Interaction of drug metabolism with the other elements of drug safety evaluation

Thresholds in Acute and Long Term Animal Studies

The possibilities to define thresholds in general pharmacology and toxicology depend upon the proper knowledge and determination of the most sensitive parameters and relevant targets leading to clinical effects or toxicity. We are generally confronted with the statistical summation of very numerous individual responses and effects at the enzymological, sub-cellular, cellular, organ, or systemic level. In consequence, biological actions of chemicals reveal typical dose-response characteristics described by simple mathematics, and more or less sharp thresholds. Thresholds and targets can change after single and repeated (cumulative) doses by means of altered kinetics and metabolism, by adaptation and modified sensitivity of targets and/or receptors. Most sensitive targets and parameters do not always determine the real thresholds for clinical toxicity. Threshold and specificity of carcinogens can also be modified by similar conditions mentioned. Nutritional factors play an important role in experimental car-dnogenesis, and also for the cancer incidence in man. Frequently, secondary mechanisms such as overloading, induction of cellular growth, mechanical and chemical irritation and damage, and other conditions have simulated real carcinogenic properties of chemical molecules. The important question on the existence and validity of thresholds in carc/nogenes/s must be connected with a solid evaluation of the mode of action of the individual compounds. Otherwise, mathematical calculations of safety limits, definitions of acceptable risks and no-effect levels lack any biological foundation. Finally, the individual perception and social acceptance of risks reveal certain thresholds.

Extrapolation from animals to man: predictions, pitfalls and perspectives

Comparative drug disposition studies can be useful in extrapolating from animals to man provided that the criteria indicating interspecies similarity in disposition reflect similar exposure to the foreign compound. Interspecies variability, on the other hand, can often be related to physiological or biochemical differences, thereby providing a rationale for the unsuitability or limitations of a species as a model for human metabolism. Retrospective evaluation of the following examples illustrates the relevance of the indicated disposition characteristics to risk and efficacy assessment: (a) oxaprozin (route of excretion, enterohepatic circulation and exposure; plasma concentrations and efficacy prediction); (b) ciramadol (species differences in presystemic elimination and major metabolic pathway); (c) acebutolol (pharmacologically active human metabolite absent in one of the toxicology species); (d) esmolol (duration of pharmacologic effect controlled by species dependent nature of blood esterases). Stereochemical preferences in the disposition of racemic drugs often differ among species. Extrapolations from one species to another cannot be made in this situation. Pharmacokinetic parameters based on measurements of the sum of the isomers are meaningless and potentially misleading. Future improvements can come from: computer assisted predictions of metabolic pathways; increased use of human tissues; and use of animal species physiologically similar to humans, e.g. the miniature swine.

Comparative metabolism of 17alpha-ethynyl steroids used in oral contraceptives

The metabolism of mestranol, ethynylestradiol, norethynodrel, norethindrone, ethynodiol diacetate, lynestrenol, and norgestrel is reviewed. The estrogenic components of the oral contraceptives, mestranol or ethynylestradiol, have nearly identical metabolic pathways since mestranol is rapidly and almost completely converted to ethynylestradiol The major fraction of the drugs plus metabolites is excreted in the urine as conjugated materials. All of the 17beta-ethynyl progestins reviewed follow similar metabolic paths. For three of these, norethynodrel, ethynodiol diacetate and lynestrenol, a principal metabolite is norethindrone. Biotransformation to more polar metabolites and conjugation proceed rapidly for these three precursor drugs and norethindrone. Norgestrel follows metabolic paths similar to those of norethindrone. However, the ethyl moiety at the C-13 position appears to slow the metabolism of this steroid so that biotransformation to more polar metabolites and the conjugation of these steroids does not proceed as rapidly as that of the other progestins. The high progestational potency of norgestrel may be attributed to this slow rate of biotransformation. Some of the pharmacokinetic parameters derived from the research reports reviewed here are summarized. The compounds appear to be readily absorbed, and they and their metabolites are excreted to a greater extent in the urine than in the feces.