Overview: Evaluation of metabolism-based drug toxicity in drug development

Drug metabolism can be a key determinant of drug toxicity. A nontoxic parent drug may be biotransformed by drug metabolizing enzymes to toxic metabolites (metabolic activation). Conversely, a toxic drug may be biotransformed to nontoxic metabolites (detoxification). The approaches to evaluate metabolism-based drug toxicity include the identification of toxic metabolites and the evaluation of toxicity in metabolically competent and metabolically compromised systems. A clear understanding of the role of drug metabolism in toxicity can aid the identification of risk factors that may potentiate drug toxicity, and may provide key information for the development of safe drugs.     

Intrinsic or acquired drug resistance and metastasis: Are they linked phenotypes?

Evidence is reviewed which suggests a linkage may exist between certain forms of de novo or acquired drug resistance and metastasis. This includes finding that expression of certain dominantly acting mutant oncogenes or tumor suppressor genes, e.g. genes which normally act to “drive” tumor progression and metastasis, can also affect the expression of drug resistance. Moreover, this can be accompanied by altered expression of certain cellular genes thought to be involved in expression of drug resistance. A direct linkage between acquired drug resistance and metastasis would suggest that tumor sublines selected for drug resistance should manifest more aggressive malignant properties than their drug-sensitive counterparts. While this does not appear to be true for drug resistant sublines selected in vitro, indeed such cell lines frequently manifest diminished in vivo tumorigenic and/or metastatic competence, there is some evidence to support such a correlation exists for tumor cell lines that are selected in vivo for drug resistance. Attention is also drawn to the fact that new linkages between metastasis and drug resistance may be uncovered by analyzing the ability of tumor subpopulations to acquire drug resistance after one or several previous exposures to chemotherapeutic drugs, as opposed to examining intrinsic drug resistance only. Furthermore, ability to detect induced or acquired drug resistance in vitro may be strongly influenced by the types of assay used to detect and monitor drug resistanc. In particular, three-dimensional cell culture systems may reveal acquired or induced “multicellular” drug resistance in situations where conventional two-dimensional culture systems may therefore reveal as yet undiscovered associations between the phenotypes of metastasis and drug resistance.     

Biomarkers in Alzheimer's disease drug development

Biomarkers may be of great value in Alzheimer's disease drug development to select the most optimal drug candidates for large and expensive phase 3 clinical trials. Biomarkers will also be important to provide evidence that a drug affects the underlying pathophysiology of the disease, which, together with a beneficial effect on the clinical course, will be essential for labeling the drug as having a disease-modifying effect.     

Intrauterine transfusion of twins.

Drug interactions are important causes of both unexpected toxic and therapeutic effects. Adverse reactions due to drug interaction are proportional to the number of drugs given and the duration of administration. Although drug interactions may be beneficial, they are most often recognized when they increase mortality or morbidity. The frequency of adverse drug interactions in clinical practice makes it mandatory for physicians to know the drugs and mechanisms involved.A drug may potentiate or antagonize the effects of another drug by direct chemical or physical combination, by altering gastrointestinal absorption, by influencing metabolism, transport, or renal clearance, by changing the activity of a drug at its receptor site, or by modifying the patient''s response to the drug by a variety of means.This article stresses the importance of avoiding multible drug therapy. When such treatment is unavoidable, patients must be carefully observed for evidence of intensified or diminished drug effect. Only this permits the detection and prevention of untoward drug interactions.     

The relationship between rational drug design and drug side effects

Previous analysis of systems pharmacology has revealed a tendency of rational drug design in the pharmaceutical industry. The targets of new drugs tend to be close with the corresponding disease genes in the biological networks. However, it remains unclear whether the rational drug design introduces disadvantages, i.e. side effects. Therefore, it is important to dissect the relationship between rational drug design and drug side effects. Based on a recently released drug side effect database, SIDER, here we analyzed the relationship between drug side effects and the rational drug design. We revealed that the incidence drug side effect is significantly associated with the network distance of drug targets and diseases genes. Drugs with the distances of three or four have the smallest incidence of side effects, whereas drugs with the distances of more than four or smaller than three show significantly greater incidence of side effects. Furthermore, protein drugs and small molecule drugs show significant differences. Drugs hitting membrane targets and drugs hitting cytoplasm targets also show differences. Failure drugs because of severe side effects show smaller network distances than approved drugs. These results suggest that researchers should be prudent on rationalizing the drug design. Too small distances between drug targets and diseases genes may not always be advantageous for rational design for drug discovery.     

Antibiotic Toxicity and Absorption in Zebrafish Using Liquid Chromatography-Tandem Mass Spectrometry

Evaluation of drug toxicity is necessary for drug safety, but in vivo drug absorption is varied; therefore, a rapid, sensitive and reliable method for measuring drugs is needed. Zebrafish are acceptable drug toxicity screening models; we used these animals with a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method in a multiple reaction monitoring mode to quantify drug uptake in zebrafish to better estimate drug toxicity. Analytes were recovered from zebrafish homogenate by collecting supernatant. Measurements were confirmed for drugs in the range of 10–1,000 ng/mL. Four antibiotics with different polarities were tested to explore any correlation of drug polarity, absorption, and toxicity. Zebrafish at 3 days post-fertilization (dpf) absorbed more drug than those at 6 h post-fertilization (hpf), and different developmental periods appeared to be differentially sensitive to the same compound. By observing abnormal embryos and LD₅₀ values, zebrafish embryos at 6 hpf were considered to be suitable for evaluating embryotoxicity. Also, larvae at 3 dpf were adapted to measure acute drug toxicity in adult mammals. Thus, we can exploit zebrafish to study drug toxicity and can reliably quantify drug uptake with LC-MS/MS. This approach will be helpful for future studies of toxicology in zebrafish.     

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.     

Adaptation to trichodermin and anisomycin in Physarum polycephalum

The effects of the protein synthesis inhibitors trichodermin and anisomycin on the growth of the eucaryotic myxomycete Physarum polycephalum have been examined. When either of these drugs is added to log phase monoxenic cultures of myxamoebae, cell division is immediately arrested, but on continued incubation, growth resumes at a rate only slightly lower than that of drug free cultures. The length of the drug induced growth lag is roughly proportional to drug concentration. When adapted cells are transferred to fresh drug containing medium, growth is not inhibited. However, if the drug concentration is increased, transient inhibition is again exhibited. Measurement of the antibiotic concentration in used media demonstrates no significant external inactivation of either drug during adaptation. The resumption of growth cannot be attributed to the selection of stable drug-resistant mutants: single amoebal colonies arising on drug plates are found to be as drug-sensitive as control colonies when retested after subculture. In addition, when adapted cells are transferred to drug free medium, the phenotypic drug-resistance is completely lost after several generations of growth. As recovery occurs in the continuous presence of drug and is not due to the accumulation of drug-resistant mutants, this response appears to be an example of drug adaptation. Cross adaptation between anisomycin and trichodermin is also demonstrated, suggesting a common system is involved in adaptation to these structurally dissimilar, but functionally similar, drugs.     

A theory of drug tolerance and dependence II: the mathematical model

The preceding paper presented a model of drug tolerance and dependence. The model assumes the development of tolerance to a repeatedly administered drug to be the result of a regulated adaptive process. The oral detection and analysis of exogenous substances is proposed to be the primary stimulus for the mechanism of drug tolerance. Anticipation and environmental cues are in the model considered secondary stimuli, becoming primary in dependence and addiction or when the drug administration bypasses the natural-oral-route, as is the case when drugs are administered intravenously. The model considers adaptation to the effect of a drug and adaptation to the interval between drug taking autonomous tolerance processes. Simulations with the mathematical model demonstrate the model's behaviour to be consistent with important characteristics of the development of tolerance to repeatedly administered drugs: the gradual decrease in drug effect when tolerance develops, the high sensitivity to small changes in drug dose, the rebound phenomenon and the large reactions following withdrawal in dependence. The present paper discusses the mathematical model in terms of its design. The model is a nonlinear, learning feedback system, fully satisfying control theoretical principles. It accepts any form of the stimulus-the drug intake-and describes how the physiological processes involved affect the distribution of the drug through the body and the stability of the regulation loop. The mathematical model verifies the proposed theory and provides a basis for the implementation of mathematical models of specific physiological processes.     

Enzyme-triggered nanomedicine: drug release strategies in cancer therapy

Nanomedicine as a field has emerged from the early success of nanoparticle-based drug delivery systems, in particular for treatment of cancer, and the advances made in nano- and biotechnology over the past decade. A prerequisite for nanoparticle-based drug delivery systems to be effective is that the drug payload is released at the target site. A large number of drug release strategies have been proposed that can be classified into certain areas. The simplest and most successful strategy so far, probably due to relative simplicity, is based on utilizing certain physico-chemical characteristics of drugs to obtain a slow drug leakage from the formulations after accumulation in the cancerous site. However, this strategy is only applicable to a relatively small range of drugs and cannot be applied to biologicals. Many advanced drug release strategies have therefore been investigated. Such strategies include utilization of heat, light and ultrasound sensitive systems and in particular pH sensitive systems where the lower pH in endosomes induces drug release. Highly interesting are enzyme sensitive systems where over-expressed disease-associated enzymes are utilized to trigger drug release. The enzyme-based strategies are particularly interesting as they require no prior knowledge of the tumour localization. The basis of this review is an evaluation of the current status of drug delivery strategies focused on triggered drug release by disease-associated enzymes. We limit ourselves to reviewing the liposome field, but the concepts and conclusions are equally important for polymer-based systems.     

A theory of drug tolerance and dependence I: a conceptual analysis

A mathematical model of drug tolerance and its underlying theory is presented. The model extends a first approach, published previously. The model is essentially more complex than the generally used model of homeostasis, which is demonstrated to fail in describing tolerance development to repeated drug administrations. The model assumes the development of tolerance to a repeatedly administered drug to be the result of a regulated adaptive process. The oral detection and analysis of exogenous substances is proposed to be the primary stimulus for the mechanism of drug tolerance. Anticipation and environmental cues are in the model considered secondary stimuli, becoming primary only in dependence and addiction or when the drug administration bypasses the natural-oral-route, as is the case when drugs are administered intravenously. The model considers adaptation to the effect of a drug and adaptation to the interval between drug taking autonomous tolerance processes. Simulations with the mathematical model demonstrate the model's behavior to be consistent with important characteristics of the development of tolerance to repeatedly administered drugs: the gradual decrease in drug effect when tolerance develops, the high sensitivity to small changes in drug dose, the rebound phenomenon and the large reactions following withdrawal in dependence. The mathematical model verifies the proposed theory and provides a basis for the implementation of mathematical models of specific physiological processes. In addition, it establishes a relation between the drug dose at any moment, and the resulting drug effect and relates the magnitude of the reactions following withdrawal to the rate of tolerance and other parameters involved in the tolerance process. The present paper analyses the concept behind the model. The next paper discusses the mathematical model.     

Simplified antimalarial therapeutic monitoring: using the day-7 drug level?

The blood concentration profiles of most antimalarial drugs vary considerably between patients. The interpretation of antimalarial drug trials evaluating efficacy and effectiveness would be improved considerably if the exposure of the infecting parasite population to the antimalarial drug treatment could be measured. Artemisinin combination treatments are now recommended as first-line drugs for the treatment of falciparum malaria. Measurement of the blood, serum or plasma concentration of the slowly eliminated partner antimalarial drug on day 7 of follow-up is simpler and might be a better determinant of therapeutic response than the area under the concentration-time curve. Measurement of the day-7 drug level should be considered as a routine part of antimalarial drug trials.     

Effect of Drug Solubility on Polymer Hydration and Drug Dissolution from Polyethylene Oxide (PEO) Matrix Tablets

The purpose of the study was to investigate the effect of drug solubility on polymer hydration and drug dissolution from modified release matrix tablets of polyethylene oxide (PEO). Different PEO matrix tablets were prepared using acetaminophen (ACE) and ibuprofen (IBU) as study compounds and Polyox WSR301 (PEO) as primary hydrophilic matrix polymer. Tablet dissolution was tested using the USP Apparatus II, and the hydration of PEO polymer during dissolution was recorded using a texture analyzer. Drug dissolution from the preparations was dependent upon drug solubility, hydrogel formation and polymer proportion in the preparation. Delayed drug release was attributed to the formation of hydrogel layer on the surface of the tablet and the penetration of water into matrix core through drug dissolution and diffusion. A multiple linear regression model could be used to describe the relationship among drug dissolution, polymer ratio, hydrogel formation and drug solubility; the mathematical correlation was also proven to be valid and adaptable to a series of study compounds. The developed methodology would be beneficial to formulation scientists in dosage form design and optimization.     

Drug discovery for malaria: a very challenging and timely endeavor

The prevalence of resistance to known antimalarial drugs has resulted in the expansion of antimalarial drug discovery efforts. Academic and nonprofit institutions are partnering with the pharmaceutical industry to develop new antimalarial drugs. Several new antimalarial agents are undergoing clinical trials, mainly those resurrected from previous antimalarial drug discovery programs. Novel antimalarials are being advanced through the drug development process, of course, with the anticipated high failure rate typical of drug discovery. Many of these are summarized in this review. Mechanisms for funding antimalarial drug discovery and genomic information to aid drug target selection have never been better. It remains to be seen whether ongoing efforts will be sufficient for reducing malaria burden in the developing world.     

An approach to the modeling of the tolerance mechanism in the drug effect. II: On the implications of compensatory regulation

In the previous paper (Peper et al., 1987: J. theor. Biol. 127, 413), a model of drug tolerance was developed based on the assumption that the decrease of drug effect after repeated administration of a drug is caused by the involved regulations in the organism adapting themselves to the presence of the drug. In the present paper, the behaviour of the model is studied with respect to the dose-response relation, the drug effect in dependent and non-dependent subjects and withdrawal symptoms. Computer simulations demonstrate the model to be highly sensitive to sudden changes of drug dose. Dependent on the open loop gain of the adaptive mechanism, a sudden decrease of drug dose might result in an effect opposite to the common drug effect. In the model, the rate of decrease of drug dose necessary for optimal drug withdrawal appears to be determined by the same mechanism as the rate of increase of dose necessary for a constant effect at the commencement of treatment. The behaviour of the model suggests the degree of drug dependence in an addicted subject to depend on the extent to which non-somatic factors are involved in the process of initiation of the adaptive mechanisms.     

Constant Pharmacologic Effect and Chronopharmacology: Theoretical Aspects

The theoretical drug infusion rates requisite to obtain a constant pharmacologic effect are determined taking into account chronopharmacologic phenomena. The introduction of chronopharmacology into pharmacokinetic theory leads to a clocktime-dependent infusion rate. The infusion modulation depends both on type of chronophenomenon, chronopharmacokinetics or chronestesy, and plasma clearance rate of the drug. In the presence of chronestesy of a biosystem the pharmacologic effect can be maintained constant only when plasma drug clearance is fast enough to allow an adequate modulation of the plasma drug concentration. Although the established equations proceed from theoretical concept they could be useful for programming drug delivery systems.     

The role of multiscale computational approaches for rational design of conventional and nanoparticle oral drug delivery systems

Multiscale computational modeling of drug delivery systems (DDS) is poised to provide predictive capabilities for the rational design of targeted drug delivery systems, including multi-functional nanoparticles. Realistic, mechanistic models can provide a framework for understanding the fundamental physico-chemical interactions between drug, delivery system, and patient. Multiscale computational modeling, however, is in its infancy even for conventional drug delivery. The wide range of emerging nanotechnology systems for targeted delivery further increases the need for reliable in silico predictions. This review will present existing computational approaches at different scales in the design of traditional oral drug delivery systems. Subsequently, a multiscale framework for integrating continuum, stochastic, and computational chemistry models will be proposed and a case study will be presented for conventional DDS. The extension of this framework to emerging nanotechnology delivery systems will be discussed along with future directions. While oral delivery is the focus of the review, the outlined computational approaches can be applied to other drug delivery systems as well.     

Visualizing drug efficacy in vivo

Many enzymes are therapeutic targets for drug discovery, whereas other enzymes are important for understanding drug metabolism and pharmacokinetics during compound testing in animals. Testing of drug efficacy and metabolism in an animal model requires the measurement of disease endpoints as well as assays of enzyme activity in specific tissues at selected time points during treatment. This requires the removal of tissue and biochemical assays. Techniques to noninvasively assess drug effects on enzyme activity using imaging technology would facilitate understanding of drug efficacy, pharmacokinetics, and drug metabolism. Using a commercially available cytochrome P-450 3A substrate whose oxidized product is a luciferase substrate, we show for the first time that cytochrome P-450 enzyme activity can be measured in vivo in real time by bioluminescent imaging. This imaging approach could be applicable to study drug effects on therapeutic target enzymes, as well as drug metabolism enzymes.     

Constant Pharmacologic Effect and Chronopharmacology: Theoretical Aspects

The theoretical drug infusion rates requisite to obtain a constant pharmacologic effect are determined taking into account chronopharmacologic phenomena. The introduction of chronopharmacology into pharmacokinetic theory leads to a clocktime-dependent infusion rate. The infusion modulation depends both on type of chronophenomenon, chronopharmacokinetics or chronestesy, and plasma clearance rate of the drug. In the presence of chronestesy of a biosystem the pharmacologic effect can be maintained constant only when plasma drug clearance is fast enough to allow an adequate modulation of the plasma drug concentration. Although the established equations proceed from theoretical concept they could be useful for programming drug delivery systems.