Grapefruit juice increases felodipine oral availability in humans by decreasing intestinal CYP3A protein expression

The increase in oral availability of felodipine and other commonly used medications when taken with grapefruit juice has been assumed to be due to inhibition of CYP3A4, a cytochrome P450 that is present in liver and intestine. To evaluate the effect of repeated grapefruit juice ingestion on CYP3A4 expression, 10 healthy men were given 8 oz of grapefruit juice three times a day for 6 d. Before and after receiving grapefruit juice, small bowel and colon mucosal biopsies were obtained endoscopically, oral felodipine kinetics were determined, and liver CYP3A4 activity was measured with the [14C N-methyl] erythromycin breath test in each subject. Grapefruit juice did not alter liver CYP3A4 activity, colon levels of CYP3A5, or small bowel concentrations of P-glycoprotein, villin, CYP1A1, and CYP2D6. In contrast, the concentration of CYP3A4 in small bowel epithelia (enterocytes) fell 62% (P = 0.0006) with no corresponding change in CYP3A4 mRNA levels. In addition, enterocyte concentrations of CYP3A4 measured before grapefruit juice consumption correlated with the increase in Cmax when felodipine was taken with either the 1st or the 16th glass of grapefruit juice relative to water (r = 0. 67, P = 0.043, and r = 0.71, P = 0.022, respectively). We conclude that a mechanism for the effect of grapefruit juice on oral felodipine kinetics is its selective downregulation of CYP3A4 in the small intestine.     

The character of inhibition of the metabolism of 1,2-benzopyrone (coumarin) by grapefruit juice in human

OBJECTIVE: Constituents of grapefruit juice are known to interfere with mammalian cytochrome P450 isozymes such as intestinal CYP3A4 and hepatic CYP2A6, lowering the biotransformation of drugs and increasing their bioavailability. The aim of this study was to investigate whether the presence of naringin is demanded for the inhibition of the coumarin 7-hydroxylase in man or other compounds are responsible for it. METHODS: In cross-over studies, doses of 10 mg coumarin, together with combinations of grapefruit juice, water and naringin, were given orally to one healthy male volunteer, We investigated increasing amounts of grapefruit juice, keeping the volume of liquid constant at 1 L; increasing doses of naringin given in water; increasing amounts of juice, keeping the dose of naringin constant; or increasing doses of naringin, keeping the amount of juice constant. Urine samples were collected up to 24 h after dosing and 7-hydroxycoumarin was quantified fluorimetrically in urine hydrolysates after HPLC separation to determine the excretion rates. RESULTS: While increasing amounts of grapefruit juice delay the excretion of 7-hydroxycoumarin by 2 h, increasing doses of naringin in water up to twofold (i.e. naringin content of 2 L grapefruit juice) do not cause any alteration in the time course of excretion. Experiments with increasing amounts of juice, keeping the dose of naringin constant, indicate that the inhibitory potency of small amounts of grapefruit juice can be amplified by naringin. The same is true when the ratio between juice constituents and naringin is enhanced up to threefold by adding naringin. CONCLUSION: As naringin alone is ineffective, the inhibitory effect of grapefruit juice on the metabolism of coumarin is caused by at least one compound other than naringin. The persistency of the primary inhibitor not identified yet can obviously be modulated by the naring(en)in-system.     

Neither dapsone hydroxylation nor cortisol 6beta-hydroxylation detects the inhibition of CYP3A4 by HIV-1 protease inhibitors

OBJECTIVE: This study examined the use of dapsone N-hydroxylation and cortisol 6beta-hydroxylation, well accepted in vivo probes of cytochrome P4503A4 (CYP3A4) activity, on defining the effect of three HIV protease inhibitors on CYP3A4 activity. METHODS: Subjects from University Hospital Infectious Disease Clinic about to be started on indinavir, and subjects from two clinical studies, one using ritonavir and the other using amprenavir, were recruited to participate in the study. Subjects received dapsone 100 mg p.o. followed by an 8-h urine collection for dapsone, dapsone N-hydroxylamine, cortisol, and 6beta-hydroxycortisol concentrations before HIV protease inhibitor administration, and 3 4 weeks into receiving HIV protease inhibitors. RESULTS: None of the HIV protease inhibitors demonstrated statistically significant alterations in dapsone recovery ratio and 6beta-hydroxycortisol/cortisol ratio. In fact, with ritonavir, the dapsone recovery ratio tended to increase rather than decrease, suggesting induction. These negative results were found despite evidence of CYP3A4 inhibition by these three HIV protease inhibitors via published drug-drug interactions with drugs that are substrates for CYP3A4. CONCLUSIONS: These in vivo assays used to probe CYP3A4 activity are suboptimal, most likely because of the presence of extrahepatic sites of metabolism for both dapsone and cortisol, and multiple CYP isozymes involved in dapsone N-hydroxylation.     

Interaction of citrus juices with pranidipine, a new 1,4-dihydropyridine calcium antagonist, in healthy subjects

OBJECTIVES: The study was conducted to investigate whether oral co-administration with citrus juices significantly affects the pharmacokinetics and/or pharmacodynamics of pranidipine, a new 1,4-dihydropyridine calcium antagonist, in healthy male subjects. Grapefruit juice and orange juice, which were both commercially available, were used in this study. METHODS: Sixteen healthy male Japanese subjects participated in this study and were divided into two groups for grapefruit juice and orange juice treatment. The study followed an open-labelled crossover design, comparing the effects of a single oral dose of 2 mg pranidipine taken together with 250 ml citrus juice or 250 ml water. Serum pharmacokinetics of pranidipine, adverse reactions, blood pressure, heart rate, 12-lead ECG, haematology, clinical chemistry and urinalysis were measured throughout the study. RESULTS: For grapefruit juice, mean Cmax and AUC0-24 h were significantly higher than those of water (P=0.0003 and 0.0005, respectively, ANOVA) with the ratios of log transformed values being 1.50 and 1.74, respectively. There were no differences in tmax and t1/2 between the juice and water treatments. A significant increase in heart rate (P=0.0240, ANOVA with repeated measurements) was observed in the juice treatment whereas there were no significant differences in systolic and diastolic blood pressure between the two treatments. For orange juice, a small decrease in mean Cmax was observed compared with water (P=0.0218, ANOVA) with the ratio being 0.86, but there was no significant difference in AUC0-24h between the two treatments. No marked differences were observed in tmax and t1/2. Oral pranidipine administration with orange juice did not affect heart rate, systolic and diastolic blood pressures or other parameters for safety evaluation. CONCLUSIONS: Oral co-administration with grapefruit juice and pranidipine was associated with increased bioavailability and changed the pharmacodynamics of pranidipine, particularly with regard to heart rate. Orange juice intake with pranidipine did not markedly affect the pharmacokinetics and no clinically significant changes were observed in the pharmacodynamics and safety evaluation.     

Drug interactions with grapefruit juice. Extent, probable mechanism and clinical relevance

Concomitant intake with grapefruit juice increases the concentrations of many drugs in humans. The effect seems to be mediated mainly by suppression of the cytochrome P450 enzyme CYP3A4 in the small intestine wall. This results in a diminished first pass metabolism with higher bioavailability and increased maximal plasma concentrations of substrates of this enzyme. The effect was most pronounced in drugs with a high first pass degradation and in many cases has the clear potential to reach clinical relevance, as shown by an occasional change in drug effects or tolerability. For felodipine, nitrendipine, nisoldipine and saquinavir, the interaction was most marked with median increases of area under the curve (AUC) and/or the maximum (peak) plasma drug concentration after single-dose administration (Cmax) values exceeding 70% of respective control periods. Less pronounced, but possibly relevant, concentration increases were found for nifedipine, nimodipine, verapamil, cyclosporin, midazolam, triazolam and terfenadine. This list is not complete because many drugs have not been studied yet. The components of grapefruit juice which are the most probable causes of the interactions are psoralen derivatives, but the flavonoid naringenin may also contribute. Concomitant grapefruit juice intake does not generally decrease the variability of drug pharmacokinetic parameters. Therefore, it is recommended that patients refrain from drinking grapefruit juice when they are taking a drug that is extensively metabolised, unless a lack of interaction has already been demonstrated for the drug. It is also recommended that drugs possibly interacting with grapefruit juice should be appropriately labelled. A place for grapefruit juice as a drug-sparing agent in treatment involving expensive medicine cannot be derived from the information currently available on grapefruit juice interactions.     

Effect of grapefruit juice on clarithromycin pharmacokinetics

To investigate whether grapefruit juice inhibits the metabolism of clarithromycin, 12 healthy subjects were given water or grapefruit juice before and after a clarithromycin dose of 500 mg in a randomized crossover study. Administration of grapefruit juice increased the time to peak concentration of both clarithromycin (82 +/- 35 versus 148 +/- 83 min; P = 0.02) and 14-hydroxyclarithromycin (84 +/- 38 min versus 173 +/- 85; P = 0.01) but did not affect other pharmacokinetic parameters.     

Antimicrobial activity produced by six dentifrices

OBJECTIVE: To examine the effect of grapefruit juice on the bioavailability of carbamazepine in patients with epilepsy. METHODS: This was a randomized crossover study consisting of 2 phases. Ten patients with epilepsy who had received therapy with 200 mg carbamazepine 3 times a day for the previous 3 to 4 weeks participated. They were given either grapefruit juice or 300 mL water at 8 am along with 200 mg carbamazepine. Each treatment was separated by 2 days; subjects continued to receive carbamazepine therapy during the 2-day period. On both occasions, blood samples were collected at different time intervals between 0 to 8 hours. Carbamazepine levels were estimated by reversed-phase HPLC technique. RESULTS: Compared with water, grapefruit significantly increased the steady peak concentration (6.55 versus 9.20 microgram/mL), trough concentration (4.51 versus 6.28 microgram/mL), and area under the plasma concentration-time curve (43.99 versus 61.95 micrograms.h/mL) of carbamazepine. No significant effect was found in the time to reach peak plasma concentration. CONCLUSION: Grapefruit juice increases the bioavailability of carbamazepine by inhibiting CYP3A4 enzymes in gut wall and in the liver.     

Time courses of hepatic injuries induced by chloroform and by carbon tetrachloride: comparison of biochemical and histopathological changes

The relationship was investigated between biochemical and morphological changes in chloroform (CHCl3)- and carbon tetrachloride (CCl4)-induced liver damage. The time courses of hepatic microsomal cytochrome P450 (CYP) content, hepatic microsomal CYP2E1 activity, hepatic reduced glutathione (GSH) content, plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities were examined in relation to the liver morphology in rats orally treated with CHCl3 or CCl4 (3.35 mmol/kg). The CYP content and the activity of CYP2E1 markedly decreased in the CCl4-treated rats 3 h after treatment compared to much lower decreases in the CHCl3-treated rats. The hepatic GSH content was decreased to a similar extent in both groups of rats at 3 h after treatment; in the CCl4-treated rats, the GSH content continued to decrease, reaching a minimum at 24 h and without attaining the normal level at 72 h after treatment. By contrast, hepatic GSH content in the CHCl3-treated rats began to increase from 6 h, attaining complete recovery 48 h after treatment. Plasma ALT and AST activities were significantly elevated by CCl4 as early as 3 h after treatment, while the activities in the CHCl3-treated rats did not increase until 6 h after treatment. In both groups of rats, ALT and AST activities reached a maximum at 24 h, and gradually decreased, remaining at abnormal levels at 72 h. Hepatic cells in the CCl4-treated rats were found to be necrotic as early as 3 h post-treatment, whereas few or no morphological changes appeared in the liver of CHCl3-treated rats. The extent of necrosis was at a maximum 24 h after treatment in both CHCl3- and CCl4-treated rats. In addition, some necrotic cells remained in the liver of CCl4-treated rats 72 h after treatment, while the necrosis in the CHCl3-treated rats was almost negligible. The present results indicate that almost the same time-courses of biochemical and morphological changes were followed in rats of both the CHCl3- and CCl4-treated groups.     

Subglottic tracheal stenosis in Wegener''s granulomatosis. Report of 3 cases]

OBJECTIVE: This study was performed to assess whether coadministration with grapefruit juice significantly affects the pharmacokinetics of amlodipine, a dihydropyridine class calcium antagonist with slow absorption, distribution and low plasma clearance. The primary objective was to evaluate whether short exposure to grapefruit juice could affect the metabolism of amlodipine to an extent similar to that previously demonstrated for other dihydropyridines (e.g. felodipine, nisoldipine, nitrendipine). METHODS: Twelve healthy male volunteers followed a randomised, open crossover study design, comparing the effect of a single oral dose of amlodipine (5 mg) taken together with a glass of grapefruit juice (250 ml) vs water. Blood samples to determine plasma concentration were taken and blood pressure (BP) and heart rate (HR) were measured throughout the study. RESULTS: When amlodipine was coadministered with grapefruit juice, Cmax was 115% and AUC(0-72 h) was 116% compared with water, but tmax was not significantly changed. There were no significant differences in BP and HR between the two treatments. A small decrease in diastolic BP, however, was observed in both treatments 4-8 h after drug administration, coinciding with Cmax, but this was normalised after 12 h. The BP reduction seen was compensated by a slight increase in HR, which remained throughout the study. CONCLUSION: An interaction between grapefruit juice and amlodipine was demonstrated. The haemodynamic data showed that a dose of 5 mg was sufficient to achieve a BP reduction in healthy subjects, but the increase in amlodipine plasma concentration seen after intake of grapefruit juice was too small to significantly affect BP or HR. The clinical significance of this food/drug interaction, however, cannot be ignored since there is considerable variation between individuals and a more extensive intake of grapefruit juice might give more pronounced effects.     

Clinical experience using gabapentin adjunctively in patients with a history of mania or hypomania

Biliary, plasma, and urinary disposition of paclitaxel and paclitaxel metabolites were determined simultaneously in a patient with percutaneous biliary drain. The complete chemical structures of the major metabolites were established by mass spectrometry and NMR spectroscopy. A nonlinear elimination model was indicated by the fact that the rate of biliary excretion of paclitaxel rose as plasma concentrations fell. Dihydroxypaclitaxel was the predominant biliary metabolite, in contrast to the barely detectable levels in two previous patients. This derivative results from hydroxylation at the C6 position of the taxane ring and at the phenyl C3'-position on the C13 side chain mediated by cytochrome P450 2C8 and 3A4, respectively. In line with this mechanism, the two other main metabolites corresponded to 6alpha-hydroxypaclitaxel and to the paclitaxel derivative hydroxylated in the para-position on the phenyl ring at the C3'-position of the C13. A high CYP3A4 activity in the patient is consistent with the repeated administration of methylprednisolone for 14 days before paclitaxel treatment, a compound known to induce the CYP3A isoform, and with the increased ratio of 6beta-hydroxycortisol/cortisol in urine, an index of CYP3A activity. These findings emphasize the influence of pretreatment with corticoids on the disposition of paclitaxel.     

Grapefruit juice does not enhance the effects of midazolam and triazolam in man

OBJECTIVES: Since grapefruit juice (Gra) inhibits hepatic P450 (CYP3A4), we studied its potential to enhance the effects of midazolam (Mid) and triazolam (Trz), which are metabolized by the CYP3A4 isoenzyme. METHODS: In Study I parallel groups of healthy students were given orally Mid 10 mg with water or grapefruit juice (GraMid), two placebo groups receiving water or Gra. The effects of Mid were measured by psychomotor tests and by self-rating on visual analogue scales before and 30 and 90 min after intake. Study II was similar, but the post-treatment tests were at 45 and 90 min, and the active drugs used were 0.250 mg Trz, GraTrz, and Mid 10 mg. In the crossover Study III, 6 subjects took Mid 10 mg alone and with Gra (GraMid) and 750 mg erythromycin (EryMid). Performance tests were made and blood was sampled before and 30, 60 and 90 min after intake. Midazolam and its active metabolite alpha-OH-midazolam were assayed by gas chromatography (GC) and radioreceptor assay (RRA). RESULTS: In Study I, both Mid and GraMid impaired digit symbol substitution (DSS), letter cancellation (LC) and flicker fusion (CFF) at 90 min. GraMid had more effect (P < 0.05) than Mid on the DSS performance. Mid caused drowsiness at 30 and 90 min. Both Mid and GraMid caused clumsiness and a feeling of impaired performance at 90 min. In Study II, the active drugs impaired objective test performances (DSS, LC, CFF) at 90 min, without having a clear subjective effect. In Study III, Mid, EryMid and GraMid impaired performance in the DSS, LC and CFF tests. EryMid proved stronger than Mid and GraMid on DSS and LC tests at 30 min. Mean values of plasma midazolam (and alpha-OH-midazolam) at 30, 60, 90 and 120 min after Mid 10 mg were 68(19), 61(19), 43(14) and 42(12) micrograms.l-1. The corresponding values after EryMid were 164(14), 137(13), 104(10) and 89(10) micrograms.l-1, and after GraMid 60(12), 69(16), 61(15) and 57 (14) micrograms.l-1. CONCLUSIONS: The grapefruit juice used did have any particular interaction with oral doses of 10 mg midazolam and 0.25 mg triazolam in healthy young subjects.     

Determinants of midterm outcome of partial left ventriculectomy in dilated cardiomyopathy

The objective of this study was to test the hypothesis that maternal CRH concentrations are elevated in women experiencing threatened preterm labor who subsequently give birth within 24 h compared to those in women who do not. We also characterized the changes in maternal plasma cortisol, ACTH, corticosteroid binding capacity (CBC), and CRH concentrations in 28 healthy pregnant women between 20-38 weeks gestation. Overall, maternal plasma CRH concentrations were significantly greater (P < 0.05) in those women giving birth within 24 h (1343.3 +/- 143.9 pg/mL; n = 81) compared to those in women who did not (714.5 +/- 64.8 pg/mL; n = 144) or those in normal subjects. This difference was present between 28-36 weeks, but not 24-28 weeks gestation. The ratio of maternal cortisol to CBC was also significantly greater (P < 0.05; 0.65 +/- 0.04; n = 82) in women giving birth within 24 h than in those who did not (0.55 +/- 0.02; n = 136). This difference was significant at all gestational ages studied. Elevated CRH concentrations and bioavailability of free cortisol may both be implicated in the pathogenesis of preterm labor in some women. Further prospective clinical trials are warranted to determine the positive and negative predictive values of maternal CRH concentrations and/or the ratio of cortisol/CBC for identifying women with threatened preterm labor destined to give birth within 24 h.     

Cytochrome P450 3A4 activity in premenopausal and postmenopausal women, based on 6-beta-hydroxycortisol:cortisol ratios

STUDY OBJECTIVE: To characterize cytochrome P450 (CYP) 3A4 activity in premenopausal and postmenopausal women by evaluating the urinary 6-beta-hydroxycortisol:cortisol ratio. DESIGN: Prospective study SUBJECTS: Thirteen premenopausal and 13 postmenopausal women who were healthy and not receiving drugs known to affect CYP3A4 activity INTERVENTIONS: Beginning on day 2 of menses, premenopausal women collected first morning urine samples every other day for a complete menstrual cycle. Postmenopausal women collected first morning urine every other day for 28 days. MEASUREMENTS AND MAIN RESULTS: Mean weekly 6-beta-hydroxycortisol:cortisol ratios did not differ during the phase (week) of the menstrual cycle. Daily ratios did not differ in postmenopausal women. No difference between premenopausal and postmenopausal women was found on comparing overall median ratios. CONCLUSION: Cytochrome P450 3A4 activity as measured by 6-beta-hydroxy cortisol:cortisol ratio did not differ by week of menstrual cycle, suggesting no menstrual cycle-related changes. Menopause does not appear to be associated with differences in CYP3A4 activity, compared with premenopause.     

Role of CYP3A4 in human hepatic diltiazem N-demethylation: inhibition of CYP3A4 activity by oxidized diltiazem metabolites

The antihypertensive agent diltiazem (DTZ) impairs hepatic drug metabolism by inhibition of cytochrome P450 (CYP). The accumulation of DTZ metabolites in serum occurs during prolonged therapy and leads to decreased DTZ elimination. Thus, DTZ metabolites may contribute to CYP inhibition. This study assessed the role of human CYPs in microsomal DTZ oxidation and the capacity of DTZ metabolites to inhibit specific CYP activities. DTZ N-demethylation varied 10-fold in microsomal fractions from 17 livers (0.33-3.31 nmol/mg of protein/min). DTZ oxidation was correlated with testosterone 6beta-hydroxylation (r = 0.82) and, to a lesser extent, tolbutamide hydroxylation (r = 0.59) but not with activities mediated by CYP1A2 or CYP2E1. CYP3A4 in lymphoblastoid cell microsomes catalyzed DTZ N-demethylation but CYP2C8 and CYP2C9 were also active (approximately 20% and 10% of the activity supported by CYP3A4); seven other CYPs produced little or no N-desmethyl DTZ from DTZ. The CYP3A4 inhibitors ketoconazole and troleandomycin decreased microsomal DTZ oxidation, but inhibitors or substrates of CYP2C, CYP2D and CYP2E1 produced no inhibition. Some inhibition was produced by alpha-naphthoflavone, a chemical that inhibits CYP1As and also interacts with CYP3A4. In further experiments, the capacities of DTZ and three metabolites to modulate human CYP 1A2, 2E1, 2C9 and 3A4 activities were evaluated in vitro. DTZ and its N-desmethyl and N,N-didesmethyl metabolites selectively inhibited CYP3A4 activity, whereas O-desmethyl DTZ was not inhibitory. The IC50 value of DTZ against CYP3A4-mediated testosterone 6beta-hydroxylation (substrate concentration, 50 microM) was 120 microM. The N-desmethyl (IC50 = 11 microM) and N,N-didesmethyl (IC50 = 0.6 microM) metabolites were 11 and 200 times, respectively, more potent. From kinetic studies, N-desmethyl DTZ and N,N-didesmethyl DTZ were potent competitive inhibitors of CYP3A4 (Ki = approximately 2 and 0.1 microM, respectively). CYP3A4 inhibition was enhanced when DTZ and N-desmethyl DTZ underwent biotransformation in NADPH-supplemented hepatic microsomes in vitro, supporting the contention that inhibitory metabolites may be generated in situ. These findings suggest that N-demethylated metabolites of DTZ may contribute to CYP3A4 inhibition in vivo, especially under conditions in which N-desmethyl DTZ accumulates, such as during prolonged DTZ therapy.     

Nutritional status modulates rat liver cytochrome P450 arachidonic acid metabolism

Alterations in nutritional status affect hepatic cytochrome P450 levels. Since cytochromes P450 participate in the metabolism of arachidonic acid, we hypothesized that changes in liver P450 arachidonic acid metabolism occur during fasting and refeeding. Male Fisher 344 rats were either fed, fasted 48 hr (F48), fasted 48 hr and then refed 6 hr (F48/R6), or fasted 48 hr and then refed 24 hr (F48/R24). F48 rats had reduced body weight, increased plasma beta-hydroxybutyrate, and reduced plasma insulin compared with the other groups. Although there was no significant change in total liver P450 content, there was a significant 20%, 48%, and 24% reduction in total hepatic microsomal arachidonic acid metabolism in F48, F48/R6, and F48/R24 rats, respectively, compared with fed rats. Epoxygenase activity decreased by 28%, 51%, and 26% in F48, F48/R6, and F48/R24 rats, respectively. In contrast, omega-1 hydroxylase activity increased by 126% in F48 rats compared with fed rats. Immunoblotting revealed that levels of CYP2C11 protein were markedly reduced, whereas levels of CYP2E1 protein were markedly increased in the F48 and F48/R6 groups. In contrast, levels of CYP1A1, CYP1A2, CYP2B1, CYP2J3, CYP4A1, and CYP4A3 were unchanged with fasting/refeeding. Northern blots revealed that levels of CYP2C11 mRNAs were decreased, whereas CYP2E1 mRNAs were increased in F48 and F48/R6 rats. Recombinant CYP2C11 metabolized arachidonic acid primarily to epoxides with preference for the 14(S),15(R)-, 11(R), 12(S)-, and 8(S),9(R)- epoxyeicosatrienoic acid enantiomers. We conclude that (1) nutritional status affects hepatic microsomal arachidonic acid metabolism, (2) reduced epoxygenase activity in F48 and F48/R6 rats is accompanied by decreased levels of CYP2C11, (3) increased omega-1 hydroxylase activity is accompanied by augmented levels of CYP2E1, and (4) the effects of fasting on CYP2C11 and CYP2E1 expression occur at the pretranslational level.     

Identification and characterization of human cytochrome P450 isoforms interacting with pimozide

Using human liver microsomes (HLMs) and recombinant human cytochrome P450 (CYP450) isoforms, we identified the major route of pimozide metabolism, the CYP450 isoforms involved, and documented the inhibitory effect of pimozide on CYP450 isoforms. Pimozide was predominantly N-dealkylated to 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI). The formation rate of DHPBI showed biphasic kinetics in HLMs, which suggests the participation of at least two activities. These were characterized as high-affinity (K(m1) and Vmax1) and low-affinity (K(m2) and Vmax2) components. The ratio of Vmax1 (14 pmol/min/mg protein)/K(m1) (0.73 microM) was 5.2 times higher than the ratio of Vmax2 (244 pmol/min/mg protein)/K(m2) (34 microM). K(m2) was 91 times higher than K(m1). The formation rate of DHPBI from 25 microM pimozide in nine human livers correlated significantly with the catalytic activity of CYP3A (Spearman r = 0.79, P = .028), but not with other isoforms. Potent inhibition of DHPBI formation from 10 microM pimozide was observed with ketoconazole (88%), troleandomycin (79%), furafylline (48%) and a combination of furafylline and ketoconazole (96%). Recombinant human CYP3A4 catalyzed DHPBI formation from 10 microM pimozide at the highest rate (V = 2.2 +/- 0.89 pmol/min/pmol P450) followed by CYP1A2 (V = 0.23 +/- 0.08 pmol/min/pmol P450), but other isoforms tested did not. The K(m) values derived with recombinant CYP3A4 and CYP1A2 were 5.7 microM and 36.1 microM, respectively. Pimozide itself was a potent inhibitor of CYP2D6 in HLMs when preincubated for 15 min (Ki = 0.75 +/- 0.98 microM) and a moderate inhibitor of CYP3A (Ki = 76.7 +/- 34.5 microM), with no significant effect on other isoforms tested. Our results suggest that pimozide metabolism is catalyzed mainly by CYP3A, but CYP1A2 also contributes. Pimozide metabolism is likely to be subject to interindividual variability in CYP3A and CYP1A2 expression and to drug interactions involving these isoforms. Pimozide itself may inhibit the metabolism of drugs that are substrates of CYP2D6.     

Growth hormone inhibits lipoprotein lipase activity in human adipose tissue

The in vitro effects of GH on human adipose tissue lipoprotein lipase (LPL) activity and messenger ribonucleic acid (mRNA) levels were studied using a tissue incubation technique. After preincubation for 3 days, abdominal sc adipose tissue pieces were exposed to cortisol (1000 nmol/L) for 3 days to induce LPL activity. Addition of GH (50 micrograms/L) to the cortisol-containing medium during the last 24 h (day 6) caused a decrease by 84 +/- 4% (P < 0.01) in heparin-releasable LPL activity and by 65 +/- 4% (P < 0.01) in total LPL activity. Moreover, the heparin-releasable fraction was reduced from 42% of the total LPL activity with cortisol alone to 17% when both GH and cortisol were present in the incubation medium during the last 24 h (P < 0.01). The reduction in LPL activity in response to GH was not accompanied by a decrease in the level of LPL mRNA measured by a solution hybridization ribonuclease protection assay. In adipose tissue incubated in the control medium for 6 days, the addition of GH alone during the last 24 h caused an insignificant decrease in heparin-releasable LPL activity. Low control activities limited the scope for further decrease. It is concluded that GH counteracts the potent stimulatory effect of glucocorticoids on LPL activity without affecting LPL mRNA levels. Therefore, the inhibition of LPL activity by GH probably occurs during translation and/or posttranslational processing of the enzyme, and the mechanism may involve a decreased channeling of the lipase to the cell surface.     

Grape juice but not orange or grapefruit juice inhibits platelet activity in dogs and monkeys

Platelet aggregation (PA) contributes to both the development of atherosclerosis and acute platelet thrombus formation (APTF) followed by embolization producing cyclic flow reductions (CFR) in stenosed and damaged dog and human coronary arteries. In seven anesthetized dogs with coronary stenosis and medial damage, CFR occurred at 7 +/- 3/30 min and were abolished 127 +/- 18 min after gastric administration of 10 mL of purple grape juice/kg. Collagen-induced ex vivo whole blood PA decreased by 49 +/- 9% after the abolishment of CFR with grape juice. Ten mL of orange juice/kg (n = 5) and 10 mL of grapefruit juice/kg (n = 5) had no significant effect on the frequency of the CFR or on ex vivo PA. In vitro studies have suggested that flavonoids bind to platelet cell membranes and thus may have an accumulative or tissue-loading effect over time. To test this we fed 5 mL of grape juice/kg to 5 cynomologous monkeys for 7 d. Collagen-induced ex vivo PA decreased by 41 +/- 17% compared to control (pre-reatment) after 7 d of feeding. In the same 5 monkeys, neither 5 mL of orange juice/kg nor 5 mL of grapefruit juice/kg given orally for 7 d produced any significant change in PA. Grape juice contains the flavonoids quercetin, kaempferol and myricetin, which are known inhibitors of PA in vitro. Orange juice and grapefruit juice, while containing less quercetin than grape juice, primarily contain the flavonoids naringin, luteolin and apigenin glucoside. The flavonoids in grapes were shown in vitro to be good inhibitors of PA, whereas the flavonoids in oranges and grapefruit to be poor inhibitors of PA. The consumption of grape juice, containing these inhibitors of PA, may have some of the protection offered by red wine against the development of coronary artery disease (CAD) and acute occlusive thrombosis, whereas orange juice or grapefruit juice may be ineffective. Thus, grape juice may be a useful alternative dietary supplement to red wine without the concomitant alcohol intake.     

Grapefruit juice and the response to warfarin

The effect if any of prepared frozen grapefruit juice on prothrombin times (PTs) in patients undergoing stabilized warfarin therapy was studied. Patients receiving low-intensity warfarin therapy (targeted International Normalized Ratio [INR], 2-3) who had two consecutive baseline PTs within 10% of each other were recruited. Patients who regularly consumed grapefruit juice or alcohol or who were taking drugs known to interact with grapefruit juice were excluded. A one-week supply of freshly prepared frozen grapefruit juice in individual 8-oz containers was given to all the subjects, who were told to drink the entire contents of on container three times a day for one week. PTs were measured and INRs calculated on the day before grapefruit juice ingestion began (day 0) and a days 2, 6, and 8. Ten men (mean age, 66 years) were enrolled; one withdrew because of diarrhea. Compliance in consuming the juice was reported to range from 85.7% to 100% among patients. There was no significant difference among PT or INR values over the course of the study in any of the nine subjects. Ingestion of grapefruit juice prepared from frozen concentrate did not change PTs in patients treated with warfarin.     

A pharmacokinetic interaction between carbamazepine and olanzapine: observations on possible mechanism

OBJECTIVE: Olanzapine is a novel antipsychotic, which is effective against both the positive and negative symptoms of schizophrenia and causes fewer extrapyramidal adverse effects than conventional antipsychotics. The purpose of the present study was to assess the potential for a pharmacokinetic interaction between olanzapine and carbamazepine, since these agents are likely to be used concomitantly in the treatment of manic psychotic disorder. METHOD: The pharmacokinetics of two single therapeutic doses of olanzapine were determined in 11 healthy volunteers. The first dose of olanzapine (10 mg) was taken alone and the second dose (10 mg) after 2 weeks of treatment with carbamazepine (200 mg BID). Measurement of urinary 6beta-hydroxycortisol/cortisol excretion was used as an endogenous marker to confirm that induction of CYP3A4 by carbamazepine had occurred. RESULTS: The dose of olanzapine given after a 2-week pretreatment with carbamazepine was cleared more rapidly than olanzapine given alone. Olanzapine pharmacokinetic values for Cmax and AUC were significantly lower after the second dose, the elimination half-life was significantly shorter, and the clearance and volume of distribution were significantly increased. CONCLUSION: Carbamazepine has been shown to induce several P450 cytochromes including CYP3A4 and CYP1A2. Since CYP1A2 plays a role in the metabolic clearance of olanzapine, the interaction may be attributed to induction of CYP1A2 by carbamazepine, leading to increased first-pass and systemic metabolism of olanzapine. The interaction is not considered to be of clinical significance because olanzapine has a wide therapeutic index, and the changes in plasma concentration of olanzapine are within the fourfold variation that occurs without concern for safety in a patient population.