Synthesis, physicochemical properties, and cytotoxicity of a series of 5'-ester prodrugs of 5-iodo-2'-deoxyuridine

Utility of transporter/receptor(s) in drug delivery to the eye

The eye is a highly protected organ, and designing an effective therapy is often considered a challenging task. The anatomical and physiological barriers result in low ocular bioavailability of drugs. Due to these constraints, less than 5% of the administered dose is absorbed from the conventional ophthalmic dosage forms. Further, physicochemical properties such as lipophilicity, molecular weight and charge modulate the permeability of drug molecules. Vision-threatening diseases such as glaucoma, diabetic macular edema, cataract, wet and dry age-related macular degeneration, proliferative vitreoretinopathy, uveitis, and cytomegalovirus retinitis alter the pathophysiological and molecular mechanisms. Understanding these mechanisms may result in the development of novel treatment modalities. Recently, transporter/receptor targeted prodrug approach has generated significant interest in ocular drug delivery. These transporters and receptors are involved in the transport of essential nutrients, vitamins, and xenobiotics across biological membranes. Several influx transporters (peptides, amino acids, glucose, lactate and nucleosides/nucleobases) and receptors (folate and biotin) have been identified on conjunctiva, cornea, and retina. Structural and functional delineation of these transporters will enable more drugs targeting the posterior segment to be successfully delivered topically. Prodrug derivatization targeting transporters and receptors expressed on ocular tissues has been the subject of intense research. Several prodrugs have been designed to target these transporters and enhance the absorption of poorly permeating parent drug. Moreover, this approach might be used in gene delivery to modify cellular function and membrane receptors. This review provides comprehensive information on ocular drug delivery, with special emphasis on the use of transporters and receptors to improve drug bioavailability.

Vitreal pharmacokinetics of biotinylated ganciclovir: role of sodium-dependent multivitamin transporter expressed on retina

PURPOSE

The objective of this study was to investigate the role of sodium-dependent multiple vitamin transporter (SMVT) on Biotin-Ganciclovir (biotin-GCV) uptake on both human retinal pigmented epithelium cell line (ARPE-19) and rabbit retina. Study also aims to delineate the vitreal pharmacokinetics of biotin-GCV.

METHOD

ARPE-19 was employed to study the in vitro uptake experiments. New Zealand white albino rabbits were used to study in vivo retinal uptake and vitreal pharmacokinetics following intravitreal administration of biotin-GCV. In vitro uptake kinetics of [3H] biotin was determined at various initial concentrations. Competitive inhibition studies were conducted in the presence of unlabelled biotin, desthiobiotin, pantothenic acid, and lipoic acid. Various other uptake studies were performed to functionally characterize the transporter. To provide the molecular evidence of this transporter, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) studies were also conducted. In vivo retinal/choroidal uptake studies were carried out with New Zealand albino rabbits. Unconscious animal ocular microdialysis studies were performed in order to evaluate intravitreal pharmacokinetics of GCV and Biotin-GCV.

RESULTS

Uptake of [3H] biotin into ARPE-19 was linear over 7 min, and found to be saturable with K(m) of 138.25 muM and Vmax of 38.85 pmol/min/mg protein. Both pantothenic acid and lipoic acid decreased significantly in uptake of biotin in the concentration-dependent manner. Uptake of biotin into ARPE-19 was found to be temperature, energy, and Na+ dependent but Cl(-)independent. Further, RT-PCR studies identified a band exhibiting presence of hSMVT on ARPE-19. Biotin-GCV is recognized by SMVT system present on the ARPE-19 and rabbit retina. Vitreal Pharmacokinetics profile reveals that most of the parameters were not significantly different for GCV and Biotin-GCV. However, use of Biotin-GCV may result in sustain levels of regenerated GCV in vitreous.

CONCLUSIONS

SMVT was identified and functionally characterized on ARPE-19 cells. Further, Biotin-GCV shares this transport system. Vitreal pharmacokinetics of the conjugate was determined in unconscious rabbit model.

Effect of lactide/glycolide ratio on the in vitro release of ganciclovir and its lipophilic prodrug (GCV-monobutyrate) from PLGA microspheres

PURPOSE

The aim of this study is to investigate the roles of the lactide/glycolide ratio and prodrug derivatization on the in vitro release of ganciclovir and its lipophilic prodrug (GCV-monobutyrate) from PLGA microspheres for the treatment of cytomegalovirus (CMV) retinitis.

METHOD

Two grades of PLGA, a higher lactide content PLGA 6535 [d,l-lactide:glycolide=65:35, MW 45,000-75,000Da] and lower lactide content PLGA 5050 [d,l-lactide:glycolide=50:50, MW 45,000-75,000Da] were employed to prepare GCV loaded microspheres. The effect of lipophilic prodrug derivatization was investigated by converting GCV to GCV-monobutyrate (GCVMB). Microspheres were prepared by the oil-in-oil (O/O) solvent evaporation method and characterized in vitro, by studying their surface/internal morphology, entrapment efficiency, particle size, drug release, true density and glass transition temperature. In vitro release data were analyzed by a model equation to estimate various parameters of the drug release curves.

RESULTS

The O/O solvent evaporation method generated a high drug payload of up to 91%. Higher entrapment efficiencies were observed in the case of hydrophilic drug (GCV) relative to the lipophilic prodrug (GCVMB). Loosely bound or surface adsorbed drug/prodrug molecules may have resulted in the very short period (about 6h) of the initial burst phase in all types of microspheres. GCV loaded microspheres utilized more time to release 50% (T(50) value) of entrapped drug than GCVMB microspheres. T(50) values estimated for GCVMB were shorter than those for GCV from microspheres with similar lactide/glycolide ratios. Lactide content in PLGA did not significantly alter GCVMB release relative to GCV release. The proposed model equation effectively estimated the drug release parameters (R(2)>0.98) with all drug/prodrug-PLGA combinations. SEM pictures have revealed that although both GCV and GCVMB microspheres were spherical but internal morphology was different, with former having uniform and dense whereas later have porous structures. Corroborating with internal morphologies, results revealed that true densities of GCV microspheres were relatively greater than corresponding GCVMB microspheres.

CONCLUSION

The proposed method of preparation yields higher efficiency of drug entrapment for the hydrophilic drug. Prodrug entrapment into microspheres could result in longer residence time at the site of administration due to multiple processes involved in drug release at infected tissue. These processes include release from microspheres and enzymatic conversion of the prodrug to parent drug.

Biochemical and pharmacokinetic evaluation of a novel pyrimidine nucleoside nitric oxide donor as a potential anticancer/antiviral agent

The objective of this study was to determine the physiochemical, biochemical and pharmacokinetic properties of 5-iodo-3'-O-nitro-2'-deoxyuridine (INUdR), a novel prodrug releasing both nitric oxide (NO) and 5-iodo-2'-deoxyuridine. The INUdR partition coefficient (log P=1.12) was determined by both the shake-flask method and by calculation using Interactive Analysis Log P Program. In vitro binding of INUdR to bovine serum albumin (BSA) was estimated using an ultrafiltration method to be 65 to 77%, depending on the INUdR concentration. INUdR was stable in phosphate buffer (pH 7.4) and in water, at both ambient temperature and at 37 degrees C. INUdR was resistant to phosphorolysis when incubated with thymidine phosphorylase. Plasma, L-cysteine and glutathione catalyzed release of NO from INUdR, as determined using the Griess reaction. In all three systems, the release of NO by INUdR was equal to or greater than that of the reference drug isosorbide dinitrate. The pharmacokinetics of INUdR following single intravenous bolus and oral doses of INUdR (40 mg/kg) to male Sprague-Dawley rats were characterized by a short elimination half-life (T(1/2) 0.27 h), a large steady-state volume of distribution (V(ss) 0.89 l/kg) and high oral bioavailability (F=0.95). In conclusion, INUdR lipophilicity, shelf-stability, and resistance towards catabolic breakdown by thymidine phosphorylase, together with its non-spontaneous, yet considerable NO release, constitute favorable characteristics of a potential anticancer/antiviral agent.

Evaluation of [76Br]FBAU 3',5'-dibenzoate as a lipophilic prodrug for brain imaging

[76Br]FBAU is a potential PET tracer for assessing proliferation. This study proposes that [76Br]FBAU 3',5'-dibenzoate has higher blood-brain-barrier permeability than [76Br]FBAU itself and thus might be better suited for applications in the brain. The brain uptake indexes of the two compounds measured after carotid injection (29.6 +/- 13.9 for [76Br]FBAU 3',5'-dibenzoate, versus 10.0 +/- 8.7 for [76Br]FBAU) support this claim. Biodistribution study also showed that the brain accumulation of activity was higher in rats injected with [76Br]FBAU 3',5'-dibenzoate than with [76Br]FBAU (0.119+/-0.023 DUR at 1 h, versus 0.061 +/- 0.006). [76Br]FBAU 3',5'-dibenzoate was relatively stable in rat plasma, gradually being hydrolyzed to [76Br]FBAU exponentially with a calculated half-life of 0.8 h. DNA incorporation of [76Br]FBAU was also confirmed. The results presented support the hypothesis that the 3',5'-dibenzoate can act as a prodrug for FBAU and deliver more radiolabeled nucleoside to the brain.

Effect of mono- and di-acylation on the ocular disposition of ganciclovir: physicochemical properties, ocular bioreversion, and antiviral activity of short chain ester prodrugs

A series of short-chain carboxylic mono- and diesters of ganciclovir were synthesized in our laboratory. Physico-chemical properties, i.e., solubility (pH 4.2), partition coefficient in 1-octanol/phosphate buffer (pH 7.4), aqueous stability at various pH values, bioreversion kinetics in various ocular homogenates and effectiveness against various Herpes viruses in vitro were determined. The compounds exhibited a decrease in solubility as the ester length ascended with a corresponding increase in the octanol/buffer partition coefficient values. All of the prodrugs exhibit stability profiles typical of a carboxylic ester with maximum stability at neutral or slight acidic pH (4.0-7.0). Apparent first-order rate constants associated with prodrug to drug hydrolysis in the ocular homogenates varied depending on the size of the promoiety, lipophilicity of the compound, and the ocular tissue studied. The acetyl and butyryl mono and diesters were screened against various Herpes viruses. The monobutyrate ester of ganciclovir exhibits excellent activity against HSV-2 and VZV and provides a very high selectivity index against most of the viruses studied.

Prodrugs in nasal drug delivery

Prodrugs have been used to overcome poor solubility, insufficient stability, incomplete absorption across biological membranes and premature metabolism to active species. This review examines the importance of various physicochemical factors affecting nasal absorption of drugs. Novel trends in nasal prodrug development in the areas of targeted delivery to the CNS and selective targeting of the nutrient transporter system of the nasal mucosa have received considerable attention.

Ocular drug delivery in veterinary medicine

This paper provides a comprehensive overview of the various approaches currently used in the development of ocular drug delivery systems for the treatment of ocular diseases in animals. It is obvious from the literature that most of the products that are currently available are derived from human medicine without consideration given to the differences which exist between the anatomy and physiology of the eye of various animal species which ultimately affect product design and performance. As a result, many of the products for animal use seem in many circumstances inappropriate for animal care. The article deals with some aspects of eye anatomy and physiology of different animals, and then provides an overview of the most commonly encountered pathologies. The paper then discusses the currently available drug products and finally reviews new delivery concepts. Several hundred references are included in the paper and provide access to further information on the subject.

Synthesis of the dioleoyl derivative of iododeoxyuridine and its incorporation into reconstituted high density lipoprotein particles

We investigated the potential use of reconstituted HDL particles (NeoHDL) as a carrier for lipophilic (pro)drugs. The antiviral drug iododeoxyuridine (IDU) was used as model compound. [3H]-IDU was derivatized with two oleoyl residues to dioleoyl[3H]iododeoxyuridine ([3H]IDU-Ol2), and the lipophilic prodrug was incorporated into NeoHDL by cosonication of [3H]IDU-Ol2 with lipids and HDL apoproteins. NeoHDL particles with the same density, size, and electrophoretic mobility as native HDL were obtained, which contained 7.3 +/- 0.8% (w/w) [3H]IDU-Ol2 (about 30 molecules of prodrug per particle). NeoHDL-associated [3H]IDU-Ol2 was stable during 2 h of incubation with human plasma; the prodrug was not appreciably hydrolyzed, nor exchanged with LDL. After intravenous injection of [3H]-IDU-Ol2-loaded 125I-NeoHDL into rats, [3H]IDU-Ol2 disappeared more rapidly from the circulation than the 125I-apoproteins (78.0 +/- 8.0% vs 30.1 +/- 4.5% of the dose cleared from plasma in 60 min, respectively). The hepatic association of the prodrug was higher than that of the apoproteins (21.6 +/- 0.5 vs 5.2 +/- 1.0% of the dose at 10 min after injection, respectively). As selective clearance and uptake of lipid esters is also observed with native HDL, this suggests that, in vivo, prodrug-loaded NeoHDL may be subject to physiological HDL-specific processing. Lactosylated [3H]IDU-Ol2-loaded 125I-NeoHDL, which contains galactose residues that can be recognized by galactose receptors on parenchymal liver cells, was rapidly cleared from plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

The physicochemical properties, plasma enzymatic hydrolysis, and nasal absorption of acyclovir and its 2'-ester prodrugs

A series of 2'-(O-acyl) derivatives of 9-(2-hydroxyethoxymethyl)guanine (acyclovir) was synthesized by acid anhydride esterification. Aqueous solubilities in isotonic phosphate buffer (pH 7.4), partition coefficients in 1-octanol/phosphate buffer, and hydrolysis kinetics in rat plasma were determined. The ester prodrugs showed consistent increases in lipophilicity with corresponding decreases in aqueous solubility as a function of side-chain length. The bioconversion kinetics of the prodrugs appear to depend on both the apolar and the steric nature of the acyl substituents. When perfused through the rat nasal cavity using the in situ perfusion technique, acyclovir showed no measurable loss from the perfusate. Nasal uptake of acyclovir prodrugs, on the other hand, were moderately improved. Furthermore, the extent of nasal absorption appears to depend on the lipophilicity of the prodrugs in the descending order hexanoate > valerate > pivalate > butyrate. Simultaneous prodrug cleavage by nasal carboxylesterase was also noted in the case of hexanoate.

Effect of acylation on the ocular disposition of acyclovir. II: Corneal permeability and anti-HSV 1 activity of 2'-esters in rabbit epithelial keratitis

In vitro permeability studies were conducted on isolated rabbit corneal membranes using aliphatic acyl esters of acyclovir to determine the effect of lipophilicity on the transcorneal diffusion. Corneal membrane permeability coefficients increased with increasing lipophilicity of the straight chain aliphatic esters. The branch chain ester, acyclovir isobutyrate, displayed an anomalously low corneal permeability when compared to acyclovir esters having similar molecular size and 1-octanol/water partition coefficient. In vivo corneal penetration studies were conducted on unanesthetized rabbits. The aqueous humor concentrations of acyclovir and the ester prodrugs were measured at twenty five minutes after the topical instillation of an aqueous solution of the appropriate compound. The concentration of acyclovir in the aqueous humor increased with increasing 1-octanol/water partition coefficient. The lipophilic modification was shown to have a greater effect on increasing productive corneal absorption than the precorneal loss pathways. The effectiveness of acyclovir butyrate as a treatment for primary herpetic keratitis was evaluated in the McKrae strain infected rabbit model. The compound did not lose activity due to the esterification of the 2' hydroxyl group.

Enhanced delivery of 5-iodo-2'-deoxyuridine to the brain parenchyma

5'-Ester derivatives of 5-iodo-2'-deoxyuridine (IDU) with varying degrees of lipophilicity were examined to evaluate the effectiveness of lipophilic ester prodrugs for enhanced and sustained delivery of IDU to the brain parenchyma. Approximately 1.0% (1.0 +/- 0.19; n = 4) of the total radioactivity was found in the brain at 30 min following intravenous administration of the lipophilic benzoyl-5'-ester of 125I-labeled IDU, whereas IDU per se yielded only 0.01% (0.01 +/- 0.06; n = 4). Since the IDU 5'-esters generated significantly higher levels of IDU in the brain, an HPLC analysis of IDU in the presence of 5'-esters and the metabolite 5-iodouracil was developed to characterize IDU uptake in the brain. The drug was detected at levels of 6.6 and 9.5 micrograms/g of brain tissue at 3 hr following intravenous administration of valeryl and benzoyl IDU, respectively, at a dose level of 40 mg/kg IDU equivalent each. IDU, on the other hand, when injected at a similar dose level, produced concentration levels below 0.01 micrograms/g of brain tissue, which was too low to be detected accurately by the HPLC assay. These results suggest that the 5'-ester derivatives cross the blood-brain barrier effectively and generate significantly higher brain levels of the parent drug in the brain parenchyma. The regenerated hydrophilic drug because of its polarity is "locked in" the brain and is subsequently metabolized by pyrimidine phosphorylase to 5-iodouracil.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain parenchymal metabolism of 5-iodo-2'-deoxyuridine and 5'-ester prodrugs

In an attempt to generate derivatives of 5-iodo-2'-deoxyuridine (IDU) with enhanced blood-brain barrier (BBB) permeability, a series of 5' ester prodrugs of IDU was synthesized and their metabolism studied in rat brain homogenate and its different subcellular fractions. The rate of hydrolysis was dependent on the steric and polar nature of the ester substituent. Ester hydrolyzing activities were associated primarily with the cytosolic fraction and were due mainly to the presence of cholinesterases as confirmed by inhibition experiments performed with different esterase inhibitors. The metabolism of IDU to 5-iodouracil (5-IU) by the cytosolic fraction, in the presence and absence of specific pyrimidine nucleoside phosphorylase inhibitors, also suggests that there are two specific enzyme systems catalyzing two different metabolic processes. IDU 5'-esters competitively inhibit the metabolism of IDU and the inhibitory effect depends on the affinity of a particular ester toward the enzyme and also on the rate by which the ester itself undergoes hydrolysis. In the absence of any 5'-ester, 95% IDU was metabolized within 6 hr. However, in the presence of an eightfold molar excess of butyryl-IDU, the hydrolysis of IDU was completely inhibited over a 6-hr time period.

Effects of 5'-ester modification on the physicochemical properties and plasma protein binding of 5-iodo-2'-deoxyuridine

A series of 5'-(O-acyl and O-benzoyl) derivatives of 5-iodo-2'-deoxyuridine (IDU) was synthesized by direct acylation of the parent nucleoside in a pyridine-N,N'-dimethylformamide mixture (1:1). Aqueous solubilities in phosphate buffer (pH 7.4), partition coefficients in 1-octanol/phosphate buffer (pH 7.4), plasma protein binding properties, and plasma reversion kinetics of these potential prodrugs were evaluated. The esters showed an expected increase in lipophilicity with a corresponding decrease in aqueous solubility relative to the parent compound. The association constants (Ka) with albumin also exhibited a good linear correlation with the lipophilicity of the compounds. However, the reversion rate constants in plasma varied with the steric and polar nature of the acyl or benzoyl substituent.

Carboxylic ester hydrolase activity in hairless and athymic nude mouse skin

The carboxylic ester hydrolase activity was compared in athymic nude mouse skin and hairless mouse skin with respect to hydrolytic ability, heat inactivation, pH optima, and substrate specificity. Five aliphatic 5'-esters of 5-iodo-2'-deoxyuridine (IDU) were incubated with skin homogenate preparations, and the effect of linear chain length and branching of the ester substituent on hydrolysis rate was evaluated. The ester hydrolase activity was three times higher in athymic mouse skin relative to hairless variety. In both mice skin preparations maximum hydrolysis rates were obtained with the valeryl ester followed by butryl, isobutyryl, propionyl and pivaloyl derivatives. Kinetic studies, however, revealed that higher ester hydrolase activity (Vmax) in athymic mouse skin is also associated with higher Km values, while the carboxylic ester hydrolases from these two different strains of mice have similar biochemical properties with respect to heat inactivation and pH optima. Athymic mouse skin resembled hairless mice skin in terms of cholinesterase content. A significant fraction (70-80%) of ester hydrolyzing activities in both strains of mice skin resulted from cholinesterases (true and/or pseudo). The remaining activity was attributed to different ester cleaving enzymes in the two strains of mice. Carbonic anhydrases and arylesterases contributed to the ester hydrolyzing activity of the athymic and normal hairless mice skins, respectively. Product inhibition by the regenerated hydrolytic product, free IDU, was also noticed which resulted in incomplete conversion of rapidly hydrolyzable 5'-esters such as the valeryl and butyryl derivatives.

Prodrugs of 5-iodo-2'-deoxyuridine for enhanced ocular transport

Problems associated with the use of 5-iodo-2'-deoxyuridine (IDU) in the treatment of herpes simplex keratitis can be attributed largely to the polar nature of IDU resulting in its poor permeability across the lipoidal epithelial layer of the corneal membrane. Five aliphatic 5'-esters of IDU were synthesized and evaluated as prodrugs for potential use in the treatment of deep ocular infections such as stromal keratitis, iritis, and even retinitis. A parabolic relationship between in vitro corneal membrane permeability and carbon chain length of prodrugs is evident. For a given prodrug, enzymatic hydrolysis proceeded most readily in iris-ciliary body, followed by cornea and aqueous humor. An increase in carbon chain length made the prodrugs more enzymatically labile but more resistant to chemical hydrolysis at pH 7.4 and 34 degrees C. The 5'-butyryl ester of IDU exhibited an approximately fourfold increase in aqueous humor IDU concentration relative to IDU at 25 min following instillation of 25-microliters 5 mM solutions.