Biotransformation of Drugs and other Xenobiotics during Postnatal development

Efforts to establish an animal model of Fanconi syndrome after ifosfamide administration to rats

About 10% of children develop Fanconi syndrome (FS) a few months after ifosfamide (IFO) treatment. To establish an animal model, IFO was injected as 4 or 5 treatment courses (TCs, once daily for 3 consecutive days), to adult female rats (AF, 8 mg 100 g(-1) body wt, 4 TCs), to young female rats (YF, 8 mg 100 g(-1) body wt, 5 TCs) and to male rats (M, 6 mg 100 g(-1) body wt, 4 TCs). In the adult female rats, polyuria with electrolyte and albumin wasting occurred acutely, 2 days after the first treatment course. After the third treatment course, 30% of the rats died, but survivors showed a reduced excretion of electrolytes and glucose. The body weight increase was significantly diminished in adult female and male rats by about 25% or 70%, respectively. Up to 5 months after 5 TCs in young female rats, 15% of the animals died but the survivors did not show any sign of renal failure. In males, 28% of the rats died and in surviving animals the excretion of electrolytes, proteins and glucose as well as GFR were reduced 7 weeks after the last treatment course. There were no pathomorphological changes in kidney and liver. Determination of renal and hepatic cytochrome P450 activities indicated that results of adult female and male rats could be caused by starving, known as a common side effect of IFO, and not by its nephrotoxicity. Altogether, it was not possible to establish a model of a Fanconi syndrome persisting after cessation of IFO treatment in our rat strain, whereas acute, FS-like IFO effects on the kidney could be shown.

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.

Role of biokinetics in risk assessment of drugs and chemicals in children

Whether children incur different risks from xenobiotics than adults will depend on the exposure, biokinetics, and dynamics of compound. In this paper, current knowledge on developmental physiology and possible effects on biokinetics are evaluated and the role of biokinetics in risk assessment both for drugs and chemicals is discussed. It is concluded that most dramatic age-related physiological changes that may affect biokinetics occur in the first 6-12 months of age. The difference in internal exposure between children and adults can generally be predicted from already known developmental physiological differences. However, for risk assessment it will also be necessary to determine whether internal exposure is within the drug's therapeutic window or if it will exceed the NOAEL of a chemical. Furthermore, the effects of internal exposure of potentially harmful compounds on developing organ systems is of utmost importance. However, knowledge on this aspect is very limited. Risk assessment in children could be improved by: (1) application of pediatric PBPK-models in order to gain insight into internal exposure in children, (2) studies in juvenile animals for studying effects on developing systems, and (3) extrapolation of knowledge on the relationship between internal exposure and dynamics for drugs to other chemicals.

In vivo quantification of ageing changes in the rat liver from early juvenile to senescent life

AIMS/METHOD

Using high resolution multifluorescence in vivo microscopy, the present study was undertaken to determine the changes in rat hepatic tissue architecture and microvasculature during the growth associated with juvenile maturation and adult senescence, i.e. the age of 1, 3, 12 and 24 months.

RESULTS

By 1 month of age the liver attained its full size and functional capacity, as assessed by relative organ weight and hepatic bile flow. Survey of liver architecture revealed a progressive growth of lobular area with postsinusoidal venules exhibiting a proportional increase in length, diameter and inter-vascular distance up to the age of 12-24 months. In regard to the 3.5-4-fold average increase of lobular units, a minor reduction of sinusoidal density to 87% over life strongly implies the recruitment or formation of new sinusoidal microvessels as contributing mechanism to meet oxygen demand due to overall tissue enlargement. The sinusoidal perfusion rate remained above 98% over the whole lifespan. Leukocytic interaction with the hepatic microvascular endothelium was found within the physiological range in all age groups. Moreover, kinetics of clearance of latex beads as well as lobular distribution of Kupffer cells did not differ between animals of different age. Hepatic stellate cell-associated area of ultraviolet vitamin A-autofluorescence increased with age and significantly correlated with increasing tissue concentrations of vitamin A metabolites. Biochemical parameters serving as measures of tissue integrity did not indicate age-associated tissue alterations.

CONCLUSION

These age-associated physiological changes should be carefully taken into account as a relevant variable in experimental research.

Transplacental passage of oxcarbazepine and its metabolites in vivo

PURPOSE

The purpose of this study was to investigate human fetal exposure to oxcarbazepine (OCBZ) in vivo.

METHODS

Transplacental passage and placental tissue concentrations of OCBZ and its metabolites were determined. Maternal venous blood, cord blood, and placental tissue samples from 12 mothers using OCBZ during pregnancy alone or in combination with other antiepileptic drugs were collected. Samples were analyzed with high-performance liquid chromatography.

RESULTS

Maternal venous concentrations of OCBZ and its major metabolites were at same range as cord blood concentrations (OCBZ in maternal serum, 0.19 +/- 0.16 microg/ml, and in cord serum, 0.21 +/- 0.19 microg/ml; 10-hydroxy-10,11-dihydrocarbamazepine (10-OH-CBZ) in maternal serum, 5.69 +/- 2.49 microg/ml, and in cord serum, 5.23 +/- 1.44 microg/ml; 10,11-trans-dihydroxy-10,11-dihydrocarbamazepine (10,11-D) in maternal serum, 0.29 +/- 0.22 microg/ml, and in cord serum, 0.28 +/- 0.14 microg/ml). OCBZ (0.17 +/- 0.16 microg/g placental tissue), 10-OH-CBZ (3.49 +/- 1.34 microg/g placental tissue) and 10,11-D (0.25 +/- 0.11 microg/g placental tissue) were detected in the placental tissue. The amount of OCBZ detected from placental tissue was 0.01% of the daily dose.

CONCLUSIONS

OCBZ, like other antiepileptic drugs, is transferred significantly through the placenta in humans.

Age-dependent changes in rat liver microsomal membrane structure and functions under benzene treatment

The influence of benzene on 3 and 24 months old rat liver microsomes was studied. Some structural and functional changes occur under benzene treatment in the cytochrome P-450 system which are more pronounced in 3 months old rat microsomes than in the 24 months ones. Glucose-6-phosphatase and glucose dehydrogenase activities indicate that 3 months old rat microsomal vesicles are more stable against benzene injury than those, of 24 months old ones. In vitro benzene hydroxylation activation by NADPH addition decreased disruptive xenobiotic's effect on 3 but not on 24 months old rat liver microsomal vesicles. This fact suggests that the rate of benzene hydroxylation is important for its membrane damaging action effect. Thus, age-related differences in xenobiotic action on liver microsomes could be related to the decrease of benzene metabolism rate with senescence.

Luminol and lucigenin amplified chemiluminescence and lipidperoxidation with brain microsomes from rats during ontogenetic development

The chemiluminescence (CL) amplifiers luminol (LM) and lucigenin (LC) react with different reactive oxygen species (ROS) in dependence on the ROS generating system used. With liver microsomes LMCL indicates predominantly superoxide anion radicals, whereas LCCL is mainly a measure for hydroxyl radical formation or of reactive organic radicals. With brain microsomes only LCCL, but not LMCL could be measured. For both brain microsomes from newborn (both sexes) and 60 day-old (male) rats LCCL is dependent on protein and NADPH concentration, activity in newborns being only 15% compared with young adult rats. As compared with liver microsomes 10-fold higher protein concentrations are needed to obtain comparable LCCL, whereas the NADPH demand is the same as with liver. A distinct ontogenetic development was demonstrated: low activities in the fetus, in newborn and 10-day-old rat are followed by higher activities with increasing age, after a maximum at an age of 60 days a decline was observed. Microsomal lipidperoxidation (LPO) was measured as formation of thiobarbituric acid reactive substances (TBARS) and was also dependent on protein and NADPH concentration. Unexpectedly, LPO with brain microsomes from newborn rats did not show any developmental variation.

Developmental expression of cytochrome P450 enzymes in human liver

Drug-metabolizing cytochrome P450 enzymes, the major phase I enzymes, are active in human liver already at very early stages of intrauterine development, although presumably at fairly low concentrations and in low numbers. During maturation, these enzymes go through various developmental programmes towards adulthood. The major increase both in abundance as well as in number of different enzymes takes place after birth, probably during the first year of life. Detailed information concerning these developmental changes is still limited. The major drug-metabolizing P450 enzymes appear to be primarily members of the CYP3A subfamily in all stages of development. The balance between different members of this subfamily, however, undergoes significant switches from the foetal predominant CYP3A7 to the major adult form CYP3A4. The ontogeny of the other cytochrome P450 enzymes is less well characterized, but the major switch-on appears to occur mainly after birth. Developmental expression of P450 enzymes is one of the key factors determining the pharmacokinetic status of developing individuals both pre- and postnatally.