Role of metabolism and pharmacokinetic studies in the discovery of new drugs--present and future perspectives

Drug metabolic stability in early drug discovery to develop potential lead compounds

Knowledge of the metabolic stability of a new drug substance eliminated by biotransformation is essential for envisaging the pharmacokinetic parameters required for deciding drug dosing and frequency. Strategies aimed at modifying lead compounds may improve metabolic stability, thereby reducing the drug dosing frequency. Replacement of selective hydrogens with deuterium can effectively enhance the drug's metabolic stability by increasing the biological half-life. Further, cyclization, change in ring size, and chirality can substantially improve the metabolic stability of drugs. The microsomal t_1/2 approach for measuring drug in vitro intrinsic clearance by automated LC-MS/MS offers sensitive high-throughput screens with reliable data. The obtained in vitro intrinsic clearance from metabolic stability data helps predict the drug's in vivo total clearance using different scaling factors and hepatic clearance models. This review summarizes all the recent approaches and technological advancements in metabolic stability studies for narrowing down the potential lead compounds in drug discovery. Further, we summarized the potential pitfalls and assumptions made during the in vivo intrinsic clearance estimation from in vitro intrinsic clearance.

Pharmacokinetic optimization of 4-substituted methoxybenzoyl-aryl-thiazole and 2-aryl-4-benzoyl-imidazole for improving oral bioavailability

Microtubules are critical components of the cytoskeleton. Perturbing their function arrests the growth of a broad spectrum of cancer cell lines, making microtubules an excellent and established target for chemotherapy. All of the U.S. Food and Drug Administration-approved antitubulin agents bind to paclitaxel or vinblastine binding sites in tubulin. Because of the complexity of their structures, it is difficult to structurally modify the vinca alkaloids and taxanes and develop orally bioavailable agents. Antitubulin agents that target the colchicine-binding site in tubulin may provide a better opportunity to be developed for oral use because of their relatively simple structures and physicochemical properties. A potent antitubulin agent, 4-(3,4,5-trimethoxybenzoyl)-2-phenyl-thiazole (SMART-H), binding to the colchicine-binding site, was discovered in our laboratory. However, the bioavailability of SMART-H was low because of its poor solubility. Structural modification of SMART-H led to the development of 2-aryl-4-benzoyl-imidazole analog (ABI-274), with improved bioavailability and potency but still considerable first-pass metabolism. A chlorine derivative (ABI-286), replacing the methyl site of ABI-274, resulted in 1.5-fold higher metabolic stability in vitro and 1.8-fold lower clearance in rats in vivo, indicating that metabolic stability of ABI-274 can be extended by blocking benzylic hydroxylation. Overall, ABI-274 and ABI-286 provided 2.4- and 5.5-fold increases in exposure (area under the curve) after oral dosing in rats compared with SMART-H. Most importantly, the structural modifications did not compromise potency. ABI-286 exhibited moderate clearance, moderate volume of distribution, and acceptable oral bioavailability. This study provided the first evidence that ABI-286 may be the first member of a new class of orally bioavailable antitubulin agents.

Pharmacologically active metabolites of currently marketed drugs: potential resources for new drug discovery and development

Biotransformation is the major clearance mechanism of therapeutic agents from the body. Biotransformation is known not only to facilitate the elimination of drugs by changing the molecular structure to more hydrophilic, but also lead to pharmacological inactivation of therapeutic compounds. However, in some cases, the biotransformation of drugs can lead to the generation of pharmacologically active metabolites, responsible for the pharmacological actions. This review provides an update of the kinds of pharmacologically active metabolites and some of their individual pharmacological and pharmacokinetic aspects, and describes their importance as resources for drug discovery and development.

Improving the decision-making process in the structural modification of drug candidates: enhancing metabolic stability

The activity-exposure-toxicity relationship, which can be described as "the rule of three", presents the single most difficult challenge in the design of drug candidates and their subsequent advancement to the development stage. ADME studies are widely used in drug discovery to optimize the balance of properties necessary to convert lead candidates into drugs that are safe and effective for humans. Metabolite characterization has become one of the key drivers of the drug discovery process, helping to optimize ADME properties and increase the success rate for drugs. Various strategies can influence drug design in the decision-making process in the structural modification of drug candidates to reduce metabolic instability.

Lack of in vitro-in vivo correlation of a novel investigational anticancer agent, SH 30

In solid tumors, the reasons for the lack of in vitro and in vivo correlation of drug activities are multifold and includes permeability to the tumor cells, interstitial hypertension and metabolic degradation. So, it is important to study the permeability and metabolic disposition of new compounds early in discovery and development of anticancer drugs. An experimental anti-cancer drug, SH 30 demonstrated highly selective and potent cytotoxic activity against a number of multi-drug resistant tumor cell lines in vitro. However, it was inactive in a murine tumor model. This study was conducted to identify the barriers that result in lack of correlation between in vitro and in vivo cytotoxic activity of novel anticancer agents. Two important barriers: physical (permeability) and metabolic (enzymatic inactivation) to poor delivery of SH 30 to solid tumors were investigated in this study. Tumors were sliced to separate the vascular and avascular sections. The concentrations of the drug at various regions of the tumor after single and multiple doses were investigated to determine the permeability barrier. The permeability barrier was also probed using two in vitro model systems, namely, matrigel films representing extracellular matrix and caco-2 multilayer cell cultures that simulate solid tumors. The drug and its metabolite concentrations were determined in the plasma and tumors to determine the metabolic barrier to the drug cytotoxic action. The metabolic barrier was further probed using in vitro mouse hepatocytes and liver microsome preparations. Our examination revealed the metabolic barrier to be the major contributor to the ineffectiveness of SH 30 in vivo. Examination of concentration of the drug across various regions of the tumor corroborated by data from in vitro permeation studies suggested that, for SH 30, permeability barrier did not exist. After single injection, the concentrations of SH 30 and its metabolites in plasma and tumor were comparable to another investigational drug with similar features (XK 469). Contrary to day 1, after 8 consecutive days of administration, SH 30 concentrations were significantly lower, while the metabolites concentrations were higher, suggesting extensive metabolism due to induction of enzyme(s). The in vitro hepatocytes and liver microsome results also showed SH 30 biotransformation to the same metabolites. Neither drug penetration, nor drug distribution into regions of the tumors distal to vasculature were impeded. The inactivity of SH 30 in vivo is primarily due to induction of extensive metabolism to inactive metabolites. This metabolism prevents adequate drug levels being achieved in the tumor.

Optimization of metabolic stability as a goal of modern drug design

Metabolism and other pharmacokinetic (PK) studies have always played a critical role in helping to optimize the bioavailability and duration of action of new drugs thereby increasing their success rate. With the advent of automated combinatorial synthesis, high-throughput pharmacological testing, and the ability to create extensive databases in the past decade, drug discovery has undergone an amazing evolution. With the increased throughput of drug discovery, metabolism and other PK studies have evolved to keep pace. Often called "early ADME" studies, these studies are characterized by parallel processing and higher throughput than before. This article focuses on a particular class of early ADME (absorption, distribution mechanism, and excretion) studies known as "metabolic stability" studies. The theoretical basis for metabolic stability and its relationship to the concept of metabolic intrinsic clearance is briefly presented. Some key relationships between structure and metabolism are summarized. Several case studies from recent medicinal chemistry literature are reviewed to exemplify how metabolic stability studies influenced drug design and led to improvements in bioavailability and half-life. Finally, future trends in drug metabolism and analytical chemistry and how they may influence metabolic stability studies are reviewed.

Absorption characteristics of sustained-release 4-aminopyridine (fampridine SR) in patients with chronic spinal cord injury

Fampridine SR (4-aminopyridine) is a potassium channel-blocking drug currently being investigated for its therapeutic efficacy in ameliorating central conduction deficits due to demyelination in patients with spinal cord injury (SCI). The present open-label pharmacokinetic trial examined the absorption characteristics of a sustained-release form of the drug in 25 SCI subjects with chronic incomplete injuries. The overall group mean Cmax of 27.7 +/- 6.2 ng/mL occurred at a tmax of 3.4 +/- 1.4 hours. AUC0-12 was 210.5 +/- 49.5 ng/mL.h. For paraplegics, AUCtmax was 76.02 +/- 33.28 and for tetraplegics was significantly less at 51.25 +/- 20.36 (p = 0.037). A statistically significant difference in the initial rate and extent of absorption, but not in total 4-AP bioavailability over the 12-hour study period, was evident between tetraplegic patients, 0.60 +/- 0.23, and paraplegic patients, 0.39 +/- 0.14 (p = 0.02). There was a linear correlation (p < 0.05) between the neurological level of injury and Cmax/AUCtmax. These results confirm and extend previous observations of different rates of drug absorption among SCI patients with lesions above and below the sympathetic outflow (T6) and provide evidence of the absorption characteristics of this sustained-release form of 4-aminopyridine, which is helpful for optimal dosing.

Cytochrome P450 enzyme activity and protein expression in primary porcine enterocyte and hepatocyte cultures

1. A method for the isolation and cultivation of porcine hepatocytes and porcine duodenal enterocytes for the investigation of drug oxidation reactions has been established. 2. Hepatocytes as well as enterocytes metabolized ethoxyresorufin (EROD) and ethoxycoumarin (ECOD) effectively, the rate being 31+/-17 pmol/h x dish (EROD) and 9530+/-4062 pmol/h x dish (ECOD) in the case of hepatocytes, and 9+/-4 pmol/h x dish (EROD) and 510+/-467 pmol/h x dish (ECOD) in the case of enterocytes. Diazepam, another CYP monooxygenase substrate, was also metabolized by porcine hepatocytes but not with porcine enterocytes, thus indicating differences in the metabolic competence of the liver and the gut. 3. The ability to induce enzymes responsible for the metabolism of ethoxyresorufin and ethoxycoumarin was investigated in vitro on treatment of the cell cultures with either 50 microM 3-methylcholanthrene (3-MC) or 50 microM beta-naphthoflavone (beta-NF). With enterocyte cultures, ECOD activity was inducible up to 20-fold, whereas EROD remained unchanged following treatment with either 3-MC or beta-NF. 4. Western blotting provided additional evidence for the expression of CYP1A1 and CYP3A4 at the protein level and treatment of cultured enterocytes with 30 microM Aroclor 1254 or 50 microM beta-NF resulted in enhanced expression of the CYP1A protein, and CYP3A4 protein expression was induced following treatment with 50 microM DEX, 2 mM PB, 30 microM Aroclor 1254 or 50 microM beta-NF. 5. The metabolism of diazepam was also investigated with baculovirus-expressed human CYP enzymes (2C8, 2C9-ARG, 2C9-CYS, 2C19, 3A4, 3A4+cytochrome b5 and 3A5) and evidence was obtained to suggest the formation of temazepam and oxazepam by enzymes of the CYP3A subfamily. Small amounts (32+/-12 ng/ml) of desmethyldiazepam were additionally recovered in microsomal preparations of all CYP-transfected cell lines. 6. In conclusion, porcine duodenal enterocytes can successfully be cultured for a short period and may be used as a tool for studying intestinal metabolism, whereas porcine hepatocytes can be cultured for prolonged periods (>10 days) reliably to investigate hepatic drug oxidation reactions.

The influence of food on the absorption and metabolism of drugs: an update

Food-drug interactions can lead to a loss of therapeutic efficacy or toxic effects of drug therapy. Generally, the effect of food on drugs results in a reduction in the drug's bioavailability; however, food can also alter drug clearance. The benefits of considering metabolism and pharmacokinetic information in the drug discovery process have been highlighted by Humphrey and Smith (79) and the process of rational drug design should include considerations of the chemistry, pharmacology and pharmacokinetics of the drug (80) and the impact of diet on these parameters.

The use of in vitro metabolism techniques in the planning and interpretation of drug safety studies

An important issue in toxicology is the suitability of the data obtained with experimental animals for human risk assessment. Because it is not possible to use humans in long-term toxicological studies, the use of animals will continue. However, the data obtained in animal studies can be better extrapolated to the patient by utilizing bridging studies with in vitro models of human drug metabolism. There are 2 basic categories of in vitro methods for the examination of human liver drug metabolism. The first group of in vitro methods consists of the cellular models, which include primary hepatocytes, liver slices, and cell lines. The second group is the use of preparations of the drug-metabolizing enzymes such as tissue homogenates, subcellular fractions, and isolated enzymes. Studies modeling both the human and experimental animal metabolism of a drug are useful in the design of toxicological studies. In vitro studies can identify metabolites, species-specific metabolic routes, and the experimental animal model that best reflects the potential human exposure to the drug and its metabolites. This information can also be useful in the design of the clinical studies by identifying human metabolites, the enzymes responsible for the metabolic clearance of the drug, the effects of genetics and other host factors on the metabolism of the drug, and potential drug-drug interactions. An example of how such information is generated and interpreted is presented.

Considerations in the design of toxicokinetic programs

The objectives of toxicokinetic (TK) studies are to evaluate systemic exposure of the toxicity species to drug and/or metabolite(s) and to relate exposure to dose level and toxicological findings. The tacit assumption is that such exposure can be related to target organ toxicity. Planning a TK program involves early development (often during discovery support) of an appropriate plasma assay, assurance that the studies conform to Good Laboratory Practices, evaluating exposure at steady state, and effective collaboration among toxicologists, kineticists, and other relevant disciplines. The 3 types of TK studies--prospective, concomitant, and retrospective--each have different goals. The various stages of TK program implementation range from those during drug discovery and selection to support of chronic toxicity studies. Exposure is best expressed as the area under the plasma concentration/time curve of drug and/or metabolite(s) and, for highly protein bound drugs, is based on the unbound fraction. Although the objectives of TK programs are generally standard and the Second International Conference on Harmonization has developed TK guidelines, the programs differ among pharmaceutical companies. Some variables in program design and implementation include properties of the drug, formulation used, characteristics of the target species, ability to develop a toxicokinetic-toxicodynamic relationship, strategies based on scientific/technical/philosophical considerations, dedicated resources, corporate support, and effectiveness of interdepartmental collaborations.

Design of drugs through a consideration of drug metabolism and pharmacokinetics

Drug metabolism input to the discovery process has been to date largely on an empirical case by case basis. Considerable advances have been made, such that basic rules can be applied to the behaviour of a compound in man based on physico-chemistry and structure. This is particularly true in the area of the cytochrome P-450 enzymes, the principal enzymes involved in the primary clearance of drugs. The major human forms, CYP2D6, CYP2C9 and CYP3A4 all have distinct substrate preferences which are being catalogued and rationalised. Such understandings will not only impact on existing drugs. Since the enzymes and systems will remain the same, these understandings can be applied to the design of molecules for the targets of the future, whilst the structure activity relationships of those targets are being researched and revealed.

Drug toxicokinetics: scope and limitations that arise from species differences in pharmacodynamic and carcinogenic responses

Toxicokinetics describes the concentration and time course of a xenobiotic in the circulation under the conditions of a toxicology study. However, the fundamental challenge to the toxicologist, of extrapolating the findings in animals to a risk assessment in humans, requires knowledge and understanding of both the pharmacokinetics and pharmacodynamic responses in each species. This paper exemplifies situations where measurement of plasma concentrations may provide information useful in the design and interpretation of the toxicity observed in a given species; it also illustrates how intrinsic interspecies differences in pharmacodynamic response limit the extrapolation of toxicity data across species. The special case of the multistage cumulative phenomenon of carcinogenicity, with the implications of daily dose, duration of dosing, and species differences in response, is also discussed.

Structure-based drug design: applications in immunopharmacology and immunosuppression

Structure-based drug design (SBDD) combines the power of many scientific disciplines, such as X-ray crystallography, nuclear magnetic resonance, medicinal chemistry, molecular modeling, biology, enzymology and biochemistry, in a functional paradigm of drug development. The current strength of SBDD lies in parlaying enzyme inhibitors into drugs, but a variety of technological advances over the past few years now makes it possible to address complex biological targets, such as those regulating immunosuppression and immunoactivation. Manual Navia and Debra Peattie discuss the SBDD paradigm and consider several of its achievements and challenges in immunopharmacology, particularly as these apply to the design of novel, potent immunosuppressants.

Structure-based drug design: applications in immunopharmacology and immunosuppression

Structure-based drug design (SBDD) combines the power of many scientific disciplines, such as X-ray crystallography, nuclear magnetic resonance, medicinal chemistry, molecular modeling, biology, enzymology and biochemistry, in a functional paradigm of drug development. The current strength of SBDD lies in parlaying enzyme inhibitors into drugs, but a variety of technological advances over the past few years now makes it possible to address complex biological targets, such as those regulating immunosuppression and immunoactivation. Manuel Navia and Debra Peattie discuss the SBDD paradigm and consider several of its achievements and challenges in immunopharmacology, particularly as these apply to the design of novel, potent immunosuppressants.