Biomedical Engineering Reference
In-Depth Information
recombinant products. At least one monoclonal antibody has been raised which specifi cally binds
only to factor IX which contains pre-bound Ca
2
(i.e. the Ca
2
-dependent conformation of factor
IX). Immobilization of this antibody allowed the development of an immunoaffi nity system in
which factor IX binds to the column in the presence of a Ca
2
-containing buffer. Subsequent elu-
tion is promoted simply by inclusion of a chelating agent (e.g. EDTA) in the elution buffer.
Some 5-25 per cent of individuals suffering from haemophilia A develop anti-factor VIII an-
tibodies, and 3-6 per cent of haemophilia B sufferers develop anti-factor IX antibodies. This
complicates treatment of these conditions and, as mentioned previously, one approach to their
treatment is direct administration of factor VIIa. The therapeutic rationale is that factor VIIa could
directly activate the fi nal common steps of the coagulation cascade, independently of either factor
VIII or IX (Figure 12.1). Factor VIIa forms a complex with tissue factor that, in the presence of
phospholipids and Ca
2
, activates factor X.
A recombinant form of factor VIIa (called 'NovoSeven' or 'eptacog alfa-activated') is marketed
by Novo-Nordisk (Table 12.2). The recombinant molecule is produced in a BHK cell line, and the
fi nal product differs only slightly (in its glycosylation pattern only) from the native molecule.
Purifi cation entails use of an immunoaffi nity column containing immobilized murine anti-
factor VII antibody. It is initially produced as an unactivated, single-chain 406 amino acid
polypeptide, which is subsequently proteolytically converted into the two-chain active factor VIIa
complex. After sterilization by fi ltration, the fi nal product is aseptically fi lled into its fi nal product
containers, and freeze-dried.
A (very rare) genetic defi ciency in the production of factor XIII also results in impaired clotting
effi cacy in affected persons. In this case, covalent links that normally characterize transformation
of a soft clot into a hard clot are not formed. Factor XIII preparations, partially purifi ed from hu-
man blood, are used to treat individuals with this condition; to date, no recombinant version of the
product has been commercialized.
12.3 Anticoagulants
Although blood clot formation is essential to maintaining haemostasis, inappropriate clotting can
give rise to serious, sometimes fatal medical conditions. The formation of a blood clot (a throm-
bus) often occurs inappropriately within diseased blood vessels. This partially or completely ob-
structs the fl ow of blood (and hence oxygen) to the tissues normally served by that blood vessel.
Thrombus formation in a coronary artery (the arteries that supply the heart muscle itself with
oxygen and nutrients) is termed coronary thrombosis. This results in a heart attack, characterized
by the death (infarction) of oxygen-deprived heart muscle; hence the term myocardial infarction.
The development of a thrombus in a vessel supplying blood to the brain can result in development
of a stroke. In addition, a thrombus (or part thereof) that has formed at a particular site in the vas-
cular system may become detached. After travelling through the blood, this may lodge in another
blood vessel, obstructing blood fl ow at that point. This process, which can also give rise to heart
attacks or strokes, is termed embolism.
Anticoagulants are substances that can prevent blood from clotting and, hence, are of thera-
peutic use in cases where a high risk of coagulation is diagnosed. They are often administered
to patients with coronary heart disease and to patients who have experienced a heart attack or
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