Showing papers in "Critical Reviews in Therapeutic Drug Carrier Systems in 1987"

Fate and behavior of liposomes in vivo: a review of controlling factors.

Abstract: Of fundamental importance in the design of a therapeutic drug carrier system is a thorough understanding of the factors which control its fate in the living animal. The use of liposomes as a carrier system able to improve the therapeutic efficacy of a wide range of drugs, requires manipulation of its physical characteristics, thereby influencing in vivo behavior. This review brings together findings of recent studies which describe how liposomal stability and clearance in vivo are controlled by the architecture of the vesicles themselves which in turn, via interaction with humoral factors, controls the fate in terms of tissue distribution of the carrier and its contents. Based on these studies, a rationale for liposome design aimed at in vivo drug delivery is discussed.

Cyclodextrins in drug carrier systems.

Abstract: One of the important characteristics of cyclodextrins is the formation of an inclusion complex with a variety of drug molecules in solution and in the solid state. As a consequence of intensive basic research, exhaustive toxic studies, and realization of industrial production during the past decade, there seem to be no more barriers for the practical application of natural cyclodextrins in the biomedical field. Recently, a number of cyclodextrin derivatives and cyclodextrin polymers have been prepared to obtain better inclusion abilities than parent cyclodextrins. The natural cyclodextrins and their synthetic derivatives have been successfully utilized to improve various drug properties, such as solubility, dissolution and release rates, stability, or bioavailability. In addition, the enhancement of drug activity, selective transfer, or the reduction of side effects has been achieved by means of inclusion complexation. The drug-cyclodextrin complex is generally formed outside of the body and, after administration, it dissociates, releasing the drug into the organism in a fast and nearly uniform manner. In the biomedical application of cyclodextrins, therefore, particular attention should be directed to the magnitude of the stability constant of the inclusion complex. In the case of parenteral application, a rather limited amount of work has been done because the cyclodextrins in the drug carrier systems have to be more effectively designed to compete with various biological components in the circulatory system. However, the works published thus far apparently indicate that the inclusion phenomena of cyclodextrin analogs may allow the rational design of drug formulation and that the combination of molecular encapsulation with other carrier systems will become a very effective and valuable method for the development of a new drug delivery system in the near future.

Nanoparticles in drug delivery.

Abstract: Alkylcyanoacrylates can be polymerized in acidified aqueous media by a process of anionic polymerization. The small particles produced tend to be monodisperse and have sizes in the range of 20 to 3000 nm depending upon the polymerization conditions and the presence of additives in the form of surfactants and other stabilizers. The polyalkylcyanoacrylate nanoparticles so produced have been studied in recent years as a possible means of targeting drugs to specific sites in the body, with particular emphasis in cancer chemotherapy. The small colloidal carriers are biodegradable and drug substances can be incorporated normally by a process of surface adsorption. The review by Davis and others considers the formulation of nanoparticles, the important physicochemical variables such as pH, monomer concentration, added stabilizers, ionic strengths, etc., as well as the characteristics of the particle so created in terms of surface charge, particle size, and molecular weight. Monodisperse particles in the range of 20 to 3000 nm can be obtained. In addition, by the use of stabilizers such as dextran and its derivatives, which can be incorporated into the nanoparticle surface by a process of polymer grafting, it is possible to make nanoparticles with interesting surface characteristics and different surface charges (sign). The stability of nanoparticles in vitro and their biodegradation in vivo are examined, and the possible formation of toxic products such as formaldehyde is highlighted. Alternative biodegradable acrylates are mentioned. Drugs can be incorporated into nanoparticles by either direct incorporation during the polymerization process or adsorption to preformed nanoparticles. The efficiency of the incorporation and the release characteristics of model compounds as well as anticancer drugs are discussed. Methods for examining these processes, including the determination of adsorption and desorption, kinetics, and isotherms, are mentioned. Selectivity in drug targeting can, in theory, be achieved by the attachment of some form of homing device, normally a monoclonal antibody or a lectin. Work in vitro and in vivo, where nanoparticles have been coated with monoclonal antibodies, is described. Finally, methods for the labeling of nanoparticles with gamma-emitting radionuclides are presented, and results obtained in animal species are given.

Intranasal drug delivery for systemic medications.

Abstract: This article is designed to provide a critical literature review on the scientific advances in intra- and transnasal drug delivery for systemic medication. The article discusses the fundamentals, developmental concepts, and biomedical assessment of the transnasal administration of systemically effective drugs which are either easily inactivated when taken orally, due to the extensive hepatic first-pass elimination, or too large in molecular size to be absorbed efficiently, except by i.v. administration.

The extracellular matrix of stratum corneum: role of lipids in normal and pathological function

Abstract: The mammalian stratum corneum, formerly treated as a homogeneous film, is now more properly viewed as a two-compartment system. The cornified cell is protein-enriched and lipid-depleted, lying embedded in an expanded extracellular matrix of highly nonpolar lipids. Because of its strategic location between the cornified layer, this lipid matrix is responsible for many phenomena related to the permeability barrier, as well as cohesion and desquamation. Thus, manipulation of this compartment could lead to enhanced drug delivery and improved lubrication, as well.

The design of cytotoxic-agent-antibody conjugates.

Abstract: The rationale for the use of antibodies as carriers of cancer chemotherapeutic agents is based upon: the presence on cells of tumor-associated cell surface antigens (TAA); the ability to obtain specific polyclonal or monoclonal antibodies against them; and the availability of methods for binding appropriate toxic agents or radionuclides with retention of activity of both antibody and agents. The general finding so far has been that both conventional polyclonal and hybridoma-derived monoclonal antibodies can deliver cytocidal amounts of toxic agents to target tumor cells both in vitro and in vivo. Moreover, various agents with different modes of antitumor activity (e.g., DNA intercalation or alkylation, enzyme inhibition, and cell surface modification) have all produced superior tumor inhibition in conjugate form compared to the individual or synergistic inhibition produced by agent and antibody. Recent studies are contributing to the understanding of the mechanism of action of drug-antibody conjugates and are thus establishing guidelines for this approach to cancer therapy.

Nonparenteral administration of peptide and protein drugs.

Abstract: Peptides/proteins are an important class of drugs which are usually administered by parenteral route. In recent years, pharmaceutical research has been directed towards developing a nonparenteral route of delivery of peptide/protein drugs. These studies report that it may be possible to administer the peptides/proteins especially insulin, by nasal, buccal, rectal, or even transdermal route. Therefore, there is a great potential for future development of a nonparenteral route of delivery of peptide/protein drugs. The primary objective of this review is to report the present status of research involving nonparenteral administration of macromolecular peptides/proteins, with special emphasis on insulin.

Transdermal drug delivery: problems and possibilities.

Abstract: Transdermal drug delivery has, in recent years, attracted considerable interest. However, despite the obvious advantages of this novel route of drug administration to achieve systemic therapeutic effect, there remain important, and sometimes severe, limitations on the physiochemical and pharmacological properties of the agent to be delivered. This article reviews the present status and accomplishments of transdermal drug delivery. The benefits, disadvantages, and, as yet, unresolved problems inherent to drug input via the skin are addressed. Deficiencies in our knowledge of the percutaneous absorption process in man are identified, and approaches to elucidate these unknowns are discussed. Finally, with respect to the future potential of transdermal drug delivery, experimental and modeling techniques, which can be employed to assess the feasibility and promise of a therapeutic candidate, are described and illustrated with pertinent examples.

Poly(hydroxy acids) in drug delivery.

Abstract: Poly(hydroxy acids) so far have been examined for use in drug delivery in limited number, while the advantageous use of the polymers has been recognized due to their biodegradability and biocompatibility. Homo- and copolymers of lactic acid and glycolic acid have been studied in drug delivery by many workers, while homo- and copolymers of epsilon-caprolactone have been studied by only one group of workers. Although poly-hydroxybutyric acid had been found to be a naturally occurring polymer, examination as to the use of the polymer in drug delivery is rather recent and reports are still limited. In the present article, the use of poly(hydroxy acids) including homo- and copolymers of lactic acid and glycolic acid, polycaprolactone, and poly-beta-hydroxybutyric acid in drug delivery is reviewed. Physicochemical properties, biodegradability, and biocompatibility of the polymers, and evaluations in vitro and in vivo of specific dosage forms using the polymers, are included. The most recent work in our laboratories on the use of polyactic acid and poly-beta-hydroxybutyric acid is also included.

Microencapsulated cells as hormone delivery systems.

Abstract: Transplantation of pancreatic islets of Langerhans has been shown to prevent the development of many of the complications associated with diabetes. Transplanted islets, however, are readily rejected by the immune system. The use of artificial membranes to isolate the transplanted islets from the immune system of the host prolongs islet allografts in experimental animals. We have developed a method for encapsulating islets in semipermeable membranes composed of alginate and polylysine. The same technique can be applied to other endocrine cell types. The capsules are 700 to 800 micron in diameter with a hydrogel membrane approximately 4 micron thick. Intraperitoneal allografts of 5 x 10(3) encapsulated islets reversed diabetes in rats for up to 21 months and intact capsules with viable beta cells could be recovered from the recipients. Microencapsulation of endocrine cells for transplantation could potentially be used in the clinical treatment of hormone deficiency diseases.

Synthetic carriers of oxygen.

Abstract: During the last decade, construction of artificial carriers of oxygen for transfusion purposes has evolved in three main directions, which can be reviewed as follows. The first approach consists of modifying hemoglobin (Hb), the natural oxygen carrier, in order to lower its oxygen affinity and increase its intravascular persistence. To achieve this aim, two basic procedures have been used: molecular and environmental modification. In the first case, Hb is modified with chemical reagents; the second requires encapsulation of Hb to obtain artificial erythrocytes. The second approach is based on the use of synthetic oxygen-carrying chelates that mimic the oxygenation function of Hb. The main products in this class are metalloporphyrins, whose chemical environment is designed to render them efficient as reversible carriers of oxygen in vivo. Finally, the third approach deals with the perfluorochemicals used in emulsified form. Perfluorochemical liquids are excellent gas solvents, but some problems remain unsolved with regard to their development as oxygen carriers in vivo: low O2 dissolving capacity, toxicity, and excretion.