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

Fragment-Based Discovery of Azocyclic Alkyl Naphthalenesulfonamides as Keap1-Nrf2 Inhibitors for Acute Lung Injury Treatment

Blocking the Kelch-like epichlorohydrin-related protein 1 (Keap1)-nuclear factor-erythroid 2 related factor 2 (Nrf2) pathway is a promising strategy to alleviate acute lung injury (ALI). A naphthalensulfonamide NXPZ-2, targeting Keap1-Nrf2 interaction to release Nrf2, was confirmed to exhibit significant anti-inflammatory activities, however, accompanying nonideal solubility and PK profiles. To further improve the properties, twenty-nine novel naphthalenesulfonamide derivatives were designed by a fragment-based strategy. Among them, compound 10u with a (R)-azetidine group displayed the highest PPI inhibitory activity (K_D2 = 0.22 μM). The hydrochloric acid form of 10u exhibited a 9-fold improvement on water solubility (S = 484 μg/mL, pH = 7.0) compared to NXPZ-2 (S = 55 μg/mL, pH = 7.0). It could significantly reduce LPS-induced lung oxidative damages and inflammations in vitro and in vivo. Furthermore, a satisfactory pharmacokinetic property was revealed. In conclusion, the novel azetidine-containing naphthalenesulfonamide represents a promising drug candidate for Keap1-targeting ALI treatment.

Amino Acids in the Development of Prodrugs

Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.

Synthesis and biological evaluation of liguzinediol mono- and dual ester prodrugs as promising inotropic agents

The potent positive inotropic effect, together with the relatively low safety risk of liguzinediol (LZDO), relative to currently available inotropic drugs, has prompted us to intensively research and develop LZDO as a potent positive inotropic agent. In this study, to obtain LZDO alternatives for oral chronic administration, a series of long-chain fatty carboxylic mono- and dual-esters of LZDO were synthesized, and preliminarily evaluated for physicochemical properties and bioconversion. Enhanced lipophilic properties and decreased solubility of the prodrugs were observed as the side chain length increased. All esters showed conspicuous chemical stability in phosphate buffer (pH 7.4). Moreover, the enzymatic hydrolysis of esters in human plasma and human liver microsomes confirmed that the majority of esters were converted to LZDO, with release profiles that varied due to the size and structure of the side chain. In vivo pharmacokinetic studies following oral administration of monopivaloyl (M5), monodecyl (M10) and monododecyl (M12) esters demonstrated the evidently extended half-lives relative to LZDO dosed alone. In particular the monopivaloyl ester M5 exhibited an optimal pharmacokinetic profile with appropriate physiochemical characteristics.

Calculation procedures and HPLC method for analysis of the lipophilicity of acyclovir esters

Acyclovir (ACV) belongs to a class of drugs with low bioavailability. Selected ACV esters including acetyl (Ac-), isobutyryl (iBut-), pivaloyl (Piv-), ethoxycarbonyl (Etc-) and nicotinoyl (Nic-) were synthesized, and their lipophilicity was determined by the high-performance liquid chromatography (HPLC) RP method. Statistical analyses of the comparative values of log P and clog P were carried out using computational methods. It was proved that the AC log P algorithm can be useful for the analysis of these compounds and has a statistically justified application in the assessment of the quantitative structure-activity relationship. Moreover, the lipophilicity determined by the HPLC method appears as follows: ACV < Ac- < Nic- < Etc- < iBut- < Piv-.

New green synthesis and formulations of acyclovir prodrugs

Different green synthesis of alkyl esters of acyclovir (acyclovir prodrugs) is described. Hexanoic, decanoic, dodecanoic and tetradecanoic acyclovir esters were synthesized reacting acyclovir and the respective acid anhydride in dimethyl sulfoxide (DMSO), in solvents from renewable sources and without solvent (T=30 °C). Yields in prodrugs after 10 min of reaction were >95% using DMSO as solvent. The purification methodology was very simple, shorter and greener than previously described. The biosolvent, N,N-dimethylamide of decanoic acid, let us to obtain >95% yield at 24 h. This oily biosolvent is not dermotoxic and the reaction crude can directly be used in topic formulations. Syntheses without solvent proceeded successfully for acyclovir esters. Indeed, dodecanoate and tetradecanoate yielding >98% conversion of reactants in 30 min. In spite of requiring mild temperature (65 °C), substrate molar ratios were lowered to 1 : 1, thus conducing to a more efficient use of raw materials. The synthetic procedures were scaled up to a 300 g batch (yield 98-99% isolated ester). These esters can be used as acyclovir prodrugs in topic formulations. The esters release from an oil/water micro-emulsion and a hydrogel formulation were tested with good results.

Effect of chitosan glutamate, carbomer 974P, and EDTA on the in vitro Caco-2 permeability and oral pharmacokinetic profile of acyclovir in rats

BACKGROUND

Chitosan glutamate and polyacrylic acid (e.g., carbomer 974P) are known to modulate the tight junctions in the intestinal wall and increase permeability and blood exposure of drugs absorbed orally by the paracellular route.

AIM

To assess the impact of chitosan glutamate and carbomer 974P on the absorption of paracellularly absorbed model drug, acyclovir, in vitro and in rat in vivo.

METHODS

The influence of chitosan glutamate and carbomer 974P (alone and in combination with EDTA-Na2) on the in vitro Caco-2 permeability and oral pharmacokinetic profile in the rat of acyclovir was investigated.

RESULTS

In the presence of chitosan glutamate, the apparent permeability of acyclovir across Caco2 monolayer increased 4.1 times relative to control. This increase was accompanied by a significant ( approximately 60%) decrease in transepithelial electrical resistance values indicating opening of the tight junctions in the cell monolayer. In rat, chitosan glutamate doubled oral bioavailability of acyclovir and tripled the amount of acyclovir excreted unchanged into urine. In contrast, the effect of carbomer 974P was not statistically significant at 5% level.

CONCLUSIONS

In conclusion, chitosan glutamate (1-3%) and chitosan glutamate (1%)/EDTA-Na2 (0.01%) are effective excipients to increase permeability of acyclovir across Caco-2 cell monolayers and the oral absorption in the rat in vivo.

Acyclovir delivery systems

Herpes viruses (herpes simplex, varicella zoster, cytomegalovirus) are the main cause of a wide variety of human infections. Although the development of successful antiviral agents against infections caused by herpes viruses had been slow until the last decade, the production of delivery systems for acyclovir are a promising alternative. The present review summarizes the principal advances made in developing carriers for the delivery of acyclovir by different routes of administration.

Acyclovir liposomes for intranasal systemic delivery: development and pharmacokinetics evaluation

Intranasal route is one of the most attractive routes for distributing drugs to systemic circulation. Liposomes are used as biocompatible carriers to improve delivery properties across nasal mucosa. The objective of the present study was to formulate acyclovir liposomes and partition into poly-N-vinyl-2-pyrrolidone. Entrapment efficiency showed that multilamellar and unilamellar liposomes were 43.2% +/- 0.83 and 21% +/- 1.01, respectively. The bioavailability of acyclovir from nasal mucoadhesive gel was 60.72% compared with intravenous route. The use of liposomes acyclovir and mucoadhesive gel not only promoted the prolonged contact between the drug and the absorptive sites in the nasal cavity, but also facilitated direct absorption through the nasal mucosa.

Self-assembled drug delivery systems: Part 3. In vitro/in vivo studies of the self-assembled nanoparticulates of cholesteryl acyl didanosine

Self-assembled drug delivery systems (SADDS) are defined as the self-assemblies of amphiphilic prodrugs, integrating prodrugs, molecular self-assembly and nanotechnology for drug targeting and controlled release. Cholesteryl-succinyl didanosine (CSD) and cholesteryl-adipoyl didanosine (CAD) nanoparticulate systems in water were previously prepared and optimized. In this paper, the in vitro and in vivo behavior of them was investigated. Precipitation occurred when they were mixed with acid solutions due to rapid production of hypoxanthine and subsequent disruption of supramolecular structures. They showed pH-dependent degradation and kept relatively stable in the neutral pH range. CSD is more stable than CAD due to the shorter spacer and poloxamer protection. CSD showed different degradation rates in various plasma with the descending order of rat, mouse, rabbit, dog and human. The half-life (t(1/2)) of CSD is 9 days in rat plasma, and 5.9 days in rat liver homogenates. CAD has a faster degradation than CSD though the t(1/2) in rat liver homogenates is long to 23 h. CSD nanoparticulates showed no significant anti-HIV effect in MT4 cell model because of very slow degradation. CSD nanoparticulates showed the distribution t(1/2) of 7.6 min after bolus intravenous (i.v.) administration to rats, and the site-specific distribution in liver, lung and spleen with the high t(1/2) of 10 days in liver. The factors affecting achievement of successful SADDS are discussed.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Intranasal delivery: physicochemical and therapeutic aspects

Interest in intranasal (IN) administration as a non-invasive route for drug delivery continues to grow rapidly. The nasal mucosa offers numerous benefits as a target issue for drug delivery, such as a large surface area for delivery, rapid drug onset, potential for central nervous system delivery, and no first-pass metabolism. A wide variety of therapeutic compounds can be delivered IN, including relatively large molecules such as peptides and proteins, particularly in the presence of permeation enhancers. The current review provides an in-depth discussion of therapeutic aspects of IN delivery including consideration of the intended indication, regimen, and patient population, as well as physicochemical properties of the drug itself. Case examples are provided to illustrate the utility of IN dosing. It is anticipated that the present review will prove useful for formulation scientists considering IN delivery as a delivery route.

Rapid-onset intranasal delivery of metoclopramide hydrochloride. Part I. Influence of formulation variables on drug absorption in anesthetized rats

Intranasal (IN) administration is a promising approach for rapid-onset delivery of medications and to circumvent their first-pass elimination when taken orally. Metoclopramide (MCP) is a potent antiemetic, effective even for preventing emesis induced by cancer chemotherapy. The feasibility of developing an efficacious intranasal formulation of metoclopramide has been undertaken in this study. The nasal absorption of MCP was studied in anesthetized rats over 60min using the in vivo in situ technique. The influence of several formulation variables, vis., pH and the addition of preservative, viscosity and absorption enhancing agents on the nasal MCP absorption was examined. The data obtained showed that MCP was well absorbed nasally where almost 90% of the drug was absorbed after 60min from the rat nasal cavity. The MCP absorption was pH-dependant such that the apparent first-order rate constant of absorption (K(app)) was almost tripled when the pH of the solution was increased from 5 to 8. However, deviation from the classical pH-partition theory was observed pointing to the role of aqueous pore pathway in MCP nasal absorption. The K(app) was significantly increased (P<0.05) by incorporation of 0.01% of the preservative benzalkonium chloride. Conversely, increasing the solution viscosity by the use of hydroxylpropyl methylcellulose adversely affected the rate of absorption. The use of enhancers namely sodium deoxycholate, sodium cholate, chitosan low and high molecular weight, protamine sulphate and poly-l-arginine resulted in significant increase in MCP absorption. The highest promoting effect was observed with the bile salt sodium deoxycholate where about 92% of the drug was absorbed in 25min from the rat nasal cavity and the K(app) showed more than two-fold increase as compared to control (from 0.0452 to 0.1017min(-1)).

Controllable synthesis of polymerizable ester and amide prodrugs of acyclovir by enzyme in organic solvent

A facile control of the acylation position at the primary hydroxyl and amino of acyclovir, respectively, was achieved and five polymerizable acyclovir prodrugs were synthesized. Various reaction conditions were studied in detail. Thus, lipase acrylic resin from Candida antarctica (CAL-B) in pyridine or acetone showed high chemo-selectivity toward the primary hydroxyl of acyclovir. However, lipase PS 'Amano' (PS) in DMSO selectively acylated the amino group. The selectivity of PS could be adjusted by changing reaction solvents. The acyclovir vinyl derivatives obtained would be important monomers used for the preparation of macromolecular nucleoside drugs.

Development and in vitro evaluation of a liposomal vaginal delivery system for acyclovir

Design of a liposome delivery system for vaginal administration of acyclovir, able to provide sustained release and improved bioavailability of the encapsulated drug for the local treatment of genital herpes was investigated. Acyclovir was encapsulated in liposomes prepared by the polyol dilution method, whereby various phospholipid compositions were used: egg phosphatidylcholin (PC)/egg phosphatidylglycerol (PG) 9:1, egg phosphatidylcholine (PC) and egg phosphatidycholine (PC)/stearylamine (SA) 9:3. All liposome preparations were characterized and compared for particle size, polydispersity, encapsulation efficiency and tested for in vitro stability in different media chosen to simulate human vaginal conditions: buffer, pH 4.5 (corresponding to normal human vaginal pH), vaginal fluid simulant (medium developed so as to mimic the fluid produced in the vagina) with or without mucin. To be closer to in vivo application of liposomes and to achieve further improvement of their stability, liposomes were incorporated in a vehicle suitable for vaginal self-administration. Bioadhesive hydrogel made from Carbopol 974P NF resin with adequate pH value and desirable viscosity was chosen as a vehicle for liposomes containing acyclovir. In vitro release studies of liposomes incorporated in the hydrogel proved their applicability as a novel vaginal delivery system with localized and sustained release of encapsulated acyclovir. Even after 24 h of incubation in vaginal fluid simulant more than 35% of the originally encapsulated drug was retained in the hydrogel.

Design and development of multivesicular liposomal depot delivery system for controlled systemic delivery of acyclovir sodium

The aim of the present study was to design a depot delivery system of acyclovir sodium using multivesicular liposomes (MVLs) to overcome the limitations of conventional therapies and to investigate its in vivo effectiveness for sustained delivery. MVLs of acyclovir were prepared by the reverse phase evaporation method. The loading efficiency of the MVLs (45%-82%) was found to be 3 to 6 times higher than conventional multilamellar vesicles (MLVs). The in vitro release of acyclovir from MVL formulations was found to be in a sustained manner and only 70% of drug was released in 96 hours, whereas conventional MLVs released 80% of drug in 16 hours. Following intradermal administration to Wistar rats, the MVL formulations showed effective plasma concentration for 48 hours compared with MLVs and free drug solution (12-16 hours). C(max) values of MVL formulations were significantly less (8.6-11.4 microg/mL) than MLV and free drug solution (12.5 microg/mL). The AUC(0-48) of the MVL formulations was 1.5- and 3-fold higher compared with conventional liposomes and free drug solution, respectively. Overall, formulations containing phosphatidyl glycerol as negatively charged lipid showed better results. The MVL delivery system as an intradermal depot offers the advantage of a very high loading and controlled release of acyclovir for an extended period of time. The increase in AUC and decrease in C(max) reflects that the MVL formulations could reduce the toxic complications and limitations of conventional iv and oral therapies.

Pharmacokinetic Evaluation of 3′-Azido-2′, 3′-Dideoxyuridine-5′-O-Valinate-Hydrochloride as a Prodrug of the Anti-HIV Nucleoside 3′-Azido-2′, 3′-Dideoxyuridine

3′-Azido-2′, 3′-dideoxyuridine (AZDU, AzddU, CS-87) has been shown to have potent anti-HIV activity in vitro. However, the compound exhibits a relatively short half-life and incomplete oral bioavailability in humans. In an effort to improve the pharmacokinetic properties of AZDU, prodrug 3′-azido-2′,3′-dideoxyuridine-5′- O-valinate hydrochloride (AZDU-VAL) was synthesized by the esterification of 5′-OH function in AZDU. The objective of this study was to investigate the biotransformation and pharmacokinetics of AZDU-VAL along with its antiviral parent compound AZDU following intravenous and oral administration to rats. Adult male Sprague-Dawley rats were administered AZDU or AZDU-VAL by intravenous injection or oral gavage. Concentrations of AZDU-VAL and AZDU were determined by HPLC. Pharmacokinetic parameters were generated by area-moment analysis. The bioavailability of AZDU after oral administration was approximately 53%. The terminal phase half-life of the nucleoside analogue ranged between 0.6 h after intravenous administration and 1 h following oral administration. In vivo the prodrug was rapidly and efficiently biotransformed to yield AZDU following intravenous and oral administration. The apparent availability of AZDU was virtually complete following oral administration of prodrug AZDU-VAL averaging 101%. The bioavailability of AZDU following intravenous administration of AZDU-VAL averaged 106%. In summary, the disposition of AZDU was dose dependent over the dose range of 25–100 mg/kg. Renal clearance and steady state volume of distribution were lower at the higher dose level. Prodrug AZDU-VAL demonstrated improved oral bioavailability as evidenced by complete absorption and efficient bioconversion to AZDU. The results suggest that AZDU-VAL may be a promising prodrug for the delivery of AZDU.

Synthesis and characterization of novel dipeptide ester prodrugs of acyclovir

Four dipeptide (Gly-Gly, Gly-Val, Val-Val, Val-Gly) ester prodrugs of 9-[(2-hydroxyethoxy)methyl]guanine (acyclovir, ACV) were synthesized. LC/MS was used to characterize the new prodrugs. Both 1H NMR and 13C NMR spectra of the four prodrugs of ACV were measured and assigned based on spectral comparison with compounds of similar structures.

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.

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.

In vitro models to predict the in vivo mechanism, rate, and extent of placental transfer of dideoxynucleoside drugs against human immunodeficiency virus

OBJECTIVE

We tested the hypothesis that the mechanism, rate, and extent of in vivo placental transfer of dideoxynucleoside drugs against human immunodeficiency virus can be predicted by the in vitro perfused human placenta and the drug octanol-water partition coefficient.

STUDY DESIGN

Near-term pregnant macaques (Macaca nemestrina ) underwent long-term catheterization for the administration of 4 dideoxynucleosides against human immunodeficiency virus: zidovudine, didanosine, zalcitabine, and stavudine. Maternal plasma, fetal plasma, and amniotic fluid concentrations were determined frequently after intravenous bolus and/or infusion of the drugs administered into the maternal or fetal circulation on separate occasions. Antipyrine was included in all experiments as a marker of placental blood flow. The mechanism, rate, and extent of placental transfer of the 4 dideoxynucleosides in perfused human placenta were determined and compared with the findings obtained by others.

RESULTS

The mechanism and rate of the antipyrine-normalized placental transfer of the 4 dideoxynucleosides in perfused human placenta were highly correlated with those observed in vivo. The extent of placental transfer (fetal/maternal steady-state plasma concentration ratio) was also highly correlated with both the antipyrine-normalized placental transfer clearance (clearance index) determined in the in vitro perfused human placenta model (r 2 = 0.95, in vitro clearance-index model) and the drug octanol-water partition coefficient (r 2 = 0.99, in vitro partition-coefficient model). To determine the predictive capacity of these correlative models, we predicted the fetal/maternal steady-state plasma concentration ratio of each drug after excluding the data on that drug from the model fit. Both in vitro models to predict in vivo placental transfer of drug models resulted in good predictions of the observed fetal/maternal steady-state plasma concentration ratio (mean error: in vitro clearance-index model = -1. 2%; in vitro partition-coefficient model = 3.9%).

CONCLUSIONS

We propose that our models will accurately predict the extent of placental transfer of dideoxynucleoside drugs against human immunodeficiency virus. The models may also be applicable to other classes of drugs, regardless of therapeutic category, provided that these drugs passively diffuse across the placenta. Such a result will expedite phase 1 clinical trials of drugs in pregnant women.

Prodrugs for nasal drug delivery

Recently, the delivery of xenobiotics via the nasal route has received increasing attention as this offers several advantages, i.e. high systemic availability, rapid onset of action. Both charged and uncharged forms of drugs can be transported across the nasal epithelium. This mucosa is rich in various metabolizing enzymes such as aldehyde dehydrogenase, glutathione transferases, epoxide hydrolases, cyt-P450-dependent monooxygenases. The presence of these enzymes may make it possible for pharmaceutical scientists to design prodrugs for better absorption and high systemic availability. Recent advances in peptide nasal delivery through prodrug modification has been thoroughly discussed in this paper. Finally, nasally delivered therapeutic agents targeted to various disease states have been examined.

Theoretical design of prodrug-enhancer combination based on a skin diffusion model: prediction of permeation of acyclovir prodrugs treated with 1-geranylazacycloheptan-2-one

PURPOSE

A theoretical design of percutaneous penetration enhancement in which prodrug derivation and enhancer application are combined is proposed based on the skin diffusion model and it is experimentally verified.

METHODS

Employing acyclovir as a model drug, the hypothesis was tested by synthesis of its prodrugs and evaluation of their in vitro permeation in the rat skin, with or without a penetration enhancer, 1-geranylazacycloheptan-2-one(GACH).

RESULTS

Among five acyclovir prodrugs, those with higher lipophilicities (propionate, butyrate, valerate, and hexanoate prodrugs) showed greater skin penetration than those of hydrophilic prodrugs (acetate), when administered in combination with GACH. Furthermore, the observed enhancement ratios were in good agreement with those predicted by theoretical consideration.

CONCLUSIONS

Thus, skin permeation of prodrugs applied with an enhancer can be predicted and optimized by model analysis.

Bile salt-fatty acid mixed micelles as nasal absorption promoters. III. Effects on nasal transport and enzymatic degradation of acyclovir prodrugs

The absorption enhancement and presystemic degradation kinetics of a homologous series of acyclovir 2'-ester prodrugs were investigated in rats using the in situ nasal perfusion technique in the presence of bile salt-fatty acid mixed micelles. In vitro incubation studies indicated that nasal perfusate containing a mixed micellar solution generated higher ester-cleaving activity than isotonic phosphate buffer washings. Inhibitor screening and substrate specificity studies demonstrated the enzyme to be most likely carboxylesterase rather than true cholinesterase. The extent of prodrug cleavage by the carboxylesterase appears to correlate well with the substrate lipophilicity for esters with linear acyl chains. On the other hand, branching of the acyl side chain significantly retards acyclovir prodrug breakdown. To estimate the nasal epithelial membrane and cytoplasmic damaging effect caused by sodium glycocholate (NaGC)-linoleic acid (15 mM:5 mM) mixed micelles, the release profiles of 5'-nucleotidase (5'-ND), lactate dehydrogenase (LDH), and carboxylesterase in the nasal perfusate were measured as a function of time. The results indicated that the activities of all three enzymes resulting from the mixed micellar solution appeared to be significantly higher than those caused by 15 mM NaGC alone. The apparent nasal absorption rate constants of acyclovir and its butyrate, valerate, pivalate, and hexanoate ester prodrugs in mixed micellar solutions containing an esterase inhibitor (1 mM phenylmethylsulfonyl fluoride) were individually calculated. Without an inhibitor, lengthening of the linear acyl side chain of the prodrug resulted in greatly accelerated degradation coupled with moderate absorption improvement. The solubilities and micellar binding constants of acyclovir prodrugs were also determined.(ABSTRACT TRUNCATED AT 250 WORDS)