David H. Rein

Transport characterization of membranes for immunoisolation

This study relates to the diffusive transport characterization of hollow fibre membranes used in implantable bio-hybrid organs and other immunoisolatory devices. Techniques were developed to accurately determine the mass transfer coefficients for diffusing species in the 10(2)-10(5) MW range, validated and then used to study one membrane type known to effectively immunoisolate both allografts and xenografts in vivo. Low-molecular-weight diffusing markers included glucose, vitamin B12 and cytochrome C; higher-molecular-weight molecules were bovine serum albumin, immunoglobulin G, apoferritin and a range of fluorescein-tagged dextrans. Overall and fractional mass transfer coefficients through the hollow fibres were determined using a resistance-in-series model for transport. A flowing dialysis-type apparatus was used for the small-molecular-weight diffusants, whereas a static diffusion chamber was used for large-molecular-weight markers. For diffusion measurements of small-molecular-weight solutes, convective artefacts were minimized and the effect of boundary layers on both sides of the membrane were accounted for in the model. In measuring diffusion coefficients of large-molecular-weight species, boundary layer effects were shown to be negligible. Results showed that for small-molecular-weight species (< 13,000 MW) the diffusion coefficient in the membrane was reduced relative to diffusion in water by two to four times. The diffusion rate of large-molecular-weight species was hindered by several thousand-fold over their rate of diffusion in water.

Polymer science for macroencapsulation of cells for central nervous system transplantation

The goal of encapsulated cell therapy research is to develop implants containing living xenogeneic cells to treat serious and disabling human conditions. The enabling concept is straightforward: cells or small clusters of tissue are surrounded by a selective membrane barrier which admits oxygen and required metabolites, releases bioactive cell secretions but restricts the transport of the larger cytotoxic agents of the bodys immune defense system. Use of a selective membrane both eliminates the need for chronic immunosuppression in the host and allows cells to be obtained from non-human sources, thus avoiding the cell-sourcing constraints which have limited the clinical application of general successful investigative trials of unencapsulated cell transplantation for chronic pain, Parkinsons disease, and type I diabetes. Target applications for encapsulated cell therapy include these same disorders as well as other disabilities caused by loss of secretory cell function which cannot be adequately treated by current organ transplantation or drug therapies and conditions potentially capable of responding to local sustained delivery of growth factors and other biologic response modifiers. Several types of device configurations are possible. Here we focus on easily retrieved, non-vascularized, macrocapsules. Such devices have four basic components: a hollow fiber or flat sheet membrane (usually thermoplastic based), cells (primary or dividing), and extracellular matrix (natural or synthetic) to promote cell viability and function, and other device components such as seals, tethers and radio-opaque markers. Choice of membrane and extracellular matrix polymers as well as issues surrounding implantation and biocompatibility evaluation are complex, inter-related, and ultimately driven by implantation site and delivery requirements. Cross species immunoisolated cell therapy has been validated small and large animal models of chronic pain, Parkinsons disease, and type 1 diabetes and is under active investigation by several groups in animal models of Huntingtons, Hemophilia, Alzheimers, ALS, and other CNS disorders.

In vivo biostability of a polymeric hollow fibre membrane for cell encapsulation

The biostability of poly(acrylonitrile-co-vinyl chloride) (P(AN/VC)) hollow fibre membranes was assessed in the rat peritoneal cavity over a 12 month period. The mechanical and chemical stabilities of the hollow fibre membrane (HFM) were characterized by measuring its tensile strength and molecular weight (by gel permeation chromatography) pre-implantation and post-explantation. The stability of the HFM transport properties was determined by molecular weight cut-off (MWCO) and hydraulic permeability (HP). Explanted HFMs were treated with 4 M NaOH to remove adsorbed protein before measuring mechanical, chemical and transport properties. The HFM was stable in vivo for at least 12 months: (i) weight average molecular weight (Mw) at t = 0 was 143,000 g mol-1 (with a polydispersity index (PDI) of 2.3) and at t = 12 months it was 128,000 g mol-1 (with a PDI of 2.8); and (ii) tensile strength at t = 0, 52 +/- 2 mdyne, did not change significantly over time and was 46 +/- 7 mdyne at t = 15 months (P > 0.05 by a two-tailed Students t-test); and (iii) no significant differences, with respect to standard deviation, were observed in the transport properties: HP was 7.4 +/- 1.5 ml min-1 m-2 mmHg-1 at t = 0 and 7.5 +/- 1.5 ml min-1 m-2 mmHg-1 at t = 12 months, while MWCO (at 90% rejection) was initially 40,000 +/- 8000 g mol-1 and then 54,000 +/- 10,000 g mol-1 at t = 12 months.

DESIGN OF MEMBRANE-BASED BIOARTIFICIAL ORGANS

The goal of encapsulated cell therapy research is to develop implants containing living xenogeneic cells to treat serious and disabling human conditions. The concept is straightforward: cells or small clusters of tissue are surrounded by a selective membrane barrier which admits oxygen and required metabolites, releases bioactive cell secretions but restricts the transport of the larger cytotoxic agents of the body’s immune defense system. Use of a selective membrane both eliminates the need for chronic immuno-suppression in the host and allows cells to be obtained from non-human sources, thus avoiding the cell-sourcing constraints which have limited the clinical application of generally successful investigative trials of unencapsulated cell transplantation for chronic pain1, Parkinson’s disease2, and type I diabetes.3–5 Cross-species immunoisolated cell therapy has been validated in small and large animal models of chronic pain6,7, Parkinson’s disease8,9, and Type 1 diabetes10–12, and is under active investigation by several groups in animal models of Huntington’s13, ALS14 and Alzheimer’s15–19. In addition, the first encapsulated therapy using xenografts in humans has been performed in chronic nain20 and ALS21.

Transplantation of Encapsulated Cells into the Central Nervous System

Targeted Delivery to the CNS Targeted delivery of active biomolecules to the central nervous system (CNS) has been applied as a potential therapy to treat a variety of disorders, including chronic pain and neurodegenerative diseases. This approach is advantageous for delivery of neuroactive agents that do not readily cross the blood-brain barrier. The implantable systems available for the targeted delivery of these substances to the CNS are either cell-based or non-cell-based. The non-cell-based systems include controlled release polymers and infusion via pumps and catheters. These systems involve relatively simple surgical procedures and allow high control over dose over the short term. The main disadvantages involve poor long-term release kinetics for the polymer delivery systems, high risk of infection for the catheters and pumps, and the need for highly purified and stable factors for both approaches. Cell-based systems involve the encapsulation of living secretory cells in either microencapsulation or macroencapsulation devices. The main advantage of cell-based delivery systems is the continuous production and secretion of the neuroactive agents from the encapsulated cells, thereby eliminating requirements for purified and stable therapeutic factors and the need for frequent refilling or replacement. Macroencapsulation devices have the additional advantage of high mechanical strength, ease of retrievability, and increased control over the manufacturing. This chapter will focus on the delivery of therapeutic factors to the CNS using macroencapsulation devices.