From the
Department of Pediatrics, Division of Cell Biology, National Jewish Medical and Research Center, Denver, Colorado 80206, Virocell Inc., Quebec City, Quebec G1V 2L2, and
Centre de Recherche en Rheumatologie et Immunologie, CHUL Research Center, Quebec City, Quebec G1V 4G2, Canada
Received for publication, January 27, 2003 , and in revised form, April 21, 2003.
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ABSTRACT |
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INTRODUCTION |
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The inactivation of endogenous LTB4 takes place through metabolism so that little, if any, escapes into the circulation or appears in urine as an intact molecule. The metabolic transformation of LTB4 has been studied in various cellular systems including the human neutrophil, which efficiently metabolizes LTB4 by a specific cytochrome P-450-dependent pathway (CYP4F3) to produce 20-hydroxy-LTB4 (20-OH-LTB4) (8). Additionally, other cells carry out this same step of -oxidation but do so with different members of this unique family of P-450 enzymes including CYP4F2 and CYP4F4 (8). The initial
-oxidized metabolite, 20-OH-LTB4, retains significant biological activity (9) but is further metabolized to biologically inactive 20-carboxy-LTB4 (20-COOH-LTB4) by both a P-450-dependent pathway (10, 11) as well as an alcohol dehydrogenase-dependent pathway in certain cells (12). Formation of these
-oxidation products has been observed after the incubation of LTB4 with human hepatocytes (13). Once
-oxidation has occurred, 20-COOH-LTB4 can be further metabolized by
-oxidation into 18-carboxy-LTB4 (18-COOH-LTB4) and 16-COOH-LTB3 (13, 14) which requires formation of the CoA ester at the
-terminal carboxyl moiety. These chain shortened products, as CoA esters, can undergo additional
-oxidation cycles from the
-terminus to form substantially less lipophilic metabolites (14).
An additional pathway of LTB4 metabolism identified in various human cells, including lung macrophages, monocytes, and keratinocytes, occurs via the 12-hydroxyeicosanoid dehydrogenase/10-reductase pathway (15, 16). This pathway leads to oxidation of the hydroxyl group at C-12 with intermediate formation of the 12-oxo product, which is then reduced at the immediately adjacent 10,11 double bond by a
10-reductase. This leads to a series of 10,11-dihydro metabolites, which can be precursors for
- or
-oxidation. Metabolism by this pathway also results in a substantial reduction in biological activity (17).
LTB4 can also participate in synthetic metabolic reactions by conjugation with polar molecules at either hydroxy or carboxyl substituents of LTB4 and oxidized metabolites. The first LTB4 conjugation metabolite observed was a taurine conjugate of 18-COOH-LTB4 formed in the rat hepatocyte (14). Additionally, when LTB4 was incubated with human keratinocytes, a glutathione adduct was observed as an intermediate metabolite (16). Finally, glucuronide conjugates of 20-COOH-LTB4, LTB4, and 10,11-dihydro-LTB4 have been observed as products following incubation of LTB4 with human hepatocytes (13).
Interest in LTB4 as an endogenous inflammatory mediator has been somewhat hampered by an inability to assess in vivo production of LTB4 either in normal individuals or individuals thought to have the leukotriene pathway of arachidonate metabolism activated as a result of pathologic events. One quite successful strategy in assessing in vivo production of prostaglandins and thromboxane A2 has been the quantitative analysis of corresponding metabolites appearing in urine using sensitive and specific assays for each unique metabolite (18, 19). This approach has also been used to monitor the production of leukotriene C4 in human subjects through measurement of leukotriene E4 excreted into urine (20). LTB4 has been observed in urine only in individuals with a deficiency of fatty aldehyde dehydrogenase (Sjögren-Larsson Syndrome) and Zellweger patients who lack peroxisomes and have limited -oxidation of lipids (21, 22).
In the course of investigations into LTB4 as a potential agent useful in the prophylaxis or treatment of infections, synthetic LTB4 was administered into healthy human volunteers in studies of safety, tolerability, pharmacokinetics, and pharmacodynamics. These experiments afforded the possibility to examine the urine for excreted LTB4 and LTB4 metabolites when known quantities of LTB4 were administered intravenously to human subjects. Detailed study of the urine was therefore undertaken to investigate whether or not previously identified metabolites of LTB4 described above were present in human subjects.
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EXPERIMENTAL PROCEDURES |
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Preparation of LTB4 for InjectionIn order to prepare LTB4 for injection into human subjects, ethanolic LTB4 was converted into the sodium salt by the addition of 1 eq of aqueous sodium hydroxide. This ethanolic solution of neutralized LTB4 was evaporated to dryness under reduced pressure at 40 °C. The clear yellowish oily residue was immediately redissolved in sodium phosphate (30 mM) buffered 0.7% sodium chloride containing 0.01% EDTA to a final concentration of 0.7 mg/ml LTB4. The pH of the solution was adjusted to 7.5 ± 0.1, and the solution was sterilized by filtration through a 0.22-µm microporous membrane filter. The sterile solution of LTB4 was then transferred into 10-ml clear glass vials, which were sealed and stored at 80 °C. Solutions of LTB4 for injection were analyzed prior to use to assess sterility, endotoxin content, concentration, and impurity levels. The purity of LTB4 for injection was also 96% with a profile of impurities similar to that of the starting material with the exception that the
-lactone of LTB4 was not present, but an impurity corresponding to the ethyl ester of LTB4 (
0.5%) was now detectable.
Injection of LTB4Subjects enrolled in this study were members of the community at large (Quebec, Canada) who were screened according to inclusion and exclusion criteria. In addition, individuals were tested for the presence of alcohol as well as human immunodeficiency virus and hepatitis B and C. Subjects abstained from alcohol for 2 days prior to inclusion in the study. Subjects also provided written informed consent and agreed to abide with the study restrictions, including collection of urine samples. Study subjects excluded were those with any clinically significant abnormalities as assessed from an electrocardiogram, clinical laboratory tests, or had abused alcohol or used illicit drugs. Other exclusion criteria included positive hepatitis B, hepatitis C, or human immunodeficiency virus or an active infection caused by herpes simplex virus, or a virus of the orthomyxovirus or rhinovirus families, and any clinically significant surgery or illness within the previous month. None of the subjects had used antiviral, antibiotic, or corticosteroids within 14 days preceding the study or had used an investigational drug within 30 days preceding this study.
After overnight fasting for 10 h, subjects were injected with LTB4 as a bolus intravenous dose of 50 µg/kg (150 nmol/kg) or an identical placebo injection of sterile saline. Both the LTB4 and the placebo were administered in a total volume of 5 ml as a bolus injection completed within
1 min. A total of four subjects received LTB4 and one subject received the placebo injection.
The subjects fasted for at least 1 h after drug administration, after which time they were served a controlled breakfast, and standard meals at 4, 9, and 13 h post-injection. Water was permitted ad libitum. Subjects remained lying down for 1 h after LTB4 injection, and vigorous physical activity was prohibited at all times during the study. Subjects were asked to empty their bladder prior to administration, and urine was then collected at two interval pools of 06 and 724 h postinjection. The urine samples were collected in polypropylene containers and kept in an ice water bath; at the end of both collection periods, the urine samples were quickly frozen and stored at 80 °C until analyzed. Throughout the study, subjects were monitored for adverse events, and a qualified medical investigator was on-site during injection of LTB4 and until 4 h after injection.
Urine ExtractionThe two urine sample pools from each of the five subjects (representing 06 and 724 h time periods after injection of LTB4 or placebo) were thawed and kept at 4 °C. The volume of urine in each sample was measured, then transferred into polypropylene 50-ml conical tubes, and centrifuged at 4 °C for 10 min at 750 x g. The urine samples were then acidified to pH 3.73.8 using formic acid and subjected to solid phase extraction using 1-g cartridges (Waters, Milford, MA). The cartridges were first conditioned with 2 volumes of methanol (40 ml) and then equilibrated with 2 volumes of water. The acidified urine samples were then loaded onto the cartridges and washed with 2 volumes of water. Solvent flow through the extraction cartridges was 20 ± 5 ml/min. Lipophilic metabolites were eluted with 1 volume of methanol directly into 20-ml glass ampoules. A maximum of 500 ml of urine was loaded on each 20-ml cartridge, and when the urine sample volume exceeded 500 ml, the extraction cartridge was reconditioned, and the remaining volume of the urine sample was extracted using the same reconditioned solid phase extraction cartridge. Two extraction blanks (0.9% saline acidified with formic acid) were also prepared. The ampoules were flame-sealed and stored at 80 °C until analysis. The urine sample volumes varied from 160 to 400 ml for the 06-h samples and 6251050 ml for the 724-h collections.
Aliquots of the methanol eluate, typically 5%, were taken for subsequent extraction, purification, and treatment with -glucuronidase. For some studies, solid phase extraction aliquots from the 06-h collection of each subject in the study were evaporated to dryness under vacuum and then reconstituted in 5 ml of 100 mM phosphate buffer, pH 7. The pH of the mixture was adjusted to 3.8 using formic acid, and the leukotrienes present in the solid phase-extracted urine were extracted using 1:1 (v/v) hexane/ethyl acetate. After vortexing 4 times for 15 s, each sample was centrifuged at 135 x g for 5 min to separate the two layers. This liquid-liquid extraction procedure was repeated 3 additional times, and the combined organic layers from each extraction were pooled and taken to dryness under vacuum.
Normal-phase HPLC and RP-HPLCThe 1:1 hexane/ethyl acetate extract was further purified by normal phase chromatography using an Ultremex 5-µm silica 250 x 4.6 mm column (Phenomenex, Torrance, CA). The normal phase solvents used were 90:10:0.1 hexane/isopropyl alcohol/acetic acid (solvent A) and 90:10:0.3 isopropyl alcohol, 20 mM ammonium acetate, acetic acid (solvent B). The initial mobile phase was 10% solvent B at a flow rate of 1 ml/min. This initial mobile phase was held for 3 min and then a linear gradient was started to 30% solvent B at 13 min. This was followed by a second linear gradient to 40% solvent B at 25 min and finally a third linear gradient to 85% solvent B at 43 min. The column effluent was monitored using UV detection at 270 and 235 nm, and one fraction was collected each minute for 43 min.
Due to the complexity of the urine matrix, it was necessary to perform reversed phase chromatography of the normal phase fractions of interest in order to increase the extent of purification. Normal phase fractions of interest were pooled, dried down under vacuum, and analyzed by reversed phase HPLC using a Synergi 10-µm Hydro-RP 250 x 4.6 mm column (Phenomenex, Torrance, CA). The reversed phase solvents used were 8.3 mM acetic acid adjusted to pH 5.7 with ammonium hydroxide (solvent A) and 65:35 acetonitrile/methanol (v/v) (solvent B). The initial mobile phase was 15% solvent B at a flow rate of 1 ml/min. This initial mobile phase was held for 3 min, and then a linear gradient was started to 70% solvent B at 43 min. This was followed by a second linear gradient to 100% solvent B at 55 min. The column effluent was monitored using UV detection at 270 and 235 nm, and one fraction was collected each minute for 55 min.
The final analysis was carried out using reversed phase fractions of interest, which had been dried under vacuum, and then subjected to a second reversed phase HPLC separation (subsequently termed the analytical RP-HPLC) using on-line RP-HPLC with electrospray mass spectrometry (LC/MS), which employed a 150 x 1.0-mm Ultremex C18 column (Phenomenex, Torrance, CA) at a flow rate of 50 µl/min. The same reversed phase solvents were used that are described above. The initial mobile phase was 15% solvent B, which was held for 3 min, and then a linear gradient was started to 60% solvent B in 30 min. This was followed by a second linear gradient to 100% solvent B at 45 min.
Electrospray Mass SpectrometryMass spectrometry was performed on a Sciex API III+ triple quadrupole mass spectrometer (PE-Sciex, Thornhill, Ontario, Canada). The mass spectrometry experiments were carried out in the negative ion mode with a spray voltage of 2800 V and an orifice voltage of 65 V. Collisional activation and multiple reaction monitoring (MRM) data were obtained using an offset potential of 20 eV and argon as the collision gas at a thickness of 230 x 1013 molecules/cm2.
Gas Chromatography/Mass SpectrometryReversed phase fractions of interest were taken to dryness under vacuum and derivatized for GC/MS analysis by the addition of 10% N,N-diisopropylethylamine in acetonitrile (50 µl) followed by the addition of 10% pentafluorobenzyl bromide in acetonitrile (50 µl). These samples were kept at room temperature for 30 min and evaporated under a stream of dry nitrogen. The samples were then dissolved in 500 µl of methylene chloride and introduced onto a silica solid phase extraction column (Supelco, Bellefonte, PA), which was conditioned with methanol and rinsed thoroughly with hexane prior to sample addition. After sample introduction, the silica SPE column was rinsed with 6 ml of hexane, and the PFB derivatives of interest were eluted with 3 ml of ethyl acetate. The ethyl acetate eluate was dried down under nitrogen and further derivatized with the addition of 50 µl of acetonitrile and 50 µl of bis(trimethylsilyl)trifluoroacetamide by incubating at 60 °C for 20 min followed by evaporation under nitrogen. The derivatized sample was dissolved in 10 µl of acetonitrile and subjected to GC/MS analysis. A gas chromatograph/mass spectrometer (Trace 2000, Thermo Finnigan, San Jose, CA) was employed for both electron ionization (EI) and negative ion chemical ionization (NCI) analysis. NCI spectra were obtained using methane as the moderating gas, and electron ionization (EI) spectra were obtained using an electron energy of 70 eV. The [M PFB] ions obtained from NCI (23) were used to determine the retention times of the -hydroxylated LTB4 compounds of interest. EI was used to provide detailed structural information regarding the hydroxyl group position from fragmentations that occur adjacent to the trimethylsilyl ether positions (24, 25).
Hydrolysis of GlucuronidesAn aliquot of methanol eluate (5%) from the 06-h pool of each subject injected with LTB4 or the placebo was dried down under vacuum and brought back up in 4.5 ml of 100 mM phosphate buffer, pH 7. This solution was adjusted to pH 7, and 5,000 units of -glucuronidase in 0.5 ml of 100 mM phosphate buffer (pH 7) was added (26). This mixture was incubated in a 37 °C shaking water bath for 16 h for the quantitation experiments. For the time course experiments, the incubation times ranged from 1 to 24 h. After treatment, the liquid-liquid extraction procedure described in the initial metabolite extraction section was carried out followed by normal and reversed phase HPLC as described.
Quantitation of LTB4 Glucuronide and 20-COOH-LTB4The quantity of LTB4 glucuronide and 20-COOH-LTB4 present in the urine of subjects injected with LTB4 and the placebo was determined using a stable isotope dilution LC/MS/MS protocol, essentially as described previously (27). The amount of LTB4 glucuronide present in urine was determined by adding 150 pmol of d4-LTB4 as internal standard to an aliquot of the solid phase extracted urine in 5 ml of 100 mM phosphate buffer, pH 7. Hydrolysis of glucuronides was then carried out using the procedure described above followed by extraction and HPLC separation. The MRM transitions monitored were m/z 335 195 for LTB4 and m/z 339
197 for d4-LTB4. A standard curve generated used various amounts of authentic LTB4 mixed with 150 pmol of d4-LTB4, which was linear over the range 9900 pmol LTB4. Additionally, the amount of 20-COOH-LTB4 was also calculated in the urine samples by using d4-LTB4 as the internal standard. A standard curve was constructed using known amounts of 20-COOH-LTB4 mixed with d4-LTB4 (150 pmol). The MRM transition monitored for 20-COOH-LTB4 was m/z 365
195.
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RESULTS |
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The urine of subjects treated with LTB4, as well as the one placebo-treated subject, was collected into two separate pools. The first pool was 06 h after administration of LTB4 or placebo, and the second collection was 724 h after the procedure. Due to the complexity of the urine matrix, a total of five purification steps were performed in order to prepare the sample for mass spectrometric analysis. The first step in purification involved a solid phase extraction using a reversed phase protocol to remove lipophilic components from the urine matrix. The pH of the urine was adjusted to pH 3.8 prior to solid phase extraction in order to maximize recovery of LTB4 metabolites (28). The next purification step was a liquid-liquid extraction carried out on separate aliquots of the methanol eluate using a hexane/ethyl acetate extraction procedure that had been shown to result in excellent recovery of LTB4 as well as several of the target metabolites (data not shown). The solvent-extracted components were then subjected to normal phase HPLC using a silica column with gradient elution maximized for the separation of LTB4, 20-OH-LTB4, and 20-COOH-LTB4 (data not shown). The retention times of these leukotrienes were 7.6, 14.9, and 16.8 min, respectively, and these specific fractions (±1 min) were collected. The normal phase fractions of interest were introduced onto a 4.6-mm reversed phase HPLC column where the retention time of LTB4, 20-OH-LTB4, and 20-COOH-LTB4 standards were 36.1, 21.8, and 17.3 min, respectively. Finally, the reversed phase fractions that eluted at the expected retention times (±1 min) were introduced onto a 1-mm analytical RP-HPLC column and analyzed by on-line mass spectrometry (LC/MS). The retention times for standards and known UV absorption characteristics of leukotrienes were used as a guide to screening additional fractions than those noted for metabolites of LTB4. For example, fractions that eluted from the normal phase column after 16.8 min were analyzed for metabolites more polar than 20-COOH-LTB4. Another experimental strategy used to verify the identity of LTB4 metabolites was to monitor two MRM transitions for each known LTB4 metabolite. This strategy was critical to identify unambiguously the metabolites listed below. The MRM transitions used in this study were obtained from a previous study of the collision-induced dissociation mass spectra derived from the corresponding metabolites (13, 29).
Collisional activation of intact LTB4 glucuronide-conjugated metabolites had been shown previously to result in a neutral loss of 176 daltons to yield the aglycone (parent) anion as well as a glucuronic acid anion at m/z 193 (13). These ions and the neutral loss event did not provide structural details concerning the aglycone portion of the metabolite but did provide relevant information as to the existence of glucuronides eluting from the HPLC column. Because glucuronide conjugates of 20-COOH-LTB4, LTB4, and 10,11-dihydro-LTB4 had been observed previously (13) following incubation of LTB4 with human hepatocytes, a constant neutral loss scan was carried out for the LC/MS analysis of the solid phase extracted urine. However, a complex mixture of components was clearly evident in the placebo control as well as in the urine from the individual receiving the LTB4 injection. Despite the solid phase extraction of this sample, it was clear that the use of a single ion transition, in this case a constant neutral loss of 176 daltons, as well as a precursor mass specific for LTB4 were not sufficiently unique criteria to identify specific glucuronides present in the complex urine mixture. The most likely reason for this was that multiple LTB4 glucuronide conjugates (see below) were present and each one at sufficiently low levels and that naturally occurring glucuronide conjugates from unrelated molecules produced signals that were greater in magnitude compared with any single LTB4 glucuronide. Thus it was necessary to carry out the second stage of reversed phase HPLC as well as hydrolyze the glucuronides prior to analysis. A summary of the metabolites that were found in urine of all the subjects injected with LTB4 is reported in Table I. The urine from the subject that received the placebo injection was also analyzed, but none of the metabolites reported (Table I) satisfied all of the criteria in chromatography as well as tandem mass spectrometry to suggest the presence of the metabolite. Specific details are provided below.
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20-COOH-LTB4 and 10,11-Dihydro-20-COOH-LTB4The normal phase and reversed phase retention times of 20-COOH-LTB4 were determined to be 16.8 and 17.1 min, respectively. Therefore, the pooled normal phase fractions 1718 from each subject treated with LTB4 were subjected to RP-HPLC on a 4.6-mm column, and a conjugated triene chromophore was observed for a component eluting at 17.3 min (Fig. 2a, inset). The RP-HPLC fractions 1718 were pooled and analyzed by LC/MS/MS. The expected ion transitions for the collisional activation of 20-COOH-LTB4 m/z 365 195 and m/z 365
169 (29) were monitored in an MRM assay. When unhydrolyzed urine was subjected to the prior normal-phase and RP-chromatographic separation, LC/MS/MS analysis revealed the elution of a component indicated by these specific ion transitions at the suggested retention time and with the appropriate relative abundant ratios (Fig. 2, a and b). In addition to monitoring the MRM transitions for 20-COOH-LTB4, the MRM transitions at m/z 367
115 and 367
169 were also monitored in order to assess whether or not 10,11-dihydro-20-COOH-LTB4 was present in these HPLC fractions. Both of these MRM transitions were significant in the unhydrolyzed urine at a slightly more lipophilic retention time (17.6 min, Fig. 2, c and d), which suggested that indeed 10,11-dihydro-20-COOH-LTB4 was present in urine samples of subjects injected with LTB4. The collisional activation of m/z 367 yielded a product ion mass spectrum (Fig. 2c, inset) that was virtually identical to that published previously (13) for 10,11-dihydro-20-COOH-LTB4. The abundant product ions observed at m/z 115 and 169 were consistent with cleavage adjacent to the hydroxyl substituents with charge retention on the corresponding moiety as reported previously (29).
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Glucuronide Conjugates of 20-, 19-, 18-, and 17-OH-LTB4 The normal phase retention time of 20-OH-LTB4 standard was 14.9 min, and the reversed phase retention time was 21.8 min using a 4.6-mm column. The pooled normal phase fractions 1516 were separated on the 4.6-mm RP column, and fractions 2123 of the -glucuronidase-treated and untreated urine aliquots were analyzed by LC/MS/MS. Any metabolites of LTB4 that were hydroxylated near the methyl terminus were detected using the MRM transition m/z 351
195 (29). This transition was clearly observed in urine that had been treated with
-glucuronidase (Fig. 3a) but was not observed in the urine not treated with
-glucuronidase. Furthermore, four separate HPLC components eluted within a 3-min time window at 21.4, 22.4, 22.9, and 23.6 min in the
-glucuronidase-treated urine. In order to establish the identity of the four components that share the MRM transition m/z 351
195, reversed phase fractions 2123 were derivatized for GC/MS analysis as the pentafluorobenzyl ester (PFB) and trimethylsilyl ether (TMS) derivative. The NCI of these derivatives had the most abundant ion at m/z 567 (Fig. 3b), which corresponded to a trihydroxy-TMS-derivatized carboxylate anion. Negative ion chemical ionization was used to determine the retention times (9.98, 10.10, 10.17, and 10.42 min) for the
-hydroxylated LTB4 compounds of interest. After the retention times were established, EI was used to provide detailed structural information regarding the hydroxyl group position from bond cleavage that occurs adjacent to the trimethylsilyl ether positions. The EI mass spectrum of the derivatized metabolite with a retention time of 9.98 min (Fig. 3c) indicated that there are hydroxyl groups at C5 from the m/z 369 fragment (TMS-O+=CH-(CH2)3CO2PFB) and C12 from the m/z 549 (TMS-O+=CH-(CH)6-CH(TMSO)-(CH2)3CO2PFB) (24). Additionally the ions at m/z 73 (TMS+) and m/z 181 (C7H2F5+) were from the derivative groups. Finally, the major fragment ion at m/z 145 (TMSO+=CH(CH2)2CH3) was indicative of a 17-hydroxylated metabolite due to
-cleavage of the C16C17 bond adjacent to the trimethylsilyl ether (25). From the EI spectrum shown in Fig. 3c, it was determined that the LTB4 metabolite with a retention time of 9.98 min was 17-OH-LTB4. The EI spectra of the hydroxylated LTB4 metabolites with retention times of 10.10 and 10.18 min (Fig. 3b) were obtained and showed major
-cleavage ions at m/z 131 (TMS-O+ = CHCH2CH3) and m/z 117 (TMS-O+ = CHCH3), respectively, as expected for
-1- and
-2-hydroxylated metabolites (25). In addition the relative abundance of other ions in the electron ionization mass spectra (data not shown) was identical to spectra published previously (30, 31) of the
-1- and
-2-hydroxylated metabolites of LTB4. These data were consistent with the identification of the metabolites with retention times of 10.10 and 10.18 min as 18-OH-LTB4 and 19-OH-LTB4. Finally, the EI spectrum of the derivatized LTB4 metabolite with a retention time of 10.42 min, identical to the gas chromatographic retention time of the 20-OH-LTB4 standard, was identical to the EI mass spectrum of 20-OH-LTB4 (24). The GC/MS data indicated that glucuronide conjugates of 20-, 19-, 18-, and 17-OH-LTB4 were present in urine of subjects injected with LTB4 and that the most abundant
-oxidized metabolite was 17-OH-LTB4.
Glucuronide Conjugates of LTB4 and 10,11-Dihydro-LTB4 The normal phase and reversed phase retention times of LTB4 were determined to be 7.6 and 36.1 min, respectively. Therefore, pooled normal phase fractions 79 were subjected to RP-HPLC. Only the -glucuronidase-treated urine revealed a conjugated triene chromophore with a
max of 270 nm at 35.8 min (Fig. 4a, inset). The pooled reversed phase fractions 3537 of the
-glucuronidase-treated and untreated samples were then subjected to LC/MS/MS analysis. In order to ascertain if LTB4 was present in the urine, the MRM transitions m/z 335
195 and m/z 335
129 were monitored, and neither of these transitions were present in the unhydrolyzed urine (Fig. 5, b and c). However, the MRM transitions for LTB4 were detected at 36.9 min when the
-glucuronidase-treated urine was analyzed (Fig. 4, a and b). In addition to monitoring the MRM transitions for LTB4, the MRM transitions m/z 337
115 and m/z 337
225 for 10,11-dihydro-LTB4 were also monitored in the unhydrolyzed and
-glucuronidase-treated urine. The most abundant of the MRM transitions for 10,11-dihydro-LTB4 was not present in the unhydrolyzed urine (Fig. 5a); however, both of these MRM transitions were present at the correct relative intensity ratio at the slightly more lipophilic retention time compared with LTB4 of 37.9 min (Fig. 6, a and b). This mass spectrometric data indicated that glucuronide conjugates of LTB4 as well as 10,11-dihydro-LTB4 were present in urine of the subjects injected with LTB4.
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Glucuronide Conjugate of 10-Hydroxy-4,6,12-octadecatrienoic Acid (10-HOTrE)Normal phase fractions eluting between 4 and 6 min were also subjected to RP-HPLC on a 4.6-mm column, and reversed phase fractions eluting between 39 and 41 min were subject to LC/MS/MS. The specific MRM transitions for 10-HOTrE that were monitored during this LC/MS/MS assay were m/z 293 137 and 293
153 (29). Neither of these transitions were present in the specific fractions of the unhydrolyzed urine, but the transitions were quite abundant at 39.4 min in the
-glucuronidase-treated urine (Fig. 7, a and b). These data were consistent with a glucuronide conjugate of 10-HOTrE present in urine of subjects injected with LTB4.
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Quantitation of LTB4 Glucuronide and 20-COOH-LTB4 The quantity of LTB4 glucuronic acid conjugates excreted into urine of subjects injected with LTB4 was determined using a stable isotope dilution mass spectrometric assay (Table II). Enzymatic hydrolysis of the LTB4 glucuronide conjugates was achieved by incubating the solid phase extracted urine with -glucuronidase for 16 h at 37 °C. After hydrolysis and purification using RP-HPLC, the MRM transitions for LTB4 and the internal standard d4-LTB4 were monitored during LC/MS/MS analysis. A time course for the hydrolysis of urine was also carried out (Fig. 8) on an aliquot of the solid phase extracted urine to follow the yield of released LTB4. The yield of free LTB4 was found to maximize between 15 and 24 h of treatment, and therefore a hydrolysis time of 16 h was routinely used in subsequent studies. The quantity of 20-COOH-LTB4 was also determined in each urine sample using the d4-LTB4 as an analog internal standard. The standard curve for this latter assay was linear over the range of 9900 pmol with an excellent linearity and correlation coefficient (data not shown). The quantity of 20-COOH-LTB4 present in each urine pool (06 h) was similar to that of LTB4 glucuronide (Table II).
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DISCUSSION |
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A separate pathway of LTB4 metabolism found in specific cells, including the lung macrophage, monocytes, and keratinocytes, involved reduction of the conjugated triene to a conjugated diene and has been termed the 12-hydroxyeicosanoid dehydrogenase/10-reductase pathway (16). This pathway is initiated through oxidation of the 12-hydroxy group at carbon 12 by 12-hydroxyeicosanoid dehydrogenase and the formation of 12-oxo-LTB4. This conjugated ketone is then reduced by
10-reductase and further reduction of the ketone moiety at carbon-12 by a keto-reductase to the hydroxyl moiety. This pathway results in a series of metabolites termed 10,11-dihydro-LTB4 metabolites, which can be further transformed by
/
-oxidation from either the methyl or carboxyl terminus (16).
The third major pathway of LTB4 metabolism has been characterized by conjugation with polar intermediates by specific enzymes that convert hydroxyl or carboxyl substituents of LTB4 into ether or ester conjugates. Conjugation with glucuronic acid was particularly prominent in the metabolism of LTB4 with human hepatocytes (13). It is important to note that not a single glucuronide of LTB4 existed, but rather a mixture of numerous glucuronide conjugate positional isomers, perhaps greater than five, was possible when considering acyl glucuronides (32). Additionally, glucuronide conjugates of LTB4 metabolites derived from the pathways described above were also possible that could result in an exceedingly complex mixture of metabolites, making identification of prominent metabolites of LTB4 excreted into the urine difficult.
Previous studies of LTB4 metabolism in intact animals had involved the use of tritium-labeled LTB4 to assist in isolation and purification of metabolites. When rats were injected with radiolabeled LTB4, both bile and urine were found to contain metabolites. The bile contained 2025% of the injected radioactivity, whereas the urine contained less than 10% of the injected dose (33). Reversed phase HPLC analysis of the bile and urine revealed that most metabolites were less lipophilic than LTB4; however, structural identification of these metabolites was not reported. In another study, the in vivo metabolism of LTB4 in the monkey was reported (34) after infusion with radiolabeled LTB4 (8 µmol/kg). In this investigation, 25% of the injected radioactivity was recovered in urine after 24 h. More than 70% of the infused radioactivity was reported as volatile metabolites, and no intact LTB4 was detected in urine. Purification of nonvolatile radiolabeled metabolites was carried out, and one metabolite was found to coelute with 20-hydroxy-LTB4. The structures of the additional urinary metabolites were not reported, although several metabolites were separated.
In the present study where LTB4 (150 nmol/kg) was injected into human subjects in a Phase I Clinical Trial, an alternative strategy was employed to identify LTB4 metabolites based upon the previous known metabolites of LTB4 using sensitive and specific LC/MS/MS assays. Because the retention times and relative intensities of ion transitions were known for several LTB4 metabolites, it was possible to take advantage of the chromatographic behavior of metabolites under both reversed and normal phase conditions. For example, it was possible to obtain data consistent with the existence of 20-COOH-LTB4, a component present in the urine of individuals treated with LTB4, but not present in the urine of the untreated control subject. The criteria for metabolite identification was based upon HPLC retention times as well as the relative abundance of several specific ion transitions. The relative intensity of m/z 365 169 compared with that of m/z 365
195 was 0.70 (Fig. 2, a and b), which was similar to that published previously (29). If a single ion transition was used, multiple components could often be detected (Figs. 1d and 7a). This was mostly the result of the presence of unrelated compounds in urine that generated signals with the same ion transition channel, most likely due to the large number of molecules present in urine at levels much higher than those observed for the true LTB4 metabolites. Therefore, criteria were established where two MRM transitions for each of the identified metabolites had to coincide at the correct retention time and, if possible, UV absorption spectra in order to report the presence of a specific metabolite. By using this strategy, it was also possible to detect the presence of 18-COOH-LTB4, 10,11-dihydro-18-COOH-LTB4, and 10,11-dihydro-20-COOH-LTB4 excreted into the urine in each of the subjects, even though authentic standards were not available to unambiguously establish the ratio of MRM transitions. In the case of 18-COOH-LTB4 and 10,11-dihydro-18-COOH-LTB4, additional information was employed in that the compound with the correct MRM transitions eluted as a less lipophilic component compared with 20-COOH-LTB4. Additionally, 10,11-dihydro-20-COOH-LTB4 had the correct MRM transitions and eluted at a slightly more lipophilic retention time than 20-COOH-LTB4. No data could be obtained consistent with the presence of 16-COOH-LTB3 as urinary metabolite of LTB4. It was very clear, however, that neither 20-OH-LTB4 nor LTB4 itself were present as excreted products in urine of subjects treated with LTB4. The fact that LTB4 was not present in urine of these subjects suggested that LTB4 was rapidly metabolized following injection and was consistent with previous primate studies where
50 times more LTB4 was injected (34).
One of the major pathways of LTB4 metabolism in human subjects in this study was glucuronic acid conjugation, which is a common means that the intact organism can enhance the excretion of lipophilic compounds (35, 36). In order to establish unambiguously that glucuronide conjugates of LTB4 were present in the urine of subjects injected with LTB4, a strategy that employed analysis of identical aliquots of -glucuronidase-treated and untreated samples were analyzed by LC/MS/MS. A further complication of multiple glucuronides for any single LTB4 metabolite reduced the overall sensitivity of the analysis of each individual metabolite as an intact glucuronide. Because
-glucuronidase can cleave ether-linked as well as 1-O-acyl-linked glucuronide conjugates, it was employed as the most applicable method to remove the glucuronic acid conjugate. Studies employing acid or based catalyzed hydrolysis led to extensive degradation of LTB4 metabolites (data not shown). The LC/MS/MS data clearly indicated the existence of several excreted glucuronide metabolites of the injected LTB4. These included 20-, 19-, 18-, and 17-OH-LTB4, 10-HOTrE, 10,11-dihydro-LTB4, and even intact LTB4 glucuronides. None of these components were present in the nonhydrolyzed urine samples nor were they present in the untreated placebo urine. The identity of each of these metabolites (after hydrolysis) could be clearly ascertained using the criteria described above.
Of some interest was the unexpected observation of 17-OH-LTB4, 18-OH-LTB4, and 19-OH-LTB4 glucuronide. Powell and Gravelle (30) found that rat polymorphonuclear leukocytes could form both 19-OH-LTB4 and 18-OH-LTB4 upon incubation with arachidonic acid and stimulation by A23187
[GenBank]
. Additionally, rat liver microsomes were found to convert LTB4 into 20-OH-LTB4 along with small amounts of 19-OH-LTB4 (31). Recently, recombinant CYP4F5 and CYP4F6 were found to convert LTB4 into 17-OH-, 18-OH-, 19-OH-, and 20-OH-LTB4 during in vitro incubation (37). These P450 isozymes were found expressed in hepatocytes of rats (37). During the LC/MS/MS assay, detection of four separate components with the ion transition of m/z 351 195 was observed after treatment with
-glucuronidase (Fig. 3a). In order to determine the position of hydroxylation of the four components, reversed phase fractions 2123 were pooled and analyzed by GC/MS as the PFB ester and TMS ether derivative. The four trihydroxy-TMS derivatized carboxylate anions (m/z 567) are shown in the NCI-GC/MS chromatogram in Fig. 3b at retention times of 9.98, 10.10, 10.18, and 10.42 min. Electron ionization was then used to provide structural information concerning the hydroxyl group position from cleavage that occurs adjacent to the TMS ether position. The EI mass spectra at retention times of 9.98, 10.10, and 10.18 min showed a major
-cleavage ion at m/z 145 (Fig. 3c), m/z 131, and m/z 117, respectively. From these data it was concluded that 17-OH-, 18-OH-, and 19-OH-LTB4 were present as glucuronide conjugates in the urine of subjects injected with LTB4. Additionally, the metabolite with a retention time of 10.42 min was identified as 20-OH-LTB4. The main
-hydroxylated LTB4 metabolite found in the
-glucuronidase-treated urine of subjects injected with LTB4 was found to be 17-OH-LTB4, whereas 20-OH-LTB4 was a minor peak. This unexpected finding might reflect a situation where 20-OH-LTB4 can undergo
-oxidation to form less lipophilic metabolites rather than forming a glucuronide (Fig. 9). Thus, the glucuronidation pathway was a more predominant fate for
-1-,
-2-, and
-3-hydroxylation products of LTB4 because subsequent
-oxidation was not possible.
The most abundant metabolites identified were LTB4 glucuronide and 20-COOH-LTB4. Therefore, quantitative assays based on stable isotope dilution or analog dilution using stable isotope-labeled LTB4 was employed to measure specifically these two metabolites of LTB4. The quantity of the LTB4 glucuronide (after hydrolysis) was found to range between 15 and 30 pmol/ml of urine. The enzymatic hydrolysis experiment was performed five separate times for one subject, and the error was determined to be ±1.7 pmol/ml of urine. The amount of 20-COOH-LTB4 was also determined in these urine samples, and it ranged from 18 to 31 pmol/ml of urine. This quantitation was found to have an analytical error of ±0.86 pmol/ml in five replicate analyses. From these results, it appeared that approximately the same amount of 20-COOH-LTB4 and LTB4 glucuronide were excreted into the urine of each subject. It is not known if a single glucuronide isomer predominated for LTB4 glucuronide; however, based on previous studies it was likely that a number of glucuronides existed. The total amount of 20-COOH-LTB4 and LTB4 glucuronide represented only 0.2% recovery of the injected LTB4 for each of the subjects. This rather low recovery was expected based on the previously described studies of LTB4 metabolism as well as the metabolism of prostanoids into individual metabolites (1820).
It is important to note that the metabolic products of LTB4 in vivo reported above were only found in the urine sample from a 0- to 6-h period after injection of LTB4. The 724-h urine sample was also analyzed, and none of these metabolites were observed. This was consistent with the time course described previously (33, 34) for appearance of LTB4 metabolites in urine in various animal experiments. These studies indicated that LTB4 underwent degradation via multiple pathways leading to extensive chain shortening typical of fatty acids as well as glucuronidation (Fig. 9). Nevertheless, some caution must be exercised in extrapolation of the observations reported here to the metabolic fate of LTB4 synthesized from endogenous arachidonic acid in vivo. This study involved administration of high doses of LTB4 that could experience a different metabolic fate in quantitative terms from LTB4 synthesized within a cell.
In summary, the metabolism of LTB4 in the human subjects is quite complex with multiple pathways responsible for the degradation of LTB4 prior to elimination into urine. Some of the most abundant products were those derived from conjugation with glucuronic acid of either intact LTB4 or other highly lipophilic metabolites. In this investigation, 11 different metabolites of LTB4 were identified in urine subjects injected with 150 nmol/kg LTB4. Four unconjugated metabolites were excreted into urine and identified by mass spectrometry as 18-COOH-LTB4, 10,11-dihydro-18-COOH-LTB4, 20-COOH-LTB4, and 1011-dihydro-20-COOH-LTB4. The glucuronide conjugates of 20-, 19-, 18-, and 17-OH-LTB4, 10-HOTrE, 10,11-dihydro-LTB4, and intact LTB4 itself were also observed. None of these metabolites exceeded 1% of the injected dose; however, it was possible to detect the glucuronide conjugates by mass spectrometry after hydrolysis to the aglycone metabolite. It is possible that these LTB4 metabolites may be relevant targets to assess the in vivo production of LTB4, but additional experiments will be required to define which of these observed metabolites from the exogenously administered LTB4 best reflects endogenous LTB4 production.
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FOOTNOTES |
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To whom correspondence should be addressed: National Jewish Medical and Research Center, 1400 Jackson St., Denver, CO 80206. Tel.: 303-398-1849; Fax: 303-398-1694; E-mail: murphyr{at}njc.org.
1 The abbreviations used are: LTB4, leukotriene B4; HPLC, high performance liquid chromatographic; RP-HPLC, reversed phase HPLC; LC/MS, liquid chromatography/mass spectrometry; GC/MS, gas-liquid chromatography/mass spectrometry; MRM, multiple reaction monitoring; 20-OH-LTB4, 20-hydroxy-LTB4; 20-COOH-LTB4, 20-carboxy-LTB4; d4-LTB4, [6,7,14,15-2H4]LTB4; EI, electron ionization; NCI, negative ion chemical ionization; TMS, trimethylsilyl ether; PFB, pentafluorobenzyl ester; 10-HOTrE, 10-hydroxy-4,6,12-octadecatrienoic acid.
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