**Kelli Gerth, Sunitha Kodidela, Madeline Mahon, Sanjana Haque, Neha Verma and Santosh Kumar \***

Department of Pharmaceutical sciences, University of Tennessee Health Science Center, 881 Madison Avenue, Memphis, TN 38163, USA; kgerth1@uthsc.edu (K.G.); skodidel@uthsc.edu (S.K.); mmahon1@uthsc.edu (M.M.); shanque8@uthsc.edu (S.H.); neha.verma@smail.astate.edu (N.V.)

**\*** Correspondence: ksantosh@uthsc.edu.com

Received: 1 October 2019; Accepted: 4 December 2019; Published: 7 December 2019

**Abstract:** The cytochrome P450 (CYP) family of enzymes is known to metabolize the majority of xenobiotics. Hepatocytes, powerhouses of CYP enzymes, are where most drugs are metabolized into non-toxic metabolites. Additional tissues/cells such as gut, kidneys, lungs, blood, and brain cells express selective CYP enzymes. Extrahepatic CYP enzymes, especially in kidneys, also metabolize drugs into excretable forms. However, extrahepatic cells express a much lower level of CYPs than hepatocytes. It is possible that the liver secretes CYP enzymes, which circulate via plasma and are eventually delivered to extrahepatic cells (e.g., brain cells). CYP circulation likely occurs via extracellular vesicles (EVs), which carry important biomolecules for delivery to distant cells. Recent studies have revealed an abundance of several CYPs in plasma EVs and other cell-derived EVs, and have demonstrated the role of CYP-containing EVs in xenobiotic-induced toxicity via cell–cell interactions. Thus, it is important to study the mechanism for packaging CYP into EVs, their circulation via plasma, and their role in extrahepatic cells. Future studies could help to find novel EV biomarkers and help to utilize EVs in novel interventions via CYP-containing EV drug delivery. This review mainly covers the abundance of CYPs in plasma EVs and EVs derived from CYP-expressing cells, as well as the potential role of EV CYPs in cell–cell communication and their application with respect to novel biomarkers and therapeutic interventions.

**Keywords:** extracellular vesicles; exosomes; cytochrome P450; extrahepatic tissues; plasma; circulatory CYPs
