Journal of Toxicology and Environmental Health
ISSN: 0098-4108 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/uteh19
Fate of silvex following oral administration to humans M. W. Sauerhoff , M. B. Chenoweth , R.J. Karbowski , W.H. Braun , J.C. Ramsey , P.J. Gehring & G. E. Blau To cite this article: M. W. Sauerhoff , M. B. Chenoweth , R.J. Karbowski , W.H. Braun , J.C. Ramsey , P.J. Gehring & G. E. Blau (1977) Fate of silvex following oral administration to humans, Journal of Toxicology and Environmental Health, 3:5-6, 941-952, DOI: 10.1080/15287397709529628 To link to this article: http://dx.doi.org/10.1080/15287397709529628
Published online: 20 Oct 2009.
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FATE OF SILVEX FOLLOWING ORAL ADMINISTRATION TO HUMANS M. W. Sauerhoff Stauffer Chemical Company, Westport, Connecticut
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M. B. Chenoweth Bio-Medical Research Laboratory, The Dow Chemical Company, Midland, Michigan R. J. Karbowski, W. H. Braun, J. C. Ramsey, P. J. Gehring Health and Environmental Research, Toxicology Research Laboratory, The Dow Chemical Company, Midland, Michigan G. E. Blau Computations Research Laboratory, The Dow Chemical Company, Midland, Michigan The fate of silvex [2-(2,4,5-trichlorophenoxy)propionic acid] was defined in seven men and one woman following oral administration of 1 mg/kg. No adverse effects were observed. Samples of blood plasma, urine, and feces were collected at designated time intervals through 168 hr. Plasma samples were analyzed for silvex only, while samples of urine and feces were analyzed for silvex, silvex conjugate(s), 2,4,5trichlorophenol, and 2,4,5-trichlorophenol conjugate(s). Apparent first-order kinetics described the biphasic clearance of silvex from plasma and excretion in urine. The half-life values for clearance of silvex from plasma were 4.0 ± 1.9 and 16.5 ± 7.3 hr for the initial and terminal phases, respectively. Peak plasma levels of silvex occurred within 2-4 hr after dosage. Within 24 hr after administration, 65% of the administered dose had been excreted in urine. Silvex was excreted in urine as silvex and silvex conjugate(s). The half-life values for excretion of silvex per se in urine were 5.0 ± 1.8 and 25.9 ± 6.3 hr for the two phases, respectively. Small amounts (3.2% or less) of silvex and/or silvex conjugate(s) were eliminated in feces. Recovery of silvex and its conjugated) in urine and feces through 168 hr ranged from 66.6 to 95.1% of the administered dose, with a mean value and standard deviation of 80.3 ± 10.5%. In humans, silvex is readily absorbed after ingestion and subsequently readily excreted, predominantly via the urine.
INTRODUCTION Silvex [2-(2,4,5-trichlorophenoxy)propionic acid] is a herbicide active against undesirable woody plants and weeds in pastures, fields, lawns, Requests for reprints should be sent to W. H. Braun, Pharmacokinetics/Metabolism, The Dow Chemical Company, 1803 Building, Midland, Michigan 48640. 941 journal of Toxicology and Environmental Health, 3:941-952,1977 Copyright © 1977 by Hemisphere Publishing Corporation
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rangeland, certain field crops, and aquatic habitats. The compound belongs to the family of chlorophenoxy-acid herbicides of which 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid (2,4-D) are members. Silvex has been manufactured and marketed under the trade name KURON 1 herbicide, which contains propylene glycol butyl ether esters of silvex (approximately 65% silvex). The acute oral LD 5 0 values for KURON herbicide are approximately 1070 and 850 mg/kg for the rat and rabbit, respectively (The Dow Chemical Co., unpublished data). The acute oral LD SO for silvex in rats is 650 mg/kg (560-760 mg/kg) (Rowe and Hymas, 1954). There is a dearth of information available in the open literature on the subchronic and chronic toxicity of silvex. Two-year studies have been conducted in Wistar rats and beagle dogs maintained on diets containing Kurosal SL (a formulation of the potassium salt of silvex) (The Dow Chemical Co., unpublished studies). Beagle dogs, four per sex per dose level, were maintained on diets containing 0.0, 0.r056, 0.019, and 0.0056% Kurosal. The daily doses in silvex acid equivalents were, respectively, 0.056% in diet: 8.2 mg/kg-day, males 9.9 mg/kg-day, females 0.019% in diet: 2.6 mg/kg-day, males and females 0.0056% in diet: 0.9 mg/kg-day, males and females One dog of each sex from each level was killed after 1 yr and the remainder were killed after 2 yr for pathological evaluation. Evidence of an untoward effect on the liver was reported in both males and females receiving the high level and in males receiving the intermediate level. Pathologically, the damage was described as mild degeneration and necrosis of hepatocytes with slight fibroblastic proliferation. Elevated levels of serum glutamic-pyruvic transaminase and serum glutamic-oxaloacetic transaminase were discerned in females receiving the high level for 18 and 24 months. No changes were observed in other parameters—growth, food consumption, hematological evaluation, blood urea nitrogen, alkaline phosphatase, bromosulfophthalein dye retention, and organ weights. The no-adverse-effect level was judged as 0.9 mg/kg-day silvex acid. Rats, 30 per sex per dose level, were maintained on diets containing sufficient Kurosal to provide doses of approximately 0.0, 0.26, 0.8, 2.6, and 7.9 mg/kg-day silvex acid equivalents. Three, four, or five rats per sex per dose level were killed at 12 and 18 months; the remainder died spontaneously or were killed after 2 yr. The only reported effect related to treatment was an increased kidney/body weight ratio in males receiving the high dose and a lower body weight in the same rats. 1 Trademark of The Dow Chemical Co.
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The kidney/body weight ratios of male rats receiving the other doses of silvex were also increased significantly when compared with controls. However, the control kidney/body weight ratio was somewhat less than that found in other experiments conducted in the laboratory. It cannot be ruled out that the kidney/body weight ratios were increased as a result of treatment. However, the absence of pathological effects, grossly and histologically, indicates that, if related to treatment, this parameter very likely reflects a physiological adaptation. Other parameters that were evaluated and judged not to have revealed an untoward effect were general appearance, mortality, tumor incidence, hematological evaluations, serum urea nitrogen, serum alkaline phosphatase, and gross and microscopic pathology. A pharmacokinetic study of silvex in humans was conducted to determine the rate at which silvex is eliminated from the body and the potential for accumulation of silvex in the body. In the study, 1 mg/kg was administered. This dose was deemed safe after a review of the toxicity studies of silvex in laboratory animals and similar studies conducted in this laboratory on other chlorophenoxy acids 2,4,5-T and 2,4-D (Gehring et al., 1973;Sauerhoff etal., 1977a, 1977b).
MATERIALS AND METHODS Subjects Seven male (ages 37-61) and one female (age 48) human volunteer (weights 56.7-94.8 kg) participated in this study. The medical history of each individual was reviewed and a physical examination was conducted before the study. Included in the examination were blood pressure, pulse rate, pulmonary function, electrocardiogram, chest X-ray, hemoglobin, packed cell volume, red blood cell count, white blood cell count and differential, sedimentation rate, serum concentration of total calcium, cholesterol, triglycerides, glucose, inorganic phosphate, albumin, total protein, uric acid, bilirubin, glutamic-oxaloacetic transaminase, lactic dehydrogenase, alkaline phosphatase, and urinary protein and sugar. A complete neurological examination was also conducted. An additional evaluation including clinical chemistry, hematology, and urinalysis was conducted 2 wk after completion of the study. A preliminary pharmacokinetic study was conducted in one male subject; the samples were analyzed for silvex only. Dosage and Sample Collection for Analysis Between 8:00 and 8:05 a.m., all subjects ingested as a powder in a teaspoon 1.0 mg/kg analytical grade free acid of silvex (sample AGR 127923, The Dow Chemical Co.) of greater than 99% purity, containing less than 0.01 ppm 2,3,7,8-tetrachlorodibenzo-p-dioxin. The mouth was rinsed
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M. W. SAUERHOFF ETAL.
repeatedly with water, which was swallowed. Food was withheld 1 hr before and 4 hr after ingestion of the dose. All subjects ate a similar breakfast, consisting of juice or citrus fruit, cereal with milk, one egg, and toast. Blood samples (10 ml) were collected in vacutainer tubes containing 4 mg of potassium oxalate at 1, 2, 4, 8, 12, 24, 36, 48, 60, 72, and 96 hr after ingestion. After collection, the whole blood samples were centrifuged, and the plasma was separated and transferred to vials for subsequent analysis of silvex. Subjects were instructed not to eat for at least 1 hr before a blood sample was to be collected. All the urine voided by each subject during successive 12 hr intervals for 168 hr after dosing was collected and the volume was recorded. The urine collected during each 12 hr interval was thoroughly mixed and 5 ml portions were analyzed for silvex. All subjects urinated immediately before ingesting the dose. All the feces excreted during successive 24 hr intervals for 168 hr after dosing were collected and frozen for subsequent analysis. Silvex Analysis Plasma, urine, and 33% fecal homogenates were analyzed for silvex by gas chromatography-mass spectrometry. Samples (5 ml) were acidified" with 1 ml 1 /V HCI and extracted three times with diethyl ether. Ether extracts were pooled, reduced in volume to about 10 ml under a stream of dry nitrogen, and methylated by bubbling diazomethane through the solution. The ether extracts were evaporated to dryness under a stream of dry nitrogen and reconstituted in 1 ml hexane. Five microliters of the hexane solution was injected into a Finnigan model 3000D Quadrupole Gas Chromatograph Mass Spectrometer equipped with a 6 ft. X 2 mm ID, 3% OV-1 on 80/100 mesh Chromosorb W column. The instrument was operated with a source temperature of 40°C, a column temperature of 220°C, helium carrier gas, and an ion energy of 70 eV. The m/e 282 (P) and m/e 284 (P+2) ratios were monitored by mass fragmentography with electronic integration of peak areas. Standards of silvex in plasma, urine, and feces and blanks of plasma, urine, and feces were analyzed with the experimental samples. Analysis of Conjugates and Metabolites Urine samples from all subjects collected during the intervals 0-12, 12-24, 24-36, 36-48, 48-60, and 60-72 hr were analyzed for silvex conjugate(s). Separate 5 ml portions of urine were adjusted to pH 1 by addition of HCI or to pH 13 by addition of NaOH and refluxed at 100°C for up to 24 hr. These samples were analyzed for silvex as described above. Standards and blanks of hydrolyzed urine were also analyzed. Separate hydrolyzed and nonhydrolyzed urine samples (5 ml) collected from the subjects (0-12, 12-24, and 24-36 hr) were analyzed for
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2,4,5-trichlorophenol, using the extraction and derivatization procedure described above. Gas chromatography-mass spectrometry was used to detect the methyl ester of 2,4,5-trichlorophenol. Nonhydrolyzed urine samples collected from the subjects (0-12 and 12-24 hr) were analyzed for the presence of the glycine conjugate of silvex. The samples were acidified, extracted, and methylated as described for analysis of silvex. The m/e ratio was monitored on the mass spectrometer for the methyl ester of the silvex glycine conjugate. Fecal samples were analyzed for conjugates of silvex and 2,4,5trichlorophenol. Fecal samples from five subjects (0-24 and 24-48 hr) were prepared as 33% aqueous homogenates in distilled water. Duplicate 5 ml portions of the homogenates were adjusted to pH 1 with acid or to pH 13 with base. Homogenates were refluxed at 100°C for 24 hr, extracted, and methylated as described for silvex. The m/e ratios of the methyl ester of silvex and 2,4,5-trichlorophenol were monitored on the mass spectrometer. RESULTS No untoward or adverse effects were detected in any of the subjects. Parameters evaluated in the follow-up clinical chemistry, hematology, and urinalysis were all in the normal range. The concentration of silvex in the plasma is shown in Fig. 1 as a function of time after administration. Peak plasma levels of silvex were reached between 2 and 4 hr after ingestion. The clearance of silvex from plasma was apparently biphasic with first-order rates of absorption and clearance as expressed in the following equation. 10 "S *n JO 0-
-?
X a> _> to
1
J E _to
a.
0.1 .04 8
16
24
32
40
48 56 Time, Hr
64
72
80
88
96
FIGURE 1. Concentration of silvex in plasma of eight human volunteers following ingestion of 1 mg/kg silvex.
M. W. SAUERHOFF ET AL.
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- a) (0 - a)
e [ot-ka)W-ka)
.-at
-0)(a-
J
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The equation can be represented by the diagram below:..
D at t = 0
Ct = plasma concentration of silvex at time t ka = first-order constant for absorption (hr" 1 ) FD = fraction of dose absorbed k12 = first-order rate constant for transfer from compartment 1 to compartment 2 /?2i = first-order rate constant for transfer from compartment 2 to compartment 1 kel = first-order rate constant for elimination by all processes from compartment 1 D = dose A i = amount in compartment 1 ("central compartment") at time t A2 = amount in compartment 2 ("tissue compartment") at time t Cj = concentration in compartment 1 at time t a = hybrid rate constant for initial clearance phase (hr" 1 ) |3 = hybrid rate constant for secondary phase of plasma clearance (hr" 1 ) Vt = volume of distribution for first compartment (ml/kg) V2 = volume of distribution for second compartment (ml/kg)
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TABLE 1. Rate Constants, Half-Life Values and Volumes of Distribution for Clearance of Silvex from Plasma Following a Single Oral Dose of 1 mg/kg Silvex to Eight Human Subjects Subject 1 2 3 4 5 6 7 86
10
Mhr-) 0.371 0.345 0.759 0.199 0.127 0.251 0.385 0.416
± 0.090 ±0.157 + 0.229 + 0.042 ±0.008 ±0.015 ± 0.504 ±0.137
* e/ (hr-) 0.100 0.128 0.281 0.115 0.076 0.116 0.135 0.069
*„ (hr1)
± 0.006 0.059 ± 0.0089 ± 0.047 0.063 ± 0.0275 0.201 ± 0.057 ± 0.032 ±0.007 0.071 ± 0.0039 ± 0.006 0.0468 ± 0.006 ±0.005 0.0691 ±0.0018 ±0.034 0.131 ±0.015 ± 0.003 0.0279 ±0.0037
Mean + SDC 0.356 + 0.191 0.206 ± 0.066 0.0837 ± 0.0558 a
*,,(hr-) 0.0246 0.0461 0.328 0.0489 0.0343 0.089 0.105 0.0569
K (ml/kg)
± 0.0061 109 ±8 ± 0.0046 129 + 44 ± 0.094 94+ 13 ± 0.0061 111.0± 18.0 + 0.0022 158.0 ± 5.09 ± 0.0044 81 ± 3 ±0.165 129 ± 124 + 0.0090 -
0.089 ±0.107
115 + 25
Vt (ml/kg) 0 45 ± 13 93 ± 52 155 ± 66 75 + 16 115 + 8 104 ± 5 163 ± 226 -
107 ± 41
a(hr-) 0.142 0.196 0.737 0.196 0.129 0.241 0.386 0.101
ty2 (hr)
±0.011 4.86 ± 0.362 ± 0.042 3.52 ± 0.763 ± 0.115 0.944 ±0.147 ±0.010 3.57 + 0.180 ±0.006 5.33 ± 0.232 ± 0.006 2.88 ± 0.080 ± 0.189 1.79 + 0.878 ± 0.004 6.86 ± 0.270
0.263 ± 0.209
4.0 ± 1.9
Pi hr" 0.0417 0.0415 0.0769 0.0429 0.0275 0.0333 0.0460 0.0210
1
)
± 0.0058 ±0.0168 ±0.0217 ± 0.0029 ±0.0014 ±0.0013 ± 0.0250 + 0.0099
U4 (hr) 16.6 16.7 9.00 16.1 25.2 20.8 15.1 33.0
+ 2.31 ± 6.76 ± 2.54 ± 1.09 ± 1.28
±0.812 ± 8.19 t 15.5
0.0413 ± 0.0167 16.5 ±7.3
V2 was calculated from £ „ V1 =k2, V%. Parameters for subject 8 were generated from the urinary excretion data. Variations in the plasma data did not allow a meaningful parameter estimation. Volumes of distribution cannot be generated from urinary excretion data. c The standard deviations in this row are for the point estimates obtained from the individual subjects. In the other rows the standard deviations are linearized confidence limits reflecting the uncertainty in the parameter estimates.
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TABLE 2. Percentages of a Single Oral Dose of 1 mg/kg Silvex Excreted in Urine as Silvex (S) and Silvex Conjugate (S-C) and in Feces as Silvex and/or Silvex Conjugate During Consecutive Time Intervals 1 Timp i line
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(hr)
S
S-C
2 T3
S
S-C
3 T
S
S-C
4 T
S
S-C
0-24 64.6 12.6 77.2 34.5 33.1 67.6 40.5 23.5 64.0 23.1 36.0 24-48 7.91 2.75 10.7 4.19 7.97 12.2 3.92 4.80 8.72 3.37 6.28 48-72 5.87 0.00 5.87 1.62 1.65 3.27 0.72 1.32 2.04 1.29 2.05 72-96 1.29 1.09 0.33 0.74 96-120 0.34 0.21 0.39 120-144 0.56 0.07 0.19 Total 79.7 15.4 42.3 42.7 45.8 29.6 29.1 44.3 Total S+SC 95 .1 85.0 75.4 73.4 Fecal —b — recovery 3.20 0.79 Total (urine and 85.0 feces) 95 .1 78.6 74.2
T 59.1 9.65 3.34
"T represents totat silvex plus silvex conjugate excreted in urine during consecutive 24 hr intervals. "Fecal samples were not analyzed where at least 85% of the administered dose was recovered in urine.
The rate constants characterizing the lines of best f i t for the plasma curves were obtained for each subject by nonlinear computer estimation. The values for these constants are shown in Table 1. The plasma clearance a and 0 phase half-life values (mean ± SD of eight subjects) were 4.0 ±1.9 and 16.5 ±7.3 hr, respectively. The mean volumes of distribution for the two compartments were 115 ±25 (l^i) and 107 ±41 (V2) ml/kg, respectively. Orally administered silvex was excreted from the body in urine as silvex and silvex conjugate(s). The silvex conjugate(s) was hydrolyzed to silvex under both acidic and basic conditions. The percentages of the dose of orally administered silvex excreted as silvex and silvex conjugate in urine are presented in Table 2. Within 24 hr after administration, 64.0 ±10.6% (mean ± SD) of the administered dose had been excreted in urine. Recovery of silvex and silvex conjugate in urine ranged from 66.6% of administered silvex for subject 6 to 95.3% for subject 1. Recovery of silvex and/or conjugate for all subjects was 79.8 ±10.7% (mean ± SD). Subject 1 excreted 79.7% as silvex and 15.4% as conjugate, while subject 7 excreted 54.2% as conjugate and 40.4% as silvex. Significant levels of 2,4,5-trichlorophenol were not detected in hydrolyzed or nonhydrolyzed urine samples. The glycine conjugate of silvex was not detected in nonhydrolyzed urine. The percentage of orally administered silvex excreted in urine as silvex per se during consecutive 12 hr intervals is presented graphically in
FATE OF SILVEX IN HUMANS
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6
5
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Time (hr)
S
S-C
T
S
S-C
8
7 T
S
S-C
T
S
S-C
T
0-24 22.8 29.2 52.0 29.7 18.0 47.7 32.4 44.4 76.8 28.3 38.9 67.2 24-48 4.87 6.36 11.2 5.70 2.76 8.46 6.47 5.00 11.5 5.63 7.58 13.2 48-72 1.68 1.50 3.18 3.10 1.54 4.64 1.50 2.23 3.73 1.26 0.56 1.82 72-96 1.50 1.25 0.53 0.19 0.12 96-120 0.88 0.22 1.05 120-144 0.33 0.11 0.50 40.4 54.2 35.6 42.2 Total 33.3 37.3 42.1 24.5 70.4 66.6 94 .6 Total S+SC 77. 8 Fecal — recovery 0.18 0.00 Total (urine and feces) 70.6 66.6 94 .6 77.8
semilogarithmic coordinates in Fig. 2. There was apparent biphasic excretion of silvex in urine. The a or rapid phase of excretion predominated through 36-48 hr. The j3 phase predominated during the 48-144 hr time interval. The rate constants and half-life values for both phases of urinary excretion are presented in Table 3. 100
10
o Q
0.1
0.01 0
8
16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 136 144 Time, Hr
FIGURE 2. Percentage of dose excreted in urine as silvex during successive 12 hr time intervals following a single oral dose of 1 mg/kg silvex.
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TABLE 3. Rate Constants and Half-Life Values for the Excretion of Silvex in Urine Following a Single Oral Dose of 1 mg/kg Silvex to Eight Human Subjects Subject
l/t
O
1 2 3 4 5 6 7 8 Mean ± SD
r-) 0.165 0.092 0.104 0.145 0.119 0.105 0.078 0.069 0.109
± 0.186 ± 0.027 + 0.029 ± ± ± ± ± ±
0.018 0.076 0.200 0.123 0.003 0.032
* . . (hr-) 0.0839 0.0426 0.0271 0.0386 0.0308 0.0354 0.0375 0.0279 0.0393
±0.0351 ±0.0117 ±0.0112 ± 0.0143 ± 0.0069 ± 0.0077 ± 0.0890 ± 0.0037 ± 0.0204
* . . (hr-)
a(hr-)
0.0932 ± 0.1740 0.0213 ±0.0139 0.0054 ± 0.0049 0.0440 ±0.0818 0.0445 ± 0.0371 0.0414 ± 0.0107 0.0797 ± 0.0970 0.0569 ± 0.0090 0.0483 ± 0.0285
0.295 ±0.261 0.128 ±0.030 0.114 ±0.029 0.199 ±0.096 0.173 ±0.082 0.158 ±0.183 0.091 ±0.191 0.101 ±0.004 0.157 ±0.067
(hr) 2.35 5.41 6.24 3.48 4.01 4.39 7.57 6.86 5.00
± 2.08 ± 1.31 ± 1.59 ± 1.69 ± 1.92 ±5.08 ± 15.80 ± 0.27 ± 1.80
PO« " ) 0.0468 0.0319 0.0254 0.0280 0.0211 0.0235 0.0319 0.210 0.0287
± 0.0461 ±0.0015 ±0.0104 ±0.0166 ± 0.0084 ±0.0184 ±0.0155 ± 0.0099 ± 0.0084
*/, (hr) 14.8 21.7 27.2 24.5 32.8 29.4 21.7 33.0 25.9
± 14.6 ±6.46 ± 11.1 ± 14.5 ± 13.1 ±23.0 ± 10.6 ± 15.5 ±6.3
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Recovery of silvex and/or silvex conjugate in feces was established in five subjects (3, 4, 5, 6, and 8) in whom urinary recovery of silvex ranged from 66.6 to 77.8% of the administered dose. These data are shown in Table 3. Recovery of silvex and/or silvex conjugate in feces accounted for not more than 3.2% of the administered dose (subject 3), while subjects 4, 5, 6, and 8 excreted 0.79, 0.18, 0.00, and 0.00% of the administered dose, respectively. Total recovery of silvex in urine and feces is also shown in Table 3. No significant ( p < 0 . 0 5 ) levels of 2,4,5-trichIorophenol were found in feces.
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DISCUSSION Interpretation of the plasma data is consistent with a twocompartment pharmacokinetic model as described above (results). The rate constants and half-life values for the first-order absorption and clearance of silvex are presented in Table 1. The data and resultant parameters were evaluated statistically by a lack-of-fit test to determine whether the two-compartment model was the simplest one adequate to describe the data. Subjects were evaluated individually. In all instances, a twocompartment representation was significantly better than a onecompartment model and was adequate to describe the data. Adding more compartments did not significantly improve the fit. There is no indication in the data that the clearance of silvex from plasma is saturated at the 1 mg/kg dose in humans. Sauerhoff et al. (1977a) have shown that in rats the clearance of silvex can be saturated but only at large doses (50 mg/kg). The terminal or |3 phase half-life value of 16.5 hr indicates that silvex would not accumulate in the body. A practical question is, what will the plasma concentration of silvex be after the /7th daily dose of 1 mg/kg, and how rapidly will steady-state concentrations be reached? Assuming that the kinetics for elimination remain unchanged with repeated dosing, multiple dosing was simulated with a digital computer program using mean parameter estimates from the eight subjects. The steady-state concentration of silvex in plasma would be 5.06 jug/g. Steady state would be attained by the fifth daily dose, after which there would be no significant additional buildup of silvex in the plasma. Excretion of silvex in urine (Fig. 2 and Table 2) is biphasic with a more rapid excretion phase predominating through 36-48 hr and a somewhat slower phase \ty2 = 2 5 . 9 hr) predominating through 48-144 hr. The excretion data are also consistent with a two-compartment pharmacokinetic model. The plasma clearance and urinary excretion data are therefore internally consistent. Silvex is excreted from the body as silvex and a silvex conjugate. Detectable levels of 2,4,5-trichlorophenol and/or 2,4,5-trichlorophenol conjugate were not found in urine by gas chromatography—mass spectrometry.
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Small amounts of silvex and/or silvex conjugate were found in feces. This may represent unabsorbed compound or absorbed compound subsequently excreted in bile and eliminated from the body in feces. Biliary excretion and enterohepatic circulation of silvex and silvex conjugate have been shown to be a major pathway in rats (Sauerhoff et al., 1977a). It may be useful to compare the data generated in this study with the data from studies on two closely related compounds, 2,4,5-T and 2,4-D. Silvex, 2,4-D, and 2,4,5-T are all well absorbed after oral administration. Clearance of 2,4,5-T from plasma (Gehring et al., 1973) could be explained by a one-compartment model with a pooled half-life value of 23.1 hr and clearance of 2,4-D from plasma could be explained by either a one- or a two-compartment model with a pooled half-life of 11.6 hr (Sauerhoff et al., 1977b), while silvex clearance was biphasic for all subjects with a half-life (/3 phase) of 16.5 hr. While the clearance of silvex from plasma was biphasic, the |3 or slow clearance component is comparable with the monophasic rate for clearance of 2,4,5-T. The chlorinated phenoxy acids 2,4-D, 2,4,5-T, and silvex are all excreted from the body predominantly via the urine. 2,4,5-T is excreted as the unchanged acid, 2,4-D is excreted mainly as the unchanged acid with smaller amounts of a 2,4-D conjugate, and silvex is excreted as free silvex and silvex conjugate. No evidence of saturation kinetics was observed at the dose levels studied (2,4,5-T, 5 mg/kg; 2,4-D, 5 mg/kg; and silvex, 1 mg/kg). REFERENCES Gehring, P. J., Kramer, C. G., Schwetz, B. A., Rose, J. Q. and Rowe, V. K. 1973. The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) following oral administration to man. Toxicol. Appl. Pharmacol. 26:352-361. Piper, W. N., Rose, J. Q., Leng, M. L. and Gehring, P. J. 1973. The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in rats and dogs. Toxicol. Appl. Pharmacol. 26:339-351. Rowe, V. K. and Hymas, T. A. 1954. Summary of toxicological information of 2,4-D and 2,4,5-T type herbicides and evaluation of the hazards to livestock associated with their use. Am. J. Vet. Res. 15:622-629. Sauerhoff, M. W., Braun, W. H. and LeBeau, J. E. 1977a. The dose-dependent pharmacokinetic profile of silvex following intravenous administration in rats. J. Toxicol. Environ. Health 2(3):605-618. Sauerhoff, M. W., Braun, W. H., Blau, G. E. and Gehring, P. J. 1977b. The fate of 2,4-dichlorophenoxyacetic acid following oral administration to man. Toxicology, in press. Tocco, D. J., Duncan, A. E. W., Deluna, F. A., Hucker, H. B., Gruber, V. F. and Vandenheurel, W. J. A. 1975. Physiological disposition and metabolism of Timolol in man and laboratory animals. Drug. Metab. Dispos. 3(5):361-370.
Received September 9, 1977 Accepted September 14, 1977
ISSN: 0098-4108 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/uteh19
Fate of silvex following oral administration to humans M. W. Sauerhoff , M. B. Chenoweth , R.J. Karbowski , W.H. Braun , J.C. Ramsey , P.J. Gehring & G. E. Blau To cite this article: M. W. Sauerhoff , M. B. Chenoweth , R.J. Karbowski , W.H. Braun , J.C. Ramsey , P.J. Gehring & G. E. Blau (1977) Fate of silvex following oral administration to humans, Journal of Toxicology and Environmental Health, 3:5-6, 941-952, DOI: 10.1080/15287397709529628 To link to this article: http://dx.doi.org/10.1080/15287397709529628
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FATE OF SILVEX FOLLOWING ORAL ADMINISTRATION TO HUMANS M. W. Sauerhoff Stauffer Chemical Company, Westport, Connecticut
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M. B. Chenoweth Bio-Medical Research Laboratory, The Dow Chemical Company, Midland, Michigan R. J. Karbowski, W. H. Braun, J. C. Ramsey, P. J. Gehring Health and Environmental Research, Toxicology Research Laboratory, The Dow Chemical Company, Midland, Michigan G. E. Blau Computations Research Laboratory, The Dow Chemical Company, Midland, Michigan The fate of silvex [2-(2,4,5-trichlorophenoxy)propionic acid] was defined in seven men and one woman following oral administration of 1 mg/kg. No adverse effects were observed. Samples of blood plasma, urine, and feces were collected at designated time intervals through 168 hr. Plasma samples were analyzed for silvex only, while samples of urine and feces were analyzed for silvex, silvex conjugate(s), 2,4,5trichlorophenol, and 2,4,5-trichlorophenol conjugate(s). Apparent first-order kinetics described the biphasic clearance of silvex from plasma and excretion in urine. The half-life values for clearance of silvex from plasma were 4.0 ± 1.9 and 16.5 ± 7.3 hr for the initial and terminal phases, respectively. Peak plasma levels of silvex occurred within 2-4 hr after dosage. Within 24 hr after administration, 65% of the administered dose had been excreted in urine. Silvex was excreted in urine as silvex and silvex conjugate(s). The half-life values for excretion of silvex per se in urine were 5.0 ± 1.8 and 25.9 ± 6.3 hr for the two phases, respectively. Small amounts (3.2% or less) of silvex and/or silvex conjugate(s) were eliminated in feces. Recovery of silvex and its conjugated) in urine and feces through 168 hr ranged from 66.6 to 95.1% of the administered dose, with a mean value and standard deviation of 80.3 ± 10.5%. In humans, silvex is readily absorbed after ingestion and subsequently readily excreted, predominantly via the urine.
INTRODUCTION Silvex [2-(2,4,5-trichlorophenoxy)propionic acid] is a herbicide active against undesirable woody plants and weeds in pastures, fields, lawns, Requests for reprints should be sent to W. H. Braun, Pharmacokinetics/Metabolism, The Dow Chemical Company, 1803 Building, Midland, Michigan 48640. 941 journal of Toxicology and Environmental Health, 3:941-952,1977 Copyright © 1977 by Hemisphere Publishing Corporation
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rangeland, certain field crops, and aquatic habitats. The compound belongs to the family of chlorophenoxy-acid herbicides of which 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid (2,4-D) are members. Silvex has been manufactured and marketed under the trade name KURON 1 herbicide, which contains propylene glycol butyl ether esters of silvex (approximately 65% silvex). The acute oral LD 5 0 values for KURON herbicide are approximately 1070 and 850 mg/kg for the rat and rabbit, respectively (The Dow Chemical Co., unpublished data). The acute oral LD SO for silvex in rats is 650 mg/kg (560-760 mg/kg) (Rowe and Hymas, 1954). There is a dearth of information available in the open literature on the subchronic and chronic toxicity of silvex. Two-year studies have been conducted in Wistar rats and beagle dogs maintained on diets containing Kurosal SL (a formulation of the potassium salt of silvex) (The Dow Chemical Co., unpublished studies). Beagle dogs, four per sex per dose level, were maintained on diets containing 0.0, 0.r056, 0.019, and 0.0056% Kurosal. The daily doses in silvex acid equivalents were, respectively, 0.056% in diet: 8.2 mg/kg-day, males 9.9 mg/kg-day, females 0.019% in diet: 2.6 mg/kg-day, males and females 0.0056% in diet: 0.9 mg/kg-day, males and females One dog of each sex from each level was killed after 1 yr and the remainder were killed after 2 yr for pathological evaluation. Evidence of an untoward effect on the liver was reported in both males and females receiving the high level and in males receiving the intermediate level. Pathologically, the damage was described as mild degeneration and necrosis of hepatocytes with slight fibroblastic proliferation. Elevated levels of serum glutamic-pyruvic transaminase and serum glutamic-oxaloacetic transaminase were discerned in females receiving the high level for 18 and 24 months. No changes were observed in other parameters—growth, food consumption, hematological evaluation, blood urea nitrogen, alkaline phosphatase, bromosulfophthalein dye retention, and organ weights. The no-adverse-effect level was judged as 0.9 mg/kg-day silvex acid. Rats, 30 per sex per dose level, were maintained on diets containing sufficient Kurosal to provide doses of approximately 0.0, 0.26, 0.8, 2.6, and 7.9 mg/kg-day silvex acid equivalents. Three, four, or five rats per sex per dose level were killed at 12 and 18 months; the remainder died spontaneously or were killed after 2 yr. The only reported effect related to treatment was an increased kidney/body weight ratio in males receiving the high dose and a lower body weight in the same rats. 1 Trademark of The Dow Chemical Co.
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The kidney/body weight ratios of male rats receiving the other doses of silvex were also increased significantly when compared with controls. However, the control kidney/body weight ratio was somewhat less than that found in other experiments conducted in the laboratory. It cannot be ruled out that the kidney/body weight ratios were increased as a result of treatment. However, the absence of pathological effects, grossly and histologically, indicates that, if related to treatment, this parameter very likely reflects a physiological adaptation. Other parameters that were evaluated and judged not to have revealed an untoward effect were general appearance, mortality, tumor incidence, hematological evaluations, serum urea nitrogen, serum alkaline phosphatase, and gross and microscopic pathology. A pharmacokinetic study of silvex in humans was conducted to determine the rate at which silvex is eliminated from the body and the potential for accumulation of silvex in the body. In the study, 1 mg/kg was administered. This dose was deemed safe after a review of the toxicity studies of silvex in laboratory animals and similar studies conducted in this laboratory on other chlorophenoxy acids 2,4,5-T and 2,4-D (Gehring et al., 1973;Sauerhoff etal., 1977a, 1977b).
MATERIALS AND METHODS Subjects Seven male (ages 37-61) and one female (age 48) human volunteer (weights 56.7-94.8 kg) participated in this study. The medical history of each individual was reviewed and a physical examination was conducted before the study. Included in the examination were blood pressure, pulse rate, pulmonary function, electrocardiogram, chest X-ray, hemoglobin, packed cell volume, red blood cell count, white blood cell count and differential, sedimentation rate, serum concentration of total calcium, cholesterol, triglycerides, glucose, inorganic phosphate, albumin, total protein, uric acid, bilirubin, glutamic-oxaloacetic transaminase, lactic dehydrogenase, alkaline phosphatase, and urinary protein and sugar. A complete neurological examination was also conducted. An additional evaluation including clinical chemistry, hematology, and urinalysis was conducted 2 wk after completion of the study. A preliminary pharmacokinetic study was conducted in one male subject; the samples were analyzed for silvex only. Dosage and Sample Collection for Analysis Between 8:00 and 8:05 a.m., all subjects ingested as a powder in a teaspoon 1.0 mg/kg analytical grade free acid of silvex (sample AGR 127923, The Dow Chemical Co.) of greater than 99% purity, containing less than 0.01 ppm 2,3,7,8-tetrachlorodibenzo-p-dioxin. The mouth was rinsed
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M. W. SAUERHOFF ETAL.
repeatedly with water, which was swallowed. Food was withheld 1 hr before and 4 hr after ingestion of the dose. All subjects ate a similar breakfast, consisting of juice or citrus fruit, cereal with milk, one egg, and toast. Blood samples (10 ml) were collected in vacutainer tubes containing 4 mg of potassium oxalate at 1, 2, 4, 8, 12, 24, 36, 48, 60, 72, and 96 hr after ingestion. After collection, the whole blood samples were centrifuged, and the plasma was separated and transferred to vials for subsequent analysis of silvex. Subjects were instructed not to eat for at least 1 hr before a blood sample was to be collected. All the urine voided by each subject during successive 12 hr intervals for 168 hr after dosing was collected and the volume was recorded. The urine collected during each 12 hr interval was thoroughly mixed and 5 ml portions were analyzed for silvex. All subjects urinated immediately before ingesting the dose. All the feces excreted during successive 24 hr intervals for 168 hr after dosing were collected and frozen for subsequent analysis. Silvex Analysis Plasma, urine, and 33% fecal homogenates were analyzed for silvex by gas chromatography-mass spectrometry. Samples (5 ml) were acidified" with 1 ml 1 /V HCI and extracted three times with diethyl ether. Ether extracts were pooled, reduced in volume to about 10 ml under a stream of dry nitrogen, and methylated by bubbling diazomethane through the solution. The ether extracts were evaporated to dryness under a stream of dry nitrogen and reconstituted in 1 ml hexane. Five microliters of the hexane solution was injected into a Finnigan model 3000D Quadrupole Gas Chromatograph Mass Spectrometer equipped with a 6 ft. X 2 mm ID, 3% OV-1 on 80/100 mesh Chromosorb W column. The instrument was operated with a source temperature of 40°C, a column temperature of 220°C, helium carrier gas, and an ion energy of 70 eV. The m/e 282 (P) and m/e 284 (P+2) ratios were monitored by mass fragmentography with electronic integration of peak areas. Standards of silvex in plasma, urine, and feces and blanks of plasma, urine, and feces were analyzed with the experimental samples. Analysis of Conjugates and Metabolites Urine samples from all subjects collected during the intervals 0-12, 12-24, 24-36, 36-48, 48-60, and 60-72 hr were analyzed for silvex conjugate(s). Separate 5 ml portions of urine were adjusted to pH 1 by addition of HCI or to pH 13 by addition of NaOH and refluxed at 100°C for up to 24 hr. These samples were analyzed for silvex as described above. Standards and blanks of hydrolyzed urine were also analyzed. Separate hydrolyzed and nonhydrolyzed urine samples (5 ml) collected from the subjects (0-12, 12-24, and 24-36 hr) were analyzed for
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2,4,5-trichlorophenol, using the extraction and derivatization procedure described above. Gas chromatography-mass spectrometry was used to detect the methyl ester of 2,4,5-trichlorophenol. Nonhydrolyzed urine samples collected from the subjects (0-12 and 12-24 hr) were analyzed for the presence of the glycine conjugate of silvex. The samples were acidified, extracted, and methylated as described for analysis of silvex. The m/e ratio was monitored on the mass spectrometer for the methyl ester of the silvex glycine conjugate. Fecal samples were analyzed for conjugates of silvex and 2,4,5trichlorophenol. Fecal samples from five subjects (0-24 and 24-48 hr) were prepared as 33% aqueous homogenates in distilled water. Duplicate 5 ml portions of the homogenates were adjusted to pH 1 with acid or to pH 13 with base. Homogenates were refluxed at 100°C for 24 hr, extracted, and methylated as described for silvex. The m/e ratios of the methyl ester of silvex and 2,4,5-trichlorophenol were monitored on the mass spectrometer. RESULTS No untoward or adverse effects were detected in any of the subjects. Parameters evaluated in the follow-up clinical chemistry, hematology, and urinalysis were all in the normal range. The concentration of silvex in the plasma is shown in Fig. 1 as a function of time after administration. Peak plasma levels of silvex were reached between 2 and 4 hr after ingestion. The clearance of silvex from plasma was apparently biphasic with first-order rates of absorption and clearance as expressed in the following equation. 10 "S *n JO 0-
-?
X a> _> to
1
J E _to
a.
0.1 .04 8
16
24
32
40
48 56 Time, Hr
64
72
80
88
96
FIGURE 1. Concentration of silvex in plasma of eight human volunteers following ingestion of 1 mg/kg silvex.
M. W. SAUERHOFF ET AL.
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- a) (0 - a)
e [ot-ka)W-ka)
.-at
-0)(a-
J
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The equation can be represented by the diagram below:..
D at t = 0
Ct = plasma concentration of silvex at time t ka = first-order constant for absorption (hr" 1 ) FD = fraction of dose absorbed k12 = first-order rate constant for transfer from compartment 1 to compartment 2 /?2i = first-order rate constant for transfer from compartment 2 to compartment 1 kel = first-order rate constant for elimination by all processes from compartment 1 D = dose A i = amount in compartment 1 ("central compartment") at time t A2 = amount in compartment 2 ("tissue compartment") at time t Cj = concentration in compartment 1 at time t a = hybrid rate constant for initial clearance phase (hr" 1 ) |3 = hybrid rate constant for secondary phase of plasma clearance (hr" 1 ) Vt = volume of distribution for first compartment (ml/kg) V2 = volume of distribution for second compartment (ml/kg)
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TABLE 1. Rate Constants, Half-Life Values and Volumes of Distribution for Clearance of Silvex from Plasma Following a Single Oral Dose of 1 mg/kg Silvex to Eight Human Subjects Subject 1 2 3 4 5 6 7 86
10
Mhr-) 0.371 0.345 0.759 0.199 0.127 0.251 0.385 0.416
± 0.090 ±0.157 + 0.229 + 0.042 ±0.008 ±0.015 ± 0.504 ±0.137
* e/ (hr-) 0.100 0.128 0.281 0.115 0.076 0.116 0.135 0.069
*„ (hr1)
± 0.006 0.059 ± 0.0089 ± 0.047 0.063 ± 0.0275 0.201 ± 0.057 ± 0.032 ±0.007 0.071 ± 0.0039 ± 0.006 0.0468 ± 0.006 ±0.005 0.0691 ±0.0018 ±0.034 0.131 ±0.015 ± 0.003 0.0279 ±0.0037
Mean + SDC 0.356 + 0.191 0.206 ± 0.066 0.0837 ± 0.0558 a
*,,(hr-) 0.0246 0.0461 0.328 0.0489 0.0343 0.089 0.105 0.0569
K (ml/kg)
± 0.0061 109 ±8 ± 0.0046 129 + 44 ± 0.094 94+ 13 ± 0.0061 111.0± 18.0 + 0.0022 158.0 ± 5.09 ± 0.0044 81 ± 3 ±0.165 129 ± 124 + 0.0090 -
0.089 ±0.107
115 + 25
Vt (ml/kg) 0 45 ± 13 93 ± 52 155 ± 66 75 + 16 115 + 8 104 ± 5 163 ± 226 -
107 ± 41
a(hr-) 0.142 0.196 0.737 0.196 0.129 0.241 0.386 0.101
ty2 (hr)
±0.011 4.86 ± 0.362 ± 0.042 3.52 ± 0.763 ± 0.115 0.944 ±0.147 ±0.010 3.57 + 0.180 ±0.006 5.33 ± 0.232 ± 0.006 2.88 ± 0.080 ± 0.189 1.79 + 0.878 ± 0.004 6.86 ± 0.270
0.263 ± 0.209
4.0 ± 1.9
Pi hr" 0.0417 0.0415 0.0769 0.0429 0.0275 0.0333 0.0460 0.0210
1
)
± 0.0058 ±0.0168 ±0.0217 ± 0.0029 ±0.0014 ±0.0013 ± 0.0250 + 0.0099
U4 (hr) 16.6 16.7 9.00 16.1 25.2 20.8 15.1 33.0
+ 2.31 ± 6.76 ± 2.54 ± 1.09 ± 1.28
±0.812 ± 8.19 t 15.5
0.0413 ± 0.0167 16.5 ±7.3
V2 was calculated from £ „ V1 =k2, V%. Parameters for subject 8 were generated from the urinary excretion data. Variations in the plasma data did not allow a meaningful parameter estimation. Volumes of distribution cannot be generated from urinary excretion data. c The standard deviations in this row are for the point estimates obtained from the individual subjects. In the other rows the standard deviations are linearized confidence limits reflecting the uncertainty in the parameter estimates.
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TABLE 2. Percentages of a Single Oral Dose of 1 mg/kg Silvex Excreted in Urine as Silvex (S) and Silvex Conjugate (S-C) and in Feces as Silvex and/or Silvex Conjugate During Consecutive Time Intervals 1 Timp i line
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(hr)
S
S-C
2 T3
S
S-C
3 T
S
S-C
4 T
S
S-C
0-24 64.6 12.6 77.2 34.5 33.1 67.6 40.5 23.5 64.0 23.1 36.0 24-48 7.91 2.75 10.7 4.19 7.97 12.2 3.92 4.80 8.72 3.37 6.28 48-72 5.87 0.00 5.87 1.62 1.65 3.27 0.72 1.32 2.04 1.29 2.05 72-96 1.29 1.09 0.33 0.74 96-120 0.34 0.21 0.39 120-144 0.56 0.07 0.19 Total 79.7 15.4 42.3 42.7 45.8 29.6 29.1 44.3 Total S+SC 95 .1 85.0 75.4 73.4 Fecal —b — recovery 3.20 0.79 Total (urine and 85.0 feces) 95 .1 78.6 74.2
T 59.1 9.65 3.34
"T represents totat silvex plus silvex conjugate excreted in urine during consecutive 24 hr intervals. "Fecal samples were not analyzed where at least 85% of the administered dose was recovered in urine.
The rate constants characterizing the lines of best f i t for the plasma curves were obtained for each subject by nonlinear computer estimation. The values for these constants are shown in Table 1. The plasma clearance a and 0 phase half-life values (mean ± SD of eight subjects) were 4.0 ±1.9 and 16.5 ±7.3 hr, respectively. The mean volumes of distribution for the two compartments were 115 ±25 (l^i) and 107 ±41 (V2) ml/kg, respectively. Orally administered silvex was excreted from the body in urine as silvex and silvex conjugate(s). The silvex conjugate(s) was hydrolyzed to silvex under both acidic and basic conditions. The percentages of the dose of orally administered silvex excreted as silvex and silvex conjugate in urine are presented in Table 2. Within 24 hr after administration, 64.0 ±10.6% (mean ± SD) of the administered dose had been excreted in urine. Recovery of silvex and silvex conjugate in urine ranged from 66.6% of administered silvex for subject 6 to 95.3% for subject 1. Recovery of silvex and/or conjugate for all subjects was 79.8 ±10.7% (mean ± SD). Subject 1 excreted 79.7% as silvex and 15.4% as conjugate, while subject 7 excreted 54.2% as conjugate and 40.4% as silvex. Significant levels of 2,4,5-trichlorophenol were not detected in hydrolyzed or nonhydrolyzed urine samples. The glycine conjugate of silvex was not detected in nonhydrolyzed urine. The percentage of orally administered silvex excreted in urine as silvex per se during consecutive 12 hr intervals is presented graphically in
FATE OF SILVEX IN HUMANS
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6
5
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Time (hr)
S
S-C
T
S
S-C
8
7 T
S
S-C
T
S
S-C
T
0-24 22.8 29.2 52.0 29.7 18.0 47.7 32.4 44.4 76.8 28.3 38.9 67.2 24-48 4.87 6.36 11.2 5.70 2.76 8.46 6.47 5.00 11.5 5.63 7.58 13.2 48-72 1.68 1.50 3.18 3.10 1.54 4.64 1.50 2.23 3.73 1.26 0.56 1.82 72-96 1.50 1.25 0.53 0.19 0.12 96-120 0.88 0.22 1.05 120-144 0.33 0.11 0.50 40.4 54.2 35.6 42.2 Total 33.3 37.3 42.1 24.5 70.4 66.6 94 .6 Total S+SC 77. 8 Fecal — recovery 0.18 0.00 Total (urine and feces) 70.6 66.6 94 .6 77.8
semilogarithmic coordinates in Fig. 2. There was apparent biphasic excretion of silvex in urine. The a or rapid phase of excretion predominated through 36-48 hr. The j3 phase predominated during the 48-144 hr time interval. The rate constants and half-life values for both phases of urinary excretion are presented in Table 3. 100
10
o Q
0.1
0.01 0
8
16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 136 144 Time, Hr
FIGURE 2. Percentage of dose excreted in urine as silvex during successive 12 hr time intervals following a single oral dose of 1 mg/kg silvex.
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TABLE 3. Rate Constants and Half-Life Values for the Excretion of Silvex in Urine Following a Single Oral Dose of 1 mg/kg Silvex to Eight Human Subjects Subject
l/t
O
1 2 3 4 5 6 7 8 Mean ± SD
r-) 0.165 0.092 0.104 0.145 0.119 0.105 0.078 0.069 0.109
± 0.186 ± 0.027 + 0.029 ± ± ± ± ± ±
0.018 0.076 0.200 0.123 0.003 0.032
* . . (hr-) 0.0839 0.0426 0.0271 0.0386 0.0308 0.0354 0.0375 0.0279 0.0393
±0.0351 ±0.0117 ±0.0112 ± 0.0143 ± 0.0069 ± 0.0077 ± 0.0890 ± 0.0037 ± 0.0204
* . . (hr-)
a(hr-)
0.0932 ± 0.1740 0.0213 ±0.0139 0.0054 ± 0.0049 0.0440 ±0.0818 0.0445 ± 0.0371 0.0414 ± 0.0107 0.0797 ± 0.0970 0.0569 ± 0.0090 0.0483 ± 0.0285
0.295 ±0.261 0.128 ±0.030 0.114 ±0.029 0.199 ±0.096 0.173 ±0.082 0.158 ±0.183 0.091 ±0.191 0.101 ±0.004 0.157 ±0.067
(hr) 2.35 5.41 6.24 3.48 4.01 4.39 7.57 6.86 5.00
± 2.08 ± 1.31 ± 1.59 ± 1.69 ± 1.92 ±5.08 ± 15.80 ± 0.27 ± 1.80
PO« " ) 0.0468 0.0319 0.0254 0.0280 0.0211 0.0235 0.0319 0.210 0.0287
± 0.0461 ±0.0015 ±0.0104 ±0.0166 ± 0.0084 ±0.0184 ±0.0155 ± 0.0099 ± 0.0084
*/, (hr) 14.8 21.7 27.2 24.5 32.8 29.4 21.7 33.0 25.9
± 14.6 ±6.46 ± 11.1 ± 14.5 ± 13.1 ±23.0 ± 10.6 ± 15.5 ±6.3
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Recovery of silvex and/or silvex conjugate in feces was established in five subjects (3, 4, 5, 6, and 8) in whom urinary recovery of silvex ranged from 66.6 to 77.8% of the administered dose. These data are shown in Table 3. Recovery of silvex and/or silvex conjugate in feces accounted for not more than 3.2% of the administered dose (subject 3), while subjects 4, 5, 6, and 8 excreted 0.79, 0.18, 0.00, and 0.00% of the administered dose, respectively. Total recovery of silvex in urine and feces is also shown in Table 3. No significant ( p < 0 . 0 5 ) levels of 2,4,5-trichIorophenol were found in feces.
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DISCUSSION Interpretation of the plasma data is consistent with a twocompartment pharmacokinetic model as described above (results). The rate constants and half-life values for the first-order absorption and clearance of silvex are presented in Table 1. The data and resultant parameters were evaluated statistically by a lack-of-fit test to determine whether the two-compartment model was the simplest one adequate to describe the data. Subjects were evaluated individually. In all instances, a twocompartment representation was significantly better than a onecompartment model and was adequate to describe the data. Adding more compartments did not significantly improve the fit. There is no indication in the data that the clearance of silvex from plasma is saturated at the 1 mg/kg dose in humans. Sauerhoff et al. (1977a) have shown that in rats the clearance of silvex can be saturated but only at large doses (50 mg/kg). The terminal or |3 phase half-life value of 16.5 hr indicates that silvex would not accumulate in the body. A practical question is, what will the plasma concentration of silvex be after the /7th daily dose of 1 mg/kg, and how rapidly will steady-state concentrations be reached? Assuming that the kinetics for elimination remain unchanged with repeated dosing, multiple dosing was simulated with a digital computer program using mean parameter estimates from the eight subjects. The steady-state concentration of silvex in plasma would be 5.06 jug/g. Steady state would be attained by the fifth daily dose, after which there would be no significant additional buildup of silvex in the plasma. Excretion of silvex in urine (Fig. 2 and Table 2) is biphasic with a more rapid excretion phase predominating through 36-48 hr and a somewhat slower phase \ty2 = 2 5 . 9 hr) predominating through 48-144 hr. The excretion data are also consistent with a two-compartment pharmacokinetic model. The plasma clearance and urinary excretion data are therefore internally consistent. Silvex is excreted from the body as silvex and a silvex conjugate. Detectable levels of 2,4,5-trichlorophenol and/or 2,4,5-trichlorophenol conjugate were not found in urine by gas chromatography—mass spectrometry.
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Small amounts of silvex and/or silvex conjugate were found in feces. This may represent unabsorbed compound or absorbed compound subsequently excreted in bile and eliminated from the body in feces. Biliary excretion and enterohepatic circulation of silvex and silvex conjugate have been shown to be a major pathway in rats (Sauerhoff et al., 1977a). It may be useful to compare the data generated in this study with the data from studies on two closely related compounds, 2,4,5-T and 2,4-D. Silvex, 2,4-D, and 2,4,5-T are all well absorbed after oral administration. Clearance of 2,4,5-T from plasma (Gehring et al., 1973) could be explained by a one-compartment model with a pooled half-life value of 23.1 hr and clearance of 2,4-D from plasma could be explained by either a one- or a two-compartment model with a pooled half-life of 11.6 hr (Sauerhoff et al., 1977b), while silvex clearance was biphasic for all subjects with a half-life (/3 phase) of 16.5 hr. While the clearance of silvex from plasma was biphasic, the |3 or slow clearance component is comparable with the monophasic rate for clearance of 2,4,5-T. The chlorinated phenoxy acids 2,4-D, 2,4,5-T, and silvex are all excreted from the body predominantly via the urine. 2,4,5-T is excreted as the unchanged acid, 2,4-D is excreted mainly as the unchanged acid with smaller amounts of a 2,4-D conjugate, and silvex is excreted as free silvex and silvex conjugate. No evidence of saturation kinetics was observed at the dose levels studied (2,4,5-T, 5 mg/kg; 2,4-D, 5 mg/kg; and silvex, 1 mg/kg). REFERENCES Gehring, P. J., Kramer, C. G., Schwetz, B. A., Rose, J. Q. and Rowe, V. K. 1973. The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) following oral administration to man. Toxicol. Appl. Pharmacol. 26:352-361. Piper, W. N., Rose, J. Q., Leng, M. L. and Gehring, P. J. 1973. The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in rats and dogs. Toxicol. Appl. Pharmacol. 26:339-351. Rowe, V. K. and Hymas, T. A. 1954. Summary of toxicological information of 2,4-D and 2,4,5-T type herbicides and evaluation of the hazards to livestock associated with their use. Am. J. Vet. Res. 15:622-629. Sauerhoff, M. W., Braun, W. H. and LeBeau, J. E. 1977a. The dose-dependent pharmacokinetic profile of silvex following intravenous administration in rats. J. Toxicol. Environ. Health 2(3):605-618. Sauerhoff, M. W., Braun, W. H., Blau, G. E. and Gehring, P. J. 1977b. The fate of 2,4-dichlorophenoxyacetic acid following oral administration to man. Toxicology, in press. Tocco, D. J., Duncan, A. E. W., Deluna, F. A., Hucker, H. B., Gruber, V. F. and Vandenheurel, W. J. A. 1975. Physiological disposition and metabolism of Timolol in man and laboratory animals. Drug. Metab. Dispos. 3(5):361-370.
Received September 9, 1977 Accepted September 14, 1977
