Vascular permeability to macromolecules in rabbit paw and skeletal muscle: a lymphatic study with a mathematical interpretation of transport processes

A microfluidic device inspired by leaky tumor vessels for hematogenous metastasis mechanism research

Endothelial intercellular pores of tumor vessels generally lead to enhanced interstitial flow and may facilitate the migration of tumor cells. The permeability of tumor vessels causes a concentration gradient of growth factors (CGGF) from blood vessels to tumor tissues, which is opposite to the direction of interstitial flow. In this work, exogenous chemotaxis under the CGGF is demonstrated as a mechanism of hematogenous metastasis. A bionic microfluidic device inspired by endothelial intercellular pores of tumor vessels has been designed to study the mechanism. A porous membrane vertically integrated into the device using a novel compound mold is utilized to mimic the leaky vascular wall. The formation mechanism of the CGGF caused by endothelial intercellular pores is numerically analyzed and experimentally verified. The migration behavior of U-2OS cells is studied in the microfluidic device. The device is divided into three regions of interest (ROI): primary site, migration zone, and tumor vessel. The number of cells in the migration zone increases significantly under the CGGF, but decreases under no CGGF, indicating tumor cells may be guided to the vascellum by exogenous chemotaxis. Transendothelial migration is subsequently monitored, demonstrating the successful replication of the key steps in vitro in the metastatic cascade by the bionic microfluidic device.

Tumour-vessel-on-a-chip models for drug delivery

Nanocarriers for drug delivery have great potential to revolutionize cancer treatment, due to their enhanced selectivity and efficacy. Despite this great promise, researchers have had limited success in the clinical translation of this approach. One of the main causes of these difficulties is that standard in vitro models, typically used to understand nanocarriers' behaviour and screen their efficiency, do not provide the complexity typically encountered in living systems. In contrast, in vivo models, despite being highly physiological, display serious bottlenecks which threaten the relevancy of the obtained data. Microfluidics and nanofabrication can dramatically contribute to solving this issue, providing 3D high-throughput models with improved resemblance to in vivo systems. In particular, microfluidic models of tumour blood vessels can be used to better elucidate how new nanocarriers behave in the microcirculation of healthy and cancerous tissues. Several key steps of the drug delivery process such as extravasation, immune response and endothelial targeting happen under flow in capillaries and can be accurately modelled using microfluidics. In this review, we will present how tumour-vessel-on-a-chip systems can be used to investigate targeted drug delivery and which key factors need to be considered for the rational design of these materials. Future applications of this approach and its role in driving forward the next generation of targeted drug delivery methods will be discussed.

Synthetic tumor networks for screening drug delivery systems

Tumor drug delivery is a complex phenomenon affected by several elements in addition to drug or delivery vehicle's physico-chemical properties. A key factor is tumor microvasculature with complex effects including convective transport, high interstitial pressure and enhanced vascular permeability due to the presence of "leaky vessels". Current in vitro models of the tumor microenvironment for evaluating drug delivery are oversimplified and, as a result, show poor correlation with in vivo performance. In this study, we report on the development of a novel microfluidic platform that models the tumor microenvironment more accurately, with physiologically and morphologically realistic microvasculature including endothelial cell lined leaky capillary vessels along with 3D solid tumors. Endothelial cells and 3D spheroids of cervical tumor cells were co-cultured in the networks. Drug vehicle screening was demonstrated using GFP gene delivery by different formulations of nanopolymers. The synthetic tumor network was successful in predicting in vivo delivery efficiencies of the drug vehicles. The developed assay will have critical applications both in basic research, where it can be used to develop next generation delivery vehicles, and in drug discovery where it can be used to study drug transport and delivery efficacy in realistic tumor microenvironment, thereby enabling drug compound and/or delivery vehicle screening.

Similarity of permeabilities for Ficoll, pullulan, charge-modified albumin and native albumin across the rat peritoneal membrane

AIM

Compared to neutral globular proteins, neutral polysaccharides, such as dextran, pullulan and Ficoll, appear hyperpermeable across the glomerular filtration barrier. This has been attributed to an increased flexibility and/or asymmetry of polysaccharides. The present study investigates whether polysaccharides are hyperpermeable also across the continuous capillaries in the rat peritoneum.

METHODS

In anaesthetized Wistar rats, FITC-Ficoll or FITC-pullulan together with (125)I-human serum albumin (RISA) or neutralized (125)I-bovine serum albumin (nBSA) were given intravenously, after which peritoneal dialysis (PD) using conventional PD fluid (Gambrosol 1.5%) was performed for 120 min. Concentrations of FITC-polysaccharides and radioactive albumin species in plasma and dialysis fluid were analysed with high-performance size exclusion chromatography and a gamma counter respectively. Transperitoneal clearance values were calculated for polysaccharides in the molecular radius range 36-150 A, and for RISA and nBSA.

RESULTS

Ficoll and pullulan showed more or less identical permeabilities, compared to RISA and nBSA, across the peritoneal membrane. Although RISA-clearance, 5.50 +/- 0.28 (microL min(-1); +/-SEM), tended to be lower than the clearances of Ficoll(36A) (6.55 +/- 0.25), pullulan(36A) (6.08 +/- 0.22) and nBSA (6.56 +/- 0.23), the difference was not statistically significant. This is in contrast to the hyperpermeability exhibited by polysaccharides across the glomerular filtration barrier and also contrasts with the charge selectivity of the latter.

CONCLUSION

The phenomenon of molecular flexibility is more important for a macromolecule's permeability through the glomerular filter than across the continuous peritoneal capillary endothelium. Furthermore, it seems that charge plays a subordinate role in the steady-state transport across the combined peritoneal capillary-interstitial barrier.

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.

Initial equilibration of albumin and IgG in rabbit hind paw skin and lymph

To determine if the interstitial distribution of plasma proteins influences transport from plasma to lymph, the initial plasma-to-extravascular and plasma-to-lymph equilibrations for 131I-labeled rabbit albumin and 125I-labeled rabbit IgG were compared in hind paw heel skin of anesthetized New Zealand White rabbits. Plasma specific activities were maintained constant with time using continuous infusions. The plasma space in skin samples taken at the end of the experiment was estimated by a 3-min accumulation of 59Fe-labeled bovine transferrin. Albumin and IgG concentrations in plasma, lymph, and skin were determined by rocket electroimmunoassay. The lymph specific activity relative to that of plasma for the larger sized IgG was significantly greater (P less than 0.01) than that for albumin during the initial 8 hr of equilibration. The tissue specific activity relative to that of plasma for IgG was significantly less (P less than 0.01) than that for albumin. The distribution space at lymph concentration for both proteins was 1 ml/g dry skin wt, indicating similar excluded volumes. These data are consistent with the hypothesis that convective movement of plasma proteins through the interstitium affects lymph equilibration while diffusion affects tissue equilibration.

Fluid and protein fluxes across small and large pores in the microvasculature. Application of two-pore equations

Treating the blood-tissue barrier as a two-pore membrane the separate fluid and solute fluxes occurring across 'small pores' and 'large pores' were modelled in continuous capillaries employing two-pore equations for the calculations together with the non-linear flux equation and theories for restricted diffusion and for the reflection coefficient (sigma). The two-pore equations derived proved useful for analyses of transvascular protein flux data obtained at low as well as at high filtration rates. These equations were applied to lymphatic protein flux data from dog paw (Renkin et al. 1977a, b) and to tracer albumin uptake data from rat skeletal muscle (Rippe et al. 1979). For both sets of data the small- and large-pore radii became closely similar, 44 vs. 45 A and 240 vs. 225 A, which also holds for the large-pore fractions of hydraulic conductivity (0.097 vs. 0.056). The main result of this analysis is that the passage of macromolecules normally occurring across the microvascular walls is almost entirely convective, and hence, dependent on the transmural hydrostatic and oncotic pressure gradients and on the hydraulic conductivity. For example, 75-90% of the transvascular passage of albumin was found to be due to convection through large pores at normal lymph flows, the remaining portion being mainly due to diffusion across small pores. Solutes larger than albumin were almost exclusively transported by convection across large pores. Two-pore heterogeneity was found to explain the previously observed variations of the apparent overall large solute diffusion capacity (PSapp) and the overall reflection coefficient (sigma f) with filtration rate and also previous overestimations of PS. Furthermore, the present results were not compatible with protein transport across any 'non-hydraulically conductive capillary pathways' as previously postulated from the lymphatic protein flux data analysed here (Renkin 1985).