Variability and Therapeutic Drug Monitoring

It is important to note that the many variables involved in a drug's effectivity create an enormous potential for interindividual variability in their clinical potency. For example, drug half-life may be a constant in a given individual in a given situation, but absorption and elimination rates vary to a great extent between individuals. Absorption was illustrated in Fig. A.6, presenting the mean values in a group of 18 volunteers. In Fig. A.8, data are presented from a few selected individuals in the same experiment. Note the variability in peak plasma levels, in the time when this peak level occurs, and in plasma levels several hours after administration of the drug. To give another example, benzodiazepines such as diazepam may have unexpectedly large and long-lasting sedative effects in the elderly because the drug's half-life can extend to days due to remarkably reduced biotransformation in some individuals. It is easy to understand that repeated dosing under such circumstances leads to accumulation of the drug that can exceed safe levels. On the other hand, drug levels should not drop below the minimum necessary for a therapeutic effect. Since it is important to maintain adequate drug levels despite the problems created by interindividual variability, plasma levels of the drug are sometimes monitored in therapeutic settings. The major presumption in therapeutic drug monitoring is that plasma levels correlate reasonably well with potency, which, fortunately, is often the case. The variation in drug sensitivity within a population is largely genetic in origin, and a new science of pharmacogenetics has recently emerged to address the many questions that arise in this area.

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