Cell encapsulation
He demonstrated that tumor cells in a polymer structure transplanted into pig abdominal cavity remained viable for a long period without being rejected by the immune system.
[2] Twenty years later, this approach was successfully put into practice in small animal models when alginate-polylysine-alginate (APA) microcapsules immobilizing xenograft islet cells were developed.
[3] The study demonstrated that when these microencapsulated islets were implanted into diabetic rats, the cells remained viable and controlled glucose levels for several weeks.
[4][5][6] Encapsulated cells expressing a cytochrome P450 enzyme to locally activate an anti-tumour prodrug were used in a trial for advanced, non-resectable pancreatic cancer.
An important reason for this is that the encapsulated cells would provide a source of sustained continuous release of therapeutic products for longer durations at the site of implantation.
Moreover, the prospect of implanting artificial cells with similar chemical composition in several patients irrespective of their leukocyte antigen could again allow reduction in costs.
[7] The potential of using cell microencapsulation in successful clinical applications can be realized only if several requirements encountered during the development process are optimized such as the use of an appropriate biocompatible polymer to form the mechanically and chemically stable semi-permeable matrix, production of uniformly sized microcapsules, use of an appropriate immune-compatible polycations cross-linked to the encapsulation polymer to stabilized the capsules, selection of a suitable cell type depending on the situation.
The polymer alginate is very commonly used due to its early discovery, easy availability and low cost but other materials such as cellulose sulphate, collagen, chitosan, gelatin and agarose have also been employed.
[8][9] Extensive work has been done using alginates which are regarded as the most suitable biomaterials for cell microencapsulation due to their abundance, excellent biocompatibility and biodegradability properties.
[20][21] Another approach to increasing the biocompatibility of the membrane biomaterial is through surface modification of the capsules using peptide and protein molecules which in turn controls the proliferation and rate of differentiation of the encapsulated cells.
[22][23] Another vital factor that controls the use of cell microcapsules in clinical applications is the development of a suitable immune-compatible polycation to coat the otherwise highly porous alginate beads and thus impart stability and immune protection to the system.
Thus several research groups have been looking for alternatives to PLL and have demonstrated promising results with poly-L-ornithine[26] and poly(methylene-co-guanidine) hydrochloride[27] by fabricating durable microcapsules with high and controlled mechanical strength for cell encapsulation.
Several groups have also investigated the use of chitosan which is a naturally derived polycation as a potential replacement for PLL to fabricate alginate-chitosan (AC) microcapsules for cell delivery applications.
[35] Gelatin is prepared from the denaturation of collagen and many desirable properties such as biodegradability, biocompatibility, non-immunogenity in physiological environments, and easy processability make this polymer a good choice for tissue engineering applications.
[49] The use of an ideal high quality biomaterial with the inherent properties of biocompatibility is the most crucial factor that governs the long term efficiency of this technology.
It is essential that the microcapsules have adequate membrane strength (mechanical stability) to endure physical and osmotic stress such as during the exchange of nutrients and waste products.
They secrete cytokines and produce a severe inflammatory reaction at the implantation site around the capsules, in turn leading to a decrease in viability of the encapsulated cells.
[77] It is necessary that the bacterial cells remain stable and healthy in the manufactured product, are sufficiently viable while moving through the upper digestive tract and are able to provide positive effects upon reaching the intestine of the host.
The potential of using bioartificial pancreas, for treatment of diabetes mellitus, based on encapsulating islet cells within a semi permeable membrane is extensively being studied by scientists.
[66] The Edmonton protocol involves implantation of human islets extracted from cadaveric donors and has shown improvements towards the treatment of type 1 diabetics who are prone to hypoglycemic unawareness.
[81] However, the two major hurdles faced in this technique are the limited availability of donor organs and with the need for immunosuppresents to prevent an immune response in the patient's body.
The first attempt towards this aim was demonstrated in 1980 by Lim et al. where xenograft islet cells were encapsulated inside alginate polylysine microcapsules and showed significant in vivo results for several weeks.
An example of this was demonstrated by Cirone et al. when genetically modified IL-2 cytokine secreting non-autologous mouse myoblasts implanted into mice showed a delay in the tumor growth with an increased rate of survival of the animals.
[93][94] However, this method of local delivery of microcapsules was not feasible in the treatment of patients with many tumors or in metastasis cases and has led to recent studies involving systemic implantation of the capsules.
[95][96] In 1998, a murine model of pancreatic cancer was used to study the effect of implanting genetically modified cytochrome P450 expressing feline epithelial cells encapsulated in cellulose sulfate polymers for the treatment of solid tumors.
On the basis of these results, an encapsulated cell therapy product, NovaCaps, was tested in a phase I and II clinical trial for the treatment of pancreatic cancer in patients[98][99] and has recently been designated by the European medicines agency (EMEA) as an orphan drug in Europe.
[42] However, solutions to issues such as immune response leading to inflammation of the surrounding tissue at the site of capsule implantation have to be researched in detail before more clinical trials are possible.
[103][104] An example of this is shown in the study by Zang et al. where genetically modified xenogeneic CHO cells expressing VEGF were encapsulated in alginate-polylysine-alginate microcapsules and implanted into rat myocardium.
[105] It was observed that the encapsulation protected the cells from an immunoresponse for three weeks and also led to an improvement in the cardiac tissue post-infarction due to increased angiogenesis.
One of the most successful approaches is an external device that acts similarly to a dialysis machine, only with a reservoir of pig hepatocytes surrounding the semipermeable portion of the blood-infused tubing.
