# Nitrosamine carcinogenesis in 5120 rodents: chronic administration of sixteen different concentrations of NDEA, NDMA, NPYR and NPIP in the water of 4440 inbred rats, with parallel studies on NDEA alone of the effect of age of starting (3, 6 or 20 weeks) and of species (rats, mice or hamsters).

##### Peto R., Gray R., Brantom P., Grasso P.

A Weibull analysis is presented of the dose and time relationships for the effects on 4 080 inbred rats of chronic ingestion in the drinking water of 16 different doses of N-nitroso-dimethylamine (NDMA) and of N-nitrosodiethylamine (NDEA). The sites chiefly affected were the liver (by both agents) and the oesophagus (by NDEA only). Since the experiment continued into extreme old age, effects became clearly measurable even at a dose of only 0.01 mg/kg per day, which is an order of magnitude lower than previously achieved. (After only two years of treatment, however, the 'TD50' doses needed to halve the proportion of tumourless survivors would have been about 0.06 mg/kg per day of NDEA and about 0.12 mg/kg per day of NDMA.) The general pattern of response was that the log probability of remaining tumourless was given by the product of two terms, the first (the 'Weibull b-value') depending on the dose-rate but not on the duration of exposure, and the second depending not on dose at all but on (approximately the seventh power of) duration. For oesophageal tumours, the 'Weibull b-value' was approximately proportional to the cube of the dose-rate of NDEA (males 21 d3, females 11 d3, where d = dose-rate in mg/kg adult body weight/day, and the background incidence was unmeasurably low. For liver tumours induced by NDEA, the b-value was approximately proportional to the fourth power of dose-rate + 0.04 mg/kg per day (males, 19 (d + 0.04)4; females, 32 (d + 0.04)4), although the relationships were slightly different for the different subsites of liver tumour. This one formula implies both approximate linearity at low doses and an approximately cubic relationship within the higher range of doses that was studied. For liver tumours induced by NDMA, the Weibull b-value was approximately proportional to the sixth power of dose rate + 0.1 mg/kg per day (males, 37 (d + 0.1)6; females, 51 (d + 0.1)6), again with variation between liver subsites, and again implying approximate linearity at low doses. The difference between the latter two relationships may represent differences in the induction of particular DNA repair enzymes by NDMA and NDEA, or in the effects of those enzymes on methylated and on ethylated DNA. These formulae should, of course, be trusted only in the range of doses from which they were derived, and particularly not for those above it.(ABSTRACT TRUNCATED AT 400 WORDS)