Prospective allometric scaling: does the emperor have clothes?

Extrapolation of praziquantel pharmacokinetics to a pediatric population: a cautionary tale

L-praziquantel (PZQ) pharmacokinetic data were analyzed from two relative bioavailability Phase 1 studies in adult, healthy subjects with two new oral dispersion tablet (ODT) formulations of L-PZQ administered under various combinations of co-administration with food, water, and/or crushing. Linear mixed effects models adequately characterized the noncompartmental estimates of the pharmacokinetic profiles in both studies. Dose, food, and formulation were found to significantly affect L-PZQ exposure in both studies. The model for AUC was then extrapolated to children 2–5 years old accounting for enzyme maturation and weight. The predicted exposures were compared to an external Phase 1 study conducted by the Swiss Tropical and Public Health Institute using a currently marketed formulation (Cesol 600 mg immediate-release tablets) and found to be substantially lower than observed. A root cause analysis was completed to identify the reason for failure of the models. Various scenarios were proposed and tested. Two possible reasons for the failure were identified. One reason was that the model did not account for the reduced hepatic clearance seen in patients compared to the healthy volunteer population used to build the model. The second possible reason was that PZQ absorption appears sensitive to meal composition and the model did not account for differences in meals between a standardized Phase 1 unit and clinical sites in Africa. Further studies are needed to confirm our hypotheses.

Interspecies allometric scaling of antimalarial drugs and potential application to pediatric dosing

Pharmacopeial recommendations for administration of antimalarial drugs are the same weight-based (mg/kg of body weight) doses for children and adults. However, linear calculations are known to underestimate pediatric doses; therefore, interspecies allometric scaling data may have a role in predicting doses in children. We investigated the allometric scaling relationships of antimalarial drugs using data from pharmacokinetic studies in mammalian species. Simple allometry (Y = a × W(b)) was utilized and compared to maximum life span potential (MLP) correction. All drugs showed a strong correlation with clearance (CL) in healthy controls. Insufficient data from malaria-infected species other than humans were available for allometric scaling. The allometric exponents (b) for CL of artesunate, dihydroartemisinin (from intravenous artesunate), artemether, artemisinin, clindamycin, piperaquine, mefloquine, and quinine were 0.71, 0.85, 0.66, 0.83, 0.62, 0.96, 0.52, and 0.40, respectively. Clearance was significantly lower in malaria infection than in healthy (adult) humans for quinine (0.07 versus 0.17 liter/h/kg; P = 0.0002) and dihydroartemisinin (0.81 versus 1.11 liters/h/kg; P = 0.04; power = 0.6). Interpolation of simple allometry provided better estimates of CL for children than MLP correction, which generally underestimated CL values. Pediatric dose calculations based on simple allometric exponents were 10 to 70% higher than pharmacopeial (mg/kg) recommendations. Interpolation of interspecies allometric scaling could provide better estimates than linear scaling of adult to pediatric doses of antimalarial drugs; however, the use of a fixed exponent for CL was not supported in the present study. The variability in allometric exponents for antimalarial drugs also has implications for scaling of fixed-dose combinations.

Learning and confirming with preclinical studies: modeling and simulation in the discovery of GDC-0917, an inhibitor of apoptosis proteins antagonist

The application of modeling and simulation techniques is increasingly common in the preclinical stages of the drug development process. GDC-0917 [(S)-1-((S)-2-cyclohexyl-2-((S)-2-(methylamino)propanamido)acetyl)-N-(2-(oxazol-2-yl)-4-phenylthiazol-5-yl)pyrrolidine-2-carboxamide] is a potent second-generation antagonist of inhibitor of apoptosis (IAP) proteins that is being developed for the treatment of various cancers. GDC-0917 has low to moderate clearance in the mouse (12.0 ml/min/kg), rat (27.0 ml/min/kg), and dog (15.3 ml/min/kg), and high clearance in the monkey (67.6 ml/min/kg). Accordingly, oral bioavailability was lowest in monkeys compared with other species. Based on our experience with a prototype molecule with similar structure, in vitro-in vivo extrapolation was used to predict a moderate clearance (11.5 ml/min/kg) in humans. The predicted human volume of distribution was estimated using simple allometry at 6.69 l/kg. Translational pharmacokinetic-pharmacodynamic (PK-PD) analysis using results from MDA-MB-231-X1.1 breast cancer xenograft studies and predicted human pharmacokinetics suggests that ED50 and ED90 targets can be achieved in humans using acceptable doses (72 mg and 660 mg, respectively) and under an acceptable time frame. The relationship between GDC-0917 concentrations and pharmacodynamic response (cIAP1 degradation) was characterized using an in vitro peripheral blood mononuclear cell immunoassay. Simulations of human GDC-0917 plasma concentration-time profile and cIAP1 degradation at the 5-mg starting dose in the phase 1 clinical trial agreed well with observations. This work shows the importance of leveraging information from prototype molecules and illustrates how modeling and simulation can be used to add value to preclinical studies in the early stages of the drug development process.

Significant species difference in amide hydrolysis of GDC-0834, a novel potent and selective Bruton's tyrosine kinase inhibitor

(R)-N-(3-(6-(4-(1,4-dimethyl-3-oxopiperazin-2-yl)phenylamino)-4-methyl-5-oxo-4,5-dihydropyrazin-2-yl)-2-methylphenyl)-4,5,6,7-tetrahydrobenzo[b]thiophene-2-carboxamide (GDC-0834) is a potent and selective inhibitor of Bruton's tyrosine kinase (BTK), investigated as a potential treatment for rheumatoid arthritis. In vitro metabolite identification studies in hepatocytes revealed predominant formation of an inactive metabolite (M1) via amide hydrolysis in human. The formation of M1 appeared to be NADPH-independent in human liver microsomes. M1 was found in only minor to moderate quantities in plasma from preclinical species dosed with GDC-0834. Human clearance predictions using various methodologies resulted in estimates ranging from low to high. In addition, GDC-0834 exhibited low clearance in PXB chimeric mice with humanized liver. Uncertainty in human pharmacokinetic prediction and high interest in a BTK inhibitor for clinical evaluation prompted an investigational new drug strategy, in which GDC-0834 was rapidly advanced to a single-dose human clinical trial. GDC-0834 plasma concentrations in humans were below the limit of quantitation (<1 ng/ml) in most samples from the cohorts dosed orally at 35 and 105 mg. In contrast, substantial plasma concentrations of M1 were observed. In human plasma and urine, only M1 and its sequential metabolites were identified. The formation kinetics of M1 was evaluated in rat, dog, monkey, and human liver microsomes in the absence of NADPH. The maximum rate of M1 formation (V(max)) was substantially higher in human compared with that in other species. In contrast, the Michaelis-Menten constant (K(m)) was comparable among species. Intrinsic clearance (V(max)/K(m)) of GDC-0834 from M1 formation in human was 23- to 169-fold higher than observed in rat, dog, and monkey.

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

Methods for prediction of hepatic clearance (CLH) in man have been evaluated. A physiologically-based in-vitro to in-vivo (PB-IVIV) method with human unbound fraction in blood (fu,bl) and hepatocyte intrinsic clearance (CLint)-data has a good rationale and appears to give the best predictions (maximum ∼2-fold errors; &lt; 25% errors for half of CL-predictions; appropriate ranking). Inclusion of an empirical scaling factor is, however, needed, and reasons include the use of cryopreserved hepatocytes with low activity, and inappropriate CLint- and fu,bl-estimation methods. Thus, an improvement of this methodology is possible and required. Neglect of fu,bl or incorporation of incubation binding does not seem appropriate. When microsome CLint-data are used with this approach, the CLH is underpredicted by 5- to 9-fold on average, and a 106-fold underprediction (attrition potential) has been observed. The poor performance could probably be related to permeation, binding and low metabolic activity. Inclusion of scaling factors and neglect of fu,bl for basic and neutral compounds improve microsome predictions. The performance is, however, still not satisfactory. Allometry incorrectly assumes that the determinants for CLH relate to body weight and overpredicts human liver blood flow rate. Consequently, allometric methods have poor predictability. Simple allometry has an average overprediction potential, &gt; 2-fold errors for ∼1/3 of predictions, and 140-fold underprediction to 5800-fold overprediction (potential safety risk) range. In-silico methodologies are available, but these need further development. Acceptable prediction errors for compounds with low and high CLH should be ∼50 and ∼10%, respectively. In conclusion, it is recommended that PB-IVIV with human hepatocyte CLint and fu,bl is applied and improved, limits for acceptable errors are decreased, and that animal CLH-studies and allometry are avoided.

Scaling pharmacodynamics from in vitro and preclinical animal studies to humans

An important feature of mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) models is the identification of drug- and system-specific factors that determine the intensity and time-course of pharmacological effects. This provides an opportunity to integrate information obtained from in vitro bioassays and preclinical pharmacological studies in animals to anticipate the clinical and adverse responses to drugs in humans. The fact that contemporary PK/PD modeling continues to evolve and seeks to emulate systems level properties should provide enhanced capabilities to scale-up pharmacodynamic data. Critical steps in drug discovery and development, such as lead compound and first in human dose selection, may become more efficient with the implementation and further refinement of translational PK/PD modeling. In this review, we highlight fundamental principles in pharmacodynamics and the basic expectations for in vitro bioassays and traditional allometric scaling in PK/PD modeling. Discussion of PK/PD modeling efforts for recombinant human erythropoietin is also included as a case study showing the potential for advanced systems analysis to facilitate extrapolations and improve understanding of inter-species differences in drug responses.

To scale or not to scale: the principles of dose extrapolation

The principles of inter-species dose extrapolation are poorly understood and applied. We provide an overview of the principles underlying dose scaling for size and dose adjustment for size-independent differences. Scaling of a dose is required in three main situations: the anticipation of first-in-human doses for clinical trials, dose extrapolation in veterinary practice and dose extrapolation for experimental purposes. Each of these situations is discussed. Allometric scaling of drug doses is commonly used for practical reasons, but can be more accurate when one takes into account species differences in pharmacokinetic parameters (clearance, volume of distribution). Simple scaling of drug doses can be misleading for some drugs; correction for protein binding, physicochemical properties of the drug or species differences in physiological time can improve scaling. However, differences in drug transport and metabolism, and in the dose-response relationship, can override the effect of size alone. For this reason, a range of modelling approaches have been developed, which combine in silico simulations with data obtained in vitro and/or in vivo. Drugs that are unlikely to be amenable to simple allometric scaling of their clearance or dose include drugs that are highly protein-bound, drugs that undergo extensive metabolism and active transport, drugs that undergo significant biliary excretion (MW > 500, ampiphilic, conjugated), drugs whose targets are subject to inter-species differences in expression, affinity and distribution and drugs that undergo extensive renal secretion. In addition to inter-species dose extrapolation, we provide an overview of dose extrapolation within species, discussing drug dosing in paediatrics and in the elderly.

Development of translational pharmacokinetic-pharmacodynamic models

Contemporary models in the field of pharmacokinetic-pharmacodynamic (PK-PD) modeling often incorporate the fundamental principles of capacity limitation and operation of turnover processes to describe the time course of pharmacological effects in mechanistic terms. This permits the identification of drug- and system-specific factors that govern drug responses. There is considerable interest in utilizing mechanism-based PK-PD models in translational pharmacology, whereby in silico, in vitro, and preclinical data may be effectively coupled with relevant models to streamline the discovery and development of new therapeutic agents. These translational PK-PD models form the subject of this review.

Pharmacokinetic prediction for intravenous β‐lactam antibiotics in pediatric patients

A method for predicting pharmacokinetics in pediatric patients for intravenous beta-lactam antibiotics is proposed. We focused on the allometric relationships of pharmacokinetic parameters with individual body weights (BW) in human including healthy adults and pediatric patients. Drug concentration data for 15 intravenous beta-lactam antibiotics were collected retrospectively from the published articles and the individual pharmacokinetic parameters were re-calculated. A mixed effect modeling (MEM) was applied for the allometric relationship for those beta-lactam antibiotics, and mean and variances of inter-drug variability for the allometric parameters and also variance for intra-drug (residual) variability were estimated. Then drug-specific allometric parameters were estimated by an empirical Bayesian method using the pharmacokinetic parameters for a drug only in healthy adults as observations, and finally the individual pharmacokinetic parameters in pediatric patients were predicted. The predictability of the method was evaluated by the leave-one-out method. We also demonstrated a method for simulating plasma concentration-time profiles in pediatric patients, and the predicted time-course curves generally coincided well with the actual plasma concentration data for the tested drugs.

Zotarolimus, a novel sirolimus analogue with potent anti-proliferative activity on coronary smooth muscle cells and reduced potential for systemic immunosuppression

Sirolimus (rapamycin) is an immunosuppressant used in preventing allograft rejection and in drug-eluting stents to prevent restenosis after angioplasty. Zotarolimus, an analogue of sirolimus, was designed to have a shorter in vivo half-life. Zotarolimus was found to be mechanistically similar to sirolimus in having high-affinity binding to the immunophilin FKBP12 and comparable potency for inhibiting in vitro proliferation of both human and rat T cells. Rat pharmacokinetic studies with intravenous dosing demonstrated terminal elimination half-lives of 9.4 hours and 14.0 hours for zotarolimus and sirolimus, respectively. Given orally, T1/2 values were 7.9 hours and 33.4 hours, respectively. Consistent with its shorter duration, zotarolimus showed a corresponding and statistically significant 4-fold reduction in potency for systemic immunosuppression in 3 rat disease models. Pharmacokinetic studies in cynomolgus monkey underpredicted the half-life difference between zotarolimus and sirolimus apparent from recent clinical data. In vitro inhibition of human coronary artery smooth muscle cell proliferation by zotarolimus was comparable to sirolimus. Drug-eluting stents for local delivery of zotarolimus to the vessel wall of coronary arteries are in clinical development. The pharmacological profile of zotarolimus suggests it may be advantageous for preventing restenosis with a reduced potential for causing systemic immunosuppression or other side effects.

Quantitative structure-pharmacokinetic/pharmacodynamic relationships

Quantitative structure-activity relationships have long been considered a vital component of drug discovery and development, providing insight into the role of molecular properties in the biological activity of similar and unrelated compounds. Recognition that in vitro bioassay and/or pre-clinical activity are insufficient for anticipating which compounds are suitable leads for further development has shifted the focus toward integrated pharmacokinetic (PK) and pharmacodynamic (PD) processes. Over the last decade, considerable progress has been made in constructing empirical and mechanistic quantitative structure-PK relationships (QSPKR), as well as diverse mechanism-based pharmacodynamic models of drug effects. In this review, traditional and contemporary approaches to developing QSPKR models are discussed, along with selected examples of attempts to couple QSPKR and pharmacodynamic models to anticipate the intensity and time-course of the pharmacological effects of new or related compounds, or quantitative structure-pharmacodynamic relationships modeling. Such models are in accordance with the goals of systems biology and the ideal of designing drugs and delivery systems from first principles.

Enzymes involved in the bioconversion of ester-based prodrugs

Enzymes are essential for the activation of many prodrugs. In this review, the most important enzymes (e.g., paraoxonase, carboxylesterase, acetylcholinesterase, cholinesterase) involved in the bioconversion of ester-based prodrugs will be discussed in terms of their biology and biochemistry. Most of these enzymes fall into the category of hydrolytic enzymes. However, nonhydrolytic enzymes, including cytochrome P450s, can also catalyze the bioconversion of ester prodrugs and thus will be discussed here. Other factors influencing the ability of these enzymes to catalyze the bioconversion of ester-based prodrugs, particularly species and interindividual differences and stereochemical and structural features of the prodrugs, will be discussed.

A Novel Strategy for Physiologically Based Predictions of Human Pharmacokinetics

BACKGROUND

The major aim of this study was to develop a strategy for predicting human pharmacokinetics using physiologically based pharmacokinetic (PBPK) modelling. This was compared with allometry (of plasma concentration-time profiles using the Dedrick approach), in order to determine the best approaches and strategies for the prediction of human pharmacokinetics.

METHODS

PBPK and Dedrick predictions were made for 19 F. Hoffmann-La Roche compounds. A strategy for the prediction of human pharmacokinetics using PBPK modelling was proposed in this study. Predicted values (pharmacokinetic parameters, plasma concentrations) were compared with observed values obtained after intravenous and oral administration in order to assess the accuracy of the prediction methods.

RESULTS

By following the proposed strategy for PBPK, a prediction would have been made prospectively for approximately 70% of the compounds. The prediction accuracy for these compounds in terms of the percentage of compounds with an average-fold error of <2-fold was 83%, 50%, 75%, 67%, 92% and 100% for apparent oral clearance (CL/F), apparent volume of distribution during terminal phase after oral administration (V(z)/F), terminal elimination half-life (t(1/2)), peak plasma concentration (C(max)), area under the plasma concentration-time curve (AUC) and time to reach C(max) (t(max)), respectively. For the other 30% compounds, unacceptable prediction accuracy was obtained in animals; therefore, a prospective prediction of human pharmacokinetics would not have been made using PBPK. For these compounds, prediction accuracy was also poor using the Dedrick approach. In the majority of cases, PBPK gave more accurate predictions of pharmacokinetic parameters and plasma concentration-time profiles than the Dedrick approach.

CONCLUSIONS

Based on the dataset evaluated in this study, PBPK gave reasonable predictions of human pharmacokinetics using preclinical data and is the recommended approach in the majority of cases. In addition, PBPK modelling is a useful tool to gain insights into the properties of a compound. Thus, PBPK can guide experimental efforts to obtain the relevant information necessary to understand the compound's properties before entry into human, ultimately resulting in a higher level of prediction accuracy.

Recent advances in pharmacokinetic extrapolation from preclinical data to humans

The early characterisation of drug metabolism and pharmacokinetic (DMPK) properties of new chemical entities plays a key role in the pharmaceutical industry's effort to reduce attrition. Specifically, a major goal of early DMPK studies is to accurately predict the behaviour of new chemical entities in humans, thus allowing likely failures to be terminated rapidly and resource to be placed on molecules most likely to succeed. The present review summarises progress over the past several years in the key technologies used in the pharmaceutical industry to achieve these goals: namely, in vivo, in vitro and in silico/computational tools. The limitations of the various assays are discussed, with attention also given to likely future directions in this field.

Population pharmacokinetic studies in pediatrics: issues in design and analysis

The current review addresses the following 3 frequently encountered challenges in the design and analysis of population pharmacokinetic studies in pediatrics: (1) body size adjustments during the development of pharmacostatistical models, (2) design and validation of limited sampling strategies, and (3) the integration of historical priors in data analysis and trial simulation. Size adjustments with empiric approaches based on body weight or body surface area have frequently proven as a pragmatic tool to overcome large size differences in a pediatric study population. Allometric size adjustments, however, provide a more mechanistic, physiologically based approach that, if used a priori, allows delineation of the effect of size from that of other covariates that show a high degree of collinearity. The frequent lack of dense data sets in pediatric clinical pharmacology because of ethical and logistic constraints in study design can be overcome with the application of D-optimality-based limited sampling schemes in combination with Bayesian and nonlinear mixed-effects modeling approaches. Empirically based dose selection and clinical trial designs for pediatric clinical pharmacology studies can be improved by applying clinical trial simulation techniques, especially if they integrate adult and pediatric in vitro and/or in vivo data as historic priors. Although integration of these concepts and techniques in population pharmacokinetic analyses is not only limited to pediatric research, their application allows researchers to overcome some major hurdles frequently encountered in pharmacokinetic studies in pediatrics and, thus, provides the basis for additional clinical pharmacology research in this previously insufficiently studied fraction of the general population.

Application of simple mathematical expressions to relate the half-lives of xenobiotics in rats to values in humans

INTRODUCTION

Previous publications from GlaxoSmithKline and University of Toledo laboratories convey our independent attempts to predict the half-lives of xenobiotics in humans using data obtained from rats. The present investigation was conducted to compare the performance of our published models against a common dataset obtained by merging the two sets of rat versus human half-life (hHL) data previously used by each laboratory.

METHODS

After combining data, mathematical analyses were undertaken by deploying both of our previous models, namely the use of an empirical algorithm based on a best-fit model and the use of rat-to-human liver blood flow ratios as a half-life correction factor. Both qualitative and quantitative analyses were performed, as well as evaluation of the impact of molecular properties on predictability.

RESULTS

The merged dataset was remarkably diverse with respect to physiochemical and pharmacokinetic (PK) properties. Application of both models revealed similar predictability, depending upon the measure of stipulated accuracy. Certain molecular features, particularly rotatable bond count and pK(a), appeared to influence the accuracy of prediction.

DISCUSSION

This collaborative effort has resulted in an improved understanding and appreciation of the value of rats to serve as a surrogate for the prediction of xenobiotic half-lives in humans when clinical pharmacokinetic studies are not possible or practicable.

A comprehensive analysis of the role of correction factors in the allometric predictivity of clearance from rat, dog, and monkey to humans

This study was conducted to comprehensively evaluate the performance of various allometric scaling methods for the prediction of human clearance. Allometric scaling was used to predict clearance for 103 compounds, for which clearance data in the rat, dog, monkey, and humans were available. Allometry was performed using all three preclinical species and with combinations of any two species. The methods employed included standard allometry and various correction factors, including brain weight, maximum lifespan potential, and glomerular filtration. Scaling was performed on all compounds universally and on segregated subsets based on allometric exponent, clearance, physicochemical property, or route of elimination. 776 allometric combinations with 27,313 individual outcomes were performed. A predicted-to-observed clearance ratio of 0.5 to twofold was preselected as the criterion for predictive success. The success rate of allometric scaling ranged from 18 to 53%; none of the correction factors resulted in substantially improved predictivity. Furthermore, none of the methods attempted in this study achieved a success rate greater than that observed by simply estimating human clearance based on monkey hepatic extraction. Prospective allometric scaling, with or without correction factors, represents a suboptimal technique for estimating human clearance based on in vivo preclinical data.

A COMPREHENSIVE QUANTITATIVE AND QUALITATIVE EVALUATION OF EXTRAPOLATION OF INTRAVENOUS PHARMACOKINETIC PARAMETERS FROM RAT, DOG, AND MONKEY TO HUMANS. I. CLEARANCE

This study was conducted to comprehensively survey the available literature on intravenous pharmacokinetic parameters in the rat, dog, monkey, and human, and to compare common methods for extrapolation of clearance, to identify the most appropriate species to use in pharmacokinetic lead optimization, and to ascertain whether adequate prospective measures of predictive success are currently available. One hundred three nonpeptide xenobiotics were identified with intravenous pharmacokinetic data in rat, dog, monkey, and human; both body weight- and hepatic blood flow-based methods were used for scaling of clearance. Allometric scaling approaches, particularly those using data from only two of the preclinical species, were less successful at predicting human clearance than methods based on clearance as a set fraction of liver blood flow from an individual species. Furthermore, commonly used prospective measures of allometric scaling success, including correlation coefficient and allometric exponent, failed to discriminate between successful and failed allometric predictions. In all instances, the monkey tended to provide the most qualitatively and quantitatively accurate predictions of human clearance and also afforded the least biased predictions compared with other species. Additionally, the availability of data from both common nonrodent species (dog and monkey) did not ensure enhanced predictive quality compared with having only monkey data. The observations in this investigation have major implications for pharmacokinetic lead optimization and for prediction of human clearance from in vivo preclinical data and support the continued use of nonhuman primates in preclinical pharmacokinetics.

Drug disposition viewed in terms of the fractal volume of distribution

PURPOSE

(i) Evaluate the predictive performance of the fractal volume of drug distribution, v(f), (Pharm. Res.18, 1056, 2001), (ii) develop the concept of the fractal clearance, CLf, which is the clearance analogue of v(f), (iii) examine the utility of CLf in allometric studies, (iv) develop allometric relationships for the elimination half-life, t1/2, and (v) evaluate the use of v(f) and CLf in predicting the volume of drug distribution, Vap, clearance, CL, and elimination half-life, t1/2.

METHODS

Estimates for v(f) of various drugs were obtained and correlated with body mass using data only from animal species. A comparison was made between the predicted and actual v(f) values for humans. For a variety of animal species CLf values were estimated from the equation: [equation: see text]. The allometric equations developed using CLf were compared with other allometric approaches. Allometric equations were also developed for t1/2 utilizing the allometric relationships of v(f) and CLf,

RESULTS

The predicted estimates of v(f) were very close to the actual values and the correlation exhibited favorable statistical properties. The values of the allometric exponents for CLf were found to be close to 0.75. The predictive performance for CL using the allometric equations for CLf in conjunction with the rule of exponents was found to be better than the currently considered most accurate allometric approaches. The values of the allometric exponents for t1/2 were found to be close to 0.25

CONCLUSION

The predictive ability of v(f) is high; predictions for Vap based on v(f) values are better than the current approaches. CLf expressed a good behavior both in prospective and retrospective analysis. The allometric exponents, 0.75, 0.25 for CLf and t1/2, respectively, agree with the theoretical expected values.

Veterinary drug delivery: potential for skin penetration enhancement

A range of topical products are used in veterinary medicine. The efficacy of many of these products has been enhanced by the addition of penetration enhancers. Evolution has led to not only a highly specialized skin in animals and humans, but also one whose anatomical structure and skin permeability differ between the various species. The skin provides an excellent barrier against the ingress of environmental contaminants, toxins, and microorganisms while performing a homeostatic role to permit terrestrial life. Over the past few years, major advances have been made in the field of transdermal drug delivery. An increasing number of drugs are being added to the list of therapeutic agents that can be delivered via the skin to the systemic circulation where clinically effective concentrations are reached. The therapeutic benefits of topically applied veterinary products is achieved in spite of the inherent protective functions of the stratum corneum (SC), one of which is to exclude foreign substances from entering the body. Much of the recent success in this field is attributable to the rapidly expanding knowledge of the SC barrier structure and function. The bilayer domains of the intercellular lipid matrices within the SC form an excellent penetration barrier, which must be breached if poorly penetrating drugs are to be administered at an appropriate rate. One generalized approach to overcoming the barrier properties of the skin for drugs and biomolecules is the incorporation of suitable vehicles or other chemical compounds into a transdermal delivery system. Indeed, the incorporation of such compounds has become more prevalent and is a growing trend in transdermal drug delivery. Substances that help promote drug diffusion through the SC and epidermis are referred to as penetration enhancers, accelerants, adjuvants, or sorption promoters. It is interesting to note that many pour-on and spot-on formulations used in veterinary medicine contain inert ingredients (e.g., alcohols, amides, ethers, glycols, and hydrocarbon oils) that will act as penetration enhancers. These substances have the potential to reduce the capacity for drug binding and interact with some components of the skin, thereby improving drug transport. However, their inclusion in veterinary products with a high-absorbed dose may result in adverse dermatological reactions (e.g., toxicological irritations) and concerns about tissue residues. These are important considerations when formulating a veterinary transdermal product when such compounds are added, either intentionally or otherwise, for their penetration enhancement ability.

Fractal volume of drug distribution: it scales proportionally to body mass

PURPOSE

To develop the physiologically sound concept of fractal volume of drug distribution, vf, and evaluate its utility and applicability in interspecies pharmacokinetic scaling.

METHODS

Estimates for vf of various drugs in different species were obtained from the relationship: vf = (v - Vpl)(Vap - Vpl)/V + Vpl where v is the total volume of the species (equivalent to its total mass assuming a uniform density Ig/mL), Vpl is the plasma volume of the species and Vap is the conventional volume of drug distribution. This equation was also used to calculate the fractal analogs of various volume terms of drug distribution (the volume of central compartment, Vc, the steady state volume of distribution, Vss, and the volume of distribution following pseudodistribution equilibrium, Vz). The calculated fractal volumes of drug distribution were correlated with body mass of different mammalian species and allometric exponents and coefficients were determined.

RESULTS

The calculated values of vf for selected drugs in humans provided meaningful and physiologically sound estimates for the distribution of drugs in the human body. For all fractal volume terms utilized, the allometric exponents were found to be either one or close to unity. The estimates of the allometric coefficients were found to be in the interval (0,1). These decimal values correspond to a fixed fraction of the fractal volume term relative to body mass in each one of the species.

CONCLUSIONS

Fractal volumes of drug distribution scale proportionally to mass. This confirms the theoretically expected relationship between volume and mass in mammalian species.

Computer-assisted drug development (CADD): an emerging technology for designing first-time-in-man and proof-of-concept studies from preclinical experiments

Computer-assisted drug development (CADD) is an emerging technology for accelerating drug development based on the integration of mathematical modelling and simulation. This methodology provides a knowledge-based decisional tool on alternative development strategies based on the evaluation of potential risks on drug safety, and the definition of experimental design of new trials with expected power and probability of success. An example of CADD implementation is presented to design the first-time-in-man (FTIM) and the proof-of-concept (PoC) study of a new CNS compound. The final objective of the example presented is not necessarily to supply a success story of a correct prediction of human data from animal studies but to define a credible strategy suitable to design FTIM and PoC studies using preclinical data without the support of any human in vivo information. Rhesus monkey and human PK were initially estimated using allometric scaling on data collected in dogs, cynomolgus monkeys and rats. A PK/PD model was derived from a study conducted in rodent and validated by comparing the model predicted response to the one observed in a PET experiment conducted in rhesus monkey. The final PK/PD model, incorporating potential variability and uncertainty on scaled human prediction together with a receptor affinity adjustment derived from in vitro binding studies, was used to design the first-time-in-man and the proof-of-concept study.