Effect of cell arrangement and interstitial volume fraction on the diffusivity of monoclonal antibodies in tissue.

We present theoretical calculations relating the effective diffusivity of monoclonal antibodies in tissue (Deff) to the actual diffusivity in the interstitium (Dint) and the interstitial volume fraction phi. Measured diffusivity values are effective values, deduced from concentration profiles with the tissue treated as a continuum. By using homogenization theory, the ratio Deff/Dint is calculated for a range of interstitial volume fractions from 10 to 65%. It is assumed that only diffusion in the interstitial spaces between cells contributes to the effective diffusivity. The geometries considered have cuboidal cells arranged periodically, with uniform gaps between cells. Deff/Dint is found to generally be between (2/3) phi and phi for these geometries. In general, the pathways for diffusion between cells are not straight. The effect of winding pathways on Deff/Dint is examined by varying the arrangement of the cells, and found to be slight. Also, the estimates of Deff/Dint are shown to be insensitive to typical nonuniformities in the widths of gaps between cells. From our calculations and from published experimental measurements of the effective diffusivity of an IgG polyclonal antibody both in water and in tumor tissue, we deduce that the diffusivity of this molecule in the interstitium is one-tenth to one-twentieth its diffusivity in water. We also conclude that exclusion of molecules from cells (an effect independent of molecular weight) contributes as much as interstitial hindrance to the reduction of effective diffusivity, for small interstitial volume fractions (around 20%). This suggests that the increase in the rate of delivery to tissues resulting from the use of smaller molecular-weight molecules (such as antibody fragments or bifunctional antibodies) may be less than expected.     

Effects of Hydration on Steric and Electric Charge-Induced Interstitial Volume Exclusion—a Model

The presence of collagen and charged macromolecules like glycosaminoglycans (GAGs) in the interstitial space limits the space available for plasma proteins and other macromolecules. This phenomenon, known as interstitial exclusion, is of importance for interstitial fluid volume regulation. Physical/mathematical models are presented for calculating the exclusion of electrically charged and neutral macromolecules that equilibrate in the interstitium under various degrees of hydration. Here, a central hypothesis is that the swelling of highly electrically charged GAGs with increased hydration shields parts of the neutral collagen of the interstitial matrix from interacting with electrically charged macromolecules, such that exclusion of charged macromolecules exhibits change due to steric and charge effects. GAGs are also thought to allow relatively small neutral, but also charged macromolecules neutralized by a very high ionic strength, diffuse into the interior of GAGs, whereas larger macromolecules may not. Thus, in the model, relatively small electrically charged macromolecules, such as human serum albumin, and larger neutral macromolecules such as IgG, will have quite similar total volume exclusion properties in the interstitium. Our results are in agreement with ex vivo and in vivo experiments, and suggest that the charge of GAGs or macromolecular drugs may be targeted to increase the tissue uptake of macromolecular therapeutic agents.     

In vivo mimicking model for solid tumor towards hydromechanics of tissue deformation and creation of necrosis

The present work addresses transvascular and interstitial fluid transport inside a solid tumor surrounded by normal tissue (close to an in vivo mimicking setup). In general, biological tissues behave like a soft porous material and show mechanical behavior towards the fluid motion through the interstitial space. In general, forces like viscous drag that are associated with the fluid flow may compress the tissue material. On the macroscopic level, we try to model the motion of fluids and macromolecules through the interstitial space of solid tumor and the normal tissue layer. The transvascular fluid transport is assumed to be governed by modified Starling’s law. The poroelastohydrodynamics (interstitial hydrodynamics and the deformation of tissue material) inside the tumor and normal tissue regions is modeled using linearized biphasic mixture theory. Correspondingly, the velocity distribution of fluid is coupled to the displacement field of the solid phase (mainly cellular phase and extracellular matrix) in both the normal and tumor tissue regions. The corresponding velocity field is used within the transport reaction equation for fluids and macromolecules through interstitial space to get the overall solute (e.g., nutrients, drug, and other macromolecules) distribution. This study justifies that the presence of the normal tissue layer plays a significant role in delaying/assisting necrosis inside the tumor tissue. It is observed that the exchange process of fluids and macromolecules across the interface of the tumor and normal tissue affects the effectiveness factor corresponding to the tumor tissue.     

Morphometry of the extremely thin pulmonary blood-gas barrier in the chicken lung

The gas exchanging region in the avian lung, although proportionally smaller than that of the mammalian lung, efficiently manages respiration to meet the high energetic requirements of flapping flight. Gas exchange in the bird lung is enhanced, in part, by an extremely thin blood-gas barrier (BGB). We measured the arithmetic mean thickness of the different components (endothelium, interstitium, and epithelium) of the BGB in the domestic chicken lung and compared the results with three mammals. Morphometric analysis showed that the total BGB of the chicken lung was significantly thinner than that of the rabbit, dog, and horse (54, 66, and 70% thinner, respectively) and that all layers of the BGB were significantly thinner in the chicken compared with the mammals. The interstitial layer was strikingly thin in the chicken lung ( approximately 86% thinner than the dog and horse, and 75% thinner than rabbit) which is a paradox because the strength of the BGB is believed to come from the interstitium. In addition, the thickness of the interstitium was remarkably uniform, unlike the mammalian interstitium. The uniformity of the interstitial layer in the chicken is attributable to a lack of the supportive type I collagen cable that is found in mammalian alveolar lungs. We propose that the surrounding air capillaries provide additional structural support for the pulmonary capillaries in the bird lung, thus allowing the barrier to be both very thin and extremely uniform. The net result is to improve gas exchanging efficiency.     

Biodistribution and anticancer activity of a new vinca alkaloid encapsulated into long-circulating liposomes

S12363 is a potent therapeutic agent with a strong in vitro activity against a variety of tumor types but also a high in vivo toxicity. Loading of this drug into long-circulating liposomes is expected to enhance its therapeutic index. Pharmacokinetics of liposomal S12363 showed that circulating S12363 was entrapped into liposomes until 24 hours after intravenous injection in mice. The liposomal formulation significantly increased the plasma concentration, half-life, and AUC and decreased the plasma clearance rates and volume of distribution of S12363. Liposome extravasation was evaluated with two tumor models by both microscopic analysis and liposome radiolabeling. Liposome accumulation was much more important in the case of B16 melanoma, compared to H460 tumor, with both inoculated subcutaneously and with comparable size. H460 tumor was also inoculated into the lung. The tumor localization did not influence liposome accumulation into the tissue. The liposomal formulation injected into mice bearing B16 melanoma allowed a 10-fold accumulation of S12363 into the tumor interstitium, as compared to the solution. Bioluminescence data, supported by the survival curves of the animals, showed that S12363-liposomes were able to significantly restrict B16 melanoma progression and increase mice survival.     

Interstitial Fluid Flow and Drug Delivery in Vascularized Tumors: A Computational Model

Interstitial fluid is a solution that bathes and surrounds the human cells and provides them with nutrients and a way of waste removal. It is generally believed that elevated tumor interstitial fluid pressure (IFP) is partly responsible for the poor penetration and distribution of therapeutic agents in solid tumors, but the complex interplay of extravasation, permeabilities, vascular heterogeneities and diffusive and convective drug transport remains poorly understood. Here we consider–with the help of a theoretical model–the tumor IFP, interstitial fluid flow (IFF) and its impact upon drug delivery within tumor depending on biophysical determinants such as vessel network morphology, permeabilities and diffusive vs. convective transport. We developed a vascular tumor growth model, including vessel co-option, regression, and angiogenesis, that we extend here by the interstitium (represented by a porous medium obeying Darcy''s law) and sources (vessels) and sinks (lymphatics) for IFF. With it we compute the spatial variation of the IFP and IFF and determine its correlation with the vascular network morphology and physiological parameters like vessel wall permeability, tissue conductivity, distribution of lymphatics etc. We find that an increased vascular wall conductivity together with a reduction of lymph function leads to increased tumor IFP, but also that the latter does not necessarily imply a decreased extravasation rate: Generally the IF flow rate is positively correlated with the various conductivities in the system. The IFF field is then used to determine the drug distribution after an injection via a convection diffusion reaction equation for intra- and extracellular concentrations with parameters guided by experimental data for the drug Doxorubicin. We observe that the interplay of convective and diffusive drug transport can lead to quite unexpected effects in the presence of a heterogeneous, compartmentalized vasculature. Finally we discuss various strategies to increase drug exposure time of tumor cells.     

Computational Modeling of Tumor Response to Drug Release from Vasculature-Bound Nanoparticles

Systemically injected nanoparticle (NPs) targeting tumor vasculature offer a venue for anti-angiogenic therapies as well as cancer detection and imaging. Clinical application has been limited, however, due to the challenge of elucidating the complex interplay of nanotechnology, drug, and tumor parameters. A critical factor representing the likelihood of endothelial adhesion is the NP vascular affinity, a function of vascular receptor expression and NP size and surface-bound ligand density. We propose a theoretical framework to simulate the tumor response to vasculature-bound drug-loaded NPs and examine the interplay between NP distribution and accumulation as a function of NP vascular affinity, size, and drug loading and release characteristics. The results show that uniform spatial distribution coupled with high vascular affinity is achievable for smaller NPs but not for larger sizes. Consequently, small (100 nm) NPs with high vascular affinity are predicted to be more effective than larger (1000 nm) NPs with similar affinity, even though small NPs have lower drug loading and local drug release compared to the larger NPs. Medium vascular affinity coupled with medium or larger sized NPs is also effective due to a more uniform distribution with higher drug loading and release. Low vascular affinity hampered treatment efficacy regardless of NP size, with larger NPs additionally impeded by heterogeneous distribution and drug release. The results further show that increased drug diffusivity mainly benefits heterogeneously distributed NPs, and would negatively affect efficacy otherwise due to increased wash-out. This model system enables evaluation of efficacy for vascular-targeted drug-loaded NPs as a function of critical NP, drug, and tumor parameters.     

Multiscale Measurements Distinguish Cellular and Interstitial Hindrances to Diffusion In Vivo

Molecular cancer therapy relies on interstitial diffusion for drug distribution in solid tumors. A mechanistic understanding of how tumor components affect diffusion is necessary to advance cancer drug development. Yet, because of limitations in current techniques, it is unclear how individual tissue components hinder diffusion. We developed multiscale fluorescence recovery after photobleaching (MS-FRAP) to address this deficiency. Diffusion measurements facilitated by MS-FRAP distinguish the diffusive hindrance of the interstitial versus cellular constituents in living tissue. Using multiscale diffusion measurements in vivo, we resolved the contributions of these two major tissue components toward impeding diffusive transport in solid tumors and subcutaneous tissue in mice. We further used MS-FRAP in interstitial matrix-mimetic gels and in vivo to show the influence of physical interactions between collagen and hyaluronan on diffusive hindrance through the interstitium. Through these studies, we show that interstitial hyaluronan paradoxically improves diffusion and that reducing cellularity enhances diffusive macromolecular transport in solid tumors.     

Fabrication of PLGA nanoparticles with a fluidic nanoprecipitation system

Particle size is a key feature in determining performance of nanoparticles as drug carriers because it influences circulating half-life, cellular uptake and biodistribution. Because the size of particles has such a major impact on their performance, the uniformity of the particle population is also a significant factor. Particles comprised of the polymer poly(lactic-co-glycolic acid) (PLGA) are widely studied as therapeutic delivery vehicles because they are biodegradable and biocompatible. In fact, microparticles comprised of PLGA are already approved for drug delivery. Unfortunately, PLGA nanoparticles prepared by conventional methods usually lack uniformity. We developed a novel Fluidic NanoPrecipitation System (FNPS) to fabricate highly uniform PLGA particles. Several parameters can be fine-tuned to generate particles of various sizes.     

Translational diffusion of globular proteins in the cytoplasm of cultured muscle cells

Modulated fringe pattern photobleaching (MFPP) was used to measure the translational diffusion of microinjected fluorescein isothiocyanate (FITC)-labeled proteins of different sizes in the cytoplasm of cultured muscle cells. This technique, which is an extension of the classical fluorescence recovery after photobleaching (FRAP) technique, allows the measurement of the translational diffusion of macromolecules over several microns. Proteins used had molecular masses between 21 and 540 kDa. The results clearly indicated that the diffusivity of the various proteins is a decreasing function of their hydrodynamic radius. This decrease is more rapid with globular proteins than with FITC-labeled dextrans (, Biophys. J. 70:2327-2332), most likely because, unlike globular proteins, dextrans are randomly coiled macromolecules with a flexible structure. These data do not exclude the possibility of a rapid diffusion over a short distance, unobservable with our experimental set-up, which would take place within the first milliseconds after bleaching and would correspond to the diffusion in restricted domains followed by impeded diffusion provoked by the network of microtubules, microfilaments, and intermediate filaments. Thus our results may complement rather than contradict those of Verkman and collaborators (, J. Cell Biol. 138:1-12). The biological consequence of the size-dependent restriction of the mobility of proteins in the cell cytoplasm is that the formation of intracellular complexes with other proteins considerably reduces their mobility.     

Hindered diffusion in polymeric solutions studied by fluorescence correlation spectroscopy

Diffusion of molecules in the crowded and charged interior of the cell has long been of interest for understanding cellular processes. Here, we introduce a model system of hindered diffusion that includes both crowding and binding. In particular, we obtained the diffusivity of the positively charged protein, ribonuclease A (RNase), in solutions of dextrans of various charges (binding) and concentrations (crowding), as well as combinations of both, in a buffer of physiological ionic strength. Using fluorescence correlation spectroscopy, we observed that the diffusivity of RNase was unaffected by the presence of positively charged or neutral dextrans in the dilute regime but was affected by crowding at higher polymer concentrations. Conversely, protein diffusivity was significantly reduced by negatively charged dextrans, even at 0.4 μM (0.02% w/v) dextran. The diffusivity of RNase decreased with increasing concentrations of negative dextran, and the amount of bound RNase increased until it reached a plateau of ∼80% bound RNase. High salt concentrations were used to establish the electrostatic nature of the binding. Binding of RNase to the negatively charged dextrans was further confirmed by ultrafiltration.     

Diffusion-limited compartmentalization of mammalian cell nuclei assessed by microinjected macromolecules

In order to investigate the accessibility of the nucleoplasm for macromolecules with different physical properties, we microinjected FITC-conjugated dextrans of different sizes as well as anionic FITC-dextrans and FITC-poly-L-lysine into mammalian cell nuclei. Small dextrans displayed a homogeneous nuclear distribution. With increasing molecular mass (42 to 2500 kDa), FITC-dextrans were progressively excluded from chromatin regions, accumulating in and thereby outlining an apparently extended interchromatin space. Anionic FITC-dextrans (500 kDa) showed complete exclusion from labeled chromatin regions, while the positively charged FITC-poly-L-lysine was to some extent present within the chromatin regions. Moreover, the FITC-poly-L-lysine preferentially localized at the nuclear periphery. We also found a size-dependent exclusion of FITC-dextrans from nucleoli regions, while the FITC-poly-L-lysine accumulated in the nucleoli. Thus, the distinct and restricted nuclear accessibility for macromolecules is dependent on molecule size and electrical charge.     

Permeabilization of the plasma membrane of L1210 mouse leukemia cells using lithotripter shock waves

Permeabilization of L1210 cells by lithotripter shock waves in vitro was monitored by evaluating the accumulation of fluorescein-labeled dextrans with a relative molecular mass ranging from 3,900–2,000,000. Incubation with labeled dextran alone caused a dose- and time-dependent increase in cellular fluorescence as determined by flow cytometry, with a vesicular distribution pattern in the cells consistent with endocytotic uptake. Shock wave exposure prior to incubation with labeled dextran revealed similar fluorescence intensities compared to incubation with labeled dextran alone. When cells were exposed to shock waves in the presence of labeled dextran, mean cellular fluorescence was further increased, indicating additional internalization of the probe. Confocal laser scanning microscopy confirmed intracellular fluorescence of labeled dextran with a diffuse distribution pattern. Fluorescence-activated cell sorting with subsequent determination of proliferation revealed that permeabilized cells were viable and able to proliferate. Permeabilization of the membrane of L1210 cells by shock waves in vitro allowed loading of dextrans with a relative molecular mass up to 2,000,000.Permeabilization of tumor cells by shock waves provides a useful tool for introducing molecules into cells which might be of interest for drug targeting in tumor therapy in vivo.This work was supported by the Deutsche Forschungsgemeinschaft grant De 531/1-1. We are particularly grateful to Dr. Ulrich Dirnagl (Department of Neurology, University of Munich, Marchioninistr. 15, 81377 Munich, Germany) for performing the confocal laser scanning microscopy and to Gerhard Adams for excellent technical assistance.     

Ultrasound-assisted powder-coating technique to improve content uniformity of low-dose solid dosage forms

An ultrasound-assisted powder-coating technique was used to produce a homogeneous powder formulation of a low-dose active pharmaceutical ingredient (API). The powdered particles of microcrystalline cellulose (MCC; Avicel^® PH-200) were coated with a 4% m/V aqueous solution of riboflavin sodium phosphate, producing a uniform drug layer on the particle surfaces. It was possible to regulate the amount of API in the treated powder. The thickness of the API layer on the surface of the MCC particles increased near linearly as the number of coating cycles increased, allowing a precise control of the drug content. The tablets (n = 950) prepared from the coated powder showed significantly improved weight and content uniformity in comparison with the reference tablets compressed from a physical binary powder mixture. This was due to the coated formulation remaining uniform during the entire tabletting process, whereas the physical mixture of the powders was subject to segregation. In conclusion, the ultrasound-assisted technique presented here is an effective tool for homogeneous drug coating of powders of irregular particle shape and broad particle size distribution, improving content uniformity of low-dose API in tablets, and consequently, ensuring the safe delivery of a potent active substance to patients.Key words: content uniformity, homogeneity, low-dose API, powder coating, ultrasound     

Chronopharmacokinetics of imipenem in the rat

There are no studies indicating a possible modification of imipenem pharmacokinetics related to the hour (i.e., circadian time) of its administration. The aim of this study was to evaluate the influence of different times of intramuscular imipenem administration on its disposition in Wistar AF EOPS rats. Four groups of eight animals were given a single intramuscular injection of 140 mg/kg of imipenem either at 10:00, 16:00, 22:00, or 04:00 h. Blood samples were collected 0.5, 1, 2, 3, 4, 6, and 8 h after drug injection, and the main pharmacokinetic parameters determined were C_max, T_max, elimination half-life (t_1/2), area under the concentration-versus-time curve (AUC), total serum clearance (CL/F), and volume of distribution (V/F). Circadian variation of C_max (49%), T_max (92%), and AUC (19%) was observed leading to variability of imipenem exposure. Clearance and volume of distribution were modified according to the circadian time of drug injection but did not reach statistical significance. The results suggest that varying the time of administration induces intra-individual variability.     

Direct interstitial infusion of NK1-targeted neurotoxin into the spinal cord: a computational model

Convection-enhanced delivery of substance P (SP) nocitoxins to the spinal cord interstitium is under consideration for the treatment of chronic pain. To characterize treatment protocols, a three-dimensional finite-element model of infusion into the human dorsal column was developed to predict the distribution of SP-diphtheria toxin fusion protein (SP-DT') within normal and target tissue. The model incorporated anisotropic convective and diffusive transport through the interstitial space, hydrolysis by peptidases, and intracellular trafficking. For constant SP-DT' infusion (0.1 microl/min), the distribution of cytotoxicity in NK1 receptor-expressing neurons was predicted to reach an asymptotic limit at 6-8 h in the transverse direction at the level of the infusion cannula tip ( approximately 60% ablation of target neurons in lamina I/II). Computations revealed that SP-DT' treatment was favored by a stable SP analog (half-life approximately 60 min), high infusate concentration (385 nM), and careful catheter placement (adjacent to target lamina I/II). Sensitivity of cytotoxic regions to NK1 receptor density and white matter protease activity was also established. These data suggest that intraparenchymal infusions can be useful for treatment of localized chronic pain.     

Transendothelial transfer of macromolecules in vitro

The transendothelial transfer of macromolecules has been difficult to study because of the complexities of the in vivo models. We have developed a model of an endothelium cultured on a permeable support and used it to characterize the transendothelial transfer of albumin. Porcine pulmonary artery endothelial cells form a single layer of cells lining the gelatin-impregnated polycarbonate micropore filters, and the cells develop junctional structures similar to endothelial tight junctions observed in vivo. The monolayer resists the flow of electrical current, and the resistance is sensitive to extracellular calcium concentrations. Albumin transfer across the cultured monolayers was found to be asymmetric, and the rate of transfer from interstitium to lumen was greater than that from lumen to interstitium. The asymmetric transfer occurred against a concentration gradient and was abolished by treating the monolayer with NaCN. Increasing albumin concentrations increased the rate of interstitial to luminal transfer, and the process demonstrated saturation at an interstitial albumin concentration of 725 microM. These data point out the usefulness of the in vitro preparation to identify potentially important aspects of transendothelial transport that would be difficult to detect in vivo.     

Numerical modeling of fluid flow in solid tumors

A mathematical model of interstitial fluid flow is developed, based on the application of the governing equations for fluid flow, i.e., the conservation laws for mass and momentum, to physiological systems containing solid tumors. The discretized form of the governing equations, with appropriate boundary conditions, is developed for a predefined tumor geometry. The interstitial fluid pressure and velocity are calculated using a numerical method, element based finite volume. Simulations of interstitial fluid transport in a homogeneous solid tumor demonstrate that, in a uniformly perfused tumor, i.e., one with no necrotic region, because of the interstitial pressure distribution, the distribution of drug particles is non-uniform. Pressure distribution for different values of necrotic radii is examined and two new parameters, the critical tumor radius and critical necrotic radius, are defined. Simulation results show that: 1) tumor radii have a critical size. Below this size, the maximum interstitial fluid pressure is less than what is generally considered to be effective pressure (a parameter determined by vascular pressure, plasma osmotic pressure, and interstitial osmotic pressure). Above this size, the maximum interstitial fluid pressure is equal to effective pressure. As a consequence, drugs transport to the center of smaller tumors is much easier than transport to the center of a tumor whose radius is greater than the critical tumor radius; 2) there is a critical necrotic radius, below which the interstitial fluid pressure at the tumor center is at its maximum value. If the tumor radius is greater than the critical tumor radius, this maximum pressure is equal to effective pressure. Above this critical necrotic radius, the interstitial fluid pressure at the tumor center is below effective pressure. In specific ranges of these critical sizes, drug amount and therefore therapeutic effects are higher because the opposing force, interstitial fluid pressure, is low in these ranges.     

Effect of crowding by dextrans and Ficolls on the rate of alkaline phosphatase-catalyzed hydrolysis: a size-dependent investigation

The cell cytosol is crowded with macromolecules such as proteins, nucleic acids, and membranes. The consequences of such crowding remain unclear. How is the rate of a typical enzymatic reaction, involving a freely diffusing enzyme and substrate, affected by the presence of macromolecules of different sizes, shapes, and concentrations? Here, we mimic the cytosolic crowding in vitro, using dextrans and Ficolls, for the first time in a variety of sizes ranging from 15 to 500 kDa, in a concentration range 0–30% w/w. Alkaline phosphatase–catalyzed hydrolysis of p‐nitrophenyl phosphate (PNPP) was chosen as the model reaction. A pronounced decrease in the rate with increase in fractional volume occupancy of dextran is observed for larger dextrans (200 and 500 kDa) in contrast to smaller dextrans (15–70 kDa). Our results indicate that, at 20% w/w, smaller dextrans (15–70 kDa) reduce the initial rate moderately (1.4‐ to 2.4‐fold slowing), while larger dextrans (>200 kDa) slow the reaction considerably (>5‐fold). Ficolls (70 and 400 kDa) slow the reaction moderately (1.3‐ to 2.3‐fold). The influence of smaller dextrans was accounted by a combination of increase in viscosity as sensed by PNPP and a minor offsetting increase in enzyme activity due to crowding. Larger dextrans apparently reduce the frequency of enzyme substrate encounter. The reduced influence of Ficolls is attributed to their compact and quasispherical shape, much unlike the dextrans. © 2006 Wiley Periodicals, Inc. Biopolymers 83: 477–486, 2006 This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Tissue distribution of cocaine in the pregnant rat

Cocaine hydrochloride was administered by single intraperitoneal (IP) doses to pregnant rats at day 18 or 19 of gestation. Plasma and tissue cocaine and norcocaine concentrations were measured by high-pressure liquid chromatography. Pharmacokinetic analysis of concentration versus time data showed rapid distribution of cocaine and its metabolite to maternal and fetal tissues. The area under the cocaine concentration versus time curve (AUC) in fetus compared to maternal plasma was 3.33. The half-life of cocaine in the maternal plasma and fetus was 46 and 55 minutes, respectively, similar to values reported for cocaine elimination half-life in human plasma. The order of cocaine concentrations was placenta greater than fetal liver greater than maternal heart greater than whole fetus greater than fetal brain greater than maternal brain = maternal plasma. Norcocaine concentrations were usually less than 20% of cocaine concentrations in plasma and tissues. These results support extensive fetal exposure to cocaine following administration to pregnant rodents. Pharmacodynamic studies of cocaine in pregnancy should consider the effects of the drug on the developing fetus.