Lack of human relevance for procymidone's developmental toxicity attributable to species difference in its kinetics and metabolism

Toxicity of procymidone to Bombyx mori based on physiological and transcriptomic analysis

Procymidone is widely used in vegetables and fruits because of its broad-spectrum and high efficiency. However, it is unclear whether procymidone can affect silkworm (Bombyx mori) growth and cocoon production. This study investigated the effects of procymidone on the growth and cocoon production of silkworms. We analyzed the growth, and cocoon quality of fifth instar larvae fed on mulberry leaves saturated with different concentrations (2.5, 5, and 10 mg/ml) of procymidone and the control. Results showed that procymidone supplementation decreased the larval growth and cocoon quality compared to the control group, suggesting that procymidone had toxicity to silkworms. Additionally, after transcriptomic analysis, we identified 396 significantly differentially expressed genes (DEGs) in the presence of procymidone. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) illustrated that these DEGs were closely related to metabolism. Taken together, these results confirmed that procymidone could cause toxicity by affecting metabolism in silkworm larvae. We believed that these results could provide important materials for the effect of procymidone on silkworms and gave us some clues for pesticides used in the mulberry garden.

Prediction of the human pharmacokinetics of epyrifenacil and its major metabolite, S-3100-CA, by a physiologically based pharmacokinetic modeling using chimeric mice with humanized liver

Human internal dosimetry of pesticides is essential in the risk assessment when toxicity has been confirmed in laboratory animals. While human toxicokinetics data of pesticides are hardly obtained intendedly, the use of physiologically based pharmacokinetic (PBPK) modeling has become important for predicting human internal dosimetry. Especially, when the compound exhibits complicated pharmacokinetics via active uptake, metabolism, and biliary excretion in liver, it is difficult to obtain these hepatic parameters only by the in vitro experiments. Epyrifenacil, a new herbicide, is rapidly metabolized to S-3100-CA (CA) in mammals and causes hepatotoxicity in mice. CA is eliminated from the systemic circulation by biliary excretion and metabolism in liver. Although uptake of CA by transporters is observed in mouse primary hepatocytes, significantly less of it is observed in human primary hepatocytes. In order to evaluate human internal dosimetry of CA, a precise PBPK model was developed. To obtain human hepatic parameters, i.e., hepatic elimination intrinsic clearance via biliary excretion and metabolism, we used chimeric mice with humanized liver as a model to reproduce the complicated pharmacokinetics of CA in humans. After we developed a mouse PBPK model, by replacing mouse parameters with those of humans, we calculated CA concentration in human liver. Comparing the predicted CA exposure in human liver with the measured values in mice, we demonstrated a clear interspecies difference of approximately 4 times lower C_max and AUC in humans. This result suggested that the risk of hepatotoxicity is less in humans than in mice.

Comparative hepatotoxicity of a herbicide, epyrifenacil, in humans and rodents by comparing the dynamics and kinetics of its causal metabolite

A new herbicide, epyrifenacil (S-3100), inhibits protoporphyrinogen oxidase (PPO) in plants. Repeated administration of epyrifenacil in laboratory animals led to some toxicological changes related to PPO inhibition, e.g., hepatotoxicity caused by porphyrin accumulation and anemia caused by the inhibition of heme biosynthesis. In vitro studies revealed that an ester-cleaved metabolite, S-3100-CA, is predominant in mammals, exhibits PPO-inhibitory activity, and thus is the cause of epyrifenacil-induced toxicity. To assess the human risk, the effects of species differences on the dynamics (PPO inhibition) and kinetics (liver uptake) of epyrifenacil were evaluated separately. The results of in vitro assays revealed an approximately tenfold weaker inhibition of PPO by S-3100-CA in humans than in rodents and six- to thirteen-fold less hepatic uptake of S-3100-CA in humans than in mice. Finally, it was suggested that humans are less sensitive to the toxicity of epyrifenacil than are rodents, although further mechanistic research is highly anticipated.

Developmental toxicity of procymidone to larval zebrafish based on physiological and transcriptomic analysis

As a broad-spectrum with low toxicity, procymidone (PCM), is widely used in agriculture and frequently observed in aquatic system, which may cause some impacts on aquatic organisms. Here, to determine the developmental toxicity of PCM, embryonic and larval zebrafish were exposed to PCM at 0, 1, 10, 100 μg/L in dehydrogenated natural water containing 0.01% acetone for 7 days. The results showed that high concentration of PCM could cause the pericardial edema and increase the heart rates in larval zebrafish, suggesting that PCM had developmental toxicity to zebrafish. We also observed that PCM exposure not only changed the physiological parameters including TBA, GLU and pyruvic acid, but also changed the transcriptional levels of glycolipid metabolism related genes. In addition, after transcriptomics analysis, a total of 1065 differentially expressed genes, including 456 up-regulated genes and 609 down-regulated genes, changed significantly in 100 μg/L PCM treated larval zebrafish. Interestingly, after GO (Gene Ontology) analysis, the different expression genes (DEGs) were mainly enriched to the three different biology processes including GABA-nervous, lipid Metabolism and response to drug. We also observed that the levels of GABA receptor related genes including gabrg2, gabbr1α, gabbr1 and gabra6α were inhibited by PCM exposure. Interestingly, the swimming distance of larval zebrafish had the tendency to decrease after PCM exposure, indicating that the nervous system was affected by PCM. Taken together, the results confirmed that the fungicide PCM could cause developmental toxicity by influencing the lipid metabolism and GABA mediated nervous system and behavior in larval zebrafish. We believed that the results could provide an important data for the influence of PCM on aquatic animals.

Researches on the evaluation of pesticide safety in humans using a pharmacokinetic approach

Similar to the pharmaceutical compounds, pesticides require human safety assessment for their registration and distribution; however, it is absolutely impossible to assess human safety by dosing humans with pesticides. Thus, how to appropriately evaluate the safety of pesticides in humans remains a great subject of debate. In this article, we present some examples of pesticide toxicity studies that identify species differences in toxicity and evaluate human safety by applying combinations of novel in vivo, in vitro, and in silico techniques to separately assess the key toxicodynamic (i.e., sensitivity) and/or toxicokinetic (i.e., exposure) factors. Because it is scientifically sound, the safety assessment strategy illustrated for three compounds in this article is expected to play an important role in the human safety assessment of agricultural compounds.

Analysis for data-derived extrapolation factors for procymidone

The derivation of Chemical Specific Adjustment Factors (CSAFs) (IPCS, 2005; U.S. EPA, 2014) depends on the choice of appropriate dose metric. EPA and IPCS guidance was applied to derive a CSAF for developmental toxicity for procymidone (PCM). Although kinetic data were not available in humans at any dose, sufficient toxicokinetic data are available in a surrogate species, primates, and from chimeric mice with both rat and human liver cells to offer insights. Alternative approaches were explored in the derivation of the CSAG based on review of the available kinetic data. The most likely dosimetric adjustment is the Cmax based on the character of the critical effect - reduced anogenital distance and increased incidence of hypospadias in male rats, which likely occurs during a small window of time during development of the rat fetus. Cmax is also the default dosimeter from U.S. EPA (1991). However, in this case, the use of Cmax is also likely more conservative than the use of area under the curve (AUC), which otherwise is the default recommendation of the IPCS (2005). Despite human data, estimated tentative CSAF value is 0.48 (range, 0.22 to 0.74). The use of any of these values would be supported by the available data.