Lipophilic Model Compound Research Articles

Development of a Novel Lipid-Based Nanosystem Functionalized with WGA for Enhanced Intracellular Drug Delivery.

Despite a considerable number of new antibiotics under going clinical trials, treatment of intracellular pathogens still represents a major pharmaceutical challenge. The use of lipid nanocarriers provides several advantages such as protection from compound degradation, increased bioavailability, and controlled and targeted drug release. Wheat germ agglutinin (WGA) is known to have its receptors on the alveolar epithelium and increase phagocytosis. The present study aimed to produce nanostructured lipid carriers with novel glycosylated amphiphilic employed to attach WGA on the surface of the nanocarriers to improve intracellular drug delivery. High-pressure homogenization was employed to prepare the lipid nanocarriers. In vitro, high-content analysis and flow cytometry assay was employed to study the increased uptake by macrophages when the nanocarriers were grafted with WGA. A lipid nanocarrier with surface-functionalized WGA protein (~200 nm, PDI > 0.3) was successfully produced and characterized. The system was loaded with a lipophilic model compound (quercetin; QU), demonstrating the ability to encapsulate a high amount of compound and release it in a controlled manner. The nanocarrier surface functionalization with the WGA protein increased the phagocytosis by macrophages. The system proposed here has characteristics to be further explored to treat intracellular pathogens.

A Customized Screening Tool Approach for the Development of a Self-Nanoemulsifying Drug Delivery System (SNEDDS)

The present study focused on establishing a novel, (pre-)screening approach that enables the development of promising performing self-nanoemulsifying drug delivery systems (SNEDDSs) with a limited number of experiments. The strategic approach was based on first identifying appropriate excipients (oils/lipids, surfactants, and co-solvents) providing a high saturation solubility for lipophilic model compounds with poor aqueous solubility. Excipients meeting these requirements were selected for SNEDDS development, and a special triangular mixture design was applied for determining excipient ratios for the SNEDDS formulations. Celecoxib and fenofibrate were used as model drugs. Formulations were studied applying a specific combination of in vitro characterization methods. Specifications for a promising SNEDDS formulation were self-imposed: a very small droplet size (< 50 nm), a narrow size distribution of these droplets (PDI < 0.15) and a high transmittance following SNEDDS dispersion in water (> 99% in comparison with purified water). Excipients that provided a nanoemulsion after dispersion were combined, and ratios were optimized using a customized mapping method in a triangular mixture design. The best performing formulations were finally studied for their in vitro release performance. Results of the study demonstrate the efficiency of the customized screening tool approach. Since it enables successful SNEDDS development in a short time with manageable resources, this novel screening tool approach could play an important role in future SNEDDS development.Graphical abstract

Development of polyoxyethylene (2) oleyl ether-gliadin nanoparticles: Characterization and in vitro cytotoxicity

Natural polymers have been widely investigated as materials for the delivery of active compounds as a consequence of their biocompatibility, low-cost and the opportunity they furnish to obtain micro- and nanostructures. In this investigation, commercial wheat gliadin was used as raw material with the aim of obtaining a vegetal protein-based nanoformulation to be used for various applications. The influence of non-ionic and anionic surfactants on the physico-chemical properties of gliadin nanoparticles was evaluated in order to propose a suitable candidate able to stabilize the colloidal structure. The use of Super Refined polyoxyethylene (2) oleyl ether gave the best results, promoting the formation of spherical-shaped nanosystems with a narrow size distribution. The oleyl ether-based emulsifier prevented the destabilization of the colloidal systems when pH- and temperature-dependent stress was applied. A freeze-dried formulation was obtained when mannose was used as a cryoprotectant. Polyoxyethylene (2) oleyl ether-stabilized nanosystems were shown to retain and release both hydrophilic and lipophilic model compounds in a controlled manner. The cytotoxicity of the surfactant-free and polyoxyethylene (2) oleyl ether-stabilized gliadin based nanosystems was assessed on human cells, both normal and tumoural, in order to investigate the concentrations of particles that can be used during in vitro experiments. Polyoxyethylene (2) oleyl ether-stabilized gliadin-based nanosystems are promising carriers for the delivery of several active compounds.

Enhancement of macrophage uptake via phosphatidylserine-coated acetalated dextran nanoparticles

Although vital to the immune system, macrophages can act as reservoirs for pathogens such as tuberculosis and human immunodeficiency virus. Limitations in the treatment of such diseases include targeting therapeutics directly to macrophages and the large systemic dosages needed. The objective of this study is to develop a nanoparticle (NP)-based drug delivery system that can provide targeted delivery into macrophages. Acetalated dextran (Ac-Dex) NP loaded with the lipophilic model compound curcumin (CUR) were synthesized and coated in 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), a phospholipid that induces phagocytosis in macrophages. DPPS-CUR NP were found to release 67.8% of encapsulated CUR within 24 h at pH 5.35 and exhibited minimal CUR release (6.3%) at pH 7.4. DPPS-CUR NP were uptaken by murine macrophages significantly more than NP without DPPS coating and NP exposure to these macrophages resulted in minimal toxicity to the cells and minimal nitric oxide production. These results suggest that the combination of the DPPS coating and pH-sensitive polymer Ac-Dex can provide a NP delivery system capable of enhanced uptake by macrophages and potential systemic stability to more effectively deliver drugs of interest. As a result, the described DPPS-CUR NP can serve as a viable delivery system for the treatment of macrophage-associated diseases.

In vitro digestion of lactoferrin-glycomacropeptide nanohydrogels incorporating bioactive compounds: Effect of a chitosan coating

The behaviour of lactoferrin (Lf) – glycomacropeptide (GMP) nanohydrogels with and without a chitosan coating was evaluated during gastrointestinal digestion. The application of a chitosan coating allowed to reduce the protein degradation from 86% to 76% for Lf and from 71% to 53% for GMP.Protein bioaccessibility results showed that in nanohydrogels with chitosan coating 23% of Lf and 40% of GMP remained intact until absorption. Based on these results, the bioaccessibility of two different bioactive compounds encapsulated in Lf-GMP nanohydrogels with chitosan coating was evaluated during gastrointestinal digestion. Curcumin was used as lipophilic model compound and caffeine as a hydrophilic model compound. Bioaccessibility of curcumin in coated nanohydrogels was 72% while the corresponding value for curcumin in free form only reached 66%. It was also observed that under simulated gastric and intestinal conditions, free curcumin lost around 68% of its antioxidant activity while when incorporated into nanohydrogels only 30% of this activity was lost. Results also showed that the bioaccessibility of caffeine encapsulated in coated nanohydrogels was 63% while caffeine in free form only reached 59%.

Effects of 6-Hydroxyceramides on the Thermotropic Phase Behavior and Permeability of Model Skin Lipid Membranes.

Ceramides (Cer) based on 6-hydroxysphingosine are important components of the human skin barrier, the stratum corneum. Although diminished concentrations of 6-hydroxyCer have been detected in skin diseases such as atopic dermatitis, our knowledge on these unusual sphingolipids, which have only been found in the skin, is limited. In this work, we investigate the biophysical behavior of N-lignoceroyl-6-hydroxysphingosine (Cer NH) in multilamellar lipid membranes composed of Cer/free fatty acids (FFAs) (C16-C24)/cholesterol/cholesteryl sulfate. To probe the Cer structure-activity relationships, we compared Cer NH membranes with membranes containing Cer with sphingosine (Cer NS), dihydrosphingosine, and phytosphingosine (Cer NP), all with the same acyl chain length (C24). Compared with Cer NS, 6-hydroxylation of Cer not only increased membrane water loss and permeability in a lipophilic model compound but also dramatically increased the membrane opposition to electrical current, which is proportional to the flux of ions. Infrared spectroscopy revealed that Cer hydroxylation (in either Cer NH or Cer NP) increased the main transition temperature of the membrane but prevented good Cer mixing with FFAs. X-ray powder diffraction showed not only lamellar phases with shorter periodicity upon Cer hydroxylation but also the formation of an unusually long periodicity phase (d = 10.6 nm) in Cer NH-containing membranes. Thus, 6-hydroxyCer behaves differently from sphingosine- and phytosphingosine-based Cer. In particular, the ability to form a long-periodicity lamellar phase and highly limited permeability to ions indicate the manner in which 6-hydroxylated Cer contribute to the skin barrier function.

Evidence for a new mechanism behind HIFU-triggered release from liposomes

A promising approach for local drug delivery is high-intensity focused ultrasound (HIFU)-triggered release of drugs from stimuli-responsive nanoparticles such as liposomes. The aim of this study was to investigate whether another release mechanism is involved with HIFU-triggered release from liposomes beside cavitation and temperature. Furthermore, it was studied whether this new release mechanism allows the release of lipophilic compounds. Therefore, both a lipophilic (Nile red) and a hydrophilic (fluorescein) compound were loaded into thermosensitive (TSL) or non-thermosensitive liposomes (NTSL) and the liposomes were subjected both to continuous wave (CW)- and pulsed wave (PW)-HIFU. The mean liposome size varied from 97 to 139nm with a polydispersity index (PDI)≤0.06 for the different formulations. The Tm of the phospholipid bilayer of the TSL was around 42°C. Approximately 80% of fluorescein was released within 15min from TSL at temperatures≥42°C. In contrast, no fluorescein release from NTSL and NR release from both TSL and NTSL was observed at temperatures up to 60°C. CW-HIFU exposure of TSL resulted in rapid temperature elevation up to 52°C and subsequently almost quantitative fluorescein release. Fluorescein release from NTSL was also substantial (~64% after 16min at 20W). Surprisingly, CW-HIFU exposure (20W for 16min) resulted in the release of NR from TSL (~66% of the loaded amount), and this was even higher from NTSL (~78%). PW-HIFU exposure did not result in temperatures above the Tm of TSL. However, nearly 85% of fluorescein was released from TSL after 32min at 20W of PW-HIFU exposure, whereas the release from NTSL was around 27%. Interestingly, NR release from NTSL was~30% after 2min PW-HIFU exposure and increased to~70% after 32min. Furthermore, addition of microbubbles to the liposomes prior to PW-HIFU exposure did not result in more release, which suggests that cavitation can be excluded as the main mechanism responsible for the triggered release of both a hydrophilic and a lipophilic model compound from liposomes. Dynamic light scattering analysis showed that the mean size and PDI of the liposomes did not significantly change after CW- and PW-HIFU exposure. Taken together, it is therefore concluded that neither temperature elevation nor inertial cavitation is essential for the release of both hydrophilic and lipophilic compounds from liposomes. It is assumed that the release originates from radiation force-induced acoustic streaming, causing the liposomes to collide at the walls of the exposure chamber leading to shear forces which in turn results in reversible liposome destabilization and release of both hydrophilic and lipophilic compounds.

The Usual Suspects—Influence of Physicochemical Properties on Lag Time, Skin Deposition, and Percutaneous Penetration of Nine Model Compounds

The influence of physicochemical properties of nine model compounds on lag time, skin deposition, and percutaneous penetration was evaluated. Static diffusion cells mounted with human skin were used as the experimental model, and experiments were carried out in accordance with Organization for Economic Cooperation and Development (OECD) guidelines. The model compounds were chosen to cover a wide spectrum of solubilities and molecular weights. The pesticides included were glyphosate, dimethoate, pirimicarb, malathion, paclobutrazol, methiocarb, prochloraz, and benzoic acid, with the ninth model compound being caffeine. The fastest dermal penetration was observed for compounds with log Pow values between 1.5 and 4. Malathion did not fit into this generalization. No clear relationship was observed between molecular weight and Kp values. The shortest lag time was observed for the most hydrophilic model compounds. With increasing molecular weight, the lag time rose. Thus, the lag time for the smallest model compound was close to 1.5 h, while the lag time exceeded 20 h for a model compound with a molecular weight of 377 g. A difference in lag time of this magnitude inevitably produces differences in the amounts of a chemical able to penetrate the skin within a limited period of time. The relative deposition in the skin was highest for the lipophilic model compounds. For log Pow values between -1 and 2, a linear relationship was observed between log Pow and log Kp. Comparisons between theoretical Kp values based on the Potts–Guy equation and experimental Kp values demonstrated good agreement. Experimental Kp values based on experiments using full-thickness human skin generally underestimated the calculated Kp values by 25%. The observations are relevant to industrial hygienists and regulatory agencies using experimental data for assessing systemic toxicity following dermal exposures as for development of structure activity relationships for dermal absorption of chemicals.

An in situ perfusion protocol of rat epididymal adipose tissue useful in metabolic studies

Experimental approaches involving the perfusion of tissues and organs offer the advantage of improved physiological relevance over the use of isolated tissues or cells while at the same time being much more controlled and tissue-specific than studies in vivo. Nevertheless, there have been few metabolic studies performed in perfused white adipose tissue, largely because of the difficulty of the surgical technique involved. Although some methods have been described, they are difficult to use as perfusion protocols and their reproducibility is poor. We have modified a rat perfusion method, based on a modification of the Ho and Meng technique, for use with epididymal white adipose tissue (eWAT), and we present it here as a protocol to be reproduced. We also offer surgical solutions for the most common variants of vessel distributions in rats. Using the protocol described here, the perfused adipose tissue is viable and metabolically active, as indicated by the maintenance of tissue ATP levels and adiponectin secretion and by endogenous lipolysis regulation. Moreover, there is a high level of lipoprotein lipase activity in the endothelium of the tissue, which is heparin-releasable. Thus, this method is a useful and reproducible tool that allows the perfusion of eWAT for use in metabolic studies.

Response surface methodology for optimization and characterization of limonene-based coenzyme Q10 self-nanoemulsified capsule dosage form

The aim of this study was to systematically obtain a model of factors that would yield an optimized self-nanoemulsified capsule dosage form (SNCDF) of a highly lipophilic model compound, Coenzyme Q10 (CoQ). Independent variables such as amount of R-(+)-limonene (X1), surfactant (X2), and cosurfactant (X3), were optimized using a 3-factor, 3-level Box-Behnken statistical design. The dependent variables selected were cumulative percentage of drug released after 5 minutes (Y1) with constraints on drug release in 15 minutes (Y2), turbidity (Y3), particle size (Y4), and zeta potential (Y5). A mathematical relationship obtained, Y1 = 78.503 + 6.058X1 + 13.738X2 + 5.986X3 - 25.831X1(2) + 9.12X1X2 - 26.03 X1X3 - 38.67 X2(2) +11.02X2X3 - 15.55 X3(3) (r2 = 0.97), explained the main and quadratic effects, and the interaction of factors that affected the drug release. Response surface methodology (RSM) predicted the levels of factors X1, X2, and X3 (0.0344, 0.216, and 0.240, respectively), for a maximized response of Y1 with constraints of >90% release on Y2. The observed and predicted values of Y1 were in close agreement. In conclusion, the Box-Behnken experimental design allowed us to obtain SNCDF with rapid (>90%) drug release within 5 minutes with desirable properties of low turbidity and particle size.

Nasal drug delivery to the cerebrospinal fluid: transport of a lipophilic compound

The aim of this study was to investigate the possibility of direct transport of drugs from the olfactory area to the cerebrospinal fluid (CSF) by intranasal administration (i.n.) of a lipophilic drug in human volunteers. In a large number of animal studies evidence has been found for drug transport via the nose to the brain [1]. Human studies are suggesting such a transport, for instance for i.n. midazolam [2], on pharmacodynamic grounds, but pharmacokinetic evidence is as yet missing. This study evaluates drug CSF concentrations in patients at the neurosurgery department after i.n. and intravenous (i.v.) administration of melatonin, used in this study as a lipophilic model compound. It has been chosen because it is an endogenous relatively safe compound, for which a very good nasal absorption has been reported [3]. In a pharmacokinetic study, patients at the neurosurgery department with an external cerebrospinal drain were recruited. Three patients were included. Each patient received on one day melatonin i.n. as a dose of 400 μg (200 μg in each nostril) and on another day 200 μg melatonin i.v. Blood samples and CSF samples were collected just before and at 5, 10, 20, 0, 40, 60, 120 and 180 min after drug administration. Melatonin concentrations were measured with h.p.l.c. Concentration–time curves of the plasma and CSF concentrations of melatonin were compared after i.n. and i.v. administration. The nasal absorption of melatonin in blood is fast and efficient. Peak plasma concentrations after i.n. are reached within 20 min and are comparable with those reached after i.v. administration. The AUCCSF:AUCplasma after i.n. administration was in all three patients not larger than after i.v. administration. In the three investigated patients the increase in CSF melatonin concentration after i.n. administration appeared to be comparable with that after i.v. administration (Table 1). Table 1 AUC (0, 180 min) of melatonin in CSF after i.n. and i.v. administration. The increase of the melatonin concentrations in the CSF does not differ largely in the period 0–180 min, whether the drug is administered i.n. or i.v., indicating that the drug enters the CSF via the systemic circulation and the blood–brain barrier. It is evident that i.n. administration of a lipophilic compound could be an interesting alternative route of administration, giving high blood and also high CSF concentrations, but the results of this study provide no indication for an additional transport mechanism of the lipophilic compound directly from the nose to the CSF.

Enhanced skin permeation of a lipophilic drug using supersaturated formulations

Supersaturation was used to enhance the permeation of a lipophilic model compound (a lavendustin derivative, LAP) through excised pig skin in vitro. The drug was dissolved in a series of liquid and semisolid vehicles (in which it had different solubilities) and which were prepared using either (i) the method of mixed cosolvents, (ii) the method of solvent evaporation, or (iii) the method of dissolving the drug with heating. Saturated formulations showed comparable permeation rates through the skin, independent of the absolute concentration of the drug in the vehicle. Supersaturated solutions at a degree of saturation of two resulted in a doubling of the drug permeation rate. These experiments show, therefore, that the percutaneous absorption of LAP may be consistently increased using supersaturated formulations, independent of the type and composition of the vehicles and independent of their method of preparation.

Demonstration of a Coupled Metabolism–Efflux Process at the Choroid Plexus as a Mechanism of Brain Protection Toward Xenobiotics

Brain homeostasis depends on the composition of both brain interstitial fluid and CSF. Whereas the former is largely controlled by the blood-brain barrier, the latter is regulated by a highly specialized blood-CSF interface, the choroid plexus epithelium, which acts either by controlling the influx of blood-borne compounds, or by clearing deleterious molecules and metabolites from CSF. To investigate mechanisms of brain protection at the choroid plexus, the blood-CSF barrier was reconstituted in vitro by culturing epithelial cells isolated from newborn rat choroid plexuses of either the fourth or the lateral ventricle. The cells grown in primary culture on semipermeable membranes established a pure polarized monolayer displaying structural and functional barrier features, (tight junctions, high electric resistance, low permeability to paracellular markers) and maintaining tissue-specific markers (transthyretin) and specific transporters for micronutriments (amino acids, nucleosides). In particular, the high enzymatic drug metabolism capacity of choroid plexus was preserved in the in vitro blood-CSF interface. Using this model, we demonstrated that choroid plexuses can act as an absolute blood-CSF barrier toward 1-naphthol, a cytotoxic, lipophilic model compound, by a coupled metabolism-efflux mechanism. This compound was metabolized in situ via uridine diphosphate glururonosyltransferase-catalyzed conjugation, and the cellular efflux of the glucurono-conjugate was mediated by a transporter predominantly located at the basolateral, i.e., blood-facing membrane. The transport process was temperature-dependent, probenecid-sensitive, and recognized other glucuronides. Efflux of 1-naphthol metabolite was inhibited by intracellular glutathione S-conjugates. This metabolism-polarized efflux process adds a new facet to the understanding of the protective functions of choroid plexuses.

The Predictive Accuracy of in Vitro Measurements for the Dermal Absorption of a Lipophilic Penetrant (Fluazifop-Butyl) through Rat and Human Skin

The Predictive Accuracy of in Vitro Measurements for the Dermal Absorption of a Lipophilic Penetrant (Fluazifop-Butyl) through Rat and Human Skin. Ramsey, J. D., Woollen, B. H., Auton, T. R., and Scott, R. C. (1994). Fundam. Appl. Toxicol. 23, 230-236. The predictive accuracy of in vitro measurements in estimating dermal absorption has been evaluated in rat and human skin using fluazifop-butyl (FB), a lipophilic model compound, at dosage rates of 2.5, 25, and 250 μg/cm 2. In vitro studies used rat and human epidermal membranes mounted in static diffusion cells with radiolabeled FB and receptor fluids of 50% aqueous ethanol (Aq Et), 6% polyethylene glycol 20 oleyl ether in saline (PEG), or tissue culture medium (TCM). In vivo rat studies with radiolabeled FB were carried out to parallel previously published human volunteer studies. For rat skin, in vitro measurements with all types of receptor fluid provide an adequate prediction (generally within a factor of 3) of in vivo absorption. Absorption data for human epidermal membranes with a receptor fluid of Aq Et were adequately predictive of the in vivo absorption. In contrast, membranes with PEG or TCM significantly underestimated the in vivo absorption. The results support the conclusion that in vitro studies are useful to predict in vivo dermal absorption in rat and man, when appropriate receptor fluids are used.

The Use of 2,4,5,2′,4′,5′‐Hexachlorobiphenyl (6‐CB) as an Unmetabolizable Lipophilic Model Compound

2,4,5,2',4'5'-Hexachlorobiphenyl (6-CB)--a polychlorinated biphenyl (PCB) congener resistant to metabolism in most species--has become a major residue in the biosphere including human adipose tissue. Its use as a model of unmetabolizable lipophilic compounds and as a tool in toxicokinetics in the last two decades is reviewed. This extremely water-insoluble compound is transported in plasma by albumin and lipoproteins. Binding to these plasma proteins appears to be important for uptake and release processes in different tissues. The redistribution kinetics of 6-CB as well as its pronounced adipose tissue storage and a very slow excretion with the faeces has been established in long-term animal studies. Excretion is strongly influenced by an increasing or diminishing adipose storage compartment size. Other minor pathways of elimination, e.g., via hair, become also important in the absence of metabolism and renal excretion. 6-CB has revealed the possibility of an almost quantitative transfer of the maternal body burden to the offspring via milk. The use of 6-CB in studies with tissue preparations in vitro is providing insight into transport mechanisms of uptake and release.

Uptake of persistent environmental chemicals by cultured human cells

Uptake of the persistent environmental chemicals 2,2′,4,4′,5,5′-hexachlorobiphenyl and 1,1,1-trichloro-2,2-di-(4-chlorophenyl)ethane (the insecticide DDT) by Chang liver cells, an established human cell line, has been investigated. Monolayer cells were incubated with culture medium to which the lipophilic model compounds had been added. The time course of uptake of either compound was biphasic, reaching equilibrium after about 5 hr of incubation. The ratio of DDT:hexachlorobiphenyl uptake was dependent on the presence of serum proteins. Increasing concentrations of serum proteins in the culture medium progressively inhibited uptake. Efflux from the cells was not entirely reversible: 10–20% of the chemicals were not released. Uptake was a linear function of the external concentration of the compounds. Absorptive binding to the outer cell plasma membrane could be determined by removing bound chemicals with fetal calf serum (“back exchange”). With this method, temperature-dependent translocation through the cell plasma membrane could directly be demonstrated. The effect of low temperature as well as the influence of metabolic inhibitors point out the contribution of energy-driven uptake pathways. Demonstration of LDL receptor-like binding protein on Chang liver cells facilitated estimation of the role of receptor-mediated uptake. This route of uptake proved to be of minor importance only, as was transport of the protein-bound chemicals via fluid pinocytosis. The results demonstrate that cellular endocytosis of plasma membrane-bound chemicals constitutes a major uptake pathway for lipophilic chemicals.

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 < pentobarbital < glutethimide, phenylbutazone < thiophental, methadone < 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 viuo has recently been shown not to correlate with octanol partition coefficients.

Pharmacokinetics in rats of 2,4,5,2',4',5'-hexachlorobiphenyl, an unmetabolizable lipophilic model compound.

1. Pharmacokinetics of the unmetabolizable lipophilic model compound, 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) was studied in rats, using g.l.c. and 14C methods. 2. After single i.v. doses of 0.6 and 3.6 mg/kg, 16% dose was excreted in 40 weeks in the faeces; the value for infinite time was 17% dose. This limited excretion was first-order with a half-life of 100 days for the terminal component. Urinary excretion was nearly complete after 1 week and amounted to 0.8% dose. 3. 6-CB was redistributed from blood to liver, muscle, skin, and adipose tissue. The latter contained a constant level of about 75% dose from 6 to 40 weeks, while the total lean tissue level fell to 6% dose; only 6-CB in the lean tissue compartment was available for excretion. 4. In rats given six oral doses of 0.6 mg/kg at weekly intervals, excretion and distribution patterns were similar to the single-dose situation, and were thus independent of dose, route of administration, and dose regimen. 5. It is concluded that in rats under physiological conditions, about 75% of every dose of 6-CB is irreversibly stored in adipose tissue and that excretion is limited to 18% dose. 6-CB in rats exhibit novel pharmacokinetics of unmetabolizable lipophilic compounds.