Determination of methylprednisolone metabolites in human urine by gas chromatography-mass spectrometry

Efficient dispersive solid-phase extraction of methylprednisolone from exhaled breath of COVID-19 patients

In the current study, bismuth ferrite nano-sorbent was synthesized and utilized as a sorbent for the dispersive solid-phase extraction of methylprednisolone from exhaled breath samples. The size and morphology of the nano-sorbent were characterized by X-ray diffraction analysis and scanning electron microscopy. Following its desorption with acetonitrile, methylprednisolone was quantified by a high-performance liquid chromatography-ultraviolet detector. Factors affecting the extraction of methylprednisolone were optimized. Under optimized experimental conditions, a linear relationship between the analytical signals and methylprednisolone concentration was obtained in the range of 0.001-0.2 μg mL^-1 for exhaled breath condensate samples and 0.002-0.4 μg per filter for filter samples. A pre-concentration factor of 6.4-fold, corresponding to an extraction recovery of 96.0%, was achieved. The validated method was applied for the determination of methylprednisolone in real samples taken from the exhaled breath of COVID-19 patients under mechanical ventilation.

A preliminary study on urinary excretion patterns of methylprednisolone after oral and intra-articular administration and effect on endogenous glucocorticosteroids profile

INTRODUCTION:

The use of glucocorticosteroids (GCs) through oral, intravenous, intramuscular, or rectal routes is prohibited in sports. Its use is permitted through inhalation, topical and intra-articular route of administration. Methylprednisolone (MP) is available for use by different routes for anti-inflammatory and immunosuppressive purposes. To discriminate its intake by permitted & forbidden routes, a reporting level of 30 ng/ml is set by World Anti-Doping Agency. The aim of this study was to compare MP's excretion profile following oral & intra-articular administration & to evaluate its effect on endogenous GCs profile.

MATERIALS & METHODS:

The MP was administered through oral and intra-articular route to different patients & urine samples were collected up to 100 h. The urine samples were hydrolyzed, extracted, and analyzed on Liquid chromatography-mass spectrometry/MS.

RESULTS:

MP levels in urine exceeded the reporting limit of 30 ng/ml after oral (8 mg) and intra-articular administration (80 mg) routes. After oral intake (8 mg), MP levels exceeded the reporting level up to 24 h. However, after intra-articular injection (80 mg), the MP could be detected above the reporting level up to 80 h.

CONCLUSION:

The findings reveal that the MP can exceed the reporting level in urine even after administration by permitted route (i.a.). Further analysis of four endogenous GCs (Cortisol, Cortisone, TH Cortisone, and 11-deoxycortisol) showed a decreased excretion following administration of MP by oral & intra-articular routes.

Urinary profile of methylprednisolone and its metabolites after oral and topical administrations

Methylprednisolone (MP) is prohibited in sports competitions when administered by systemic routes; however its use by topical administration is allowed. Therefore, analytical approaches to distinguish between these different administration pathways are required. A reporting level of 30ng/mL was established for this purpose. However, the suitability of that reporting level for MP is not known. In the present work, excretion profiles of MP and different metabolites after oral and topical administrations have been compared. A method for the quantification of MP and the qualitative detection of fifteen previously reported metabolites has been validated. The method involved an enzymatic hydrolysis, liquid-liquid extraction and analysis by liquid chromatography coupled to tandem mass spectrometry. The method was found to be linear, selective, precise and accurate. The high sensitivity (limit of detection 0.1ng/mL) and linear range (0.1-250ng/mL) achieved allowed for the quantification of MP at both the low concentrations present after topical administration and the high concentrations detected after oral intake. The method was applied to samples collected after oral (4 or 40mg) and topical administration (10mg of MP aceponate/day for 5 consecutive days) to healthy volunteers. After oral administration, MP and all metabolites were detected in urines collected up to at least 36h. Only MP and five metabolites were detected in samples obtained after topical treatment. As expected, concentrations of MP after topical administration were well below current reporting level (30ng/mL), however 3 out of 4 samples in range 8-24h after the low oral dose (4mg) were also below that concentration. Taking into account metabolites detected after both administration routes, metabolites 16β,17α,21-trihydroxy-6α-methylpregna-1,4-diene-3,11,20-trione (M8) and 17α,20α,21-trihydroxy-6α-methylpregna-1,4-diene-3,11-dione (M11) are best markers to differentiate between topical and oral administrations. Their signals after topical administration were lower than those obtained in the first 48h after all oral doses.

Identification of budesonide metabolites in human urine after oral administration

Budesonide (BUD) is a glucocorticoid widely used for the treatment of asthma, rhinitis, and inflammatory bowel disease. Its use in sport competitions is prohibited when administered by oral, intravenous, intramuscular, or rectal routes. However, topical preparations are not prohibited. Strategies to discriminate between legal and forbidden administrations have to be developed by doping control laboratories. For this reason, metabolism of BUD has been re-evaluated using liquid chromatography-tandem mass spectrometry (LC-MS/MS) with different scan methods. Urine samples obtained after oral administration of 3 mg of BUD to two healthy volunteers have been analyzed for metabolite detection in free and glucuronide metabolic fractions. Structures of the metabolites have been studied by LC-MS/MS using collision induced dissociation and gas chromatography-mass spectrometry (GC/MS) in full scan mode with electron ionization. Combination of all structural information allowed the proposition of the most comprehensive picture for BUD metabolism in humans to this date. Overall, 16 metabolites including ten previously unreported compounds have been detected. The main metabolite is 16α-hydroxy-prednisolone resulting from the cleavage of the acetal group. Other metabolites without the acetal group have been identified such as those resulting from reduction of C20 carbonyl group, oxidation of the C11 hydroxyl group and reduction of the A ring. Metabolites maintaining the acetal group have also been identified, resulting from 6-hydroxylation (6α and 6β-hydroxy-budesonide), 23-hydroxylation, reduction of C6-C7, oxidation of the C11 hydroxyl group, and reduction of the C20 carbonyl group. Metabolites were mainly excreted in the free fraction. All of them were excreted in urine during the first 24 h after administration, and seven of them were still detected up to 48 h after administration for both volunteers.

Using complementary mass spectrometric approaches for the determination of methylprednisolone metabolites in human urine

RATIONALE

The metabolism of methylprednisolone is revisited in order to find new metabolites that could be important for distinguishing between different routes of administration. Recently developed liquid chromatography/tandem mass spectrometry (LC/MS/MS) strategies for the detection of corticosteroid metabolites have been applied to the study of methylprednisolone metabolism.

METHODS

The structures of these metabolites were studied using two complementary mass spectrometric techniques: LC/MS/MS in product ion scan mode with electrospray ionization and gas chromatography/mass spectrometry (GC/MS) in full scan mode with electron ionization. Metabolites were also isolated by semipreparative liquid chromatography fractionation. Each fraction was divided into two aliquots; one was studied by LC/MS/MS and the other by GC/MS after methoxyamine-trimethylsilyl derivatization.

RESULTS

The combination of all the structural information allowed us to propose a comprehensive picture of methylprednisolone metabolism in humans. Overall, 15 metabolites including five previously unreported compounds have been detected. Specifically, 16β,17α,21-trihydroxy-6α-methylpregna-1,4-diene-3,11,20-trione, 17α,20β,21-trihydroxy-6α-methylpregna-1,4-diene-3, 11-dione, 11β,17α,21-trihydroxy-6α-hydroxymethylpregna-1,4-diene-3,20-dione, 11β,17α,20ξ,21-tetrahydroxy-6α-hydroxymethylpregna-1,4-diene-3-one, and 17α,21-dihydroxy-6α-hydroxymethylpregna-1,4-diene-3,11,20-trione are proposed as feasible structures for the novel metabolites. In addition to the expected biotransformations: reduction of the C20 carbonyl, oxidation of the C11 hydroxy group, and further 6β-hydroxylation, we propose that hydroxylation of the 6α-methyl group can also take place.

CONCLUSIONS

New metabolites have been identified in urine samples collected after oral administration of 40 mg of methylprednisolone. All identified metabolites were found in all samples collected up to 36 h after oral administration. However, after topical administration of 5 g of methylprednisolone aceponate, neither the parent compound nor any of the metabolites were detected.

A sensitive and specific precursor ion scanning approach in liquid chromatography/electrospray ionization tandem mass spectrometry to detect methylprednisolone acetate and its metabolites in rat urine

A new, simple, sensitive and specific liquid chromatography/electrospray ionization tandem mass spectrometric (LC/ESI-MS/MS) method in precursor ion scanning (PIS) mode has been developed for the rapid detection of methylprednisolone acetate (MPA) and its metabolites in rat urine. A suitable product ion specific for methylprednisolone (MP) and MPA was selected after a fragmentation study on 20 (cortico)steroids at different collision energies (5-40 eV). Urine samples were simply treated with acetonitrile then dried in a SpeedVac system. The method was validated and compared with other PIS methods for detecting corticosteroids in human urine. It was more sensitive, with limit of detection (LOD) and lower limit of quantitation (LLOQ), respectively, of 5 and 10 ng mL(-1). The method was applied for the analysis of rat urine collected before and after (24, 48, 72 h) intra-articular (IA) injection of a marketed formulation of MPA (Depo-Medrol(R)). MS/MS acquisitions were taken at different collision energies for the precursor ions of interest, detected in PIS mode, to verify the MP-related structure. Six different metabolites were detected in rat urine, and their chemical structures were assigned with a computational study.

Evaluation of different scan methods for the urinary detection of corticosteroid metabolites by liquid chromatography tandem mass spectrometry

Different approaches for the non-target detection of corticosteroids in urine have been evaluated. As a result of previous studies about the ionization (positive/negative) and fragmentation of corticosteroids, several methods based on both precursor ion (PI) and neutral loss (NL) scans are proposed. The applicability of these methods was checked by the injection of a standard solution containing 19 model compounds. Five of the studied methods (NL of 76 Da; PI of 77, 91 and 105; PI of 237; PI of 121, 147 and 171; and NL of 38 Da) exhibited satisfactory results at the concentration level checked (corresponding to 20 ng/ml in sample). Some other methods in negative ionization mode such as the NL of 104 Da were found to lack sufficient sensitivity. Some of the applied methods were found to be specific for a concrete structure (NL of 38 Da for fluorine containing corticosteroids) while others showed a wide range applicability (PI of 77, 91 and 105 showed response in all model compounds). Interference by endogenous compounds was also tested by the analysis of negative urines and urines spiked with different corticosteroids. The suitability of these methods for the detection of corticosteroid metabolites was checked by the analysis of urine samples collected after the administration of methylprednisolone and triamcinolone. A combination of the reported methods seems to be the approach of choice in order to have a global overview about the excreted corticosteroid metabolites.

Determination of methylprednisolone acetate in biological fluids at gold nanoparticles modified ITO electrode

The electrochemical behavior of a corticosteroid methylprednisolone (MP), used for doping, has been studied at gold nanoparticles modified indium tin oxide (nanoAu/ITO) electrode. The nanoAu/ITO electrode exhibited an effective catalytic response towards its oxidation and lowered its oxidation potential by approximately 127 mV when compared with bare ITO electrode. Oxidation of MP has been carried out in phosphate containing electrolyte in the pH range 2.13-10.00 and a well-defined oxidation peak was noticed. Linear concentration curves are obtained over the concentration range 0.01-1.0 microM with a detection limit of 2.68 x 10(-7)M at nanoAu/ITO electrode. A diffusion coefficient of 2.36 x 10(-6)cm(2)/s is calculated for MP using chronoamperometry. The proposed method is effectively applied to detect the concentration of MP in pharmaceutical formulations and human blood plasma and urine samples. A comparison of MP concentration determined in blood plasma and urine by the proposed method and GC/MS indicated that the results are essentially similar. It is believed that the method will be useful in determining this drug in case of doping.

High-performance liquid-chromatographic-atmospheric-pressure chemical-ionization ion-trap mass-spectrometric identification of isomeric C6-hydroxy and C20-hydroxy metabolites of methylprednisolone in the urine of patients receiving high-dose pulse therapy

Fourteen metabolites of methylprednisolone have been analysed by gradient-elution high-performance liquid chromatography coupled with tandem mass spectrometry (LC-MS-MS). The compounds were separated on a Cp Spherisorb 5 microm ODS column connected to a guard column packed with pellicular reversed phase. The mobile phase was an acetonitrile- 1.0% aqueous acetic acid gradient at a flow rate of 1.5 mL min(-1) The analysis gave a complete picture of parent drug, prodrugs and metabolites, and the alpha/beta stereochemistry was resolved. The short (1-2 h) elimination half-life of methylprednisolone is explained by extensive metabolism. The overall picture of the metabolic pathways of methylprednisolone is apparently simple-reduction of the C20 carbonyl group and further oxidation of the C20,C21 side chain (into C21COOH and C20COOH), in competition with or in addition to oxidation at the C6 position.

Isolation and identification of the C6-hydroxy and C20-hydroxy metabolites and glucuronide conjugate of methylprednisolone by preparative high-performance liquid chromatography from urine of patients receiving high-dose pulse therapy

In the present study metabolites of methylprednisolone were detected using gradient elution high-performance liquid chromatography. Separation was performed by a Cp Spherisorb ODS 5 microm (250 mmx4.6 mm I.D.) column, connected to a guard column, packed with pellicular reversed phase. The mobile phase was a mixture of acetonitrile and 1% acetic acid in water. At t = 0, this phase consisted of 2% acetonitrile and 98% acetic acid 1% in water (v/v). During the following 35 min the phase changed linearly until it attained a composition of acetonitrile-buffer (50:50, v/v). At 40 min (t = 40) the mobile phase was changed over 5 min to the initial composition, followed by equilibration during 2 min. The flow-rate was 1.5 ml/min. UV detection was achieved at 248 nm. We have isolated the respective compounds with the most abundant concentration and suggested their chemical structure based on NMR, IR, UV, MS, retention behaviour and melting points. The c/, stereochemistry could not be solved in this study. The overall picture of the metabolic pathways of methylprednisolone is apparently simple: reduction of the C20 carbonyl group and further oxidation of the C20-C21 side chain (into C21-COOH and C20-COOH), in competition with or additional to the oxidation at the C6-position.

Derivatization procedures for gas chromatographic-mass spectrometric determination of xenobiotics in biological samples, with special attention to drugs of abuse and doping agents

The development of low cost MS detectors in recent years has promoted an important increase in the applicability of GC-MS system to analyze for the presence of foreign substances in the human body. Drugs and toxic agents are in vivo metabolized in such a way that more polar compounds are usually formed. Derivatization of these metabolites is often an unavoidable requirement for gas chromatographic analysis. Application of derivatization methods in recent years has been relevant, especially for silylation, acylation, alkylation and the formation of cyclic or diastereomeric derivatives. Given the relevance of drug of abuse testing in modern toxicology, main derivatization procedures for opiates, cocaine, cannabis, amphetamines, benzodiazepines and LSD have been reviewed. Papers describing the analyses of drugs of abuse in matrixes other than blood, such as hair or sweat, have received special attention. Advances in derivatization for sports drug testing have been particularly relevant for anabolic steroids, diuretics and corticosteroids. Among the several methodologies applied, the formation of trimethylsilyl, perfluoroacyl or methylated derivatives have proved to be both versatile and extensively used. Further advances in derivatization for GC-MS applications in clinical and forensic toxicology will depend on the one hand on the degree of further use of GC-MS for routine applications and, on the other hand, on the alternative progress made for developments in LC-MS or CE-MS. Last but not least, the appearance of comprehensive libraries in which reference spectra for different derivatives of many drugs and their metabolites are collected will have an important impact on the expansion of derivatization in GC-MS for toxicological applications.

Gender-based effects on methylprednisolone pharmacokinetics and pharmacodynamics

The pharmacokinetics and selected pharmacodynamic responses to methylprednisolone were investigated in six men and six premenopausal women after a dose of 0.6 mg/kg ideal body weight. Women (luteal phase) exhibited a greater methylprednisolone clearance (0.45 versus 0.29 L/hr/kg) and shorter elimination half-life (1.7 versus 2.6 hours) than men. The volume of distribution of methylprednisolone was similar when normalized for ideal body weight. Pharmacodynamic models were used to examine the methylprednisolone suppressive effects on cortisol secretion and basophil and helper T lymphocyte trafficking. A significantly smaller 50% inhibitory concentration (IC50) value (0.1 versus 1.7 ng/ml) was seen in the women for suppression of cortisol secretion, indicating increased sensitivity. However, the area under the concentration-time curve of effect was similar for both groups. The IC50 values for effects of methylprednisolone on basophil trafficking related to estradiol concentrations in a log-linear fashion in women, with increased sensitivity found at higher estradiol concentrations. Men displayed a greater 24-hour net suppression in blood basophil numbers, but no difference was observed in net cortisol and helper T lymphocyte suppression between the sexes. These findings suggest that methylprednisolone dosages should be based on ideal body weight. Although women are more sensitive to methylprednisolone as measured by cortisol suppression, they eliminate the drug more quickly, generally producing a similar net response.

Pharmacokinetics of methylprednisolone hemisuccinate and methylprednisolone in chronic liver disease

The disposition of methylprednisolone (MP) and its prodrug hemisuccinate (MPHS) was assessed in six middle-aged patients with chronic liver disease (CLD) and compared with six younger, healthy subjects after a single IV dose of 25.4 mg of MPHS. Blood and urine samples were collected over 12 hours. Plasma and urine concentrations of MPHS and MP and plasma cortisol were measured by HPLC. MPHS clearance (CL) was significantly reduced in the CLD group (495 vs. 1389 mL/hr/kg) whereas volume of distribution (Vss) of MPHS (about 0.35 1/kg) did not differ. The elimination half-life, t1/2 beta, was significantly longer in CLD (0.61 vs. 0.32 hr). The percent recovery of unchanged MPHS in urine was similar (about 9%) in both groups. The kinetic parameters of MP did not differ between the two groups for: clearance (about 370 L/hr/kg IBW), Vss (about 1.3 L/kg), and t1/2 beta (about 3.0 hr). The suppression t1/2 of cortisol after MPHS was longer (3.9 vs. 1.9 hr) indicating metabolic pathways for cortisol and MP are affected differently in CLD. Reduction in MPHS CL may reflect altered hepatic blood flow due to both cirrhosis and age effects. However, good availability of MP from MPHS and lack of perturbation of MP pharmacokinetics in CLD patients may provide therapeutic advantages in selection of this glucocorticoid. This is the first study that characterizes the disposition of the prodrug MPHS and the formation of MP simultaneously in CLD patients.

Bioavailability and nonlinear disposition of methylprednisolone and methylprednisone in the rat

Bioavailability of low (10 mg/kg) and high (50 mg/kg) doses of methylprednisolone was determined after oral administration of the free alcohol of methylprednisolone and iv administration of methylprednisolone sodium succinate. Plasma concentrations of methylprednisolone and methylprednisone (reversible metabolite) were measured by HPLC. Methylprednisolone systemic availability (F) was 49-57% after iv administration and approximately 35% after oral administration. Solubilization of steroids with PEG:ethanol had no effect on their disposition. Apparent systemic clearance (CL) of methylprednisolone was 21 mL/min (low dose), approximately twice the liver blood flow. Dose-dependent changes in steady-state volume of distribution (Vdss) and central volume of distribution (Vdc), volumes, and apparent CL were observed. The methylprednisolone-to-methylprednisone AUC ratio decreased with dose due to saturation of methylprednisone formation clearance (CL12), but this is a minor metabolic pathway. The mean residence time (MRT) increased threefold with dose. Graphical estimates of the Michaelis-Menten capacity (Vmax) and affinity (Km) constants were in reasonable agreement with CL values for the low-dose experimental data. Low systemic availability of iv methylprednisolone sodium succinate was in part due to sequential first-pass hepatic metabolism of the methylprednisolone formed. Methylprednisolone disposition is complex in the rat due to extensive first-pass effects, nonlinear elimination, nonlinear distribution, and reversible metabolism.

Characterization of prednisone, prednisolone and their metabolites by gas chromatography-mass spectrometry

Human urinary metabolites of the synthetic corticosteroids prednisone and prednisolone were detected in the course of gas chromatographic steroid profiling as methoxime-trimethylsilyl derivatives. Metabolites were provisionaly identified by combined gas chromatography-mass spectrometry. The major metabolites were 11-keto/11-hydroxy conversion products, 20-hydroxy and 4,5-dihydro analogues of the parent drugs. Cortisone, 6-hydroxy and fully saturated A-ring compounds were minor metabolites. Retention indices and mass spectral data are presented.

Simultaneous determination of beclomethasone, beclomethasone monopropionate and beclomethasone dipropionate in biological fluids using a particle beam interface for combining liquid chromatography with negative-ion chemical ionization mass spectrometry

A new simple and sensitive assay has been developed for the simultaneous quantitative measurement of beclomethasone dipropionate and its hydrolysis products in human plasma and urine. Beclomethasone 17.21-dipropionate, beclomethasone 17-monopropionate, beclomethasone and the internal standard, dexamethasone 21-acetate, were measured by combined liquid chromatography and negative-ion chemical ionization mass spectrometry with methane as the reagent gas. A particle beam interface from Hewlett Packard was used. Under mild operating conditions, abundant and stable characteristic high-mass ions were generated in the ion source of the mass spectrometer by a resonance electron-capture mechanism. The fast extraction procedure requires 1 ml of plasma or urine, and the quantification limit of the method is 1 ng ml-1 for the three tested compounds.

A rapid and highly sensitive method for the quantitative determination of dexamethasone in plasma, synovial fluid and tissues by combined gas chromatography/negative ion chemical ionization mass spectrometry

A new, simple and highly sensitive assay is developed for the quantitative measurement of very low levels of dexamethasone in human plasma, synovial fluid and tissues following a topical administration of the drug. Dexamethasone and the internal standard, flumethasone, are measured by gas chromatography/negative ion chemical ionization mass spectrometry with methane as the reagent gas. After a three-step extraction procedure, the two compounds of interest are converted to their trimethylsilyl ether derivatives using trimethylsilylimidazole and formamide as the base catalyst. Under soft derivatization conditions only one chromatographic peak corresponding to the trisubstituted derivative is observed. The mass spectrometer is focused to monitor abundant and stable characteristic high-mass ions (m/z 446 and 464) which are generated in the ion source by an electron capture process. This assay requires only 1 ml of plasma or 0.5 ml of synovial fluid and the detection limit of the method is equal to 0.1 ng ml-1 with a relative standard deviation lower than 6%.

Urinary excretion of prednisolone following intravenous administration in humans

The urinary excretion of prednisolone was studied in eight normal human volunteers (two women and six men) following intravenous (16, 32, 48 and 64 mg) doses. Urine prednisolone concentrations were determined by a high performance thin layer chromatographic method (HPTLC). The overall mean prednisolone elimination half life in urine following all the intravenous doses as determined by the rate and sigma minus plots was 1.13 +/- 0.25 hour. This was independent of dose and shorter than that found in plasma (4.10 +/- 1.00 s.d. hour). The overall mean percentage of dose excreted unchanged in urine was 16.7 +/- 5.8% following all intravenous and oral doses respectively. About 80% of this amount was excreted within the first 4 hours of the intravenous administration. Renal clearance of prednisolone decreased with time by the first order kinetic (r = 0.790) and its overall value following all IV doses was 0.0183 +/- 0.0103 (s.d.) l/h/kg. The metabolic clearance remained constant with increasing doses from 16 to 64 mg (0.0883 +/- 0.0306 s.d. l/h/kg). From this study it was concluded that a definitive account of the renal elimination of prednisolone and its possible metabolites warrant further investigation. The fraction of the dose excreted unchanged was relatively small and variable suggesting that prednisolone elimination occurs mainly by metabolism.

Two chromatographic methods for the determination of corticosteroids in human biological fluids: pharmacokinetic applications

Two techniques, high-performance liquid chromatography (HPLC) and quantitative high-performance thin-layer chromatography coupled with densitometry (HPTLC), were developed for the determination of prednisolone (PL), methylprednisolone (MP), and methylprednisolone sodium succinate (MPSS) in human plasma, saliva, and urine. The HPLC and HPTLC methods shared a single and simple step of an organic extraction procedure and separation of steroids using a normal-phase column or HPTLC plate. The methods allow simultaneous measurement of endogenous cortisol in plasma following the administration of PL and MP. The calibration curves of steroids in all biological fluids were linear over a wide range of concentrations of PL and MP in all biological fluids (0.025-4 micrograms/mL). The limit of detection of both assays for PL and MP was 10 ng/mL in plasma and saliva and 25 ng/mL in urine, and of MPSS was 50 ng/mL in plasma. Both methods were reproducible with an inter- and intra-assay coefficient of variation of less than 10% for all steroids over a wide range of concentrations in all biological fluids. No interference from endogenous steroids was found. The presented methods are simple, rapid, specific, sensitive, reproducible, and economical for the pharmacokinetic study of these steroids. The application of these methods for the pharmacokinetic study of both MP and PL in vivo and the in vitro hydrolysis of MPSS is discussed.