Review and update of leukemia risk potentially associated with occupational exposure to benzene

Since the 1980 U.S. Supreme Court decision on the Occupational Safety and Health Administration's (OSHA) proposal to lower the occupational benzene standard from 10 ppm to 1 ppm, numerous quantitative assessments of the leukemia risk of benzene exposure have been prepared. The primary difference between these risk assessments has been in the way in which benzene exposure has been estimated and in the models applied to describe the dose-response relationship. The more recent assessments, in attempting to estimate benzene exposures on an individual basis, and in applying models which make maximal use of the available data points, represent a substantial improvement over earlier assessments. In this paper, we will review the available risk assessments and the data upon which they are based and will present our own assessment, which builds on prior efforts. Our reevaluation of the underlying data on the cohort that we judged to be most suitable for quantitative risk analysis suggested that past assessments may have overestimated risk by a factor of 3 to 24. In addition, we will present some recently made available data of relevance to the benzene exposure histories of cohort of concern. These data provide additional suggestion that the total benzene exposure of certain members of this cohort has likely been seriously underestimated, the extent to which remains to be determined. Further analysis of these data and pursuit of additional sources to improve the characterization of the benzene exposure of this cohort appear to be warranted in order to define more precisely the benzene-leukemia dose-response relationship.

The use of repeated measures analysis of variance for plaque and gingival indices

Clinical trials for anti-gingivitis and anti-plaque agents commonly use the mean of Silness and Löe plaque indices and Löe and Silness gingival indices as response variables. The aim of this report is to determine if data from anti-plaque and anti-gingivitis clinical trials using Silness and Löe plaque indices and Löe and Silness gingival indices satisfy conditions necessary for the use of the univariate or multivariate approach to repeated measures. These conditions are multivariate normality, homogeneity of variance-covariance matrices, and for the univariate approach, a type-H variance-covariance matrix. Data from 5 separate clinical trials representing a wide range in sample size, pretreatment mean gingival and plaque indices and treatment effects were used to test these conditions. Either the univariate or multivariate approach to repeated measures was found to be appropriate for both responses of the 5 clinical trials. Thus, means of Silness & Löe and Löe and Silness gingival indices meet the necessary conditions for use of either the univariate and/or multivariate approach to repeated measures. However, significant time-treatment interactions are a common occurrence in these types of clinical trials and must be evaluated carefully. The analyses in this study were carried out using SAS. Other mainframe statistical software packages and many micro-computer statistical software packages have routines to analyze repeated measures experiments with analysis of variance methods. However, some of the packages may omit the multivariate approach to repeated measures or may not include interactions between within-subject and between-subject effects. These packages should be used with caution.

Trend tests for proportional responses in developmental toxicity experiments

The data from developmental toxicity experiments usually are very difficult to analyze statistically because of the lack of independence among littermates and the random nature of the litter size. Only a few of the models that have been proposed in the literature have accounted for both of these features. One of the models proposed by Van Ryzin is invoked to construct a test of trend (dose response). The construction is achieved via a statistical technique called isotonic regression, which is applied to the moment estimators derived by Van Ryzin. The trend test based on isotonic regression is relatively straightforward to calculate, and when the number of dose groups (including control) is four or less, the significance of the observed result is easily determined. An example, in which fetolethality is the end point of interest, demonstrates the test.

The role of aging and chronic disease in sexual dysfunction

Sexual dysfunction is so highly prevalent in elderly males that it is often considered an inevitable consequence of "normal aging." To determine if other factors are related to an age-associated decline in sexual function, we surveyed two groups of elderly male veterans in a geriatric ambulatory care clinic: aged 65 to 75 years ("young-old") and aged over 75 ("old-old"). We compared their survey responses with responses from a general medical clinic for unstable medical patients, aged under 65 ("old-young"). Of 347 subjects surveyed, 225 completed a health and sexual function questionnaire (response rate = 65%). Absent libido was reported by 30% of old-young, 31% of young-old, and 47% of old-old. Erectile dysfunction was reported in 26% of old-young, 27% of young-old, and 50% of old-old (P less than .01). We used ordinal logistic regression and found overall sexual dysfunction to be significantly related to subjective poor health, diabetes mellitus, and incontinence (P less than .05), while controlling for age. These data suggest that, although sexual dysfunction is more common in the aged, it is often related more to comorbid illness than aging alone.

Evaluation of the interaction of three genotoxic agents in eliciting sister-chromatid exchanges using response surface methodology

Chinese hamster cells (V79) were treated with ethylnitrosourea (ENU), bischloroethylnitrosourea (BCNU), and cis-diamminedichloroplatinum(II) (DDP) alone and in combination. Sister chromatid exchanges (SCEs) were quantitated as measures of genotoxicity of the three agents. The combination experiment employed a factorial design in which cells were treated, in various concentration combinations, with all agents simultaneously. Response surface methodology using a polynomial model in treatment variables to approximate the mean the distribution of SCE events was employed for analysis of the interactions of the three genotoxic agents. Due to unequal variances of the number of SCEs in the various treatment groups, a weighted least-squares analysis was used to estimate the parameters of the dose-response relationship. The single-agent results suggest that the DDP concentration-response curve has a much steeper slope than the ENU and BCNU curves, and is concave downward as compared to the relatively linear concentration-response curves of ENU and BCNU. The combination results suggest that ENU and DDP are involved in a negative interaction. The BCNU/DDP interaction, the ENU/BCNU interaction, and the three-factor interaction are not statistically significant. The analysis of these data demonstrates the usefulness of a statistical procedure for evaluating the biological effects resulting from exposure to multiple cytotoxic agents. The methodology can be used with many other types of endpoints and is not limited by the number of treatment agents.

The evaluation of biological interactions using response surface methodology

Response surface methodology was employed in the statistical analysis of the combination exposures of genotoxic agents, bischloroethylnitrosourea with cis-diamminedichloroplatinum (II) and cis-diamminedichloroplatinum (II) with X rays. The measured endpoint in each case was sister chromatid exchanges in V79 Chinese hamster cells. The combination experiments employed a factorial design in which cells were treated, in various concentration combinations, with two agents simultaneously. Bis-chloroethylnitrosourea and cis-diamminedichloroplatinum (II) each exhibited curvilinear concentration-related increases in sister chromatid exchanges. X rays exhibited a dose-dependent increase in sister chromatid exchanges. For the cis-diamminedichloroplatinum (II)/X ray combinations, response surface methodology indicates a less-than-additive interaction, suggested by the non-parallel concentration-response curves of one agent at varying concentrations of the other, and a slight dose-dependent increase in sister chromatid exchanges due to X rays alone. Both cis-diamminedichloroplatinum and bis-chloroethylnitrosourea exhibited concentration-related increases in sister chromatid exchanges, cis-diamminedichloroplatinum (II) being 8-10 times (dependent on what level of effect was compared) more potent than bis-chloroethylnitrosourea. For the cis-diamminedichloroplatinum (II)/bis-chloroethylnitrosourea combinations, an increasingly less-than-additive interaction was detected. The analysis of these combinations demonstrates the strength of response surface methodology, a collection of mathematical and statistical techniques for detecting, analyzing and describing the biological effects resulting from exposures to multiple cytotoxic agents. The descriptive ability of these procedures is shown to be useful in that it leads to the suggestion of hypotheses regarding mechanisms of action.

Renal hypophosphatemic rickets. Growth and mineral metabolism after treatment with calcitriol (1,25-dihydroxyvitamin D3) and phosphate supplementation

To delineate the mineral metabolism of renal hypophosphatemic rickets and to update progress in linear growth after calcitriol (1,25-dihydroxyvitamin D3) therapy, the medical records of 19 patients were examined retrospectively from January 1978 through December 1985. With a mean (+/- SD) follow-up period of 42.0 +/- 5.4 months after calcitriol had been administered for at least 12 months, the growth measurements were as follows: the percentile weight (mean +/- SD) remained unchanged, with the initial being 12.3% +/- 17.3% and the final being 15.3% +/- 18.6%, and the length/height percentiles were -2.7% +/- 5.9% and -2.4% +/- 4.4%. The growth velocity index showed a significant improvement from mean values of 61.7% (at age 2 years) to mean values of 101.2% (at age 11.2 years). Serum phosphate concentrations rose from the initial value of 2.9 +/- 0.6 to 3.5 +/- 0.8 mg/dL (0.9 +/- 0.2 to 1.1 +/- 0.3 mmol/L). The effects of calcitriol on renal function were tested by creatinine clearance values, which were 127 +/- 22 mL/min/1.73 m2 (2.12 +/- 0.37 mL/s/1.73 m2) at the conclusion of the study, compared with 128 +/- 25 mL/min/1.73 m2 (2.13 +/- 0.41 mL/s/1.73 m2) obtained at the initiation of calcitriol therapy. We conclude that calcitriol treatment of renal hypophosphatemic rickets in children results in improvement of growth velocity and serum phosphate concentration without deterioration of renal function.

Estimation and analysis of the concentration-response surfaces associated with multiple-agent combinations

Chinese hamster cells (V79) were treated with ethylnitrosourea (ENU) and cis-diamminedichloroplatinum(II) (DDP) alone and in combination. Sister chromatid exchanges (SCEs) were quantified as measures of genotoxicity of the two agents. The combination experiment employed a factorial design in which cells were treated, in various concentration combinations, with both agents simultaneously. Response surface methodology, using a polynomial model based on a negative binomial distribution of SCE events, was employed for analysis of the interactions of the two genotoxic agents. The negative binomial distribution, a generalization of the Poisson distribution, is required since SCEs are discrete variables which, under the conditions of these experiments, have a distribution which exhibits extra-Poisson variability. The model of the ENU/DDP combinations indicated an increasingly less-than-additive effect resulting from increasing concentrations of each agent in the combination. The analysis of these experiments demonstrates the usefulness of a powerful statistical procedure for evaluating the biological effects resulting from exposure to multiple cytotoxic agents. The methodology can be used with many other types of endpoints and is not limited by the number of treatment agents.

Genetic predisposition to diabetes mellitus is associated with impaired humoral immunity to coxsackievirus B4

Experiments were performed to determine whether genetic predisposition to diabetes mellitus (DM) or clinical DM or both exert an influence on the production of neutralization antibodies to coxsackievirus B4 (CB4). The homozygous diabetic mutant mouse db+/db+, on the inbred C57BL/KsJ genetic background, develops a diabetes-like disease when maintained on ad libitum diet but restriction of excess food intake prevents overt disease. The doubly heterozygote db+/+m or the homozygote +m/+m misty coat color mutant, on the C57BL/KsJ genetic background, do not develop DM and served as controls. Animals infected with one-half a previously determined LD50 of CB4 were bled prior to infection and at 3, 5, 7, 14, 21 days and at 1, 2, 3, 4 and 5 months after infection. Serum neutralization antibody (NA) levels were determined from the percent CB4 plaque reduction. Until 2 months following infection, NA levels were not significant in either of the homozygous diabetic mutant groups, db+/db+. In the diabetic mutant group db+/db+, without overt disease, neutralization of CB4 when observed, was low, short-lived, and apparently not specific. However, in the homozygous diabetic mutants with spontaneous diabetes, CB4 NA became evident at 2 months after infection. By 3 months post-infection, serum NA levels were sufficient to cause 90% virus plaque reduction. These observations demonstrate that hereditary DM as characterized by the mutation diabetes, db, in the C57BL/KsJ mouse, is associated with a marked impaired humoral immune response to a diabetogenic human CB4. Specifically, there is an inability to develop an adequate level of anti-CB4 antibodies. The type and degree of immunological impairment are apparently different prior to and after onset of diabetes mellitus.

Prediction of carrier status in Duchenne dystrophy by creatine kinase measurement

Serum creatine kinase (CK) was measured in 515 healthy white women and 28 obligate carriers for Duchenne muscular dystrophy. There was substantial overlap between the control and carrier populations. To analyze the impact of the degree of overlap, the predictive value of a CK result was determined by (1) using sensitivity and specificity analysis, which assumes dichotomization into a positive or negative result based on a particular cut-off value; and (2) using likelihood ratio analysis, which evaluates an individual result based on the continuum observed for control and carrier populations. There was no clinically important difference whether an observed 57% or a hypothetical 33% overlap between control and carrier results was used. Because of the substantial overlap, the CK test utility is limited to those suspected carriers whose results fall above the healthy population interval. A low CK result does not provide sufficient assurance of noncarrier status.

Calcium and phosphate metabolism in children with idiopathic hypoparathyroidism or pseudohypoparathyroidism: effects of 1,25-dihydroxyvitamin D3

Two children with congenital hypoparathyroidism and two children with pseudohypoparathyroidism were given maintenance doses of 15 to 45 ng/kg/day 1,25-dihydroxyvitamin D3 for a total of 255 months. The urinary calcium excretion showed an upward elevation after the first 2 years of treatment but was not significantly higher than that in 10 normal control subjects. The renal threshold for phosphate excretion stayed within the normal ranges compared with control values. Two hypercalcemic and two hypocalcemic episodes occurred during this period of treatment. Hypercalcemia was reversed within 1 week after withdrawal of 1,25-dihydroxyvitamin D3. Hypocalcemia was countered by increasing the dose of 1,25-dihydroxyvitamin D3. Renal functions were not adversely affected, as estimated by creatinine clearance and reciprocals of serum creatinine concentrations. The mean serum calcium concentration during 1,25-dihydroxyvitamin D3 treatment was significantly higher (P = 0.001) compared with that obtained during vitamin D2 treatment at a dose of 500 to 3000 IU/kg/day. These data provide additional support for the long-term use of 1,25-dihydroxyvitamin D3 in idiopathic hypoparathyroidism and pseudohypoparathyroidism.

Confidence interval about the response at the stationary point of a response surface, with an application to preclinical cancer therapy

A method of calculating a confidence interval about the response at the stationary point of a response surface is presented. We show that the technique can also be used to calculate a confidence region about the location of the stationary point. The procedure is applied, via Cox's proportional-hazards model, to the analysis of survival data from a preclinical cancer chemotherapy experiment involving the combination of two drugs. It is seen that the results can be useful in determining the existence of a therapeutic synergism.

Influence of diabetes mellitus heredity on susceptibility to coxsackievirus B4

Using the criteria of virus susceptibility as defined by the 50 percent lethal dose response and the percent cumulative mortality response it was shown that the diabetic mutation db, located on chromosome 4, exerted a particular influence on the host response to CB4 challenge. Neither the yellow obese mutation Ay on chromosome 2 nor the misty coat color mutation located one centimorgan from the db mutation had the same effect on CB4 response. The obese diabetic mutation ob located on chromosome 6 appeared to enhance susceptibility to CB4. However, the high susceptibility of the inbred C57BL/6J line on which the ob mutation is found was apparently a significant contributing factor to the ob mutant high virus susceptibility. The response to CB4 was also a useful criteria to discern differences in the genetic background of closely related inbred lines. Based on the CB4 LD50 values the C57BL/6J inbred line was the most susceptible while the C57BL/Ks inbred line was the most resistant. However, using the percent cumulative mortality response as an index of host resistance, the C57BL/KsJ was the most susceptible and the C57BL/Ks the least. These findings further support the thesis that genetic predisposition to diabetes mellitus, as characterized by the mutation db on chromosome 4 is associated with a particular susceptibility and host response to coxsackie-virus B4. It also illustrates that under specific conditions, comparison of the response to virus challenge can be used as an indicator of genetic differences between closely related inbred lines.

Are reference limits for serum creatine kinase valid in detection of the carrier state for Duchenne muscular dystrophy?

We evaluated serum creatine kinase (CK) as an index to heterozygosity in Duchenne muscular dystrophy. When the 97.5th percentile of the CK normal reference interval was selected as the cutoff point, only 31% of 28 obligate carrier mothers and 24% of 43 mothers of simplex cases (those with only one occurrence of dystrophy in the kindred) exceeded this cutoff value. The outcome depended to some degree on the method used for establishing the reference limit for 379 presumably non-carrier, ambulatory women. The considerable overlap of CK activities between the control and carrier population as well as the non-gaussian distribution of the data permitted no satisfactory approach for differentiating these two populations. Neither the application of likelihood ratios, which evaluates a continuum of results without the dichotomy of a cutoff point, nor the application of predictive value based on sensitivity and specificity, which involves use of a cutoff value for decision making, provided a reliable estimate of carrier status. There was no significant difference (2 alpha = .19) between the median CK activity of obligate carrier mothers and mothers of simplex cases. The serum CK test does not provide data that either support or reject the Haldane hypothesis.

Are reference limits for serum creatine kinase valid in detection of the carrier state for Duchenne muscular dystrophy?

We evaluated serum creatine kinase (CK) as an index to heterozygosity in Duchenne muscular dystrophy. When the 97.5th percentile of the CK normal reference interval was selected as the cutoff point, only 31% of 28 obligate carrier mothers and 24% of 43 mothers of simplex cases (those with only one occurrence of dystrophy in the kindred) exceeded this cutoff value. The outcome depended to some degree on the method used for establishing the reference limit for 379 presumably non-carrier, ambulatory women. The considerable overlap of CK activities between the control and carrier population as well as the non-gaussian distribution of the data permitted no satisfactory approach for differentiating these two populations. Neither the application of likelihood ratios, which evaluates a continuum of results without the dichotomy of a cutoff point, nor the application of predictive value based on sensitivity and specificity, which involves use of a cutoff value for decision making, provided a reliable estimate of carrier status. There was no significant difference (2 alpha = .19) between the median CK activity of obligate carrier mothers and mothers of simplex cases. The serum CK test does not provide data that either support or reject the Haldane hypothesis.

A likelihood ratio test for a patterned covariance matrix in a multivariate growth-curve model

In a multivariate growth-curve model, the estimator of the parameter matrix is a function of the matrix of the sums of squares and of the cross-products due to error. However, if the assumption of a patterned covariance matrix is valid, then the parameter estimator does not depend on the error matrix. A likelihood ratio test of this patterned covariance matrix is constructed and its distribution is discussed. A numerical example is provided in which the design consists of two treatment groups, with three repeated measures being taken of the three response variables.

Sampling from a skewed population distribution as exemplified by estimation of the creatine kinase upper reference limit

Creatine kinase (EC 2.7.3.2) was measured in sera from 580 females, ages 1-77 years, and 550 males, ages 1-63 years. The distribution of results for male and female groups shows pronounced skewing toward higher values. The observed distribution of results could not be described by any of six mathematical formulas for skewed distributions, an indication of the unsuitability of such formulas to transform these data for parametric analysis. The range of 97.5 percentile estimates produced by six independent samples of 100, 200, and 400 observations randomly selected from a mathematical model defined by the adult female distribution showed progressive narrowing from the 150-380 U/L interval for the samples of 100 observations to 200-265 U/L for the samples of 400 observations; no further improvement was seen when 800 observations were used. The samples of 100 and 200 observations contained extreme value points that might appear as "outliers" but were shown to be valid members of the population distribution when larger sample sizes were collected.