Cytochrome P450 4B1 (CYP4B1) as a target in cancer treatment.

Cytochrome P450 4B1 (CYP4B1) plays crucial roles in biotransforming of xenobiotics. Its predominant extrahepatic expression has been associated with certain tissue-specific toxicities. However, the expressions of CYP4B1 in various cancers and hence their potential roles in cancer development were inclusive. In this work, existing knowledge on expression and regulation of CYP4B1 gene and protein, catalysis of CYP4B1, association of CYP4B1 with cancers, contradicting findings about human CYP4B1 activities as well as the employing CYP4B1 in suicide gene approach for cancer treatment were reviewed. To date, it appears that there is a wide spectrum of tissue distribution of CYP4B1 with lungs as the predominant sites. Several nuclear receptors are possibly responsible for regulating its gene expression. The involvement of CYP4B1 in cancer was considered via activation of procarcinogens and neovascularization. However, human CYP4B1 was found to be inactive due to a substitution of proline with serine at position 427. Suicide gene approach combining reengineered CYP4B1 and prodrug 4-ipomeanol (4-IPO) has shown a promising potential for targeted cancer therapy. Further studies should focus on the verification of human CYP4B1 catalytic activities. More compounds with similar structure as 4-IPO should be tested to identify more alternative agents for the suicide gene approach in cancer treatment.

In vitro modulation of naturally occurring flavonoids on cytochrome P450 2A6 (CYP2A6) activity.

1. The effect of flavonoids on coumarin 7-hydroxylation, an activity marker of an important human liver cytochrome P450 isoform, cytochrome P450 2A6 (CYP2A6), was investigated in this study. 2. Coumarin 7-hydroxylase activity was measured fluorometrically in reaction mixtures containing cDNA-expressed CYP2A6, nicotinamide adenine dinucleotide phosphate generating system and 10 uM coumarin, at various concentrations of flavonoids. 3. Among the 23 compounds tested, most of the active members were from flavonol group of hydroxylated flavonoids, with myricetin being the most potent inhibitor followed by quercetin, galangin, and kaempferol. 4. Further exploration of the inhibition mechanism of these compounds revealed that myricetin, galangin, and kaempferol exhibited mixed-type of inhibition pattern while quercetin was observed to exhibit competitive mode of inhibition. 5. Structure-function analyses revealed that degree of inhibition was closely related to the number and location of hydroxyl groups, glycosylation of the free hydroxyl groups, degree of saturation of the flavane nucleus as well as the presence of the alkoxylated function.

Genetic polymorphism of CYP2C8 in three Malaysian ethnics: CYP2C8*2 and CYP2C8*3 are found in Malaysian Indians.

BACKGROUND: CYP2C8 is genetically polymorphic. Four variants, CYP2C8*2, CYP2C8*3, CYP2C8*4 and CYP2C8*5, which contain mutations in the coding regions have been reported to exhibit different enzyme activity as compared with CYP2C8*1. OBJECTIVE: To determine the allele frequency of three codon-changing variants (CYP2C8*2, CYP2C8*3 and CYP2C8*4) in the Malaysian population. METHOD: Healthy unrelated volunteers from three major races in Malaysia were recruited. The study was approved by the local Research Ethics Committee. DNA was extracted using a standard protocol. A two-step multiplex PCR method was developed to detect three alleles of CYP2C8. PCR results were confirmed by subsequent direct DNA sequencing. RESULT: Only the Indians showed CYP2C8 polymorphism with allele frequency of 98% for CYP2C8*1, 0.8% for CYP2C8*2 and 1.2% for CYP2C8*3. CYP2C8*4 was not detected in any of the ethnic groups. CONCLUSION: To the best of our knowledge, the current study described, for the first time polymorphisms of CYP2C8 in Malaysian Indians.

A simple multiplex PCR method for the concurrent detection of three CYP2C8 variants.

BACKGROUND: Cytochrome P450 (CYP) 2C8 is a principle enzyme responsible for the metabolism of many clinically important drugs as well as endogenous compounds such as arachidonic acid. The enzyme is genetically polymorphic but a simple method is not available to study its genetic polymorphism. We developed and optimized a variant-specific PCR techniques to detect CYP2C8*2, CYP2C8*3 and CYP2C8*4. METHOD: Genomic DNA was extracted from blood using standard extraction methods. A two-step PCR method was developed to detect simultaneously three CYP2C8 variants. In the first PCR (PCR1), specific regions from exons 3, 5 and 8 of the CYP2C8 gene were amplified. The products were used as templates in parallel alleles-specific PCR (PCR2). This method was tested against DNA samples obtained from 57 healthy Malaysian volunteers. RESULT: The bands of interest were successfully amplified. This method showed specific and reproducible results when tested on healthy volunteers. DNA sequencing further confirmed genotype results obtained from current method. CONCLUSION: We have successfully developed and optimized a multiplex PCR method suitable for use in population studies of CYP2C8 polymorphism.

Involvement of cytochromes p450 in drug-drug interactions: an overview.

Drug interactions can cause iatrogenic disease. If concurrent medications are taken, the potential exists for a drug interaction to occur. Renewed interest in the topic interactions has been generated by the fatal interactions involving non-sedating histamine H-1 antagonists and the recent intriduction of two therapeutic agents, the selective serotonin reuptake inhibitors (SSRIs) and HIV protease inhibitors, for the treatment of depression and AIDS, respectively. These three therapeutic agents have been implicated in clinically significant drug interactions. The consequences of these interactions vary in clinical significance, extent, and effect. Some interactions are theoretical whereas others may lead to severe iatrogenic adverse experiences including lethal consequences.The purpose of this review is to alert the medical practioner to potential drug interactions that may occur when these drugs are prescribed to patients. The pharmacological basis and clinical signficance of these interactions are reviewed. The pharmacological mechanisms underlying these interactions are illustrative of those that may be involved for many other medications. Doctors should be aware of the potential pitfall that may occur when certain groups of drugs are prescribed with concurrent medications.

The xenobiotic inhibitor profile of cytochrome P4502C8.

AIMS: To investigate inhibition of recombinant CYP2C8 by: (i) prototypic CYP isoform selective inhibitors (ii) imidazole/triazole antifungal agents (known inhibitors of CYP), and (iii) certain CYP3A substrates (given the apparent overlapping substrate specificity of CYP2C8 and CYP3A). METHODS: CYP2C8 and NADPH-cytochrome P450 oxidoreductase were coexpressed in Spodoptera frugiperda (Sf21) cells using the baculovirus expression system. CYP isoform selective inhibitors, imidazole/triazole antifungal agents and CYP3A substrates were screened for their inhibitory effects on CYP2C8-catalysed torsemide tolylmethylhydroxylation and, where appropriate, the kinetics of inhibition were characterized. The conversion of torsemide to its tolylmethylhydroxy metabolite was measured using an h.p.l.c. procedure. RESULTS: At concentrations of the CYP inhibitor 'probes' employed for isoform selectivity, only diethyldithiocarbamate and ketoconazole inhibited CYP2C8 by > 10%. Ketoconazole, at an added concentration of 10 microM, inhibited CYP2C8 by 89%. Another imidazole, clotrimazole, also potently inhibited CYP2C8. Ketoconazole and clotrimazole were both noncompetitive inhibitors of CYP2C8 with apparent Ki values of 2.5 microM. The CYP3A substrates amitriptyline, quinine, terfenadine and triazolam caused near complete inhibition (82-91% of control activity) of CYP2C8 at concentrations five-fold higher than the known CYP3A Km. Kinetic studies with selected CYP3A substrates demonstrated that most inhibited CYP2C8 noncompetitively. Apparent Ki values for midazolam, quinine, terfenadine and triazolam ranged from 5 to 25 microM. CONCLUSIONS: Inhibition of CYP2C8 occurred at concentrations of ketoconazole and diethyldithiocarbamate normally employed for selective inhibition of CYP3A and CYP2E1, respectively. Some CYP3A substrates have the capacity to inhibit CYP2C8 activity and this may have implications for inhibitory drug interactions in vivo.

Baculovirus-mediated expression of cytochrome P4502C8 and human NADPH-cytochrome P450 reductase: optimization of protein expression.

1. High expression levels of cytochrome P450 (CYP) 2C8 and NADPH-cytochrome P450 oxidoreductase (OxR) in Spodoptera frugiperda (Sf21) cells have been achieved using the baculovirus expression system. 2. The baculovirus dual expression plasmid, pAcUW31, was used to insert CYP2C8 and OxR cDNAs downstream of the polyhedrin (polh) or p10 promoters, either separately or together, generating four recombinant baculoviruses; two expressing single proteins (CYP2C8 driven by the p10 promoter, bVp10.2C8 or OxR driven by the polh promoter, bVpolh.OxR) with another two coexpressing both CYP2C8 and OxR under reciprocal control of the polh and p10 promoters (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR). 3. High levels of singly expressed CYP2C8 and OxR were achieved from bVp10.2C8 and bVpolh.OxR, with levels of 0.7-1.2 nmol CYP/mg protein and 400-500 nmol cytochrome c reduced/min/mg protein respectively. 4. The two dual gene clones (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR) showed, in general, greater variation in CYP content and OxR activity than single gene clones. Screening was therefore necessary for the selection of dual gene clones expressing both proteins optimally. 5. Sf21 microsomes infected by selected dual gene clones were, on average, 14 times more active in tolbutamide hydroxylase activity than those expressing CYP2C8 alone, with a mean spectral CYP content of 79 pmol/mg cell lysate protein and a mean OxR level of 600 nmol/min/mg cell lysate protein.

Insulinoma in Auckland 1970-1985.

This paper presents summaries of the eight cases of insulinoma which occurred in the Auckland area (annual incidence 1 in 1.5 x 10(6) between 1 January 1970 and 31 December 1985. These were five males (ages 28, 28, 31, 55, 57) and three females (ages 46, 49, 54). Seven patients proceeded to operation. Six tumours were removed successfully at laparotomy, one was malignant. There was no instance of synchronous multiple tumours.