1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution in the other position of the imidazole decreases the binding affinity. strong class=”kwd-title” Keywords: Retinoic Acid Metabolism blocking brokers, RAMBAs, Cancer differentiation Introduction Although significant advances have been made in the treatment of some malignancies, the prognosis of patients with metastasis tumors remains poor. activity against RA metabolism. Most of them ITIC are nitrogen-containing compounds. The results showed that among these compounds only six compounds ( em N /em -benzyl-2-phenylethanamine, itraconazole, chlorpromazine, 5-chloro-1,3-benzoxazol-2-amine, proadifen and furazolidone) showed inhibition of RA metabolism which was 50%. Ketoconazole and liarozole were also screened as standard potent inhibitors in the same system and gave 87.5% and 89% inhibition, respectively. The results indicate that mostly azoles with substituents in positions other than the 1-position on the ring are very weak inhibitors of RA metabolism. The most effective inhibitors (ketoconazole, itraconazole, bifonazole and clotrimazole) are 1-substituted and possess relatively large aromatic groups in the molecule. 1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution ITIC in the other position of the imidazole decreases the binding affinity. strong class=”kwd-title” Keywords: Retinoic Acid Metabolism blocking brokers, RAMBAs, Cancer differentiation Introduction Although significant advances have been made in the treatment of some malignancies, the prognosis of patients with metastasis tumors remains poor. Differentiating brokers redirect ITIC cells toward their normal phenotype and therefore may reverse or suppress evolving malignant lesions or prevent cancer invasion. In addition, they offer a potential alternative to the classic cytostatic drugs and indeed represent an attractive target for medicinal intervention. Retinoids (vitamin A and its natural metabolites and synthetic analogs) are currently the subject of intense biological interest stimulated by the discovery and characterization of retinoid receptor and the realization of these compounds as nonsteroidal small-molecule hormones [1, 2]. em All-trans /em -retinoic acid (ATRA), the biologically most active metabolite of vitamin A, plays a major role in cellular differentiation and proliferation of epithelial tissue. ATRA is being used in differentiation therapy of cancer, in cancer chemoprevention and for the treatment of acne [3C5]. Recently, ATRA has confirmed useful in cancer chemotherapy [6C8]. One of the most impressive effects of ATRA is usually on acute promyelocytic leukaemia. Treatment of acute promyelocytic leukaemia patients with high dose of ATRA resulted in complete remission [9, 10]. Furthermore, several experiments in animals ITIC have exhibited that ATRA inhibited the induction and caused the disappearance of prostate tumors [11]. In spite of these encouraging results, the effects of prolonged ATRA therapy on human cancers in the clinic has been scarce and disappointing [12]. It has been suggested that this therapeutic effects of ATRA are undermined by its rapid in vivo metabolism and catabolism by cytochrome P450 enzyme (CYPs) [13, 14]. One of the strategies for preventing in vivo catabolism of ATRA is usually to inhibit the P450 enzyme(s) responsible for this process. Indeed, this seems to be an emerging approach that may yield effective brokers for the chemoprevention and/or treatment of cancers [15]. This may create a novel class of brokers known as retinoic acid metabolism blocking brokers (RAMBAs). Liarozole, a P-450 inhibitor (17, 20 steroid lyase) and the first RAMBA to undergo clinical investigation, preferentially increases intratumor levels of endogenous RA, resulting in antitumor activity [16]. This has opened up the possibility of developing more specific inhibitors of ATRA metabolism as a novel approach to cancer treatment. In the present study a wide range of established P450 inhibitors has been screened to examine their inhibitory activity on ATRA metabolism. Materials and Methods Reagents em N /em -benzyl-2-phenylethanamine was purchased from Aldrich Chemical Company Ltd, 2-methyl-5-phenyl-1,3-benzoxazole, 4-(4-bromophenyl)-1,2,3-thiadiazole, 5-(4-methyl-phenyl)-1,2,4-thiadiazole, 5-(3-chlorophenyl)-1,3-oxazole and 2-(thiophen-2-yl)-1,3,4-oxadiazole were obtained from Maybridge Chemical Co Ltd. (Tintagel Cornwall). Liarozole was donated by Janssen Research Foundation (Bearse Belgium). em All-trans- /em retinoic acid, NADPH, butylated hydroxyanisole and all the other P-450 inhibitors were purchased from Sigma Chemical Company. [11,12-3H]- em All-trans- /em retinoic acid (ATRA) was from DuPont (UK) Ltd. Formic acid, ammonium acetate and Hisafe III scintillation fluid (optiphase III) were obtained from Fisons Ltd. All solvents IEGF used for chromatography were of HPLC grade and were obtained from Rathburn Chemicals Ltd.UK. All other laboratory reagents were of analytical grade and obtained from British Drug House. Animals Healthy male wistar rats were fasted overnight and killed by stunning. Preparation of rat liver microsomes Rat liver microsomes were prepared by a previously described method [17] and stored at ?80C. RA Metabolism Assay The incubation system contained RA.