Esters of hydantoic acids as prodrugs of hydantoins

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.

Targeting drugs to the brain by redox chemical delivery systems

Chemical delivery systems (CDSs) based on the redox conversion of a lipophilic dihydropyridine to an ionic, lipid-insoluble pyridinium salt have been developed to improve the access of therapeutic agents to the central nervous system. A dihydropyridinium-type CDS or a redox analog of the drug is sufficiently lipophilic to enter the brain by passive transport, then undergoes an enzymatic oxidation to an ionic pyridinium compound, which promotes retention in the CNS. At the same time, peripheral elimination of the entity is accelerated due to facile conversion of the CDS in the body. This review discusses chemical, physicochemical, biochemical, and biological aspects in relation to the principles and practical implementation of the redox brain-targeting approach to various classes of drugs. Representative examples to the brain-enhanced delivery of neurotransmitters, steroids, anticonvulsants, antibiotics, antiviral, anticancer and antidementia agents, and neuropeptides and their analogs are presented in detail. In vivo and in vitro studies and preliminary clinical data of several novel derivatives have been promising, which could lead to a practical use of the redox CDSs after proper pharmaceutical development. The investigations accentuate the need for considering physicochemical, metabolic, and pharmacokinetic properties in designing of carrier systems that are able to target drugs into the central nervous system.

Phenytoin penetration into brain after administration of phenytoin or fosphenytoin

PURPOSE

This study was designed to measure the brain penetration of phenytoin (PHT) after intravenous (i.v.) administration of either standard PHT or fosphenytoin (FPHT), a PHT prodrug. The study was formulated to answer the question whether the time required for FPHT to be converted to PHT in the bloodstream would delay the accumulation of PHT in brain.

METHODS

Four rats were sampled at various times after intravenous infusion of 30 mg/kg PHT i.v. or 30 mg/kg PHT equivalents of FPHT i.v. PHT was measured in serum, protein-free ultrafiltrate, and in brain, by using high-performance liquid chromatography.

RESULTS

Although the initial PHT-free fraction was significantly higher for FPHT-treated rats than it was for PHT-treated rats, brain PHT levels were significantly reduced after infusion of FPHT.

CONCLUSIONS

When FPHT is used for treatment of generalized status epilepticus, it should be anticipated that lower initial brain PHT levels will be achieved than are typically found with standard PHT.

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.

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)

Acetylacroninium salts as soluble prodrugs of the antineoplastic agent acronine

The low aqueous solubility of acronine (approximately 2 mg/liter) has been overcome by identification of quaternary prodrug salts exhibiting apparent molar solubilities two to five orders of magnitude greater than acronine. The synthesis and kinetics of hydrolysis of the various prodrug acetylacroninium salts were studied, and the half-life for hydrolysis under conditions approximating the in vivo situation was estimated to be about 5 min. Such rapid reversion, together with the greatly increased solubility, appears to qualify the prodrug for intravenous use.

Rational development of a soluble prodrug of a cytotoxic nucleoside: preparation and properties of arabinosyladenine 5'-formate

The 5'-O-formate ester of arabinosyladenine was synthesized and shown to be suitable as a water-soluble prodrug, being at least 60 times more soluble than the parent cytotoxic nucleoside. This increased solubility was ascribed to a decrease in intermolecular interaction in the crystalline state, as evidenced by an similar 90 degrees lower melting point for the ester relative to the parent compound. The prodrug reverted to the parent compound in aqueous solution, its pH-rate profile being V-shaped with maximum stability at a pH similar 4.2, corresponding to a half-life of about 10 days. The rate of hydrolysis of the prodrug at 37degrees in 91% human serum and 91% whole blood was studied. At an initial concentration of 0.4 mg of prodrug/ml of 91% whole blood, reversion to arabinosyladenine appeared to be essentially complete in about 15 min. The prodrug did not appear to be subject to enzymatic deamination. This feature, together with the good solubility of the prodrug, makes possible the effective formulation of arabinosyladenine for intravenous purposes. The rationale involved in the general design of a prodrug and the specific considerations necessitated in the case of adenine arabinoside are discussed.