In vitro metabolism of dexamethasone (DEX) in human liver and kidney: the involvement of CYP3A4 and CYP17 (17,20 LYASE) and molecular modelling studies

Dexamethasone (DEX) has previously been shown to be extensively metabolised to 6-hydroxylated and side-chain cleaved metabolites in human liver in vitro. CYP3A4 is responsible for 6alpha- and 6beta-hydroxylation of DEX and CYP17 is thought to mediate side-chain cleavage to generate 9alphafluoro-androsta-1,4-diene-11beta-hydroxy-16alpha-methyl-3,17-dione (9alphaF-A). Although 9alphaF-A has not previously been isolated as a metabolite in its unhydroxylated form in human liver incubations, it is formed as an intermediate metabolite, which is subsequently rapidly hydroxylated to OH-9alphaF-A. A main part of this study has been to conclusively show that DEX undergoes extensive side-chain cleavage to form 9alphaF-A in human kidney fractions, which is in contrast to profiles obtained for DEX metabolism in parallel human liver microsomal incubations where 6-hydroxylation is the predominant pathway. Furthermore, molecular models of CYP3A4 and CYP17 (17,20 lyase) have been used to model the enzyme fits of DEX. From these modelling studies it has been shown that DEX complements both putative enzyme active sites in orientations likely to lead to the formation of the metabolites identified in vitro. We have also been able to rationalise the preferential formation of the 6betaOH-DEX isomer.

The relationship between the disposition and immunogenicity of sulfamethoxazole in the rat

Idiosyncratic toxicity associated with sulfamethoxazole (SMX) is thought to be a consequence of bioactivation to the hydroxylamine metabolite (SMX-NOH) and further oxidation to the ultimate reactive metabolite, nitroso-sulfamethoxazole (SMX-NO). To establish the link between the formation of the ultimate reactive metabolite and SMX hypersensitivity, we have undertaken metabolism and immunogenicity studies in the rat by use of SMX and its metabolites. SMX was excreted in urine as N4-acetyl SMX and SMX-NOH, with approximately 10% remaining unchanged as parent amine. After administration of SMX-NOH (54 mg x kg(-1)) and SMX-NO (10 mg x kg(-1)), 38.3% and 46.1% of the doses, respectively, were excreted in urine as SMX and N4-acetyl SMX, which indicated extensive reduction of these metabolites in vivo. The immunogenic potential of SMX and its metabolites, SMX-NOH and SMX-NO, were assessed in rats by analyzing serum samples for the presence of anti-SMX IgG antibodies during a 4-week dosing period. No antibodies to SMX were detected in either control or SMX-treated rats. In contrast, a high titer of SMX-specific IgG antibody was present in sera from all the rats administered SMX-NO, reaching a maximum 14 to 21 days after the initial dose. Rats administered SMX-NOH only produced a weak IgG response after 3 weeks of dosing. These findings indicate that SMX-NO is highly immunogenic and may be responsible for the hypersensitivity reactions associated with SMX. Both SMX-NOH and SMX-NO undergo extensive reduction in vivo which may afford protection against SMX toxicity.

Dexamethasone metabolism in vitro: species differences

Dexamethasone (DEX) is extensively metabolized to 6-hydroxyDEX (6OH-DEX) and side-chain cleaved metabolites in human liver both in vitro and in vivo with CYP3A4 responsible for the formation of 6-hydroxylated products. In the present study, the metabolism of [3H]DEX has been examined in the liver fractions from various mammalian species and metabolite profiles compared with those obtained with human liver microsomes. Metabolites were quantified by radiometric high-pressure liquid chromatography (HPLC) and characterized by liquid chromatography-mass spectrometry (LC-MS) and co-chromatography with chemical standards, where available. 6OH-DEX formation was quantified for each species and the inhibitory potency of ketoconazole at 1 and 20 microM determined. Glycyrrhetinic acid, a specific inhibitor of 11-dehydrogenase, was also used to determine the extent of reductive DEX metabolism. Species differences in metabolite profiles obtained from microsomal incubations were both quantitative and qualitative. 6-Hydroxylation was variable (highest in the hamster) and was not always the major route of metabolism, and formation was sex-specific in the rat (male >> female). The inhibition of 6-hydroxylation (CYP3A) by ketoconazole was variable, and indicates that ketoconazole cannot be regarded as a selective inhibitor of CYP3A proteins in all species. Cytosolic incubations produced similar profiles in different species with the formation of a metabolite (M5) which was inhibited by glycyrrhetinic acid and tentatively identified in this study as 11-dehydro-side-chain cleaved DEX (11DH-9alphaF-A). In conclusion, the male rat gave a metabolite profile which was closest to that seen in the human. However, 6-hydroxylation was most extensive in the hamster which may therefore be a suitable model to use for further studies on DEX metabolism by CYP3A.

Gait electromyography in children with myelomeningocele at the sacral level

OBJECTIVE

Patients with sacral level myelomeningocele can be expected to maintain a high level of ambulatory status long into adulthood. Gait deterioration and knee pain reported in this population may be attributed to compensatory movements and increased recruitment of less affected muscle groups to achieve this desired level of ambulation. The objective of this study was to analyze the effect of the solid ankle-foot-orthoses (AFOs) on the muscular activity of selected muscles during walking.

DESIGN

Cohort/outcome.

SETTING

Laboratory.

PATIENTS

Twenty four patients with sacral level myelomeningocele between 4 to 17 years of age.

INTERVENTION

Electromyographic activity of selected muscle groups were studied during barefoot walking and walking with solid AFOs at a self-selected walking velocity.

MAIN OUTCOME MEASURES

Timing of electromyographic activity and sagittal plane knee kinematics. Comparison to normal electromyographic patterns and changes between barefoot and AFO walking conditions.

RESULTS

With the AFOs there was significantly less prolonged stance phase quadriceps activity compared with barefoot walking, although greater than normal activity persisted. There was no change between conditions for the other monitored muscle groups. All muscles elicited greater duration of activity over the course of the gait cycle.

CONCLUSIONS

Our results show that solid AFOs improve the prolonged knee extensor activity evident for barefoot walking. This is clinically relevant to the gait deterioration and knee pain sometimes seen in this patient population. We espouse early and persistent orthotic intervention to reduce compensatory muscular overactivity and maintain gait quality.

Indirect fluorescent IgM antibody response of pigs infected with porcine reproductive and respiratory syndrome syndrome virus

IgG and IgM antibody responses were examined by an indirect fluorescent antibody method in pigs following inoculation with different porcine reproductive and respiratory syndrome virus (PRRSV) isolates or a vaccine virus. Viremia was also examined in the pigs. The IgG antibody was first detected between 9 and 14 days post inoculation (PI) and maintained high titers for at least 7 weeks PI. No change in IgG antibody titers was observed when the pigs were reinoculated with PRRSV 35 days PI. IgM antibody was detected between 5 and 28 days PI in the pigs. Reinoculation at 35 days PI caused a short term rise of IgM antibody. Virus was isolated from sera collected between 2 and 21 days PI. The IgM antibody was detected regularly in sera collected during viremia and up to 1-2 weeks after the viremic periods. These results suggest that pigs with detectable IgM antibody are probably pigs with recent infection and that routine testing of IgM antibody in purchased breeding pigs from seropositive farms may be useful in identification of pigs with recent infection.

Evaluation of the effects of nursery depopulation on the persistence of porcine reproductive and respiratory syndrome virus and the productivity of 34 farms

Nursery depopulation has been described as an effective strategy for improving the performance of weaned pigs. In order to assess whether the strategy was effective under a wide range of conditions, a study was carried out on 34 farms in the USA. Four groups with different depopulation protocols were designed on the basis of the location of the depopulated facility (on site vs off site) and the period for which the nursery remained empty (seven days vs 14 days). The changes in average daily liveweight gain, percentage mortality, feed efficiency and treatment cost per pig produced were assessed 12 months before and after nursery depopulation. The ability to eliminate porcine reproductive and respiratory syndrome (PRRS) virus was examined by indirect fluorescent antibody testing of the nursery pigs. Significant improvements (P < 0.0001) were detected in both average daily gain and percentage mortality after depopulation when the differences within an individual group were analysed, but no significant differences (P > 0.14) were observed between the study groups. Serological testing indicated that antibodies to PRRS virus were still present in 14 of the 34 farms after depopulation.

Induction of dual infections in newborn and three-week-old pigs by use of two plaque size variants of porcine reproductive and respiratory syndrome virus

OBJECTIVE

To determine whether pigs can be dually infected with different plaque variants of porcine reproductive and respiratory syndrome virus (PRRSV).

ANIMALS

2 pregnant sows, 20 newborn pigs, and 20 three-week-old pigs.

PROCEDURE

The 2 late-term pregnant sows were inoculated with the PRRSV small-plaque variant MN-Hs, and their pigs were challenge exposed at 7 days of age with the PRRSV large-plaque variant MN-HI. In addition, twelve 3-week-old pigs were inoculated with MN-Hs virus. Two groups of the pigs were challenge exposed with MN-HI virus at 14 and 42 days after initial inoculation. Virus was isolated from the pigs at various intervals, plaque sizes were examined, and serologic testing was performed.

RESULTS

From the 2 groups of ten 7-day-old pigs, small-plaque PRRSV was isolated form 8 and 10 pigs. After subsequent challenge exposure, dual infections were diagnosed in 3 of 10 inoculated and 4 of 10 contact control pigs. In 3-week-old pigs infected with MN-Hs virus, dual infections were documented in 2 of 6 pigs when challenge exposed 14 days after initial infection. Virus was not isolated from sera of another 6 pigs challenge exposed 42 days after initial infection. Serum neutralization antibody was detected in all 6 pigs at challenge exposure.

CONCLUSIONS

Dual infections were documented in viremic newborn pigs and 3-week-old pigs, using 2 plaque variants of PRRSV.

Characterization of the metabolites of carbamazepine in patient urine by liquid chromatography/mass spectrometry

The urinary metabolites of carbamazepine (CBZ) in epileptic patients receiving long-term drug treatment have been characterized by LC/MS. CBZ-10,11-epoxide (9.6-15.0 micrograms/ml), trans-10, 11-dihydrodiol-CBZ (273.0-400.00 micrograms/ml), and CBZ (2.4-3.8 micrograms/ml) were measured by HPLC. The secondary N-glucuronide of CBZ, four phenolic O-glucuronides (including those of 2- and 3-OH-CBZ), two additional OH-CBZ O-glucuronides, and the N-glucuronide of CBZ-10,11-epoxide constituted the products of either direct conjugation or preliminary monoxygenation. Derivatives of these monoxygenated compounds, which were characterized as O-glucuronides, were represented by dihydroxylated (catechol) CBZ and its putative O-methyl metabolite and by 10,11-dihydrodiol-CBZ. 10,11-Dihydro-10-OH-CBZ O-glucuronide, a metabolite thought to be excreted only by uremic subjects, was not found. More complicated biotransformations of the 10,11-ene moiety were revealed by two carbinol products of azepine ring contraction: 9-OH-methyl-10-carbamoyl acridan and an hydroxylated derivative thereof, which were excreted as O-glucuronides. No polar sulfur-containing metabolites that might serve as indicators of reactive intermediate formation were found in human urine.

Development of a screening method for anti-6 beta-hydroxycortisol antibody using an enzyme-linked immunosorbent assay (ELISA) and its applications

An indirect enzyme-linked immunosorbent assay (ELISA) for the detection of anti-6 beta-hydroxycortisol (6 beta-OHC) antibody has been developed. After immunization of 6 beta-OHC protein conjugates in New Zealand White rabbits, specific polyclonal antibody to 6 beta-OHC was detected by ELISA in which the wells of microtiter plates were coated with 6 beta-OHC conjugated to protein. The rabbit anti-6 beta-OHC antibody titer was above 1:500,000 dilution. Cross-reactivity with other structurally related steroids such as cortisol hydrocortisone was less than 5%. The sensitivity of the polyclonal antibody was comparable to previous studies reported, and was within the accepted detection limit for 6 beta-OHC in man and in laboratory animals. The assay has a low detection limit of 1 ng/ml, an intraassay variation of 3.1% and an interassay variation of 5.2%. The application of the anti-6 beta-OHC-based-ELISA to detect urinary 6 beta-OHC was tested by measuring the concentration of 6 beta-OHC in man before and after enzyme induction by rifampicin treatment. The mean 24 h urine output of 6 beta-OHC in man subjects was 370 +/- 105 micrograms and 1350 +/- 201 micrograms before and after rifampicin administration, respectively. This polyclonal anti-6 beta-OHC antibody-based ELISA can be modified to detect mouse anti-6 beta-OHC IgG with equally good precision and specificity which should be useful in screening positive clones of a 6 beta-OHC IgG secreting mouse hybridoma currently being developed for detecting enzyme induction of CYP3A4 in man and laboratory animals.

Structural basis for the haemotoxicity of dapsone: the importance of the sulphonyl group

The structural basis of dapsone (4,4'-diaminodiphenyl sulphone) haemotoxicity has been determined by investigation of the in vitro bioactivation of a series of 4-substituted arylamines. In the presence of rat liver microsomes, dapsone (100 microM) was the most potent former of methaemoglobin in human erythrocytes (44.8 +/- 6.7%). Substitution of the sulphone group with sulphur (11.6 +/- 1.4% methaemoglobin), oxygen (4.5 +/- 1.1%), nitrogen (0.0 +/- 3.2%), carbon (13.6 +/- 0.8%) or a keto group (34.0 +/- 6.1%) resulted in a decrease in methaemoglobin formation. Only one compound, 4,4'-diaminodiphenylamine, generated significant (P < 0.001) amounts of methaemoglobin (25.6 +/- 2.5%) in the absence of NADPH. To assess further the role of the 4-substituent in methaemoglobinaemia, the toxicity of a series of 4-substituted aniline derivatives was also studied. Of the anilines studied, 4-nitroaniline caused the most methaemoglobin (36.5 +/- 8.0%), whilst aniline caused the least (0.3 +/- 0.5%). Overall, there was a significant correlation (r2 = 0.83) between the haemotoxicity and the Hammett constant, sigma(p), suggesting that it is the electron-withdrawing properties of the substituent that influence the methaemoglobin formation. In the presence of microsomes prepared from two human livers, dapsone was the most haemotoxic bis arylamine, whereas 4-iodoaniline was the most potent methaemoglobin former (60.6 and 73.6%) and aniline the least potent (1.1 and 2.4%). As a whole, these results indicate that the sulphonyl group, which is essential for the pharmacological activity of dapsone, is also largely responsible for the haemotoxicity seen with this drug.

Synthesis, antimalarial activity, and molecular modeling of tebuquine analogues

Tebuquine (5) is a 4-aminoquinoline that is significantly more active than amodiaquine (2) and chloroquine (1) both in vitro and in vivo. We have developed a novel more efficient synthetic route to tebuquine analogues which involves the use of a palladium-catalyzed Suzuki reaction to introduce the 4-chlorophenyl moiety into the 4-hydroxyaniline side chain. Using similar methodology, novel synthetic routes to fluorinated (7a, b) and a dehydroxylated (7c) analogue of tebuquine have also been developed. The novel analogues were subjected to testing against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. Tebuquine was the most active compound tested against both strains of Plasmodia. Replacement of the 4-hydroxy function with either fluorine or hydrogen led to a decrease in antimalarial activity. Molecular modeling of the tebuquine analogues alongside amodiaquine and chloroquine reveals that the inter-nitrogen separation in this class of drugs ranges between 9.36 and 9.86 A in their isolated diprotonated form and between 7.52 and 10.21 A in the heme-drug complex. Further modeling studies on the interaction of 4-aminoquinolines with the proposed cellular receptor heme revealed favorable interaction energies for chloroquine, amodiaquine, and tebuquine analogues. Tebuquine, the most potent antimalarial in the series, had the most favorable interaction energy calculated in both the in vacuo and solvent-based simulation studies. Although fluorotebuquine (7a) had a similar interaction energy to tebuquine, this compound had significantly reduced potency when compared with (5). This disparity is possibly the result of the reduced cellular accumulation (CAR) of fluorotebuquine when compared with tebuquine within the parasite. Measurement of the cellular accumulation of the tebuquine analogues and seven related 4-aminoquinolines shows a significant relationship (r = 0.98) between the CAR of 4-aminoquinoline drugs and the reciprocal of drugs IC50.

Bioactivation and inactivation of aflatoxin B1 by human, mouse and rat liver preparations: effect on SCE in human mononuclear leucocytes

The purpose of this study was to investigate the use of human and animal subcellular liver fractions in an in vitro evaluation of carcinogenic risk. The bioactivation and bioinactivation of the known genotoxic carcinogen aflatoxin B1 by human, mouse and rat liver preparations was investigated using the SCE assay in human lymphocytes as a genotoxic endpoint. There was a 10-fold variation in SCE response (1.1-11.6 SCE/Cell) in human mononuclear leucocytes (MNLs) after aflatoxin B1 was activated by human liver microsomes (n = 6). Activation correlated with the CYP1A2 phenotype of livers (r = 0.8; p < 0.05), but there was no correlation with either GST M1 genotype or epoxide hydrolase phenotype. Mouse liver microsomes activated aflatoxin B1 to a greater extent [(1 micro M) 12.8 +/- 2.51 SCE/Cell] than either rat [(10 micro M) 12.0 +/- 3.84 SCE/Cell or human (L25) [(10 micro M) 8.8 +/- 2.00 SCE/Cell liver microsomes. The addition of mouse liver cytosol and reduced glutathione (GSH) significantly (p < 0.001) reduced aflatoxin B1-dependent genotoxicity, whereas the addition of either human or rat cytosol (+GSH) was without effect. These data indicate that species variation in both bioactivation and bioinactivation can exist. Therefore there is a necessity for careful selection of activation systems from species whose biochemical profile reflects that of man.

Disposition of amodiaquine and related antimalarial agents in human neutrophils: implications for drug design

The development and clinical use of 4-aminoquinoline antimalarial agents such as amodiaquine have been limited by toxicity to neutrophils. We have investigated the chemical basis of amodiaquine-induced toxicity and compared the findings with those for established antimalarial drugs proposed for human use. Amodiaquine, like chloroquine, mefloquine and halofantrine, was lysosomotropic and accumulated in human neutrophils. Amodiaquine did not lead to impairment of either cellular function or cell viability at therapeutic levels. In contrast to other antimalarial agents, amodiaquine (because it contains a 4-aminophenol function) depleted glutathione in activated neutrophils, by formation of an electrophilic quinoneimine metabolite. Bioactivation was accompanied by the expression of a drug-related antigen on the cell surface, which was recognized by drug-specific antibodies, suggesting that a type II hypersensitivity reaction is responsible for the observed toxicity. Similar bioactivation and accumulation were observed for the structurally related amopyroquine. The effects of chemical modifications at the 3'- and 5'-positions, which are known to enhance antimalarial activity, were also investigated. The introduction of a lipophilic 5'-chlorophenyl group and 3'-t-butyl group blocked bioactivation but enhanced cellular accumulation, with resultant impairment of function and neutrophil viability, whereas introduction of a second cationic dialkylamino group (bis-mannich compounds) blocked bioactivation and reduced cellular accumulation, without producing noticeable effects on cellular function and viability. These data provide a chemical rationale for the idiosyncratic agranulocytosis observed with amodiaquine, and they suggest that similar toxicity might be anticipated for amopyroquine but is less likely with bis-mannich antimalarial agents such as pyronaridine.

Characterisation of the toxic metabolite(s) of naphthalene

The toxicity of naphthalene and its metabolites has been investigated in vitro. Both naphthalene and its metabolite 1-naphthol were bioactivated by human hepatic microsomes to metabolite(s) which were toxic to mononuclear leucocytes (MNL). However 1-naphthol was more cytotoxic than naphthalene (49.8 +/- 13.9% vs. 19.0 +/- 10.0% cell death; P < 0.01), indicating that the toxicity of naphthalene is dependent on the bioactivation of 1-naphthol. CYP2E1-induced rat liver microsomes increased metabolism of naphthalene by 13% compared to control microsomes with a concomitant increase in both 1-naphthol and dihydrodiol formation. The cytotoxicity of naphthalene but not of 1-naphthol was increased by CYP2E1 induction, indicating that separate enzymes are involved in the bioactivation of 1-naphthol. The metabolites of 1-naphthol, 1,2-naphthoquinone (51.4 +/- 6.6% cell death) and 1,4-naphthoquinone (49.1 +/- 3.4% cell death) were directly toxic to MNL and depleted glutathione to 1.0% of the control levels. Both quinones were also genotoxic to human lymphocytes. In contrast, the primary metabolite of naphthalene, the 1,2-epoxide (0-100 microM) was neither cytotoxic nor genotoxic, and did not deplete glutathione. In conclusion, our data suggests that the cytotoxicity and genotoxicity of naphthalene is associated with the formation of quinones from 1-naphthol rather than naphthalene-1,2-epoxide.

Chronologic change of serum IgG antibody response in chickens reinfected with Cryptosporidium baileyi

Eight 2-day-old SPF chickens were each inoculated orally with a single dose of 5 x 10(5) oocysts of Cryptosporidium baileyi, and immunoglobulin G (IgG) antibody responses were chronologically measured by indirect immunofluorescent antibody (IFA) assay. Anti-C. baileyi IgG antibody levels remained high (1:106.67 to 1:512.00) for at least 4 months with 330 days of a detectable period. Ten days after the negative conversion, each chicken was re-challenged with 1 x 10(7) oocysts of the same species. Subsequent infection in 340-day-old individuals caused sudden elevated IgG antibody levels and the titer peaked on day 28 postchallenge inoculation (PCI), at 1:1.024 with a 65 days of detection period. Chickens in primary infection showed oocyst shedding profiles, but did not exhibit any oocyst shedding before or after experimental reinfection.

Mechanism-based design of parasite-targeted artemisinin derivatives: synthesis and antimalarial activity of benzylamino and alkylamino ether analogues of artemisinin

Several artemisinin derivatives linked to benzylamino and alkylamino groups were synthesized in order to enhance accumulation within the malaria parasite. The in vitro antimalarial activity was assessed against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. In general the incorporation of amino functionality enhances the activity relative to artemisinin. The most potent analogue in the series was compound 6 which was severalfold more active than artemisinin against both strains of P. falciparum used in the study.

Manipulation of the N-alkyl substituent in amodiaquine to overcome the verapamil-sensitive chloroquine resistance component

Aminoquinoline resistance correlates with lipid solubility at pH 7.2. Consequently, the in vivo dealkylation of amodiaquine, to the less lipid-soluble desethylamodiaquine, is a likely contributor to therapeutic failure in vivo. Therefore, 4-aminoquinoline drugs with lipid solubilities similar to that of amodiaquine, but which are not subject to side chain modification in vivo, should be superior antimalarial agents. In this study, we have identified amopyroquine and N-tertbutylamodiaquine as two such compounds. The values for the logarithms of the partition coefficients for amopyroquine and N-tertbutylamodiaquine are between those for amodiaquine and its dealkylated metabolite, desethylamodiaquine. Both amopyroquine and N-tertbutylamodiaquine possess levels of antimalarial activity greater than that of desethylamodiaquine and significantly reduced cross-resistance patterns; i.e., the former two compounds are not subject to the verapamil-sensitive resistance mechanism. Simple in vitro markers of direct toxicity and potential reactive metabolite formation suggest that these two compounds are no more toxic than amodiaquine and desethylamodiaquine.

The role of drug accumulation in 4-aminoquinoline antimalarial potency. The influence of structural substitution and physicochemical properties

We have investigated a series of novel 4-aminoquinoline analogues related to amodiaquine, that possess side chain modifications designed to influence both drug pKa and lipophilicity. These compounds have been used to determine the influence of physicochemical properties on antimalarial activity against, and accumulation by, both chloroquine-susceptible and chloroquine-resistant isolates of Plasmodium falciparum. The compounds tested exhibited a 500-fold range of absolute antimalarial potency. Absolute drug potency and drug accumulation were found to be significantly correlated in each of the four isolates of Plasmodium falciparum studied. The level of accumulation was unrelated to lipophilicity and was significantly greater than the predicted levels of accumulation based on drug pKa, compartmental pH, and Henderson-Hasselbach considerations. Further analysis of the relationship between 4-aminoquinoline accumulation and activity implicated the involvement of additional forces in the accumulation process.

Interindividual and interspecies variation in hepatic microsomal epoxide hydrolase activity: studies with cis-stilbene oxide, carbamazepine 10, 11-epoxide and naphthalene

Microsomal epoxide hydrolase (HYL1) is a single-gene enzyme responsible for the hydrolysis of epoxides derived from the oxidative metabolism of xenobiotics. Variation in HYL1, therefore, may be an important determinant of drug toxicity. We have investigated HYL1 enzyme kinetics in six different species including man, for which a liver bank genotyped for polymorphisms in exons 3 and 4 of the HYL1 gene was used. Activity was measured by radiochromatography with high specific activity radiolabeled substrates, cis-stilbene oxide (CSO) and carbamazepine 10,11-epoxide (CBZ-E). In addition, naphthalene was used to investigate the hydrolysis of an epoxide (naphthalene 1,2-epoxide [N-E] generated in situ. There was marked species variation in enzyme activity that was substrate dependent. CSO was rapidly hydrolyzed by microsomes from all species, the rank order of specific activity being human > rabbit > dog > rat > hamster > mouse. In contrast, hydrolysis of CBZ-E was only observed with human liver microsomes. CBZ-E was only a weak (IC50 = 1 mM) inhibitor of CSO hydrolysis. The hydrolysis of N-E, determined as the diol-to-total metabolite ratio, was human > rabbit > dog > hamster > mouse > rat. Intraspecies variation in man was 4-fold, 7-fold and 2-fold for CSO, CBZ-E and N-E, respectively: none of this variation could be directly accounted for by the HYL1 polymorphisms in exons 3 and 4. These data emphasize the need for careful toxicokinetic evaluation of species used in the safety evaluation of compounds likely to form epoxide intermediates in vivo.

Amodiaquine accumulation in Plasmodium falciparum as a possible explanation for its superior antimalarial activity over chloroquine

Amodiaquine is a 4-aminoquinoline antimalarial whose structure is similar to chloroquine. In contrast to the wealth of information available about chloroquine accumulation and its relationship to activity, little is known about the uptake characteristics of amodiaquine, a drug that is inherently more active against malaria parasites. In this study we have investigated the accumulation of amodiaquine in Plasmodium falciparum in vitro, in order to gain an insight into the mechanisms responsible for its superior activity over chloroquine. The driving force for parasite accumulation of the 4-aminoquinolines is proposed to be a transmembrane proton gradient maintained by a vacuolar ATPase. In the present study, amodiaquine accumulation was greatly reduced, at steady state, in the absence of glucose and at 0 degrees C indicating a clear energy dependence of uptake. Amodiaquine accumulation in Plasmodium falciparum was shown to be 2- to 3-fold greater than chloroquine accumulation. This observation probably accounts for amodiaquine's greater inherent activity but is surprising given that amodiaquine is a weaker base than chloroquine. With this in mind we present evidence for an intraparasitic binding component in the accumulation of the 4-aminoquinolines. Differences in binding affinity of this 'receptor' for amodiaquine and chloroquine may partially explain the greater accumulation and in vitro potency of amodiaquine compared to chloroquine.

Idiosyncratic drug reactions. Metabolic bioactivation as a pathogenic mechanism

The metabolism of drugs to chemically reactive metabolites may play a pivotal role in the pathogenesis of idiosyncratic drug toxicity. A large number of in vitro studies and a limited number of in vivo studies have demonstrated that many drugs are not toxic per se, but produce toxicity after undergoing enzyme-mediated bioactivation to chemically reactive species. Such reactive species may inflict a toxic insult on the cell either directly or indirectly by acting as a hapten and initiating an immune-mediated reaction. The enzymes responsible for bioactivation have been widely studied, both quantitatively and qualitatively, the most important being the enzymes of the cytochrome P450 (CYP) mixed function oxidase system. CYP enzymes are the most predominant drug metabolising enzymes in the liver and are also present in most other tissues of the body. The diversity of this enzyme system means that a wide range of xenobiotic substrates can be bioactivated by either a single CYP isoform or multiple isoforms of this enzyme superfamily. Other enzymes do, however, play an important role in drug bioactivation. In white blood cells, for example, myeloperoxidase has been shown to bioactivate a wide range of drugs. In other tissues low in CYP activity, prostaglandin H synthase may also be responsible for bioactivation; e.g. in the kidney paracetamol (acetaminophen) toxicity is though to result from activation via this enzyme. The phase II or conjugation enzymes may also be important in the ultimate bioactivation of drug molecules. Whilst activation by these enzymes is, to date, apparently confined to chemicals, most drugs are also substrates for these enzymes and bioactivation by them must remain a possibility.

The effect of fluconazole and ketoconazole on the metabolism of sulphamethoxazole

1. Cytochrome P450-mediated bioactivation of sulphamethoxazole to a hydroxylamine has been implicated in the hypersensitivity reactions associated with co-trimoxazole administration. Inhibiting the formation of the hydroxylamine may be one method of preventing the high frequency of toxicity which is observed in HIV-infected patients. Therefore, in this study, we have investigated the ability of fluconazole and ketoconazole, known cytochrome P450 inhibitors, to inhibit the formation of sulphamethoxazole hydroxylamine. 2. Ten healthy male volunteers were given co-trimoxazole (800 mg sulphamethoxazole and 160 mg trimethoprim) alone or 1 h after either fluconazole (150 mg) or ketoconazole (200 mg) in a randomized fashion with a washout period of at least 1 week between each phase. Urine was collected for 24 h, and sulphamethoxazole and its metabolites were quantified by electrospray LC-MS. 3. Ketoconazole had no effect on the urinary recovery of sulphamethoxazole or any of its metabolites. In contrast, fluconazole significantly (P < 0.001) inhibited the formation of sulphamethoxazole hydroxylamine by 50.0 +/- 15.1%. Fluconazole also inhibited the oxidation of sulphamethoxazole to the 5-methylhydroxy and 5-methylhydroxy acetate metabolites by 69.9 +/- 15.8% and 64.0 +/- 12.0%, respectively, but had no effect on the amount of sulphamethoxazole, N4-acetyl sulphamethoxazole, or sulphamethoxazole N1-glucuronide excreted in urine. 4. The potential clinical benefit of using fluconazole to prevent hypersensitivity to co-trimoxazole in patients with AIDS needs to be assessed in a prospective study using both metabolite formation and the clinical occurrence of adverse reactions as end-points.

Survival of Cryptosporidium muris (strain MCR) oocysts under cryopreservation

We have successfully maintained Cryptosporidium muris by cryopreservation. Oocysts were suspended in distilled water, stored at -20 degrees C for 24 hrs, and then cryopreserved at -70 degrees C. Cryopreserved specimens were slowly thawed at 5 degrees C. Oocysts, which had been cryopreserved for 15 months without cryoprotective agents, retained their infectivity by the mouse titration method. Oocysts stored at 5 degrees C in 2.5% potassium dichromate failed to retain their infectivity beyond 6.5 months.

Relevance of induction of human drug-metabolizing enzymes: pharmacological and toxicological implications

1. Human drug-metabolizing systems can be induced, or activated, by a large number of exogenous agents including drugs, alcohol, components in the diet and cigarette smoke, as well as by endogenous factors. 2. Such perturbation of enzyme activity undoubtedly contributes to both intra-and inter-individual variation both with respect to the rate and route of metabolism for a particular drug. Induction may, in theory, either attenuate the pharmacological response or exacerbate the toxicity of a particular drug, or both. 3. The clinical impact of enzyme induction will depend upon the number of different enzyme isoforms affected and the magnitude of the inductive response within an individual, and also on the therapeutic indices of the affected substrates. 4. The toxicological implications will be determined either by any change in the route of metabolism, or by a disturbance of the balance between activation and detoxication processes, which may be isozyme selective.