Pharmacokinetic and Pharmacodynamic Events

Figure 21.2 The exposure-response road map passes through pharmacokinetics and pharmacodynamics. This sequence of events is essentially the same as that which informs compnter simulation of clinical trials, with the addition of complicating, bnt important, factors snch as protocol adherence and dropouts.

Phase III studies represent the confirmatory phase of drug development, which takes several years and usually involves several thousand patients at multiple trial centers. Large patient numbers are required in these trials to provide convincing documentation of clinical efficacy and safety, a more complete adverse event profile and covariates and estimates of variability in dose response relationship due to individual differences in pharmacokinetics and pharmacodynamics. They are aimed at definitively determining a drug s effectiveness and side-effect profile. Most of these studies are double-blind and placebo-controlled, sometimes with the option of open-label long-term extensions. 

Fig. 8. Scheme representing a drug and a biological system acting upon each other through pharmacokinetic events and pharmacodynamic events (Testa, 1987). 

As was noted in Chapter 4, pharmacokinetic and pharmacodynamic effects are studied in nonclinical research. These topics are also of critical importance in clinical investigations. A drug s pharmacokinetics and pharmacodynamics are of considerable interest to clinicians who may prescribe the drug to patients once it is approved. Meaningful decisions about a drug s optimal use can only be made with an understanding of the time course of events that occur after the drug s administration, and both pharmacokinetics and pharmacodynamics are concerned with this time course. By consideration of the pharmacokinetic processes of absorption, distribution, metabolism, and excretion (ADME), the... 

In both pharmacokinetic and pharmacodynamic considerations, an important emphasis concerns the rate at which events occur and the rate at which circumstances change. The pharmacokinetic phase covers the relationship between drug input and the concentration achieved over time. The pharmacodynamic phase covers the relationship between concentration and the therapeutic effect over time (toxicodynamics is concerned with the relationship between concentration and adverse effects over time). 

To evaluate the biological consequences of antibodies, patients with positive responses should be monitored for their impact on the pharmacokinetic and pharmacodynamic effect of the therapeutic protein. Also the effect on adverse events and possible neutralization of the endogenous protein, in cases where human proteins are administered at pharmacological doses, should be monitored. When the consequences of the immunogenicity are known, the risk can be established. 

The endpoints for phase 0 trials are pharmacokinetics, pharmacodynamics, and biomarker responses to the new drug. Phase I trials test for safety along with the pharmacokinetics and pharmacodynamics. In phase II through IV trials, common endpoints are clinical measurements of response, toxicity, time to progression or the progression-free interval, and overall survival. Several cooperative oncology groups have objective criteria to measure the severity of adverse events or serious adverse events. 

Pharmacodynamics is the study of dmg action primarily in terms of dmg stmcture, site of action, and the biochemical and physiological consequences of the dmg action. The availabiUty of a dmg at its site of action is deterrnined by several processes (Fig. 1), including absorption, metaboHsm, distribution, and excretion. These processes constitute the pharmacokinetic aspects of dmg action. The onset, intensity, and duration of dmg action are deterrnined by these factors as well as by the avadabihty of the dmg at its receptor site(s) and the events initiated by receptor activation (see Drug delivery). 

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