*Review* **Drug–Drug Interactions Involving Intestinal and Hepatic CYP1A Enzymes**

#### **Florian Klomp 1, Christoph Wenzel 2, Marek Drozdzik 3 and Stefan Oswald 1,\***


Received: 9 November 2020; Accepted: 8 December 2020; Published: 11 December 2020

**Abstract:** Cytochrome P450 (CYP) 1A enzymes are considerably expressed in the human intestine and liver and involved in the biotransformation of about 10% of marketed drugs. Despite this doubtless clinical relevance, CYP1A1 and CYP1A2 are still somewhat underestimated in terms of unwanted side effects and drug–drug interactions of their respective substrates. In contrast to this, many frequently prescribed drugs that are subjected to extensive CYP1A-mediated metabolism show a narrow therapeutic index and serious adverse drug reactions. Consequently, those drugs are vulnerable to any kind of inhibition or induction in the expression and function of CYP1A. However, available in vitro data are not necessarily predictive for the occurrence of clinically relevant drug–drug interactions. Thus, this review aims to provide an up-to-date summary on the expression, regulation, function, and drug–drug interactions of CYP1A enzymes in humans.

**Keywords:** cytochrome P450; CYP1A1; CYP1A2; drug–drug interaction; expression; metabolism; regulation
