Pharmacokinetics of Fluazifop-butyl in Human Volunteers II: Dermal Dosing

Percutaneous absorption in man: in vitro-in vivo correlation

AIMS

To examine the existing literature to determine the degree to which percutaneous absorption data obtained using the excised human skin model match those obtained from living man.

METHODS

The scientific literature was reviewed to collect data on compounds whose percutaneous absorption through human skin had been measured under both in vitro and in vivo conditions. The in vitro-in vivo (IVIV) correlation was evaluated by computing the in vitro/in vivo ratio using total absorption (percent of applied dose) as the metric for comparison.

RESULTS

A total of 92 data sets were collected from 30 published studies. The average IVIV ratio across all values was 1.6, though for any single data set there could be a nearly 20-fold difference between the in vitro and in vivo values. In 85% of the cases, however, the difference was less than 3-fold. The correlation was significantly improved when data were excluded from studies in which the protocols for both studies were not fully harmonized. For harmonized data sets the average IVIV ratio was 0.96 and there was a less than 2-fold difference between the in vitro and in vivo results for any one compound, with IVIV ratios ranging from 0.58 to 1.28. The dominant factors leading to exclusion of data were the use of skin from different anatomical sites and vehicles of differing composition.

CONCLUSIONS

Percutaneous absorption data obtained from the excised human skin model closely approximate those obtained from living man when the two study protocols are appropriately matched.

The ethics of human volunteer studies involving experimental exposure to pesticides: unanswered dilemmas

The controversy about the use of data from human volunteer studies involving experimental exposure to pesticides as part of regulatory risk assessment has been widely discussed, but the complex and interrelated scientific and ethical issues remain largely unresolved. This discussion paper, generated by authors who comprised a workgroup of the ICOH Scientific Committee on Rural Health, reviews the use of human experimental studies in regulatory risk assessment for pesticides with a view to advancing the debate as to when, if ever, such studies might be ethically justifiable. The discussion is based on three elements: (a) a review of discussion papers on the topic of human testing of pesticides and the positions adopted by regulatory agencies in developed countries; (b) an analysis of published and unpublished studies involving human testing with pesticides, both in the peer-reviewed literature and in the JMPR database; and (c) application of an ethical analysis to the problem. The paper identifies areas of agreement which include general principles that may provide a starting point on which to base criteria for judgements as to the ethical acceptability of such studies. However, the paper also highlights ongoing unresolved differences of opinion inherent in ethical analysis of contentious issues, which we propose should form a starting point for further debate and the development of guidelines to achieve better resolution of this matter.

Percutaneous absorption of an insect repellent p-menthane-3,8-DIOL: a model for human dermal absorption

p-Menthane-3,8-diol(38DIOL) was recently introduced as a natural topical insect repellent in the commercial product "OFF! Botanicals" lotion. The objective of this study was to provide an estimate of the potential for 38DIOL systemic absorption in humans. Carbon-14-labeled 38DIOL formulated in the lotion and in an ethanol solution was applied to excised pig skin in an in vitro flow-through test system predictive of skin absorption in humans. Twenty-four hours after application, radiolabel recovered from the dermis and receptor fluid was summed to determine percent absorption. At a dose of approximately 80 microg/cm(2) of 38DIOL in the lotion, a value of 3.5 +/- 0.8% of applied dose was obtained with pig skin. The corresponding value for 38DIOL in ethanol (90 microg/cm(2)) was not significantly different (3.0 +/- 1.2%). Most of the applied dose of 38DIOL was found to evaporate from pig skin (77 +/- 8% for the lotion and 87 +/- 1% for ethanol solution), thus limiting percutaneous absorption values. For reference purposes, the pig skin absorptions of piperonyl butoxide (PBO) at 100 microg/cm(2) in isopropanol, N,N-diethyl-m-toluamide (DEET) at 500 microg/cm(2) in ethanol, and neat isododecane at 650 microg/cm(2) (in order of increasing volatility) were 15 +/- 6%, 23 +/- 3%, and 0.09 +/- 0.05% of applied dose respectively. Isododecane was lost almost exclusively from the skin surface by evaporation. For additional reference, absorptions of PBO, DEET, and 38DIOL were found to be higher with excised rat skin.

Comparative evaluation of absorbed dose estimates derived from passive dosimetry measurements to those derived from biological monitoring: validation of exposure monitoring methodologies

Passive dosimetry (PD) methods for measuring and estimating exposure to agricultural workers (i.e., persons handling agricultural chemicals and working in treated crops) have been in use since the 1950s. A large number of studies were conducted in the 1950s through 1970s to characterize exposure. Since the 1980s quantitative dermal PD methods are used in conjunction with inhalation PD methods to measure whole-body exposure. These exposure or absorbed dose estimates are then compared to "no effect" exposure levels for hazards identified in toxicology studies, and have become the standard for risk assessment for regulatory agencies. The PD methods used have never been validated. Validation in the context of human exposure monitoring methods means that a method has been shown to measure accurately a delivered dose in humans. The most practical alternative to isolating parts of the body for validating recovery methods is to utilize field exposure studies in which concurrent or consecutive measurements of exposure and absorbed dose have been made with PD and biomonitoring in the same cohorts of individuals. This ensures that a direct comparison can be made between the two estimates of absorbed dose, one derived from PD and the other from biomonitoring. There are several studies available (published and proprietary) employing both of these approaches. Reports involving 14 concurrent or consecutive PD-biomonitoring studies were quantitatively evaluated with 18 different methods of application or reentry scenarios for eight different active ingredients for which measured human kinetics and dermal absorption data existed. This evaluation demonstrated that the total absorbed dose estimated using PD for important handler and reentry scenarios is generally similar to the measurements for those same scenarios made using human urinary biomonitoring methods. The statistical analysis of individual worker PD:biomonitoring ratios showed them to be significantly correlated in these studies. The PD techniques currently employed yield a reproducible, standard methodology that is valid and reliably quantifies exposure.

Comparison of the human skin grafted onto nude mouse model with in vivo and in vitro models in the prediction of percutaneous penetration of three lipophilic pesticides

The evaluation of the degree of percutaneous penetration of agrochemicals is a key part of risk assessment for operators. The availability of suitable and predictive experimental models is crucial, in particular in the case of lipophilic compounds which persist in the stratum corneum (SC). Regulatory models (rat in vivo, human and rat in vitro) and the innovative human skin grafted onto nude mice (HuSki) model were compared for their ability to predict the human skin absorption. Radiolabelled malathion, lindane and cypermethrin (4microg/cm(2)) were topically applied to each model. The % of applied dose absorbed and that present in skin and SC were evaluated at 24h. Additionally, the absorption profile of cypermethrin was evaluated in the in vivo rat and HuSki models for up to 11 days. We found that the human in vitro and HuSki models closely predicted the human skin absorption at 24h, while rat models overestimated the human skin absorption. Furthermore, our experiments with cypermethrin indicated that evaluation of % percutaneous absorption over extended periods of time was feasible with the HuSki model. In our studies the HuSki model overcame the limitations of the regulatory models and is promising to realistically refine the dermal absorption assessment of topically applied chemicals.

Human volunteer studies with non-pharmaceutical chemicals: metabolism and pharmacokinetic studies

1. Human volunteer studies are an essential part of drug development but their use in the area of non-pharmaceutical chemicals has so far been very limited. Such studies can have considerable value in the assessment and improvement of the safe use of chemicals. 2. Once metabolic pathways and target metabolites have been identified in volunteers this information can be used in studies in the workplace or in the general population. Studies should be performed selectively only if there is both a toxic hazard and a significant exposure potential. In addition, they should only be carried out if the required information cannot be obtained in any other way. 3. Volunteer studies with non-pharmaceuticals have become increasingly acceptable in the light of established international guidelines, no-fault compensation, improvements in study design and technical developments which allow the use of very low dose levels. The final decision on whether to carry out a study must always rest with an independent ethical committee. 4. The practical aspects of the study should be specified in a detailed protocol conforming with the principles of good clinical practice. The safety of volunteers must be of paramount concern throughout. Depending on the nature of the chemical and the study, it may be advisable to carry out studies in a clinical facility where equipment is available for the treatment of any emergencies that might occur. 5. Numerous investigators have now shown that human volunteer studies are ethically acceptable, practicable and yield important information. The risk to volunteers is minimal and this approach can lead to an improved foundation for occupational hygiene standards, more accurate risk assessment and thus better protection of the workforce and the general population.

Comparison of two methods for determining the toxicokinetics of fluazifop-butyl after intravenous dosing in rats

1. We have described and compared the use of two blood sampling techniques to measure the kinetics of fluazifop-butyl, a selective herbicide. Following intravenous administration of radiolabelled compound, blood samples were collected from female rats either by tail vein puncture or from chronically implanted catheters inserted in tethered rats. Urine samples were also collected from tethered animals. 2. Both techniques indicate that fluazifop-butyl is rapidly eliminated from blood into urine (t(1/2)3-4.5 h) and the overall blood concentration profiles were comparable between the two methods. However, by using cannulated rats, kinetic data were obtained from individual animals, providing evidence of inter-animal variation and allowing compartmental and statistical analysis. 3. The tethered rat technique is relatively simple and reliable. Compared to tail vein bleeding, results obtained from cannulated animals are more informative, providing comprehensive data from a small number of rats. It is therefore the preferred method for our kinetic based research studies using compounds known to exhibit multicompartmental elimination kinetics.

A physiologically based mathematical model of dermal absorption in man

A sound understanding of the mechanisms determining percutaneous absorption is necessary for toxicological risk assessment of chemicals contacting the skin. As part of a programme investigating these mechanisms we have developed a physiologically based mathematical model. The structure of the model parallels the multi-layer structure of the skin, with separate surface, stratum corneum and viable tissue layers. It simulates the effects of partitioning and diffusive transport between the sub-layers, and metabolism in the viable epidermis. In addition the model describes removal processes on the surface of the skin, including the effects of washing and desquamation, and rubbing off onto clothing. This model is applied to data on the penetration of the herbicide fluazifop-butyl through human skin in vivo and in vitro. Part of this dataset is used to estimate unknown model parameter values and the remainder is used to provide a partial validation of the model. Only a small fraction of the applied dose was absorbed through the skin; most of it was removed by washing or onto clothing. The model provides a quantitative description of these loss processes on the skin surface.

Modelling dermal pharmacokinetics using in vitro data. Part II. Fluazifop-butyl in man

In a previous paper it was demonstrated that dermal absorption of the herbicide fluazifop-butyl in the rat could be modelled by combining a knowledge of the pharmacokinetics following intravenous and oral dosing with in vitro measurements of dermal absorption. This paper demonstrates the validation of a similar model for the dermal absorption of fluazifop-butyl in man. Pharmacokinetic parameters derived from an oral dosing study are combined in a mathematical model with in vitro measurements of dermal absorption of fluazifop-butyl. Model predictions of the rate and extent of dermal absorption of fluazifop-butyl are compared with the results of dermal absorption studies in human volunteers. Good agreement is found between the model predictions and the experimental measurements. These results have implications for improved risk assessment. The model provides a tool for risk assessment based on both internal dose (e.g. peak plasma concentration, plasma area under the curve) as well as total absorbed dose. However, further work is required to evaluate whether the same techniques are applicable to a wider range of compounds.

Biological monitoring for pesticide exposure--the role of human volunteer studies

Predictions of human pesticide metabolism which are needed for the interpretation of biological monitoring data are frequently made from animal studies. Consequently, assumptions have to be made about the relationship between absorbed dose and metabolite excretion. The results from two human volunteer studies highlight the problems associated with extrapolating from animal studies in this way. The pyrethroid insecticide cypermethrin shows markedly different metabolite patterns when administered orally or dermally in man. Following dermal dosing the ratio of trans/cis cyclopropane acids is approximately 1:1, compared to 2:1 after oral administration. The ratio of total cyclopropane acids to phenoxybenzoic acids also differs depending on the route (dermal 1:4, oral 1:0.8). A knowledge of human metabolism by these two routes enables a much more meaningful interpretation of biological monitoring measurements. The herbicide molinate forms a mercapturate conjugate as a major urinary metabolite in the rat (35%). In volunteers at low dose levels this metabolite is present at insignificant levels (< 1%) and 4-hydroxymolinate is a much more abundant metabolite (39%). This shows that extrapolation between species can be very misleading. It is concluded that the benefits of using human volunteers for metabolism studies at low doses far outweigh the minimal risks involved. As a basis for biological monitoring such studies can lead to a greatly improved risk assessment for pesticides in use.