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Bioartificial Organs

Risks and Requirements

DAVID HUNKELER

Laboratory of Polymers and Biomaterials, Swiss Federal Institute of Technology,CH-1015, Lausanne, Switzerland

INTRODUCTION

The development of therapies based on bioartificial organs requires the ap-propriate tissue, material, and process selections. While the requirements for tis-sue engineered products, such as bioartificial skin and cartilage, differ fromimmunoisolation-based devices such as the bioartificial liver and pancreas, sixkey issues must be addressed, concomitantly, as one integrated clinical feedbackwith basic research and product development. These are summarized below.

Appropriate Material Selection

Sterilizability, residual endotoxin level and biocompatibility1,a are generally cat-egorized as material preparation and purification operations. While their require-ments are well documented1 they are not usually coupled with questions related toquality control and raw material sourcing. Specifically, as bioartificial organs moveinto commerce2 a consistent set of polymeric precursors will be required. Therefore,while tissue engineered products based on polylactic-co-glycolic acid can be regu-lated via the synthesis procedure, as can hollow fibers based on acrylics, diffusionchambers and microcapsules made of or filled with naturally occurring polysaccha-rides present challenges in terms of batch to batch repeatability. Recent innovationsin controlled degradation of polyanions, such as alginate and carrageenans, as wellas chitosan based polycations are discussed in the third chapter of Part II (Novel Ma-terials) of this book. It is anticipated that only through the development of processeswhich regulate molecular parameters, principally the molar mass distribution andcharge spacing, along with preparation protocols, which govern rheological and in-terfacial properties, and purification methods, can biomaterials be commercially ap-plied as bioartificial organs. Material selection, therefore, relies on informationrelated to the ability to purify polymer solutions via filtration, and centrifugation, thesterilizability of materials by irradiation and autoclaving, as well as endotoxin deac-tivation or removal. The use of blends or copolymers has also been shown to offeradvantages in simultaneously controlling membrane properties such as permeabilityand mechanical strength.3

aWilliams’ Dictionary of Biomaterials, to be published in 1999 defines material biocompati-bility as its ability to perform with an appropriate host response in a specific application.

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Appropriate Tissue Sourcing and Harvesting

The ethical issues related to tissue selection will be debated in the discussion sec-tion of this chapter. However, with the exception of autografts, which have a limiteduse in bioartificial organs (for example the re-transplantation of encapsulated isletsfrom non-diabetic patients having full or partial pancreatotomies) tissue selection re-mains controversial. For several potential bioartificial organs the amount of allografttissue available is significantly below demand and, as is the case for whole organtransplants, most national waiting lists are in the thousands for the kidney, liver, andpancreas. The ultimate substitution with xenografts or genetically modified cells isa topic of investigation and several sections of this monograph detail recent advances(Part IV: Islet Isolation, Transplantation and the Bioartificial Pancreas). To date xe-nograft-based bioartificial organs have had outstanding early successes4 followed bytwo decades of minimal advancement. As a community, the physicians and biolo-gists involved in tissue harvesting and selection need to assess contradictory data.For example, certain hormones secreted from discordant xenografts, such as porcineinsulin, can be genetically quite similar to what is indigenously produced by the host(porcine insulin and human insulin differ by only one amino acid). However, in thecase of porcine islets, tissue fragility is extreme and the xenotransplant cannot, de-spite the presence of a ingress- and egress-specific membranes, hide the graft fromthe host. Therefore, methods to improve graft survival, such as co-culture, and co-immobilization with various tissues, are viewed as possible means to induce a localimmune protection (Part V: Biological Aspects). Clearly, this implicates transplantsite selection as well as device recoverability. The latter presents a tradeoff betweenvascularization, to improve the supply oxygen and nutrients, and retrievability (PartIII: Encapsulation).

Appropriate Process Selection

The process of producing bioartificial organs is clearly a function of device ge-ometry, with the appropriate material and method of preparation closely coupled.5

Processes can vary from extrusion (liquid-liquid and liquid-air6) to spinning disksand mechanical cutting of fluid streams to generate discrete microcapsules. Thetradeoffs associated with various immunoisolation technologies is treated in Part III(Encapsulation) with the aspects of the enzymatic process of cell harvesting, and tis-sue digestion, covered in Part IV. Clearly, the bioartificial organ, as a process, in-volves the synchronization of biological, technological and material quality controlprotocols. While this is not expected to be insurmountable, it has not been addressedto date.

Clinical Adaptability of the Device

An aspect not normally addressed in bioartificial organ development is the abilityto design materials which can cross platforms following clinical feedback. For ex-ample, most polysaccharides function only as components of microcapsular systemsbased on polyelectrolyte complexation, a reaction between oppositely charged mol-ecules. The potential to use such systems in various transplantation sites is often lim-

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ited by an inability to cast hollow fibers or structurally satisfactory flat membranesfrom the same material. This problem is exacerbated for typical phase inversionmembranes which serve well in macro-sized bioartificial organs (e.g., Hybrid Bio-artificial Pancreas) but cannot be formulated into microcapsules, and the acrylic hol-low fibers which are easy to co-extrude, though only in a tubular configuration.Therefore, a hollow fiber based bioartificial pancreas, which may work intraperito-neally, would be impossible to inject intraportally in the liver. The development ofnovel materials which do not require device definition prior to clinical evaluation re-lieves several design constraints and, for the first time, provides surgeons with site-independent device alternatives (Part II).

Development of Science and Products

Technology management requires the coordination of science, both existingand emerging, with product and process development, and marketing. For bioar-tificial organs this is complicated by the clinical trial sequence required to obtainregulatory approval. Although not specifically part of this compilation volume,BIO+AO II’s Scale-Up panel discussion revealed that start-ups could technolog-ically advance faster then the science justified and that this disequilibrium rip-pled through to the large corporate and venture capital supporters, oftentransiently destabilizing stock price. The predictability of marketing products,without regulatory approval, as well as the challenges in coordinating biological,material science and engineering activities, renders commercial biotechnologyvolatile at the best of times. Bioartificial organ development has not been an ex-ception for the cartilage, skin and liver products (Parts VI and VII) in clinicaltrials, nor should one expect it to be for the next generation of devices treatingother hormone deficient, or neurodegenerative, diseases.

Adequately Addressing Risks

Bioartificial organs, as a therapy, present clinical risks. On a product level,technological development is coupled with financial and marketing uncertaintiesrelated to Phase I/II/III trials. Societal and ethical risks include those related to theuse of xenografts. Recently Bach and Fineberg have called for a moratorium onxenotransplantation7 which comprises four points:

• Public risk requires a public mechanism.

• Public risk requires an iterative legislation.

• Individual informed consent will have to be modified to include patientsclose relatives and sexual partners.

• Viruses can overcome the species barrier (e.g., return to pigs after infectinghumans).

The discussion section of this chapter summarizes a response from eight partici-pants at the BIO+AO II conference distinguishing the risks in whole-organ and im-munoisolated xenotransplantation, as well as recommending a course of action.8

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DISCUSSIONb

The authors of a recent paper8 have noted the relevance of Bach and Fineberg’smoratorium7 to whole-organ transplantation and agreed, in general, with their firsttwo statements, although the third point could be applied less strictly for immunoiso-lated xenografts for reasons that will be further elaborated herein. The following sec-tion is a verbatim copy of parts of the jointly issued response to the call for amoratorium.

“Bach himself has noted that there has never been a reported case of a humaninfected with a porcine-derived pathogen due to porcine tissue exposure. Further-more, as previously discussed, two recent studies clearly demonstrate no evidence ofPERV infection following short9 and long term10 exposure to porcine tissues. Assuch, the risk is a potential risk and not a documented one. Therefore, society is putin the position to calculate an undefined risk. This is clearly distinct from definedinfections transmitted by organ/tissue allografts. While this seems to reflect positive-ly on cellular xenograft transplantation, we must remain concerned with our abilityto limit exposure of the host to a potential porcine-derived pathogen. Since we do notknow what the actual threat is, with some exceptions such as PERVs, it is difficult toquantify the risks, although one may speculate that an immune barrier will preventegress of a pathogen to the recipient. In any case, the risks of whole-organ and cel-lular transplantation clearly differ, and the respective technologies should be exam-ined and regulated (legislatively) separately. Bioartificial organs should follow acautious path of implementation, as will be outlined in the recommendations sectionof this chapter.

A more general issue is that Bach and Fineberg’s argument must also make thecase that patient infection is due specifically to porcine xenografts. Since we raise,slaughter and eat pigs, our societal exposure to porcine tissues is considerable, in-cluding the aforementioned porcine tissue transplants. We have also implanted dia-betics with porcine pancreatic extracts, prior to the development of geneticallyengineered human insulin, as well as porcine islets, and have no evidence ofzoonoses contracted by this route. Thus, from a safety point of view (i.e., moratori-um), one needs to argue that contracting a porcine-derived pathogen, if via a xe-nograft, could not also occur via another route (e.g., food supply).

Furthermore, the Karolinska Institute in Stockholm has grafted fetal pig isletsinto patients with insulin dependent diabetes mellitus while undertaking general im-munosuppression.11 Recently, the sera from these patients has been sent to the Cen-ter for Disease Control in Atlanta, GA. Although little, if any, function of the fetaltissue was detected, the naked islets caused no contamination, following intrahepat-ic implantation, in patients.

Cellular immunoisolated xenografts entail less risk than does whole-organ trans-plantation, since bioartificial organs are:

• Viral protective: membrane controls ingress/egress

• Antigen blocking: surface modifications change the host-contacting surface,

bThe text in the discussion is a highly condensed, verbatim, version of a paper, in press, usedwith permission of the publisher.8 The DISCUSSION is not presented as the author of this chapter’ssole work and full reference is given to all co-authors.

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blocking the antigen attachment-endothelial cell activation-hyperacute rejec-tion cascade

• Retrievable: immunoisolation devices/bioartificial organs can be designed forretrievability

• Isolating: macro/micro device cascades can be constructed to provide cellisolation

• Controllable: cells can be genetically modified with suicide genes

• Robust: the transplantation site can be varied for bioartificial organs (e.g.,use an immunoprivileged site)

Additionally, risk assessment can be performed prior to transplantation for cel-lular grafts such as islets. Indeed, preliminary evaluations have already been car-ried out, with extracorporeal xenograft-based bioartificial livers (BAL) in Phase IIIclinical trials (Part VI of this Book). Although the FDA has approved the use of theBAL as a bridge for comatose patients awaiting whole-organ transplantation, we be-lieve that less threatening diseases require a cautious approach to implementation.Moreover, a clinical evaluation of cellular xenotransplantation should proceed onlyafter it has been demonstrated that autografts and allografts of the bioartificial or-gan in question (e.g., pancreas, parathyroid) are safe and effective. Specifically, theauthors of this paper advocate the systematic development of cellular xenotrans-plantation and application of bioartificial organs.”

RECOMMENDATIONS

(1) Research on xenotransplantation in concordant and discordant animal models should proceed.

(2) Cellular xenograft transplantation should follow the clinical demonstration of concept and evaluation of bioartificial organ function in auto- and allografts.

(3) Cellular- and whole-organ xenotransplants involve unique recipient and societal risks and should, therefore, be regulated independently.

(4) The risks of cellular xenotransplantation can be assessed without threaten-ing the public health. The proposed moratorium on whole-organ xenotrans-plantation should not be extended to xenograft-based bioartificial organs.8

REFERENCES

1. PROKOP, A. & T. WANG. 1997. Purification of polymers used for fabrication of animmunoisolation barrier. Ann. N.Y. Acad. Sci. 831: 223–231.

2. HUNKELER, D. Chameleons: bioartificial organs move from the clinic to the market.Nature Biotechnol. In press.

3. PROKOP, A., D. HUNKELER, M. HARALSON, S. DIMARI & T. WANG. 1998. Water solu-ble polymers for immunoisolation II: evaluation of multicomponent encapsulationsystems. Adv. Polym. Sci. 136: 55.

4. LIM, F. & A.M. SUN. 1980. Microencapsulated islets as bioartificial endocrine pan-creas. Science 210: 908.

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5. RENKEN, A. & D. HUNKELER. 1998. Polimery 9: 530.6. HUNKELER, D. 1997. Polymers for bioartificial organs. Trends Polym. Sci. 5: 286.7. BACH, F. H. & H.V. FINEBERG. 1998. Call for moratorium on xenotransplants. Nature

391: 326.8. HUNKELER, D., A. SUN, D. SCHARP, G. KORBUTT, R. RAJOTTE, R. GILL, R. CALAFIORE

& PH. MOREL. Risks Involved in the xenotransplantation of immunoisolated tissue.Nature In press.

9. PATIENCE, C., G.S. PATTON, Y. TAKEUCHI, R.A. WEISS, M.O. MCCLURE, L. RYDBER

& M.E. BREIMER. 1998. Lancet 352: 699.10. HENEINE, W.H., A. TIBELL, M.W. SWITZER, P. SANDSTROM, G.V. ROSALES, A.

MATHEWS, O. KORSGREN, L.E. CHAPMAN, T.M. FOLKS & C.G. GROTH. 1998. Lancet352: 695.

11. GROTH, C.G., O. KORSGREN, A. TIBELL, et al. 1994. Lancet 344: 1402.