The November 2021 EMM Editor's Choice article is “Effect of life stage and target tissue on dose–response assessment of ethyl methane sulfonate-induced genotoxicity” (https://onlinelibrary.wiley.com/doi/10.1002/em.22465) by Roberta Mittelstaedt, Azra Dad, Mason Pearce, Robert Heflich and Xuefei Cao.
The protease inhibitor drug nelfinavir mesylate (Viracept®) can be used to treat human immunodeficiency virus (HIV) infection, which causes acquired immunodeficiency syndrome (AIDS) in HIV infected patients. In 2007, the manufacturer of Viracept® issued a recall of the drug due to contamination with mutagenic ethyl methane sulfonate (EMS). The EMS contamination resulted from a reaction between methane sulfonic acid used to manufacture Viracept® with residual ethanol used to clean a storage tank. EMS is a direct-acting, DNA reactive, monofunctional ethylating agent that is mutagenic in numerous genotoxicity tests. EMS is also classified as possibly carcinogenic to humans (Group 2B carcinogen) by the International Agency for Research on Cancer. Following the Viracept® recall, studies were conducted in rat, for in vitro chromosome damage, and in a Muta™Mouse transgenic rodent (TGR) assay. Data from these studies suggested that EMS posed no significant cancer risk for adult humans treated with EMS-contaminated Viracept® at the estimated daily dose. However, as Viracept® is prescribed to prevent vertical transmission of the HIV virus from pregnant mothers to their progeny, the EMS-exposed population could include adults, children, newborns, and possibly the unborn.
In their November 2021 article, Mittelstaedt and colleagues evaluated EMS-induced mutations in another TGR, gpt-delta mice, which have lower spontaneous mutant frequencies (MFs) than the Muta™Mouse model used in the initial risk assessment. In addition, pharmacokinetic modeling for the EMS impurity risk assessment was based on adult exposures, and the previous Muta™Mouse experiments were conducted in young adult rodents. Therefore, Mittelstaedt et. al. expanded the life-stage of the EMS exposures to neonate gpt-delta mice for comparisons of EMS-induced genotoxicity to that in gpt-delta mice treated as adults. In their study, the Investigators evaluated micronucleus (MN) induction in peripheral blood and gpt gene mutation in liver, lung, bone marrow, small intestine, spleen, and kidney. MN frequencies in the peripheral blood of EMS-exposed neonates were not substantially different when compared to the EMS-exposed adult gpt-delta mice. However, gpt mutations were greater in the kidneys of EMS-treated neonates, while in the EMS-treated adults, higher mutant frequencies were observed in bone marrow, liver and spleen. Benchmark dose potency ranking (a method that identifies the dose that produces a predetermined change in the response rate of an adverse effect) indicated that the MF differences for kidney, although small, were significant. The results of this study further add to the body of evidence for an effect of life-stage on MFs observed in TGR neonates treated with other mutagens as compared to MFs in dosed adults. The Investigators determined that overall, the results from this study suggest that the MN frequencies and MFs in EMS-treated gpt-delta neonate and adult mice were similar. Thus, the Investigators concluded that the data derived from the adult Muta™Mouse TGR study used to estimate the EMS cancer risk associated with the exposure to contaminated Viracept® adequately described the risk to both the exposed patients and their progeny.