## 3 Discussion  
This study was performed to compare the efficacy of two different adsorbent resins to reduce IS and PCS by an in vitro study, and to test the efficacy of IS and PCS removal by the DVB-PVP cartridge in vivo, by a small-scale randomized controlled trial in HD patients. Furthermore, the efficacy of the symbiotic NATUREN G, singularly for 7 weeks, and in combination with the innovative DVB-PVP HD, for 1 week, was tested. The aim of this strategy was to reduce PBUTs production by gut microbiota modulation and to increase their removal by improving blood purification therapy inpatients undergoing HD, for their better survival [5; 7; 9] and quality of life. Sorbents, routinely used for many years in drug intoxication [33] and poisoning, have been more recently employed in other clinical application, such as sepsis [34; 35], acute kidney injury [36], and, lately, to remove molecules not easily cleared by HD, such as PBUTs [20; 21; 37; 38; 39]. In one of the first attempts to assess the capacity of the sorbent system to increase protein-bound solutes' removal by HD, it was demonstrated how the addition of activated charcoal to the dialysate could increase the clearance of indican, p-cresol, and PCS. In these in vitro dialysis experiments, for each protein-bound solutes, the clearance with charcoal added to the dialysate, doubled the clearance without charcoal, while it had no effect on the clearance of unbound solutes, such as urea [39]. Our in vitro data showed that DVB-PVP resin was more effective in adsorbing IS and PCS than cellulose, removing a mean of 56% PCS and around 54% IS, after 6 h of perfusion, from an experimental solution containing mean IS and PCS concentrations comparable to those present in hemodialysis patients [1]. In the in vivo study, the DVB-PVP cartridge showed its adsorbing efficacy only for IS plasma levels if compared to the baseline, as evidenced in the placebo group, after the first two DVB-HD sessions (Figure 4B). Two of the five patients in the placebo group did not complete the third DVB-PVP HD session owing to the lowering of blood pressure during the treatment, not allowing the observation of the trend towards a reduction in IS and PCS levels also after the third experimental session. We are unable, actually, to explain the discrepancy between in vitro and in vivo data on PCS removal. We can only assume that the in vivo relationship between PCS and resin is affected by a variety of conditions hard to reproduce in an in vitro setting, where standardized conditions are an effort to eliminate every kind of interference that might influence the ability by the resins to remove IS and PCS and to characterize the resins' behavior. Differently, many factors could affect the IS and PCS concentration in vivo, such as their binding with proteins other than albumin. We are aware that longer follow up and more cases would provide a better assessment of PCS removal. Similarly, a recent study reported that hexadecyl-immobilized cellulose beads (HICB) adsorbed IS free (55.4% of 1 mM IS solution) in vitro, much more than polysulfone dialysis membrane and polymethyl methacrylate membrane, but the clinical trial to use a (HICB)-containing column did not show an effect in reducing serum PBUTs levels in HD patients [21]. [PERSON] and coworkers [37] tested the in vitro efficacy of IS and PCS adsorption by nanoporous activated carbon monolith prototype finding a reduction of IS (\(-\)85%) and PCS (\(-\)78%) concentration after 60 minutes perfusion through it. Furthermore, they showed that nanoporous monoliths were able to remove IS (\(-\)83%) and PCS (\(-\)75%) after 30 min of continuous perfusion of 20 mL of the post high flux HD patient blood samples, observing, in addition, an albumin drop of 23%. In the final conclusions, the authors recognized that further work is necessary to develop and assess a scaled-up device that can be used in conjunction with a dialysis circuit [37].