ABSTRACT
The word xenobiotic, a combination of two Greek words (“xenos” and “bio¯tkós”), literally means “stranger to life” and is referred mostly to an exogenous chemical or substance foreign to the human body or other organisms. Xenobiotics may act as therapeutic agents, toxins, chemical carcinogens, food additives, pollutants, etc. Pharmacokinetics (PK) and toxicokinetics (TK) are the quantitative science of physiological and biochemical processes involved in absorption, distribution, metabolism, and excretion (ADME) of xenobiotics, and their corresponding pharmacological or toxicological response in vivo and in vitro (Figure 1.1). The overlapping goals of both disciplines are to understand the behavior of xenobiotics in the body, how the body handles them, and predict their pharmacological or toxicological response. The end result of PK/TK analysis is establishing appropriate criteria and approaches to avoid undesirable outcomes, and safeguard human or animal life. The desirable outcome can be as practical as developing appropriate dosage forms or a dosage regimen for the purpose of achieving an optimum therapeutic outcome or pharmacological response, establishing regulatory guidelines/policies for consistency of the outcomes through bioequivalence and biosimilar evaluations, or as challenging as low dose extrapolation of chemical carcinogens and the understanding of safe doses in hazardous environmental pollutants.
