In Silico Prediction of the Site of Oxidation by Cytochrome P450 3A4 That Leads to the Formation of the Toxic Metabolites of Pyrrolizidine Alkaloids

Predictive Minisci late stage functionalization with transfer learning

Structural diversification of lead molecules is a key component of drug discovery to explore chemical space. Late-stage functionalizations (LSFs) are versatile methodologies capable of installing functional handles on richly decorated intermediates to deliver numerous diverse products in a single reaction. Predicting the regioselectivity of LSF is still an open challenge in the field. Numerous efforts from chemoinformatics and machine learning (ML) groups have made strides in this area. However, it is arduous to isolate and characterize the multitude of LSF products generated, limiting available data and hindering pure ML approaches. We report the development of an approach that combines a message passing neural network and 13C NMR-based transfer learning to predict the atom-wise probabilities of functionalization for Minisci and P450-based functionalizations. We validated our model both retrospectively and with a series of prospective experiments, showing that it accurately predicts the outcomes of Minisci-type and P450 transformations and outperforms the well-established Fukui-based reactivity indices and other machine learning reactivity-based algorithms.

Inflammation Intensifies Monocrotaline-Induced Liver Injury

Pyrrolizidine alkaloids (PAs) are the most common toxins of plant origin, and it is evident that PAs pollute soil, water, nearby plants, and derived foods. Cases of human poisoning due to ingestion of PA-contaminated foods have been reported in several countries. Monocrotaline (MCT) is a pyrrolizidine alkaloid from the plants of Crotalaria genus that causes hepatic and cardiopulmonary toxicities, and the exhibition of the toxicities requires the metabolic activation by CYP3A4 to form electrophilic dehydro-monocrotaline (DHM). The present study demonstrated that myeloperoxidase (MPO) also participated in the bioactivation of MCT. N-Chloromonocrotaline was detected in both HClO/MCT incubations and MPO/H2O2/MgCl2/MCT incubations. DHM-derived N-acetylcysteine (NAC) conjugates were detected in the above incubations fortified with NAC. Lipopolysaccharide-induced inflammation in mice resulted in an elevated level of hepatic MPO activity, increased metabolic activation of MCT, and intensified elevation of serum ALT and AST activity induced by MCT. MPO inhibitor 4-aminobenzoic acid hydrazide was found to reverse these alterations. Mpo-KO mice were resistant to the observed potentiating effect of inflammation on MCT-induced liver injury. In conclusion, inflammation intensified MCT-induced liver injury. MPO participated in the observed potentiating effect of inflammation on the hepatotoxicity induced by MCT.

Structure-Dependent Toxicokinetics of Selected Pyrrolizidine Alkaloids In Vitro

Phytochemicals like pyrrolizidine alkaloids (PAs) can affect the health of humans and animals. PAs can occur for example in tea, honey or herbs. Some PAs are known to be cytotoxic, genotoxic, and carcinogenic. Upon intake of high amounts, hepatotoxic and pneumotoxic effects were observed in humans. This study aims to elucidate different toxicokinetic parameters like the uptake of PAs and their metabolism with in vitro models. We examined the transport rates of differently structured PAs (monoester, open-chained diester, cyclic diester) over a model of the intestinal barrier. After passing the intestinal barrier, PAs reach the liver, where they are metabolized into partially instable electrophilic metabolites interacting with nucleophilic centers. We investigated this process by the usage of human liver, intestinal, and lung microsomal preparations for incubation with different PAs. These results are completed with the detection of apoptosis as indicator for bioactivation of the PAs. Our results show a structure-dependent passage of PAs over the intestinal barrier. PAs are structure-dependently metabolized by liver microsomes and, to a smaller extent, by lung microsomes. The detection of apoptosis of A549 cells treated with lasiocarpine and monocrotaline following bioactivation by human liver or lung microsomes underlines this result. Conclusively, our results help to shape the picture of PA toxicokinetics which could further improve the knowledge of molecular processes leading to observed effects of PAs in vivo.

Sinusoidal obstruction syndrome secondary the intake of Senecio brasiliensis: A case report

Hepatic sinusoidal obstruction syndrome (HSOS) is a hepatic vascular disease histologically characterized by edema, necrosis, detachment of endothelial cells in small sinusoidal hepatic and interlobular veins and intrahepatic congestion, which leads to portal hypertension and liver dysfunction. In the Western world, most HSOS cases are associated with myeloablative pretreatment in a hematopoietic stem cell transplantation setting. Here we report a case of a 54 years old female patient, otherwise healthy, with no history of alcoholic ingestion, who presented with jaundice and signs of portal hypertension, including ascites and bilateral pleural effusion. She had no history of liver disease and denied any other risk factor for liver injury, except Senecio brasiliensis ingestion as a tea, prescribed as a therapy for menopause. Acute viral hepatitis and thrombosis of the portal system were excluded in complementary investigation, as well as sepsis, metastatic malignancy and other liver diseases, setting a RUCAM score of 6. Computed tomography demonstrated a diffuse liver parenchymal heterogeneity (in mosaic) and an extensive portosystemic collateral venous circulation, in the absence of any noticeable venous obstruction. HSOS diagnosis was confirmed through a liver biopsy. During the following-up period, patient developed refractory pleural effusion, requiring hemodialysis. Right before starting anticoagulation, she presented with abdominal pain and distention, with findings compatible of mesenteric ischemia by computed tomography. A laparotomy was performed, showing an 80cm segment of small bowel ischemia, and resection was done. She died one day after as a result from a septic shock refractory to treatment. The presented case was related to oral intake of S. brasiliensis, a plant containing pyrrolidine alkaloids, which are one of the main causes of HSOS in the East, highlighting the risk of liver injury with herbs intake.

Metabolic Toxification of 1,2-Unsaturated Pyrrolizidine Alkaloids Causes Human Hepatic Sinusoidal Obstruction Syndrome: The Update

Saturated and unsaturated pyrrolizidine alkaloids (PAs) are present in more than 6000 plant species growing in countries all over the world. They have a typical heterocyclic structure in common, but differ in their potential toxicity, depending on the presence or absence of a double bond between C1 and C2. Fortunately, most plants contain saturated PAs without this double bond and are therefore not toxic for consumption by humans or animals. In a minority of plants, however, PAs with this double bond between C1 and C2 exhibit strong hepatotoxic, genotoxic, cytotoxic, neurotoxic, and tumorigenic potentials. If consumed in error and in large emouns, plants with 1,2-unsaturated PAs induce metabolic breaking-off of the double bonds of the unsaturated PAs, generating PA radicals that may trigger severe liver injury through a process involving microsomal P450 (CYP), with preference of its isoforms CYP 2A6, CYP 3A4, and CYP 3A5. This toxifying CYP-dependent conversion occurs primarily in the endoplasmic reticulum of the hepatocytes equivalent to the microsomal fraction. Toxified PAs injure the protein membranes of hepatocytes, and after passing their plasma membranes, more so the liver sinusoidal endothelial cells (LSECs), leading to life-threatening hepatic sinusoidal obstruction syndrome (HSOS). This injury is easily diagnosed by blood pyrrolizidine protein adducts, which are perfect diagnostic biomarkers, supporting causality evaluation using the updated RUCAM (Roussel Uclaf Causality Assessment Method). HSOS is clinically characterized by weight gain due to fluid accumulation (ascites, pleural effusion, and edema), and may lead to acute liver failure, liver transplantation, or death. In conclusion, plant-derived PAs with a double bond between C1 and C2 are potentially hepatotoxic after metabolic removal of the double bond, and may cause PA-HSOS with a potential lethal outcome, even if PA consumption is stopped.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

An in vitro study on interaction of anisodine and monocrotaline with organic cation transporters of the SLC22 and SLC47 families

Current study systematically investigated the interaction of two alkaloids, anisodine and monocrotaline, with organic cation transporter OCT1, 2, 3, MATE1 and MATE2-K by using in vitro stably transfected HEK293 cells. Both anisodine and monocrotaline inhibited the OCTs and MATE transporters. The lowest IC₅₀ was 12.9 µmol·L^-1 of anisodine on OCT1 and the highest was 1.8 mmol·L^-1 of monocrotaline on OCT2. Anisodine was a substrate of OCT2 (K_m = 13.3 ± 2.6 µmol·L^-1 and V_max = 286.8 ± 53.6 pmol/mg protein/min). Monocrotaline was determined to be a substrate of both OCT1 (K_m = 109.1 ± 17.8 µmol·L^-1, V_max = 576.5 ± 87.5 pmol/mg protein/min) and OCT2 (K_m = 64.7 ± 14.8 µmol·L^-1, V_max = 180.7 ± 22.0 pmol/mg protein/min), other than OCT3 and MATE transporters. The results indicated that OCT2 may be important for renal elimination of anisodine and OCT1 was responsible for monocrotaline uptake into liver. However neither MATE1 nor MATE2-K could facilitate transcellular transport of anisodine and monocrotaline. Accumulation of these drugs in the organs with high OCT1 expression (liver) and OCT2 expression (kidney) may be expected.

Antibody-based detection of lysine modification of hepatic protein in mice treated with retrorsine

Many pyrrolizidine alkaloids (PAs), an important class of natural products, are hepatotoxic and carcinogenic. Increased attention has been paid to PA poisoning cases worldwide. Generally, most PAs themselves are not toxic. However, reactive intermediates formed from PAs by metabolic oxidation have been linked to toxicity and carcinogenesis. PAs themselves are generally not toxic, and their reactive metabolites resulting from metabolic oxidation are considered to be an essential responsible for PA toxicities. Protein modification by the electrophilic metabolites is proposed to play a key role in PA-induced cytotoxicity. The present study investigated the interaction of lysine residues of proteins with reactive metabolites of toxic PAs. Antibodies selectively recognizing lysine-based protein adduction were prepared and characterized. ELISA and immunoblot methods, in the presence and absence of synthetic model PA adducts, were used to test specific binding of the antibodies to modified lysine residues of BSA and to hepatic proteins extracted from mice treated with retrorsine. The lysine residue adduction was also detected in the tissues of retrorsine-treated mice by use of an immunohistochemical approach. In conclusion, the prepared antibodies selectively recognized the lysine adducts and may be used for the investigation of mechanisms of toxic action of PAs.

Clinical characteristics and treatment of pyrrolizidine alkaloid-related hepatic vein occlusive disease

BACKGROUND AND AIMS

Hematopoietic stem cell transplantation related hepatic vein occlusive disease (HSCT-HVOD) has been researched extensively; however, little is known about the clinical features and treatment of pyrrolizidine alkaloid-induced HVOD (PA-HVOD). This retrospective single centre study examined the clinical and laboratory characteristics of 108 patients with acute PA-HVOD and explored the efficacy of anticoagulation and TIPS therapy.

METHODS

The study included 108 consecutive patients with PA-HVOD between July 2008 and June 2016. The clinical manifestations and the results of laboratory and imaging tests were evaluated. The survival rates of patients treated with different approaches were recorded.

RESULTS

Serum total bilirubin was <34.2 μmol/L (2 mg/dL) in approximately 40% of patients. More than 90% of patients were presented with hepatomegaly, uneven liver perfusion in the balance phase, compressive stenosis of the hepatic segmental inferior vena cava and decreased peak velocity of portal vein blood flow. Severe portal hypertension was observed in all patients undergoing HVPG examination or TIPS operation. Anticoagulation therapy with low molecular weight heparin combined with warfarin was significantly more effective than liver protection and supportive therapy, and TIPS further improved the prognosis of patients who did not respond to anticoagulation therapy. The total effective rate of the anticoagulation-TIPS ladder therapeutic strategy was 91%.

CONCLUSIONS

Patients with PA-HVOD had different characteristics than those with HSCT-HVOD. Anticoagulation and TIPS treatment may be effective for patients with PA-HVOD.

New glutathione conjugate of pyrrolizidine alkaloids produced by human cytosolic enzyme-dependent reactions in vitro

RATIONALE

The toxic metabolites of pyrrolizidine alkaloids (PAs) are initially formed by cytochrome P450-mediated oxidation reactions and primarily eliminated as glutathione (GSH) conjugates. Although the reaction between the reactive metabolites and GSH can occur spontaneously, the role of the cytosolic enzymes in the process has not been studied.

METHODS

The toxic metabolites of selected PAs (retrorsine, monocrotaline, senecionine, lasiocarpine, heliotrine or senkirkine) were generated by incubating them in 100 mM phosphate buffer (pH 7.4) containing liver microsomes of human, pig, rat or sheep, NADPH and reduced GSH in the absence or presence of human, pig, rat or sheep liver cytosolic fraction. The supernatants were analyzed using liquid chromatography connected to Finnigan LTQ ion-trap, Agilent QTOF or Thermo Scientific Q Exactive Focus quadrupole-orbitrap mass spectrometers.

RESULTS

Retrorsine, senecionine and lasiocarpine yielded three GSH conjugates producing [M - H]^- ions at m/z 439 (7-GSH-DHP (CHO)), m/z 441 (7-GSH-DHP (OH)) and m/z 730 (7,9-diGSH-DHP) in the presence of human liver cytosolic fraction. 7-GSH-DHP (CHO) was a novel metabolite. Monocrotaline, heliotrine and senkirkine did not produce this novel 7-GSH-DHP (CHO) conjugate. 7-GSH-DHP (CHO) disappeared when incubated with hydroxylamine, and a new oxime derivative was formed. This metabolite was formed only by the human liver cytosolic enzymes but not in the presence of rat or sheep liver cytosolic fractions under otherwise identical reaction conditions.

CONCLUSIONS

7-GSH-DHP (CHO) has not been reported before, and thus it was considered as a novel metabolite of PAs. This may clarify the mechanisms involved in PA detoxification and widely observed but less understood species differences in response to PA exposure.

Editor's Highlight: Identification of Any Structure-Specific Hepatotoxic Potential of Different Pyrrolizidine Alkaloids Using Random Forests and Artificial Neural Networks

Pyrrolizidine alkaloids (PAs) are characteristic metabolites of some plant families and form a powerful defense mechanism against herbivores. More than 600 different PAs are known. PAs are ester alkaloids composed of a necine base and a necic acid, which can be used to divide PAs in different structural subcategories. The main target organs for PA metabolism and toxicity are liver and lungs. Additionally, PAs are potentially genotoxic, carcinogenic and exhibit developmental toxicity. Only for very few PAs, in vitro and in vivo investigations have characterized their toxic potential. However, these investigations suggest that structural differences have an influence on the toxicity of single PAs. To investigate this structural relationship for a large number of PAs, a quantitative structural-activity relationship (QSAR) analysis for hepatotoxicity of over 600 different PAs was performed, using Random Forest- and artificial Neural Networks-algorithms. These models were trained with a recently established dataset specific for acute hepatotoxicity in humans. Using this dataset, a set of molecular predictors was identified to predict the hepatotoxic potential of each compound in validated QSAR models. Based on these models, the hepatotoxic potential of the 602 PAs was predicted and the following hepatotoxic rank order in 3 main categories defined (1) for necine base: otonecine > retronecine > platynecine; (2) for necine base modification: dehydropyrrolizidine ≫ tertiary PA = N-oxide; and (3) for necic acid: macrocyclic diester ≥ open-ring diester > monoester. A further analysis with combined structural features revealed that necic acid has a higher influence on the acute hepatotoxicity than the necine base.

Pyrrolizidine alkaloids: occurrence, biology, and chemical synthesis

Covering: 2013 up to the end of 2015This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

Impairment of Host Liver Repopulation by Transplanted Hepatocytes in Aged Rats and the Release by Short-Term Growth Hormone Treatment

Hepatocyte transplantation is an alternative to whole liver transplantation. Yet, efficient liver repopulation by transplanted hepatocytes is low in livers of old animals. This restraint might be because of the poor proliferative capacity of aged donor hepatocytes or the regenerative impairment of the recipient livers. The age-dependent liver repopulation by transplanted wild-type hepatocytes was investigated in juvenile and senescent rats deficient in dipeptidyl-peptidase IV. Repopulation was quantified by flow cytometry and histochemical estimation of dipeptidyl-peptidase IV enzyme activity of donor cells in the negative host liver. As a potential pathway involved, expression of cell cycle proteins was assessed. Irrespective of the age of the donor hepatocytes, large cell clusters appeared in juvenile, but only small clusters in senescent host livers. Because juvenile and senescent donor hepatocytes were likewise functional, host-derived factor(s) impaired senescent host liver repopulation. Growth hormone levels were significantly higher in juvenile than in senescent rats, suggesting that growth hormone might promote host liver repopulation. Indeed, short-term treatment with growth hormone augmented senescent host liver repopulation involving the growth hormone-mediated release of the transcriptional blockade of genes associated with cell cycle progression. Short-term growth hormone substitution might improve liver repopulation by transplanted hepatocytes, thus augmenting the therapeutic benefit of clinical hepatocyte transplantation in older patients.