The widespread usage of methylphenidate (MPH) in the pediatric population has received considerable attention due to its potential effect on child development. For the first time a physiologically ...based pharmacokinetic (PBPK) model has been developed in juvenile and adult humans and nonhuman primates to quantitatively evaluate species- and age-dependent enantiomer specific pharmacokinetics of MPH and its primary metabolite ritalinic acid. The PBPK model was first calibrated in adult humans using in vitro enzyme kinetic data of MPH enantiomers, together with plasma and urine pharmacokinetic data with MPH in adult humans. Metabolism of MPH in the small intestine was assumed to account for the low oral bioavailability of MPH. Due to lack of information, model development for children and juvenile and adult nonhuman primates primarily relied on intra- and interspecies extrapolation using allometric scaling. The juvenile monkeys appear to metabolize MPH more rapidly than adult monkeys and humans, both adults and children. Model prediction performance is comparable between juvenile monkeys and children, with average root mean squared error values of 4.1 and 2.1, providing scientific basis for interspecies extrapolation of toxicity findings. Model estimated human equivalent doses in children that achieve similar internal dose metrics to those associated with pubertal delays in juvenile monkeys were found to be close to the therapeutic doses of MPH used in pediatric patients. This computational analysis suggests that continued pharmacovigilance assessment is prudent for the safe use of MPH.
An association between increased serum concentrations of perfluoroalkyl substances (PFAS) such as perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) and early menopause has been reported ...(Knox et al., 2011; Taylor et al., 2014). This association may be explained by the fact that women who underwent menopause no longer excrete PFAS through menstruation. Our objective was to assess how much of the epidemiologic association between PFAS and altered timing of menopause might be explained by reverse causality. We extended a published population life-stage physiologically-based pharmacokinetic (PBPK) model of PFOS and PFOA characterized by realistic distributions of physiological parameters including age at menopause. We then conducted Monte Carlo simulations to replicate the Taylor population (Taylor et al., 2014) and the Knox population (Knox et al., 2011). The analysis of the simulated data overall showed a pattern of results that was comparable to those reported in epidemiological studies. For example, in the simulated Knox population (ages 42–51) the odds ratio (OR) for menopause in the fifth quintile of PFOA compared to those in the first quintile was 1.33 (95% CI 1.26–1.40), whereas the reported OR was 1.4 (95% CI 1.1–1.8). Using our model structure, a substantial portion of the associations reported can be explained by pharmacokinetics.
•Blood PFAS concentrations increase in menopausal women because of reduced loss in menses.•The PFAS Monte Carlo PBPK model reproduces epidemiological population characteristics.•Pharmacokinetics explain a substantial portion of the associations between serum PFAS and onset of menopause.
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An association between serum levels of perfluoroalkyl substances (PFAS) and endometriosis has recently been reported in an epidemiologic study. Oral contraceptive use to treat dysmenorrhea (pelvic ...pain associated with endometriosis) could potentially influence this association by reducing menstrual fluid loss, a route of excretion for PFAS. In this study, we aimed to evaluate the influence of differential oral contraceptive use on the association between PFAS and endometriosis. We used a published life-stage physiologically based pharmacokinetic (PBPK) model to simulate plasma levels of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from birth to age at study participation (range 18–44years). In the simulated population, PFAS level distributions matched those for controls in the epidemiologic study. Prevalence and geometric mean duration (standard deviation SD) of oral contraceptive use in the simulated women were based on data from the National Health and Nutrition Examination Survey; among the women with endometriosis the values were, respectively, 29% and 6.8 (3.1) years; among those without endometriosis these values were 18% and 5.3 (2.8) years. In simulations, menstrual fluid loss (ml/cycle) in women taking oral contraceptives was assumed to be 56% of loss in non-users. We evaluated the association between simulated plasma PFAS concentration and endometriosis in the simulated population using logistic regression. Based on the simulations, the association between PFAS levels and endometriosis attributable to differential contraceptive use had an odds ratio (95% CI) of 1.05 (1.02, 1.07) for a loge unit increase in PFOA and 1.03 (1.02, 1.05) for PFOS. In comparison, the epidemiologic study reported odds ratios of 1.62 (0.99, 2.66) for PFOA and 1.25 (0.87, 1.80) for PFOS. Our results suggest that the influence of oral contraceptive use on the association between PFAS levels and endometriosis is relatively small.
•We performed a quantitative bias analysis of a study of PFAS and endometriosis•PFAS levels were simulated in women with and without endometriosis.•Simulated levels of PFOS and PFOA were affected by oral contraceptive (OC) use.•Failure to consider OC use caused bias in the PFAS-endometriosis association
A mechanistic tissue composition Model incorporating passive and active transport for the prediction of steady-state tissue:plasma partition coefficients (Kt:pl) of chemicals in multiple mammalian ...species was used to assess interindividual and interspecies variability. This approach predicts Kt:pl using chemical lipophilicity, pKa, phospholipid membrane binding, and the unbound plasma fraction, together with tissue fractions of water, neutral lipids, neutral and acidic phospholipids, proteins, and pH. Active transport Kt:pl is predicted using Michaelis–Menten transport parameters. Species-specific biological properties were identified from 126 peer reviewed journal articles, listed in the Supporting Information, for Mouse, rat, guinea pig, rabbit, beagle dog, pig, Monkey, and human species. Means and coefficients of variation for biological properties were used in a Monte Carlo analysis to assess variability. The results show Kt:pl interspecies variability for the brain, fat, heart, kidney, liver, lung, muscle, red blood cell, skin, and spleen, but uncertainty in the estimates obscured some differences. Compounds undergoing active transport are shown to have concentration-dependent Kt:pl. This tissue composition-based mechanistic Model can be used to predict Kt:pl for organic chemicals across eight species and 10 tissues, and can be an important component in drug development when scaling Kt:pl from animal Models to humans.
Organophosphorus (OP) pesticides and nerve agents have been designed to inhibit the hydrolysis of the neurotransmitter acetylcholine by covalently binding to the active site serine of ...acetylcholinesterase while Alzheimer drugs and prophylactics, such as tacrine, are characterized by reversible binding. Historically, the guinea pig has been believed to be the best non-primate model for OP toxicology and medical countermeasure development because, similarly to humans, guinea pigs have low amounts of circulating OP metabolizing carboxylesterase. To explore the hypothesis that guinea pigs are the appropriate responder species for OP toxicology and medical countermeasure development, guinea pig acetylcholinesterase (gpAChE) was cloned into pENTR/D-TOPO, recombined into pT-Rex-DEST30 and expressed in Human Embryonic Kidney 293 cells. Recombinant gpAChE was purified to a specific activity of 800 U/mg using size exclusion and immobilized nickel affinity chromatography, with purity confirmed by gel electrophoresis. Ellman's assay was used to enzymatically characterize gpAChE, identifying a K(M) of 154±18.7 µmol L(-1) and a k(cat) of 4.79x10(4)±5.26x10(2) /sec. Apparent gpAChE IC50's for diisopropylfluorophosphate, dicrotophos, paraoxon, and an Alzheimer's drug, tacrine, were found to be 10.1±1.98, 337±108, 1.02±0.29 and 0.30±0.01 µmol L(-1), respectively. Apparent gpAChE inhibition constants for diisopropylfluorophosphate, dicrotophos, paraoxon, and tacrine were found to be 8.40±0.60, 4.50±0.30, 0.29±0.01 and 0.42±0.07 µmol L(-1), respectively. Lineweaver-Burk plots confirmed tacrine as a mixed inhibitor and paraoxon, dicrotophos and diisopropylfluorophosphate as irreversible non-competitive inhibitors. gpAChE bimolecular rate constants for diisopropylfluorophosphate, dicrotophos and paraoxon were found to be 1.44±0.33x10(4), 1.56±0.12x10(3) and 4.57± 0.23x10(5) L µmol(-1) min(-1), respectively. Although the blood levels of OP metabolizing carboxylesterases in the guinea pig are similar to the low levels in human blood, the gpAChE is different in its enzymology. Therefore, medical countermeasures against OP intoxication should be tested for efficacy with the recombinant form of gpAChE prior to initiating animal studies.
Multiple oximes have been synthesized and evaluated for use as countermeasures against chemical warfare nerve agents. The current U.S. military and civilian oxime countermeasure, ...2-(hydroxyimino)methyl-1-methylpyridin-1-ium chloride (2-PAM), is under consideration for replacement with a more effective acetylcholinesterase reactivator, 1,1’-methylenebis{4-hydroxyiminomethyl}pyridinium dimethanesulfonate (MMB-4). Kinetic data in the scientific literature for MMB-4 are limited; therefore, a physiologically based pharmacokinetic (PBPK) model was developed for a structurally related oxime, 1,1’-trimethylenebis{4-hydroximinomethyl}pyridinium dibromide. Based on a previous model structure for the organophosphate diisopropylfluorophosphate, the model includes key sites of acetylcholinesterase inhibition (brain and diaphragm), as well as fat, kidney, liver, rapidly perfused tissues and slowly perfused tissues. All tissue compartments are diffusion limited. Model parameters were collected from the literature, predicted using quantitative structure–property relationships or, when necessary, fit to available pharmacokinetic data from the literature. The model was parameterized using rat plasma, tissue and urine time course data from intramuscular administration, as well as human blood and urine data from intravenous and intramuscular administration; sensitivity analyses were performed. The PBPK model successfully simulates rat and human data sets and has been evaluated by predicting intravenous mouse and intramuscular human data not used in the development of the model. Monte Carlo analyses were performed to quantify human population kinetic variability in the human evaluation data set. The model identifies potential pharmacokinetic differences between rodents and humans, indicated by differences in model parameters between species. The PBPK model can be used to optimize the dosing regimen to improve oxime therapeutic efficacy in a human population.
Organophosphates are a group of pesticides and chemical warfare nerve agents that inhibit acetylcholinesterase, the enzyme responsible for hydrolysis of the excitatory neurotransmitter acetylcholine. ...Numerous structural variants exist for this chemical class, and data regarding their toxicity can be difficult to obtain in a timely fashion. At the same time, their use as pesticides and military weapons is widespread, which presents a major concern and challenge in evaluating human toxicity. To address this concern, a quantitative structure–activity relationship (QSAR) was developed to predict pentavalent organophosphate oxon human acetylcholinesterase bimolecular rate constants. A database of 278 three-dimensional structures and their bimolecular rates was developed from 15 peer-reviewed publications. A database of simplified molecular input line entry notations and their respective acetylcholinesterase bimolecular rate constants are listed in Supplementary Material, Table I. The database was quite diverse, spanning 7 log units of activity. In order to describe their structure, 675 molecular descriptors were calculated using AMPAC 8.0 and CODESSA 2.7.10. Orthogonal projection to latent structures regression, bootstrap leave-random-many-out cross-validation and y-randomization were used to develop an externally validated consensus QSAR model. The domain of applicability was assessed by the William’s plot. Six external compounds were outside the warning leverage indicating potential model extrapolation. A number of compounds had residuals >2 or <−2, indicating potential outliers or activity cliffs. The results show that the
HOMO
–
LUMO energy gap
contributed most significantly to the binding affinity. A mean training
R
2
of 0.80, a mean test set
R
2
of 0.76 and a consensus external test set
R
2
of 0.66 were achieved using the QSAR. The training and external test set RMSE values were found to be 0.76 and 0.88. The results suggest that this QSAR model can be used in physiologically based pharmacokinetic/pharmacodynamic models of organophosphate toxicity to determine the rate of acetylcholinesterase inhibition.
The current standard of care for treatment of organophosphate (OP) poisoning includes pretreatment with the weak reversible acetylcholinesterase (AChE) inhibitor pyridostigmine bromide. Because this ...drug is an AChE inhibitor, similar side effects exist as with OP poisoning. In an attempt to provide a therapeutic capable of mitigating AChE inhibition without such side effects, high-throughput screening was performed to identify a compound capable of increasing the catalytic activity of AChE. Herein, two such novel positive allosteric modulators (PAMs) of AChE are presented. These PAMs increase AChE activity threefold, but they fail to upshift the apparent IC50 of a variety of OPs. Further development and optimization of these compounds may lead to pre- and/or postexposure therapeutics with broad-spectrum efficacy against pesticide and nerve agent poisoning. In addition, they could be used to complement the current therapeutic standard of care to increase the activity of uninhibited AChE, potentially increasing the efficacy of current therapeutics in addition to altering the therapeutic window.
Organophosphate (OP) nerve agents such as sarin, soman, tabun, and O-ethyl S-2-(diisopropylamino) ethyl methylphosphonothioate (VX) do not react solely with acetylcholinesterase (AChE). Evidence ...suggests that cholinergic-independent pathways over a wide range are also targeted, including serine proteases. These proteases comprise nearly one-third of all known proteases and play major roles in synaptic plasticity, learning, memory, neuroprotection, wound healing, cell signaling, inflammation, blood coagulation, and protein processing. Inhibition of these proteases by OP was found to exert a wide range of noncholinergic effects depending on the type of OP, the dose, and the duration of exposure. Consequently, in order to understand these differences, in silico biologically based dose-response and quantitative structure-activity relationship (QSAR) methodologies need to be integrated. Here, QSAR were used to predict OP bimolecular rate constants for trypsin and α-chymotrypsin. A heuristic regression of over 500 topological/constitutional, geometric, thermodynamic, electrostatic, and quantum mechanical descriptors, using the software Ampac 8.0 and Codessa 2.51 (SemiChem, Inc., Shawnee, KS), was developed to obtain statistically verified equations for the models. General models, using all data subsets, resulted in R
2
values of .94 and .92 and leave-one-out Q
2
values of 0.9 and 0.87 for trypsin and α-chymotrypsin. To validate the general model, training sets were split into independent subsets for test set evaluation. A y-randomization procedure, used to estimate chance correlation, was performed 10,000 times, resulting in mean R
2
values of .24 and .3 for trypsin and α-chymotrypsin. The results show that these models are highly predictive and capable of delineating the complex mechanism of action between OP and serine proteases, and ultimately, by applying this approach to other OP enzyme reactions such as AChE, facilitate the development of biologically based dose-response models.
Precision agriculture informed by electromagnetic induction surveys could reduce groundwater withdrawals and nitrogen leaching from coarse soils. However, coarse, nonsaline soils often have extremely ...narrow ranges of mapped apparent electrical conductivity (ECa) and the efficacy of ECa for predicting soil physical properties is uncertain in this context. For this reason, it is also uncertain as to whether electromagnetic induction surveys are valuable for guiding precision agriculture on coarse, nonsaline soils. Additionally, the need to ground-truth electromagnetic induction surveys for individual agricultural fields with soil sampling and statistical model development hampers adoption of precision agriculture at the regional scale. Our research objectives were to quantify the variation in mapped ECa and develop statistical relationships between ECa and soil physical properties both within and across several agricultural fields in the Wisconsin Central Sands, a distinct hydropedological region with coarse, glaciolacustrine soils. We used nonparametric correlation analyses to identify associations and quantile regression, a statistical approach with no assumptions of normality or homoscedasticity, to identify predictive relationships between ECa and soil physical properties. We found strong, significant (p < 0.05) correlative and predictive relationships between ECa and topsoil (0–0.3 m) particle size fraction, organic matter content, and field capacity within and across several fields. Yet, we did not observe many significant relationships between ECa and subsoil (0.5–0.6 m) physical properties, which we attribute to heterogeneous soil layering and the low depth resolution of our soil sampling approach. Our findings demonstrate that proximal sensing of ECa can identify intrafield variability in soil properties under extremely narrow observed ECa ranges (0–11 mS m−1). Moreover, we found that interfield quantile regression models predicted soil physical properties across several agroecosystems. Heteroscedasticity was present in interfield ECa relationships with physical properties, which resulted in the need for different quantile regression models across the conditional distribution. The flexibility for accommodating heteroscedasticity in soils and simplicity of modeled functions make quantile regression a promising approach for developing interfield or regional models of ECa to predict soil physical properties in distinct, hydropedological regions with coarse soils.
•Glaciolacustrine entisols and alfisols often have extremely narrow ECa ranges.•Narrow ECa ranges still predict texture, field capacity, and organic matter content.•ECa had strong relationships with soil properties across several fields in a region.•There is potential for using quantile regression to develop regional ECa models.
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