New horizons in drug metabolism,pharmacokinetics and drug discovery

Along with minimal toxicity, good drug metabolism and pharmacokinetic (DMPK) properties are essential for the clinical success of a drug candidate. A major cause of failure of orally administered drugs during their development is the discovery that in humans they have low intestinal absorption and/or high clearance causing low and variable bioavailability. In addition, drug interactions and the presence of active metabolites can prevent or complicate their successful development. With poor pharmacokinetics it can be difficult to achieve a suitable dosage regimen for the required pharmacodynamic action. The main role of DMPK in discovery is, therefore, the prediction of human pharmacokinetics and metabolism. Reducing the rate of attrition during drug discovery and development is now considered essential, particularly as it is now possible to screen an ever-greater number of compounds.     

The right compound in the right assay at the right time: an integrated discovery DMPK strategy

The high rate of attrition during drug development and its associated high research and development (R&D) cost have put pressure on pharmaceutical companies to ensure that candidate drugs going to clinical testing have the appropriate quality such that the biological hypothesis could be evaluated. To help achieve this ambition, drug metabolism and pharmacokinetic (DMPK) science and increasing investment have been deployed earlier in the R&D process. To gain maximum return on investment, it is essential that DMPK concepts are both appropriately integrated into the compound design process and that compound selection is focused on accurate prediction of likely outcomes in patients. This article describes key principles that underpin the contribution of DMPK science for small-molecule research based on 15 years of discovery support in a major pharmaceutical company. It does not aim to describe the breadth and depth of DMPK science, but more the practical application for decision making in real-world situations.     

Drug metabolism and pharmacokinetics in drug discovery

The discovery and development of new drugs seems to be an inefficient process, since too few new chemical entities (NCEs) successfully make it to the market. Because one of the main reasons for failure in development is thought to be poor pharmacokinetics (PK), drug metabolism and PK (DMPK) have assumed a central role within the field of drug discovery. A good development candidate requires a balance of potency, safety and PK; therefore, techniques that can help understand these characteristics are employed to enable researchers to design more robust candidates. A number of new in silico, in vitro and in vivo techniques are available to screen compounds for key absorption, distribution, metabolism and excretion (ADME) characteristics, which, when applied within a rational strategy, can make a major contribution to the design and selection of successful NCEs.     

药代动力学人体预测及其在新药研发中的应用

药物代谢和药代动力学(DMPK)通过揭示药物的体内代谢处置过程,理解药物药理效应和毒副反应的体内物质基础,是连接药物分子及其性质与生物学效应的桥梁。DMPK人体预测应用模型拟合技术,由人体外试验数据和动物体内外数据预测人体药代动力学性质,并与药效动力学和毒性评价相关联,可提高新药研发效率、降低临床失败率和节省资源。经典的异速放大法和体外-体内外推法主要用于预测人体清除率和稳态表观分布容积等重要的药代动力学参数。近10年来,基于生理的药代动力学模型(PBPK)的快速发展和应用实践,推动了DMPK人体预测在新药研发、药物监管、临床合理和个体化用药中的应用。PBPK模型不仅能预测消除和分布等参数,还能用于药物人体药代动力学行为的预测,包括血药浓度-时间曲线和药物-药物相互作用,以及不同人群体内药代动力学和药代-药效预测。作为新药研发的转化科学技术以及个体化用药的指导工具,DMPK人体预测将具有更为广泛的应用价值。     

The changing role of drug metabolism and pharmacokinetics

The Pharmacokinetics and Drug Metabolism for Chemists meeting held November 13, 2001, in Stevenage, U.K., provided an interesting overview of how the drug metabolism and pharmacokinetics (DMPK) field has developed and the increasing importance, now recognized by most major companies, of obtaining DMPK data early in the drug discovery process. In addition to the repositioning of DMPK as a critical element of drug discovery, it was also made clear that the future should see fewer development compounds terminated because of DMPK factors.     

Prediction of oral drug absorption in humans--from cultured cell lines and experimental animals

BACKGROUND: Over the past decade, the use of drug metabolism and pharmacokinetic (DMPK) parameters to optimize selection of candidates for drug development has become one of the primary focuses of research organizations involved in new drug discovery. OBJECTIVE: This review will focus on the feasibility of predicting human absorption using data from a cultured Caco-2 cell system and non-clinical animals. METHODS: In-house Caco-2 permeability data were compared with human absorption data from the literature. Animal absorption data obtained from current literature were also compared with human data. RESULTS/CONCLUSION: Using a combination of rapid in-vivo and in-vitro DMPK screens on a large array of compounds during the lead optimization process has resulted in the streamlined development of compounds that have acceptable DMPK properties. Since it is well recognized that human intestinal absorption cannot be precisely predicted by a single screening assay, it is important to utilize various in-vitro and in-vivo non-clinical studies during lead optimization in drug discovery.     

In vivo rat PK profiling in drug discovery: new challenges

The importance of evaluating drug metabolism and pharmacokinetic (DMPK) properties very early in the drug discovery process in order to reduce attrition during development is now well recognised. In this paper we illustrate an approach for PK screening that provides a range of parameters that would not be available from conventional PK profiling. In combination with an assessment of physicochemical and in vitro properties, the in vivo PK protocol described provides better mechanistic understanding of the PK behaviour of a compound or class of compounds. The higher level of interpretation and use of in vitro and in vivo data better describe the disposition properties and give an estimation of the biophase concentration of the drug, providing a clear guidance for the design of higher quality molecules. Moreover, the collection of a broader set of in vivo and in vitro PK data improves the predictability of the DMPK science and it can allow an integrated safety risk assessment.     

High-throughput screening approaches for investigating drug metabolism and pharmacokinetics

1. High-throughput screening approaches have been adopted throughout the pharmaceutical industry to aid in the rapid discovery of new chemical entities. Because it is now well recognized that the selection of a robust candidate requires a balance of potency, safety and pharmacokinetics, the role of drug metabolism departments has widened from their traditional one of supporting drug development to include the screening of compounds during the discovery process. To put drug metabolism and pharmacokinetic (DMPK) studies in context, the evolving role of DMPK screening in the drug discovery strategy of pharmaceutical companies will be discussed and a generalized approach will be presented. 2. With the increasing numbers of compounds requiring screening, DMPK optimization methods have had to be adapted for high throughput. There have been many developments in this field over the past decade and this review will focus on the high-throughput DMPK screening methodologies used today and in the recent past. 3. In vitro and in silico (computer-based) methods have proven most amenable to high-throughput approaches and these will firm the bulk of the review, but some advances with in vivo methods will also be discussed. As there has been a vast increase in published material on the topic of high-throughput DMPK methodologies in the past 10 years, it would be impossible to cover every method in detail, so this review will concentrate on the key areas and refer the reader to other, more detailed reviews wherever possible. 4. Most high-throughput methods would not be possible without the enabling technologies of computing, automation, new sample preparation technologies, and highly sensitive and selective detection systems, and these will also be reviewed. 5. The advantages and disadvantages of the screening methods will be presented, in particular the issue of handling the false-positives and -negatives that arise. 6. In concluding the review, future developments in this field will be discussed along with key issues that will need to be addressed.     

High-throughput screening approaches for investigating drug metabolism and pharmacokinetics

1. High-throughput screening approaches have been adopted throughout the pharmaceutical industry to aid in the rapid discovery of new chemical entities. Because it is now well recognized that the selection of a robust candidate requires a balance of potency, safety and pharmacokinetics, the role of drug metabolism departments has widened from their traditional one of supporting drug development to include the screening of compounds during the discovery process. To put drug metabolism and pharmacokinetic (DMPK) studies in context, the evolving role of DMPK screening in the drug discovery strategy of pharmaceutical companies will be discussed and a generalized approach will be presented. 2. With the increasing numbers of compounds requiring screening, DMPK optimization methods have had to be adapted for high throughput. There have been many developments in this field over the past decade and this review will focus on the high-throughput DMPK screening methodologies used today and in the recent past. 3. In vitro and in silico (computer-based) methods have proven most amenable to high-throughput approaches and these will form the bulk of the review, but some advances with in vivo methods will also be discussed. As there has been a vast increase in published material on the topic of high-throughput DMPK methodologies in the past 10 years, it would be impossible to cover every method in detail, so this review will concentrate on the key areas and refer the reader to other, more detailed reviews wherever possible. 4. Most high-throughput methods would not be possible without the enabling technologies of computing, automation, new sample preparation technologies, and highly sensitive and selective detection systems, and these will also be reviewed. 5. The advantages and disadvantages of the screening methods will be presented, in particular the issue of handling the false-positives and-negatives that arise. 6. In concluding the review, future developments in this field will be discussed along with key issues that will need to be addressed.     

ADME of Biologics—What Have We Learned from Small Molecules?

Thorough characterization and in-depth understanding of absorption, distribution, metabolism, and elimination (ADME) properties of a drug candidate have been well recognized as an important element in small molecule (SM) drug discovery and development. This has been the area of focus for drug metabolism and pharmacokinetics (DMPK) scientists, whose role has been evolving over the past few decades from primarily being involved in the development space after a preclinical candidate was selected to extending their involvement into the discovery stage prior to candidate selection. This paradigm shift has ensured the entry into development of the best candidates with optimal ADME properties, and thus has greatly impacted SM drug development through significant reduction of the failure rate for pharmacokinetics related reasons. In contrast, the sciences of ADME and DMPK have not been fully integrated into the discovery and development processes for large molecule (LM) drugs. In this mini-review, we reflect on the journey of DMPK support of SM drug discovery and development and highlight the key enablers that have allowed DMPK scientists to make such impacts, with the aim to provide a perspective on relevant lessons learned from SM drugs that are applicable to DMPK support strategies for LMs.     

Utility of human/human-derived reagents in drug discovery and development: An industrial perspective

The shift to combinatorial chemistry and parallel synthesis in drug discovery has resulted in large numbers of compounds entering the lead seeking and lead development phases of the process. To support this, higher throughput computational (in silico) and in vitro approaches have become the forefront of the drug metabolism and pharmacokinetic (DMPK) input into drug discovery. This has been accompanied by a shift in focus from animal-derived data to human based studies, reflecting the realisation that extrapolation from animals to human has its limitations. In silico approaches may be regarded as human derived tools for DMPK, since models (template/pharmacophore and protein homology modelling), for example, for the human CYP enzymes, are widely used for identifying qualitatively enzyme/substrate interactions. Quantitative assessment of drug metabolism using human hepatocytes or sub-cellular fractions provide a valuable tool both for the screening out of high metabolic lability and in estimations of human intrinsic clearance. In terms of drug absorption, the human colon adenocarcinoma cell line, Caco-2, offers a versatile human derived system for measuring drug permeability, despite over expression of the efflux transporter P-glycoprotein (P-gp). The importance of P-gp can then be further assessed in recombinant systems expressing the human P-gp, where substrate affinity and inhibition potency can be measured, important factors when considering transporter mediated drug-drug interactions. The primary cause of pharmacokinetic-based drug-drug interactions (DDIs) is through enzyme inhibition or induction, with the CYP enzymes being of major importance. Human liver microsomes and hepatocytes are invaluable tools in assessment of DDI vulnerability of new chemical entities, having the capacity to identify enzymes responsible for specific routes of metabolism, and hence areas of vulnerability for a DDI. In addition, human-based screening tools can be used to identify the perpetrator of a DDI through enzyme inhibition/induction. Large differences in the nature of enzymes induced and the extent of induction when comparing animals to man are known. Thus, in vitro models allowing assessment of induction potential in human tissue, establishes some relevance to the clinical situation.     

Metabolism and toxicity of drugs. Two decades of progress in industrial drug metabolism

The science of drug metabolism and pharmacokinetics (DMPK) has developed significantly over the past 20 years, and its functional role in today's pharmaceutical industry has matured to the point where DMPK has become an indispensable discipline in support of drug discovery and development. While contributions to the lead optimization phase of discovery efforts have been particularly noteworthy in helping to select only the best drug candidates for entry into development, it should be recognized that the scope of DMPK spans the continuum of discovery through clinical evaluation and even into the post-marketing phase; as such, the breadth of DMPK's involvement is almost unique in contemporary pharmaceutical research. This perspective summarizes notable advances in the field, many of which have been made possible by technological developments in areas such as molecular biology, genetics, and bioanalytical chemistry, and highlights the critical nature of key partnerships between Drug Metabolism, Medicinal Chemistry, and Safety Assessment groups in attempting to advance drug candidates with a low potential for causing adverse events in humans. Finally, some speculative predictions are made of the future role of DMPK in pharmaceutical research, where current advances in our mechanistic understanding of the molecular processes that control the absorption, disposition, metabolism, elimination, and toxicity of drugs and their biotransformation products will combine to further enhance the impact of DMPK in drug discovery and development.     

In vivo rat PK profiling in drug discovery: new challenges

The importance of evaluating drug metabolism and pharmacokinetic (DMPK) properties very early in the drug discovery process in order to reduce attrition during development is now well recognised. In this paper we illustrate an approach for PK screening that provides a range of parameters that would not be available from conventional PK profiling. In combination with an assessment of physicochemical and in vitro properties, the in vivo PK protocol described provides better mechanistic understanding of the PK behaviour of a compound or class of compounds. The higher level of interpretation and use of in vitro and in vivo data better describe the disposition properties and give an estimation of the biophase concentration of the drug, providing a clear guidance for the design of higher quality molecules. Moreover, the collection of a broader set of in vivo and in vitro PK data improves the predictability of the DMPK science and it can allow an integrated safety risk assessment.     

Utility of mass spectrometry for in-vitro ADME assays

A balance between pharmacological activity, safety and drug metabolism and pharmacokinetics (DMPK) attributes determines the fate of a new chemical entity (NCE) in drug discovery. Because of the increased number of NCEs requiring DMPK evaluation, several in vitro higher-throughput screens and counter screens designed to evaluate DMPK attributes have been introduced in drug discovery. The DMPK screens evaluate NCEs for potential absorption, metabolism, drug-drug interactions, brain penetration, protein binding and pharmacokinetics. Higher-throughput analytical methodologies for the determination of either a common end product of a screen or the parent compound (and/or possible metabolites) are essential for successful DMPK screens. Because of its speed, sensitivity and specificity, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has become the technology of choice for sample analysis. In this review, several in vitro screening assays that we employ in drug discovery are discussed with an emphasis on LC-MS/MS role in accelerating them.     

药物代谢清除率体外预测模型研究进展与问题分析

本文介绍了药物代谢清除率临床前预测的相关理论和数学模型，从而将所得的体外代谢清除率数据用于肝清除率的推算，并根据国内外学者对该预测方法准确度评估及体内体外相关性考察，指出该研究领域中出现的难点和可能的改进方案；以期使体内代谢行为的预测更加准确，并对尽早确定创新药物在人体内药物代谢动力学行为是否适合进行进一步研究开发具有重要的指导意义，并为今后创新药物的研发提供新的思路。     

Metabolic stability for drug discovery and development: pharmacokinetic and biochemical challenges

Metabolic stability refers to the susceptibility of compounds to biotransformation in the context of selecting and/or designing drugs with favourable pharmacokinetic properties. Metabolic stability results are usually reported as measures of intrinsic clearance, from which secondary pharmacokinetic parameters such as bioavailability and half-life can be calculated when other data on volume of distribution and fraction absorbed are available. Since these parameters are very important in defining the pharmacological and toxicological profile of drugs as well as patient compliance, the pharmaceutical industry has a particular interest in optimising for metabolic stability during the drug discovery and development process. In the early phases of drug discovery, new chemical entities cannot be administered to humans; hence, predictions of these properties have to be made from in vivo animal, in vitro cellular/subcellular and computational systems. The utility of these systems to define the metabolic stability of compounds that is predictive of the human situation will be reviewed here. The timing of performing the studies in the discovery process and the impact of recent advances in research on drug absorption, distribution, metabolism and excretion (ADME) will be evaluated with respect to the scope and depth of metabolic stability issues.Quantitative prediction of in vivo clearance from in vitro metabolism data has, for many compounds, been shown to be poor in retrospective studies. One explanation for this may be that there are components used in the equations for scaling that are missing or uncertain and should be an area of more research. For example, as a result of increased biochemical understanding of drug metabolism, old assumptions (e.g. that the liver is the principal site of first-pass metabolism) need revision and new knowledge (e.g. the relationship between transporters and drug metabolising enzymes) needs to be incorporated into in vitro-in vivo correlation models. With ADME parameters increasingly being determined on automated platforms, instead of using results from high throughput screening (HTS) campaigns as simple go/no-go filters, the time saved and the many compounds analysed using the robots should be invested in careful processing of the data. A logical step would be to investigate the potential to construct computational models to understand the factors governing metabolic stability. A rational approach to the use of HTS assays should aim to screen for many properties (e.g. physicochemical parameters, absorption, metabolism, protein binding, pharmacokinetics in animals and pharmacology) in an integrated manner rather than screen against one property on many compounds, since it is likely that the final drug will represent a global average of these properties.     

Preclinical pharmacokinetics: an approach towards safer and efficacious drugs

Lack of efficacy and toxicity are considered to be major reasons for drug failures and pharmacokinetics governs them to a large extent. Compound with favorable pharmacokinetics is more likely to be efficacious and safe. Therefore, the preclinical pharmacokinetic evaluation should be comprehensive enough to ensure that compounds do not fail in the clinic. Preclinical ADME screening facilitates early elimination of weak candidates and directs the entire focus of the drug development program towards fewer potential lead candidates. Hence, it is mandatory that the pre-clinical candidates are subjected to as many possible reality checks. Reliance on in-vitro tests should be minimized because they do not represent the real physiological environment but rather slow down the pace of a drug discovery program. Compounds can be straight away subjected to in-vivo high throughput screens such as cassette dosing, cassette analysis or rapid rat screen etc. Candidates with the desired in-vivo pharmacokinetic profile may be further profiled in-vitro, using assays such as metabolic stability, reaction phenotyping, CYP-450 inhibition and induction, plasma protein binding etc. in human microsomes, human recombinant CYP-450 enzymes and human plasma. This also provides an early indication of whether the compound which worked in animals would work in human as well. In-vitro metabolic stability profile is a qualitative as well as quantitative comparison of metabolism of a compound in human and animal models. It helps in identifying the right model for toxicity studies. Extensive metabolism is generally considered a liability as it limits the systemic exposure and shortens the half-life of a compound. Several strategies such as reduction of lipophilicity, modification and / or blocking of metabolically soft spots and use of enzyme inhibitors; have been developed to combat metabolism. In spite of several concerns, the fact that active metabolites of several marketed drugs have been developed as drugs with better efficacy, safety and pharmacokinetics profile; cannot be denied. Therefore, instead of considering metabolic instability a liability it can be exploited as a tool for discovering better drugs. It is equally important to identify the metabolic pathways of the drug candidates by conducting in-vitro CYP450 reaction phenotyping assays. The identification of drug metabolizing enzymes involved in the major metabolic pathways of a compound helps in predicting the probable drug-drug interactions in human. Compounds with more than one metabolic pathway have less likelihood of clinically significant drug interactions. In-vitro CYP450 inhibition and induction screens are used to evaluate the potential of compound towards drug - drug interactions and the most prone candidates may either be discarded or taken ahead with a caution. It is known that only unbound drug is pharmacologically active and therefore the assessment of bound fraction by the estimation of plasma protein binding of a compound is another important parameter to be explored in-vitro. In addition to the process of 'weeding out' weak candidates early in the drug discovery process, it is equally important to identify the probable causes of poor ADME exhibited by some compounds as this information is useful to medicinal chemists for improving upon backbones that exhibit un favorable pharmacokinetic profile. Toxicity study is the foundation of an INDA (Investigational new drug application) and therefore, the final selection of a compound can be performed only after proper toxicological evaluation in animal models. Toxicokinetics forms an integral part of toxicity study and is used to assess the exposure of candidates in toxicity models and correlate the drug levels in blood and various tissues with the toxicological findings. Although in-vivo screening of compounds in animal models and in-vitro assays in human recombinant CYP-450 enzymes help in drug candidate selection, both approaches have their own limitations. There is no certainty that the selected candidates will exhibit the desired target PK profile in human and real human PK remains suspense until the compound enters Phase-1 clinical trial. The recognition of human micro dosing, (HMD) by medicines and healthcare products regulatory agency (MHRA) and European agency for evaluation of medicinal products [EMEA] is a stepping stone in the direction of obtaining human PK data early in the preclinical stage. This would gradually shift the focus of early drug development away from animal studies directly towards safe and ethical studies in human yielding more relevant and reliable pharmacokinetic data. HMD would provide an answer to the growing public demand for a reduction in the use of animals for pharmaceutical development.     

Trends in early drug safety

Successful introduction of a new drug to the market is not only an extremely costly and complicated process, but also fraught with a substantial risk of failure. The number of new drugs launched each year from 1990 to 2000 has stayed relatively constant, while the cost of pharmaceutical research and development has risen by almost 2.5-fold over the same period. What is not revealed by these figures is that the chance of success for a drug candidate passing through the various hurdles in pharmaceutical development is at best 1 in 10 and has barely changed despite advancing technology in other areas of research and development. While we expect high failure rates in drug discovery, it is of substantial concern that most candidates in development on which large investments have already been made are probably not going to make any return. A major stumbling block is the absorption, distribution, metabolism, excretion and toxicology profile of drug candidates. These issues were discussed at the Society for Medicines Research symposium held September 18, 2003, in London, United Kingdom. Recent SMR symposia have focused on the ADME and pharmacokinetic aspects of drug discovery and development. Indeed, it is now uncommon for drug discovery projects not to address these issues early in their lifetimes. Although it is less common to address drug safety early in a project, it is being utilized more frequently to help select the best clinical candidates for further development. This meeting report summarizes some of the key aspects of early drug safety issues facing the drug discoverer today. Classical approaches to toxicology, p450-mediated safety, cardiovascular safety, "omics" approaches and their impact upon clinical safety will be discussed.