Dose-Response Assessment

"All substances are poisons: there is none which is not a poison. The right dose differentiates a poison and a remedy." Paracelsus (1493-1541)

Dose-response assessment is the process of characterizing the relation between the dose of an agent administered or received, and the incidence of an adverse health effect in exposed populations, and estimating the incidence of the effect as a function of human exposure to the agent. 'Dose' is commonly used to indicate the amount of the agent while 'response' refers to the effect of the agent once administered. Dose-response relationships are determined graphically by determining the effect of varying the administered dose on the response. Generally, increasing the dose of a harmful agent will result in a proportional increase in both the incidence of an adverse effect as well as the severity of the effect.

A dose-response curve defines the relationship between dose and response based on the following assumptions: 1) response increases as dose increases 2) there is a threshold dose- a dose below which there is no effect. This simple model is useful to develop basic dose-response relationships however, more complex dose-response relationships would be predicted for many endocrine toxicants depending on the target organ and the species exposed.

The issue of dose-response relationships is of central importance to the debate regarding endocrine toxicants. Endocrine toxicants often act by mimicking or antagonizing (blocking) the actions of endogenous hormones that are already present at physiologically functional concentrations. Ambient concentrations of endocrine toxicants are very low, resulting in the terminology 'low-dose effects' for investigations of endocrine disruption. Dose-response relationships, when determined, have used much higher concentrations of potential endocrine toxicants than would normally be found in the environment. Identifying a causal association at ambient, low-dose concentrations between endocrine toxicants and adverse health effects in humans would be highly supportive of the endocrine disrupter hypothesis. Unfortunately, exposure data is not available in many epidemiological studies. Some of the most striking examples of adverse health effects in humans and wildlife have been as a result of accidental exposure to very high concentrations of chemicals. Attempts to replicate these effects in animals at environmental dosages have not always been successful.

Metabolism and Pharmacokinetics
Both endogenous and exogenous substances, including hormones and endocrine toxicants, are 'processed' by the body, thereby affecting the distribution and elimination of the substance. 'Pharmacokinetics' are the movements, and/or rates of movements of substances within biological systems as affected by uptake, distribution, binding, elimination, and biotransformation. Endogenous hormones, once synthesized, are transported via the blood to various tissues, bind to receptors, may be transformed by biochemical processes such as phosphorylation and are eventually eliminated by a process termed 'metabolism'. These processes, distribution, binding, transformation, elimination, are important not only in the determination of the mechanism of action of these hormones, but also determine the effective amount or concentration of hormone that is 'bioavailable' (available for biological activity).

The pharmacokinetics of exogenous substances, including endocrine toxicants, are particularly important in the assessment of their biological potential. Exposure to a chemical toxicant, for example, that is immediately metabolized (eliminated) prior to interaction with endocrine receptors, is unlikely to act as an endocrine disrupter. Similarly, if insufficient amounts of the chemical toxicant are available at the site of the target receptor, it may be possible for endogenous hormones to compete with the exogenous ligands, thereby preventing the action of the exogenous chemical.

Potency
Potency is a measure of drug activity that is expressed in terms of the amount required to produce an effect of given intensity. Potency varies inversely with the amount of drug that is required to produce this effect- the more potent the drug the less required to induce the effect. Different endocrine toxicants will have different potencies, represented by unique dose-response curves. Assessment of the potency of an endocrine toxicant is a necessary step for the characterization of the risk posed following exposure to the toxicant. Many factors determine the potency of a given contaminant including bioavailability, affinity for target receptor, metabolism of the contaminant, half-life of the contaminant and the relative potency of endogenous hormones.

Following exposure to an endocrine toxicant, only a fraction of the total amount of toxicant will be available to interact with receptors. This fraction is termed 'bioavailable'. As discussed in further detail below, endocrine toxicants and other chemicals may be bound to plasma proteins or sequestered within tissues. It is only the unbound or free fraction of the toxicant that is bioavailable.

Affinity is a term used to describe the ability of a chemical to bind to a receptor or molecule. The affinity of a contaminant is determined by its chemical structure and biochemical properties. A small amount of a compound may be able to successfully compete with larger amounts of a different compound if its affinity for the receptor is greater. Shown on the left, Chemical A (blue curve) is more potent than Chemical B (red curve). This is evident by examining the dose required to produce a half-maximal response.

Chemical A and B are both able to produce the same response, however Chemical A triggers this response at a lower dose (1) compared to Chemical B (2). The simple explanation for this outcome is that Chemical A has a greater affinity for the receptor controlling the response compared to Chemical B. However, there are many additional factors which also contribute to the potency of an agent.
Compounds are broken down or metabolized once inside the body. The metabolites formed may have different biological activity compared to the original compound. For example, some metabolites may be reactive species that cause non-specific tissue toxicity. Other metabolites may be able to mimic endogenous compounds and trigger or block normal intracellular signaling pathways. Assessment of the potency of an endocrine toxicant must therefore take into consideration the potencies of the metabolites formed and their half-life.

'Half-life' is used in pharmacology as a measure of the lifespan of a chemical or compound. Exogenous chemicals can be degraded and eliminated by the body or may be stored in tissues, depending on the biochemical properties of the chemical. The half-life of a chemical is therefore a measure of its bioavailability. The persistence of chemicals such as organochlorines which accumulate in the body contributes to their relative potency compared to chemicals which are rapidly degraded and eliminated before producing a biological response.

Studies must continue to develop dose-response relationships for chemical agents suspected to have endocrine disrupting properties. Wide dose ranges should be used to encompass both toxic as well as mechanistic end-points. Credible dose-response relationships will be obtained from several sources including toxicity data, mechanistic end-points, epidemiological studies and field studies.

Several issues must be considered in the evaluation of dose-response assessments.

1. Experimental Model: It is neither feasible, nor ethical to expose human subjects to serial doses of potential hazardous chemicals to measure adverse effects, and thus an experimental model is used. The validity of the experimental model (animal) is critical to extrapolate effects in animals to effects in humans following exposure to endocrine disrupters.
   
   
2. Physiology of Endocrine System: While the dose-response relationship may characterize an association between two variables (dose of chemical agent and response), the response or adverse effect is most likely the result of many processes that are interdependent and necessary for maintaining homeostasis of the tissue, organ, or function being studied
   
   
3. Homeostasis: Homeostasis is the maintenance of a biological system that is achieved by numerous feedback mechanisms. For an individual cell, intracellular pH, ion balance, water balance and many other processes are regulated within a narrow range. Larger systems such as tissues, organs and entire organisms also maintain homeostasis of hormone levels, blood cell counts, body temperature, metabolic rates and many other processes. It is necessary to understand how perturbations in the homeostasis of a system (ie. endocrine system) can result in disease or dysfunction. Quantification of these changes in homeostasis may be reflected in the dose-response relationship.
   
   
4. Discrimination of Effects of Endocrine Disrupters: It is necessary to be able to discriminate the effects of endogenous hormone levels on the endocrine system from the effects of exogenous toxicants. This requires an understanding of the mechanisms through which endocrine disruptors perturb homeostasis and endocrine function.
   
   
5. Individual Susceptibility: It is commonly known that many diseases are affected by both modifiable risk factors (lifestyle, diet, socio-economic factors) as well as non-modifiable factors (genetics, gender, race, age). These interindividual factors may affect the susceptibility of some populations to the effects of endocrine disrupters. These factors should be considered in the dose-response relationship.


Thus, the assessment of dose-response relationships is very complex. A single dose-response relationship cannot model all adverse effects for all endocrine mechanisms and all populations. As discussed, many mechanisms of action have been proposed and many chemical agents may act as endocrine toxicants. Dose-response relationship for individual endocrine toxicants can, however, be an important piece of evidence in the determination of the risk posed by exposure to the toxicant.

Other steps in risk assessment consist of: hazard identification, exposure assessment and risk characterization.