Enhanced brain delivery of 2′-fluoro-5-methylarabinosyluracil

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.

Recent advances in retrometabolic design approaches

The retrometabolic drug design approaches simultaneously incorporate structure activity (SAR) and structure metabolism (SMR) relationships in the design process. Two major approaches were developed, the chemical delivery systems (CDS), which allow chemical-enzymatic targeting of drugs via strategic sequential enzymatic activation of the inactive CDSs. On the opposite end of the retrometabolic design loop are the soft drugs (SD), which are designed to have highly improved therapeutic indeces by controlling their metabolism, after they achieve their therapeutic role. One of the most successful SD class is the 'inactive metabolite approach', where the design starts from an inactive metabolite of a drug. Its strategic manipulation yields an isosteric/isoelectronic drug analog, which is enzymatically deactivated to the very inactive metabolite at the desired compartment and with controlled rate. Overall, retrometabolic approaches represent a complex collection of chemical-enzymatic means for the design of safer drugs and for their controlled release. Most recent advances involve FDA approval of a soft steroid, as well as the first successful brain targeting of various neuropeptides and their brain-targeted analogs.

Recent advances in the brain targeting of neuropharmaceuticals by chemical delivery systems

Brain-targeted chemical delivery systems represent a general and systematic method that can provide localized and sustained release for a variety of therapeutic agents including neuropeptides. By using a sequential metabolism approach, they exploit the specific trafficking properties of the blood-brain barrier and provide site-specific or site-enhanced delivery. After a brief description of the design principles, the present article reviews a number of specific delivery examples (zidovudine, ganciclovir, lomustine benzylpenicillin, estradiol, enkephalin, TRH, kyotorphin), together with representative synthetic routes, physicochemical properties, metabolic pathways, and pharmacological data. A reevaluated correlation for more than 60 drugs between previously published in vivo cerebrovascular permeability data and octanol/water partition coefficients is also included since it may be useful in characterizing the properties of the blood-brain barrier, including active transport by P-glycoprotein.

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.