Interspecies scaling of clearance and volume of distribution for horse antivenom F(ab')2

Developing a computational pharmacokinetic model of systemic snakebite envenomation and antivenom treatment

Snakebite envenomation is responsible for over 100,000 deaths and 400,000 cases of disability annually, most of which are preventable through access to safe and effective antivenoms. Snake venom toxins span a wide molecular weight range, influencing their absorption, distribution, and elimination within the body. In recent years, a range of scaffolds have been applied to antivenom development. These scaffolds similarly span a wide molecular weight range and subsequently display diverse pharmacokinetic behaviours. Computational simulations represent a powerful tool to explore the interplay between these varied antivenom scaffolds and venoms, to assess whether a pharmacokinetically optimal antivenom exists. The purpose of this study was to establish a computational model of systemic snakebite envenomation and treatment, for the quantitative assessment and comparison of conventional and next-generation antivenoms. A two-compartment mathematical model of envenomation and treatment was defined and the system was parameterised using existing data from rabbits. Elimination and biodistribution parameters were regressed against molecular weight to predict the dynamics of IgG, F(ab')₂, Fab, scFv, and nanobody antivenoms, spanning a size range of 15-150 kDa. As a case study, intramuscular envenomation by Naja sumatrana (equatorial spitting cobra) and its treatment using Fab, F(ab')₂, and IgG antivenoms was simulated. Variable venom dose tests were applied to visualise effective antivenom dose levels. Comparisons to existing antivenoms and experimental rescue studies highlight the large dose reductions that could result from recombinant antivenom use. This study represents the first comparative in silico model of snakebite envenomation and treatment.

Developing Small Molecule Therapeutics for the Initial and Adjunctive Treatment of Snakebite

The World Health Organization (WHO) recently added snakebite envenoming to the priority list of Neglected Tropical Diseases (NTD). It is thought that ~75% of mortality following snakebite occurs outside the hospital setting, making the temporal gap between a bite and antivenom administration a major therapeutic challenge. Small molecule therapeutics (SMTs) have been proposed as potential prereferral treatments for snakebite to help address this gap. Herein, we discuss the characteristics, potential uses, and development of SMTs as potential treatments for snakebite envenomation. We focus on SMTs that are secretory phospholipase A2 (sPLA2) inhibitors with brief exploration of other potential drug targets on venom molecules.

Snakebite: When the Human Touch Becomes a Bad Touch

Many issues and complications in treating snakebite are a result of poor human social, economic and clinical intervention and management. As such, there is scope for significant improvements for reducing incidence and increasing patient outcomes. Snakes do not target humans as prey, but as our dwellings and farms expand ever farther and climate change increases snake activity periods, accidental encounters with snakes seeking water and prey increase drastically. Despite its long history, the snakebite crisis is neglected, ignored, underestimated and fundamentally misunderstood. Tens of thousands of lives are lost to snakebites each year and hundreds of thousands of people will survive with some form of permanent damage and reduced work capacity. These numbers are well recognized as being gross underestimations due to poor to non-existent record keeping in some of the most affected areas. These underestimations complicate achieving the proper recognition of snakebite’s socioeconomic impact and thus securing foreign aid to help alleviate this global crisis. Antivenoms are expensive and hospitals are few and far between, leaving people to seek help from traditional healers or use other forms of ineffective treatment. In some cases, cheaper, inappropriately manufactured antivenom from other regions is used despite no evidence for their efficacy, with often robust data demonstrating they are woefully ineffective in neutralizing many venoms for which they are marketed for. Inappropriate first-aid and treatments include cutting the wound, tourniquets, electrical shock, immersion in ice water, and use of ineffective herbal remedies by traditional healers. Even in the developed world, there are fundamental controversies including fasciotomy, pressure bandages, antivenom dosage, premedication such as adrenalin, and lack of antivenom for exotic snakebites in the pet trade. This review explores the myriad of human-origin factors that influence the trajectory of global snakebite causes and treatment failures and illustrate that snakebite is as much a sociological and economic problem as it is a medical one. Reducing the incidence and frequency of such controllable factors are therefore realistic targets to help alleviate the global snakebite burden as incremental improvements across several areas will have a strong cumulative effect.

Scale-up of a physiologically-based pharmacokinetic model to predict the disposition of monoclonal antibodies in monkeys

Preclinical assessment of monoclonal antibody (mAb) disposition during drug development often includes investigations in non-human primate models. In many cases, mAb exhibit non-linear disposition that relates to mAb-target binding [i.e., target-mediated disposition (TMD)]. The goal of this work was to develop a physiologically-based pharmacokinetic (PBPK) model to predict non-linear mAb disposition in plasma and in tissues in monkeys. Physiological parameters for monkeys were collected from several sources, and plasma data for several mAbs associated with linear pharmacokinetics were digitized from prior literature reports. The digitized data displayed great variability; therefore, parameters describing inter-antibody variability in the rates of pinocytosis and convection were estimated. For prediction of the disposition of individual antibodies, we incorporated tissue concentrations of target proteins, where concentrations were estimated based on categorical immunohistochemistry scores, and with assumed localization of target within the interstitial space of each organ. Kinetics of target-mAb binding and target turnover, in the presence or absence of mAb, were implemented. The model was then employed to predict concentration versus time data, via Monte Carlo simulation, for two mAb that have been shown to exhibit TMD (2F8 and tocilizumab). Model predictions, performed a priori with no parameter fitting, were found to provide good prediction of dose-dependencies in plasma clearance, the areas under plasma concentration versu time curves, and the time-course of plasma concentration data. This PBPK model may find utility in predicting plasma and tissue concentration versus time data and, potentially, the time-course of receptor occupancy (i.e., mAb-target binding) to support the design and interpretation of preclinical pharmacokinetic-pharmacodynamic investigations in non-human primates.

The Effect of a Polyvalent Antivenom on the Serum Venom Antigen Levels of Naja sputatrix (Javan Spitting Cobra) Venom in Experimentally Envenomed Rabbits

The treatment protocol of antivenom in snake envenomation remains largely empirical, partly due to the insufficient knowledge of the pharmacokinetics of snake venoms and the effects of antivenoms on the blood venom levels in victims. In this study, we investigated the effect of a polyvalent antivenom on the serum venom antigen levels of Naja sputatrix (Javan spitting cobra) venom in experimentally envenomed rabbits. Intravenous infusion of 4 ml of Neuro Polyvalent Snake Antivenom [NPAV, F(ab')2 ] at 1 hr after envenomation caused a sharp decline of the serum venom antigen levels, followed by transient resurgence an hour later. The venom antigen resurgence was unlikely to be due to the mismatch of pharmacokinetics between the F(ab')2 and venom antigens, as the terminal half-life and volume of distribution of the F(ab')2 in serum were comparable to that of venom antigens (p > 0.05). Infusion of an additional 2 ml of NPAV was able to prevent resurgence of the serum venom antigen level, resulting in a substantial decrease (67.1%) of the total amount of circulating venom antigens over time course of envenomation. Our results showed that the neutralization potency of NPAV determined by neutralization assay in mice may not be an adequate indicator of its capability to modulate venom kinetics in relation to its in vivo efficacy to neutralize venom toxicity. The findings also support the recommendation of giving high initial dose of NPAV in cobra envenomation, with repeated doses as clinically indicated in the presence of rebound antigenemia and symptom recurrence.

Safety, potential efficacy, and pharmacokinetics of specific polyclonal immunoglobulin F(ab')₂ fragments against avian influenza A (H5N1) in healthy volunteers: a single-centre, randomised, double-blind, placebo-controlled, phase 1 study

BACKGROUND

Human infection with the avian influenza A H5N1 virus results in disease with a high fatality rate, against which antiviral treatments have limited efficacy. We aimed to investigate the safety, pharmacokinetics, and therapeutic potential of specific polyclonal immunoglobulin equine F(ab')₂ fragments raised against influenza A/Vietnam/1194/2004 virus (H5N1 subtype) in healthy volunteers.

METHODS

We did a randomised, double-blind, placebo-controlled, single-centre phase 1 study. In stage 1 (one infusion) and stage 2 (five infusions) of the trial, we randomly assigned healthy male volunteers to receive once-daily intravenous infusions of 0·85 U/kg body weight of F(ab')₂ or once-daily saline placebo. Randomisation was done centrally, with one block of four patients and one block for substitutes (three actives, one placebo) in stage 1, and two blocks of six patients (five actives and one placebo) and the same block for substitutes in stage 2. The primary objective was assessment of the clinical and laboratory safety of F(ab')₂, which was monitored for 22 days in the group that received one dose (assessments on days 0-2, 4, 8, 15, and 22) and 33 days in the group that received five doses (days 0-6, 8, 10, 12, 19, 26, and 33). A final post-study safety assessment was done at 120 days. We also assessed pharmacokinetic outcomes, and assayed haemagglutination and seroneutralisation activity. Analysis was done according to intention-to-treat. This trial is registered with ClinicalTrials.gov, number NCT02295813.

FINDINGS

We enrolled 16 healthy Asian men between Sept 28 and Dec 28, 2012, and randomly assigned 13 to one or five doses of F(ab')₂ and three to placebo. F(ab')₂ was well tolerated, and no deaths or serious adverse events occurred. Three patients had mild adverse events (one each of blepharospasm, sinusitis, and pyrexia). The pyrexia (38°C) was regarded as probably related to the infusion, and resolved after 37 min. Our laboratory assessments of blood and urine samples and physical examinations of heart rate, electrocardiogram readings, and weight showed no clinically significant safety issues. Mean peak plasma concentrations were 19·3 μg/mL (SD 3·5) with the one dose schedule and 23·0 μg/mL (4·5) with the five-dose schedule. F(ab')₂ were still detectable in plasma on average up to 5 days after five doses. Haemagglutination inhibition was only increased after the third dose, but in-vitro seroneutralisation activity was transiently increased after each of the five doses to concentrations regarded as clinically beneficial in infected patients.

INTERPRETATION

F(ab')₂ showed good safety, tolerability, and therapeutic potential for managing of H5N1 exposed patients.

FUNDING

Fab'entech.

IgY pharmacokinetics in rabbits: implications for IgY use as antivenoms

This paper presents the first study of chicken IgY pharmacokinetics (PK) in rabbits. We measured IgY blood serum concentrations using a specific high sensitivity ELISA method. The fast initial component observed when studying horse Fab, F(ab')2 or IgG was absent from IgY PK. During the first 80 min of observation there was only a single slow exponential decay, which sped up afterward to the point that IgY became undetectable after 216 h of observation; due to this time course, PK parameters were determined with trapezoidal integration. The most significant IgY pharmacokinetic parameters determined were (all presented as medians and their 95% confidence interval): Area Under the Curve = 183.8 (135.2, 221.5) mg·h·L(-1); Distribution volume of the central compartment·[Body Weight (BW)](-1) = 46.0 (21.7, 70.3) mL·kg(-1); Distribution volume in steady state·BW(-1) = 56.8 (44.4, 68.5) mLkg(-1); Mean Residence Time = 40.1 (33.6, 48.5) h; Total plasma clearance·BW(-1) = 1.44 (1.15, 1.66) mL·h(-1)·kg(-1). Anti IgY IgG titers determined by ELISA increased steadily after 72 h, and reached 2560 (1920, 5760) dilution(-1) at 264 h; anti-chicken IgG concentrations rose up to 3.19 (2.31, 6.17) μg/mL in 264 h. Our results show that IgY PK lacks the fast initial decay observed in other PK studies using horse IgG, F(ab')2 or Fab, remains in the body 39.0 (28.7, 47.2) % much as IgG and is ≈3 times more immunogenic that horse IgG in rabbits.

Specific polyclonal F(ab')2 neutralize a large panel of highly pathogenic avian influenza A viruses (H5N1) and control infection in mice

AIM

There is still no specific therapy for infection with the highly pathogenic avian influenza A virus (HPAI) H5N1, which caused 39 human cases with a 64% fatality rate in 2013.

MATERIALS & METHODS

We prepared highly purified specific equine polyclonal immunoglobulin fragments (F(ab')2) against H5N1 and tested them for efficacy in vitro and with different administration schedules in H5N1-challenged BALB/c mice.

RESULTS

in vitro, F(ab')2 neutralized 21 different H5N1 strains from different areas, representative of 11 different clades and sub-clades and 9 years of evolution of the virus. In vivo mouse experiments identified that the most efficient administration protocol consists of five consecutive daily injections after infection; 10 mg/kg giving a 60% increase in survival.

CONCLUSION

These data demonstrate the ability of anti-H5N1 F(ab')2 to markedly reduce the mortality and morbidity associated with infection of mice with HPAI H5N1 virus, and their potential for human therapy.

ADME of monoclonal antibody biotherapeutics: knowledge gaps and emerging tools

Absorption, distribution, metabolism and excretion (ADME) data are pivotal for small-molecule drug development, with well-developed in vitro and in vivo correlation tools and guidances from regulatory agencies. In the past two decades, monoclonal antibody (mAb) biotherapeutics have been successfully approved, including derived novel conjugates of active molecules (toxins or bioactive peptides) for specific target delivery or half-life extension. However, ADME information of mAb therapeutics lags behind that of small molecules due to the complex nature of the molecules and lack of appropriate tools to study drug exposure, biotransformation, and target engagement in the vascular and tissue spaces. In this perspective, the current knowledge gaps on ADME of mAb-related therapeutics are reviewed with potential solutions from emerging analytical technologies.

Pharmacokinetics, pharmacodynamics and physiologically-based pharmacokinetic modelling of monoclonal antibodies

Development of monoclonal antibodies (mAbs) and their functional derivatives represents a growing segment of the development pipeline in the pharmaceutical industry. More than 25 mAbs and derivatives have been approved for a variety of therapeutic applications. In addition, around 500 mAbs and derivatives are currently in different stages of development. mAbs are considered to be large molecule therapeutics (in general, they are 2-3 orders of magnitude larger than small chemical molecule therapeutics), but they are not just big chemicals. These compounds demonstrate much more complex pharmacokinetic and pharmacodynamic behaviour than small molecules. Because of their large size and relatively poor membrane permeability and instability in the conditions of the gastrointestinal tract, parenteral administration is the most usual route of administration. The rate and extent of mAb distribution is very slow and depends on extravasation in tissue, distribution within the particular tissue, and degradation. Elimination primarily happens via catabolism to peptides and amino acids. Although not definitive, work has been published to define the human tissues mainly involved in the elimination of mAbs, and it seems that many cells throughout the body are involved. mAbs can be targeted against many soluble or membrane-bound targets, thus these compounds may act by a variety of mechanisms to achieve their pharmacological effect. mAbs targeting soluble antigen generally exhibit linear elimination, whereas those targeting membrane-bound antigen often exhibit non-linear elimination, mainly due to target-mediated drug disposition (TMDD). The high-affinity interaction of mAbs and their derivatives with the pharmacological target can often result in non-linear pharmacokinetics. Because of species differences (particularly due to differences in target affinity and abundance) in the pharmacokinetics and pharmacodynamics of mAbs, pharmacokinetic/pharmacodynamic modelling of mAbs has been used routinely to expedite the development of mAbs and their derivatives and has been utilized to help in the selection of appropriate dose regimens. Although modelling approaches have helped to explain variability in both pharmacokinetic and pharmacodynamic properties of these drugs, there is a clear need for more complex models to improve understanding of pharmacokinetic processes and pharmacodynamic interactions of mAbs with the immune system. There are different approaches applied to physiologically based pharmacokinetic (PBPK) modelling of mAbs and important differences between the models developed. Some key additional features that need to be accounted for in PBPK models of mAbs are neonatal Fc receptor (FcRn; an important salvage mechanism for antibodies) binding, TMDD and lymph flow. Several models have been described incorporating some or all of these features and the use of PBPK models are expected to expand over the next few years.

Role of the animal model on the pharmacokinetics of equine-derived antivenoms

Antivenom pharmacokinetics has been studied in heterologous models in which the animal species used as immunoglobulin source is different from that used as recipient. In these models, after intravenous administration of antivenom, the plasma concentration of immunoglobulins shows a rapid initial declining-phase followed by a slower terminal-phase, which has been associated with antivenom distribution and elimination, respectively. We have compared pharmacokinetic parameters for equine-derived antivenom in homologous (horse) and heterologous (cow) models. It was found that the maximum concentration is lower in cows than in horses. Additionally, the steady-state distribution volume is higher in cows as compared to horses. On the other hand, models were not different in the time required to reach the maximum concentration, the area under the concentration/time curve, the half-life of decay during the slowest phase, the systemic clearance and the mean residence time. Similar results were obtained in a rabbit model, in which the pharmacokinetics was also affected by passive immunization of rabbits with anti-equine IgG. We conclude that, in addition to other physiological differences (e.g. cardiac frequency, plasmatic volume, glomerular filtration rate, etc.) between animal models, the ability to remove foreign immunoglobulins might influence the way in which the plasma concentration of antivenom decreases over time, thereby distorting the pharmacokinetic predictions based on non-compartmental models.

Interspecies scaling and prediction of human clearance: comparison of small- and macro-molecule drugs

Human clearance prediction for small- and macro-molecule drugs was evaluated and compared using various scaling methods and statistical analysis. Human clearance is generally well predicted using single or multiple species simple allometry for macro- and small-molecule drugs excreted renally. The prediction error is higher for hepatically eliminated small-molecules using single or multiple species simple allometry scaling, and it appears that the prediction error is mainly associated with drugs with low hepatic extraction ratio (Eh). The error in human clearance prediction for hepatically eliminated small-molecules was reduced using scaling methods with a correction of maximum life span (MLP) or brain weight (BRW). Human clearance of both small- and macro-molecule drugs is well predicted using the monkey liver blood flow method. Predictions using liver blood flow from other species did not work as well, especially for the small-molecule drugs.

Prediction of human clearance of therapeutic proteins: simple allometric scaling method revisited

In this report, the utility of a commonly used interspecies scaling method to predict the systemic clearance (CL) of therapeutic proteins in humans was evaluated. Based on analysis of a pharmacokinetic data set of 34 therapeutic proteins, including 12 monoclonal antibodies (mAbs) and Fc fusion proteins, human CL can generally be predicted reasonably well with simple allometric scaling and a fixed exponent of 0.8: approximately 95% of the cases predicted values within 2-fold of the observed values when using CL data from multiple species, or approximately 90% simply using CL from monkeys. Specific to mAbs/Fc fusion proteins, scaling from monkey CL using a fixed exponent of 0.8 gave an excellent prediction; all predicted CL values were within 2-fold of the corresponding observed values. Compared with the simple allometric scaling method that uses a fitted exponent from CL data of > or =3 preclinical species, the fixed exponent approach with 1-2 preclinical species is simple, resource-saving and minimizes systematic bias. Together with its overall satisfactory prediction accuracy, especially in the absence of non-linear pharmacokinetics and species-specific clearance mechanisms, this fixed exponent method affords a viable alternative to other published allometric methods, including the Rule of Exponents (ROE).

Pharmacokinetic allometric scaling of antibodies: application to the first-in-human dose estimation

The objectives of this study are: (i) to evaluate the predictive performance of interspecies scaling for antibodies to predict clearance, volume of distribution at steady state, and half-life in humans from animal data and (ii) to estimate first-in-human dose based on the predicted human clearance. Four methods were used to predict clearance in humans: (1) clearance versus body weight (simple allometry), (2) the product of clearance and maximum life-span potential (MLP) versus body weight, (3) the product of clearance and brain weight versus body weight, and (4) the application of a fixed exponent of 0.75. Based on the predicted human clearance, six methods were used to estimate the first-in-human dose. The predicted pharmacokinetic parameters and the estimated first-in-human dose of antibodies were compared with the observed human values. The results of the study indicated that the clearance of antibodies can be predicted with reasonable accuracy in humans and a good estimate of first human dose can be obtained from the predicted human clearance.

Hybrid in vivo FMT-CT imaging of protease activity in atherosclerosis with customized nanosensors

OBJECTIVE

Proteases are emerging biomarkers of inflammatory diseases. In atherosclerosis, these enzymes are often secreted by inflammatory macrophages, digest the extracellular matrix of the fibrous cap, and destabilize atheromata. Protease function can be monitored with protease activatable imaging probes and quantitated in vivo by fluorescence molecular tomography (FMT). To address 2 major constraints currently associated with imaging of murine atherosclerosis (lack of highly sensitive probes and absence of anatomic information), we compared protease sensors (PS) of variable size and pharmacokinetics and coregistered FMT datasets with computed tomography (FMT-CT).

METHODS AND RESULTS

Coregistration of FMT and CT was achieved with a multimodal imaging cartridge containing fiducial markers detectable by both modalities. A high-resolution CT angiography protocol accurately localized fluorescence to the aortic root of atherosclerotic apoE(-/-) mice. To identify suitable sensors, we first modeled signal kinetics in-silico and then compared 3 probes with oligo-L-lysine cleavage sequences: PS-5, 5 nm in diameter containing 2 fluorochromes, PS-25, a 25-nm version with an elongated lysine chain and PS-40, a polymeric nanoparticle. Serial FMT-CT showed fastest kinetics for PS-5 but, surprisingly, highest fluorescence in lesions of the aortic root for PS-40. PS-40 robustly reported therapeutic effects of atorvastatin, corroborated by ex vivo imaging and qPCR for the model protease cathepsin B.

CONCLUSIONS

FMT-CT is a robust and observer-independent tool for noninvasive assessment of inflammatory murine atherosclerosis. Reporter-containing nanomaterials may have unique advantages over small molecule agents for in vivo imaging.

Pharmacokinetics of a F(ab')2 scorpion antivenom in healthy human volunteers

This paper presents the first study of F(ab')2 scorpion antivenom pharmacokinetics in humans. We have studied the pharmacokinetics of an antiscorpion venom preparation (Alacramyn) in eight human healthy volunteers. The fabotherapic was administered as a 47.5 mg i.v. bolus. Blood samples were drawn at 0, 5, 15, 30, 45, 60, 90, 120, 180 and 360 min after antivenom administration. Subsequently, the volunteers made seven visits to the hospital. Four of them at 24 h intervals, one at day 10, and one at day 21. We measured antivenom plasmatic concentrations using a specific high sensitivity ELISA method for F(ab')2. The time course of F(ab')2 in serum of seven subjects was well described by a lineal combination of three exponential components; a four exponential component model was necessary to fit the eighth subject. The most significant antivenom pharmacokinetic parameters determined were: AUC(infinity)=596.9 (369.3, 891.2) mg h l(-1); V(c) = 3.1 (2.3, 4.3)l; V(ss) = 15.4 (12.8, 39.9)l; MRT = 250.0 (218.8, 310.2) h; CL = 96.6 (58.0, 139.2) ml h(-1); t(1/2,tau1) (also called t(1/2,alpha)) = 0.25 (0.13, 0.37) h; t(1/2,tau(z)) (corresponding to the slowest component) = 161.3 (141.0, 212.0) h.

Antibody pharmacokinetics and pharmacodynamics

The U.S. Food and Drug administration (FDA) has approved several polyclonal antibody preparations and at least 18 monoclonal antibody preparations (antibodies, antibody fragments, antibody fusion proteins, etc.). These drugs, which may be considered as a diverse group of therapeutic proteins, are associated with several interesting pharmacokinetic characteristics. Saturable binding with target antigen may influence antibody disposition, potentially leading to nonlinear distribution and elimination. Independent of antigen interaction, concentration-dependent elimination may be expected for IgG antibodies, due to the influence of the Brambell receptor, FcRn, which protects IgG from catabolism. Antibody administration may induce the development of an endogenous antibody response, which may alter the pharmacokinetics of the therapeutic antibody. Additionally, the pharmacodynamics of antibodies are also complex; these drugs may be used for a wide array of therapeutic applications, and effects may be achieved by a variety of mechanisms. This article provides an overview of many of the complexities associated with antibody pharmacokinetics and pharmacodynamics.

Allometric principles for interspecies extrapolation in toxicological risk assessment--empirical investigations

Four types of data (toxicokinetic data of pharmaceuticals from six species including humans, LD(50) values from eight animal species, long-term NOAEL values of pesticides from mice, rats, and dogs, and toxicity data on anti-neoplastic agents from six species including humans) were used for interspecies comparisons. Species differences with regard to kinetic parameters and toxicity were evaluated and the concordance with predictions by allometric scaling according to caloric demand (allometric exponent 0.75) or to body weight (allometric exponent 1) was checked. For LD(50) values, agreement was poor for both allometric concepts. Recently reported concordance of LD(50) species differences with body weight scaling could be traced back to biased data selection. The other three datasets are clearly in agreement with the allometric scaling according to caloric demand. Caloric demand scaling is thus proposed as a generic interspecies extrapolation method in the absence of substance-specific data. Moreover, the evaluated data make it possible to describe uncertainty associated with the process of interspecies extrapolation by allometric rules.

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.