International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 18. Review of the genotoxicity and carcinogenicity of antischistosomal drugs; is there a case for a study of mutation epidemiology? Report of a task group on mutagenic antischistosomals

Praziquantel use in aquaculture - Current status and emerging issues

Parasitic diseases are major constraints in fish mariculture. The anthelmintic praziquantel (PZQ) can effectively treat a range of flatworm parasites in a variety of fish species and has potential for broader application than its current use in the global aquaculture industry. In this review we report on PZQ's current use in the aquaculture industry and discuss its efficacy against various flatworm parasites of fish. Routes of PZQ administration are evaluated, along with issues related to palatability, pharmacokinetics and toxicity in fish, while PZQ's effects on non-target species, environmental impacts, and the development of drug-resistance are discussed.

Praziquantel for Schistosomiasis: Single-Drug Metabolism Revisited, Mode of Action, and Resistance

Schistosomiasis, a major neglected tropical disease, affects more than 250 million people worldwide. Treatment of schistosomiasis has relied on the anthelmintic drug praziquantel (PZQ) for more than a generation. PZQ is the drug of choice for the treatment of schistosomiasis; it is effective against all major forms of schistosomiasis, although it is less active against juvenile than mature parasites. A pyrazino-isoquinoline derivative, PZQ is not considered to be toxic and generally causes few or transient, mild side effects. Increasingly, mass drug administration targeting populations in sub-Saharan Africa where schistosomiasis is endemic has led to the appearance of reduced efficacy of PZQ, which portends the selection of drug-resistant forms of these pathogens. The synthesis of improved derivatives of PZQ is attracting attention, e.g., in the (i) synthesis of drug analogues, (ii) rational design of pharmacophores, and (iii) discovery of new compounds from large-scale screening programs. This article reviews reports from the 1970s to the present on the metabolism and mechanism of action of PZQ and its derivatives against schistosomes.

Effectiveness of hyperbaric oxygen for experimental treatment of schistosomiasis mansoni using praziquantel-free and encapsulated into liposomes: assay in adult worms and oviposition

The treatment of schistosomiasis depends on a single drug: praziquantel (PZQ). However, this treatment presents limitations such as low and/or erratic bioavailability that can contribute to cases of tolerance. Improvements to the available drug are urgently needed and studies with a controlled system of drug release, like liposomes, have been gaining prominence. The present study evaluated the activity and synergy between liposomal-praziquantel (lip.PZQ) and hyperbaric oxygen therapy (HBO). Mice received doses of 60 or 100mg/kg PZQ or lip.PZQ, 50 days post-infection, and after the treatment, were exposed to HBO (3 atmosphere absolute - ATA) for 1h. The viability of adult worms and oviposition were analyzed, by necropsy and Kato-Katz examination performed after 15 days of treatment. A concentration of 100mg/kg of lip.PZQ+HBO was more effective (48.0% reduction of worms, 83.3% reduction of eggs/gram of feces) and 100% of the mice had altered of oograms (indicating interruption of oviposition) compared to other treatments and to the Control group (infected and untreated). It is known that PZQ requires participation of the host immune system to complete its antischistosomal activity and that HBO is able to stimulate the immune system. The drug became more available in the body when incorporated into liposomes and, used with HBO, the HBO worked as an adjuvant. This explains the decreases of oviposition and worms recovered form hepatic portal system.

Establishing the level of safety concern for chemicals in food without the need for toxicity testing

There is demand for methodologies to establish levels of safety concern associated with dietary exposures to chemicals for which no toxicological data are available. In such situations, the application of in silico methods appears promising. To make safety statement requires quantitative predictions of toxicological reference points such as no observed adverse effect level and carcinogenic potency for DNA-reacting chemicals. A decision tree (DT) has been developed to aid integrating exposure information and predicted toxicological reference points obtained with quantitative structure activity relationship ((Q)SAR) software and read across techniques. The predicted toxicological values are compared with exposure to obtain margins of exposure (MoE). The size of the MoE defines the level of safety concern and should account for a number of uncertainties such as the classical interspecies and inter-individual variability as well as others determined on a case by case basis. An analysis of the uncertainties of in silico approaches together with results from case studies suggest that establishing safety concern based on application of the DT is unlikely to be significantly more uncertain than based on experimental data. The DT makes a full use of all data available, ensuring an adequate degree of conservatism. It can be used when fast decision making is required.

Praziquantel for the treatment of schistosomiasis: its use for control in areas with endemic disease and prospects for drug resistance

Praziquantel became available for the treatment of schistosomiasis and other trematode-inflicted diseases in the 1970s. It was revolutionary because it could be administered orally and had very few unwanted side effects. As a result of marked reductions in the price of praziquantel, the rate at which it is used has accelerated greatly in recent years. For the foreseeable future it will be the mainstay of programs designed to control schistosome-induced morbidity, particularly in sub-Saharan Africa where schistosomiasis is heavily endemic. There is currently no evidence to suggest that any schistosomes have developed resistance to praziquantel as a result of its widespread use. Nevertheless, while resistance may not pose an obvious or immediate threat to the usefulness of praziquantel, complacency and a failure to monitor developments may have serious consequences in the longer term since it will be the only drug that is readily available for large-scale treatment of schistosomiasis.

Albendazole, mebendazole and praziquantel. Review of non-clinical toxicity and pharmacokinetics

The pharmacokinetics and toxicity of albendazole, mebendazole and praziquantel are extensively reviewed, drawing on original published work and reviews in the open scientific literature and on assessments by international agencies and official regulatory bodies in Europe and the USA. Information about human and veterinary medical uses and adverse reactions is evaluated. The totality of the non-clinical information available about these long-established drugs may not comply with current official guidelines for new medicines but reasons are given why the "deficiencies" are only apparent and the data gaps can be replaced by other results, largely obtained from the target species and the many years of clinical experience of safe use of these drugs in humans and animals.

Analysis of lacI mutations in Big Blue transgenic mice subjected to parasite-induced inflammation

Parasite infections have long been associated with specific types of human cancers. Schistosoma hematobium is an inducer of urinary bladder cancer, Helicobacter pylori is a gastric carcinogen, and hepatitis B virus and Opisthorchis viverrini are causative agents of hepatocellular carcinoma. Another liver fluke, Fasciola hepatica, has also been identified as a neoplastic risk agent, primarily in animals. We used F. hepatica-induced inflammation in mice to determine if the presence of an aggressive liver fluke could induce mutagenic events in mammalian tissue. This provides a perspective on the relationship between chronic inflammation and cancer and may be a model for future studies on this complex association. In previous studies using the Big Blue transgenic mouse assay, we demonstrated an increase in lacI mutations in liver cells harvested from mice harboring F. hepatica flukes when compared to uninfected control animals. In these studies, we report on the types of mutations associated with this parasite infection. The observed mutational spectrum roughly corresponded to the spectrum of spontaneous mutations in liver cells when compared to control (uninfected) animals. However, the spectrum of mutations from parasitized animals showed a significant increase in complex changes and multiple mutations (18.2%) when compared to what would be expected from control animals (2.8%).

Enhanced liver cell mutations in trematode-infected Big Blue transgenic mice

Parasite infections in humans have long been associated with specific types of cancers. Schistosoma hematobium is a known inducer of urinary bladder cancer, Helicobacter pylori is a gastric carcinogen, and hepatitis B virus and Opisthorchis viverrini are causative agents of liver cell cancers. Another liver fluke, Fasciola hepatica, has also been identified as a neoplastic risk agent, primarily in animals. We used F. hepatica as a model agent to determine if the presence of an aggressive liver fluke could induce mutagenic events in mammalian tissue. Using the Big Blue(R) transgenic mouse assay, we found a two-fold increase in lacI mutations in cells harvested from mice harboring F. hepatica worms when compared to uninfected control animals. These data indicate that biological infections can cause increased genetic damage in surrounding host tissue.

O6-methylguanine and O6-methylguanine-DNA [corrected] methyltransferase activity in tissues of BDF-1 mice treated with antiparasitic drugs

Levels of the DNA promutagenic methylation damage, O6-methylguanine (O6-MeG) and the activity of the O6-methylguanine-DNA methyltransferase (MGMT), the enzyme responsible for repairing O6-MeG, were measured at various time intervals in tissues of BDF-I mice administered a single therapeutic dose of the antischistosomal agents hycanthone, oxaminiquine and metrifonate. Hycanthone increased O6-MeG in the liver-DNA after 6 h, then decreased by 3-fold after 48 h. Lower levels of the adduct and a slower rate of formation were found in the intestine and bladder. MGMT activities were significantly lower in the liver (74%) and bladder (25%) compared to control animals after 6 h, then restored by 48 h. Oxaminiquine increased O6-MeG in all tissues, but spleen, after 6 h and persisted only in the bladder after 48 h. Liver and bladder tissues of these animals exhibited a pattern of alteration in the MGMT activity similar to that observed for hycanthone. Metrifonate induced a profile of O6-MeG comparable to that of oxaminiquine but the levels of the adduct were about 2-fold lower. Hepatic MGMT in these animals was significantly lower (approximately 38%) than the control values after 6 h, then restored by 48 h. A significant negative correlation was obtained between O6-MeG and MGMT activity in the liver (r=- 0.85), intestine (r=- 0.62) and bladder (r=- 0.59). These results demonstrate that treatment with antischistosomal agents may lead to the formation of promutagenic alkylation damage in the tissue DNA and alterations in the DNA repair capacity.

Genotoxic activity of praziquantel

Praziquantel is a synthetic drug with a remarkable activity against parasites, particularly treamatodes and cestodes. Initial genotoxicity tests used a spectrum of endpoints including tests in bacteria, yeasts, mammalian cells and Drosophila and each one gave negative results. Effects on reproductive cells of mice were negative as well. However, host mediated studies in mice and humans were contradictory and a comutagenic effect with several mutagens and carcinogens was found. Later studies, including monitoring in humans and pigs have shown that Praziquantel induces a greater frequency of hyperploid lymphocytes as well as structural chromosomal aberrations, but not in all the individuals treated. In vitro studies have demonstrated that Praziquantel can induce micronuclei in syrian hamster embryonic (SHE) cells and in lymphocytes of some individuals. The same was found about structural chromosomal aberrations. Fetal death and fetal resorption were found when Praziquantel was administered in high doses to pregnant rats between the 6th and 10th day of gestation. Due to its efficiency as a parasiticide, Praziquantel is in use in Latin-American, Asiatic, African and East-European countries where infections by trematodes and cestodes are frequent. However, the extensive use of Praziquantel in multiple reinfections, in non-infected and non-diagnosed individuals for prevention, in higher doses or repeated doses for cysticercosis treatment and in individuals exposed to environmental mutagens, in conjunction with new findings about its metabolism and genotoxic properties, make it necessary to further evaluate the potential of this drug not only to be mutagenic per se, but to contribute in the development of neoplasm.

Electrophoretical and histochemical characterization of Schistosoma mansoni phenol oxidases

The phenol oxidase system, which is thought to play a central role in eggshell formation, was analyzed by means of electrophoretic and histochemical techniques. In contrast to current beliefs, our data show that males also express the phenol oxidase activity. The analysis of the electrophoretic pattern from males and females shows differences: adult males present a major band and a faint band, migrating slightly below. Adult females express a more complex pattern composed of four bands. Histochemical studies revealed that female phenol oxidase is concentrated in the vitelline cells, is inhibited by DDC, and this inhibition is correlated with disruption of female egg production. The present studies, in agreement with other reports, reveal that female phenol oxidase may be involved in sclerotization of the S. mansoni eggshell.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Immune response impairment, genotoxicity and morphological transformation induced by Taenia solium metacestode

In chronic helminthic infections such as cysticercosis, where the parasites live for years, profound modulation of the host immune response has been reported. To evaluate the genotoxicity of a drug used to treat cysticercosis, we observed the occurrence of genetic damage in cultured lymphocytes from cysticercotic swine and patients who had not been exposed to the drug. The human lymphocytes also showed a slower proliferation. These data suggested that the disease itself was promoting genetic damage in host lymphocytes which, in part, could explain the retardation of the lymphocyte proliferation observed in cysticercotic patients. Pigs infected with Taenia solium cysticerci showed an increased lymphocyte proliferation for 6-8 weeks post infection, followed by an impaired proliferation after this period. Significant induction of sister-chromatid exchanges was also observed in lymphocytes from infected pigs after the 6th week post infection. Additionally, it was found that a factor secreted by the cysticerci morphologically transformed primary fibroblasts in culture. The results strongly suggest that the parasite produces genetic instability in the host cells, which could result in immunosuppression and malignant transformation of target cells.

Implications for the involvement of the immune system in parasite-associated cancers

Biological factors can be carcinogenic risk factors in humans and in animals. Numerous theories have been developed to explain the causal link between biologically-associated disease and the ensuing neoplasia. In this paper we discuss the merits of one of these theories, the possible association between the mammalian inflammatory response and cancer.

Evaluation of the carcinogenic and genotoxic potential of praziquantel in the Syrian hamster embryo cell transformation assay

Praziquantel, a drug used for the treatment of neurocysticercosis, was tested for its ability to induce morphological transformation of Syrian hamster embryo fibroblasts. Results indicate that praziquantel transforms these cells without affecting their viability. Further experiments were carried out to investigate its possible mechanism of action in the same cell system. Micronucleus formation was observed in cultures treated with concentrations which induced morphological transformation, about 40% of these micronuclei were positive to a kinetochore antibody. No induction of DNA repair synthesis was observed even at cytotoxic concentrations. These results suggest that praziquantel has an aneugenic effect which could be responsible for its ability to transform morphologically these cells. Risk-benefit analysis should be carried out whenever this drug is utilized.

Mutation at the HPRT locus in patients with neurocysticercosis treated with praziquantel

Praziquantel (PZQ) is the drug of choice in the treatment of neurocysticercosis (NC), a parasitic disease caused by Taenia solium larvae. Variant frequencies at the hprt locus were analyzed in a group of NC patients before and after treatment with PZQ as well as in two control groups: healthy donors and non-parasitic neurological patients. Data show that PZQ does not induce hprt mutations, but that cysticerci by themselves or together with palliative treatment administered to NC patients could induce mutations in some patients.

Possible mechanisms of alteration in the capacities of carcinogen metabolizing enzymes during schistosomiasis and their role in bladder cancer induction

Carcinoma of the urinary bladder is the most common malignancy in many tropical and subtropical countries. There is a well documented association with chronic urinary schistosomal infection, and bladder cancer associated with schistosomiasis is a major cause of morbidity and mortality in the endemic areas. Many factors have been suggested as possible causative agents in schistosome-associated bladder carcinogenesis but theories concerning the possible role of schistosomal infection in altering host metabolism of chemical carcinogens have received most attention. In experimental schistosomiasis there is a common pattern of changes in the activities of several hepatic Phase I and Phase II enzymes. Phase I enzymes show increased activities in the early stages of infection but these activities are reduced to below their preinfection levels in the intermediate and late chronic stages of the disease. The activities of Phase II enzymes are altered in favour of the deconjugation pathways in the later stages of the disease. The possible basic mechanisms that might be involved in such changes during parasitism and their potential role in the induction of bladder neoplasia are discussed.

Sister-chromatid exchange frequencies induced by metrifonate in mammalian in vivo and in vitro systems

Metrifonate is a drug widely used to cure several helminthiases in man including cysticercosis. In this study we determine its capacity to induce SCEs in vivo and in vitro using the fluorescence-plus-Giemsa technique. In the human lymphocyte system we tested 10, 20, 30, 40, 50 and 60 micrograms/ml added at 24 h of culture with a duration of 72 h. The results showed a SCE inducing effect with the exception of the first tested dose (10 micrograms/ml), but without a dose-response expression. A cell cycle progression delay was seen with the three highest dosages. In the mouse bone marrow we tested 30, 60 and 120 mg/kg in a 24-h experiment, and the results indicated an SCE increase only with the highest dose. No modification of the cell proliferation kinetics was observed.

Direct interception of mutagens and carcinogens by biomolecules

Five points are emphasized: 1. Chemical interception and mere physical exclusion of mutagens and carcinogens constitute the major means by which mutations in cellular DNA are prevented. DNA repair processes comprise critical, but relatively minor, modes of genetic protection. 2. Disruption of a mutagen-interception defense mechanism can lead to substantial increases in mutagenesis and can preordain sites to eventual tumor formation. 3. Quantitation of the relative contributions of various blocking molecules is often simplified by the fact that protection can be calculated merely through knowledge of the measured concentration of the antimutagen and its rate of reaction with specific mutagens as measured in straightforward in vitro tests. 4. Two recently recognized defensive molecules, carnosine and ergothioneine, are put ++forward as examples of interesting chemical interceptor molecules. 5. Essentially all antimutagens are in fact "double-edged swords." Situations can be artificially constructed that can lead to generation of toxic species from molecules that are normally antimutagens; in isolated cases some of these interactions can be pictured as having deleterious consequences in vivo. This may be one reason why a number of important antimutagens are often sequestered, either in different tissues or by binding to dispensable macromolecules.

Female-specific mutagenic response of mice to hycanthone

Male and female gametogeneses differ markedly in all mammals. While male germ cells are continuously being produced from stem cells throughout the reproductive life span, the number of female germ cells is fixed during prenatal development and, soon after birth, all of the oocytes are arrested in a modified diplotene, or dictyate, stage. Following puberty, dictyate oocytes are hormonally triggered to mature either singly or in groups, resulting in ovulation and the completion of the first meiotic division. It has been hypothesized that female mice are more susceptible to dominant lethal effects of intercalating agents than male mice because oocyte chromosomes, which are arrested in a diffuse state, are generally more accessable to intercalation than are the more condensed chromosomes present within most male germ cell stages. This hypothesis was further tested using the intercalating agent hycanthone methane-sulfonate. Effects of hycanthone were studied in maturing and primordial oocytes and in male germ cells throughout spermatogenesis. No induction of dominant lethality was observed for treated males while a significant increase in embryonic death, expressed around the time of implantation, was observed in females that mated within 4.5 days after treatment. These effects were the result of dominant lethal mutations induced in maturing oocytes and not of maternal toxicity as indicated by the presence of chromosomal aberrations observed at first-cleavage metaphase of zygotes obtained from treated females. These results add support to the hypothesis that certain intercalating chemicals, which are not mutagenic to male mice, may be mutagenic to females and point to a need for more in-depth studies of female-specific mutagenesis.