**4. Nonclinical pharmacokinetics**

understanding the drug and indexing its exposure, determining the clinical dose, and designing the final marketed dosage form. The PK parameters obtained from non-compartmental analyses are illustrated in **Figure 4**. Cmax, the peak concentration gives researchers a maximum drug exposure and is also dependent on the absorption rate for extravascular administrations, while the time of Cmax, Tmax, is also indicative of the rate of absorption, but one must understand drug elimination is also occurring at this time. The log-linear slope at the end of the concentration-time curve can be used to estimate the terminal elimination rate constant and the terminal elimination half-life, assuming the curve is well characterized and the PK exhibits first-order elimination. Too short of a sampling interval or limitations of the bioanalytical method may result in missing the terminal elimination phase, so in some cases this slope may be more representative of drug distribution. By calculating an area under the concentration-time curve, called AUC, an index of overall exposure is obtained, and this exposure is independent of the shape of the curve, be it the sharp increase of an intravenous injection with a high Cmax, or lower concentrations observed over a longer amount of time after a slow-release oral formulation. From AUC calculations and the terminal elimination rate constant, estimations volume of distribution and clearance, abbreviated as V and CL, for intravenous doses, or after extravascular doses abbreviated V/F and CL/F, unadjusted for the

64 Pharmacokinetics and Adverse Effects of Drugs - Mechanisms and Risks Factors

**Figure 4.** Non-compartmental model parameters. Cmax = peak concentration, Tmax = time of peak concentration, kel = negative terminal slope from ln concentration versus time regression, T1/2 = 0.693/kel (apparent terminal elimination half-life) AUC0-t = Area un the concentration-time curve from 0 to the last quantifiable concentration estimated by the trapezoidal rule, AUC0-∞ = Area under the curve extrapolated to infinity (AUC0-t + Cp(t)/kel, CL/F = Dose/AUC0-∞ (after a single dose), V/F = apparent volume of distribution after an extravascular dose, calculated by CL/F / kel.

bioavailable, F, can be made.

Before an investigational drug is ever administered to a human subject, an immense amount of animal and in vitro data are gathered. For example, tests in cell lines and/or animal models are used to determine the potential of the drug's therapeutic action. Other in vitro tests can screen for safety, such as in hERG (Human ether-a-go-go Related Gene) expressed cells, to determine a drug's potential to interact with the potassium channel, IKR, and cause cardiac arrhythmias [21]. Ultimately, single- and repeat-dose toxicity studies, also called 'toxicokinetic or TK' studies, in rodents and at least 1 non-rodent species are needed to support the investigation of the drug in humans [14]. While these studies are mandated by regulatory agencies, they are also useful in the design of the FIH study for a drug's development program. Depending on the type of drug and its apparent risk, several methods of determining the starting dose based on observed toxicity at dose levels can be used. These methods range from simple adjustment and allometric scaling of the non- observed adverse effect level (NOAEL) in the most sensitive animal species studied, with a safety margin [22]. to complex scaling modeling to predict human exposures from animal data. For particularly risky compounds, the starting dose is sometimes carefully based on the minimum biologically active concentration and its associated dose level, also called the minimum effective dose (MED) [23].

Sometimes detailed PK in animals is available, but generally the PK data from animal studies come from the toxicoketinetic studies. In these studies, the goal is to determine exposure for correlation with toxicity, but qualitative expectations of how the drug will behave in a human are conceived. It would be expected, but not guaranteed, that a quickly absorbed and quickly eliminated drug would also act similarly in humans. Useful predictions of a drug's human PK can be made using computer modeling techniques, called PBPK (physiological based pharmacokinetic modeling) interspecies scaling, which take different species' capacities of absorption, body distribution, and metabolic/excretion into account and simulate PK concentrations based on an analogous human model [21, 24–26].

Nonclinical studies are also important for providing an idea of the mechanism of the drug's metabolism, whether any cytochrome P 450 enzymes are involved, and identification of metabolites which could be important in humans [27]. Metabolites identified in animals that represent 10% of drug circulating material need to be monitored in toxicology studies and later in clinical studies if still a significant metabolite, is disproportionately produced in humans, or if it is biologically active [28]. In vitro experiments with hepatic enzyme preparations and various chemical probes identify which CYP 450 enzymes are potentially active in the metabolism of a drug. Once these are determined, potential drug-drug interaction pathways are realized; this information is then used to design Phase I drug-drug interaction studies to characterize the clinical significance of these possible interactions in. In vitro experiments also provide the identities of drug-transporters which may move drug into or out of various organs in the body. Drug–drug interactions can also be mediated by inhibition or competition within these transporter systems [29].
