Carotid artery injection technique: bounds for bolus mixing by plasma and by brain

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Strategies for therapy of retinal diseases using systemic drug delivery: relevance of transporters at the blood-retinal barrier

INTRODUCTION

There is an increasing need for managing rapidly progressing retinal diseases because of the potential loss of vision. Although systemic drug administration is one possible route for treating retinal diseases, retinal transfer of therapeutic drugs from the circulating blood is strictly regulated by the blood-retinal barrier (BRB).

AREAS COVERED

This review discusses the constraints and challenges of drug delivery to the retina. In addition, this article discusses the properties of drugs and the conditions of the BRB that affect drug permeability. The reader will gain insights into the strategies for developing therapeutic drugs that are able to cross the BRB for treating retinal diseases. Further, the reader will gain insights into the role of BRB physiology including barrier functions, and the effect of influx and efflux transporters on retinal drug delivery.

EXPERT OPINION

When designing and selecting optimal drug candidates, it's important to consider the fact that they should be recognized by influx transporters and that efflux transporters at the BRB should be avoided. Although lipophilic cationic drugs are known to be transported to the brain across the blood-brain barrier, verapamil transport to the retina is substantially higher than to the brain. Therefore, lipophilic cationic drugs do have a great ability to increase influx transport across the BRB.

Involvement of an influx transporter in the blood-brain barrier transport of naloxone

Naloxone, a potent and specific opioid antagonist, has been shown in previous studies to have an influx clearance across the rat blood-brain barrier (BBB) two times greater than the efflux clearance. The purpose of the present study was to characterize the influx transport of naloxone across the rat BBB using the brain uptake index (BUI) method. The initial uptake rate of [(3)H]naloxone exhibited saturability in a concentration-dependent manner (concentration range 0.5 microM to 15 mM) in the presence of unlabeled naloxone. These results indicate that both passive diffusion and a carrier-mediated transport mechanism are operating. The in vivo kinetic parameters were estimated as follows: the Michaelis constant, K(t), was 2.99+/-0.71 mM; the maximum uptake rate, J(max), was 0.477+/-0.083 micromol/min/g brain; and the nonsaturable first-order rate constant, K(d), was 0.160+/-0.044 ml/min/g brain. The uptake of [(3)H]naloxone by the rat brain increased as the pH of the injected solution was increased from 5.5 to 8.5 and was strongly inhibited by cationic H(1)-antagonists such as pyrilamine and diphenhydramine and cationic drugs such as lidocaine and propranolol. In contrast, the BBB transport of [(3)H]naloxone was not affected by any typical substrates for organic cation transport systems such as tetraethylammonium, ergothioneine or L-carnitine or substrates for organic anion transport systems such as p-aminohippuric acid, benzylpenicillin or pravastatin. The present results suggest that a pH-dependent and saturable influx transport system that is a selective transporter for cationic H(1)-antagonists is involved in the BBB transport of naloxone in the rat.

Major involvement of low-density lipoprotein receptor-related protein 1 in the clearance of plasma free amyloid beta-peptide by the liver

PURPOSE

To identify the molecules responsible for amyloid beta-peptide (1-40) (Abeta(1-40)) uptake by the liver, which play a major role in the systemic clearance of Abeta(1-40).

METHODS

The liver uptake index method was used to examine the mechanisms of Abeta(1-40) uptake by the liver in vivo.

RESULTS

[125I]Abeta(1-40) uptake by the rat liver was concentration-dependent (50% saturation concentration = 302 nM). The inhibitory spectrum of Abeta fragments indicated that 17-24 in Abeta (LVFFAEDV) was the putative sequence responsible for hepatic Abeta(1-40) uptake. Receptor-associated protein (RAP) inhibited [125I]Abeta(1-40) uptake by 48%. RAP-deficient mice, in which low-density lipoprotein receptor-related protein 1 (LRP-1) expression was suppressed, showed a 46% reduction in [125I]Abeta(1-40) uptake by the liver. siRNA-mediated suppression of LRP-1 expression in the liver resulted in a reduction in [125I]Abeta(1-40) uptake by 64%. Both the expression of LRP-1 in the liver and the hepatic Abeta(1-40) uptake were significantly reduced in 13-month-old rats compared with 7-week-old rats.

CONCLUSIONS

LRP-1 is the major receptor responsible for the saturable uptake of plasma free Abeta(1-40) by the liver. Reduction of LRP-1 expression will play a role in the age-related reduction in hepatic Abeta(1-40) clearance.

How to measure drug transport across the blood-brain barrier

The extent to which a substance in the circulation gains access to the CNS needs to be determined for potential neuropharmaceuticals as well as for drug candidates with primary targets in the periphery. Characteristics of the in vivo methods, ranging from classical pharmacokinetic techniques (intravenous administration and tissue sampling) over brain perfusions to microdialysis and imaging techniques, are highlighted. In vivo measurements remain unmatched with respect to sensitivity and for the characterization of carrier-mediated uptake, receptor-mediated transport, and active efflux. Isolated microvessels are valuable tools for molecular characterization of transporters. Endothelial cell culture models of the blood-brain barrier (BBB) are pursued as in vitro systems suitable for screening procedures. Recent applications of conditionally immortalized cell lines indicate that a particular weakness of culture models because of downregulation of BBB-specific transporter systems can be overcome. In silico approaches are being developed with the goal of predicting brain uptake from molecular structure at early stages of drug development. Currently, the predictive capability is limited to passive, diffusional uptake and predominantly relies on few molecular descriptors related to lipophilicity, hydrogen bonding capacity, charge, and molecular weight. A caveat with most present strategies is their reliance on surrogates of BBB transport, like CNS activity/inactivity or brain-to-blood partitioning rather than actual BBB permeability data.

Differential contributions of rOat1 (Slc22a6) and rOat3 (Slc22a8) to the in vivo renal uptake of uremic toxins in rats

PURPOSE

Evidence suggests that uremic toxins such as hippurate (HA), indoleacetate (IA), indoxyl sulfate (IS), and 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) promote the progression of renal failure by damaging tubular cells via rat organic anion transporter 1 (rOat1) and rOat3 on the basolateral membrane of the proximal tubules. The purpose of the current study is to evaluate the in vivo transport mechanism responsible for their renal uptake.

METHODS

We investigated the uremic toxins transport mechanism using the abdominal aorta injection technique [i.e., kidney uptake index (KUI) method], assuming minimal mixing of the bolus with serum protein from circulating serum.

RESULTS

Maximum mixing was estimated to be 5.8% of rat serum by measuring estrone sulfate extraction after addition of 0-90% rat serum to the arterial injection solution. Saturable renal uptake of p-aminohippurate (PAH, K(m) = 408 microM) and benzylpenicillin (PCG, K(m) = 346 microM) was observed, respectively. The uptake of PAH and PCG was inhibited in a dose-dependent manner by unlabeled PCG (IC(50) = 47.3 mM) and PAH (IC(50) = 512 microM), respectively, suggesting that different transporters are responsible for their uptake. A number of uremic toxins inhibited the renal uptake of PAH and PCG. Excess PAH, which could inhibit rOat1 and rOat3, completely inhibited the saturable uptake of IA, IS, and CMPF by the kidney, and by 85% for HA uptake. PCG inhibited the total saturable uptake of HA, IA, IS, and CMPF by 10%, 10%, 45%, and 65%, respectively, at the concentration selective for rOat3.

CONCLUSIONS

rOat1 could be the primary mediator of the renal uptake of HA and IA, accounting for approximately 75% and 90% of their transport, respectively. rOat1 and rOat3 contributed equally to the renal uptake of IS. rOat3 could account for about 65% of the uptake of CMPF under in vivo physiologic conditions. These results suggest that rOat1 and rOat3 play an important role in the renal uptake of uremic toxins and the induction of their nephrotoxicity.

Blood-brain barrier efflux transport

Efflux transport at the blood-brain barrier (BBB) limits the brain tissue exposure to a variety of potential therapeutic agents, including compounds that are relatively lipophilic and would be predicted to permeate the endothelial lining of the brain microvasculature. Recent advances in molecular and cell biology have led to identification of several specific transport systems at the blood-brain interface. Refinement of classical pharmacokinetic experimentation has allowed assessment of the structural specificity of transporters, the impact of efflux transport on brain tissue exposure, and the potential for drug-drug interactions at the level of BBB efflux transport. The objective of this minireview is to summarize efflux transporter characteristics (location, specificity, and potential inhibition) for transport systems identified in the BBB. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on net brain tissue uptake of substrates also are presented. The potential impact of efflux transport on the pharmacodynamics of agents acting in the central nervous system are illustrated. Finally, general issues regarding the role of identifying efflux transport as part of the drug development process are discussed.

Investigation on the influx transport mechanism of pentazocine at the blood-brain barrier in rats using the carotid injection technique

The influx transport mechanism of pentazocine (PTZ) at the blood-brain barrier (BBB) was investigated in rats using the carotid injection technique. The uptake kinetics of PTZ into the rat brain exhibited saturability, which occurred by both nonsaturable and carrier-mediated transport processes. The in vivo kinetic parameters were estimated as follows: the maximal uptake rate (Jmax), 3.6 +/- 1.2 micromol/min/g brain and the apparent Michaelis constant (K1), 3.7 +/- 1.7 mM for the saturable component of PTZ into the brain, and the nonsaturable uptake rate constant (Kd), 0.06 +/- 0.04 ml/min/g brain. The uptake of PTZ by the brain was strongly inhibited by lidocaine, imipramine and propranolol, and also by H1-antagonists such as mepyramine, diphenhydramine. In addition, narcotic-antagonist analgesic (buprenorphine, butorphanol or eptazocine) and an opioid antagonist (naloxone) significantly inhibited PTZ transport. These results suggest that PTZ permeates into the brain via a carrier-mediated transport system, which may widely recognize the cationic drugs.

Acute upregulation of blood-brain barrier glucose transporter activity in seizures

Brain extraction of (18)F-labeled 2-fluoro-2-deoxy-D-glucose (FDG) was significantly higher in pentylene tetrazole (PTZ)-treated rats (32 +/- 4%) than controls (25 +/- 4%). The FDG permeability-surface area product (PS) was also significantly higher with PTZ treatment (0.36 +/- 0.05 ml. min(-1). g(-1)) than in controls (0.20 +/- 0.06 ml. min(-1). g(-1)). Cerebral blood flow rates were also elevated by 50% in seizures. The internal carotid artery perfusion technique indicated mean [(14)C]glucose clearance (and extraction) was increased with PTZ treatment, and seizures increased the PS by 37 +/- 16% (P < 0.05) in cortical regions. Because kinetic analyses suggested the glucose transporter half-saturation constant (K(m)) was unchanged by PTZ, we derived estimates of 1) treated and 2) control maximal transporter velocities (V(max)) and 3) a single K(m). In cortex, the glucose transporter V(max) was 42 +/- 11% higher (P < 0.05) in PTZ-treated animals (2.46 +/- 0.34 micromol. min(-1). g(-1)) than in control animals (1.74 +/- 0.26 micromol. min(-1). g(-1)), and the K(m) = 9.5 +/- 1.6 mM. Blood-brain barrier (BBB) V(max) was 31 +/- 10% greater (P < 0.05) in PTZ-treated (2.36 +/- 0. 30 micromol. min(-1). g(-1)) than control subcortex (1.80 +/- 0.25 micromol. min(-1). g(-1)). We conclude acute upregulation of BBB glucose transport occurs within 3 min of an initial seizure. Transporter V(max) and BBB glucose permeability increase by 30-40%.

Selective expression of the large neutral amino acid transporter at the blood-brain barrier

Amino acid supply in brain is regulated by the activity of the large neutral amino acid transporter (LAT) at the brain capillary endothelial cell, which forms the blood-brain barrier (BBB) in vivo. Bovine BBB poly(A)(+) RNA was isolated from 2.0 kg of fresh bovine brain and size fractionated on a sucrose density gradient, and a size-fractionated bovine BBB cDNA library in the pSPORT vector was prepared. The full-length cDNA encoding the bovine BBB LAT was isolated from this library, and the predicted amino acid sequence was 89-92% identical to the LAT1 isoform. The bovine BBB LAT1 mRNA produced a 10-fold enhancement in tryptophan transport into frog oocytes coinjected with bovine BBB LAT1 mRNA and the mRNA for 4F2hc, which encodes the heavy chain of the heterodimer. Tryptophan transport into the mRNA-injected oocytes was sodium independent and was specifically inhibited by other large neutral amino acids, and the K(m) of tryptophan transport was 31.5 +/- 5.5 microM. Northern blotting with the bovine BBB LAT1 cDNA showed that the LAT1 mRNA is 100-fold higher in isolated bovine brain capillaries compared with C6 rat glioma cells or rat brain, and the LAT1 mRNA was not detected in rat liver, heart, lung, or kidney. These studies show that the LAT1 transcript is selectively expressed at the BBB compared with other tissues, and the abundance of the LAT1 mRNA at the BBB is manyfold higher than that of transcripts such as the 4F2hc antigen, actin, or the Glut1 glucose transporter.

Blood-brain barrier transport and protein binding of flumazenil and iomazenil in the rat: implications for neuroreceptor studies

The calculated fraction of receptor ligands available for blood-brain barrier passage in vivo (f(avail)) may differ from in vitro (f(eq)) measurements. This study evaluates the protein-ligand interaction for iomazenil and flumazenil in rats by comparing f(eq) and f(avail). Repeated measurements of blood-brain barrier permeability for two benzodiazepine antagonists were performed in 44 rats by the double-indicator technique. Cerebral blood flow was measured by intracarotid Xe-injection. The apparent permeability-surface product (PSapp) was measured while CBF or bolus composition was changed. Comparison of PSapp obtained in the absence and presence of 5% albumin in the injectate yielded f(avail), whereas f(eq) was measured by equilibrium dialysis. Iomazenil and flumazenil f(avail) was 62% and 82%, respectively, whereas f(eq) was significantly lower, 42% and 61%. The PSapp for iomazenil and flumazenil increased significantly by 89% and 161% after relative CBF increases of 259% and 201%, respectively. The results demonstrate that application of f(eq) in neuroreceptor studies underestimates the plasma input function to the brain. Model simulations render possible that the differences between f(avail) and f(eq) as well as the effect of CBF on PSapp can be caused by capillary heterogeneity.

[Transport of drugs across the blood-brain barrier]

The blood-brain barrier prevents an indifferent medicine existing in the blood to enter also in the brain. This barrier has got an anatomical base: it is first consisting in a cerebrovascular layer of endothelial capillary vessels of the peripheral tissue. It is moreover covered by outgrowths of the flial cells, which are called astrocytes. There are, for that reason, important limits to a size of molecules which can reach the cerebral tissue through a paracellular way (through what is called in English "tight-junctions"). Most medicines must use the transcellular way. Lipophily is necessary to follow that way. Year after year, it appeared, thanks to a comparative study of the substances, that there exists--grosso modo--a positive correlation between the lipophilic level and the permeation-level of a substance in the cerebral tissue. There are, however, several exceptions: it is so that hydrophilic substances, possessing an important nourishing function (such as glucosis, amino-acids) seem to penetrate much more easily than we could expect when we consider their physicochemical characteristics. This is the result of the fact that there exist specifical transport-mechanisms for these substances at the level of the endothelial cell-membranes, allowing the penetration of such substances. There exist, on the contrary, lipophilic components that penetrate the cerebral tissue much less strongly than we should expect. This happens because there also exist pumping-mechanisms at the level of the hemato-encephalic barrier. The concerning substance, which was recently discovered is the "glycoprotein P", which is also responsible for the "multi-drug-resistance" and for the resistance of tumors to cytostatics. This phenomenon relies on a very efficient pumping of substances which have penetrated cells in which this protein expressed itself in the membranous structure. In order to obtain a better understanding of the function of the hemato-encephalic barrier, comprising the transport of medicines, it is most important to have reliable experimental models. It is to that aim that, during former years, the technique of cultivating endothelial cerebrovascular cells was developed. These cells are isolated from brains of calves or rats and, subsequently, cultivated on a laboratory medium; about a week later, they have grown a single and confluent layer. This layer represents a kint of "hemato-encephalic barrier" in vitro, which allows us to study the transfer of substances through the layer and thus also the details concerning the transport mechanisms, as well as the factors influencing the permeability of the cells-layer (for instance the inflammatory stimuli). Concerning the "in vivo" research, the technique of intracerebral microdialysis in lab-animals proved to be very promising. In order to effect this microdialysis, a semipermeable microcannula is introduced in the brain tissue, across which an iso-osmotic liquid is being injected continuously. The substances staying in the interstitial liquid of the cerebral tissue will diffuse under the influence of a concentration gradient, into the dialysing liquid and they will also be ready to be analysed. Thanks to this technique, it is possible to follow, in the same animal, the evolution of the concentration in the brain of a substance which has, for instance been injected in a peripheral region. In this way, we obtain, indirectly and in vivo, informations about the functioning-process of the "hemato-encephalic barrier". We can, moreover, effect measures on a specific spot, for instance in tumoral brain tissue: this allows us to study the influence of specific transport-mechanisms. These rather recent techniques, as well in vitro as in vivo, will allow us, in consequence, to increase, during the next years, our understanding of the way the hemato-encephalic barrier functions as to the transfer of medicines towards the central nervous system. This understanding may lead us to new strategies allowing

Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids

The transport of neutral amino acids through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is an important control point for the overall regulation of cerebral metabolism, including protein synthesis and neurotransmitter production. The Michaelis-Menten kinetics of BBB amino acid transport have been investigated in vivo with the brain uptake index (BUI) technique, and in vitro with the isolated human brain capillary preparation. The only amino acid that is albumin-bound is tryptophan, and the majority of albumin-bound tryptophan in the plasma is available for transport through the BBB via an enhanced dissociation mechanism that operates at the surface of the brain capillary endothelium. The availability in brain of amino acids is predicted from the BBB Km values to be sharply influenced by supra-physiological concentrations of phenyalanine in the 200-500 microM range. Moreover, the measurement of cerebral protein synthesis with an internal carotid artery perfusion technique and HPLC-based measurements of aminoacyl-transfer RNA specific activities shows an inverse relationship between cerebral protein synthesis and plasma phenyalanine concentrations in the 200-500 microM range. These findings indicate the neurotoxicity of hyperphenylalninemia is not restricted to the phenylketonuria range of approximately 2000 microM, but is exerted in the supra-physiological range of 200-500 microM.

Arginine vasopressin reduces the blood-brain transfer of L-tyrosine and L-valine: further evidence of the effect of the peptide on the L-system transporter at the blood-brain barrier

Arginine vasopressin (AVP) coinjected into the carotid artery in physiological concentrations (0.1 nmol/l), with either L-[3H]tyrosine or L-[3H]valine, induced changes in the kinetic parameters of the blood-to-brain transfer of both large neutral amino acids (LNAA) without alterations in brain haemodynamics. The half-saturation constant (Km), the maximum velocity of transport (V(max)) and Kd, the nonsaturable transport constant, were estimated in 9 brain regions of male Wistar rats anaesthetized with ether. Apart from Kd, significant changes in Km and V(max) were observed in all brain regions investigated. On average Km decreased from 0.17 to 0.048 mmol/l for tyrosine, and from 0.61 to 0.059 mmol/l for valine, whereas V(max) declined from 22 to 9.9 nmol/min/g for tyrosine, and from 29 to 3.2 nmol/min/g for valine, respectively. The results provide further evidence that vasopressin-receptor interactions at the blood-brain barrier (BBB) induce changes in the properties of the common transporter, the L-system, which eventually result in a suppression of the blood-to-brain transfer of LNAA. Data analysis of the 5 LNAA tested so far reveals a significant negative correlation (R = 0.98, P < 0.05) between the respective substrate affinity for the transporter and the corresponding magnitude of transport reduction induced by circulating AVP. Calculations of the unidirectional influx (J) of the LNAA indicate that AVP (1) reduces J by approximately one-third for every LNAA, but (2) does not change the relative contribution for each single LNAA to the total influx across the BBB.

Down-regulation of blood-brain glucose transport in the hyperglycemic nonobese diabetic mouse

The intracarotid injection method has been utilized to examine blood-brain barrier (BBB) glucose transport in hyperglycemic (4-6 days) mice. In anesthetized mice, Brain Uptake Indices were measured over a range of glucose concentrations from 0.010-50 mmol/l; glucose uptake was found to be saturable and kinetically characterized. The maximal velocity (Vmax) for glucose transport was 989 +/- 214 nmol.min-1.g-1. and the half-saturation constant estimated to be 5.80 +/- 1.38 mmol/l. The unsaturated Permeability Surface area product (PS) is = 171 + 8 microliters.min.-1.g-1. A rabbit polyclonal antiserum to a synthetic peptide encoding the 13 C-terminal amino acids of the human erythrocyte glucose transporter immunocytochemically confirmed the presence of the GLUT1 isoform in non-obese diabetic (NOD) mouse brain capillary endothelia. These studies indicate that a down-regulation of BBB glucose transport occurs in these spontaneously hyperglycemic mice; both BBB glucose permeability (as indicated by PS product) and transporter maximal velocity are reduced (in comparison to normoglycemic CD-1 mice), but the half-saturation constant remains unchanged.

Developmental modulation of blood-brain-barrier glucose transport in the rabbit

Blood-brain barrier (BBB) glucose transport rates were measured using the intracarotid injection method in newborn, 14-day-old suckling, 28-day-old weanling and adult rabbits, and compared with membrane transporter density. Light microscope immunochemistry confirmed the presence of the GLUT1 glucose transporter isoform in these rabbits. Quantitative electron microscopic immunogold analyses of GLUT1-immunoreactive sites per micrometer of capillary membrane indicated GLUT1 density increased with age, and correlated with in vivo measurements of Vmax. Maximal transport velocities (Vmax) of glucose transfer (an indicator of the activity and relative number of transporter proteins) increased significantly (P = 0.05) with age: in neonates Vmax = 0.61 mumol.min-1.g-1, in sucklings Vmax = 0.68 mumol.min-1.g-1, in weanlings Vmax = 0.88 mumol.min-1.g-1, and in adults Vmax = 1.01 mumol.min-1 g-1. Cerebral blood flow (CBF) rates, increased with age from 0.19 and 0.26 ml.min-1.g-1 in neonates and sucklings to 0.51 (weanlings) and 0.70 (adults) ml.min-1.g-1. Non-linear regression analyses indicated the half-saturation constant (Km) for glucose transport ranged from 13 mM in adult rabbits to 19 mM in 14-day-old sucklings: differences in Km were not significant. Age-related changes in the Permeability-Surface Area product (PS +/- S.E.) of both water and glucose were also seen. At all ages studied, the diffusion component (Kd) of glucose uptake was not distinguishable from zero. We conclude developmental up-regulation of the rabbit BBB glucose transporter is characterized by no changes in transporter affinity, and provide the first demonstration of increased membrane transporter proteins correlating with an age-related increase (65%) in glucose transporter maximal velocity.

Uptake of drugs and expression of P-glycoprotein in the rat 9L glioma

Two weeks after the inoculation of 1.5 x 10(5) 9L glioma cells into the rat brain, the uptake of radiolabelled drugs into the brain and the experimental 9L glioma during the first cerebral circulation was measured with a liquid scintillation counter and analyzed by the method of Oldendorf (1970). The expression of P-glycoprotein, which is known to be associated with the efflux of drugs, was also studied, using anti-P-glycoprotein monoclonal antibody, C-219. Furthermore, the ultrastructure of brain capillaries, tumor vessels, and glioma cells was studied by conventional and immunoelectron microscopy. Sucrose (control), the transport of which through the blood-brain barrier is known to be negligible, accumulated to fivefold higher levels in the tumor than in normal brain. Ranimustine (MCNU), 5-fluorouracil (5-FU), and doxorubicin showed little accumulation in the normal brain, whereas nimustine (ACNU) showed an increased accumulation. MCNU and doxorubicin showed negligible accumulation in the glioma cells despite diffusion into the tumor interstitial space. In contrast, ACNU and 5-FU showed an increased accumulation in tumor cells. The accumulation of 5-FU in the cultured 9L glioma cells was decreased by ATP inhibitors or by low temperature. Although both brain capillary endothelial cells and glioma cell membrane were immunohistochemically positive for P-glycoprotein, the tumor vasculature showed low expression of P-glycoprotein. The endothelial cells of tumor vessels ultrastructurally showed increased fenestrations, swelling, and disrupted junctions. Accordingly, it is suggested that hydrophobic drugs such as doxorubicin, being pumped out by P-glycoprotein, do not accumulate in 9L glioma cells as do other lipophilic drugs such as ACNU, or drugs such as 5-FU, which accumulate by a carrier-mediated mechanism.

Blood-brain glucose transfer in the mouse

The intracarotid injection method has been utilized to examine blood-brain barrier (BBB) glucose transport in normal mice, and after a 2-day fast. In anesthetized mice, cerebral blood flow (CBF) rates were reduced from 0.86 ml.min-1 x gm-1 in control to 0.80 ml.min-1 x gm-1 in fasted animals (p > 0.05). Brain Uptake Indices were significantly (p < 0.05) higher in fasted (plasma glucose = 4.7 mM) than control (plasma glucose = 6.5 mM) mice, while plasma glucose was significantly lower. The maximal velocity (Vmax) for glucose transport was 1562 +/- 303 nmoles.min-1 x g-1, and the half-saturation constant (Km =) 6.67 +/- 1.46 mM in normally fed mice. In fasted mice the Vmax was 2053 +/- 393 nmoles.min-1 x g-1 (p > 0.05), and the half-saturation constant (Km =) 7.40 +/- 1.60 mM (not significant, P > 0.05). A rabbit polyclonal antiserum to a synthetic peptide encoding the 13 C-terminal amino acids of the human erythrocyte glucose transporter (GLUT-1) immunocytochemically confirmed that the mouse brain capillary endothelial glucose transporter is a GLUT-1 transporter, and immunoreactivity was similar in brain endothelia from fed and fasted animals. In conclusion, after a 2-day fast in the mouse, we saw significant reductions in forebrain weight (7%), and plasma glucose levels (27%). Increased brain glucose extraction (25%, p < 0.05), and a 22% increase in the unsaturated permeability-surface area product (p < 0.05) was also observed.

Blood-brain barrier penetration of felbamate

The brain uptake index (BUI) method of Oldendorf was used to examine blood-brain barrier (BBB) drug transport in mice, rats, and rabbits; felbamate (FBM) extraction (E) in a single transcapillary passage was 5-20%, and drug uptake in rat brain was not concentration-dependent. Like diazepam, FBM was retained in mouse brain. To ensure that radioactivity measurements reflected the disposition of parent drug and not some metabolite, extracts of mouse brain were prepared for further analysis. No FBM metabolites were detected in brain 5 min after administration: In silica gel thin-layer chromatography (TLC), a single [14C]FBM peak was detected--Rf = 0.504 (70:30 acetone:hexane). Confirmatory high-performance liquid chromatography (HPLC) separations [30% methanol, 1.3 ml/min, C18 column, ultraviolet (UV) detection 254 nm] indicated a single peak containing greater than 93% of the radioactivity in the FBM fraction (12-min retention time). In a single transit through the liver (a nonbarrier tissue with fenestrated capillaries), FBM E was 82%. The octanol:buffered saline partition coefficient of FBM was (log PFBM =) 0.54 +/- 0.01. Thus, lipid-mediated BBB penetration of FBM is similar to that of phenytoin (PHT) and phenobarbital (PB). Plasma proteins do not affect FBM entry to the brain: neither human serum, nor bovine or human serum albumin (BSA, HSA), nor human alpha 1 acid glycoprotein (orosomucoid) significantly modified BBB FBM extraction. Erythrocyte-borne FBM may also dissociate and gain access to the brain in a single transcapillary passage. Differences between newborn and adult rabbit BBB FBM extraction and between different anesthetic agents are attributable to cerebral blood flow (CBF) rates. The permeability-surface area products (PS = [CBF].[E]) for FBM in rats, rabbits, and mice were 0.09, 0.16 and 0.30 ml/min/g, respectively. Preliminary autoradiographic analyses of frozen brain sections suggest that [14C]FBM distributes relatively uniformly throughout the brain and that minor variations apparently are a function of differing CBF rates.

Comparison of the blood-brain barrier and liver penetration of acridine antitumor drugs

The blood-brain barrier penetration of amsacrine and its analogs 9-([2-methoxy-4-[(methylsulfonyl)-amino]phenyl]amino)-,5-dimethyl- 4-acridine carboxamide (CI-921) and M-[2-(dimethylamino)ethyl]-acridine-4-carboxamide (AC) was measured in the barbiturate-anesthetized mouse. After intracarotid administration, AC was almost completely extracted (90%) in a single transit through the brain capillaries, whereas CI-921 (20%) and amsacrine (15%) were moderately extracted. AC is retained in the brain; no loss of AC from the brain was apparent at 1, 2, 4, or 8 min after injection. In contrast, after intraportal administration, 75% of the AC, 94% of the CI-921, and 57% of the amsacrine was extracted in a single transit through the hepatic vasculature. Rather than being retained in the mouse liver, these acridine antitumor agents show time-dependent loss (t1/2 = 10 min for amsacrine and AC, 24 min for CI-921). We conclude that unlike most antitumor agents, these acridine drugs appear to penetrate the blood-brain barrier readily.

Transport of tryptophan into brain from the circulating, albumin-bound pool in rats and in rabbits

Tryptophan is the only amino acid in the circulation that is bound by albumin, and previous studies have suggested that the brain tryptophan supply is a function of either the free or the albumin-bound pool of tryptophan in blood. Since the albumin molecule per se does not cross the brain capillary wall, i.e., the blood-brain barrier (BBB), the transport of tryptophan from the circulating albumin-bound pool may involve enhanced dissociation of tryptophan from the albumin binding sites within the cerebral microcirculation. This hypothesis was confirmed in the present studies wherein the dissociation constant (KaD) of albumin binding of tryptophan in the rat or rabbit brain microcirculation was measured in vivo. Brain extraction data for [14C]tryptophan determined with the carotid artery injection technique were fit to the Kety-Renkin-Crone equation modified for protein-bound solute. The KaD of albumin binding in the rat or rabbit brain microcirculation under pentobarbital anesthesia was 1.7 +/- 0.1 and 3.9 +/- 1.0 mM, respectively, as compared to the KD value measured in vitro with equilibrium dialysis, 0.13 +/- 0.03 mM. In contrast, the KaD value of albumin binding of tryptophan in vivo in the rabbit brain microcirculation was reduced by ether anesthesia to a value of 2.1 +/- 0.4 mM. This reduction in the KaD under ether anesthesia was associated with a 2.5-fold increase in cerebral blood flow. In addition, dialyzed rabbit serum caused a statistically significant inhibition in [14C]tryptophan influx during ether, but not pentobarbital, anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of plasma protein binding on the brain uptake of an antifungal agent, terbinafine, in rats

The intracarotid injection technique has been used to determine the unidirectional brain uptake of an antifungal, lipophilic agent, terbinafine (Lamisil, Sandoz Basle), in the rat. Ultrafiltration showed it to be highly bound to human plasma, human serum albumin (HSA), alpha 1-acid glycoprotein (AAG) and lipoproteins (VLDL, LDL, HDL). The effect of plasma protein binding of the drug on brain uptake was also examined with the technique. The lowest brain uptake was observed in the presence of plasma (6%); it varied from 23 to 30% with physiological concentrations of VLDL, LDL and HSA and was significantly higher (43-45%) in the presence of physiological concentrations of AAG and HDL. The free fraction as determined in-vitro and the brain uptake of the drug varied inversely with the plasma protein concentrations; however, the brain uptake was higher than expected from in-vitro measurements. These data indicate that the amount of circulating Lamisil available for brain penetration exceeds its free fraction; they also show that plasma proteins differently reduce the brain transport of the drug.

Plasma binding and transport of diazepam across the blood-brain barrier. No evidence for in vivo enhanced dissociation

The tissue uptake of extensively plasma-bound compounds is reportedly inconsistent with the conventional free-drug hypothesis limiting transport to unbound moiety in rapid intracapillary equilibrium with bound complex. Instead, protein-mediated/cell surface enhancement of dissociation has been postulated to occur in the microvasculature. This possibility was investigated by studying the passive transport of diazepam across the blood-brain barrier. Microdialysis probes placed within the vena cava and brain cortex were used to directly compare steady-state, interstitial unbound diazepam levels in both Wistar and genetically analbuminemic rats. The absence of albumin in the latter increased the unbound fraction of diazepam by almost fivefold; however, in both groups, the ratio of unbound concentrations in brain and blood at equilibrium was equal to unity. If enhanced dissociation occurred in the microvasculature, then the unbound brain level should have been greater than that in the systemic circulation. It is probable that earlier findings suggestive of protein-mediated transport reflect a nonequilibrium phenomenon. Comparison of the extent of diazepam's in vivo binding in blood by microdialysis to that estimated in vitro using conventional equilibrium dialysis with microcells showed good agreement, thus validating a widely accepted assumption of equivalency of these two values.

Pipequaline transport from blood to brain and liver: role of plasma protein-bound drug

Brain uptake of pipequaline (45319 RP) has been studied in-vivo after a single capillary transit by intracarotid injection to rats. Pipequaline is extensively bound to plasma proteins: i.e. human serum albumin (HSA), alpha-1-acid glycoprotein (AAG), lipoproteins and blood cells, mainly erythrocytes. The dialysable drug fraction as measured in-vitro by equilibrium dialysis at 37 degrees C, was inversely related to the concentration of binding component. Similarly, the brain uptake of pipequaline was inversely related to the protein concentration of the injected solution. However, the measured brain uptake of pipequaline was higher than those predicted by in-vitro measurements of dialysable drug for all proteins and erythrocytes, except HSA. These results show that a fraction of bound pipequaline as measured in-vitro is available for transport through the blood brain barrier. HSA-bound pipequaline is an exception as it is restricted to the vascular space. Pipequaline was totally cleared by the liver through a single passage.

The unit impulse response procedure for the pharmacokinetic evaluation of drug entry into the central nervous system

The unit impulse response theory has been adapted to characterize the transport profile of drugs into the central nervous system (CNS). From the obtained input function, the cumulative plasma volume (V) cleared by transport into the CNS in time can be calculated. Simulation studies demonstrated that transport governed by passive diffusion resulted in a linear relationship between V and time, while the slope of the line, the blood-brain barrier (BBB) clearance, proved to be an adequate and model independent parameter to characterize drug transport into the CNS. The error in the result of the numerical procedure could be limited to less than 10% of the theoretically predicted value. Superposition of 5 or 10% random noise on simulated data did not result in significant differences between the calculated and theoretically predicted clearance values. Simulations of carrier-mediated transport resulted in nonlinear transport curves; the degree of nonlinearity, and thus the detectability, was dependent on the initial degree of saturation of the system, the rate of desaturation, as caused by drug elimination processes and the noise level on the data. In vivo experiments in the rat were performed, using atenolol, acetaminophen, and antipyrine as model drugs. Linear transport relationships were obtained for all drugs, indicating that transport was dependent on passive diffusion or a low affinity carrier system. BBB-clearance values were 7 +/- 1 microliters/min for atenolol, 63 +/- 7 microliters/min for acetaminophen and 316 +/- 25 microliters/min for antipyrine. These experiments validate the applicability of the presented technique in in vivo studies.

Triiodothyronine bound to red blood cells is not available for transport through the blood-brain barrier

Many steroid and thyroid hormones and some drugs are bound by circulating red cells. Red cell-bound ligand may not be physiologically inert, as recent studies show that red cell-bound drug is available for uptake by brain. To investigate whether triiodothyronine (T3) is available for uptake by brain in vivo from the circulating red cell pool, the present investigations measure the effects of human erythrocytes on rat brain uptake of [125I]T3 in vivo. The fraction of circulating T3 available for uptake in vivo in the presence of 0, 2, 5, 10, 22, or 44% red cells was essentially identical to the fraction of [125I]T3 unbound in vitro. Therefore, [125I]T3 bound to red cells obtained from normal volunteers is not available for uptake by brain in vivo.

Dialyzable serum cofactor(s) required for the protein-mediated transport of DL-propranolol into rat brain

To elucidate the characteristics of promotion factor(s) in rat serum required for the protein-mediated transport of drugs into the brain, we examined the brain uptake of DL-propranolol as a model drug using the in vivo brain uptake index (BUI) method in rats. The protein-mediated transport was not observed in rats injected with the buffer solution containing either various concentrations of purified rat alpha 1-acid glycoprotein (alpha 1-AGP) or rat albumin. When the filtrate from rat serum was used as an injection vehicle to which a physiological concentration of purified rat serum protein(s) was added, the protein-mediated transport of DL-propranolol was observed in the rat brain. Moreover, the ability of protein-mediated transport of DL-propranolol was reduced in rats injected with the dialyzed serum compared with the undialyzed serum. These results suggest that the dialyzable promotion factor in serum is required for the protein-mediated transport of DL-propranolol into the brain.

pH dependence of histidine affinity for blood-brain barrier carrier transport systems for neutral and cationic amino acids

The effects of pH (3.5-7.5) on the brain uptake of histidine by the blood-brain barrier (BBB) carriers for neutral and cationic amino acids were tested, in competition with unlabeled histidine, arginine, or phenylalanine, with the single-pass carotid injection technique. Cationic amino acid ( [14C]arginine) uptake was increasingly inhibited by unlabeled histidine as the pH of the injection solution decreased. In contrast, the inhibitory effect of unlabeled histidine on neutral amino acid ( [14C]phenylalanine) uptake decreased with decreasing pH. Brain uptake indices with varying histidine concentrations indicated that the neutral form of histidine inhibited phenylalanine uptake whereas the cationic form competed with arginine uptake. Since phenylalanine decreased [14C]histidine uptake at all pH values whereas arginine did not, it was concluded that the cationic form of histidine had an affinity for the cationic carrier, but was not transported by it. We propose that the saturable entry of histidine into brain is, under normal physiological circumstances, mediated solely by the carrier for neutral amino acids.

TeBG- and CBG-bound steroid hormones in rabbits are available for influx into uterus in vivo

The metabolic clearance rate (MCR) of gonadal or adrenal steroid hormones in rabbits often does not bear the expected inverse relationship with hormone binding to testosterone-binding globulin (TeBG) or corticosteroid-binding globulin (CBG). This suggests TeBG or CBG may not impede steroid hormone delivery to tissues. The effects of rabbit plasma proteins on the influxes of 3H-labeled steroids from the circulation into the rabbit uterus were measured in vivo using a tissue sampling single-injection technique. In the absence of plasma proteins, estradiol (E2) and testosterone (T) were freely diffusible through the uterine microvasculature (i.e., extraction greater than 80%). The extractions of dihydrotestosterone (DHT) and corticosterone (B) ranged from 60 to 72%, while that of cortisol (F) was reduced at 40%. Rabbit serum exerted no inhibition of the influxes of the steroids tested. The influxes of T and B greatly exceeded the rates that would be expected if only the free and albumin-bound fractions estimated in vitro were diffusible in vivo. However, the extraction of [3H]corticosteroid-binding globulin or bovine [3H]albumin were low, consistent with little, if any, extravascular uptake of the plasma proteins. The results indicate both albumin-bound and globulin-bound steroid hormone are available for transport into the uterus in the rabbit in vivo without significant exodus of the plasma protein, per se.

Inhibition of blood-brain barrier permeability to DL-propranolol by serum from acute renal failure rats

The effect of uranyl nitrate-induced acute renal failure on the brain uptake of DL-propranolol was investigated in rats with a series of tissue-sampling single-carotid injection techniques. When the buffer solution was used as an injection solution, the brain uptake index (BUI), the extraction ratio (ET), and the blood-brain barrier (BBB) permeability-surface area product (PSapp) and PSu (corrected PSapp for the unbound fraction) in uremic rats were significantly lower than those in control rats. These parameters for DL-propranolol were decreased significantly in both control and uremic rats receiving injection of the uremic serum. The PSu values in both of the control and uremic rats injected with either control or uremic rat serum were significantly higher than those in rats injected with the buffer solution, suggesting the presence of a protein-mediated transport mechanism; that is, the conventional assumption that the fraction of the drug which is available for the uptake in vivo is equal to the unbound fraction as measured in vitro may not hold. In contrast, the brain extraction of D-[14C]glucose, [3H]inulin and [3H]water, which show no binding to serum protein, was not affected by the coinjection of either control or uremic rat serum. On the other hand, using either the ultrafiltrate from serum (control and uremic) or supernatant fraction from heat-treated serum (control and uremic) as the injection solution, no significant difference in the PSu value for DL-propranolol was observed between control and uremic serum. These results suggest that (1) the decrease in the PSu value for DL-propranolol in uremic rats may be attributed mainly to the presence of an endogenous inhibitory substance(s) for the brain uptake or to the decrease in the exchangeable fraction in vivo in the uremic serum; (2) the decrease in the PSu value for DL-propranolol may also be partly attributed to the change in the BBB permeability and/or surface area; (3) the inhibitor for the brain uptake may be characterized as a temperature-sensitive and nonfiltrable substance(s) at physiological pH; and (4) the ability of protein-mediated transport for DL-propranolol into brain was decreased in uremic rats.

Selective availability of protein bound estrogen and estrogen conjugates to the rat kidney

Estrogen glucuronides are selectively cleared by the kidney as compared to estrogen sulfates. The selective trafficking of circulating serum estrogen conjugates to kidney and urine may arise from differential transport properties of the various estrogen conjugates in the renal microcirculation. In the present study, the effects of glomerular and peritubular permeability barriers, and plasma protein binding on the influx of unconjugated and conjugated estrogens into rat kidney were studied. Experiments were carried out utilizing an in vivo double isotope, single injection, timed tissue sampling technique. The extractions of these steroids by the renal cortex were examined utilizing inulin as the reference substance, as it is freely permeable through the glomerular and tubular capillary permeability barriers, but not extracted by tubular epithelial cells. The method was validated by studying the extraction of para-amino hippuric acid (PAH) before and after probenecid treatment. In the absence of plasma proteins, all the estrogens and estrogen conjugates readily diffused through both the glomerular and peritubular capillary permeability barriers and were extracted by tubular epithelia. The addition of 4% albumin to the injection solution led to significant inhibitions of extraction of estradiol (E2) and estrone sulphate (E1-S) only. The extraction of E1-S was reduced to a value less than that of inulin; the extraction of E2 was less than that of control value but significantly more than that of inulin. The addition to the injection solution of human pregnancy sera containing sex hormone binding globulin and albumin was associated with a reduction in the extraction of all estrogens and estrogen conjugates except estriol and E1-S to values approximating that of inulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylalanine transport across the blood-brain barrier as studied with the in situ brain perfusion technique

Unidirectional L-phenylalanine transport into six brain regions of pentobarbital-anesthetized rats was studied using the in situ brain perfusion technique. This technique allows both accurate measurements of cerebrovascular amino acid transport and complete control of perfusate amino acid composition. L-Phenylalanine influx into the brain was sodium independent and could be described by a model with a saturable and a nonsaturable component. Best-fit values for the kinetic constants in the parietal cortex equaled 6.9 X 10(-4) mumol/s/g for Vmax, 0.011 mumol/ml for Km, and 1.8 X 10(-4) ml/s/g for KD during perfusion with fluid that did not contain competing amino acids. D-Phenylalanine competitively inhibited L-phenylalanine transport with a Ki approximately 10-fold greater than the Km for L-phenylalanine. There were no significant regional differences in Km, KD, or Ki, whereas Vmax was significantly greater in the cortical lobes than in the other brain regions. L-Phenylalanine influx during plasma perfusion was only 30% of that predicted in the absence of competing amino acids. Competitive inhibition increased the apparent Km during plasma perfusion by approximately 20-fold, to 0.21 mumol/ml. These data provide accurate new estimates of the kinetic constants that describe L-phenylalanine transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer site affinity (1/Km) for L-phenylalanine is three- to 12-fold greater than previously estimated in either awake or anesthetized animals.

Brain extraction of a calcium channel blocker

Dihydropyridine calcium channel blockers may be effective treatment for acute cerebral ischemia, but the uptake of these drugs into the brain is unknown. A 0.2-ml bolus of [14C]nicardipine hydrochloride and [3H]water was injected into the common carotid arteries of 7 normal and 7 ischemic rats. The corrected first-pass extraction of nicardipine, compared to water, was calculated to be 30.7% into the hemispheres and 42.3% into the hippocampi. The uptake was greater into the ischemic hemispheres (p less than 0.001). These data suggest that dihydropyridines are available to binding sites and calcium channels in neurons.

Niacinamide transport through the blood-brain barrier

The unidirectional influx of niacinamide across cerebral capillaries, the anatomical locus of the blood-brain barrier, was measured with an in situ rat brain perfusion technique employing [14C]niacinamide. Niacinamide was transported rapidly across the blood-brain barrier by a system that was not saturable with 10 mM niacinamide in the perfusate. However, with periods of perfusion longer than 30 seconds, there was substantial backflow of [14C]niacinamide into the perfusate. Niacinamide (1.7 microM) transport through the blood-brain barrier was not significantly inhibited by 3-acetylpyridine. Thus, niacinamide is transported rapidly and bidirectionally through the blood-brain barrier by a high capacity transport system. Although involved in the transfer of niacinamide between blood and brain, this transport system does not play an important regulatory role in the synthesis of NMN, NAD, and NADP from niacinamide in brain.

Pantothenic acid transport through the blood-brain barrier

The unidirectional influx of D-pantothenic acid (PA) across cerebral capillaries, the anatomical locus of the blood-brain barrier, was measured with an in situ rat brain perfusion technique using [3H]D-PA (1.1 Ci/mmol). PA was transported across the blood-brain barrier by a saturable system that could be described by a Michaelis-Menten transport model with a half-saturation concentration and maximal influx rate of 19 microM and 0.21 nmol/g of brain/min, respectively. PA (0.3 microM) transport through the blood-brain barrier was significantly inhibited by probenecid, nonanoic acid, and biotin (all less than or equal to 0.25 mM), but not by penicillin G, pyruvate, beta-hydroxybutyrate, L-leucine (all 1 mM), or poly-L-lysine HBr (1 mg/ml). Probenecid (0.25 mM), nonanoic acid (0.5 mM), and PA (1.0 mM) did not inhibit [3H]L-leucine transport through the blood-brain barrier, whereas 30 microM-L-leucine inhibited [3H]leucine transport to 23% of control values. Thus, PA is transported through the blood-brain barrier by a low-capacity, saturable transport system with a half-saturation concentration approximately 10 times the plasma PA concentration. Although involved in the transfer of PA from blood into brain, this system does not play an important regulatory role in the synthesis of CoA from PA in brain.

CBG does not restrict blood-brain barrier corticosterone transport in rabbits

The metabolic clearance rate of corticosterone in rabbits is unrelated to the physiological concentration of corticosteroid binding globulin (CBG) in rabbit plasma. This suggests that corticosterone is available for transport into peripheral tissues in rabbits from the circulating CBG-bound pool, similar to what is known to occur in rat liver. This hypothesis was tested in the present studies, which investigate the transport of corticosterone into rabbit brain from the circulating rabbit or human CBG-bound pool. Corticosterone was readily exchangeable in brain capillaries in vivo from the circulating albumin-bound and rabbit or human CBG-bound pools. The involvement of specific CBG receptors on brain capillary endothelia in this process was investigated with [3H]-labeled human CBG prepared by reductive methylation. The transport of [3H]CBG across rabbit brain capillaries in vivo was immeasurably low, and no specific binding of this radiolabeled plasma protein to isolated brain capillaries in vitro was observed at 37 degrees C during incubations up to 120 min. These studies indicate that the rabbit is a novel system for assessing the role of CBG in delivering corticosterone to peripheral tissues in vivo and that specific endothelial CBG receptors may not participate in the transport process.

Serum bioavailability of sex steroid hormones

This chapter has reviewed the factors underlying the transport of testosterone and oestradiol into tissues in vivo. The following points have been emphasized. Albumin-bound testosterone is nearly freely available for transport into brain and liver and is partially available for transport into salivary gland and lymph node; testosterone transport into hair follicles has not been measured thus far. SHBG-bound testosterone is not available for transport into tissues; SHBG-bound oestradiol is available for transport into liver, salivary gland, and lymph node, bug not into brain under normal conditions. The transport of hormone from the circulating plasma protein-bound pool involves tissue-mediated enhanced dissociation of the hormone from the protein without significant exodus of the plasma protein from the microcirculation compartment. The tissue-mediated enhanced dissociation mechanism varies in activity between different organs and is a much more important factor than organ differences in capillary transit times in regulating the amplification of hormone delivery to different tissues. The concentration of free testosterone inside cells in the absence of significant cellular metabolism of the hormone is nearly ten times greater than the concentration of free testosterone in vitro, but is nearly equal to the concentration of free plus albumin-bound hormone. In the presence of active tissue metabolism of hormone, the concentration of cellular free testosterone may be much less than the albumin-bound hormone and may fortuitously approximate the concentration of free testosterone in vitro. This is the situation in salivary gland; the low concentration of testosterone in saliva appears to be due to active salivary metabolism of the hormone, since both free and albumin-bound testosterone are available for transport into salivary gland.