1. Introduction
This invention relates generally to apparatus for maintaining and growing biological cells ex vivo and, more particularly, to apparatus of this kind that maintains and grows the cells in a portable cassette while maintaining a sterile system that is closed to the external environment.
Many medical disorders can now be resolved by using transplanted cells, tissues, or organs. Transplantation has evolved from the surgical transfer of tissue from one part of a patient's body to another, to the surgical transfer of organs and tissues between individuals, to the transplantation of blood and immune systems between individuals. With increased demonstration of the medical benefit of tissue transplantation, the demand for organs and tissues suitable for these procedures has far exceeded the availability. Furthermore, in those cases where availability is less an issue, e.g., bone marrow, the procedure is cost prohibitive and can be invasive for the donor or the patient.
As an evolution of the clinical need, two related fields have evolved, which have been termed "cell therapy" and "tissue engineering." Cell therapy generally refers to the use of living cells, rather than drugs, to treat a clinical disorder or disease. Perhaps the most widely practiced form of cell therapy today is with bone marrow or hematopoietic stem cell transplantation in patients who have received hematopoietic toxic chemotherapy or radiation. This procedure involves the reinfusion of early stage cells that originate in the bone marrow, so that these cells can reestablish a patient's blood and immune system, and often the bone marrow tissue as well. Through this cell therapy process, the hematopoietic toxicity from cancer treatment is remedied.
Tissue engineering generally refers to the utilization of different disciplines between engineering, physiology, and cell biology, to develop at least a partially living tissue that is capable of normal tissue function. Once produced, this tissue may be transplanted into humans to restore or improve normal tissue or organ function. Numerous biotechnology companies are engaged in projects to engineer human tissues for transplantation.
For both cell therapy and tissue engineering procedures, there is a critical need to be able to process and/or produce ex vivo the cells that will be used for the therapeutic transplant. Biological science has now progressed such that, for many of the human tissues, methodology has been developed so that the key cells of that tissue can be grown outside the body. As a result, a clinically useful amount of tissue can be generated from a small amount of starting material, which is obtained with a minimally invasive technique. With this achievement, the opportunity for increasing and more diverse use of tissue transplantation is offered.
In parallel with this advancement, and largely dependent upon its success, are the numerous gene therapy approaches being advanced to initial clinical trials that involve the ex vivo genetic manipulation of cells and tissue. Gene therapy involves transduction of the genome of the cell to achieve correction of a defective gene, regulation of a disease condition, or production of a beneficial molecule. Those gene therapy procedures that will benefit from ex vivo administration of a gene vector to an expanded or donor tissue in order to enhance the targeting of the gene and avoid this systemic administration (likely to include most conceivable gene therapies for the next decade or longer) will be well served by the above advancements in tissue genesis and production.
The particular physical and biological requirements for the production of cells and tissues of blood, skin, cartilage, bone, pancreas, the nervous system, and various other endothelial and mesenchymal tissues of interest to cell and genetic therapists, will vary. However, two key components are necessary in order to grow cells and tissues ex vivo: 1) cells of self or donor origin that are capable of replicating and differentiating, as needed, for the formation of functioning tissue; and 2) an ex vivo system comprised of biocompatible materials that provide for the physiological requirements (e.g., surface attachment, medium exchange, and oxygenation) for the above cells to grow.
An excellent example of the merging interface of cell therapy with tissue engineering is the ex vivo production of human bone marrow. This process illustrates as well the interrelationship between the cell/tissue production methodology and the medical device requirements to properly implement the tissue production.
Although lacking the physical geometry that is a feature of other tissues or organs, bone marrow is a tissue comprised of many different cell types, ranging from different stromal fibroblasts, mesenchymal cells, to stem cells and the other cells of the hematopoietic system. The ex vivo process found to be needed for ex vivo bone marrow growth, was to mimic the natural functional environment of the bone marrow, providing for the controlled nutrient perfusion and oxygenation of the stem and stromal cell components under precise conditions of temperature and medium composition. Key to the success was to provide culture conditions that were concurrently amenable for each of the many different cell types that are found in human bone marrow.
Using this approach of tissue engineering, for the first time, the human stem cells that are found in the bone marrow were able to not only survive in culture, but also replicate to produce more stem as well as more mature progenitor cells. This result is in direct contrast to when hematopoietic stem/progenitor cells have been isolated (e.g., CD-34 selection) prior to the culture process. In this case the stem cells do not grow and the cultures die off over a short period, presumably because heterogeneous tissue interactions have been eliminated.
With the successful production of these bone marrow tissue cells, they can be available to be used as a substitute for bone marrow transplantation. This example is an excellent demonstration of how the lost function of a damaged or destroyed tissue, e.g., bone marrow, can be repaired or restored with ex vivo engineered tissue-specific cells.
Once the basic cell/tissue production process is identified, the next requirement for therapeutic utilization is the need for clinical systems to implement the process. These systems should be amenable for routine use by the thousands of hospitals and clinics in the developed and developing world that serve the patients intended to benefit from the transplantation cells and tissues in native or genetically altered form.
2. Critical Requirements for an Ex-Vivo Cell Production Process
Cell and organ transplantation therapy to date has relied on the clinical facility to be able to handle and process cells or tissues through the use of laboratory products and processes, governed to varying degrees by standard operating procedures, and with varying FDA and other regulatory authority involvement. The procedures to date, however, generally have not required extensive manipulation of the cells or tissue beyond providing standard incubation solutions, short term storage or containment, or--as in the case of bone marrow or peripheral blood stem cells for stem cell transplants--cryopreservation. With the addition of steps that require the actual growth and production of cells or tissues for transplantation, there are many considerations that need to be addressed in order for a reliable and clinically safe process to result. This issue is the same regardless of whether the cell production is occurring at the patient care location (as might be the case for the production of cells for a stem cell transplant), or at some distant manufacturing site (as might be the case for the production of a biosynthetic device).
A. Process Reliability for an Ex Vivo Cell Production Process
Perhaps the most critical of all issues to be addressed is the technical art that is inherent with most cell culture processes. Site-to-site differences in cell culture are often sensitive. For an acceptable clinical cell culture process, the technical art or sophistication must be eliminated such that the cell product will be the same when the process is used in different physical locations.
This problem can best be handled by implementing a well characterized, robust process with automation. While the variable human factor is eliminated from the technical process, human oversight should be maintained for the quality monitoring and control process. Alternatively, highly controlled training and standard procedures can be used to address inherent variability in the practice of cell production processes as well, and in certain cases will be required when the technical steps cannot be automated.
However, if a controlled process can lead to the same result repeatedly, then it can and should be automated. From a strategic point of view, automation via a medical device is desirable because it eliminates variability due to human error or human initiative, it reduces the need for highly skilled labor and thus cost of the process, and it makes the process amenable for widespread practice. Ultimately, any manual process remains vulnerable to an ineffective automation strategy. Automation also meets a general desire by clinicians to provide better quality assurance to their patients.
B. Process Sterility--Closed Systems
With any cell culture procedure, a major concern is sterility. When the product cells are to be transplanted into patients--often at a time when the patient is immunocomprised, as is the case with stem cell transplants and with organ transplants--absence of microorganisms is mandated. Most laboratory cell culture procedures are carried out under aseptic conditions with the technician practicing so-called sterile technique. Many of the bioreactor systems that have been developed offer advantages over the manual processes in that once the culture is initiated, the culture chamber and the fluid pathway is maintained in a sterile, closed environment. However, even with these systems, the initial setup and takedown steps, such as the medium priming and collection of the cells at the completion of the process, requires non-sterile manual procedures.
Accordingly, laboratory cell culture systems are only partly closed, i.e., they involve numerous aseptic connections, are mostly operated in a controlled-environment hood, and have pre- and post-processing steps requiring open centrifuge tubes and the like. The most optimal objective is to have the culture process be carried out in a system where the culture chamber and fluid path is functionally closed to the external environment, with the sterile integrity maintained from the time the device is manufactured until it has been disposed of.
C. Cell Recovery
For cell therapy, the product of the cell culture process is the cells. Accordingly, efficient collection of the cells at the completion of the culture process is an important feature of an effective cell culture system. Recovery of cells from most cell production processes is a challenge. They are either: 1) packed into the interstices of a make-do dialysis cartridge, or 2) suspended in many liters of culture medium. The first case necessitates unreasonable physical force to dislodge cells (being neither reliable nor easily automated), and the second case requires significant time, patience, and a certain degree of good fortune (being neither reliable nor closed).
The better approach for production of cells as the product is to culture cells in a defined, reasonable space, without physical barriers to recovery, so that simple elution of product results in a manageable, concentrated volume of cells amenable to final washing in a commercial, closed system cell washer designed for the purpose. An ideal system would allow for the efficient and complete removal of all cells produced, including both adherent and nonadherent cells. Furthermore, the harvest process should be able to be completed without breaking the sterile barrier of the fluid path of the culture chamber.
D. Optimization of Key Culture Parameters by Design
With any large volume cell culture, several fundamental parameters require almost constant control. Cultures must be provided with the medium that allows for normal metabolic functions and growth, and typically this medium is delivered to the cell by a pumping mechanism (e.g., in a bioreactor) or by a technician manually feeding or exchanging the medium on a regular basis. As an additional part of this exchange process, culture byproducts are also removed from the culture.
Growing cells or tissue also requires a source of oxygen. Different cell types have different oxygen requirements, and all cell cultures have differing oxygen delivery requirements depending on the density of the culture. Accordingly, a controllable and flexible means for providing oxygen to the cells is a necessary component of the culture system.
Even distribution of the cell population and medium supply in the culture chamber is an important process control. This control is often achieved by use of a suspension culture design, which can be effective where cell-to-cell interactions are not as important as cell-to-medium interactions. Examples of suspension culture systems include various tank reactor designs and gas-permeable plastic bags. Aside from mature blood cells, such as T-cells, such designs are often deleterious as they impede the development of three dimensional structure in tissue. The growth of bone marrow stem cells is precluded in environments favoring single cell suspensions, because stem cells appear to desire contact with stromal and other accessory cells in order to replicate.
E. Status Feedback and Production Record Capability
The essence of "good manufacturing practice" is: 1) equipment and facilities that are capable of reliably providing the desired product, 2) process control that through validation demonstrate an ability to produce product within desired specifications, and 3) final product and process documentation controls that prevent mix-ups or inappropriate release of product before complete review of test and production records is conducted and accepted.
In a purely manual manufacturing environment, this is accomplished by selecting well-qualified personnel, providing them with extensive training, and developing a system of standard operating procedures and extensive documentation that provides for quality assurance. In an automated manufacturing environment, the principles of process validation are used to demonstrate that the process is stable and capable of meeting specifications. The principles of statistical process control are then implemented to monitor the process to assure consistent conformance to specifications.
In today's environment, as cell culture becomes prominent in clinical care, much of the process control and record keeping will depend upon trained operators, maintaining detailed records. The future lies in the availability of equipment and materials that are designed and manufactured for the intended purpose and automation that allows the process to be validated and controlled. Only under these conditions can cellular therapy be delivered cost effectively to the market.
3. Cell Culture Devices and Procedures
In nature, tissue function and viability depends upon the life support process that is mediated by the vascular system. Nutrients, physiological salts, and oxygen are all brought to the tissue through arteries. The waste products produced by the tissue, which can often be toxic to the tissue, are carried away by the veins. The other major component of tissue maintenance and repair is cellular--the pool of progenitor or stem cells that can replace cells lost or damaged.
Accordingly, for tissue to be developed ex vivo, these same elements are required to be managed by the culture devices and procedures. In other words, the stem/progenitor cells needed to expand the cellular component of the tissue must be maintained in a physical and biological environment that is biocompatible and provides a means to control delivery of nutrients and oxygen to the cells, and carry away waste or other byproducts of the growing cell populations.
A. Traditional Cell Culture Processes
Over the past decade, increasing numbers of medical researchers have sought to develop treatments based in part on the culture of human blood cells, including lymphocytes, monocytes, neutrophil precursors, and immature blood cells including stem and progenitor cells. The evolution of technologies adapted or developed to meet these needs provides an excellent demonstration of the need for clinical systems for the production of human cells for therapy.
1. Laboratory Environment
Traditional cell culture technologies depend upon controlled environments for cell handling. Cell culture laboratories incorporate such features as laminar flow hoods controlled access to the laboratory by gowned personnel, and regular sterilization procedures to decontaminate laboratory surfaces. Personnel require extensive training to practice sterile technique, to avoid contamination of open containers and cell transfer devices by contact with non-sterile materials. In spite of these prophylactic measures, outbreaks of contamination in traditional cell culture laboratories, e.g., fungus contamination, is a common occurrence, often with the impact of halting operations for days or weeks while the source of the contamination is determined and resolved.
Traditional cell culture technologies further depend upon incubation in an environment providing controlled gas mixtures and controlled temperature, usually satisfied by the use of commercial incubators ranging in size from large benchtop units to large floor-standing units.
Therapeutic requirements for numbers of blood cells (typically 10 to 100 billion per patient treatment) and limitations in maximum cell culture density (typically one billion per liter of medium), together with space requirements for major laboratory hardware (e.g., hoods, incubators, refrigerators) and personnel activity, have resulted in considerable laboratory space requirements per patient therapy. Laboratory support operations, including preparation of media and the practice of various assays expand these space requirements and associated capital investments and labor costs. Use of traditional cell culture technology for patient therapy thus results in relatively high costs per patient treatment.
Such a laboratory environment is not conducive to the reliable and routine production of large numbers of cells for patient therapy, given its reliance on manual, highly skilled technique. Achieving "good manufacturing practices" in such an environment is a daunting challenge, requiring the development and adherence to massive volumes of standard operating procedures to eliminate inherent variability in laboratory practices.
2. Tissue Culture Flasks and Roller Bottles
The earliest cell cultures were achieved in glass petri dishes, which were largely supplanted by pre-sterilized plastic tissue culture flasks in the 1960's and 1970's. Early attempts at large scale culture of human cells for therapy in the mid-1980's involved the use of numerous glass roller bottles in a room-sized mechanized facility. Even today, most cell therapies have their genesis in plastic tissue culture flasks, and process scale-up involves use of more, larger so-called T-flasks, which are fed manually in a laminar flow hood.
Manufacturers of tissue culture media used in human therapy have gradually moved away from glass bottle packaging to plastic bottles, and most recently have been developing flexible plastic container systems for their media, similar to those used for decades for intravenous (IV) solutions. The use of flexible containers has been driven in part by the desire by a few customers to eliminate open transfer steps for culture media, which can introduce potential contamination.
As described earlier, one preferred objective is to provide a culture process that can deliver medium and oxygenation at uniform and controlled rates that mimic serum perfusion of tissue in vivo. In order to achieve these relatively slow delivery rates, a means of internally oxygenating the cells is often required. This is one requirement that is ideally met using the simplest of cell culture processes--a culture dish. Here the surface of the culture is uniformly exposed to oxygen, and oxygen is available to the cells as needed. A similar situation can be produced in a flatbed bioreactor, with a gas permeable/liquid impermeable membrane a short distance from the cell bed. This allows for a system to have variable medium perfusion rates from stagnant to high, with the oxygenation of the culture remaining constant and uniform.
3. Flexible Tissue Culture Containers
Flexible tissue culture containers, or culture bags, were developed in the mid-1980's, in response to clinicians' desire to perform culture of cells for human therapy in a reproducible and reliable manner across multiple laboratories and institutions. The use of aseptic tubing connections technology, used commonly in the medical device industry (e.g., for blood collection and transfusion containers), rather than conventional sterile technique in laminar flow hoods, reduces the probability of contamination to less than one chance in a thousand per connection. Flexible containers fitted with aseptic connectors were appropriated from blood banking, where blood platelet concentrates were stored for several days in incubators in gas-permeable, liquid-impermeable plastic containers, which permitted bicarbonate pH buffering of the platelets by the carbon dioxide gas in the incubator.
In the late 1980's, extensive trials of various forms of lymphocyte therapy were conducted using such culture bags, with low incidence of contamination. Today, these culture containers continue to find use in experimental cell therapies where oxygen consumption requirements are minimal and non-adherent cells grow satisfactorily in suspension culture. Such containers do not, however, support the growth of human stem cells that require contact with a heterogeneous population of adherent stromal cells. Advantages of culture bags include relative simplicity of use, reduce skill level requirements, and potential use without laminar flow hoods. However, to date, processes utilizing culture bags remain labor- and space-intensive and are limited in their clinical applicability.
4. Bioreactors
Platform-operated culture systems, typically referred to as bioreactors, have been available commercially for many years and employ a variety of types of culture technologies. Of the different bioreactors used for mammalian cell culture, most have been designed to allow for the production of high density cultures of a single cell type. Typical application of these high density systems is to produce as the end-product, a conditioned medium produced by the cells. This is the case, for example, with hybridoma production of monoclonal antibodies and with packaging cell lines for viral vector production. These applications differ from applications where therapeutic end-product is the harvested cells themselves.
These systems have made an important first step towards a usable clinical system. Once set up and running, the systems provide automatically regulated (not necessarily uniform) medium flow, oxygen delivery, and temperature and pH controls, and they allow for production of large numbers of cells. While bioreactors thus provide some economies of labor and minimization of the potential for mid-process contamination, the set-up and harvest procedures involve considerable labor requirements and open processing steps, which require laminar flow hood operation (some bioreactors are sold as large benchtop environmental containment chambers to house the various individual components that must be manually assembled and primed). Further, such bioreactors are optimally designed for use with a homogeneous cell mixture, and not the mixture of cell types that exists with tissues such as bone marrow.
Many bioreactors have a high medium flow rate requirement for operation. The reason of this feature is that the oxygenation mechanism is to oxygenate the medium outside of the growth chamber, immediately before the medium is perfused into the growth chamber. Since a high density culture will quickly deplete the medium of oxygen, the medium must have a short residence time in the chamber, in order to be reoxygenated and recirculated back into the culture chamber. Furthermore, this process results in an absence of uniformity in oxygenation of the growing tissue, since cells proximal to the medium inlet see much higher concentrations of oxygen than do the cells proximal to the medium outlet. This results in the different cells growing in different areas of the bioreactor.
An additional limitation is that many of the bioreactor designs, such as the various three-dimensional matrix-based designs (e.g., hollow fiber cartridges or porous ceramics), can impede the successful recovery of expanded cells and/or tissues, particularly when culture growth is vigorous, and also can limit mid-procedure access to cells for purposes of process monitoring.
The various trade-offs described have limited the utility of these systems and, in general, such bioreactors have not been used for human cell therapy as much as the less automated but often more user-friendly culture bag systems.
It should, therefore, be appreciated that there is a need for a cell production system that can maintain and grow selected biological cells without being subject to the foregoing deficiencies. There is a particular need for such a system that can receive, maintain and grow such cells in a sterile system within a portable cassette without exposing that sterile system to the external environment. The present invention fulfills that need.