directly access the product levels and quality within the production steps as de-

monstrated by the two studies published recently for monoclonal antibody titer and

glycosylation monitoring [43–45]. Such tools are highly promising if applied to

viral production processes for the in-line detection of total viral particles.

Capacitance spectroscopy aims at monitoring live biomass based on the prin-

ciple of dielectric spectroscopy. Capacitance probes consist of small electrodes

immersed in a cell suspension which will allow for its polarization under specific

frequencies. The polarization occurs in a volume of 1 cm³ around the probe thus

implying a good homogeneity of the suspension. This technology then measures the

charge and discharge of the viable and intact cells which then behave like small

capacitors. Thus, it can detect all the cells present within the cell suspension culture

volume, allowing to use this technology as a cell counting probe or a biomass

quantification tool. This is why such technology was successfully applied to cell

growth and death monitoring of a wide range of cell types used in manufacturing

processes including both suspension cells and adhered cells on suspended micro-

carriers [46–50]. The acquisition time being below seconds allows the integration in

feedback control loops as for fed-batch feeding strategies [51], [52].

Such technology that analyses the specific dielectric properties of cells also al-

lows access to several cell biological properties indirectly. Namely, cell size and

volume, cell membrane thickness and villosities, and cell cytoplasm composition

would impact the dielectric measurement. This is of great interest in the context of

viral production processes. Indeed, it has repeatedly been proven that viral re-

plication within a cell affects their biological properties including cytoplasm

composition with the accumulation of viral particles components (antigens or

capsids) or the membrane structure (effect of viral budding). Thus, this technology

was also exploited to monitor indirectly viral cycle biological events within the

production phase. For several virus and cell pairs, capacitance spectroscopy was

proven to be a valuable tool, allowing for inline monitoring of viral production

phases [47], [53–55].

Process analytical technology tools available for viral production processes are

not yet at the stage to provide information directly on the viral product in-line or

within the production process equipment. Nevertheless, regarding the advances

performed for similar cell-based processes for large-scale recombinant protein

production, we can expect that new tools will be soon available to reach this

stage. Spectroscopic tools are for now some of the most promising analytical

equipment to fulfill such needs but the connection of at-line analytics using au-

tomated and sterile sample loops could allow reaching faster the target proposed

by regulatory agencies, the holy grail being access to a direct quantification of

active viral particles.

REFERENCES

[1] FDA, “Bioanalytical method validation guidance for industry,” 2018.

[2] P. J. Klasse, “Molecular determinants of the ratio of inert to infectious virus par-

ticles,” Prog. Mol. Biol. Transl. Sci., vol. 129, pp. 285–326, 2015.

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Bioprocessing of Viral Vaccines