May 1, 2014 (Vol. 34, No. 9)

Engineering Risk Reduction and Patient Safety during Manufacturing

Production of patient-specific cell-based therapies (PSCT) presents unique scale and cost challenges not seen in pharmaceuticals and biologics manufacturing. Rather than scaling up to increase batch size (and thus gain efficiencies), processes supporting PSCT must be “scaled out” to deliver a large number of individual batches. And with each dose representing a separate batch, the necessary requirements for batch records, QC testing, and quality assurance release must be repeated with every patient.

Different approaches are also required to assure sterility of the final product. By definition, the sterility assurance level (SAL) is the probability of a single unit being nonsterile (e.g., viable organisms are present) after the sterilization process. SAL is expressed on a log scale; an SAL of 10-6, for example, indicates a 1 in 1,000,000 probability of a nonsterile unit. Manufacturers of cell therapies are faced with the need to provide a sufficiently high degree of assurance that each batch of a cell therapy product is not contaminated by the environment in which it was processed or by other batches processed previously or in parallel.

In traditional manufacturing processes, terminal sterilization and filter sterilization are used. Terminal sterilization destroys contaminants and can deliver an SAL of 10-6 using validated methods such as steam, gamma irradiation, electron beam, or ethylene oxide. Filter sterilization removes contaminants and, while the SAL is not directly specified, this process should support a product SAL of 10-3 or better.

With cell-based therapies, the cells themselves are the product and, as such, terminal sterilization or filtration cannot be used. Cells cannot withstand terminal methods and are too large to pass through sterilizing-grade filter membranes. In this manufacturing setting, all product contact materials (e.g., containers, tubing, syringes, and reagents) are presterilized using validated terminal or filter sterilization methods. Aseptic processing is used to prevent the ingress of contaminants throughout the manufacturing process. As with filter sterilization, an SAL is not specified but generally should be 10-3 or better.

Open Processing

Aseptic processing can be open or closed. Open processing is typically conducted in a biological safety cabinet (BSC) within a Class 10,000/Grade B cleanroom. An isolator or glove box is sometimes used to reduce cleanroom requirements as well as provide a higher degree of isolation from the operator. In any of these cases, an open process is directly exposed to the immediate surrounding environment.

To prevent cross-contamination between batches, only one batch is processed at a time and the work zone (interior of BSC, isolator, or glove box) must be disinfected (i.e., line clearance) before each subsequent batch.

Isolators and glove boxes are sometimes mischaracterized as closed systems. These approaches do provide greater isolation of the process from the general environment, and most notably humans, but are not closed systems. The surrounding environment for the process is the interior of the isolator/glove box and does not isolate one batch from another. In fact, for an isolator or glove box, the line clearance process is often carried out via a hydrogen peroxide sterilization cycle.

With these constraints, open processing cannot efficiently and robustly support scale out of PSCT in which multiple lots are optimally manufactured in parallel, in a shared environment.

Closed Systems

In a closed system, there is no exposure of the process to the surrounding environment. Closed processing prevents ingress of microbes from the environment and incorporates single-use disposable components for all materials that come into contact with the product. All components and reagents are presterilized by terminal or filter sterilization. This approach enables processing of multiple patient batches in parallel in the same area; processing in a BSC within a high-grade cleanroom (Grade B/Class 10,000) or isolator is not required.

Use of closed aseptic processing can have a significant impact on the bottom line. This approach can deliver improved process robustness, reduced cost of goods due to labor reduction, reduce the amount of gowning materials needed, and lower facility costs (operating costs and capital expense amortization). While the advantages are clear, implementation of a closed system requires careful consideration of all components that may contact the product and precise engineering of the presterilized, single-use flow path. Areas of potential risk for ingress of microbes in a closed system include:

  • Material transfer: No open material transfer such as pipetting or pouring
  • Connections: No open connections such as spike, luer, needle/septum
  • Sliding seals: No unguarded sliding seals such as a syringe plunger
  • Gas/liquid exchange: No direct gas/fluid exchange that does not exclude microbial contaminants

When developing a closed processing system, a number of design considerations can help mitigate the risk of ingress including:

  • Use of sterile barrier filters for introduction of liquid or gas (e.g., venting) that has been exposed to surrounding environment
  • Closed connections (e.g., sterile tube welding, disposable sterile dock connectors) in place of open connections (e.g., spikes, luers, needle/septum)
  • Use of design approaches that avoid moving seals (e.g., pinch valves instead of stop cock valves, skip rope design instead of rotating seal for fluid flow into/out of spinning chamber)
  • Limiting moving seals to rotating seals and elimination of sliding seals (e.g., syringes)
  • Closed cell processing apparatus, e.g., WFEVR (wash/fluid exchange, volume reduction), selection, sorting, culture, transduction

Given these stringent requirements, it can be quite challenging to implement a completely closed process. Complicating the task is the fact that many reagents are only available in formats that cannot be directly interfaced to a closed process. For example, many supplements are supplied in vials that must be withdrawn with a syringe and needle or are only available as solutions in a bag without sterile connection capability.
A viable alternative to a fully closed system for manufacturing cell-based therapies is a hybrid “functionally closed” solution (Figure 1). Closed processing is implemented for the cell journey portion of the process where the risk of cross-contamination exists. Reagent preparation is performed in a separate environment suitable for open processing that includes adaptations for close connectivity into the main, closed portion. Reagents for multiple batches can be processed in parallel with no risk of cross-contamination.

As cell therapies continue to advance toward the clinic, the need to engineer robust, sustainable, and cost-effective manufacturing processes becomes increasingly important. Functionally closed systems enable the high degree of sterility needed to assure patient safety while being practical to implement.

Figure 1. Schematic of a functionally closed processing system.

Brian Hampson ([email protected]) is vice president, engineering and innovation, PCT.

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