Automated bidirectional coupling of multiscale models of aerosol dosimetry: validation with subject-specific deposition data.

Assessing the toxicity of airborne particulate matter or the efficacy of inhaled drug depends upon accurate estimates of deposited fraction of inhaled materials. In silico approaches can provide important insights into site- or airway-specific deposition of inhaled aerosols in the respiratory system. In this study, we improved on our recently developed 3D/1D model that simulate aerosol transport and deposition in the whole lung over multiple breath cycles (J. Aerosol Sci 151:105647). A subject-specific multiscale lung model of a healthy male subject using computational fluid-particle dynamics (CFPD) in a 3D model of the oral cavity through the large bronchial airways entering each lobe was bidirectionally coupled with a recently improved Multiple Path Particle Dosimetry (MPPD) model to predict aerosol deposition over the entire respiratory tract over multiple breaths for four conditions matching experimental aerosol exposures in the same subject from which the model was developed. These include two particle sizes (1 and 2.9 µm) and two subject-specific breathing rates of ~300 ml/s (slow breathing) and ~750 ml/s (fast breathing) at a target tidal volume of 1 L. In silico predictions of retained fraction were 0.31 and 0.29 for 1 µm and 0.66 and 0.62 for 2.9 µm during slow and fast breathing, respectively, and compared well with experimental data (1 µm: 0.31±0.01 (slow) and 0.27±0.01 (fast), 2.9 µm: 0.63±0.03 (slow) and 0.68±0.02 (fast)). These results provide a great deal of confidence in the validity and reliability of our approach.

Influence of alveolar mixing and multiple breaths of aerosol intake on particle deposition in the human lungs.

Predictive dosimetry models play an important role in assessing health effect of inhaled particulate matter and in optimizing delivery of inhaled pharmaceutical aerosols. In this study, the commonly used 1D Multiple-Path Particle Dosimetry model (MPPD) was improved by including a mechanistically based model component for alveolar mixing of particles and by extending the model capabilities to account for multiple breaths of aerosol intake. These modifications increased the retained fraction of particles and consequently particle deposition predictions in the deep lung during tidal breathing. Comparison with an existing dataset (J. Aerosol Sci., 99:27-39, 2016) obtained under two breathing conditions referred to as slow and fast breathing showed significant differences in 1 µm particle deposition between predictions based on subject-specific breathing patterns and lung volume (slow: 30 ± 1%, fast: 21 ± 1%, (average ± standard deviation), N = 7) and measurements (slow: 43 ± 9%, fast: 30 ± 5%) when the prior version of MPPD (single breath and no mixing, J. Aerosol Sci., 151:105647, 2021) was used. Adding a mixing model and multiple breaths moved the predictions (slow: 34 ± 2%, fast:25 ± 2%) closer to the range of deposition measurements. For 2.9 µm particles, predictions from both the original (slow: 70 ± 2%, fast: 57 ± 2%) and the revised MPPD model (slow: 71 ± 2%, fast: 59 ± 3%) compared well with experiments (slow: 67 ± 8%, fast: 58 ± 10%). This was expected as suspended fraction of 2.9 µm particles was small and thus the addition of alveolar mixing and multi breath capability only slightly increased the retained fraction for particles of this size and greater. The revised 1D model improves dose predictions in the deep lung and support human risk assessment from exposure to airborne particles.

Baseline brain function in the preadolescents of the ABCD Study.

The Adolescent Brain Cognitive Development (ABCD) Study® is a 10-year longitudinal study of children recruited at ages 9 and 10. A battery of neuroimaging tasks are administered biennially to track neurodevelopment and identify individual differences in brain function. This study reports activation patterns from functional MRI (fMRI) tasks completed at baseline, which were designed to measure cognitive impulse control with a stop signal task (SST; N = 5,547), reward anticipation and receipt with a monetary incentive delay (MID) task (N = 6,657) and working memory and emotion reactivity with an emotional N-back (EN-back) task (N = 6,009). Further, we report the spatial reproducibility of activation patterns by assessing between-group vertex/voxelwise correlations of blood oxygen level-dependent (BOLD) activation. Analyses reveal robust brain activations that are consistent with the published literature, vary across fMRI tasks/contrasts and slightly correlate with individual behavioral performance on the tasks. These results establish the preadolescent brain function baseline, guide interpretation of cross-sectional analyses and will enable the investigation of longitudinal changes during adolescent development.

Efficient bi-directional coupling of 3D Computational Fluid-Particle Dynamics and 1D Multiple Path Particle Dosimetry lung models for multiscale modeling of aerosol dosimetry.

The development of predictive aerosol dosimetry models has been a major focus of environmental toxicology and pharmaceutical health research for decades. One-dimensional (1D) models successfully predict overall deposition averages but fail to accurately predict local deposition. Computational fluid-particle dynamics (CFPD) models provide site-specific predictions but at a computational cost that prohibits whole lung predictions. Thus, there is a need for developing multiscale strategies to provide a realistic subject-specific picture of the fate of inhaled aerosol in the lungs. CT-based 3D/CFPD models of the large airways were bidirectionally coupled with individualized 1D Navier-Stokes airflow and particle transport based upon the widely used Multiple Path Particle Dosimetry Model (MPPD). Distribution of airflows among lobes was adjusted by measured lobar volume changes observed in CT images between FRC and FRC + 1.5 L. As a test of the effectiveness of the coupling procedures, deposition modeling of previous 1 µm aerosol exposure studies was performed. The complete coupled model was run for 3 breaths, with the computation-intense portion being the 3D CFPD Lagrangian particle tracking calculation. The average deposition per breath was 11% in the combined multiscale model with site-specific doses available in the CFPD portion of the model and airway- or region-specific deposition available for the MPPD portion. In conclusion, the key methods developed in this study enable predictions of ventilation heterogeneities and aerosol deposition across the lungs that are not captured by 3D or 1D models alone. These methods can be used as the foundation for multi-scale modeling of the full respiratory system.

Genome-wide autozygosity is associated with lower general cognitive ability.

Inbreeding depression refers to lower fitness among offspring of genetic relatives. This reduced fitness is caused by the inheritance of two identical chromosomal segments (autozygosity) across the genome, which may expose the effects of (partially) recessive deleterious mutations. Even among outbred populations, autozygosity can occur to varying degrees due to cryptic relatedness between parents. Using dense genome-wide single-nucleotide polymorphism (SNP) data, we examined the degree to which autozygosity associated with measured cognitive ability in an unselected sample of 4854 participants of European ancestry. We used runs of homozygosity-multiple homozygous SNPs in a row-to estimate autozygous tracts across the genome. We found that increased levels of autozygosity predicted lower general cognitive ability, and estimate a drop of 0.6 s.d. among the offspring of first cousins (P=0.003-0.02 depending on the model). This effect came predominantly from long and rare autozygous tracts, which theory predicts as more likely to be deleterious than short and common tracts. Association mapping of autozygous tracts did not reveal any specific regions that were predictive beyond chance after correcting for multiple testing genome wide. The observed effect size is consistent with studies of cognitive decline among offspring of known consanguineous relationships. These findings suggest a role for multiple recessive or partially recessive alleles in general cognitive ability, and that alleles decreasing general cognitive ability have been selected against over evolutionary time.

Computational fluid dynamics modeling of Bacillus anthracis spore deposition in rabbit and human respiratory airways.

Three-dimensional computational fluid dynamics and Lagrangian particle deposition models were developed to compare the deposition of aerosolized Bacillus anthracis spores in the respiratory airways of a human with that of the rabbit, a species commonly used in the study of anthrax disease. The respiratory airway geometries for each species were derived respectively from computed tomography (CT) and µCT images. Both models encompassed airways that extended from the external nose to the lung with a total of 272 outlets in the human model and 2878 outlets in the rabbit model. All simulations of spore deposition were conducted under transient, inhalation-exhalation breathing conditions using average species-specific minute volumes. Two different exposure scenarios were modeled in the rabbit based upon experimental inhalation studies. For comparison, human simulations were conducted at the highest exposure concentration used during the rabbit experimental exposures. Results demonstrated that regional spore deposition patterns were sensitive to airway geometry and ventilation profiles. Due to the complex airway geometries in the rabbit nose, higher spore deposition efficiency was predicted in the nasal sinus compared to the human at the same air concentration of anthrax spores. In contrast, higher spore deposition was predicted in the lower conducting airways of the human compared to the rabbit lung due to differences in airway branching pattern. This information can be used to refine published and ongoing biokinetic models of inhalation anthrax spore exposures, which currently estimate deposited spore concentrations based solely upon exposure concentrations and inhaled doses that do not factor in species-specific anatomy and physiology for deposition.

In vitro metabolism of benzo[a]pyrene and dibenzo[def,p]chrysene in rodent and human hepatic microsomes.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and often carcinogenic contaminants released into the environment during natural and anthropogenic combustion processes. Benzo[a]pyrene (B[a]P) is the prototypical carcinogenic PAH, and dibenzo[def,p]chrysene (DBC) is a less prevalent, but highly potent transplacental carcinogenic PAH. Both are metabolically activated by isoforms of the cytochrome P450 enzyme superfamily to form reactive carcinogenic and cytotoxic metabolites. Metabolism of B[a]P and DBC was studied in hepatic microsomes of male Sprague-Dawley rats, naïve and pregnant female B6129SF1/J mice, and female humans, corresponding to available pharmacokinetic data. Michaelis-Menten saturation kinetic parameters including maximum rates of metabolism (VMAX, nmol/min/mg microsomal protein), affinity constants (KM, µM), and rates of intrinsic clearance (CLINT, ml/min/kg body weight) were calculated from substrate depletion data. CLINT was also estimated from substrate depletion data using the alternative in vitro half-life method. VMAX and CLINT were higher for B[a]P than DBC, regardless of species. Clearance for both B[a]P and DBC was highest in naïve female mice and lowest in female humans. Clearance rates of B[a]P and DBC in male rat were more similar to female human than to female mice. Clearance of DBC in liver microsomes from pregnant mice was reduced compared to naïve mice, consistent with reduced active P450 protein levels and elevated tissue concentrations and residence times for DBC observed in previous in vivo pharmacokinetic studies. These findings suggest that rats are a more appropriate model organism for human PAH metabolism, and that pregnancy's effects on metabolism should be further explored.

A Bidirectional Coupling Procedure Applied to Multiscale Respiratory Modeling.

In this study, we present a novel multiscale computational framework for efficiently linking multiple lower-dimensional models describing the distal lung mechanics to imaging-based 3D computational fluid dynamics (CFD) models of the upper pulmonary airways in order to incorporate physiologically appropriate outlet boundary conditions. The framework is an extension of the Modified Newton's Method with nonlinear Krylov accelerator developed by Carlson and Miller [1, 2, 3]. Our extensions include the retention of subspace information over multiple timesteps, and a special correction at the end of a timestep that allows for corrections to be accepted with verified low residual with as little as a single residual evaluation per timestep on average. In the case of a single residual evaluation per timestep, the method has zero additional computational cost compared to uncoupled or unidirectionally coupled simulations. We expect these enhancements to be generally applicable to other multiscale coupling applications where timestepping occurs. In addition we have developed a "pressure-drop" residual which allows for stable coupling of flows between a 3D incompressible CFD application and another (lower-dimensional) fluid system. We expect this residual to also be useful for coupling non-respiratory incompressible fluid applications, such as multiscale simulations involving blood flow. The lower-dimensional models that are considered in this study are sets of simple ordinary differential equations (ODEs) representing the compliant mechanics of symmetric human pulmonary airway trees. To validate the method, we compare the predictions of hybrid CFD-ODE models against an ODE-only model of pulmonary airflow in an idealized geometry. Subsequently, we couple multiple sets of ODEs describing the distal lung to an imaging-based human lung geometry. Boundary conditions in these models consist of atmospheric pressure at the mouth and intrapleural pressure applied to the multiple sets of ODEs. In both the simplified geometry and in the imaging-based geometry, the performance of the method was comparable to that of monolithic schemes, in most cases requiring only a single CFD evaluation per time step. Thus, this new accelerator allows us to begin combining pulmonary CFD models with lower-dimensional models of pulmonary mechanics with little computational overhead. Moreover, because the CFD and lower-dimensional models are totally separate, this framework affords great flexibility in terms of the type and breadth of the adopted lower-dimensional model, allowing the biomedical researcher to appropriately focus on model design. Research funded by the National Heart and Blood Institute Award 1RO1HL073598.

Stability and change of genetic and environmental effects on the common liability to alcohol, tobacco, and cannabis DSM-IV dependence symptoms.

This study investigated the stability of genetic and environmental effects on the common liability to alcohol, tobacco, and cannabis dependence across adolescence and young adulthood. DSM-IV symptom counts from 2,361 adolescents were obtained using a structured diagnostic interview. Several sex-limited longitudinal common pathway models were used to examine gender differences in the magnitude of additive genetic (A), shared environment, and non-shared environmental effects over time. Model fitting indicated limited gender differences. Among older adolescents (i.e., age > 14), the heritability of the latent trait was estimated at 0.43 (0.05, 0.94) during the first wave and 0.63 (0.21, 0.83) during the second wave of assessment. A common genetic factor could account for genetic influences at both assessments, as well as the majority of the stability of SAV over time [rA = 1.00 (0.55, 1.00)]. These results suggest that early genetic factors continue to play a key role at later developmental stages.

Nominal association with CHRNA4 variants and nicotine dependence.

Nicotine binds to nicotinic acetylcholine receptors and studies in animal models have shown that α4ß2 receptors mediate many behavioral effects of nicotine. Human genetics studies have provided support that variation in the gene that codes for the α4 subunit influences nicotine dependence (ND), but the evidence for the involvement of the ß2 subunit gene is less convincing. In this study, we examined the genetic association between variation in the genes that code for the α4 (CHRNA4) and ß2 (CHRNB2) subunits of the nicotinic acetylcholine receptor and a quantitative measure of lifetime DSM-IV ND symptom counts. We performed this analysis in two longitudinal family-based studies focused on adolescent antisocial drug abuse: the Center on Antisocial Drug Dependence (CADD, N = 313 families) and Genetics of Antisocial Drug Dependence (GADD, N = 111 families). Family-based association tests were used to examine associations between 14 single nucleotide polymorphisms (SNPs) in CHRNA4 and CHRNB2 and ND symptoms. Symptom counts were corrected for age, sex and clinical status prior to the association analysis. Results, when the samples were combined, provided modest evidence that SNPs in CHRNA4 are associated with ND. The minor allele at both rs1044394 (A; Z = 1.988, P = 0.047, unadjusted P-value) and rs1044396 (G; Z = 2.398, P = 0.017, unadjusted P-value) was associated with increased risk of ND symptoms. These data provide suggestive evidence that variation in the α4 subunit of the nicotinic acetylcholine receptor may influence ND liability.

Extension of a PBPK model for ethylene glycol and glycolic acid to include the competitive formation and clearance of metabolites associated with kidney toxicity in rats and humans.

A previously developed PBPK model for ethylene glycol and glycolic acid was extended to include glyoxylic acid, oxalic acid, and the precipitation of calcium oxalate that is associated with kidney toxicity in rats and humans. The development and evaluation of the PBPK model was based upon previously published pharmacokinetic studies coupled with measured blood and tissue partition coefficients and rates of in vitro metabolism of glyoxylic acid to oxalic acid, glycine and other metabolites using primary hepatocytes isolated from male Wistar rats and humans. Precipitation of oxalic acid with calcium in the kidneys was assumed to occur only at concentrations exceeding the thermodynamic solubility product for calcium oxalate. This solubility product can be affected by local concentrations of calcium and other ions that are expressed in the model using an ion activity product estimated from toxicity studies such that calcium oxalate precipitation would be minimal at dietary exposures below the NOAEL for kidney toxicity in the sensitive male Wistar rat. The resulting integrated PBPK predicts that bolus oral or dietary exposures to ethylene glycol would result in typically 1.4-1.6-fold higher peak oxalate levels and 1.6-2-fold higher AUC's for calcium oxalate in kidneys of humans as compared with comparably exposed male Wistar rats over a dose range of 1-1000 mg/kg. The converse (male Wistar rats predicted to have greater oxalate levels in the kidneys than humans) was found for inhalation exposures although no accumulation of calcium oxalate is predicted to occur until exposures are well in excess of the theoretical saturated vapor concentration of 200mg/m(3). While the current model is capable of such cross-species, dose, and route-of-exposure comparisons, it also highlights several areas of potential research that will improve confidence in such predictions, especially at low doses relevant for most human exposures.

The heritability of general cognitive ability increases linearly from childhood to young adulthood.

Although common sense suggests that environmental influences increasingly account for individual differences in behavior as experiences accumulate during the course of life, this hypothesis has not previously been tested, in part because of the large sample sizes needed for an adequately powered analysis. Here we show for general cognitive ability that, to the contrary, genetic influence increases with age. The heritability of general cognitive ability increases significantly and linearly from 41% in childhood (9 years) to 55% in adolescence (12 years) and to 66% in young adulthood (17 years) in a sample of 11 000 pairs of twins from four countries, a larger sample than all previous studies combined. In addition to its far-reaching implications for neuroscience and molecular genetics, this finding suggests new ways of thinking about the interface between nature and nurture during the school years. Why, despite life's 'slings and arrows of outrageous fortune', do genetically driven differences increasingly account for differences in general cognitive ability? We suggest that the answer lies with genotype-environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part based on their genetic propensities.

Magnetic resonance imaging and computational fluid dynamics (CFD) simulations of rabbit nasal airflows for the development of hybrid CFD/PBPK models.

The percentages of total airflows over the nasal respiratory and olfactory epithelium of female rabbits were calculated from computational fluid dynamics (CFD) simulations of steady-state inhalation. These airflow calculations, along with nasal airway geometry determinations, are critical parameters for hybrid CFD/physiologically based pharmacokinetic models that describe the nasal dosimetry of water-soluble or reactive gases and vapors in rabbits. CFD simulations were based upon three-dimensional computational meshes derived from magnetic resonance images of three adult female New Zealand White (NZW) rabbits. In the anterior portion of the nose, the maxillary turbinates of rabbits are considerably more complex than comparable regions in rats, mice, monkeys, or humans. This leads to a greater surface area to volume ratio in this region and thus the potential for increased extraction of water soluble or reactive gases and vapors in the anterior portion of the nose compared to many other species. Although there was considerable interanimal variability in the fine structures of the nasal turbinates and airflows in the anterior portions of the nose, there was remarkable consistency between rabbits in the percentage of total inspired airflows that reached the ethmoid turbinate region (approximately 50%) that is presumably lined with olfactory epithelium. These latter results (airflows reaching the ethmoid turbinate region) were higher than previous published estimates for the male F344 rat (19%) and human (7%). These differences in regional airflows can have significant implications in interspecies extrapolations of nasal dosimetry.

SNPs in CHRNA6 and CHRNB3 are associated with alcohol consumption in a nationally representative sample.

The CHRNA6 and CHRNB3 genes have been associated with nicotine dependence and early subjective response to nicotine. Here we present evidence, using a nationally representative sample of adults, that this region is also associated with alcohol behaviors. Six SNPs (single nucleotide polymorphisms) spanning the CHRNB3/A6 genes were analyzed using the statistical genetics software FBAT-PC, which allows one to examine a collection of multiple phenotypes to generate a maximally heritable composite phenotype for each SNP. The six SNPs were tested using FBAT-PC including four alcohol phenotypes: average number of drinks, blackouts, total number of DSM-IV abuse and dependence symptoms endorsed, and quit attempts. Three SNPs in CHRNA6 (rs1072003, P = 0.015; rs892413, P = 0.0033 and rs2304297, P = 0.012) and one SNP in CHRNB3 (rs13280604, P = 0.0053) were associated with a composite of the alcohol phenotypes. The association was primarily driven by the average number of drinks.

Developmental epidemiology of drug use and abuse in adolescence and young adulthood: Evidence of generalized risk.

Past studies highlight a narrowing gender gap and the existence of a shared etiology across substances of abuse; however, few have tested developmental models using longitudinal data. We present data on developmental trends of alcohol, tobacco, and marijuana use, abuse and dependence assessed during adolescence and young adulthood in a community-based Colorado twin sample of 1733 respondents through self-report questionnaires and structured psychiatric interviews. Additionally, we report on the rates of multiple substance use and disorders at each developmental stage, and the likelihood of a substance use disorder (SUD; i.e., abuse or dependence) diagnosis in young adulthood based on adolescent drug involvement. Most notably, we evaluate whether the pattern of multiple substance use and disorders and likelihood ratios across substances support a model of generalized risk. Lastly, we evaluate whether the ranked magnitudes of substance-specific risk match the addiction liability ranking. Substance use and SUDs are developmental phenomena, which increase from adolescence to young adulthood with few and inconsistent gender differences. Adolescents and young adults are not specialized users, but rather tend to use or abuse multiple substances increasingly with age. Risk analyses indicated that progression toward a SUD for any substance was increased with prior involvement with any of the three substances during adolescence. Despite the high prevalence of alcohol use, tobacco posed the greatest substance-specific risk for developing subsequent problems. Our data also confirm either a generalized risk or correlated risk factors for early onset substance use and subsequent development of SUDs.

Dosimetry considerations in the enhanced sensitivity of male Wistar rats to chronic ethylene glycol-induced nephrotoxicity.

Male Wistar rats have been shown to be the most sensitive sex, strain and species to ethylene glycol-induced nephrotoxicity in subchronic studies. A chronic toxicity and dosimetry study was therefore conducted in male Wistar rats administered ethylene glycol via the diet at 0, 50, 150, 300, or 400 mg/kg/day for up to twelve months. Subgroups of animals were included for metabolite analysis and renal clearance studies to provide a quantitative basis for extrapolating dose-response relationships from this sensitive animal model in human health risk assessments. Mortality occurred in 5 of 20 rats at 300 mg/kg/day (days 111-221) and 4 of 20 rats at 400 mg/kg/day (days 43-193), with remaining rats at this dose euthanized early (day 203) due to excessive weight loss. Increased water consumption and urine volume with decreased specific gravity occurred at 300 mg/kg/day presumably due to osmotic diuresis. Calculi (calcium oxalate crystals) occurred in the bladder or renal pelvis at > or =300 mg/kg/day. Rats dying early at > or =300 mg/kg/day had transitional cell hyperplasia with inflammation and hemorrhage of the bladder wall. Crystal nephropathy (basophilic foci, tubule or pelvic dilatation, birefringent crystals in the pelvic fornix, or transitional cell hyperplasia) affected most rats at 300 mg/kg/day, all at 400 mg/kg/day, but none at < or =150 mg/kg/day. No significant differences in kidney oxalate levels, the metabolite responsible for renal toxicity, were observed among control, 50 and 150 mg/kg/day groups. At 300 and 400 mg/kg/day, oxalate levels increased proportionally with the nephrotoxicity score supporting the oxalate crystal-induced nephrotoxicity mode of action. No treatment-related effects on the renal clearance of intravenously infused (3)H-inulin, a marker for glomerular filtration, and (14)C-oxalic acid were observed in rats surviving 12 months of exposure to ethylene glycol up to 300 mg/kg/day. In studies with naïve male Wistar and F344 rats (a less sensitive strain), a significant difference was observed in oxalate clearances between young rats (i.e. Wistar clearance < F344) but not in age-matched old rats. Regardless, the ratios of oxalate:inulin clearances in these two strains of rats, including those exposed to ethylene glycol, were all < 1, suggesting that a fraction of the filtered oxalate is reabsorbed. Other species, including humans, typically have clearance ratios >1 and are more effective at clearing oxalic acid by both glomerular filtration and active secretion. Thus, the lower renal clearance and kidney accumulation of oxalates in male Wistar rats enhances their sensitivity, which will be a factor in human risk assessments. The benchmark dose values (BMD05, BMDL05) were 170 mg/kg/day and 150 mg/kg/day for nephropathy, and 170 mg/kg/day and 160 mg/kg/day for birefringent crystals, using incidence times severity data in each case. The NOAEL of 150 mg/kg/day is the same as that reported after 16-week exposure and appears to be a threshold dose below which no renal toxicity occurs, regardless of exposure duration.

Genetic and environmental influences on age at sexual initiation in the Colorado Adoption Project.

Whereas the majority of research on adolescent sexual initiation has focused solely on environmental factors, the present study used behavioral genetic analyses to investigate the relative contributions of genetic and environmental influences. Structural equation models were fitted to data from adoptive and non-adoptive sibling pairs (231 biologically related pairs and 169 unrelated pairs) from the Colorado Adoption Project. Information from censored individuals who had not yet experienced sexual initiation was maximized by adapting the twin survival analysis method of Pickles et al. (Behav Genet 24(5):457-468, 1994) to accommodate adoptive and non-adoptive siblings. Point estimates of variance components from an ACE model, including additive genetic (A), shared environmental (C), and non-shared environmental (E) influences were 28%, 24%, and 48%, respectively. Despite the lower point estimate for shared environmental effects than additive genetic effects, a CE model provided the best fit to the data. However, because adoptive siblings provide a direct estimate of shared environmental influences there is greater power to detect shared environmental effects in adoption designs. Evidence for genetic influences from our data were somewhat lower than those obtained in previous twin studies, possibly reflecting a return to more socially conservative sexual attitudes, changing sexual behaviors, or ambiguities in the wording of questions commonly used in research on adolescent sexuality.

IgM accelerates affinity maturation.

Secreted IgM is a potent adjuvant that concentrates antigen into secondary lymphoid organs and initiates antibody responses, germinal centre formation and is crucial in resolving infections. The current studies investigated the influence of specific IgM on both the quantity and quality of antibody produced in response to T-dependent and T-independent antigens. The addition of IgM to either antigen had no significant effect on the titre or duration of antibody responses. However, the presence of specific IgM led to accelerated affinity maturation when mice were challenged with low doses of IgM-containing immune complexes (IgM-IC) of T-dependent antigens compared with antigen alone. Interestingly, the administration of higher concentrations of IgM-IC increased follicular deposition of antigen but did not result in accelerated affinity maturation or in higher antibody affinities. The administration of IgM complexed with T-independent antigens had no effect on antibody titre, duration or affinity maturation, despite increased antigen deposition in lymphoid follicles. Together, these results demonstrate that IgM accelerates affinity maturation in immune responses to T-dependent antigens and implies an antigen optimum exists for the generation of high affinity antibodies. The data also suggest IgM plays an important role in the induction of CD4 T cells, facilitating germinal centre formation and function.

Using physiologically-based pharmacokinetic modeling to address nonlinear kinetics and changes in rodent physiology and metabolism due to aging and adaptation in deriving reference values for propylene glycol methyl ether and propylene glycol methyl ether acetate.

Reference values, including an oral reference dose (RfD) and an inhalation reference concentration (RfC), were derived for propylene glycol methyl ether (PGME), and an oral RfD was derived for its acetate (PGMEA). These values were based on transient sedation observed in F344 rats and B6C3F1 mice during a two-year inhalation study. The dose-response relationship for sedation was characterized using internal dose measures as predicted by a physiologically-based pharmacokinetic (PBPK) model for PGME and its acetate. PBPK modeling was used to account for changes in rodent physiology and metabolism due to aging and adaptation, based on data collected during Weeks 1, 2, 26, 52, and 78 of a chronic inhalation study. The peak concentration of PGME in richly perfused tissues (i.e., brain) was selected as the most appropriate internal dose measure based on a consideration of the mode of action for sedation and similarities in tissue partitioning between brain and other richly perfused tissues. Internal doses (peak tissue concentrations of PGME) were designated as either no-observed-adverse-effect levels (NOAELs) or lowest-observed-adverse-effect levels (LOAELs) based on the presence or the absence of sedation at each time point, species, and sex in the two-year study. Distributions of the NOAEL and LOAEL values expressed in terms of internal dose were characterized using an arithmetic mean and standard deviation, with the mean internal NOAEL serving as the basis for the reference values, which was then divided by appropriate uncertainty factors. Where data were permitting, chemical-specific adjustment factors were derived to replace default uncertainty factor values of 10. Nonlinear kinetics, which was predicted by the model in all species at PGME concentrations exceeding 100 ppm, complicate interspecies, and low-dose extrapolations. To address this complication, reference values were derived using two approaches that differ with respect to the order in which these extrapolations were performed: (1) default approach of interspecies extrapolation to determine the human equivalent concentration (PBPK modeling) followed by uncertainty factor application, and (2) uncertainty factor application followed by interspecies extrapolation (PBPK modeling). The resulting reference values for these two approaches are substantially different, with values from the latter approach being seven-fold higher than those from the former approach. Such a striking difference between the two approaches reveals an underlying issue that has received little attention in the literature regarding the application of uncertainty factors and interspecies extrapolations to compounds where saturable kinetics occur in the range of the NOAEL. Until such discussions have taken place, reference values based on the former approach are recommended for risk assessments involving human exposures to PGME and PGMEA.

Derivation of a human equivalent concentration for n-butanol using a physiologically based pharmacokinetic model for n-butyl acetate and metabolites n-butanol and n-butyric acid.

The metabolic series approach for risk assessment uses a dosimetry-based analysis to develop toxicity information for a group of metabolically linked compounds using pharmacokinetic (PK) data for each compound and toxicity data for the parent compound. The metabolic series approach for n-butyl acetate and its subsequent metabolites, n-butanol and n-butyric acid (the butyl series), was first demonstrated using a provisional physiologically based pharmacokinetic (PBPK) model for the butyl series. The objective of this work was to complete development of the PBPK model for the butyl series. Rats were administered test compounds by iv bolus dose, iv infusion, or by inhalation in a recirculating closed chamber. Hepatic, vascular, and extravascular metabolic constants for metabolism were estimated by fitting the model to the blood time course data from these experiments. The respiratory bioavailability of n-butyl acetate (100% of alveolar ventilation) and n-butanol (50% of alveolar ventilation) was estimated from closed chamber inhalation studies and measured ventilation rates. The resulting butyl series PBPK model successfully reproduces the blood time course of these compounds following iv administration and inhalation exposure to n-butyl acetate and n-butanol in rats and arterial blood n-butanol kinetics following inhalation exposure to n-butanol in humans. These validated inhalation route models can be used to support species and dose-route extrapolations required for risk assessment of butyl series family of compounds. Human equivalent concentrations of 169 ppm and 1066 ppm n-butanol corresponding to the rat n-butyl acetate NOAELs of 500 and 3000 ppm were derived using the models.