Lymphatic versus nonlymphatic fluid absorption from the peritoneal cavity as related to the peritoneal ultrafiltration capacity and sieving properties

Multiphysics modeling and simulation of local transport and absorption kinetics of intramuscularly injected lipids nanoparticles

Recent clinical applications of mRNA vaccines highlight the critical role of drug delivery, especially when using lipid nanoparticles (LNPs) as the carrier for genetic payloads. However, kinetic and transport mechanisms for locally injected LNPs, such as lymphatic or cellular uptake and drug release, remain poorly understood. Herein, we developed a bottom-up multiphysics computational model to simulate the injection and absorption processes of LNPs in muscular tissues. Our purpose was to seek underlying connections between formulation attributes and local exposure kinetics of LNPs and the delivered drug. We were also interested in modeling the absorption kinetics from the local injection site to the systemic circulation. In our model, the tissue was treated as the homogeneous, poroelastic medium in which vascular and lymphatic vessel densities are considered. Tissue deformation and interstitial fluid flow (modeled using Darcy's Law) were also implemented. Transport of LNPs was described based on diffusion and advection; local disintegration and cellular uptake were also integrated. Sensitivity analyses of LNP and drug properties and tissue attributes were conducted using the simulation model. It was found that intrinsic tissue porosity and lymphatic vessel density affect the local transport kinetics; diffusivity, lymphatic permeability, and intracellular update kinetics also play critical roles. Simulated results were commensurate with experimental observations. This study could shed light on the development of LNP formulations and enable further development of whole-body pharmacokinetic models.

Peritoneal Fluid and Tracer Albumin Kinetics in the Rat. Effects of Increases in Intraperitoneal Hydrostatic Pressure

Objectives

To study the peritoneal fluid loss rate, the clearance (CI) of radioactive tracer albumin (RISA) eliminated from the peritoneal cavity (PC), as well as the peritoneal-to-plasma RISA clearance (CI -+ P) during acute peritoneal dialysis (PD) at large elevations in intraperitoneal hydrostatic pressure (IPP).

Design

Experimental study in anesthetized Wistar rats.

Methods

The intraperitoneal volume (IPV) was assessed using RISA dilution, correcting for the RISA CI from the PC. Volume recovery at termination of the dwells was obtained using graduated cylinders and preweighed gauze tissues. Measurements of CI and CI -+ P were obtained by repeated micro-sampling of dialysate and plasma, respectively. The IPP was continuously measured, and could be varied by external concentric abdominal compression using an inflatable cuff. On termination of the experiments, samples from tissues lining the PC were analyzed with respect to their content of RISA and edema, the latter being assessed from wet/dry weight ratios.

Results

At 2 mm Hg of IPP (control) the RISA CI was 27.1:1:2.0(:1:SE)μL.min-l, whereas CI→ Pwasonly 8.07:1:0.67 μL.min-l, at a total fluid loss rate of 10.1:1:5.4μL.min-1 for 1.36% Dianeal. At an IPP of 14 mm Hg, the CI increased to 55.3±4.1 μL.min -1 and the peritoneal fluid absorption rate was 34.4±5.6 μL.min -l, whereas CI -+ P was just moderately increased as compared to control (11.2:1:1.4 μL. min -I). Furthermore, a pleural effusion of 1.16:1:0.08 mL was detectable at elevated IPPs. The degree of edema formation in the anterior abdominal muscles (AAM) and the diaphragm (DIA) was largely insignificant during 150 min at 2 mm Hg of IPP, but increased markedly at 14 mm Hg, as did the RISA uptake to the AAM and DIA. The discrepancy between CI and CI -+ P was largely accounted for by tracer entrance into tissues lining the peritoneal cavity, mainly the AAM.

Conclusions

At a nearly unchanging capillary Starling equilibrium, the losses of fluid and of RISA from the PC were markedly elevated at increased IPPs. However, the RISA clearance to the plasma appeared to be only moderately altered at elevated IPP and represented only a minor fraction of the RISA clearance out of the PC. Tissues lining the PC apparently act as a variable ‘sink’ for intraperitoneal proteins and fluid during peritoneal dialysis (PD).

Clearance of Tracer Albumin from Peritoneal Cavity to Plasma at Low Intraperitoneal Volumes and Hydrostatic Pressures

Objective

To assess the clearance of radiolabeled tracer albumin (RISA) from peritoneal cavity to plasma (CI → P) in rats under essentially “normal” conditions, that is, when intraperitoneal hydrostatic pressure (IPP) is subatmospheric and the intraperitoneal (IP) “free” fluid volume (IPV) is low.

Methods

A volume of 0.3 mL of RISA was injected IP into anesthetized Wistar rats (wt = 300 g) when the IPV was approximately 2 mL (normal) or the IPV was approximately 10 mL, and IPP was either -1.8 mmHg (normal) or +1.5 mmHg (produced by an external cuff). Plasma samples (25 μL) were obtained repeatedly during the dwell, which lasted 30 300 min, after which the peritoneal cavity was opened to recover the IPV and residuallP RISA activity. The CI → P was assessed as the mass transfer of RISA into plasma, occurring per unit time,-divided by the calculated mean IP RISA concentration (CD). The interstitial RISA space was measured as the mass of RISA accumulated, per unit tissue weight, in peritoneal tissue samples divided by the CD.

Results

A markedly lower CI → P (2.47 ± 0.67 μL/min), as well as total RISA clearance out of the peritoneal cavity (CI), was found under “normal” conditions (an IPV of approximately 2 mL and an IPP of approximately -1.8 mmHg) compared to the situation during peritoneal dialysis (an IPV of approximately 20 mL and an IPP of +1 mmHg). Furthermore, the interstitial RISA space increased linearly over time even at negative IPPs and at an unchanging peritoneal interstitial fluid volume. At a low (normal) IPV the CI → P did not increase significantly with elevating IPP, and increased only marginally when tracer distribution was improved by artificial vibration of the rats. However the CI → P increased when larger volumes were infused to increase the totallPV.

Conclusions

It is concluded that the CI → P and CI at low IPPs and IPVs are not as high as during peritoneal dialysis. Increases in CI → P were, however, coupled to increases in IPV. This highlights the importance of the IPV per se and of a sufficient IP tracer distribution for direct lymphatic absorption to be efficient. This study was presented in part at the XVIth Annual Conference on Peritoneal Dialysis, Denver, Colorado, U.S.A., 1997 (33).

Intraperitoneal Addition of Hyaluronan Improves Peritoneal Dialysis Efficiency

Background

It has been shown that hyaluronan (HA) can decrease peritoneal fluid absorption. It is not known, however, how various molecular weights and various concentrations of hyaluronan affect peritoneal fluid absorption rate.

Methods

A study of 4-hour dwells, with frequent dialysate and blood sampling, was performed in male SpragueCawley rats (6 7 rats in each group) with 1311 albumin as an intraperitoneal volume marker. Each rat was infused intraperitoneally with 25 mL of 1.5% glucose solution alone or 1.5% glucose solution containing hyaluronan at various molecular weights (MW -85 kC, 280 kC, 500 kC, and 4 MC) or containing hyaluronan of MW 500 kC at various concentrations (0.01%,0.05%,0.1%,0.5%). Two additional groups were infused with 40 mL of 1.36% glucose dialysate alone or 1.36% glucose dialysate with 0.01 % hyaluronan (MW 500 kC) to test the effect of hyaluronan when high dialysate fill volume was used.

Results

Addition of 0.01% hyaluronan significantly decreased peritoneal fluid absorption rate (KE) (by 22%, p < 0.01). The decrease was more marked with hyaluronan at high MW or high concentration, or with high dialysate fill volume. The net ultrafiltration tended to be higher in all hyaluronan groups compared to their control groups except in the 4 MC group; this difference was mainly due to a lower KE in all the hyaluronan groups. The direct lymphatic flow was significantly decreased in the 0.5% HA group. The transcapillary ultrafiltration rate (au) was significantly lower in the 4 MC group as compared to the control group. No difference in au was found between the other groups as compared to their control groups.

Conclusions

(1) Intraperitoneal addition of hyaluronan may increase net peritoneal fluid removal, mainly because hyaluronan decreases peritoneal fluid absorption rate. The decrease was more marked when high dialysate fill volume was used, indicating that intraperitoneal addition of hyaluronan can prevent the decreased net ultrafiltration caused by an increase in dialysate fill volume. (2) The decrease in peritoneal fluid absorption rate may be both MW-dependent and concentration-dependent: that is, a higher MW as well as a higher concentration of hyaluronan result in a more marked decrease in peritoneal fluid absorption rate. (3) Low concentrations of high MW hyaluronan may also decrease au. However, au did not decrease when high concentrations of hyaluronan were used despite a significant decrease in peritoneal fluid absorption rate.

Efficient Targeting of Adipose Tissue Macrophages in Obesity with Polysaccharide Nanocarriers

Obesity leads to an increased risk for type 2 diabetes, heart disease, stroke, and cancer. The causal link between obesity and these pathologies has recently been identified as chronic low-grade systemic inflammation initiated by pro-inflammatory macrophages in visceral adipose tissue. Current medications based on small-molecule drugs yield significant off-target side effects with long-term use, and therefore there is a major need for targeted therapies. Here we report that nanoscale polysaccharides based on biocompatible glucose polymers can efficiently target adipose macrophages in obese mice. We synthesized a series of dextran conjugates with tunable size linked to contrast agents for positron emission tomography, fluorophores for optical microscopy, and anti-inflammatory drugs for therapeutic modulation of macrophage phenotype. We observed that larger conjugates efficiently distribute to visceral adipose tissue and selectively associate with macrophages after regional peritoneal administration. Up to 63% of the injected dose remained in visceral adipose tissue 24 h after administration, resulting in >2-fold higher local concentration compared to liver, the dominant site of uptake for most nanomedicines. Furthermore, a single-dose treatment of anti-inflammatory conjugates significantly reduced pro-inflammatory markers in adipose tissue of obese mice. Importantly, all components of these therapeutic agents are approved for clinical use. This work provides a promising nanomaterials-based delivery strategy to inhibit critical factors leading to obesity comorbidities and demonstrates a unique transport mechanism for drug delivery to visceral tissues. This approach may be further applied for high-efficiency targeting of other inflammatory diseases of visceral organs.

Inflammatory ascites formation induced by macromolecules in mice and rats

Different macromolecules were administered intraperitoneally to stimulate formation of protein-rich ascitic fluid in rodents. Stimulatory effect of plant lectins depended on the attachment to cell surface carbohydrates, Canavalia ensiformis (ConA) lectin was used in the majority of experiments. The time course of ConA-induced ascites was divided into an early (up to 4 h) and a late (from 6 h on) phase, with a transitional period between the two. Water and protein accumulation showed parallel time courses: volume of the ascitic fluid peaked at around 3 h, and fibrin threads appeared after 6 h. Viscosity of the ascitic fluid and its supernatant increased with time, reaching maximal fibrinogen concentration at around 16 h. Peritoneal permeability, followed by pleural and pericardial effusions, was elicited only by lectins that form soluble complexes with serum glycoproteins, whereas the effect of serum-precipitating lectins was restricted to the peritoneum. Macromolecules with serial positive charges (e.g., polylysine or polyethyleneimine) enhanced peritoneal permeability by ionic interactions with cell surface molecules. Viscosity of the polycation-induced ascitic fluid did not tend to increase with time and corresponded to the early phase of the ConA-induced ascites. Polyglutamate, a polyanionic macromolecule, inhibited the effect of polycations, but not that of ConA. The most efficient stimulatory macromolecules appear to induce ascites by noncovalent cross-linking of cell surface glycoproteins or glycosaminoglycans or both. A similar mechanism may operate in the maintenance of basal secretion to prevent eventual desiccation. Noncovalent cross-linking appears to be a common denominator of both basal and enhanced permeability.

Dynamics of albumin in plasma and ascitic fluid in patients with cirrhosis

BACKGROUND/AIMS

To determine dynamics of albumin in plasma and ascitic fluid of patients with cirrhosis.

METHODS

Forty-seven patients were classified in four groups: I--patients without fluid retention; II--patients with ascites not resistant to subsequent diuretic treatment; III--recompensated patients during diuretic treatment; and IV--patients with diuretic-resistant ascites. Transvascular and transperitoneal albumin transports were quantified by 131I-/125I-labelled human albumin.

RESULTS

TER(P) (i.e. the fraction of intravascular albumin (IVM) passing from plasma into the interstitial space per hour) was increased in all groups. In group IV patients the transport rate of albumin from plasma into the ascitic fluid (TER(PA)) was significantly higher than the transport rate from the ascitic fluid back into the plasma: TER(AP) (0.45 vs. 0.26% IVM/h, P < 0.002). In group II patients TER(PA) was similar to TER(AP) (0.27 vs. 0.25% IVM/h, ns). A direct correlation was found between TER(PA) and TER(AP) in both groups of patients (r = 0.78, P < 0.001).

CONCLUSION

In non-resistant ascites, there is a steady state between the transport of albumin into the peritoneal cavity and back into the plasma, but in resistant ascites the former transport is elevated. Thus, local factors may be important to treatment of ascites.

Hyaluronan decreases peritoneal fluid absorption: effect of molecular weight and concentration of hyaluronan

BACKGROUND

We have recently shown that the addition of hyaluronan to peritoneal dialysis solution could decrease the peritoneal fluid absorption rate, possibly through decreasing peritoneal tissue hydraulic conductivity. The physical-chemical properties of hyaluronan were found to be both molecular weight and concentration dependent. In this study, we investigated the effects of different molecular weight as well as different concentrations of hyaluronan on the peritoneal fluid kinetics.

METHODS

A four-hour dwell study was performed in 48 male Sprague-Dawley rats (6 rats in each group) with 131I albumin (RISA) as an intraperitoneal volume marker. Each rat was intraperitoneally injected with 25 ml of 1.36% glucose dialysate alone (control) or with 0.01% hyaluronan (HA) with different molecular weights [85,000 (HA85K group), 280,000 (HA280K group), 500,000 (HA500K group), and 4,000,000 (HA4M group) molecular wt] or with a different concentrations of hyaluronan [(molecular wt 500,000); 0.01% (0.01% HA group), 0.05% (0.05% HA group), 0.1% (0.1% HA group), and 0.5% (0.5% HA group) hyaluronan].

RESULTS

The peritoneal fluid absorption rate (as assessed by the RISA elimination rate, KE) was significantly decreased in the HA500K and H4M groups as well as in all the different concentration groups (with molecular wt 500,000) as compared with the control group, resulting in significantly higher net fluid removal in these groups (except for the H4M group) as compared with the control group. In the 0.5% HA group (but not in the other hyaluronan groups), the direct lymphatic absorption (KEB) was also significantly decreased. The transcapillary ultrafiltration rate (Qu) was significantly lower in the HA4M group as compared with the control group but significantly higher in the 0.05% HA (and tended to be higher in the 0.1% HA group) as compared with the other groups. No difference in Qu was found between the 0.5% HA group as compared with the control group, despite a more marked decrease in KE in this group as compared with the H4M group. There were no significant differences in KE, Qu, and net fluid removal between the HA85K and HA280K groups and the control group.

CONCLUSIONS

Our results suggest that (a) the addition of hyaluronan to dialysate could decrease peritoneal fluid absorption and thus increase the net ultrafiltration; this effect appears to be both size dependent and concentration dependent. (b) High molecular weight fraction of hyaluronan may also decrease the transcapillary Qu by decreasing tissue hydraulic conductivity. (c) A higher concentration of hyaluronan in dialysate resulted in a more marked decrease in peritoneal fluid absorption (absorption to peritoneal tissues as well as direct lymphatic absorption), possibly through both decreasing tissue hydraulic conductivity and increasing fluid viscosity. (d) Decreasing tissue hydraulic conductivity by adding a high concentration of hyaluronan to dialysate does not decrease the transcapillary ultrafiltration, possibly because the osmotic effect of hyaluronan may counterbalance the decrease in transcapillary ultrafiltration because of the decrease in tissue hydraulic conductivity.

Effect of increased dialysate fill volume on peritoneal fluid and solute transport

It has recently been recommended that the peritoneal dialysate volume should in general be increased to increase the peritoneal small solute clearances. However, the net ultrafiltration volume may decrease due to higher intraperitoneal hydrostatic pressure (IPP) and higher peritoneal fluid absorption induced by higher fill volume. In the present study, we investigated the effects of increasing the fill volume on peritoneal fluid and solute transport. A four-hour dwell study with frequent dialysate and blood sampling was performed in 32 male Sprague-Dawley rats using 16 ml, 25 ml, 30 ml or 40 ml (8 rats in each group) of 3.86% glucose solution with 131I albumin as an intraperitoneal volume marker. The peritoneal transport of fluid, glucose, urea, sodium, potassium, phosphate and total protein as well as IPP with different fill volume were evaluated. The IPP and peritoneal fluid absorption rate (as estimated from the 131I albumin elimination coefficient, KE) significantly increased with increase in fill volume (P < 0.05), whereas the direct lymphatic absorption did not change with increasing fill volume. There was a strong correlation between IPP and KE. However, the net ultrafiltration volume was significantly higher in the high fill volume groups compared to the low fill volume groups, mainly due to a better maintenance of the dialysate to plasma glucose concentration gradient in the high fill volume groups. There was no significant difference in the diffusive mass transport coefficients (KBD) and sieving coefficients for any of the investigated solutes, although KBD values tended to be lower in the 16 ml group. The clearances for small solutes increased with increased fill volume, although these increases were slightly smaller than predicted from the increase in fill volume. We conclude that: (1) An increase in dialysate fill volume using 3.86% glucose solution results in higher intraperitoneal hydrostatic pressure and higher peritoneal fluid absorption, but, on the other hand, a higher net ultrafiltration; (2) The increase in net ultrafiltration with increased fill volume is mainly due to a better maintenance of glucose concentration in the dialysate, inducing an increased transcapillary ultrafiltration rate; (3) Solute clearances increase although not quite to the same extent as predicted from the increase in fill volume. Our results indicate that decreased net ultrafiltration volume associated with higher dialysate fill volume (due to higher IPP and higher peritoneal fluid absorption) could be avoided if hypertonic glucose solutions are used.

Albumin-based solutions for peritoneal dialysis: investigations with a rat model

To evaluate albumin, an osmotic agent for peritoneal dialysis, the peritoneal fluid and solute transport were investigated during a 4-h single cycle peritoneal dialysis with albumin-based dialysis solutions. Two different albumin solutions were used in 15 normal Sprague-Dawley rats: isotonic 7.5% albumin solution (ADS 1, n = 7) and a combined 7.5% albumin and 1.35% glucose solution (ADS 2; n = 8). A standard 1.36% Dianeal solution was used to provide control values (n = 6). The rate of the intraperitoneal volume change (Qv) was positive during the initial 90 min with ADS 2 and during the initial 60 min with Dianeal 1.36% solution but negative with ADS 1. The peritoneal bulk flow reabsorption rate, Qa, was similar in all three groups. The estimated rate of transcapillary ultrafiltration (Qu = Qv + Qa) was positive with all three solutions throughout the dialysis. With ADS 1, Qu increased gradually during the initial 90 min and then remained stable, but it decreased with ADS 2 and Dianeal 1.36% solution. Qu with ADS 2 did not differ from that with Dianeal 1.36% solution during the initial 60 min, but it was significantly higher during the latter part of dialysis. The value of Qu during the last 2 h of dialysis was 0.026 +/- 0.010 and 0.025 +/- 0.009 ml/min with ADS 1 and ADS 2, respectively, and it was significantly higher than that with Dianeal 1.36% solution (0.005 +/- 0.007 ml/min; p < 0.017).(ABSTRACT TRUNCATED AT 250 WORDS)