Paracetamol (N-Acetyl-Para-Aminophenol) Disrupts Early Embryogenesis by Cell Cycle Inhibition

A large percentage of pregnancies end in miscarriage or stillbirth, with estimates showing that 10% to 40% of embryos fail to implant and an additional 10% implanting but failing before clinical recognition of the pregnancy. Genetic studies have shown that up to 50% of losses can be linked to aneuploidy and other chromosomal abnormalities, demonstrating that environmental factors also play a role. N-acetyl-para-aminophenol (APAP) is a commonly used over the counter pain relieving medication (also known as acetaminophen); as many as 50% of pregnant women globally report using APAP, and high environmental background exposure through rivers and waterways has been found in several countries in Europe. This study was designed to assess whether APAP interferes with cell division and preimplantation development (PID). The mechanism of APAP’s influence on PID and cell division is thought to be related to DNA replication; DNA replication stress can be caused by the inhibition of ribonucleotide reductase (RNR), as reported in several previous studies. In this study, the effect of APAP on RNR was tested using 3 strains of fission yeast, and results showed an increased sensitivity to chemical RNR inhibition compared with controls as well as decreased proliferation. Subsequent to these experiments, further models were performed in recombinant S. pombe strain models, more correctly simulating APAP interactions with its target cells in humans, and these experiments likewise showed sensitivity to APAP, providing evidence for an inhibition of RNR. In experiments using human somatic cells and embryonic stem cells, exposure to APAP resulted in decreased total cell numbers without affecting cell function, as well as a reduction in DNA synthesis compared with controls. This was similarly true in in vitro experiments, which also showed that NAPQI is unlikely to be responsible for disruption of meiosis, thus indicating a direct effect of APAP exposure. Mouse models of APAP exposure add further evidence to this claim, as early PID in mice was disrupted at 24 and 48 hours after exposure to APAP. This disruption mainly consisted of reduced cell number and slowed development to the blastocyst stage. This effect was also dose-dependent in these models, with higher exposure dosage resulting in more drastic effects. Late PID was also disrupted with APAP exposure, but this disruption consisted of reduced numbers of cells in the ICM compared with controls. Further experiments were conducted in mouse models of pregnancy, and APAP exposure resulted in significantly decreased implantation rates as well as the number of live fetuses, and this relationship was again dose-dependent. In addition, APAP exposure was associated with an increased number of resorption points compared with controls, indicating a higher post-implantation embryonic mortality rate. Human embryos exposed to therapeutic doses of APAP experienced a slight delay in development compared with controls, as well as a reduced number of cells, indicating that these findings may be generalizable to humans. These results indicate that exposure to APAP may be related to early pregnancy loss due to the disruption of PID. This study was not adequate in scope to elucidate the exact mechanism of this disruption, though there are several possibilities that cannot yet be ruled out. Future research should focus on characterizing this mechanism as well as understanding if there are ways to reduce the impact of APAP on PID. In addition, future research should assess similar mechanisms of inhibition caused by other triggers that may contribute to early pregnancy loss. (Abstracted from Hum Reprod. 2025 Oct 1;40(10):1860-1876. doi: 10.1093/humrep/deaf116)