Application of modelling techniques to the planning of in vitro arsenic kinetic studies

A kinetic model describing the hepatic methylation of arsenite [As(III)] was developed on the basis of limited data from in vitro mechanistic studies. The model structure is as follows: sequential enzymic methylation of arsenite to its monomethylated (MMA) and dimethylated (DMA) products by first-order and Michaelis-Menten kinetics, respectively; uncompetitive inhibition of the formation of DMA by As(III); and first-order reversible binding of As(III), MMA and DMA to cytosolic proteins. Numerical sensitivity analysis was used to evaluate systematically the impact of changes in input parameters on model responses. Sensitivity analysis was used to investigate the possibility of designing experiments for robust testing of the uncompetitive inhibition hypothesis, and for further refining the model. Based on the sensitivity analysis, the MMA concentration is the most important response on which to focus. The parameters V(max) and k(i) can be reliably estimated by using the same concentration time-course data at intermediate initial arsenite concentrations of 1--5microM at 30 +/- 5 minutes. K(m) must be estimated independently of V(max), since the two parameters are highly correlated at all times, and the optimal experimental conditions would include lower initial concentrations of arsenite (0.1--0.5microM) and earlier time-points (about 8--18 minutes). The use of initial arsenite concentrations much above 5microM would not yield additional useful information, because the sensitivity coefficients for MMA, protein-bound MMA, DMA and protein-bound DMA tend to become extremely small or exhibit erratic trends. Overall trends in the sensitivity analysis indicated the desirability of performing measurements at times shorter than 60 minutes. This work demonstrates that physiological modelling and sensitivity analysis can be efficient tools for experimental planning and hypothesis testing when applied in the earliest phases of kinetic model development, thus allowing more-efficient and more-directed experimentation, and minimising the use of laboratory animals.

Early neuropsychological impairment in HIV-seropositive intravenous drug users: evidence from the Italian Multicentre Neuropsychological HIV Study

The aim of the Italian Multicentre Neuropsychological HIV Study is to assess the prevalence and natural history of cognitive deficit in intravenous drug users (i.v.DUs) during the asymptomatic phase of HIV infection. The study is currently being conducted in four centres (Napoli, Benevento, Verona and Pavia) whose catchment areas are characterized by different levels of prevalence of HIV infection. Cognitive evaluation is being performed by means of a standardized neuropsychological test battery. A total of 251 subjects (167 males and 84 females) have been recruited in the cross-sectional phase of the study, including 75 asymptomatic HIV-seropositive i.v.DUs (HIV+/i.v.DUs), 97 HIV-seronegative i.v.DUs (HIV-/i.v.DUs) and 79 non-i.v.DU seronegative controls matched to i.v.DUs with regard to sex, age and educational level. The prevalence of global cognitive impairment (performance at least 1.5 standard deviations worse than the average of the control group, on at least two out of five tests) was significantly higher in HIV+/i.v.DUs than in either HIV-/i.v.DUs (22.7% vs. 8.2%; P < 0.01) or healthy controls (22.7% vs. 2.5%; P < 0.001). The difference between HIV-/i.v.DUs and healthy controls was not statistically significant (8.2% vs. 2.5%; P = 0.19). The results of this study lend further support to the 'cerebral reserve' model. The cerebral reserve could indeed be reduced in i.v.DUs as a consequence of chronic exposure to the substance of abuse, so that these subjects become more vulnerable to direct and indirect neurotoxic effects of HIV.

Longitudinal neuropsychological evaluation of HIV-infected intravenous drug users

The present study aimed to describe the cognitive status of a group of HIV-positive asymptomatic intravenous drug users (IVDU) and changes which occurred over a 12-month follow-up period. Forty-two HIV positive IVDU were selected and matched for age, sex, educational level and pattern of drug abuse with 39 seronegative IVDU controls. Baseline and follow-up evaluation included neuropsychological tests exploring attention, language, memory, logic and visuomotor abilities, biological markers and clinical parameters. About one-third of both seropositive and seronegative subjects showed at baseline slight cognitive deficits, which did not change during the follow-up period.

Granulocyte defects and opioid receptors in chronic exposure to heroin or methadone in humans

In order to elucidate better the immunological effect of opioid abuse in the absence of HIV infection as a confounding factor, granulocyte function was investigated in three groups of HIV-negative subjects, including 20 active parenteral heroin abusers (H), 20 long-term methadone-maintained former opiate abusers (M) and 20 healthy controls (C). Chemotaxis to N-formyl methionyl-leucyl-phenylalanine (fMLP), casein and activated plasma were markedly and similarly reduced (approx. 50%) in both H and M groups, as was true for superoxide production after fMLP and PMA stimulation, 47% decrease of C values. Polymorphonuclear (PMN) of H and M subjects also exhibited a very marked and similar reduction in the expression of CD11b/CD18 integrin receptors after fMLP treatment, with values that were less than 10% of those in controls, as observed by flow cytometry. In parallel, PMN of H and M individuals presented an approximately four-fold increase in opioid receptors numbers compared to controls, a significant inverse correlation existing between the increase in opiate receptors and defective chemotaxis. The possible mechanism underlying the observed changes in PMN of H and M individuals is discussed.