Clinical Pharmacology and Therapeutics

CHAPTER 16
CELL-BASED AND RECOMBINANT
DNA THERAPIES
● Gene therapy 94 ● Human stem cell therapy 95
The term ‘biotechnology’ encompasses the application of
advances in our knowledge of cell and molecular biology since
the discovery of DNA to the diagnosis and treatment of disease.
Recent progress in molecular genetics, cell biology and
the human genome has assisted the discovery of the mechanisms
and potential therapies of disease. The identification of
a nucleotide sequence that has a particular function (e.g. production
of a protein), coupled with our ability to insert that
human nucleotide sequence into a bacterial or yeast chromosome
and to extract from those organisms large quantities of
human proteins, has presented a whole array of new opportunities
in medicine. (Human gene sequences have also been
inserted into mice to develop murine models of human disease.)
In 1982, the first recombinant pharmaceutical product,
human recombinant insulin, was marketed. Since then, more
than 100 medicines derived via biotechnology have been
licensed for use in patients, whilst hundreds more are currently
undergoing clinical trials. Successes include hormones,
coagulation factors, enzymes and monoclonal antibodies,
extending the range of useful therapeutic agents from low
molecular weight chemical entities to macromolecules. Once
discovered, some biotechnology products are manufactured
by chemical synthesis rather than by biological processes.
Examples of recombinant products are listed in Table 16.1. In
parallel with these advances, the human genome project is
establishing associations between specific genes and specific
diseases. Detailed medical histories and genetic information
are being collected and collated from large population samples.
This will identify not only who is at risk of a potential disease
and may thus benefit from prophylactic therapy, but also
who may be at risk of particular side effects of certain drugs.
This carries potentially momentous implications for selecting
the right drug for the individual patient – a ‘holy grail’ known
as personalized medicine. Achieving this grail is not imminent.
It is not just the physical presence but, more importantly,
the expression of a gene that is relevant. Often a complex interaction
between many genes and the environment gives rise to
disease. Despite these complexities, the human genome project
linked with products of recombinant DNA technology,
including gene therapy, offers unprecedented opportunities
for the treatment of disease.
Most recombinant proteins are not orally bioavailable, due
to the efficiency of the human digestive system. However, the
ability to use bacteria to modify proteins systematically may
aid the identification of orally bioavailable peptides. Nucleic
acids for gene therapy (see below) are also inactive when
administered by mouth. Drug delivery for such molecules is
very specialized and at present consists mainly of incorporating
the gene in a virus which acts as a vector, delivering the
DNA into the host cell for incorporation into the host genome
and subsequent transcription and translation by the cellular
machinery of the host cell.
Human proteins from transgenic animals and bacteria are
used to treat diseases that are caused by the absence or
impaired function of particular proteins. Before gene cloning
permitted the synthesis of these human proteins in large
quantities, their only source was human tissues or body fluids,
carrying an inherent risk of viral (e.g. hepatitis B and C
and HIV) or prion infections. An example in which protein
replacement is life-saving is the treatment of Gaucher’s disease,
a lysosomal storage disease, which is caused by an
inborn error of metabolism inherited as an autosomal recessive
trait, which results in a deficiency of glucocerebrosidase,
which in turn results in the accumulation of glucosylceramide
in the lysosomes of the reticulo-endothelial system, particularly
the liver, bone marrow and spleen. This may result in
hepatosplenomegaly, anaemia and pathological fractures.
Originally, a modified form of the protein, namely alglucerase,
had to be extracted from human placental tissue. The deficient
enzyme is now produced by recombinant technology.
The production of recombinant factor VIII for the treatment
of haemophilia has eliminated the risk of blood-borne viral
infection. Likewise, the use of human recombinant growth
hormone has eliminated the risk of Creutzfeldt–Jakob disease
that was associated with human growth hormone extracted
from bulked cadaver-derived human pituitaries.
Recombinant technology is used to provide deficient proteins
(Table 16.1) and can also be used to introduce modifications
of human molecules. In the human insulin analogue,
lispro insulin, produced using recombinant technology, the
order of just two amino acids is reversed in one chain of the
insulin molecule, resulting in a shorter duration of action than