The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe.

Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

A pharmacokinetic study of intermittent rifabutin dosing with a combination of ritonavir and saquinavir in patients infected with human immunodeficiency virus.

AIM: Our primary aim was to evaluate the plasma exposures and safety of rifabutin and its active 25-O-desacetyl metabolite during concomitant therapy of intermittent rifabutin dosing regimens with a combination of ritonavir and saquinavir. METHODS: Twenty-four patients without mycobacterial infection who were human immunodeficiency virus seropositive and who were receiving 400 mg each of ritonavir and saquinavir twice daily participated in a 3-period, 2-group longitudinal pharmacokinetic study. Patients were equally randomized to receive 300 mg of rifabutin every 7 days (group 1) or 150 mg of rifabutin every 3 days (group 2) for 8 weeks. Blood samples were collected over the dosing intervals of the protease inhibitors at baseline (period 1) and of the 3 drugs after 4 weeks (period 2) and 8 weeks (period 3) for HPLC measurement of plasma concentrations of the 3 drugs and 25-O-desacetylrifabutin. RESULTS: Nineteen patients (group 1, n = 10; group 2, n = 9) completed the study. Five individuals withdrew from the study; 3 of them experienced side effects, and 2 were lost to follow-up. For combined groups, mean saquinavir and ritonavir overall (area under the concentration-time curve [AUC]) and peak (C(max)) plasma exposures averaged over periods 2 and 3 did not change significantly (8% to 19%; P > .05) compared with those in period 1 (90% confidence intervals, -7% to 26% for ritonavir and -2% to 38% for saquinavir). Rifabutin and metabolite AUC and C(max) exposures were stable over the 8 weeks, with intraindividual coefficients of variation of 12% to 19%. Oral clearance of rifabutin was similar in both groups (321 mL/min in group 2 versus 372 mL/min in group 1; P = .34). Rifabutin C(max) values were significantly lower in group 2 (310 ng/mL versus 496 ng/mL in group 1; P = .004). Rifabutin and metabolite predose levels were significantly higher in group 2 (rifabutin: 54 ng/mL versus 17 ng/mL; desacetyl rifabutin: 55 ng/mL versus 28 ng/mL; P < .002). CONCLUSIONS: Rifabutin exposures were similar at 4 and 8 weeks and had minimal effect on ritonavir and saquinavir exposures. Intermittent rifabutin dosing over 8 weeks provided a safe and manageable regimen for concurrent therapy with a combination of ritonavir and saquinavir.

Pharmacokinetic interaction between mefloquine and ritonavir in healthy volunteers.

AIMS: To evaluate the pharmacokinetic interaction between ritonavir and mefloquine. METHODS: Healthy volunteers participated in two separate, nonfasted, three-treatment, three-period, longitudinal pharmacokinetic studies. Study 1 (12 completed): ritonavir 200 mg twice daily for 7 days, 7 day washout, mefloquine 250 mg once daily for 3 days then once weekly for 4 weeks, ritonavir restarted for 7 days simultaneously with the last mefloquine dose. Study 2 (11 completed): ritonavir 200 mg single dose, mefloquine 250 mg once daily for 3 days then once weekly for 2 weeks, ritonavir single dose repeated 2 days after the last mefloquine dose. Erythromycin breath test (ERMBT) was administered with and without drug treatments in study 2. RESULTS: Study 1: Ritonavir caused less than 7% changes with high precision (90% CIs: -12% to 11%) in overall plasma exposure (AUC(0,168 h)) and peak concentration (Cmax) of mefloquine, its two enantiomers, and carboxylic acid metabolite, and in the metabolite/mefloquine and enantiomeric AUC ratios. Mefloquine significantly decreased steady-state ritonavir plasma AUC(0,12 h) by 31%, Cmax by 36%, and predose levels by 43%, and did not affect ritonavir binding to plasma proteins. Study 2: Mefloquine did not alter single-dose ritonavir pharmacokinetics. Less than 8% changes in AUC and Cmax were observed with high variability (90%CIs: -26% to 45%). Mefloquine had no effect on the ERMBT whereas ritonavir decreased activity by 98%. CONCLUSIONS: Ritonavir minimally affected mefloquine pharmacokinetics despite strong inhibition of CYP3A4 activity from a single 200 mg dose. Mefloquine had variable effects on ritonavir pharmacokinetics that were not explained by hepatic CYP3A4 activity or ritonavir protein binding.

Single and multiple dose pharmacokinetics of nelfinavir and CYP2C19 activity in human immunodeficiency virus-infected patients with chronic liver disease.

AIMS: To evaluate the single-dose and multiple-dose pharmacokinetics of nelfinavir and its active M8 metabolite in eight HIV-seropositive patients with liver disease, and to examine the relationship between CYP2C19 activity (genotype and plasma M8/nelfinavir metabolic ratio) and the severity of liver disease in these patients. METHODS: Nelfinavir was given as a single dose (500 or 750 mg) to patients beginning therapy and twice (500, 750 or 1000 mg) or three times (250 or 750 mg) daily during chronic therapy. Single-dose pharmacokinetic values were used to predict multiple-dose regimens. Peak and total plasma exposures between 2-4 microg ml-1 and 45-75 microg ml-1 h, respectively, and predose levels > 0.7 microg ml-1 were targeted for multidose nelfinavir. Genotype was determined by analysis for CYP2C19*1, CYP2C19*2, and CYP2C19*3. Individuals were grouped according to their genotype, molar M8/nelfinavir AUC ratio (low: < 0.1, intermediate: 0.1-0.3, high > 0.3), and Child-Pugh classification for severity of liver disease. RESULTS: Nelfinavir pharmacokinetics were characterized by wide interindividual variability, low clearance (181-496 ml min-1 70 kg-1, n = 7), and prolonged half-life (5-20 h, n = 7). M8/nelfinavir AUC ratio increased 58% (n = 4) and alpha 1-acid glycoprotein levels decreased up to 39% (n = 5) from single to multiple dosing. CYP2C19 activity was low (metabolic AUC ratio < 0.1) in four patients with moderate to severe liver disease even though they were genetically extensive CYP2C19 metabolizers (*1/*1 or *1/*2). Three patients required lower daily doses than the standard regimen of 750 mg every 8 h to achieve target concentrations and maintain virologic suppression at < 50 RNA copies ml-1 (up to 20 months). CONCLUSIONS: Acquired CYP2C19 deficiency from moderate or severe liver disease resulted in decreased M8 formation. Long-term HIV suppression is possible using low nelfinavir doses in patients with liver disease.

Chemical composition measurements of the atmosphere of Jupiter with the Galileo Probe mass spectrometer.

The Galileo Probe entered the atmosphere of Jupiter on December 7, 1995. Measurements of the chemical and isotopic composition of the Jovian atmosphere were obtained by the mass spectrometer during the descent over the 0.5 to 21 bar pressure region over a time period of approximately 1 hour. The sampling was either of atmospheric gases directly introduced into the ion source of the mass spectrometer through capillary leaks or of gas, which had been chemically processed to enhance the sensitivity of the measurement to trace species or noble gases. The analysis of this data set continues to be refined based on supporting laboratory studies on an engineering unit. The mixing ratios of the major constituents of the atmosphere hydrogen and helium have been determined as well as mixing ratios or upper limits for several less abundant species including: methane, water, ammonia, ethane, ethylene, propane, hydrogen sulfide, neon, argon, krypton, and xenon. Analysis also suggests the presence of trace levels of other 3 and 4 carbon hydrocarbons, or carbon and nitrogen containing species, phosphine, hydrogen chloride, and of benzene. The data set also allows upper limits to be set for many species of interest which were not detected. Isotope ratios were measured for 3He/4He, D/H, 13C/12C, 20Ne/22Ne, 38Ar/36Ar and for isotopes of both Kr and Xe.

The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer.

The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C2 and C3 nonmethane hydrocarbons, and possibly PH3 and Cl. 4He/H2 in the Jovian atmosphere was measured to be 0.157 +/- 0.030. 13C/C12 was found to be 0.0108 +/- 0.0005, and D/H and 3He/4He were measured. Ne was depleted, < or = 0.13 times solar, Ar < or = 1.7 solar, Kr < or = 5 solar, and Xe < or = 5 solar. CH4 has a constant mixing ratio of (2.1 +/- 0.4) x 10(-3) (12C, 2.9 solar), where the mixing ratio is relative to H2. Upper limits to the H2O mixing ratio rose from 8 x 10(-7) at pressures <3.8 bars to (5.6 +/- 2.5) x 10(-5) (16O, 0.033 +/- 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10(-6) at pressures less than 3.8 bars but rose from about 0.7 x 10(-5) at 8.7 bars to about 7.7 x 10(-5) (32S, 2.5 solar) above 15 bars. Only very large upper limits to the NH3 mixing ratio have been set at present. If PH3 and Cl were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitude-dependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr.

Pharmacokinetic and pharmacodynamic interactions between diltiazem and quinidine.

OBJECTIVES: To examine the pharmacokinetic and pharmacodynamic interactions between quinidine and diltiazem because both drugs can inhibit drug metabolism. METHODS: Twelve fasting, healthy male volunteers (age, 24 +/- 5 years; weight, 75 +/- 10 kg) received a single oral dose of diltiazem (60 mg) or quinidine (200 mg), alone and on a background of the other drug, in a crossover study. Background treatment consisted of 100 mg quinidine twice a day or 90 mg sustained-release diltiazem twice a day for 2 day before the study day. RESULTS: Pretreatment with diltiazem significantly (p < 0.05) increased the area under the curve of quinidine from 7414 +/- 1965 to 11,213 +/- 2610 ng.hr/ml and increased its terminal elimination half-life (t1/2) from 6.8 +/- 1.1 to 9.3 +/- 1.5 hours. Its oral clearance was decreased from 0.39 +/- 0.1 to 0.25 +/- 0.1 L/hr/kg, whereas the maximal concentration was not significantly affected. Diltiazem disposition was not significantly affected by pretreatment with quinidine. Diltiazem pretreatment increased QTc and PR intervals and decreased heart rate and diastolic blood pressure. No significant pharmacodynamic differences were shown for diltiazem alone versus quinidine pretreatment. CONCLUSION: Diltiazem significantly decreased the clearance and increased the t1/2 of quinidine, but quinidine did not alter the kinetics of diltiazem with the dose used. No significant pharmacodynamic interaction was shown for the combination that would not be predicted from individual drug administration.

The Galileo probe mass spectrometer: composition of Jupiter's atmosphere.

The composition of the jovian atmosphere from 0.5 to 21 bars along the descent trajectory was determined by a quadrupole mass spectrometer on the Galileo probe. The mixing ratio of He (helium) to H2 (hydrogen), 0.156, is close to the solar ratio. The abundances of methane, water, argon, neon, and hydrogen sulfide were measured; krypton and xenon were detected. As measured in the jovian atmosphere, the amount of carbon is 2.9 times the solar abundance relative to H2, the amount of sulfur is greater than the solar abundance, and the amount of oxygen is much less than the solar abundance. The neon abundance compared with that of hydrogen is about an order of magnitude less than the solar abundance. Isotopic ratios of carbon and the noble gases are consistent with solar values. The measured ratio of deuterium to hydrogen (D/H) of (5 +/- 2) x 10(-5) indicates that this ratio is greater in solar-system hydrogen than in local interstellar hydrogen, and the 3He/4He ratio of (1.1 +/- 0.2) x 10(-4) provides a new value for protosolar (solar nebula) helium isotopes. Together, the D/H and 3He/4He ratios are consistent with conversion in the sun of protosolar deuterium to present-day 3He.

Simultaneous determination of diltiazem and quinidine in human plasma by liquid chromatography.

A novel method for simultaneous determination of diltiazem and quinidine in human plasma is described. Plasma is alkalinized and extracted with methyl tert.-butyl ether. The ether phase is separated and evaporated. The residue is reconstituted in 0.2 ml of mobile phase containing 56 mM octanesulfonic acid then washed twice with n-hexane. Aliquots are chromatographed on a silanol-deactivated reversed-phase column using a mobile phase containing aqueous H2SO4 (0.01 M, pH 2)-methanol-acetonitrile (45:45:10) and 10 mM octanesulfonic acid. Peaks are monitored with a UV detector set at 237 nm and a fluorescence detector using an excitation set at 247 nm and a 270 nm UV cut-off filter at the emission. Calibration and standard curves were linear from 1 to 130 ng on-column for diltiazem and from 2 to 600 ng on-column for quinidine. Limits of quantitation were 2 and 4 ng/ml for diltiazem and quinidine, respectively. Recoveries from spiked plasma were 94.0 to 102.5% (R.S.D. 6.0-11.4%) for diltiazem and 98.5% to 104.1 (R.S.D. 7.7-8.7%) for quinidine over the ranges studied. In vitro stability was studied in spiked plasma samples stored at -80 degrees C for sixteen months. Both diltiazem and quinidine remained within 10% from nominal values. For ex vivo stability at -80 degrees C, a plasma sample obtained from a volunteer 2 h after oral administration of diltiazem (60 mg) was analysed for two days after sampling and eighteen months later. The mean deviation from initial measured was 4.7%.

Separation, identification and determination of lumichrome in swine feed and kidney.

During surveillance of hog carcasses from Manitoba for antibiotic residues by the Health of Animals Laboratory, Agriculture Canada, Saskatoon, an unknown substance was found which produced tetracycline-like results with the methods used. This same substance was found in an implicated swine feed premix. Using various HPLC systems and columns, UV spectroscopy, reverse-phase TLC, and mass spectrometry, the substance was isolated from the feed premix, and identified as lumichrome, a photodegradation product of riboflavin. Traces of the same substance were found in riboflavin standard. Analysis of swine kidney, previously found to contain the unknown, showed the same substance was present at a level of about 1 ppm.

Assay of oxolinic acid residues in salmon muscle by liquid chromatography with fluorescence detection: interlaboratory study.

A previously developed method that uses a simplified sample preparation and fluorometric detection of liquid chromatographic eluates for the determination of oxolinic acid in salmon muscle has been collaboratively studied. Five laboratories participated in the study to analyze, in quintuplicate, blank salmon muscle fortified at 10, 20, 50, and 100 micrograms/kg (ppb), and 2 incurred samples from salmon given feed with medicated oxolinic acid. The tissue, 2 g mixed with 2 g Na2SO4, is extracted with ethyl acetate and centrifuged, and the solvent is evaporated. The residue is partitioned in a mixture of hexane and 0.01 M oxalic acid, and the aqueous phase is chromatographed using fluorescence detection at 327 nm excitation and 369 nm emission. Mean recoveries ranged from 77.2 to 84.5% in spiked samples with reproducibility relative standard deviation (RSDR) ranging from 11.5 to 18.3%. Treated salmon were found to contain 8.71 and 53.8 micrograms/kg with RSDR of 18.6 and 16.7%, respectively. The corresponding repeatability relative standard deviations (RSDR) were 5.8-12.2%, and 7.7 and 6.2%. The method is recommended for regulatory purposes in Canada.

The identification and assay of 2-methyl-5-nitroimidazole in pigs treated with dimetridazole.

An unidentified metabolite of dimetridazole (DMZ), found in pig plasma, muscle and kidney, was shown by chromatography and spectroscopy to be 2-methyl-5-nitroimidazole (2-MNI), resulting from N-demethylation of DMZ. This route of degradation competes with the oxidation pathway previously described. The concentration of 2-MNI in the plasma of pig fed medicated diet (DMZ 0.0125%) ranged from 29 to 83 ppb, 2 hours after the morning meal, similar to DMZ, but lower than that of the major metabolite, 2-hydroxymethyl-1-methyl-5-nitroimidazole (HMMNI). Its elimination profile in plasma was biphasic, similar to those of HMMNI and DMZ. Early and terminal half lives were 2.6 and 9.1 h respectively. None of the metabolites could be detected in any of the tissues studied 49 hours after withdrawal.

Quantitation and confirmation of sulfamethazine residues in swine muscle and liver by LC and GC/MS.

The present paper describes a liquid chromatographic (LC) method for purification of crude swine tissue extracts before gas chromatographic/mass spectrometric (GC/MS) quantitation and confirmation of sulfamethazine at low ppb levels. Fractions corresponding to sulfamethazine were collected, evaporated to dryness, N-methylated with diazomethane, concentrated, and analyzed by GC/MS. A mass spectrometer was set to selected ion monitoring (SIM) mode. Ions 233, 227, 228, and 92 m/z were detected. Ratio 227/233 m/z (sulfamethazine/internal standard, [phenyl 13C6] sulfamethazine) was used for quantitation, while ratios 228/227 and 92/227 m/z, respectively, were used for confirmation. Quantitation in spiked blank muscle tissue was tested from 100 to 1 ppb and found acceptable at all concentrations studied; coefficients of variations ranged from 4.9 to 14.4%. Similar results were obtained for liver tissue from 5 to 20 ppb; coefficients of variation ranged from 1.2 to 9.1%.

Chromatographic and spectroscopic characterization of sulphur-bound dimetridazole and ranidazole derivatives.

The 5-nitroimidazoles, dimetridazole and ronidazole, two important veterinary drugs, were reacted under reductive conditions with the sulfhydryl-containing substrates cysteine and glutathione to yield 5-amino-4-S-substituted imidazoles. After purification by reversed-phase liquid chromatography (RP-LC), the four adducts were characterized by RP-LC with photodiode array detection using conditions where their parent drugs were not eluted from the column. Structural identification was conducted by spectroscopic techniques, mainly 1-dimensional and 2-dimensional NMR. While the dimetridazole adducts were found to be monosubstituted at the C-4 position, the two ronidazole products contained two units of the sulfhydryl substrate, located at the C-4 and C-6 positions.

Sulfamethazine (sulfadimidine) residues in Canadian consumer milk.

A survey on the presence of sulfamethazine (sulfadimidine) residues in consumer milk has been conducted in 10 cities across Canada. In each city, homogenized milk was purchased at 3 different retail outlets, each supplied by different processing plants. A total of 30 samples was analyzed by a liquid chromatographic method. The limit of quantitation was 5 ppb. In addition to automatic integration, visual inspection of the chromatograms was required to distinguish between low concentrations of sulfamethazine and 2 unknown interfering peaks. Two samples, from different cities, contained 11.4 and 5.24 ppb of the drug. Drug identity was confirmed by mass spectrometry. All other samples appeared to be free of the drug.

Dimetridazole residues in pork tissue. I. Assay by liquid chromatography with electrochemical detector.

A liquid chromatographic (LC) method with electrochemical detection in the reductive mode was developed for the quantitative determination of dimetridazole (DMZ) and its major metabolite (HMMNI) at residue levels in pork tissue. For blood plasma, a sample is precipitated with 2 volumes of acetonitrile and centrifuged, and a diluted aliquot of the supernatant liquid is chromatographed. For muscle, a 10 g sample is extracted 3 times with dichloromethane. After evaporation of the combined extracts, the residue is redissolved in a mixture of hexane and mobile phase (0.3% TEA in 0.6M ammonium acetate pH 5.0 and acetonitrile, 85 + 15) and centrifuged, and an aliquot of the lower phase is chromatographed. Chromatography is accomplished using valve switching with 2 liquid circuits, employing the same mobile phase for both. The sample is deaerated by sparging with helium under slight positive pressure to prevent rediffusion of the oxygen. The sample is first loaded into a deoxygenator and the flow is stopped for complete deoxygenation. The flow is then resumed to transfer the sample into the first, low back-pressure column (ODS, 10 microns, 4.6 x 200 mm). Switching the valve at this point removes the deoxygenator from the circuit and connects the first column to a second one (ODS, 5 microns, 4.6 x 150 mm) in tandem. After the effluent is passed through a second deoxygenator to reduce the residual oxygen in the mobile phase, it is monitored by an electrochemical detector with a screened wall jet cell and a gold mercury electrode, set at -1.2 V.(ABSTRACT TRUNCATED AT 250 WORDS)

Dimetridazole residues in pork tissue. II. Application of liquid chromatographic method to monitor elimination of drug and its major metabolite.

A study was conducted to monitor the elimination of dimetridazole (DMZ) and its major metabolite 2-hydroxymethyl-1-methyl-5-nitroimidazole (HMMNI) in swine plasma and tissue, using a liquid chromatographic method with electrochemical detector sensitive to 0.5 ppb. The study consisted of 2 experiments. In the preliminary experiment, one young female piglet was fed medicated ration containing 125 ppm dimetridazole (DMZ) for 2 weeks, followed by a withdrawal period using regular ration for 5 days. Another, control, piglet was given regular diet throughout. Plasma concentrations of DMZ and its most important residue, HMMNI, were measured daily at 2 h after the morning feeding and, on days 8 and 15, several times during the day. The 2 h concentrations after 3 days loading ranged from 47 to 77 ppb for DMZ and 424 to 1081 ppb for HMMNI. A daily cycle in the plasma levels was seen for both substances. Upon withdrawal of medication, elimination of drug and metabolite was biexponential with a terminal half-life of 6.7 h. In the second experiment, 5 piglets were medicated as above and slaughtered 2, 6, 12, 25, and 49 h after withdrawal of the medication; the concentration of DMZ and HMMNI was measured in plasma, muscle, kidney, and liver. DMZ in the plasma amounted to 22 and 1.8 ppb at 2 and 6 h, while HMMNI declined from 535 ppb at 2 h to 0.75 ppb at 25 h. Most values for both substances found in muscle were close to those in the plasma; in kidney they amounted to 9-17% of the plasma levels.(ABSTRACT TRUNCATED AT 250 WORDS)