Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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CLINICAL TRIALS 87<br />
• Discovery<br />
• Screening<br />
• Preclinical testing<br />
• Phase I (usually healthy volunteers)<br />
• Phase IIa<br />
• Phase IIb<br />
• Phase III (1000–5000 patients)<br />
* Registration<br />
• Phase IV<br />
of practice, namely Good Manufacturing Practice (GMP),<br />
Good Laboratory Practice (GLP) <strong>and</strong> Good <strong>Clinical</strong> Practice<br />
(GCP). Good <strong>Clinical</strong> Practice is an international ethical <strong>and</strong><br />
scientific quality st<strong>and</strong>ard for designing, conducting, recording<br />
<strong>and</strong> reporting trials that involve the participation of<br />
human subjects. The stages of drug development are outlined<br />
in Figure 15.1.<br />
DRUG DISCOVERY, DESIGN AND SYNTHESIS<br />
Whilst r<strong>and</strong>om screening <strong>and</strong> serendipity remain important<br />
in the discovery of new drugs, new knowledge of the role of<br />
receptors, enzymes, ion channels <strong>and</strong> carrier molecules in<br />
both normal physiological processes <strong>and</strong> disease now permits<br />
a more focused approach to drug design. Using advances in<br />
combinatorial chemistry, biotechnology, genomics, high output<br />
screening <strong>and</strong> computer-aided drug design, new drugs<br />
can now be identified more rationally.<br />
PRECLINICAL STUDIES<br />
Proof of principle<br />
Proof of concept<br />
Cost is approximately £500 million, 60% of which is spent in<br />
clinical trials. Time from discovery to registration approximately<br />
10–13 years<br />
Figure 15.1: Stages of drug development.<br />
Early (exploratory)<br />
development<br />
Late (confirmatory)<br />
development<br />
New chemical entities are tested in animals to investigate their<br />
pharmacology, toxicology, pharmacokinetics <strong>and</strong> potential<br />
efficacy in order to select drugs of potential value in humans.<br />
Although there is considerable controversy concerning the<br />
value of some studies performed in animals, human drug development<br />
has an excellent safety record, <strong>and</strong> there is underst<strong>and</strong>able<br />
reluctance on the part of the regulatory authorities<br />
to reduce requirements. At present, the European guidelines<br />
require that the effects of the drug should be assessed in two<br />
mammalian species (one non-rodent) after two weeks of dosing<br />
before a single dose is administered to a human. In addition,<br />
safety pharmacology <strong>and</strong> mutagenicity tests will have been<br />
assessed. Additional <strong>and</strong> longer duration studies are conducted<br />
before product licence approval. The timing, specific tests <strong>and</strong><br />
duration of studies may relate to the proposed human usage<br />
in both the clinical trials <strong>and</strong> eventual indications.<br />
CLINICAL TRIALS<br />
Physicians read clinical papers, review articles <strong>and</strong> pharmaceutical<br />
advertisements describing clinical trial results. Despite<br />
peer review, the incompetent or unscrupulous author can conceal<br />
deficiencies in design <strong>and</strong> possibly publish misleading<br />
data. The major medical journals are well refereed, although<br />
supplements to many medical journals are less rigorously<br />
reviewed for scientific value. An underst<strong>and</strong>ing of the essential<br />
elements of clinical trial design enables a more informed<br />
interpretation of published data.<br />
Assessment of a new treatment by clinical impression is<br />
not adequate. Diseases may resolve or relapse spontaneously,<br />
coincidental factors may confound interpretation, <strong>and</strong> the<br />
power of placebo <strong>and</strong> enthusiastic investigators are a major<br />
influence on subjective response. In order to minimize these<br />
factors <strong>and</strong> eliminate bias, any new treatment should be rigorously<br />
assessed by carefully designed, controlled clinical trials.<br />
All physicians involved in clinical trials must follow the<br />
guidelines of the Declaration of Helsinki <strong>and</strong> subsequent<br />
amendments.<br />
OBJECTIVES<br />
The first step in clinical trial design is to determine the questions<br />
to be addressed. Primary <strong>and</strong> achievable objectives must<br />
be defined. The question may be straightforward. For example,<br />
does treatment A prolong survival in comparison with treatment<br />
B following diagnosis of small-cell carcinoma of the<br />
lung? Survival is a clear <strong>and</strong> objective end-point. Less easily<br />
measured end-points such as quality of life must also be<br />
assessed as objectively as possible. Prespecified subgroups of<br />
patients may be identified <strong>and</strong> differences in response determined.<br />
For example, treatment A may be found to be most<br />
effective in those patients with limited disease at diagnosis,<br />
whereas treatment B may be most effective in those with<br />
widespread disease at diagnosis. Any physician conducting a<br />
clinical trial must not forget that the ultimate objective of all<br />
studies is to benefit patients. The patients’ welfare must be of<br />
paramount importance.<br />
RANDOMIZATION<br />
Patients who agree to enter such a study must be r<strong>and</strong>omized<br />
so that there is an equal likelihood of receiving treatment A or B.<br />
If treatment is not truly r<strong>and</strong>omized, then bias will occur. For<br />
example, the investigator might consider treatment B to be<br />
less well tolerated <strong>and</strong> thus decide to treat particularly frail<br />
patients with treatment A. Multicentre studies are often necessary<br />
in order to recruit adequate numbers of patients, <strong>and</strong> it is<br />
essential to ensure that the treatments are fairly compared. If<br />
treatment A is confined to one centre/hospital <strong>and</strong> treatment<br />
B to another, many factors may affect the outcome of the<br />
study due to differences between the centres, such as interval