Volume 27, Issue 4

Cytochrome P450 (CYP) 2B6 belongs to the set of important hepatic drug-metabolizing CYPs. It makes up roughly 3%–6% of total hepatic CYP content and metabolizes several pharmaceuticals including bupropion, efavirenz, cyclophosphamide, pethidine, ketamine and propofol. The enzyme is susceptible to drug-drug interactions by enzyme induction and inhibition. In addition to drugs, CYP2B6 is able to both detoxify and bioactivate a number of procarcinogens and environmental agents including pesticides and herbicides. There is an extensive interindividual variability in the expression of CYP2B6, which is in part explained by extensive genetic polymorphism. CYP2B6 is one of the most polymorphic CYP genes in humans with over 100 described SNPs, numerous complex haplotypes and distinct ethnic and racial frequencies. This review summarizes the basic properties of CYP2B6 and the main characteristics of clinical relevance.

The process of bringing new and innovative drugs, from conception and synthesis through to approval on the market can take the pharmaceutical industry 8–15 years and cost approximately $1.8 billion. Two key technologies are improving the hit-to-drug timeline: high-throughput screening (HTS) and rational drug design. In the latter case, starting from some known ligand-based or target-based information, a lead structure will be rationally designed to be tested in vitro or in vivo . Computational methods are part of many drug discovery programs, including the assessment of ADME (absorption-distribution-metabolism-excretion) and toxicity (ADMET) properties of compounds at the early stages of discovery/development with impressive results. The aim of this paper is to review, in a simple way, some of the most popular strategies used by modelers and some successful applications on computational chemistry to raise awareness of its importance and potential for an actual multidisciplinary drug discovery process.

Background: Paraoxonase 1 (PON1) is important in organophosphates and xenobiotic metabolism and as an antioxidant bio-scavenger. PON1 activity was shown to significantly decrease in depressed patients after antidepressant treatment instauration. Our aim was to investigate the in vitro inhibitory effects of three antidepressants (imipramine, amitriptyline and fluoxetine) on PON1 activity. Methods: Plasma from healthy volunteers was spiked with antidepressant drugs. The working solutions were then diluted with plasma to obtain concentrations that covered the therapeutic margin. PON1 was tested by a kinetic method in triplicate after incubation at 37°C for 2 h. Results: Tricyclic antidepressants significantly inhibited PON1. Fluoxetine had no effect. The inhibition percentage for imipramine was 15.6% at 100 μg/L after incubation for 1 h (131±1 vs. 155±2 IU/L; p<0.01). At 350 μg/L, the inhibition percentage for imipramine 19.2% after 1 h and 20.2% after 2 h. Amitriptyline was a stronger inhibitor: 26% after 30 min at 125 μg/L. At 250 μg/L, the inhibition percentage for amitriptyline was 36.5% after 30 min (100±4 vs. 159±2 IU/L; p<0.01). Conclusions: The tested tricyclic antidepressants significantly inhibit PON1 activity in a concentration-dependent manner. Amitriptyline had a higher inhibition potency than imipramine.

Background: Losartan is metabolized to losartan carboxylic acid (E-3174) by the polymorphic cytochrome CYP2C9. The aim of the study was to develop a high-performance liquid chromatographic (HPLC) method with fluorescence detection for simultaneously measuring losartan and its metabolite E-3174 in urine to evaluate the losartan urinary metabolic ratio (MR: losartan/E-3174) for CYP2C9 phenotyping in humans. Methods: The compounds were separated in a reversed-phase chromatographic column and detected by fluorescence at a wavelength of 250 nm for excitation and of 370 nm for emission. Results: No analytical interferences with endogenous compounds were found, and the extraction recoveries were over 88%. Limits of quantification of 2 ng mL –1 for losartan and 5 ng mL –1 for E-3174 were achieved, as well as good reproducibility with coefficients of variation of <9% in all cases. Analyses with the present HPLC method show significant differences (p<0.05) in losartan MRs between the four CYP2C9 genotype groups in 13 Spanish healthy volunteers. Conclusions: The method developed is simple and affordable, as well as sensitive and reliable to calculate the MR. Therefore, it appears to be useful for CYP2C9 phenotyping using losartan as a drug test in populations, such as Hispanics with different allele combinations.

Background: Methocarbamol is a skeletal muscle relaxant and is widely used to relieve pain in muscles. Many drugs may have interactions with each other when used at the same time. Yeast sucrase is taken as a drug by patients with congenital sucrase-isomaltase deficiency (CSID). Methods: In this study, the interaction between methocarbamol and yeast sucrase was investigated. Results: Our results showed that methocarbamol can inhibit sucrase activity and reduce the maximum reaction velocity (Vmax) of the enzyme by a non-competitive pattern. Measurement of IC 50 and Ki of the drug revealed that methocarbamol did not bind the enzyme with high affinity. Fluorescence measurement showed that the drug binds to free enzyme and enzyme-substrate complexes that were accompanied by structural changes on the enzyme. Guaifenesin, which has a similar structure to methocarbamol, does not affect the activity of sucrase. Conclusions: Methocarbamol inhibits sucrase activity and its carbamate group plays a main role in the binding of drug to sucrase.

Background: Statins and coumarins are prescribed in combination on a regular basis. Some case reports suggested that statins might affect the dose requirements of coumarins. The aim of the study was to investigate whether acenocoumarol and phenprocoumon maintenance doses are influenced by statin use. Methods: The Pre-EU-PACT database was used, which contains information on 471 acenocoumarol and 624 phenprocoumon users. The influence of individual statins on the acenocoumarol and phenprocoumon maintenance dose was investigated by comparing unadjusted and adjusted mean differences of the maintenance dose between statin and non-statin users. Results: Lower adjusted acenocoumarol dose requirements were observed for patients using atorvastatin, simvastatin, pravastatin, and rosuvastatin. These patients had a reduction in adjusted mean acenocoumarol maintenance dose of 0.11, 0.29, 0.38, and 0.69 mg/day, respectively, compared with a mean adjusted dose of 2.60 mg/day for the patients not using a statin. There was no significant effect of statin use on unadjusted and adjusted phenprocoumon maintenance dose (p=0.23 and p=0.35, respectively). Conclusions: Mean acenocoumarol maintenance dosages were decreased when acenocoumarol is co-administered with the different statins. Statin use does not affect phenprocoumon maintenance doses significantly.

Background: An increasing number of crystal structures for eukaryotic P450s have been published, which provided the chance to explore more structural features and construct some knowledge-based methods to facilitate modeling. Methods: The crystal structures of 14 cytochrome P450s (CYP450) were selected to extract generic spatial anchors typical for three-dimensional (3D) structures of eukaryotic P450s. Multiple sequence alignment and structural superimposition were applied to recognize evolutionarily conserved regions. Results: Regions containing uninterrupted helical components were identified as structurally conservative blocks (SCBs). The reliability and robustness of the SCBs were further evaluated by sequence entropy and structural deviation. Finally, these SCBs were applied and tested directly in constructing the homology model of the P450 1B1. Conclusions: SCBs could potentially be applied as universal template to standardize the homology modeling procedure and help predict drug metabolism preferences for eukaryotic P450s.