Regulatory Considerations

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Evaluation of an NMEs drug-drug interaction potential is an integral part of the regulatory review prior to its market approval [1, 7]. The clinical pharmacology and biopharmaceutic review of an NDA focuses on key questions relevant to the review and integrates information across various studies [46]. For example, in addition to questions addressing how the following intrinsic factors (age, gender, race, weight, height, disease, genetic polymorphism, pregnancy, and organ dysfunction) may influence exposure and/or response, the reviewers also ask questions related to extrinsic factors:

• What extrinsic factors (co-administered drugs, herbal products, diet, smoking, and alcohol use) influence exposure and/or response and what is the impact of differences, if any, in exposure on pharmacodynamics of an NME?

• Based upon what is known about exposure-response relationships and their variability, what dosage regimen adjustments, if any, do you recommend for each of these factors?

Among drug-drug interaction questions, the following may be addressed via in vitro studies:

• Is there an in vitro basis to suspect in vivo drug-drug interaction?

• Is the drug a substrate of CYP enzymes?

• Is the drug an inhibitor and/or an inducer of CYP enzymes?

• Is the drug a substrate and/or an inhibitor of P-glycoprotein transport processes?

• Are there other metabolic/transporter pathways that may be important?

Depending on the answers to the above questions, additional studies may be conducted to fully assess the interaction potential of an NME with other drugs, herbal products, and/or food/juices. Figure 3 illustrates one algorithm in the evaluation of CYP enzyme-based drug-drug interactions of an NME; starting with in vitro evaluations of the metabolic profile and the CYP enzyme-modulating effects of the NME using human enzymes. Based on the outcomes of these in vitro evaluations, which are reviewed along with additional in vivo clearance information, further clinical studies may be conducted (Fig. 3).

The appropriate use of in vitro metabolism and drug interaction information can provide the basis for the design of subsequent in vivo studies, or obviate the need for further in vivo studies, as illustrated in the following two cases. For example, Drug A's effects on various cytochrome P450 enzyme activities have been evaluated with the following probe reactions (phenacetin O-deethylation for CYP1A2; tolbutamide 4'-hydroxylation for CYP2C9, S-mephenytoin 4'-hydroxylation for CYP2C19, bufuralol l'-hydroxylation for CYP2D6 and testosterone 6^-hydroxylation for CYP3A) using human liver microsomes. The data show

FIGURE 3 An algorithm for evaluating drug-drug interactions [21].

that Drug A does not inhibit CYP1A2, CYP2C9, CYP2C19, and CYP2D6 at concentrations 100-fold the mean steady state Cmax level achievable after the administration of the highest proposed clinical dose. Based on this information, no further in vivo studies on Drug A's inhibitory effects on CYP1A2, 2D6, 2C9, and 2C19 will be needed. Drug A inhibits CYP3A. Further analysis indicates the Ki value to be 1/100 of the Cmax level; suggesting Drug A to be a strong CYP3A inhibitor. A follow-up clinical study with oral midazolam administration confirmed its effect on substrates of CYP3A. The focus of the clinical evaluation on CYP3A has provided data useful for risk/benefit evaluation of Drug A and subsequent product labeling. Similarly, Drug B has been evaluated using in vitro methods and shown to have Ki values in the following rank order: CYP1A2=CYP2C9>CYP3A>CYP2C19>CYP2D6. As many of these I/Ki ratios fall within the gray area between "low risk" and "high risk" (21), an in vivo study focused on CYP2D6 was performed. By focusing on the CYP enzyme that appeared to be affected most by Drug B, the lack of interaction from this latter in vivo study would eliminate the need to study Drug B's effects on the other CYP enzymes.


In a proposed revision of physician labeling format and content, significant (or evidence of no) drug-drug interactions would appear in the Highlights section, in addition to having this information in the main body of the labeling [48]. In vitro and in vivo information on the metabolic pathways and metabolites, including contribution of specific enzymes, and known or expected effects of inducers or inhibitors of the pathway, is described in the clinical pharmacology section of the labeling. Any information on pathways or interactions that have been ruled out by in vitro data is also included in this section. Important clinical consequences of this information would be placed in drug interactions, warnings, precautions, boxed warning, contraindications, and dosage and administration sections of the main labeling, as appropriate. Examples of appropriate labeling language are provided in italic below:

[Case 1] In vitro interaction has been studied for the new drug and no interactions have been demonstrated; no in vivo studies have been conducted to confirm or refute the in vitro finding.

In vitro drug interaction studies reveal no inhibition of the metabolism of the new drug by the CYP3A4 inhibitor ketoconazole. No clinical studies have been performed to evaluate this finding. However, based on the in vitro findings, a metabolic interaction with ketoconazole, itraconazole, and other CYP3A4 inhibitors is not anticipated.

Recent examples, such as rosiglitazone (inhibitory effect on CYP enzymes), and sildenafil (inhibitory effects on CYP1A2, 2C9, 2C19, 2D6, 2E1, and 3A4), are listed in Table 2.

[Case 2] Through in vitro investigations, specific enzymes have been identified as metabolizing the test drug, but no in vivo or in vitro drug interaction studies have been conducted.

In vitro drug metabolism studies reveal that the new drug is a substrate of the CYP_enzyme. No in vitro or clinical drug interaction studies have been performed. However, based on the in vitro data, blood concentrations of the new drug are expected to increase in the presence of inhibitors of the CYP_enzyme such as_,_, or.

Recent examples, such as pimozide (substrate of CYP3A, ventricular arrhythmia observed in patients also taking CYP3A inhibitors, macrolide antibiotics) and Ketoconazole are listed in Table 2.

Recently approved product labels have reflected the increased understanding of metabolic pathways and consequences of drug

TABLE 2 Labeling Examples of Metabolism and Drug-Drug Interaction Information o

Drug name (brand name)

Labeling section

Labeling statement



VIAGRA (sildenafil)


ORAP (pimozide)




In vitro data demonstrate that rosiglitazone is predominantly metabolized by Cytochrome P450 (CYP) isoenzyme 2C8, with CYP2C9 contributing as a minor pathway.

Drugs metabolized by cytochrome P450

In vitro drug metabolism studies suggest that rosiglitazone does not inhibit any of the major P450 enzymes at clinically relevant concentrations. In vitro data demonstrate that rosiglitazone is predominantly metabolized by CYP2C8, and to a lesser extent, 2C9.

Effects of other drugs on VIAGRA

In vitro studies: Sildenafil metabolism is principally mediated by the cytochrome P450 (CYP) isoforms 3A4 (major route) and 2C9 (minor route). Therefore, inhibitors of these isoenzymes may reduce sildenafil clearance.

Effects of VIAGRA on other drugs

In vitro studies: Sildenafil is a weak inhibitor of the cytochrome P450 isoforms 1A2, 2C9, 2C19, 2D6, 2E1, and 3A4 (IC50> 150|iM). Given sildenafil peak plasma concentrations of approximately 1 nM after recommended doses, it is unlikely that VIAGRA will alter the clearance of substrates of these isoenzymes.

Metabolism and pharmacokinetics

Pimozide is extensively metabolized, primarily by N-dealkylation in the liver. This metabolism is catalyzed mainly by the cytochrome P450 3A (CYP 3A) enzymatic system and to a lesser extent by cytochrome P450 1A2 (CYP 1A2).

Ventricular arrhythmias have been rarely associated with the use of macrolide antibiotics in patients with prolonged QT intervals, as might be produced by ORAP. Specifically, two sudden deaths have been reported when clarithromycin was added to ongoing pimozide therapy. Furthermore, some evidence suggests that pimozide is metabolized partly by the enzyme system cytochrome P450 3A (CYP 3A). Macrolide antibiotics are inhibitors of CYP 3A, and thus could potentially impede pimozide metabolism. For these reasons, ORAP is contraindicated in patients receiving the macrolide antibiotics clarithromycin, erythromycin, azithromycin, dirithromycin, and troleandomycin.

TABLE 2 Continued.

Drug name (brand name)

Labeling section

Labeling statement


Because azole antifungal agents are also inhibitors of the CYP 3A enzymes and thus may likewise impair pimozlde metabolism, ORAP is contraindicated in patients receiving the azole antifungal agents itraconazole and ketoconazole. Similarly, protease inhibitor drugs are also inhibitors of CYP 3A, and thus ORAP is contraindicated in patients receiving protease inhibitors such as ritonavir, saquinavir, indinavir, and nelfinavir. (See PRECAUTIONS—Drug Interactions.)

Nefazone is a potent inhibitor of CYP 3A, and its concomitant use with ORAP is also contraindicated.

Other drugs that are relatively less potent inhibitors of CYP 3A should also be avoided, in view of the risks, e.g., zileuton.

Coadministration of terfenadine or astemizole with ketoconazole tablets is contraindicated. (See BOX WARNING, WARNINGS and PRECAUTIONS sections.)

Concomitant administration of NIZORAL " Tablets with cisapride is contraindicated. (See BOX WARNING, WARNINGS, and PRECAUTIONS sections.)

Concomitant administration of NIZORAL" Tablets with oral triazolam is contraindicated. (See PRECAUTIONS section.)

interactions by health care practitioners. Newer labels frequently include in vitro parameters evaluating the drug's effect on specific cytochrome P450 metabolism and the clinical consequences of the changes in these enzyme activities have on co-administered drugs. In addition, the labels also include the influence of concomitantly administered drugs on the drug itself. Table 2 lists some examples of the labeling language based on in vitro information. Less frequently included in the labels today are transporter information and metabolic interactions based on other noncytochrome P450 enzymes. As the science progresses and technologies in the evaluation become standard, future labeling should include these other types of information.


As many of the new drugs are to be indicated for patients who receive other drugs or biologies, it is necessary to know the drug interaction potential early on in the development. For compounds eliminated by a single pathway, there is a high probability of drug interaction. The appropriate use of in vitro metabolism (including isozyme characterization) and drug interaction information can provide the basis for the design of confirmatory in vivo studies or obviate the need for further in vivo studies. Further improvement in the in vitro methodologies evaluating other, noncytochrome P450-based metabolilsm/drug interactions and transporterbased interactions should improve our abilities to assess drug-drug interactions for risk/benefit evaluation during drug development and regulatory review.


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