James A Dykens

Pfizer Inc., New York, New York, United States

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Publications (38)94.95 Total impact

  • Walter H Moos · James A Dykens

    No preview · Article · Mar 2015 · Drug Development Research
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    Yvonne Will · James Dykens

    Preview · Article · Aug 2014 · Expert Opinion on Drug Metabolism & Toxicology
  • James A. Dykens · Yvonne Will
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    ABSTRACT: This chapter contains sections titled: IntroductionMagnitude of the ProblemMitochondrial PhysiologyDrug-Induced Mitochondrial Dysfunction (DIMD) Has Been OverlookedNovel Methods to Detect Mitochondrial Dysfunction in VitroAn Emerging Model of Idiosyncratic Drug ToxicityMitochondrial DiseasesPotential Biomarkers of Mitochondrial DysfunctionAnimal ModelsSummary Points
    No preview · Chapter · Nov 2010
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    ABSTRACT: IntroductionReactive MetabolitesMitochondrial ToxicityOxidative StressInhibition of Bile Salt Effiux Protein and Drug-Induced CholestasisBiomarkersConclusions References
    No preview · Article · Mar 2010
  • Yvonne Will · James A. Dykens

    No preview · Chapter · Dec 2009
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    James W Simpkins · Kun Don Yi · Shao-Hua Yang · James A Dykens
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    ABSTRACT: Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.
    Full-text · Article · Nov 2009 · Biochimica et Biophysica Acta
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    ABSTRACT: 17-Estradiol is a less feminizing isomer of the potent hormonal estrogen, 17β-estradiol. 17-Estradiol is an orally active small molecule with conflicting reports of efficacy in preclinical models of degenerative diseases. A number of studies suggest neuroprotective potential in human neurodegenerative disorders, including Alzheimer's disease (AD) and Parkinson's disease. Several studies have established an antioxidant effect of 17-estradiol in humans. The sodium salt of 17-estradiol 3-sulfate is a minor component (2.5–9.5%) of several widely marketed estrogen hormone replacement products, such as Premarin®, that are approved by the U.S. Food and Drug Administration and have been prescribed and studied in women and men for more than 65 years. Most of the more than 100 published reports on the neurological effects of feminizing estrogens found positive responses in at least one measure relating to cognition or prevention and treatment of AD, notwithstanding the negative results in the Women's Health Initiative studies. Whether these limited, and often not statistically significant, findings are clinically meaningful remains unknown. In many in vitro and in vivo preclinical neuroprotection and related studies, 17-estradiol and 17β-estradiol are active at similar concentrations and doses. However, 17-estradiol is less pleiotropic than 17β-estradiol, and thus its potential toxicity might be lower. Given decades of mixed reports regarding the potential efficacy and safety of strongly feminizing hormones in neurodegenerative diseases, the weakly feminizing 17-estradiol might be a suitable candidate for clinical testing of the neuroprotective potential of this chemical class because it avoids, or significantly reduces, the adverse effects of potent hormonal compounds. Drug Dev Res 70:1–21, 2009. © 2009 Wiley-Liss, Inc.
    No preview · Article · Feb 2009 · Drug Development Research
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    ABSTRACT: The ability to predict ocular side effects of systemically delivered drugs is an important issue for pharmaceutical companies. Although animal models involving standard clinical ophthalmic examinations and postmortem microscopic examinations of eyes are still used to identify ocular issues, these methods are being supplemented with additional in silico, in vitro, and in vivo techniques to identify potential safety issues and assess risk. The addition of these tests to a development plan for a potential new drug provides the opportunity to save time and money by detecting ocular issues earlier in the program. This review summarizes a current practice for minimizing the potential for systemically administered, new medicines to cause adverse effects in the eye.
    Full-text · Article · Feb 2009 · Cutaneous and Ocular Toxicology
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    ABSTRACT: As a class, the biguanides induce lactic acidosis, a hallmark of mitochondrial impairment. To assess potential mitochondrial impairment, we evaluated the effects of metformin, buformin and phenformin on: 1) viability of HepG2 cells grown in galactose, 2) respiration by isolated mitochondria, 3) metabolic poise of HepG2 and primary human hepatocytes, 4) activities of immunocaptured respiratory complexes, and 5) mitochondrial membrane potential and redox status in primary human hepatocytes. Phenformin was the most cytotoxic of the three with buformin showing moderate toxicity, and metformin toxicity only at mM concentrations. Importantly, HepG2 cells grown in galactose are markedly more susceptible to biguanide toxicity compared to cells grown in glucose, indicating mitochondrial toxicity as a primary mode of action. The same rank order of potency was observed for isolated mitochondrial respiration where preincubation (40 min) exacerbated respiratory impairment, and was required to reveal inhibition by metformin, suggesting intramitochondrial bio-accumulation. Metabolic profiling of intact cells corroborated respiratory inhibition, but also revealed compensatory increases in lactate production from accelerated glycolysis. High (mM) concentrations of the drugs were needed to inhibit immunocaptured respiratory complexes, supporting the contention that bioaccumulation is involved. The same rank order was found when monitoring mitochondrial membrane potential, ROS production, and glutathione levels in primary human hepatocytes. In toto, these data indicate that biguanide-induced lactic acidosis can be attributed to acceleration of glycolysis in response to mitochondrial impairment. Indeed, the desired clinical outcome, viz., decreased blood glucose, could be due to increased glucose uptake and glycolytic flux in response to drug-induced mitochondrial dysfunction.
    No preview · Article · Dec 2008 · Toxicology and Applied Pharmacology
  • Walter H. Moos · James A. Dykens · Neil Howell
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    ABSTRACT: Alzheimer's disease (AD) is a major neurodegenerative disorder in the elderly and is the most common form of dementia. Approved AD drugs like donepezil and memantine provide some cognitive benefits to a subset of patients. However, the cost burden of AD will continue to increase in the absence of more effective drugs that treat its symptoms and ultimately slow its progression or delay or prevent its onset. A substantial number of studies report that 17β-estradiol (E2; estrogen), a potent endogenous feminizing hormone, is neuroprotective. Unfortunately, negative results from the Women's Health Initiative (WHI) and its ancillary Memory Study (WHIMS) have overshadowed the positive data collected over several decades. Importantly, there are a number of reasons why the WHIMS results should not be generalized to less- or non-feminizing estrogens. For the sake of argument, consider “non-hormonal” or non-feminizing estrogens to be worthwhile drug development candidates for AD and other neurodegenerative conditions because they have the potential to retain the positive neuroprotective activities of E2 while reducing the potential complications associated with feminizing hormones. Because the literature comparing estrogenicity of various estrogens is complex and spans multiple decades, we summarize herein the available hormonal comparisons of a common less-feminizing estrogen, 17-estradiol (17E2), with E2. 17E2 is less feminizing than E2 in every published study where reasonable comparisons can be made. One-half of these studies report a difference of two orders of magnitude or greater. In practice, compounds like 17E2 may prove to be largely non-feminizing at doses to be explored in human clinical trials. Moreover, other novel E2 analogues have been identified that are essentially devoid of feminizing actions. If the positive effects of feminizing estrogens on neurodegeneration prove useful in treating neurodegenerative diseases, less-feminizing and non-feminizing estrogen analogues may be even more effective, safer, and better tolerated. Drug Dev Res 69:177–184, 2008. © 2008 Wiley-Liss, Inc.
    No preview · Article · Aug 2008 · Drug Development Research
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    ABSTRACT: Cardiovascular disease has recently been suggested to be a significant complication of cancer treatment with several kinase inhibitors. In some cases, the mechanisms leading to cardiotoxicity are postulated to include mitochondrial dysfunction, either as a primary or secondary effect. Detecting direct effects on mitochondrial function, such as uncoupling of oxidative phosphorylation or inhibition of electron transport chain components, as well as identifying targets within the mitochondrial electron transport chain, can be accomplished in vitro. Here, we examined the effects of the tyrosine kinase inhibitor drugs imatinib, dasatinib, sunitinib, and sorafenib on ATP content in H9c2 cells grown under conditions where cells are either glycolytically or aerobically poised. Furthermore, we measured respiratory capacity of isolated rat heart mitochondria in the presence of the four kinase inhibitors and examined their effect on each of the oxidative phosphorylation complexes. Of the four kinase inhibitors examined, only sorafenib directly impaired mitochondrial function at clinically relevant concentrations, potentially contributing to the cytotoxic effect of the drug. For the other three kinase inhibitors lacking direct mitochondrial effects, altered kinase and other signaling pathways, are a more reasonable explanation for potential toxicity.
    Full-text · Article · Aug 2008 · Toxicological Sciences
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    ABSTRACT: Mitochondrial toxicity is increasingly implicated in a host of drug-induced organ toxicities, including hepatotoxicity. Nefazodone was withdrawn from the U.S. market in 2004 due to hepatotoxicity. Accordingly, we evaluated nefazodone, another triazolopyridine trazodone, plus the azaspirodecanedione buspirone, for cytotoxicity and effects on mitochondrial function. In accord with its clinical disposition, nefazodone was the most toxic compound of the three, trazodone had relatively modest effects, whereas buspirone showed the least toxicity. Nefazodone profoundly inhibited mitochondrial respiration in isolated rat liver mitochondria and in intact HepG2 cells where this was accompanied by simultaneous acceleration of glycolysis. Using immunocaptured oxidative phosphorylation (OXPHOS) complexes, we identified Complex 1, and to a lesser amount Complex IV, as the targets of nefazodone toxicity. No inhibition was found for trazodone, and buspirone showed 3.4-fold less inhibition of OXPHOS Complex 1 than nefazodone. In human hepatocytes that express cytochrome P450, isoform 3A4, after 24 h exposure, nefazodone and trazodone collapsed mitochondrial membrane potential, and imposed oxidative stress, as detected via glutathione depletion, leading to cell death. Our results suggest that the mitochondrial impairment imposed by nefazodone is profound and likely contributes to its hepatotoxicity, especially in patients cotreated with other drugs with mitochondrial liabilities.
    Full-text · Article · Jul 2008 · Toxicological Sciences
  • James W Simpkins · James A Dykens
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    ABSTRACT: Oxidative stress, bioenergetic failure and mitochondrial dysfunction are all implicated in the etiology of neurodegenerative diseases such as Alzheimer's disease (AD). The mitochondrial involvement in neurodegenerative diseases reflects the regulatory role mitochondrial failure plays in both necrotic cell death and apoptosis. The potent feminizing hormone, 17 beta-estradiol (E2), is neuroprotective in a host of cell and animal models of stroke and neurodegenerative diseases. The discovery that 17alpha-estradiol, an isomer of E2, is equally as neuroprotective as E2 yet is >200-fold less active as a hormone, has permitted development of novel, more potent analogs where neuroprotection is independent of hormonal potency. Studies of structure-activity relationships and mitochondrial function have led to a mechanistic model in which these steroidal phenols intercalate into cell membranes where they block lipid peroxidation reactions, and are in turn recycled. Indeed, the parental estrogens and novel analogs stabilize mitochondria under Ca(2+) loading otherwise sufficient to collapse membrane potential. The neuroprotective and mitoprotective potencies for a series of estrogen analogs are significantly correlated, suggesting that these compounds prevent cell death in large measure by maintaining functionally intact mitochondria. This therapeutic strategy is germane not only to sudden mitochondrial failure in acute circumstances, such as during a stroke or myocardial infarction, but also to gradual mitochondrial dysfunction associated with chronic degenerative disorders such as AD.
    No preview · Article · Mar 2008 · Brain Research Reviews
  • J.A. Dykens
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    ABSTRACT: James A Dykens earned his PhD from the University of Maine studying photosensitized oxidative physiology of corals on Australia's Great Barrier Reef. He pursued postdoctoral research in the Department of Pharmacology at New York University Medical Center, where he studied tryptophan metabolism and hemoglobin biochemistry. His work at NYU was among the first to propose the oxidative pathology of excitotoxicity and its role in the etiology of neurodegenerative diseases. He was on faculty at Grinnell College prior to joining the Immunopathology Department at Warner-Lambert, Parke-Davis Pharmaceutical Research, in Ann Arbor (now Pfizer). At Parke-Davis, he continued his work on oxidative neuropathology, using electron paramagnetic resonance spectroscopy to characterize free radical production during mitochondrial failure. In 1996, he joined MitoKor in San Diego (now Migenix) in order to focus on the mitochondrial etiology of both chronic degenerative diseases and the cytotoxicity resulting from acute ischemia-reperfusion. Dr Dykens founded EyeCyte Therapeutics to develop novel, nonhormonal, cytoprotective steroids to preserve retinal function and vision in blinding diseases.
    No preview · Chapter · Dec 2007
  • James A Dykens · Yvonne Will
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    ABSTRACT: Mitochondrial dysfunction is increasingly implicated in the etiology of drug-induced toxicities. Members of diverse drug classes undermine mitochondrial function, and among the most potent are drugs that have been withdrawn from the market, or have received Black Box warnings from the FDA. To avoid mitochondrial liabilities, routine screens need to be positioned within the drug-development process. Assays for mitochondrial function, cell models that better report mitochondrial impairment, and new animal models that more faithfully reflect clinical manifestations of mitochondrial dysfunction are discussed in the context of how such data can reduce late stage attrition of drug candidates and can yield safer drugs in the future.
    No preview · Article · Oct 2007 · Drug Discovery Today
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    ABSTRACT: Mitochondrial impairment is increasingly implicated in the etiology of toxicity caused by some thiazolidinediones, fibrates, and statins. We examined the effects of members of these drug classes on respiration of isolated rat liver mitochondria using a phosphorescent oxygen sensitive probe and on the activity of individual oxidative phosphorylation (OXPHOS) complexes using a recently developed immunocapture technique. Of the six thiazolidinediones examined, ciglitazone, troglitazone, and darglitazone potently disrupted mitochondrial respiration. In accord with these data, ciglitazone and troglitazone were also potent inhibitors of Complexes II+III, IV, and V, while darglitazone predominantly inhibited Complex IV. Of the six statins evaluated, lovastatin, simvastatin, and cerivastatin impaired mitochondrial respiration the most, with simvastatin and lovastatin impairing multiple OXPHOS Complexes. Within the class of fibrates, gemfibrozil more potently impaired respiration than fenofibrate, clofibrate, or ciprofibrate. Gemfibrozil only modestly inhibited Complex I, fenofibrate inhibited Complexes I, II+III, and V, and clofibrate inhibited Complex V. Our findings with the two complementary methods indicate that (1) some members of each class impair mitochondrial respiration, whereas others have little or no effect, and (2) the rank order of mitochondrial impairment accords with clinical adverse events observed with these drugs. Since the statins are frequently co-prescribed with the fibrates or thiazolidinediones, various combinations of these three drug classes were also analyzed for their mitochondrial effects. In several cases, the combination additively uncoupled or inhibited respiration, suggesting that some combinations are more likely to yield clinically relevant drug-induced mitochondrial side effects than others.
    No preview · Article · Oct 2007 · Toxicology and Applied Pharmacology
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    ABSTRACT: Many highly proliferative cells generate almost all ATP via glycolysis despite abundant O(2) and a normal complement of fully functional mitochondria, a circumstance known as the Crabtree effect. Such anaerobically poised cells are resistant to xenobiotics that impair mitochondrial function, such as the inhibitors rotenone, antimycin, oligomycin, and compounds like carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), that uncouple the respiratory electron transfer system from phosphorylation. These cells are also resistant to the toxicity of many drugs whose deleterious side effect profiles are either caused, or exacerbated, by impairment of mitochondrial function. Drug-induced mitochondrial toxicity is shown by members of important drug classes, including the thiazolidinediones, statins, fibrates, antivirals, antibiotics, and anticancer agents. To increase detection of drug-induced mitochondrial effects in a preclinical cell-based assay, HepG2 cells were forced to rely on mitochondrial oxidative phosphorylation rather than glycolysis by substituting galactose for glucose in the growth media. Oxygen consumption doubles in galactose-grown HepG2 cells and their susceptibility to canonical mitochondrial toxicants correspondingly increases. Similarly, toxicity of several drugs with known mitochondrial liabilities is more readily apparent in aerobically poised HepG2 cells compared to glucose-grown cells. Some drugs were equally toxic to both glucose- and galactose-grown cells, suggesting that mitochondrial impairment is likely secondary to other cytotoxic mechanisms.
    Preview · Article · Jul 2007 · Toxicological Sciences
  • James A Dykens · Lisa D Marroquin · Yvonne Will
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    ABSTRACT: Mitochondrial dysfunction is increasingly implicated in the etiology of drug-induced toxicities and negative side-effect profiles. Early identification of mitochondrial liabilities for new chemical entities is therefore crucial for avoiding late-stage attrition during drug development. Limitations of traditional methods for assessing mitochondrial dysfunction have discouraged routine evaluation of mitochondrial liabilities. To circumvent this bottleneck, a high-throughput screen has been developed that measures oxygen consumption; one of the most informative parameters for the assessment of mitochondrial status. This technique has revealed that some, but not all, members of many major drug classes have mitochondrial liabilities. This dichotomy encourages optimism that efficacy can be disassociated from mitochondrial toxicity, resulting in safer drugs in the future.
    No preview · Article · Apr 2007 · Expert Review of Molecular Diagnostics
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    ABSTRACT: Neuroprotective effects of estrogens have been shown in various in vitro and in vivo models, but the mechanisms underlying protection by estrogen are not clear. Mounting evidence suggests antioxidant effects contribute to the neuroprotective effects of estrogens. In the present study, we assessed the protective effects of estrogens against H2O2-induced toxicity in human neuroblastoma cells and the potential mechanisms involved in this protection. We demonstrate that 17beta-estradiol (17beta-E2) increases cell survival against H2O2 toxicity in human neuroblastoma cells. 17beta-E2 effectively reduced lipid peroxidation induced by 5-min H2O2 exposure. Furthermore, 17beta-E2 exerts the protective effects by maintaining intracellular Ca2+ homeostasis, attenuating ATP depletion, ablating mitochondrial calcium overloading, and preserving mitochondrial membrane potential. Two nonfeminizing estrogens, 17alpha- and ent-estradiol, were as effective as 17beta-E2 in increasing cell survival, alleviating lipid peroxidation, preserving mitochondrial function, and maintaining intracellular glutathione levels and Ca2+ homeostasis against H2O2 insult. Moreover, the estrogen receptor antagonist fulvestrant (ICI 182,780) did not block effects of 17beta-E2, but increased cell survival and blunted intracellular Ca2+ increases. However, these estrogens failed to reduce cytosolic reactive oxygen species, even at concentrations as high as 10 microM. In conclusion, estrogens exert protective effects against oxidative stress by inhibiting lipid peroxidation and subsequently preserving Ca2+ homeostasis, mitochondrial membrane potential, and ATP levels.
    Full-text · Article · Aug 2006 · Molecular Pharmacology
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    Meharvan Singh · James A Dykens · James W Simpkins
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    ABSTRACT: Estrogens are gonadal steroid hormones that are present in the circulation of both males and females and that can no longer be considered within the strict confines of reproductive function. In fact, the bone, the cardiovascular system, and extrahypothalamic regions of the brain are now well-established targets of estrogens. Among the numerous aspects of brain function regulated by estrogens are their effects on mood, cognitive function, and neuronal viability. Here, we review the supporting evidence for estrogens as neuroprotective agents and summarize the various mechanisms that may be involved in this effect, focusing particularly on the mitochondria as an important target. On the basis of this evidence, we discuss the clinical applicability of estrogens in treating various age-related disorders, including Alzheimer disease and stroke, and identify the caveats that must be considered.
    Full-text · Article · Jun 2006 · Experimental Biology and Medicine