Downstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 133

Downstream processing

6.1 Introduction

Downstream processing serves to (a) recover the therapeutic protein from its producer cell source
upon completion of the upstream processing phase, (b) purify the protein and (c) formulate the
protein into fi nal product format.
An overview of the steps normally undertaken during downstream processing is presented in
Figure 6.1. Details of the exact steps undertaken during the downstream processing of any specifi c
biopharmaceutical product are usually considered confi dential by the manufacturer. Such details
are thus rarely made generally available. However, a potential downstream processing procedure
for recombinant tPA is presented in Figure 6.2, and other examples are provided at various stages
through the remainder of this text.
Downstream processing is undertaken under clean-room conditions in order to protect the prod-
uct stream from environmental contamination ( Figure 6.3). In addition, the water used as solvent
during downstream processing (and, indeed, often during upstream processing) is highly puri-
fi ed 'water for injections' (WFI). Standard po table (drinkable) water contains contaminants (e.g.
microorganisms, dissolved organic and particulate matter, etc.) that could either react with the
protein directly or that would have an adverse effect upon patient health if present in the fi nal
product. Generation of 'purifi ed water' (often used to make up media for microbial fermentation)
along with even purer WFI, is summarized in Figure 6.4.
All proteins retain their structural integrity and biological activity only over characteristic
pH ranges. Proteins become denatured outside these ranges, losing their characteristic three-
dimensional structure, and hence activity (Chapter 2). Most biopharmaceuticals are s table only
at pH values approaching neutrality (approximately pH 5-8 for many). As such, downstream
processing is carried out using not WFI per se as solvent but in buffer solutions made from WFI.
A buffer is a solution that resists a change in its pH value even with the addition of small amounts
of either acid or alkali, and hence effectively controls the pH environment of the protein.