Uptake in vitro of lipophilic model compounds into adipose tissue preparations and lipids

In vitro uptake of 11 lipophilic model compounds into rat epididymal adipose tissue slices, adipocytes, triglycerides, and lecithin was studied. Relative uptake at equilibrium into adipose tissue slices increased from 6 to 87% in the following sequence: phenazone, morphine less than pentobarbital less than glutethimide, phenylbutazone less than thiopental, methadone less than chlorpromazine, imipramine. In the presence of albumin a similar sequence was obtained at lower uptake levels, with DDE and 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) on top with 95% uptake. However, the time to reach equilibrium was unproportionately greater for DDE and 6-CB (16-40 hr) than for other compounds (1-4 hr). A linear positive correlation was found between relative uptake and partition coefficient (octanol/water). Relative uptake was independent of drug concentration. There were no significant differences between uptake values measured with adipose tissue slices, adipocytes, triolein, and a saturated short-chain triglyceride. In contrast, uptake into lecithin was not correlated with the octanol partition coefficient. Thiopental, imipramine, and 6-CB were taken up into lean tissue slices (liver, lung, skin) in excess of their lipid content, suggesting additional binding sites. Release from preloaded adipose tissue slices followed first order kinetics, was accelerated by albumin, and was much slower for 6-CB and DDE than for thiopental and imipramine. The results indicate that uptake of lipophilic xenobiotics in vitro is a partition process between the aqueous medium and the triglyceride of the adipose tissue preparation. In contrast, the extent of adipose tissue storage of drugs in vivo has recently been shown not to correlate with octanol partition coefficients.     

Prediction of skin permeation of highly lipophilic compounds; in vitro model with a modified receptor phase

A skin permeation in vitro model for highly lipophilic compounds was developed, containing bovine serum albumin (BSA) in the receptor phase. Skin permeation rates of three homologous series of compounds varying within a wide range of hydrophobicity were measured. For the more water-insoluble compounds (log P_IM 3), it was demonstrated that the modified in vitro model proposed ensures a more efficient collection of the permeant in the receptor phase.     

Measurement and prediction of hydrophobicity parameters for highly lipophilic compounds: application of the HPLC column-switching technique to measurement of log P of diarylpyrazines

In the preparatory stage of structure-activity relationship (QSAR) studies of anti-platelet aggregant pyrazine derivatives, log P values (P: 1-octanol/water partition coefficient) of diarylpyrazines were measured by a newly developed HPLC column-switching technique. The system consists of two processes: (1) adsorption of the sample at the top end of a short precolumn, and then (2) quantifying the enriched analyte by a conventional analytical column. By using the log P values thus obtained, the correction factor for the steric hindrance caused by the vicinal diphenyl groups was estimated. The log k values (k; retention factor) were also measured with methanol-buffer (pH 7.4) eluents and related to log P. The eluent of 50% methanol content (M50) gave a good linear relationship over a wide range of log P (-0.3< log P < 5.2), indicating that log k(M50) parameter is useful for predicting the log P value.     

Engineered tissue as a model to study cell and tissue function from a biophysical perspective

Cells, tissues and organs function in a three-dimensional (3D) environment. Ideally, cell-based models that capture both the 3D organization and multi-cellular complexity of the native system provide the most powerful tools for screening the effects of therapeutic candidates. This approach to drug discovery bridges tissue engineers, who are constructing 3D tissues, with biologists, who are studying healthy versus diseased states and to pharmacologists, who are developing screening assays. Within this context, an innovative biophysical perspective of tissue morphogenesis, malignancy and treatment responsiveness has been established recently. Numerous experimental studies have shown that mechanical loading regulates the anabolic and catabolic metabolism of cells. Anabolic mechanisms, in particular, are of vital importance in the process of tissue engineering, which is of increasing scientific and clinical interest. Cells seeded and cultured in appropriate constructs should be mechanically stimulated to produce and to structure the required constituents of the extracellular matrix. However, the determination of the most effective type of loading, the appropriate load history and the mechanical field variables responsible for the stimulation of the cell activity, as well as the pathways of communication among cells, are still subject of contrary discussions and motivation of recent investigation. In this review we discuss the tissue-level response to mechanical signalling, we provide an overview of prominent techniques currently used for exerting mechanical stresses on engineered tissue and an overview of numerical mechanics studies providing information on mechanical field variables potentially triggering the biological activity.     

The prediction of drug metabolism, tissue distribution, and bioavailability of 50 structurally diverse compounds in rat using mechanism-based absorption, distribution, and metabolism prediction tools.

The aim of this study was to assess a physiologically based modeling approach for predicting drug metabolism, tissue distribution, and bioavailability in rat for a structurally diverse set of neutral and moderate-to-strong basic compounds (n = 50). Hepatic blood clearance (CL(h)) was projected using microsomal data and shown to be well predicted, irrespective of the type of hepatic extraction model (80% within 2-fold). Best predictions of CL(h) were obtained disregarding both plasma and microsomal protein binding, whereas strong bias was seen using either blood binding only or both plasma and microsomal protein binding. Two mechanistic tissue composition-based equations were evaluated for predicting volume of distribution (V(dss)) and tissue-to-plasma partitioning (P(tp)). A first approach, which accounted for ionic interactions with acidic phospholipids, resulted in accurate predictions of V(dss) (80% within 2-fold). In contrast, a second approach, which disregarded ionic interactions, was a poor predictor of V(dss) (60% within 2-fold). The first approach also yielded accurate predictions of P(tp) in muscle, heart, and kidney (80% within 3-fold), whereas in lung, liver, and brain, predictions ranged from 47% to 62% within 3-fold. Using the second approach, P(tp) prediction accuracy in muscle, heart, and kidney was on average 70% within 3-fold, and ranged from 24% to 54% in all other tissues. Combining all methods for predicting V(dss) and CL(h) resulted in accurate predictions of the in vivo half-life (70% within 2-fold). Oral bioavailability was well predicted using CL(h) data and Gastroplus Software (80% within 2-fold). These results illustrate that physiologically based prediction tools can provide accurate predictions of rat pharmacokinetics.     

Effect of fat tissue volume on the distribution kinetics of biperiden as a function of age in rats

The relationship between the volume of fat tissue and variations in the time course of plasma biperiden concentration in rats has been examined in three different groups (4-, 10-, and 50-week-old rats). The plasma concentrations at 24 hr after iv injection of 3.2 mg/kg varied between 0.8 ng/ml (4-week-old rats) and 5.0 ng/ml (50-week-old rats). The rank order of the steady state distribution volume of biperiden was: 50-week-old rats greater than 10-week-old rats greater than 4-week-old rats. The fat volume of the whole body, extracted from the dried carcass with ether, varied between 42 g/kg (4-week-old rats) and 167 g/kg (50-week-old rats). There was a good correlation between the steady state distribution volume of biperiden per lean mass body weight and the fat volume per lean mass body weight (r = 0.987). The fat/plasma concentration ratios at 8 hr after the iv injection varied between 600 (4-week-old rats) and 200 (50-week-old rats), whereas the brain/plasma concentration ratios were identical to those at steady state among the three groups. The time courses of biperiden concentration in plasma, brain, and fat were simulated using a physiological pharmacokinetic model. There was reasonable agreement between the model predictions and the observed data, suggesting that the change in the fat volume is a dominant determinant of the distribution volume of biperiden in rats. Age-related changes in tissue and plasma concentrations are discussed in relation to the clinical usefulness of the blood level monitoring.     

Transporters as a determinant of drug clearance and tissue distribution

Transporters play an important role in the processes of drug absorption, distribution and excretion. In this review, we have focused on the involvement of transporters in drug excretion in the liver and kidney. The rate of transporter-mediated uptake and efflux determines the rate of renal and hepatobiliary elimination. Transporters are thus important as a determinant of the clearance in the body. Even when drugs ultimately undergo metabolism, their elimination rate is sometimes determined by the uptake rate mediated by transporters. Transporters regulate the pharmacological and/or toxicological effect of drugs because they limit their distribution to tissues responsible for their effect and/or toxicity. For example, the liver-specific distribution of some statins via organic anion transporters helps them to produce their high pharmacological effect. On the other hand, as in the case of metformin taken up by organic cation transporter 1, drug distribution to the tissue(s) may enhance its toxicity. As transporter-mediated uptake is a determinant of the drug elimination rate, drug–drug interactions involving the process of transporter-mediated uptake can occur. In this review, we have introduced some examples and described their mechanisms.

More recently, some methods to analyze such transporter-mediated transport have been reported. The estimation of the contributions of transporters to the net clearance of a drug makes it possible to predict the net clearance from data involving drug transport in transporter-expressing cells. Double transfected cells, where both uptake and efflux transporters are expressed on the same polarized cells, are also helpful for the analysis of the rate of transporter-mediated transcellular transport.     

Evaluation of skin permeation and accumulation profiles of a highly lipophilic fatty ester

The aim was to evaluate the skin permeation and accumulation profiles of a highly lipophilic fatty ester using the combination of various permeation enhancing techniques to study the potential of highly lipophilic fatty esters as local topical agents. Permeation and accumulation profiles of ketorolac stearate (C18:0) were studied using solubility improved formulation, supersaturated solution of permeant in enhancer vehicle, lipophilic receptor solution, enhancer pretreatment, and the removal of stratum corneum and delipidization of skins. Impermeability and minimal skin accumulation of ketorolac stearate could delineate a preliminary possibility for designing safer topical agents without systemic absorption.     

Evaluation of perfused porcine skin as a model system to quantitate tissue distribution of fullerene nanoparticles

Nanomaterials are increasingly playing a role in society for uses ranging from biomedicine to microelectronics, however pharmacokinetic studies, which will be necessary for human health risk assessments, are limited. Tissue distribution, one component of pharmacokinetics, can be assessed by quantifying arterial extraction of materials in an isolated perfused porcine skin flap (IPPSF). The objective of this study was to assess the IPPSF as a model system to quantitate the distribution of fullerene nanoparticles (nC₆₀) from the vascular space into tissues. IPPSFs were perfused for 4 h with 0.885 μg/mL nC₆₀ in media with immunoglobulin G present (IgG⁺) or absent (IgG⁻) followed by a 4 h perfusion with media only during a washout phase. Arterial and venous concentrations of nC₆₀ were measured in the media by HPLC–UV/vis chromatography. Steady state differences in the arterial and venous nC₆₀ concentrations were compared to determine extraction from the vascular space of the IPPSF, and the venous nC₆₀ concentration versus time profiles were used to calculate compartmental pharmacokinetic parameters. The steady state differences in the arterial and venous concentrations in the IPPSF were small with extraction percentages (mean ± sd) of 8.2 ± 5.7% and 4.2 ± 6.7% for IgG⁺ and IgG⁻ media, respectively, and were not significantly different between the types of media. The venous concentrations of nC₆₀ in both types of media were best fit with a 2 compartment model with terminal half lives (harmonic mean) of 17.5 and 28.0 min for IgG⁺ and IgG⁻ media, respectively. The apparent volumes of distribution at steady state were 0.12 ± 0.047 and 0.10 ± 0.034 L/kg, for IgG⁺ and IgG⁻ media, respectively. By 4 h following infusion of nC₆₀, the recovery of nC₆₀ in the venous effluent was 94 ± 5.5% and 97 ± 6.8% of the infused nC₆₀ for IgG⁺ and IgG⁻ media, respectively. Based on the apparent volume of distribution, the low extraction during the perfusion, and the high percentage recovery following the washout phase, there was limited distribution of nC₆₀ from the vascular space into the extracellular space and negligible intracellular uptake of nC₆₀ in this system.     

Differences in adipose tissue distribution of basic lipophilic drugs between intraperitoneal and other routes of administration

1. Distribution of 14C-methadone in male rats was studied after administration of single i.p. doses. Highest concentrations were attained after 30 min in the decreasing order of abdominal adipose tissues, liver, lung, other organs. Concentrations in abdominal adipose tissues were 20 times higher than in subcutaneous adipose tissue. This is in contrast with what occurs with other routes of administrations, where only low concentrations are attained in both subcutaneous and abdominal adipose tissues. 2. The situation in the peritoneal cavity after i.p. injection was simulated in vitro by incubation of whole, excised abdominal adipose tissues of rats with methadone and other basic drugs (dibenzepine, alprenolol, opipramol, propranolol, chlorpheniramine, desipramine, imipramine and chlorpromazine) for 6 h at pH 7.4. These drugs were readily taken up (25 to 74% at equilibrium). 3. There was a positive correlation between uptake and lipophilicity of the drugs as measured by log P (octanol/water). Less lipophilic drugs such as amphetamine, morphine and chlorphentermine were not appreciably taken up from the incubation medium. The threshold log P-value for appreciable adipose tissue uptake is around 2. 4. It is concluded from these data and related studies that basic lipophilic drugs are not stored in adipose tissues in vivo, except when given via the i.p. route. In the latter case the storage appears to result from non-systemic, diffusional uptake from the peritoneal cavity.     

Carbon dioxide supercritical fluid extracts from yarrow and rose hip herbal dust as valuable source of aromatic and lipophilic compounds

The present study was designed to demonstrate the potential of supercritical extracts from Achillea millefolium and Rosa canina herbal dust, and their mixtures, as a source of valuable aromatic and lipophilic compounds. The supercritical carbon dioxide extraction (SFE-CO₂) was performed at the pressures from 10-30 MPa, providing the total extraction yields (EY) in the range from 0.12 to 10.57%, being the highest when pure R. canina herbal dust was extracted using SFE-CO₂ at 30 MPa for 5 h. Chemical profiles of SFE-CO₂ extracts were determined by GC-MS and GC-FID. Oxygenated monoterpenes and sesquiterpene hydrocarbons were among the most abundant compounds in the extracts produced from A. millefolium and mixtures with a higher share of A. millefolium herbal dust. In the same mixtures, at the pressure of 10 MPa, a cosolvent effect was observed, which provided enhanced extraction of eucalyptol. The major tocol in A. millefolium and R. canina mixtures was α-tocopherol (589.49 mg/L). By investigating the influence of extraction pressure, it has been determined that higher compound recoveries could be obtained at lower pressures. The results clearly demonstrate that SFE-CO₂ extracts of the A. millefolium and R. canina and their herbal dust mixtures are a promising source of valuable compounds to be used in pharmaceutical formulations.     

Xenopus laevis is a potential alternative model animal species to study reproductive toxicity of phytoestrogens

This study investigated effects of phytoestrogen quercetin on the gonadal development in Xenopus laevis. X. laevis at Nieuwkoop and Faber stage 46/47 were exposed to 50, 100 and 200 microg/L quercetin till 1 month postmetamorphosis. Gonads from frogs at 1 and 3 months postmetamorphosis were examined in gross morphology and histology. The highest dose of quercetin as well as estradiol (E2) significantly increased the percentages of phenotypic females. Exposure to quercetin at all doses induced abnormal testes with certain ovarian characteristics to some degree in gross morphology, including ovotestes. The abnormality rate exceeded 10% in each quercetin treatment. Histologic examination revealed that some abnormal testes exhibited intersexuality with testicular structure and ovarian structure or oocytes interspersed in testicular structure at 1 month postmetamorphosis. At 3 months postmetamorphosis, testicular abnormalities were more obvious, such as necrosis or apoptosis of spermatogonia, occurrence of developed or undeveloped oocytes, delay of the development of seminiferous tubes without or less late stage spermatocytes. The results have shown that quercetin cannot only feminize but also impair testicular development of X. laevis, i.e. X. laevis is sensitive to phytoestrogen. It is suggested that X. laevis might be an alternative model species to study reproductive toxicity of phytoestrogens.     

Tubulin as a major determinant of tissue distribution of vincristine

The tissue distribution of vincristine was correlated with the tubulin content in different mouse tissues. The concentration of tubulin in the mouse tissues was determined by estimation of the tissue binding capacities for colchicine. Significant differences in the tubulin concentration were observed among the tissues. The comparison of the apparent tissue-to-plasma partition coefficients (KPapp) of vincristine (taken from the literature) with the tubulin concentration (expressed by the binding capacities for colchicine) showed a good correlation. These results suggest that the in vivo distribution of vincristine is predicted by the tissue tubulin concentration.     

Investigation of alternative compounds to poly(E-MA) as a polymeric surfactant for preparation of microcapsules by phase separation method

Various water-soluble polymers were used to examine an alternative emulsifier for poly(ethylene-alt-maleic anhydride), used in the preparation of crosslinked polyurea microcapsules. Microcapsules were successfully prepared by using the water-soluble polymers with large molecular weight alternating copolymers, namely poly(olefin-maleic anhydride), poly(olefin-maleic acid), and poly(acrylic acid). On the other hand, no microcapsule resulted from olefin-maleic acid with small molecular weight alternating copolymers. From these results, the following guidelines were obtained for the selection of polymeric surfactants suitable for crosslinked polyurea microcapsule. A polymeric surfactant must have maleic acid or a carboxyl group in order to form a crosslinked polyurea microcapsule membrane. Furthermore, to form a stronger capsule membrane it is desirable to have a maleic anhydride group. It is also important for membrane formation that the polymeric surfactant has a suitable molecular weight.     

Investigation of alternative compounds to poly(E-MA) as a polymeric surfactant for preparation of microcapsules by phase separation method

Various water-soluble polymers were used to examine an alternative emulsifier for poly(ethylene-alt-maleic anhydride), used in the preparation of crosslinked polyurea microcapsules. Microcapsules were successfully prepared by using the water-soluble polymers with large molecular weight alternating copolymers, namely poly(olefin-maleic anhydride), poly(olefin-maleic acid), and poly(acrylic acid). On the other hand, no microcapsule resulted from olefin-maleic acid with small molecular weight alternating copolymers. From these results, the following guidelines were obtained for the selection of polymeric surfactants suitable for crosslinked polyurea microcapsule. A polymeric surfactant must have maleic acid or a carboxyl group in order to form a crosslinked polyurea microcapsule membrane. Furthermore, to form a stronger capsule membrane it is desirable to have a maleic anhydride group. It is also important for membrane formation that the polymeric surfactant has a suitable molecular weight.     

Physiologically based pharmacokinetic modelling: a sub-compartmentalized model of tissue distribution

We present a sub-compartmentalized model of drug distribution in tissue that extends existing approaches based on the well-stirred tissue model. It is specified in terms of differential equations that explicitly account for the drug concentration in erythrocytes, plasma, interstitial and cellular space. Assuming, in addition, steady state drug distribution and by lumping the different sub-compartments, established models to predict tissue-plasma partition coefficients can be derived in an intriguingly simple way. This direct link is exploited to explicitly construct and parameterize the sub-compartmentalized model for moderate to strong bases, acids, neutrals and zwitterions. The derivation highlights the contributions of the different tissue constituents and provides a simple and transparent framework for the construction of novel tissue distribution models.     

Single daily dose and simplified dosing regimens as a method to improve antibiotic therapy

Pharmacists can be an important member of the patient care team by assisting with the development of dosing regimens. By optimizing the pharmacokinetic properties of the antimicrobial agents, regimens can be developed that are simple to manage. Newer approaches to simplifying dosing regimens include once-daily aminoglycoside therapy, continuous infusion beta-lactams, and utilizing agents with long half-lives such as ceftriaxone and azithromycin. These efforts could result in improved compliance and in some instances decrease costs and toxicities associated with antibiotic therapy.