Regional pharmacokinetics. I. Physiological and physicochemical basis

Better prediction of the local concentration-effect relationship: the role of physiologically based pharmacokinetics and quantitative systems pharmacology and toxicology in the evolution of model-informed drug discovery and development

Model-informed drug discovery and development (MID3) is an umbrella term under which sit several computational approaches: quantitative systems pharmacology (QSP), quantitative systems toxicology (QST) and physiologically based pharmacokinetics (PBPK). QSP models are built using mechanistic knowledge of the pharmacological pathway focusing on the putative mechanism of drug efficacy; whereas QST models focus on safety and toxicity issues and the molecular pathways and networks that drive these adverse effects. These can be mediated through exaggerated on-target or off-target pharmacology, immunogenicity or the physiochemical nature of the compound. PBPK models provide a mechanistic description of individual organs and tissues to allow the prediction of the intra- and extra-cellular concentration of the parent drug and metabolites under different conditions. Information on biophase concentration enables the prediction of a drug effect in different organs and assessment of the potential for drug-drug interactions. Together, these modelling approaches can inform the exposure-response relationship and hence support hypothesis generation and testing, compound selection, hazard identification and risk assessment through to clinical proof of concept (POC) and beyond to the market.

The Influence of Site of Collection on Postmortem Morphine Concentrations in Heroin Overdose Victims

When assaying for postmortem morphine concentration, significant site sampling variability exists between central and peripheral sampling sites and even within sampling regions of the body. To study the variation, 76 suspected heroin overdoses were identified. Each had femoral artery (FA) and vein (FV), left and right ventricle and pooled heart blood samples obtained at autopsy. Forty-four tested positive for morphine. Morphine concentrations were determined by gas chromatography/mass spectrometry, with sampling site differences reported as log-transformed ratios and compared by signed rank test. The mean FA to FV ratio for total morphine was 1.2 (range 0-4.5). The ratio for left heart to right heart total morphine was 1.1 (range 0.4-3.2). Left ventricular to FV total morphine ratio was 2.0 (range 0.6-6.9). In these opioid overdose deaths, FA and FV morphine concentrations are usually similar, although up to 4.5-fold differences were noted. Centrally obtained morphine concentrations are on average twice as high compared with peripheral morphine concentrations. Left and right ventricular morphine concentrations were usually similar, although up to 3.2-fold differences were noted (left side higher).

Human subcutaneous tissue distribution of fluconazole: comparison of microdialysis and suction blister techniques

AIMS

To investigate uptake of fluconazole into the interstitial fluid of human subcutaneous tissue using the microdialysis and suction blister techniques.

METHODS

A sterile microdialysis probe (CMA/60) was inserted subcutaneously into the upper arm of five healthy volunteers following an overnight fast. Blisters were induced on the lower arm using gentle suction prior to ingestion of a single oral dose of fluconazole (200 mg). Microdialysate, blister fluid and blood were sampled over 8 h. Fluconazole concentrations were determined in each sample using a validated HPLC assay. In vivo recovery of fluconazole from the microdialysis probe was determined in each subject by perfusing the probe with fluconazole solution at the end of the 8 h sampling period. Individual in vivo recovery was used to calculate fluconazole concentrations in subcutaneous interstitial fluid. A physiologically based pharmacokinetic (PBPK) model was used to predict fluconazole concentrations in human subcutaneous interstitial fluid.

RESULTS

There was a lag-time (approximately 0.5 h) between detection of fluconazole in microdialysate compared with plasma in each subject. The in vivo recovery of fluconazole from the microdialysis probe ranged from 57.0 to 67.2%. The subcutaneous interstitial fluid concentrations obtained by microdialysis were very similar to the unbound concentrations of fluconazole in plasma with maximum concentration of 4.29 +/- 1.19 microg ml(-1) in subcutaneous interstitial fluid and 3.58 +/- 0.14 microg ml(-1) in plasma. Subcutaneous interstitial fluid-to-plasma partition coefficient (Kp) of fluconazole was 1.16 +/- 0.22 (95% CI 0.96, 1.35). By contrast, fluconazole concentrations in blister fluid were significantly lower (P < 0.05, paired t-test) than unbound plasma concentrations over the first 3 h and maximum concentrations in blister fluid had not been achieved at the end of the sampling period. There was good agreement between fluconazole concentrations derived from microdialysis sampling and those estimated using a blood flow-limited PBPK model.

CONCLUSIONS

Microdialysis and suction blister techniques did not yield comparable results. It appears that microdialysis is a more appropriate technique for studying the rate of uptake of fluconazole into subcutaneous tissue. PBPK model simulation suggested that the distribution of fluconazole into subcutaneous interstitial fluid is dependent on tissue blood flow.

An in vitro-in vivo validation of the isolated perfused tumor and skin flap preparation as a model of cisplatin delivery to tumors

The isolated perfused tumor and skin flap (IPTSF) is a unique model system in which drug disposition is evaluated in tumor tissue and surrounding normal tissue, both of which are supplied by the same vascular system. We compared tissue Pt concentrations obtained following systemic administration of cisplatin (CDDP) to whole pigs bearing tumored skin flaps with data obtained from IPTSF treated similarly. During the in vivo study, CDDP was administered intravenously to six pigs that had tumor and skin flaps. IPTSF were created in four pigs and isolated in a perfusion chamber and perfused with medium containing CDDP for 180 min. Venous plasma or perfusate samples were serially collected throughout perfusion. Tissue samples were collected after perfusion was complete. All samples were assayed for Pt by atomic absorption spectroscopy. Area under the curve of Pt profiles from IPTSF and in vivo perfused flaps were not significantly different. Pt concentrations were significantly higher in tumor samples from in vivo perfused flaps than in samples from IPTSF. Pt concentrations in skin and subcutaneous tissue were not significantly different. When consideration is given to all of the potential variables that were operative in these experiments, the results of this study demonstrate that Pt distribution within the IPTSF was comparable to that obtained in vivo.

Compartment model to describe peripheral arterial-venous drug concentration gradients with drug elimination from the venous sampling compartment

The central compartment of an n-compartment mammillary model was concatenated with a gradient compartment (i.e., venous sampling compartment) and with a conventional effect compartment to model arterial-venous drug concentration gradients and the arterial drug concentration vs effect relationship, respectively. To model drug metabolism during transit between the peripheral arterial and peripheral venous circulations, the gradient compartment included a first-order elimination path. Simulations of the model were employed to demonstrate the impact of sampling site (e.g., arterial vs peripheral venous) on analyses of the concentration vs effect relationship.

From piecewise to full physiologic pharmacokinetic modeling: applied to thiopental disposition in the rat

Physiologically based pharmacokinetic modeling procedures employ anatomical tissue weight, blood flow, and steady tissue/blood partition data, often obtained from different sources, to construct a system of differential equations that predict blood and tissue concentrations. Because the system of equations and the number of variables optimized is considerable, physiologic modeling frequently remains a simulation activity where fits to the data are adjusted by eye rather than with a computer-driven optimization algorithm. We propose a new approach to physiological modeling in which we characterize drug disposition in each tissue separately using constrained numerical deconvolution. This technique takes advantage of the fact that the drug concentration time course, CT(t), in a given tissue can be described as the convolution of an input function with the unit disposition function (UDFT) of the drug in the tissue, (i.e., CT(t) = (Ca(t)QT)*UDFT(t) where Ca(t) is the arterial concentration, Q tau is the tissue blood flow and * is the convolution operator). The obtained tissue until disposition function (UDF) for each tissue describes the theoretical disposition of a unit amount of drug infected into the tissue in the absence of recirculation. From the UDF, a parametric model for the intratissue disposition of each tissue can be postulated. Using as input the product of arterial concentration and blood flow, this submodel is fit separately utilizing standard nonlinear regression programs. In a separate step, the entire body is characterized by reassembly of the individuals submodels. Unlike classical physiologic modeling the fit for a given tissue is not dependent on the estimates obtained for other tissues in the model. Additionally, because this method permits examination of individual UDFs, appropriate submodel selection is driven by relevant information. This paper reports our experience with a piecewise modeling approach for thiopental disposition in the rat.

An assessment of methods for sampling blood to characterize rapidly changing blood drug concentrations

The accuracy of different blood sampling methods used to characterize rapidly changing blood drug concentrations was examined both in vitro and in vivo. It was shown in vitro that blood sampling methods based on the fraction collection principle failed to characterize a "square wave" change in drug concentration, and there was a 9-16-s delay before achieving 95% of the expected drug concentration. Varying the catheter size and length did not improve the response. This observation is consistent with laminar and/or turbulent flow producing dispersion and mixing of blood of different drug concentrations in the catheter. A sampling method (flush and withdrawal) was developed to minimize these effects. In vivo studies showed that peak blood drug concentrations obtained using this method after an iv bolus of a drug were approximately 25-28% higher than those simultaneously obtained by methods based on fraction collection principles. It is concluded that blood sampling methods based on fraction collection principles can produce significant errors in measured blood drug concentrations. The error is greater the greater the rate of change of the blood drug concentrations.

Regional pharmacokinetics. III. Modelling methods

Regional pharmacokinetics is the study of drug concentrations in specific regions of the body due to drug uptake and elution. Mathematical methods of interpreting regional pharmacokinetic data can vary greatly in their complexity depending on their intended use (i.e. to describe or predict), but must reinforce rather than replace experimental pharmacokinetics. 'Black box' analysis provides and empirical method for the study of complex pharmacokinetic systems using either statistical moment or linear systems analysis. However, these methods are only applicable to linear and time-invariant systems, and ignore the large body of information concerning the physiological and physiochemical basis of regional pharmacokinetics. Clearance concepts are suitable for describing linear drug uptake processes, but mass balance principles have wider applications in describing the rate and extent of both drug uptake and elution. Compartmental models of a region can vary from single compartment descriptions based on the concept of venous equilibrium to complex multi-compartmental models of the intravascular, interstitial, and intracellular spaces, in which drug transport between compartments is a function of drug binding and ionization. Ultimately, as more regional pharmacokinetic information is obtained, more complex three dimensional models may be necessary such as those used to describe the uptake of oxygen from capillaries.

The in vivo blood, fat and muscle concentrations of lignocaine and bupivacaine in the hindquarters of sheep

1. A method was developed for sampling muscle and fat from the hindquarters of sheep undergoing spinal anaesthesia. The method was used to measure the concentrations of lignocaine and bupivacaine in the blood, muscle and fat of the hindquarters of sheep during and after 180 min constant-rate infusions of the drugs. 2. For both drugs the muscle drug concentrations were a relatively constant ratio of the simultaneous arterial blood drug concentrations during and after the infusion. 3. There was uptake of both lignocaine and bupivacaine into subcutaneous fat during the infusions. At the end of the infusion the ratio of the fat: arterial blood drug concentrations were 1.54 (SD = 0.57, n = 4) and 3.1 (SD = 1.4, n = 4) for lignocaine and bupivacaine, respectively. 4. The drug concentrations in fat declined relatively slowly after the infusion. The ratio of the fat: arterial blood drug concentrations 180 min after the end of the infusion was 21.5 (SD 4.0, n = 3) and for lignocaine, and 120 min after the end of the infusion was 9.54 (SD 5.2, n = 3) for bupivacaine. 5. It was concluded that the concentrations of lignocaine and bupivacaine in muscle were essentially in equilibrium with the arterial concentrations during and after the infusion. However, the concentrations of lignocaine and bupivacaine in fat were not in equilibrium with the arterial concentrations in the post-infusion period.