Phenytoin Prodrugs V: In Vivo Evaluation of Some Water-Soluble Phenytoin Prodrugs in Dogs

Additive Anticonvulsant Profile and Molecular Docking Analysis of 5,5'-Diphenylhydantoin Schiff Bases and Phenytoin

Four 5,5'-diphenylhydantoin Schiff bases possessing different aromatic species (SB1-SB4) were recently synthesized and characterized using spectroscopic and electrochemical tools. The present study aimed to ascertain the anticonvulsant activity of the novel phenytoin derivatives SB1-Ph, SB2-Ph, SB3-Ph, and SB4-Ph, containing different electron-donor and electron-acceptor groups, and their possible mechanism of action. The SB2-Ph exhibited the highest potency to suppress the seizure spread with ED50 = 8.29 mg/kg, comparable to phenytoin (ED50 = 5.96 mg/kg). While SB2-Ph did not produce neurotoxicity and sedation, it decreased locomotion and stereotypy compared to control. When administered in combination, the four Schiff bases decreased the phenytoin ED50 by more than 2× and raised the protective index by more than 7× (phenytoin+SB2-Ph). The strongest correlation between in-vivo and docking study results was found for ligands' interaction energies with kappa and delta receptors. These data, combined with the worst interaction energies of our ligands with the mu receptor, suggest that the primary mechanism of their action involves the kappa and delta receptors, where the selectivity to the kappa receptor leads to higher biological effects. Our findings suggest that the four Schiff bases might be promising candidates with potential applications as a safe and effective adjuvant in epilepsy.

Drug/Lead Compound Hydroxymethylation as a Simple Approach to Enhance Pharmacodynamic and Pharmacokinetic Properties

Hydroxymethylation is a simple chemical reaction, in which the introduction of the hydroxymethyl group can lead to physical–chemical property changes and offer several therapeutic advantages, contributing to the improved biological activity of drugs. There are many examples in the literature of the pharmaceutical, pharmacokinetic, and pharmacodynamic benefits, which the hydroxymethyl group can confer to drugs, prodrugs, drug metabolites, and other therapeutic compounds. It is worth noting that this group can enhance the drug’s interaction with the active site, and it can be employed as an intermediary in synthesizing other therapeutic agents. In addition, the hydroxymethyl derivative can result in more active compounds than the parent drug as well as increase the water solubility of poorly soluble drugs. Taking this into consideration, this review aims to discuss different applications of hydroxymethyl derived from biological agents and its influence on the pharmacological effects of drugs, prodrugs, active metabolites, and compounds of natural origin. Finally, we report a successful compound synthesized by our research group and used for the treatment of neglected diseases, which is created from the hydroxymethylation of its parent drug.

Synthesis and Biological Evaluation of Amino Acid Based Mutual Amide Prodrugs of Phenytoin as Anticonvulsant Agents

BACKGROUND

Phenytoin (5,5-diphenyl hydantoin) has poor water solubility, which results in incomplete oral availability. Other problems associated with the oral and intramuscular administration of phenytoin are gastric irritation and inflammation at the site of injection.

OBJECTIVE

The purpose of this study was to synthesize mutual amide prodrugs of phenytoin by using amino acids like glycine, L-tryptophan, L-lysine and taurine.

METHODS

These prodrugs were synthesized and characterized by Fourier Transform Infrared (FTIR), Proton nuclear magnetic resonance (1H NMR) and Mass Spectra. Physical and spectral characterization was performed by determination of solubility, maximum wavelength, partition coefficient (log P), ionization constant (pKa), specific (α) and molar rotation (μ), refractive index (n), specific refraction (RS) and molar refraction (RM).

RESULTS

The results obtained from solubility and log P values determination indicated that phenytoin prodrugs can be administered by oral as well as a parenteral route by minimizing the limitations associated with phenytoin. Anticonvulsant activity of prodrugs (4a-4d) was evaluated by using maximal electroshock (MES) and strychnine induced seizure test on albino mice of either sex weighing 25-30 g in which 4b and 4d were found to have significant anticonvulsant activity for MES and strychnine induced seizure test. In vitro enzymatic hydrolysis study of 4b and 4d was performed on liver, intestinal mucosa and plasma sample of male Sprague Dawley rats weighing 280-300 g in which phenytoin was eluted at 10.13 to 10.68 minutes at 220 nm.

CONCLUSION

The results obtained from the present work showed that amino acid-based mutual prodrug strategy can be a promising method to increase the solubility and anticonvulsant activity of phenytoin for the development of anticonvulsant agents.

Prodrugs: My Initial Exploration and Where It Led

This review presents my early exploration in the area of prodrugs and specifically prodrugs of the anticonvulsant, phenytoin, also called diphenylhydantoin. My journey started in graduate school with an introduction to the prodrug concept and continued for much of my career as I remain fascinated by the topic/technique. I have also included some backstories that the reader might find noteworthy. Prodrug intervention is now recognized as one of the better tools for taking a challenging small molecule drug from un-developable to developable.

The Need for Restructuring the Disordered Science of Amorphous Drug Formulations

The alarming numbers of poorly soluble discovery compounds have centered the efforts towards finding strategies to improve the solubility. One of the attractive approaches to enhance solubility is via amorphization despite the stability issue associated with it. Although the number of amorphous-based research reports has increased tremendously after year 2000, little is known on the current research practice in designing amorphous formulation and how it has changed after the concept of solid dispersion was first introduced decades ago. In this review we try to answer the following questions: What model compounds and excipients have been used in amorphous-based research? How were these two components selected and prepared? What methods have been used to assess the performance of amorphous formulation? What methodology have evolved and/or been standardized since amorphous-based formulation was first introduced and to what extent have we embraced on new methods? Is the extent of research mirrored in the number of marketed amorphous drug products? We have summarized the history and evolution of amorphous formulation and discuss the current status of amorphous formulation-related research practice. We also explore the potential uses of old experimental methods and how they can be used in tandem with computational tools in designing amorphous formulation more efficiently than the traditional trial-and-error approach.

Use of IV fosphenytoin pharmacokinetics to determine the loading dose for a clinical trial of canine status epilepticus

OBJECTIVE

Canine status epilepticus (CSE) has potential as a translational platform to evaluate the safety and efficacy of novel compounds and inform human status epilepticus trials. The aim of this study was to determine the intravenous dose of fosphenytoin (FOS) needed for dogs in a CSE clinical trial to attain phenytoin (PHT) concentrations similar to those used for human status epilepticus and monitor PHT concentrations.

METHODS

Four healthy dogs were used to characterize PHT pharmacokinetics. Each received either 15 mg/kg or 25 mg/kg of PHT equivalent intravenously. Blood samples were collected and FOS (total) and derived PHT (total and unbound) plasma concentrations were measured using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Noncompartmental pharmacokinetics (PK) parameter values were determined and compartmental PK modeling and simulations were used to select the dose for the clinical trial with a target goal of 1-2 μg/ml unbound PHT at 30-60 min postinfusion. Predicted total and unbound PHT concentrations were compared with concentrations in blood collected from dogs treated for CSE in the clinical trial.

RESULTS

Initial estimates suggested that a loading dose of 25 mg/kg would attain unbound concentrations of 1-2 μg/ml; however, this dose produced concentrations above 3-6 μg/ml, which resulted in clinically significant toxicity. A two-compartment model best fit the PHT concentration data with alpha-phase half-life of 2-5 min and elimination half-life of ~5 h. Based on the simulations, a dose of 15 mg/kg was selected and used in the clinical trial and 15 of 16 dogs randomized to the treatment arm had PHT plasma concentrations within the goal range.

SIGNIFICANCE

This study demonstrates that characterization of pharmacokinetics in a small number of dogs is useful in determining dosage regimens designed to attain targeted concentrations in clinical trials. Using this approach, we were able to determine a safe and effective dose of FOS for a clinical trial of CSE.

Prodrug strategies to overcome poor water solubility

Drug design in recent years has attempted to explore new chemical spaces resulting in more complex, larger molecular weight molecules, often with limited water solubility. To deliver molecules with these properties, pharmaceutical scientists have explored many different techniques. An older but time-tested strategy is the design of bioreversible, more water-soluble derivatives of the problematic molecule, or prodrugs. This review explores the use of prodrugs to effect improved oral and parenteral delivery of poorly water-soluble problematic drugs, using both marketed as well as investigational prodrugs as examples. Prodrug interventions should be considered early in the drug discovery paradigm rather than as a technique of last resort. Their importance is supported by the increasing percentage of approved new drug entities that are, in fact, prodrugs.

Anticonvulsant Therapy in Dogs and Cats

This article reviews anticonvulsant therapies in current use for dogs and cats and briefly describes new modes of anticonvulsant therapy that are being investigated or pending publication. Most of the information contained within the article is based on published information. Some of the information, however, is based on the author's clinical experience and is identified as such.

Pharmacokinetics of phenytoin following intravenous and intramuscular administration of fosphenytoin and phenytoin sodium in the rabbit

The purpose of this study was to evaluate and compare plasma phenytoin concentration versus time profiles following intravenous (i.v.) and intramuscular (i.m.) administration of fosphenytoin sodium with those obtained following administration of standard phenytoin sodium injection in the rabbit. Twenty-four adult New Zealand White rabbits (2.1 +/- 0.4 kg) were anaesthetized with sodium pentobarbitone (30 mg/kg) followed by i.v. or i.m. administration of a single 10 mg/kg phenytoin sodium or fosphenytoin sodium equivalents. Blood samples (1.5 ml) were obtained from a femoral artery cannula predose and at 1, 3, 5, 7, 10, 15, 20, 30, 45, 60, 90, 120, 180, 240 and 300 min after drug administration. Plasma was separated by centrifugation (1000 g; 5 min) and fosphenytoin, total and free plasma phenytoin concentrations were measured using high performance liquid chromatography (HPLC). Following i.v. administration of fosphenytoin sodium plasma phenytoin concentrations were similar to those obtained following i.v. administration of an equivalent dose of phenytoin sodium. Mean peak plasma phenytoin concentrations (Cmax) was 158% higher (P = 0.0277) following i.m. administration of fosphenytoin sodium compared to i.m. administration of phenytoin sodium. The mean area under the plasma total and free phenytoin concentration-time curve from time zero to 120 min (AUC(0-120)) following i.m. administration was also significantly higher (P = 0.0277) in fosphenytoin treated rabbits compared to the phenytoin group. However, there was no significant difference in AUC(0-180) between fosphenytoin and phenytoin-treated rabbits following i.v. administration. There was also no significant difference in the mean times to achieve peak plasma phenytoin concentrations (Tmax) between fosphenytoin and phenytoin-treated rabbits following i.m. administration. Mean plasma albumin concentrations were comparable in both groups of animals. Fosphenytoin was rapidly converted to phenytoin both after i.v. and i.m. administration, with plasma fosphenytoin concentrations declining rapidly to undetectable levels within 10 min following administration via either route. These results confirm the rapid and complete hydrolysis of fosphenytoin to phenytoin in vivo, and the potential of the i.m. route for administration of fosphenytoin delivering phenytoin in clinical settings where i.v. administration may not be feasible.

Water-soluble phosphate prodrugs of 1-propargyl-8-styrylxanthine derivatives, A(2A)-selective adenosine receptor antagonists

Water-soluble prodrugs of potent, A(2A)-selective adenosine receptor (AR) antagonists were prepared. 8-(m-Bromostyryl)-3, 7-dimethyl-1-propargylxanthine (BS-DMPX, 11) and the analogous 8-(m-methoxystyryl)xanthine derivative (MS-DMPX, 5b) were used as starting points. It was found that polar functional groups suitable for the attachment of a prodrug moiety were tolerated on the styryl ring and even better on the 3-substituent. 8-(m-Hydroxystyryl)-DMPX (7) and 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-1-propargylxanthine (5e, MSX-2) were the most potent and A(2A)-selective compounds and were selected for prodrug formation. For the preparation of 5e a new ring-closure method was applied. Treatment of 6-amino-1-(3-hydroxypropyl)-5-(m-methoxycinnamoylamino)-3-propa rgylur acil with hexamethyldisilazane at high temperature resulted in higher yields of the target xanthine than the standard ring-closure procedure using sodium hydroxide. Phosphate prodrugs were prepared by classical phosphorylation using phosphorus oxychloride and alternatively by using a phosphoramidite method. Phosphates of the aliphatic alcohol 5e could be obtained by both methods in similar yields. The phenolic compound 7, however, could be phosphorylated only by using the phosphoramidite method. The disodium salts of the phosphate prodrugs exhibited high water solubility (8-(m-methoxystyryl)-7-methyl-3-[3-O-phosphatylpropyl]-1- propargylxan thine disodium salt, 9b: 17 mM, 9 mg/mL). Prodrug 9b was found to be stable in aqueous solution (pH 7) but readily cleaved by phosphatases to liberate 5e (MSX-2). Compound 5e showed high affinity for rat A(2A) AR (K(i) = 8 nM), human recombinant A(2A) AR (K(i) = 5 nM), and human native A(2A) AR (K(i) = 15 nM) and was highly selective versus rat A(1) AR (110-fold), human recombinant A(2A) AR (500-fold), human A(2B) AR (>2000-fold), and human A(3) AR (>2000-fold).

Aqueous solubility and dissolution rate does not adequately predict in vivo performance: a probe utilizing some N-acyloxymethyl phenytoin prodrugs

Some physicochemical properties of N-acyloxyalkyl prodrugs of phenytoin were reported previously.(1,2) It was shown that despite their lower aqueous solubilities relative to phenytoin, these lower-melting prodrugs with apparently disrupted crystalline structures gave either comparable or enhanced in vitro solubility and dissolution rate in simulated intestinal media made up of bile salts and lecithin (SIBLM).(2) The current objective was to compare the in vivo behavior of two of these prodrugs to phenytoin in dogs and attempt to correlate the in vitro behavior to their in vivo behavior. The oral bioavailability of phenytoin after administration of phenytoin (1) and the selected prodrugs, 3-pentanoyloxymethyl 5, 5-diphenylhydantoin (2) and 3-octanoyloxymethyl 5, 5-diphenylhydantoin (3), in fed and fasted beagle dogs were compared to intravenously administered phenytoin. Phenytoin and its prodrugs showed improvement in fed-state phenytoin bioavailability relative to the fasted state indicating that food enhanced the delivery of phenytoin from phenytoin and its prodrugs. The increased bioavailability in the fed state may be due to stimulation of bile release by food and, for the prodrugs, possible catalysis of their dissolution by lipases.(3) In both, fasted and fed states, prodrugs 2 and 3 gave higher AUC values of phenytoin than the parent compound. The enhanced bioavailability of phenytoin after oral administration were more obvious in fed dogs. Although enhanced, AUC values of phenytoin from the prodrugs relative to phenytoin were not statistically different (at 95% confidence level) in fasted state, but were different in fed state. Although the aqueous solubilities and dissolution of both prodrugs were lower than phenytoin, dissolution of 2 and 3 was equivalent and greater, respectively, relative to phenytoin in SIBLM. As expected, the in vivo behavior correlated better with the in vitro SIBLM dissolution behavior. These results indicate that aqueous solubility per se does not adequately predict in vivo behavior.

Synthesis of water-soluble phenytoin prodrugs

The synthesis of novel water-soluble phenytoin derivatives, bearing an ionizable group, and a preliminary study for in vitro blood hydrolysis are reported. The results show that hydrolysis of amino esters 8 is very fast, much more than that of fosphenytoin.

Development of anti-influenza virus drugs I: improvement of oral absorption and in vivo anti-influenza activity of Stachyflin and its derivatives

PURPOSE

Stachyflin and its derivatives which are active against the influenza virus in vitro, were studied to improve their reduced in vivo activity after oral administration by chemical modification and some vehicles.

METHODS

The solubility was examined for different vehicles. The improvement of gastrointestinal absorption was evaluated by the plasma concentration after oral administration to mice or the in situ loop method with rats. The in vivo anti-influenza activity was examined using mice infected with the influenza virus and evaluated based on the virus titer in the lung by TCID50.

RESULTS

PEG 400 showed the highest solubility of Stachyflin and its derivative among the vehicles studied. While no viral inhibition was found in the lung after oral administration of 0.5% HPMC suspension of Stachyflin, in vivo anti-influenza virus activity was found with the PEG 400 solution. The absorption of Stachyflin by PEG 400 showed about a fifty-fold increase in AUC compared with that of 0.5% HPMC suspension. Improving the oral absorption of Stachyflin led to an increase in the in vivo anti-influenza virus activity. When the Stachyflin derivative in PEG 4000 was administered orally, there was more enhancement of the oral absorption than with PEG 400. When the aqueous solution of the phosphate ester prodrugs of Stachyflin and its derivative was administered orally, the absorption of the parent compound was improved and in vivo anti-influenza virus activity was found.

CONCLUSIONS

When Stachyflin and its derivatives were administered orally to mice with a solution in PEG and an aqueous solution of their phosphate ester, their oral absorption was improved and in vivo anti-influenza virus activity was observed.

Phenytoin pharmacokinetics following oral administration of phenytoin suspension and fosphenytoin solution to rats

The administration of phenytoin suspension in conjunction with enteral nutrition supplements through nasogastric (NG) feeding tubes to humans has been associated with suboptimal phenytoin absorption, subtherapeutic concentrations, and breakthrough seizures. Postulated mechanisms include chelation to proteins and electrolytes in the enteral feeding, binding to NG tubing, and alterations in gastrointestinal pH resulting in precipitation of phenytoin. The purpose of this pilot study was to evaluate the oral absorption of commercially available fosphenytoin injectable solution compared to phenytoin suspension in the rat to determine whether equivalent oral fosphenytoin and phenytoin suspension doses should be used for future human studies of fosphenytoin oral absorption in the presence of concomitant enteral nutrition. A single oral 30 mg/kg phenytoin equivalents dose of either commercially available fosphenytoin or phenytoin suspension was administered to male Wistar rats following an overnight fast. Blood samples (0.3 ml) for phenytoin plasma concentration were obtained from a jugular vein catheter at baseline and 0.5, 1, 1.5, 2, 3, 4, 5, 8, 12 and 24 h post-study drug administration and analyzed by high performance liquid chromatography (HPLC) (CV% < 6). Mean phenytoin Cmax was 85% [corrected] (P = 0.010) higher in fosphenytoin vs phenytoin treated rats. Tmax was 2.4 h (62%, P=0.021) shorter in fosphenytoin vs phenytoin treated rats. No significant differences in AUClast were found. The presence of a phosphate ester moiety does not appear to inhibit the appearance of phenytoin following oral administration of fosphenytoin. Phenytoin plasma concentration profiles following oral administration of fosphenytoin are characterized by higher Cmax and shorter Tmax values relative to oral administration of phenytoin suspension.

Elevated free fosphenytoin concentrations in uremic sera: uremic toxins hippuric acid and indoxyl sulfate do not account for the impaired protein binding of fosphenytoin

Fosphenytoin is a new phosphate ester prodrug of phenytoin. Impaired protein binding of phenytoin in uremia has been extensively documented, which prompted us to investigate the protein binding of fosphenytoin in uremic sera. Also studied was the role of uremic toxins hippuric acid and indoxyl sulfate as potential inhibitor of the protein binding of fosphenytoin because these compounds impair protein binding of phenytoin in uremia. Five serum pools were prepared from normal volunteers and five pools from patients with uremia. None of them received phenytoin. The normal serum pools were diluted with saline to mimic the albumin concentration of uremic pool. Both the diluted normal pool and the uremic pool were supplemented with fosphenytoin; after incubation at room temperature for 30 minutes, total and free fosphenytoin concentrations as phenytoin equivalents were measured using fluorescence polarization immunoassay (Abbott Laboratories; Abbott Park, IL, U.S.A.). The authors observed significantly elevated free fosphenytoin concentration in uremic sera compared with that of normal sera in all cases. Because both normal and uremic sera had the same concentrations of albumin, the elevated free fosphenytoin concentration in uremic sera was not caused by hypoalbuminemia. Both indoxyl sulfate and hippuric acid cause significant displacement of phenytoin from protein binding. In contrast, none caused any displacement of fosphenytoin from protein binding.

Bioavailability and anticonvulsant activity of a monoglyceride-derived prodrug of phenytoin after oral administration to rats

The plasma levels of phenytoin after oral administration of phenytoin and phenytoin 2-monoglyceride, a phenytoin prodrug, to rats were determined by gas chromatography. Compared to the application of the parent drug, administration of the prodrug resulted in a 3-fold increase of Cmax and a 4-fold increase of the AUC. This correlated with an earlier onset and peaking of the anticonvulsant activity determined in the maximal electroschock (MES) test. The peak effect was reached 1 h after dosing the monoglyceride compared to 2 h after application of phenytoin itself. On the basis of the median effective dose, the prodrug was 3 times more effective antagonizing MES-induced seizures than the parent drug. It is concluded that phenytoin 2-monoglyceride might be a useful prodrug for the oral delivery of phenytoin.

New anticonvulsant drugs. Focus on flunarizine, fosphenytoin, midazolam and stiripentol

In the past decade, several new antiepileptic drugs have been tested. Most recently, 5 new antiepileptic drugs have been launched onto European and US markets. These include vigabatrin, oxcarbazepine and lamotrigine in Europe, and felbamate and gabapentin in the US. In addition to these, 3 additional drugs are in the clinical investigational stage: flunarizine, fosphenytoin and stiripentol. A fourth agent is midazolam, which was originally introduced in 1986, but recently has shown effectiveness in the treatment of status epilepticus. Flunarizine is a selective calcium channel blocker that has shown anticonvulsant properties in both animal and human studies. It is a long-acting anticonvulsant that clinical studies have shown to have effects similar to those of phenytoin and carbamazepine in the treatment of partial, complex partial and generalised seizures. Fosphenytoin was developed to eliminate the poor aqueous solubility and irritant properties of intravenous phenytoin. It is rapidly converted to phenytoin after intravenous or intramuscular administration. In clinical studies, this prodrug showed minimal evidence of adverse events and no serious cardiovascular or respiratory adverse reactions. It may have a clear advantage over the present parenteral formulation of phenytoin. Midazolam is a benzodiazepine that is more potent than diazepam as a sedative, muscle relaxant and in its influence on electroencephalographic measures. It has been shown to be an effective treatment for refractory seizures in status epilepticus. Stiripentol has anticonvulsant properties as well as the ability to inhibit the cytochrome P450 system. There are significant metabolic drug interactions between stiripentol and phenytoin, carbamazepine and phenobarbital (phenobarbitone). Stiripentol has been studied in patients with partial seizures, refractory epilepsy and refractory absence seizures with some efficacious results.

Chemical systems for delivery of antiepileptic drugs to the central nervous system

The chemical delivery system (CDS) approach, a recently developed procedure conceived to enhance the specific central nervous system (CNS) uptake of drugs, has been applied to several antiepileptic agents. CDSs based on dihydropyridine<-->pyridinium salt type redox targetors, reversibly linked to the drug, were designed, synthesized and tested for some traditional (phenytoin, valproate) and potential (stiripentol) antiepileptic drugs, as well as some compounds (GABA, adenosine) with important roles in epileptogenesis. Physicochemical, in vitro stability, in vivo tissue distribution, activity and toxicity studies were performed for the new derivatives. The results of these investigations indicated that selected CDSs possessed properties required for delivering the drugs to the CNS. In vivo experiments indicated improved brain uptake and enhanced pharmacologic activity in some of the examined cases. On the other hand, no toxic side effects were registered during the studies. Properly developed CDSs could enhance the therapeutic indexes of the anticonvulsant drugs.

Phenytoin prodrug 3-phosphoryloxymethyl phenytoin (ACC-9653): pharmacokinetics in patients following intravenous and intramuscular administration

A phenytoin prodrug, 3-phosphoryloxymethyl phenytoin (ACC-9653; 1), has been developed with more favorable physicochemical properties than phenytoin for parenteral administration. The purpose of this study was to evaluate the pharmacokinetic profile of 1 following iv and im administration in adult patients receiving chronic oral phenytoin monotherapy. Each patient (9 males, 1 female) received a single iv dose of undiluted 1 equivalent to their twice daily phenytoin dose (100-200 mg). An equivalent dose of im 1 was administered in the gluteus maximus muscle one week later. Serial blood samples were obtained after each dose. Phenytoin and 1 concentrations were measured using HPLC. Compartmental analysis using weighted nonlinear least squares, and noncompartmental pharmacokinetic analysis were performed on each patient's concentration-time data. Data following iv 1 in eight of ten patients were best described using a two-compartment model. Mean pharmacokinetic parameter estimates for iv 1 in these patients were central volume of distribution (Vdc) of 0.040 +/- 0.0084 L/kg and plasma disappearance half-life (t1/2 alpha) of 8.0 +/- 2.9 min ("conversion" t1/2). Overall mean clearance (CL) was 0.24 +/- 0.080 L/kg/h in the 10 patients. Mean pharmacokinetic parameter estimates for im 1 were a rate constant (ka) of 2.47 +/- 1.41 h-1 and an absolute bioavailability (F) of 100.5 +/- 20.3%. Mean observed tmax values for phenytoin were 0.57 +/- 0.26 and 1.46 +/- 0.76 h following iv and im 1, respectively. Model-independent estimates of clearance agreed well with the compartmental analyses. Steady-state predose phenytoin concentrations did not significantly vary from the comparable concentrations following iv 1 administration (p = 0.22).(ABSTRACT TRUNCATED AT 250 WORDS)

Improved anticonvulsant activity of phenytoin by a redox brain delivery system I: Synthesis and some properties of the dihydropyridine derivatives

Nine chemical delivery systems (CDSs) were synthesized for the efficient transport of phenytoin (DPH) across the blood-brain barrier. The CDSs were based on a dihydropyridine in equilibrium quaternary pyridinium ion redox system which relies on chemistry similar to the NADH in equilibrium NAD interconversion for activity. The chemical carriers, derivatives of trigonelline, 1-alkylcarboxynicotinamide, 3-pyridylacetic acid, and N-methylpicolinic acid, were esterified with 3-(hydroxymethyl)phenytoin. The CDSs proved to be more lipophilic (5-23 times) than DPH. The 1-alkylcarboxydihydronicotinamide CDSs, excluding the sterically hindered one (11e), were quite unstable in rat tissue homogenates and hydrolyzed to release DPH. In human blood, however, they were found to be much more stable (75 times) toward hydrolysis. All other CDSs were oxidized quantitatively to the corresponding pyridinium ion in rat brain homogenates. These compounds were found to possess the required physicochemical characteristics for delivering DPH into rat brain.

Pancreatic lipase-catalyzed hydrolysis of esters of hydroxymethyl phenytoin dissolved in various metabolizable vehicles, dispersed in micellar systems, and in aqueous suspensions

Lipase-catalyzed hydrolysis of fatty acid esters of 3-hydroxymethyl phenytoin was studied in various triglyceride and ethyl oleate emulsions, dispersed in micellar solutions, and suspended in an aqueous buffered solution. Phenytoin release from ethyl oleate emulsions of the prodrugs show apparent first-order kinetics with the pentanoate to nonanoate derivatives and sigmoidal kinetics with the long-chain fatty acid derivatives (stearate and oleate). A transition in the kinetic behavior, between the short- and the long-chain acyl prodrugs, was observed with the decanoate derivative. These observations are accounted for by a proposed kinetic model. Phenytoin release from the solid prodrugs follows zero-order kinetics and is independent of the total amounts of suspended material but directly proportional to the lipase concentration. Lipolysis of the solid suspended prodrugs was dependent on the length of the acyl side chain of the prodrug, with maxima for the pentanoate and the octanoate derivatives. The short-chain derivatives, acetate and propionate, as well as the long-chain prodrug, stearate, showed the slowest lipolysis rate when present as solid dispersions. The zero-order rate is qualitatively correlated with the melting point of the prodrugs. This result might be expected if the melting point is taken as a measure of the cohesivity or packing of the molecules at the surface of a crystal.

Phenytoin prodrug: preclinical and clinical studies

The currently available phenytoin (PHT) solution has many disadvantages stemming from poor aqueous solubility of PHT. A novel approach to solve the problem has been the synthesis of a phosphate ester of PHT (PHT prodrug ACC-9653). This water-soluble compound is metabolized rapidly into PO4 and PHT. A four center open-label, baseline-controlled study of 43 patients with epilepsy maintained on oral twice-daily PHT monotherapy was performed to evaluate the safety and pharmacokinetic profile of the prodrug. Patients received an i.v. or i.m. dose of ACC-9653 at a dose equivalent to the patients' morning dose of PHT. Intravenous dosages were infused at a rate of 75 mg/min, and i.m. dosages were given as one or two injections. After a period of 6 days, during which patients were again maintained with oral PHT, they were given a dose of ACC-9653 via whichever route they had not yet received. The Tmax of the prodrug averaged 5.7 and 36 min (0.095 and 0.606 h) after i.v. and i.m. administrations, respectively. The elimination half-life of ACC-9653 (conversion from prodrug to PHT) after i.v. and i.m. administration was 8.4 and 32.7 min (0.140 and 0.545 h), respectively, and both were independent of the dose. The plasma clearance of ACC-9653 was not dependent on dose or route of administration and averaged 19.8 +/- 1.16 and 17.8 +/- 0.83 L/h after i.v. and i.m. administrations, respectively. The area under curve ratio of PHT after i.m. and i.v. ACC-9653 was 1.17 +/- 0.13 which was not significantly different from 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of ACC-9653 (phenytoin prodrug)

ACC-9653, the disodium phosphate ester of 3-hydroxymethyl-5,5-diphenylhydantoin, is a prodrug of phenytoin with advantageous physicochemical properties. ACC-9653 is rapidly converted enzymatically to phenytoin in vivo. ACC-9653 and phenytoin sodium have equivalent anticonvulsant activity against seizures induced by maximal electroshock (MES) in mice following i.p., oral, or i.v. administration. The ED50 doses were 16 mg/kg for i.v. ACC-9653 and 8 mg/kg for i.v. phenytoin sodium. ACC-9653 and phenytoin sodium have similar antiarrhythmic activity against ouabain-induced ventricular tachycardia in anesthetized dogs. The total doses of ACC-9653 or phenytoin sodium necessary to convert the arrhythmia to a normal sinus rhythm were 24 +/- 6 and 14 +/- 3 mg/kg, respectively. Only phenytoin sodium displayed in vitro antiarrhythmic activity against strophanthidin-induced arrhythmias in guinea pig right atria. In anesthetized dogs, a high dose of ACC-9653 (31 mg/kg) was infused over 15, 20, and 30 min and the responses were compared to an equimolar dose of phenytoin sodium (21 mg/kg). The ACC-9653 and phenytoin sodium treatments produced similar marked reductions in diastolic blood pressure and contractile force (LVdP/dt). The maximum effects of each treatment occurred at the time of maximum phenytoin sodium levels. Acute toxicity studies of ACC-9653 and phenytoin sodium were carried out in mice, rats, rabbits, and dogs by i.v., i.m., and i.p. routes of administration. The systemic toxic signs of both agents were similar and occurred at approximately equivalent doses. Importantly, the local irritation of ACC-9653 was markedly less than phenytoin sodium following i.m. administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Safety, tolerance and pharmacokinetics of intravenous doses of the phosphate ester of 3-hydroxymethyl-5,5-diphenylhydantoin: a new prodrug of phenytoin

A new prodrug of phenytoin, the disodium phosphate ester of 3-hydroxymethyl-5,5-diphenylhydantoin (ACC-9653), was administered intravenously over 30 minutes to four different groups of volunteers at doses of 150, 300, 600, and 1200 mg. The prodrug and phenytoin were measured in plasma samples, collected at specified times, by specific high performance liquid chromatography (HPLC) assays. The prodrug, after achieving a maximum concentration at the end of the 30-minute infusion (Cmax 20, 36, 75, 129 micrograms/mL) declined rapidly with a half-life (t1/2) of about 8 minutes. The area under the plasma concentration-time curve (10, 19, 43, 77 mg.hr/L) was proportional to dose whereas the total clearance, 14 L/hr, was independent of dose. The volume of distribution of the prodrug, a polar, water-soluble molecule was about 2.6 L, indicating that most of the dose remained in the plasma. The concentration of phenytoin reached 90% of its maximum about 12 minutes after the end of the infusion of ACC-9653. At the dose of 1200 mg of prodrug, the average peak concentration of phenytoin was about 17 micrograms/mL, near the upper limit of the therapeutic range. Adverse reactions (lightheadedness, nystagmus, incoordination) were minor and attributed to phenytoin. No significant abnormalities in ECG, Holter monitoring, or EEG were noted after the infusion of ACC-9653.

Prodrugs. Do they have advantages in clinical practice?

Prodrugs are pharmacologically inactive chemical derivatives of a drug molecule that require a transformation within the body in order to release the active drug. They are designed to overcome pharmaceutical and/or pharmacokinetically based problems associated with the parent drug molecule that would otherwise limit the clinical usefulness of the drug. The scientific rationale, based on clinical, pharmaceutical and chemical experience, for the design of various currently used prodrugs is presented in this review. The examples presented are by no means comprehensive, but are representative of the different ways in which the prodrug approach has been used to enhance the clinical efficacy of various drug molecules.

Phenytoin prodrugs III: water-soluble prodrugs for oral and/or parenteral use

Various bioreversible derivatives of phenytoin, a poorly water soluble and erratically absorbed drug after both oral and parenteral dosing, were synthesized. Initial evaluation of these expected prodrugs, i.e., their aqueous solubility, cleavage in the presence of various animal tissues, and anticonvulsant activity in mice, confirmed that a number of the derivatives did indeed behave as prodrugs. The more promising prodrugs were the disodium phosphate ester and various amino groups containing acyl esters of 3-(hydroxymethyl)-5,5-diphenylhydantoin.

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.

Phenytoin Prodrugs IV: Hydrolysis of Various 3-(Hydroxymethyl)phenytoin Esters

The aqueous chemical stability of various bioreversible derivatives or prodrugs of phenytoin, a poorly water-soluble and erratically absorbed drug after both oral and intramuscular parenteral dosing, were evaluated. This study, together with assessments of other physiochemical properties including cleavage in the presence of various animal tissues and anticonvulsant activity in mice, helped identify a number of promising candidate prodrugs. Various amino groups containing acyl esters of 3-(hydroxymethyl)phenytoin [3-(hydroxymethyl)-5,5-diphenylhydantoin] were identified as potential orally and perhaps parenterally useful prodrugs, while the disodium phosphate ester of 3-(hydroxymethyl)phenytoin appears to be ideally suited as a parenteral form of phenytoin.