# Impact of Cytochrome P450 Enzymes on the Phase I Metabolism of Drugs
## 1 Introduction  
Cytochrome P450 enzymes (CYPs or P450s) represent a superfamily of heme-containing proteins, predominantly present within the endoplasmic reticulum of hepatocytes, responsible for the oxidative metabolism of a broad range of endogenous substances and xenobiotics. Many members of the CYP class are currently known, and the numbers continue to grow as more genomes are sequenced. Until now, 57 human CYP isoforms have been recognized [1] and progresses in the development of molecular probes for human CYPs have been recently reviewed [2]. CYPs are present in many tissues, such as the liver, intestine, lung, heart, and brain, and catalyze the phase I metabolism of conventional drugs (Figure 1) [3, 4]. Indeed, drug metabolism involves the (bio)chemical modification of a drug that may occur in the body [5] and plays a crucial role in pharmaceutical development and clinical practice, and contributes to toxicology, carcinogenesis and endocrinology [6]. Drug metabolism consists mainly of two phases, phase I and phase II [7, 8]. Phase I metabolism reactions generally do not determine a significant modification in the molecular weight or aqueous solubility of the substrate. At this stage, a functional group, generally a polar group, such as -OH, -SH or NH\({}_{2}\), is unmasked or introduced into the parent molecule, thereby rendering it accessible to further metabolism. Phase II metabolism occurs directly on the parent compounds that incorporate specific structural motifs, or, more often, on the functional groups added or exposed by phase I oxidation. Phase II conjugation generally results ina significant enhancement in molecular weight and aqueous solubility, thus favoring the process of elimination. Most common CYPs are represented by monooxygenases, which bind and reductively activate molecular oxygen, O\({}_{2}\), splitting it into its two atoms, one of which is then inserted into the substrate RH that is bound in the P450 active site, contiguous to the heme cofactor. Thus, they are called monooxygenases as they use one oxygen atom to obtain the product ROH and reduce the second oxygen atom to water, following the general reaction:  
\[\text{RH}+\text{O}_{2}+2\text{e}^{-}+2\text{H}^{+}\rightarrow\text{ROH}+\text {H}_{2}\text{O}\]  
CYP monooxygenases play a pivotal role in drug pharmacokinetics and are frequently involved in drug-drug interactions [9]. CYPs metabolize many xenobiotics including several environmental carcinogens. Particularly, among the 57 known human CYPs, 12 metabolize 90% of xenobiotics [10] and 6 CYP isoenzymes are able to metabolize the most common drugs used in therapy, namely CYP3A4, CYP3A5, CYP2D6, CYP2C9, CYP2C19 and CYP1A2. CYPs are often upregulated by their own substrates, leading to improved metabolism and plasma concentrations of the concurrent drugs. On the contrary, CYP inhibition by several drugs provides high therapeutic drug levels and drug-induced toxicity, preventing CYPs from fulfilling their protective role in detoxification [11]. Moreover, metabolic reactions may be often directly related to food, with some foods exerting their activity by interacting with CYPs. For instance, St. John's worth deeply induces CYP3A4[12] and, therefore, it lowers drug bioavailability and exposure. Pomegranate (_Punica grantum_ L.) juice, grapefruit and cranberries effectively inhibit both CYP3A4 and/or CYP2C9 [13; 14; 15]. Moreover, interindividual variability in CYP-mediated drug metabolism may exist, with some isoforms, including CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP3A4/5, being highly polymorphic [16]. The influence of CYPs on drug toxicity may be mainly exerted by either reducing the exposure to the parent compound or by transforming the drug into a toxic compound [17]. Several factors involved in this mechanism are repre  
Figure 1: A cartoon depicting the main intracellular biotransformations in the xenobiotic metabolism. sented by enzyme induction or inhibition, reversible or irreversible, and pharmacogenetics.