**6. Early clinical studies with primary PK endpoints**

Once the SAD and MAD studies have confirmed acceptable safety to proceed further into Phase 1, several types of studies are conducted where PK endpoints are the primary objective, and continued collection of safety data is only secondary. These studies determine the effects of other drugs, diseases, and patient qualities on the PK of the drug in relatively small numbers of subjects, reducing the risk to patients in the Phase 2 and 3 studies, and ultimately informing the marketed use of the drug.

### **6.1. Food effect studies**

One of the most important PK studies for an orally administered (and sometimes with inhaled drugs where some drug is swallowed) is the food-effect study. An experienced clinical pharmacokineticist will say that the absence of a food effect on the rate or extent of a drug's absorption is rare, and looking at many drugs, most have some difference in absorption between fed and fasted states. Food-effect information is typically not clear from nonclinical studies in most programs, as animals are usually fed ad libitum or on a regular schedule in toxicology studies. A food effect can be somewhat predicted for a specific drug, with knowledge of its solubility, its lipophilicity, and pH dependence on ionization and partitioning, but a human study is required to confirm the extent of the food-effect [34]. Since food in the stomach can affect gastric pH and potentially bind to a drug, and food and/or its fat content can affect gastric emptying time, the probability of some effect on absorption is high. A food effects range from very subtle changes in just Tmax or Cmax, to several-fold increases or decreases in overall exposure, to ultimately where a lipophilic drug might be totally unabsorbed without a minimum of dietary fat. PK parameters, especially Cmax, Tmax, and AUC, can characterize this difference with only a single dose of drug, in a crossover study design, where each subject is administered the drug with and without food. Many drug development programs strive to get this information as early as possible to determine the optimal dosing conditions, and is often part of the SAD, MAD, or SAD/MAD study protocol.

The purpose of the food-effect PK study is to determine if a difference occurs, and if this difference is clinically significant. If found to be clinically significant, that is that food decreases absorption enough to make it less effective, or that it increases absorption enough to cause toxicity. If the PK shows a clinically significant food effect, adjustment of the therapeutic dose and/or instructions on how the drug should be administered will be included in the approved drug labeling. The type of drug is also important in this decision, as commonly food may delay Tmax and decrease Cmax, but if only the extent of exposure (AUC) is important for the drug's efficacy, then the food effect might not be clinically relevant.

### **6.2. PK studies in special populations**

It should be noted that the single- and multiple-dose studies are not always run in two separate studies. Depending upon the sponsor, type of drug, its PK qualities, and how much dose-limiting toxicity is expected, these assessments may all be performed under a single study protocol [33]. These studies are termed SAD/MAD studies, and may be designed in two parts, a single-dose and a multiple-dose part to follow when the first is completed or partially completed. Sometimes the study is designed for the sequential groups to get a single dose followed by a washout period where they are monitored, and then the same group will be started on multiple doses at the same level as the first dose for a period of time expected to reach steady state. Safety and or PK is examined for that group, and if deemed safe, then the

Once the SAD and MAD studies have confirmed acceptable safety to proceed further into Phase 1, several types of studies are conducted where PK endpoints are the primary objective, and continued collection of safety data is only secondary. These studies determine the effects of other drugs, diseases, and patient qualities on the PK of the drug in relatively small numbers of subjects, reducing the risk to patients in the Phase 2 and 3 studies, and ultimately

dose is escalated in the next group.

**Figure 5.** Single dose and steady-state pharmacokinetics.

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

informing the marketed use of the drug.

**6. Early clinical studies with primary PK endpoints**

Once the pharmacokinetic behavior of a drug and its initial safety is confirmed in normal healthy volunteers in the early Phase 1 studies, additional Phase 1 studies are performed to determine if PK differs in various special populations [35]. A simple special population study can be used to bridge the entire drug development program of a drug for one population to apply to another population. An example would be a drug developed in Japanese populations that is then intended to also be marketed in the US. Most small molecule drugs are investigated in subjects with hepatic or renal impairment [36, 37], and depending on the drug's intended use, additional studies in elderly, obese, certain racial/ethnic groups, or others are performed. While safety is monitored in these studies, the PK endpoints allow inference of safety and efficacy that has been determined in previous studies. In other words, if age does not appear to affect the PK of a drug, it is well accepted that the previous safety findings will also likely apply, in general, if the drug is used without regard to age. In hepatic and renal impairment, plasma proteins, such as albumin, can be lower than in healthy subjects, so free drug concentrations are often examined to determine if any PK differences are related to the differences in binding, or if increased free drug concentrations might result in any drug effect differences [38]. These studies are often single-dose PK studies in the special group and in healthy volunteers of similar demographics. PK parameters of exposure are key in the between-group comparisons; however, elimination rates and absorption rates are also important.

so is whether the drug might accumulate in specific tissues in an undesired way. Along with the question of how a drug is eliminated, another question is what metabolites are formed and how are they excreted. These questions are answered with PK studies using radiolabeled drug, which is commonly called the 'ADME' or 'Mass Balance' study [41]. These studies usually include only 6–8 healthy males given a single dose and confined to a research clinic until most of the drug-related radioactivity has been recovered in urine and/or feces. An easily measurable dose of the study drug is administered along with a small amount of the drug that is radio-

and radioactivity in blood and plasma are measured, and total radioactivity are measured in complete urine and feces collections. These studies can be quite long, as the measurements continue and subjects are confined until only small amounts of radioactivity are excreted in urine/ feces each day. The advantage of measuring radioactivity is that it represents the total amounts of drug-related material in blood/plasma, urine, and feces. The drawback for total radioactivity measurements is that it is non-specific. However, comparing unlabeled unchanged drug levels and total radioactivity levels allows the scientist to gauge the amount of metabolites that are circulating and their collective PK behavior. A second aspect of these studies are the determination of the identity of the metabolites and their quantities in plasma, urine and feces by radio chromatography, also known as metabolic profiling. Since the drug was radiolabeled, different chemical entities resulting from the breakdown of the drug in the body can be identified as they will also be radiolabeled. The amounts of radioactivity recovered in urine and feces are totaled, and summed, for the determination of mass balance, i.e. the amount of radioactivity

The distribution of the drug and its metabolites (as measured by radioactivity) into erythrocytes is another important aspect of the ADME study, and goes along with the question of drug accumulating in tissues. By measuring radioactivity concentrations in whole blood and in plasma, it is possible to determine if the drug-related material binds to or collects in erythrocytes [42].

The above overview has shown that pharmacokinetics is an integral part of drug development, and a critical part of early drug development. At the beginning of Phase 1 in the development program only animal data is available; hence, what is known through pharmacokinetic measurements in those animal studies is applied in designing Phase 1 human studies. This chapter outlined the importance of pharmacokinetic data in drug development overall and in specific types of early clinical studies. The PK measurements in the FIH study provide confirmation of safety at the measured exposures from the tested dose levels. Even if exposures from a given dose change due to food-effects, drug–drug interaction, drug-disease interactions, or use in a special population, safety can be associated and risks assessed by bridging these results to the initial safety or efficacy exposures. Throughout the drug development program, pharmacokinetics is a tool used to link exposure to efficacy and safety, and it assists in the determination of dosages of marketed drugs; for this reason, PK data are an

H). PK of unlabeled drug

Application of Pharmacokinetics in Early Drug Development

http://dx.doi.org/10.5772/intechopen.74189

71

labeled, usually with carbon 14 (<sup>14</sup>C), and sometimes with tritium (<sup>3</sup>

administered is expected to be nearly equal to the radioactivity excreted.

important part of the information provided to clinicians.

**7. Summary**

### **6.3. Drug-Drug interaction studies**

As mentioned previously, nonclinical studies are key in screening a drug for potential drug– drug interactions (DDIs). Once the cytochrome P450 enzymes and transporters for which a new drug is a substrate, an inhibitor, or can induce expression, are identified, clinical studies are performed to confirm, quantitate, and determine clinical significance of any DDI. It would be tedious and cost prohibitive to test every possible DDI, so appropriate probes [39] (other drugs which are known CYP or transporter substrates, inhibitors, or inducers) are chosen to be co-administered and PK measured to determine if a clinically relevant DDI through a specific metabolism or transporter pathway exists [40]. Without PK measurements in this type study, it would take large numbers of subjects to study a drug–drug interaction with safety endpoints only, however with PK, a small number of subjects' exposures to one or both drugs can determine if there is a safety risk by examining previous PK measurements and correlated safety findings. These study designs may differ depending upon the potential interaction, but usually the drug under development is dosed to steady-state at a therapeutic dose. Often these studies are conducted with 12–24 healthy subjects confined to a research clinic, and consist of a fixed treatment sequence for all subjects. An example of a common design would be where the probe drug is administered alone, followed by a washout period, then the multiple doses of the investigational drug are given until steady-state levels are reached, then the probe drug is co-administered. PK of the probe drug is measured to determine if the PK is affected. The sequence could be reversed if the investigational drug is hypothesized to be affected by the probe drug. Two-way DDI designs are also used to determine the two drugs affect each other. The primary PK endpoints are generally Cmax and AUCs to determine if peak or overall exposure differ due to a DDI, but Tmax and elimination rates are helpful in determining if the mechanism is due to decreased metabolism or a change in absorption.

Interpretation of the PK data for these studies is often straightforward; if a DDI increases exposure of a drug, depending upon its safety profile, to a degree that toxicity could develop, or decreases exposure enough that efficacy would be lost, then warnings will be issued in the approved labeling. Occasionally unexpected results in these studies are seen such that relating the results to specific enzymes or transporters becomes difficult, especially when multiple enzymes or unknown transporters are involved. In such instances, the characterization of DDIs helps in the design of Phase 2 and 3 studies where a study protocol excludes patients taking certain medications, and allow the safe investigation in patient populations.

### **6.4. Radiolabeled drug studies**

A concern for a small molecule drug in development is the question of whether the drug is readily removed from the body completely, and how that complete elimination occurs. Less so is whether the drug might accumulate in specific tissues in an undesired way. Along with the question of how a drug is eliminated, another question is what metabolites are formed and how are they excreted. These questions are answered with PK studies using radiolabeled drug, which is commonly called the 'ADME' or 'Mass Balance' study [41]. These studies usually include only 6–8 healthy males given a single dose and confined to a research clinic until most of the drug-related radioactivity has been recovered in urine and/or feces. An easily measurable dose of the study drug is administered along with a small amount of the drug that is radiolabeled, usually with carbon 14 (<sup>14</sup>C), and sometimes with tritium (<sup>3</sup> H). PK of unlabeled drug and radioactivity in blood and plasma are measured, and total radioactivity are measured in complete urine and feces collections. These studies can be quite long, as the measurements continue and subjects are confined until only small amounts of radioactivity are excreted in urine/ feces each day. The advantage of measuring radioactivity is that it represents the total amounts of drug-related material in blood/plasma, urine, and feces. The drawback for total radioactivity measurements is that it is non-specific. However, comparing unlabeled unchanged drug levels and total radioactivity levels allows the scientist to gauge the amount of metabolites that are circulating and their collective PK behavior. A second aspect of these studies are the determination of the identity of the metabolites and their quantities in plasma, urine and feces by radio chromatography, also known as metabolic profiling. Since the drug was radiolabeled, different chemical entities resulting from the breakdown of the drug in the body can be identified as they will also be radiolabeled. The amounts of radioactivity recovered in urine and feces are totaled, and summed, for the determination of mass balance, i.e. the amount of radioactivity administered is expected to be nearly equal to the radioactivity excreted.

The distribution of the drug and its metabolites (as measured by radioactivity) into erythrocytes is another important aspect of the ADME study, and goes along with the question of drug accumulating in tissues. By measuring radioactivity concentrations in whole blood and in plasma, it is possible to determine if the drug-related material binds to or collects in erythrocytes [42].
