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Substantial differences in response to drugs commonly exist among patients. Such between or interindividual variability is often reflected by various marketed dose strengths of a drug. Because variability in response within a subject from one occasion to another (intraindividual variability) is generally smaller than interindividual variability, there is usually little need to subsequently adjust an individual’s dosage regimen, once well-established, unless the condition or treatment of the patient changes. Clearly, if intraindividual variability were large and unpredictable, finding and maintaining dosage for an individual would be an extremely difficult task, particularly for a drug with a low therapeutic index (e.g., warfarin).

Many patients stabilized on one medicine receive another for the treatment of the same or concurrent condition or disease. Sometimes, the second drug affects the response to the first. The change in response may be clinically insignificant for most of the patient population, with the recommendation that no adjustment in dosage be made. However, a few individuals may exhibit an exaggerated response, which could prove fatal unless the dosage of the first drug given to them is reduced. The lesson is clear: Average data are useful as a guide; but ultimately, information pertaining to the individual patient is all-important.

PS: Evidence for interindividual differences in drug response

  • Variability in the dosage required to produce a given response – daily dose of warfarin
  • Variability in pharmacokinetics – phenytoin’s wide scatter in plateau plasma concentration
  • Variability in pharmacodynamics – levels of endogenous agonists or antagonists

Clearly, variability exists in both pharmacokinetics and pharmacodynamics, and measurement of drug in plasma is a prerequisite for separating the two. The characterization of pharmacokinetic and pharmacodynamic variabilities within the population is called population pharmacokinetics and population pharmacodynamics, respectively.

The dependence on dose and time in the assignment of variability is minimized by expressing variability not in terms of observations but rather in terms of the parameter values defining pharmacokinetics and pharmacodynamics, that is, in F, ka, Cl, and V for pharmacokinetics, and in Emax, C50, and the factor defining the steepness of the concentration-response relationship for pharmacodynamics.

Why People Differ

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The reasons why people differ in their responsiveness to a given dose of a drug are manifold and include genetics, disease, age, gender, body weight, drugs given concomitantly, and various behavioral and environmental factors. Age, body weight, disease, and concomitantly administered drugs are important because they are measurable sources of variability that can be taken into account. Gender-linked differences in hormonal balance, body composition, and activity of certain enzymes manifest themselves in differences in both pharmacokinetics and responsiveness, but overall, the effect of gender is small. Although inheritance accounts for a substantial part of the differences in response among individuals for many drugs, much of this variability is still largely unpredictable, particularly in regard to pharmacodynamics.
 
Pharmaceutical formulation and the process used to manufacture a product can be important because both can affect the rate and extent of release, and hence entry, into the body. A well-designed formulation diminishes the degree of variability in the release characteristics of a drug in vivo.
 
Heavy cigarette smoking tends to reduce clinical and toxic effects of some drugs, including theophylline, caffeine, and olanzapine. The drug affected are extensively metabolized by hepatic oxidation catalyzed by CYP1A2; induction of this enzyme is the likely cause.
 
Although on average the body maintains homeostasis, many biological functions and many endogenous substances undergo temporal rhythms. The period of the cycle is often circadian, approximately 24 hr, although there may be both shorter and longer cycles upon which the daily one is superimposed. The menstrual cycle and seasonal variations in the concentrations of some endogenous substances are examples of cycles with a long period. Drug responses and pharmacokinetics may therefore change with time of the day, day of the month, or season of the year.
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