Selective inhibitory effects of suberosin on CYP1A2 in human liver microsomes

Suberosin is a natural phytoconstituent isolated from Citropsis articulata, especially employed for its anticoagulant properties. Although metabolic studies assessing suberosin have been conducted, it is possible interactions with drugs and food have not yet been investigated. In the present study, we analyzed the selective inhibitory effects of suberosin on cytochrome P450 (CYP) enzymes using a cocktail probe assay. Various concentrations of suberosin (0-50 μM) were incubated with isoform-specific CYP probes in human liver microsomes (HLMs). We found that suberosin significantly inhibited CYP1A2-catalyzed phenacetin O-deethylation, exhibiting IC50 values of 9.39 ± 2.05 and 3.07 ± 0.45 μM with and without preincubation in the presence of β-NADPH, respectively. Moreover, suberosin showed concentration-dependent, but not time-dependent, CYP1A2 inhibition in HLMs, indicating that suberosin acts as a substrate and reversible CYP1A2 inhibitor. Using a Lineweaver-Burk plot, we found that suberosin competitively inhibited CYP1A2-catalyzed phenacetin O-deethylation. Furthermore, suberosin showed similar inhibitory effects on recombinant human CYP1A1 and 1A2. In conclusion, suberosin may elicit herb-drug interactions by selectively inhibiting CYP1A2 during the concurrent administration of drugs that act as CYP1A2 substrates.

Acetaminophen Has Lipid Composition-Dependent Membrane Interactivity That Could Be Related to Nephrotoxicity but Not to Analgesic Activity and Hepatotoxicity

OBJECTIVE

Although acetaminophen is one of the most widely used over-the-counter drugs, the mechanisms by which this classical drug exerts analgesic, hepatotoxic, and nephrotoxic effects remain unclear. We hypothesized that acetaminophen might act on cellular membranes of nerves, liver, and kidneys. In order to verify this hypothesis, we studied the interactivity of acetaminophen with biomimetic lipid bilayer membranes by comparing with structurally related phenacetin.

METHODS

Liposomal membranes (unilamellar vesicles suspended in the buffer of pH 7.4) were prepared with phospholipids and cholesterol to mimic the membrane lipid composition of neuronal cells, hepatocytes, and nephrocytes. They were subjected to reactions with acetaminophen and phenacetin at clinically relevant concentrations, followed by measuring fluorescence polarization to determine their membrane interactivity to modify membrane fluidity.

RESULTS

Acetaminophen and phenacetin interacted with neuro-mimetic and hepato-mimetic membranes to increase membrane fluidity at 10-100 μM. Both drugs were more effective in fluidizing hepato-mimetic membranes than neuro-mimetic membranes. Although the relative membrane-interacting potency was phenacetin >> acetaminophen in neuro-mimetic and hepato-mimetic membranes, such membrane effects conflicted with their relative analgesic and hepatotoxic effects. Acetaminophen and phenacetin strongly interacted with nephro-mimetic membranes to increase membrane fluidity at 2-100 μM and 0.1-100 μM, respectively. Phenacetin interacted significantly with nephro-mimetic membranes at lower concentrations (<2 μM) than acetaminophen, which was consistent with their relative nephrotoxic effects.

CONCLUSION

In comparison with phenacetin, lipid composition-dependent membrane interactivity of acetaminophen could be related to nephrotoxicity but not to analgesic activity and hepatotoxicity.

Low-diluted Phenacetinum disrupted the melanoma cancer cell migration

Dynamic and reciprocal interactions generated by the communication between tumor cells and their matrix microenvironment, play a major role in the progression of a tumor. Indeed, the adhesion of specific sites to matrix components, associated with the repeated and coordinated formation of membrane protrusions, allow tumor cells to move along a determined pathway. Our study analyzed the mechanism of action of low-diluted Phenacetinum on murine cutaneous melanoma process in a fibronectin matrix environment. We demonstrated a reduction of dispersed cell migration, early and for as long as 24 h, by altering the formation of cell protrusions. Moreover, low-diluted Phenacetinum decreased cell stiffness highly on peripheral areas, due to a disruption of actin filaments located just under the plasma membrane. Finally, it modified the structure of the plasma membrane by accumulating large ordered lipid domains and disrupted B16 cell migration by a likely shift in the balance between ordered and disordered lipid phases. Whereas the correlation between the excess of lipid raft and cytoskeleton disrupting is not as yet established, it is clear that low-diluted Phenacetinum acts on the actin cytoskeleton organization, as confirmed by a decrease of cell stiffness affecting ultimately the establishment of an effective migration process.

Combined use of filtered and edited 1 H NMR spectroscopy to detect 13 C-enriched compounds in complex mixtures

In conventional metabolism and pharmacokinetic studies, radioactive isotopes are used to identify and quantify the breakdown products of xenobiotics. However, the stable isotope (13) C provides a cheaper and less hazardous alternative. Metabolites of (13) C-enriched xenobiotics can be detected, quantified and identified by (13) C-filtered NMR spectroscopy. However, one obstacle to using (13) C is its 1.1% natural abundance that produces a background signal in (13) C-filtered NMR spectra of crude biological extracts. The signal makes it difficult to distinguish between (13) C-enriched xenobiotics resonances from endogenous metabolites unrelated to the xenobiotic. This study proposes that the (13) C background signal can be distinguished from resonances of (13) C-enriched xenobiotics by the absence of a (12) C component in the xenobiotic. This is detected by combined analysis of (13) C-filtered and -edited NMR spectra. The theory underlying the approach is described and the method is demonstrated by the detection of sub-microgram amounts of (13) C-enriched phenacetin in crude extracts of hepatocyte microsomes.

Investigation of cytochrome P450 1A2 and 3A inhibitory properties of Danshen tincture

Danshen (Salvia miltiorrhiza Bunge) as a famous Traditional Chinese medicine is widely used in the treatment of cardiovascular and cerebrovascular diseases in the world. Danshen tincture (DT), extracted from Danshen root with a mixture of water and alcohol, is a commonly used preparation method for human consumption. The aim of this study was to investigate the effects of DT on the cytochrome P450 (CYP) 1A2 and 3A activities by human and rat liver microsomes. Effects of DT were assessed with use of Danshen ethanolic extract (DEE) and selective substrates, markers of CYP activities. DEE (0.5-10 μg/ml) competitively inhibited human and rat liver microsomal CYP1A2 activity with inhibition constant (K(i)) values at 3.40 and 5.16 μg/ml, respectively. At the same time, DEE (2.5-20 μg/ml) not only noncompetitively inhibited human liver microsomal CYP3A4/5 activity with a K(i) of 11.9 μg/ml, but also competitively inhibited rat liver microsomal CYP3A1/2 activity with a K(i) of 52.1 μg/ml. The data indicate that DEE inhibited the metabolism of CYP1A2 and 3A substrates in human and rat liver in vitro with different mode of inhibition. This study may be helpful for clinical application of Danshen tincture.

Evaluation of the activity of P450 enzymes in rats: use of the single marker or combined drug administration

OBJECTIVES

A "cocktail" of several substrates is frequently used to assess metabolic activity of multiple cytochrome P450 enzymes in one session. Some interactions among substrates can appear and may influence the rate of biotransformation of other ones. Our current work was aimed on the influence of tolbutamide on cytochrome P450-mediated metabolism of phenacetin and vice versa.

DESIGN

In the presented work, the biotransformation rates of phenacetin and tolbutamide (markers of rat CYP1A2 and CYP2C6/11 metabolic activities, respectively) administered either separately or both simultaneously were compared. The model of isolated perfused rat liver was used.

RESULTS

Phenacetin had no significant effect on tolbutamide hydroxylation. Tolbutamide addition to the perfusion medium significantly increased the rate of O-deethylation of phenacetin.

CONCLUSION

Some differences in the rate of P450-mediated metabolism can be observed when comparing assessment using combination of two model substrates with the common way (single marker administration). Due to these differences, results obtained by the mentioned methodologies might not be fully comparable.

Effect of sodium ozagrel on the activity of rat CYP2D6

The aim of the study was to investigate the influence of sodium ozagrel on CYP2D6 (cytochromeP450 2D6) activity. The studies were performed with rat urine and liver microsomes and chemical inhibitors. The metabolism of dextromethorphan (dextrophan/dextromethorphan, dextrophan is a metabolite of dextromethorphan) and phenacetin (paracetamol/phenacetin, paracetamol is a metabolites of phenacetin) was used as probe to measure CYP2D6 and CYP1A2 (cytochromeP450 1A2) activity, respectively, determined by high-performance liquid chromatography (HPLC). The results showed that the metabolism of dextrophan/dextromethorphan in the sodium ozagrel-treated group (37 mg/kg) was higher than that of the control (P<0.05/6) in both in vivo and in vitro studies (r=0.9811). The rate of dextromethorphan metabolism was inhibited by sodium ozagrel and cimetidine in rat liver microsomes prepared from sodium ozagrel-treated rats and control rats group (sodium ozagrel IC(50)=26.5 microM, cimetidine IC(50)=86.3 microM in sodium ozagrel-treated group; sodium ozagrel IC(50)=13.9 microM, cimetidine IC(50)=24.8 microM in control group). The inhibitory effect of sodium ozagrel on CYP2D6 activity was noncompetitive with dextromethorphan with a K(i) of 324.94 microM. Kinetic parameters of the reactions were established by using Lineweaver-Burk with K(m)=0.67 mM and V(max)=2.13 pm/min/mg protein for the sodium ozagrel-treated group and K(m)=0.29 mM, and V(max)=0.91 pm/min/mg protein for the control group, respectively. The expression of CYP2D6 protein in the treated group was higher than that of the control group, as determined by Western blotting. The activity and expression of CYP1A2 did not show obvious differences in the control group and sodium ozagrel treated group. In conclusion, sodium ozagrel metabolism in rats is mediated primarily through CYP2D6, and sodium ozagrel can induce CYP2D6 activity.

Effect of acetaminophen, a cyclooxygenase inhibitor, on Morris water maze task performance in mice

Although the mechanism of action of acetaminophen (AAP) is not fully understood, some studies suggest that AAP and phenacetin (PHE) are selective cyclooxygenase (COX)-3 inhibitors. To examine the participation of COX-3 in memory formation, water maze performance was studied in mice treated with AAP, PHE or other COX inhibitors. Mice received intraperitoneal injections of drugs immediately after each training session. Administration of high-dose AAP [302.3 mg/kg (IC50 for COX-2)] or PHE [179.2 mg/kg (IC50 for COX-2)] and of non-specific (indomethacin: 20 mg/kg) or specific COX-2 (NS-398: 10 mg/kg) inhibitor impaired the performance in hidden platform (HP) not visible platform (VP) tasks, whereas low-dose (15.1 mg/kg) AAP facilitated performance in HP and VP tasks. The facilitation of performance by low-dose AAP was reversed by co-administration with a 5-HT(1/2) receptor antagonist (methysergide: 0.47 mg/kg). The middle-dose [69.5 mg/kg (IC50 for COX-3)] of AAP, the PHE [17.9 mg/kg (IC50 for COX-3)] and a specific COX-1 inhibitor (piroxicam: 10-20 mg/kg) did not influence performance in either task. These results suggest that the memory impairment by high-dose AAP and PHE and facilitation of performance by low-dose AAP could involve endogenous COX-2 and serotonergic neuronal activity, but not COX-3, respectively.

Acetaminophen, phenacetin and dipyrone do not modulate pressor responses to arachidonic Acid or to pressor agents

In contrast to nonsteroidal anti-inflammatory drugs (NSAIDs), the nonopioid analgesics phenacetin, acetaminophen and dipyrone exhibit weak anti-inflammatory properties. An explanation for this difference in pharmacologic activity was provided by the recent discovery of a new cyclooxygenase isoform, cyclooxygenase (COX)-3, that is reported to be inhibited by phenacetin, acetaminophen and dipyrone. However, COX-3 was found to be a spliced variant of COX-1 and renamed COX-1b. Although recent studies provide evidence for the existence of this new COX isoform, it is uncertain whether this COX-3 (COX-1b) isoform, or putative acetaminophen-sensitive pathway, plays a role in the generation of vasoactive prostaglandins. NSAIDs increase systemic blood pressure by inhibiting the formation of vasodilator prostanoids. Angiotensin II, norepinephrine and other vasoconstrictor agents have been reported to release prostaglandins. It is possible that this acetaminophen-sensitive pathway also modulates pressor responses to these vasoconstrictor agents. Therefore, the purpose of the present study was to determine whether this acetaminophen-sensitive pathway plays a role in the generation of vasoactive products of arachidonic acid or in the modulation of vasoconstrictor responses in the pulmonary and systemic vascular bed of the intact-chest rat. In the present study, the nonopioid analgesics did not attenuate changes in pulmonary or systemic arterial pressure in response to injections of the prostanoid precursor, arachidonic acid, to the thromboxane A(2) mimic, U46619, or to angiotensin II or norepinephrine. The results of the present study do not provide evidence in support of a role of a functional COX-3 (COX-1b) isoform, or an acetaminophen-sensitive pathway, in the generation of vasoactive prostanoids or in the modulation of responses to vasoconstrictor hormones in the intact-chest rat.

In vitro interactions of water-soluble garlic components with human cytochromes p450

Eight water-soluble components of aged garlic extract were evaluated to assess their potential to inhibit the activity of human cytochrome-P450 (CYP) enzymes. The in vitro model consisted of human liver microsomes with index reactions chosen to profile the activity of the following six CYP isoforms: CYP1A2, 2B6, 2C9, 2C19, 2D6, and 3A. With only 2 exceptions, none of the 8 garlic components produced >50% inhibition even at high concentrations (100 micromol/L). S-methyl-L-cysteine and S-allyl-L-cysteine at 100 micromol/L produced modest inhibition of CYP3A, reducing activity to 20-40% of control. However available clinical evidence does not indicate CYP3A inhibition in vivo. The findings suggest that drug interactions involving inhibition of CYP3A enzymes by aged garlic extract are very unlikely.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Effects of phenacetin and its metabolite p-phenetidine on COX-1 and COX-2 activities and expression in vitro

The present study was aimed to test the possible cyclooxygenase (COX)-1/COX-2 selectivity of the old analgesic drug phenacetin and its metabolite p-phenetidine, which exhibits high renal toxicity. Paracetamol (acetaminophen), the main metabolite of phenacetin with low renal toxicity, and indomethacin were selected as reference compounds. Collagen-stimulated platelet thromboxane B2 (TxB2) production and phorbol 12-myristate-13-acetate (PMA)-induced neutrophil prostaglandin E2 (PGE2) synthesis were used as indicators for COX-1 and COX-2 activity, respectively. Phenacetin was even less potent than paracetamol to reduce the production of both TxB2 and PGE2, and no clear preference for either of the COX-enzymes was seen. P-phenetidine was a more potent inhibitor, already at nanomolar level, of the synthesis of these prostanoids than indomethacin and showed some preference to COX-2 inhibition. Somewhat higher, micromolar, concentrations of p-phenetidine also reduced COX-2 expression in neutrophils. We suggest that the very potent inhibitory activity of p-phenetidine on PGE2 synthesis combined with the reduction of COX-2 expression could explain the renal papillary necrosis in phenacetin kidney.

Identification of cytochrome P450 1A2 as enzyme involved in the microsomal metabolism of Huperzine A

Huperzine A is a reversible and selective cholinesterase inhibitor approved for the treatment of Alzheimer's disease. To identify which cytochrome P450 (CYP) isoenzymes are involved in the metabolism of Huperzine A, an in vitro study was performed with rat liver microsomes and immunoinhibition and chemical inhibition methods. Huperzine A metabolism was analyzed with high-performance liquid chromatography (HPLC) and expressed as Huperzine A disappearance rate. Result showed that 76.2% of Huperzine A metabolism was inhibited by CYP1A2 antibody and 17.8% by CYP3A1/2 antibody. Inhibitory effects produced by CYP2C11 and 2E1 antibodies were minor. The CYP1A2 substrate phenacetin showed an inhibitory effect of 70.3%. In conclusion, Huperzine A metabolism in rat liver microsomes is mediated primarily by CYP1A2, with a probable secondary contribution of CYP3A1/2. CYP2C11 and 2E1 are likely not involved in Huperzine A metabolism.

Comparison of vasoconstrictor responses to selected NSAIDs in rabbit renal and femoral arteries

BACKGROUND

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been shown to induce adverse renal effects, which are closely related to physiological inhibition of renal prostaglandin synthesis.

AIM

This study was aimed to evaluate the effect of drugs inhibiting both cyclooxygenase (COX) isoforms COX-1 and COX-2 on vasoconstrictor responses to noradrenaline in the rabbit renal artery and to compare these responses with femoral artery as a systemic vessel.

METHODS

Rabbit femoral and renal arteries were perfused with a constant flow. Vascular responses to drugs were measured and registered as changes in perfusion pressure.

RESULTS

It was found that the vasoconstrictor responses to noradrenaline were significantly enhanced after administration of all NSAIDs in both the renal and femoral arteries. The effect of indomethacin on renal vasoconstrictor responses was more pronounced compared to ibuprofen or phenacetin. Comparison of NSAIDs effects on renal and femoral arteries did not show significant differences.

CONCLUSIONS

These results demonstrate an increase of vasoconstrictor activity after NSAIDs administration without significant differences between the renal and femoral arteries. The strongest potentiation of the vasoconstrictor responses in the renal artery was found with indomethacin. (Tab. 1, Fig. 4, Ref. 20.)

Renal PGE2 production in the human and rat following phenacetin, acetaminophen and p-aminophenol

Co-oxidation of phenacetin, acetaminophen (APAP) and p-aminophenol (p-AP) by prostaglandin H synthase (PHS) was investigated in human and rat renal microsomes. The formation of prostaglandin E2 (PGE2) was assessed in cortex, outer and inner medulla following phenacetin, APAP and p-AP (0-5 mM) incubations. For all compounds and concentrations tested, a significantly higher PGE2 production was observed in inner medulla compared to cortex. Rat inner medulla incubated with phenacetin resulted in an increased formation of PGE2 at all concentrations compared to control (1 mM phenacetin increased production by 243%). Human inner medulla demonstrated an increased PGE2 production at 1, 3 and 5 mM phenacetin versus control (136% increase at 1 mM). An increase in PGE2 formation in rat and human inner medulla was observed at low APAP concentrations (0.1, 0.3, 0.5 and 1 mM) compared to control (216% and 396% in human and rat respectively following 1 mM APAP). 5 mM APAP inhibited PGE2 formation in the rat inner medulla but not in human inner medulla. An inhibition of PGE2 production by 5 mM p-AP was observed in both the rat and human inner medulla. In the rat PGE2 production was inhibited 69% by 5 mM, whereas in the human the inhibition was 76% at 5 mM. These studies demonstrate a species-specific PHS-mediated renal metabolism of APAP, with the human kidney demonstrating a continous formation of reactive metabolites at high concentrations of APAP. However, phenacetin and p-AP are metabolized in a similar manner in these 2 species.

Differential inhibition of human CYP1A1 and CYP1A2 by quinidine and quinine

1. The inhibition of recombinant CYP1A1 and CYP1A2 activity by quinidine and quinine was evluated using ethoxyresorutin O-deethylation, phenacetin O-deethylation and propranolol desisopropylation as probe catalytic pathways. 2. With substrate concentrations near the Km of catalysis, both quinidine and quinine potently inhibited CYP1A1 activity with [I](0.5) approximately 1-3 microM, whereas in contrast, there was little inhibition of CYP1A2 activity. The Lineweaver-Burk plots with varying inhibitor concentrations suggested that inhibition by quinidine and quinine was competitive. 3. There was only trace metabolism of quinidine by recombinant CYP1A1, whereas rat liver microsomes as a control showed extensive consumption of quinidine and metabolite production. 4. This work suggests that quinidine is a non-classical inhibitor of CYP1A1 and that it is not as highly specific at inhibiting CYP2D6 as previously thought.

Phenacetin, acetaminophen and dipyrone: analgesic and rewarding effects

The antinociceptive and rewarding effects of phenacetin, a mild analgesic with abuse liability, were compared with those of acetaminophen, dipyrone and indomethacin in the formalin and conditioned place preference tests. Phenacetin, acetaminophen and dipyrone attenuated the pain response, beginning at 50, 50 and 100 mg/kg, respectively. Systemically active drugs produced weaker antinociception when injected into the paw or intracerebroventricularly at doses approximately 10(3) lower than those required for systemic effects; dose effect relations were bell-shaped by the intracerebroventricular route. By all three routes, there was a clear ceiling to the effects of acetaminophen that is consistent with its clinical efficacy. Systemic phenacetin and acetaminophen produced a conditioned place preference at doses that produced antinociception. Dipyrone produced a place aversion by the systemic route and a place preference intracerebroventricularly. The latter had a bell-shaped dose effect relationship identical to that in the formalin test. Indomethacin was inactive in all tests, except for mild hyperalgesia by the intraventricular route. The results indicate that the antinociceptive effects of phenacetin, acetaminophen and dipyrone reflect a combination of peripheral and central actions, neither of which involves inhibition of cyclooxygenase. The central component of the antinociceptive effects may be related to activation of brain mechanisms that are involved in reinforcement.

Expression and induction of CYP1A1/1A2, CYP2A6 and CYP3A4 in primary cultures of human hepatocytes: a 10-year follow-up

1. The aims were to refine experimental conditions (using 76 human hepatocyte preparations) in terms of the selection of enzyme inducers and their optimal concentration, the treatment duration with inducers and the choice of specific cytochrome P450 isoform(s) probes to optimize the use of primary hepatocytes for predicting the potential induction by new chemical entities of cytochrome P450 isoforms in vivo in man. 2. In the absence of any inducer, basal cytochrome P450 isoform(s)-mediated activities decreased to 20% of their initial activity (end of the seeding period) by 72-96 h. In contrast, UGT-dependent enzyme activities remained at a constant level (+/- 20%) up to the fifth day of culture. 3. Beta-naphthoflavone, at an optimal concentration of 50 microM and after a 3-day treatment, specifically and potently induced 7-ethoxyresorufin (10.4 +/- 10.4-fold, n = 74) and phenacetin (6.6 +/- 6.4-fold, n = 60) O-deethylation processes, markers for CYP1A1 and CYP1A2 isoforms respectively. Only a 2-fold increase was noted following treatment with 2 mM phenobarbitone, whereas dexamethasone and rifampicin had no effect at all. 4. A 3-day treatment of human hepatocytes with 50 microM dexamethasone was associated with a major induction of both coumarin 7-hydroxylation (9.4 +/- 11.4-fold, n = 49) and nifedipine dehydrogenation (4.7 +/- 3.8-fold, n = 61), markers for CYP2A6 and CYP3A4 respectively. Phenobarbitone, however, exhibited a broad but moderate inducing effect on 7-ethoxyresorufin (2.2 +/- 1.5-fold, n = 55) and phenacetin (1.7 +/- 0.9-fold, n = 54) O-deethylation, coumarin 7-hydroxylation (3.9 +/- 9.2-fold, n = 50) and nifedipine dehydrogenation (2.1 +/- 2.0-fold, n = 47). 5. Km obtained for the different cytochrome P450 isoform substrates in untreated hepatocytes were in the same range of magnitude that those determined on human hepatic microsomal fractions. Enzyme induction processes were characterized by a large increase in apparent Vmax whereas apparent Km were not affected. 6. These studies demonstrate that human hepatocytes in primary culture can respond specifically and quantitatively to model inducers. This in vitro system offers a useful approach to study the regulation of human hepatic biotransformation activities and should facilitate the demand for a reproducible method for addressing cytochrome P450 induction.

Oxidation of histamine H1 antagonist mequitazine is catalyzed by cytochrome P450 2D6 in human liver microsomes

Mequitazine [10-(3-quinuclidinylmethyl) phenothiazine] is a long-acting and selective histamine H1-receptor antagonist that is mainly biotransformed by human liver microsomes to yield hydroxylated and S-oxidized metabolites. Mequitazine hydroxylase was inhibited by propranolol and quinidine. Lineweaver-Burk plots for the hydroxylation and the S-oxidation indicated that the hydroxylation occurred with a low Km (0.72 +/- .26 microM) in human liver microsomes. Microsomes from genetically engineered human B-lymphoblastoid cells expressing cytochrome P450 2D6 (CYP2D6) efficiently metabolized mequitazine to the hydroxylated and S-oxidized metabolites. The results indicate that CYP2D6 isozyme is a major form of CYP responsible for the metabolism of mequitazine in human liver microsomes. Inhibition of CYP3A-catalyzed midazolam 1'-hydroxylase by various histamine H1 antagonists, including mequitazine, suggested that mequitazine and some other histamine H1 antagonists could also be inhibitors of CYP3A in human liver microsomes.

Inhibition of human platelet cathepsin A by non-steroidal anti-inflammatory drugs--in vitro study

Since the lysosomal proteases appear at site of inflammation they may be a target for non-steroidal anti-inflammatory drugs (NSAIDs). In in vitro study the inhibition of human platelet cathepsin A by NSAIDs was found. Indomethacin, phenacetin and aminophenazone were the most potent inhibitors of cathepsin A. Acetylsalicylic acid added to platelet lysate inhibited cathepsin A in the same extent as salicylate. The inhibition was time-dependent and reversed after dialysis. Mixed type of inhibition by salicylate was shown.

Identification of human CYP isoforms involved in the metabolism of propranolol enantiomers--N-desisopropylation is mediated mainly by CYP1A2

1. Studies using human liver microsomes and six recombinant human CYP isoforms (i.e. CYP1A2, 2A6, 2B6, 2D6, 2E1 and 3A4) were performed to identify the cytochrome P450 (CYP) isoform(s) involved in the ring 4-hydroxylation and side-chain N-desisopropylation of propranolol enantiomers in humans. 2. alpha-Naphthoflavone and 7-ethoxyresorufin (selective inhibitors of CYP1A1/2) inhibited the N-desisopropylation of R- and S-propranolol by human liver microsomes by 20 and 40%, respectively, while quinidine (a selective inhibitor of CYP2D6) abolished the 4-hydroxylation of both propranolol enantiomers almost completely. In contrast, sulphaphenazole (CYP2C8/9 inhibitor), S-mephenytoin (CYP2C19 inhibitor), troleandomycin (CYP3A3/4 inhibitor) and diethyldithiocarbamate (CYP2E1 inhibitor) elicited only weak inhibitory effects on propranolol metabolism via the two measured metabolic pathways. 3. Significant (P < 0.01) correlations were observed between the microsomal N-desisopropylation of both propranolol enantiomers and that for the O-deethylation of phenacetin among the 11 different human liver microsome samples (r = 0.98 and 0.77 for R- and S-propranolol, respectively). A marginally significant (r = 0.60, P congruent to 0.05) correlation was also observed between N-desisopropylation of S-, but not of R-propranolol and the 4'-hydroxylation of S-mephenytoin. No significant correlations were observed between the N-desisopropylation of propranolol enantiomers and the 2-hydroxylation of desipramine, the hydroxylation of tolbutamide or the 6 beta-hydroxylation of testosterone. 4. Significant (P < 0.01) correlations were observed between the microsomal 4-hydroxylation of R- and S-propranolol and the 2-hydroxylation of desipramine (r = 0.85 and 0.98, respectively). A weak (r = 0.66), albeit significant (P < 0.05) correlation was observed between the 4-hydroxylation of R-, but not of S-propranolol and the hydroxylation of tolbutamide. No significant correlations were observed between the 4-hydroxylation of propranolol enantiomers and the oxidation of other substrates for CYP1A2, 2C19, and 3A3/4. 5. Recombinant human CYP1A2 and CYP2D6 exhibited comparable catalytic activity with respect to the N-desisopropylation of both propranolol enantiomers; only expressed CYP2D6 exhibited a marked catalytic activity with respect to the 4-hydroxylation of both propranolol enantiomers.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification by analgesics of lesion development in the urinary tract and various other organs of rats pretreated with dihydroxy-di-N-propylnitrosamine and uracil

Effects of the analgesics phenacetin, acetaminophen and antipyrine on lesion development in the urinary tract and other organs in male F344 rats were investigated. Animals were concurrently administered with 0.1% dihydroxy-di-N-propylnitrosamine (DHPN) in drinking water and 3.0% uracil in the diet for 4 weeks and then, starting 1 week after the cessation of this treatment, received basal diet or diet containing phenacetin, acetaminophen or antipyrine for 35 weeks. The occurrences of renal cell tumors were increased in the groups given phenacetin or antipyrine, as compared with the DHPN + uracil alone controls. Antipyrine, but not the two other compounds, also enhanced development of hyperplastic lesions in the renal pelvis and ureter. In the urinary bladder, phenacetin and antipyrine treatments were both associated with increased incidence of preneoplastic or neoplastic lesions. Furthermore, phenacetin alone, without the initiating agent pretreatments, induced simple hyperplasias of the urinary bladder at high incidence. Antipyrine enhanced induction of hyperplastic lesions in the ureter and was also found to increase the incidences of preneoplastic and neoplastic lesions in the liver. Although decreased incidences of tumor development of lung and thyroid were observed for the group given phenacetin, this might have been linked to the decreased weight gain. The results confirmed that combination treatment with DHPN + uracil is effective for wide-spectrum initiation of carcinogenesis in the urological tract and demonstrated significant modification potential for both phenacetin and antipyrine.

Taxol metabolism in rat hepatocytes

Metabolism of the anticancer drug taxol was investigated in freshly isolated rat hepatocytes. Two main metabolites were separated by reversed-phase HPLC and shown by tandem mass spectrometry to be monohydroxylated metabolites. Kinetic studies revealed apparent Km values of 68 and 61 microM with identical Vmax values for the two metabolites. Verapamil and midazolam, but not phenacetin, showed concentration-dependent inhibition of taxol metabolism with both metabolites being affected equally. The IC50 was about 100 microM for verapamil and 25 microM for midazolam. These observations demonstrate for the first time in vitro metabolism of taxol and suggest that the metabolism may be subject to potentially important interactions with numerous other drugs.

Effects of the phenacetin metabolite 4-nitrosophenetol on the glutathione status and the transport of glutathione S-conjugates in human red cells

The extent of ferrihemoglobin formation in human erythrocytes by 4-nitrosophenetol and its metabolisation rate strongly depended on the availability of cellular GSH. Ferrihemoglobin formation rate was increased by inhibition of the red cell glutathione reductase, and 4-nitrosophenetol disappeared more slowly. When red cells were completely depleted from SH groups, ferrihemoglobin formation was retarded, despite 4-nitrosophenetol was hardly metabolized. In turn, the glutathione status of human red cells was strongly affected by 4-nitrosophenetol. GSSG, which was produced in large amounts, was reduced, as long as the reducing system was intact. The decreased total glutathione content, however, did not recover completely, indicating formation of stable glutathione S-conjugates. The active export of the stable model glutathione thioether S-(2,4-dinitrophenyl)glutathione was strongly inhibited by 4-nitrosophenetol. A Lineweaver-Burk plot of the transport data suggested a competitive inhibition mechanism, presumably caused by glutathione adducts. The results indicate that the strong pi-donor substituent in 4-nitrosophenetol enables metabolic reactions with glutathione, producing biological effects hitherto not observed with nitrosobenzene. Bicyclic arylamines and glutathione S-conjugates may cause ferrihemoglobin formation that is not brought about by the diaphorase reaction. The latter may be responsible for transport inhibition of GSSG and other glutathione S-conjugates.

Caffeine, estradiol, and progesterone interact with human CYP1A1 and CYP1A2. Evidence from cDNA-directed expression in Saccharomyces cerevisiae

Heterologous expression of cytochrome P-450 cDNAs in yeast is a potent instrument for the study of enzyme-specific parameters and can be used to answer questions with regard to substrate specificity as well as drug interaction in a background with no interfering activities. Two cDNAs of human CYP1A1 and CYP1A2 were expressed in yeast Saccharomyces cerevisiae, and microsomes of transformed strains contained substantial amounts of functional heterologous enzymes. Enzyme kinetics with 7-ethoxyresorufin as substrate resulted in KM values of 0.017 and 1.67 microM and Vmax values of 840 and 387 pmol/mg/min for CYP1A1 and CYP1A2, respectively. Both heterologous enzymes showed an overlapping substrate specificity pattern assayed with different phenoxazone ethers and caffeine. Caffeine was shown to be metabolized by CYP1A2 and CYP1A1. Both enzymes formed paraxanthine and minor amounts of theobromine; however, trimethyluric acid was exclusively formed by CYP1A1. The fact that theophylline was not formed by either enzyme anticipates the involvement of additional enzyme(s) in the primary metabolism of caffeine. Inhibition studies with caffeine, phenacetin, 17 beta-estradiol, and progesterone as inhibitors of the CYP1A1 and CYP1A2 catalyzed O-deethylation of 7-ethoxyresorufin suggest all compounds as possible substrates of CYP1A enzymes. 17 beta-estradiol inhibited CYP1A1-catalyzed paraxanthine and trimethyluric acid formation. In contrast 17 beta-estradiol did not inhibit CYP1A2-catalyzed formation of primary caffeine metabolites. These data clearly demonstrate the capacity of human CYP1A1 and CYP1A2 to metabolize caffeine. Furthermore, possible consequences of CYP1A enzyme inhibition by caffeine, phenacetin, 17 beta-estradiol, and progesterone will be discussed.

Effects of the phenacetin metabolite 4-nitrosophenetol on glycolysis and pentose phosphate pathway in human red cells

Human erythrocytes exposed to 4-nitrosophenetol showed marked alterations of their endogenous metabolism. Rapid ferrihemoglobin formation mediated by the NADPH-dependent enzymic cycling of the nitrosoarene ("Kiese cycle") and extensive GSSG production caused an immediate drain of G-6-P into the pentose phosphate pathway at maximal flow. Despite a 2.4-fold increase in glucose phosphorylation rate and a branching ratio of 97:3 between pentose phosphate pathway and Embden-Meyerhof pathway, the G-6-P supply was obviously insufficient to meet the immense NADPH demand. Thus, a significant recycling of pentose phosphate pathway-derived F-6-P was observed in the order of 65%. Comparison of NADPH regeneration and ferrihemoglobin formation indicates the "Kiese cycle" to be a minor mechanism in ferrihemoglobin production in the case of high 4-nitrosophenetol concentrations. Most probably, reactive intermediates of 4-nitrosophenetol other than N-hydroxy-4-phenetidine, i.e. bicyclic arylamines and glutathione S-conjugates are formed which produce ferrihemoglobin without involvement of NADPH. The experiments have shown that red cells are remarkable robust to tackle the massive oxidative stress as elicited by 4-nitrosophenetol. The immediate metabolic response of the pentose phosphate pathway allows rapid regeneration of reduced glutathione. Thereby, SH-containing enzymes are effectively protected and/or regenerated and hemolysis is kept minimal. Hence, red cells are favourably suited for clearing the blood from N-oxygenated arylamines before they can reach more sensitive target organs.

Nature of cytochromes P450 involved in the 2-/4-hydroxylations of estradiol in human liver microsomes

Kinetics of the 2- and 4-hydroxylations of estradiol (E2) by human liver microsomal samples were studied to determine the major P450 isoform involved in these endogenous reactions. Thirty human liver microsomal samples were analysed. Metabolism of 25 microM [14C]E2 produced 2-hydroxy and 4-hydroxy derivatives with a ratio of 3.2 +/- 1.5 and a great inter-individual variation. Kinetic analysis of the 2- and 4-hydroxylations of E2 exhibited a curvilinear double reciprocal plot with an apparent Km of 15 microM. Further experiments demonstrated that alpha-naphthoflavone, testosterone and progesterone increased the 2-hydroxylation activity, suggesting the involvement of a substrate activation mechanism. These two hydroxylations of E2 were shown to be catalysed by cytochrome P450 with an apparent dissociation constant Ks of 0.8 microM. These 2- and 4-hydroxylations inter-correlated significantly (r = 0.93; N = 30). The 2-hydroxylation of E2 correlated with four monooxygenase activities known to be supported by P450 3A4/3A5, namely nifedipine oxidation (r = 0.78; N = 29); erythromycin N-demethylation (r = 0.69; N = 27), testosterone 6 beta-hydroxylation (r = 0.66; N = 25) and tamoxifen N-demethylation (r = 0.64; N = 29). On the other hand, E2-hydroxylations did not correlate with activities supported by P450 1A2 and P450 2E1. Furthermore, drugs as cyclosporin, diltiazem, triacetyl-oleandomycin and 17 alpha-ethynylestradiol inhibited more than 90% of the E2-hydroxylations at concentrations < 250 microM, while weak inhibition was shown with 500 microM cimetidine and no significant inhibition with caffeine, phenacetin and omeprazole. Finally, 2- and 4-hydroxylations of E2 correlated significantly with the content of P450 3A4/3A5 immunodetected by a monoclonal antibody anti-human P450-nifedipine (r = 0.84; N = 28). E2-hydroxylation activities were inhibited by more than 80% with polyclonal anti-human anti-P450-nifedipine. Preincubation of human liver microsomes with 100 microM gestodene (a suicide substrate of P450 3A4) inactivated this P450 isoform and accordingly allowed evaluation of the contribution of other P450 isoforms to the E2 metabolism to about 21% (+/- 17%, N = 29). All these results taken together suggest that P450 3A4/3A5 are the major forms involved in the formation of catecholestrogens in the human liver microsomes.

Uroporphyrinogen oxidation catalyzed by reconstituted cytochrome P450IA2

Previous work suggested that the oxidation of uroporphyrinogen to uroporphyrin is catalyzed by cytochrome P450IA2. Here we determined whether purified reconstituted mouse P450IA1 and IA2 oxidize uroporphyrinogen. Cytochromes P450IA1 and IA2 were purified from hepatic microsomes from 3-methylcholanthrene (MC)-treated C57BL/6 mice, using a combination of affinity chromatography and high performance liquid chromatography. Reconstituted P450IA1 was more active than P450IA2 in catalyzing ethoxyresorufin-O-deethylase (EROD) activity, whereas P450IA2 was more active than P450IA1 in catalyzing uroporphyrinogen oxidation (UROX). Both reactions required NADPH, NADPH-cytochrome P450 reductase, and either P450IA1 or IA2. Ketoconazole competitively inhibited both EROD and UROX activities, in microsomes from MC-treated mice. Ketoconazole also inhibited UROX catalyzed by reconstituted P450IA2. In contrast, ketoconazole did not inhibit UROX catalyzed by xanthine oxidase in the presence of iron-EDTA. Superoxide dismutase, catalase, and mannitol inhibited UROX catalyzed by xanthine oxidase/iron-EDTA, but did not affect UROX catalyzed by either microsomes or reconstituted P450IA2. These results suggest that UROX catalyzed by P450IA2 in microsomes and reconstituted systems does not involve free reactive oxygen species. Two known substrates of cytochrome P450IA2, 2-amino-3,4-dimethylimidazole[4,5-f]quinoline and phenacetin, were shown to inhibit the microsomal UROX reaction, suggesting that uroporphyrinogen binds to a substrate-binding site on the cytochrome P450.

Evidence for the involvement of several cytochromes P-450 in the first steps of caffeine metabolism by human liver microsomes

Caffeine biotransformation and four monooxygenase activities involving cytochrome P-450IA2, namely ethoxy- and methoxyresorufin O-dealkylases, phenacetin O-deethylase, and acetanilide 4-hydroxylation were studied in 25 human liver microsomes. All these activities were highly significantly intercorrelated (r greater than 0.72, p less than 0.001) and correlated with the level of immunoreactive P-450IA2 content (r greater than 0.65; p less than 0.001). P-450IA content was measured by immunoblotting with anti-rat P-450 beta-naphthoflavone-B, an antibody that detects only a single band corresponding to P-450IA2. The formation rate of two caffeine metabolites, namely paraxathine and theobromine, was correlated with the four monooxygenase activities measured and P-450IA2-specific content (r greater than 0.75). However, inhibition studies of caffeine metabolism by phenacetin, a specific substrate of P-450IA2, clearly indicated that only the N-3 demethylation of caffeine was supported by this enzyme. These in vitro data demonstrate that P-450IA2 is predominantly responsible for the major metabolic pathway of caffeine and that the formation of other demethylated metabolites is mediated, at least partly, by other P-450 enzymes.

Characterization of metronidazole metabolism by human liver microsomes

The metabolism of metronidazole was studied in microsomes isolated from livers of human kidney donors. The formation of the major in vivo metabolite, hydroxymetronidazole, proceeded according to biphasic kinetics, suggesting the involvement of at least two enzymatic sites. The affinity constant (Km) of the high affinity site ranged from 140 to 320 microM and metabolism at this site contributed more than 75% of the intrinsic clearance. Thus, at therapeutic doses of metronidazole most of the hydroxylation in vivo should be associated with this site. Antipyrine, cimetidine, alpha-naphthoflavone, caffeine, theophylline, mephenytoin, tolbutamide, quinidine, acetone and nifedipine were poor inhibitors of the formation of hydroxymetronidazole by human liver microsomes. Propranolol (500 microM) inhibited the hydroxylation rate by 70%. Phenacetin inhibited metronidazole hydroxylation with a competitive inhibition constant (Ki) of 4-5 microM. However, metronidazole did not inhibit the O-deethylation of phenacetin. It is concluded that cytochromes P450 IA2, IIC9, IIC10, IID6, IIE1 and IIIA3 do not contribute significantly to the high affinity hydroxylation of metronidazole in man.

Effects of multiple dosing of phenacetin in the micronucleus test

As a part of the international cooperative study to identify the most sensitive regimen in the micronucleus test, phenacetin was given i.p. to male CD-1 mice at doses of 37.5, 75, 150, 300, 400, and 600 mg/kg once, twice, thrice or four times and the bone marrow cells were harvested 24 h after the final dosing. Positive responses were seen at 600 mg/kg after single and triple dosing and at 400 and 600 mg/kg after double dosing. No dose level gave a positive response after quadruple dosing. A repeated-dosing effect was detected at double and triple dosing. Although triple dosing gave the highest magnitude of micronuclei at 600 mg/kg, double dosing showed a sufficient sensitivity and was more convenient from the viewpoint of selecting a suitable test dose and carrying out the micronucleus test.

Study of the ability of phenacetin, acetaminophen, and aspirin to induce cytotoxicity, mutation, and morphological transformation in C3H/10T1/2 clone 8 mouse embryo cells

Use of the analgesic compounds acetylsalicylic acid (aspirin), phenacetin, and acetaminophen has been correlated with increased risk of renal cancer in humans. Hence, we studied these compounds for ability to induce cytotoxicity, mutation to ouabain resistance, and morphological transformation in cultured C3H/10T1/2 clone 8 (10T1/2) mouse embryo cells. All three compounds were cytotoxic from 0.5-mg/ml to 2-mg/ml concentrations as evidenced by decreased plating efficiency. None of the compounds induced detectable base substitution mutations to ouabain resistance even at cytotoxic concentrations. Aspirin did not induce morphological transformation. Both phenacetin and acetaminophen induced low but concentration-dependent numbers of atypical, weak type II morphologically transformed foci; at equimolar concentrations, phenacetin was 1.1- to 3.0-fold more active in inducing these foci. Neither phenacetin nor acetaminophen was cotransforming with 3-methylcholanthrene, and neither compound promoted cell transformation when added to 3-methylcholanthrene-initiated 10T1/2 cells. The focus-inducing potency of both compounds was increased by addition of an Arochlor-induced hamster liver S9 fraction as an exogenous metabolizing system. However, seven putative metabolites of phenacetin and acetaminophen that were tested--N-hydroxyphenacetin, p-phenetidine, p-aminophenol, p-nitrosophenol, benzoquinone, acetamide, and N-acetyl-p-benzoquinoneimine--were inactive in transformation assays at the concentrations reducing plating efficiency of treated cells to 50% of the plating efficiency of nontreated (control) cells. Several acetaminophen- and phenacetin-induced foci were cloned, expanded into cell lines, and characterized. These cell lines stably formed type II foci when maintained at confluence for 2 to 4 wk in reconstruction experiments with nontransformed 10T1/2 cells; however, they did not exhibit significantly increased saturation density compared to 10T1/2 cells, and they did not grow in soft agarose. These results suggest that metabolic intermediates of high concentrations of phenacetin and acetaminophen induce a low frequency of nonneoplastic morphological transformation of 10T1/2 mouse embryo cells.

The effects of acetaminophen, antipyrine and phenacetin on rat urothelial cell proliferation

Abuse of combination analgesics containing phenacetin, antipyrine (phenazone) and caffeine have been associated with urinary tract tumors. Phenacetin and antipyrine have been shown to be promoters of urinary tract carcinogenesis and antipyrine is also a weak urinary tract carcinogen. Acetaminophen, the main metabolite of phenacetin, is one of the most commonly used analgesics in the USA. In the present study, the dose-related effect on the cell proliferation of the urothelium was evaluated in male Sprague-Dawley rats by autoradiography. Nine groups of twenty, 6-week old rats were treated with 0.5%, 1.0% or 1.5% of acetaminophen, antipyrine or phenacetin in the diet. A tenth group of rats received control diet without added chemicals. Ten rats from each group were killed after each of 6 and 12 weeks of feeding. There was a dose-related increase in the labeling index in the urothelium of the bladder and kidney, particularly after 6 weeks of drug administration. In particular, the 1.0% and 1.5% dose levels of antipyrine and phenacetin showed a marked proliferative effect on the urothelium. In the bladder after 6 weeks, the labeling indices were significantly increased. After 12 weeks, although numerically increased, the indices were not statistically significant. In the renal pelvic urothelium the labeling index was significantly increased in antipyrine and phenacetin treated rats at doses of 1.0% and 1.5%. After 12 weeks the majority of rats treated with 1.5% antipyrine and phenacetin had labeling indices greater than or equal to 2-fold than the control rats both in the kidney and bladder. The increased labeling indices were associated with urothelial hyperplasia, in particular after 6 weeks. In the rats treated with antipyrine there were significant degenerative changes in the urothelial cells expressed as marked vacuolization. The vacuolization is considered to be a toxic effect and the beginning of cell death. Thus cell death with regeneration may be responsible for the increased labeling index in the antipyrine groups. High doses of antipyrine were also associated with renal papillary necrosis in 50% of the rats.

Metabolic activation of phenacetin in rat nasal mucosa: dose-dependent binding to the glands of Bowman

The metabolism and binding of the analgetic drug [ring-3H]phenacetin in the nasal mucosa were studied in vitro and in vivo in male Sprague-Dawley rats. As shown by whole-body and light microscopic autoradiography there was an irreversible binding of metabolites to the glands of Bowman in the olfactory mucosa after high but not after low doses of [3H]phenacetin. In the other tissues, the distribution of radioactivity was not changed when the dose was increased. Autoradiography of [3H]-acetaminophen showed no preferential uptake of radioactivity in the olfactory mucosa. At incubation of nasal septa with [3H]phenacetin in vitro, a binding of metabolites to the glands of Bowman was observed indicating that the metabolism occurred in situ. In rats, glutathione (GSH) depleted by pretreatment with phorone, there was a binding to the glands of Bowman in the olfactory mucosa also after a trace dose of [3H]phenacetin. Addition of GSH decreased the irreversible binding of [3H]phenacetin metabolites that occurred in 9000 X g nasal mucosa supernatants incubated with [3H]phenacetin. There was a moderate decrease in the level of nonprotein sulfhydryl groups, mainly GSH, in the olfactory mucosa after administration of 100-300 mg/kg phenacetin. Collectively, these data suggest that phenacetin is metabolized and subsequent to GSH depletion, bound preferentially in the glands of Bowman. The data also suggest that in situ metabolic activation and binding of phenacetin in the rat nasal mucosa at high doses may play a role in the pathogenesis of the nasal tumors induced by high doses of phenacetin in the rat.

Inhibition of DNA, RNA and protein synthesis and chromatin alteration by N-hydroxyphenacetin

The effects of N-hydroxyphenacetin on DNA function and structure were investigated to elucidate the involvement of phenacetin in analgesic nephropathy and transitional cell carcinoma. N-Hydroxyphenacetin or a metabolite inhibited synthesis of DNA, RNA and protein; DNA inhibition was greater at higher pH. No single-strand breaks were detectable in DNA after N-hydroxyphenacetin treatment and no appreciable effect on cell viability was observed at concentrations up to 5 mM. N-Hydroxyphenacetin-induced alteration to chromatin structure was detected using nucleoid sedimentation analysis. Direct binding to plasmid DNA was not observed. These observations are consistent with a role for phenacetin metabolites in renal disease.

Effect of phenacetin and caffeine on N-butyl-N-(4-hydroxybutyl)-nitrosamine-initiated urothelial carcinogenesis in rats

The present experimental study was undertaken to clarify whether phenacetin and caffeine exert a cocarcinogenic and/or promoting effect on N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN)-initiated urothelial carcinogenesis. BBN was initially administered to female Wistar rats by gavage in 3 consecutive fractionated doses of 100 mg/kg body weight each at 24-hour intervals. Phenacetin was continuously fed at a daily dose of 500 mg/kg body weight, and caffeine was given in the drinking water at a dose of 110 mg/kg body weight/day throughout the experiment. After an experimental period of 21 months the incidence of BBN-induced tumors in the urinary bladder (number of rats with a bladder tumor) had not increased following additional administration of phenacetin alone (47%) or in combination with caffeine (48%) compared with the control group, the animals of which received exclusively BBN (44%). However, there was a significant enhancement of a multifocal tumor development (number of rats with more than 1 tumor in the bladder), when additionally phenacetin was fed alone (44% of the tumor-bearing animals) or in combination with caffeine (47%) compared with the control rats treated with BBN alone which showed only solitary tumors. Similarly, the incidence of a multicentric tumor development had increased, although not significantly, following administration of phenacetin alone or simultaneously with caffeine for 15 months. Caffeine revealed no complete initiating carcinogenic potential for the resting as well as the regenerating bladder urothelium stimulated to proliferate by either a partial cystectomy or cyclophosphamide. Furthermore, no cocarcinogenic and/or promoting activity of caffeine on BBN-initiated bladder tumor development was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

The 3-methylcholanthrene-mimetic effect of 4,4'-dichlorobiphenyl-treatment on phenacetin-induced hepatic glutathione depletion and liver microsomal phenacetin O-deethylation in rats

Both 4,4'-dichlorobiphenyl (4,4'-DCB) and 3-methylcholanthrene (3-MC) caused a substantial increase of phenacetin-induced hepatic glutathione (GSH) depletion, whereas phenobarbital (PB) had no effect, suggesting that 4,4'-DCB possesses cytochrome P-448 inducing activity. The O-deethylation of phenacetin by liver microsomes from control and PB- and 4,4'-DCB-treated rats showed biphasic Michaelis-Menten kinetics, in contrast to the monophasic course after pretreatment with 3-MC. Hepatic phenacetin levels indicated that in vivo interaction with only a high affinity site is involved in the O-deethylation of phenacetin. 4,4'-DCB and 3-MC caused marked increases in intrinsic clearance and extraction ratio of phenacetin, whereas control values were obtained after PB-treatment. Because of an absence of a spectral change at low phenacetin concentrations, it could not be demonstrated whether the observed differences in metabolism should be ascribed to a change in binding of phenacetin to cytochrome P-450. The results of this study indicate that after pretreatment with various enzyme inducers the phenacetin-induced hepatic GSH depletion strongly correlates with microsomal phenacetin O-deethylation. Further, these findings suggest a discrepancy between 4,4'-DCB and PB in cytochrome P-450 inducing activity, as 4,4'-DCB mimics 3-MC in the induction of phenacetin O-deethylase. The difference between 4,4'-DCB and PB is discussed in relation to the multiplicity and induction of cytochrome P-450 isoenzymes.

Urinary enzyme excretion after a single dose of phenacetin and paracetamol (acetaminophen) during antidiuresis and during water diuresis

2 g phenacetin or paracetamol in a single oral dose were administered to five healthy persons under the conditions of antidiuresis and subsequent water diuresis. Excretion of the brush border enzyme GGT, the cytoplasm enzyme LDH, and the lysosomal enzymes, NAG and GAL, was analysed before, during and after ingestion of the analgesics. Increased excretion of LDH and GGT indicated a similar moderate damage of the tubular epithelia after phenacetin and paracetamol. The state of diuresis appeared to have no influence.

Effects of kerosene hydrocarbons on tissue metabolism in rats

Kerosene hydrocarbons inhibit respiration of liver and kidney tissues in vitro simultaneously impairing biotransformation of hexobarbital and phenacetin in vivo. An increase in concentration of lactate and pyruvate in the blood and liver tissue, and a decrease in glucose concentration in the blood, reduction of glycogen content in the liver and in the skeletal muscle, with concomitant increase of lactate dehydrogenase activity in the liver were observed. These effects were noted mainly in rats acutely poisoned, while in subchronic poisoning they were less pronounced.

N-hydroxy-2-acetylaminofluorene as effective inhibitor of aldehyde oxidase

N-Hydroxy-2-acetylaminofluorene has been found to be an effective inhibitor of aldehyde oxidase. At concentrations of 1 X 10(-6) M and 1 X 10(-5) M, 38% and 88% inhibition was observed on the oxidase activity towards N1-methylnicotinamide. The inhibition was of noncompetitive type and had a Ki value of 4.4 X 10(-6) M. In contrast, little inhibition of the enzyme was observed with 2-aminofluorene, 2-acetylaminofluorene and acetohydroxamic acid even at a concentration of 1 X 10(-4) M.

N-acetyltransferase multiplicity and the bioactivation of N-arylhydroxamic acids by hamster hepatic and intestinal enzymes

The mechanism-based inactivation (suicide inactivation) by N-hydroxyphenacetin (NHP) of N-arylhydroxamic acid N,O-acyltransferase (AHAT) and p-aminobenzoic acid N-acetyltransferase (PABA NAT) activities of a partially purified hamster liver preparation was investigated. The inactivation of both enzyme activities was irreversible, but a partial protection of PABA NAT could be achieved by inclusion of the nucleophile cysteine in the incubation mixture; cysteine did not reduce the extent of inactivation of AHAT by NHP. Hepatic AHAT and PABA NAT activities were separated by affinity chromatography, and the resolved enzyme activities were subjected to incubation in the presence of NHP, N-hydroxy-2-acetamidofluorene (N-OH-AAF), and N-hydroxy-4-acetamidobiphenyl (N-OH-AABP); AHAT, but not PABA NAT, was inactivated by NHP, N-OH-AAF and N-OH-AABP. Incubation of hamster heptic PABA NAT with radiolabeled N-OH-AAF resulted in the formation of only 15% as much fluorenylamine-tRNA adduct as was formed when N-OH-AAF was bioactivated with hamster hepatic AHAT. Hamster intestinal AHAT and PABA NAT activities also were resolved by affinity chromatography; the intestinal AHAT fractions were much more effective than the PABA NAT fractions in bioactivating N-OH-AAF. These results demonstrate that hamster liver and intestine contain at least two arylamine transacetylating activities, one of which is much more effective than the other in the bioactivation of toxic and carcinogenic N-arylhydroxamic acids.

Paracetamol and phenacetin

Since their synthesis in the late 1800s paracetamol (acetaminophen) and phenacetin have followed divergent pathways with regard to their popularity as mild analgesic/antipyretic drugs. Initially, paracetamol was discarded in favour of phenacetin because the latter drug was supposedly less toxic. Today the opposite is true, and paracetamol, along with aspirin, has become one of the two most popular 'over-the-counter' non-narcotic analgesic agents. This marked increase in the wide approval attained by paracetamol has been accompanied by the virtual commercial demise of phenacetin because of its role, albeit somewhat circumstantial, in causing analgesic nephropathy. Both paracetamol and phenacetin are effective mild analgesics, suitable for treating mild to moderate pain, and their actions are broadly comparable with those of aspirin and related salicylates, although they do not appear to possess significant anti-inflammatory activity. Since a major portion of a dose of phenacetin is rapidly metabolised to paracetamol, it seems possible that phenacetin owes some of its therapeutic activity to its main metabolite, paracetamol, whereas its most troublesome side effect (methaemoglobinaemia) is due to another metabolite, p-phenetidine. The mechanism of action of paracetamol is poorly defined, although it has been speculated that it may selectively inhibit prostaglandin production in the central nervous system, which would account for its analgesic/antipyretic properties. The lack of any significant influence on peripheral cyclooxygenase would explain the absence of anti-inflammatory activity. At therapeutic doses paracetamol is well tolerated and produces fewer side effects than aspirin. The most frequently reported adverse effect associated with paracetamol is hepatotoxicity, which occurs after acute overdosage (usually doses greater than 10 to 15g are needed) and, very rarely, during long term treatment with doses at the higher levels of the therapeutic range. Paracetamol damages the liver through the formation of a highly reactive metabolite which is normally inactivated by conjugation with glutathione. Overdoses of paracetamol exhaust glutathione stores, thus allowing the accumulation of this toxic metabolite which covalently binds with vital cell elements and can result in liver necrosis. Glutathione precursors (notably intravenous N-acetylcysteine) have proved remarkably successful in treating paracetamol overdose, as long as treatment is initiated within 10 hours.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of nonsteroidal anti-inflammatory drugs on prostaglandins and renal function

In the past 15 years, there has been an explosion in the number of nonsteroidal anti-inflammatory drugs on the market. Along with this explosion have come increasing reports of the physiologic and pathologic changes seen in the kidneys. This report reviews the effects of prostaglandins on the kidney and the physiologic changes that result when prostaglandin synthesis is blocked. The world literature on renal complications of nonsteroidal anti-inflammatory drugs is reviewed and 274 cases of acute renal disease associated with their use are reported. The following cases are described: nephrotic syndrome (34); acute interstitial nephritis (51); acute tubular necrosis (29); papillary necrosis (53); poor perfusion with renal failure (40); acute glomerulitis or vasculitis (13); and unspecified renal failure (102). Fenoprofen appeared to be more nephrotoxic than other nonsteroidal anti-inflammatory drugs and resulted in multiple renal lesions in the same patient.

Pitfalls in the interpretation of whole-body autoradiograms: long-time retention in brain and adrenal cortex caused by metabolic incorporation of 14C from various labelled xenobiotics

The long-time retention of radioactivity in mice was studied by whole-body autoradiography after administration of 1-14C-alkanes (C12 and C16), 1-14C-polychlorododecanes (56 and 68% Cl w/w), U-14C-polychlorohexadecane (23% Cl w/w), ethyl-14C-phenacetin and ring-3H-phenacetin. All the labelled xenobiotics, except the high-chlorinated (68% Cl) polychloroalkane and the ring-3H-phenacetin, gave rise to long-time (12-60 days) retention of radioactivity in the central nervous system and in the adrenal cortex; the distribution of radioactivity within the brain corresponded to the stain intensity of myelin stained sections. Administration of 1-14C-fatty acids (C12 and C16) and 1-14C-acetylcoenzyme A gave a similar distribution pattern. The lipophilic radioactivity in brain and adrenal tissue was extracted and separated with thin-layer chromatography. In the adrenal extracts, the label co-chromatography mainly with cholesteryl ester, and in the brain extracts with cholesterol and with more polar lipids (mainly phosphatidyl-choline and -ethanolamine). The brain homogenate contained a non-extracted, probably proteinaceous, residue, with comparably high radioactivity. The results show that several 14C-labelled xenobiotics which give long time retention of radioactivity in the adrenal cortex and brain, are degraded to intermediates with the possibility to become incorporated into endogenous substances. The high-chlorinated alkane (1) and ring-3H-phenacetin (2) did not give such long time retention due to its persistance towards degradation (1), and lack of labelling of the degradable part of the molecule (2). It is concluded that erroneous interpretations can be drawn from distribution studies if the routes of degradation and positions of label of the 14C-labelled compounds are not considered.