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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

    WORLD HEALTH ORGANIZATION



    Toxicological evaluation of certain veterinary drug
    residues in food



    WHO FOOD ADDITIVES SERIES 39





    Prepared by:
    The forty-eighth meeting of the Joint FAO/WHO Expert
    Committee on Food Additives (JECFA)



    World Health Organization, Geneva 1997


    FLUMEQUINE (addendum)

    First draft prepared by
    Professor F.R. Ungemach
    Institute of Pharmacology, Pharmacy and Toxicology
    Veterinary Faculty, University of Leipzig, Leipzig, Germany


    1. Explanation
    2. Biological data
       2.1 Toxicological studies
           2.1.1  Short-term toxicity
                  2.1.1.1  Arthropathy in dogs
                  2.1.1.2  Hepatotoxicity in mice
                  2.1.1.3  Mechanism of tumorigenicity in mice
           2.1.2  Special studies on human intestinal flora
    3. Comments
    4. Evaluation
    5. References


    1.  EXPLANATION

         Flumequine is a fluoroquinolone antimicrobial agent. This
    substance was evaluated by the Committee at its forty-second meeting
    (Annex 1, reference 110). At that time, an ADI could not be
    established owing to a lack of adequate information on the
    toxicological and microbiological hazards of flumequine: (i) necessary
    to identify a NOEL for hepatotoxicity; (ii)  on the mechanism of
    tumorigenesis; (iii) on the possible induction of arthropathy; and
    (iv) on the microbiological safety of residues.

         The additional information that was provided on these issues is
    summarized in this monograph addendum.

    2.  BIOLOGICAL DATA

    2.1.  Toxicological data

    2.1.1  Short-term toxicity

    2.1.1.1  Arthropathy in dogs

         Erosive arthropathy is a characteristic toxic reaction to
    quinolones and has been observed in growing animals and in particular
    young dogs after moderate doses of various fluoroquinolones (Gough 
     et al., 1979; Brown, 1996). Transient joint damage has also been
    reported in children (Norrby & Lietman, 1993). An early three-month
    study of the toxicity of flumequine in young adult beagle dogs did not
    address arthropathic lesions of weight-bearing joints in particular
    (Nelson  et al., 1972).

         Flumequine was administered twice daily as tablets by gavage to
    groups of 10 three-month-old beagle dogs of each sex at doses of 0,
    15, 30, 60, or 150 mg/kg bw per day for 13 consecutive weeks. Four
    animals of each group were killed after three weeks. The absorption of
    the test compound was checked periodically by high-performance liquid
    chromatography analysis for flumequine and its 7-hydroxy metabolite in
    plasma. The animals were observed daily for clinical signs, with
    special attention to lameness and locomotor activity. Body weight was
    recorded weekly. The serum activity of alkaline phosphatase was
    measured on three occasions during the study. Twenty animals were
    killed after three weeks of treatment, and the remainder were killed
    at 13 weeks. All animals were submitted to necropsy and checked for
    gross abnormalities and alterations of the articular surfaces of
    weight-bearing joints of the fore- and hindlimbs; the shoulder and hip
    joints were submitted to histopathological examination. The study was
    conducted according to good laboratory practice (GLP) guidelines.

         No deaths and only a few signs of adverse reactions were
    observed, including vomiting and reduced food consumption which
    increased in frequency with dose. No clinical signs of arthropathy,
    such as staggering gait and reduced locomotor activity, were reported.
    Females at the higher doses had markedly reduced weight gain. The
    serum activity of alkaline phosphatase remained unchanged. Gross
    necropsy revealed erosions of the joint surfaces in two of 10 dogs at
    the highest dose and in one of 10 females at the lowest dose. Slight
    histopathological lesions of the articular cartilage were observed in
    one of 10 dogs given 60 mg/kg bw per day and three of 10 dogs given
    150 mg/kg bw per day. These lesions were characterized by erosions,
    cavities, a fibrillary appearance of the cartilage, and synovial
    hyperplasia. The severity of the arthropathic lesions was similar at
    three and 13 weeks. The gross lesions of the hip joints of one animal
    after 13 weeks at the lowest dose were not accompanied by
    histopathological alterations (Woehrle, 1996).

         The grossly observed erosions in the hip joints of one female at
    the lowest dose were considered to be spontaneous, since no related
    histopathological alterations were found and no gross lesions were
    observed at the next two higher doses. The NOEL for induction by
    flumequine of arthropathy in young dogs was thus 30 mg/kg of bw per
    day.

    2.1.1.2  Hepatotoxicity in mice

         In short-term and long-term studies of toxicity evaluated
    previously by the Committee (Annex 1, reference 110), oral
    administration of flumequine caused dose-related hepatotoxic effects
    in rats and mice. Hypertrophy, degenerative changes, and focal
    necrosis of hepatocytes were observed in rats at 800 mg/kg bw per day
    in a three-month study (Nelson  et al., 1972) and at 400 and 800
    mg/kg bw per day in a two-year study (Sibinski  et al., 1977a) and in
    CD-1/ICR mice at 400 and 800 mg/kg bw per day in an 18-month study
    (Sibinski  et al., 1977b, 1979). The prevalence of hepatotoxic
    lesions increased with the duration of treatment. After cessation of
    flumequine administration, the liver damage was reversed (Sibinski 
     et al., 1979). Male mice were the most sensitive to 
    flumequine-induced liver damage (Sibinski  et al., 1977b, 1979).
    There was no NOEL for the hepatotoxic effects of flumequine in CD-1
    mice.

         In a 13-week study designed to investigate the hepatotoxic
    lesions and the activity of hepatic drug-metabolizing enzymes,
    flumequine was administered orally to groups of 16 CD-1 mice of the
    same strain as used in the previous studies. The animals were fed
    diets containing flumequine at concentrations providing doses of 0,
    25, 50, 100, 400, or 800 mg/kg bw per day to males and 0, 100, 400, or 
    800 mg/kg bw per day to females. The concentration of the test
    compound in the diet was checked periodically. The animals were
    observed daily for clinical signs, and body weight, while food
    consumption and food conversion efficiency were recorded weekly.
    Plasma enzyme activities were measured once, after 12 weeks. On the
    last day of treatment, the test compound was shown to be absorbed by
    high-performance liquid chromatography analysis of flumequine and its
    7-hydroxy metabolite in plasma. At the end of treatment, all animals
    were necropsied, and complete macroscopic examinations were conducted.
    The weights of the livers were recorded, and samples of the liver and
    other grossly abnormal tissues were submitted to histopathology. At
    the end of the experiment, liver microsomes were prepared to test the
    activity of the xenobiotic-metabolizing enzyme system by measuring
    total protein and cytochrome P450 content, P450-dependent dealkylation
    of resorufin and coumarin derivatives, and 1-naphthol glucuronidation.
    Microsomes from Aroclor-treated animals were used as positive
    controls. The study was performed in compliance with GLP.

         No deaths and no clinical signs of treatment-related adverse
    reactions were observed. The body-weight gains of the animals at the
    highest dose were reduced during the first week. This effect was more
    pronounced in males, which also showed slightly lower food consumption

    and efficiency of food conversion. No differences from the controls
    were seen in the other groups during the study. Plasma enzyme
    activities indicated liver damage at high doses, with a significant
    increase in the activities of alanine aminotransferase and alkaline
    phosphatase at doses of 400 and 800 mg/kg bw per day flumequine and of
    lactic dehydrogenase and aspartate aminotransferase at 800 mg/kg bw
    per day. Liver weights were increased at the two highest doses.
    Histopathological examination of the livers revealed dose-dependent
    degenerative alterations of hepatocytes, with hypertrophy and fatty
    vacuolation (in males at doses greater than 25 mg/kg bw per day and in
    females at doses greater than 100 mg/kg bw per day), increased ploidy,
    intranuclear inclusions, and centrilobular necrosis (at doses greater
    than 100 mg/kg bw per day). The effects were more pronounced in male
    animals. Increased mitotic activity was observed in males at the
    highest dose. Flumequine at doses up to 800 mg/kg bw per day had
    little or no effect on P450-dependent hepatic drug-metabolizing
    enzymes or on glucuronyltransferase activity.

         It was concluded that (i) flumequine has no remarkable inducing
    or inhibitory effect on the hepatic P450-dependent 
    xenobiotic-metabolizing enzyme system or on glucuronidation, and (ii)
    the liver is the target organ of flumequine in mice. The degeneration
    of hepatocytes with focal necrosis, accompanied by increased mitotic
    activity indicating regenerative processes, was seen only in male mice
    at the highest dose. The slight hypertrophic alterations of liver
    cells with minimal degenerative alterations in males at doses of 50
    and 100 mg/kg bw per day were regarded as signs of hepatotoxic lesions
    rather than metabolic overload. The NOEL was thus 25 mg/kg bw per day
    on the basis of hepatotoxic lesions in males (Stewart, 1995).

    2.1.1.3  Mechanism of tumorigenicity in mice

         The results of long-term studies with rodents previously
    evaluated by the Committee (Annex 1, reference 110) showed no
    carcinogenic effects in rats (Sibinski  et al., 1977a), but a 
    dose-related increase in the incidence of benign and malignant liver
    tumours was observed in CD-1 mice at doses greater than 100 mg/kg bw
    per day. The tumour incidence was parallelled by hepatotoxic changes
    and was significantly higher in male mice, which are known to be
    sensitive to liver tumour induction (Sibinski  et al., 1977b, 1979;
    McClain, 1990). As the compound was inactive in a range of tests for
    genotoxicity, including assays for gene mutation in bacteria and
    mammalian cells  in vitro and for chromosomal aberrations in
    mammalian cells  in vivo, the mechanism of tumorigenicity was
    unclear. 

         The available toxicological database on flumequine and data from
    the open literature were reviewed in order to discern the genotoxic or
    non-genotoxic ('epigenetic') mechanism of the hepatocarcinogenicity of
    flumequine (Marzin, 1996). By definition, a genotoxic carcinogen acts
    directly on DNA in the target tissues, inducing DNA damage, strand
    breaks, or mutations, which can be assessed  in vitro in assays for
    genotoxicity and in short-term assays in rodents. A non-genotoxic

    carcinogen is devoid of such activity. The neoplastic response to a
    non-mutagenic carcinogen is characterized by a steep dose-response
    curve and a threshold dose. Non-genotoxic carcinogenicity is
    considered to be brought about by potent induction of cytotoxicity and
    cell proliferation, which may increase the frequency of tumours in the
    target organs by virtue of sustained mitogenic stimuli. Demonstration
    of a lack of genotoxic potential and of the induction of a 
    dose-related increase in cell proliferation thus remains critical for
    identifying non-genotoxic carcinogens (Faccini  et al., 1992;
    Purchase, 1994; Shaw & Jones, 1994). 

         As negative results were obtained in various assays for
    genotoxicity, including reverse mutation in Salmonella typhimurium,
    gene mutation at the hprt locus in mouse lymphoma cells, gene mutation
    in Chinese hamster cells, and chromosomal aberrations in bone-marrow
    cells of rats  in vivo, it is unlikely that flumequine or its
    metabolites have direct genotoxic or mutagenic activity. Flumequine,
    like other 4-quinolones, exerts its antibacterial effects at the level
    of DNA by inhibiting bacterial topoisomerase II (DNA gyrase) (Hussy
     et al., 1986; Sato  et al., 1989). Although bacterial and
    eukaryotic topoisomerases II share some structural homology, they
    differ markedly in structure and function, which may explain the very
    different sensitivities to the inhibitory activity of 4-quinolones
    (Liu & Wang, 1991). Various 4-quinolones, including fluoroquinolones,
    have an affinity for the mammalian enzyme that is several orders of
    magnitude lower than that for the bacterial gyrase. Thus, the median
    inhibitory dose (ID50) for calf thymus topoisomerase II exceeds the
    ID50 for gyrase of  Escherichia coli by at least 100 to 2460-fold
    (in the case of loxacin) (Hussy  et al., 1986; Sato  et al., 1989).
    This low affinity is assumed to be a common feature of all
    fluoroquinolones. Although the inhibition of mammalian topoisomerase
    II by flumequine was not investigated, it is unlikely to exist, even
    at therapeutic doses. This conclusion is supported by the lack of
    mutagenic potential of flumequine, whereas specific inhibitors of the
    eukaryotic topoisomerase II are genotoxic and induce frameshift-type
    mutations (Huff & Kreuzer, 1991).

         Non-genotoxic tumorigenesis in the rodent liver can occur by
    various mechanisms, including compound-related hormonal activity,
    peroxisomal proliferation, induction of hepatic drug-metabolizing
    enzymes, and hepatotoxicity.

         The toxicological database, including the results of studies of
    reproductive toxicity, revealed no evidence for any hormonal activity
    of flumequine; and in short- and long-term assays for toxicity in
    mice, no histopathological alterations of liver cells indicating
    peroxisomal proliferation were reported.

         Various inducers of the hepatic P450-dependent xenobiotic
    biotransformation system, such as phenobarbital and halogenated cyclic
    hydrocarbons, act as tumour promoters in rodent liver when
    administered at high doses over a prolonged period (Diwan  et al., 
    1990; McClain, 1990; Grasso & Hinton, 1991). Since flumequine had only

    negligible effects on the hepatic P450 enzyme system in the 13-week
    study of toxicity in CD-1 mice at doses up to 800 mg/kg bw per day
    (Stewart, 1995), induction of the hepatic xenobiotic-metabolizing
    enzyme system can be excluded as a non-genotoxic mechanism of the
    hepatocarcinogenicity of flumequine.

         Flumequine is hepatotoxic, causing hepatocellular degeneration
    and focal necrosis in male and female mice, followed by a mitogenic
    response in male mice at the highest dose (Stewart, 1995). The 
    dose-related severity of these hepatotoxic lesions parallelled the
    incidence of benign and malignant liver tumours (Sibinski  et al., 
    1977b). Various non-genotoxic hepatotoxins have been shown to induce
    liver tumours in rodents (Drinkwater  et al., 1990; Butterworth &
    Goldsworthy, 1991; Grasso & Hinton, 1991). High doses and prolonged
    exposure increase the frequency of mutations and the likelihood of
    neoplastic transformation at the cellular level by still hypothetical
    mechanisms, such as expression of protooncogenes and growth factors
    (Thompson  et al., 1986; Dubois, 1990). In rodents, the mechanism is
    believed to be increased liver-cell proliferation due to repeated
    hepatocellular necrosis-regeneration cycles, leading to the
    development of foci of phenotypically altered hepatocytes (so-called
    'preneoplastic lesions'), which finally progress to neoplasms (Pitot
     et al., 1990; Butterworth & Goldsworthy, 1991; Grasso & Hinton,
    1991).

         The occurrence of foci of altered hepatocytes is an important
    link in the cascade of hepatotoxicity-induced liver tumorigenesis.
    Although the different types of preneoplastic lesions can readily be
    detected by conventional haematoxylin and eosin staining of liver
    tissue sections (Bannasch & Zerban, 1994), as was done in the studies
    of the toxicity of flumequine, no such lesions were reported in
    previously evaluated studies, including the 18-month study of
    carcinogenicity in CD-1 mice (Sibinski  et al., 1977b; Annex 1,
    reference 111). In the 13-week study in CD-1 mice, clear-cell foci of
    altered hepatocytes, which are one type of preneoplastic lesion, were
    observed in one male at 400 mg/kg bw per day and in one male and one
    female at 800 mg/kg bw per day (Stewart, 1995). The lesions were not
    characterized histochemically. 

         In a special study to assess marker enzymes of hepatic
    preneoplastic lesions, the effect of flumequine on the activity of 
    gamma-glutamyltransferase and glutathione  S-transferase, which in
    its placental form is a characteristic marker of foci of
    phenotypically altered hepatocytes (Bannasch & Zerban, 1994), was
    investigated in homogenates of livers from some of the CD-1 mice used
    in the 13-week study. The activity of gamma-glutamyltransferase
    remained unchanged. Administration of flumequine at doses of 400 and
    800 mg/kg bw per day resulted in marked stimulation of glutathione 
     S-transferase activity towards 1-chloro-2,4-dinitrobenzene in
    females, whereas the enzyme activity in males was only slightly
    affected. Doses up to 100 mg/kg bw per day had no effect (Stewart,
    1996). Because of its inadequate design, this study did not allow a
    valid assessment of marker enzymes of preneo-plastic lesions, which

    are confined to a small number of phenotypically altered hepatocytes.
    Therefore, this study was not considered further in the  evaluation.

         In conclusion, flumequine was considered to be a non-genotoxic
    hepatocarcinogen, and the induction of hepatocellular 
    necrosis-regeneration cycles by hepatotoxicity was considered to be
    the relevant mechanism for induction of liver tumours. Since cytotoxic
    effects are a prerequiste of hepatocarcinogenicity, tumours are
    induced only at hepatotoxic doses (Cohen & Ellwein, 1990; Pitot 
     et al., 1990). Therefore, the NOEL for the hepatotoxicity of
    flumequine, 25 mg/kg bw per day, was considered to be the threshold
    for both the hepatotoxicity and the associated carcinogenicity of
    flumequine. In evaluating the safety of flumequine, it must be kept in
    mind that the NOEL for hepatoxic lesions was derived from a short-term
    (13 weeks) study and was extrapolated to the level required for tumour
    formation observed at the end of a lifetime study (18 months) in mice.

    2.1.2  Special studies on human intestinal flora

         No experimental data on the effect of flumequine on the bacteria
    of the human gut microflora were available at the time of the previous
    evaluation (Annex 1, reference 110). Additional studies have been
    provided to assess the effects of flumequine and its 7-hydroxy
    metabolite on bacterial isolates from human intestinal microflora 
     in vitro. Studies  in vivo have not been performed.

         In the first study, the minimum concentrations resulting in 50%
    inhibition (MIC50) and 90% inhibition (MIC90) and the geometric
    mean of the MIC50 were determined for 100 bacterial strains isolated
    from the faeces of healthy volunteers, comprising 10 isolates of 10
    aerobic and anaerobic bacterial genera typical of the human gut
    microflora. The tests were performed in agar with serial dilutions
    under anaerobic and aerobic  (Escherichia coli) conditions. Three
    bacterial tester strains were tested for reference. The inoculum
    density was 107 colony forming units (cfu) per ml. The GLP status of
    the study was not reported, but the protocol and conduct met accepted
    standards for such studies. The results of the MIC determinations are
    summarized in Table 1.

          E. coli was the most sensitive species, the mean MIC50 value
    for the 10 strains tested being 0.33 µg/ml. The mean MIC50 values
    for the most sensitive predominant species isolated from the human
    gastrointestinal tract,  Clostridium and  Fusobacterium, were 0.95
    and 1.0 µg/ml, respectively (Richez, 1994a).

         In a second set of experiments under similar experimental
    conditions, the MIC values of 7-hydroxyflumequine were determined
    against 10 strains each of  E. coli, Clostridium spp., and
     Fusobacterium spp.  E. coli was much less sensitive to the
    metabolite than to the parent substance, with no inhibition at a
    concentration of 2 µg/ml 7-hydroxyflumequine and 100% inhibition at 
    4 µg/ml. The strains of  Clostridium and  Fusobacterium spp. were
    not sensitive to the highest concentration tested (16 µg/ml) (Richez,
    1995).

        Table 1.   Susceptibility of human intestinal bacteria to flumequine in vitro

                                                                                     

    Bacterial species         MIC (µg/ml)

    (10 strains each)                                                                

                              Range          MIC50      MIC90      Geometric mean
                                                                                     

    E. coli (aerobic)         0.25-0.50      0.33         0.48       0.47

    E. coli (anaerobic)       0.25-0.50      0.33         0.48       0.47

    Streptococcus spp.        16- > 32       > 32         > 32       > 32

    Proteus spp.              > 32           > 32         > 32       > 32

    Lactobacillus spp.        > 32           > 32         > 32       > 32

    Bifidobacterium spp.      > 32           > 32         > 32       > 32

    Bacteroides fragilis      16- > 32       > 32         > 32       > 32

    Eubacterium spp.          > 32           > 32         > 32       > 32

    Clostridium spp.          0.50-2.0       0.95         1.70       1.32

    Fusobacterium spp.        1.0-32         1.0          5.1        3.25

    Peptostreptococcus spp.   > 32           > 32         > 32       > 32

                                                                                     
    
         In a further experiment, the influence of inoculum size on the
    MIC was investigated for 10 strains of  E. coli isolated from human
    faeces. Each strain was tested under anaerobic and aerobic conditions
    with an inoculum of either 107 or 109 cfu/ml. No effect of inoculum
    density was seen (Richez, 1994b).

         In a study reported in the open literature, the effects of
    gastrointestinal factors and pH on the MIC50 of flumequine were
    studied with relevant bacterial species. The tests were performed by
    the broth dilution technique in the presence of cooked meat or a
    combination of meat and milk, at pH values of 3-7.5. The obligate
    anaerobes remained insensitive to flumequine (MIC50 > 40 µg/ml),
    whereas the MIC50 values for the  E. coli strains were increased by
    two- to eightfold (Nouws  et al., 1994).

         The intestinal bioavailability of flumequine to enteric bacteria
    was tested by giving 830 mg 14C-flumequine orally to five healthy
    volunteers.  The levels of radiolabel were then monitored in plasma,
    urine, and faeces for up to five days. A total of 84% (76-92%)
    radiolabel was recovered in excreta, with  9% (5.7-13%) in faeces and
    75% (70-81%) in urine. It was concluded that about 10% of a dose of
    flumequine is available to the gut microflora (Riker Laboratories,
    Inc., 1994).

         No studies were submitted on the selection of intestinal
    bacterial resistance or on the inhibitory effects on microorganisms
    used in industrial processing of foodstuffs of animal origin.

         It was concluded that  E. coli is the most sensitive of the
    relevant bacterial strains of human gut microflora tested  in vitro. 
    The absence of an effect of inoculum size indicates that flumequine
    has similar reactivity even at the high bacterial density in the human
    colon. The metabolite 7-hydroxyflumequine has markedly less
    antibacterial activity and can be considered to exert no relevant
    adverse effects on the human intestinal microflora. About 10% of
    ingested flumequine is available to the microflora in the human gut. 

         The thirty-eighth Committee concluded that the most relevant
    parameter  in vitro for assessing the risk to human intestinal flora
    is the geometric mean MIC against the most sensitive intestinal
    microorganism (Annex 1, reference 97). The MIC50 of flumequine for
     E. coli of 0.33 µg/ml should thus be considered the concentration
    that has no effect on human intestinal microflora and be used to
    establish the ADI.  E. coli, which is very sensitive to
    fluoroquinolones in general, is, however, a minor component of the
    gastrointestinal flora (Moore & Moore, 1995). It is therefore more
    appropriate to consider the effects of flumequine on the most
    sensitive obligate anaerobes, which are the bacterial species that
    predominate in the human gut (Moore & Moore, 1995). The MIC50 values
    for  Clostridium and  Fusobacterium spp., 0.95-1.0 µg/ml, were
    considered to be the concentrations with no effect on human intestinal
    microflora.

         Flumequine is a fluoroquinolone and thus has a broad spectrum of
    activity against aerobic gram-negative bacteria. In humans, this class
    of antimicrobial agents is used clinically for selective elimination
    of potential aerobic and facultative anaerobic pathogens from the
    gastrointestinal tract while preserving the predominant anaerobic
    bacterial gut flora. Furthermore the administration of therapeutic
    oral doses of fluoroquinolones such as ciprofloxacin and norfloxacin
    to humans has not been shown to alter the intestinal bacterial ecology
    or to weaken the barrier effect. Anaerobic bacteria such as
     Bifidobacterium, Bacteroides, Eubacterium, Fusobacterium, and
     Peptostreptococcus spp., the main components of the flora in the
    human gastrointestinal tract, are largely unaffected by these
    compounds (Midtvedt, 1990; Nord, 1995). When assessing the effects of
    flumequine on human gastrointestinal flora, it is important to
    interpret the MIC50 values for selected intestinal bacterial species

    in the context of the overall gut ecosystem. Since the obligate
    anaerobic bacteria that are predominantly isolated from the
    gastrointestinal tract are relatively insensitive to fluoroquinolones,
    disturbance of the human gut ecosystem by residues of flumequine is
    unlikely.

    3.  COMMENTS

         The Committee considered additional information on the induction
    of arthropathy in young dogs, the hepatotoxic and liver 
    enzyme-inducing effects of flumequine in mice, the possible mechanism
    of the hepatocarcinogenicity of flumequine, and its effect on human
    gut microflora. The studies were carried out according to appropriate
    standards for study protocol and conduct.

         In order to test the effects of flumequine on articular
    cartilage, it was administered as tablets to groups of 10 
    three-month-old beagle dogs at doses of 0, 15, 30, 60, or 150 mg/kg 
    bw per day for 13 weeks; four animals from each group were killed
    after three weeks. The animals showed no overt clinical signs of
    arthropathy. Gross necropsy revealed erosions of the joint surfaces in
    two of 10 dogs at the highest dose and in one of 10 animals at the
    lowest dose. Histopathological lesions of the articular cartilage were
    observed in one of 10 dogs given 60 mg/kg bw per day and three of 10
    dogs given 150 mg/kg bw per day. The severity of the lesions was
    similar at three and 13 weeks. The Committee considered that the gross
    lesions in the one animal at the lowest dose were not 
    compound-related, since no histopathological alterations were found
    and no gross lesions were observed at the next two higher doses.
    Therefore, the NOEL for induction of arthropathy in young dogs was 30
    mg/kg bw per day.

         In a 13-week study designed to investigate hepatotoxic lesions
    and the activities of hepatic drug-metabolizing enzymes, flumequine
    was administered to male CD-1 mice in the feed at doses equal to 0,
    25, 50, 100, 400, or 800 mg/kg bw per day and to females at 0, 100,
    400, or 800 mg/kg bw per day. The effects observed were reduced body
    weight, significantly increased plasma activities of alanine and
    aspartate aminotransferases, alkaline phosphatase and lactic
    dehydrogenase, and increased liver weights at 400 and 800 mg/kg bw per
    day. Histopathological examination of the liver revealed 
    dose-dependent hypertrophy, degenerative alterations, and
    centrilobular hepatocellular necrosis. The hepatotoxic lesions were
    more pronounced in male than in female mice and were observed at all
    doses greater than 25 mg/kg bw per day. Increased mitosis was observed
    only in males at the highest dose. Flumequine caused little or no
    induction of hepatic cytochrome P450-dependent drug-metabolizing
    enzymes or glucuronyltransferase when given at doses up to 800 mg/kg
    bw per day. The NOEL was 25 mg/kg bw per day on the basis of induction
    of hepatotoxic lesions in male mice.

         At its forty-second meeting, the Committee noted that there was
    evidence of compound-related tumorigenic effects in the livers of CD-1
    mice. The hepatotumorigenic activity of flumequine was more pronounced
    in male mice, which are known to be sensitive to liver tumour
    induction. As the compound was inactive in a range of tests for
    genotoxicity, including assays for gene mutation in bacteria and
    mammalian cells  in vitro and for chromosomal aberrations in
    mammalian cells  in vivo, the mechanism of this tumorigenesis was
    unclear.

         The present Committee noted that, although an inhibitory effect
    of flumequine on mammalian topoisomerase II, leading to DNA damage,
    was not investigated, bibliographical data on structurally related
    fluoroquinolones indicate that this mechanism is unlikely to operate.
    The Committee concluded that there is no evidence that flumequine has
    genotoxic potential.

         Non-genotoxic tumorigenesis in the liver can be due to various
    mechanisms, including compound-related hormonal activity, peroxisomal
    proliferation, induction of hepatic drug-metabolizing enzymes, and
    hepatotoxicity.

         The toxicological database, including studies of reproductive
    toxicity, revealed no evidence for any hormonal activity of
    flumequine. In short- and long-term studies of toxicity in mice, no
    histopathological alterations of liver cells that indicate peroxisomal
    proliferation were reported. Induction of the hepatic cytochrome P450
    enzyme system can be excluded by the results of the 13-week study in
    CD-1 mice.

         Flumequine is hepatotoxic, causing hepatocellular degeneration
    and focal necrosis in male and female mice, which was followed by a
    mitogenic response in male mice at the highest dose in the 13-week
    study described above. The dose-related severity of these hepatotoxic
    lesions parallelled the incidence of benign and malignant liver
    tumours. Various non-genotoxic hepatotoxins have been shown to induce
    liver tumours. The mechanism is believed to be increased liver-cell
    proliferation due to repeated hepatocellular necrosis-regeneration
    cycles, leading to the development of foci of phenotypically altered
    hepatocytes (so-called 'preneoplastic lesions'), which finally
    progress to neoplasms. In the 13-week study in CD-1 mice, clear-cell
    foci of altered hepatocytes, which are one type of preneoplastic
    lesion, were observed in one male at 400 mg/kg bw per day and in one
    male and one female at 800 mg/kg bw per day.

         In a study to assess marker enzymes of hepatic preneoplastic
    lesions, the effects of flumequine on the activity of 
    gamma-glutamyltransferase and glutathione  S-transferase were
    investigated in homogenates of livers from some of the mice used in 
    the 13-week study. Because of its inadequate design, including the
    lack of histochemical characterization of the foci of altered
    hepatocytes, this study was not considered further in the evaluation.

         The Committee considered that induction of hepatocellular
    necrosis-regeneration cycles by hepatotoxicity is the relevant
    mechanism for induction of liver tumours by flumequine. Therefore, the
    NOEL for the hepatotoxicity of flumequine, 25 mg/kg of bw per day, was
    considered to be the threshold for both the hepatotoxicity of
    flumequine and its associated carcinogenicity. The Committee noted
    that hepatotoxicity would have been better explored in a study of
    longer duration.

         The effect of flumequine on human intestinal microflora was
    assessed by determining the MIC50 values for 100 bacterial strains
    isolated from human faeces, comprising 10 isolates from 10 aerobic and
    anaerobic bacterial genera typical of the human gut microflora. These
    included  Escherichia coli, Streptococcus spp.,  Proteus spp.,
     Lactobacillus spp.,  Bifidobacterium spp.,  Bacteroides fragilis, 
     Eubacterium spp.,  Clostridium spp.,  Fusobacterium spp., and
     Peptostreptococcus spp. The inoculum density was 107 colony
    forming units per ml.  E. coli was the most sensitive bacterial
    species tested, with an MIC50 value of 0.33 µg/ml. The MIC50 value
    was not dependent on the size of the inoculum.  E. coli was markedly
    less sensitive to 7-hydroxyflumequine, with an MIC50 value of 4
    µg/ml. The mean MIC50 values for the most sensitive predominant
    species typically isolated from the human gastrointestinal tract,
     Fusobacterium and  Clostridium, were 1.0 and 0.95 µg/ml,
    respectively. In a study of the influence of gastrointestinal factors
    and pH on the MIC50 values of flumequine for relevant bacterial
    species of the human gastrointestinal tract, the values for obligate
    anaerobes were unaffected, whereas the MIC50 values for  E. coli 
    strains were increased by two- to eightfold.

         The upper limit of the ADI based on the antimicrobial activity of
    flumequine on human gut flora was calculated on the basis of the
    formula described on p. 12 as follows:


    Upper limit       1 µg/ga × 220 g
       of ADI     =                            
                      0.1b   ×   1c   ×   60 kg

                  =   37 µg/kg bw


    a    Mean MIC50 for the most sensitive predominant bacterial
         species,  Fusobacterium and  Clostridium
    b    Fraction of oral dose available to act on microorganisms in the
         colon, based on a study in which 830 mg 14C-flumequine were
         given orally to five healthy volunteers. The levels of radiolabel
         were then monitored in plasma, urine, and faeces for up to five
         days. A total of 84% (78-92%) of the radiolabel was recovered in
         the excreta, with 9% (5.7-13%) in faeces and 75% (70-81%) in
         urine. The Committee concluded that approximately 10% of
         flumequine is available to the gut microflora.

    c    A safety factor of 1 was used because relevant and sufficient
         microbiological data were provided.

    4.  EVALUATION

         The Committee noted that flumequine belongs to the group of
    antimicrobial fluoroquinolones that are active against aerobic 
    gram-negative bacteria. In humans, this class of antimicrobial agents
    is used clinically for selective elimination of potential aerobic and
    facultative anaerobic pathogens from the gastrointestinal tract while
    preserving the predominant anaerobic bacterial gut flora. The
    Committee also recognized that administration of therapeutic oral
    doses of fluoroquinolones such as ciprofloxacin and norfloxacin to
    humans has no appreciable effect on the intestinal bacterial ecology
    or on the barrier effect. In addition, anaerobic bacteria such as
     Bifidobacterium, Bacteroides, Eubacterium, Fusobacterium, and
     Peptostreptococcus spp., the main components of the human gut flora,
    are largely unaffected by these compounds.  E. coli, however, which
    is very sensitive to fluoroquinolones in general, is a minor component
    of the gastrointestinal flora. The Committee considered that, in
    assessing the effects of flumequine on the bacteria of the human
    gastrointestinal flora, the MIC50 values for the selected intestinal
    bacterial species should be interpreted in the context of the overall
    ecosystem of the gastrointestinal tract. Since the obligate anaerobic
    bacteria that predominate in the gastrointestinal tract are relatively
    insensitive to fluoroquinolones, disturbance of the human gut
    ecosystem by residues of flumequine is unlikely. Therefore the
    Committee decided to base the ADI on the toxicological properties of
    flumequine and not on its effect on the intestinal microflora.

         The Committee considered the NOEL of 25 mg/kg bw per day for
    hepatotoxicity in male CD-1 mice in the 13-week study to be the most
    appropriate toxicological end-point for consumer safety. An ADI of 
    0-30 µg/kg bw was established by applying a 1000-fold safety factor,
    which was chosen to account for the short duration of the study and
    the lack of histochemical characterization of the foci of altered
    hepatocytes.

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    See Also:
       Toxicological Abbreviations
       Flumequine (JECFA Food Additives Series 51)
       Flumequine (WHO Food Additives Series 53)
       Flumequine (WHO Food Additives Series 33)
       FLUMEQUINE (JECFA Evaluation)