**1. Introduction**

Loxoprofen (LOX) is an anti-inflammatory prodrug (NSAID) with potential antipyretic and analgesic properties, but its side effects are less defined when compared to other NSAIDs [1–3]. It is one of the most clinically prescribed NSAIDs in Japan, and it is also popular in Eastern Asia, the Middle East, Latin America, and Africa [1]. It is a well-known cyclo-oxygenase (COX1 and COX2) inhibitor that reduces the synthesis of inflammatory agents such as prostaglandins [2,4,5]. It is also widely used for the managemen<sup>t</sup> of pain and inflammation in chronic and transient conditions (e.g., toothache, headache, menstrual cramps, common cold, etc.) [1,2,6].

Although LOX has fewer sides e ffects than other NSAIDs, many adverse e ffects have been reported such as GI disorders (erosive gastritis and bleeding), renal disorders, cardiovascular disorders, headaches, anaphylaxis, and abdominal pain [1,2,6,7]. Moreover, the use of LOX with drugs such as methotrexate, warfarin, aspirin, and valacyclovir is contraindicated [6,8–10]. Generally, the pharmacokinetic (PK) parameters of many drugs are influenced by the inhibition or induction of cytochrome P450 enzymes (CYPs) [11], which can result in various drug interactions, toxicity, and either an increase or decrease in drug activity [12].

Previous studies have extensively investigated the metabolic activities of LOX in various organisms such as rats, mice, monkeys, rabbits, and humans [1,13,14]. However, to the best of our knowledge, there is still a large gap in knowledge regarding the interactions of LOX with other drugs that may cause simple to severe adverse drug interactions. The alcoholic metabolites of LOX (cis-LOX and trans-LOX) are mainly produced by carbonyl reductase (CR), and trans-LOX is the only active metabolite derived from LOX, which LOX is a pharmacologically inactive drug unless it is metabolized to trans-LOX [1,5]. In addition to CR, LOX is also metabolized by CYP450. In our previous study, we reported that the CYP-mediated metabolism of LOX was catalyzed by CYP3A4 and CYP3A5 to form hydroxylated LOX (OH-LOX). We also found that an inactive metabolite of LOX catalyzed by CYP was significantly higher in dexamethasone (DEX)-induced liver microsomes [15].

It is well known that CYP3A4 is most abundant in the liver, and it can catalyze approximately 50% of commercially available drugs [16,17]. Many commercial drugs are known to regulate the activity of CYP3A, and they may be clinically administered with LOX. The CYP3A/LOX metabolic pathway produces an inactive metabolite and competes with another pathway for LOX that produces the active metabolite. If LOX is co-administered with a drug that modulates the activity of CYP3A, there may be possible drug interactions a ffecting the concentration of LOX or the active metabolite of LOX. Thus, we investigated the interaction of LOX with CYP3A by monitoring the e ffects of DEX (a CYP3A inducer) or ketoconazole (KTC, a CYP3A inhibitor) pretreatment on the PK parameters of LOX, cis-LOX, and trans-LOX in an ICR mouse model.

#### **2. Materials and Methods**
