Flumethrin, (R,S)-a -cyano-4-fluoro-3-phenoxybenzyl 3-(b,4-dichlorostyryl)-2,2-dimethylcyclopropanecarboxylate, is a pyrethroid acaracide composed of a mixture of two diasterioisomers (trans-Z-1 and trans-Z-2, with an approximate ratio 55:45) formed by the reaction of 4-fluoro-3-phenoxybenzaldehyde and trans-(E)-3-[2-chloro-2-(4-chlorophenyl)vinyl]-2,2-dimethylcyclopropanecarboxylic acid chloride in the presence of cyanide. It is widely used as a topical pesticide for the control of ectoparasites such as ticks and buffalo flies on farm animals by spraying, dipping or other treatments. It was reviewed by the present Meeting for the first time. The focus was on the uses against animal ectoparasites, although flumethrin residues in honey and beeswax from supervised trials on honey bee colonies were also provided and reviewed.
The Meeting agreed that data on environmental fate were not required in relation to potential flumethrin residues in animal products from uses as an ectoparasiticide, but considered such information to be desirable for assessing the potential for undesirable environmental effects.
The mammalian metabolism of flumethrin was reported for rats and cattle. Flumethrin administered orally, i.v. and duodenally showed ester hydrolysis to 3-(B,4-dichlorostyryl)-2,2-dimethylcyclopropanecarboxylic acid (flumethrin acid, BFN 5533A) and (through the probable cyanohydrin (FCR 1271) and 4-fluoro-3-phenoxybenzaldehyde (FCR 1260) intermediates) to the other main identified metabolite 4-fluoro-3-phenoxybenzoic acid. Flumethrin acid is conjugated to form the glucuronide and the fluorophenoxybenzoic acid component is further oxidized to 4-fluoro-3-(4-hydroxyphenoxy)benzoic acid, the last two compounds being conjugated to glycine. The studies indicate substantially greater 14C elimination in the faeces than in the urine from chlorophenyl-labelled flumethrin and roughly equal elimination in faeces and urine from the fluorophenyl-labelled compound, with faster elimination of the fluorophenyl label.
Although the rat metabolism studies with labelled or unlabelled flumethrin are useful for identifying metabolites and provide useful information on the mammalian metabolism of orally, i.v. or duodenally administered flumethrin, they do not fully reflect exposure from topical application which is relevant to the approved uses on cattle, horses, goats or sheep.
A 1986 material balance and distribution study on cattle was based on the administration of fluorophenyl-labelled 14C-flumethrin as a back treatment, approximately at approved rates. After 48 hours 71% of the administered dose remained in or on the skin in the application area with <10 ng flumethrin equivalents/g in the tissues and <3 ng equivalents/ml in the milk through 31 hours, demonstrating slow absorption within this short test period.
In a similar study on cattle in 1994 chlorophenyl-labelled flumethrin was administrated at approximately an approved rate intravenously as opposed to topically and samples were analysed to study metabolism. Relatively little radioactivity was eliminated within the short period of 8 hours before slaughter, but significant amounts of the administered dose (32% in a lactating cow, 13% in a steer) were detected in edible tissues in the decreasing order liver (in the cow) or muscle (in the steer), fat and kidney. Residues as mg/kg flumethrin equivalents in the tissues were highest in liver (13.4 mg/kg cow, 3.4 mg/kg steer), with 0.2-0.3 mg/kg in muscle, 0.2-0.4 mg/kg in fat and 0.3 mg/kg in milk. The liver residues suggest greater metabolic activity in the steer (lower radioactivity in the liver) than in the cow. This is reinforced by the ratios of flumethrin to BNF 5533A of 87:7 and 29:40 in cow and steer livers respectively and by the higher levels of glucuronide found in the liver and kidney of the steer. The opposite may be seen in male and female rats if flumethrin to metabolite ratios in faeces are compared: the proportion of the metabolite is higher in female faeces.
Of the measured radioactivity, 76 to 95% of the residues were identified in tissues and 68% in milk. Only flumethrin was identified in the milk but an unidentified metabolite constituted almost 12% of the milk radioactivity. BNF 5533A glucuronide was identified only in the liver and kidney. Residue levels of BNF 5533A were 1-1.5 times those of flumethrin in muscle and fat with no pronounced difference between the cow and steer. While the log POW of 6 for flumethrin indicates fat-solubility, residue levels of flumethrin per se from the i.v. injections in this study are comparable in the muscle and fat of both steers and cows, actually slightly higher in the muscle. However, as will be discussed later, residues from topical applications in the supervised trials were higher in the fat.
Analytical methods are available for the determination of flumethrin and flumethrin acid (BNF 5533A) in the tissues of cattle and sheep and of flumethrin in milk. Only flumethrin and, at lower levels, an unidentified metabolite were reported in milk in cattle metabolism studies. In recent analytical methods homogenized tissues are generally extracted with acetonitrile or acetonitrile/phosphoric acid solution, partitioned into dichloromethane and/or hexane, cleaned up on silica gel columns and determined by HPLC using UV detectors. In some cases the flumethrin acid metabolite is further cleaned up on C-18 solid phase cartridges after separation from flumethrin on silica gel before HPLC determination. Analysis of milk is similar, although in some cases milk solids are removed by the addition of acetone before partitioning into acetonitrile.
Multi-residue methods for organochlorine compounds have also been modified for the determination of pyrethroids, including flumethrin, in animal fat. The modified method involves the partition of rendered finely sliced fat between acetonitrile and hexane, dilution of the acetonitrile, Florisil column clean-up and determination by GLC with EC detection. Information was not sufficient for an independent estimate of a limit of determination for this method, although satisfactory recoveries were achieved at fortification levels of 0.02 mg/kg.
Generally, analytical recoveries are 80% or better by the more recent methods in tissues and milk with fortification at or near the reported limits of determination. Limits of determination of 0.005 mg/kg for flumethrin in milk, 0.01 mg/kg for flumethrin acid in milk, 0.01 mg/kg for flumethrin in tissues, and 0.01 or 0.02 mg/kg for flumethrin acid in tissues are generally reported, depending on the method. For the most part these limits are supported by sample chromatograms from the authors' laboratories, although in some cases sample chromatograms do not convincingly support a 0.01 mg/kg limit of determination in liver and kidney.
While the reported limits of determination may be achievable in the authors' laboratories, the Meeting concluded that limits of determination of 0.01 mg/kg for flumethrin in milk and 0.02 mg/kg for flumethrin and flumethrin acid in tissues are more realistic for routine enforcement among different laboratories. However, for dietary intake estimates the use of half these levels would be appropriate where no residues are detected.
Analytical methods have also been reported for residues of flumethrin in honey and beeswax, with recoveries generally about 75% or better. The reported limits of detection and determination were 0.001 and 0.003 mg/kg for honey and 0.02 and 0.026 mg/kg for wax, and the authors' sample chromatograms were consistent with these levels.
The manufacturer's working paper considered information on the stability of residues in stored analytical samples not to be necessary and no such studies were submitted, except relevant information incidentally included in one supervised trial report which showed flumethrin residues in milk to be stable for 40 days at -18°C. On the basis of this report and the persistence of flumethrin residues in fat even in live cows, the Meeting considered that the information provided was adequate to support estimates of maximum residue levels in cattle meat (fat) and milk. The Meeting further concluded that information on the stability of flumethrin in stored samples of other tissues (liver, kidney) was needed before maximum residue levels estimated for these tissues could be recommended for use as MRLs.
Data were available on supervised trials in a number of countries of ectoparasite control in cattle, sheep or goats using a variety of flumethrin formulations, as well as on mite control in beehives.
Data on supervised trials of ectoparasite control on cattle were available from Australia, Germany, South Africa and Japan. Approved uses (including labels) were provided only for Australia (on cattle and horses). The most recent, comprehensive and best described studies of flumethrin residues in cattle from plunge dipping or pour-on applications are 1994-1995 Australian trials submitted by the Australian government, but there was no information on whether GLP was followed in them. For example, no information was provided on the interval from slaughter to analysis, and actual storage conditions were not reported although the protocol called for storage at -40°C. An exception was a GLP study to determine the potential for residues of flumethrin acid from the treatment of cattle with a flumethrin pour-on formulation.
The 1994-95 Australian studies did not include data from spray applications, which are approved in Australia, although older Australian (and other) studies submitted to the Meeting included data from some types of approved spray applications. The older studies for the most part were also not reported to have been conducted under GLP, although in many cases essential information was available to give a reasonable degree of confidence in the data. Because only Australian approved uses for ectoparasite control in cattle were available, the Meeting based its analysis of the cattle data primarily on the Australian trials. That situation was not ideal since in the most recent and best documented studies residues were determined only in fat whereas some of the older trials included analyses of fat, liver, muscle, kidney and milk.
In the 1994-95 Australian trials low residues (0.008 mg/kg from one dip, <0.013 mg/kg from two dips) were reported for plunge dip treatments, except in one of 84 test animals which showed 0.04 and 0.05 mg/kg flumethrin in loin and renal fat respectively. The treatments were in accordance with approved uses, except that the interval between the two treatments was 3 days compared to the recommended minimum of 10 days. Higher residues were reported from approved pour-on applications to a total of 56 animals, with maximum residues of 0.04 mg/kg in loin fat from one application and 0.05 mg/kg from two applications 7 days apart, as compared with an implied minimum approved interval of 14 days. The maximum residues in renal fat were 0.11 mg/kg from one treatment and 0.14 mg/kg from two applications at the 7-day interval. The combined data from dip and pour-on trials at approved rates in the 1994-95 and 1981-84 Australian trials are shown below. The numbers of samples with the same residue or within the same ranges are shown in parentheses.
Single dips
|
Fat |
<0.005 (67), 0.006, 0.007, 0.008, 0.041, 0.047 mg/kg. |
|
Liver |
<0.005 (6) mg/kg. |
|
Muscle |
<0.005 (5), 0.01 mg/kg. |
|
Kidney |
<0.005 (6) mg/kg. |
2-dips
|
Fat |
<0.005 (69), 0.009, <0.011, <0.013 mg/kg (3-day interval as compared with the approved 10-day). |
Single pour-on
|
Fat |
<0.005 (24), 0.005 (9), 0.006-0.01 (11), 0.011-0.015 (13). 0.017-0.020 (6), 0.023-0.029 |
|
|
(9), 0.032, 0.034, 0.04, 0.042, 0.11 mg/kg. Total number = 77. |
|
Liver |
0.005 (23), 0.01 mg/kg. |
|
Muscle |
0.005 (22), 0.005, 0.01 mg/kg. |
|
Kidney |
0.005 (23), 0.01 mg/kg. |
2 pour-ons
|
Fat |
0.011-0.015 (8), 0.016-0.020 (9), 0.022-0.025 (9), 0.026-0.03 (9), 0.031-0.051 (15), |
|
|
0.052-0.058 (3), 0.097, 0.14 mg/kg (7-day interval as compared with the approved 14-day). Total number = 55. |
"Fat" includes renal and subcutaneous fat.
The double-underlined ranges within which the median residues fall.
In the Australian spray trials in 1981 at 0.7-2.6 times GAP rates, the residues in fat, liver, muscle and kidney (24 samples of each) were all 0.05 mg/kg.
On the basis of the single pour-on applications according to GAP the Meeting estimated an STMR of 0.01 mg/kg for the fat of meat and 0.005 mg/kg for whole meat.
As noted above, information on approved uses on cattle was provided only for Australia, but it is useful and of interest to relate the results of trials in other countries to Australian approved uses. In such trials the maximum residues from applications approximating Australian approved uses were 0.08 mg/kg (or 0.1 mg/kg depending on the study) in fat, and 0.01 to 0.1 mg/kg, again depending on the study, in liver, muscle and kidney. In one German study residues in liver were 0.03 mg/kg. While such a comparison may be questionable, it suggests that the maximum flumethrin residues in cattle are likely to be similar if approved uses in those countries are similar to those in Australia. The German studies also show that flumethrin residues in fat from pour-on applications reach their highest level after about 4 days and stay at or near that level for up to 28 days. This confirms the finding in the Australian trials.
It is clear that the potential for residues in cattle tissues is greater from approved pour-on uses than from spray or dip applications and, in contrast to metabolism studies with i.v. administration, field trials indicate that flumethrin residues from topical applications are likely to be significantly greater in fat than in other tissues. It is also of interest to note that the maximum residues of 0.11 to 0.14 mg/kg found in cattle fat in the pour-on trials are consistent with residues up to 0.2 mg/kg found in random Australian monitoring and less than some residues (as high as 1.1 mg/kg) found in follow-up investigations prompted by the finding of residues in random monitoring.
The supervised trials data are consistent with MRLs of 0.2 mg/kg in the carcase fat of cattle and 0.01 mg/kg in cattle muscle and kidney. The Meeting noted maximum flumethrin residues of 0.01 mg/kg in liver in the Australian trials, took into account residues up to 0.03 mg/kg in German trials approximating approved Australian uses and <0.05 or <0.1 mg/kg in other non-Australian trials, and concluded that prudence required a 0.05 mg/kg level for liver.
In the absence of studies of the storage stability of residues in tissues other than fat and in view of differences between the ratios of flumethrin residues in fat to those in non-fatty tissues found in metabolism studies and supervised trials, the Meeting was unwilling to recommend the use of the maximum residue levels estimated for liver and kidney as MRLs. This could be reconsidered at a future JMPR if relevant studies of storage stability with tissues other than fat become available.
The monitoring data suggest that residues in fat may occasionally exceed 0.2 mg/kg, especially from pour-on applications. For dietary intake purposes a level of 0.005 mg/kg (generally the lowest reported limit of determination) would be reasonable for flumethrin in the muscle, liver, fat and kidney of cattle.
A ratio of 1.9 (0.84 correlation coefficient) was reported for the residues in perirenal to those in subcutaneous fat arising from pour-on applications. Residues were also reported to be up to 32% lower in the fat of animals with greater fat deposits, presumably indicating fat dilution of the residues. In selected samples, analysis of extracted fat from core samples from cartons of frozen carcases correlate well with renal and loin fat samples taken at slaughter from the same animals.
Flumethrin residues in loin and renal fat from single approved pour-on applications increased rapidly from 2 days after treatment through the fourth day, then declined slowly until a second increase after 21 days, then declined gradually to 45 days. A similar pattern of two peaks was noted for two applications, although residues were higher and the second peak later owing to the second application. The pattern confirms the persistence of flumethrin in animal fat.
Supervised trials of ectoparasite control in sheep were available from Australia, South Africa and the U.K. Because information on approved uses was available only from the UK, the Meeting based its conclusions on sheep primarily on the single UK study. Sheep were dipped once (re-dipping is permitted after 14 days) approximately according to approved uses and samples of fat, liver, muscle and kidney were taken for analysis at intervals from 0.5 to 4 days after treatment. Although residues were low, they tended to be higher in subcutaneous than in omental fat. The maximum residues were 0.03 and <0.01 mg/kg in subcutaneous and omental fat respectively, 0.02 mg/kg in liver and <0.01 mg/kg in muscle and kidney.
Maximum residues in the relatively old Australian dip trials at 0.9 to 1.3 times UK approved use rates were 0.04 mg/kg in fat and <0.005 mg/kg in liver, kidney and muscle. Relatively old data were also available from Australia and South Africa from pour-on applications, but no relevant approved uses were provided. The maximum residues found in the Australian sheep dipping trials were comparable to those found in the dip treatments of Australian cattle, and reasonably consistent with the UK dipping results when the use rates were similar. However, because only one well-documented sheep study was available which reflected approved uses, because only one dip was represented and because sheep are generally expected to have higher residues than cattle, the Meeting concluded that the data were insufficient to estimate maximum residue levels for sheep.
Data on residues in milk were available from supervised trials on cattle, sheep and goats. Data on residues in cattle milk were available from Australia, Germany, Japan and South Africa. As with cattle tissues information on approved uses was available only for Australia and the Meeting placed most emphasis on the Australian trials. Although the Australian studies were relatively old, they were for the most part acceptably documented, included pour-on and spray applications and covered a range of intervals after treatment. The maximum residues approximately reflecting Australian approved uses after various intervals were as follows.
|
Single Pour-on
|
Ratio to GAP rate |
9 h |
24 h |
72 h |
|
0.5 |
0.01 |
0.01 |
0.01 |
|
|
1.0 |
0.04 |
0.02 |
0.01 |
|
|
0.5 |
0.04 |
0.04 |
0.02 |
|
|
1.0 |
0.03 |
0.03 |
0.01 |
|
|
Single Spray
|
|
|
|
|
|
1.0 |
<0.1 |
<0.1 |
|
|
|
1.0 |
0.01 |
(2 applications) |
|
|
|
1.3 |
<0.01 |
<0.01 |
|
The combined results from the pour-on treatments at 0.5 and 1 times the GAP rate gave the following residues at 9-72 hours: <0.01 (14), 0.01 (13), 0.02, 0.02, 0.03 (4), 0.04 (3).
The Meeting estimated an STMR for flumethrin in milk of 0.01 mg/kg.
Although no information on German approved uses was provided, two pour-on applications at approved Australian use rates resulted in maximum residues in 2 trials of 0.04 and 0.06 mg/kg after 2 days, decreasing to 0.02 and 0.04 mg/kg after 4 days and then continuing to decrease slowly.
As in the case of tissues it is clear that higher residues result from pour-on applications than from other types of application. The results suggest that multiple applications produce higher residues and point to the need for additional trials with multiple applications at approved use rates.
Supervised trials data on residues of flumethrin in sheep and goat milk were also available, but without information on relevant approved uses. At rates approved for pour-on applications to cattle in Australia, the maximum residues were 0.04 mg/kg and 0.01 mg/kg in goat and sheep milk respectively.
On the basis of the available information a maximum residue level of 0.05 mg/kg for cattle milk is reasonable, although additional data from trials with multiple treatments at approved use rates are needed to confirm that estimate.
In addition to residues of flumethrin per se in cattle tissues, data were also available from one trial on residues of flumethrin acid in tissues at intervals of 1 to 35 days after the second of two pour-on applications of flumethrin at approved application rates. The maximum flumethrin acid residues, found after 2 days were 0.04 mg/kg in fat, 0.06 mg/kg in liver, 0.01 mg/kg in muscle, and 0.05 mg/kg in kidney. These indicate that the acid metabolite is less soluble than the parent compound in fat.
Honey and beeswax. Eleven supervised trials were conducted in Switzerland, the UK, and Germany (mostly Germany) to determine the potential for flumethrin residues in honey and beeswax from the treatment of bee colonies for mite control. Applications were in the form of flumethrin-impregnated strips. The trials varied in duration from 4 to 56 weeks and covered a variety of periods of honey production, including pre-winter storage periods and before or during nectar flow. At the recommended rate of 4 strips/frame (3.6 mg ai/strip) approved in the UK, no residues (<0.001 or <0.002 mg/kg, depending on the analytical method) were measured in any of the 34 honey samples analysed. The Meeting concluded that a maximum residue level of 0.005 mg/kg (limit of determination) would be suitable for use as an MRL for honey.
Residues in beeswax were <0.02 (2), 0.02 (2), <0.03 (4), 0.03, 0.04 (3), 0.05 (3), 0.06, 0.07 (2), 0.1 (2), 0.13 and 0.2 mg/kg. The maximum residues in each of the four trials were 0.02, 0.05, 0.13 and 0.2 mg/kg with a mean of 0.1 mg/kg and an estimated median of 0.09 mg/kg. In one trial with treatment at ten times the recommended rate the maximum residue was 0.15 mg/kg. Information was lacking on many aspects of the Swiss trial which gave the maximum residue of 0.2 mg/kg.
Residue Definition. The flumethrin acid metabolite BNF 5533A was found in cow metabolism studies (8 hours after the i.v. injection of flumethrin) to occur at 1 to 1.5 times the level of flumethrin in animal tissues, but was not reported in milk. The Meeting assumed that flumethrin would be of significantly greater toxicological concern than the metabolite, noted that only flumethrin per se was reported in milk and was the main residue in tissues (especially in fat) found in supervised trials, and concluded that flumethrin was the preferred indicator residue for regulatory purposes.
For the estimation of dietary intake, it is useful to note that metabolism studies suggest that the total residues (or flumethrin per se) in meat (muscle) could be similar to or slightly higher than in fat, although that did not occur in the supervised trials where flumethrin residues were higher in fat than in muscle, essentially in all instances. The total residues of flumethrin and flumethrin acid in tissues could be expected to be at most about three times those of flumethrin.
Fat-solubility and expression of residues in meat. The log POW of 6 for flumethrin indicates high fat-solubility. This is supported by a metabolism study with back treatments of a lactating cow, where measurable residues were found in renal fat but not in muscle or subcutaneous fat. Metabolism studies with i.v. dosing, however, indicate that once flumethrin residues enter the blood stream, levels of flumethrin per se or of total radioactivity are similar in the muscle and fat of both steers and cows. If there is a difference residues in muscle under these conditions appear to be slightly higher. The same is true for BNF 5533A, the flumethrin acid metabolite. Analytical methods are available for the determination of flumethrin residues in carcase meat or fat. Residues in edible offal can conveniently be on a whole-commodity basis.
The manufacturer expects to propose limits to the European Union for liver (0.04 mg/kg), milk (0.12 mg/kg) and fat (0.1 mg/kg), but none for muscle or kidney, since no residues were reported in these tissues in cattle or sheep. However, the procedures for sampling and analysis in the field trials and for the regulation of residues in meat (muscle) are key factors in determining how residues in meat should be expressed.
The most relevant, recent and comprehensive supervised trials (Australian 1994-95) involved the determination of residues in fat rendered from finely sliced loin (subcutaneous) and renal fat. In the older studies, even though mean residues were at or below the limit of detection or determination, residues in the meat (muscle) of individual cows were measurable in some cases (up to 0.02 mg/kg). In older trials on cattle and sheep the type of fat was not defined, except in a 1980 South African trial where it was renal fat. No information on approved uses was provided for the older (1980-1989) trials, except those in Australia.
Since meat is often regulated at the international level on the basis of residues in subcutaneous fat, as a practical matter it is convenient to propose limits for meat on a fat basis (in the carcase fat) derived from residues in loin and subcutaneous fat found in supervised trials, noting that residues can be higher in renal than in subcutaneous fat. For these reasons the Meeting recommended that limits for flumethrin in meat be expressed on the carcase fat.
