Wednesday, April 24, 2013

Viral Inactivation of Cell Culture Media HTST

For all this talk of bioreactor sterility, the vast majority of this contamination talk refers to bacterial contamination.

What about viral contamination?

Viral contamination is mitigated with HTST treatment of cell culture media. This is where you put the cell culture media in continuous flow while subject to a high temperature for a short period time.
  • High-temperature inactivates the virus.
  • Short-time ensures that cell culture media components do not denature.
The best time to put the media in continuous flow is when you pump the media from the media prep tank to the bioreactor, so viral inactivation often happens during this transfer. The HTST unit is essentially:
  • Heater
  • Hold-tube (insulated pipe)
  • Cooler
At large-scale, the first plug of media that goes through the HTST may not meet the specification, so this plug of media cannot be permitted to be delivered to the bioreactor. This plug may be sent to drain or recycled through the HTST unit until the HTST unit reaches steady-state.

Once at steady-state, the remainder of the media is pumped (through a sterile-filter and subsequent sterile pipes) into the bioreactor; when batch volume is reached, the remaining media is sent to drain.

Simple enough, right?

The hard part is when your HTST performance begins to degrade:
  • Perhaps your sterile filter starts clogging
  • Perhaps your heater controller output maxes out
  • Perhaps your media-prep post-use is showing problems
As recently as January of this year, Amgen's Drug Product Development published a paper titled, "Identification and root cause analysis of cell culture media precipitates in the viral deactivation treatment with high-temperature/short-time method."

I haven't read the paper, but my manufacturing sciences consulting experience predicts it to say the following:

The calcium phosphate in the cell culture media becomes insoluble at the high temperatures during the HTST. This calcium phosphate precipitate may collect on the surface of the holding-tubes thereby decreasing the heat-transfer coefficient, sporadically causing the HTST to fail.

This calcium phosphate (sandy white stuff) may also clog up the 0.1 micron sterile filter causing a high delta-pressure across the sterile filter, maybe even diminishing the media flow rate.

The problem is that calcium phosphate stays in solution except when both the temperature is high and the pH is high.

Unfortunately, high-temperature is a requirement of HTST; which means the only solution to preventing calcium phosphate precipitation and the ensuing HTST performance degradation and filter clogging is to run the media through at low pH.

See Our Fix

This is a classic multivariate problem where operating in a different range will solve the problem.

See also:

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