The phenomenon and rationale of marked dependence of drug concentration on blood sampling site. Implications in pharmacokinetics, pharmacodynamics, toxicology and therapeutics (Part II)

Validation of cardiac image-derived input functions for functional PET quantification

PURPOSE

Functional PET (fPET) is a novel technique for studying dynamic changes in brain metabolism and neurotransmitter signaling. Accurate quantification of fPET relies on measuring the arterial input function (AIF), traditionally achieved through invasive arterial blood sampling. While non-invasive image-derived input functions (IDIF) offer an alternative, they suffer from limited spatial resolution and field of view. To overcome these issues, we developed and validated a scan protocol for brain fPET utilizing cardiac IDIF, aiming to mitigate known IDIF limitations.

METHODS

Twenty healthy individuals underwent fPET/MR scans using [18F]FDG or 6-[18F]FDOPA, utilizing bed motion shuttling to capture cardiac IDIF and brain task-induced changes. Arterial and venous blood sampling was used to validate IDIFs. Participants performed a monetary incentive delay task. IDIFs from various blood pools and composites estimated from a linear fit over all IDIF blood pools (3VOI) and further supplemented with venous blood samples (3VOIVB) were compared to the AIF. Quantitative task-specific images from both tracers were compared to assess the performance of each input function to the gold standard.

RESULTS

For both radiotracer cohorts, moderate to high agreement (r: 0.60-0.89) between IDIFs and AIF for both radiotracer cohorts was observed, with further improvement (r: 0.87-0.93) for composite IDIFs (3VOI and 3VOIVB). Both methods showed equivalent quantitative values and high agreement (r: 0.975-0.998) with AIF-derived measurements.

CONCLUSION

Our proposed protocol enables accurate non-invasive estimation of the input function with full quantification of task-specific changes, addressing the limitations of IDIF for brain imaging by sampling larger blood pools over the thorax. These advancements increase applicability to any PET scanner and clinical research setting by reducing experimental complexity and increasing patient comfort.

A Complete Extension of Classical Hepatic Clearance Models Using Fractional Distribution Parameter fd in Physiologically Based Pharmacokinetics

The classical organ clearance models have been proposed to relate the plasma clearance CLp to probable mechanism(s) of hepatic clearance. However, the classical models assume the intrinsic capability of drug elimination (CLu,int) that is physically segregated from the vascular blood but directly acts upon the unbound drug concentration in the blood (fubCavg), and do not handle the transit-time delay between the inlet/outlet concentrations in their closed-form clearance equations. Therefore, we propose unified model structures that can address the internal blood concentration patterns of clearance organs in a more mechanistic/physiological manner, based on the fractional distribution parameter fd operative in PBPK. The basic partial/ordinary differential equations for four classical models are revisited/modified to yield a more complete set of extended clearance models, i.e., the Rattle, Sieve, Tube, and Jar models, which are the counterparts of the dispersion, series-compartment, parallel-tube, and well-stirred models. We demonstrate the feasibility of applying the resulting extended models to isolated perfused rat liver data for 11 compounds and an example dataset for in vitro-in vivo extrapolation of the intrinsic to the systemic clearances. Based on their feasibilities to handle such real data, these models may serve as an improved basis for applying clearance models in the future.

Clinical Bridging Studies and Modeling Approach for Implementation of a Patient Centric Sampling Technique in Padsevonil Clinical Development

Volumetric absorptive microsampling (VAMS) techniques have gained popularity these last years as innovative tool for collection of blood pharmacokinetic (PK) samples in clinical trials as they offer many advantages over dried blood spot and conventional venous blood sampling. The use of Mitra®, a blood collection device based on volumetric absorptive microsampling (VAMS) technology, was implemented during clinical development of padsevonil (PSL), an anti-seizure medication (ASM) candidate. The present study describes the approach used to bridge plasma (obtained from conventional venous blood sampling) and blood exposures (obtained with Mitra®) to support the use of Mitra as sole blood PK sampling method in clinical trials. Paired blood (using Mitra®) and plasma samples (using conventional venous blood sampling) were collected in healthy volunteers as well as in patients with epilepsy. PSL concentration in plasma and blood were analyzed using different approaches which included evaluation of blood-to-plasma ratios (B/P) over time, linear regression, Bland-Altman analysis as well as development of a linear-mixed effect model based on clinical pharmacology studies. Results showed that the observed in vivo B/P and the measured bias between the 2 collection methods were consistent with the measured in vitro B/P. Graphical analysis demonstrated a clear time effect on the B/P which was confirmed in the linear mixed effect model with sampling time identified as significant covariate. Finally, the built-in model was validated using independent datasets and was shown to adequately predict plasma concentration based on blood concentration with a mean bias of less than 9% (predicted versus observed plasma concentration).

Validation of a dried blood spot method to measure tacrolimus concentrations in small volumes of mouse blood

Background: The small blood volume of mice complicates tacrolimus pharmacokinetic studies in these animals. Here we explored dried blood spot (DBS) as a novel method to measure tacrolimus blood concentrations in mice. DBS samples were collected from three sampling sites (cheek, tail and heart) and compared with heart whole blood samples measured via LC-MS/MS. Results: Tacrolimus concentrations in the whole blood samples ranged from 2.56 to 27.64 μg/l. DBS of cheek vein blood was the most reliable sampling site, with a mean bias of 0.15 μg/l (95% CI: -4.20 to 4.50). Conclusion: The DBS cheek method can be used for serial monitoring of tacrolimus blood concentrations in mice, offering an animal-friendly method for tacrolimus pharmacokinetic studies in mice.

Influence of Sampling Site and Fluid Flow on the Accuracy of Total Body Clearance Calculation

Studies have showed that by assuming arteriovenous drug concentrations are homogenous after intravenous injection, the determination of total body clearance based on venous drug concentrations is often inaccurate. This study considers the use of a fluidic pharmacokinetic profile generator where 28 different profile types were generated corresponding to a physiological model with varying sampling sites, administration locations, and fluid flow rates. Clearance was calculated using established equations, commercial software, and recently proposed models. The results show large differences in clearance values calculated with published equations and commercial software relative to the actual value of clearance. Alterations in sampling site, administration location, and fluid flow rates each influence the extent of calculation errors. The data show that a significant drug concentration gradient exists within the central circulatory system. The results show that the best way to address this issue would be to inject the drug at a peripheral location to allow for sufficient mixing and then sample from a large vein. Extrapolating for missing data can also lead to large errors in clearance calculation; this can be addressed by collecting more samples early after IV bolus administration or by collecting data during steady state conditions for an IV infusion.

Population Pharmacokinetics of Dexmedetomidine After Short Intravenous Infusion in Chinese Children

BACKGROUND AND OBJECTIVE

Dexmedetomidine is a highly selective alpha2-adrenoceptor agonist with sedative and analgesic properties which is also used in pediatric anesthesia. Although the pharmacokinetics of dexmedetomidine have been studied in pediatric patients, there are no data for Chinese children available. As alterations in pharmacokinetics due to ethnicity cannot be ruled out, it was the aim of this study to characterize the pharmacokinetics of dexmedetomidine in Chinese pediatric patients.

METHODS

Thirty-nine children aged 1-9 years undergoing surgery were enrolled in the study. Dexmedetomidine was administered as short intravenous infusion of 1-2 µg/kg in 10 min. Venous blood samples were drawn until 480 min after stopping of infusion. Dexmedetomidine plasma concentrations were measured with high-performance liquid chromatography and mass spectrometry. Pharmacokinetic modeling was performed by population analysis using linear compartment models.

RESULTS

Data of 36 patients (age 1-9 years, weight 10-27 kg) were analyzed. The pharmacokinetics of dexmedetomidine were best described by a two-compartment model with an allometric power model and estimates standardized to 70 kg body weight. The population estimates (95 % CI) per 70 kg bodyweight were: clearance 36.2 (33.3-41.1) l/h, central volume of distribution 84.3 (70.3-91.4) l, intercompartmental clearance 82.8 (63.6-136.6) l/h, peripheral volume of distribution 114 (95-149) l, and terminal half-life 4.4 (3.6-5.3) h. Age did not show any influence on weight-adjusted parameters.

CONCLUSIONS

Chinese children showed a similar clearance, but larger volumes of distribution and longer terminal half-life when compared to studies in Caucasians.

TRIAL REGISTRATION

ChiCTR-OPC-14005659.

Pharmacokinetics of dexmedetomidine in isoflurane-anesthetized New Zealand White rabbits

OBJECTIVE

To characterize the pharmacokinetics of dexmedetomidine when administered as a short intravenous (IV) infusion to isoflurane-anesthetized rabbits.

STUDY DESIGN

Experimental study.

ANIMALS

A total of six healthy adult female New Zealand White rabbits.

METHODS

Rabbits were anesthetized with isoflurane in oxygen. Following determination of isoflurane minimum alveolar concentration (MAC), the anesthetic dose was reduced to 0.7 × MAC, and dexmedetomidine hydrochloride (20 μg kg^-1) was infused IV over 5 minutes. Arterial blood samples were obtained immediately before and at 1, 2, 5, 6, 7, 10, 15, 30, 60, 90, 120, 240 and 360 minutes following termination of the infusion. Samples were transferred into tubes containing ethylenediaminetetraacetic acid and centrifuged immediately. The plasma was harvested and stored at -80 °C until analyzed. Concentrations of dexmedetomidine in plasma were determined by liquid chromatography mass spectrometry. Compartment models were fitted to the time and concentration data using nonlinear regression.

RESULTS

A three-compartment model best fit the data set. Median volume of distribution at steady state and terminal half-life were 3169 mL kg^-1 (range, 2182-3859 mL kg^-1) and 80 minutes (range, 72-88 minutes), respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

The pharmacokinetics of dexmedetomidine in isoflurane-anesthetized, healthy, New Zealand White rabbits were characterized in this study. Data from this study can be used to determine dosing regimens for dexmedetomidine in isoflurane-anesthetized rabbits.

Comparison of Capillary and Venous Drug Concentrations After Administration of a Single Dose of Risperidone, Paliperidone, Quetiapine, Olanzapine, or Aripiprazole

Risperidone, paliperidone, quetiapine, olanzapine, and aripiprazole are antipsychotic drugs approved for treating various psychiatric disorders, including schizophrenia. The objective of this randomized, parallel‐group, open‐label study was to compare finger‐stick‐based capillary with corresponding venous whole‐blood and plasma concentrations for these drugs after administration of a single dose to healthy volunteers. All whole‐blood and plasma drug concentrations were measured with validated liquid chromatography–tandem mass spectrometry methods. Capillary and venous concentrations (both in plasma and whole blood) were in close agreement, although a time‐dependent difference was observed, most obviously for olanzapine and paliperidone, with slightly higher capillary versus venous drug concentrations during the first hours after administering a single dose. The observed difference between capillary and venous plasma drug concentrations is expected not to be relevant in clinical practice, considering the wide window of therapeutic concentrations and the wide range of drug concentrations in the patient population for a given dose. Based on these results, finger‐stick‐based capillary drug concentrations have been shown to approximate venous drug concentrations.

Comparison of Capillary and Venous Plasma Drug Concentrations After Repeated Administration of Risperidone, Paliperidone, Quetiapine, Olanzapine, or Aripiprazole

Quantification of blood levels of antipsychotic drugs may be useful for managing medication therapy. This open‐label, parallel‐group study was performed to compare finger‐stick‐based capillary with corresponding venous plasma concentrations for risperidone, paliperidone, quetiapine, olanzapine, and aripiprazole and their major metabolites after repeated dosing in patients with schizophrenia or related illnesses. Finger‐stick‐based capillary and venous blood samples were collected at various times within a dosing interval. All drug concentration measurements in the derived plasma samples were performed with validated liquid chromatography–tandem mass spectrometry methods. Finger‐stick‐based capillary and venous plasma drug concentrations after repeated dosing were generally similar. Olanzapine capillary plasma concentrations, however, were on average approximately 20% higher than venous concentrations, with a trend for a relatively greater difference occurring shortly after dosing. In addition, smaller capillary–venous differences were observed for extended‐release and long‐acting intramuscular formulations and for aripiprazole, a drug with a long half‐life, compared with drugs administered as an immediate‐release formulation (risperidone, olanzapine). After repeated dosing, plasma derived from finger‐stick‐based blood was observed to be predictive of the venous concentrations. Capillary sampling may be an appropriate alternative to venous sampling to readily evaluate systemic drug concentrations.

Plasma radiometabolite correction in dynamic PET studies: Insights on the available modeling approaches

Full kinetic modeling of dynamic PET images requires the measurement of radioligand concentrations in the arterial plasma. The unchanged parent radioligand must, however, be separated from its radiometabolites by chromatographic methods. Thus, only few samples can usually be analyzed and the resulting measurements are often noisy. Therefore, the measurements must be fitted with a mathematical model. This work presents a comprehensive analysis of the different models proposed in the literature to describe the plasma parent fraction (PPf) and of the alternative approaches for radiometabolite correction. Finally, we used a dataset of [(11)C]PBR28 brain PET data as a case study to guide the reader through the PPf model selection process.

Development and application of a human PBPK model for bromodichloromethane to investigate the impacts of multi-route exposure

As a result of its presence in water as a volatile disinfection byproduct, bromodichloromethane (BDCM), which is mutagenic, poses a potential health risk from exposure via oral, dermal and inhalation routes. We developed a refined human physiologically based pharmacokinetic (PBPK) model for BDCM (including new chemical-specific human parameters) to evaluate the impact of BDCM exposure during showering and bathing on important measures of internal dose compared with oral exposure. The refined model adequately predicted data from the published literature for oral, dermal and bathing/showering exposures. A liter equivalency approach (L-eq) was used to estimate BDCM concentration in a liter of water consumed by the oral route that would be required to produce the same internal dose of BDCM resulting from a 20-min bath or a 10-min shower in water containing 10 µg l(-1) BDCM. The oral liter equivalent concentrations for the bathing scenario were 605, 803 and 5 µg l(-1) BDCM for maximum venous blood concentration (Cmax), the area under the curve (AUCv) and the amount metabolized in the liver per hour (MBDCM), respectively. For a 10-min showering exposure, the oral L-eq concentrations were 282, 312 and 2.1 µg l(-1) for Cmax, AUC and MBDCM, respectively. These results demonstrate large contributions of dermal and inhalation exposure routes to the internal dose of parent chemical reaching the systemic circulation, which could be transformed to mutagenic metabolites in extrahepatic target tissues. Thus, consideration of the contribution of multiple routes of exposure when evaluating risks from water-borne BDCM is needed, and this refined human model will facilitate improved assessment of internal doses from real-world exposures. Published 2015. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

A new DBS card with spot sizes independent of the hematocrit value of blood

Background: DBS cards have been a big promise for decades. However, blood with low hematocrit (Ht) values results on regular cellulose-based DBS cards in larger spot sizes, compared with blood with high Ht-values. A new material has been developed to solve this problem. Results: This material, based on hydrophilic-coated woven polyester fibers, shows spot sizes independent of the Ht-value of blood. Homogeneity over the spot is within 10% RSD. Conclusion: Quantitative measurements over a broad Ht range show nonbiased results compared with whole spot analysis. The cards are experienced as reproducible, robust and easy to use on aspects of punchability and extractability.

Are Physiologically Based Pharmacokinetic Models Reporting the Right C(max)? Central Venous Versus Peripheral Sampling Site

Physiologically based pharmacokinetic (PBPK) models can over-predict maximum plasma concentrations (C_max) following intravenous administration. A proposed explanation is that invariably PBPK models report the concentration in the central venous compartment, rather than the site where the samples are drawn. The purpose of this study was to identify and validate potential corrective models based on anatomy and physiology governing the blood supply at the site of sampling and incorporate them into a PBPK platform. Four models were developed and scrutinised for their corrective potential. All assumed the peripheral sampling site concentration could be described by contributions from surrounding tissues and utilised tissue-specific concentration-time profiles reported from the full-PBPK model within the Simcyp Simulator. Predicted concentrations for the peripheral site were compared to the observed C_max. The models results were compared to clinical data for 15 studies over seven compounds (alprazolam, imipramine, metoprolol, midazolam, omeprazole, rosiglitazone and theophylline). The final model utilised tissue concentrations from adipose, skin, muscle and a contribution from artery. Predicted C_max values considering the central venous compartment can over-predict the observed values up to 10-fold whereas the new sampling site predictions were within 2-fold of observed values. The model was particularly relevant for studies where traditional PBPK models over-predict early time point concentrations. A successful corrective model for C_max prediction has been developed, subject to further validation. These models can be enrolled as built-up modules into PBPK platforms and potentially account for factors that may affect the initial mixing of the blood at the site of sampling.

Impact of the blood sampling site on time-concentration drug profiles following intravenous or buccal drug administration

The aim of this study was to examine the effect of the sampling site on the drug concentration-time profile, following intravenous or buccal (often called 'oral transmucosal') drug administration. Buprenorphine (20 μg/kg) was administered IV or buccally to six cats. Blood samples were collected from the carotid artery and the jugular and medial saphenous veins for 24 h following buprenorphine administration. Buprenorphine concentration-time data were examined using noncompartmental analysis. Pharmacokinetic parameters were compared using the Wilcoxon signed rank test, applying the Bonferroni correction. Significance was set at P < 0.05. Following IV administration, no difference among the sampling sites was found. Following buccal administration, maximum concentration [jugular: 6.3 (2.9-9.8), carotid: 3.4 (1.9-4.9), medial saphenous: 2.5 (1.7-4.1) ng/mL], area under the curve [jugular: 395 (335-747), carotid: 278 (214-693), medial saphenous: 255 (188-608) ng·min/mL], and bioavailability [jugular: 47 (34-67), carotid: 32 (20-52), medial saphenous: 23 (16-55)%] were higher in the jugular vein than in the carotid artery and medial saphenous vein. Jugular venous blood sampling is not an acceptable substitute for arterial blood sampling following buccal drug administration.

Image-derived input function derived from a supervised clustering algorithm: methodology and validation in a clinical protocol using [11C](R)-rolipram

Image-derived input function (IDIF) obtained by manually drawing carotid arteries (manual-IDIF) can be reliably used in [¹¹C](R)-rolipram positron emission tomography (PET) scans. However, manual-IDIF is time consuming and subject to inter- and intra-operator variability. To overcome this limitation, we developed a fully automated technique for deriving IDIF with a supervised clustering algorithm (SVCA). To validate this technique, 25 healthy controls and 26 patients with moderate to severe major depressive disorder (MDD) underwent T1-weighted brain magnetic resonance imaging (MRI) and a 90-minute [¹¹C](R)-rolipram PET scan. For each subject, metabolite-corrected input function was measured from the radial artery. SVCA templates were obtained from 10 additional healthy subjects who underwent the same MRI and PET procedures. Cluster-IDIF was obtained as follows: 1) template mask images were created for carotid and surrounding tissue; 2) parametric image of weights for blood were created using SVCA; 3) mask images to the individual PET image were inversely normalized; 4) carotid and surrounding tissue time activity curves (TACs) were obtained from weighted and unweighted averages of each voxel activity in each mask, respectively; 5) partial volume effects and radiometabolites were corrected using individual arterial data at four points. Logan-distribution volume (V_T/f_P) values obtained by cluster-IDIF were similar to reference results obtained using arterial data, as well as those obtained using manual-IDIF; 39 of 51 subjects had a V_T/f_P error of <5%, and only one had error >10%. With automatic voxel selection, cluster-IDIF curves were less noisy than manual-IDIF and free of operator-related variability. Cluster-IDIF showed widespread decrease of about 20% [¹¹C](R)-rolipram binding in the MDD group. Taken together, the results suggest that cluster-IDIF is a good alternative to full arterial input function for estimating Logan-V_T/f_P in [¹¹C](R)-rolipram PET clinical scans. This technique enables fully automated extraction of IDIF and can be applied to other radiotracers with similar kinetics.

Blood microsampling using capillaries for drug-exposure determination in early preclinical studies: a beneficial strategy to reduce blood sample volumes

Background: Capillary microsampling (CMS) of blood with subsequent blood analysis offers a potential strategy to deal with increased demand to reduce blood sample volumes in animal discovery and preclinical studies. Results: A generic approach is presented allowing PK analysis in 15 µl blood samples. CMS blood exposure data were compared with the traditional plasma exposure results in rats and dogs. Blood PK profiles obtained for two different compounds were in agreement with profiles obtained in plasma. From these studies ex vivo blood to plasma ratios were also obtained. In a mouse study, blood PK profiles that were obtained following automatic sampling overlay with the blood PK profiles obtained with CMS. Conclusion: CMS in 15 µl glass capillaries allows collection and handling of small and exact volumes of blood. Although CMS can also be applied for plasma collection, the full benefit is only achieved with blood collection and analysis.

Kinetic analysis of [11C]befloxatone in the human brain, a selective radioligand to image monoamine oxidase A

BACKGROUND

[11C]Befloxatone measures the density of the enzyme monoamine oxidase A (MAO-A) in the brain. MAO-A is responsible for the degradation of different neurotransmitters and is implicated in several neurologic and psychiatric illnesses. This study sought to estimate the distribution volume (VT) values of [11C]befloxatone in humans using an arterial input function.

METHODS

Seven healthy volunteers were imaged with positron emission tomography (PET) after [11C]befloxatone injection. Kinetic analysis was performed using an arterial input function in association with compartmental modeling and with the Logan plot, multilinear analysis (MA1), and standard spectral analysis (SA) at both the regional and voxel level. Arterialized venous samples were drawn as an alternative and less invasive input function.

RESULTS

An unconstrained two-compartment model reliably quantified VT values in large brain regions. A constrained model did not significantly improve VT identifiability. Similar VT results were obtained using SA; however, the Logan plot and MA1 slightly underestimated VT values (about -10%). At the voxel level, SA showed a very small bias (+2%) compared to compartmental modeling, Logan severely underestimated VT values, and voxel-wise images obtained with MA1 were too noisy to be reliably quantified. Arterialized venous blood samples did not provide a satisfactory alternative input function as the Logan-VT regional values were not comparable to those obtained with arterial sampling in all subjects.

CONCLUSIONS

Binding of [11C]befloxatone to MAO-A can be quantified using an arterial input function and a two-compartment model or, in parametric images, with SA.

Hemato-critical issues in quantitative analysis of dried blood spots: challenges and solutions

Dried blood spot (DBS) sampling for quantitative determination of drugs in blood has entered the bioanalytical arena at a fast pace during the last decade, primarily owing to progress in analytical instrumentation. Despite the many advantages associated with this new sampling strategy, several issues remain, of which the hematocrit issue is undoubtedly the most widely discussed challenge, since strongly deviating hematocrit values may significantly impact DBS-based quantitation. In this review, an overview is given of the different aspects of the ‘hematocrit problem’ in quantitative DBS analysis. The different strategies that try to cope with this problem are discussed, along with their potential and limitations. Implementation of some of these strategies in practice may help to overcome this important hurdle in DBS assays, further allowing DBS to become an established part of routine quantitative bioanalysis.

Population-based input function and image-derived input function for [¹¹C](R)-rolipram PET imaging: methodology, validation and application to the study of major depressive disorder

Quantitative PET studies of neuroreceptor tracers typically require that arterial input function be measured. The aim of this study was to explore the use of a population-based input function (PBIF) and an image-derived input function (IDIF) for [(11)C](R)-rolipram kinetic analysis, with the goal of reducing - and possibly eliminating - the number of arterial blood samples needed to measure parent radioligand concentrations.

METHODS

A PBIF was first generated using [(11)C](R)-rolipram parent time-activity curves from 12 healthy volunteers (Group 1). Both invasive (blood samples) and non-invasive (body weight, body surface area, and lean body mass) scaling methods for PBIF were tested. The scaling method that gave the best estimate of the Logan-V(T) values was then used to determine the test-retest variability of PBIF in Group 1 and then prospectively applied to another population of 25 healthy subjects (Group 2), as well as to a population of 26 patients with major depressive disorder (Group 3). Results were also compared to those obtained with an image-derived input function (IDIF) from the internal carotid artery. In some subjects, we measured arteriovenous differences in [(11)C](R)-rolipram concentration to see whether venous samples could be used instead of arterial samples. Finally, we assessed the ability of IDIF and PBIF to discriminate depressed patients (MDD) and healthy subjects.

RESULTS

Arterial blood-scaled PBIF gave better results than any non-invasive scaling technique. Excellent results were obtained when the blood-scaled PBIF was prospectively applied to the subjects in Group 2 (V(T) ratio 1.02±0.05; mean±SD) and Group 3 (V(T) ratio 1.03±0.04). Equally accurate results were obtained for two subpopulations of subjects drawn from Groups 2 and 3 who had very differently shaped (i.e. "flatter" or "steeper") input functions compared to PBIF (V(T) ratio 1.07±0.04 and 0.99±0.04, respectively). Results obtained via PBIF were equivalent to those obtained via IDIF (V(T) ratio 0.99±0.05 and 1.00±0.04 for healthy subjects and MDD patients, respectively). Retest variability of PBIF was equivalent to that obtained with full input function and IDIF (14.5%, 15.2%, and 14.1%, respectively). Due to [(11)C](R)-rolipram arteriovenous differences, venous samples could not be substituted for arterial samples. With both IDIF and PBIF, depressed patients had a 20% reduction in [(11)C](R)-rolipram binding as compared to control (two-way ANOVA: p=0.008 and 0.005, respectively). These results were almost equivalent to those obtained using 23 arterial samples.

CONCLUSION

Although some arterial samples are still necessary, both PBIF and IDIF are accurate and precise alternatives to full arterial input function for [(11)C](R)-rolipram PET studies. Both techniques give accurate results with low variability, even for clinically different groups of subjects and those with very differently shaped input functions.

Minimally invasive input function for 2-18F-fluoro-A-85380 brain PET studies

PURPOSE

Quantitative neuroreceptor positron emission tomography (PET) studies often require arterial cannulation to measure input function. While population-based input function (PBIF) would be a less invasive alternative, it has only rarely been used in conjunction with neuroreceptor PET tracers. The aims of this study were (1) to validate the use of PBIF for 2-(18)F-fluoro-A-85380, a tracer for nicotinic receptors; (2) to compare the accuracy of measures obtained via PBIF to those obtained via blood-scaled image-derived input function (IDIF) from carotid arteries; and (3) to explore the possibility of using venous instead of arterial samples for both PBIF and IDIF.

METHODS

Ten healthy volunteers underwent a dynamic 2-(18)F-fluoro-A-85380 brain PET scan with arterial and, in seven subjects, concurrent venous serial blood sampling. PBIF was obtained by averaging the normalized metabolite-corrected arterial input function and subsequently scaling each curve with individual blood samples. IDIF was obtained from the carotid arteries using a blood-scaling method. Estimated Logan distribution volume (V(T)) values were compared to the reference values obtained from arterial cannulation.

RESULTS

For all subjects, PBIF curves scaled with arterial samples were similar in shape and magnitude to the reference arterial input function. The Logan V(T) ratio was 1.00 ± 0.05; all subjects had an estimation error <10%. IDIF gave slightly less accurate results (V(T) ratio 1.03 ± 0.07; eight of ten subjects had an error <10%). PBIF scaled with venous samples yielded inaccurate results (V(T) ratio 1.13 ± 0.13; only three of seven subjects had an error <10%). Due to arteriovenous differences at early time points, IDIF could not be calculated using venous samples.

CONCLUSION

PBIF scaled with arterial samples accurately estimates Logan V(T) for 2-(18)F-fluoro-A-85380. Results obtained with PBIF were slightly better than those obtained with IDIF. Due to arteriovenous concentration differences, venous samples cannot be substituted for arterial samples.

Image-derived input function for brain PET studies: many challenges and few opportunities

Quantitative positron emission tomography (PET) brain studies often require that the input function be measured, typically via arterial cannulation. Image-derived input function (IDIF) is an elegant and attractive noninvasive alternative to arterial sampling. However, IDIF is also a very challenging technique associated with several problems that must be overcome before it can be successfully implemented in clinical practice. As a result, IDIF is rarely used as a tool to reduce invasiveness in patients. The aim of the present review was to identify the methodological problems that hinder widespread use of IDIF in PET brain studies. We conclude that IDIF can be successfully implemented only with a minority of PET tracers. Even in those cases, it only rarely translates into a less-invasive procedure for the patient. Finally, we discuss some possible alternative methods for obtaining less-invasive input function.

Plasma levels of bupivacaine and its metabolites after subacromial infusions in concentrations 2.5 or 5.0 mg/ml

BACKGROUND

We studied plasma bupivacaine concentrations in patients with a continuous subacromial bupivacaine infusion after an ambulatory arthroscopic shoulder surgery to evaluate whether it is feasible to discharge patients with an on-going infusion early on the operation day.

METHODS

Sixteen ASA I-III patients undergoing elective arthroscopic shoulder surgery were randomized in 1:1 ratio to receive a continuous infusion of either 2.5 or 5.0 mg/ml bupivacaine subacromially for 48 h post-operatively. Before the commencement of the infusion, 20 ml of 5.0 mg/ml bupivacaine was injected subacromially in both groups. Plasma bupivacaine concentrations were defined as the primary endpoint and concentrations of its metabolites, side effects and pain scores as the secondary endpoints.

RESULTS

The mean total plasma bupivacaine concentration increased up to 48 h, the highest mean being 0.87 (SD 0.30) μg/ml during the 5.0 mg/ml treatment and 0.24 (0.10) μg/ml during the 2.5 mg/ml bupivacaine treatment. After 48 h, there was a significant difference between the groups in the plasma levels. The highest mean 4-hydroxy-bupivacaine and desbutylbupivacaine concentrations were 0.11 and 0.22 μg/ml, respectively. In the pain scores, no significant difference was found. No clear signs of toxicity were observed.

CONCLUSIONS

The concentrations of total bupivacaine and its metabolites remained below toxic levels. Excluding patients with renal or liver diseases, both 2.5 and 5.0 mg/ml bupivacaine as subacromial infusion 2 ml/h for 48 h following shoulder arthroscopy seem to be well tolerated, enabling patient discharge with an on-going infusion on the operation day. Because of similar side effects and pain scores in both groups, 2.5 mg/ml may be preferable.

Paradigm shift for the alcohol breath test

The alcohol breath test (ABT) has been used for quantification of ethyl alcohol in individuals suspected of driving under the influence for more than 50 years. In this time, there has been little change in the concepts underlying this single breath test. The old model, which assumes that end-exhaled breath alcohol concentration is closely related to alveolar air alcohol concentration, is no longer acceptable. This paper reviews experimental research and mathematical modeling which has evaluated the pulmonary exchange processes for ethyl alcohol. Studies have shown that alcohol exchanges dynamically with the airway tissue both during inspiration and expiration. The airway tissue interaction makes it impossible to deliver air with alveolar alcohol concentration to the mouth. It is concluded that the ABT is dependent on physiological factors that need to be assessed for accurate testing.

Pharmacokinetic comparisons of tail-bleeding with cannula- or retro-orbital bleeding techniques in rats using six marketed drugs

INTRODUCTION

The evaluation of drug disposition properties of chemical entities in drug discovery research typically involves the conduct of pharmacokinetic studies in rodents that requires blood sampling over several time points, preferably without disrupting the physiological status of the animals. Several blood withdrawal methods have been employed throughout the industry, yet these methods have not been comprehensively evaluated with regard to their effects on pharmacokinetic profiles of the drug investigated to recommend best practices.

METHODS

In this paper, the pharmacokinetics of six marketed drugs from four distinct therapeutic classes were compared using tail-vein, femoral-artery cannula-, and retro-orbital sinus bleeding techniques. The marketed drugs used in these studies were pentoxifylline, gemfibrozil, glipizide, methotrexate, clonidine, and fluoxetine.

RESULTS

Following oral administration, peak plasma concentration (C(max)), and area under the curve (AUC(0-24)) values for all compounds were not significantly different with the tail-vein method when compared to cannula- or retro-orbital sinus bleeding, except for fluoxetine and gemfibrozil for which minor, but statistically significant differences were observed. The effect of arterial versus venous tail-bleeding on the pharmacokinetics of pentoxifylline indicated no statistical differences in either C(max) or AUC(0-24) values. However, for fluoxetine, higher exposures were observed with tail arterial than venous sampling (2-fold with respect to C(max) and 1.7-fold with respect to AUC(0-24), p<0.05).

DISCUSSION

The observed differences with fluoxetine may be due to its pharmacological effects on thermoregulatory responses that influence tail blood flow, a hypothesis that remains to be tested. Based on these observations, we recommend the tail-bleeding technique for pharmacology or toxicology exposure and F% studies, particularly in early discovery work. Retro-orbital bleeding is controversial and is no longer considered a humane method. Cannula-bleeding, especially coupled with automated blood-collection techniques, has become the most efficient way for pharmaceutical industry to perform rat bioavailability studies.

Intranasal midazolam: a comparison of two delivery devices in human volunteers

Bidirectional nasal drug delivery is a new administration principle with improved deposition pattern that may increase nasal drug uptake. Twelve healthy subjects were included in this open, non-randomized 3-way crossover study: midazolam (3.4 mg) intravenously (1 mg mL (-1)), or nasally by bidirectional or traditional spray (2 x100 microL of a 17 mg mL(-1) nasal midazolam formulation). The primary outcome was bioavailability. Blood samples were drawn for 6 h for determination (gas-chromatography-mass-spectrometry) of midazolam and 1-OH-midazolam. Pharmacokinetic calculations were based on non-compartmental modelling, sedation assessed by a subjective 0-10 NRS-scale, and nasal dimensions by non-invasive acoustic rhinometry. Mean bioavailabilities were 0.68-0.71, and Tmax 15 min for the sprays, which also were bioequivalent (ratio geometric means (90%) CI: 97.6% (90% CI 83.5; 113.9)). Sedation after bidirectional spray followed intravenous sedation closely, while sedation after the traditional spray was less pronounced. A negative correlation between Cmax and smallest cross-sectional area was seen. Adverse effects such as local irritation did not differ significantly between the sprays. Apparently bidirectional delivery did not increase systemic bioavailability of midazolam. We cannot disregard that only the traditional spray caused less sedation than intravenous administration. This finding needs to be confirmed in trials designed for this purpose.

Interpretation of analytical toxicology results in life and at postmortem

Interpretation of analytical toxicology results from live patients is sometimes difficult. Possible factors may be related to: (i) the nature of the poison(s) present; (ii) sample collection, transport and storage; (iii) the analytical methodology used; (iv) the circumstances of exposure; (v) mechanical factors such as trauma or inhalation of stomach contents; and (vi) pharmacological factors such as tolerance or synergy. In some circumstances, detection of a drug or other poison may suffice to prove exposure. At the other extreme, the interpretation of individual measurements may be simplified by regulation. Examples here include whole blood alcohol (ethanol) in regard to driving a motor vehicle and blood lead assays performed to assess occupational exposure. With pharmaceuticals, the plasma or serum concentrations of drugs and metabolites attained during treatment often provide a basis for the interpretation of quantitative measurements. With illicit drugs, comparative information from casework may be all that is available. Postmortem toxicology is an especially complex area since changes in the composition of fluids such as blood depending on the site of collection from the body and the time elapsed since death, amongst other factors, may influence the result obtained. This review presents information to assist in the interpretation of analytical results, especially regarding postmortem toxicology. Collection and analysis of not only peripheral blood, but also other fluids/tissues is usually important in postmortem work. Alcohol, for example, can be either lost from, or produced in, blood especially if there has been significant trauma, hence measurements in urine or vitreous humour are needed to confirm the reliability of a blood result. Measurement of metabolites may also be valuable in individual cases.

Impact of peripheral elimination on the concentration-effect relationship of remifentanil in anaesthetized dogs

BACKGROUND

This study elucidates the impact of sampling site when estimating pharmacokinetic-pharmacodynamic (PK-PD) parameters of drugs such as remifentanil that undergo tissue extraction in the biophase. The interrelationship between the concentrations of remifentanil predicted for the effect compartment and those measured in arterial, venous, and cerebrospinal fluid were investigated under steady-state conditions.

METHODS

Following induction of anaesthesia with pentobarbital, an arterial cannula (femoral) and two venous catheters (jugular and femoral) were inserted. Electrodes were placed for EEG recording of theta wave activity. Each dog received two consecutive 5-min infusions for the PK-PD study and a bolus followed by a 60-min infusion was started for the steady-state study. Cerebrospinal fluid, arterial and venous blood samples were drawn simultaneously after 30, 40, and 50 min. At the end of the infusion, arterial blood samples were collected for pharmacokinetic analysis.

RESULTS

Remifentanil PK-PD parameters based on theta wave activity were as follows: apparent volume of distribution at steady-state (V(ss)) (231+/-37 ml kg(-1)), total body clearance (Cl) (63+/-16 ml min(-1) kg(-1)), terminal elimination half-life (t(1/2 beta)) (7.71 min), effect compartment concentration at 50% of maximal observed effect (EC(50)) (21+/-13 ng ml(-1)), and equilibration rate constant between plasma and effect compartment (k(e0)) (0.48+/-0.24 min). The mean steady-state cerebrospinal fluid concentration of 236 ng ml(-1) represented 52 and 74% of that in arterial and venous blood, respectively.

CONCLUSIONS

Our study re-emphasizes the importance of a sampling site when performing PK-PD modelling for drugs undergoing elimination from the effect compartment. For a drug undergoing tissue elimination such as remifentanil, venous rather than arterial concentrations will reflect more exactly the effect compartment concentrations, under steady-state conditions.

Physiologically based pharmacokinetic modeling of arterial - antecubital vein concentration difference

BACKGROUND

Modeling of pharmacokinetic parameters and pharmacodynamic actions requires knowledge of the arterial blood concentration. In most cases, experimental measurements are only available for a peripheral vein (usually antecubital) whose concentration may differ significantly from both arterial and central vein concentration.

METHODS

A physiologically based pharmacokinetic (PBPK) model for the tissues drained by the antecubital vein (referred to as "arm") is developed. It is assumed that the "arm" is composed of tissues with identical properties (partition coefficient, blood flow/gm) as the whole body tissues plus a new "tissue" representing skin arteriovenous shunts. The antecubital vein concentration depends on the following parameters: the fraction of "arm" blood flow contributed by muscle, skin, adipose, connective tissue and arteriovenous shunts, and the flow per gram of the arteriovenous shunt. The value of these parameters was investigated using simultaneous experimental measurements of arterial and antecubital concentrations for eight solutes: ethanol, thiopental, 99Tcm-diethylene triamine pentaacetate (DTPA), ketamine, D2O, acetone, methylene chloride and toluene. A new procedure is described that can be used to determine the arterial concentration for an arbitrary solute by deconvolution of the antecubital concentration. These procedures are implemented in PKQuest, a general PBPK program that is freely distributed http://www.pkquest.com.

RESULTS

One set of "standard arm" parameters provides an adequate description of the arterial/antecubital vein concentration for ethanol, DTPA, thiopental and ketamine. A significantly different set of "arm" parameters was required to describe the data for D2O, acetone, methylene chloride and toluene - probably because the "arm" is in a different physiological state.

CONCLUSIONS

Using the set of "standard arm" parameters, the antecubital vein concentration can be used to determine the whole body PBPK model parameters for an arbitrary solute without any additional adjustable parameters. Also, the antecubital vein concentration can be used to estimate the arterial concentration for an arbitrary input for solutes for which no arterial concentration data is available.

Magnitude and Time-Course of Arterio-Venous Differences in Blood-Alcohol Concentration in Healthy Men

BACKGROUND AND OBJECTIVE

Human studies of arterio-venous (AV) differences in drug concentrations and the consequences for pharmacokinetic modelling and concentration-effect relationships are very limited. We therefore investigated the intravenous and intra-arterial concentrations of alcohol (ethanol) during the absorption, distribution and elimination stages of alcohol metabolism in healthy men.

STUDY PARTICIPANTS AND METHODS

Nine male volunteers aged 26-67 years drank 0.6 g alcohol/kg bodyweight in 2-15 minutes. The drink was prepared from 95% v/v alcohol, which was diluted with an alcohol-free beverage to 20% v/v. Before the start of drinking and for 6-7 hours post-administration, blood samples were drawn at 15- to 20-minute intervals from indwelling catheters in a radial artery and a cubital vein on the same arm. The blood-alcohol concentration (BAC) was determined by headspace gas chromatography, and blood-water content was measured by desiccation.

RESULTS

The peak concentration (Cmax) of alcohol in arterial blood was 0.98 g/L (SD 0.209) compared with 0.84 g/L (SD 0.176) for venous blood (p < 0.001), whereas median time to reach Cmax (tmax) was the same (35 minutes). The AV difference was greatest at 10 minutes after the end of drinking (mean 0.20 g/L [range 0.09-0.40 g/L]), decreasing as the absorption of alcohol continued. At a median time of 90 minutes post-administration (range 45-105 minutes), the AV difference was momentarily zero. At later times, the AV differences became increasingly negative and at 280 minutes post-administration the mean was -0.051 g/L (range -0.025 to -0.078 g/L). The slope of the post-absorptive phase (k0) was 0.116 g/L/h (SD 0.0167) for arterial blood compared with 0.109 g/L/h (SD 0.0185) for venous blood (p < 0.001). The extrapolated time to reach zero BAC was 391 minutes (SD 34) for arterial blood and 420 minutes (SD 41) for venous blood; the difference of 29 minutes was statistically highly significant (p < 0.001). The apparent volume of distribution of alcohol, the area under the concentration-time curves (AUC) and the water content of arterial and venous blood samples were not significantly different for the two sampling compartments.

CONCLUSION

The arterial and venous blood-alcohol profiles were shifted in time owing to the time it takes for alcohol to equilibrate between arterial blood and tissue water. Alcohol is metabolised in the liver but not in muscle tissue, which acts as a reservoir for alcohol. The concentrations of alcohol in arterial and venous blood were the same at only one timepoint, which signifies complete equilibration of alcohol in total body water. During the entire post-absorptive phase, the concentration of alcohol in venous blood draining skeletal muscles was slightly greater than the arterial blood concentration; therefore, the AV differences were negative.

The effect of the addition of epinephrine on early systemic absorption of epidural ropivacaine in humans

The addition of epinephrine to ropivacaine has not been recommended because ropivacaine has intrinsic vasoconstrictor properties. However, few pharmacokinetic data are available on the addition of epinephrine to epidural ropivacaine in humans. In this prospective, double-blinded study, we randomized patients having elective abdominal hysterectomy to receive epidural ropivacaine 1.5 mg/kg, diluted in 15 mL, either with (epinephrine group, n = 12) or without (plain group, n = 12) epinephrine 5 microg/mL and then measured arterial and venous plasma concentrations of ropivacaine at intervals up to 180 min. We found that arterial and venous plasma ropivacaine concentrations were smaller in the epinephrine group compared with the plain group in the first 60 min after the drug administration (P < 0.01). Mean (+/- SD) maximum total plasma ropivacaine concentration was smaller in the epinephrine group (arterial, 0.92 +/- 0.32 microg/mL; venous, 0.82 +/- 0.33 microg/mL) compared with the plain group (1.31 +/- 0.39 microg/mL and 1.31 +/- 0.50 microg/mL, respectively; P = 0.01). Time to maximum total plasma ropivacaine concentration was not significantly different between groups (mean +/- SD; arterial, 16 +/- 2 min; venous, 23 +/- 2 min in the epinephrine group versus 9 +/- 2 min and 12 +/- 3 min, respectively, in the plain group; P = 0.08). Arterial plasma ropivacaine concentrations were larger than venous concentrations during the first hour (P < 0.01); the arterio-venous difference decreased exponentially, and the rate and magnitude of this decrease was unaffected by epinephrine. We conclude that the addition of epinephrine 5 microg/mL to ropivacaine reduced the early systemic plasma concentrations of ropivacaine after epidural injection and may be useful for decreasing the risk of toxicity from systemic absorption of epidural ropivacaine.

IMPLICATIONS

The addition of epinephrine 5 microg/mL to epidural ropivacaine reduced the systemic arterial and venous plasma concentrations of ropivacaine in the first hour and the maximum plasma concentration of ropivacaine. Epinephrine may be a useful additive for reducing the risk of systemic toxicity when large doses of ropivacaine are given epidurally.

Calculation of steady-state distribution delay between central and peripheral compartments in two-compartment models with infusion regimen

A lag time may exist between blood drug concentration and drug effect. Various factors can contribute to the lag time, among which the drug distribution delay is a significant one. The drug distribution delay can also exist between different compartments. An equation was derived to calculate the steady-state drug concentration delay between central and peripheral compartments in a two-compartment model with infusion regimen.

Modeling nicotine arterial-venous differences to predict arterial concentrations and input based on venous measurements: application to smokeless tobacco and nicotine gum

Significant arterio-venous differences in nicotine concentrations have been observed during and after cigarette smoking, nicotine nasal spray, and intravenous nicotine administration. In this paper we describe a novel mathematical method for estimating arterial blood levels from venous blood level data. The model allows to quantify: (i) the influence of the microcirculation in the hands and forearm on the distribution of nicotine, and (ii) the influence of disregarding the venous to arterial circulation in the estimate of systemic inputs. We also (iii) propose a general method to predict arterial concentrations and inputs given venous data. The basic model we adopt is based on the relationship Cv = T * Ca, where Cv and Ca are the concentration in the venous and arterial site, respectively, T is the arterio-venous transfer function and * indicates convolution. We use empirical data to estimate T. We then compare estimates of systemic inputs to the venous site obtained taking into account the transfer function or, as usually done, disregarding it. The relationship we use to compare estimated inputs are: Cv = T * ka * A (where Ka is the arterial disposition function and A the systemic input), and Cv = Kv * A (where Kv is the venous disposition function), respectively. Finally, the estimated transfer function allows to estimate (average) Ca or A given arbitrary venous data. (i) Our analysis suggests that a bi-exponential T is needed to describe observed arterial-venous differences. The estimated transfer function indicates that no elimination of nicotine is involved in the forearm. (ii) Disregarding T, as usually done, erroneously obtains too complex venous input functions (because these input functions incorporate T). (iii) Disregarding T erroneously estimates significantly higher total inputs. (iv) Using the proposed model and previously published venous nicotine level data we predict substantial arterial-venous differences in blood nicotine levels for smokeless tobacco and nicotine gum. The use of disposition functions obtained from venous data may lead to erroneous estimation of the rates of entry into the circulation and systemic bioavailability for many drugs.

Arteriovenous serum cocaine concentration difference after intravenous bolus injection and constant-rate infusions: relation to pharmacodynamic estimates in rats

We characterized the pharmacokinetics of cocaine using both arterial and venous serum data after a bolus dose (2 mg/kg) and two constant-rate infusions (12.24 and 24.48 microg/min) for 2 h in rats. A published behavioral effect was used to investigate the effects of arteriovenous serum concentration differences on pharmacodynamic estimates for the 2 mg/kg dose. Significant temporal arteriovenous serum cocaine and benzoylecgonine (the major metabolite) concentration differences existed after cocaine administrations. The AUCs for arterial serum data were greater than the AUCs for venous data, indicating that cocaine was metabolized more extensively in the venous sampling site. Cocaine's behavioral effect could be directly related to serum concentrations with no hysteresis observed between the effects and arterial or venous serum concentrations. The pharmacodynamic estimates derived from arterial serum data approximated those from the venous data due to the most decline of cocaine's effect occurred in the elimination phase during which serum cocaine concentrations were not significantly different between the two sampling sites.

Quantification of doxorubicin in plasma--a comparative study of capillary and venous blood sampling

Doxorubicin, an anthraquinone glycoside, is currently one of the clinically most important antineoplastic drugs. The aim of the present study was to identify potential concentration differences of doxorubicin in plasma from capillary and venous blood samples. Sixteen patients (seven females and nine males; median age: 37 years, range: 1-77 years) were included. The quantitative analysis of doxorubicin was carried out by reversed-phase liquid chromatography with fluorometric detection. The concentration of doxorubicin in capillary and venous samples were closely correlated (r=0.98; p<0.0001). The median plasma concentration ratio capillary/venous was 1.13 (95% confidence interval: 1.06-1.20) and not affected either by plasma drug concentration, age or the body mass index of the patient. The concentration ratio was significantly higher in males (median: 1.18) than in females (median: 1.01). The observed concentration differences of doxorubicin in plasma from capillary and venous samples are of minor importance only. Capillary blood sampling is recommended for use in pharmacokinetic studies of doxorubicin, especially in pediatric patients, in order to avoid sometimes traumatic venous blood sampling.

The rate and extent of oral bioavailability versus the rate and extent of oral absorption: clarification and recommendation of terminology

In the literature, the meanings of the terms oral absorption and oral bioavailability of drugs vary greatly. Absorption has been considered to take place at the mucosal membrane of the gastrointestinal (GI) tract. It has also been defined as the process from the site of drug administration to the site of measurement. In the latter definition, the extent of oral absorption depends on the extent of first-pass elimination in the gut wall and liver even though a drug may be completely absorbed from the GI tract. Moreover, these two terms have also been used interchangeably. Inconsistency in the definition of these two terms has led to varying interpretations of these terms among students, researchers and laymen, and such an inconsistency seems undesirable. Apparently because of these inconsistencies, improper correlations between the Caco-2 permeability or intestinal permeability and the oral bioavailability of drugs subject to extensive first-pass effect may have occurred. It is suggested that absorption be defined as movement of drug across the outer mucosal membranes of the GI tract, while bioavailability be defined as availability of drug to the general circulation or site of pharmacological actions. Since transit times (this may range from about 1 min to several hours) across enterocytes, liver, lungs, and the peripheral venous sampling tissue are virtually unknown for all drugs, this factor alone would favor the use of "oral bioavailability rate" rather than "oral absorption rate" in all routine studies.

Relationships between steady state blood concentrations and cardiac output during intravenous infusions

The blood concentration of a drug at steady state can be calculated from the ratio of infusion rate over the total body drug clearance. When the blood is assumed to be a homogenous pool, the clearance is not usually referenced to any particular site within the circulation. In contrast, in recirculating systems, the relative sites of an infusion, the lungs and organs of elimination, and dilution in the cardiac output, produce concentration gradients in the blood. Equations that describe the relationship between the steady state concentrations of a drug in arterial, pulmonary artery, mixed venous and eliminating organ (e.g. liver or kidney) blood and the intrinsic clearance of the drug in the lungs and the eliminating organ were derived analytically. The concentrations at all sites were shown to be cardiac output dependent with the following exceptions: (1) in arterial blood, when the drug is highly extracted by the lungs, and (2) in eliminating organ blood when there is no lung clearance of the drug. The arterial and pulmonary artery concentrations will be least affected by cardiac output in the absence of lung clearance, and if the intrinsic clearance in the eliminating organ is substantially less than cardiac output.

Do plasma concentrations obtained from early arterial blood sampling improve pharmacokinetic/pharmacodynamic modeling?

In pharmacokinetic/pharmacodynamic (PK/PD) modeling the first blood sample is usually taken 1 to 2 min after drug administration (late sampling). Therefore, investigators have to extrapolate the plasma concentration to Time 0. Extrapolation, however, erroneously assumes instantaneous and complete mixing of drug in the central volume of distribution. We investigated whether plasma concentrations obtained from early arterial blood sampling would improve PK/PD modeling. In 14 pigs, one of five neuromuscular blocking agents (NMBAs) was administered into the right ventricle within 1 sec and arterial sampling was performed every 1.2 sec (1st min). The response of the tibialis muscle was measured mechanomyographically. The influence of inclusion of data from early arterial sampling on PK/PD modeling was determined. Furthermore, the concentrations in the effect compartment at 50% block (EC50) derived from modeling were compared to the measured concentration in plasma during a steady state 50% block. A very high peak in arterial plasma concentration was seen within 20 sec after administration of the NMBA. Extensive modeling revealed that plasma concentrations obtained from early arterial blood sampling improve PK/PD modeling. Independent of the type of modeling, the EC50 and KeO based on data sets that include early arterial blood sampling were, for all five NMBAs, significantly higher and lower respectively, than those based on data sets obtained from late sampling. Early arterial sampling shows that the mixing of the NMBA in the central volume of distribution is incomplete. A parametric PD (sigmoid Emax) model could not describe the time course of effect of the NMBAs adequately.

Pharmacokinetics in organs and the intact body: model validation and reduction

Physiological pharmacokinetic models are based on the structure of the circulatory system reflecting the convective transport of drug by blood flow to the various organs and tissues. Distribution kinetics at the organ level is mostly simplified as transfer between well-stirred compartments neglecting a priori the effects of intravascular dispersion and diffusion within tissue parenchyma. Recirculatory models based on residence time theory overcome these structural limitations since they allow in a most general way the decomposition of the body into its natural subsystems. Because of the unidentifiability of the global multi-organ model on the basis of plasma concentration-time curves the following methods/experimental designs will be discussed which provide quantitative information regarding the subsystems under in vivo conditions: (i) determination of tissue concentration-time profiles (destructive sampling), (ii) estimation of the organ transit time density from input/output profiles and (iii) application of a recirculatory model with reduced complexity to clinical pharmacokinetic data.