amount of vector genomes (1E12 viral genomes (vg) per injection), it has been

found that direct injection for treatment of e.g., muscular dystrophy would require

even higher dose inputs (up to 1E14 adeno-associated virus (AAV) vg per kg of

patient bodyweight). With such high-dose requirements, the current state-of-the-art

AAV manufacturing process could provide enough material to treat two patients

with one production run [3]. What the final demand might be, will be evident in the

short future, but still new ways to produce large amounts of viruses will be needed.

Over the last years, many promising bioreactor concepts have been established and

process options are now available to significantly increase the yield in virus particle

production. In addition, not only the production processes might change, but equally

the formulation and the administration of products. In this chapter, process in-

tensification towards higher volumetric virus productivity (VVP), the amount of virus

particles produced considering the total volume of spent medium during cell growth

and virus replication phase and the total process time, is discussed. Many lessons can

be learned from improvements made in recombinant protein production, but likewise

many specific requirements for virus production need to be considered additionally.

Clearly, this is not a complete coverage of all relevant literature, only a selection of

examples highlighting this topic is discussed, mainly from recent studies of our group.

6.2

MOTIVATION FOR PROCESS INTENSIFICATION

Following up on the proposed calculation exercise of chapter 5 to estimate how

many vaccine doses per year can be obtained for a chosen production method and

the respective virus-host cell system, it is obvious that even advanced production

platforms are quickly limited for pandemic scenarios. While implementation of new

technologies (e.g., mRNA vaccines) could mitigate problems arising from the

limited production capacity of viral vectors, the current coronavirus disease 2019

(COVID-19) pandemic revealed that not only good manufacturing practice (GMP)-

production capacities are limited, but also the supply of equipment and consum-

ables, delivery, transport, and the availability of qualified and trained personnel.

Streamlining, organization, resource management, and fair distribution are equally

needed, whereas panic-buying and hoarding policies have to be avoided. For in-

fectious diseases, where cell-specific virus yields (CSVY) are as low as those

identified for SARS-CoV-2 [4] or Zika virus [5] (about 20 infectious virions/cell) or

even lower as for hepatitis C virus (HCV) [6] and no alternatives such as mRNA

vaccines are available yet, process intensification is a must. In the case of gene

therapies, which require a highly specific delivery of genomic material into the

target cell, viral vectors are often the preferred choice. Although several non-viral

gene delivery systems, e.g., DNA or mRNA, have been developed in the last three

decades, efficiency of gene transduction is usually less than in viral systems [7].

Process options and technologies for process intensification described in the

following relate mainly to small-scale experiments performed by academic groups.

While licensing new and innovative processes often involves high costs, in parti-

cular for the case additional clinical studies are required, we are convinced that the

ever-increasing demand for virus particle-based products will force vaccine man-

ufacturers to implement new technologies and consider advanced process options.

Process intensification

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