Exploring Ester Prodrugs: A Comprehensive Review of Approaches, Applications, and Methods

The review provides an overview of the approaches, applications, and methods for ester prodrugs. Ester prodrugs are pharmacologically inactive compounds in their original form but become active drugs on biotransformation within the body, which offers advantages concerning the solubility, stability, and targeted delivery of the active drug. Several approaches of ester prodrugs have been reviewed in this review, including simple ester prodrugs, amino acid ester prodrugs, sugar ester prodrugs, lipid ester prodrugs, and polymeric ester prodrugs. This review incorporates in vitro and in vivo methods as well as the characterization of physical and chemical properties for ester prodrugs, cell culture systems, enzymatic assays, and animal models—all of these having a very important bearing on the evaluation of stability, bioavailability, and efficacy for ester prodrugs. While the benefits of using ester prodrugs are significant, there are also disadvantages like instability, poor or variable enzymatic hydrolysis, and toxicity from released promoieties or by-products. This review discusses solutions to the various limitations that include enhancing stability with ionizable promoieties and using physiologically-based pharmacokinetic modeling. The review also highlights the application of ester prodrugs in neurological disorders, such as Parkinson’s disease, and the ongoing efforts to address the critical limitations in treatment efficacy. Future prodrug strategies are poised to advance significantly by harnessing diverse transport mechanisms across the blood-brain barrier and integrating nanotechnology.

Synthesis and biological evaluation of lipid-soluble prodrugs of anethole dithiolthione

16 ADT carboxylate esters were prepared by means of esterification and these compounds were expected to increase the bioavailability of 4-hydroxyanehole trithione.In vivo studies showed that ADT concentration of 3a in plasma was much higher than that of ATT during 120 min.Compound 3a could reach blood peak values of ADT at 660.6 ng/mL which was about 14 times of that by ATT.Additionally,the acute toxicity assay indicated high safety of compound 3a that the maximum tolerated dose was no less than 3.25 g/kg.

Synthesis and Cytotoxic Activity of Novel Water-soluble Prodrugs of Combretastatin A-4

Novel water-soluble prodrugs of combretastatin A-4 （5-8） were synthesized and evaluated for their in vitro cytotoxicity against lung carcinoma A549. Compound 5, bearing phosphoryl choline （PC） moiety, showed 90% inhibition at 32 ktg/mL concentration after 24 h. The findings showed the PC derivative would be a promising candidate for the development of new water-soluble prodrug of cytotoxic combretastatin A-4,

Preparation of Curcumin Prodrugs and Their in Vitro Anti-tumor Activities

The curcumin prodrugs, which could be selectively activated in tumor cells, were prepared to establish a basis for the targeted chemotherapy for cancer. On the basis of the molecular structure of curcumin, the N-maleoyl-L-valine-curcumin （NVC）, N-maleoyl- glycine-curcumin （NGC） were chemically synthesized and identified by IR and NMR spectroscopy. After treatment with these two prodrugs for 6--24 h, the rates of growth inhibition on human bladder cancer EJ cells and renal tubular epithelial （HKC） cells were detected by MTT colorimetry. Our results showed that after the treatment with 20 μmol/L--40 μmol/L NVC and NGC for 6--24 h, the growth inhibitory effects on EJ cells were 6.71%-65.13 % （P〈0.05）, 10. 96 %-73.01 % （p〈0. 05）, respectively, in both dose- and time-dependent manners. When compared with the curcumin of same concentrations, the growth inhibitory effects of these two prodrugs on HKC cells were significantly decreased （P〈0.01）. It is concluded that activation of curcumin prodrugs via hydrolysis functions of cellular esterase could inhibit the growth activities of tumor cells, and reduce the side effects on normal diploid cells. This provided a novel strategy for further exploration of tumor targeted chemotherapeutic drugs.

Design, synthesis and in vitro evaluation of L-amino acid esters prodrugs of acyclic nucleoside phosphonates as anti-HBV agent

A series of novel L-amino acid esters prodrugs of acyclic nucleoside phosphonates was synthesized and their anti-HBV activity was evaluated in HepG2 2.2.15 cells. Compound 1d exhibited more potent anti-HBV activity and lower cytotoxicity than those of adefovir dipivoxil with EC50 and CC50 values of 0.207 μmol/L and 2530 μmol/L, respectively.

Bioreductive prodrugs as cancer therapeutics:targeting tumor hypoxia

Hypoxia, a state of low oxygen, is a common feature of solid tumors and is associated with disease progression as well as resistance to radiotherapy and certain chemotherapeutic drugs. Hypoxic regions in tumors, therefore, represent attractive targets for cancer therapy. To date, five distinct classes of bioreactive prodrugs have been developed to target hypoxic cells in solid tumors. These hypoxia-activated prodrugs, including nitro compounds, N-oxides, quinones, and metal complexes, generally share a common mechanism of activation whereby they are reduced by intracellular oxidoreductases in an oxygensensitive manner to form cytotoxins. Several examples including PR-104, TH-302, and EO9 are currently undergoing phase II and phase III clinical evaluation. In this review, we discuss the nature of tumor hypoxia as a therapeutic target, focusing on the development of bioreductive prodrugs. We also describe the current knowledge of how each prodrug class is activated and detail the clinical progress of leading examples.

Synthesis and Evaluation of Glyceride Prodrugs of Naproxen

The glyceride ester derivatives 6a and 6b were prepared by reacting 1,2,3-trihydroxy propane 1,3-dipalmitate/stearate with (S)-naproxen as potential prodrugs. The synthesis was achieved successfully with the aid of N,N’-dicyclohexyl- carbodiimide. These prodrugs were evaluated for anti inflammatory, analgesic and gastroprotective activity. It was found that prodrugs 6a and 6b showed less irritation to gastric mucosa as indicated by ulcer index. The synthesized glyceride esters were found to possess good pharmacological profile as shown by results of anti inflammatory and analgesic activity. The aqueous studies were performed in order to ensure the release of prodrugs. Both prodrugs were found to stable at acidic pH while undergoes hydrolysis at pH 7.4. These findings suggest that the glyceride prodrugs 6a and 6b might be used as potential biolabile derivatives.

Prodrugs incorporated into nanotechnology-based drug delivery systems for possible improvement in bioavailability of ocular drugs delivery

Numerous systems have been designed during the past three decades to improve bioavailability of ophthalmic drug delivery,including:ocular prodrugs and nanotechnology-based drug delivery system.The former can improve the efficacy of ocular drug via enhancing corneal penetration of ocular drugs,prolonging their duration of action and/or reducing the systemic side-effects,unfortunately,some characteristics of the pro-drugs,such as poorly aqueous stability,poorly aqueous solubility and severe eye irritation probably,limit their clinical practice and cannot be ignored.As we all know,nanotech-nology for ocular drug delivery can carry poorly soluble drugs,protect the encapsulated molecules from hydrolysis,control the rate of drug delivery and prolong the precorneal retention of drugs.All of these merits may solve the problems in the utilization of ocular prodrugs and increase the bioavailability of ocular drug delivery.By reviewing recent ad-vances of prodrugs and nanostructures in ocular drug delivery,this paper focus specifically on the promising prospects of nanocarriers overcoming the drawbacks of prodrugs for ophthalmic drug delivery by precorneal routes.

Investigation of mitigating effect of colon-specific prodrugs of boswellic acid on 2,4,6-trinitrobenzene sulfonic acidinduced colitis in Wistar rats: Design, kinetics and biological evaluation

To develop a colon-targeting bioreversible delivery system for β-boswellic acid (BBA) and explore utility of its prodrugs in 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats.METHODSSynthesis of 4 co-drugs of BBA with essential amino acids was achieved by CDI coupling, followed by their spectral characterization. In vitro kinetics were studied by HPLC in aqueous buffers, homogenates of gastrointestinal tract and fecal matter. In vivo kinetic studies were performed in Wistar rat plasma, urine and feces. The prodrugs were screened in TNBS-induced colitis modeled Wistar rats. Statistical significance was assumed at P < 0.05, P < 0.01, P < 0.001 when compared with disease controls using one-way and two-way ANOVAs.RESULTSProdrugs were stable in 0.05 mol/L HCl buffer (pH 1.2) and stomach homogenates. Negligible hydrolysis was observed in phosphate buffer and intestinal homogenates. Substantial release (55%-72% and 68%-86%) of BBA was achieved in rat fecal matter and homogenates of colon. In vivo studies of BBA with L-tryptophan (BT) authenticated colon-specific release of BBA. But, surprisingly substantial concentration of BBA was seen to reach the systemic circulation due to probable absorption through colonic mucosa. Site-specifically enhanced bioavailability of BBA could be achieved in colon, which resulted in demonstration of significant mitigating effect on TNBS-induced colitis in rats without inducing any adverse effects on stomach, liver and pancreas. Prodrug of BT was found to be 1.7% (P < 0.001) superior than sulfasalazine in reducing the inflammation to colon among all prodrugs tested.CONCLUSIONThe outcome of this study strongly suggests that these prodrugs might have dual applicability to inflammatory bowel disease and chronotherapy of rheumatoid arthritis.

Colon-specific prodrugs of 4-aminosalicylic acid for inflammatory bowel disease

Despite the advent of biological products, such as anti-tumor necrosis factor-&#x003b1; monoclonal antibodies (infliximab and adalimumab), for treatment of moderate to severe cases of inflammatory bowel disease (IBD), most patients depend upon aminosalicylates as the conventional treatment option. In recent years, the increased knowledge of complex pathophysiological processes underlying IBD has resulted in development of a number of newer pharmaceutical agents like low-molecular-weight heparin, omega-3 fatty acids, probiotics and innovative formulations such as high-dose, once-daily multi-matrix mesalamine, which are designed to minimize the inflammatory process through inhibition of different targets. Optimization of delivery of existing drugs to the colon using the prodrug approach is another attractive alternative that has been utilized and commercialized for 5-aminosalicylic acid (ASA) in the form of sulfasalazine, balsalazide, olsalazine and ipsalazine, but rarely for its positional isomer 4-ASA - a well-established antitubercular drug that is twice as potent as 5-ASA against IBD, and more specifically, ulcerative colitis. The present review focuses on the complete profile of 4-ASA and its advantages over 5-ASA and colon-targeting prodrugs reported so far for the management of IBD. The review also emphasizes the need for reappraisal of this promising but unexplored entity as a potential treatment option for IBD.

Ocular prodrugs: Attributes and challenges

Ocular drug delivery is one of the most attention-grabbing and challenging endeavors among the numerous existing drug delivery systems.From a drug delivery point of view,eye is an intricate organ to investigate and explore.In spite of many limitations,advancements have been made with the intention of improving the residence time or permeation of the drug in the ocular region.Poor bioavailability of topically administered drugs is the major issue pertaining to ocular drug delivery.Several efforts have been made towards improving precorneal residence time and corneal penetration,e.g.iontophoresis,prodrugs and ionpairing,etc.Prodrug approach(chemical approach)has been explored by the formulation scientists to optimize the physicochemical and biochemical properties of drug molecules for improving ocular bioavailability.Formulation of ocular prodrugs is a challenging task as they should exhibit optimum chemical stability as well as enzymatic liability so that they are converted into parent drug after administration at the desired pace.This review will encompass the concept of derivatization and recent academic and industrial advancements in the field of ocular prodrugs.The progression in prodrug designing holds a potential future for ophthalmic drug delivery.

Synthesis, characterization and in vivo evaluation of honokiol bisphosphate prodrugs protects against rats’ brain ischemia-reperfusion injury

Honokiol(HK)usage is greatly restricted by its poor aqueous solubility and limited oral bioavailability.We synthesized and characterized a novel phosphate prodrug of honokiol(HKP)for in vitro and in vivo use.HKP greatly enhanced the aqueous solubility of HK(127.54±15.53 mg/ml)and the stability in buffer solution was sufficient for intravenous administration.The enzymatic hydrolysis of HKP to HK was extremely rapid in vitro(T 1/2=8.9±2.11 s).Pharmacokinetics studies demonstrated that after intravenous administration of HKP(32 mg/kg),HKP was converted rapidly to HK with a time to reach the maximum plasma concentration of^5 min.The prodrug HKP achieved an improved T 1/2(7.97±1.30 h)and terminal volume of distribution(26.02±6.04 ml/kg)compared with direct injection of the equimolar parent drug(0.66±0.01 h)and(2.90±0.342 ml/kg),respectively.Furthermore,oral administration of HKP showed rapid and improved absorption compared with the parent drug.HKP was confirmed to maintain the bioactivity of the parent drug for ameliorating ischemia-reperfusion injury by decreasing brain infarction and improving neurologic function.Taken together,HKP is a potentially useful aqueous-soluble prodrug with improved pharmacokinetic properties which may merit further development as a potential drug candidate.

Triphenylmethanol Conjugates of Triptorelin as Anti-Lipid Peroxidation Prodrugs

Antioxidants are substances that can prevent or slow damage to cells caused by free radicals, unstable molecules that the body produces as a reaction to environmental and other pressures. Free radicals may play a role in heart disease, cancer and other diseases. If the body cannot process and remove free radicals efficiently, oxidative stress can result. This can harm cells and body function. Free radicals are also known as reactive oxygen species (ROS). In this research, Triptorelin&#174;(TRP) conjugates of triphenylmethanol derivatives (TPMs) were synthesized to evaluate their in vitro lipid peroxidation potency. Comparative lipid peroxidation assays between TRP-TPMs conjugates and the corresponding TPMs derivatives were measured using thiobarbituric reactive substance (TBARS) in a dose- and time-dependent manner following the Fenton’s pathway. Overall, TBARS decreased between 20% - 30% for the treated samples of synthesized conjugates compared to their respective control physical mixtures. These data suggest that TRP-TPMs derivatives can be used to improve the biological activity of TRP.

Triphenylmethanol Conjugates of Triptorelin as Cell-Penetrating Anti-Cancer Prodrugs

Some Triptorelin® (TRP) conjugates of triphenylmethanol derivatives (TPMs) with optimized hydrophobicity were synthesized by reacting 2-substituted methoxy benzenes with 1,3,5-trioxane, followed by the conjugation with TRP and sebacic acid to produce TRP-TPMs derivatives. Comparative antiproliferative assays between TRP-TPMs conjugates and the corresponding non-covalent physical mixtures of the TPMs derivatives and TRP were used to treat human acute lymphoblastic leukemia (CCRF-CEM), human ovarian adenocarcinoma (SK-OV-3) and mouse preadipocytes (3T3-L1) cells. TRP-TPMs conjugates at the 50 μM inhibited cell proliferation in CCRF-CEM, SK-OV-3 and 3T3-L1 cells by 21% - 37%, 24% - 73%, 37% - 56%, respectively following incubation for 72 h. These findings indicate that TRP-TPMs derivatives have the potential to enhance the biological activity of TRP.

Synthesis and Stability of Hydroxyfasudil Derivatives as Prodrugs

Fasudil (HA-1077) is the first small-molecule inhibitor of Rho-kinase and has been employed for clinical treatment of cerebral vasospasm. Hydroxyfasudil, as a metabolite of fasudil, exhibited better activity than fasudil. However, it also suffered from quick metabolisation, weak lipotropy and worse penetration of the blood–brain barrier. Thus, some hydroxyfasudil derivatives such as hydroxyfasudil acetate, hydroxyfasudil phosphate and 1-methoxyfasudil as the prodrugs of hydroxyfasudil were designed and synthesised. Meanwhile, the stability of these three compounds was also investigated. Furthermore, the reason and mechanism of hydrolysis of these compounds were discussed. This work could provide a useful guide for future research. © 2016, Tianjin University and Springer-Verlag Berlin Heidelberg.

Glucocorticoids-based prodrug design:Current strategies and research progress

Attributing to their broad pharmacological effects encompassing anti-inflammation,antitoxin,and immunosuppression,glucocorticoids(GCs)are extensively utilized in the clinic for the treatment of diverse diseases such as lupus erythematosus,nephritis,arthritis,ulcerative colitis,asthma,keratitis,macular edema,and leukemia.However,longterm use often causes undesirable side effects,including metabolic disorders-induced Cushing's syndrome(buffalo back,full moon face,hyperglycemia,etc.),osteoporosis,aggravated infection,psychosis,glaucoma,and cataract.These notorious side effects seriously compromise patients'quality of life,especially in patients with chronic diseases.Therefore,glucocorticoid-based advanced drug delivery systems for reducing adverse effects have received extensive attention.Among them,prodrugs have the advantages of low investment,low risk,and high success rate,making them a promising strategy.In this review,we propose the strategies for the design and summarize current research progress of glucocorticoid-based prodrugs in recent decades,including polymer-based prodrugs,dendrimer-based prodrugs,antibody-drug conjugates,peptide-drug conjugates,carbohydrate-based prodrugs,aliphatic acid-based prodrugs and so on.Besides,we also raise issues that need to be focused on during the development of glucocorticoid-based prodrugs.This review is expected to be helpful for the research and development of novel GCs and prodrugs.

Cytochrome P450 Directed Prodrug Activation Therapy in Research of Cancer Enzymology

Cancer enzymology is a promising filiation of bio-medical sciences. In thepast decades, enzymes, such as GST(glutathione S-transferase) , PKC(protein kinase C) , Topo(DNAtopoisomerases), TK(tyrosine kinase), CD (bacterial cytosine deaminase), CPG2(carboxypeptidase G2) ,and PNP (purine nucleoside phosphorylase), have been known to bear close relations to cancer. Theirspecific expression and influence on the process of tumor initiation, promotion and progressionattract scientists to apply them as a biochemical marker of certain malignant tumor, a predictor ofresponse in cancer chemotherapy; to apply them to drug design, tumor prevention and as adjuvant toradiotherapy or surgery.

PEPT1-mediated prodrug strategy for oral delivery of peramivir

Peramivir was a novel and highly potent neuraminidase(NA) inhibitor for the treatment of influenza A and B. However, it exhibited a very low oral bioavailability(only 3%) due to the high polarity(log P of-1.4) and the low membrane permeability across the intestine. To utilize the PEPT1-mediated prodrug strategy to improve the oral absorption and develop the oral alternative, seven amino acid ester prodrugs and seven amino acid amide prodrugs have been synthesized. The permeability of these prodrugs across Caco-2 cells were screened. Peramivr-(CH_2)_2-l-Val and Peramivir-l-Ile were of the highest permeability in ester prodrugs and amide prodrugs, respectively, and then they were selected for further studies. Glycylsarcosine(gly-sar) uptake by Caco-2 could be inbihited by Peramivir-(CH_2)_2-l-Val and Peramivir-l-Ile in a concentration-dependent manner, and the IC 50 was 1.34 ± 0.31 m M and 1.78 ± 0.48 m M, respectively. The direct uptake of Peramivir-(CH_2)_2-l-Val and Peramivirl-Ile in MDCK-PEPT1 cells were significantly higher than in MDCK mock cells, and could be markedly inhibited by gly-sar. The uptake of Peramivir-(CH_2)_2-l-Val and Peramivir-l-Ile(0.01 to 50 m M) in MDCK-hPEPT1 cells conformed to Michaelis–Menten Equation. The oral bioavailability of peramivir was 65.3% and 37.3% after the oral administration of Peramivir-(CH_2)_2-l-Val and Peramivir-l-Ile to rats, respectively. The oral absorption and bioactivation of Peramivir-(CH_2)_2-l-Val was rapid and extensive, and no Peramivir-(CH_2)_2-l-Val was found in plasma. Because the amide bond was relatively stable, Peramivir-l-Ile could not be totally converted to the parent drug in vivo. Peramivir-(CH_2)_2-l-Val with good oral profiles and rapid bioactivation might be a promising prodrug for the further clinic development. The present study also corroborated the idea that the PEPT1-mediated prodrug approach has enormous promise for improving the oral absorption of poorly absorbed drug.

Synthesis and Characterization of PEG-scuteilarin Conjugates,a Potential PEG Ester Prodrug for the Oral Delivery of Scutellarin

Highly water soluble esters of scutellarin with variable molecular weight polyethylene glycol （PEG） were prepared via PEGylation. The physicochemical properties and the stabilities under different conditions were investigated. By PEG modification, the greatly increased water solubility and desirable partition coefficient of scuteUarin were obtained, and the results showed that these conjugates were potential prodrugs for the oral delivery of scuteUarin.

Simultaneous determination of doxorubicin and its dipeptide prodrug in mice plasma by HPLC with fluorescence detection

A simple and sensitive high performance liquid chromatography with fluorescence detection （HPLC-FD） has been developed for simultaneous quantification of doxorubicin （DOX） and its dipeptide conjugate prodrug （PDOX） in mice plasma. The chromatographic separation was carried out on an Amethyst C18-H column with gradient mobile phase of 0.1% formic acid and 0.1% formic acid in acetonitrile at a flow rate of 1.0 mL/min. The excitation and emission wavelengths were set at 490 and 550 nm, respectively. The method was comprehensively validated. The limits of detection were low up to 5.0 ng/mL for DOX and 25.0 ng/mL for PDOX. And the limits of quantification were low up to 12.5 ng/mL for DOX and 50 ng/mL for PDOX, which were lower than those for most of the current methods. The calibration curves showed good linearity （R2 〉 0.999） over the concentration ranges. The extraction recoveries ranged from 84.0% to 88.2% for DOX and from 85.4% to 89.2% for PDOX. Satisfactory intra-day and inter-day precisions were achieved with RSDs less than 9.1%. The results show that the developed HPLC-FD method is accurate, reliable and will be helpful for preclinical pharmacokinetic study of DOX and PDOX.