ABSTRACT:Time-dependent inhibition (TDI) of cytochrome P450 (P450) enzymes caused by new molecular entities (NMEs) is of concern because such compounds can be responsible for clinically relevant drug-drug interactions (DDI). Although the biochemistry underlying mechanism-based inactivation (MBI) of P450 enzymes has been generally understood for several years, significant advances have been made only in the past few years regarding how in vitro time-dependent inhibition data can be used to understand and predict clinical DDI. In this article, a team of scientists from 16 pharmaceutical research organizations that are member companies of the Pharmaceutical Research and Manufacturers of America offer a discussion of the phenomenon of TDI with emphasis on the laboratory methods used in its measurement. Results of an anonymous survey regarding pharmaceutical industry practices and strategies around TDI are reported. Specific topics that still possess a high degree of uncertainty are raised, such as parameter estimates needed to make predictions of DDI magnitude from in vitro inactivation parameters. A description of follow-up mechanistic experiments that can be done to characterize TDI are described. A consensus recommendation regarding common practices to address TDI is included, the salient points of which include the use of a tiered approach wherein abbreviated assays are first used to determine whether NMEs demonstrate TDI or not, followed by more thorough inactivation studies for those that do to define the parameters needed for prediction of DDI.Pharmacokinetic drug-drug interactions (DDIs) can occur when one drug alters the metabolism of a coadministered drug. The outcome is an increase or decrease in the systemic clearance and/or bioavailability, and a corresponding change in the exposure to a coadministered drug. The clinical consequences of DDIs range from lack of therapeutic efficacy of a life saving drug to severe adverse drug reactions, including fatalities. Significant drug-drug interactions can lead to termination of development of otherwise promising new therapies, withdrawal of a drug from the market, or severe restrictions/limitations on its use (Wienkers and Heath, 2005). Because of the impact on patient health and safety, DDI was the subject of a position paper in 2003 by scientists from member companies of the Pharmaceutical Research and Manufacturers of America (PhRMA) that focused on Article, publication date, and citation information can be found at

Physiologically based pharmacokinetic (PBPK) modeling and simulation can be used to predict the pharmacokinetic behavior of drugs in humans using preclinical data. It can also explore the effects of various physiologic parameters such as age, ethnicity, or disease status on human pharmacokinetics, as well as guide dose and dose regiment selection and aid drug–drug interaction risk assessment. PBPK modeling has developed rapidly in the last decade within both the field of academia and the pharmaceutical industry, and has become an integral tool in drug discovery and development. In this mini-review, the concept and methodology of PBPK modeling are briefly introduced. Several case studies were discussed on how PBPK modeling and simulation can be utilized through various stages of drug discovery and development. These case studies are from our own work and the literature for better understanding of the absorption, distribution, metabolism and excretion (ADME) of a drug candidate, and the applications to increase efficiency, reduce the need for animal studies, and perhaps to replace clinical trials. The regulatory acceptance and industrial practices around PBPK modeling and simulation is also discussed.

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The uptake properties of taurocholate (TC) and estradiol 17beta-D-glucuronide (E(2)17betaG) were examined in freshly isolated and cryopreserved human hepatocytes to discover if active transport is retained in cryopreserved human hepatocytes. Firstly, the uptake of TC and E(2)17betaG was measured before and after cryopreservation. The uptake of TC was found to be Na(+)-dependent both in fresh and cryopreserved hepatocytes. The uptake activity in cryopreserved hepatocytes was found to range from 10 to 200% of that observed in freshly isolated cells. A kinetic analysis was performed to evaluate the transport activity of TC and E(2)17betaG and revealed that the Michaelis constant (K(m)) for these compounds in cryopreserved human hepatocytes was 2-8 and 3-18 microM, respectively. This was within the range of K(m) values previously found in human Na(+)-taurocholate cotransporting polypeptides (NTCP) and organic anion transporting polypeptides (OATP) 2 and 8, respectively. The kinetic analyses also showed that the species difference between human and rat hepatocytes was more marked for the maximal uptake rate (V(max)) (>22 and >22 times higher for TC and E(2)17betaG in rats than in humans, respectively) than that for K(m) (2-12 and 0.7-4 times higher, respectively), compared with earlier data we obtained in primary cultured rat hepatocytes. Hence, we conclude that cryopreserved human hepatocytes, at least in part, retain their transporter functions and, therefore, can be a useful experimental system for examining the mechanism of the hepatic uptake of drugs.

ABSTRACT:Apparent intrinsic clearance (CL int,app ) of 7-ethoxycoumarin, phenacetin, propranolol, and midazolam was measured using rat and human liver microsomes and freshly isolated and cryopreserved hepatocytes to determine factors responsible for differences in rates of metabolism in these systems. The cryopreserved and freshly isolated hepatocytes generally provided similar results, although there was greater variability using the latter system. The CL int,app values in hepatocytes are observed to be lower than that in microsomes, and this difference becomes greater for compounds with high CL int,app . This could partly be attributed to the differences in the free fraction (f u ). The f u in hepatocyte incubations (f u,hep-inc ) was influenced not only by the free fraction of compounds in the incubation buffer (f u,buffer ) but also by the rate constants of uptake (k up ) and metabolism (k met ). This report provides a new derivation for f u,hep-inc , which can be expressed as, where the C hep , C buffer , V hep , and V buffer represent the concentrations of a compound in hepatocytes and buffer and volumes of hepatocytes and buffer, respectively. For midazolam, the f u,hep-inc was calculated, and the maximum metabolism rate in hepatocytes was shown to be limited by the uptake rate.The determination of in vitro intrinsic clearance (CL int ) for drug candidates in the early discovery stage is a common practice in the pharmaceutical industry (Houston, 1994;Lave et al., 1997;Obach et al., 1997). The CL int values of drug candidates can help to confirm whether metabolism is the main clearance pathway when it is compared with the total body clearance in vivo. It is also helpful in rank-ordering drug candidates based on their metabolic stabilities, assessing species and gender differences in metabolic clearance, and projecting the metabolic clearance of drug candidates in humans. The in vitro CL int may be derived from enzyme kinetic data such as V max /K m (Lin et al., 1996;Tan and Pang, 2001;Griffin and Houston, 2004) or from the in vitro t 1/2 values where subK m substrate concentrations are used (Lave et al., 1997, Obach, 1999Lau et al., 2002;Jones and Houston, 2004). The CL int can be calculated from the experimental apparent intrinsic clearance, CL int,app , by correcting for free fraction of test compounds in the incubations. To further predict the in vivo hepatic clearance from the in vitro intrinsic clearance, a well stirred model is often used (Naritomi et al., 2001;Ito and Houston, 2004). A survey of literature revealed that in hepatocyte incubations, the free fraction of test compound has not been well defined. Simply assuming a steady state where the intracellular free concentration equals the extracellular free concentration may allow one to roughly estimate CL int for some compounds. However, clearance, after a dose in vitro or in vivo, is actually a dynamic system such that at any given time the amount of compound getting into a cell typically equals the amount of compound leaving the cell by diffus...

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ABSTRACT:VELCADE (bortezomib, PS-341), reversibly inhibits the 20S proteasome and exhibits cytotoxic and antitumor activities. Pretreatment of cancer cells with bortezomib increases the chemosensitivity of these cells, suggesting that bortezomib may be used in combination chemotherapy. The relative contributions of the five major human cytochromes P450 (P450s), 1A2, 2C9, 2C19, 2D6, and 3A4 (the focus of the present study), to the metabolism of bortezomib are an important aspect of potential drug interactions. Relative activity factor (RAF), chemical inhibition, and immunoinhibition using monoclonal antibodies were three approaches employed to determine the relative contributions of the major human P450s to the net hepatic metabolism of bortezomib. RAFs for the P450 isoform-selective substrates were determined; the ratio of the rate of metabolism of bortezomib with cDNA-expressed P450s versus rate of metabolism with human liver microsomes was normalized with respect to the RAF for each P450 isoform to determine the percentage contributions of the P450s to the net hepatic metabolism of bortezomib. CYP3A4 followed by CYP2C19 were determined to be the major contributors to the metabolism of bortezomib. Chemical inhibition and immunoinhibition confirmed that CYP3A4 and CYP2C19 were the major P450s responsible for the hepatic metabolism of bortezomib. The studies were conducted with 2 M bortezomib, and the disappearance of bortezomib, rather than appearance of a specific metabolite, was quantified to determine the contributions of the P450s to the overall hepatic metabolism of bortezomib in humans.Boronic acids as protease inhibitors were first synthesized in the early 1970s and were demonstrated to act as potent transition state analogs of serine proteases (Koehler and Leinhard, 1971;Philipp and Bender, 1971;Kettner and Shenvi, 1984). Throughout the 1980s, peptide boronic acids were shown to be effective inhibitors of trypsin, chymotrypsin, ␣-lytic protease, pancreatic elastase, leukocyte elastase, thrombin, and ␤-lactamases (Kettner and Shenvi, 1984;Crompton et al., 1988) and have been explored for use as therapeutic agents in various disease states (Snow and Bachovchin, 1995;Groziak, 2001). More recently, peptidyl boronic acids were demonstrated as potent proteasome inhibitors, and antitumor and anti-inflammatory efficacy was observed both in vitro and in animal models (Adams et al., 1998). The approval of VELCADE (bortezomib, PS-341; Fig. 1), a dipeptidyl boronic acid, by the United States Food and Drug Administration in 2003 for the treatment of relapsed refractory multiple myeloma made it the first boronic acid and the first in a new class of drugs, proteasome inhibitors, to be marketed as a therapeutic agent.Bortezomib, an N-pyrazinylcarbonylated derivative of the dipeptide boronic acid Phe-Leu-B(OH) 2 , is a potent, selective, and reversible inhibitor (K i ϳ 0.62 nM) of the 26S proteasome in mammalian cells (Adams et al., 1998). The molecular mechanisms by which bortezomib exerts its effects include inhibition...