Different teratogenic efficacy to mouse fetal CNS of 5‐azacytidine in combination with X‐irradiation depends on the sequence of successive application

Abstract

The single treatment of pregnant mice on day 12 post conception with 5-azacytidine (AzaCr), followed by a single irradiation dose of 200 rad two hours later, is exclusively neurotoxic to the fetus, as shown by a severe hypoplasia of the parieto-occipital regions of the telencephalon. This effect is explicable by the specific function of the mitotic cell population for the integrity of the cortex wall. Combining these two hazards in the reverse manner, i.e., irradiation followed by AzaCr, resulted in no general hypoplastic effect in the forebrain and only caused a depletion of cells in the marginal cortex. This indicates a significantly diminished AzaCr sensitivity of fetal cortical cells subsequent to X-irradiation. In addition, rosette-like cell clustering in the cortex of all X-irradiated animals occurs to a similar degree, irrespective of any additional AzaCr-treatment. The only conformity between these different schedules is that a great portion of the surviving cells is most likely in the DNA synthesizing phase at the time of irradiation. It is therefore concluded that rosette formation starts perferentially from cells injured during the S-phase.

Sequence-dependent toxicity and small bowel mucosal injury in neonatal mice treated with low doses of 5-azacytidine and X-irradiation at the late organogenesis stage

“…Although the doses of both azacytidine and X-irradiation were substantially lower than those previously reported (Schmahl 1979), the main features of CNS abnormalities were similar to the earlier results:…”

“…The pathogenesis of these sequence-dependent effects can be attributed to a selective vulnerability of cells in different stages of the generation cycle. This comprises a high degree of cytolethality affecting the S/Ge-stage cells in azacytidine/X-irradiation treatment and the GJS-stage cells in the reverse combinations (Schmahl 1979). The present observations show the validity of a teratological assay in providing a detailed analysis of the cell kinetic responses after combined noxious influences.…”

“…Similar effects of drug sequence-dependent toxicity are also known from combination experiments in mice, using only drugs with different sequences and intervals (Paterson et al 1979). Some years ago, we already referred to the validity of a teratological assay system for evaluating drug-radiation interactions (Schmahl 1979). Besides its practical importance in public health politics (UNSCEAR 1982), prenatal pathology also provides a good possibility of studying cell kinetic responses in an in vivo-model system (Schmahl 1982), since in rodents the early fetal cell populations of the different organs are almost homogeneous with approximately constant cycle times until day 14-15 of gestation (Kauffman 1968(Kauffman , 1975Schultze et al 1974).…”

“…In the Aza----> X sequence the preferential destruction of those neuroepithelial cells passing through the S-, G2-, and M-stages during treatment led to severe disruptions of the telencephalic inner limiting membrane and subsequently to a marked hypoplasia of the forebrain. In the X --~ Aza sequence, however, which was mainly effective against cells in the Gl-stage and the subsequent S-stage, only minor events were observed such as a mild reduction of cortex layers 1 and 2 (Schmahl 1979).…”

“…In a previous report (Schmahl 1979) we described the different teratogenic efficacy of a combined treatment of pregnant mice on day 12 of gestation with azacytidine ("Aza") and X-rays ("X") either in an Aza ~ X sequence or in a X ~ Aza sequence at a two hour interval. As azacytidine is readily accumulated in the fetal CNS (Seifertova et al 1972(Seifertova et al , 1974, we observed the main pathological effects of the combined treatment only in the preterm CNS.…”