Sources and upstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 120

• the genetics of many potential systems remain poorly characterized, leading to ineffi cient

transformation systems and low expression levels.

CaroRX and Merispace are (at the time of writing) two of the lead plant-produced biopharmaceu-
ticals. Both are in phase II clinical trials. Neither is destined for parenteral administration. CaroRX is
a recombinant antibody that targets Streptococcus mutans , a major causative agent of bacterial tooth
decay. Binding prevents bacterial adherence to teeth, and the product is being developed for regular
topical administration. Merispace is a recombinant mammalian gastric lipase enzyme produced in
transgenic corn. It is intended to be used orally to counteract lipid malabsorption relating to exocrine
pancreatic insuffi ciency caused by conditions such as cystic fi brosis and chronic pancreatitis.
In order to overcome environmental concerns in particular, some companies are investigat-
ing the use of engineered plant cell lines as opposed to intact transgenic plants in the context of
biopharmaceutical production. One company (DowAgroSciences) gained approval in 2006 for a
veterinary subunit vaccine against Newcastle disease in poultry produced by such means.

5.2.3.5 Insect cell-based systems

A wide range of proteins have been produced at laboratory scale in recombinant insect cell culture
systems. The approach generally entails the infection of cultured insect cells with an engineered
baculovirus (viral family that naturally infect insects) carrying the gene coding for the desired
protein placed under the infl uence of a powerful viral promoter. Amongst the systems most com-
monly employed are:

• the silkworm virus Bombyx mori nuclear polyhedrovirus (BmNPV) in conjunction with cul-

tured silkworm cells (i.e. Bombyx mori cells);

• the virus Autographa californica nuclear polyhedrovirus (AcNPV), in conjunction with cul-

tured armyworm cells ( Spodoptera frugiperda cells).

Baculovirus/insect cell-based systems are cited as having a number of advantages, including:

• High-level intracellular recombinant protein expression. The use of powerful viral promoters,

such as promoters derived from the viral polyhedrin or P10 genes, can drive recombinant pro-
tein expression levels to 30-50 per cent of total intracellular protein.

• Insect cells can be cultured more rapidly and using less expensive media compared with mam-

malian cell lines.

• Human pathogens (e.g. HIV) do not generally infect insect cell lines.

However, a number of disadvantages are also associated with this production system, including:

• Targeted extracellular recombinant production generally results in low-level extracellular accu-

mulation of the desired protein (often in the milligram per litre range). Extracellular production
simplifi es subsequent downstream processing, as discussed later in this chapter.