Absorption and mass balance of piperonyl butoxide following an 8-h dermal exposure in human volunteers.

Dermal absorption, metabolism and excretion of piperonyl butoxide (PBO) was studied using 14C-PBO either by itself as a 3% (w/w) solution in isopropyl alcohol or as a 4% (w/w) solution in an aqueous end-use formulation. Each of these two formulations were tested on four young, healthy male volunteers, using a single topical application on the ventral forearm under non-occlusive conditions for an 8-h period. The application sites were thoroughly cleaned with cotton swabs moistened with isopropyl alcohol, then rinsed with isopropyl alcohol. Blood from the ipsilateral and contralateral arms, urine and feces were collected at selected intervals during the 8-h application and through a 120-h post-application period. The application area was also tape-stripped to determine if any of the test material accumulated in the stratum corneum. These samples provided data which permitted insight into the kinetics of penetration and elimination processes of PBO. The absorption of PBO either by itself or formulated was very poor, as demonstrated by the radioactivity excreted in the urine, and radioactivity in the ipsilateral plasma. When dosed by itself, approximately 1.78% of the dose was excreted in the urine. In contrast, only 0.47% of the formulated PBO was excreted in the urine. Trace radioactivity was detected in the feces from both formulations. The absorbed radioactivity was rapidly eliminated in the urine. There was no evidence of accumulation of PBO in the skin as evidenced by low amounts of radioactivity in the tape-strippings. The majority of the applied radioactivity was recovered from the skin surface. Total recovery of the applied radioactivity was 100.86 and 104.22% for PBO and the formulated product respectively. Absorbed PBO was completely metabolized to at least three major metabolites prior to its excretion in the urine. The three metabolites represented over 70% of the excreted radioactivity for PBO. The HPLC retention times for these metabolites are different than that seen in rats. The structures of these metabolites have not been elucidated.

Oncogenicity studies of piperonyl butoxide in rats and mice.

1. The oncogenicity of Piperonyl butoxide (PBO) has been studied in the mouse and rat. CD-1 mice were administered PBO in the diet at target doses of 0, 30, 100 and 300 mg/kg/day for 79 weeks and Sprague-Dawley rats 0, 30, 100 and 500 mg/kg/day for 104/105 weeks. 2. At termination of the study in the mouse there was evidence of increased liver weights and an increased incidence of eosinophilic adenomas at 100 and 300 mg/kg/day in males and 300 mg/kg/day in females. 3. In rats there was increased liver weights at 100 and 500 mg/kg/day associated with hepatocyte hypertrophy in both male and female rats. There was no increased incidence of neoplasia at any site. Hypertrophy and hyperplasia of thyroid follicles was observed at 500 mg/kg/day in both sexes. 4. The observations reflect the expected changes related to the induction of the mixed function oxygenase group of enzymes. In the mouse the increased incidence of eosinophilic adenomas is not considered relevant for human risk evaluation.

Effect of piperonyl butoxide on cell replication and xenobiotic metabolism in the livers of CD-1 mice and F344 rats.

Male CD- 1 mice were fed diets containing 0 (control), 10, 30, 100, and 300 mg/kg/day piperonyl butoxide (PBO) and 0.05% sodium phenobarbital (NaPB) and male F344 rats were fed diets containing 0 (control), 100, 550, 1050, and 1850 mg/kg/day PBO and 0.5% NaPB for periods of 7 and 42 days. In both species PBO and NaPB increased relative liver weight and whereas PBO produced a midzonal (mouse) or periportal/midzonal (rat) hypertrophy, NaPB produced a centrilobular hypertrophy. In the rat, individual cell necrosis was also observed at 42 days after high doses of PBO. Replicative DNA synthesis, assessed as the hepatocyte labeling index following implantation of 7-day osmotic pumps containing 5-bromo-2'-deoxyuridine during Study Days 0-7 and 35-42, was increased in mice given 300 mg/kg/day PBO and NaPB for 7 days and in rats given 550 and 1050 mg/kg/day PBO and NaPB for 7 days and 1050 mg/kg/day PBO for 42 days. While PBO had no effect on body weights in mice, the body weights of rats given 550, 1050, and 1850 mg/kg/day PBO for 42 days were reduced to 92, 89, and 70% of control, respectively. PBO induced microsomal cytochrome P450 content and mixed function oxidase activities in the mouse and rat, although the effects were less marked than those produced by NaPB. In summary, this data demonstrates that PBO can produce liver enlargement in the mouse and the rat which is associated with induction of xenobiotic metabolism, hypertrophy, and hyperplasia. The hepatic effects of PBO in the mouse were similar to but less marked than those produced by NaPB. In the rat high doses of PBO were hepatotoxic and resulted in a marked reduction in body weight. Thus while the reported formation of eosinophilic nodules in mouse liver by PBO may occur by a mechanism(s) similar to that of NaPB and other nongenotoxic enzyme inducers, the reported tumor formation in rats at greater than the maximum tolerated dose is most likely associated with marked enzyme induction in conjunction with a regenerative hyperplasia resulting from PBO-induced hepatotoxicity.

Lack of genotoxicity of piperonyl butoxide.

The genotoxicity of piperonyl butoxide has been investigated in bacterial mutation assays using tester strains TA98, TA100, TA1535, TA1537 and TA1538. The assays were conducted both with and without metabolic activation. Piperonyl butoxide was tested for mutation with and without metabolic activation in the CHO/HGPRT assay. Chromosomal aberrations were investigated also using Chinese hamster ovary (CHO) cells and effects on DNA were evaluated by in vitro unscheduled DNA synthesis (UDS) test using rat liver primary cell cultures. Piperonyl butoxide was not shown to be genotoxic in any assay system. The data presented supports the view that the liver tumors observed in rodents at dose levels above the maximally tolerated dose (MTD) result from a secondary non-genotoxic mechanism.

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices.

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.