Pharmacokinetic enhancers or boosting agents are used in combination with a primary therapeutic agent, not for their direct effects on the disease or condition for which they are used, but because they enhance the activity, increase drug levels and/or prolong the half-life of the primary agent. Pharmacokinetic enhancers typically inhibit enzymes such as human cytochrome P450 (CYP 3A) and thereby block the rate-controlling steps in the metabolism of the primary drug or inhibit its inactivation. While pharmacokinetic enhancers are usually well tolerated, they can be problematic and may account for side effects including liver injury of the drug combination. Furthermore, pharmacokinetic enhancers are often used as a fixed combination with the primary agent in one tablet, capsule, powder or solution for injection. For this reason, their role in causing adverse events may be difficult to define unless there are direct comparisons to subjects treated with the primary agent alone.
Pharmacokinetic enhancers in current use can be categorized into three groups: (1) beta-lactamase inhibitors used in combination with beta-lactam antibiotics such as the penicillins and cephalosporins (clavulanate, tazobactam, sulbactam, avibactam); (2) broad-spectrum inhibitors of hepatic drug-metabolizing enzymes such as CYP 3A4 (ritonavir, cobicistat); and (3) inhibitors of drug-specific metabolizing enzymes (carbidopa, cilastatin, tipiracil). Undoubtedly, other categories of pharmacokinetic enhancers will be developed as they can play an essential role in optimizing pharmacologic and therapeutic effects of important medications.
The major pharmacokinetic enhancers in clinical use in the United States are listed in the table below, along with the primary agent(s) with which they are combined, major brand name(s) of the combination, the enzyme activity that they inhibit or enhance, year of approval, and the hepatotoxicity score of the combination. Specific information and references to their hepatotoxicity are given in the sections on the primary agents rather than separately for the pharmacokinetic enhancer.
† The likelihood score is for the combination, as the role of the primary agent vs the pharmacokinetic enhancer cannot always be clearly demarcated. In the case of clavulanic acid, it is usually considered the primary cause of the liver injury rather than amoxicillin or ticarcillin with which it is combined.