common Fund, has invested $70 million over a 5-year period to launch the Microphysiological Systems (MPS) Program or Organs-on-Chips Program.

The NIH Microphysiological Systems Program targets novel human cell-based systems for predictive drug toxicity
Th e NIH MPS Program is part of a coordinated eff ort between the NIH, the Defense Advanced Research Projects Agency (DARPA), and the FDA to accelerate the development of human MPS that will improve the reliability to identify human drug toxicities and predict the potential effi cacy of a drug in a human population prior to use of the drug in late-stage clinical studies. While the NIH and DARPA independently fund and manage their own MPS programs, coordination of these programs occurs through biannual meetings with NIH and DARPA investigators. Th ese meetings provide opportunities not only to share advances in platform development, cell source characterization, but also to establish collaborations that will help accelerate the development, validation and integration of robust human MPS.
Th e goal of the NIH MPS Program is to create and integrate MPS that utilize human primary or stem cell sources, which are sustainable over a 4-week period and functionally represent the 10 major organ systems: circulatory, respiratory, integumentary, reproductive, endo crine, gastrointestinal, nervous, urinary, musculoskeletal, and immune. Th e 19 investigators funded under the MPS Program are supported through 2-year (Table 1) and 5-year (Table 2) cooperative agree ments, where annual milestones determine the success ful progression

Abstract
The National Institutes of Health has partnered with the US Food and Drug Administration and the Defense Advanced Research Projects Agency to accelerate the development of human microphysiological systems (MPS) that address challenges faced in predictive toxicity assessment and effi cacy analysis of new molecular entities during the preclinical phase of drug development. Use of human MPS could provide better models for predicting the effi cacy of new molecular entities in clinical trials. It is also anticipated that improvements in predicting drug toxicities early in the drug development process through the use of MPS or human organs-on-a-chip will decrease the need to withdraw new therapies from the market and minimize or eliminate deaths due to unidentifi ed drug toxicities.  of each project. Th e MPS cooperative agreement mechanism also facilitates monthly reviews of progress on individual projects where scientifi c program staff , representing 15 NIH institutes, provide advice on system metrics and assist in the identifi cation of available resources that can help achieve the aggressive timelines of the program. To further serve the objectives of the cooperative agreement, the MPS Program has imple mented a communications interface sponsored by NCATS, which is design to facilitate information exchange and the development of MPS organ and cell source standards among the investigators. Th e concept of developing standards around the microphysiological organ systems under development by NIH investigators is driven by the need to provide a MPS consensus view on the minimal functional requirements of a system to accurately mimic the function of the human organ. A similar approach is being applied to the induced pluripotent stem cell sources being utilized in MPS development, wherein any induced pluripotent stem cell source must: demonstrate a normal karyotype; be free of bacterial and mycoplasma contamination; demonstrate expression of stem cell markers; demon strate successful silencing of transgenes used in the reprogramming phase; and have the ability to diff er entiate along mesoderm, ectoderm and endoderm germ layer lineages. Th rough the development of MPS and cell source standards, collaborations and information ex change, it is anticipated that within the next 5 years the collective eff orts of the NIH grantees will culminate in MPS that will provide improved research tools for effi cacy and toxicology screening of new molecular entities.

I N T R O D U C T I O N Open Access
To evaluate the potential improvement MPS provide over traditional in vivo animal toxicology, a set of training compounds has been selected based on known toxicities in the human population. Th is training set of compounds addresses organ-specifi c toxicities that will be tested across the various MPS platforms. A second set of drugs, where the identity is blinded to the investigators, will be used for validation to ensure an unbiased evaluation of the reliability and predictability of these systems. Th e global application of MPS in new drug toxicity and effi cacy testing, and recognition of these systems by regulatory agencies such as the FDA, will require coordination with standards agencies like the International Conference on Harmonization. Th e International Confer ence on Harmonization has developed a comprehensive set of safety guidelines to uncover potential risks such as carcinogenicity, genotoxicity and reprotoxicity, and these safety guidelines have been adopted by international regulatory agencies.

Defense Advanced Research Projects Agency and US Food and Drug Administration
To provide advanced engineering and platform development that will support the integration of 10 human organ systems onto a single platform with a biofl uidics infrastructure that can link the various organ systems, a parallel and complementary eff ort to the NIH MPS Program is funded by DARPA. Dr Linda Griffi th (Massachusetts Institute of Technology, MA) and Dr Don Ingber (Wyss Institute, Harvard University, MA) are DARPA investigators tasked with the stepwise integration and validation of 10 human organ systems onto a shared The emphasis of these 2-year grants is the incorporation of human embryonic, induced pluripotent stem cell and progenitor cell sources into organ-specifi c microphysiological systems. The multicellular models developed will represent the three-dimensional architecture and cellular composition of the organ being modeled, and will lead to the development of microphysiological systems that represent the function of the target organ under normal physiological conditions and disease states.
platform that supports the viability of the integrated systems for a 4-week period. Like the NIH MPS Program, the DARPA Program represents a 5-year, $75 million commitment.
As defi ned in a memorandum of understanding between the NIH and the FDA, for the NIH MPS Program the FDA provides key guidance for development of robust compound libraries and physiological readouts that refl ect toxicities both within a single organ system and across multiple organ systems.

Why animal models can fail
Th e challenge of accurately predicting drug toxicities and effi cacies is in part due to inherent species diff erences in drug metabolizing enzyme activities [4,5] and cell typespecifi c sensitivities to toxicants [5]. For example, a dramatic diff erence between rat and human drug metabo lism is demonstrated by the metabolism of coumarin. Species diff erences in coumarin toxicity appear to be mediated through two major phase I metabolic pathways. Th e fi rst involves the conversion of coumarin by cytochrome P450 CYP2A enzymes to the nontoxic metabolite 7-hydroxycoumarin. Whereas in humans this reaction is very effi cient, in rats CYP2A enzymes preferentially catalyze 7α-hydroxylation of testosterone rather than coumarin 7-hydroxylation [6] and therefore the formation of 7-hydroxycoumarin is extremely low in rats and the lack of 7-hydroxycoumarin is thought to render rats more susceptible to hepatotoxicity [7]. Th e second pathway involves detoxifi cation of the epoxide intermediate coumarin 3,4-epoxide. In humans, coumarin 3,4-epoxide spontaneously rearranges to o-hydroxyphenyl acetaldehyde, which is further detoxifi ed by oxidation to ohydroxyphenylacetic acid. In rats, this conversion to ohydroxyphenylacetic acid is 50 times lower than in humans, and represents a second major species diff erence in coumarin-mediated heptotoxicity [8]. Certainly, a clear understanding of the role of drug metabolism in

Primary UH2 investigators
Institution Title toxicity and potential species diff erences in response to toxic metabolites will aid in the development of safer drugs. Inherent diff erences in cell type-specifi c sensitivities between species may also be an important diff erentiating factor for the prediction of drug toxicities. An example of diff erential cell toxicities across species is observed with bizelesin, a potent synthetic derivative of the anticancer agent CC-1065 that preferentially alkylates and binds the minor groove of DNA. Results with myelopoietic cells in vitro, in the absence of liver metabolism, reproduced in vivo species diff erences in myelosuppression, hence showing that murine cells are 1,000-fold more sensitive than human or canine cells [9]. In the case of the antidiabetic drug troglitazone, metabolism by CYP3A4 and CYP3a1 in human and rat, respectively, results in a reactive quinone intermediate that can bind to proteins and nucleic acids and can potentially produce oxidative stress through the generation of reactive oxygen species via redox cycling or depletion of the oxidative stress protective tripeptide, glutathione. Using rat and human hepatocyte monolayer cultures, Lauer and colleagues demonstrated strong species diff erences and marked alterations in expressions of genes involved in xenobiotic metabolism as well as oxidative stress after treatment with troglitazone [6]. Th e higher sensitivity of human hepatocytes towards troglitazone treatment in contrast to the weaker eff ects observed in rat hepatocytes at least partially ex plains why the strong hepatotoxic eff ects of troglitazone in the human population could not be predicted from regulatory animal studies [6]. Th e results of these in vitro studies suggest that in addition to diff erences in drug metabolism, target cell sensitivity diff erences may also account for species diff erences in toxicity.
A review of FDA-approved drugs released from 1975 to 1999 estimated that 2.9% of these marketed drugs were withdrawn from the market due to severe adverse drug eff ects [10]. According to Temple and Himmel, hepatotoxicity was the most frequent single reason for removing drugs from the market during this timeframe, and probably will still be the most important single toxicity leading to withdrawal or signifi cant modifi cation of labeling for FDA-approved drugs going forward [11]. Examples of drugs removed from the market due to hepatoxicity during this time period include ticry nafen, benoxaprofen, bromfenac, and troglitazone. In addition to hepatoxicity, six drugs were withdrawn from the market because of hemolytic anemia (nomifensine maleate), hemolytic anemia, renal and hepatic injury (temafl oxacin), thromboembolism (azaribine), anaphylaxis (zomepirac sodium), acute but reversible renal failure (suprofen), and increased mortality (fl osequinan). Based on estimates from Lasser and colleagues, the probability of a new drug acquiring black box warnings or being withdrawn from the market over 25 years is approximately 20% [12]. Th rough development of human cell-based MPS, it is hoped that toxicities as mentioned above will be detected earlier, preferably during the preclinical phase of drug development.
Given the list of FDA drugs withdrawn due to unpredicted toxicities in the human population, a key goal of the NIH MPS Program is to accurately evaluate the predictive value of these systems in determining drug toxicity and effi cacy. Th is goal can be achieved through either development of a single microphysiological system, such as a liver organoid system that can reliably predict human hepatoxicity, or through a more complex platform where multiple MPS predict multiorgan toxicities to a given challenge drug. Th e multisystem platform approach will also assist in understanding the time dependence and magnitude of drug-drug interactions; for instance, how one drug may increase the blood concentration of another, as seen in the eff ect of mibefradil on midazolam blood concentrations [12]. Understanding the limitations of these model systems will also be important. For instance, in cases where recognition of toxicity in the human population has been delayed -such as seen in the marked hypo tension with clozapine, the pulmonary fi brosis and marrow tox icity with tocainide, the valvulopathy asso ciated with fenfl uramine hydrochloride and the subarachnoid hemorrhage associated with phenyl propanolamine [13] -the application of relatively short-term exposures of drugs in the MPS may limit the predictive value of the system for delayed toxicities.

National Center for Advancing Translational Sciences and the Microphysiological Systems Program
Th e MPS Program is an excellent fl agship program for NCATS, established in 2011 with a mission to catalyze the generation of innovative methods and technologies that will enhance the development, testing, and implementation of diagnostics and therapeutics across a wide range of diseases and conditions. Th e opportunities for signifi cant advancements in the prediction of human drug toxicities through the development of MPS requires a multidisciplinary approach that relies on an under standing of human physiology, stem cell biology, material sciences and bioengineering. At the end of the 5-year funding period it is anticipated that the availability of these systems to a broader research community will foster a multitude of new research applications including, but not limited to, studies in environment exposures, reproduction and development, infectious diseases, cancer, countermeasures for chemical warfare, immune responses and neuroinfl ammation.