Re: Waalkes et al.: Lung tumors in mice induced by “whole-life” inorganic arsenic exposure at human-relevant doses, Arch Toxicol, 2014

Abstract

and associates for the facilities in which their studies have been performed. Charles River Laboratories reported historical controls (Giknis and Clifford 2005) for males: adenoma incidences ranging between 2 and 42 %, and carcinomas between 1.43 and 26 %. In females, the ranges are 1.67–26.6 % for adenomas and 0.77–18.37 % for carcinomas. The variability in background incidences of these tumors in control mice is evident in the two studies reported by Waalkes’ group (Tokar et al. 2011; present paper) (see attached Table 1). Comparing the incidence of adenoma plus carcinoma at 50 ppb in males to the controls of the previous study (51 vs. 34 %) utilizing a Fisher exact test (oneor two-sided) gives a p value >0.05. p is also >0.05 for a similar comparison for females. Furthermore, the incidences of adenomas and of carcinomas in all groups in Waalkes et al. (2014) are within historical controls reported by Charles River. Thus, it is difficult to make a case for them being treatment related. In addition, the statistical criterion of significance used by Waalkes et al. does not follow the criteria of Haseman et al. (1986). When evaluating the incidence of common tumors (background incidence ≥1 %), Haseman et al. (1986) from the National Toxicology Program (NTP) recommended a p value <0.01 be utilized for statistical significance, not p < 0.05 as used by Waalkes. Using this criterion, the Waalkes tumor data are not significant, even using concurrent controls for comparison. The lack of other tumors in Waalkes et al. (2014) such as liver, adrenal, and ovary, which was reported by the Waalkes group utilizing the same model (Tokar et al. 2011), is also noteworthy. An additional complication in the present experiment was the differences in survival between groups, especially the low-dose groups being considerably less than other groups. For example, in the low-dose females only 2 of The article by Waalkes et al. (2014) on lung tumors in mice induced by inorganic arsenic following “whole-life” exposure is problematic, especially when compared to these investigators’ previous publications (specifically, Tokar et al. 2011). The major difficulties with this model of Waalkes are the lack of consistency and reproducibility. In the present study, Waalkes et al. saw a statistically significant increase (p ≤ 0.05) in lung adenomas in males only at 500 ppb inorganic arsenic, but not at 5,000 ppb, and in females, only at 50 ppb and not at higher doses. A statistically significant increase in lung carcinomas was found only in males and only at 50 ppb, not at higher doses. The authors note the absence of a “typical dose response”. In this context, it is important to compare the current data with the authors’ previous study (Tokar et al. 2011) in which doses of 6,000, 12,000, and 24,000 ppb inorganic arsenic did not increase adenomas in any group, and a statistically significant increase in carcinomas was seen in males only at 24,000 ppb and in females at 12,000 and 24,000 ppb. The peculiar, conflicting results are likely due to the high variability in incidence of lung tumors in CD-1 mice. The well-known wide variability in background incidences of lung tumors in CD-1 mice needs to be incorporated into the interpretation of the results of these studies. Reference to historical controls has not been provided by Dr. Waalkes