Toxins Removal and Inflammatory State modulAtion During Online Hemodiafiltration: Comparison of Two Different Dialyzers

Patients with end-stage renal disease (ESRD) have a significantly increased cardiovascular mortality rate compared to the general population.

According to the USRDS (United States Renal Data System), in 2017 the prevalence of cardiovascular disease in the United States was 65.8% in patients with chronic kidney disease (CKD), compared to 31.9% of the subjects with stable renal function (https://www.usrds.org/2017/view/v1_04.aspx).

In the natural history of nephropathy, besides the traditional cardiovascular risk factors, an increasingly important role is played by the nontraditional factors related to uremia and dialysis treatment, namely anemia, hyperhomocysteinemia, malnutrition, hyperparathyroidism, electrolyte imbalance, and above all chronic inflammatory state and oxidative stress. The main components of dialysis-related inflammatory response include neutrophil and monocyte activation, cytokine release, oxidative stress with production of free radicals, lipid and protein oxidation, and alterations in blood redox state.

There is much evidence to indicate that ESRD patients, especially those on dialysis, are exposed to increased oxidative and carbonyl stress, that cause chemical modifications of proteins and accumulation of AGEs (advanced glycation end products).

The formation of AGEs occurs through Maillard reaction, a non-enzymatic glycation of peptide amino groups, resulting in protein structural and functional alteration and production of reactive carbonyl species .

Several studies have demonstrated a correlation between the levels of tissue AGEs and the development of various complications, since the accumulation of AGEs constitutes an index of hyperglycemia, oxidative stress and inflammation. The activation and up-regulation of the AGE receptors, including the main one RAGE (AGE Receptor), leads to the initiation of inflammatory cascades involved in the formation of atherosclerotic plaques and endothelial dysfunction, and lastly in the pathogenesis of cardiovascular disease. AGEs are also related to the thickening of the intima-media tunica of the carotid arteries, considered a marker of arteriosclerosis, as clinically confirmed by the close relationship between coronary artery disease and atherosclerotic complications in diabetes and renal failure.

Besides AGEs, a huge pattern of protein-bound toxins and of middle molecular weight uremic toxins are known to contribute to the uremic syndrome in terms of cardiovascular complications, inflammation and fibrosis . Concerning protein-bound uremic toxins, p-cresyl sulfate and indoxyl sulfate have received considerable attention recently. Among middle molecules, B2M, free light chains and FGF23 have been identified as markers of disease progression and mortality in CKD and hemodialysis.

Furthermore, the activation of leukocytes, both in response to the exposure to dialysis membranes and as a consequence of the retrotransportation of substances that promote the expression of cytokines from the dialysate, increases the release of proinflammatory cytokines, such as IL-1β, TNF-α and IL-6.

The biocompatibility and flow properties of the different filters can influence the inflammatory and oxidative response to dialysis and the levels of oxidized molecules. In general, short-term dialysis with high-flow membranes has been shown to decrease plasma levels of oxidized low-density lipoproteins, regardless of the filter material.

Nonetheless, the introduction of high-flux membranes, which increased clearance of middle molecules, appears to be inadequate to improve patient survival, as reported by two major randomized trials, the HEMO study [24] and the MPO study.

The use of on-line HDF, that combines convective and diffusive solute removal, can counterbalance the rapid decrease of middle molecule diffusion obtained with high flux hemodialysis. The integration of diffusion with convection allows a more modest sieving coefficient decrease for middle molecular weight solutes. To date, evidence from four randomized controlled trials on HDF suggests, although not definitely, that HDF with convection volumes >20 L results in better patient outcomes .

Moreover, a correlation of the membrane typology with inflammation and oxidative stress in the medium-long term was found in patients undergoing chronic dialysis treatment. A prospective study in a cohort of 50 patients who started hemodialysis with a high-flux polysulfone membrane (HF80 series, Fresenius Medical Care, St. Wendel, Germany) evaluated the variation of the inflammation and oxidative stress biomarkers. The results showed significantly increased circulating levels of IL-6, C-reactive protein (CRP) and higher carbonylation rate in dialysis patients compared to healthy controls in the first year of observation, and this trend was maintained in the following 12 months.

Successive studies investigated the long-term effects on inflammation and oxidative stress of dialysis with Helixone (Fresenius Medical Care), another type of high-flux polysulfone membrane designed to maximize the clearance of uremic toxins of average molecular weight (> 500 Daltons, Da) compared to conventional high-flux membranes. The introduction of changes as reduced hollow fiber wall thickness (≈35 μm), smaller inner diameter (≈185 μm) and higher average pore size (≈3.3 nm) has improved the clearance of medium molecular weight molecules, including β2-microglobulin (B2M), limiting the passage of larger molecules like albumin, without significant effects on inflammatory parameters .

The features of synthetic membranes related their chemical composition and the asymmetric structure of the fibers make them preferable for new highly-convective therapies, such as high-volume hemodiafiltration (HDF), only partially allowed with cellulose.

However, cellulose triacetate (CTA) and modified cellulose membranes remain the best option for the patients with hypersensitivity reactions to synthetic fibers. Although synthetic membranes, including those based on polysulfone, have a lower risk of hypersensitization than non-biocompatible ones (such as Cuprophane), in some cases adverse reactions have been reported, plausibly due to the contact between the dialyzer itself with plasma proteins and platelets.

Moreover, the choice of polysulfone membranes has been proven to affect the intracellular and serum levels of bisphenol A (BPA, molecular weight 228.3 kDa), a ubiquitous environmental toxicant found in plastic food and beverage containers, as well as in some dialyzers, with several detrimental effects. BPA is considered an endocrine-disrupting chemical that acts mainly as a female hormone: it is associated with reproductive dysfunction, immune abnormality and increased incidence of cancer. Patients on hemodialysis are a high-risk population for BPA overload, because the normal urinary elimination of the conjugated molecule decreases with low renal function. For these reasons, BPA can be thought as a uremic toxin, similarly to p-cresol, and a particular attention should be given to BPA-containing medical devices, including dialysis membranes.

Mas et al. compared BPA-free and BPA-containing dialyzers in 72 patients under on-line HDF, using a prospective 9-month, crossover study design. BPA-free dialyzers determined a greater long-term decrease in BPA levels compared to BPA-containing dialyzers, although at three months the difference was not significant. This may be due to the use of the on-line HDF technique that has been related to lower BPA levels than conventional hemodialysis.

Recently, a new generation filter in asymmetric triacetate (ATA) has been introduced, born from a technology that combines the requirements of synthetic membranes for the possibility of being used with high-volume HDF and the advantages of natural fibers (cellulose) in terms of reduced allergic reactions.

These novel ATA membranes have been demonstrated to achieve satisfactory Kt and convection volume, with good biocompatibility and inflammatory profiles, indicating them as a valuable option for patients allergic to synthetic membranes under HDF treatment.