Environmental and Molecular Mutagenesis 10:439-440 (1987)

Letter to the Editor

Choice of Route of Exposure in Mammalian Genotoxicity Experiments

Sheu et a1 [1987] recently described the results of a series of germ cell UDS experiments in which MMS and TEM were evaluated following either oral gavage or intraperitoneal (ip) injection; TEM was also evaluated following direct testicular injection. The authors concluded that use of the oral route of exposure could be justified, and further, that direct testicular injection could prove useful for the detection of weak UDS responses. They also established important mouse strain sensitivity differences, a topic also raised last year by de Serres in connection with hycanthone [de Serres, 19861.

Recently a discussion took place in Mutation Research on the relative merits and disadvantages of the oral gavage and ip injection routes of exposure [Ashby, 1986a,b; Shelby, 19861. The data presented by Sheu et a1 [1987] might be seen as presenting the chance for an honorable truce in those discussions, but I do not believe this to be so. Sheu et al used two electrophilic alkylating agents, and almost any proposal can be justified with such materials. For example, alkylating agents are, almost without exception, active in all in vitro genotoxicity assays, but one hopefully would not use that evidence to support the need for only a single mutagenicity assay in genetic toxicology.

Sheu et a1 clearly intended their paper to influence and to be helpful to those conducting in vivo mutagenicity tests on agents of environmental importance, but in this it will fail if data for a wider range of chemicals are not also considered. Dramatic quantitative and sometimes qualitative differences in assay responses to many chemi- cals, dependent upon the route of exposure selected, are to be expected as more data become available. The critical factor was identified by Shelby [1986]; namely, that the purpose for which data are generated should be central to the choice of route of exposure; in particular, the question of whether the derived data are to be used for hazard definition or hazard assessment. In my original paper on this subject [Ashby, 1986a1, I suggested that the limiting case of the ip injection route of exposure for germ cell studies could be considered, hypothetically, as intratesticular injection; such an experiment was described for TEM by Sheu et al. Not surprisingly, a positive result was obtained, but its relevance to hazard assessment is dubious. The authors quoted a dose level of 5 mg/kg of TEM as being lethal, yet they were able to assess the UDS activity of a dose level of 30 mg/kg testis following intratesticular injection. Assuming equal distribution of TEM in all tissues after whole-body administration, the dose level of 30 mg/kg testis is equivalent to a whole-body dose of 10 g/kg, 2 X lo3 of the lethal dose level. By analogy, intraocular injection of TEM would probably produce retinal tumours, but what would be the value of such data?

The choice of route of exposure is one of the most important variables in mammalian genotoxicity testing; I suggest that the article by Sheu et a1 does not

0 1987 Alan R. Liss, Inc.

440 Ashby

resolve the issue, but contributes usefully to it, as do the reports by Tice and his colleagues and by Chidiac and Goldberg in the same issue of Environmental Mutagen- esis [Tice et al, 1987; Chidiac and Goldberg, 19871.

John Ashby ICI Central Toxicology Laboratory Maccles field Cheshire, England

REFERENCES

Ashby J (1986a): Is there a continuing role for the intraperitoneal route of exposure in short-term rodent

Ashby J (1986b): Reply to letter by MD Shelby. Mutat Res 170:173. Chidiac P and Goldberg MT (1987): Lack of induction of nuclear aberrations by Captan in mouse

de Serres FJ (1986): Hycanthone, an unresolved case study in risk assessment. Mutat Res 164: 199-201. Shelby MD (1986): A case for the continued use of the intraperitoneal route of exposure. Mutat Res

Sheu CW, Sega GA, Owens JG (1987): Effect of mode of administration of methyl methanesulphonate and triethylenemelamine on induction of unscheduled DNA synthesis in mouse germ cells. Environ Mutagen 9:281-288.

Tice RR, Boucher R, Luke CA, Shelby MD (1987): Comparative cytogenetic analysis of bone marrow damage induced in male B6C3F1 mice by multiple exposure to gaseous 1,3-butadiene. Environ Mutagen 9:235-250.

genotoxicity assays? Mutat Res 156:239-243.

duodenum. Environ Mutagen 9:297-306.

170:169-171.