Developmental pharmacology

The Effect of Experimental Protocol on the Toxicity of Saxitoxin in Mice

Regulatory limits for toxins in shellfish are required to ensure the health of consumers. However, these limits also impact the profitability of shellfish industries making it critical that they are fit for purpose. Since human toxicity data is rarely available, the setting of regulatory limits is dependent on animal data which can then be extrapolated for use in the assessment of human risk. The dependence on animal data to keep humans safe means that it is critical that the toxicity data used is robust and of high quality. Worldwide, the protocols used in toxicity testing are varied, making it hard to compare results and adding confusion over which results better reflect the true toxicity. In this study, we look at the effect of mouse gender, i.p. dose volume, mouse body weight and feeding protocols (both acute and sub-acute) on the toxicity of saxitoxin. This allowed the effect of different variables used in toxicity testing to be understood and showed that the feeding protocol used in both acute and sub-acute studies greatly influenced the toxicity of saxitoxin in mice. Therefore, the adoption of a standard protocol for the testing of shellfish toxins is recommended.

A Tribute to the Pioneers of Fetal Pharmacology

Clinical pharmacology is a branch of the field of pharmacology that evolved following the recognition that the nature, duration, and intensity of drug action depend on both the intrinsic properties of the drug and an interaction with the host to whom the drug is given. Advances in drug development have placed highly specific and extremely potent therapeutic agents in the marketplace. While these advances have progressed rapidly in adult medicine, pediatric clinical pharmacology has not kept pace and until very recently has lagged behind the research and attention paid to the proper use of therapeutic and diagnostic drugs in adults. Recognition that advances in the science of developmental pharmacology and pediatric clinical pharmacology were essential in the development of new drugs to treat children came in the 1950s and 1960s mostly through the work of 2 pioneering scientists in fetal and perinatal clinical pharmacology, Drs Sumner Yaffe and Bernard Mirkin. Here we pay a tribute to these most influential pioneers in the United States who were instrumental in paving the path for advancing the field of fetal and perinatal pharmacology concepts and their incorporation into pediatric drug development programs.

The pharmacokinetics and pharmacodynamics of rifampicin in adults and children in relation to the dosage recommended for children

The dosages of antituberculosis agents recommended for treatment of childhood tuberculosis often reflect those for adult patients with similar mg/kg body weight dosages and ranges advised. Literature relating to the pharmacokinetics and pharmacodynamics of rifampicin (RMP) is reviewed and the serum concentrations reached by adults, both patients and healthy volunteers and children, established or not established on RMP, compared. Straight line regression of maximum RMP serum concentrations (C(max)) on dosage, weighted for the number of individuals, found slopes (SE) of 1.025 (0.067) and 0.881 (0.046) respectively for adult volunteers not established and established on RMP (P = 0.076), and similarly 0.748 (0.057) and 0.684 (0.038) respectively for adult patients (P < 0.001) and 0.622 (0.050) and 0.368 (0.041) respectively for children (P < 0.001). These results indicate that for equivalent RMP dosages adult patients reach a lower C(max) than adult volunteers and that adults, both volunteers and patients established on RMP reach higher C(max) values than children; children established on RMP require approximately twice the mg/kg body weight dosage of RMP to reach serum concentrations equivalent to those of adults. It is noteworthy that many adult patients receiving currently recommended RMP dosages also do not reach the often recommended RMP 2 h serum concentration of 8 μg/mL.

Approaches for assessing risks to sensitive populations: lessons learned from evaluating risks in the pediatric population

Assessing the risk profiles of potentially sensitive populations requires a "tool chest" of methodological approaches to adequately characterize and evaluate these populations. At present, there is an extensive body of literature on methodologies that apply to the evaluation of the pediatric population. The Health and Environmental Sciences Institute Subcommittee on Risk Assessment of Sensitive Populations evaluated key references in the area of pediatric risk to identify a spectrum of methodological approaches. These approaches are considered in this article for their potential to be extrapolated for the identification and assessment of other sensitive populations. Recommendations as to future research needs and/or alternate methodological considerations are also made.

Developmental pharmacology and toxicology: biotransformation of drugs and other xenobiotics during postnatal development

The following conclusions can be drawn: Depending on reaction type, species (strain), organ (tissue) and sex (in rats and mice) both phase I and phase II ractions show developmental patterns with maximum activities in juvenile or young adult animals. In man drug disposition in newborns is also generally much lower than in children and young adults. Mammals with long gestation periods are born with considerable activities and adult values are reached earlier than in animals with short gestation periods. Highest activities (with very few exceptions) are observed in liver postnatally. With increasing age, in all laboratory animals and in man, a decline of biotransformation capacity is observed. Most of these reactions are inducible by inducers of the phenobarbital-, 3-methylcholanthrene- and tetrachlorodibenzo-p-dioxine-, steroid- or alcohol-type. Inducibility also depends on reaction type, species (strain), organ (tissue) and sex (in rats and mice). Inducibility begins in the earliest embryonic stage and reaches high rates in species with long gestation periods before birth, and at or after birth in species with short gestation periods. The beginning of high inducibility depends also on inducer types and the induced parameter. Inducible reactions can be increased in immature as well as in very old animals up to or above the level of adult animals. Isozymes may show different developmental patterns and inducibilities. Development and induction of isozymes catalyzing different reactions can be triggered in clusters. There is increasing evidence that basic biotransformation activities as well as their inducibilities by foreign compounds are essentially influenced by a kind of temporal genetic control during the whole life span. When we compared different monooxygenase reactions and their inducibility (Klinger et al., 1968) both the presence of P450 isozymes and their different inducibility as well as their different developmental pattern became evident, the direct proof of which was given 10 years later by the Nebert group (Atlas et al. 1977). Functional heterogeneity was demonstrated by differential development and inducibility by glucocorticoids also for UDP-glucuronosyl transferase (Wishart, 1978): in the rat the increased fetal glucocorticoid activity between days 17 and 20 of gestation triggers the surge to adult or higher than adult activities. This cluster of the so-called late fetal glucuronyltransferase group is distinctly different from the neonatal or postnatal cluster with peak values after birth and a pronounced inducibility by 3-methylcholanthrene, but not by steroids. These different transferase clusters can be differentiated by different substrates, too. To get an overview on the postnatal development of the most important phase I and phase II reactions and also for the heme biosynthetic pathway as a prerequisite for the P450 synthesis, a compilation of literature data similar to a score was constructed with the aim to recognize parallel developments, possible common control and regulation mechanisms, cp. Klinger et al. 1987, cp. also Figs. 1-3. Evidently different developmental patterns can be observed, direct connections or dependencies cannot be detected. Thus different concentrations resp. activities of different P450 forms during ontogenetic development are influenced by many factors on the transscriptional and posttransscriptional level. Reviews on concepts and therories of development and aging have been published by several authors respectively editors (cp. Klinger, 1996). But there is no convincing concept or proof on the molecular-biological basis of an internal clock which regulates and controls individual development, aging and death.

Utilization of juvenile animal studies to determine the human effects and risks of environmental toxicants during postnatal developmental stages

BACKGROUND

Toxicology studies utilizing animals and in vitro cellular or tissue preparations have been used to study the toxic effects and mechanism of action of drugs and chemicals and to determine the effective and safe dose of drugs in humans and the risk of toxicity from chemical exposures. Testing in animals could be improved if animal dosing using the mg/kg basis was abandoned and drugs and chemicals were administered to compare the effects of pharmacokinetically and toxicokinetically equivalent serum levels in the animal model and human. Because alert physicians or epidemiology studies, not animal studies, have discovered most human teratogens and toxicities in children, animal studies play a minor role in discovering teratogens and agents that are deleterious to infants and children. In vitro studies play even a less important role, although they are helpful in describing the cellular or tissue effects of the drugs or chemicals and their mechanism of action. One cannot determine the magnitude of human risks from in vitro studies when they are the only source of toxicology data.

METHODS

Toxicology studies on adult animals is carried out by pharmaceutical companies, chemical companies, the Food and Drug Administration (FDA), many laboratories at the National Institutes of Health, and scientific investigators in laboratories throughout the world. Although there is a vast amount of animal toxicology studies carried out on pregnant animals and adult animals, there is a paucity of animal studies utilizing newborn, infant, and juvenile animals. This deficiency is compounded by the fact that there are very few toxicology studies carried out in children. That is one reason why pregnant women and children are referred to as "therapeutic orphans."

RESULTS

When animal studies are carried out with newborn and developing animals, the results demonstrate that generalizations are less applicable and less predictable than the toxicology studies in pregnant animals. Although many studies show that infants and developing animals may have difficulty in metabolizing drugs and are more vulnerable to the toxic effects of environmental chemicals, there are exceptions that indicate that infants and developing animals may be less vulnerable and more resilient to some drugs and chemicals. In other words, the generalization indicating that developing animals are always more sensitive to environmental toxicants is not valid. For animal toxicology studies to be useful, animal studies have to utilize modern concepts of pharmacokinetics and toxicokinetics, as well as "mechanism of action" (MOA) studies to determine whether animal data can be utilized for determining human risk. One example is the inability to determine carcinogenic risks in humans for some drugs and chemicals that produce tumors in rodents, When the oncogenesis is the result of peroxisome proliferation, a reaction that is of diminished importance in humans.

CONCLUSIONS

Scientists can utilize animal studies to study the toxicokinetic and toxicodynamic aspects of drugs and environmental toxicants. But they have to be carried out with the most modern techniques and interpreted with the highest level of scholarship and objectivity. Threshold exposures, no-adverse-effect level (NOAEL) exposures, and toxic effects can be determined in animals, but have to be interpreted with caution when applying them to the human. Adult problems in growth, endocrine dysfunction, neurobehavioral abnormalities, and oncogenesis may be related to exposures to drugs, chemicals, and physical agents during development and may be fruitful areas for investigation. Maximum permissible exposures have to be based on data, not on generalizations that are applied to all drugs and chemicals. Epidemiology studies are still the best methodology for determining the human risk and the effects of environmental toxicants. Carrying out these focused studies in developing humans will be difficult. Animal studies may be our only alternative for answering many questions with regard to specific postnatal developmental vulnerabilities.

Development of acute exposure guideline levels for airborne exposures to hazardous substances

Hazardous substances can be released into the atmosphere due to industrial and transportation accidents, fires, tornadoes, earthquakes, and terrorists, thereby exposing workers and the nearby public to potential adverse health effects. Various enforceable guidelines have been set by regulatory agencies for worker and ambient air quality. However, these exposure levels generally are not applicable to rare lifetime acute exposures, which possibly could occur at high concentrations. Acute exposure guideline levels (AEGLs) provide estimates of concentrations for airborne exposures for an array of short durations that possibly could cause mild (AEGL-1), severe, irreversible, potentially disabling adverse health effects (AEGL-2), or life threatening effects (AEGL-3). These levels can be useful for emergency responders and planners in reducing or eliminating potential risks to the public. Procedures and methodologies for deriving AEGLs are reviewed in this paper that have been developed in the United States, with direct input from international representatives of OECD member-countries, by the National Advisory Committee for Acute Exposure Guidelines for Hazardous Substances and reviewed by the National Research Council. Techniques are discussed for the extrapolation of effects across different exposure durations. AEGLs provide a viable approach for assisting in the prevention, planning, and response to acute airborne exposures to toxic agents.

Incorporating children's toxicokinetics into a risk framework

Children's responses to environmental toxicants will be affected by the way in which their systems absorb, distribute, metabolize, and excrete chemicals. These toxicokinetic factors vary during development, from in utero where maternal and placental processes play a large role, to the neonate in which emerging metabolism and clearance pathways are key determinants. Toxicokinetic differences between neonates and adults lead to the potential for internal dosimetry differences and increased or decreased risk, depending on the mechanisms for toxicity and clearance of a given chemical. This article raises a number of questions that need to be addressed when conducting a toxicokinetic analysis of in utero or childhood exposures. These questions are organized into a proposed framework for conducting the assessment that involves problem formulation (identification of early life stage toxicokinetic factors and chemical-specific factors that may raise questions/concerns for children); data analysis (development of analytic approach, construction of child/adult or child/animal dosimetry comparisons); and risk characterization (evaluation of how children's toxicokinetic analysis can be used to decrease uncertainties in the risk assessment). The proposed approach provides a range of analytical options, from qualitative to quantitative, for assessing children's dosimetry. Further, it provides background information on a variety of toxicokinetic factors that can vary as a function of developmental stage. For example, the ontology of metabolizing systems is described via reference to pediatric studies involving therapeutic drugs and evidence from in vitro enzyme studies. This type of resource information is intended to help the assessor begin to address the issues raised in this paper.

Differences in sensitivity of children and adults to chemical toxicity: the NAS panel report

The National Academy of Sciences (NAS) Committee on Pesticides in the Diets of Infants and Children worked for some 4 years to evaluate the extent and the health-related consequences of exposure of infants and children to pesticides. The focus of this paper is on deliberations and recommendations of the committee relevant to protection of infants and children from toxic effects of pesticides. The most comprehensive data available for contrasting the toxicity of chemicals in the young and adults were compilations of rodent mortality studies. Age-dependent differences in chemical lethality were less than 1 order of magnitude and usually varied no more than 2- to 3-fold. Findings in studies of pesticides and other chemicals revealed that toxicity was age- and compound-dependent. The younger and more immature the subject, the more different its response from that of an adult. Substantial anatomical, biochemical, and physiological changes occur during infancy, childhood, and adolescence. These maturational changes can substantially affect the absorption, distribution, metabolism, and elimination of chemicals. The net effect of immaturity on pharmacokinetics and pharmacodynamics is difficult to predict. Measurements of physiological functions in different age groups can be made and input into physiologically based pharmacokinetic (PBPK) models. The committee felt that PBPK models could be effectively utilized for different exposure scenarios, to predict the time course of potentially toxic chemicals and metabolites in different organs of children. The committee recognized that maturing organ systems of infants and children may be susceptible to injury by chemicals. There may be developmental periods (i.e., windows of vulnerability) when the endocrine, reproductive, immune, visual, or nervous systems are particularly sensitive to certain chemicals. The committee recommended early assessments using sensitive indices of injury to these organ systems of test animals. Only limited information was available on the therapeutic efficacy and toxicity of drugs in pediatric populations. The most definitive data were maximally tolerated doses (MTDs) of chemotherapeutic agents. MTDs were frequently higher for children than adults, though the differences between age groups were usually < or =2. It was concluded by the NAS committee that immaturity does not necessarily entail greater sensitivity to chemical toxicity; age-dependent toxicity is chemical-dependent; and the existing 10-fold interspecies uncertainty factor provides adequate protection of infants and children, based on current knowledge.

Molecular basis of bilirubin UDP-glucuronosyltransferase induction in spontaneously diabetic rats, acetone-treated rats and starved rats

The co-ordinated induction of several hepatic drug-metabolizing enzymes is a common feature in the regulation of drug biotransformation under normal and pathological conditions. In the present study the activity and expression of bilirubin UDP-glucuronosyltransferase (UGT1A1) were investigated in livers of BioBreeding/Worcester diabetic, fasted and acetone-treated rats. Bilirubin glucuronidation was stimulated by all three treatments; this was correlated with an increase in the UGT1A1 protein concentration in hepatic microsomes. Transcriptional induction of UGT1A1 was also observed in diabetes and starvation but not with acetone treatment, which apparently caused translational stabilization of the enzyme protein. The hormonal/metabolic alterations in diabetes and starvation might be a model for postnatal development. The sudden interruption of maternal glucose supply signals the enhanced expression of UGT1A1, giving a novel explanation for the physiological induction of bilirubin glucuronidation in newborn infants.

Cytochrome-P450-dependent biotransformation of xenobiotics in human and rodent embryonic tissues

Profound species differences and developmental stage differences as well as a lack of solid data prevent broad, sweeping generalizations in terms of statements that can be made concerning the prenatal expression of individual P450 isoforms. It is clear, however, that several of such isoforms are expressed at levels that can be toxicologically significant. At present, the greatest interest appears to be in P450s 1A1, 1B1, 2E1, and 3A7, each of which has been reported to be expressed at toxicologically significant levels or at least at potentially toxicologically significant levels during organogenesis. Reports of the expression of other P450 isoforms at later stages of gestation also have appeared in the recent literature.

Drinking water contaminants (arsenic, cadmium, lead, benzene, and trichloroethylene). 1. Interaction of contaminants with nutritional status on general performance and immune function in broiler chickens

The objective of this study was to examine possible interactions between drinking water contaminants and suboptimal nutritional status for performance and immune function in male broiler chickens. Experimental drinking water contained a mixture of arsenic, benzene, cadmium, lead, and trichloroethylene (TCE) at low concentrations (0.80, 1.3, 5.0, 6.7, and 0.65 ppm) and high concentrations (8.6, 13, 50, 67, and 6.5 ppm). These chemicals were selected because they are among the most common contaminants found in ground water near hazardous waste sites. The experimental diets included feed containing 50% added vitamins and minerals (V&M) and feed without added V&M. Increasing levels of drinking water contaminants and decreasing levels of V&M in diet resulted in significantly (P < or = 0.05) decreased water and feed intake, decreased weight gain, and suppression of natural, humoral, and cell-mediated immune response. In a paired-water study, feed consumption, body weight, and immune function were decreased in chickens provided low and high concentrations of the chemical mixture in drinking water compared with chickens given control drinking water equal to the volumes consumed by the chickens given the low and high concentration of mixture, respectively. A deficiency of dietary V&M caused increased sensitivity to adverse effects of drinking water contaminants.

Methods for implanting steroid-containing cannulae into the paraventricular nucleus of neonatal rats

Implantation of cannulae into brains of neonatal rats presents methodological difficulties. We discuss such issues as avoiding tissue injury, and describe successful techniques. Cannulae size, methods of preparation, insertion, and securing are evaluated. We present a modified cannula holder applicable to the soft neonatal brain. Application of these methods to the study of glucocorticoid receptors in the neonatal rat hypothalamus is illustrated.

Experimental treatment protocols for scorpion envenomation: a review of common therapies and an effect of kallikrein-kinin inhibitors

Nine fatal cases from the sting of the scorpion Leiurus quinquestriatus are presented. All victims showed association of CNS and cardiovascular manifestations. Either the CNS or the cardiovascular effects could occur first in the early phases of the scorpion envenoming syndrome; the CNS manifestations, however, always preceded the terminal hypotension and cardiac arrest. Pharmacokinetic studies in rabbits following s.c. injection of the labelled venom showed that rapid absorption took place with about 70% of the maximum blood concentration reached within 15 min. Intramuscular injection of antivenom did not significantly affect the absorption of the venom or the other pharmacokinetic parameters. The total area under concentration time curve was not significantly different from that following i.v. injection, showing that nearly complete absorption of the venom from the s.c. site would occur in 7-8 hr. The i.v. infusion of venom into anaesthetized rats, at a rate comparable to the absorption rate from s.c. sites, allowed the determination of the minimum lethal dose (MLD) with reasonable accuracy. In rescue experiments, anaesthetized rats were injected s.c. with multiple MLD of venom and infused i.v. with drugs commonly used in the treatment of scorpion envenomation. The prepared potent specific antivenoms, but not the commercial polyvalent antivenom, rescued all animals from the lethal effect of the venom, even when injected late. Atropine, atropine+phentolamine, chlorpromazine, hydrocortisone and indomethacin were able, in varying degrees, to rescue some rats injected with 2 MLD of venom. Phentolamine, propranolol, hydralazine and calcium gluconate significantly prolonged the survival time, but did not rescue any animals. Chlorpheniramine, saline and 1/4 saline + 5% dextrose were without any effect. Aprotinin, the kallikrein-kinin inhibitor, was able to rescue half of the animals from the lethal action of the venom. Electrocardiographic studies showed that L. quinquestriatus venom, irrespective of the route of administration, causes myocardial ischaemia and either inferior or anterior wall infarction. This was associated with an initial moderate and a terminal severe bradycardia together with a variety of rhythm and conduction defects. Except for minor and transient electrocardiographic changes, either the prepared antivenoms or aprotinin protected rabbits and rats from the cardiac effects of the venom.

Selecting exposure parameters in developmental neurotoxicity assessments

Numerous factors must be considered in selecting exposure parameters for developmental neurotoxicity investigations. Whether employing a single dose during pregnancy, or continuous exposure from prepregnancy through early postnatal developmental periods, the following primary factors should be addressed: 1) Purpose of the study; 2) pharmacokinetics/pharmacodynamics; 3) biotransformation; 4) genotypic variables; 5) limiting factors, including the availability of test compounds for evaluation; and, 6) several general, miscellaneous factors. Whether a single, large dose of an exogenous agent is more toxic to the developing nervous system than a series of smaller doses depends upon the interaction of the physicochemical, pharmacokinetic, and pharmacodynamic properties of the agent with the genotypic features of the test organism.

Drug glucuronidation in humans

Glucuronidation is a major metabolic pathway for a large number of drugs in humans. Conjugation of drugs and other chemicals with glucuronic acid is catalyzed by the multigene UDP-glucuronosyltransferase family. It is believed that a number (unspecified at present) of glucuronosyltransferase isozymes, which probably differ in terms of substrate specificity and regulation, contribute to drug glucuronidation. Factors known to influence the pharmacokinetics of glucuronidated drugs in man, presumably via an effect on specific glucuronosyltransferases, include age (especially the neonatal period), cigarette smoking, diet, certain disease states, coadministered drugs, ethnicity, genetics and hormonal effects.

Extrapolation of animal toxicity data to man

Laboratory animals are used as models for humans in toxicity studies. This use is based on the assumption that extrapolation of biological data from animals to humans is valid. Three methods of extrapolation are considered: the use of body mass equivalence, caloric scaling across species, and the use of the surface area equivalence. Allometry, defined as the study of size and its consequences, is considered. There is still controversy whether there is an allometric relationship for energy metabolism. Allometry offers, among others, the concept that not all of the mass of the animal is equally involved in metabolism. In recent years the principles of pharmacokinetics have been applied to interspecies scaling; pharmacokinetic short-term studies can be used to determine whether allometric scaling is justified. Considerations, however, should be given to (pharmacokinetic) differences in the same species and to species variability. It would be useful to develop a set of criteria for deciding when the pharmacokinetic model is needed and when simpler models will suffice.

Factors associated with the development of neoplasia

A number of factors have been associated with the development of the phenotypic expression of neoplastic disease. The following article reviews the evidence implicating these factors in carcinogenesis and considers them under the groupings of familial or genetic factors, dietary factors, hormonal factors, age and gender.

Comparative pharmacokinetics of gentamicin, neomycin and oxytetracycline in newborn calves

The pharmacokinetics of three antibiotics--gentamicin, neomycin and oxytetracycline were determined in newborn calves. The kinetic determinations, using two-compartment open models, were made at increasing ages from 1 day to 42 days and compared with those made from older calves (250+ days). Although all three antibiotics are eliminated unchanged primarily by glomerular filtration, there were marked differences in the development of elimination processes for individual drugs. The pharmacokinetics of neomycin were not influenced by age. Although the elimination half-life of gentamicin appeared to decrease with age, the changes were not significant and were due to an increased elimination rate in only one calf. There was no change with age in the remaining three calves. Oxytetracycline elimination was significantly reduced in newborn calves. This was exemplified by a decrease in the half-life of elimination t1/2 (beta) from 672.5 +/- 99.4 in the newborn to 385.6 +/- 76.8 at 6 weeks of age, and 377.3 +/- 40.8 min in the 250-day-old calf. These changes were consistent in all four calves. The rate of elimination remained low for the first 4 weeks of life. The volume of distribution Vd, area was not changed after the first week of life. Based on pharmacokinetic changes, an adjustment of dosage is indicated for oxytetracycline in the newborn calf as compared to the older calf or adult.

Biological basis for extrapolation across mammalian species

The rationale for extrapolation or "scaling" across species is founded in the commonality of anatomic characteristics and the universality of physiologic functions and biochemical reactions. The development of the allometric equation, Y = aWn, relating species body size (W) with various morphological, physiological, biochemical, pharmacological, and toxicological characteristics, as the fundamental basis for extrapolation of biological data from laboratory animals to man is outlined. The familiar methods of extrapolation on the basis of "milligrams per kilogram body weight" and "body surface area" are simply examples, W1.0 and W0.67, respectively, of this equation. The experimental observations used to support these two, and other extrapolation bases, are reviewed. Criteria for the selection of an appropriate base for transfer of specific biologic data from laboratory animals to man, and the expected reliability of the extrapolation, are discussed with the enunciation of four guiding principles. The application of these principles to the extrapolation to man of dose-tumor incidence data from carcinogenicity bioassays of laboratory animals is discussed. The components are identified, and illustrative examples are given.

Relation between renal and hepatic excretion of drugs. II. Age-dependence of phenol red excretion in comparison with those of p-aminohippurate and indocyanine green

The excretory functions of kidney and liver do not develop simultaneously during the maturation of an individual. Therefore age related differences in the relation between renal and hepatic drug excretion could be expected. In this study the excretion of p-aminohippurate (PAH) and indocyanine green (ICG) as model substances for nearly exclusive excretion via kidney or liver, respectively, have been compared with that of phenol red eliminated both via kidneys and liver (3:1). Experiments were performed on rats between the 10th and 105th days of life. For PAH and ICG the typical age courses of renal or hepatic excretion have been confirmed. Both urinary and biliary phenol red excretion show an influence of age, however, renal elimination reaches adult values as early as at the 20th day of life. Furthermore the age relation concerning compensation of the loss of kidney or liver excretory functions has been studied. Neither after nephrectomy (NX) nor after bile duct ligation (DL) the PAH or ICG elimination via the alternative pathway, respectively, were quantitatively increased. Thus a compensation of the interruption of the main elimination route does not occur in all ages. In contrast, phenol red excretion into urine and bile increases distinctly after DL or NX. This increase becomes even significant after administration of suitable doses of phenol red saturating transport capacities of liver or kidney. The compensation is first of all caused by passive pharmacokinetic changes. Active compensatory mechanisms have not been proved.

Aspects of pharmacology in the neonatal foal

Other therapeutic agents used in foals for specific diseases are discussed elsewhere. The marked effect of species, age, and degree of maturity on drug metabolism in the neonate reinforces the danger of interspecies extrapolation of pharmacology, the need for information specific for the foal, and the necessity for monitoring drug levels in the individual. Suggested antimicrobial doses are listed in Tables 3, 4, and 6. Recommended doses of anticonvulsants and sedatives are listed in Table 8 and in the article "Intensive Care of the Neonatal Foal." The following are recommendations for drug therapy in the neonate: Avoid unnecessary administration of drug to the dam at parturition because of possible placental transfer of the drug with subsequent effects on the neonate. If possible, avoid unnecessary drug therapy in foals under 30 days of age. Select a drug that undergoes minimal biotransformation (hepatic metabolism) and is not highly protein bound. Owing to probable immunodeficiency in the neonate, broad-spectrum, bactericidal drugs are preferred for treatment of life-threatening infections. Every attempt should be made to identify the etiologic agent so that drug therapy can be based on cultures and sensitivity test results to maximize the benefit-risk ratio. Parenteral (intramuscular or intravenous) drug administration is preferable to oral. Avoid drugs that are known oxidants, which may produce hemolysis or methemoglobinemia. In general, the same or a slightly higher initial dose should be employed in the neonate, but it should be given less frequently than in the adult if it has a high potential to cause toxicity. When possible, individual monitoring of serum levels of potentially toxic drugs should be employed in premature and newborn foals unless specific drug pharmacokinetics are known for that age group.

The development of drug-metabolizing enzymes in the neonatal guinea-pig

In the guinea-pig, drug-metabolizing enzyme activities, with the exception of glucuronylation and possibly hydrolysis, tend to develop more rapidly after birth than in the other commonly investigated rodent species. This anomalous nature of the guinea-pig during its neonatal period should be borne in mind when assessing drug-metabolizing capacity and the potential toxic effects of chemicals in developing animals.

Morphine glucuronidation in human fetal and adult liver

The glucuronyltransferase activity towards morphine was measured in microsomes isolated from liver specimens obtained from human fetuses and cancer patients. All the fetal livers investigated had measurable UDP-glucuronyltransferase activity towards morphine. There was no correlation between the gestational age (15 to 27 weeks) and the glucuronidation rate. The mean value of the enzymatic activities was higher in fetal livers obtained by hysterotomy (0.20 nmoles x min-1 x mg-1) than in livers obtained after induced abortion (0.11 nmoles x min-1 x mg-1). The average rate of glucuronidation in microsomes from adult liver (mean 1.15 nmoles x min-1 x mg-1) was 6 to 10 times higher than in the fetal liver microsomes. Together with previous investigations on human adult and fetal liver glucuronidation, the present results support the theory of heterogeneity of human UDP-glucuronyltransferase.

Clinical pharmacokinetics of cerebrospinal fluid

The distribution of drugs into the cerebrospinal fluid has long been considered a challenging field of investigation in 2 major respects: (a) understanding how the physicochemical properties (molecular weight, pKa, plasma protein binding) of various molecules influence their movements across such a specific structure as the blood-brain barrier; and (b) defining the relationship between cerebrospinal fluid concentrations of various drugs and their central (side) effects. An attempt has been made to review the very dispersed information presently available to offer a clinically orientated picture of this area of pharmacokinetics. Drugs acting on the central nervous system (benzodiazepines, tricyclic antidepressants, anticonvulsants, opioids), antibacterial agents, cardiovascular drugs (beta-adrenoceptor blockers and digoxin), antineoplastic drugs (mainly methotrexate), and other miscellaneous agents (corticosteroids, cimetidine, methylxanthines) are reviewed. The available evidence seems to support the conclusion that only for methotrexate and antibacterial agents does knowledge of cerebrospinal fluid pharmacokinetics have direct therapeutic implications, while the mosaic of information available for other drugs does little more than provide a partially satisfactory picture.

Pharmacology and toxicology of infant skin

Cutaneous metabolism and pharmacology have been the focus of increased scientific inquiry in the past 2 decades. However, in the past few years, attention has been focused specifically on the effects of topically applied drugs in infants as different qualitatively or quantitatively from their effects in adults. Prior to 1972, it was known that brain damage occurred in animals with prolonged blood levels of 2 microgram/ml hexachlorophene, and that washing newborn babies with a standard 3% hexachlorophene liquid soap for 3-5 days resulted in significant blood levels of the compound. However, this knowledge was not disseminated widely enough to prevent the tragic deaths of infants after the use of baby powder contaminated with 6.6% hexachlorophene [1]. This incident highlighted the need for increased understanding of drug effects not only from the viewpoint of the skin as a target organ, but also of percutaneous penetration and resultant blood levels; the affinity of other body tissues for drugs and their metabolites, metabolites which may result from the effect of the skin itself acting on the drug; and the infant's much greater ratio of surface area to body weight, allowing the infant to percutaneously absorb proportionately greater quantities of topical medication than an adult. Although tissue distribution of most drugs has not been studied in infants, it is known that such distribution often depends on age. For example, in infants and children with a given plasma level, of drugs such as barbiturates, morphine and tetracycline, the brain tissue level may exceed that of the adult. Thus, drugs and chemicals that penetrate infant skin may produce effects different than those penetrating adult skin.

Developmental changes in amphetamine-induced taste aversions

In this study a conditioned taste aversion paradigm was employed to examine the ontogenetic trend in psychopharmacological responsiveness to amphetamine among infant (18 days of age), periadolescent (35 days of age), and young adult (52 days of age) rats. The ability of amphetamine to alter taste preference increased with dosage level and this effect interacted with age. Infant rats demonstrated greater sensitivity to the taste aversion inducing properties of amphetamine than either periadolescent or young adult animals. In contrast, periadolescent animals demonstrated a marked resistance to amphetamine's taste aversion inducing properties when compared with either infant or young adult animals. This developmental pattern in amphetamine drug responsiveness seen utilizing the taste aversion paradigm parallels the previously examined ontogenetic trend in amphetamine response using locomotor activity as a response measure.