Bioavailability of ACC-9653 (phenytoin prodrug)

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.

Bioavailability of intravenous fosphenytoin sodium in healthy Japanese volunteers

To compare and evaluate the bioavailability for intravenous fosphenytoin sodium with that of intravenous phenytoin sodium in Japanese subjects. In study 1, healthy Japanese male volunteers received a 30-min infusion of 375 mg fosphenytoin sodium or an equimolar dose of 250 mg phenytoin by a double-blind, crossover method. In study 2, other healthy Japanese male volunteers received a 30-min or 10-min infusion of 563 mg fosphenytoin sodium, followed by a dose of 750 mg after 2 weeks in an unblinded manner. Comparing with 250 mg phenytoin sodium, 375 mg fosphenytoin sodium exhibited lower total plasma phenytoin C max, whereas the geometric mean ratio of the AUC of total and free phenyotoin for fosphenytoin sodium at a dose of 375 mg was very similar to phenytoin sodium at a equimolar dose of 250 mg (AUC0-t ratio: 0.98 and 1.02, respectively). Therefore, fosphenytoin is almost completely converted to phenytoin in subjects. Fosphenytoin sodium was rapidly converted to phenytoin at doses of 375, 563, and 750 mg. The maximum concentration (C max) of total plasma phenytoin increased in a dose-dependent manner. The area under the plasma concentration-time curve (AUC) increased slightly more than proportionally with the administered dose, and clearance (CL) decreased with increasing dose. Pain and other infusion-site reactions were reported by all 12 subjects with phenytoin sodium, whereas very few symptoms were observed with fosphenytoin sodium. In conclusion, fosphenytoin sodium is considered to be a useful substitute for phenytoin sodium with almost no associated injection-site reactions.

Phenytoin and phenobarbital stable isotope studies in neonates

A pharmacokinetic study of phenytoin and phenobarbital with nonradioactive isotopes was performed in nine neonates in an intensive care unit setting. A single-pulse dose of either labeled phenobarbital (1,3-(15)N, 2-(13)C) or labeled phenytoin (2-(13)C, 1, 3-(15)N) was administered to neonates who manifested gestation between 25 and 40 weeks and were receiving maintenance medication. Blood samples were collected at fixed intervals, and with a computerized gas chromatography mass spectrometry system, plasma concentrations of the labeled and unlabeled drug in relation to time administered were obtained. According to the calculations obtained from labeled analogue, several kinetic characteristics related to drug absorption, clearance, and elimination were determined. The use of a nonradioactive labeled isotope overcomes the limitations of conventional pharmacokinetic methodology and can be specifically useful in neonates and infants in whom volumes of distribution are rapidly changing and steady state is not achieved.

Clinical review: status epilepticus

Status epilepticus (SE) has an annual incidence exceeding 100,000 cases in the United States alone, of which more than 20% result in death. Thus, increased awareness of presentation, etiologies, and treatment of SE is essential in the practice of critical care medicine. This review discusses current definitions of SE, as well as its clinical presentation and classification. The recent literature on epidemiology is reviewed, including morbidity and mortality data. An overview of the systemic pathophysiologic effects of SE is presented. Finally, significant studies on the treatment of acute SE and refractory SE are reviewed, including the use of anticonvulsants, such as benzodiazepines, and other drugs.

Absorption of rectally administered phenytoin: a pilot study

OBJECTIVE

To test the hypothesis that rectally administered phenytoin is absorbed in healthy volunteers.

DESIGN

This single-center, open-label crossover pilot study compared rectal absorption with intravenous administration of phenytoin injectable solution (7 mg/kg) in healthy volunteers. Twelve serial blood samples were taken from each volunteer beginning at time zero until 36 hours following administration. These were analyzed for presence of phenytoin by immunoassay.

SETTING

The study took place at St. Paul's Hospital, a tertiary care center.

PARTICIPANTS

Funding permitted for a sample size of five healthy participants, two men and three women, aged 21-45 years. Selection was by volunteer sample. Inclusion criteria were as follows: no known medical conditions, not receiving medication, no history of adverse drug reactions or allergies, not known to be pregnant, and normal liver function as determined per study protocol.

MAIN OUTCOME MEASURES

Signs of absorption as indicated by presence of phenytoin in blood samples, maximum concentrations (Cmax), time to Cmax (tmax), AUC, and apparent bioavailability.

RESULTS

Maximum mean concentrations of 2.4+/-1.1 mg/L (mean +/- SD) following rectal administration and 11.2+/-1.6 mg/L following intravenous administration were achieved during the first one to two hours (tmax in both treatment arms). Mean apparent bioavailability of the rectally administered phenytoin was 24.4+/-13.4% (15.8-44.1%).

CONCLUSIONS

Results from this pilot study demonstrate that rectal absorption of phenytoin begins within 30 minutes following single-dose administration and was reported by four out of five volunteers to be the preferred route. Further studies are required before extrapolation can be made to the patient population.

Advances in the medical treatment of epilepsy

Treatment options for epilepsy, especially using antiepileptic drugs, have increased substantially in the past five years. Since 1993, four novel antiepileptic drugs have been approved and marketed in the United States: felbamate, gabapentin, lamotrigine, and topiramate. Two others, tiagabine and vigabatrin, are likely to be approved in the near future. For many patients, these agents offer the realistic promise of improved seizure control, often with fewer adverse effects and less significant drug interactions compared with older agents. In addition, fosphenytoin, a water-soluble phenytoin prodrug with a number of advantages over intravenous phenytoin, has been released. There are new administration options for carbamazepine, diazepam, and valproic acid. For drug-resistant or -intolerant patients, there has been renewed interest in alternative therapies, especially the ketogenic diet. Taken together, these represent significant therapeutic advances that are benefiting patients with epilepsy. At the same time, improved understanding of the basic mechanisms of epileptogenesis, and of the cellular and molecular actions of available antiepileptic drugs, creates a framework for designing unique therapeutic strategies that are targeted at key sites of vulnerability involved in the development and maintenance of the epileptic state.

Anticonvulsant effect of fosphenytoin in amygdala-kindled rats: comparison with phenytoin

Phenytoin has been reported to exert variable anticonvulsant effects in the kindling model of complex partial seizures. Phenytoin is only water soluble at a pH of more than 10, and it has been suspected that poor absorption of the drug is responsible for its lack of effect in some experiments. Recently, fosphenytoin, a prodrug of phenytoin, has been developed by phosphorylating phenytoin which makes the drug water soluble at physiological pH while it is rapidly transformed to phenytoin after injection. This study examined the anticonvulsant profile and the absorption after intraperitoneal injection of fosphenytoin, compared to its parental drug phenytoin. The pharmacokinetic parameters of phenytoin and fosphenytoin were compared by determining plasma levels of phenytoin after i.p. injection of 50 mg/kg phenytoin or the equivalent dose of 84 mg/kg of fosphenytoin in non-kindled female Wistar rats. After both injections the maximal plasma concentration of phenytoin was about 30 microg/ml. The relative bioavailability of fosphenytoin was 83%. In contrast to phenytoin, failed injections resulting in non-detectable plasma concentration of phenytoin were almost absent after fosphenytoin. In fully kindled female Wistar rats, fosphenytoin dose-dependently increased the focal seizure (afterdischarge) threshold. Seizure severity and duration at threshold were reduced only after the highest does of fosphenytoin tested (84 mg/kg). Thus, fosphenytoin showed anticonvulsant properties similar to phenytoin in amygdala kindled rats. We conclude that fosphenytoin is an adequate and reliable substitute for the parenteral injection of phenytoin in experimental seizure models of rats.

New drug therapy for acute seizure management

Several new agents have recently become available for the long-term treatment of epilepsy. Until now, there has been little change for the acute management of seizures. Three new agents may alter our present practice. Fosphenytoin has recently been approved as a substitute for parenteral phenytoin. It provides similar efficacy without the risk for infusion site injury while allowing greater flexibility in intravenous solutions. Intravenous valproate adds flexibility for patients on valproate, allowing patients to be rapidly loaded. In addition, it will prevent patients from having to change seizure medications when intervening medical illness or surgery do not allow medications by mouth. Viscous diazepam solution for rectal administration will allow for safe and effective treatment for seizures at home and will potentially decrease emergency department services and hospitalization.

The clinical pharmacokinetics of the newer antiepileptic drugs. Focus on topiramate, zonisamide and tiagabine

Following the introduction of felbamate, gabapentin, lamotrigine, oxcarbazepine and vigabatrin in the early 1990s, other new antiepileptic drugs have been advancing in clinical development. Those most extensively evaluated to date include topiramate, zonisamide and tiagabine. Topiramate, licensed recently in the UK, acts multifactorially through the blockade of sodium channels and kainate/AMPA receptors, enhancement of gamma-aminobutyric acid (GABA)ergic transmission and inhibition of carbonic anhydrase. It is well absorbed from the gastrointestinal tract and negligibly bound to plasma proteins. When used as a monotherapy, topiramate is eliminated primarily in the urine in an unchanged form with a half-life of 20 to 30 hours; elimination is faster in patients receiving concurrent medication with enzyme-inducing anticonvulsants, in whom the extent of biotransformation becomes more prominent. Zonisamide, which has been commercially available in Japan for some years, also has a multifactorial mode of action, possibly involving the blockade of sodium channels, T-type calcium channels and inhibition of carbonic anhydrase. It is rapidly absorbed, 50% bound to plasma proteins and is eliminated predominantly by biotransformation; zonisamide has a half-life of 50 to 70 hours in monotherapy patients, or 25 to 35 hours in patients comedicated with enzyme-inducing anticonvulsants. Tiagabine, a nipecotic acid derivative which inhibits GABA reuptake, is rapidly and completely absorbed after oral intake. It is highly (96%) bound to plasma proteins and it is eliminated primarily by cytochrome P450 3A-mediated oxidation, with a half-life of about 7 hours in healthy volunteers. Tiagabine metabolism is also enhanced by concurrent medication with enzyme-inducing anticonvulsants, resulting in a need to use dosages larger than those required in monotherapy or valproic acid (sodium valproate)-treated patients. Additional investigational antiepileptic agents included in this article are rufinamide (CGP 33101), fosphenytoin, levetiracetam, losigamone, remacemide and stiripentol. All these drugs have undergone early characterisation with respect to pharmacokinetic features and interaction potential.

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.

Bioavailability studies of drugs with nonlinear pharmacokinetics: II. Absolute bioavailability of intravenous phenytoin prodrug at therapeutic phenytoin serum concentrations determined by double-stable isotope technique

Measurement of the absolute bioavailability of phenytoin (PHT) derived from test doses of phenytoin prodrug (PPD) at therapeutic PHT serum concentrations is complicated by two problems: 1) the area under the serum concentration versus time curve (AUC) produced by a given size of test dose will vary directly with background PHT serum concentration due to the nonlinear pharmacokinetic properties of PHT; 2) PPD is more water soluble than PHT, making renal excretion of PPD more likely. The authors describe a double-stable isotope method that obviates these two problems. Using only six subjects, the authors were able to demonstrate bioequivalence of PHT derived from intravenous PPD with intravenous PHT by current FDA standards for AUC ratio of test/reference formulation (90% confidence intervals between 0.80 and 1.20; ratio > or = 0.80 in > or = 80% of subjects; statistical power to detect a difference of 0.20 with a probability of 0.80).

Status epilepticus in children. The acute management

Status epilepticus is a common pediatric emergency that may result in significant morbidity and mortality. This article provides a clinical update on generalized tonic-clonic status epilepticus in children and a practical approach to their initial stabilization and pharmacologic management. Only an organized approach to the initial stabilization and management of the child in status epilepticus will help prevent unnecessary complications and death.

Emergency management of seizures: an overview

The drugs currently used in the emergency management of seizures are chiefly phenytoin, phenobarbital, diazepam, lorazepam, and paraldehyde. The combination of intravenous phenytoin and lorazepam has the advantages of rapid onset of action, sustained efficacy, and freedom from drug interactions. The intermittent oral or rectal administration of diazepam is especially useful for acute home treatment of recurrent seizures. Phenytoin prodrug (ACC-9653), an investigational new drug, is promptly absorbed after intramuscular injection. Unlike phenytoin, it does not require propylene glycol and high alkalinity for solubility and therefore does not produce soft-tissue injury after parenteral administration. It appears to be close to an ideal drug for the emergency management of seizures.