Carboxylesterases 1 and 2 hydrolyze phospho-nonsteroidal anti-inflammatory drugs: relevance to their pharmacological activity

The ester-containing prodrug NT-0796 enhances delivery of the NLRP3 inflammasome inhibitor NDT-19795 to monocytic cells expressing carboxylesterase-1

NT-0796 is an ester prodrug which is metabolized by carboxylesterase-1 (CES1) to yield the carboxylic acid NDT-19795, an inhibitor of the NLR family pyrin domain-containing protein 3 (NLRP3) inflammasome. When applied to human monocytes/macrophages which express CES1, however, NT-0796 is much more potent at inhibiting NLRP3 inflammasome activation than is NDT-19795. Comparison of the binding of NDT-19795 and NT-0796 in a cell-based NLRP3 target engagement assay confirms that NDT-19795 is the active species. Moreover, microsomes expressing CES1 efficiently convert NT-0796 to NDT-19795, confirming CES1-dependent activation. To understand the basis for the enhanced potency of the ester prodrug species in human monocytes, we analyzed the accumulation and de-esterification of NT-0796 in cultured cells. Our studies reveal NT-0796 rapidly accumulates in cells, achieving estimated cellular concentrations above those applied to the medium, with concomitant metabolism to NDT-19795 in CES1-expressing cells. Using cells lacking CES1 or a poorly hydrolysable NT-0796 analog demonstrated that de-esterification is not required for NT-0796 to achieve high cellular levels. As a result of a dynamic equilibrium whereby NDT-19795 formed intracellularly is subsequently released to the medium, concentrations of NT-0796 sufficient to inhibit NLRP3 can be completely metabolized to NDT-19795 resulting in a transient pharmacodynamic response. In contrast, when NDT-19795 is applied directly to cells, observed cell-associated levels are below those present in the medium and remain stable over time. Dynamics observed within the context of a closed tissue culture system highlight the utility of NT-0796 as a vehicle for delivering the NDT-19795 acid payload to CES1 expressing cells.

Evaluation of the inhibitory effects of antigout drugs on human carboxylesterases in vitro

Gout is an immune-metabolic disease that frequently coexists with multiple comorbidities such as chronic kidney disease, cardiovascular disease and metabolic syndrome, therefore, it is often treated in combination with these complications. The present study aimed to evaluate the inhibitory effect of antigout drugs (allopurinol, febuxostat, topiroxostat, benzbromarone, lesinurad and probenecid) on the activity of the crucial phase I drug-metabolizing enzymes, carboxylesterases (CESs). 2-(2-benzoyl-3-methoxyphenyl) benzothiazole (BMBT) and fluorescein diacetate (FD) were utilized as the probe reactions to determine the activity of CES1 and CES2, respectively, through in vitro culturing with human liver microsomes. Benzbromarone and lesinurad exhibited strong inhibition towards CESs with Ki values of 2.16 and 5.15 μM for benzbromarone towards CES1 and CES2, respectively, and 2.94 μM for lesinurad towards CES2. In vitro-in vivo extrapolation (IVIVE) indicated that benzbromarone and lesinurad might disturb the metabolic hydrolysis of clinical drugs in vivo by inhibiting CESs. In silico docking showed that hydrogen bonds and hydrophobic interactions contributed to the intermolecular interactions of antigout drugs on CESs. Therefore, vigilant monitoring of potential drug-drug interactions (DDIs) is imperative when co-administering antigout drugs in clinical practice.

Phosphosulindac (OXT-328) prevents and reverses chemotherapy induced peripheral neuropathy in mice

Background

Chemotherapy-induced peripheral neuropathy (CIPN), a side effect of chemotherapy, is particularly difficult to treat. We explored whether phosphosulindac (PS), a modified NSAID, could treat CIPN.

Methods

CIPN was induced in male C57BL/6 J mice by paclitaxel, vincristine or oxaliplatin. Mechanical allodynia was measured with the von Frey test and cold allodynia with the acetone test. To determine the preventive effect of PS, it was administered 2 days before the induction of CIPN. Mouse Lewis lung carcinoma xenografts were used to determine if PS altered the chemotherapeutic efficacy of paclitaxel. Cultured cell lines were used to evaluate the effect of PS on neuroinflammation.

Results

Treatment with each of the three chemotherapeutic agents used to induce CIPN lowered the mechanical allodynia scores by 56 to 85% depending on the specific agent. PS gel was applied topically 3x/day for 16-22 days to the hind paws of mice with CIPN. This effect was dose-dependent. Unlike vehicle, PS returned mechanical allodynia scores back to pre-CIPN levels. PS had a similar effect on paclitaxel-induced CIPN cold allodynia. Sulindac, a metabolite of PS, had no effect on CIPN. PS significantly prevented CIPN compared to vehicle. Given concomitantly with paclitaxel to mice with lung cancer xenografts, PS relieved CIPN without affecting the anticancer effect of paclitaxel. The enantiomers of PS were equally efficacious against CIPN, suggesting the therapeutic suitability of the racemate PS. There were no apparent side effects of PS. PS suppressed the levels of IL-6, IL-10, CXCL1, and CXCL2 induced by paclitaxel in a neuroblastoma cell line, and macrophage activation to the M1 proinflammatory phenotype.

Conclusion

Topically applied PS demonstrated broad therapeutic and preventive efficacy against CIPN, preserved the anticancer effect of paclitaxel, and was safe. Its anti-CIPN effect appears to be mediated, in part, by suppression of neuroinflammation. These data support further evaluation of topical PS for the control of CIPN.

Mechanism of non-steroidal anti-androgen-induced liver injury: Reactive metabolites of flutamide and bicalutamide activate inflammasomes

Flutamide is a non-steroidal anti-androgen agent, which is mainly used for the treatment of prostate cancer. Flutamide is known to cause severe adverse events, which includes idiosyncratic liver injury. However, details of the mechanism of these adverse reactions have not been elucidated. We investigated whether flutamide induces the release of damage-associated molecular patterns (DAMPs) that activate inflammasomes. We also tested bicalutamide, enzalutamide, apalutamide, and darolutamide for their ability to activate inflammasomes in differentiated THP-1 cells. The supernatant from the incubation of flutamide and bicalutamide with human hepatocarcinoma functional liver cell-4 (FLC-4) cells increased caspase-1 activity and production of IL-1ß by differentiated THP-1 cells. In the supernatant of FLC-4 cells with flutamide and bicalutamide, the heat shock protein (HSP) 40 or 60 was significantly increased. Addition of a carboxylesterase or a CYP inhibitor to the FLC-4 cells prevented release of HSPs from the FLC-4 cells. These results suggested that the reactive metabolites of flutamide and bicalutamide can cause the release of DAMPs from hepatocytes and activate inflammasomes. Inflammasome activation may be an important step in the activation of the immune system by flutamide or bicalutamide, which in some patients, can cause immune-related adverse events.

The Antiangiogenic Effect and Ocular Pharmacology of Novel Modified Nonsteroidal Anti-Inflammatory Drugs in the Treatment of Oxygen-Induced Retinopathy

Purpose: To evaluate the hypothesis that 3 novel compounds, OXT-328, Q-922, and CL-717 show efficacy in the treatment of oxygen-induced retinopathy (OIR) and whether or not their route of administration is intravitreal, topical, or systemic. Methods: The OIR mouse model, characterized by an avascular area (AVA) and a neovascular area (NVA) of the retina, was used to study retinopathy of prematurity and other retinal diseases characterized by abnormal vessel growth. We measured the effect of our compounds on both the AVA and NVA in whole mounts of mouse retinal tissue. We also evaluated their ability to prevent new vessel formation in chicken chorioallantoic membranes (CAMs). Finally, we measured the in vitro uptake and biodistribution of topically applied CL-717 in human eye explants. Results: In mice with OIR, compared to controls, a single intravitreal administration of Q-922 or OXT-328 significantly reduced both AVA and NVA. CL-717 administered as eye drops over 5 days also reduced AVA and NVA, whereas OXT-328 eye drops had no effect. Q-922 given intraperitoneal (150 mg/kg/day × 5 days) reduced AVA and NVA. Remarkably, explanted human eyes bathed in CL-717 show rapid uptake and biodistribution in ocular tissues. In the chicken CAM model, all 3 compounds reduced the formation of new blood vessels by about one-third. No side effect in mice was observed, except for mild ocular surface irritation with Q-922. Conclusions: Systemic administration of Q-922 or topical administration of CL-717 holds particular promise for a simplified treatment of proliferative retinopathies without the necessity of intravitreal injections.

Selective detection of carboxylesterase 2 activity in cancer cells using an activity-based chemiluminescent probe

Carboxylesterase 2 (CES2) has crucial roles in both xenobiotic metabolism and formation of pathogenic states including cancer. Thus, it is highly critical to monitor intracellular CES2 activity in living cancer cells. Here, we report a CES2 activatable phenoxy 1,2-dioxetane based chemiluminescent agent (CL-CES2). The probe exhibited a selective turn-on response in the presence of CES2 enzyme and enabled detection of CES2 activity in three different cancer cells that possess varying enzyme concentrations with high signal to noise ratios. In contrast no signal was obtained with CES1, an isoform of CES2 enzyme. CL-CES2 marks the first ever example of a CES2-responsive chemiluminescent luminophore and holds a great potential in further understanding the roles of CES2 activity in tumorogenesis.

Animal models of dry eye disease: Useful, varied and evolving (Review)

Dry eye disease (DED), which is a prevalent disease that still lacks successful treatment options, remains a major challenge in ophthalmology. Multiple animal models of DED have been used to decipher its pathophysiology and to develop novel treatments. These models use mice, rats, rabbits, cats, dogs and non-human primates. Each model assesses aspects of DED by focusing on elements of the lacrimal functional unit, which controls the homeostasis of the tear film. The present review outlines representative DED animal models and assesses their contribution to the study of DED. Murine models are the most extensively used, followed by rabbit models; the latter offer the advantage of larger eyes, a favorable biochemical profile for drug studies, experimental ease and relatively low cost, contrasting with non-human primates, which, although closer to humans, are not as accessible and are expensive. No comprehensive ‘ideal’ animal model encompassing all aspects of human DED exists nor is it feasible. Investigators often choose an animal model based on their experimental needs and the following four features of a given model: The size of the eye, its biochemical composition, the available research reagents and cost. As research efforts in DED expand, more refined animal models are needed to supplement the enormous contribution made to date by existing models.

Carboxylesterase inhibitors from clinically available medicines and their impact on drug metabolism

Mammalian carboxylesterases (CES), the key members of the serine hydrolase superfamily, hydrolyze a wide range of endogenous substances and xenobiotics bearing ester or amide bond(s). In humans, most of identified CES are segregated into the CES1A and CES2A subfamilies. Strong inhibition on human CES (including hCES1A and hCES2A) may modulate pharmacokinetic profiles of CES-substrate drugs, thereby changing the pharmacological and toxicological responses of these drugs. This review covered recent advances in discovery of hCES inhibitors from clinically available medications, as well as their impact on CES-associated drug metabolism. Three comprehensive lists of hCES inhibitors deriving from clinically available medications including therapeutic drugs, pharmaceutical excipients and herbal medicines, alongside with their inhibition potentials and inhibition parameters, are summarized. Furthermore, the potential risks of hCES inhibitors to trigger drug/herb-drug interactions (DDIs/HDIs) and future concerns in this field are highlighted. Potent hCES inhibitors may trigger clinically relevant DDIs/HDIs, especially when these inhibitors are co-administrated with CES substrate-drugs with very narrow therapeutic windows. All data and knowledge presented here provide key information for the clinicians to assess the risks of clinically available hCES inhibitors on drug metabolism. In future, more practical and highly specific substrates for hCES1A/hCES2A should be developed and used for studies on CES-mediated DDIs/HDIs both in vitro and in vivo.

Phospho-Sulindac (OXT-328) Inhibits Dry Eye Disease in Rabbits: A Dose-, Formulation- and Structure-Dependent Effect

Purpose: Inflammation of the ocular surface is central to dry eye disease (DED). The anti-inflammatory agent phospho-sulindac (PS) at a high dose was efficacious against DED in a rabbit model. We assessed the dose, formulation and structure dependence of PS's effect. Methods: In rabbits with concanavalin A-induced DED we evaluated a range of PS concentrations (0.05%-1.6%) and dosing frequencies, assessed the duration of its effect with PS in 2 solution formulations and one emulsion formulation, and compared the efficacy of PS to that of sulindac, and of the structurally similar phospho-ibuprofen amide. We determined tear breakup time (TBUT) (tear stability), Schirmer's tear test (tear production), and by esthesiometry corneal sensitivity (symptoms). We also determined the biodistribution in the eye of topically applied PS. Results: PS in a solution formulation, given as eye drops q.i.d. was efficacious starting at a dose of 0.1%. The effect was apparent after 2 days of treatment and lasted at least 8 days after the last dose. Both signs (evidenced by TBUT and Schirmer's test) and symptoms (measured by corneal sensitivity) improved significantly. The best formulation was the solution formulation; a cyclodextrin-based formulation was also successful but the emulsion formulation was not. PS and its metabolites were essentially restricted to the anterior chamber of the eye. Sulindac and phospho-ibuprofen amide had no efficacy on DED. Conclusions: PS is efficacious against DED. Its effect, encompassing signs, and symptoms, are dose, formulation, and structure dependent. PS has therapeutic promise and merits further development.

Discovery of hCES2A inhibitors from Glycyrrhiza inflata via combination of docking-based virtual screening and fluorescence-based inhibition assays

An integrated strategy via combination of chemical profiling, docking-based virtual screening and fluorescence-based high-throughput inhibitor screening assays was used to efficiently identify natural hCES2A inhibitors from herbal medicines.

FRET-based intracellular investigation of nanoprodrugs toward highly efficient anticancer drug delivery

In order to overcome unpredictable side-effects and increased cytotoxicity of conventional carrier-based anticancer drug delivery systems, several systems that consist exclusively of the pure drug (or prodrug) have been proposed. The behavior and dynamics of these systems after entering cancer cells are, however, still unknown, hindering their progress towards in vivo and clinical applications. Here, we report a comprehensive in cellulo study of carrier-free SN-38 nanoprodrugs (NPDs), previously developed by our group. The work shows the intracellular uptake, localization, and degradation of the NPDs via FRET microscopy. Accordingly, new FRET-NPDs were chemically synthesized and characterized. Prodrug to drug conversion and therapeutic efficiency were also validated. Our work provides crucial information for the application of NPDs as drug delivery systems and demonstrates their outstanding potential as next-generation anticancer nanomedicines.

A pharmacokinetic-pharmacodynamic model based on multi-organ-on-a-chip for drug-drug interaction studies

In drug discovery, the emergence of unexpected toxicity is often a problem resulting from a poor understanding of the pharmacokinetics of drug-drug interactions (DDI). Organ-on-a-chip (OoC) has been proposed as an in vitro model to evaluate drug efficacy and toxicity in pharmacology, but it has not been applied to DDI studies yet. In this study, we aim to evaluate whether organ-on-a-chip technologies can be applied to DDI studies. To assess the usefulness of OoC for DDI studies, we proposed a multi-organ-on-a-chip (MOoC) with a liver part as the metabolic model and a cancer part as the drug target model, and a pharmacokinetic-pharmacodynamic (PK-PD) model describing the MOoC. An anticancer prodrug, CPT-11, was used to evaluate the drug efficacy of the metabolite in the liver part of the MOoC. To evaluate DDI using the MOoC, the inhibitory effect of simvastatin and ritonavir on the metabolism of CPT-11 was tested. The DDI estimation method was evaluated by comparing the results of the concomitant administration experiment using the MOoC and the results of simulation using the proposed PK-PD model with the estimated parameters. The results were similar, suggesting that the combination of the PK-PD model and the MOoC is a useful way to predict DDI. We conclude that OoC technologies could facilitate a better understanding of pharmacokinetic mechanisms with DDI.

Inhibition of human carboxylesterases by ginsenosides: structure-activity relationships and inhibitory mechanism

BACKGROUND

Human carboxylesterases (hCES) are key serine hydrolases responsible for the hydrolysis of a wide range of endogenous and xenobiotic esters. Although it has been reported that some ginsenosides can modulate the activities of various enzymes, the inhibitory effects of ginsenosides on hCES have not been well-investigated.

METHODS

In this study, more than 20 ginsenosides were collected and their inhibitory effects on hCES1A and hCES2A were assayed using the highly specific fluorescent probe substrates for each isoenzyme. Molecular docking simulations were also performed to investigate the interactions between ginsenosides and hCES.

RESULTS

Among all tested ginsenosides, Dammarenediol II (DM) and 20S-O-β-(d-glucosyl)-dammarenediol II (DMG) displayed potent inhibition against both hCES1A and hCES2A, while protopanaxadiol (PPD) and protopanaxatriol (PPT) exhibited strong inhibition on hCES2A and high selectivity over hCES1A. Introduction of O-glycosyl groups at the core skeleton decreased hCES inhibition activity, while the hydroxyl groups at different sites might also effect hCES inhibition. Inhibition kinetic analyses demonstrated that DM and DMG functioned as competitive inhibitors against hCES1A-mediated d-luciferin methyl ester (DME) hydrolysis. In contrast, DM, DMG, PPD and PPT inhibit hCES2A-mediated fluorescein diacetate (FD) hydrolysis via a mixed manner.

CONCLUSION

The structure-inhibition relationships of ginsenosides as hCES inhibitors was investigated for the first time. Our results revealed that DM and DMG were potent inhibitors against both hCES1A and hCES2A, while PPD and PPT were selective and strong inhibitors against hCES2A.

Identification of topoisomerases as molecular targets of cytosporolide C and its analog

Cytosporolide (Cytos) A-C, isolated from the fungus Cytospora sp., have anti-microbial activity, but their molecular targets in mammalian cells are unknown. We have previously reported the total synthesis of Cytos A by biomimetic hetero-Diels-Alder reaction. In this study, to examine the novel bioactivity of Cytos, we synthesized Cytos C and measured cell growth-inhibiting activities of 7 compounds, including Cytos A and C, in several human cancer cell lines. Among these compounds, Cytos C and tetradeoxycytosporolide A (TD-Cytos A), a model compound for the synthesis of Cytos A, had anti-proliferative effects on cancer cells, and TD-Cytos A exhibited stronger activity than Cytos C. In vitro topoisomerase-mediated DNA relaxing experiments showed that TD-Cytos A inhibited the activities of topoisomerase I and II, whereas Cytos C targeted only topoisomerase I. These data suggest that the anti-proliferative activities of Cytos correlate with the inhibition of topoisomerases and implicated TD-Cytos A as a novel anti-cancer drug that suppresses the activities of topoisomerase I and II.

Phosphosulindac is efficacious in an improved concanavalin A-based rabbit model of chronic dry eye disease

Dry eye disease (DED) currently has no satisfactory treatment partly because of the lack of informative animal models. We evaluated the anti-inflammatory phosphosulindac (PS) for the treatment of DED using a new rabbit model of DED based on the concanavalin A (Con A) acute DED model: we injected all lacrimal glands with Con A weekly under ultrasound guidance, which prolonged DED to >3 weeks, and thoroughly assessed efficacy with tear break-up time (TBUT), tear osmolarity, Schirmer test, and tear lactoferrin levels. Rabbits with DED (n = 8-10 eyes per group) were treated topically with PS or vehicle 3×/day for 21days. PS restored TBUT, tear osmolarity, and lactoferrin levels (P < 0.0001-0.04) to normal but did not significantly improve the results of the Schirmer test. PS showed no side effects and was much more efficacious than cyclosporine or lifitegrast. In the cornea, PS suppressed the activation of nuclear factor kappa-B, the levels of transforming growth factor beta, interleukin-1 beta, interleukin-6, and interleukin-8, and the levels of matrix metalloproteinase (MMP)-1 and MMP-9, and MMP activity. Levels of prostaglandin E₂ (PGE₂) in tears and cornea were preserved in PS-treated rabbits. Ketorolac and diclofenac, two ophthalmic NSAIDs causing corneal melt, nearly completely suppressed PGE₂ levels but had no effect on MMPs. The effects of PS on PGE₂ and MMPs likely account for its apparent ocular safety. Our results establish an animal model for acute and chronic DED suitable for drug efficacy studies and indicate that PS merits evaluation for DED.

B-esterase determination and organophosphate insecticide inhibitory effects in JEG-3 trophoblasts

The placenta and trophoblasts express several B-esterases. This family includes acethylcholinesterase (AChE), carboxylesterase (CES) and butyrylcholinesterase (BChE), which are important targets of organophosphate insecticide (OP) toxicity. To better understand OP effects on trophoblasts, B-esterase basal activity and kinetic behavior were studied in JEG-3 choriocarcinoma cell cultures. Effects of the OP azinphos-methyl (Am) and chlorpyrifos (Cp) on cellular enzyme activity were also evaluated. JEG-3 cells showed measurable activity levels of AChE and CES, while BChE was undetected. Recorded Km for AChE and CES were 0.33 and 0.26 mM respectively. Native gel electrophoresis and RT-PCR analysis demonstrated CES1 and CES2 isoform expression. Cells exposed for 4 and 24 h to the OP Am or Cp, showed a differential CES and AChE inhibition profiles. Am inhibited CES and AChE at 4 h treatment while Cp showed the highest inhibition profile at 24 h. Interestingly, both insecticides differentially affected CES1 and CES2 activities. Results demonstrated that JEG-3 trophoblasts express AChE, CES1 and CES2. B-esterase enzymes were inhibited by in vitro OP exposure, indicating that JEG-3 cells metabolization capabilities include phase I enzymes, able to bioactivate OP. In addition, since CES enzymes are important for medicinal drug activation/deactivation, OP exposure may interfere with trophoblast CES metabolization, probably being relevant in a co-exposure scenario during pregnancy.

An in vitro screening with emerging contaminants reveals inhibition of carboxylesterase activity in aquatic organisms

Pharmaceuticals and personal care products (PPCPs) form part of the new generation of pollutants present in many freshwater and marine ecosystems. Although environmental concentrations of these bioactive substances are low, they cause sublethal effects (e.g., enzyme inhibition) in non-target organisms. However, little is known on metabolism of PPCPs by non-mammal species. Herein, an in vitro enzyme trial was performed to explore sensitivity of carboxylesterase (CE) activity of aquatic organisms to fourteen PPCPs. The esterase activity was determined in the liver of Mediterranean freshwater fish (Barbus meridionalis and Squalius laietanus), coastal marine fish (Dicentrarchus labrax and Solea solea), middle-slope fish (Trachyrhynchus scabrus), deep-sea fish (Alepocephalus rostratus and Cataetix laticeps), and in the digestive gland of a decapod crustacean (Aristeus antennatus). Results showed that 100μM of the lipid regulators simvastatin and fenofibrate significantly inhibited (30-80% of controls) the CE activity of all target species. Among the personal care products, nonylphenol and triclosan were strong esterase inhibitors in most species (36-68% of controls). Comparison with literature data suggests that fish CE activity is as sensitive to inhibition by some PPCPs as that of mammals, although their basal activity levels are lower than in mammals. Pending further studies on the interaction between PPCPs and CE activity, we postulate that this enzyme may act as a molecular sink for certain PPCPs in a comparable way than that described for the organophosphorus pesticides.

The evolving role of nonsteroidal anti-inflammatory drugs in colon cancer prevention: a cause for optimism

Colorectal cancer (CRC) is a serious yet preventable disease. The low acceptance and cost of colonoscopy as a screening method or CRC make chemoprevention an important option. Nonsteroidal anti-inflammatory drugs (NSAIDs), not currently recommended for CRC prevention, have the potential to evolve into the agents of choice for this indication. Here, we discuss the promise and challenge of NSAIDs for this chemopreventive application.Multiple epidemiologic studies, randomized clinical trials (RCTs) of sporadic colorectal polyp recurrence, RCTs in patients with hereditary colorectal cancer syndromes, and pooled analyses of cardiovascular-prevention RCTs linked to cancer outcomes have firmly established the ability of conventional NSAIDs to prevent CRC. NSAIDs, however, are seriously limited by their toxicity,which can become cumulative with their long-term administration for chemoprevention, whereas drug interactions in vulnerable elderly patients compound their safety. Newer, chemically modified NSAIDs offer the hope of enhanced efficacy and safety.Recent work also indicates that targeting earlier stages of colorectal carcinogenesis, such as the lower complexity aberrant crypt foci, is a promising approach that may only require relatively short use of chemopreventive agents. Drug combination approaches exemplified by sulindac plus difluoromethylornithine appear very efficacious. Identification of those at risk or most likely to benefit from a given intervention using predictive biomarkers may usher in personalized chemoprevention. Agents that offer simultaneous chemoprevention of diseases in addition to CRC, e.g., cardiovascular and/or neurodegenerative diseases,may have a much greater potential for a broad clinical application.

Fenofibrate subcellular distribution as a rationale for the intracranial delivery through biodegradable carrier

Fenofibrate, a well-known normolipidemic drug, has been shown to exert strong anticancer effects against tumors of neuroectodermal origin including glioblastoma. Although some pharmacokinetic studies were performed in the past, data are still needed about the detailed subcellular and tissue distribution of fenofibrate (FF) and its active metabolite, fenofibric acid (FA), especially in respect to the treatment of intracranial tumors. We used high performance liquid chromatography (HPLC) to elucidate the intracellular, tissue and body fluid distribution of FF and FA after oral administration of the drug to mice bearing intracranial glioblastoma. Following the treatment, FF was quickly cleaved to FA by blood esterases and FA was detected in the blood, urine, liver, kidney, spleen and lungs. We have also detected small amounts of FA in the brains of two out of six mice, but not in the brain tumor tissue. The lack of FF and FA in the intracranial tumors prompted us to develop a new method for intracranial delivery of FF. We have prepared and tested in vitro biodegradable poly-lactic-co-glycolic acid (PLGA) polymer wafers containing FF, which could ultimately be inserted into the brain cavity following resection of the brain tumor. HPLC-based analysis demonstrated a slow and constant diffusion of FF from the wafer, and the released FF abolished clonogenic growth of glioblastoma cells. On the intracellular level, FF and FA were both present in the cytosolic fraction. Surprisingly, we also detected FF, but not FA in the cell membrane fraction. Electron paramagnetic resonance spectroscopy applied to spin-labeled phospholipid model-membranes revealed broadening of lipid phase transitions and decrease of membrane polarity induced by fenofibrate. Our results indicate that the membrane-bound FF could contribute to its exceptional anticancer potential in comparison to other lipid-lowering drugs, and advocate for intracranial delivery of FF in the combined pharmacotherapy against glioblastoma.

Phospho-NSAIDs have enhanced efficacy in mice lacking plasma carboxylesterase: implications for their clinical pharmacology

PURPOSE

The purpose of the study was to evaluate the metabolism, pharmacokinetics and efficacy of phospho-NSAIDs in Ces1c-knockout mice.

METHODS

Hydrolysis of phospho-NSAIDs by Ces1c was investigated using Ces1c-overexpressing cells. The rate of phospho-NSAID hydrolysis was compared between wild-type, Ces1c+/- and Ces1c-/- mouse plasma in vitro, and the effect of plasma Ces1c on the cytotoxicity of phospho-NSAIDs was evaluated. Pharmacokinetics of phospho-sulindac was examined in wild-type and Ces1c-/- mice. The impact of Ces1c on the efficacy of phospho-sulindac was investigated using lung and pancreatic cancer models in vivo.

RESULTS

Phospho-NSAIDs were extensively hydrolyzed in Ces1c-overexpressing cells. Phospho-NSAID hydrolysis in wild-type mouse plasma was 6-530-fold higher than that in the plasma of Ces1c-/- mice. Ces1c-expressing wild-type mouse serum attenuated the in vitro cytotoxicity of phospho-NSAIDs towards cancer cells. Pharmacokinetic studies of phospho-sulindac using wild-type and Ces1c-/- mice demonstrated 2-fold less inactivation of phospho-sulindac in the latter. Phospho-sulindac was 2-fold more efficacious in inhibiting the growth of lung and pancreatic carcinoma in Ces1c -/- mice, as compared to wild-type mice.

CONCLUSIONS

Our results indicate that intact phospho-NSAIDs are the pharmacologically active entities and phospho-NSAIDs are expected to be more efficacious in humans than in rodents due to their differential expression of carboxylesterases.

Phospho-aspirin-2 (MDC-22) inhibits estrogen receptor positive breast cancer growth both in vitro and in vivo by a redox-dependent effect

Phospho-aspirin (PA-2) is a novel aspirin derivative that exhibits promising anticancer properties and is considerably safer than conventional aspirin. In this study, we investigated the chemotherapeutic efficacy of PA-2 in preclinical models of estrogen receptor positive (ER+) breast cancer and elucidated its mechanism of action. PA-2 inhibited the growth of ER+ cells more potently than aspirin in vitro, and exerted a triple cytokinetic effect that includes induction of apoptosis and cell cycle arrest as well as the inhibition of cell proliferation. PA-2 is highly efficacious in vivo, as treatment of established MCF7 xenografts with PA-2 induced tumor stasis (98.2% inhibition, p<0.01). PA-2 triggered the activation of p53-dependent apoptosis via two distinct mechanisms: 1) acetylation of p53 (at K373), which disrupts its interaction with its transcription repressor MDM2, and 2) translocation of p53 to the mitochondria leading to the dissipation of mitochondrial transmembrane potential (ΔΨ(m)). Consistent with these observations, both the RNAi-mediated knockdown of p53 and forced deactylation via HDAC1 over-expression attenuated the anticancer effect of PA-2 in MCF7 cells. An upstream mediator of the signaling effects of PA-2 is RONS. PA-2 induced oxidative stress in vitro and in mice bearing MCF7 xenografts; its induction effect appears to be tumor-specific. Crucially, administration of N-acetylcysteine, a ROS scavenger, abrogated the effect of PA-2 on p53 acetylation and mitochondria translocation, thus identifying RONS as proximal molecules mediating the anticancer effect of PA-2. In summary, our findings demonstrate that PA-2 is a promising antineoplastic compound against ER+ breast cancer, warranting further evaluation as an anticancer agent.

A novel ibuprofen derivative with anti-lung cancer properties: synthesis, formulation, pharmacokinetic and efficacy studies

Phospho-non-steroidal anti-inflammatory drugs (phospho-NSAIDs) are a novel class of NSAID derivatives with potent antitumor activity. However, phospho-NSAIDs have limited stability in vivo due to their rapid hydrolysis by carboxylesterases at their carboxylic ester link. Here, we synthesized phospho-ibuprofen amide (PIA), a metabolically stable analog of phospho-ibuprofen, formulated it in nanocarriers, and evaluated its pharmacokinetics and anticancer efficacy in pre-clinical models of human lung cancer. PIA was 10-fold more potent than ibuprofen in suppressing the growth of human non-small-cell lung cancer (NSCLC) cell lines, an effect mediated by favorably altering cytokinetics and inducing oxidative stress. Pharmacokinetic studies in rats revealed that liposome-encapsulated PIA exhibited remarkable resistance to hydrolysis by carboxylesterases, remaining largely intact in the systemic circulation, and demonstrated selective distribution to the lungs. The antitumor activity of liposomal PIA was evaluated in a metastatic model of human NSCLC in mice. Liposomal PIA strongly inhibited lung tumorigenesis (>95%) and was significantly (p<0.05) more efficacious than ibuprofen. We observed a significant induction of urinary 8-iso-prostaglandin F2αin vivo, which indicates that ROS stress probably plays an important role in mediating the antitumor efficacy of PIA. Our findings suggest that liposomal PIA is a potent agent in the treatment of lung cancer and merits further evaluation.

Pegylation improves the pharmacokinetics and bioavailability of small-molecule drugs hydrolyzable by esterases: a study of phospho-Ibuprofen

Esterase hydrolysis of drugs can accelerate their elimination, thereby limiting their efficacy. Polyethylene glycol (PEG) covalently attached to drugs (pegylation) is known to improve the efficiency of many drugs. Using as a test agent the novel phospho-ibuprofen (PI), we examined whether pegylation of PI could abrogate its hydrolytic degradation by esterases; PI, known to inhibit colon cancer growth, has a carboxylic ester hydrolyzable by carboxylesterases (CES). We covalently attached mPEG-2000 to PI (PI-PEG) and studied its stability by exposing it to cells overexpressing CES and by administering it to mice. We also evaluated PI-PEG's anticancer efficacy in human colon cancer xenografts and in Apc(min/+) mice. PI-PEG was stable in the presence of cells overexpressing CES1 or CES2, whereas PI was extensively hydrolyzed (90.2 ± 0.7%, 14.3 ± 1.1%, mean ± S.E.M.). In mice, PI was nearly completely hydrolyzed. Intravenous administration of PI-PEG resulted in significant levels in blood and in colon cancer xenografts (xenograft values in parentheses): area under the curve for 0-24 hours = 2351 (2621) (nmol/g) × h; Cmax = 1965 (886) nmol/g; Tmax = 0.08 (2) hour. The blood levels of ibuprofen, its main hydrolytic product, were minimal. Compared with controls, PI-PEG inhibited the growth of the xenografts by 74.8% (P < 0.01) and reduced intestinal tumor multiplicity in Apc(min/+) mice by 73.1% (P < 0.01), prolonging their survival (100% versus 55.1% of controls; P = 0.013). Pegylation protects PI from esterase hydrolysis and improves its pharmacokinetics. In preclinical models of colon cancer, PI-PEG is a safe and efficacious agent that merits further evaluation.

The in vitro metabolism of phospho-sulindac amide, a novel potential anticancer agent

Phospho-sulindac amide (PSA) is a novel potential anti-cancer and anti-inflammatory agent. Here we report the metabolism of PSA in vitro. PSA was rapidly hydroxylated at its butane-phosphate moiety to form two di-hydroxyl-PSA and four mono-hydroxyl-PSA metabolites in mouse and human liver microsomes. PSA also can be oxidized or reduced at its sulindac moiety to form PSA sulfone and PSA sulfide, respectively. PSA was mono-hydroxylated and cleared more rapidly in mouse liver microsomes than in human liver microsomes. Of eight major human cytochrome P450s (CYPs), CYP3A4 and CYP2D6 exclusively catalyzed the hydroxylation and sulfoxidation reactions of PSA, respectively. We also examined the metabolism of PSA by three major human flavin monooxygenases (FMOs). FMO1, FMO3 and FMO5 were all capable of catalyzing the sulfoxidation (but not hydroxylation) of PSA, with FMO1 being by far the most active isoform. PSA was predominantly sulfoxidized in human kidney microsomes because FMO1 is the dominant isoform in human kidney. PSA (versus sulindac) is a preferred substrate of both CYPs and FMOs, likely because of its greater lipophilicity and masked-COOH group. Ketoconazole (a CYP3A4 inhibitor) and alkaline pH strongly inhibited the hydroxylation of PSA, but moderately suppressed its sulfoxidation in liver microsomes. Together, our results establish the metabolic pathways of PSA, identify the major enzymes mediating its biotransformations and reveal significant inter-species and inter-tissue differences in its metabolism.

Phospho-aspirin (MDC-22) inhibits breast cancer in preclinical animal models: an effect mediated by EGFR inhibition, p53 acetylation and oxidative stress

Background

The anticancer properties of aspirin are restricted by its gastrointestinal toxicity and its limited efficacy. Therefore, we synthesized phospho-aspirin (PA-2; MDC-22), a novel derivative of aspirin, and evaluated its chemotherapeutic and chemopreventive efficacy in preclinical models of triple negative breast cancer (TNBC).

Methods

Efficacy of PA-2 was evaluated in human breast cancer cells in vitro, and in orthotopic and subcutaneous TNBC xenografts in nude mice. Mechanistic studies were also carried out to elucidate the mechanism of action of PA-2.

Results

PA-2 inhibited the growth of TNBC cells in vitro more potently than aspirin. Treatment of established subcutaneous TNBC xenografts (MDA-MB-231 and BT-20) with PA-2 induced a strong growth inhibitory effect, resulting in tumor stasis (79% and 90% inhibition, respectively). PA-2, but not aspirin, significantly prevented the development of orthotopic MDA-MB-231 xenografts (62% inhibition). Mechanistically, PA-2: 1) inhibited the activation of epidermal growth factor receptor (EGFR) and suppressed its downstream signaling cascades, including PI3K/AKT/mTOR and STAT3; 2) induced acetylation of p53 at multiple lysine residues and enhanced its DNA binding activity, leading to cell cycle arrest; and 3) induced oxidative stress by suppressing the thioredoxin system, consequently inhibiting the activation of the redox sensitive transcription factor NF-κB. These molecular alterations were observed in vitro and in vivo, demonstrating their relevance to the anticancer effect of PA-2.

Conclusions

Our findings demonstrate that PA-2 possesses potent chemotherapeutic efficacy against TNBC, and is also effective in its chemoprevention, warranting further evaluation as an anticancer agent.

Curcumin enhances the lung cancer chemopreventive efficacy of phospho-sulindac by improving its pharmacokinetics

Phospho-sulindac (PS) is a safe sulindac derivative with promising anticancer efficacy in colon cancer. We evaluated whether its combination with curcumin could enhance the efficacy in the treatment of lung cancer. Curcumin, the principal bioactive component in turmeric, has demonstrated versatile capabilities to modify the therapeutic efficacy of a wide range of anticancer agents. Here, we evaluated the effect of co-administration of curcumin on the anticancer activity of PS in a mouse xenograft model of human lung cancer. Curcumin enhanced the cellular uptake of PS in human lung and colon cancer cell lines. To assess the potential synergism between curcumin and PS in vivo, curcumin was suspended in 10% Tween-80 or formulated in micellar nanoparticles and given to mice by oral gavage prior to the administration of PS. Both formulations of curcumin significantly improved the pharmacokinetic profiles of PS, with the 10% Tween-80 suspension being much more effective than the nanoparticle formation. However, curcumin did not exhibit any significant modification of the metabolite profile of PS. Furthermore, in a mouse subcutaneous xenograft model of human lung cancer, PS (200 mg/kg) in combination with curcumin (500 mg/kg) suspended in 10% Tween-80 (51% inhibition, p<0.05) was significantly more efficacious than PS plus micelle curcumin (30%) or PS (25%) or curcumin alone (no effect). Consistent with the improved pharmacokinetics, the combination treatment group had higher levels of PS and its metabolites in the xenografts compared to PS alone. Our results show that curcumin substantially improves the pharmacokinetics of PS leading to synergistic inhibition of the growth of human lung cancer xenografts, representing a promising drug combination.

Aerosol administration of phospho-sulindac inhibits lung tumorigenesis

Phospho-sulindac is a sulindac derivative with promising anticancer activity in lung cancer, but its limited metabolic stability presents a major challenge for systemic therapy. We reasoned that inhalation delivery of phospho-sulindac might overcome first-pass metabolism and produce high levels of intact drug in lung tumors. Here, we developed a system for aerosolization of phospho-sulindac and evaluated the antitumor efficacy of inhaled phospho-sulindac in an orthotopic model of human non-small cell lung cancer (A549 cells). We found that administration by inhalation delivered high levels of phospho-sulindac to the lungs and minimized its hydrolysis to less active metabolites. Consequently, inhaled phospho-sulindac (6.5 mg/kg) was highly effective in inhibiting lung tumorigenesis (75%; P < 0.01) and significantly improved the survival of mice bearing orthotopic A549 xenografts. Mechanistically, phospho-sulindac suppressed lung tumorigenesis by (i) inhibiting EGF receptor (EGFR) activation, leading to profound inhibition of Raf/MEK/ERK and PI3K/AKT/mTOR survival cascades; (ii) inducing oxidative stress, which provokes the collapse of mitochondrial membrane potential and mitochondria-dependent cell death; and (iii) inducing autophagic cell death. Our data establish that inhalation delivery of phospho-sulindac is an efficacious approach to the control of lung cancer, which merits further evaluation.

Topically applied phospho-sulindac hydrogel is efficacious and safe in the treatment of experimental arthritis in rats

PURPOSE

Formulate phospho-sulindac (P-S, OXT-328) in a Pluronic hydrogel to be used as a topical anti-inflammatory agent and study its efficacy, safety and pharmacokinetics in an arthritis model.

METHODS

LEW/crlBR rats with Freund's adjuvant-induced arthritis were treated with P-S formulated in Pluronic hydrogel (PSH). We determined the clinical manifestations of arthritis including the locomotor activity of the rats; evaluated joints for inflammation, bone resorption, cartilage damage, COX-2 expression and NF-κB activation; assayed plasma IL-6 and IL-10 levels; and studied the pharmacokinetics of P-S in rats after topical or oral administration.

RESULTS

PSH applied at the onset of arthritis or when arthritis was fully developed, suppressed it by 56-82%, improved the locomotor activity of the rats 2.1-4.4 fold, suppressed synovial inflammation, bone resorption, cartilage damage, NF-κB activation and COX-2 expression but not plasma IL-6 and IL-10 levels. There were no side effects. PSH produced rapidly high local levels of P-S with <14% of P-S reaching the circulation, while orally administered P-S was rapidly metabolized generating much lower joint levels of P-S.

CONCLUSIONS

Topical application of PSH is efficacious and safe in the treatment of Freund's adjuvant-induced arthritis; has a favorable pharmacokinetic profile; and likely acts by suppressing key pro-inflammatory signaling pathways.

Comparative in vitro metabolism of phospho-tyrosol-indomethacin by mice, rats and humans

Phospho-tyrosol-indomethacin (PTI; MPI 621), a novel anti-cancer agent, is more potent and safer than conventional indomethacin. Here, we show that PTI was extensively metabolized in vitro and in vivo. PTI was rapidly hydrolyzed by carboxylesterases to generate indomethacin as its major metabolite in the liver microsomes and rats. PTI additionally undergoes cytochromes P450 (CYP)-mediated hydroxylation at its tyrosol moiety and O-demethylation at its indomethacin moiety. Of the five major human CYPs, CYP3A4 and CYP2D6 catalyze the hydroxylation and O-demethylation reactions of PTI, respectively; whereas CYP1A2, 2C9 and 2C19 are inactive towards PTI. In contrast to PTI, indomethacin is primarily O-demethylated by CYP2C9, which prefers acidic substrates. The hydrolyzed and O-demethylated metabolites of PTI are further glucuronidated and sulfated, facilitating drug elimination and detoxification. We observed substantial inter-species differences in the metabolic rates of PTI. Among the liver microsomes from various species, PTI was the most rapidly hydrolyzed, hydroxylated and O-demethylated in mouse, human and rat liver microsomes, respectively. These results reflect the differential expression patterns of carboxylesterase and CYP isoforms among these species. Of the human microsomes from various tissues, PTI underwent more rapid carboxylesterase- and CYP-catalyzed reactions in liver and intestine microsomes than in kidney and lung microsomes. Together, our results establish the metabolic pathways of PTI, reveal significant inter-species differences in its metabolism, and provide insights into the underlying biochemical mechanisms.

The anticancer effect of phospho-tyrosol-indomethacin (MPI-621), a novel phosphoderivative of indomethacin: in vitro and in vivo studies

We have synthesized a novel derivative of indomethacin, phospho-tyrosol-indomethacin (PTI; MPI-621), and evaluated its anticancer efficacy in vitro and in vivo. PTI inhibited the growth of human colon, breast and lung cancer cell lines 6-30-fold more potently than indomethacin. In vivo, in contrast to indomethacin that was unable to inhibit colon cancer xenograft growth, PTI inhibited the growth of colon (69% at 10mg/kg/day, P < 0.01) and lung (91% at 15mg/kg/day, P < 0.01) subcutaneous cancer xenografts in immunodeficient mice, suppressing cell proliferation by 33% and inducing apoptosis by 75% (P < 0.05, for both). Regarding its pharmacokinetics in mice, after a single intraperitoneal injection of PTI, its plasma levels reached the maximum concentration (Cmax = 46 μM) at 2h (Tmax) and became undetectable at 4h. Indomethacin is the major metabolite of PTI, with plasma Cmax = 378 μM and Tmax = 2.5h; it became undetectable 24h postadministration. The cellular uptake of PTI (50-200 μM) at 6h was about 200-fold greater than that of indomethacin. Regarding its safety, PTI had no significant genotoxicity, showed less gastrointestinal toxicity than indomethacin and presented no cardiac toxicity. Mechanistically, PTI suppressed prostaglandin E2 production in A549 human lung cancer cells and strongly inhibited nuclear factor-κB activation in A549 xenografts. These findings indicate that PTI merits further evaluation as an anticancer agent.

A novel NSAID derivative, phospho-ibuprofen, prevents AOM-induced colon cancer in rats

The cancer chemopreventive properties and gastrointestinal toxicity of ibuprofen are well documented. Modification of existing NSAIDs has improved on the chemopreventive efficacy of this agent and reduced its toxicity. In this study, ibuprofen and a modified derivative (phospho-modified ibuprofen or p-ibuprofen) were used in a chemically induced model of colon cancer. Fisher 344 rats were injected with azoxymethane then treated with either ibuprofen (500 ppm) or p-ibuprofen (900 ppm) for 20 weeks to observe aberrant crypt foci (ACF) or 40 weeks to evaluate tumor incidence and multiplicity. β-catenin and p65 were measured in colonic tissues by immunofluorescence staining. Equal molar doses of ibuprofen (75 and 670 mg/kg) and p-ibuprofen (135 and 1,215 mg/kg) were administered to rats for 7 days to assess acute toxicity. The in vitro effect of p-ibuprofen on COX-2 and PGE(2) synthesis, β-catenin expression and NF-κB activity were examined in RAW 264.7 macrophage and HCT 116 colon cancer cells. At week 20, p-ibuprofen and ibuprofen significantly reduced the multiplicity of ACF compared with control (p<0.05); 31.2 and 37.9%, respectively. At week 40, p-ibuprofen and ibuprofen reduced the multiplicity of colon tumors compared with control (p<0.01) by 47.2 and 56.6%, respectively. Equal molar concentrations of ibuprofen (670 mg/kg) and p-ibuprofen (1,215 mg/kg) resulted in stomach ulceration in 85.7% (6 out of 7) and 14.3% (1 out of 7) of rats, respectively, with p<0.01. Immunofluoresence staining and western blot analysis demonstrated that both ibuprofen and p-ibuprofen suppressed β-catenin nuclear translocation in colon cancer cells. In addition, p-ibuprofen but not ibuprofen inhibited NF-κB activation in colon cancer cells. Collectively, these results suggest that p-ibuprofen is a potential effective novel drug for long-term use in colon cancer prevention.

Prodrugs of nonsteroidal anti-inflammatory drugs (NSAIDs), more than meets the eye: a critical review

The design and the synthesis of prodrugs for nonsteroidal anti-inflammatory drugs (NSAIDs) have been given much attention by medicinal chemists, especially in the last decade. As a therapeutic group, NSAIDs are among the most widely used prescribed and over the counter (OTC) medications. The rich literature about potential NSAID prodrugs clearly shows a shift from alkyl, aryalkyl or aryl esters with the sole role of masking the carboxylic acid group, to more elaborate conjugates that contain carefully chosen groups to serve specific purposes, such as enhancement of water solubility and dissolution, nitric oxide release, hydrogen sulfide release, antioxidant activity, anticholinergic and acetylcholinesterase inhibitory (AChEI) activity and site-specific targeting and delivery. This review will focus on NSAID prodrugs that have been designed or were, later, found to possess intrinsic pharmacological activity as an intact chemical entity. Such intrinsic activity might augment the anti-inflammatory activity of the NSAID, reduce its side effects or transform the potential therapeutic use from classical anti-inflammatory action to something else. Reports discussed in this review will be those of NO-NSAIDs, anticholinergic and AChEI-NSAIDs, Phospho-NSAIDs and some miscellaneous agents. In most cases, this review will cover literature dealing with these NSAID prodrugs from the year 2006 and later. Older literature will be used when necessary, e.g., to explain the chemical and biological mechanisms of action.

Topical phospho-sulindac (OXT-328) is effective in the treatment of non-melanoma skin cancer

Phospho-sulindac (P-S, OXT-328), a novel sulindac derivative, has shown superior anticancer efficacy and safety compared to sulindac. In this study, we investigated the efficacy of topical P-S hydrogel in the treatment of non-melanoma skin cancer in preclinical models. P-S is a potent inhibitor of A431 epidermoid carcinoma in vitro and achieves this effect by inhibiting cell proliferation and inducing apoptosis. The anticancer efficacy of topical and oral P-S was further evaluated in mice bearing A431 intradermal xenografts. Compared to the controls, topical P-S hydrogel inhibited the A431 xenografts by 70.5% (p<0.01), while oral P-S inhibited it by 43.4% (p<0.05), being significantly less effective than topical P-S (p= 0.017). Topical P-S hydrogel generated significant levels (>500 nmol/g tumor tissue) of intact P-S in the tumors, accounting for 92.5% of the total metabolites in the A431 xenografts. This local delivery of high levels of intact P-S to the A431 xenografts is an important contributor to the potent activity of topical P-S and no local or systemic side effects were noted in the treatment group. Thus, topical P-S is a promising treatment modality against non-melanoma skin cancer and merits further evaluation.

In vitro and in vivo metabolic studies of phospho-aspirin (MDC-22)

PURPOSE

To investigate the metabolism of phospho-aspirin (PA, MDC-22), a novel anti-cancer and anti-inflammatory agent.

METHODS

The metabolism of PA was studied in the liver and intestinal microsomes from mouse, rat and human.

RESULTS

PA is rapidly deacetylated to phospho-salicylic acid (PSA), which undergoes regioselective oxidation to generate 3-OH-PSA and 5-OH-PSA. PSA also can be hydrolyzed to give salicylic acid (SA), which can be further glucuronidated. PA is far more stable in human liver or intestinal microsomes compared to those from mouse or rat due to its slowest deacetylation in human microsomes. Of the five major human cytochrome P450 (CYP) isoforms, CYP2C19 and 2D6 are the most active towards PSA. In contrast to PSA, conventional SA is not appreciably oxidized by the CYPs and liver microsomes, indicating that PSA is a preferred substrate of CYPs. Similarly, PA, in contrast to PSA, cannot be directly oxidized by CYPs and liver microsomes, indicating that the acetyl group of PA abrogates its oxidation by CYPs.

CONCLUSIONS

Our findings establish the metabolism of PA, reveal significant inter-species differences in its metabolic transformations, and provide an insight into the role of CYPs in these processes.