Activity of iPMS and nPMS in mouse bone marrow micronucleus assays: comparison with mouse dominant lethal assay data

Poor recognition of O6-isopropyl dG by MGMT triggers double strand break-mediated cell death and micronucleus induction in FANC-deficient cells

Isopropyl methanesulfonate (IPMS) is the most potent genotoxic compound among methanesulfonic acid esters. The genotoxic potential of alkyl sulfonate esters is believed to be due to their alkylating ability of the O6 position of guanine. Understanding the primary repair pathway activated in response to IPMS-induced DNA damage is important to profile the genotoxic potential of IPMS. In the present study, both chicken DT40 and human TK6 cell-based DNA damage response (DDR) assays revealed that dysfunction of the FANC pathway resulted in higher sensitivity to IPMS compared to EMS or MMS. O6-alkyl dG is primarily repaired by methyl guanine methyltransferase (MGMT), while isopropyl dG is less likely to be a substrate for MGMT. Comparison of the cytotoxic potential of IPMS and its isomer n-propyl methanesulfonate (nPMS) revealed that the isopropyl moiety avoids recognition by MGMT and leads to higher cytotoxicity. Next, the micronucleus (MN) assay showed that FANC deficiency increases the sensitivity of DT40 cells to MN induction by IPMS. Pretreatment with O6-benzyl guanine (OBG), an inhibitor of MGMT, increased the MN frequency in DT40 cells treated with nPMS, but not IPMS. Lastly, IPMS induced more double strand breaks in FANC-deficient cells compared to wild-type cells in a time-dependent manner. All together, these results suggest that IPMS-derived O6-isopropyl dG escapes recognition by MGMT, and the unrepaired DNA damage leads to double strand breaks, resulting in MN induction. FANC, therefore, plays a pivotal role in preventing MN induction and cell death caused by IPMS.

Evaluation of in vivo mutagenicity of isopropyl methanesulfonate by RBC Pig-a and PIGRET assays

A comparison between the original red blood cell (RBC) Pig-a assay, which measures Pig-a mutant cells in RBCs, and the PIGRET assay, which uses reticulocytes, was conducted using the in vivo mutagenesis assay with isopropyl methanesulfonate (iPMS) as a part of a collaborative study by the Mammalian Mutagenicity Study Group in the Japanese Environmental Mutagen Society. Three dose levels of iPMS (50, 100, and 200mg/kg) were administered once intraperitoneally to 8-week-old male Crl:CD(SD) rats, and peripheral blood was sampled at 0 (1 day before dosing), and 1, 2, and 4 weeks after dosing with iPMS. As a result, a time-dependent increase in the mutant frequency of Pig-a mutant RBCs was observed in the RBC Pig-a assay, and a statistically significant increase was observed from 2 weeks after dosing. In the PIGRET assay, on the other hand, a statistically significant increase in Pig-a mutant frequency was obtained from 1 week after dosing at all dose levels, and the Pig-a mutant frequency at the highest dose level had already reached a plateau on week 1. The maximum Pig-a mutant frequency induced by a single treatment with iPMS at 200mg/kg in the PIGRET assay was approximately two times higher than that in the RBC Pig-a assay. These results indicate that the PIGRET assay can detect Pig-a mutants much earlier and with a higher value in Pig-a mutant frequency compared with the original RBC Pig-a assay, and it can enable judgement of mutagenicity of iPMS within 1 week after a single dose.

Evaluation of the in vivo mutagenicity of isopropyl methanesulfonate in acute and 28-day studies

Understanding the mutagenic dose response could prove beneficial in the management of pharmaceutically relevant impurities. For most alkyl ester impurities, such as isopropyl methanesulfonate (IPMS), little in vivo mutagenicity data exist for dose analysis. The likelihood of a sublinear dose response for IPMS was assessed by comparing the Swain Scott constant, the SN 1/SN 2 reaction mechanism and the O(6) :N(7) guanine adduct ratio to that of more well-known alkyl esters. Based on available information, IPMS was predicted to have a mutagenic profile most like ethyl nitrosourea. To test this hypothesis, mature male Wistar Han rats were administered IPMS using acute (single administration at 3.5 to 56 mg/kg) or subchronic (28 days at 0.125 to 2 mg/kg/day) exposures. The in vivo Pig-a mutation assay was used to identify mutant phenotype reticulocyte (Ret) and red blood cell (RBC) populations. The maximum mutant response occurred approximately 15 and 28 days after the last dose administration in the mutant Ret and RBC populations respectively in the acute study and on Day 29 and 56 in the mutant Ret and RBC populations, respectively, in the subchronic study. A comparison of RBC mutant frequencies from acute and subchronic protocols suggests a sublinear response; however, this was not substantiated by statistical analysis. A No Observed Effect Level (NOEL) of 0.25 mg/kg/day resulted in a Permitted Daily Exposure equivalent to the Threshold of Toxicological Concern. An estimate of the NOEL based on the previously mentioned factors, in practice, would have pre-empted further investigation of the potent mutagen IPMS.

Detection of the two germ cell mutagens ENU and iPMS using the LacZ/transgenic mouse mutation assay

We have investigated the mutagenic effects of ENU, a potent mutagen in mouse spermatagonia, and of the two postmeiotic germ cell mutagens MMS and iPMS in germ cells, using a transgenic mouse mutation assay (lacZ/Muta Mouse, positive selection system). The test compounds were administered to 6-week-old animals by a single intraperitoneal injection. Seminiferous tubule germ cells were isolated from the testes after an expression time of 52 days and genomic DNA was extracted to examine induced mutations in the lacZ target gene. The spontaneous mutant frequencies observed in the control animals (n = 7) ranged from 3.5 to 17.9 x 10(-5) (mean value 9.5 +/- 5.3 x 10(-5). ENU (150 mg/kg; n = 8) induced a 6.9-fold increase over controls, iPMS (100 mg/kg; n = 7) a 2.4-fold increase, and no effect at all was found following MMS treatment (80 mg/kg; n = 8). The study demonstrates that the transgenic mouse mutation assay is able to detect the germ cell mutagens ENU and iPMS in the target tissue. The critical steps of the assay, however, seem to be dosing and sampling time. In contrast, MMS has failed to induce germ cell mutations in seminiferous tubules of transgenic mice at the tested dose and after an expression period of 52 days.

Mutagenicity to the mouse bone marrow by the mouse germ cell mutagen N-propyl-N-nitrosourea

N-Propyl-N-nitrosourea (PNU) is shown to be active in male mouse bone marrow micronucleus assays when dosed at either 100 or 200 mg/kg in saline. Activity was observed following either intraperitoneal (i.p.) injection or oral gavage. This observation is consistent with the demonstration by Murota and Shibuya of the specific-locus mutagenicity caused by PNU in male mouse spermatogonia when dosed at 200 mg/kg by i.p. injection. These data strengthen further the observation that rodent germ cell mutagens are also mutagenic to rodent somatic cells.