Innovations in approaches to remove uraemic toxins

Regulation of the blood-brain barrier function by peripheral cues in health and disease

The blood-brain barrier (BBB) is formed by microvascular endothelial cells which are ensembled with pericytes, astrocytes, microglia and neurons in the neurovascular unit (NVU) that is crucial for neuronal function. Given that the NVU and the BBB are highly dynamic and regulated structures, their integrity is continuously challenged by intrinsic and extrinsic factors. Herein, factors from peripheral organs such as gonadal and adrenal hormones may influence vascular function also in CNS endothelial cells in a sex- and age-dependent manner. The communication between the periphery and the CNS likely takes place in specific areas of the brain among which the circumventricular organs have a central position due to their neurosensory or neurosecretory function, owing to physiologically leaky blood vessels. In acute and chronic pathological conditions like liver, kidney, pulmonary disease, toxins and metabolites are generated that reach the brain via the circulation and may directly or indirectly affect BBB functionality via the activation of the immunes system. For example, chronic kidney disease (CKD) currently affects more than 840 million people worldwide and is likely to increase along with western world comorbidities of the cardio-vascular system in continuously ageing societies. Toxins leading to the uremic syndrome, may further lead to neurological complications such as cognitive impairment and uremic encephalopathy. Here we summarize the effects of hormones, toxins and inflammatory reactions on the brain vasculature, highlighting the urgent demand for mechanistically exploring the communication between the periphery and the CNS, focusing on the BBB as a last line of defense for brain protection.

Structure-based discovery of a new type of scaffold compound as binding competitors for protein-bound Uremic Toxins

Protein-bound uremic toxins (PBUTs) are the main cause of uremia, but traditional hemodialysis is ineffective in removing them because of their strong ability to bind to human serum albumin (HSA), highlighting the need for new treatments. In this study, first, structure-based docking was used to screen a diverse library of 200,376 virtual compounds against the active sites I and II. After two rounds of docking screening, 3944 candidate molecules were obtained. Second, 23 candidate molecules were obtained after ADMET prediction and toxicity analysis. Five candidate molecules were finally obtained after visual analysis and MM-PBSA calculations. We subsequently assessed their competitive displacement efficiency through a microdialysis experiment, and the results revealed that ZINC000008791789, ZINC000012297018, and ZINC000012296493 are promising binding competitors for PBUTs, as they have higher dialysis efficiency than the optimal displacer LA, approximately double the dialysis efficiency. The other two molecules, ZINC000031161007 and ZINC000004090361, although less efficient than LA, still outperformed the control group. Notably, four of them shared the same molecular scaffold, and three of them contained a flavonoid group. These findings provide a foundation for the development of more effective PBUT binding competitors, potentially benefiting uremia patients in the future.

Blood-brain barrier perturbations by uremic toxins: key contributors in chronic kidney disease-induced neurological disorders?

Chronic kidney disease is multifactorial and estimated to affect more than 840 million people worldwide constituting a major global health crisis. The number of patients will continue to rise mostly because of the ageing population and the increased prevalence of comorbidities such as diabetes and hypertension. Patients with advanced stages display a loss of kidney function leading to an accumulation of, a.o. protein-bound uremic toxins that are poorly eliminated by renal replacement therapies. This systemic retention of toxic metabolites, known as the uremic syndrome, affects other organs. Indeed, neurological complications such as cognitive impairment, uremic encephalopathy, and anxiety have been reported in chronic kidney disease patients. Several factors are involved, including hemodynamic disorders and blood-brain barrier (BBB) impairment. The BBB guarantees the exchange of solutes between the blood and the brain through a complex cellular organization and a diverse range of transport proteins. We hypothesize that the increased exposure of the brain to protein-bound uremic toxins is involved in BBB disruption and induces a perturbation in the activity of endothelial membrane transporters. This phenomenon could play a part in the evolution of neurological disorders driven by this kidney-brain crosstalk impairment. In this review, we present chronic kidney disease-induced neurological complications by focusing on the pathological relationship between the BBB and protein-bound uremic toxins. The importance of mechanistically delineating the impact of protein-bound uremic toxins on BBB integrity and membrane drug transporter expression and function in brain endothelial capillary cells is highlighted. Additionally, we put forward current knowledge gaps in the literature.

Strategies for optimizing urea removal to enable portable kidney dialysis: A reappraisal

BACKGROUND

Portable hemodialysis has the potential to improve health outcomes and quality of life for patients with kidney failure at reduced costs. Urea removal, required for dialysate regeneration, is a central function of any existing/potential portable dialysis device. Urea in the spent dialysate coexists with non-urea uremic toxins, nutrients, and electrolytes, all of which will interfere with the urea removal efficiency, regardless of whether the underlying urea removal mechanism is based on urease conversion, direct urea adsorption, or oxidation. The aim of the current review is to identify the amount of the most prevalent chemicals being removed during a single dialysis session and evaluate the potential benefits of an urea-selective membrane for portable dialysis.

METHODS

We have performed a literature search using Web of Science and PubMed databases to find available articles reporting (or be able to calculate from blood plasma concentration) > 5 mg of individually quantified solutes removed during thrice-weekly hemodialysis sessions. If multiple reports of the same solute were available, the reported values were averaged, and the geometric mean of standard deviations was taken. Further critical literature analysis of reported dialysate regeneration methods was performed using Web of Science and PubMed databases.

RESULTS

On average, 46.0 g uremic retention solutes are removed in a single conventional dialysis session, out of which urea is only 23.6 g. For both urease- and sorbent-based urea removal mechanisms, amino acids, with 7.7 g removal per session, could potentially interfere with urea removal efficiency. Additionally for the oxidation-based urea removal system, plentiful nutrients such as glucose (24.0 g) will interfere with urea removal by competition. Using a nanofiltration membrane between dialysate and oxidation unit with a molecular weight cutoff (MWCO) of ~200 Da, 67.6 g of non-electrolyte species will be removed in a single dialysis session, out of which 44.0 g are non-urea molecules. If the membrane MWCO is further decreased to 120 Da, the mass of non-electrolyte non-urea species will drop to 9.3 g. Reverse osmosis membranes have been shown to be both effective at blocking the transport of non-urea species (creatinine for example with ~90% rejection ratio), and permissive for urea transport (~20% rejection ratio), making them a promising urea selective membrane to increase the efficiency of the oxidative urea removal system.

CONCLUSIONS

Compiled are quantified solute removal amounts greater than 5 mg per session during conventional hemodialysis treatments, to act as a guide for portable dialysis system design. Analysis shows that multiple chemical species in the dialysate interfere with all proposed portable urea removal systems. This suggests the need for an additional protective dialysate loop coupled to urea removal system and an urea-selective membrane.

The Intestinal Microbiota and Metabolites in the Gut-Kidney-Heart Axis of Chronic Kidney Disease

Emerging evidences demonstrate the involvement of gut microbiota in the progression of chronic kidney disease (CKD) and CKD-associated complications including cardiovascular disease (CVD) and intestinal dysfunction. In this review, we discuss the interactions between the gut, kidney and heart in CKD state, and elucidate the significant role of intestinal microbiota in the gut-kidney-heart axis hypothesis for the pathophysiological mechanisms of these diseases, during which process mitochondria may serve as a potential therapeutic target. Dysregulation of this axis will lead to a vicious circle, contributing to CKD progression. Recent studies suggest novel therapies targeting gut microbiota in the gut-kidney-heart axis, including dietary intervention, probiotics, prebiotics, genetically engineered bacteria, fecal microbiota transplantation, bacterial metabolites modulation, antibiotics, conventional drugs and traditional Chinese medicine. Further, the identification of specific microbial communities and their corresponding pathophysiological metabolites and the illumination of the gut-kidney-heart axis may contribute to innovative basic research, clinical trials and therapeutic strategies against CKD progression and uremic complications in CKD patients.

Uraemic solutes as therapeutic targets in CKD-associated cardiovascular disease

Chronic kidney disease (CKD) is characterized by the retention of a myriad of solutes termed uraemic (or uremic) toxins, which inflict damage to several organs, including the cardiovascular system. Uraemic toxins can induce hallmarks of cardiovascular disease (CVD), such as atherothrombosis, heart failure, dysrhythmias, vessel calcification and dysregulated angiogenesis. CVD is an important driver of mortality in patients with CKD; however, reliance on conventional approaches to managing CVD risk is insufficient in these patients, underscoring a need to target risk factors that are specific to CKD. Mounting evidence suggests that targeting uraemic toxins and/or pathways induced by uraemic toxins, including tryptophan metabolites and trimethylamine N-oxide (TMAO), can lower the risk of CVD in patients with CKD. Although tangible therapies resulting from our growing knowledge of uraemic toxicity are yet to materialize, a number of pharmacological and non-pharmacological approaches have the potential to abrogate the effects of uraemic toxins, for example, by decreasing the production of uraemic toxins, by modifying metabolic pathways induced by uraemic toxins such as those controlled by aryl hydrocarbon receptor signalling and by augmenting the clearance of uraemic toxins.

Effects of Fecal Microbiota Transplantation on Composition in Mice with CKD

Background: Chronic kidney disease (CKD) is a renal disorder characterized by the accumulation of uremic toxins with limited strategies to reduce their concentrations. A large amount of data supports the pivotal role of intestinal microbiota in CKD complications and as a major source of uremic toxins production. Here, we explored whether fecal microbiota transplantation (FMT) could be attenuated in metabolic complication and uremic toxin accumulation in mice with CKD. Methods: Kidney failure was chemically induced by a diet containing 0.25% (w/w) of adenine for four weeks. Mice were randomized into three groups: control, CKD and CKD + FMT groups. After four weeks, CKD mice underwent fecal microbiota transplantation (FMT) from healthy mice or phosphate buffered saline as control. The gut microbiota structure, uremic toxins plasmatic concentrations, and metabolic profiles were explored three weeks after transplantation. Results: Associated with the increase of alpha diversity, we observed a noticeable improvement of gut microbiota disturbance, after FMT treatment. FMT further decreased p-cresyl sulfate accumulation and improved glucose tolerance. There was no change in kidney function. Conclusions: These data indicate that FMT limited the accumulation of uremic toxins issued from intestinal cresol pathway by a beneficial effect on gut microbiota diversity. Further studies are needed to investigate the FMT efficiency, the timing and feces amount for the transplantation before, to become a therapeutic option in CKD patients.

The current and future landscape of dialysis

The development of dialysis by early pioneers such as Willem Kolff and Belding Scribner set in motion several dramatic changes in the epidemiology, economics and ethical frameworks for the treatment of kidney failure. However, despite a rapid expansion in the provision of dialysis — particularly haemodialysis and most notably in high-income countries (HICs) — the rate of true patient-centred innovation has slowed. Current trends are particularly concerning from a global perspective: current costs are not sustainable, even for HICs, and globally, most people who develop kidney failure forego treatment, resulting in millions of deaths every year. Thus, there is an urgent need to develop new approaches and dialysis modalities that are cost-effective, accessible and offer improved patient outcomes. Nephrology researchers are increasingly engaging with patients to determine their priorities for meaningful outcomes that should be used to measure progress. The overarching message from this engagement is that while patients value longevity, reducing symptom burden and achieving maximal functional and social rehabilitation are prioritized more highly. In response, patients, payors, regulators and health-care systems are increasingly demanding improved value, which can only come about through true patient-centred innovation that supports high-quality, high-value care. Substantial efforts are now underway to support requisite transformative changes. These efforts need to be catalysed, promoted and fostered through international collaboration and harmonization.

Dialysis is a life-saving therapy; however, costs of dialysis are high, access is inequitable and outcomes are inadequate. This Review describes the current landscape of dialysis therapy from an epidemiological, economic, ethical and patient-centred framework, and describes initiatives that are aimed at stimulating innovations in the field to one that supports high-quality, high-value care.

The global dialysis population is growing rapidly, especially in low-income and middle-income countries; however, worldwide, a substantial number of people lack access to kidney replacement therapy, and millions of people die of kidney failure each year, often without supportive care.

The costs of dialysis care are high and will likely continue to rise as a result of increased life expectancy and improved therapies for causes of kidney failure such as diabetes mellitus and cardiovascular disease.

Patients on dialysis continue to bear a high burden of disease, shortened life expectancy and report a high symptom burden and a low health-related quality of life.

Patient-focused research has identified fatigue, insomnia, cramps, depression, anxiety and frustration as key symptoms contributing to unsatisfactory outcomes for patients on dialysis.

Initiatives to transform dialysis outcomes for patients require both top-down efforts (that is, efforts that promote incentives based on systems level policy, regulations, macroeconomic and organizational changes) and bottom-up efforts (that is, patient-led and patient-centred advocacy efforts as well as efforts led by individual teams of innovators).

Patients, payors, regulators and health-care systems increasingly demand improved value in dialysis care, which can only come about through true patient-centred innovation that supports high-quality, high-value care.