Filtering out the coronaries?

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an enzyme other than the 'classical" nitroreductase (the one that acts on nitrofurans and other simple nitrated PAHs). Frameshift mutation is often due to the intercalation of a chemical between DNA base pairs. However, the authors suggest that the potent activity exhibited by the nitropyrenes (or their metabolites) is more consistent with the formation of adducts between DNA and the chemicals. This view is supported by the decreased mutagenicity observed with strains TA1977 and 1978 which are uvrB + analogues of TA1537 and 1538, respectively. These analogue strains respond in an undiminished fashion to chemicals that cause frameshift mutation as a result of intercalation, but show a greatly decreased mutagenic response when exposed to chemicals that form adducts recognized by UV endonuclease, the product of the uvrB + gene (Rosenkranz & Mermelstein, Ioc. cit.). The authors conclude that nitropyrenes are potent mutagens in some strains of S. typhimurium, causing frameshift mutations by the formation of adducts between DNA and nitropyrene metabolites. They also postulate that the penultimate intermediates for mutagenic activity (hydroxylamines) are not obtained in all instances by reduction of the nitro function by the 'classical' nitroreductase but by another nitroreductase which appears to be specific for higher nitrated PAHs. [The authors do not record having used "positive' and "negative" controls in this study. Several of the suggestions put forward are based partly on unpublished observations, making appraisal somewhat difficult. Some of these unpublished results are reported to show that the "new' nitroreductases may be unique to bacteria.]

Filtering out the coronaries? Castelli, W. P., Garrison, R. J., Dawber, T. R., McNamara, P. M., Feinleib, M. & Kannel, W. B. (1981). The filter cigarette and coronary heart disease: the Framingham study. Lancet ii, 109. The smoker receives regular advice on his particular habit from people in all walks of life. Of th.ese, politicians and clinicians probably contribute most strongly to any observed alteration in smoking behaviour. Certainly, the introduction of filtered cigarettes in 1950 led many people to change brands perhaps on the assumption that this might reduce the risk of developing cancer, coronary heart disease or pulmonary disease. However, these assumptions can rarely be tested. The paper cited above discusses one test that has been made using the cohort of people taking part in the long-running study at the National Heart, Lung, and Blood Institute, Framingham, Massachusetts. Regular examinations have been carried out on this group every 2yr since 1948 and the results presented are from data collected at the first and at the seventh to fourteenth examinations. Examination 12 was the most recent for which smoking data were available. It was impossible to analyse the data for the women involved in the study since only the youngest smoked

at the time of the seventh examination, carried out in 1963-1964. The 1605 men who smoked at the seventh examination were divided into two groups according to age (above or below 55). A greater proportion of the younger than of the older group smoked cigarettes (60 and 42°,o, respectively) but the proportion of smokers using filters was similar (58 and 57°4, respectively). The filter smokers typically had slightly lower smoking exposure prior to examination 7 than did smokers of non-filter cigarettes but otherwise there were few differences between the two groups in overall smoking behaviour. The smoking data were compared with the rates of occurrence of myocardial infarction, coronary heart disease and death from coronary heart disease over the 14-yr period (examinations 7-14). No clear trends were observed in the older group but in the younger, the non-smokers had the lowest rates of heart disease and the filter-cigarette smokers the highest. However. no statistically significant differences were observed. These findings were unchanged after adjustment for age, systolic blood pressure and serum cholesterol levels. Filter-cigarette smokers have been observed to have a lower cancer mortality, a finding presumably due to the reduction in tar exposure (Bross & Gibson, Am. J. publ. HIth 1968, 58, 1396; Wynder & Stellman, d. natn. Cancer Inst. 1979, 62, 471). Two other components of cigarette smoke, nicotine and carbon monoxide, have been studied in connection with coronary heart disease. Data on trends in the nicotine content of cigarettes during the study period indicated that the smokers of filter cigarettes had lower nicotine exposure, and therefore it appears that nicotine lowering does not have much effect on the risk of coronary heart disease in smokers of filter cigarettes. On the other hand carbon monoxide has previously been shown to be associated with athersclerosis, a common cause of heart failure (Cited in F.C.T. 1974, 12, 150), and carbon monoxide levels may actually be increased by some filters, particularly unperforated ones. The possibility that the slightly higher rates of coronary heart disease in filter-cigarette smokers was due to a selection of such cigarettes by men who had had subclinical 'warning' of the disease seems unlikely, since most of the men had been smoking filter cigarettes for a number of years before examination 7. Suspicion about the benefits of filters is also raised by the fact that the men in the filter-cigarette group had had more favourable smoking histories prior to examination 7. This study indicates that the original philosophy that many of the risks associated with smoking would be reduced by the use of filters may well be erroneous, at least as far as coronary heart disease is concerned. Amongst the older group of men, less than one third continued to smoke cigarettes until examination 12, and those who continued to smoke had reduced their daily consumption. Significant numbers of men in the younger age group had also given up smoking. A considerable number of men (11-42°,o) were lost to follow-up at examination 12. In commenting on the study described above, Lee (Lancet 1981, !I, 642) cites conflicting findings from a recent review (Lee & Garfinkel, J. Epidem. commun. HIth 1981, 35, 16) of available evidence on mortality

Abstracts and comments--Fd Chem. Toxic. Vol. 20, No. 4 and type of cigarette smoked, which suggested that coronary heart disease mortality was 10--20~ lower in smokers of filter cigarettes than in smokers of plain cigarettes. Lee points out that the numbers involved in the Framingham study put severe limitations on the conclusions that can be drawn from the data. The studies considered in his review involved much larger numbers of deaths. However Castelli (Lancet 1981, II, 642) replies that the Framingham study did allow consideration of other potentially confounding variables such as diet and exercise.]

Carcinogenicity confounded Salsburg, D. (1980). The effects of lifetime feeding studies on patterns of senile lesions in mice and rats. Dru9 Chem. Toxicol. 3(1), I. The evaluation of the long-term toxicity of compounds in man is beset with problems, not the least of which is the interpretation of data that arise from animal bioassays. The paper cited here questions the validity of conventional methods of analysis. It is argued that the statistical models traditionally applied to chronic toxicity studies to not take account of essential biological considerations. Current methods of analysis are based on a procedure in which, taking each lesion in turn, all of the animals are assigned to one of only two categories--those that develop the lesion and those that do not. For comparison between treated groups and controls, the incidence of one lesion is treated as independent of that of any other. Salsburg suggests that this ignores a fundamental toxicological principle. He believes that consideration should be given to the premise that a biologically active compound will actually produce a dose-related shift in the pattern of lesions. In subchronic tests, that shift is easily followed, but lifetime studies are more difficult to interpret since the pattern of lesions in old control animals is itself complex. More effective analysis should begin with a search for patterns of lesions that differ between treatment groups. If independent clusters of lesions are located, it is possible to decide whether the treatment induces a shift of animals from one cluster to another. Using carcinogenicity data generated at the Huntingdon Research Centre on xylitol, sorbitol and sucrose, Salsburg illustrates his point by applying a multivariate analysis to data previously analysed by conventional univariate methods. The method of multivariate analysis used allows the patterns of statistical correlations among various lesions to cluster animals into groups that appear to have common syndromes and, finally, the relationships between the clusters and treatment are examined. One important point arises from this reanalysis. It seems to Salsburg too simplistic to report, without qualification, that xylitol is a carcinogen in the rat. Although xylitol increases, in a dose-related manner, the incidence of adrenal phaeochromocytomas, it can be shown from the same experimental data that it also decreases, again in a clearly defined dose-related manner, the incidence of some other types of tumour, such as cutaneous fibrosarcoma. Other (fairly convincing) reasons are offered in support of the multivariate approach. Salsburg concludes that some fresh thought should

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be given either to the way in which carcinogenicity bioassays are conducted at present or to the evaluation of the data that conventional bioassays generate. Because of the complexities of the pathology found in experiments involving animals in advanced old age, some consideration could be given to reducing the length of studies so that the treated animals can be compared with a relatively uncomplicated baseline of lesions in the controls. If it is unacceptable to turn the clock back--carcinogenicity studies conducted two decades or so ago often did not exceed 12 months--then more complex methods of data analysis must be used. [This is bad news for non-statistically-minded toxicologists. Multivariate analysis, if it does become the acceptable way to proceed, will bring down the curtain on an era of accessible carcinogenicity verdicts. The days when a summary of data from a study would allow an 'expert" to evaluate potential carcinogenicity may well be numbered.]

Screening out the irritations in life Burton, A. B. G., York M. & Lawrence, R. S. (1981). The in vitro assessment of severe eye irritants. Fd Cosmet. Toxicol. 19, 471. The urgent--some would say vital--search for more humane methods of testing new or everyday chemicals for their likely toxicological hazards has encouraged the development of new approaches to applied toxicological research. One area of activity has been in methods of assessing potential eye irritancy to replace the successful but often painful Draize test (Draize et al. J. Pharmac. exp. Ther. 1944, 82, 377). A method that has now been devised involves the careful removal of the eyes from newly killed New Zealand white rabbits previously confirmed as healthy and free from ocular abnormalities. Each eye is held by means of a perspex clamp in a temperaturecontrolled 'superfusion' chamber and isotonic saline is dripped onto the cornea at the limbus. The temperature is maintained, by means of a water jacket, at 32 _+ I:C, the pre-ocular temperature in the rabbit. The test substance is applied to the eye instead of the saline for a set time (usually 10sec) and is then removed by rinsing, after which the saline superfusion is continued. The effects are assessed for up to 4 hr after this treatment using a slit-lamp biomicroscope, the walls of the compartments being of black perspex to facilitate these observations. Corneal thickness is also measured. A series of chemicals were tested in this way, including known severe irritants such as sodium hydroxide and formaldehyde, moderate irritants such as allyl alcohol and acetone, and slightly or negligibly irritating compounds such as glycerine and propylene glycol. The results were generally comparable with those of a series of in vivo tests reported by Carpenter & Smyth (Am. J. Ophthal. 1946, 29, 1363), particularly for the severely irritating chemicals, which must cause the greatest pain when tested in live animals. There were, however, some notable differences. Toluene, for example, was moderately to severely irritating (grade 7) to the rabbit eye in the in vivo test undertaken by