Permeability of interstitial space of muscle (rat diaphragm) to solutes of different molecular weights

The Importance of the Interstitium in Peritoneal Transport

The peritoneal capillary exchange vessels are located within all the tissues which surround the peritoneal cavity and are separated from the peritoneal dialysis fluid by the tissue interstitium. The interstitium adds an additional barrier to transcapillary transport resistance and slows the diffusion of solutes from the blood to the dialysis fluid. The interstitium also alters the pressure environment of the blood capillary and has profound effects on water transport, causing fluid loss from the cavity to the body during dialysis.

Improving delivery and efficacy of nanomedicines in solid tumors: role of tumor priming

Effectiveness of nanomedicines in cancer therapy is limited in part by inadequate delivery and transport in tumor interstitium. This article reviews the experimental approaches to improve nanomedicine delivery and transport in solid tumors. These approaches include tumor vasculature normalization, interstitial fluid pressure modulation, enzymatic extracellular matrix degradation, and apoptosis-inducing tumor priming technology. We advocate the latter approach due to its ease and practicality (accomplished with standard-of-care chemotherapy, such as paclitaxel) and tumor selectivity. Examples of applying tumor priming to deliver nanomedicines and to design drug/RNAi-loaded carriers are discussed.

Multiscale measurements distinguish cellular and interstitial hindrances to diffusion in vivo

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

In vivo determination of diffusive transport parameters in a superfused tissue

To address the hypothesis that functional changes in tissue transport can be related to structural alterations, we combined mathematical modeling with in vivo experimentation. The model concept includes interstitial diffusion and removal by a distributed microvasculature. Transport of solute and water across the peritoneum is measured via a plastic chamber affixed to the abdominal wall of anesthetized Sprague-Dawley rats. Solutions containing [14C]mannitol, with or without vasoactive compounds [control (C; n = 10), C + nitroprusside (NP; n = 10), C + norepinephrine (NE; n = 10)], were infused into the chamber, and the volume and tracer concentrations were determined over 60 min to calculate the mass transfer coefficient (MTC) and the water flux. At 60 min, FITC-dextran (500 kDa) was given to mark the perfused vasculature. After euthanasia, the tissue under the chamber was frozen, dried, sliced with a cryomicrotome, and examined with fluorescent microscopy and quantitative autoradiography. The microvessel density (×103/cm2: NE, 50 ± 10; C, 180 ± 7.0; NP, 225 ± 15) resulted in marked differences ( P < 0.05) in water flux (μl·min−1·cm−2: NE, 0.1 ± 0.1; C, 1.6 ± 0.4; NP, 1.0 ± 0.2) and in mannitol MTC (×103cm/min: NE, 0.9 ± 0.3; C, 3.8 ± 0.3; NP, 3.6 ± 0.6). Concentration profiles and calculated capillary permeability and tissue diffusivity were significantly different among the groups. These results demonstrate a direct correlation of mass transfer, diffusion, capillary permeability, and water flux with peritoneal vascular density and validate a method by which mechanistic changes in transport may be measured.

Intraprostatic chemotherapy: distribution and transport mechanisms

PURPOSE

The present study evaluated the tissue distribution and targeting advantage of intraprostatic chemotherapy.

EXPERIMENTAL DESIGN

We studied the delivery and spatial distribution of a fluorescent drug, doxorubicin, in the prostate of beagle dogs, after intraprostatic or i.v. administration. Drug concentrations were measured using high-performance liquid chromatography and confocal fluorescence microscopy.

RESULTS

I.v. and intraprostatic injections yielded qualitatively and quantitatively different doxorubicin distribution in the prostate. A relatively homogeneous distribution was found after i.v. administration, whereas intraprostatic injection yielded a highly heterogeneous distribution with >10-fold higher concentrations localized in a cone-shaped glandular lobule bound by fibromuscular stroma, compared with other parts of the prostate. Compared with i.v. injection, intraprostatic injection yielded, on average, approximately 100-fold higher tissue-to-plasma concentration ratio, ranging from 963-fold near the injection site to 19-fold in the contralateral half of the prostate. The drug distribution within the prostate further suggests an important role for acinar flow in intraprostatic drug transport.

CONCLUSIONS

Intraprostatic administration represents a viable option to deliver high drug concentrations within the prostate. The results further suggest the fibromuscular stroma separating the prostatic lobules as a major barrier to drug transport and convective flow as an important drug transport mechanism in the prostate.

The transport barrier in intraperitoneal therapy

The peritoneal cavity is important in clinical medicine because of its use as a portal of entry for drugs utilized in regional chemotherapy and as a means of dialysis for anephric patients. The barrier between the therapeutic solution in the cavity and the plasma does not correspond to the classic semipermeable membrane but instead is a complex structure of cells, extracellular matrix, and blood microvessels in the surrounding tissue. New research on the nature of the capillary barrier and on the orderly array of extracellular matrix molecules has provided insights into the physiological basis of osmosis and the alterations in transport that result from infusion of large volumes of fluid. The anatomic peritoneum is highly permeable to water, small solutes, and proteins and therefore is not a physical barrier. However, the cells of the mesothelium play an essential role in the immune response in the cavity and produce cytokines and chemokines in response to contact with noncompatible solutions. The process of inflammation, which depends on the interaction of mesothelial, interstitial, and endothelial cells, ultimately leads to angiogenesis and fibrosis and the functional alteration of the barrier. New animal models, such as the transgenic mouse, will accelerate the discovery of methods to preserve the functional peritoneal barrier.

Axially symmetric semi-infinite domain models of microdialysis and their application to the determination of nutritive flow in rat muscle

Theoretical models for the description of microdialysis outflow:inflow (O/I) ratio for 3H2O and [14C]ethanol were developed, taking into account the nutritive fraction of total blood flow in muscle. The models yielded an approximately exponential decay expression for the O/I ratio, dependent on the physical dimensions of a linear probe (length and radius), the flow rate through the probe, muscle blood flow (including the nutritive fraction) and the diffusion coefficients for the tracer in the probe and muscle. The models compared favourably with experimental data from the constant-flow perfused rat hindlimb. Estimates of the nutritive fraction of total blood flow from experimental data were determined by minimizing the error between model and experimental data. The nutritive fraction was found to be 0.22 +/- 0.04 under basal perfusion conditions. When 70 nM noradrenaline (norepinephrine) was included in the perfusion medium, the nutritive fraction was 0.91 +/- 0.06 (P < 0.05). The inclusion of 300 nM serotonin, decreased the nutritive fraction to 0.05 +/- 0.01 (P < 0.05). This model can be applied to the determination of nutritive fraction of skeletal muscle blood flow in physiologically relevant microvascular conditions such as during exercise and in disease states.

Drug delivery and transport to solid tumors

PURPOSE

The purpose of this review is to provide an overview of the principles of and barriers to drug transport and delivery to solid tumors.

METHODS

This review consists of four parts. Part I provides an overview of the differences in the vasculature in normal and tumor tissues, and the relationship between tumor vasculature and drug transport. Part II describes the determinants of transport of drugs and particles across tumor vasculature into surrounding tumor tissues. Part III discusses the determinants and barriers of drug transport, accumulation, and retention in tumors. Part IV summarizes the experimental approaches used to enhance drug delivery and transport in solid tumors.

RESULTS

Drug delivery to solid tumors consists of multiple processes, including transport via blood vessels, transvascular transport, and transport through interstitial spaces. These processes are dynamic and change with time and tumor properties and are affected by multiple physicochemical factors of a drug, multiple tumor biologic factors, and as a consequence of drug treatments. The biologic factors, in turn, have opposing effects on one or more processes in the delivery of drugs to solid tumors.

CONCLUSION

The effectiveness of cancer therapy depends in part on adequate delivery of the therapeutic agents to tumor cells. A better understanding of the processes and contribution of these factors governing drug delivery may lead to new cancer therapeutic strategies.

Effect of concentration on restriction and diffusion of albumin in the excised rat diaphragm

In tissue samples of rat diaphragm mounted between two chambers, we measured the flow of albumin solution (0-5 g/dl) containing radioactive tracer (125)I-albumin in response to a driving pressure of 20 cmH(2)O. The ratio of the albumin concentration of the output solution to that of the input (sieving ratio, C(out)/C(in)) was measured from solution radioactivity. C(out)/C(in) increased monotonically from 0.5 with the flow of approximately 0 g/dl albumin solution (tracer) to 0.9 with the flow of 5 g/dl albumin solution. We modeled the tissue as a membrane subjected to flows of high Peclet No. with a reflection coefficient sigma = 1 - C(out)/C(in). Values of sigma decreased from 0.5 with Ringer solution to 0.1 with 5 g/dl albumin solution. Hydraulic conductivity measured with the flow of Ringer solution increased with the flow of 5 g/dl albumin solution. Wet-to-dry weight ratio and radioactivity of tissue samples immersed in 0.01-5 g/dl albumin solutions indicated a 40% increase in tissue water, associated with an albumin volume fraction of 0.3 measured at 0.5-2 h. The slower rate of albumin uptake occurring up to 20-30 h indicated intracellular diffusion that was equal with 1 and 5 g/dl albumin solution but reduced with a 0.01 g/dl albumin solution. The results suggest that interstitial pores increase in size in response to an increase in albumin concentration. We postulate a two-pore model made of intracellular pores that coalesce into a set of larger pores by osmotic flow.

Time- and concentration-dependent penetration of doxorubicin in prostate tumors

The penetration of paclitaxel into multilayered solid tumors is time- and concentration-dependent, a result of the drug-induced apoptosis and changes in tissue composition. This study evaluates whether this tissue penetration property applies to other highly protein-bound drugs capable of inducing apoptosis. The penetration of doxorubicin was studied in histocultures of prostate xenograft tumors and tumor specimens obtained from patients who underwent radical prostatectomy. The kinetics of drug uptake and efflux in whole tumor histocultures were studied by analyzing the average tumor drug concentration using high-pressure liquid chromatography. Spatial drug distribution in tumors and the drug concentration gradient across the tumors were studied using fluorescence microscopy. The results indicate that drug penetration was limited to the periphery for 12 hours in patient tumors and to 24 hours in the more densely packed xenograft tumors. Subsequently, the rate of drug penetration to the deeper tumor tissue increased abruptly in tumors treated with higher drug concentrations capable of inducing apoptosis (i.e., = 5 microm), but not in tumors treated with lower concentrations. These findings indicate a time- and concentration-dependent penetration of doxorubicin in solid tumors, similar to that of paclitaxel. We conclude that doxorubicin penetration in solid tumors is time- and concentration-dependent and is enhanced by drug-induced cell death.

Effect of intraperitoneal pressures on tissue water of the abdominal muscle

A major factor that affects solute and water transport through tissue is the state of tissue hydration. The amount of interstitial water directly affects the transport coefficients for both diffusion and convection. To investigate the effect of simultaneous exposure of tissue to hydrostatic and osmotic pressures on the state of tissue hydration and the pattern of distribution of tissue water, we dialyzed rats with isotonic (290 mosmol/kg) or hypertonic (510 mosmol/kg) solution at intraperitoneal pressures (P(ip)) between 0 and 6 mmHg, and we infused isotopic markers intravenously and determined their equilibrium distribution volumes (V(D)) in the anterior abdominal muscle (AAM) by quantitative autoradiography. Total tissue water volume (theta(TW)) was determined from dry-to-wet weight ratios. theta(urea), the V(D) of [(14)C]urea, equals the sum of the extracellular water volume (theta(EC), V(D) of [(14)C]mannitol) and intracellular water volume (theta(IC) = theta(urea) - theta(EC)). If theta(if) = interstitial water volume and theta(IV) = vascular water volume (V(D) of (131)I-labeled IgG), then theta(EC) = theta(if) + theta(IV). AAM hydrostatic pressure profiles were measured by a micropipette/servo-null system and demonstrated that elevation of P(ip) above 3 mmHg significantly (P < 0.05) increases mean tissue pressure (P(T)) to the same level regardless of intraperitoneal osmolality. The increase in P(T) resulted in a nonlinear tissue expansion primarily in the interstitium regardless of osmolality. From 0 to 6 mmHg, theta(if) (in ml/g dry tissue) increased from 0.59 +/- 0.02 to 1.7 +/- 0.05 and to 1.5 +/- 0.05 after isotonic and hypertonic dialysis, respectively, whereas theta(IC) increased from 2.8 +/- 0.08 to 3.0 +/- 0.1 after isotonic dialysis and decreased to 2.6 +/- 0.1 after hypertonic dialysis. After dialysis at 6 mmHg with isotonic or hypertonic solutions, theta(IV) increased from 0.034 +/- 0.001 to 0. 049 +/- 0.001 and 0.042 +/- 0.002, respectively. theta(urea) during hypertonic dialysis at P(ip) between 0 and 6 mmHg increased in a nonlinear fashion (F = 26.3, P < 0.001), whereas theta(IC) invariably decreased (F = 11.1, P < 0.001) and theta(if) doubled from its control value at low P(ip). In conclusion, elevation of intraperitoneal hydrostatic pressure causes tissue expansion, primarily in interstitium, irrespective of osmolality of the bathing solution. Tissue hydrostatic pressure is therefore the primary determinant of tissue properties with respect to hydration, which in turn affects diffusive and convective transport.

In vivo diffusion of immunoglobulin G in muscle: effects of binding, solute exclusion, and lymphatic removal

Previously, we demonstrated that immunoglobulin G (IgG), dissolved in an isotonic solution in the peritoneal cavity, transported rapidly into the abdominal wall when the intraperitoneal (ip) pressure was > 2 cmH2O. We hypothesized that this was chiefly caused by convection and that diffusion of IgG was negligible. To investigate the role of diffusion, we dialyzed rats with no pressure gradient across the abdominal wall muscle for 2 or 6 h with an ip isotonic solution containing 125I-labeled IgG. At the end of the experiment, the animal was euthanized and frozen and abdominal wall tissue was processed to produce cross-sectional autoradiograms. Quantitative densitometric analysis resulted in IgG concentration profiles with far lower magnitude than profiles from experiments in which convection dominated. In other in vivo experiments, we determined the lymph flow rate to be 0.8 x 10(-4) ml.min-1.g-1 and the fraction of extravascular tissue (theta s) available to the IgG to be 0.041 +/- 0.001. An in vitro binding assay was used to determine the time-dependent, nonsaturable binding constant: 0.0065 min-1 x duration of exposure. A non-steady-state diffusion model that included effects of theta s, time-dependent binding, and lymph flow was fitted to the diffusion profile data, and the IgG diffusivity within the tissue void was estimated to be 2 x 10(-7) cm2/s, a value much higher than that published by other groups. We also demonstrate from our previous data that convection of IgG through tissue dominates over diffusion at ip pressures > 2 cmH2O, but diffusion may not be negligible. Furthermore, nonsaturable binding must be accounted for in the interpretation of tissue protein concentration profiles.

An estimate of the rate of direct drug diffusion from the surface of heart and kidney--implications for their representation as compartments

In many regional pharmacokinetic experiments and models, the anatomical boundaries of the heart and kidney are intrinsically assumed to be barriers for drug diffusion such that these organs can be represented as one or more compartments. To test this, an experimental preparation was developed in which the heart and kidney of anaesthetized sheep were surrounded with 0.9% saline. The rate of drug diffusion from the surface of the organs into the saline was examined during constant-rate i.v. drug infusions. It was found that the maximum clearances of lidocaine and procainamide into the pericardial saline were 10.3-11.6 and 0.6-2.1 ml min-1 respectively, and the values for the kidney were 0.3-0.6, 0.1-1.0 and 0.4-1.3 ml min-1, for lidocaine, procainamide, and meperidine respectively. These corresponded to calculated times of 4-481 min to reach the steady-state saline concentration depending on the drug and the organ. The steady-state ratio of the saline concentrations over the arterial blood drug concentrations usually ranged from 0.5-1.0. It is concluded that drugs can rapidly enter regions of low or no perfusion surrounding these organs, and that the concept of treating the heart and kidney as compartments may not be valid in certain 'worst-case' situations.

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

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

Comparison of the hepatic uptake clearances of fifteen drugs with a wide range of membrane permeabilities in isolated rat hepatocytes and perfused rat livers

The hepatic uptake clearances of 15 ligands with a wide range of permeabilities were determined in rats using two techniques: centrifugal filtration with isolated hepatocytes and the multiple indicator dilution (MID) method with isolated perfused livers. Some of the uptake clearance values were taken from the literature. Uptake clearance values obtained from isolated hepatocytes were extrapolated to that per gram liver (PSinf.cell), assuming that 1 g of liver has 1.3 x 10(8) cells. The values of PSinf.cell varied from approximately 0.1 to 72 (mL/min/g liver). The values of PSinf.cell were similar to those (PSinf.MID) determined by the MID method for ligands with uptake clearances below approximately 1 mL/min/g liver. However, for the ligands with larger uptake clearances, the PSinf.MID values were lower than the PSinf.cell values and appeared to reach an upper limit (approx. 15-20 mL/min/g liver). The PSinf.cell values of 1-propranolol, tetraphenylphosphonium (TPP+), and diazepam were 72, 43, and 22 mL/min/g liver, respectively, whereas their uptake clearances (PSinf.MID) determined by the MID method were 4 to 10 times lower. One of the possible mechanisms for this discrepancy is that an unstirred water layer, which may exist in Disse's space in isolated perfused livers (and probably under in vivo condition), limits the hepatic uptake rate of ligands with extremely high membrane permeabilities.

The effect of convection on bidirectional peritoneal solute transport: predictions from a distributed model

A distributed model of the peritoneum has been proposed as an alternative to the standard membrane model for describing peritoneal solute transport. The effect of convection on bidirectional peritoneal solute transport is studied theoretically using the distributed model. Approximate analytical and exact numerical solutions to the distributed model yield predictions similar to those when using a membrane model of peritoneal solute transport. Difficulties in interpretation of the membrane transport parameters may arise, however, when interstitial tissue, not the capillary wall, is the dominant diffusive solute transport resistance. Under such conditions the effect of convection on peritoneal solute transport is dependent on the transport direction. Moreover, predictions from the distributed model are similar to those for a membrane model containing two transport barriers in series. Thus, both the distributed model and a membrane model containing two serial transport barriers equivalently describe the effect of convection on bidirectional peritoneal solute transport.

Concentration-dependent disappearance of fluorouracil from peritoneal fluid in the rat: experimental observations and distributed modeling

The rate of disappearance of fluorouracil from peritoneal fluid has been experimentally measured and mathematically modeled. The experimental data were obtained following the instillation of 50 ml of dialysis fluid which contained an initial fluorouracil concentration ranging from 24 microM to 12 mM. The rate of disappearance was strongly dependent upon concentration. A distributed model has been formulated which incorporates concepts of diffusion with saturable metabolism and nonsaturable capillary uptake in the tissue surrounding the peritoneal fluid. This model successfully describes the experimental observations and also suggests that the effective penetration depth into tissue is highly dependent upon concentration.