Interspecies metabolism and pharmacokinetic scaling of theophylline disposition

Interspecies Differences and Scaling for the Pharmacokinetics of Xanthine Derivatives

Pharmacokinetic characteristics of the new xanthine bronchodilators, enprofylline and 1-methyl-3-propylxanthine (MPX), were investigated in mice, rats, guinea-pigs, rabbits and dogs. The possibility of an interspecies pharmacokinetic scale was also evaluated. The concentration of these two drugs in plasma and urine was determined by HPLC. Pharmacokinetic parameters were calculated using model-independent methods. The disappearance curves of the two drugs from plasma varied markedly among animal species. Interspecies differences in the plasma protein binding of each drug were observed for all animals in the study. Differences in the biotransformation of enprofylline and MPX were also confirmed among the various animal species: enprofylline is mainly excreted in an unchanged form in urine while MPX follows a non-renal route of elimination. In all animals, the renal clearance for enprofylline was greater than the glomerular filtration rate, indicating active tubular secretion. Significant allometric relationships were seen between the values of total body clearance and steady state volume of distribution for both total and unbound enprofylline and species body weight, but similar correlations could not be recognized for MPX. Renal clearance of enprofylline was also closely correlated with species body weight, suggesting no interspecies difference with relation to affinity and/or capacity for the active tubular secretion mechanism of enprofylline. Our findings suggest that xanthine derivatives, including enprofylline, are mainly eliminated via the kidney, and an estimate of the basic pharmacokinetics in man can be obtained from data in experimental animals.

Application of fixed exponent 0.75 to the prediction of human drug clearance: an inaccurate and misleading concept

Considering the controversy surrounding the exponent of 0.75 for the prediction of human drug clearance and lack of any systematic evaluation of the aforementioned proposal, the objective of this study was to determine whether the exponent 0.75 is indeed the most suitable exponent for the prediction of human drug clearance as compared to allometric scaling using the rule of exponents (ROE). Three methods were used to predict human drug clearance. Besides evaluating the exponent of 0.75, an arbitrarily selected exponent of 0.65 was also tested. ROE was also used to predict human drug clearance, and predicted values by all three methods were compared with observed human drug clearance. The results indicate that the exponent 0.75 is not the best approach for the prediction of human drug clearance. Both exponents 0.75 and 0.65 predicted human drug clearance with uncertainty, although on average the prediction of human drug clearance by 0.65 was better than the exponent 0.75. ROE provided far more accurate prediction of human drug clearance than either of the exponents. Although exponent 0.75 occasionally provided a good prediction of human drug clearance for a given drug for a given species, overall, the method is highly erratic and unreliable.

A global examination of allometric scaling for predicting human drug clearance and the prediction of large vertical allometry**This work was presented at the American Association of Pharmaceutical Scientists Annual meeting, Salt Lake City, USA, Oct. 26, 2003

Allometrically scaled data sets (138 compounds) used for predicting human clearance were obtained from the literature. Our analyses of these data have led to four observations. (1) The current data do not provide strong evidence that systemic clearance (CL(s); n = 102) is more predictable than apparent oral clearance (CL(po); n = 24), but caution needs to be applied because of potential CL(po) prediction error caused by differences in bioavailability across species. (2) CL(s) of proteins (n = 10) can be more accurately predicted than that of non-protein chemicals (n = 102). (3) CL(s) is more predictable for compounds eliminated by renal or biliary excretion (n = 33) than by metabolism (n = 57). (4) CL(s) predictability for hepatically eliminated compounds followed the order: high CL (n = 11) > intermediate CL (n = 17) > low CL (n = 29). All examples of large vertical allometry (% error of prediction greater than 1000%) occurred only when predicting human CL(s) of drugs having very low CL(s). A qualitative analysis revealed the application of two potential rules for predicting the occurrence of large vertical allometry: (1) ratio of unbound fraction of drug in plasma (f(u)) between rats and humans greater than 5; (2) C logP greater than 2. Metabolic elimination could also serve as an additional indicator for expecting large vertical allometry.

A novel model for prediction of human drug clearance by allometric scaling

Sixty-one sets of clearance (CL) values in animal species were allometrically scaled for predicting human clearance. Unbound fractions (f(u)) of drug in plasma in rats and humans were obtained from the literature. A model was developed to predict human CL: CL=33.35 ml/min x (a/Rf(u))(0.770), where Rf(u) is the f(u) ratio between rats and humans and a is the coefficient obtained from allometric scaling. The new model was compared with simple allometric scaling and the "rule of exponents" (ROE). Results indicated that the new model provided better predictability for human values of CL than did ROE. It is especially significant that for the first time the proposed model improves the prediction of CL for drugs illustrating large vertical allometry.

Prediction of Human Drug Clearance from in Vitro and Preclinical Data Using Physiologically Based and Empirical Approaches

PURPOSE

The aim of this study is to compare the accuracy of five methods for predicting in vivo intrinsic clearance (CL(int)) and seven for predicting hepatic clearance (CL(h)) in humans using in vitro microsomal data and/or preclinical animal data.

METHODS

The human CL(int) was predicted for 33 drugs by five methods that used either in vitro data with a physiologic scaling factor (SF), with an empirical SF, with the physiologic and drug-specific (the ratio of in vivo and in vitro CL(int) in rats) SFs, or rat CL(int) directly and with allometric scaling. Using the estimated CL(int), the CL(h) in humans was calculated according to the well-stirred liver model. The CL(h) was also predicted using additional two methods: using direct allometric scaling or drug-specific SF and allometry.

RESULTS

Using in vitro human microsomal data with a physiologic SF resulted in consistent underestimation of both CL(int) and CL(h). This bias was reduced by using either an empirical SF, a drug-specific SF, or allometry. However, for allometry, there was a substantial decrease in precision. For drug-specific SF, bias was less reduced, precision was similar to an empirical SF. Both CL(int) and CL(h) were best predicted using in vitro human microsomal data with empirical SF. Use of larger data set of 52 drugs with the well-stirred liver model resulted in a best-fit empirical SF that is 9-fold increase on the physiologic SF.

CONCLUSIONS

Overall, the empirical SF method and the drug-specific SF method appear to be the best methods; they show lower bias than the physiologic SF and better precision than allometric approaches. The use of in vitro human microsomal data with an empirical SF may be preferable, as it does not require extra information from a preclinical study.

Prediction of human pharmacokinetics from animal data and molecular structural parameters using multivariate regression analysis: volume of distribution at steady state

The aim of this study was to develop a regression equation for predicting volume of distribution at steady state (Vd(ss)) in humans to enable application to various types of drugs using animal experimental data for rats and dogs and some molecular structural parameters. The Vd(ss) data for rats, dogs and humans of 64 drugs were obtained from literature. The compounds have various structures, pharmacological activities and pharmacokinetic characteristics. In addition, the molecular weight, calculated partition coefficient (clogP), and the number of hydrogen bond acceptors were used as possible descriptors related to the Vd(ss) in humans. Multivariate regression analyses, multiple linear regression analysis and the partial least squares (PLS) method were used to predict Vd(ss) in humans. Interaction terms were also introduced into the regression analysis to evaluate the non-linear relationship. For the data set used in the present study, PLS with quadratic term descriptors gave the best predictive performance. The PLS model using Vd(ss) data for only two animal species and using easily calculated structural parameters could generally predict Vd(ss) in humans better than an allometric method. In addition, the PLS model with only animal data gave almost the same predictive performance as the PLS model with quadratic term descriptors. This model may be easier to use and be practical in a realistic situation, and could predict Vd(ss) in humans better than the allometric method.

Prediction of human clearance from animal data and molecular structural parameters using multivariate regression analysis

The aim of the study reported here was to develop a method for predicting human clearance that can be applied to various kinds of drugs using clearance values for rats and dogs and some molecular structural parameters. The clearance data for rats, dogs, and humans of 68 drugs were obtained from literature. The compounds have various structures, pharmacological activities, and pharmacokinetic characteristics. In addition, molecular weight, c log P, and the number of hydrogen bond acceptors were used as possible descriptors related to the human clearance value for each drug. Three types of regression methods, multiple linear regression (MLR) analysis, partial least squares (PLS) method, and artificial neural network (ANN), were used to predict human clearance, and their predictive performances were compared with allometric approaches, which have been widely used in interspecies scaling. In MLR and PLS analyses, interaction terms were introduced to evaluate the nonlinear relationships. For the data sets used in the present study, MLR and PLS with quadratic terms gave the same equation and the best predictive performance. The value of the squared cross-validated correlation coefficient (q(2)) was 0.682. In conclusion, the MLR method using animal clearance data from only two species and using easily calculated structural parameters can generally predict human clearance better than allometric methods. This approach can be applied to drugs with various characteristics.

Allometric scaling of xenobiotic clearance: uncertainty versus universality

Statistical analysis and Monte Carlo simulation were used to characterize uncertainty in the allometric exponent (b) of xenobiotic clearance (CL). CL values for 115 xenobiotics were from published studies in which at least 3 species were used for the purpose of interspecies comparison of pharmacokinetics. The b value for each xenobiotic was calculated along with its confidence interval (CI). For 24 xenobiotics (21%), there was no correlation between log CL and log body weight. For the other 91 cases, the mean +/- standard deviation of the b values was 0.74 +/- 0.16; range: 0.29 to 1.2. Most (81%) of these individual b values did not differ from either 0.67 or 0.75 at P = 0.05. When CL values for the subset of 91 substances were normalized to a common body weight coefficient (a), the b value for the 460 adjusted CL values was 0.74; the 99% CI was 0.71 to 0.76, which excluded 0.67. Monte Carlo simulation indicated that the wide range of observed b values could have resulted from random variability in CL values determined in a limited number of species, even though the underlying b value was 0.75. From the normalized CL values, four xenobiotic subgroups were examined: those that were (i) protein, and those that were (ii) eliminated mainly by renal excretion, (iii) by metabolism, or (iv) by renal excretion and metabolism combined. All subgroups except (ii) showed a b value not different from 0.75. The b value for the renal excretion subgroup (21 xenobiotics, 105 CL values) was 0.65, which differed from 0.75 but not from 0.67.

Prediction of drug clearance in humans from laboratory animals based on body surface area

The object of the study was to develop a new allometric equation for clearance from laboratory animals to humans based on body surface area (BSA allometric method). Human clearances for 30 drugs were predicted from animal data obtained from the literature. The results predicted with the method were compared with those observed. The results were also compared with values predicted with clearance versus body weight (BW simple allometric method), the product of brain weight and clearance versus body weight (Cl x BRW method) and the product of maximum life span potential and clearance versus body weight (Cl x MLP method), respectively. Good predictions were found in 21 out of 30 with the BAS allometric method. Both BSA allometric method and BW simple allometric method can give good predictions of clearance in humans for many drugs. Similarly to BW simple allometric method, Cl x BRW method and Cl x MLP method, BSA allometric method may be used to accurately predict human clearance from laboratory animal data.

Uncertainty factors for chemical risk assessment: interspecies differences in the in vivo pharmacokinetics and metabolism of human CYP1A2 substrates

The 100-fold default uncertainty factor is used to convert a no-observed-adverse-effect level (NOAEL) from a animal toxicity study, to a "safe" value for human intake. The composite uncertainty factor (100) has to allow for interspecies (10-fold) and interindividual (10-fold) differences in toxicokinetics and toxicodynamics. The aim of the current study was to assess the validity of the interspecies default for toxicokinetics (4.0) for each of the test species (dog, rabbit, rat and mouse), using published data for compounds eliminated by CYP1A2 in humans (caffeine, theobromine, theophylline and paraxanthine). An analysis of the published literature showed that the absorption, bioavailability and route of excretion were generally similar between humans and the test species, for each probe substrate. However, interspecies differences in the route of metabolism, and the enzymes involved in this process, were identified. The magnitude of difference in the internal dose, between species, showed that values for the mouse (10.6) and rat (5.4) exceed the 4.0-fold default, whereas the rabbit (2.6) and dog (1.6) were below this value. This work supports the need to replace the generic default factors by a compound-related value derived from specific, relevant, quantitative data; this would result in more relevant and reliable non-cancer risk assessments.

Influence of hepatic regeneration after partial hepatectomy on theophylline pharmacokinetics in rats

The influence of hepatic regeneration after partial hepatectomy on theophylline pharmacokinetics has been studied on the rat. At different times after partial hepatectomy, theophylline was administered intravenously as a single dose of 6 mg/Kg. Drug plasma levels were determined by HPLC and pharmacokinetic parameters were obtained. Physiological parameters were also measured. Following hepatectomy, an increase in mass liver was observed and 15 days after surgery, liver mass was 78% of nonhepatectomized rats. Initial theophylline concentrations varied during the regeneration period, as well as the distribution volume at steady-estate (Vss). Elimination half-life (t 1/2), notably increased after hepatectomy (7.27+/-1.38 h), decreased with time (6.70+/-1.18 h, 6.47+/-0.69 and 5.17+/-0.87 h after 24 h, 3 days and 15 days post-hepatectomy, respectively) to reach a value close to that of the control group (4.30+/-1.37 h). The increase in elimination half-life led to a decrease in the mean residence time during the period of liver regeneration. However, the intrinsic clearance hardly varied during regeneration period. We could establish the following relationship between liver weight (LW) and the elimination half-life: t 1/2 (h)=-0.358*LW (g)+8.6168 (R2=0.9906). For the mean residence time (MRT) this relationship was: MRT (h) =-0.5173*LW (g)+12.433 (R2=0.991).

Calculation of the dimensions of dosage forms with release controlled by diffusion for in vivo use

Using numerical models and data obtained from in vitro experiments, the dimensions of diffusion controlled release dosage forms to achieve desired in vivo levels are predicted. Monolithic polymer-drug devices are considered, the release of the drug being controlled by transient diffusion with constant diffusivity. The dimensions of the devices are calculated for various shapes (e.g. spheres, parallelepipeds, cylinders), so that 85% of the drug is released within 6 or 24 h, respectively. Caffeine, diltiazem HCl, and theophylline are studied in ethylcellulose (EC), plasticized with dibutyl sebacate (DBS) or acetyltributyl citrate (ATBC), respectively. The dosage forms are to be administered orally once a day. The resulting drug levels in the plasma are calculated using a numerical model that takes into account: the kinetics of drug release and the pharmacokinetic data of these dosage forms and drugs. Plasma levels resulting from immediate release dosage forms are also calculated, serving as reference.

Molecular modelling of CYP1 family enzymes CYP1A1, CYP1A2, CYP1A6 and CYP1B1 based on sequence homology with CYP102

Molecular modelling of a number of CYP1 family enzymes from rat, plaice and human is described based on amino acid sequence homology with the haemoprotein domain of CYP102, a unique bacterial P450 of known structure. The interaction of various substrates and inhibitors within the putative active sites of rat CYP1A1, human CYP1A2, a fish CYP1 enzyme CYP1A6 (from plaice) and human CYP1B1, is shown to be consistent with P450-mediated oxidation in each example or, in the case of inhibitors, mechanism of inhibition. It is reported that relatively small changes between the enzymes' active site regions assist in the rationalization of CYP1 enzyme preferences for particular substrate types, and a template of superimposed CYP1A2 substrates is shown to fit the putative active site of the human CYP1A2 enzyme.

Biotransformation of theophylline in cirrhotic rats induced by biliary obstruction

The object of this work was to study the pharmacokinetic differences and the cause of these differences in cirrhotic rats induced by biliary obstruction when aminophylline (8 mg/kg as theophylline, i.v.) was administered. The concentrations of theophylline and its major metabolite (1,3-dimethyluric acid) in plasma were determined by HPLC. In addition, formation of 1,3-dimethyluric acid from theophylline in microsomes and the changes in the activity of drug metabolizing enzymes, which are suggested to be involved in theophylline metabolism, were determined. In cirrhotic rats, the systemic clearance of theophylline was reduced to 30% of the control value while AUC (area under the plasma concentration-time curve) and (t1/2)beta were increased 1.3 fold and 3.5 fold, respectively. The formation of 1,3-dimethyluric acid was decreased to 30% of the control value in microsomes of cirrhotic rat liver. In cirrhotic rat liver, activities of aniline hydroxylase (CYP2E1 related), erythromycin-N-demethylase (CYP3A related), and methoxyresorufin-O-demethylase (CYP1A2 related), which were reported to be related with theophylline metabolism, were decreased to 67%, 53%, and 76% that of normal rat liver, respectively. From the results, it can be concluded that in cirrhotic rats induced by biliary obstruction, the total body clearance of theophylline is markedly reduced and it may be due to decreased activity of drug metabolizing enzymes in liver.

A novel pathway for the metabolism of caffeine by a mixed culture consortium

A new oxidative pathway for the degradation of caffeine(1,3,7-Trimethylxanthine, I) by a mixed culture consisting of strains belonging to the genera Klebsiella and Rhodococcus is presented. The mixed culture does not initiate degradation by N-demethylation either complete or partial, but instead carries out oxidation at the C-8 position resulting in the formation of 1,3,7-trimethyluric acid (TMU, II) which further gets degraded to 3,6,8-trimethylallantoin (TMA, III). Both TMU and TMA are hitherto not shown to be formed in the microbial system. Further degradation of TMA (III) by caffeine grown cells yields dimethylurea (VII) as one of the metabolites. Oxygen uptake studies indicated that caffeine(I) grown cells oxidized TMU(II), TMA (III), glyoxalic acid (VI), dimethylurea(VII), and monomethylurea(V), but not monomethyl and dimethyluric acids. The mixed culture does not accept theophylline(1,3-dimethylxanthine), theobromine(3,7-dimethylxanthine), and paraxanthine(1,7-dimethylxanthine) as the carbon source.

Effects of DU-6859a, a new quinolone antimicrobial, on theophylline metabolism in in vitro and in vivo studies

In vitro and in vivo studies were conducted to investigate the drug interaction between a new quinolone antimicrobial, DU-6859a, and theophylline (TP). The effect of DU-6859a on TP metabolism was evaluated in vitro by measuring the rate of TP metabolite formation by using human liver microsomes. DU-6859a inhibited the metabolism of TP, especially the formation of 1-methylxanthine, in vitro, but to a lesser extent than other drugs that are known to interact with TP. TP was administered alone (200 mg twice a day [b.i.d.] for 9 days) or in combination with DU-6859a (50 or 100 mg b.i.d. for 5 days) to six healthy subjects. DU-6859a administered at a dose of 50 mg resulted in no changes in serum TP concentrations, and slight increases in serum TP concentrations were observed at a dose of 100 mg. Moreover, the administration of 100 mg of DU-6859a resulted in decreases in all urinary TP metabolites, with significant differences. It appears that although DU-6859a has a weak inhibitory effect on TP metabolism in vitro, its concomitant use with TP at clinical dosage levels does not cause any adverse effects, showing only a slight increase in blood TP concentrations and a decrease in urinary metabolites.

Integration of in vitro data into allometric scaling to predict hepatic metabolic clearance in man: application to 10 extensively metabolized drugs

In this study, we investigated rational and reliable methods of using animal data to predict in humans the clearance of drugs which are mainly eliminated through hepatic metabolism. For 10 extensively metabolized compounds, adjusting the in vivo clearance in the different animal species for the relative rates of metabolism in vitro dramatically improved the predictions of human clearance compared to the approach in which clearance is directly extrapolated using body weight. Using hepatocyte data to normalize the in vivo clearances led to lower median deviations between the observed and predicted clearances in man compared to the approach normalizing data with brain weight (30-40% vs 60-80%, respectively). In addition, the approach integrating in vitro data appeared to be superior with respect to the range of deviations: approximately 2-fold underestimation, in the worst case, was observed by using in vitro data, whereas normalizing data by brain weight led to up to 10-fold underestimation of clearance in man. In addition, the integration of in vitro data provides a more rational basis to predict the metabolic clearance in man and may be applicable to compounds undergoing phase I and phase II metabolism as well.

The use of human hepatocytes to select compounds based on their expected hepatic extraction ratios in humans

PURPOSE

The present investigation retrospectively evaluates the use of human hepatocytes to classify compounds into low, intermediate or high hepatic extraction ratio in man.

METHODS

A simple approach was used to correlate the in vivo hepatic extraction ratio of a number of compounds in man (literature and in-house data) with the corresponding in vitro clearance which was determined in human hepatocytes. The present approach assumes that, for compounds eliminated mainly through liver metabolism, intrinsic clearance is the major determinant for their in vivo hepatic extraction ratio and subsequently their bioavailability in man. The test compounds were selected to represent a broad range of extraction ratios and a variety of metabolic pathways.

RESULTS

The present data show that in vitro clearances in human hepatocytes are predictive for the hepatic extraction ratios in vivo in man. Most of the test compounds (n = 19) were successfully classified based upon human hepatocyte data into low, intermediate or high hepatic extraction compounds, i.e. compounds with potential for high, intermediate or low bioavailabilities in humans.

CONCLUSIONS

The present approach, validated so far with 19 test compounds, appears to be a valuable tool to screen for compounds with respect to liver first-pass metabolism at an early phase of drug discovery.

Influence of partial hepatectomy on theophylline pharmacokinetics in rats

The influence of partial hepatectomy on the pharmacokinetics of theophylline was determined in rats. The pharmacokinetics of intravenous theophylline was studied in unhepatectomized rats (control group: CG) and in hepatectomized rats (HG) 12 h after 70% hepatectomy. Liver function was monitored in both groups by measurements of total and direct bilirubin, transaminases GOT and GPT, and plasma protein. Seventy per cent hepatectomy caused significant liver dysfunction: transaminase levels (GOT and GPT) increased by 118 and 683%, respectively, and the total and direct bilirubin levels increased by 28.6 and 9.1%, respectively. At the same time, plasma concentrations of theophylline decreased significantly and half-life increased from 4.16 +/- 0.57 h (CG) to 7.08 +/- 0.69 h (HG), as did the distribution volumes values of central (Vc) and peripheral (Vp) compartments (Vc: CG, 0.18 +/- 0.03 L; HG, 0.24 +/- 0.03 L) (Vp: CG, 0.08 +/- 0.05 L; HG, 0.13 +/- 0.05 L). The percentage of theophylline binding to plasma proteins decreased from 44.3% in CG to 33.8% in HG. The theophylline intrinsic clearance (CLint) dropped from 1.35 +/- 0.43 mL/min (CG) to 0.93 +/- 0.10 mL/min (HG)1, which can be attributed to a significant fall in the quantity of hepatic microsomal enzymes. These modifications on the pharmacokinetics of drugs with low hepatic extraction coefficients, such as theophylline, should be considered when dosage regimens during the posthepatectomy hepatic regeneration period are planned.

The allometric approach for interspecies scaling of pharmacokinetics and toxicity of anti-cancer drugs

1. The rationale for extrapolation or 'scaling' across animal species is based on their underlying anatomical, physiological and biochemical similarities. 2. Research carried out in the 19th and early 20th century resulted in Benedict's famous 'mouse-to-elephant' graph which showed that the log of the basal metabolic rate plotted against the log of bodyweight (W) produced a straight line with a slope of 0.76. Since then it has become apparent that a number of other physiological variables (Y) exhibit a similar relationship which can be represented by the general allometric equation, Y = alpha W beta; where beta is the slope of the log-log plot and alpha is the intercept on the y axis. 3. The major pharmacokinetic parameters such as clearance and volume of distribution of many drugs are also related to W in a similar manner. 4. This empirical approach does not require a strong mathematical background and offers a relatively simple method of predicting the kinetics of anti-cancer drugs in patients from pre-clinical animal data. 5. The occurrence of major qualitative and quantitative differences in the metabolism of drugs between species is probably the single greatest complicating factor in the use of animals as predictors of drug toxicity and kinetics in patients. 6. Despite this, the allometric approach is useful for allowing the estimation of a more appropriate starting dose for some drugs in a Phase I trial, which might result in potential savings in escalation steps and maximize the chance that the dose which an individual receives has the potential for therapeutic value.

Interspecies scaling of cimetidine-theophylline pharmacokinetic interaction: interspecies scaling in pharmacokinetic interactions

The aim of the present study was the use of an interspecies scaling approach to predict drug interactions during preclinical drug disposition studies. Theophylline and cimetidine were selected because of their documented interaction. The literature was searched for pharmacokinetic data of intravenously administered theophylline alone and in the presence of cimetidine in humans, dogs and rats. Further, we determined the theophylline-cimetidine drug interaction in rabbits. Application of allometric equations to the pharmacokinetic parameters and the conversion of chronological time into pharmacokinetic time allowed us to obtain the complex Dedrick plot for theophylline when administered alone or in combination with cimetidine. A superimposable kinetic profile was obtained for the plasma levels of theophylline in all species studied, both with and without cimetidine. From the terminal phase of the curves it is possible to calculate the elimination half-life: 2.69 apolisychrons for theophylline when it is administered alone and 3.86 apolisychrons when it is administered in combination with cimetidine. This 43% increase in t1/2 is similar to the increase in the elimination half-life of theophylline in humans when it is administered after pretreatment with cimetidine. These results show that an interspecies scaling approach may be useful to predict the effect of interactions in humans from the results obtained in preclinical research with new drugs.

Pentachlorophenol carcinogenicity: extrapolation of risk from mice to humans

1. Pentachlorophenol (PCP) has been found to be carcinogenic in mice. The objective of this study was to extrapolate to humans the risk of cancer from data obtained in mice using information on disposition, serum protein binding and metabolism of PCP across species. 2. A review of the literature indicates that neither PCP nor a mutagenic metabolite, tetrachlorohydroquinone (TCHQ), has been specifically identified as responsible for the carcinogenicity. In addition, the occurrence of TCHQ as a metabolite of PCP in humans is still questionable. Therefore, cancer risk assessment is performed on the assumption that PCP itself is responsible for the carcinogenicity. 3. For interspecies extrapolation, a new method in which interspecies differences in clearance and serum protein binding are taken into account is used. The method gives estimates of equivalent human doses of PCP which are up to 4 times smaller than those obtained using body surface area. For both interspecies extrapolation methods, the estimated virtually-safe doses of PCP are smaller than the average daily intakes reported in groups of subjects nonspecifically exposed to PCP. Corresponding extra risks of cancer for lifetime exposure are from 20 to 140 times greater than the acceptable extra risk (10(-6)). The results obtained with this approach indicate that PCP is a possible public health hazard.

Placental transfer of theophylline during in situ perfusion in the rabbit

Many physiological changes take place during pregnancy, and the disposition profile of endogenous and exogenous compounds may change, too. Thus knowledge of the disposition pattern of a compound may be useful in relation to its therapeutic effect(s) and its potential toxicity on the fetus and the newborn. Because the amount of a compound received by the fetus is a product of placental transfer rate, and available maternal amount, and because it is difficult to control and evaluate the factors that may affect such a transfer in women, we set up an in situ perfused placental model in the rabbit. The reliability of the model was borne out by comparing the placental transfer of theophylline with antipyrine, a commonly used marker of placental exchange, at steady state after a two-step infusion at mean arterial plasma concentrations of 8 and 5 mg/L, respectively for theophylline and antipyrine. The rabbit placenta was perfused in situ with a modified Earle's buffer at a 1-mL/min flow rate. During perfusion, maternal plasma, placental perfusate, biochemical parameters, gas exchange, body temperature, and electrocardiogram were carefully monitored. The maternal plasma and perfusate drug concentrations over time were fitted by appropriate models and kinetic parameters were calculated. Umbilical vein/maternal artery concentration ratios reached equilibrium soon after the loading infusion was stopped for both drugs. Placental clearance averaged 0.62 and 0.77 mL/min for theophylline and antipyrine, respectively, and the clearance index of theophylline was 0.81 +/- 0.07. Although human and rabbit placentas are structurally dissimilar, the rabbit placenta perfused in situ appears to be a useful preparation for measuring the transfer processes and the related and governing factors, of different compounds.