Recombinant blood products and therapeutic enzymes - Pharmaceutical Biotechnology: Concepts and Aapplications - page 360

• The liver, in which bile ducts can become clogged.

• The small intestine, which can become obstructed by mucus mixed with digesta.

These clinical features are dominated by those associated with the respiratory tract. The physi-
ological changes induced in the lung of cystic fi brosis sufferers render this tissue susceptible to
frequent and recurrent microbial infection, particularly by Pseudomonas species. The presence of
microorganisms in the lung attracts immune elements, particularly phagocytic neutrophils. These
begin to ingest the microorganisms, and large quantities of DNA are released from damaged
microbes and neutrophils at the site of infection. High molecular mass DNA is itself extremely
viscous and increases substantially the viscosity of the respiratory mucus.
The genetic basis of this disease was underlined by the fi nding of a putative cystic fi brosis gene
in 1989. Specifi c mutations in this gene, which resides on human chromosome 7, were linked
to the development of cystic fi brosis, and the gene is expressed largely by cells present in sweat
glands, the lung, pancreas, intestine and reproductive tract.
Some 70 per cent of all cystic fi brosis patients exhibit a specifi c three-base-pair deletion in the
gene, which results in the loss of a single amino acid (phenylalanine 508) from its fi nal polypep-
tide product. Other cystic fi brosis patients display various other mutations in the same gene.
The gene product is termed cystic fi brosis transmembrane conductance regulator (CFTR), and it
codes for a chloride ion channel. It may also carry out additional (as yet undetermined) functions.
Although therapeutic approaches based upon gene therapy (Chapter 14) may well one day cure
cystic fi brosis, current therapeutic intervention focuses upon alleviating cystic fi brosis symptoms,
particularly those relating to respiratory function. Improved patient care has increased life ex-
pectancy of cystic fi brosis patients to well into their 30s. The major elements of cystic fi brosis
management include:

• chest percussion (physically pounding on the chest) in order to help dislodge respiratory tract

mucus, rendering the patient better able to expel it;

• antibiotic administration, to control respiratory and other infections;

• pancreatic enzyme replacement;

• attention to nutritional status.

The relatively recent innovation in cystic fi brosis therapy is the use of DNase to reduce the viscosity of
respiratory mucus. Scientists had been aware since the 1950s that free DNA concentrations in the lung of
cystic fi brosis sufferers were extremely high (3-14 mg ml 1 ). They realized that this could contribute to
the mucus viscosity. Pioneering experiments, entailing inhalation of DNase-enriched extracts of bovine
pancrease, were undertaken, but both product safety and effi cacy were called into question. The ob-
served toxicity was probably due to trypsin, or other contaminants, which were damaging to the underly-
ing lung tissue. The host immune system was also probably neutralizing much of the bovine DNase.
The advent of genetic engineering and improvements in chromatographic methodology facili-
tated the production of highly purifi ed recombinant human DNase (rhDNase) preparations. Initial
in vitro studies proved encouraging: incubation of the enzyme with sputum derived from a cystic