Qualifying Replenishment Working Cell Banks

Cell banks represent the fundamental starting substrates for biological drug substance/drug substance intermediate (DS/DSi) manufacturing. Therefore, the availability of well-characterized cell banks is imperative to ensure an uninterrupted drug product supply. To this end, two-tier cell banking systems are developed, consisting of master cell banks (MCBs) and working cell banks (WCBs).

Typically, WCBs are derived from an MCB. Although WCB inventories (e.g., vials) are generally developed in plentiful supply, it is sometimes necessary to replenish WCB inventories to meet product demand. Recent experience shows that regulatory requirements for introducing replenishment WCBs are highly variable in many countries.

Using the collective experience of BioPhorum’s “Making and Moving Operational Cell Banks” workstream, this article examines the various scenarios under which replenishment WCBs are manufactured. It assesses the likely impact on product quality from these variations and proposes an appropriate qualification and regulatory filing strategy in each case. The goal is to ensure a consistent cross-industry expectation for an acceptable replenishment WCB qualification strategy using a risk-based scientific approach.

As the product of a replenishment WCB is a viable cell mass, the standard assessment for cell culture comparability includes growth and viability within the thaw passage. Where no changes or minor changes are proposed for subsequent WCBs, growth and viability should be assessed; however, no formal studies assessing product quality are required. For moderate changes, the impact on the cell bank attributes and product quality can be determined using data from a representative small-scale model. [Editor’s Note: Check out BioPhorum’s article series on small-scale models in biopharma here.] However, if replenishment WCBs are generated that result in the WCB going beyond the filed limit of in vitro cell age (LIVCA) or if MCB clonality and/or stability is in question, commercial-scale process confirmation may be needed.

Cell Banks As A Critical Starting Material

Cell lines, referred to as cell substrates, used to manufacture biologics can be of mammalian, yeast, insect, or bacterial origin. They are derived using recombinant DNA technology or through the propagation of native microbial strains. Mammalian and microbial/bacterial cell substrates represent the input into the cell bank system, which is the fundamental starting material for the DS/DSi manufacturing process.

To provide a consistent cell source to initiate the manufacturing process, cell banks must be established and maintained according to strict quality, manufacturing, and regulatory standards. They represent a critical starting material for manufacturing biotechnological DS/DSi and drug product and ensure a consistent cell source is available for the product’s lifetime. [Editor’s Note: Check out BioPhorum’s recent article on how to test master cell banks as a starting material for gene therapies here.]

The two-tiered cell banking system is the best approach for the continued manufacture of a product. This is inclusive of a single MCB, derived from an initial clone or preliminary cell bank, and a subsequent WCB or multiple WCBs derived from the MCB.

Regulatory Guidance For Cell Bank Registration

Overarching regulatory guidance for cell banks is described in ICH Q5D, which outlines recommendations for preparing and characterizing cell banks for production. It also describes requirements for preparing the cell substrate and any parental cell line that contributes to the cell substrate manufacture.

It is expected that an MCB is generated from a clonal population of (mammalian) cells derived from a single cell or a single well-isolated (bacterial) colony and can be used to support early clinical material supply. As a biologic product moves through early clinical development and into late-phase development, a WCB is established from the MCB to support late-phase development through to commercial supply. The cell bank system is qualified by characterization to demonstrate the cell banks are free of detectable non-host contaminants and viral adventitious agents and to confirm cell line identity.

Genetic stability of the cell bank system must also be demonstrated to de-risk the possibility of genetic drift, which may result in a loss of genotypic/phenotypic stability of the cell substrate and could result in changes to product quality.

Demonstrating genetic stability for cell lines containing recombinant DNA expression constructs includes an analysis of the product transgene coding sequences in the MCB. For complex natural products, direct characterization of the product coding sequences may not be necessary. Cell substrate stability in these cases may be assessed by alternate means, such as morphological characteristics.

Replenishing WCBs

The two-tier cell bank structure assures an adequate supply of equivalent, well-characterized cells for DS/DSi production over the product’s expected lifetime. This can span decades; therefore, WCBs must be replenished to support continued product demand. The manufacturer is responsible for a strategy to provide a continued supply of cells from their cell bank.

Regulatory Considerations When Replenishing A WCB

The path to global approval of replenishment WCBs is not clearly defined and may depend on the regulatory strategy the manufacturer pursues during its initial marketing authorization application. Generally, any proposed changes impact the filing strategy employed in major markets. However, neither the U.S. nor EU regulatory guidance provides specific details for data requirements that apply when companies need to manufacture a replenishment WCB.

For example, in the U.S. and the EU, introducing a replenishment WCB would be classified as a change requiring a regulatory action before implementation. Following the guidance of ICH Q12, another option is to submit a post-approval change management protocol. This specifies the data to be collected to support the introduction of a replenishment WCB. Introducing a replenishment WCB using the conditions described in an approved qualification protocol results in a lower reporting category.

Each company is responsible for proposing the data package to support the introduction of a replenishment WCB, which may be further negotiated with the health authority (HA)/agency before or during the review process. A company could propose various approaches, e.g., cell line testing combined with small-scale studies. Cell bank growth and viability assessments, and possibly small-scale product quality assessments, using appropriate small-scale models are sufficient in most cases.

HA regulatory requirements for qualifying replenishment WCBs have recently become more extensive. Yet, guidance is also becoming inconsistent among HAs/agencies worldwide. As a result, uncertainties regarding HA expectations may result in delays to new WCB approvals and the potential delay or interrupted supply of product to patients.

Our Experience With HAs/Agencies

Historically, approved data packages to implement replenishment WCBs contained culture performance data for measuring key performance indicators (e.g., viability and growth) and genetic characterization to demonstrate genetic stability. WCB testing for sterility, adventitious agents (safety), and identity have also been conducted.

Our experience is that HAs have begun requiring at least one commercial-scale lot of DS/DSi to support the comparability of replenishment WCBs. This exercise often includes release testing, extended characterization testing, and stability studies. These are the same requirements needed to support the post-approval addition of a new DS/DSi manufacturing process or even a new DS/DSi manufacturing site. Collecting this information may be resource-intensive and of limited value.

The WCB represents only one part of the entire DS manufacturing process. Therefore, the initial marketing authorization control strategy should be sufficient to ensure process consistency, product quality, and safety. It would be beneficial to understand regulators’ concerns and considerations for why these comparability exercises are required.

Feedback from HAs has been variable. Consistent regulatory guidance and global alignment on regulatory requirements for manufacturing replenishment WCBs would streamline the post-approval process and help prevent delays that can lead to drug shortages and supply chain constraints.

Proposed Approaches To Qualifying Replenishment WCBs

We have developed a set of classed scenarios to qualify replenishment WCBs, based on the potential risk to cell bank growth, viability performance, and product quality. Taking a quality risk management approach, and given the lack of technical drivers and the possible risk to continuity of product supply, we make the following recommendations:

• When no changes are made to the cell banking process, equipment, or facility location, the replenishment WCB can be implemented with minor regulatory actions if it meets the originally filed specifications. We are defining this as a Class 1 approach.
• When analytical methods are changed, execute a risk assessment to evaluate if methods are equivalent and if the replenishment WCB can be implemented with minor regulatory actions if it meets the originally filed specifications. We are defining this as a Class 1a approach.
• When minor changes to the process, equipment, or facility location are made, implement an appropriate change control and risk assessment process. Formal studies assessing cell bank viability upon thaw and/or growth rate data should be executed, and data evaluated against defined cell bank acceptance criteria. The replenishment WCB can be implemented with minor regulatory actions if it meets the originally filed specifications. We are defining this as a Class 2 approach.
• When raw material changes are introduced, execute an appropriate change control and risk assessment process using a representative small-scale model. The replenishment WCB can be implemented with minor regulatory actions if it meets the originally filed specifications. We are defining this as a Class 3 approach.
• When the limit of in vitro cell age (LIVCA) is impacted or MCB clonality is in question, execute an appropriate change control and risk assessment process and commercial-scale process and product quality confirmation. In addition to commercial-scale confirmation, these changes require major regulatory actions and HA approvals. We are defining this as a Class 4 approach.

Harmonization or convergence toward a risk-based categorization of post-approval changes is encouraged as an important step toward achieving the objectives of ICH Q12. Such a system provides inherent, valuable flexibility in the regulatory approach and a framework that can support lower regulatory submission requirements.

This article is a summary of a recent BioPhorum publication on the topic. To read more, check out the full report, A science-based approach to qualifying replenishment working cell banks, an industry view.