For many compounds (neurotrophic factors, antibodies, growth factors, genetic vectors, enzymes) slow diffusion in the brain severely limits drug distribution and effect after direct drug admistration into brain parenchyma.We investigated convection as a means to enhance the distribution of the large and small molecules 51In-labeled trserrin (111In-Tf; Mr, 80,00)

No abstract

High-flow microinfusion provides a means for delivering macromolecules to large volumes of brain in easily obtainable time intervals. Slowly degraded approximately 180-kDa macromolecules, delivered at a constant volumetric flow rate of 3 microliters/min into homogeneous brain tissue (e.g., gray matter), would penetrate to a 1.5-cm radius in 12 h. The predicted concentration profile is relatively flat until it declines precipitously at the flow front. Hence, tissues are dosed rather uniformly, providing control over the undesired toxicity that may occur with alternative methods that depend on large concentration gradients for tissue transport. The penetration advantage of high-flow (convective) over low-flow (diffusive) microinfusion has been assessed at fixed pharmacodynamic effect. A 12-h high-flow microinfusion of a macromolecule degraded with a characteristic time of 33.5 h would provide 5- to 10-fold increases in volume over low-flow infusion and total treatment volumes > 10 cm3. Slower degradation rates would result in larger treatment volumes; more rapid degradation rates would reduce the volume but still favor convective over diffusive administration. This technique may be applicable to a variety of diagnostic and therapeutic agents such as radioimmunoconjugates, immunotoxins, enzymes, growth factors, and oligonucleotides.

Both theory and clinical studies demonstrate that drug concentrations in the peritoneal cavity can greatly exceed concentrations in the plasma following intraperitoneal administration. This regional advantage has been associated with clinical activity, including surgically documented complete responses in ovarian cancer patients with persistent or recurrent disease following systemic therapy, and has produced a survival advantage in a recent phase III trial. Two pharmacokinetic problems appear to limit the effectiveness of intraperitoneal therapy: poor tumor penetration by the drug and incomplete irrigation of serosal surfaces by the drug-containing solution. We have examined these problems in the context of a very simple, spatially distributed model. If D is the diffusivity of the drug in a tissue adjacent to the peritoneal cavity and k is the rate constant for removal of the drug from the tissue by capillary blood, the model predicts that (for slowly reacting drugs) the characteristic penetration distance is (D/k)1/2 and the apparent permeability of the surface of a peritoneal structure is (Dk)1/2. The permeability-area product used in classical pharmacokinetic calculations for the peritoneal cavity as a whole is the sum of the products of the tissue-specific permeabilities and the relevant superficial surface areas. Since the model is mechanistic, it provides insight into the expected effect of procedures such as pharmacologic manipulation or physical mixing. We observe that large changes in tissue penetration may be difficult to achieve but that we have very little information on the transport characteristics within tumors in this setting or their response to vasoactive drugs. Enhanced mixing is likely to offer significant potential for improved therapy; however, procedures easily applicable to the clinical setting have not been adequately investigated and should be given high priority. Clinical studies indicate that an increase in irrigated area may be achieved in many patients by individualizing the dialysate volume and consideration of patient position.

No abstract

Artificial capillaries perfused with culture medium provide a matrix in which cells can attain tissue-like densities in vitro. Products secreted into the medium can be measured as indicators of cell function or may be recovered for other purposes without disturbing the culture.

The pharmacokinetics of PCBs are complicated by numerous factors, not the least of which is the existence of up to 209 different chlorinated biphenyls. Whereas all PCB congeners are highly lipophilic and most are readily absorbed and rapidly distributed to all tissues, PCBs are cleared from tissues at very different rates, and the same congeners may be cleared at different rates by different species. With the exception of special situations in which PCBs may be passively eliminated in lipid sinks, e.g. milk or eggs, clearance is minimal prior to metabolism to more polar compounds. Rates of PBC metabolism vary greatly with species and with the degree and positions of chlorination. Mammals metabolize these compounds most rapidly, but even among mammalian species rates of metabolism vary greatly. In all species studied, the more readily metabolized chlorinated biphenyls have adjacent unsubstituted carbon atoms in the 3-4 positions. Congeners that do not have adjacent unsubstituted carbon atoms may be metabolized very slowly and are therefore cleared very slowly. These PCBs not readily cleared concentrate in adipose tissue. A physiologic pharmacokinetic model best illustrates how the concentrations of PCBs in all tissues approach equilibrium with the blood and with one another. Thus, the model illustrates how a depot of PCBs in any tissue, e.g. adipose tissue, will result in exposure of all tissues in proportion to the respective tissue/blood ratios and the body burden. The disposition of a number of PCBs in the rate has been accurately described by a physiologic model, and the model has been extrapolated to predict the disposition of these same PCBs in the mouse (58). Therefore, the physiologic pharmacokinetic model is believed to offer the best opportunity to extrapolate data obtained with laboratory animals to predict the disposition of PCBs in other species, including man. Most of the parameters of a model of PCB disposition in man are available or could be estimated. The major limitation to the construction of such a model is the absence of accurate estimates of metabolic clearance of individual PCBs by man. Accurate estimates of metabolic clearance depend on development of suitable in vitro methods to accurately predict clearance in vivo.

Spatial solute concentration profiles resulting from in vivo microdialysis were measured in rat caudate-putamen by quantitative autoradiography. Radiolabeled sucrose was included in the dialysate, and the tissue concentration profile measured after infusions of 14 min and 61.5 min in an acute preparation. In addition, the changes in sucrose extraction fraction over time were followed in vivo and in a simple in vitro system consisting of 0.5% agarose. These experimental results were then compared with mathematical simulations of microdialysis in vitro and in vivo. Simulations of in vitro microdialysis agreed well with experimental results. In vivo, the autoradiograms of the tissue concentration profiles showed clear evidence of substantial differences between 14 and 61.5 min, even though the change in extraction fraction was relatively small over that period. Comparison with simulated results showed that the model substantially underpredicted the observed extraction fraction and overall amount of sucrose in the tissue. A sensitivity analysis of the various model parameters suggested a tissue extracellular volume fraction of approximately 40% following probe implantation. We conclude that the injury from probe insertion initially causes disruption of the blood-brain barrier in the vicinity of the probe, and this disruption leads to an influx of water and plasma constituents, causing a vasogenic edema.