Ocular pharmacokinetics of acyclovir amino acid ester prodrugs in the anterior chamber: evaluation of their utility in treating ocular HSV infections

PURPOSE: To evaluate in vivo corneal absorption of the amino acid prodrugs of acyclovir (ACV) using a topical well model and microdialysis in rabbits. METHODS: Stability of L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) prodrugs was evaluated in various ocular tissues. Dose-dependent toxicity of these prodrugs was also examined in rabbit primary corneal epithelial cell culture (rPCEC) using 96-well based cell proliferation assay. In vivo ocular bioavailability of these compounds was also evaluated with a combination of topical well infusion and aqueous humor microdialysis techniques. RESULTS: Among the amino acid ester prodrugs, SACV was most stable in aqueous humor. Enzymatic degradation of EACV was the least compared to all other prodrugs. Cellular toxicity of all the prodrugs was significantly less compared to trifluorothymidine (TFT) at 5mM. Absorption rate constants of all the compounds were found to be lower than the elimination rate constants. All the prodrugs showed similar terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately two-fold increase in area under the curve (AUC) relative to ACV (p<0.05). Clast (concentration at the last time point) of SACV was observed to be 8+/-2.6microM in aqueous humor which is two and three times higher than VACV and ACV, respectively. CONCLUSIONS: Amino acid ester prodrugs of ACV were absorbed through the cornea at varying rates (ka) thereby leading to varying extents (AUC). The amino acid ester prodrug, SACV owing to its enhanced stability, comparable AUC and high concentration at last time point (Clast) seems to be a promising candidate for the treatment of ocular HSV infections.     

三环苦参酯类前药的设计、合成及其药动学研究

目的设计合成全新三环苦参酯类前药衍生物,旨在改善其体内药动学行为。方法以苦参碱为原料,经碱水解开环、羧基成酯、12N-苄基取代、甲酯还原得到三环苦参醇母体化合物。采用两种前药制备策略,合成11个酯类前药和2个含有二硫键的前药化合物。选用SD大鼠测定化合物的口服药动学(PK)参数。结果与结论合成了13个未见文献报道的全新结构的三环苦参酯类前药,其结构经HR-MS、~1H-NMR、~(13)C-NMR谱确证。药动学研究表明,不同类型前药的主要药动学参数均低于原药,cLogP值在合理范围内(0~4)的原药可能不适合使用前药策略来改善其药动学行为。     

Synthesis, metabolism and cellular permeability of enzymatically stable dipeptide prodrugs of acyclovir

The objective of this study was to synthesize and evaluate novel enzymatically stable dipeptide prodrugs for improved absorption of acyclovir. l-Valine-l-valine-acyclovir (LLACV), l-valine-d-valine-acyclovir (LDACV), d-valine-l-valine-acyclovir (DLACV) and d-valine-d-valine-acyclovir (DDACV) were successfully synthesized. The uptake and transport studies were conducted on a Caco-2 cell line. Buffer stability and metabolism of the prodrugs in Caco-2, rat intestine and liver homogenates were studied. Structure and purity of the all compounds were confirmed with LC-MS/MS and NMR spectroscopy. Uptake and transport of [(3)H] glycylsarcosine was inhibited by all prodrugs except DDACV. DLACV and DDACV exhibited no measurable degradation in Caco-2 homogenate. Except DDACV other three prodrugs were hydrolyzed in rat intestine and liver homogenates. The order of permeability across Caco-2 was LDACV>LLACV>DDACV>DLACV. A linear correlation between the amount of prodrug transported and over all permeability of acyclovir was established. This study shows that the incorporation of one d-valine in a dipeptide did not abolish its affinity towards peptide transporters (PEPT). Moreover, it enhanced enzymatic stability of prodrug to a certain extent depending on the position in a dipeptide conjugate. This strategy improved both the cellular permeability and the amount of intact prodrug transported which would enable targeting the nutrient transporters at blood ocular barrier (BOB).     

微渗析技术研究依诺沙星眼用缓释凝胶剂兔眼内药动学

目的:研究依诺沙星家兔眼内药动学.方法:采用微渗析技术进行依诺沙星眼用缓释凝胶剂在体药动学的研究,数据经非隔室模型处理,以方差分析和双单侧t检验评价各参数.结果:眼用缓释凝胶剂的药动学参数为:AUC=(1.615±0.900)μg·h·mL-1,Cmax=(2.726±1.790)μg·mL-1,Tmax=(0.33±0.24)h,Ke=(0.270±0.140);滴眼液的药动学参数为:AUC=(0.414±0.210)μg·h·mL-1,Cmax=(0.689±0.890)μg·mL-1,Tmax=(0.25±0.10)h,Ke=(0.548±0.500).结论:眼用缓释凝胶剂明显提高了药物在眼内的生物利用度,并且延长了作用时间,达到了缓释制剂的设计要求.     

Validation of an ocular microdialysis technique in rabbits with permanently implanted vitreous probes: systemic and intravitreal pharmacokinetics of fluorescein

The purpose of this work is to validate a novel ocular microdialysis sampling technique in rabbits with permanently implanted vitreous probes. This objective is achieved by studying the vitreous pharmacokinetics of fluorescein following systemic and intravitreal administration. The rabbits were divided into two groups (groups I and II) based on whether or not they were allowed a recovery period following surgical implantation of probes. The integrity of the blood-retinal barrier was determined by the vitreal protein concentrations and the fluorescein permeability index. Vitreal protein concentrations returned to baseline 48 h after probe implantation and therefore experiments were conducted 72 h post-implantation of probes in rabbits where recovery period was allowed. The permeability indices for fluorescein after systemic administration in group I (without recovery period) and group II (with recovery period) indicated that the integrity of the blood-retinal barrier was maintained and were found out to be 0.55 +/- 0.27 and 0.71 +/- 0.38%, respectively, for the vitreous chamber. Following microdialysis probe implantation in the group II rabbits, the blood-retinal barrier integrity was not compromised. A novel microdialysis technique in rabbits with permanently implanted probes for studying the pharmacokinetics of posterior segment has been developed and characterized.     

Nonpolymeric nanoassemblies for ocular administration of acyclovir: Pharmacokinetic evaluation in rabbits

The aim of this study was to increase bioavailability of the antiviral drug acyclovir (ACV) when administered by the ocular route. For this purpose, a new lipophilic derivative of acyclovir was synthesized, both possessing greater lipophilicity and providing the formation of a homogeneous water dispersion with higher amount of ACV than the aqueous solution of the parent drug. This was done by chemically linking acyclovir to the isoprenoid chain of squalene, obtaining 4′-trisnorsqualenoylacyclovir (SQACV), in which squalene is covalently coupled to the 4′-hydroxy group of acyclovir. This new prodrug was then formulated as nonpolymeric nanoassemblies through nanoprecipitation; the resulting particles were characterized in terms of mean diameter, zeta potential, and stability. The pharmacokinetic profile of the prodrug in the tear fluid and in the aqueous humor of rabbits was evaluated and compared to that of the parent drug.Data showed that SQACV nanoassemblies increased the amount of ACV in the aqueous humor of rabbits compared to free ACV solution. This new amphiphilic prodrug of acyclovir is a very promising tool to increase the ocular bioavailability of the parent drug.     

Transport of acyclovir ester prodrugs through rabbit cornea and SIRC-rabbit corneal epithelial cell line.

The purpose of this study is to assess the permeability of acyclovir (ACV) prodrugs through the rabbit corneal cell line (SIRC) as well as the cornea, and characterize the SIRC cell line for transport and metabolism studies of ester prodrugs. Prodrug derivatization of an acycloguanosine antiviral agent, acyclovir, was employed to improve its permeability across the cornea. New Zealand albino rabbits were used as an animal model for corneal studies. The SIRC cell line grown on polyester membranes was used for transport of these prodrugs. SIRC cells grown on the membrane support for 10 days developed four to six layers of epithelial cells, and this is comparable to the normal rabbit corneal epithelial layer. Transport experiments were conducted across the rabbit cornea and confluent SIRC cells using side-by-side diffusion-cell apparatus. Enzymatic hydrolysis of these compounds was evaluated in SIRC cell lysates. Appropriate reversed phase HPLC method(s) were employed for quantitation of both the prodrug and ACV simultaneously. Corneal permeabilities of some of these prodrugs (Malonyl ACV and Acetyl ACV) were higher relative to ACV. The SIRC cell line permeability values of all the prodrugs were higher compared to that of the intact cornea. The total amount of ACV-prodrugs transported, i.e., unhydrolyzed prodrugs and regenerated ACV, across the SIRC cell line was more relative to ACV. Hydrolytic studies in the SIRC cell line homogenate demonstrated the bioreversion potential of the prodrugs and the presence of enzymes, particularly the cholinesterase in the SIRC cell line. It may be concluded that the SIRC cell line is leakier compared to the cornea. Keeping in mind the limitations, the SIRC cell line after further characterization may be used for transport and metabolism studies of ester prodrugs.     

Chemical stability, enzymatic hydrolysis, and nasal uptake of amino acid ester prodrugs of acyclovir

The objective of this work was to improve nasal absorption of relatively impermeable small drug molecules via an amino acid prodrug approach. Acyclovir was selected as a model drug. L-Aspartate beta-ester, L-lysyl, and L-phenylalanyl esters of acyclovir were synthesized to investigate their effectiveness in enhancing nasal absorption of acyclovir. A stability study was conducted in phosphate buffer under various pH conditions at 25 and 37 degrees C. Enzymatic hydrolysis in rat nasal washings and plasma was conducted at 37 degrees C. A rat in situ nasal perfusion technique was utilized in this investigation to examine the rate and extent of nasal absorption of amino acid prodrugs. The remaining analyte concentrations in the nasal perfusate were quantitated by reversed-phase high-performance liquid chromatography. The results revealed that the L-lysyl and L-phenylalanyl esters were less stable than L-aspartate beta-ester. The stability of all three esters decreased with increasing pH and temperature. L-phenylalanyl ester is highly susceptible to plasma esterases, with an in vitro half-life 1.33 min. The rat in situ nasal perfusion study revealed that the extent of nasal absorption of acyclovir, L-lysyl and L-phenylalanyl esters was not significant (p < 1%). L-Aspartate beta-ester was absorbed to the extent of approximately 8% over 90 min of perfusion at an initial drug concentration of 100 microM. Nasal absorption of L-aspartate beta-ester of acyclovir was inhibited by L-asparagine but not by a dipeptide glycylsarcosine (Gly-Sar). The enhancement of acyclovir nasal absorption from the L-aspartate beta-ester prodrug suggests that nasal uptake of this prodrug probably involves an active transport system.     

Alkoxycarbonyloxyethyl ester prodrugs of FR900098 with improved in vivo antimalarial activity

FR900098 represents a derivative of the new antimalarial drug fosmidomycin with enhanced activity. The mechanism of action is the inhibition of the 1-desoxy-D-xylulose 5-phosphate (DOXP) reductoisomerase, an essential enzyme of the mevalonate independent pathway of isoprenoid biosynthesis. Prodrugs with increased oral activity in mice infected with the rodent malaria parasite Plasmodium vinckei were obtained by masking the phosphonate moiety of FR900098 as alkoxycarbonyloxyethyl esters.     

Kinetics of degradation and oil solubility of ester prodrugs of a model dipeptide (Gly-Phe)

Oil-based depot formulations may constitute a future delivery method for small peptides. Thus, a requirement is attainment of sufficient oil solubility for such active compounds. A model dipeptide (Gly-Phe) has been converted into lipophilic prodrugs by esterification at the C-terminal carboxylic acid group. The decomposition kinetics of octyl ester of Gly-Phe (IV) has been investigated at pH 7.4 (37 degrees C) and IV was shown to degrade by first-order kinetics via two parallel pathways (1) intramolecular aminolysis resulting in formation of a 2,5-diketopiperazine and (2) hydrolysis of the ester bond producing the dipeptide. The cyclisation reaction was dominating in the decomposition of methyl (II) butyl (III) octyl (IV) decyl (V) and dodecyl (VI) esters of Gly-Phe at pH 7.4. However, this degradation pathway was almost negligible for pH below 6. During degradation of the dipeptide esters in 80% human plasma pH 7.4 (37 degrees C) a minimal amount of cyclo(-Gly-Phe) was formed. A faster degradation of the esters in 80% human plasma pH 7.4 compared to those in aqueous solution pH 7.4 was suggested to be due to fast cleavage of the peptide bond. Low oil solubilities for Gly-Phe and the hydrochlorides of the dipeptide esters III and VI were observed. Although the solubility of Gly-Phe in oil solutions was enhanced by hydrophobic ion pairing with sodium decyl sulfonate the oil solubility was still less than 1 mg Gly-Phe/ml. By addition of a solubiliser, 10% N,N-dimethylacetamide (DMA), to Viscoleo the solubility of the HIP complexes increased significantly. The present study indicates that sufficient oil solubility might only be obtained for relatively small peptides by using the prodrug approach in combination with solubility enhancing organic solvents like DMA.     

Phenytoin prodrugs V: In vivo evaluation of some water-soluble phenytoin prodrugs in dogs

Phenytoin bioavailability was evaluated in beagle dogs after oral and intravenous administrations of sodium phenytoin and two amino acyl esters and a disodium phosphate ester of 3-(hydroxymethyl)phenytoin (three prodrugs of phenytoin). Phenytoin displayed nonlinear pharmacokinetics in the dogs, complicating the determination of the absolute bioavailability of phenytoin from sodium phenytoin and the prodrugs. All three prodrugs essentially released phenytoin after intravenous administration in a quantitative manner, and all gave plasma levels of phenytoin after oral administration greater than those found after administration of sodium phenytoin. Based on the behavior in dogs and the earlier determination of the physicochemical properties of the prodrugs, it was concluded that one of the amino acyl esters, 3-(hydroxymethyl)-5,5-diphenylhydantoin N,N-dimethylglycine ester methanesulfonate, would be the most useful prodrug for oral administration, while 3-(hydroxymethyl)-5,5-diphenylhydantoin disodium phosphate ester would be the most useful for parenteral administration.     

Skin permeation and ex vivo skin metabolism of O-acyl haloperidol ester prodrugs

Ethyl (HE), propyl (HP), butyl (HB), octyl (HO) and decyl (HD) O-acyl esters of haloperidol (HA) were evaluated for permeation across full-thickness human and guinea pig skin. The inclusion of 0.5mgmL(-1) cetrimide as a receptor phase solubilising agent did not significantly alter the barrier properties of the membranes. The permeation of the parent drug, HA, across guinea pig skin was found to be greater than that of its derivatives. Prodrug hydrolysis by cutaneous esterases was minimal. The permeation of HE, HP and HB across freshly excised guinea pig skin was subsequently investigated, however, prodrug hydrolysis remained low. Hydrolysis studies using a skin extract revealed only limited prodrug metabolism. However, in the presence of a liver extract, hydrolysis of all prodrugs was rapid. It was proposed that GGGX esterases, required for the hydrolysis of tertiary esters, were not present at a sufficiently high concentration within the skin for substantial prodrug hydrolysis to occur. This does not necessarily detract from the system as post-transdermal delivery liberation of HA in vivo is an equally useful mode for delivering this drug to the systemic circulation.     

Study of ocular pharmacokinetics of in situ gel system for S(-)-satropane evaluated by microdialysis

S(-)-Satropane is currently being developed to in situ forming ophthalmic gel, a new ophthalmic delivery system, for the treatment of glaucoma. To evaluate the pharmacokinetic profiles of S(-)-satropane, the microdialysis method was employed. The concentration of S(-)-satropane in dialysates was measured by using liquid chromatography/tandem mass spectrometry (LC-MS/MS). Unlike the common solution prepared in normal saline, in which the level of S(-)-satropane in aqueous humor increased rapidly after instillation and reached the maximal level (C(max) of 1.508+/-0.297 microg ml(-1)) within 1h, S(-)-satropane exhibited 3.2-fold greater C(max) and 2.2-fold greater AUC(0-3h) (p<0.05) in the in situ forming gel. The results showed that the in situ forming gel system could improve the ocular bioavailability of S(-)-satropane.     

Phenytoin prodrugs VI: In vivo evaluation of a phosphate ester prodrug of phenytoin after parenteral administration to rats

Tissue damage caused by subcutaneous and intramuscular administration of three phenytoin prodrugs to rats was assessed. Since two of the prodrugs caused significant irritation, only 3-(hydroxymethyl)-5,5-diphenylhydantoin disodium phosphate ester might be useful as a nonirritant phenytoin prodrug suitable for parenteral administration. To confirm the release of phenytoin from this prodrug, phenytoin availability after intramuscular and intravenous administrations of the phosphate prodrug was evaluated in rats and compared with sodium phenytoin. The prodrug quantitatively released phenytoin after intravenous administration, and phenytoin levels from intramuscular administration of the prodrug were far superior to those generated from similarly administered sodium phenytoin. Based on this and earlier studies, it was concluded that this prodrug should be further assessed as a parenteral form of phenytoin.     

Disposition kinetics of a dipeptide ester prodrug of acyclovir and its metabolites following intravenous and oral administrations in rat

The objective of this work was to study the disposition kinetics of valine-valine–acyclovir (VVACV), a dipeptide ester prodrug of acyclovir following intravenous and oral administrations in rat. A validated LC-MS/MS analytical method was developed for the analysis VVACV, Valine-Acyclovir (VACV), and Acyclovir (ACV) using a linear Ion Trap Quadrupole. ACV was administered orally for comparison purpose. In the VVACV group, both blood and urine samples and in the ACV group only blood samples were collected. All the samples were analyzed using LC-MS/MS. The LLOQ for ACV, VACV, and VVACV were 10, 10, and 50?ng/ml, respectively. Relevant pharmacokinetic parameters were obtained by non-compartmental analyses of data with WinNonlin. Following i.v. administration of VVACV, AUC_0-inf (min*μM) values for VVACV, VACV, and ACV were 55.06, 106, and 466.96, respectively. The AUC obtained after oral administration of ACV was 178.8. However, following oral administration of VVACV, AUC_0-inf values for VACV and ACV were 89.28 and 810.77, respectively. Thus the exposure of ACV obtained following oral administration of VVACV was almost 6-fold higher than ACV. This preclinical pharmacokinetic data revealed that VVACV has certainly improved the oral bioavailability of ACV and is an effective prodrug for oral delivery of ACV.     

Disposition kinetics of a dipeptide ester prodrug of acyclovir and its metabolites following intravenous and oral administrations in rat

The objective of this work was to study the disposition kinetics of valine-valine-acyclovir (VVACV), a dipeptide ester prodrug of acyclovir following intravenous and oral administrations in rat. A validated LC-MS/MS analytical method was developed for the analysis VVACV, Valine-Acyclovir (VACV), and Acyclovir (ACV) using a linear Ion Trap Quadrupole. ACV was administered orally for comparison purpose. In the VVACV group, both blood and urine samples and in the ACV group only blood samples were collected. All the samples were analyzed using LC-MS/MS. The LLOQ for ACV, VACV, and VVACV were 10, 10, and 50 ng/ml, respectively. Relevant pharmacokinetic parameters were obtained by non-compartmental analyses of data with WinNonlin. Following i.v. administration of VVACV, AUC(0-inf) (min*μM) values for VVACV, VACV, and ACV were 55.06, 106, and 466.96, respectively. The AUC obtained after oral administration of ACV was 178.8. However, following oral administration of VVACV, AUC(0-inf) values for VACV and ACV were 89.28 and 810.77, respectively. Thus the exposure of ACV obtained following oral administration of VVACV was almost 6-fold higher than ACV. This preclinical pharmacokinetic data revealed that VVACV has certainly improved the oral bioavailability of ACV and is an effective prodrug for oral delivery of ACV.     

盐酸阿扎司琼在家兔体内的药代动力学研究

目的:进行盐酸阿扎司琼在家兔体内的药代动力学研究。方法:采用 HPLC 法测定家兔经腹腔注射盐酸阿扎司琼氯化钠注射液(5 mg·kg~(-1)体重)后在家兔体内的血药浓度,得出药-时曲线图及相关药动学参数。结果:盐酸阿扎司琼在家兔体内过程符合二室模型。结论:家兔单剂量腹腔注射盐酸阿扎司琼氯化钠注射液后,能迅速被吸收,达峰时间短,吸收速度快,分布快,消除速度快。本研究结果可为本品临床合理用药提供参考,并为盐酸阿扎司琼的相关剂型研究提供药物体内研究资料。     

In vivo microdialysis technique on rat myocardium]

The microdialysis procedure is a technique that has been established for some years. The heart, however, has several difficulties in the application of this technique. The heart is the beating in vivo, in contrast to other organs. I developed the flexibly mounted microdialysis technique, which involves the synchronized movement of the tip of the probe with the beating heart to reduce tissue injury. With this technique, it is feasible to make stable and long-term measurements of interstitial biological substances. By employing the flexibly mounted microdialysis technique, I will demonstrate that the monitoring of free radical generation and the level of adenosine measured during AMP perfusion that gives an index of the activity of ecto-5'-nucleotidase in the tissue. In the future, this technique will have versatile applications for useful studies to elucidate the actual mechanism in the pathogenesis of heart disorders.     

In vitro and in vivo study of water-soluble prodrugs of dexanabinol

Trialkylammonium acetoxymethyl esters of dexanabinol were synthesized and evaluated as water-soluble prodrugs. Syntheses were performed by conventional methods; solubility in water and stability in buffers and human plasma were determined by HPLC, and in vivo tissue distribution studies were performed in a rat model. Most of the new derivatives were soluble in water (approximately 50 mg/mL). They were relatively stable in water, while rapidly hydrolyzed in human plasma. Distribution studies indicated that peak concentrations of drug both in blood (30 microg/mL) and brain (2 microg/mL) were rapidly (5 min) achieved after iv administration of a selected prodrug to rats. The blood concentration decreased faster than brain levels which were detectable even after 24 h. Some of the examined esters could be further developed as water soluble prodrugs of dexanabinol.