Design, Synthesis, and Biological Evaluation of Pyridazinone-Containing Derivatives As Novel Protoporphyrinogen IX Oxidase Inhibitor.

Protoporphyrinogen IX oxidase (PPO, E.C. 1.3.3.4) plays a pivotal role in chlorophyll biosynthesis in plants, making it a prime target for herbicide development. In this study, we conducted an investigation aimed at discovering PPO-inhibiting herbicides. Through this endeavor, we successfully identified a series of novel compounds based on the pyridazinone scaffold. Following structural optimization and biological assessment, compound 10ae, known as ethyl 3-((6-fluoro-5-(6-oxo-4-(trifluoromethyl)pyridazin-1(6H)-yl)benzo[d]thiazol-2-yl)thio)propanoate, emerged as a standout performer. It exhibited robust activity against Nicotiana tabacum PPO (NtPPO) with an inhibition constant (Ki) value of 0.0338 µM. Concurrently, we employed molecular simulations to obtain further insight into the binding mechanism with NtPPO. Additionally, another compound, namely, ethyl 2-((6-fluoro-5-(5-methyl-6-oxo-4-(trifluoromethyl)pyridazin-1(6H)-yl)benzo[d]thiazol-2-yl)thio)propanoate (10bh), demonstrated broad-spectrum and highly effective herbicidal properties against all six tested weeds (Leaf mustard, Chickweed, Chenopodium serotinum, Alopecurus aequalis, Poa annua, and Polypogon fugax) at the dosage of 150 g a.i./ha through postemergence application in a greenhouse. This work identified a novel lead compound (10bh) that showed good activity in vitro and excellent herbicidal activity in vivo and had promising prospects as a new PPO-inhibiting herbicide lead.

Design, Synthesis, and Herbicidal Evaluation of Pyrrolidinone-Containing 2-Phenylpyridine Derivatives as Novel Protoporphyrinogen Oxidase Inhibitors.

In this work, a series of pyrrolidinone-containing 2-phenylpyridine derivatives were synthesized and evaluated as novel protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) inhibitors for herbicide development. At 150 g ai/ha, compounds 4d, 4f, and 4l can inhibit the grassy weeds of Echinochloa crus-galli (EC), Digitaria sanguinalis (DS), and Lolium perenne (LP) with a range of 60 to 90%. Remarkably, at 9.375 g ai/ha, these compounds showed 100% inhibition effects against broadleaf weeds of Amaranthus retroflexus (AR) and Abutilon theophrasti (AT), which were comparable to the performance of the commercial herbicides flumioxazin (FLU) and saflufenacil (SAF) and better than that of acifluorfen (ACI). Molecular docking analyses revealed significant hydrogen bonding and π-π stacking interactions between compounds 4d and 4l with Arg98, Asn67, and Phe392, respectively. Additionally, representative compounds were chosen for in vivo assessment of PPO inhibitory activity, with compounds 4d, 4f, and 4l demonstrating excellent inhibitory effects. Notably, compounds 4d and 4l induced the accumulation of reactive oxygen species (ROS) and a reduction in the chlorophyll (Chl) content. Consequently, compounds 4d, 4f, and 4l are promising lead candidates for the development of novel PPO herbicides.

Sumatran fleabane (Conyza sumatrensis [Retz.] E. Walker) control in soybean with combinations of burndown and preemergence herbicides applied in the off-season

Sumatran fleabane (Conyza sumatrensis [Retz.] E. Walker) can be found in many different agricultural environments and impact different crops, such as soybeans and corn. It is believed that the application of burndown and preemergence herbicides in the off-season are effective in controlling Sumatran fleabane in soybean crops. The objective was to evaluate the effectiveness of burndown and preemergence herbicides in the off-season, with one or two applications, in the control of Sumatran fleabane in soybean cultivation. Five field experiments were conducted in Maripá, state of Paraná (PR), Brazil. The treatments consisted of the application of burndown herbicides in combinations with preemergence ones, with one or two applications. Control of Sumatran fleabane and soybean yield were evaluated. With the set of experiments, it is highlighted that the strategy combining more applications, with different herbicides, burndown and preemergence, is more promising in the control of Sumatran fleabane. When comparing synthetic auxins, dicamba and triclopyr stand out. For sequential application, worse performance was observed for diquat. Combinations between burndown and preemergence herbicides were effective in controlling Sumatran fleabane, for pre sowing application in soybean. With emphasis on managements with sequential applications of saflufenacil with glufosinate or glyphosate. The strategy combining more applications, with different herbicides, burndown and preemergence herbicides, is more promising in the control of Sumatran fleabane.

Discovery of a Potent Thieno[2,3-d]pyrimidine-2,4-dione-Based Protoporphyrinogen IX Oxidase Inhibitor through an In Silico Structure-Guided Optimization Approach.

A key objective for herbicide research is to develop new compounds with improved bioactivity. Protoporphyrinogen IX oxidase (PPO) is an essential target for herbicide discovery. Here, we report using an in silico structure-guided optimization approach of our previous lead compound 1 and designed and synthesized a new series of compounds 2-6. Systematic bioassays led to the discovery of a highly potent compound 6g, 1-methyl-3-(2,2,7-trifluoro-3-oxo-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-benzo[b][1,4]oxazin-6-yl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione, which exhibited an excellent and wide spectrum of weed control at the rates of 30-75 g ai/ha by the postemergence application and is relatively safe on maize at 75 g ai/ha. Additionally, the Ki value of 6g to Nicotiana tabacum PPO (NtPPO) was found to be 2.5 nM, showing 3-, 12-, and 18-fold higher potency relative to compound 1 (Ki = 7.4 nM), trifludimoxazin (Ki = 31 nM), and flumioxazin (Ki = 46 nM), respectively. Furthermore, molecular simulations further suggested that the thieno[2,3-d]pyrimidine-2,4-dione moiety of 6g could form a more favorable π-π stacking interaction with the Phe392 of NtPPO than the heterocyclic moiety of compound 1. This study provides an effective strategy to obtain enzyme inhibitors with improved performance through molecular simulation and structure-guided optimization.

Identification of the organic anion transporting polypeptides responsible for the hepatic uptake of the major metabolite of epyrifenacil, S-3100-CA, in mice.

Epyrifenacil is a novel herbicide that acts as an inhibitor of protoporphyrinogen oxidase (PPO) and produces hepatotoxicity in rodents by inhibiting PPO. Our previous research revealed that the causal substance of hepatotoxicity is S-3100-CA, a major metabolite of epyrifenacil, and that human hepatocyte uptake of S-3100-CA was significantly lower than rodent one, suggesting less relevant to hepatotoxicity in humans. To clarify the species difference in the uptake of S-3100-CA, we focused on organic anion transporting polypeptides (OATPs) and carried out an uptake assay using human, rat, and mouse OATP hepatic isoforms-expressing 293FT cells. As a result, all the examined OATPs were found to contribute to the S-3100-CA uptake, suggesting that the species difference was not due to the differences in selectivity toward OATP isoforms. When [14 C]epyrifenacil was administered to mice, the liver concentration of S-3100-CA was higher in males than in females. Furthermore, when [14 C]epyrifenacil was administered with OATP inhibitors, the liver/plasma ratio of S-3100-CA was significantly decreased by rifampicin, an Oatp1a1/Oatp1a4 inhibitor in mice, but not by digoxin, an Oatp1a4-specific inhibitor. This result indicates that Oatp1a1, the predominant transporter in male mice, is the main contributor to the hepatic transport of S-3100-CA, and consequently to the gender difference. Moreover, we conclude that the species difference in the hepatic uptake of S-3100-CA observed in our previous research is not due to differences in the selectivity toward OATP isoforms but rather to the significantly higher expression of OATPs which mediate uptake of S-3100-CA in rodents than in humans.

Design and synthesis of acrylate and acrylamide substituted pyrimidinediones as potential PPO herbicides.

PPO herbicides emerge to be widely use in the agricultural field and a focus of research to many scientists due to its environmentally-friendly properties. In lieu with this, this study presents acrylate and acrylamide substituted pyrimidinediones as PPO herbicide candidates. Most synthesized compounds exhibits herbicidal activities against both monocot and dicot weeds, especially, compound 5a which showed non-selective superior activity against the commercialized, Saflufenacil. Compound 5a was further tested for residual effect and showed promising results as shorter period is needed to cultivate the next crops. The synthesized acrylate and acrylamide substituted pyrimidinediones, especially, 5a could potentially be utilized in the development of commercial protoporphyrinogen oxidase inhibitors with further tests and studies.

Different effects of an N-phenylimide herbicide on heme biosynthesis between human and rat erythroid cells.

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects and wavy ribs) were produced by S-53482, an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. The sequence of key biological events in the mode of action has been elucidated as follows: inhibition of PPO interferes with normal heme synthesis, which causes loss of blood cells leading to fetal anemia, embryolethality and the development of malformations. In this study we investigated whether the rat is a relevant model for the assessment of the human hazard of the herbicide. To study effects on heme biosynthesis, human erythroleukemia, human cord blood, and rat erythroleukemia cells were treated with the herbicide during red cell differentiation. Protoporphyrin IX, a marker of PPO inhibition, and heme were determined. We investigated whether synchronous maturation of primitive erythropoiesis, which can contribute to massive losses of embryonic blood, occurs in rats. The population of primitive erythroblasts was observed on gestational days 11 through 14. Heme production was suppressed in rat erythroid cells. In contrast, heme reduction was not seen in both human erythroid cells when PPO was inhibited. Rats underwent synchronous maturation in primitive erythropoiesis. Our results combined with epidemiological findings that patients with deficient PPO are not anemic led us to conclude that human erythroblasts are resistant to the herbicide. It is suggested that the rat would be an inappropriate model for assessing the developmental toxicity of S-53482 in humans as rats are specifically sensitive to PPO inhibition by the herbicide.

Genetic variation associated with PPO-inhibiting herbicide tolerance in sorghum.

Herbicide application is crucial for weed management in most crop production systems, but for sorghum herbicide options are limited. Sorghum is sensitive to residual protoporphyrinogen oxidase (PPO)-inhibiting herbicides, such as fomesafen, and a long re-entry period is required before sorghum can be planted after its application. Improving sorghum for tolerance to such residual herbicides would allow for increased sorghum production and the expansion of herbicide options for growers. In this study, we observed sorghum tolerance to residual fomesafen. To investigate the underlying tolerance mechanism a genome-wide association mapping study was conducted using field-collected sorghum biomass panel (SBP) data, and a greenhouse assay was developed to confirm the field phenotypes. A total of 26 significant SNPs (FDR<0.05), spanning a 215.3 kb region on chromosome 3, were detected. The ten most significant SNPs included two in genic regions (Sobic.003G136800, and Sobic.003G136900) and eight SNPs in the intergenic region encompassing the genes Sobic.003G136700, Sobic.003G136800, Sobic.003G137000, Sobic.003G136900, and Sobic.003G137100. The gene Sobic.003G137100 (PPXI), which encodes the PPO1 enzyme, one of the targets of PPO-inhibiting herbicides, was located 12kb downstream of the significant SNP S03_13152838. We found that PPXI is highly conserved in sorghum and expression does not significantly differ between tolerant and sensitive sorghum lines. Our results suggest that PPXI most likely does not underlie the observed herbicide tolerance. Instead, the mechanism underlying herbicide tolerance in the SBP is likely metabolism-based resistance, possibly regulated by the action of multiple genes. Further research is necessary to confirm candidate genes and their functions.

The Environmental Degradation and Distribution of Saflufenacil, a Fluorinated Protoporphyrinogen IX Oxidase-Inhibiting Herbicide, on a Canadian Winter Wheat Field.

Saflufenacil when applied to a field is susceptible to transport, degradation, and transformation. We used a laboratory-based approach to model the fate of saflufenacil in the environment, the results of which are compared directly with those observed in a field study where saflufenacil was applied to a crop of winter wheat at a standard rate of 63 g of active ingredient/hectare. The water solubility of 2.1 g/L for saflufenacil allows for vertical transport through soil at a rate of 4.3 cm/mL of rainwater, and a soil adsorption coefficient KOC of 28.8 suggests that some of the herbicide will absorb to the soil. Of the saflufenacil in the soil, 78 ± 2.1% (n = 18) partitioned into plants, including nontargeted crop species, where it was found primarily in leaves (78 ± 2.1%, n = 18) and roots (22 ± 1.7%, n = 18). The saflufenacil that does not partition into plants or undergo vertical transport followed a degradation pathway into 3 metabolites: a uracil-ring N-demethylated metabolite (Saf-µCH3 ), a doubly N-demethylated metabolite (Saf-2CH3 ), and a ring-cleavage metabolite (Saf-RC), identified using nontargeted mass spectrometry. In the field, saflufenacil was observed to degrade over 212 d to the persistent metabolite Saf-RC. This metabolite was found at a concentration that was 1/10th of that applied to the field, suggesting that the majority of saflufenacil had undergone transport through the soil, or uptake into the winter wheat crop. Field samples were further examined using F-19 nuclear magnetic resonance and nontargeted mass spectrometry to rule out the potential of other degradation products. Environ Toxicol Chem 2020;39:1918-1928. © 2020 SETAC.

Design, Synthesis, and Herbicidal Activity of Novel Diphenyl Ether Derivatives Containing Fast Degrading Tetrahydrophthalimide.

To seek new protoporphyrinogen oxidase (PPO) inhibitors with better biological activity, a series of novel diphenyl ether derivatives containing tetrahydrophthalimide were designed based on the principle of substructure splicing and bioisomerization. PPO inhibition experiments exhibited that 6c is the most potential compound, with the half-maximal inhibitory concentration (IC50) value of 0.00667 mg/L, showing 7 times higher activity than Oxyfluorfen (IC50 = 0.0426 mg/L) against maize PPO and similar herbicidal activities to Oxyfluorfen in weeding experiments in greenhouses and field weeding experiments. In view of the inspected bioactivities, the structure-activity relationship (SAR) of this series of compounds was also discussed. Crop selection experiments demonstrate that compound 6c is safe for soybeans, maize, rice, peanuts, and cotton at a dose of 300 g ai/ha. Accumulation analysis experiments showed that the accumulation of 6c in some crops (soybeans, peanuts, and cotton) was significantly lower than Oxyfluorfen. Current work suggests that compound 6c may be developed as a new herbicide candidate in fields.

Design and Synthesis of N-phenyl Phthalimides as Potent Protoporphyrinogen Oxidase Inhibitors.

Protoporphyrinogen oxidase (PPO) has been identified as one of the most promising targets for herbicide discovery. A series of novel phthalimide derivatives were designed by molecular docking studies targeting the crystal structure of mitochondrial PPO from tobacco (mtPPO, PDB: 1SEZ) by using Flumioxazin as a lead, after which the derivatives were synthesized and characterized, and their herbicidal activities were subsequently evaluated. The herbicidal bioassay results showed that compounds such as 3a (2-(4-bromo-2,6-difluorophenyl) isoindoline-1,3-dione), 3d (methyl 2-(4-chloro-1,3-dioxoisoindolin-2-yl)-5-fluorobenzoate), 3g (4-chloro-2-(5-methylisoxazol-3-yl) isoindoline-1,3-dione), 3j (4-chloro-2-(thiophen-2-ylmethyl) isoindoline-1,3-dione) and 3r (2-(4-bromo-2,6-difluorophenyl)-4-fluoroisoindoline-1,3-dione) had good herbicidal activities; among them, 3a showed excellent herbicidal efficacy against A. retroflexus and B. campestris via the small cup method and via pre-emergence and post-emergence spray treatments. The efficacy was comparable to that of the commercial herbicides Flumioxazin, Atrazine, and Chlortoluron. Further, the enzyme activity assay results suggest that the mode of action of compound 3a involves the inhibition of the PPO enzyme, and 3a showed better inhibitory activity against PPO than did Flumioxazin. These results indicate that our molecular design strategy contributes to the development of novel promising PPO inhibitors.

Discovery of Novel N-Isoxazolinylphenyltriazinones as Promising Protoporphyrinogen IX Oxidase Inhibitors.

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is a promising target for herbicide discovery. Search for new compounds with novel chemotypes is a key objective for agrochemists. Here, we describe the discovery and systematic SAR-based structure optimization of novel N-isoxazolinylphenyltriazinones 5-9 as PPO inhibitors. The in vivo herbicidal activity and in vitro Nicotiana tabacum PPO (NtPPO) inhibitory activity were explored in detail. A number of the new synthetic compounds displayed strong PPO inhibitory activity with Ki values in the nanomolar range. Some compounds exhibited excellent and broad-spectrum weed control at the rate of 9.375-37.5 g ai/ha by postemergence application and showed improved monocotyledonous weed control compared to saflufenacil. Most promisingly, ethyl 3-(2-chloro-5-(3,5-dimethyl-2,6-dioxo-4-thioxo-1,3,5-triazinan-1-yl)-4-fluorophenyl)-5-methyl-4,5-dihydroisoxazole-5-carboxylate, 5a, with a Ki value of 4.9 nM, displayed over 2- and 6-fold higher potency than saflufenacil (Ki = 10 nM) and trifludimoxazin (Ki = 31 nM), respectively. Moreover, 5a showed excellent and broad-spectrum weed control against 32 kinds of weeds at 37.5-75 g ai/ha. Rice exhibited relative tolerance to 5a at 150 g ai/ha by postemergence application, indicating that 5a could be a potential herbicide candidate for weed control in paddy fields.

Design, Herbicidal Activity, and QSAR Analysis of Cycloalka[d]quinazoline-2,4-dione-Benzoxazinones as Protoporphyrinogen IX Oxidase Inhibitors.

In continuation of our search for potent protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) inhibitors, we designed and synthesized a series of novel herbicidal cycloalka[d]quinazoline-2,4-dione-benzoxazinones. The bioassay results of these synthesized compounds indicated that most of the compounds exhibited very strong Nicotiana tabacum PPO (NtPPO) inhibition activity. More than half of the 37 synthesized compounds displayed over 80% control of all three tested broadleaf weeds at 37.5-150 g ai/ha by postemergent application, and a majority of them showed no phytotoxicity toward at least one kind of crop at 150 g ai/ha. Promisingly, 17i (Ki = 6.7 nM) was 6 and 4 times more potent than flumioxazin (Ki = 46 nM) and trifludimoxazin (Ki = 31 nM), respectively. Moreover, 17i displayed excellent, broad-spectrum herbicidal activity, even at levels as low as 37.5 g ai/ha, and it was determined to be safe for wheat at 150 g ai/ha in postemergent application, indicating the great potential for 17i development as a herbicide for weed control in wheat fields.

Investigating target-site resistance mechanism to the PPO-inhibiting herbicide fomesafen in waterhemp and interspecific hybridization of Amaranthus species using next generation sequencing.

BACKGROUND: Waterhemp (Amaranthus tuberculatus (Moq.) J. D. Sauer) is one of the most pernicious weeds in cropping systems of the USA due to its evolved resistance against several herbicide sites-of-action, including protoporphyrinogen oxidase inhibitors (PPO-R). Currently, the only source of PPO-R documented in waterhemp is ΔG210 of PPX2. Gene flow may not only lead to a transfer of herbicide-resistant alleles, but also produce a hybrid genotype more competitively fit than one or both parents. However, investigating gene flow of Amaranthus species has been of interest in the past two decades with limited evidence. RESULTS: Here, a high-throughput MiSeq amplicon sequencing method was used to investigate alterations of the PPX2 gene in 146 PPO-R waterhemp populations across five Midwest states of the USA. Five R128 codons of PPX2, novel to waterhemp, were found including AGG (R), GGA (G), GGG (G), AAA (K) and ATA (I). R128G, R128I, and R128K were found in 11, 3, and 2 populations, respectively. R128G and R128I, but not R128K, conferred fomesafen resistance in a bacterial system. Sequence alignment of the R128 region of PPX2 identified a tumble pigweed (Amaranthus albus)-type and Palmer amaranth (Amaranthus palmeri)-type PPX2 allele to be present and widespread in the surveyed waterhemp populations, thus providing strong evidence of gene flow between Amaranthus species. CONCLUSION: Using a next-generation sequencing method, we identified two PPO target-site mutations R128G/I novel to waterhemp and provided evidence of gene flow of Amaranthus species in a large group of screened waterhemp populations from five Midwest states of the USA. © 2019 Society of Chemical Industry.

Biochemical and physiological mode of action of tiafenacil, a new protoporphyrinogen IX oxidase-inhibiting herbicide.

We conducted biochemical and physiological experiments to investigate the mode of action of tiafenacil (Terrad'or™), a new protoporphyrinogen IX oxidase (PPO)-inhibiting pyrimidinedione herbicide. Analysis of the half-maximal inhibitory concentration (IC50) against recombinant PPO enzymes from various plant species, including amaranth (Amaranthus tuberculatus), soybean (Glycine max), arabidopsis (Arabidopsis thaliana), and rapeseed (Brassica napus), showed that tiafenacil had an IC50 of 22 to 28 nM, similar to the pyrimidinedione herbicides butafenacil and saflufenacil and the N-phenylphthalimide herbicide flumioxazin. By contrast, tiafenacil exhibited 3- to 134-fold lower IC50 values than the diphenyl ether herbicides fomesafen, oxyfluorfen, and acifluorfen. Tiafenacil is non-selective and is herbicidal to both dicots and monocots, such as the weeds velvetleaf (Abutilon theophrasti), amaranth, and barnyardgrass (Echinochloa crus-galli) as well as the crops soybean, rapeseed, rice (Oryza sativa), and maize (Zea mays) at concentrations ranging from 1 to 50 µM. Treatment of plant tissue with tiafenacil in darkness resulted in the accumulation of protoporphyrin IX. Subsequent exposure to light increased the content of malondialdehyde and significantly decreased the Fv/Fm values of chlorophyll fluorescence. The results suggest that tiafenacil is a new PPO-inhibiting pyrimidinedione herbicide.

True gene-targeting events by CRISPR/Cas-induced DSB repair of the PPO locus with an ectopically integrated repair template.

In recent years, several tools have become available for improved gene-targeting (GT) in plants. DNA breaks at specific sites activate local DNA repair and recombination, including recombination with ectopic sequences leading to GT. Large-scale transformation with the repair template can be avoided by pre-insertion of the repair template in the genome and liberation by sequence-specific nucleases (in planta GT procedure). Here, we tested whether release of the repair template was required for GT. Plants were transformed with constructs encoding a CRISPR/Cas nuclease with a recognition site in the endogenous PPO gene and a repair template harboring a 5' truncated PPO gene with two amino acid substitutions rendering the enzyme insensitive to the herbicide butafenacil. Selection resulted in so-called true GT events, repaired via homologous recombination at both ends of the gene and transmitted to the next generation. As the template was surrounded by geminiviral LIR sequences, we also tested whether replication of the template could be induced by crossing-in an integrated geminivirus REP gene. However, we could not find evidence for repair template replication by REP and we obtained similar numbers of GT events in these plants. Thus, GT is possible without any further processing of the pre-inserted repair template.

Flumioxazin metabolism in pregnant animals and cell-based protoporphyrinogen IX oxidase (PPO) inhibition assay of fetal metabolites in various animal species to elucidate the mechanism of the rat-specific developmental toxicity.

Flumioxazin, an N-phenylimide herbicide, inhibits protoporphyrinogen oxidase (PPO), a key enzyme in heme biosynthesis in mammals, and causes rat-specific developmental toxicity. The mechanism has mainly been clarified, but no research has yet focused on the contribution of its metabolites. We therefore conducted in vivo metabolism studies in pregnant rats and rabbits, and found 6 major known metabolites in excreta. There was no major rat-specific metabolite. The most abundant component in rat fetuses was APF, followed by flumioxazin and 5 identified metabolites. The concentrations of flumioxazin and these metabolites in fetuses were lower in rabbits than in rats. In vitro PPO inhibition assays with rat and human liver mitochondria showed that flumioxazin is a more potent PPO inhibitor than the metabolites. There were no species differences in relative intensity of PPO inhibition among flumioxazin and these metabolites. Based on the results of these in vivo and in vitro experiments, we concluded that flumioxazin is the causal substance of the rat-specific developmental toxicity. As a more reliable test system for research on in vitro PPO inhibition, cell-based assays with rat, rabbit, monkey, and human hepatocytes were performed. The results were consistent with those of the mitochondrial assays, and rats were more sensitive to PPO inhibition by flumioxazin than humans, while rabbits and monkeys were almost insensitive. From these results, the species difference in the developmental toxicity was concluded to be due to the difference in sensitivity of PPO to flumioxazin, and rats were confirmed to be the most sensitive of these species.

Synthesis and Herbicidal Activity of Pyrido[2,3-d]pyrimidine-2,4-dione-Benzoxazinone Hybrids as Protoporphyrinogen Oxidase Inhibitors.

To search for new protoporphyrinogen oxidase (PPO, EC 1.3.3.4) inhibitors with improved bioactivity, a series of novel pyrido[2,3-d]pyrimidine-2,4-dione-benzoxazinone hybrids, 9-13, were designed and synthesized. Several compounds with improved tobacco PPO (mtPPO)-inhibiting and promising herbicidal activities were found. Among them, the most potent compound, 3-(7-fluoro-3-oxo-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-benzo[b][1,4] oxazin-6-yl)-1-methylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, 11q, with a Ki value of 0.0074 µM, showed six times more activity than flumioxazin (Ki = 0.046 µM) against mtPPO. Compound 11q displayed a strong and broad spectrum of weed control at 37.5-150 g of active ingredient (ai)/ha by both post- and pre-emergence application, which was comparable to that of flumioxazin. 11q was safe to maize, soybean, peanut, and cotton at 150 g ai/ha, and selective to rice and wheat at 75 g ai/ha by pre-emergence application, indicating potential applicability in these fields.

ß-catenin Mutations Are Not Involved in Early-stage Hepatocarcinogenesis Induced by Protoporphyrinogen Oxidase Inhibitors in Mice.

We previously reported the contribution of constitutive androstane receptor (CAR) in cytotoxicity-related hepatocarcinogenesis induced by oxadiazon (OX) or acifluorfen (ACI), two pesticides categorized as protoporphyrinogen oxidase (PROTOX) inhibitors. The molecular characteristics of preneoplastic and neoplastic lesions induced by OX and ACI were immunohistochemically compared to those by phenobarbital (PB), a typical CAR activator, in wild-type (WT) and CAR knockout (CARKO) mice after diethylnitrosamine initiation. We focused on changes in ß-catenin and its transcriptional product glutamine synthetase (GS). In PB-promoted foci and adenomas, nuclear accumulation of mutated ß-catenin was increased with high frequency. PB treatment also increased the multiplicity and area of GS-positive foci and adenomas in WT mice. No foci and adenomas showed nuclear accumulation of ß-catenin and expression of GS in CARKO mice, similar to both genotypes of mice treated with OX and ACI. Interestingly, hepatocellular carcinoma induced in ACI-treated WT mice showed nuclear accumulation of ß-catenin and was positive for GS. Our results indicated that ß-catenin mutations were not involved in early-stage hepatocarcinogenesis induced by PROTOX inhibitors in mice, although activation of ß-catenin and CAR is important in PB-induced tumorigenesis. The significant differences in molecular profiles suggested involvements of multiple mode of actions for hepatocarcinogenesis induced by PROTOX inhibitors.

Computational Discovery of Potent and Bioselective Protoporphyrinogen IX Oxidase Inhibitor via Fragment Deconstruction Analysis.

Tuning the binding selectivity through appropriate ways is a primary goal in the design and optimization of a lead toward agrochemical discovery. However, how to achieve rational design of selectivity is still a big challenge. Herein, we developed a novel computational fragment generation and coupling (CFGC) strategy that led to a series of highly potent and bioselective inhibitors targeting protoporphyrinogen IX oxidase. This enzyme plays a vital role in heme and chlorophyll biosynthesis, which has been proven to be associated with many drugs and agrochemicals. However, existing agrochemicals are nonbioselective, resulting in a great threat to nontargeted organisms. To the best of our knowledge, this is the first bioselective inhibitor targeting the tetrapyrrole biosynthesis pathway. In addition, the candidate showed excellent in vivo bioactivity and much better safety toward humans.