As per the guideline, such interactions are of particular importance to geriatric patients, who are more likely to be using concomitant medications than younger patients, but of course are not limited to this age group. Therefore it is a general principle, not specific to these guidelines, that in cases where the therapeutic range (i.e., a range of toxic to therapeutic doses) of the drug or likely concomitant drugs is narrow, and the likelihood of the concomitant therapy is great, that specific drug-drug interaction studies be considered. The studies needed must be determined case-by-case, but the following are ordinarily recommended:
• Digoxin and oral anticoagulant interaction studies, because so many drugs alter serum concentrations of these drugs, they are widely prescribed in the elderly, and they have narrow therapeutic ranges.
• For drugs that undergo extensive hepatic metabolism, determination of the effects of hepatic-enzyme inducers (e.g., phenobarbital) and inhibitors (e.g., cimetidine).
• For drugs metabolized by cytochrome P-450 enzymes, it is critical to examine the effects of known inhibitors, such as quinidine (for cytochrome P-450 2D6) or ketoconazole and macrolide antibiotics (for drugs metabolized by cytochrome P-450 3A4). There is a rapidly growing list of drugs that can interfere with other drugs via metabolism, and sponsors should remain aware of it.
• Interaction studies with other drugs that are likely to be used with the test drug (unless important interactions have been ruled out by a Pharmacokinetic Screen).
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