FURFURAL
First draft prepared by
Dr B. Priestly
University of Adelaide
Australia
1. EXPLANATION
Furfural has not been previously evaluated by the Committee.
Furfural occurs naturally and is formed during the processing and
domestic preparation of a broad range of foods. It is also carried
over into food from its use as an extraction solvent or as a
component of flavour mixtures. Food additive or processing use
provides an intake estimated to be no more than 0.5-1% of the intake
from other food sources.
2. BIOLOGICAL DATA
2.1 Biochemical aspects
2.1.1 Absorption, distribution, and excretion
Furfural is well absorbed by all routes of exposure. In rats,
85% of 14C-furfural administered by gavage in corn oil was
recovered in urine within 72 h (NTP, 1987). In humans, approximately
78% was retained by the lungs during 8 h inhalation of vapour
(7-30 mg/m3), and 20-30% of this amount was absorbed through the
skin of a hand immersed in the liquid (Flek & Sedivec, 1978). The
biological half-life in man was estimated to be 2-2.5 h.
2.1.2 Biotransformation
The major route of metabolism is oxidation to furoic acid,
followed by conjugation with glycine (NTP 1987, Flek & Sedivec,
1978). A minor pathway involves condensation with acetate, followed
by conjugation with glycine. The resultant furfurylacryluric acid
has been detected in human urine after exposure to furfural (Flek &
Sedivec, 1978).
2.1.3 Effects on enzymes and other biochemical
parameters
No data available.
2.2 Toxicological studies
2.2.1 Acute toxicity
Furfural can irritate exposed mucosa and cause degenerative
effects on epithelial cells. Acute toxic doses cause CNS depression,
lung congestion and haemorrhage, and eye/nasal discharge. Delayed
toxic effects and those associated with repeat dosage include
hepatic and renal tubular necrosis, hypochromic anaemia and
leukopenia (Castellino et al., 1963; Jenner et al., 1964).
Markedly increased hepatic mitogenesis (without necrosis) was noted
within 6 h after a single oral dose of 50 mg/kg furfural in rats
(Shimizu & Kanisawa, 1986).
The acute lethal dose is comparable across several species and
by different routes of exposure.
Table 1. Acute toxicity of furfural
Species Route Lethal dose Reference
mg/kg bw
Rat oral 127 Jenner et al., 1964
Rat i.p. 120 Tiunov et al., 1970
Mouse oral 333 Boyland, 1940
Mouse s.c. 200 Tiunov et al., 1970
Mouse s.c. 223 (1 day survival) Castellino et al.,
119 (10 day survival) 1963
Rabbit dermal > 310 Moreno, 1976
Hamster inhalation 2500 mg/m3 Kruysse, 1972
Avian oral > 98 Schafer et al., 1983
2.2.2 Short-term studies
2.2.2.1 Mice
In range-finding dosage studies for the NTP carcinogenesis
bioassay, B6C3F1 mice were dosed by gavage with 25, 50, 100,
200, or 400 mg/kg bw/day for 16 days, and 75, 150, 300, 600 or
1200 mg/kg bw/day for 13 weeks. Relative liver weights were
increased in males at 300 and in females at 75, 150 and
300 mg/kg/day (NTP, 1990). The following table shows incidence of
centrilobular coagulative necrosis observed:
Table 2. Incidence of centrilobular coagulative necrosis
Dose level 0 75 150 300 600 1200
mg/kg bw/day)
males 0 0 1 1 9 8
females 0 0 0 0 0 2
(n=10 for all groups)
Mortality at the top two dose rates was 100 per cent for both
males and females. Most deaths occurred during the first week of
dosing.
2.2.2.2 Rats
A group of 48 male Wistar rats were fed a dietary regime of
furfural (20 ml/kg, equivalent to approximately 23 000 ppm or
1150 mg/kg bw/day) for 7 days; 30 ml/kg for 7 more days; 40 ml/kg
for a further 76 days; then 40 ml/kg for 5 days/week for a further
30 days. Two rats died, four rats were killed at 90 days, one at 12
days, and all were found to have developed a hepatic cirrhosis,
characterized by red nodules interspersed with white fibrous
bundles. The white areas displayed marked proliferation of atypical
cholangioepithelial cells and the lesion was described by the
authors as "cholangiofibrosis". There were increased numbers of
cells undergoing mitosis; none of the rats developed ascites or
hyperbilirubinaemia (Shimizu & Kanisawa, 1986).
In range-finding dosage studies for the NTP carcinogenesis
bioassay, F344/N rats were dosed by gavage with 15, 30, 60, 120, or
240 mg/kg bw/day for 16 days, and 11, 22, 45, 90 or 180 mg/kg bw/day
for 13 weeks. Only the top dose rates were significantly lethal.
Centrilobular hepatocytic vacuoles were found in all treated rats in
the 13 week study (and in 4/10 controls as well), the absolute and
relative liver and kidney weights were higher at the 90 mg/kg/day
dose level, but there was no other compound-related toxicity (NTP,
1990).
2.2.2.3 Hamsters
Groups of 10 Syrian golden hamsters of each sex were exposed to
furfural vapour (0, 77, 448, 2165 mg/m3) for 6 h/day, 5 days/week
over 13 weeks. The main findings were mild growth retardation,
irritation of the eyes and nose, and hyperplastic atrophy of the
nasal epithelium (all at the highest dose). The NEL was 77 mg/m3,
since some mild nasal epithelial degeneration was observed at 448
mg/m3 (Feron et al., 1979)
2.2.3 Long-term/carcinogenicity studies
2.2.3.1 Mice
Groups of 50 B6C3F1 mice of each sex were dosed by gavage
with 0, 50, 100 or 175 mg/kg bw/day furfural (99% pure) for 2 years.
Growth and survival were not affected by the treatment. The main
non-neoplastic findings related to treatment were an increased
incidence of hepatic multifocal pigmentation and subserosal
inflammation in both sexes (m>f) (NTP, 1990).
The principal neoplastic effects are summarized in Table 3.
Table 3. Neoplastic effects observed in mice
Dose group (n=50 except for male 100 where n=49)
Males Females
0 50 100 175 0 50 100 175
Hepatocellular 9 13 11 19 1 3 5 8
adenoma
Hepatocellular 7 12 6 21 4 0 2 4
adenoma
Adenoma/ 16 22 17 32 5 3 7 12
cardinoma
combined
Renal cortical 0 1 1 1
adenoma/
carcinoma
Forestomach 0 5 5 3
hyperplasia
Squamous cell 1 0 1 6
papilloma
The NTP conclusions (confirmed by the Peer Review Panel) were:
there was clear evidence of carcinogenicity in male mice, based on
the increased incidence of hepatocellular adenomas and carcinomas;
and there was some evidence of carcinogenicity in female mice
based upon the increased incidence of hepatocellular adenomas.
2.2.3.2 Rats
Groups of 50 F344/N rats of each sex were dosed by gavage with
0, 30, or 60 mg/kg bw/day furfural (99% pure) for 2 years. Growth
was not affected by the treatment, but survival was reduced in the
female high-dose group due to gavage-related deaths. The main
non-neoplastic findings related to treatment were an increased
incidence of hepatic centrilobular necrosis in males (3/50, 9/50,
12/50 in controls, low- and high-dose groups, respectively). Two
high-dose males had bile duct dysplasia with fibrosis and two others
had cholangio-carcinomas. The historical incidence of bile duct
neoplasms was 3/2,145, so that these lesions were considered to be
treatment-related (NTP, 1990).
The NTP conclusions (confirmed by the Peer Review Panel) were
that there was some evidence of carcinogenicity in male rats,
based on the rarity of the biliary pathology. There was no
evidence of carcinogenicity in female rats. It was noted that poor
survival in the high-dose female rats may have compromised the
detection of carcinogenesis in this group.
2.2.4 Reproduction studies
No data available.
2.2.5 Special studies on carcinogenicity
Citral and heptaldehyde are two aldehydes found to inhibit the
growth of spontaneous mammary tumours in mice. However, furfural and
its furfuracrylic acid metabolite (at a daily dose rate of 2.5 mg -
approximately 25% of the LD50) had no effect on the growth of
spontaneous mammary tumours in mice, although they were weakly
active against grafted sarcomata (Boyland, 1940).
Syrian Golden hamsters were exposed to furfural vapour for
1 year (1550 mg/m3 weeks 0-9; 1280 mg/m3 weeks 10-20;
970 mg/m3 weeks 21-52; 7 h/day; 5 days/week). Dosage reduction was
necessary to avoid substantial toxicity. Some groups were also
treated with carcinogens (benzo[a]pyrene by weekly intratracheal
instillation; total dose 18.2 or 36.4 mg; diethylnitrosamine
injected s.c every three weeks; total dose 2.1 µl). All rats were
observed for a further 29 weeks after cessation of dosing. While
furfural produced marked nasal irritation, cysts of the propria and
degeneration of the olfactory epithelium and Bowmans glands, it did
not induce respiratory tract cancers by itself, nor did it
potentiate the effects of either of the respiratory carcinogens
(Feron & Kruysse, 1978).
Half of the surviving rats from an experiment in which furfural
was used to produce hepatic cirrhosis were fed a diet containing
0.03% N-2-fluorenylacetamide for three weeks, followed by one week
of normal diet. The other received only the normal diet. All
surviving rats were killed 12 weeks later and their livers were
examined. Rats which had received furfural only did not have any
hyperplastic changes in the liver. Rats which had been treated with
N-2-fluorenylacetamide developed multiple hyperplastic nodules that
stained positive for alpha-fetoprotein, and this response was
markedly potentiated in the rats previously treated with furfural.
The authors concluded that furfural-induced hepatic cirrhosis
increases susceptibility to a potent hepatocarcinogen in the rat
(Shimizu & Kanisawa, 1986).
Samples of mouse liver tumours from the NTP Carcinogenesis
Bioassay had their transforming gene activity assessed by Southern
blot analysis of H- ras oncogene sequences. It was found that the
pattern of mutations in the ras gene differed in spontaneous
tumours compared to furfural-induced tumours. While all of the
activated oncogenes in tumours from the control mice were H-ras,
with point mutations at codon 61, 50% of the oncogenes activated in
furfural-treated mouse liver tumours were K-ras or H-ras genes
bearing point mutations other than at codon 61. The authors
concluded that furfural has a direct genotoxic effect in mouse liver
(NTP 1990).
2.2.6 Special studies on genotoxicity
Furfural is generally negative in bacterial mutagenicity
studies, but is positive in Drosophila and in some mammalian cells
in vitro. The results are summarised in Table 4.
Few in vivo genotoxicity studies have been reported for
furfural. Furfural did not induce SCEs or chromosomal aberrations in
bone marrow cells of B6C3F1 mice injected i.p. with furfural
at 50, 100 or 200 mg/kg bw (NTP 1990).
Table 4. Genotoxicity studies on furfural
Test system Test object Concentration or Result Reference
dose
Ames test TA98, 100 up to 8 µl negativea Zdienicka et
(cytotoxic) al., 1978
Ames test TA98, 1535 0.05-60 µmol negativea Loquet et
al., 1981
Ames test TA104 1 µmole negative Marnett et
al., 1985
Ames test TA97, 98, 199, equivocal positive Mortelmans et
1535, 1537 in TA100 in 1 of 2 al., 1986
test centres
Ames test TA100, 102, 104 up to 500 µg negative Shane et al.,
1988
Ames test TA98, 100, 104 negative Kato et al.,
E. coli reversion 1984
Wp2uvrA/pKM101
Umu gene S. typhimurium 1932 µg/ml negative Nakamura et al.,
expression TA1535/pSK 1002 1987
Rec-assay B. subtilis up to 1 mg negative Osawa & Namiki,
Ames test TA100 1982
Clastogenicity CHO cells up to 40 mM positiveb Stich et al.,
1981a,b
Table 4. cont'd
Test system Test object Concentration or Result Reference
dose
SCE human lymphocytes up to 140 µM positive Gomez-Arroyo &
Souza, 1985
Gene conversion S. cerevisiae D7 16 mg/ml positive Stich et al.,
1981b
Mouse lymphoma 200 µg/ml positive McGregor et al.,
L5178Ytk+/tk 1988
Forward mutation
assay
DNA strand break calf thymus DNA positive Hadi et al.,
and alkaline 1989
unwinding
Sex-linked Drosophila positive Woodruff et al.,
recessive melanolgaster (recessive) 1985
Lethal mutation lethals only
and reciprocal
translocation
a) results in two studies with TA100 were positive (Zdzienicka et al., 1978;
Loquet et al, 1981), both in the presence and absence of metabolic
activation. This strain is supposedly more sensitive to aldehydes. However,
negative results have also been recorded in TA100, and in T104, a strain
even more sensitive to aldhyde mutagens (Marnett et al., 1985)
b) positive without metabolic activation. In contrast, the parent alcohol,
furfuryl alcohol, is more potent as a clastogen and its activity was
enhanced by S9 activation.
2.3 Observations in humans
The frequency of SCE in lymphocytes cultured from 6 Mexican
factory workers purportedly exposed occupationally to furfural and
furfuryl alcohol was not different from that in 6 controls
(Gomez-Arroyo & Souza, 1985).
3. COMMENTS
The Committee considered data on the absorption, metabolism,
acute and chronic toxicity, genotoxicity, and carcinogenicity of
furfural.
Furfural is absorbed by all routes of exposure. It is
metabolized by oxidation to furoic acid and by subsequent
conjugation with glycine. The half-life in humans is 2-2.5 h.
The liver is the primary target for furfural toxicity in rats
and mice. In short-term studies, it produced liver enlargement at
doses > 90 mg/kg bw/day in rats and at 75-300 mg/kg bw/day in
mice. At higher doses, it produced centrilobular necrosis and
cholangiofibrosis.
In a 2-year gavage study, using dose levels of 0, 50, 100, or
175 mg/kg bw/day in B6C3F1 mice and 0, 30 or 60 mg/kg bw/day
in F344/N rats, furfural induced a statistically significant
increase in the incidence of hepatocellular adenomas and carcinomas
in male mice (34-64% compared with 32% in controls), hepatocellular
adenomas in female mice (6-16% compared with 2% in controls), and
bile-duct dysplasias (4%) plus cholangiocarcinomas (4%) in male rats
at the highest dose level (compared with zero incidence in controls
and a historical control incidence of 3 in 2145 (0.14%) for
cholangiocarcinomas). While furfural was generally negative in
bacterial mutagenicity tests, it was positive in a range of other
tests for genotoxicity. The pattern of oncogene expression in liver
tumours in furfural-treated mice differed from that in the
spontaneous liver tumours of the controls.
4. EVALUATION
While taking into consideration the relatively high
concentrations of furfural in some foods as normally prepared and
consumed, the Committee considered that it could not allocate an ADI
to furfural because of the evidence of genotoxicity and
carcinogenicity. The Committee considered that its direct addition
as a flavour was not appropriate, and that its use as a solvent
should be restricted to situations when alternatives were not
available, e.g., for the purification of food oil extraction of
unsaturated components. Carry-over into food should be reduced to
the lowest extent technologically feasible.
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