Current Topics in Medicinal Chemistry - Volume 13, Issue 12, 2013

Cytochrome P450 17A1 (CYP17A1; also P450c17and P450sccII) is a critically important enzyme in humans that catalyzes the formation of all endogenous androgens. It is an atypical cytochrome P450 enzyme in that it catalyzes two distinct types of substrate oxidation. Through its hydroxylase activity, it catalyzes the 17α-hydroxylation of pregnenolone to 17 α-OH pregnenolone. Subsequently, through its C17,20lyase activity, it can further convert 17α-OH pregnenolone to the androgen dehydroepiandrosterone, which is a precursor to androstenedione, testosterone, and dihydrotestosterone. The importance of androgens in diseases such as prostate cancer has been appreciated for decades and the discovery of extra-testicular formation of androgens has helped clarify the pathology of the disease, especially the castrate- resistant disease. Therefore, specific inhibition of CYP17A1 by therapeutic intervention has been an area of considerable effort in several research laboratories. This basic research has led to the discovery of several promising drug candidates followed by the conduct of several clinical trials. Recently, all these efforts have culminated in the first approval by FDA of an inhibitor of CYP17A1 for the treatment of castrate-resistant prostate cancer. Ongoing clinical trials are now evaluating the agent in earlier stages of prostate cancer and even rare forms of androgen–dependent breast cancer. Accordingly, this review focuses on the biochemistry, chemistry, and clinical inhibitors of CYP17A1.

Owing to the high degree of similarity between aldosterone synthase (CYP11B2) and cortisol synthase (CYP11B1), the design of selective inhibitors of one or the other of these two enzymes was, at one time, thought to be impossible. Through development of novel enzyme screening assays and significant medicinal chemistry efforts, highly potent inhibitors of CYP11B2 have been identified with selectivities approaching 1000-fold between the two enzymes. Many of these molecules also possess selectivity against other steroidogenic cytochromes P450 (e.g. CYP17A1 and CYP19A1) as well as hepatic drug metabolizing P450s. Though not as well developed or explored, inhibitors of CYP11B1, with selectivities approaching 50-fold, have also been identified. The therapeutic benefits of affecting the renin-angiotensin-aldosterone system have been well established with the therapeutically useful angiotensin-converting enzymes inhibitors, angiotensin receptor blockers, and mineralocorticoid receptor antagonists. Data regarding the additional benefits of an aldosterone synthase inhibitor (ASi) are beginning to emerge from animal models and human clinical trials. Despite great promise and much progress, additional challenges still exist in the path towards development of a therapeutically useful ASi.

Retinoic acid (RA), the active metabolite of vitamin A, is an important endogenous signaling molecule regulating cell cycle and maintenance of epithelia. RA isomers are also used as drugs to treat various cancers and dermatological diseases. However, the therapeutic uses of RA isomers are limited due to side effects such as teratogenicity and resistance to treatment emerging mainly from autoinduction of RA metabolism. To improve the therapeutic usefulness of retinoids, RA metabolism blocking agents (RAMBAs) have been developed. These inhibitors generally target the cytochrome P450 (CYP) enzymes because RA clearance is predominantly mediated by P450s. Since the initial identification of inhibitors of RA metabolism, CYP26 enzymes have been characterized as the main enzymes responsible for RA clearance. This makes CYP26 enzymes an attractive target for the development of novel therapeutics for cancer and dermatological conditions. The basic principle of development of CYP26 inhibitors is that endogenous RA concentrations will be increased in the presence of a CYP26 inhibitor, thus, potentiating the activity of endogenous RA in a cell-type specific manner. This will reduce side effects compared to administration of RA and allow for more targeted therapy. In clinical trials, inhibitors of RA metabolism have been effective in treatment of psoriasis and other dermatological conditions as well as in some cancers. However, no CYP26 inhibitor has yet been approved for clinical use. This review summarizes the history of development of RAMBAs, the clinical and preclinical studies with the various structural series and the available knowledge of structure activity relationships of CYP26 inhibitors.

The Cytochrome P450 4 (CYP4) family of enzymes in humans is comprised of thirteen isozymes that typically catalyze the ω-oxidation of endogenous fatty acids and eicosanoids. Several CYP4 enzymes can biosynthesize 20- hydroxyeicosatetraenoic acid, or 20-HETE, an important signaling eicosanoid involved in regulation of vascular tone and kidney reabsorption. Additionally, accumulation of certain fatty acids is a hallmark of the rare genetic disorders, Refsum disease and X-ALD. Therefore, modulation of CYP4 enzyme activity, either by inhibition or induction, is a potential strategy for drug discovery. Here we review the substrate specificities, sites of expression, genetic regulation, and inhibition by exogenous chemicals of the human CYP4 enzymes, and discuss the targeting of CYP4 enzymes in the development of new treatments for hypertension, stroke, certain cancers and the fatty acid-linked orphan diseases.

Cytochrome P450 isozymes (CYPs) from the CYP1 and CYP2 families located primarily in extra-hepatic tissues represent ideal candidates for chemotherapeutic drug development because: 1.) They are usually involved in the metabolism of endogenous substrates that are important for cell homeostasis and growth 2.) The over-expression of certain CYPs has been reported in various malignancies 3.) There has been much clinical success with inhibitors of CYPs involved in hormone synthesis. The most ideal candidates for chemotherapeutic drug development will be discussed in terms of their biological importance and relevant substrates. This review will focus on: 1.) CYP1A1 and CYP1B1 from the CYP1 family because of the dual role these enzymes play in the bioactivation of known carcinogens and endogenous compounds. 2.) The targeting of CYPs in hypoxic environments as a therapeutic strategy. 3.) CYP2J2 and its role in the metabolism of arachidonic acid to epoxyeicosatrienoic acids and angiogenesis will also be examined. While much progress has been made towards understanding the role of CYPs in extrahepatic tissue, future studies focused on the development of selective inhibitors coupled with appropriate delivery systems that would target the tumor micro-environments could lead to significant advancement in chemotherapeutic strategies.

Arachidonic acids are converted to eicosanoid mediators by the distinct enzyme systems: cyclooxygenase, lipoxygenase and cytochrome P450 (CYP) monooxygenase pathways (ω/ω-1-hydroxylases and epoxygenases). CYP2J2, CYP2C8 and CYP2C9 are the predominant epoxygenase isoforms abundantly expressed in the endothelium, myocardium, and kidney in human. The primary epoxidation products by epoxygenases are four regioisomers of cisepoxyeicosatrienoic acids (EETs): 5,6-, 8,9-, 11,12-, and 14,15-EETs. Numerous studies demonstrated that the cardiovascular protective effects of CYP epoxygenases and EETs range from vasodilation, anti-hypertension, pro-angiogenesis, anti-atherosclerosis, and anti-inflammation to anti-injury caused by ischemia-reperfusion. The roles of arachidonic acids and its metabolites in cancer biology have recently attracted great attentions. However, CYP epoxygenase derived EETs and cancer has received little attention. It was demonstrated that CYP epoxygenases and EETs are significantly upregulated in human tumors and promote tumor progression and metastasis. Additionally, specific inhibitors of CYP2J2, derived from terfenadine, exhibit strong anti-tumor activity in vitro and in vivo. It is implicated that CYP2J2 may be a therapeutic target for treating human cancers.

Cytochrome P450 (CYP) bioreactors play a major role in establishing the practical use of this enzyme family in academia and industry. The current demand for enzymatic hydroxylations of unactivated carbons in the parmaceutical industry includes the preparation of drug metabolites and various hydroxylated synthetic precursors as well as the enzyme mediated lead diversification and natural product synthesis, most of which require multigram scale synthesis. To date, the large scale application of CYPs in the synthesis of oxygenated compounds is limited by many challenges. This review describes relevant examples of CYP oxidations and also presents the strategies available to overcome such challenges. At present, P450 catalyzed reactions can only be performed at substrate concentrations ranging from 1-25 mM, unlike other biocatalytic redox reactions like ketone reductases, typically performed at substrate loads greater than 500 mM. The emergence of powerful expression methods and a large number of CYP mutants developed for specific applications holds the promise for future industrial applications. The search for higher volumetric productivities is however a task that needs to be addressed not only through the use of protein engineering as the primary tool but significant emphasis needs to be placed on process development through exploring multiple operating schemes, optimizing reaction media and modifying microbial strains needed for heterologous expression.