Browsing by Subject "Experimental & clinical pharmacology"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Application of pharmacometrics for covariate selection and dose optimization of tacrolimus in adult kidney transplant recipients
    (2012-12) Passey, Chaitali
    In spite of rigorous dose adjustments by way of therapeutic drug monitoring, a large proportion of kidney transplant recipients are unable to achieve the target tacrolimus trough concentrations. This is attributed to the narrow therapeutic window of the drug (10-15 ng/mL) and large inter-individual variability in pharmacokinetic parameter such as clearance. There is a need for development of clinical dosing models that can help prospectively predict the dose for an individual, especially in the critical period immediately post-transplant. Therefore, we established and quantified the effect of clinical and genetic factors on tacrolimus clearance (CL/F) using a large population of adult kidney transplant recipients. Tacrolimus troughs (n=11823) from 681 transplant recipients over the first 6-months post-transplant were analyzed using non-linear mixed effects modeling approach in NONMEM®. The troughs were characterized by a steady state infusion model. Covariates were analyzed using a forward selection (p<0.0.1) backward elimination (p<0.001) approach. We formulated an equation that predicts the CL/F of an individual based on the days post-transplant, presence of the highly influential CYP3A5*1 genotype, transplant at a steroid sparing center, age and concomitant use of a calcium channel blocker at the time of trough collection. The CL/F was seen to decrease with increasing days post transplant, transplant at a steroid sparing center and use of a calcium channel blocker. Transplant recipients with the CYP3A5*1/*3 and *1/*1 genotypes had a CL/F that was 70% and 100% higher, respectively, than those with the CYP3A5*3/*3 genotype. The dose required in order to achieve a particular target trough can be prospectively determined from this equation. The above equation was validated in a separate cohort of adult kidney transplant recipients. The equation was assessed by predictive performance in 795 transplant recipients (n=13,968 troughs) receiving tacrolimus using bias and precision. Assessment was done for the initial troughs as well as for all troughs over the entire 6 months. The equation has low bias (0.2 ng/ml) and good precision (within ± 20% for a typical trough of 10 ng/mL) in predicting initial troughs and could be safely used to predict initial doses. This is critical as an accurate initial dose will help the recipient to get to therapeutic range faster and reduce the number of out-of-range troughs. For all the troughs, over the 6 months post-transplant, the equation did better than a basic model with no covariates but had higher bias and imprecision than the prediction of initial troughs. We were presented with 119 single nucleotide polymorphisms (SNP) in this study. Due to software limitations and impracticalities associated with such a large number of covariates, we developed and validated a novel "winnowing method" of covariate selection that is able to test and select SNPs in combination. This method uses random selection, repetitions of generalized additive modeling in the R statistical package and post-hoc estimates from NONMEM®. The salient feature of this method is the creation of an index, ranging from 0-1, that defines the relative importance of the SNP when tested in a combination. With this method, we were able to select 26 SNPs out of the 119 SNPs, which included the well-established CYP3A5*1 SNP. We validated this method using a simulated dataset. In the validation dataset, the winnowing method was able to select all the important SNPs. The type I and type II error rates were 9% and 0% respectively. Although NONMEM® is the oldest and most widely used population pharmacokinetics software, several other software packages are now becoming available such as the Phoenix® NLMETM. One desirable feature in this new software package is a graphical user interface and menu-driven covariate selection options. Therefore, we compared these two software packages in terms of covariates selected and predictive performance using both clinical and simulated data. For the tacrolimus data, NONMEM® predictions had lower bias and imprecision as compared to Phoenix® NLMETM. For the clinical data, NONMEM® predictions had higher bias but were more precise than the Phoenix® NLMETM predictions.>
  • Thumbnail Image
    Item
    The effect of obesity and high fat diet on cardiovascular risk and intestinal drug metabolism
    (2015-01) Tam, Harrison Kin Chi
    Introduction: Childhood obesity is a major epidemic for many industrialized countries, especially in countries where a high fat diet is prevalent. The purpose of this thesis was to examine the effect of obesity and high fat diet on cardiovascular risk factors (e.g. xanthine oxidase activity and uric acid formation) and on small intestinal drug metabolism. Methods: An assay measuring the production of 7-hydroxy-lumazine was developed as a probe for xanthine oxidase activity. This lumazine assay was used to study xanthine oxidase as a risk factor for cardiovascular disease in obese children compared to normal-weight children, and this assay was also utilized to study xanthine oxidase activity in obese adolescents before and after weight loss to determine xanthine oxidase's role in mediating changes in blood pressure due to weight loss. In order to study the effects of obesity and high fat diet on small intestinal drug metabolism, UGT activity was assessed in a rat model of diet-induced obesity using in vitro phenotyping methodologies. Results: Plasma xanthine oxidase activity was increased in obese children and was associated with BMI z-score and waist circumference, but was not associated with blood pressure. Weight loss via meal replacement therapy led to decreases in both uric acid production and excretion, but no significant change in plasma levels. Increases in UGT activity were observed in both obesity resistant and obesity prone rats when they were fed a high fat diet, even though the obesity resistant rats didn't gain a significant amount of weight when fed a high fat diet. Conclusions: Changes in xanthine oxidase activity and intestinal UGT activity may be due to changes to diets and gut microbiota. High fat diets have been shown to alter the gut microbiota and increase plasma LPS levels. Therefore, a high fat diet and not obesity may be responsible for both the changes in xanthine oxidase and UGT activities, and future studies on obesity and CVD risk or alterations in drug metabolizing enzymes should focus on monitoring diets and changes to the gut microbiota.
  • Thumbnail Image
    Item
    The impact of gene transfer of arginine decarboxylase to the central nervous system on opioid analgesic tolerance
    (2014-12) Churchill, Caroline Catharine
    Opioid-based pharmacotherapy remains the most commonly prescribed treatment for patients suffering from neuropathic pain conditions. Although opioids are effective for treating neuropathic pain, when used chronically the development of adverse side effects, such as opioid analgesic tolerance, can develop. Previous studies have shown that exogenously delivered agmatine, (decarboxylated L-arginine) can prevent the development of opioid analgesic tolerance, dependence, and self-administration. This study investigated the impact of intrathecal adeno-associated virus serotype 5-human arginine decarboxylase (AAV5-hADC) in models of opioid analgesic tolerance. Pharmacological dose-response curves were collected from two cohorts using two different models of opioid analgesic tolerance. Tissues from the central nervous system of the subjects were further analyzed for hADC gene expression and for spinal agmatine levels. Investigations of the choroid plexus as a target for intrathecal gene therapeutics were also conducted. Region-directed genetic modulation of the agmatine metabolic pathway within the central nervous system could be a highly innovative approach for the control of opioid tolerance and other neuroplasticity disorders.
  • Thumbnail Image
    Item
    Mitigating oxidative stress in childhood cerebral adrenoleukodystrophy -an investigation of N-acetylcysteine pharmacology
    (2014-02) Zhou, Jie
    Adrenoleukodystrophy (ALD) is an X-linked genetic disorder which affects the adrenal glands, peripheral neuronal system, the spinal cord and white matter of central nervous system (CNS). It is a progressive neurology disorder with incidence of 1 in 17,000 newborns. ALD is caused by mutations in the ABCD1 gene, which encodes the peroxisomal membrane transporter for transporting very long chain fatty acids (VLCFAs) into peroxisomes for degradation. As a result, VLCFAs accumulate in the plasma and tissues of ALD patients. Elevated VLCFAs along with ABCD1 gene mutations are used for the diagnosis of ALD. ALD has various clinical phenotypes. Childhood cerebral adrenoleukodystrophy (CCALD) is the cerebral form of ALD that affects young boys (4~10 years of age), causing progressive, debilitating effects on the CNS leading to death within a few years. The pathophysiology of CCALD is only partially understood, but it is known that VLCFAs accumulate in the plasma, brain and other tissues in CCALD patients, which can cause oxidative stress and downstream neurodegeneration. Recently, oxidative stress, the accumulation of free radicals (reactive molecules), has been shown to cause CNS neurodegeneration and play a major role in CCALD pathophysiology. Currently, the most successful treatment for CCALD is hematopoietic stem cell transplantation (HSCT), which halts disease progression and extends life when CCALD is treated early. But it is much less effective for late-stage CCALD. Based on evidence that oxidative stress plays a role in the disease, the Blood and Marrow transplantation group at University of Minnesota has utilized N-acetylcysteine (NAC) as adjunctive therapy together with HSCT in late-stage CCALD. This combinatorial approach has improved survival rate from 36% to 84% compared to HSCT only in a cohort study (Miller et al., 2011). However, NAC's mechanisms of action are still unclear in CCALD patients. As an FDA-approved drug, NAC is used as an antidote for acetaminophen overdose and as a mucolytic agent to reduce symptoms associated with cystic fibrosis. It has gained renewed attention as a potential therapy for a number of conditions including pulmonary, neurological, psychiatric, and cardiovascular diseases. With a long history of clinical use, several mechanisms including antioxidative and anti-inflammatory activities have been proposed as the basis for its therapeutic effects. However, the exact molecular mechanism by which NAC improves the survival rate of CCALD patients is still unclear. And this missing piece of information, which is the basis for my research work, is required to further optimize the therapy. In my thesis, four research projects were designed and implemented to address the pharmacology of NAC in CCALD related biological models. The first study was to investigate the downstream signaling molecules induced by NAC in the plasma of CCALD patients. Heme oxygenase-1 (HO-1) and ferritin were examined in CCALD patients before and after NAC exposure. Based on the clinical study results that the expression of HO-1 and downstream ferritin were induced by NAC, the second study was further designed in oligodendrocytes, which are CNS glial cells and closely related to demyelination and neurodegneration, to investigate the cytoprotective role of HO-1 induced by NAC. Moreover, we also tried to delineate the role of accumulation of VLCFAs in CCALD and its relationship with oxidative stress and mitochondria. The third study was designed in oligodendrocytes to investigate whether mitochondria and oxidative stress status are affected by pathophysiological concentrations of VLCFAs and if so, whether NAC could be used to reverse this condition. Finally, the fourth pharmacokinetic/pharmacodynamics study was designed and implemented in wild-type mice to address the relationship between NAC concentration and pharmacodynamic endpoints in vivo. This study is also critical to determine the biotransformation of NAC in vivo.The results from my studies indicate HO-1 as the newly discovered downstream mediators for NAC action. Studies also show for the first time that depletion of mitochondrial glutathione (mtGSH) is the pathological cause for CCALD, and that targeting mitochondrial dysfunction can be a potential effective intervention for CCALD patients. In addition, GSH levels, redox ratio, HO-1 and ferritin levels can serve as biomarkers or pharmacodynamic endpoints to evaluate NAC efficacy. In the long term, characterization of NAC mechanisms of action will help to optimize therapy in CCALD patients. In addition, the information generated from my studies on the efficacy of NAC in CCALD is also applicable to other neurodegenerative disorders sharing similar pathologies such as Gaucher's disease, multiple Sclerosis, Alzheimer's disease etc.