Antibodies, vaccines and adjuvants - Pharmaceutical Biotechnology: Concepts and Aapplications - page 395

Table 13.4 The serum half-life values of some IgG
antibody preparations when administered to humans

Antibody type Serum half-life

Intact human monoclonal 14-21 days

Intact murine monoclonal 30-40 h

Chimaeric antibody 200-250 h

Murine F(ab) 2 fragment 20 h

Murine Fab fragment 2 h

It was hoped that such chimaeric antibodies, when compared with murine antibodies, would
be:

• signifi cantly less immunogenic;

• display a prolonged serum half-life;

• allow activation of various F c -mediated functions.

Reduced immunogenicity was expected, as only a minor part of the chimaeric antibodies is murine
in origin. Furthermore, the HAMA response is normally directed largely at epitopes on the antibody's
F c domains. The variable region appears inherently less immunogenic. In practice, the expected re-
duced immunogenicity was observed. Early clinical trials with chimaerics have shown them to be
generally safe and non-toxic. The rate of immune responses observed after single dose administra-
tion dropped from almost 80 per cent (murine) to in the region of 5 per cent (chimaeric). However,
repeated administration of chimaerics did eventually raise an immune response in most recipients.
When compared with human monoclonals (half-life 14-21 days), murine monoclonals admin-
istered to humans display a relatively short half-life (30-40 h). Chimaerization increased serum
half-life by fi vefold, with typical values of 230 h being recorded (Table 13.4). A prolonged half-life
is desirable if the antibody is to be used therapeutically, as it decreases the required frequency of
product administration. Chimaeric antibodies also allow activation of F c -mediated functions (e.g.
activation of complement, etc.), as this domain displays human sequence.
Although chimaeric antibodies contain an entire murine-derived variable region, it is only the
CDRs within this variable domain that actually dictate antigen specifi city (Box 13.2). A method
of reducing still further the antigenicity of murine antibodies is to 'humanize' them. This entails
transferring the nucleotide sequences coding for the six CDR regions of the murine antibody of
the desired specifi city into a human antibody gene. The resulting hybrid antibody will, obviously,
be entirely human in nature except for the CDRs (Figure 13.9).
Transfer of murine-derived CDR sequences into human antibody framework regions (Box 13.2),
sometimes generates an antibody with greatly reduced antigen binding affi nity. Selected murine frame-
work sequences are often also included in the humanized antibody. This process (known as reshaping the
human antibody) facilitates folding of the CDRs into their true native conformations. This, in turn, nor-
mally restores antibody-antigen-binding affi nity. Over 95 per cent of antibodies are human in sequence.
Clinical trials indicate that such proteins do indeed behave similarly to native human antibodies.
Humanization has overcome many of the major factors that limited the therapeutic effectiveness of
fi rst-generation (murine) monoclonals as therapeutic agents. Several such humanized products have
now gained marketing authorization (Table 13.2), and one such product is featured in Box 13.3.