Strategies in the design of solution-stable, water-soluble prodrugs I: a physical-organic approach to pro-moiety selection for 21-esters of corticosteroids

Self-Assembly Properties of an Amphiphilic Phosphate Ester Prodrug Designed for the Treatment of COVID-19

PF-07304814 is a water-soluble phosphate ester prodrug of a small molecule inhibitor for the SARS CoV-2 3CL protease designed for the treatment of COVID-19. The amphiphilicity and self-assembly behavior of the prodrug was investigated computationally and experimentally via multiple orthogonal techniques to better design formulations for intravenous infusion. The self-assembly of PF-07304814 into micellar structures enabled an increase in the solubility of lipophilic impurities by up to 1900x in clinically relevant formulations. The observed solubilization could help extend the drug product shelf-life and in use stability through inhibition of precipitation, without the need for solubilizing excipients. The work presented in this manuscript provides a roadmap for the characterization of prodrug self-assembly and highlights the potential for prodrug modifications to enhance solubility of both active ingredients and impurities and to extend drug product shelf-life.

Pregn-5-en-3β-ol and androst-5-en-3β-ol dicarboxylic acid esters as potential therapeutics for NMDA hypofunction: In vitro safety assessment and plasma stability

Neurosteroids are endogenous steroidal compounds that can modulate neuronal receptors. N-Methyl-D-aspartate receptors (NMDARs) are glutamate-gated, calcium-permeable ion channels that are of particular interest, as they participate in synaptic transmission and are implicated in various processes, such as learning, memory, or long-term neuronal potentiation. Positive allosteric modulators that increase the activity of NMDARs may provide a therapeutic aid for patients suffering from neuropsychiatric disorders where NMDAR hypofunction is thought to be involved, such as intellectual disability, autism spectrum disorder, or schizophrenia. We recently described a new class of pregn-5-ene and androst-5-ene 3β-dicarboxylic acid hemiesters (2-24) as potent positive modulators of NMDARs. Considering the recommended guidelines for the early stage development of new, potent compounds, we conducted an in vitro safety assessment and plasma stability screening to evaluate their druglikeness. First, compounds were screened for their hepatotoxicity and mitochondrial toxicity in a HepG2 cell line. Second, toxicity in primary rat postnatal neurons was estimated. Next, the ability of compounds 2-24 to cross a Caco-2 monolayer was also studied. Finally, rat and human plasma stability screening revealed an unforeseen high stability of the C-3 hemiester moiety. In summary, by using potency/efficacy towards NMDARs data along with toxicity profile, Caco-2 permeability and plasma stability, compounds 14 and 15 were selected for further in vivo animal studies.

Immediate hypersensitivity reactions to corticosteroids

OBJECTIVE

To review the literature on immediate hypersensitivity reactions to corticosteroids and classify them according to manifestations, routes of exposure, causative preparations, diagnostic tests, and management.

DATA SOURCES

PubMed search for English-language publications from January 1, 2004 through December 31, 2014 using search terms corticosteroid, glucocorticoid, or steroid combined with hypersensitivity, allergy, or anaphylaxis.

STUDY SELECTION

Only reports of immediate hypersensitivity reactions that occurred sooner than 24 hours after administration of a corticosteroid were included. Excluded were reports on patients with delayed reactions, including contact dermatitis.

RESULTS

Forty-eight articles fulfilled the criteria, reporting 120 reactions in 106 patients 2 to 90 years of age (55 male and 51 female). The most commonly reported manifestation was anaphylaxis in 60.8% (73 of 120) followed by urticaria and/or angioedema in 26.7%. Exposure to corticosteroid was through any route, with intravenous being the most common (44.2%, 53 of 120), followed by oral in 25.8% and intra-articular in 11.7%. Methylprednisolone was the most commonly implicated (40.8%) followed by prednisolone (20.0%). Some reacted to more than 1 preparation. Pharmacologically-inactive ingredients were implicated in 28.3%. Diagnosis was based primarily on medical history and in most cases was confirmed by challenge testing. Skin tests were positive in 74.1%. The vast majority of patients tolerated at least 1 alternative preparation, and very rarely desensitization was required.

CONCLUSION

Corticosteroids seem to be rare causes of immediate hypersensitivity reactions but possibly are misdiagnosed or under-reported relative to their worldwide use. Physicians should be cognizant of this entity and identify safe alternative preparations.

Stabilization of resveratrol in blood circulation by conjugation to mPEG and mPEG-PLA polymers: investigation of conjugate linker and polymer composition on stability, metabolism, antioxidant activity and pharmacokinetic profile

Resveratrol is naturally occurring phytochemical with diverse biological activities such as chemoprevention, anti-inflammatory, anti-cancer, anti-oxidant. But undergoes rapid metabolism in the body (half life 0.13h). Hence Polymer conjugation utilizing different chemical linkers and polymer compositions was investigated for enhanced pharmacokinetic profile of resveratrol. Ester conjugates such as α-methoxy-ω-carboxylic acid poly(ethylene glycol) succinylamide resveratrol (MeO-PEGN-Succ-RSV) (2 and 20 kDa); MeO-PEG succinyl ester resveratrol (MeO-PEGO-Succ-RSV) (2 kDa); α-methoxy poly(ethylene glycol)-co-polylactide succinyl ester resveratrol (MeO-PEG-PLAO-Succ-RSV) (2 and 6.6kDa) were prepared by carbodiimide coupling reactions. Resveratrol-PEG ethers (2 and 5 kDa) were synthesized by alkali-mediated etherification. All polymer conjugates were fully characterized in vitro and the pharmacokinetic profile of selected conjugates was characterized in rats. Buffer and plasma stability of conjugates was dependent on polymer hydrophobicity, aggregation behavior and PEG corona, with MeO-PEG-PLAO-Succ-RSV (2 kDa) showing a 3h half-life in rat plasma in vitro. Polymer conjugates irrespective of linker chemistry protected resveratrol against metabolism in vitro. MeO-PEG-PLAO-Succ-RSV (2 kDa), Resveratrol-PEG ether (2 and 5 kDa) displayed improved pharmacokinetic profiles with significantly higher plasma area under curve (AUC), slower clearance and smaller volume of distribution, compared to resveratrol.

Amino acids as promoieties in prodrug design and development

Prodrugs are biologically inactive agents that upon biotransformation in vivo result in active drug molecules. Since prodrugs might alter the tissue distribution, efficacy and the toxicity of the parent drug, prodrug design should be considered at the early stages of preclinical development. In this regard, natural and synthetic amino acids offer wide structural diversity and physicochemical properties. This review covers the use of amino acid prodrugs to improve poor solubility, poor permeability, sustained release, intravenous delivery, drug targeting, and metabolic stability of the parent drug. In addition, practical considerations and challenges associated with the development of amino acid prodrugs are also covered.

Preparation and characterization of inclusion complexes of a hemisuccinate ester prodrug of delta9-tetrahydrocannabinol with modified beta-cyclodextrins

Delta(9)-Tetrahydrocannabinol hemisuccinate (THC-HS), an ester prodrug of Delta(9)-tetrahydrocannabinol (THC) has been investigated for its potential to form inclusion complexes with modified synthetic beta-cyclodextrins (CDs). Phase solubility studies were performed to determine the stoichiometric ratio of complexation of THC-HS with random methylated beta-cyclodextrin (RAMEB) and 2-hydroxypropyl beta-cyclodextrin (HPBCD). THC-HS/RAMEB and THC-HS/HPBCD solid systems were prepared by lyophilization and the lyophilized complexes were characterized by Fourier transform infrared (FT-IR) spectroscopy, proton nuclear magnetic spectroscopy, and molecular modeling techniques. The formation of inclusion complexes of THC-HS/RAMEB and THC-HS/HPBCD was demonstrated by an A(L) type curve with the slopes less than unity by the phase solubility method. The association constants for THC-HS/RAMEB and THC-HS/HPBCD were found to be 562.48 and 238.83 M(-1), respectively. The stoichiometry of both of the complexes was found to be 1:1 as determined from the Job's plot. This was confirmed by (1)H NMR and FT-IR techniques. The results obtained from the molecular modeling studies were in accordance with the data obtained from nuclear magnetic resonance and FT-IR. The docking studies revealed the most probable mode of binding of THC-HS with RAMEB in which the alkyl chain was submerged in the hydrophobic pocket of the CD molecule and hydrogen bonding interactions were observed between the hemisuccinate ester side chain of THC-HS and the rim hydroxy groups of RAMEB. The solubility of THC-HS was significantly higher in RAMEB compared to HPBCD. Solid dispersions of THC-HS with CDs will be further utilized to develop oral formulations of THC-HS with enhanced bioavailability.

Prodrug strategies to overcome poor water solubility

Drug design in recent years has attempted to explore new chemical spaces resulting in more complex, larger molecular weight molecules, often with limited water solubility. To deliver molecules with these properties, pharmaceutical scientists have explored many different techniques. An older but time-tested strategy is the design of bioreversible, more water-soluble derivatives of the problematic molecule, or prodrugs. This review explores the use of prodrugs to effect improved oral and parenteral delivery of poorly water-soluble problematic drugs, using both marketed as well as investigational prodrugs as examples. Prodrug interventions should be considered early in the drug discovery paradigm rather than as a technique of last resort. Their importance is supported by the increasing percentage of approved new drug entities that are, in fact, prodrugs.

Kinetics and mechanisms of activation of alpha-amino acid ester prodrugs of camptothecins

The alpha-amino acid ester prodrugs of the antitumor agent camptothecin and a more potent, lipophilic silatecan analogue, DB-67, have been shown by NMR spectroscopy and quantitative kinetic analyses to undergo quantitative conversion to their pharmacologically active lactones via a nonenzymatic mechanism that at pH 7.4 is favored over direct hydrolysis. The alternate pathway involves the reversible intramolecular nucleophilic amine attack at the camptothecin E-ring carbonyl to generate a lactam (I) followed by a second intramolecular reaction to produce a bicyclic hemiortho ester (I'). The intermediates were isolated and shown to exist in an apparent equilibrium dominated by the hemiortho ester in DMSO using NMR spectroscopy. The conversion of prodrugs of camptothecin or DB-67 containing either alpha-NH(2) or alpha-NHCH(3) and their corresponding hemiortho esters were monitored versus time in aqueous buffer (pH 3.0 and 7.4) at 37 degrees C, and the kinetic data were fit to a model based on the proposed mechanism. The results indicated that while the prodrugs are relatively stable at pH 3, facile lactone release occurs from both the prodrugs and their corresponding hemiortho ester intermediates under physiological conditions (pH 7.4). The glycinate esters and their hemiortho esters were found to be more cytotoxic than the N-methylglycinates or their corresponding hemiortho ester intermediates in vitro using a human breast cancer cell line (MDA-MB-435S), consistent with their more rapid conversion to active lactone. The pH dependence of the nonenzymatic pathway for conversion of these alpha-amino acid ester prodrugs suggests that they may be useful for tumor-targeting via liposomes, as they can be stabilized in an acidic environment in the core of liposomes and readily convert to the active lactone following their intratumoral release.

Investigations into the chromatographic behavior of a doxorubicin-peptide conjugate

HPLC impurity profile method development for a doxorubicin-heptapeptide conjugate included significant changes of the separation profile with diluent, eluent and pH. These separation variables were also temperature-dependent with a shift in retention from 35 to 45 degrees C. There was also a direct relationship of temperature with LC retention, and a pH minimum at 5.9. Atypical dependence of the impurity profile on diluent at a k' of 18 led to further investigation. A large change in retention by several minutes was a function of both the organic eluent composition and temperature between 15 and 30 degrees C. Several Van't Hoff temperature studies from 5 to 65 degrees C on several column types resulted in non-linear plots. Analysis of the molecular subunits suggested that the peptide portion of the analyte influenced the non-linear retention behavior. The stationary phase type was not a significant factor causing non-linearity. Circular dichroism-temperature studies indicated a notable transition in ellipticity for the amine regions (198-202 nm) that occurred between 39 and 44 degrees C. This transition temperature range coincided with the results of the Van't Hoff analysis, between 35 and 44 degrees C, to indicate that these effects were not primarily stationary phase induced.

IgE-mediated anaphylactic reaction induced by succinate ester of methylprednisolone

BACKGROUND

In systemic administration the prevalence of anaphylactic reactions attributable to corticosteroids is approximately 0.3%. Positive prick tests with different corticosteroids have been reported suggesting an immunoglobulin (Ig)E-mediated mechanism.

OBJECTIVE

A 42-year-old man with multiple sclerosis developed flush, erythema, and itching a few minutes after the begin of an intravenous infusion of methylprednisolone-21-sodium succinate. DIAGNOSTIC AND RESULTS: Prick tests were found to be positive with methylprednisolone-21-sodium succinate and prednisolone-21-sodium succinate, whereas prick tests with prednisolone without ester and betamethasone-21-dihydrogen phosphate showed negative results. Oral challenge with prednisolone without ester and intravenous challenge with betamethasone-21-dihydrogen phosphate were well tolerated. Specific IgE-antibodies against methylprednisolone-21-sodium succinate were found in the serum of the patient. Because of the positive prick test and specific IgE antibodies against methylprednisolone-21-sodium succinate, the diagnosis of IgE-mediated anaphylactic reaction could be proven. Succinate ester was suspected to be immunogenic, as other corticosteroids without this particular ester or with other substitutions at the C21 remained negative both in the prick and the challenge tests.

CONCLUSIONS

This patient showed an adverse reaction caused by methylprednisolone-21-sodium succinate. The uniqueness in this case was the presence of specific IgE antibodies against this esterified corticosteroid in the patient's serum proving that this reaction was based upon a true IgE-mediated mechanism.

Preparation of prednisolone-appended alpha-, beta- and gamma-cyclodextrins: substitution at secondary hydroxyl groups and in vitro hydrolysis behavior

The carboxyl group of prednisolone 21-hemisuccinate was conjugated to one of the hydroxyl groups of alpha-, beta-, and gamma-cyclodextrins using a coupling agent, carbonyldiimidazole. The direct coupling produced prednisolone-appended cyclodextrin conjugates in which the drug is selectively introduced at one of the secondary hydroxyl groups of cyclodextrins through an ester linkage. The aqueous solubility (> 50% w/v at 25 degrees C) of these conjugates was much higher than those of prednisolone and its 21-hemisuccinate. Prednisolone was slowly released from the conjugate: the percents of prednisolone and its hemisuccinates released from the alpha-, beta-, and gamma-cyclodextrin conjugates were 49, 57, and 85%, respectively, for 24-h. The release pathway is proposed to be via two fast acyl migrations between the 2- and 3-hydroxyl groups of cyclodextrins and between the 21- and 17-hydroxyl groups of prednisolone. The slow release of prednisolone from the ester conjugates was in sharp contrast to the fast release of the prednisolone amide conjugate reported previously. Because of relatively slow and/or site-specific release properties, the present prednisolone-appended cyclodextrin conjugates may be of value as an orally administered delayed-release and/or colon-specific prodrug.

Prodrug esters of the indolocarbazole CEP-751 (KT-6587)

Prodrug esters of the indolocarbazole CEP-751 (KT-6587) were prepared with the goal of identifying water soluble, stable but cleavable forms for intravenous dosing. A dipeptide proform Lys-beta-Ala (16, CEP-2563/KT-8391) was identified for advancement to clinical trials.

Photodegradation products of methylprednisolone suleptanate in aqueous solution--evidence of a bicyclo[3.1.0]hex-3-en-2-one intermediate

Two major photodegradation products of methylprednisolone suleptanate (sodium 11 beta, 17 alpha-dihydroxy-6 alpha-methyl-21-[[8-[methyl(2-sulfoethyl)amino]-1,8-dioxooctyl+ ++]oxy]-pregna-1,4-diene-3,20-dione) in aqueous solution irradiated with white fluorescent light at 2000 lux for 28 days at 25 degrees C were isolated by preparative high-performance liquid chromatography and characterized. Structures of the photodegradation products were determined as sodium 11 beta,17 alpha-dihydroxy-5 alpha, 6 alpha-dimethyl-21-[[8-[methyl(2-sulfoethyl)amino]-1,8-dioxoocty l]-oxy]-19-norpregna-1(10),3-diene-2,20-dione and sodium 1 beta, 11 beta-epoxy-17 alpha-hydroxy-6 alpha-methyl-21-[[8-[methyl(2-sulfoethyl)amino]-1, 8-dioxooctyl]oxy]-19 alpha-pregn-4-ene-2.20-dione. Formation of these compounds, especially the 1,11-epoxy analogue, strongly supports the existence of the bicyclo[3.1.0]hex-3-en-2-one intermediate in the photorearrangement of steroidal cross-conjugated dienones. The intermediate can account for all the major ketonic and phenolic products; these products are generated from the intermediate after subsequent transformations and/or further skeletal rearrangements. The 1,11-epoxy analogues were obtained from the prednisolone steroids in high yields by irradiation with a high-pressure mercury lamp.

Effect of ionic strength on solution stability of PNU-67590A, a micellar prodrug of methylprednisolone

PURPOSE

PNU-67590A is a water-soluble micellar prodrug of methylprednisolone (MP). The major products of degradation of PNU-67590A are MP by hydrolysis and methylprednisolone 17-suleptanate (17-E) by 21-->17 acyl migration. The effect of ionic strength on micelle formation and stability of PNU-67590A in aqueous solution was examined.

METHODS

PNU-67590A solutions at pH 2 and 8 and ionic strength of 0.05, 0.1, 0.2, and 0.4 M were maintained at 25 degrees C in the dark to measure MP and 17-E levels over time.

RESULTS

The rate of degradation of micellar PNU-67590A at pH 8 was less than that of monomeric PNU-67590A, and vice versa at pH 2. Increase in ionic strength decreased both the critical micelle concentration of PNU-67590A and the degradation of micelle PNU-67590A at both pHs, resulting in improved overall stability of PNU-67590A.

CONCLUSIONS

Formulation of PNU-67590A in a concentrated solution with high ionic strength will maximize stability and shelf-life.

Marked antinephritic action and less adverse effects of methylprednisolone suleptanate by intermittent administration in rats

To establish the regimen for beneficial prolonged treatment with glucocorticoids on nephritis, we investigated the antinephritic effect of methylprednisolone suleptanate (MPS) and its influence on adrenal function by intermittent administration (IA) in comparison with daily administration (DA) in crescentic-type anti-GBM nephritic rats. In IA, MPS (0.25, 1.0 and 3.0 mg/kg) was injected for 3 successive days followed by a 3-day withdrawal during a 40-day period. MPS inhibited the elevation of urinary protein and serum cholesterol and glomerular alterations by both IA and DA. The effect of MPS on these parameters was more potent by IA than by DA. MPS significantly suppressed the increment of the number of ED-1(+) cells and TH-1(+) cells in nephritic glomeruli. DA, but not IA, caused atrophy of the adrenal glands. IA prevented the remarkable decrease in corticosterone level provoked in nephritic rats. In conclusion, for the treatment of nephritis, IA seems to be a better regimen for the administration of MPS. MPS may exert an antinephritic action by inhibiting mesangial cell proliferation and infiltration of monocytes/macrophages into glomeruli in addition inhibiting antibody production.

Immediate inhibitory effect of methylprednisolone suleptanate (U-67590A) on antigen-induced cutaneous and airway anaphylactic responses in guinea-pigs

1. Inhibitory effects of water-soluble glucocorticoids administered intravenously were examined on skin and airway reactions caused by antigen challenge or chemical mediators in guinea-pigs. 2. Methylprednisolone suleptanate (U-67590A) which is an analogue of methylprednisolone, produced immediate inhibition of 3-h and 7-day homologous passive cutaneous anaphylaxis (PCA) reactions, but not of histamine- or bradykinin-induced cutaneous vascular permeability, when administered 10 min before antigen challenge. In contrast, methylprednisolone succinate (MP) or dexamethasone (DXM) administered 10 min before antigen challenge failed to show an immediate inhibitory effect on the PCA or mediator-induced reactions. When administered 1 to 5 h before antigen challenge, all the steroids used in this study reduced both PCA and mediator-induced reactions. 3. Pretreatment with cycloheximide almost completely abolished the late inhibition of 3-h PCA and histamine reactions produced by U-67590A or MP, but it did not affect the immediate inhibition of 3-h PCA produced by U-67590A. 4. U-67590A also demonstrated immediate inhibitory effects on antigen-induced bronchoconstriction in guinea-pigs actively sensitized with ovalbumin even when administered 10 min before antigen challenge, whereas MP and DXM failed to show the immediate inhibitory effect. When administered 3 h before antigen challenge, all the steroids used in this study reduced the response to antigen. 5. The late inhibitory effect of U-67590A administered 3 h before antigen challenge was almost completely abolished by treatment with cycloheximide or 17 alpha-methyltestosterone, whereas the immediate inhibition produced by U-67590A administered 10 min before challenge was not affected by this treatment. 6. U-67590A administered 10 min or 3 h before challenge did not affect the bronchoconstriction induced by histamine or leukotriene D4.7. Release of histamine from lung fragments of sensitized guinea-pigs in vitro was inhibited by U-67590A.8. The present experiments indicate that U-67590A demonstrated dual, immediate and late, inhibitory effects. The former are independent of protein synthesis and may be associated with non-genomic direct action on the mediator-releasing process without affecting mediator-induced reactions. The latter share common inhibitory actions with other glucocorticoids which are dependent on protein synthesis through gene expression.

Chemical and enzymatic degradation of ganciclovir prodrugs: enhanced stability of the diadamantoate prodrug under acid conditions

We report the chemical and enzymatic hydrolysis of two hydrophobic prodrugs of ganciclovir (3 = dipropionate ester; 4 = diadamantoate ester). Both prodrugs undergo hydrolysis showing a pH dependence of kobs = kH+aH+ + ko + kHO-aHO- and a pH of maximum stability near pH 5. Only 4 exhibited a shelf life (t90) greater than 2 years. Compound 4 reacts significantly slower than ganciclovir in acidic media, even though the adamantyl esters provide additional reaction sites (which would be expected to increase the rate of degradation) that are distally removed from the guanine ring system, offering negligible steric or electronic substituent effects. Both 3 and 4 hydrolyzed in tissue homogenate, where kobs followed liver greater than intestine much greater than skin. Based on these findings of chemical stability, lipophilicity, and acceptable rate of enzymatic cleavage by skin esterases, 4 meets several of the criteria required for the topical sustained delivery of ganciclovir.

Simultaneous determination of methylprednisolone and methylprednisolone 21-[8-[methyl-(2-sulfoethyl)amino]-8-oxooctanoate] sodium salt in human urine by high-performance liquid chromatography with ultraviolet detection

A reversed-phase high-performance liquid chromatographic assay with ultraviolet detection at 243 nm has been developed for the quantitative determination of methylprednisolone (MP) and methylprednisolone 21-[8-[methyl-(2-sulfoethyl)amino]-8-oxooctanoate] sodium salt (MPSO) in human urine following therapeutic doses in humans. The assay procedure involves stabilization of urine samples by addition of disodium ethylenediaminetetraacetic acid (Na2EDTA) and ion-pair extractions of MPSO using tetraethylammonium chloride (TEACl) as the counter ion. After extracting both drugs and internal standard into chloroform, the extract was evaporated to dryness under nitrogen. The resulting residue was reconstituted in 200-500 microliters of mobile phase and chromatographed on an IBM C18 reversed-phase column (5 microns). The mobile phase was a mixture of water-acetonitrile-isopropanol (71.2:18.8:10.0, v/v) containing 75 microliters of 0.1 M hydrochloric acid and 0.450 g of TEACl per liter. Propyl p-hydroxybenzoate was used as an internal standard. The extraction efficiencies of MP and MPSO were greater than 90% using the ion-pairing agent TEACl. The chromatographic responses were linear up to about 200 micrograms/ml for MP and 80 micrograms/ml for MPSO and had sufficient precision and accuracy to provide quantitative data from human urine. The assay detection limit was about 8 ng/ml for MP and 25 ng/ml for MPSO in human urine. Stability studies in urine indicated that without Na2EDTA stabilization and at room temperature, rapid degradation of MPSO occurred in urine. Addition of EDTA to the urine specimen and storage at -70 degrees C increased the stability of MPSO, and little or no degradation was observed in urine stored for more than 60 days. The method has been used in the simultaneous determination of MP and MPSO in urine specimens obtained from a single-dose tolerance study of MPSO in normal male volunteers.

Methylprednisolone pharmacokinetics after intravenous and oral administration

1. The pharmacokinetics of methylprednisolone (MP) were studied in five normal subjects following intravenous doses of 20, 40 and 80 mg methylprednisolone sodium succinate (MPSS) and an oral dose of 20 mg methylprednisolone as 4 x 5 mg tablets. Plasma concentrations of MP and MPSS were measured by both high performance thin layer (h.p.t.l.c.) and high pressure liquid chromatography (h.p.l.c.). 2. The mean values (+/- s.d.) of half-life, mean residence time (MRT), systemic clearance (CL) and volume of distribution at steady state (Vss) of MP following intravenous administration were 1.93 +/- 0.35 h, 3.50 +/- 1.01 h, 0.45 +/- 0.12 lh-1 kg-1 and 1.5 +/- 0.63 1 kg-1, respectively. There was no evidence of dose-related changes in these values. The plasma MP concentration-time curves were superimposable when normalized for dose. 3. The bioavailability of methylprednisolone from the 20 mg tablet was 0.82 +/- 0.11 (s.d.). 4. In vivo hydrolysis of MPSS was rapid with a half-life of 4.14 +/- 1.62 (s.d.) min, and was independent of dose. In contrast, in vitro hydrolysis in plasma, whole blood and red blood cells was slow; the process continuing for more than 7 days. Sodium fluoride did not prevent the hydrolysis of MPSS.

The role of protein and lipid domains in the uptake of solutes by human stratum corneum

The uptake of a series of hydrocortisone esters varying in lipophilicity from water into untreated and delipidized human stratum corneum has been determined. The partition coefficients of solutes into fully hydrated stratum corneum are postulated to represent the separate contributions of three structurally distinct domains--the extractable lipids, protein, and the solvent domain. The solvent domain was assumed to have the properties of bulk water. The relative affinities of the protein and lipid domains of stratum corneum for solutes varying in structure were determined by comparing solute uptake in untreated and delipidized stratum corneum. Partitioning into the extracted lipids was also examined. Solute uptake into stratum corneum may be governed by the protein domain, the lipid domain, or a combination of the two, depending on solute lipophilicity. Due to differences in the selectivity of the two domains, a change in uptake mechanism occurs with increasing solute lipophilicity from protein-dominated uptake for hydrophilic solutes to lipid domain-dominated uptake for lipophilic solutes. The stratum corneum lipid content, which varies dramatically from individual to individual (3-46% in this study), is an important determinant of the affinity of the stratum corneum for highly lipophilic solutes but has no effect on the uptake of hydrophilic solutes.

High-performance liquid chromatographic determination of methylprednisolone and methylprednisolone 21-[8-[methyl-(2-sulfoethyl)amino]-8-oxoctanoate] sodium salt in human plasma

A high-performance liquid chromatographic assay method with ultraviolet detection at 243 nm has been developed for the quantitative determination of methylprednisolone (MP) and methylprednisolone 21-[8- [methyl-(2-sulfoethyl)amino]-8-oxoctanoate] sodium salt (MPSO) in human plasma. The method is simple, rapid and sensitive to detect MP and MPSO in human plasma following administration of therapeutic doses of MPSO. The assay procedure involved stabilization of plasma samples by addition of disodium ethylenediaminetetraacetic acid and ion-pair extractions of MPSO with tetraethylammonium chloride. After extracting both drugs and internal standard into chloroform, the extract was evaporated to dryness under nitrogen. The resulting residue was reconstituted in 200-500 microliter of mobile phase and chromatographed on a C18 IBM reversed-phase column (5 micron). The mobile phase was a mixture of water-actetonitrile-isopropanol (71:19.9:10) containing 50 microliter of 0.1 M hydrochloric acid and 0.497 g tetraethylammonium chloride. Propyl p-hydroxybenzoate was used as an internal standard. The chromatographic responses were linear up to about 200 micrograms/ml for MP and 80 micrograms/ml for MPSO in human plasma. The assay detection limit was approximately 7 ng/ml for MP and 25 ng/ml for MPSO in human plasma. Statistical analysis indicated an average recovery of 102.0 +/- 4.71% for MP and 75.2 +/- 2.88% for MPSO. Human plasma levels are reported for MP and MPSO following single-dose intravenous administration of 100-mg equivalents of MPSO.

Oral absorption of 21-corticosteroid esters: a function of aqueous stability and intestinal enzyme activity and distribution

The intestinal absorption of hydrocortisone and prednisolone are compared with three water-soluble derivatives (succinate, phosphate, and lysinate) in experiments at two levels of biological system complexity. Rates of absorption are compared by measuring permeabilities from rat intestinal perfusions of drugs and derivatives in solution. Extents of absorption are compared over a 10-fold dose range of parent steroid and with the steroid derivatives by measuring plasma levels from solid oral dosage in dogs. While the parent steroids are well absorbed over the entire length of the intestinal tract, variability in plasma levels is observed at higher doses. Limited solubility and resultant dissolution rate variability are likely to be playing a role in the early erratic blood level profiles found at higher doses. While the soluble prodrugs have a dissolution rate advantage which results in a greater concentration gradient, their absorption is limited by their aqueous luminal stability, their polarity and resultant passive membrane permeability, and the distribution and activity of enzyme reconversion sites in the intestinal tract. The unstable lysinate ester, targeted for aminopeptidase, has an absorption profile and permeability similar to that of the parent steroid. The absorption of the moderately stable succinate ester is limited by its polarity and the activity of intestinal esterases. The stable phosphate derivative is well absorbed in the upper intestine, where high levels of alkaline phosphatase exist, while the prodrug polarity and drop-off of enzyme activity limit its absorption from the lower gastrointestinal (GI) tract.

Strategies in the design of solution-stable, water-soluble prodrugs III: influence of the pro-moiety on the bioconversion of 21-esters of corticosteroids

Succinate esters and many other carboxylic acid esters utilized as water-soluble prodrugs have limited utility due to their aqueous solution instability. In an earlier study, a strategy for the design of solution-stable 21-carboxylic acid esters of corticosteroids was developed from a consideration of various physical organic factors which influence ester hydrolysis. Several 21-esters of methylprednisolone were synthesized, and their stability in aqueous solution was monitored to demonstrate the feasibility of the approach. In this study, the bioconversion of representative examples of 21-esters of methylprednisolone exhibiting shelf lives of greater than or equal to 2 years at 25 degrees C was monitored to evaluate their utility as prodrugs in comparison to a commercially marketed sodium succinate ester. Ester hydrolysis studies conducted in human and rhesus monkey serum suggest that derivatives having an anionic solubilizing moiety (sulfonate or carboxylate) are not hydrolyzed in serum, while compounds having a cationic (tertiary amino) solubilizing moiety are hydrolyzed rapidly by serum esterases. In vivo pharmacokinetic studies in rhesus monkeys were also conducted to compare the bioconversion rates and overall bioavailability of several solution-stable prodrugs with the 21-succinate ester. Derivatives having solution stability exceeding 2 years at 25 degrees C with a faster bioconversion rate and an overall bioavailability equal to or higher than that of the succinate ester have been identified. Relative bioavailability appears to be highly sensitive to the charge of the solubilizing pro-moiety and pro-moiety chain length.

Strategies in the design of solution-stable, water-soluble prodrugs II: properties of micellar prodrugs of methylprednisolone

In a previous study, a physical-organic approach to the design of solution-stable, water-soluble prodrugs of the corticosteroid methylprednisolone was outlined, and several 21-esters were synthesized to test the approach. Compounds exhibiting dilute solution stabilities approaching 2 years at 25 degrees C were reported. A complicating factor in more concentrated aqueous solutions of water-soluble prodrugs, however, is the limited extent to which hydrolysis can occur before the solution becomes saturated with respect to the relatively insoluble parent drug. In this study the advantages of micellar prodrugs as water-soluble delivery systems for parenteral administration of relatively insoluble parent drugs are explored. Micellar prodrugs, besides being highly water soluble, have additional advantages in that their micelles solubilize poorly soluble degradation products which may otherwise precipitate and may act as a self-stabilizing influence due to protection of the hydrolytically labile prodrug linkage within the micelle interior. Two 21-esters of methylprednisolone previously identified as having promising dilute solution stability have now been shown to self-associate in aqueous solution at higher concentrations, as determined by solubility, kinetic, and light-scattering measurements. One consequence of self-association is that free methylprednisolone, the product of prodrug hydrolysis, is solubilized in concentrated prodrug formulations. In addition, acid- and base-catalyzed hydrolysis rate constants are altered in the micelles, resulting in further prolongation of shelf life in concentrated solutions. Due to the added benefits of self-micellization, the water-soluble 21-esters investigated exhibit shelf lives exceeding 2 years at 30 degrees C, the upper limit of the controlled room temperature range.