Progression from selective to general involvement of hippocampal subfields in schizophrenia

Volume deficits of the hippocampus in schizophrenia have been consistently reported. However, the hippocampus is anatomically heterogeneous; it remains unclear whether certain portions of the hippocampus are affected more than others in schizophrenia. In this study, we aimed to determine whether volume deficits in schizophrenia are confined to specific subfields of the hippocampus and to measure the subfield volume trajectories over the course of the illness. Magnetic resonance imaging scans were obtained from Data set 1: 155 patients with schizophrenia (mean duration of illness of 7 years) and 79 healthy controls, and Data set 2: an independent cohort of 46 schizophrenia patients (mean duration of illness of 18 years) and 46 healthy controls. In addition, follow-up scans were collected for a subset of Data set 1. A novel, automated method based on an atlas constructed from ultra-high resolution, post-mortem hippocampal tissue was used to label seven hippocampal subfields. Significant cross-sectional volume deficits in the CA1, but not of the other subfields, were found in the schizophrenia patients of Data set 1. However, diffuse cross-sectional volume deficits across all subfields were found in the more chronic and ill schizophrenia patients of Data set 2. Consistent with this pattern, the longitudinal analysis of Data set 1 revealed progressive illness-related volume loss (~2-6% per year) that extended beyond CA1 to all of the other subfields. This decline in volume correlated with symptomatic worsening. Overall, these findings provide converging evidence for early atrophy of CA1 in schizophrenia, with extension to other hippocampal subfields and accompanying clinical sequelae over time.

Impact of duration of untreated psychosis and premorbid intelligence on cognitive functioning in patients with first-episode schizophrenia

BACKGROUND

The neurotoxic hypothesis suggests that psychosis is toxic to the brain leading to clinical consequences. In this study, we hypothesized that a longer duration of untreated psychosis (DUP) in first episode schizophrenia (FES) patients is associated with poorer cognitive functioning, and that higher premorbid intelligence buffers against DUP-related cognitive impairment.

METHOD

Eighty-one FES patients completed a neuropsychological battery, the Brief Assessment of Cognition in Schizophrenia (BACS). Composite scores of the BACS, which were normalized to a matched healthy control of seventy-three subjects, were used as an index of general cognition. A median split using the Wide Range Achievement Test-Reading Test scores was used to divide the patients into low versus high premorbid IQ groups. Hierarchical linear regression was performed to examine predictors of general cognition, including DUP.

RESULTS

Longer DUP was found to be a significant predictor of poorer general cognition. In addition, DUP predicted general cognition in the low premorbid IQ group but not in the high premorbid IQ group.

CONCLUSIONS

Our findings demonstrate that longer DUP in FES patients is associated with worse cognitive scores, and that this association is more pronounced in a subgroup of patients who have lower premorbid intelligence. Our results suggest the importance of earlier identification and management of patients with low premorbid IQ, given that their cognition may be more vulnerable to the toxicity of psychosis.

Physicochemical determinants of passive membrane permeability: role of solute hydrogen-bonding potential and volume

The relationship of solute structure with cellular permeability was probed. Two series of dipeptide mimetics consisting of glycine, alanine, valine, leucine, phenylalanine, and cyclohexylalanine with amino acids in the D-configuration were prepared. Partition coefficients for the peptidemimetics were obtained in the octanol/water (log P(octanol/water)), hydrocarbon/octanol (Delta log P), and heptane/ethylene glycol (log P(heptane/glycol)) systems in order to explore the contributions of solute volume, or surface area, and hydrogen-bond potential to the permeability of the solutes. Permeability coefficients were obtained in Caco-2 cell monolayers as a model of the human intestinal mucosa. The results were interpreted in terms of a partition/diffusion model for solute transport where membrane partitioning into the permeability-limiting membrane microdomain is estimated from the solvent partition coefficients. Neither log P(octanol/water) nor Delta log P alone correlated with cellular permeability for all the solutes. In contrast, log P(heptane/glycol) gave a qualitatively better correlation. With regard to solute properties, log P(octanol/water) is predominantly a measure of solute volume, or surface area, and hydrogen-bond acceptor potential, while Delta log P is principally a measure of hydrogen-bond donor strength. Log P(heptane/glycol) contains contributions from all these solute properties. The results demonstrate that both hydrogen-bond potential and volume of the solutes contribute to permeability and suggests that the nature of the permeability-limiting microenvironment within the cell depends on the properties of a specific solute. Collectively, these findings support the conclusion that a general model of permeability will require consideration of a number of different solute structural properties.

Transport of model peptides across Ascaris suum cuticle

Several FMRFamide-related peptides (FaRPs) found in nematodes exert potent excitatory or inhibitory effects on the somatic musculature of Ascaris suum and other nematode species when injected into the pseudocoelom or applied directly to isolated neuromuscular preparations. These peptides, however, generally fail to induce detectable effects on the neuromusculature when applied externally to intact nematodes. The apparent lack of activity for these peptides when administered externally in whole-organism assays is likely a function of both absorption and metabolism. To delineate the factors that govern transport of peptides across the cuticle/hypodermis complex of nematodes, we measured the rates of absorption of a series of structurally related model peptides using isolated cuticle/hypodermis segments from A. suum and two-chamber diffusion cells. [14C]-Labeled peptides were prepared from D-phenylalanine, with the amide nitrogens sequentially methylated to give AcfNH2, Acf3NH2, Acf(NMef)2NH2, and Ac(NMef)3NHMe. These model peptides were designed to allow systematic analysis of the influence of peptide size, hydrogen bonding and lipophilicity on transport. Results of these studies show that, within this series, permeability across the cuticle increases with addition of each methyl group. The permeability coefficient of Ac(NMef)3NHMe, with four methyl groups, was 10-fold greater than that of the smaller peptide, AcfNH2, even though both peptides contain five hydrogen bonds. When compared with vertebrate membranes, transport of the model peptides across A. suum cuticle was about 10-fold slower. A biophysical model for transcuticular transport of peptides predicted that nematode FaRPs, which are larger, less methylated and less lipophilic than the model peptides tested, would not be absorbed across the cuticle of nematodes. This prediction was confirmed for the excitatory FaRP, AF2 (KHEYLRFamide), which did not diffuse across the cuticle/hypodermis complex, but diffused rapidly across lipid-extracted cuticle preparations.

Increased lipophilicity and subsequent cell partitioning decrease passive transcellular diffusion of novel, highly lipophilic antioxidants

Oxidative stress is considered a cause or propagator of acute and chronic disorders of the central nervous system. Novel 2, 4-diamino-pyrrolo[2,3-d]pyrimidines are potent inhibitors of iron-dependent lipid peroxidation, are cytoprotective in cell culture models of oxidative injury, and are neuroprotective in brain injury and ischemia models. The selection of lead candidates from this series required that they reach target cells deep within brain tissue in efficacious amounts after oral dosing. A homologous series of 26 highly lipophilic pyrrolopyrimidines was examined using cultured cell monolayers to understand the structure-permeability relationship and to use this information to predict brain penetration and residence time. Pyrrolopyrimidines were shown to be a more permeable structural class of membrane-interactive antioxidants where transepithelial permeability was inversely related to lipophilicity or to cell partitioning. Pyrrole substitutions influence cell partitioning where bulky hydrophobic groups increased partitioning and decreased permeability and smaller hydrophobic groups and more hydrophilic groups, especially those capable of weak hydrogen bonding, decreased partitioning, and increased permeability. Transmonolayer diffusion for these membrane-interactive antioxidants was limited mostly by desorption from the receiver-side membrane into the buffer. Thus, in this case, these in vitro cell monolayer models do not adequately mimic the in vivo situation by underestimating in vivo bioavailability of highly lipophilic compounds unless acceptors, such as serum proteins, are added to the receiving buffer.

Biophysical model for organic acid excretion in Ascaris suum

To develop a model for the mechanisms of organic acid excretion in nematodes, we measured the concentrations of volatile fatty acids (VFAs), pH and electrical potentials across hypodermal and muscle membranes and across the composite body wall (consisting of hypodermis, muscle and cuticle) of Ascaris suum using standard chromatographic and microelectrode recording techniques. In incubates containing one parasite in 20 ml modified Ascaris Ringer's solution, the level of combined VFAs excreted into the medium increased linearly for about 18 h, then plateaued at a concentration of 4.2 mM; the medium acidified rapidly to a plateau at about pH 5.0 within 4-6 h. Following 24 h incubations, the concentrations of VFAs in the hypodermis, muscle, and pseudocoelomic compartments were 62.4 +/- 8.1, 62.3 +/- 7.8 and 74.4 +/- 3.2 mM, respectively. The pseudocoelomic fluid was more acidic (pH 6.52 +/- 0.06) than the hypodermis (pH 6.78 +/- 0.03) or muscle (pH 6.77 +/- 0.03). These data and the electrical potentials across hypodermal (-57.9 +/- 6.3 mV) and muscle (-30.3 +/- 0.8 mV) membranes were used to determine the equilibrium concentrations for protonated (HVFA) and anionic (VFA-) forms of the acids across these membranes and across the cuticle. Under these conditions, little transmembrane or transmural excretion of HVFAs is expected to occur in A. suum. However, a 16-27 mV driving force for VFA- excretion exists across hypodermal and muscle membranes, and a larger driving force is predicted to exist for these anions across the cuticle. This driving force could provide potential energy for VFA- excretion through anion channels which exist in muscle and hypodermal membranes of this parasite, or for facilitated transport systems.

Absorption and intestinal metabolism of purine dideoxynucleosides and an adenosine deaminase-activated prodrug of 2',3'-dideoxyinosine in the mesenteric vein cannulated rat ileum

This study investigates the mechanisms of absorption and the role of intestinally localized purine salvage pathway enzymes on the ileal availabilities of 2',3'-dideoxyinosine (ddI), a substrate for purine nucleoside phosphorylase (PNP); 2'-fluoro-2',3'-dideoxyinosine (F-ddI), a non-PNP substrate; and 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an adenosine deaminase (ADA) activated prodrug of ddI. The potential for increasing the intestinal availability of 6-Cl-ddP through the use of ADA inhibitors, namely, 2'-deoxycoformycin (DCF) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), is also explored. Drug permeability coefficients across the intestinal epithelium were determined in in situ perfusions in the mesenteric vein cannulated rat ileum based on both drug appearance in blood (Pblood) and disappearance from the lumen (Plumen) and their paracellular and transcellular components were estimated by comparison to the permeabilities of two paracellular markers, mannitol and urea. Values of Pblood for ddI were determined to be (1.1 +/- 0.3) x 10(-6) cm/s, in close agreement with the value of (1.0 +/- 0.3) x 10(-6) cm/s obtained for F-ddI, a PNP resistant analogue of ddI having virtually the same molecular size and lipophilicity as ddI. This indicates that PNP may not play an important role in the low intestinal absorption of ddI. The Pblood for 6-Cl-ddP, (19 +/- 2) x 10(-6) cm/s, was 4.5-fold lower than Plumen, (84 +/- 12) x 10(-6) cm/s, which means that 77 +/- 6% of 6-Cl-ddP was metabolized during its intestinal transport, thus qualitatively accounting for the low oral bioavailability (7%) of 6-Cl-ddP observed in vivo in rats. Extensive intracellular metabolism of 6-Cl-ddP by ADA was confirmed by the high concentrations of ddI found both in the intestinal lumen and blood during 6-Cl-ddP perfusions and by a rate of ddI appearance in blood which was approximately 10-fold higher than ddI controls. Co-perfusion of the potent, hydrophilic ADA inhibitor DCF (Ki = 0. 001-0.05 nM) with 6-Cl-ddP led to only partial inhibition of intestinal ADA, while complete inhibition was obtained using the less potent but more lipophilic inhibitor EHNA (Ki = 1-20 nM). Hence, EHNA may be used to improve intestinal absorption of 6-Cl-ddP in vivo.

Paracellular diffusion in Caco-2 cell monolayers: effect of perturbation on the transport of hydrophilic compounds that vary in charge and size

We applied the principles of molecular-size-restricted diffusion within a negative electrostatic field of force to follow the changes in the aqueous pore radius of tight junctions (TJs) induced by perturbants and the accompanying influence on the permeation of neutral (urea and mannitol), cationic (methylamine and atenolol), and anionic (formate and lactate) compounds that vary in size. The perturbants included palmitoyl-DL-carnitine (PC), which opens TJs by an unknown Ca++-independent mechanism, and ethyleneglycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), a Ca++ chelator. Mass transfer resistances of the collagen-coated filter support and the aqueous boundary layers were factored out to yield paracellular permeability coefficients (P[P]). As viewed from the P(P) values of urea and mannitol, EGTA exhibited insignificant effects on pore size at low concentrations compared with control, and then caused a dramatic opening of the TJs over a narrow concentration range (1.35-1.4 mM). The P(P) values for urea and mannitol remained constant at >1.4 mM EGTA. However, PC produced dose-dependent responses from O to 0.15 mM that plateaued at >0.15 mM. In general, cations permeated the cellular TJs faster and anions slower than their neutral images. The effects of changes in pore size (4.6 to 14.6 A in effective radius) on the ability of these solutes to permeate the TJs were analyzed by the Renkin molecular sieving function. These studies established an experimental, theoretical, and quantitative template to assess perturbants of the TJ and define the limits, short of detrimental effects, at which the TJs may be sufficiently perturbed for maximal enhancement of permeation of solutes varying in size and charge.

Mechanistic roles of neutral surfactants on concurrent polarized and passive membrane transport of a model peptide in Caco-2 cells

The transport of the model peptide Acf(NMef)2NH2 across Caco-2 cell monolayers was studied in the apical (AP) to basolateral (BL) and the BL to AP direction in the presence of Polysorbate 80 or Cremophore EL in the AP compartment. Increasing surfactant concentrations resulted in increasing AP-->BL peptide permeability and decreasing BL-->AP permeability. In either direction, limiting permeabilities were achieved at concentrations less than the critical micellar concentrations (cmc's) of the surfactants, and remained constant at much higher concentrations. These plateau permeabilities were not equivalent in the two directions. This residual assymetry was abolished by increasing the peptide concentration. Altogether, the observations support the presence of at least two pumps in Caco-2 cells for this peptide, polarized in the BL-->AP direction. These experimental results were analyzed within the context of a quantitative biophysical model incorporating concurrent passive diffusion across the AP and BL membranes accompanied by surfactant-inhibitable active polarized efflux across the AP membrane. The model was also used to locate the additional transport activity at the BL membrane as an uptake pump. Under conditions of complete inhibition, the intrinsic passive diffusional permeability of Acf(NMef)2NH2 was found to be 13 x 10(-6) cm/s, essentially identical with results reported earlier with this peptide utilizing verapamil as an inhibitor. With respect to the mechanism of surfactant inhibition of the apical efflux transport, the monomeric species was found to be responsible with no contribution from micelles. Modeling the mode of inhibition as a noncompetitive Michaelis-Menten process gave identical Kis of 0.5 microM for the two surfactants. Finally, increase of either surfactant beyond 750 microM resulted in a decrease of peptide permeability in the AP-->BL direction. This was attributed to weak association of the peptide with micelles in the AP compartment, which effectively decreased the thermodynamic activity of the peptide at surfactant concentrations greater than 20 times their cmc. Both the experimental approach and accompanying theoretical model demonstrated in this work will allow for further characterization of the inhibitory potencies of surfactants for the nonpassive efflux pathway in vitro and in vivo.

Intestinal absorption of sodium dodecyl sulfate in the rodent: evidence for paracellular absorption

Experiments were performed to define the mechanism of intestinal absorption of dodecyl sulfate (DS), an amphipathic organic anion whose chemical structure resembles that of dodecanoate, a C12 fatty acid anion. With jejunal segments perfused in single-pass fashion in the anesthetized rat, steady-state absorption of DS was concentration dependent, with the apparent permeability constant (P(app)) ranging from 4 to 22 x 10(-5) cm/s. When DS concentration was held constant and net water absorption was induced by decreasing perfusate osmolality, DS absorption increased in direct proportion to water absorption, suggesting absorption by solvent drag via the paracellular route. However, DS absorption continued even when water secretion was induced by a hypertonic perfusate. Consequently, for all experiments, DS absorption could be empirically described as the sum of two terms: 1) absorption in the absence of water absorption (P(app) = 5.6 x 10(-5) cm/s) and 2) absorption induced by water movement [(delta P(app)/delta water absorption) = 0.2 x 10(-5) cm x s(-1) x microl segment(-1) x min(-1)]. In a polarized epithelial monolayer of renal epithelial cells (Madin-Darby canine kidney cells), DS was absorbed predominantly by a paracellular pathway, as the absorption rate increased threefold when paracellular junction pore size was increased by the addition of cytochalasin D. The calculated apparent radius was 2.9 A, indicating that the cross section of the molecule, not its length, determined the rate of absorption. It is concluded that absorption of DS in the intact animal occurs slowly and mostly via the paracellular route, because the fixed negative charge on the molecule retards rapid passive entry into the enterocyte, as occurs with protonated fatty acids. That absorption of DS persisted despite net water secretion suggests a low level of transcellular absorption across the jejunal enterocyte also occurs.

How structural features influence the biomembrane permeability of peptides

Successful drug development requires not only optimization of specific and potent pharmacological activity at the target site, but also efficient delivery to that site. Many promising new peptides with novel therapeutic potential for the treatment of AIDS, cardiovascular diseases, and CNS disorders have been identified, yet their clinical utility has been limited by delivery problems. Along with metabolism, a major factor contributing to the poor bioavailability of peptides is thought to be inefficient transport across cell membranes. At the present time, the reasons for this poor transport are poorly understood. To explore this problem, we have designed experiments focused on determining the relationship between peptide structure and peptide transport across various biological membranes both in vitro and in vivo. Briefly, peptides that varied systematically in chain length, lipophilicity, and amide bond number were prepared. Permeability results with these solutes support a model in which the principal determinant of peptide transport is the energy required to desolvate the polar amides in the peptide for the peptide to enter and diffuse across the cell membrane. Further impacting on peptide permeability is the presence of active, secretory transport systems present in the apical membrane of intestinal epithelial and brain endothelial cells. In Caco-2 cell monolayers, a model of the human intestinal mucosa, this pathway displayed substrate specificity, saturation, and inhibition. Similar results have been shown in vivo in both rat intestinal and blood-brain barrier absorption models. The presence of such systems serves as an additional transport barrier by returning a fraction of absorbed peptide back to the lumen.

Influence of organic acid excretion on cuticle pH and drug absorption by Haemonchus contortus

To determine if a cuticle microenvironment pH is maintained by adult Haemonchus contortus, organic acid excretion kinetics and absorption kinetics of selected model weak acids and a weak base were measured in incubation media that varied in buffer capacity (0.25-20 mM HEPES or 5 mM glycine) and initial pH (7.5 or 3.5). To evaluate the importance of the cuticle as a pathway for organic acid excretion and drug absorption the pharynx was paralyzed with 1 nM ivermectin. H. contortus changed the media pH from initial values of 7.5 or 3.25 to an asymptotic value of approximately 5.6. The rate of pH change depended on the buffer capacity, but was not affected by chemical ligation with ivermectin. The intrinsic rate of excretion of organic acids (0.045 +/- 0.016 micromol/cm2 x h) was constant during the first 8-12 h of incubation and was independent of initial pH, buffer capacity or ivermectin ligation. The rates of absorption of the model weak acids, benzoic acid and p-nitrophenol, and the model weak base, aniline, were not affected by initial pH, buffer capacity or ivermectin ligation. These results suggest that H. contortus excretes organic acid endproducts of carbohydrate metabolism across its cuticle, and that these acids maintain a microenvironment pH within the water-filled pores of the cuticle that controls the rate of adsorption of weakly acidic or basic drugs.

Passive diffusion of weak organic electrolytes across Caco-2 cell monolayers: uncoupling the contributions of hydrodynamic, transcellular, and paracellular barriers

A systematic approach was used to demonstrate the quantitative interplay of pH, pKa, lipophilicity, charged and uncharged molecular species, molecular size, aqueous diffusivity, and stirring in passive transport across the aqueous boundary layer, microporous filter support, and transcellular and paracellular barriers in Caco-2 cell monolayers. The relationship of permeability of the aqueous boundary layer and hydrodynamic stirring was elucidated from transmonolayer fluxes of testosterone. Adrenergic receptor antagonists including propranolol (PPL), alprenolol (APL), pindolol (PDL), and atenolol (ATL) represented the model series of structurally similar weak bases with pKa values between 8.8 and 9.65. Although intrinsically lipophilic, their apparent log PC (n-octanol/water) at pH 7.4 and 6.5 ranged from -2.6 to 1.3. Effective permeability coefficients (Pe) correlated with log PC at both pH 7.4 and 6.5 showing a single sigmoidal-like curve: PPL > APL > PDL > or = ATL. The Pe approached a minimum plateau value established by the protonated ATL for the paracellular route (pore radius of 12 A) by molecular size-restricted diffusion within a negative electrostatic field of force. The Pe of the weak bases was delineated into component permeability coefficients of the aqueous boundary layer and porous filter support, the intrinsic permeabilities of charged and uncharged species for the transcellular and paracellular routes, and the extent to which the routes were utilized at each pH. This study emphasized a generally applicable approach to quantitatively analyze passive transport data on weak organic electrolytes and neutral molecules generated using cell culture monolayers.

A biophysical model of passive and polarized active transport processes in Caco-2 cells: approaches to uncoupling apical and basolateral membrane events in the intact cell

This report is aimed at the biophysical modeling of transmembrane events involving a passive diffusion and directional pumplike mechanism at the apical (AP) and basolateral (BL) membranes of cultured cell monolayers. The essence of the model is based on experimental evidences for the existence of a saturable, apically polarized transport system in Caco-2 cells for peptides which hindered apical to basolateral flux, enhanced basolateral to apical flux, and showed substrate specificity. This system was further inhibited by verapamil, suggesting some homology with P-glycoprotein, the principal mediator of drug resistance in multidrug resistant cancer cells. Preliminary evidence was also obtained suggesting an additional polarized uptake system for the same peptides in the basolateral membrane. Upon saturation and/or inhibition of the active transport mechanisms with verapamil, the peptide fluxes in apical-to-basolateral direction and the basolateral-to-apical direction converged and became controlled by the passive mechanism. Since the intent of the modeling was to provide useful templates for the design of probing experiments and to delineate and quantify mass transfer mechanisms at the AP and BL membranes and their interrelationships, theoretical equations were developed for a host of kinetic boundary conditions: (a) AP-->BL and BL-->AP transfluxes, (b) bidirectional effluxes from substrate-preloaded cells, (c) undirectional efflux across the AP or BL membrane from preloaded cells, and (d) uptake kinetics via the AP or BL membrane leading to equilibrium. Furthermore, flux expressions were reduced to membrane permeability coefficients to accommodate passive diffusion, saturation, inhibition, and directionality. The diffusional mass transport resistances of the aqueous boundary layers and microporous filter support of the cell monolayer were necessarily included.

Quantitative approaches to delineate paracellular diffusion in cultured epithelial cell monolayers

When using cultured cell monolayers to determine the mechanism of transcellular diffusion of molecules, it may be important to identify the fraction that moves through the paracellular route or passively diffuses through tight junctions. We characterized the apparent diameter of the junctional pore in a variety of epithelial cell monolayers (Caco-2, MDCK, alveolar). Using hydrophilic extracellular permeants varying in molecular radii and charge (neutral, anionic, cationic, zwitterionic), rate-determining steps and factors of the paracellular route were quantitatively delineated by the model for molecular size-restricted diffusion within a negative electrostatic field of force. Protonated amines permeated the pores faster than their neutral images while organic anions were slower. With increasing molecular size the influence of charge diminished. This approach was used to quantify the relationship between permeant radius and transepithelial electrical resistance and to analyze changes in junctional pore size as a function of pharmacological perturbation, such as in the use of absorption promoters or adjuvants.

Theoretical perspectives on anthelmintic drug discovery: interplay of transport kinetics, physicochemical properties, and in vitro activity of anthelmintic drugs

This multidisciplinary study demonstrates the utility of the biophysical model approach to assess biological activity of anthelmintics in light of drug-delivery principles. The relationships between drug absorption and efficacy for a set of structurally disparate anthelmintics were determined in cultures of Haemonchus contortus, a nematode that parasitizes the ruminant gastrointestinal tract. Uptake, parameterized by the permeability coefficient, Pe, was shown to occur by absorption across the cuticle. Rates of drug appearance in nematode carcasses paralleled rates of drug disappearance from the medium, and absorption reached an apparent equilibrium within a few hours. The parasite/medium partition coefficient, K, was derived from the ratio of drug concentration in the parasite vs the medium at equilibrium. Pe and K values for each anthelmintic were correlated with lipophilicity (as measured by the partition coefficient (PC) in n-octanol/water) and both parameters plateaued at log PC approximately 2.5, with maximum Pe approximately 8 x 10(-4) cm/min and log K < or = 2.0. Absorption kinetics were related to in vitro potency by monitoring motility of H. contortus. The time required to reduce motility by 50% (t* 50) and Pe were used to calculate Cn*, the drug concentration in the parasite at t* 50, as an indicator of intrinsic potency. The quantitative interplay of apparent biological activity expressed as t* 50, dose, and intrinsic potency highlights the important contribution of drug-uptake kinetics.

Transcellular permeability of chlorpromazine demonstrating the roles of protein binding and membrane partitioning

Transcellular permeability of the neuroleptic-anesthetic chlorpromazine (CPZ) was examined using a cell type (MDCK) that forms a confluent monolayer of polarized cells resulting in distinct apical (AP) and basolateral (BL) membrane domains separated by tight junctions. Because CPZ is membrane interactive, transmonolayer flux was analyzed as two kinetic events: cell uptake from the AP donor solution and efflux into the BL side receiver. Using the rate of cell uptake in the presence of different concentrations of BSA, an intrinsic cell partition coefficient of 3700 +/- 130 and an operational dissociation binding constant of 0.4 +/- 0.05 mM were calculated. In contrast to uptake, efflux of CPZ from either the AP or the BL side of the cell monolayer was approximately 10(4)-fold slower and was dependent upon the avidity of CPZ for the protein acceptor in the receiver solution. These results emphasized the importance of simultaneously measuring disappearance of a lipophilic molecule from the donor solution and its appearance in the receiver and demonstrated how interactions with proteins on either side of the cellular barrier influence permeability. Appearance kinetics showed that the composition of the receiving environment is critical to model a particular in vivo situation and implied that the intrinsic permeability of membrane-interactive molecules in vitro does not necessarily predict penetration beyond the initial cellular barrier in vivo.

Biophysical model of the transcuticular excretion of organic acids, cuticle pH and buffer capacity in gastrointestinal nematodes

A biophysical model was developed, using Ascaris suum as a model gastrointestinal nematode, to provide quantitative perspectives into the microenvironmental pH within the water-filled, porous, negatively charged cuticle matrix of gastrointestinal nematodes. The central features of the model include (a) the constant rate of excretion of organic acid metabolites across the cuticle, (b) the relationship between cuticle pH and pKa of the organic acids that determines the fraction of unionized and ionized species, and (c) the concentration gradient, mean concentration and buffer capacity within the cuticle that maintain the cuticle pH. The model may be used to predict the extent to which transcuticular absorption of weakly basic and acidic anthelmintics will be affected by transcuticular excretion of organic acid metabolites. Coupled with established models for drug absorption by nematodes and the host gastrointestinal tract, the cuticle pH model provides new insights to the design of drugs with physicochemical properties that favor absorption by nematodes.

Haemonchus contortus: ivermectin-induced paralysis of the pharynx

How the avermectins cause the elimination of gastrointestinal nematodes from host animals has not yet been clearly identified. Using visual and radiometric parameters to measure oral ingestion in Haemonchus contortus, we showed that ivermectin (IVM) rapidly inhibited ingestion at concentrations > or = 10(-10) M. Motility, monitored quantitatively with an automated motility meter, was unaffected by IVM at concentrations < or = 10(-8) M, while ATP levels were unaffected at concentrations < or = 10(-6) M. Since motility and ATP levels, independent measures of short-term viability, are unaffected by concentrations of IVM that effectively block oral ingestion, the drug can be used as a chemical ligature. Although H. contortus was shown to be dependent upon an exogenous supply of glucose for survival in culture, IVM (10(-9) and 10(-7) M) altered neither the uptake of 3-O-[3H]methylglucose nor the metabolism of [13C]glucose by the parasite. These data suggest that H. contortus depends upon the transcuticular uptake of glucose in culture. If oral ingestion of other nutrients is essential for long-term survival in vivo, disruption of this process may represent the primary mechanism of IVM action.

Evidence for a polarized efflux system for peptides in the apical membrane of Caco-2 cells

The transport of two model peptides across confluent monolayers of human colon adenocarcinoma (Caco-2) cells was studied. In the case of AcPhe(NMePhe)2NH2, transport in the apical to basolateral direction was increased with increasing peptide concentration in the apical compartment. Transport was also increased in the presence of verapamil. In contrast, the flux of AcPheNH2 was neither concentration dependent nor affected by verapamil. Further, in the presence of verapamil, transport in the basolateral to apical direction was showed for AcPhe(NMePhe)2NH2 and again unchanged for AcPheNH2. These results are consistent with the presence of a saturable, apically polarized transport system in Caco-2 cells which serves to hinder transport in the apical to basolateral direction, increase flux in the basolateral to apical direction and shows substrate specificity for these model peptides.

Mechanistic approaches to quantitate anthelmintic absorption by gastrointestinal nematodes

In this article, David Thompson, Norman Ho, Sandra Sims and Timothy Geary look at the problem o f quantitating drug absorption by gastrointestinal nematodes.

Use of a biophysical-kinetic model to understand the roles of protein binding and membrane partitioning on passive diffusion of highly lipophilic molecules across cellular barriers

The novel antioxidants U-78517F and U-74006F, or lazaroids, are highly lipophilic organic molecules with poor brain uptake. To understand this paradoxical behavior better, continuous monolayers of Madin-Darby canine kidney (MDCK) epithelial cells with distinct apical (AP) and basolateral (BL) plasma membrane domains grown on polycarbonate membrane filters and plastic were used to examine the mechanism of transcellular diffusion. Independent kinetic experiments were used to quantify AP to BL flux, efflux from the AP and BL membranes and AP membrane partitioning as functions of bovine serum albumin (BSA) concentration. Fluxes were appropriately reduced to permeability coefficients (Pe) for the membrane, aqueous boundary layer (ABL) and filter, BSA-drug binding constants, and effective (Ke) and intrinsic (Kintr) membrane partition coefficients in the absence of metabolism. Both Pe and Ke decreased exponentially with increased BSA concentration and a concomitant decrease in free drug concentration. Uptake was ABL-controlled under the conditions used and its Pe was 1,000-fold faster than that for efflux due to a large Kintr. Therefore, diffusion across the cellular barrier was limited kinetically by the equilibrium between protein-bound drug and free drug partitioned into the cell membrane and the rate-limiting desorption of drug from the cell membrane into the aqueous receiver. This suggests that brain uptake of these lipophilic antioxidants is limited by interactions with plasma proteins and, possibly, by unfavorable partitioning from the endothelium into the underlying tissue. The present biophysical kinetic model is proposed as generally useful in studying the penetrative ability of other membrane interacting molecules.

Mechanisms of microenvironmental pH regulation in the cuticle of Ascaris suum

The excretion kinetics of various organic acids by Ascaris suum were quantified to determine if the excretion of these metabolic end-products could generate and maintain a microclimate pH within the aqueous compartment of the cuticle. Ligated and nonligated A. suum were incubated in media buffered with 0.25 or 2.5 mM Hepes (initial pH 7.5) or 0.5 or 5 mM glycine (initial pH 3.25). The concentration of organic acids and the pH of the media were followed for 24 h. Several volatile fatty acids, including acetic, 2-methylbutyric, 2-methylvaleric, n-valeric, and n-butyric, were excreted at relatively high rates. Propionic, n-caproic, 2-methylcaproic, tiglic acid, and the non-volatile organic acids, lactic and succinic, were excreted more slowly. The organic acids were excreted at a constant rate and in apparently fixed molar concentration ratios. The accumulation of organic acids was associated with changes in pH of the medium until a limiting constant pH, in the vicinity of the pKa of the volatile fatty acids, was reached. The rate of organic acid excretion was not affected by initial medium pH, buffer capacity, or parasite ligation. The rate of pH change induced by the excretion of organic acids was also insensitive to whether ligated or nonligated A. suum were used, but was dependent on the initial buffer capacity of the medium. These results suggest that A. suum excrete the end-products of carbohydrate metabolism across the cuticle. The presence of organic acids in the aqueous pores of the cuticle creates and maintains a microclimate pH of about 5.0 +/- 0.3. This pH will influence the transport properties of weak acids and bases and should be considered in the design of delivery systems for anthelmintics.

Mechanistic studies in the transcuticular delivery of antiparasitic drugs. II: Ex vivo/in vitro correlation of solute transport by Ascaris suum

Using live, intact Ascaris suum and a closed perfusion system, the absorption kinetics and tissue distribution of selected radiolabeled permeants were measured to determine the importance of the transcuticular pathway for drug absorption. The data support the conclusions established by previous in vitro transport studies which utilized excised cuticle-hypocuticle tissue preparations. The external surface of A. suum can be breached by drugs and the rate-determining barrier is the lipoidal hypocuticle tissue, provided the permeant is sufficiently small to traverse the aqueous-filled, negatively charged collagen matrix of the cuticle. The ex vivo permeability coefficients of the model permeants for the cuticle-hypocuticle barrier were in good quantitative agreement with the in vitro permeability coefficients. The lipophilic permeants hydrocortisone and p-nitrophenol were preferentially distributed in the gut tissue, whereas the hydrophilic permeant urea was distributed evenly throughout the organism and was extensively metabolized. Ligated and nonligated A. suum showed no significant differences in either uptake kinetics or tissue distribution of the permeants. This indicates that the transcuticular pathway is the major route of drug absorption as compared to oral ingestion.

The influence of peptide structure on transport across Caco-2 cells. II. Peptide bond modification which results in improved permeability

In order to study the influence of hydrogen bonding in the amide backbone of a peptide on permeability across a cell membrane, a series of tetrapeptide analogues was prepared from D-phenylalanine. The amide nitrogens in the parent oligomer were sequentially methylated to give a series containing from one to four methyl groups. The transport of these peptides was examined across confluent monolayers of Caco-2 cells as a model of the intestinal mucosa. The results of these studies showed a substantial increase in transport with each methyl group added. Only slight difference in the octanol-water partition coefficient accompanied this alkylation, suggesting that the increase in permeability is not due to lipophilicity considerations. These observations are, however, consistent with a model in which hydrogen bonding in the backbone is a principal determinant of transport. Methylation is seen to reduce the overall hydrogen bond potential of the peptide and increases flux by this mechanism. These results suggest that alkylation of the amides in the peptide chain is an effective way to improve the passive absorption potential for this class of compounds.

The influence of peptide structure on transport across Caco-2 cells

The relationship between structure and permeability of peptides across epithelial cells was studied. Using confluent monolayers of Caco-2 cells as a model of the intestinal epithelium, permeability coefficients were obtained from the steady-state flux of a series of neutral and zwitterionic peptides prepared from D-phenylalanine and glycine. Although these peptides ranged in lipophilicity (log octanol/water partition coefficient) from -2.2 to +2.8, no correlation was found between the observed flux and the apparent lipophilicity. However, a strong correlation was found for the flux of the neutral series and the total number of hydrogen bonds the peptide could potentially make with water. These results suggest that a major impediment to peptide passive absorption is the energy required to break water-peptide hydrogen bonds in order for the solute to enter the cell membrane. This energy appears not to be offset by the favorable introduction of lipophilic side chains in the amino acid residues.

Biophysical transport properties of the cuticle of Ascaris suum

The transport properties of isolated cuticle from Ascaris suum were studied using standard two-chamber diffusion cells and a number of radiolabeled permeants which varied in molecular size, lipophilicity and electrical charge. The permeability coefficient of the collagen matrix (lipid-extracted cuticle) vs. molecular radius relationship showed the interdependence of molecular size and electrical charge of the permeants with respect to the aqueous pores of the negatively charged matrix. The permeability of neutral solutes decreased monotonically with size. Protonated amines permeated the aqueous pores faster than neutral solutes of comparable size, while the permeation of anions was slower. The average pore size was estimated to be 1.5 nm in radius. A biophysical model which accounted for diffusion of molecules within a fixed electrostatic field of force and for molecular sieving by the pore channels was used in the mechanistic interpretation of the data. The effective permeability coefficient of the non-lipid-extracted cuticle was delineated into the permeability coefficients of the water-filled collagen matrix and the lipoidal component of the cuticle to determine which layer was the rate-controlling barrier. While each solute was capable of penetrating the water-filled collagen matrix, the rate-determining step for the majority of compounds was passive diffusion across the lipid component, which controlled 75-99% of transport. The exception was water, for which transport kinetics was 75% matrix-controlled. In general, permeation across the lipid-filled tissue was more favorable for small lipophilic compounds because of molecular restriction not only in the aqueous pores, but also in the lipid-filled pores.

Local topical delivery of drugs: a model incorporating simultaneous diffusion and metabolic interconversion between drug and a single metabolite in the skin

A comprehensive biophysical model for the topical delivery of a drug and its single, locally active metabolite is proposed. This elaboration of the simpler case, in which the drug converts irreversibly to a pharmacologically active metabolite in the tissue, allows for enzymatic interconversion between drug and metabolite. Exact mathematical expressions give concentration-distance relationships of drug and metabolite as well as fluxes of the two molecules in terms of concentration of drug applied to the stratum corneum, permeability coefficient of drug in the stratum corneum, diffusion coefficients of drug and metabolite in the viable tissues (epidermis and dermis), rate constants for the two enzyme systems, and the thickness of the viable tissue. Constants included in the mathematical expressions can be evaluated independently by appropriate in vitro experiments with freshly excised animal skin. The model can then predict what physiochemical drug constants will lead to maximal levels of active metabolite at the site of activity within the skin.

Human buccal absorption of flurbiprofen

The buccal absorption of flurbiprofen was studied in normal men to quantify the transport from the oral cavity in humans and to evaluate the closed-perfusion cell apparatus as a means to study drug transport across externally accessible biologic membranes. Flurbiprofen was buccally absorbed by a passive diffusional mechanism and the rate of absorption was pH dependent. Membrane permeability coefficients for flurbiprofen were 4.3 x 10(-4) cm/sec at pH 5.5 and 2.1 x 10(-5) cm/sec at pH 7.0. These findings are in agreement with the pH relationship for buccal transport observed in dog experiments. Delineation of the effective permeability coefficients into components for the aqueous boundary layer and the lipoidal buccal membrane allowed for the prediction of the extent of absorption of the drug over a period of time. It was concluded that the buccal membranes of the human and dog were essentially lipoidal membranes with equivalent permeabilities and no evident aqueous pore pathways.

Utilizing bile acid carrier mechanisms to enhance liver and small intestine absorption

On the basis of the enterohepatocycling phenomenon of bile acids involving the intestines, liver, and gallbladder, it was conceptualized that bile acids could serve as a molecular carrier of drugs by taking advantage of the bile acid active transport mechanism. It was further proposed that derivatization or analogation of bile acids at the C3-OH position was the desired route because of the reactive hydroxyl group and, moreover, because of the active transport requirement of retaining the C17 side chain with a single terminal acidic function. Using 3-tosylcholic, 3-benzoylcholic, and 3-iodocholic acids, in situ liver absorption, biliary excretion, and intestinal absorption studies in the rat were successful in establishing the concept that C3-derivatives and analogs of bile acids are, potentially, novel molecular delivery systems for intestinal and liver-site directed absorption.

Permeation of mouse skin and silicone rubber membranes by phenols: relationship to in vitro partitioning

A discrepancy has been noted in the relationship between the relative skin permeabilities of phenols and their lipophilicities as expressed in commonly used octanol:water partition coefficients (PCoctanol:water). The lack of correlation between partitioning and permeability is seen with the nitrophenols, particularly 4-nitrophenol. In the present study, the permeability coefficients-of 4-nitrophenol and several other phenols through skin and a model lipophilic membrane made of silicone rubber were found to be independent of concentration, ruling out concentration-dependent molecular aggregation as the cause of the partitioning-permeability incongruity. An unexpectedly low permeation rate was observed for the diffusion of 4-nitrophenol through the synthetic, silicone rubber membrane, confirming the anomalous position of this phenol in permeability relative to the octanol:water partitioning scale. However, when oil:water (o/w) partition coefficients for the phenolic compounds based on either n-hexane, CH2Cl2, CHCl3, or silicone rubber as the water immiscible phase are used, permeability coefficients for the skin and the synthetic membrane followed expected permeation-partitioning dependencies. On this basis, it appears that PCoctanol:water does not properly reflect the lipophilicity of the phenols with respect to partitioning into skin and silicone rubber during mass transfer.

Cholesterol monohydrate dissolution rate studies in aqueous micellar solutions of alpha-(nonylphenyl)-omega-hydroxydeca(oxyethylene), n-alkylamines, and fatty acids

The influences of electrical and nonelectrical factors in the interfacially controlled dissolution of cholesterol monohydrate by mixed micelles of alpha-(nonylphenyl)-omega-hydroxydeca(oxyethylene) (polyoxyethylene [10]-nonylphenol ether; POEPE; 1) in combination with long-chain n-alkylamines or fatty acids were investigated and quantified under pH conditions where the amines and fatty acids exist in their charged and uncharged forms. The experimental findings were generally consistent with a mechanism involving micelle collision with the cholesterol surface. A significant interfacial barrier was found with neutral micelles; however, the magnitude of the barrier was smaller with uncharged mixed micelles than with the simple micelles of 1. With charged micelles, the interfacial mass transfer resistance decreased with the addition of sodium chloride to the extent that the interfacial barrier was essentially abolished and the dissolution kinetics became convective/diffusion controlled. The results are consistent with the phenomena of diffusion of charged micelles toward a charged surface, following the classical collision kinetic theory of colloids. The ease in the transfer of cholesterol monohydrate molecules from the crystal surface and into the micelles during the collision step was examined by considering factors affecting the crystal surface as well as those associated with the hydrophobic-hydrophilic structure of the micelle of 1 and the distribution of charged and uncharged amphipathic additives within the basic micelle structure of 1.

Cholesterol monohydrate dissolution rate studies in aqueous micellar sodium chenodeoxycholate solutions

The dissolution rate of cholesterol monohydrate in various concentrations of sodium chenodeoxycholate (1) was significantly influenced by the addition of strong electrolytes. The mass transfer resistances decreased with increasing electrolyte concentrations and attained an asymptotic minimum value predicted and experimentally established for the convective/diffusion-controlled situation. Reduction of the interfacial barrier to dissolution was many times more sensitive to Mg2+ than to Na+ at equimolar concentrations. Cholesterol monohydrate solubilities increased nonlinearly with increasing 1 in 0.01 M phosphate buffer at pH 8.0 and was not influenced by the presence of strong electrolytes. Measured diffusion coefficients gave supporting evidence that the effective micellar size remained the same within the various experimental systems up to 116 mM chenodeoxycholate. The experimental findings indicated that the interfacial barrier is electrostatic in character. They are consistent with the phenomenon of diffusion of negatively charged micelles toward a negatively charged cholesterol monohydrate surface and the subsequent collision complex transfer of cholesterol molecules at the crystal surface. The results and mechanistic interpretations are also in accord with the previous model studies on cholesterol monohydrate dissolution in the presence of mixed micelles composed of nonionic and ionic surfactants.

Influence of 1-dodecylazacycloheptan-2-one (Azone) on the topical therapy of cutaneous herpes simplex virus type 1 infection in hairless mice with 2',3'-di-O-acetyl-9-beta-D-arabinofuranosyladenine and 5'-O-valeryl-9-beta-D-arabinofuranosyladenine

The predictive value of a recently developed physical model was tested in the topical treatment of cutaneous infections caused by herpes simplex virus type 1 in hairless mice with two ester prodrugs of 9-beta-D-arabinofuranosyladenine (ara-A) (1). The tests were conducted with 2',3'-di-O-acetyl-ara-A (4) and 5'-O-valeryl-ara-A (3) topically applied with and without 15% 1-dodecylazacycloheptan-2-one (2) (Azone), a percutaneous penetration enhancer. In addition to the in vivo studies, in vitro diffusion cell experiments with excised, full-thickness skin from hairless mice were conducted to determine the penetration enhancement effects of 2. As previously observed, 2 was able to induce remarkably large (100- to 1000-fold) flux enhancements in these in vitro experiments. Consistent with predictions based on the physical model studies, formulations of 3 and 4 without 2 had little or no influence on the pathogenesis of the herpes simplex virus type 1 infections; when 2 was present in the formulations, both 3 and 4 had dramatic therapeutic effects consistent with the predictions made with the physical model. Prodrug 4 with 2 was especially efficacious in the prevention of virus-induced lesions and in the survival of all animals. Similar results were obtained with acyclovir plus 2 in this model system.

Quantitative analysis of the interfacial barrier to membrane transport of cholesterol solubilized in a charged micellar system

The transport of cholesterol solubilized in polyoxyethylene(10)-nonylphenol ether with benzyldimethyltetradecylammonium chloride additive (4:1, w/w) was studied in a diffusion cell separated by a silicone rubber membrane. Overall kinetics revealed the presence of an interfacial barrier. Additions of NaCl or Na2SO4 abolished the barrier to the extent that total cholesterol flux was essentially limited by membrane diffusion considerations. The results are consistent with the concept of micelles diffusing in an electrical field followed by a collision-complex transfer of cholesterol in the aqueous-membrane interfacial region. The electrostatic force of repulsion arises from the overlap of diffuse electrical double layers emanating from the charged mixed micelle and the cationic surfactant adsorbed on the membrane. The influence of surfactant concentration on cholesterol transport kinetics was consistent with electrostatic phenomena. The derived physical model focused on interfacial electrical properties in the donor chamber by maintaining a high concentration (greater than or equal to 0.1 M) of strong electrolyte in the receiver. A linear regression of the logarithm of theoretical transport resistance, total resistance less membrane and receiver boundary layer resistances, versus (ionic strength)1/2 in the donor resulted in a reasonable estimate of the total surface potential of the micelle and membrane surfaces as well as the net dispersion attraction constant.

Physical model approach to gastrointestinal absorption of prostaglandins II: In situ rat intestinal absorption of dinoprost

In situ absorption studies with dinoprost in the rat jejunum were carried out using a modified Doluisio technique. The absorption rate was first order. There was a sigmoidal decrease in the rate with increasing buffer pH (from 3.5 to 9.5), which strongly indicated the partitioning of weak acid species into the lipoidal membrane. An asymptotic minimum rate was attained from buffer pH 7.5 to 9.5, operationally indicative of transport of anions across aqueous pores. The importance of the aqueous diffusion layer on the mucosal side of the membrane was evident; rates at pH 3.5 and 4.5 were faster at high agitation hydrodynamics in the lumen solution. Preliminary studies showed that there was no metabolism in the lumenal solution and that metabolism occurred within the membrane. The transport mechanism involved simultaneous passive diffusion and bioconversion in the membrane because (a) a 1.5 X 10(4)-fold range in dinoprost concentration (0.014-210 microM) showed no saturable carrier-mediated tendency on the rate, (b) iodoacetic acid and indomethacin did not inhibit the absorption rate, and (c) the shape of the absorption-pH profiles was suggestive of passive diffusion. The prostaglandin did not have apparent adverse membrane and vascular effects under the conditions employed. The quantification and factorization of the physically meaningful transport parameters were accomplished using the physical model previously described. The permeability coefficients of the aqueous diffusion layer for the oscillation and static hydrodynamic situations were 0.8 X 10(-4) and 1.7 X 10(-4) cm/s, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical model approach to gastrointestinal absorption of prostaglandins III: In situ rat intestinal absorption of dinoprostone

In situ absorption studies with dinoprostone in the rat jejunum were carried out to provide a quantitative mechanistic insight of the absorption process. The variables included buffer pH (3.5-9.5), buffer capacity, hydro-dynamics in the lumen, and concentration. The disappearance kinetics from the lumen was first order. The rate decreased with increasing pH in a sigmoidal manner and reached a minimum at about pH 9. These results indicate the effects of the partitioning of nondissociated species in the lipoidal membrane and transport across aqueous pores. The rate was higher with the higher degree of agitation of the lumenal solution. Between two hydrodynamic situations, the differences in the rates were large at pH 4.5 where the transport was largely aqueous diffusion controlled and then tended to become smaller with increasing pH where the transport became effectively membrane controlled. The 15-oxo- and 13,14-dihydro-15-oxo metabolites of dinoprostone were found. The physical model was applied to quantify the permeability coefficients of the aqueous diffusion layer and the aqueous pores of the membrane and the effective membrane transport-bioconversion permeability coefficient at various pH values. The overall absorption dinoprostone was similar to that of the less lipophilic dinoprost reported earlier and also more rapid. Hence, baseline absorption studies were completed with two major reference prostaglandins from which estimations of intestinal absorption can be made for their analogues and derivatives.

Physical model approach to taste studies of drugs and pharmaceutical formulations

This paper explores the physical model approach in studying the taste of drugs and assessing physical formulation factors for improving the undesirable taste of drugs. Various aspects of physiology relevant to the taste phenomenon have been reviewed to provide the biophysical basis for mathematical modeling. The model involves non-steady-state mass transport across the aqueous boundary layer and buildup of solute concentration at the essentially impermeable tongue surface. The theoretical predictions are consistent with experimental studies on the lag time to taste perception by the electrophysiological method and also with "instantaneous" psychophysical taste perception when solute concentrations much greater than the taste threshold are applied on the tongue. Within the framework of the non-steady-state model, novel experimental studies involving the use of a porous half-diffusion cell placed on the surface of an extended human tongue and the recording of the times for psychophysical taste response are proposed to quantify and provide mechanistic understanding of the taste of drugs and also physical formulation factors in overcoming undesirable taste properties.

Topical vaginal drug delivery I: effect of the estrous cycle on vaginal membrane permeability and diffusivity of vidarabine in mice

Preliminary studies showed that the vaginal membrane permeability coefficients for vidarabine (9-beta-D-arabinofuranosyladenine) varied widely within a group of mice of the same species and age. This finding prompted an investigation of the influence of the female mouse sexual cycle on the vidarabine permeability. By means of a vaginal smear technique, the sexual cycle, which was approximately 5 days in duration, was divided into five phases. The vaginal membrane permeability of vidarabine was determined during each phase. The results revealed that the permeability coefficients for vidarabine during the diestrus phase (3 X 10(-6)-4 X 10(-5) cm/sec) were 10-100 times higher than those obtained at the early estrus or estrus phases (1-3 X 10(-7) cm/sec). Further permeation studies on membranes at early estrus and estrus were performed by separating the cornified layer from the noncornified portion of the membrane. The low permeability coefficient of vidarabine across the cornified layer (4 X 10(-7) cm/sec) suggests that this layer may be the major diffusion barrier for vidarabine when the drug is topically applied. Collectively, the data also suggest that during estrus a three-layer diffusion model is appropriate, that during early diestrus a single-layer diffusion model may apply, and that during proestrus and postestrus the situations are intermediate and more complicated.

Permeation of skin and eschar by antiseptics II: influence of controlled burns on the permeation of phenol

The safe antiseptic use of phenol over the burn-traumatized surface depends on knowledge of how the systemic accumulation of phenol is affected by burn processes. To gain insight into the underlying permeation phenomenon, the diffusion of phenol and a reference cosolute, methanol, through both scalded and branded dorsal skin sections of the hairless mouse was studied as a function of burn temperature using in vitro diffusion cells. Temperatures up to 100 and 150 degrees were used for scalding and branding, respectively, using a 60-sec; exposure time. Permeability coefficients of the traumatized skins were assessed at 37 degrees and compared with control values. Coefficients of both permeating species were not increased significantly by burn temperatures up to 70 degrees applied either by scalding or branding, however, at higher temperatures exaggerated increases in permeation rates were noted. A limiting increase of approximately 7 times the control value was noted for phenol irrespective of the burn method. Permeability of methanol was altered even more dramatically and at 100 degrees by scalding and 150 degrees by branding was over 50 times the control rate. At 80 and 100 degrees for methanol and at 80 degrees for phenol, scalding produced larger increases in the permeability coefficients than branding. Since contact for 1 min at 60 degrees is capable of producing a full-thickness burn injury, it is clear that eschar permeability to phenol immediately postburn is not related to the clinical degree of burning, but is a function of the thermal intensity (hotness) of the burn stimulus. Full-thickness wounds can be expected to have highly variable rates of systemic absorption as a direct consequence of the wide-ranging permeability possible for such burns, with the risks of topical application varying accordingly.

Permeation of skin and eschar by antiseptics I: baseline studies with phenol

To assess how the permeability of phenol is altered by thermal injury, it was first necessary to have baselines of comparison on normal skin. Using in vitro diffusion cells and the skin of the hairless mouse, [14C]phenol was applied to skin in an aqueous medium with a reference copermeating species, [3H]methanol, and 37 degrees permeability coefficients of the pair were evaluated as functions of animal age, skin hydration, stripping of the skin, dermis isolation, and phenol concentration. Age proved to be of little consequence to permeability over a wide age range. Prolonged aqueous soaking of the skins was also without much effect. Stripping of the skin and isolating the dermis by soaking techniques allowed assessment of individual skin strata diffusion resistances. When applied to skin in trace radiochemical concentrations, phenol behaved diffusionally as an alkanol with a chain length of six. But at concentrations greater than 2% w/v, phenol facilitated the permeation rates of itself and methanol; the effect was markedly concentration sensitive and only fractionally reversible. Concentration studies using silicone rubber membranes proved that the effects on the skin were the results of destroyed barrier integrity. At 6% phenol concentration there was an essentially instantaneous, 10-fold increase in the phenol permeability coefficient, raising it to two-thirds that observed with fully stripped skin. Overall, the data suggest that the stratum corneum is proportionally impaired as the phenol concentration is increased.

Hydration and percutaneous absorption III: Influences of stripping and scalding on hydration alteration of the permeability of hairless mouse skin to water and n-alkanols

The influence of hydration on the permeability of stripped and scalded skins of hairless mice was investigated in vitro using water and n-alkanols as test permeants. Irrespective of pretreatment, the permeation rates of water, methanol, and ethanol were unaffected by aqueous immersion of skin sections in a diffusion cell, consistent with earlier data on unprocessed skins. The permeation rates of butanol and hexanol also were insensitive to hydration, differing from earlier studies on normal, intact skin in which both solutes' rates doubled after 10 hr of soaking. Following both pretreatments, the permeability of octanol declined over the first 5-10 hr of maceration, but remained invariant thereafter. The decline was most pronounced for the scalded skins. With untreated skin, octanol permeability initially increased and then declined before assuming a constant value. This study indicates that the barrier properties of the epidermis and dermis are not particularly sensitive to extended hydration except in the case of octanol. Scalding at 60 degrees for 60 sec rapidly hydrates the skin, altering tissue permeability to about the same extent as a 10-hr (or longer) immersion in water at 37 degrees. Octanol's unique hydration profile is explained by locating the origin of permeability decline in tissue beneath the horny exterior of the skin.

Theoretical and experimental studies of transport of micelle-solubilized solutes

A physical model describing the simultaneous diffusion of free solute and micelle-solubilized solute across the aqueous boundary layer, coupled with partitioning and diffusion of free solute through a lipoidal membrane, is derived. In vitro experiments utilizing progesterone and polysorbate 80 showed excellent agreement between theoretical predictions based on independently determined parameters and experimental results. The physical model predicts that micelles can assist the transport of solubilized solute across the aqueous diffusion layer, resulting in a higher solute concentration at the membrane surface than would be predicted if micelle diffusion is neglected. At high surfactant concentrations, the aqueous diffusion layer resistance can be eliminated and the activity of the solute at the membrane can approach the bulk solute activity. This mechanism could explain observed enhanced absorption rates in vivo when both micelle solubilization occurs and the aqueous diffusion layer is an important transport barrier. The importance of determining and defining the thermodynamic activity of the diffusing solute is emphasized.

Permeability of thermally damaged skin v: permeability over the course of maturation of a deep partial-thickness wound

The permeability of eschar is an important factor governing rational approaches to topical control of burn wound sepsis. Previous work has shown the burn wound to have a highly variable permeability immediately after burning depending on the manner of burning. But the burn is also a dynamic wound and its physical state changes during the process of maturation. The present studies are an early attempt to characterize wound permeability as a function of maturation. Hairless mice were burned dorsally for 15 seconds on a metal surface maintained at 80 degree C. The time and temperature conditions were chosen to effect a deep partial-thickness to full-thickness injury on the animal. Mice were sacrificed daily post burn over a 2-week period and the permeabilities of 3H-methanol and 14C-butanol through the excised eschar were measured. Te eschar permeability coefficients were directly compared to permeability coefficients for the same compounds found with abdominal skin sections taken concurrently from each animal. It was observed that the branding initially caused a 50 per cent increase in the permeability of methanol and a 300-400 per cent increase in the permeability of maturation. Thereafter permeabilities tended to increase, gradually at first, but accelerating to a maximum which was observed at approximately 10 days. At the maximum, methanol's permeability was 20 times and butanol's 12 times their normal values. For both compounds permeability of eschar decreased past the maximum until termination of the studies at 14 days.

Factors affecting diazepam infusion: solubility, administration-set composition, and flow rate

The sorption of diazepam in large-volume i.v. admixtures to administration-set components and in i.v. containers was analyzed quantitatively. Solubility of diazepam in phosphate buffer at various pH levels and in i.v. fluids was measured. Partition coefficients of diazepam into components of i.v. administration sets and i.v. containers were studied by shaking a solution of diazepam in 0.9% sodium chloride, with finely cut components and measuring the change in diazepam in the aqueous phase. Flow studies through an administration set of a 0.04-mg/ml diazepam solution in 5% dextrose injection were done, varying both the flow rate and the length of tubing. The maximum free-base solubility of diazepam in phosphate buffer was 0.048 mg/ml; its solubility was 0.058, 0.050, and 0.064 mg/ml in lactated Ringer's, 0.9% sodium chloride, and 5% dextrose injections, respectively. Equilibrium partition coefficients were highest for polyvinyl chloride tubing and flexible bags. Volume-control sets made of cellulose propionate had lower but sufficiently large partition coefficients to cause diazepam loss. Polyolefin semi-rigid and glass containers had low partition coefficients. In the flow studies, the amount of solution-contact time correlated with the extent of absorption. As flow rate decreased or tubing length increased, the amount of diazepam absorbed increased proportionately. A nomogram and a predictive dosing chart are presented for calculation of actual diazepam doses delivered at various flow rates and tubing lengths. Diazepam can be administered safely and effectively by i.v. infusion. The use of volume-control sets and flexible polyvinyl chloride bags should be avoided with diazepam solutions. Polyolefin semi-rigid containers are acceptable alternatives to glass. The concentration of diazepam infusions should not exceed 0.04 mg/ml.

Permeability of thermally damaged skin: I. Immediate influences of 60 degrees C scalding on hairless mouse skin

Freshly sacrificed hairless mice were burned dorsally by direct contact with 60 degrees C water for periods ranging from 15 seconds to 8 min. Wounds ranging in degree from superficial epidermis damage to injury penetrating well into subcutaneous musculature were inflicted. Burned skin sections and reference abdominal skin sections were excised, placed in diffusion cells and investigated with regards to their permeabilities to water, methanol, ethanol, n-butanol and n-octanol. The data were couched in terms of ratios of permeability coefficients of burned skin to normal skin (scalding coefficients) for the same animal. Scalding increased permeability of skin to all compounds studied but the effects leveled out by 60 seconds. Protracted scalding was without great effect despite progressively increased depth of damage to the tissue as noted in histological sections. The degree of lost barrier competency attributable to 60 degrees C scalding was not marked for any compound but was definitely different for different alkanols. An approximately 3-fold permeability increase was noted with n-butanol, the most affected compound. The data demonstrate that near instantaneous alterations in permeability of skin accompany scalding, that decreased barrier competency does not correlate with the severity of a burn as measured in depth of the burn, and that thermal alteration of permeabilities is dependent on the physicochemical characteristics of the permeants.

Hydration and percutaneous absorption: I. Influence of hydration on alkanol permeation through hairless mouse skin

A method to study the influence of hydration on skin permeability where the skin is immersed in saline for up to 30 hr and under circumstances where a steady state rate of permeation can be established in several minutes is indicated. These circumstances allow multiple, sequential runs over a period where the permeability coefficients of some chemicals are gradually changing. It has been found that the permeabilities of water, methanol and ethanol are little affected by such hydration. However, there is a doubling of the permeability coefficients of butanol and hexanol during the first 10 hr of immersion. More hydrophobic alkanols seem to be less sensitive to the protracted aqueous conditioning. In general the results indicate that there are complex molecular structure-permeability relationships operating in skin. More specifically, the hydration effects are insightful with respect to developing barrier models for skin as they are further indications that different parallel diffusional paths are followed by polar and semi- and nonpolar species.

Physical model evaluation of topical prodrug delivery--simultaneous transport and bioconversion of vidarabine-5'-valerate III: Permeability differences of vidarabine and n-pentanol in components of hairless mouse skin

The permeation behavior of 3H-vidarabine (3H-9-beta-D-ara-binofuranosyladenine) and 14C-n-pentanol through different strata of hairless mouse skin was studied using a diffusion cell at 37 degrees under steady-state conditions. Partition coefficients for the skin components verus 0.9% aqueous NaCl solution also were obtained. Various skin preparations including full-thickness skin, cellophane-stripped skin, and dermis membranes of different thicknesses were employed. The dermis membranes were considered to be diffusionally homogenous, and the product of the permeability coefficient and the thickness was taken as the apparent diffusivity. The apparent diffusivities for both compounds investigated were independent of thickness. Therefore, it was concluded that the molecular diffusivity is constant throughout the dermis. Comparisons of permeability coefficients in various strata of the skin revealed that, while the stratum corneum is the major diffusional barrier, the epidermis appears to be significantly less permeable than the dermis.

Physical model evaluation of topical prodrug delivery--simultaneous transport and bioconversion of vidarabine-5'-valerate V: Mechanistic analysis of influence of nonhomogeneous enzyme distributions in hairless mouse skin

The mathematical problem of simultaneous transport and metabolism in the case of nonuniform enzyme distributions in the skin was solved, and the solutions were used for analyzing experimental data. Experimental data were obtained from permeation experiments with 3H-vidarabine and its 5'-valerate using cellophane tape-stripped hairless mouse skin. Results of the analyses revealed that the esterase activity was four to nine times higher in the epidermis than in the dermis, whereas the deaminase activity was about the same in the two strata. These results were in good agreement with independent experiments using tissue homogenates. The enzyme distributions and the previously reported diffusivities were employed in generating concentration profiles for the prodrug and the drug in the skin. These results may be used in predicting the possible therapeutic effect of the prodrug when it is topically applied.