Question 14 of 20 Intermediate Concept
Why do biologics use a two-tiered cell bank, and how is each tier tested?
Short answer
A master cell bank is made once from the clone, and working cell banks are made from it as needed, so every production run starts from cells of the same origin and age. Under ICH Q5D, the master bank gets full identity and purity testing, each working bank gets purity and limited identity tests, and stability is shown to the limit of cell age.
Why
ICH Q5D calls the two-tiered system, a master cell bank used to generate working cell banks, the most practical approach to a continued supply of cells. Each production campaign thaws a working bank vial. When the working bank runs low, a new one is made from the master.
This keeps cells consistent for decades and limits how many generations separate the clone from production. Testing follows the tiers. Identity and purity tests are done once for each master bank, purity and limited identity tests once for each working bank, and stability during cultivation once per product.
Purity means freedom from adventitious agents. For animal cells, Q5D asks for bacterial and fungal bioburden testing on individual containers, at 1 percent of the containers but not fewer than two, and mycoplasma testing on both banks. Viral testing follows ICH Q5A(R2).
Stability is tested at two points, cells with minimal subcultivation and cells at or beyond the limit of in vitro cell age for production, using pilot or commercial scale data. That shows the clone still makes the same product after expansion. If a working bank's cells drift, for example in glycosylation, the stability data and the master bank give a reference point.
Losing a master bank is a serious business risk, so banks are usually split across separate storage locations.
A strong answer covers
- Master bank once, working banks from it
- Consistent cell origin and age
- Full identity and purity on master, limited on working
- Bioburden and mycoplasma testing, viral per Q5A(R2)
- Stability to the limit of in vitro cell age
Common mistakes
- Making new working banks from production cells instead of the master bank.
- Skipping purity testing on a new working bank because the master passed.
- Ignoring the limit of in vitro cell age when extending a culture.
The interviewer might ask next
What would you do if a new working cell bank showed lower titer than the previous one?