IV. DISCUSSION AND CONCLUSIONS 



considered to be malignant (tubular cell adeno- 

 mas or mixed tumors), granulosa cell tumors 

 may progress to malignancy in mice (Beamer et 

 al., 1985). Progression from complex tubular 

 adenomas or granulosa cell tumors to malignant 

 neoplasms may occur in B6C3Fi mice as in other 

 strains (Murphy and Russell, 1963; Alison et al., 

 1987). Nitrofurantoin toxicity observed in the 

 gonads of female mice in these studies is difficult 

 to interpret; it may initiate genetic events lead- 

 ing to tumor development, or it may upset hor- 

 monal balance between the pituitary gland and 

 the gonads and indirectly cause or enhance tu- 

 mor development as described in studies with 

 7,12-dimethylbenz[alanthracene (DMBA) (Ta- 

 guchi et al., 1988). In another study of a 5-nitro- 

 furan (nitrofurazone), there were dose-related 

 increased incidences of ovarian tumors of these 

 same types (NTP, 1988). 



Ovarian abscesses and suppurative inflamma- 

 tion of the uterus were observed in control but 

 not in dosed female mice (Table D5) and are be- 

 lieved to be related to indigenous microbial in- 

 fections. Adenocarcinomas of the uterus were 

 seen in two dosed mice (Table Dl); uterine ade- 

 nomas and adenocarcinomas are observed rarely 

 in historical controls (6/2,010, 0.3%). 



Spindle cell ("Type A" cell) hyperplasia of the 

 adrenal gland was observed in dosed female 

 mice. A spindle cell adenoma was seen in one 

 low dose female mouse, and a spindle cell carci- 

 noma was seen in one low dose male mouse. 

 Since gonadectomy of mice is known to result in 

 hyperplasia and neoplasia of the "Type A" cells 

 in the adrenal cortex (Turusov, 1979), the 

 spindle cell hyperplasia in female mice given ni- 

 trofurantoin is likely related to the ovarian atro- 

 phy and disruption of normal hormone levels. 



Nitrofurantoin was mutagenic for Salmonella 

 strains TA98 and TAIOO, with and without 

 metabolic activation, but not for strains TA1535 

 or TA1537 (Table El). Nitrofurantoin induced 

 forward mutations at the TK locus of mouse 

 L5178Y lymphoma cells in the absence of met- 

 abolic activation (Table E2), induced increased 

 numbers of sister chromatid exchanges (SCEs) 

 (Table E3) and chromosomal aberrations in cul- 

 tured Chinese hamster ovary (CHO) cells with 

 and without metabolic activation (Table E4), 

 and was negative for sex-linked recessive lethal 



mutations in Drosophila (Table E5). The mu- 

 tagenicity of nitrofurantoin has been attributed 

 to a reduced nitre group on the furan ring as a 

 result of the metabolic action of either a bac- 

 terial nitroreductase or a eukaryotic cell enzyme 

 system. In bacteria, anaerobic conditions favor- 

 ing rapid action of the nitroreductase enzyme 

 system have been shown to enhance the muta- 

 genicity of nitrofurantoin (Rosenkranz and 

 Speck, 1976). Availability of at least one alter- 

 nate nitroreductive pathway has been demon- 

 strated in Salmonella, in that the nitroreductase 

 deficient strain TAIOO-FRI is mutated by nitro- 

 furantoin in the absence of oxygen but not in its 

 presence. Kramers (1982) has also shown that 

 Drosophila are able to carry out a presumably 

 similar metabolism. Several compounds with 

 nitro groups which are known to be Salmonella 

 mutagens have also been shown to induce muta- 

 tions in germ-free (lacking gut fiora) Drosophila. 

 Hence, the files were able to appropriately me- 

 tabolize nitro compounds to active intermedi- 

 ates. Further, extracts from germ-free files en- 

 abled a nitroreductase deficient Escherichia coli 

 strain to reduce the related 5-nitrofuran, nitro- 

 furazone, to a mutagenic form. 



Olive (1980) demonstrated that the reduction po- 

 tential (electron affinity) of nitroheterocyclic 

 compounds including nitrofurantoin is directly 

 correlated with the mutagenic activity of these 

 chemicals in hamster V79 spheroids. Further, 

 Shirai and Wang (1980) investigated the rela- 

 tionship between the strength of the mutagenic 

 response of eight different nitrofurans, including 

 nitrofurantoin, in Salmonella and their ability 

 to induce SCEs in cultured CHO cells. The mag- 

 nitude of the responses in both test systems cor- 

 related well; i.e., weak Salmonella mutagens 

 were weak inducers of SCEs, and vice versa. 

 Thus, nitrofurantoin is mutagenic in cultured 

 mammalian cells as well as in bacteria, both of 

 which apparently have the capacity to transform 

 these chemicals into mutagenic intermediates. 



The reductive metabolism of nitrofurantoin in 

 mammalian cell systems is mediated by 

 NADPH-cytochrome c reductase and xanthine 

 oxidase. The presumed active intermediate is 

 the hydroxylamino compound that has been iso- 

 lated by some investigators but is extremely 

 oxygen-labile and difficult to detect. Another 



65 



Nitrofurantoin, NTP TR 341 



