A personal view of skin permeation (1960-2013).

I want to thank Mike Roberts for arranging this issue of the journal and Annette Bunge and Richard Guy for reviewing this paper. This is, first, a history of my introduction to the field of skin permeability, and then an attempt to recall (with the inaccuracies that implies) the highlights of my research (1960-1975) on skin permeation, and finally a reflection by an old-timer on more recent concepts.

Safety profile of a food dextrin: acute oral, 90-day rat feeding and mutagenicity studies.

Nutriose is a glucose polysaccharide produced by the chromatographic separation of a dextrin fraction derived from maize, wheat or other edible starches. Animal safety studies conducted on Nutriose FB are reported. They include an acute oral and a 90-day study in rats and short-term in bacteria (Ames test) and a mutation assay at the TK locus in L5178Y mouse lymphoma cells. An acute oral study in Sprague-Dawley rats established the LD(50) as greater than 2000 mg/kg. In a 90-day, oral subchronic study, Sprague-Dawley rats were administered Nutriose FB in their diet at doses of 0, 1.25%, 2.5% or 5% for 13 weeks. Neither mortality nor significant behavioral changes occurred during the study. The consumption of Nutriose FB did not have any effect on body weight or on feed or water consumption. Blood coagulation and hematology and blood and urine biochemistry did not reveal any toxic effect of the compound. No treatment-related histopathological differences were observed between control and test groups. Adverse clinical observations, including ophthalmological observations, were marginal and not considered treatment-related. There was no effect of Nutriose FB on relative or absolute organ weight of rats of either sex, except for the increase in caecum content and caecum mucosa. The increase in caecum weight is considered a physiological adaptation seen after the ingestion of indigestible carbohydrates and is not considered a toxicological effect. The No-Observed-Adverse-Effect-Levels (NOAELs) were established by the highest tested doses: 4.4 g/kg bw/day in males and 6.5 g/kg bw/day in females. Mutation assays in bacteria (Ames tests) and in mammalian cells (tk locus in mouse lymphoma cells) were negative with Nutriose FB.

Risk analysis of dietary lead exposure.

Distribution of intake and lead levels in dietary and non-dietary sources and of lead absorption were used in a Monte-Carlo simulation to predict blood lead levels in populations of concern. Blood lead levels were determined with and without particular dietary sources, and added risk was estimated for each source. These calculations permit comparisons of relative risk used to evaluate and limit dietary exposure to lead. Added risk of exposure to lead in wine, calcium supplements and ceramic-ware, and drinking water were calculated for adult men, pregnant women, and children, respectively.

Information needed to support hazard identification and risk assessment of toxic substances.

Today pharmacokinetic data are not routinely required for food safety evaluation. The new Red Book, the Center's outline of animal testing protocols, does suggest some pharmacokinetic studies, but their use is still limited. Primarily this is because of our current reliance on animal studies only and the use of safety factors (SFs) to bridge the human to animal response. But the situation is changing and the role of pharmacokinetics and physiologically based pharmacokinetic modeling will probably increase for several reasons. (1) The increasing role of quantitative risk assessment and regulations acknowledging and permitting some level of risk. This places a demand of greater quantitation of risk and emphasizes the need for better measurement of effective dose. We made need to consider more carefully the possible nonlinear pharmacokinetic effects in high-to-low dose extrapolation. (2) The increasing need to understand more about a chemical's mechanism of action prior to a major corporate commitment. The increasing cost of mistakes in judgement regarding a chemical's prospects in the commercial and regulatory arena are demanding a better and deeper understanding of possible toxic effects. (3) The increasing desire to expand the use of a successful additive beyond the spectrum of the uses covered in the original approval. This usually means the request for a higher ADI. This must be based on more refined toxicity testing and better estimate of effective dose in order to permit the reduction of the SF. (4) The advent of novel foods for which conventional toxicological methods are inappropriate. For example, noncaloric fat substitutes that may comprise a large portion of the daily diet cannot adequately be tested in conventional animal studies. The doses cannot be exaggerated enough to permit a large enough SF. These substances may well require human clinical investigations similar to those used for drugs.

Use of 'secondary mechanism' in the regulation of carcinogens; a chronology.

Significantly more willingness to depart from worst case assumptions in the evaluation of carcinogenic risk has developed in the last 30 years. Scientists within regulatory agencies and in the academic community are reaching a consensus on the inadequacy of current test methods and on the need for more mechanistic data to help interpret the carcinogen bioassay. The slow regulatory acceptance of the concept of 'secondary mechanism' in the evaluation of cancer risk from chemicals in the U.S. is illustrated by a chronology of laws, policies, and consensus scientific positions adopted through the years.

Dioxin--an analysis of the major human studies: comparison with animal-based cancer risks.

Several major epidemiological studies have reported significant mortality rates (SMRs) for both rare cancers (soft tissue sarcoma, non-Hodgkin's, lymphoma, liver) and the more common cancers (lung, colon, etc), all allegedly caused by TCDD. In this paper, we use the potency of TCDD in animals to establish a plausible worst case cancer risk and ask whether its likely that TCDD is responsible for the epidemiological findings assuming the animal carcinogenic potency is applicable to the conditions of human exposure. Two new features of the technique are the use of measured TCDD blood levels in both animals and humans for dose scale-up and the calculation of an integrated life-time exposure for the exposed workers using measured blood levels. On the basis of the stated assumptions it appears unlikely that any of the major epidemiological studies, with the possible exception of the NIOSH study have adequate power to detect the common cancers potentially caused by TCDD.

Point estimates of cancer risk at low doses.

There has been considerable discussion regarding the conservativeness of low-dose cancer risk estimates based upon linear extrapolation from upper confidence limits. Various groups have expressed a need for best (point) estimates of cancer risk in order to improve risk/benefit decisions. Point estimates of carcinogenic potency obtained from maximum likelihood estimates of low-dose slope may be highly unstable, being sensitive both to the choice of the dose-response model and possibly to minimal perturbations of the data. For carcinogens that augment background carcinogenic processes and/or for mutagenic carcinogens, at low doses the tumor incidence versus target tissue dose is expected to be linear. Pharmacokinetic data may be needed to identify and adjust for exposure-dose nonlinearities. Based on the assumption that the dose response is linear over low doses, a stable point estimate for low-dose cancer risk is proposed. Since various models give similar estimates of risk down to levels of 1%, a stable estimate of the low-dose cancer slope is provided by s = 0.01/ED01, where ED01 is the dose corresponding to an excess cancer risk of 1%. Thus, low-dose estimates of cancer risk are obtained by, risk = s x dose. The proposed procedure is similar to one which has been utilized in the past by the Center for Food Safety and Applied Nutrition, Food and Drug Administration. The upper confidence limit, s., corresponding to this point estimate of low-dose slope is similar to the upper limit, q1., obtained from the generalized multistage model. The advantage of the proposed procedure is that s provides stable estimates of low-dose carcinogenic potency, which are not unduly influenced by small perturbations of the tumor incidence rates, unlike q1.

Use of cell proliferation data in cancer risk assessment: FDA view.

The possible uses of cell proliferation data in cancer risk assessment can be divided into three categories: direct use of mathematical models that incorporate rates of cell proliferation, use of experimental data on secondary mechanisms produced by cell proliferation, and using studies of cellular growth rates to extend the dose range of bioassay data. These three approaches are briefly discussed and some indication of their potential application to cancer risk assessment is outlined.

Perspectives on toxicological risk--an example: foodborne carcinogenic risk.

Epidemiologists estimate that approximately one third of all cancer deaths can be attributed to diet. It is instructive to attempt to apportion this dietary carcinogenic risk to the specific classes of foodstuffs and food additives, pesticides, etc., that are typically regulated. When this is done it is evident that virtually all the calculated risk can be attributed to naturally occurring carcinogens in the diet. This article indicates that both epidemiological data and the simplest kind of risk assessment agree that foodborne carcinogenic risk probably overwhelmingly originates from the food itself and not from additives, pesticides, or contaminants.

The effect of cosmetic vehicles on the penetration of N-nitrosodiethanolamine through excised human skin.

Cosmetic products are frequently applied to the skin by a large number of people, but some contain compounds that are potentially toxic, if absorption through the skin is sufficient. The percutaneous absorption of N-nitrosodiethanolamine (NDELA), an impurity in many cosmetic products, has been evaluated in diffusion cells using excised human skin. The nitrosamine was applied to the skin in vehicles with different solubility properties. The permeability constants for water (5.5 X 10(-6) cm hr-1) and propylene glycol (3.2 X 10(-6) cm hr-1) were small and similar. In isopropyl myristate, the permeability constant increased approximately 250-fold to 1.1 X 10(-3) cm hr-1. The NDELA membrane:vehicle partition coefficients were determined using trypsin-treated stratum corneum as the membrane. These coefficients were 1.8 and 1.0, respectively, for water and propylene glycol and 230 for isopropyl myristate. The permeability of NDELA through skin is apparently increased greatly when applied from sufficiently lipoidal formulations; this is primarily due to the favorable partition coefficients for NDELA from such formulations. The amount of NDELA penetrating the skin from 3 types of cosmetic products was calculated based on their different conditions of use. Products applied over large areas of the body that remain on the skin for long periods of time (i.e., sun tanning lotion) will result in the greatest absorption of NDELA if all other factors are equal.