The Kidney Project, a national effort to develop an implantable bio-artificial kidney that could eliminate the need for dialysis, will announce a key milestone in a November 7, 2019 presentation at the American Society of Nephrology Kidney Week 2019 conference in Washington, DC.
The team will report that UC San Francisco scientists have successfully implanted a prototype kidney bioreactor containing functional human kidney cells into pigs without significant safety concerns.
The device, which is about the size of a deck of cards, did not trigger an immune reaction or cause blood clots in the animals, an important milestone on the road to future human trials.
“This is the first demonstration that kidney cells can be implanted successfully in a large animal without immunosuppression and remain healthy enough to perform their function.
This is a key milestone for us,” said Kidney Project co-lead Shuvo Roy, Ph.D., a faculty member in the Department of Bioengineering and Therapeutic Sciences, a joint department of the UCSF Schools of Pharmacy and Medicine.
“Based on these results, we can now focus on scaling up the bioreactor and combining it with the blood filtration component of the artificial kidney.”
UCSF-Vanderbilt Kidney Project Aims to Eliminate Dialysis
Nearly 750,000 Americans – and two million people around the world – are treated for end-stage renal disease (ESRD), and rates of kidney disease are growing rapidly, leading to an urgent shortage of kidneys for transplant.
As of 2016 there were only 21,000 donor kidneys available for transplant in the U.S. on a waiting list of nearly 100,000 and extending five to ten years.
Most patients awaiting a kidney transplant survive by undergoing long and cumbersome dialysis treatments multiple times a week to clear toxins from their blood, but dialysis does not replace many essential kidney functions and on average, only 35 percent of dialysis patients remain alive after five years.
Dialysis and other treatments for ESRD, which are universally covered by Medicare, cost $35 billion in 2016, representing seven percent of Medicare’s annual budget.
The Kidney Project [pharm.ucsf.edu/kidney] is led by Roy and Vanderbilt University Medical Center nephrologist William H. Fissell, MD, who for more than a decade have been working to develop an implantable bio-artificial kidney with the goal of eliminating dialysis and easing the shortage of donor kidneys.
The implantable device being developed by The Kidney Project consists of two components: an blood filtration system called the hemofilter, which removes toxins from the blood by passing it through silicon membranes fabricated with precisely shaped nanometer-scale pores; and a bioreactor, which contains cultured human kidney cells intended to perform other kidney functions, such as maintaining adequate fluid volume and blood pressure, adjusting salt levels, and producing essential hormones.
Following promising studies in large animals, The Kidney Project’s hemofiltration system is currently awaiting FDA approval for an initial clinical trial to evaluate its safety.
The bioreactor technology has been tested in laboratory experiments but so far had not been implanted into animals.
Bioreactor Containing Human Kidney Cells Implanted in Pigs Without Immune Reaction or Blood Clots
In The Kidney Project’s November 7 Kidney Week presentation, Rebecca Gologorsky, MD, a UCSF Surgical Innovations Fellow on the team, will show how silicon membranes inside the implanted bioreactor protect the enclosed human kidney cells from the host immune system by keeping blood-borne immune cells and proteins out of the device.
“It has been a holy grail of transplant therapies to find ways to avoid the need for lifelong immunosuppressive drugs that are often required to prevent immune rejection,” Roy said.
“These drugs not only expose patients to infection and other harmful side-effects but have been shown to directly harm transplanted cells and organs, eroding the therapeutic benefit of transplants over time.”
Another key benefit of avoiding immunosuppression is its cost to patients, Roy says: “Medicare currently covers dialysis for life, but immunosuppressive drugs are covered for just the first three years following transplant.
Many patients who receive kidney transplants ultimately lose the new organ because they weren’t able to afford the immunosuppressive drugs needed to keep it healthy.”
Roy’s team also carefully engineered the prototype bioreactor to avoid triggering blood clots that could lead to pulmonary embolism or stroke, a major challenge faced by all patients with long-term medical implants.
They achieved this by coating the silicon membrane filters that contact the blood with biologically friendly molecules and engineering the device to avoid the turbulent blood flow that can also trigger clotting.
“We couldn’t use the standard blood-friendly coatings that have been developed for heart valves, catheters, and other devices because they are so thick that they would completely block the pores of our silicon membranes,” Roy said.
“One of our accomplishments has been to engineer a suitable surface chemistry on our silicon membranes that makes them look biologically friendly to blood.”
The results, Roy says, demonstrate progress towards The Kidney Project’s hoped-for clinical “trifecta”: a heart-powered device that runs without batteries or other external connections that could introduce infection risk, and which can clean the blood without anti-rejection drugs or blood thinners.
The researchers now aim to scale up the prototype bioreactor to contain more cells in order to test whether the implanted device can supplement kidney function in animals with kidney failure, with the ultimate goal of eventually moving the device to human safety trials.
“Advancing a complex cell therapy like this into the clinic will not be a trivial task—for instance, it will require substantial investments in cell production and characterization in controlled GMP facilities to avoid any possibility of contamination,” Roy said. “Now we’ve confirmed that we’re on the right track to move forward with these efforts.”
The kidney was the first solid organ whose function was approximated by a machine and a synthetic device. In fact, renal substitution therapy with hemodialysis (HD) or peritoneal dialysis (PD) has been the only successful long-term ex vivo organ substitution therapy to date.
The kidney was also the first organ to be successfully transplanted from a donor individual to recipient patient as an isograft. However, the lack of wide-spread availability of suitable transplantable organs has kept kidney transplantation from becoming a practical solution in most cases of chronic renal failure.
Although long-term chronic renal replacement therapy (RRT) with either hemodialysis or peritoneal dialysis has dramatically changed the prognosis of renal failure, it is not complete replacement therapy, because it provides only filtration function (usually on an intermittent basis) and does not replace the homeostatic, regulatory, metabolic, and endocrine functions
of the kidney.
Because of the non physiologic manner in which dialysis performs or does not perform the most critical renal functions, patients with end-stage renal disease (ESRD) on
dialysis continue to have major medical, social, and economic problems [1].
Accordingly, dialysis should be considered as partial substitution rather than renal replacement therapy.
Tissue engineering of an implantable bioartificial kidney composed of both biologic and synthetic components could result in substantial benefits for patients by increasing life expectancy, mobility, and quality of life; with less risk of infection and reduced costs. This approach could also be considered a cure rather than a treatment for patient.
Wearable bioartificial kidney (WEBAK)
A BAK for long-term use in ESRD, similar to short-term use in AKI, would integrate tubular cell therapy and the filtration function of a hemofilter. ESRD patients on con- ventional RRT are at high risk for cardiovascular and infec- tious complications. A clinical trial failed to show survival benefit from increased doses of hemodialysis above what is now standard care [29], suggesting that there are important metabolic derangements not adequately treated with con- ventional dialytic treatment.
Data from the survival of renal transplant recipients, which far exceed those from the sur- vival of age-, sex-, and risk-matched controls awaiting transplant, also suggest that there is some metabolic func- tion provided by the kidney that transcends this organ’s filtration function.
Patients with ESRD display elevated levels of C-reactive protein (CRP), an emerging clinical marker, and pro-inflammatory cytokines, including IL-1, IL-6, and tumor necrosis factor alpha (TNFα) [30]. All these parameters are associated with enhanced mortality in ESRD patients. Specifically, IL-6 has been identified as a single predictive factor closely correlated with mortality in hemo- dialysis patients [30].
Although all ESRD patients could conceivably benefit from a BAK, patients in the inflamma- tory stage who display elevated levels of certain markers of chronic inflammation (most notably IL-6 and CRP) would likely benefit most and will be the target population for clinical study in the near future.
For the ESRD patient population, however, there are obvious limitations in using an extracorporeal RAD connected to a hemofiltration circuit. Ideally, a BAK suit- able for long-term use in ESRD patients would be capable of performing continuously, like the native kidney, to reduce risks from fluctuations in volume status, electrolytes and solute concentrations; and to maintain acid–base and uremic toxin regulation.
Such treatment requires the design and manufacture of a compact implantable or wearable dialysis apparatus and the development of compact renal tubule cell devices with long service lifetimes.
The ideal design of the next-generation RAD would be like that of an implantable device similar to the pacemaker.
As an intermediary approach to a fully implantable BAK, a WEBAK formulation has been recently evaluated in preclin- ical large animal models utilizing either CVVH or PD as the therapeutic circuit delivery route.
The WEBAK is comprised of the use of sorbent-based technologies to replace the excre- tory function of the kidney and the compact BRECS described above to replace the metabolic function of the kidney. This approach in a PD circuit is displayed in Fig. 2.
Sorbent-based hemodialysis was developed in the late 1960s and introduced into the clinic in the 1970s. Sorbents provide the ability to reduce the large volumes of dialysis solutions utilizing highly purified water from 100 to 200 l per dialysis sessions to as little as 6 l of potable water.
With sorbent dialysis, spent dialysate from the dialyzer cartridge is not discarded but is regenerated by processing the dialy- sate through the sorbent cartridge.
The sorbent cartridge has several layers of sorbent com- pounds which regenerate used dialysate into fresh, bicar- bonate dialysate. The cartridge is based upon carbon binding, enzyme conversion, and ion exchange.
This compact and disposable regeneration process promotes this sor- bent system to remove key uremic toxins and regenerate dialysate. Sorbent cartridges operate without a connection to a water supply or drain, promoting system mobility, porta- bility, and measurable RRT.
Attempts have been made to develop wearable dialysis systems to improve the portability of renal replacement therapies. Gura et al. [31] have published research into a lightweight, wearable, continuous ambulatory ultrafiltration device consisting of a hollow fiber hemofilter, a battery- operated pulsatile pump, and two micropumps to control heparin administration and ultrafiltration.
This device re- generates dialysate with activated carbon, immobilized ure- ase, zirconium hydroxide and zirconium phosphate; similar to the once commercially available REDY dialysis system. This approach, however, requires continuous blood access to allow adequate ultrafiltration.
An entirely different approach would utilize continuous regeneration of PD fluid relying on continuous-flow PD systems [32–34].
As the development of a sorbent-based, wearable, continuous recycling flow PD is being made, the integration of renal epithelial cell therapy in the PD circuit to treat patients with ESRD is also being evaluated in large animal models of uremia [33].
Although a cell therapy device requires a continuous source of nutrients and oxygen, the use of blood circuits for this nutrient stream has been avoided due to clotting and infection risks. Accordingly, the use of recycling peritoneal fluid to maintain viability of a renal cell device, the BRECS, has been conceived and successfully tested in a uremic sheep model [33].
Similar to acute applications of cell-based therapies, the compact, cryopreservable BRECS design and the enhanced propaga- tion methods used to isolate and expand the renal cells in the BRECS are enabling advancements toward the development of a WEBAK for chronic applications which integrates a wearable sorbent dialysis system and BRECS with recycling peritoneal fluid. Further refinement and testing of this WEBAK system is underway.
Implanted artificial kidney: successes and barriers
An implantable “biohybrid” or bioartificial device has the potential to avoid both supply limitations to renal transplant and the burden of therapy of intensive maintenance dialysis [35].
The innovative combination of an implanted hemofilter and a bioreactor of renal cells would provide continuous small solute clearance along with metabolic functions of the proximal tubule, without the morbidity and burden of catheters and dialysis (Fig. 3).
Two factors limit miniaturization and implantation of the hemodialysis circuit: the size and pump requirements of modern dialyzers, and the water volume required for dialytic therapy. Hollow-fiber polymer membranes have been immensely successful in treating renal failure with extracorporeal therapies but require super-physiologic driving pressures for blood circulation through the cartridge.
The long cylindrical hollow fibers present a high resistance to blood flow, which rises further in the distal portion of the fiber as ultrafiltration increases hematocrit and viscosity, necessitating energy requiring roller pumps to circulate blood through the device.The pores in hollow-fiber dialyzers are irregular, approximately cylindrical, and non-uniform in size; typically described as having a log-normal pore size distribution.
This polydispersity compromises the trade-off between permeability and selectivity.
To prevent albu- min leakage through the largest pores in the membrane, the majority of the membrane’s pores need to be maintained much smaller than the desired cut-off target of the membrane.
The second limitation in the develop- ment of an implantable BAK, dialytic water volume, is most readily demonstrated by the estimated water volume required during a 4-h dialysis treatment with a dialysate flow of 600 ml/min, consuming 144 l of dialysate, being as much as an additional 200 l of tap water [36, 37].
Control of pores enables high-efficiency filtration
The natural filtration–reabsorption process of renal solute clearance as a model for a new approach to RRT includes the examination of the structure of the glomerular filter for clues to its performance.
The kidney’s filters appear to be uniform, elongated, slit-shaped structures, rather than the ir- regular and more cylindrically shaped pores of polymer membranes.
his particular geometry of uniform slits appears to optimize the permeability-selectivity trade-off [36].
To develop this type of slit pore design, silicon nanotechnology was used to fabricate a membrane incorporating these advantages.
Microelectromechanical systems (MEMS) is a toolkit that applies industrially mature manufacturing techniques from the semiconductor industry to produce precise miniature electromechanical devices such as pumps, valves, and sensors at low unit cost.
MEMS technology can be used for the production of silicon-nanopore membranes with slit-shaped pores that are tailored for implementation in a BAK.
The fundamental membrane engineering challenge for the implantable BAK is to simultaneously maximize water permeability, while minimizing leakage of albumin and other important macromolecules.
Prototyped ultrathin bio- mimetic silicone-nanopore membranes using bulk and surface micromachining techniques derived from MEMS technology have been fabricated and evaluated for macro- molecular transport [36–38].
In brief, membranes with pore sizes from 5 to 90 nm were produced and perfused with solutions of a polydisperse tracer (Ficoll) and albumin to characterize their transport characteristics.
The silicon- nanopore membranes showed a distinct and predictable size-dependent and charge-dependent reduction in sieving coefficient [37]. Albumin transmission through membranes with different pore sizes was measured to map the relation- ship between pore size and albumin retention.
The observed sieving coefficients matched a combined electrical/steric hindrance model. Further, an analysis of the trade-off between selectivity (ability to reject albumin) and permeability (ability to transmit water) suggests that a membrane of slit- shaped pores outperforms a comparable membrane of cy- lindrical pores [36, 37].
Data from the silicon-nanopore membranes, when compared to data from polymer mem branes, confirms that this theoretical advantage is experi- mentally observed as well.
Silicon readily forms a thin oxide coating upon exposure to atmospheric oxygen. This silica film is negatively charged at physiologic pH and both, absorbs plasma pro- teins and activates the coagulation cascade. A strategy to mitigate adverse blood–material interactions is to modify the silicon surface with a highly hydrated polymer.
Various organic polymers grafted to silicon surfaces have been suc- cessful in demonstrating the feasibility of using silicon- nanopore membranes coated with antifouling films for hemofiltration [39], allowing for progress towards an implantable BAK. Despite advancements in biocompatibility of silicon membranes, it is anticipated that long-term anticoagulation will be necessary for implantable BAK success.
Vascular perfusion of the BAK will be necessary to maintain filtration and cell viability. The first site of implan- tation is planned to be in the pelvis, utilizing iliac vessels similar to a kidney transplant (Fig. 3).
Significant preliminary data have been published documenting the technologies necessary for an implant- able bioartificial device that can provide enough small solute clearance to allow the patient with failing kidneys to avoid dialysis.
Significant engineering challenges re- main. The design of the implanted device will have to be guided by the planned therapeutic strategy. Specific means for patients to self-monitor and reprogram device function will be critical for truly independent self-care using an implanted device.
Lifecycle management of the device, including recognition of impending device fail- ure and a minimally invasive approach to renewing or replacing failed components, modules, or cartridges, seems essential. Selection of implant site will be guided by the paramount need to preserve vascular sites for future allografts.
Conclusions
Despite all the advances in renal replacement therapies, a portable, continuous, dialysate-free bioartificial kidney has not yet been achieved and still remains the ultimate goal of renal tissue engineering.
The enabling platform technologies discussed in this review advance this goal from a dream to the laboratory bench and even to the bedside.
Future research in renal tissue engineering will need to focus on reproducing mechanisms of whole-body homeostasis. A high priority must be given to sensing and regulating extracellular fluid volume, even if only at the crude level of having the patient weigh daily and adjust ultrafiltration and reabsorption by the bioartificial kidney.
Chemical-field effect transistors (ChemFETs) offer the possibility of measuring electrolyte levels in a protein-free ultrafiltrate, with a readout of potassi- um levels to the patient, who could then alter diet or treat him- or herself with potassium-absorbing resins.
The critical building blocks of an autonomous bioartificial kidney are advancing. The technology with which to adapt these advances to a more autonomous, dialysate-free system is under development.
In addition, progress has been made in the field of cryopreservation and thus the ability to manufacture, store, and distribute bioartificial organs is advancing.
The next decade, like the previous, will likely see substantive advances in renal tissue engineering.
More information: An Immunoprotected Bioreactor for Implanted Renal Cell Therapy, Abstract TH-OR033, November 07, 2019 | 04:54 PM – 05:06 PM ET, 146 A/B, Walter E. Washington Convention Center, www.asn-online.org/education/k … px?controlId=3232240
Tunable Stiffness Polyacrylamide Hydrogels with Functionalized Matrigel for Renal Tissue Culture, Abstract SA-PO038, November 09, 2019 | 10:00 AM – 12:00 PM ET, Exhibit Hall, Walter E. Washington Convention Center,
Activation of AMPK and Inhibition of TGFß Stimulate In Vitro Transport in Human Renal Epithelial Cells, Abstract SA-PO043, November 09, 2019 | 10:00 AM – 12:00 PM ET, Exhibit Hall, Walter E. Washington Convention Center , www.asn-online.org/education/k … px?controlId=3233432
Next-Generation Renal Replacement Therapies (RRT): How Do Patients Weigh the Risks and Benefits? Abstract SA-PO057, November 09, 2019 | 10:00 AM – 12:00 PM ET, Exhibit Hall, Walter E. Washington Convention Center, www.asn-online.org/education/k … px?controlId=3235261
Provided by University of California, San Francisco
