Absorption, distribution, metabolism drug discovery studies

Fig. 1 The dual role of RNAi technology in drug development process. RNAi compounds are being extensively used as a drug target discovery and validation tool. At the same time, they hold the promise of being used as drugs themselves. HTS, high-throughput screening of small molecules ADMET, absorption, distribution, metabolism, excretion, toxicity studies.

This chapter will review some of the important methods for carrying out in vivo absorption and bioavailability studies, as well as attempt to provide an overview of how the information may be used in the drug discovery process. The chapter is aimed at medicinal chemists and thus will focus on the use of animals in discovery phase absorption, distribution, metabolism, and excretion/pharmacokinetic (ADME/PK) studies, rather than the design of studies that are for regulatory submission, or part of a development safety package. 

PK studies in typical laboratory animals (typically mice, rats, dogs, or monkeys) are useful because they directly determine the various PK parameters discussed above, which affords an understanding of the whole-body characteristics of absorption, distribution, metabolism, and elimination. Because these translate with some fidelity to humans, animal PK commonly is used to assess the PK acceptability of discovery compounds. Unfortunately, these studies tend to be too slow to permit rapid evaluation of dozens or hundreds of discovery compounds. Therefore, two modifications have been introduced that provide whole-body relevance with rates of screening that are sufficient to keep pace with the production of favorable discovery drug candidates (16). 

The sensitivity and selectivity brought to drug discovery by the routine use of HPLC/MS/MS has revolutionized biopharmaceutical capabilities. This impact has been realized in the form of increased throughput and decreased cycle time. In a highly significant sense, these new analytical methods have enabled the introduction of absorption distribution metabolism excretion studies much earlier in discovery than previously possible. At one time the investment of lengthy assay development for transport studies on a discovery candidate was nearly unthinkable. With HPLC/MS/MS, this barrier has been removed. Prime ADME information can be provided at the earliest stages of discovery to aid in the selection of lead candidates. 

If the main objective is the discovery of new drug candidates, medicinal chemistry is also concerned with the fate of drugs in living organisms ( ADME studies absorption, distribution, metabolism, excretion), and with the study of bioactive compoimds not related to medicine (agrochemicals, food additives, etc.). 

NMR spectroscopy is a powerful tool for chemistry and structural biology, especially when NMR is applied to study protein-ligand interactions. In the drug discovery process, NMR is used to study whether a compound binds to a protein up to the determination of the full three-dimensional structure of the complex. Rational drug discovery requires an early appraisal of all factors impacting on the likely success of a drug candidate in the subsequent preclinical, clinical and commercial phases of dug development. The study of absorption, distribution, metabolism and excretion/pharmacokinetics (ADME/PK) has de-... 

In vitro biochemical studies of drug metabolism, particularly studies associated with pharmacokinetic (PK) properties and potential toxicological consequences, are essential in drug discovery and development. For over a decade, the major pharmaceutical companies have been promoting such efforts for incorporation into early stages in discovery (Obach, 2001 Smith and van de Waterbeemd, 1999). Consequently, the failure rate of new chemical entities (NCEs) due to absorption, distribution, metabolism, and excretion (ADME) shortcomings in clinical trials, in contrast to some of the other major hurdles including poor efficacy and intolerable toxicity, has continuously decreased (Apic et ah, 2005). 

Various in vitro assays are widely available for profiling distribution, metabolism, and pharmacokinetics (DMPK, also referred to as ADME absorption, distribution, metabolism, and excretion). Such properties of molecules are measured to ultimately predict their in vivo behavior. The metabolic stability of molecules is assessed routinely in drug discovery units by way of medium- to high-through-put assays using hepatic microsomes or hepatocytes obtained from different species (usually rat and/or human). Permeability assays (e.g., utilizing Caco-2 or MDCK cells) together with an assessment of efflux potential are also useful to troubleshoot unexpectedly low cell activity or can help select candidates for subsequent in vivo studies. 

In a more traditional pharmaceutical setting, this characterization would be done during preformulation studies. With the availability of automation and the ability to conduct most of these experiments with small quantities of material, more preformulation activities are being shifted earlier into drug discovery. Recently, Balbach and Korn37 reported a "100 mg approach" to pharmaceutical evaluation of early development compounds. Additional absorption, metabolism, distribution, elimination, and toxicity38 screens may also be conducted at this stage. 

In vitro assays play a very important role in drug discovery. First, these assays provide a simple, convenient, and fast way to test the potency and drug properties of chemical entities to help advance them rapidly. The drug-like properties commonly refer to respectable absorption, adequate distribution, low metabolism, and complete elimination (ADME) from body and minimal toxicological risk. Second, the amount of compound available is often limited in the early drug discovery stage and it is not always feasible for preclinical animal studies. Thus, in vitro assays could be a more rapid alternative to screen compounds. Third, in vitro assays are also designed to answer specific... 

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