Cellular carriers

Microspheres and nanoparticles often consist of biocompatible polymers and belong either to the soluble or the particle type carriers. Besides the aforementioned HPMA polymeric backbone, carriers have also been prepared using dextrans, ficoll, sepharose or poly-L-lysine as the main carrier body. More recently alginate nanoparticles have been described for the targeting of antisense oligonucleotides [28]. As with other polymeric carrier systems, the backbone can be modified with e.g. sugar molecules or antibody fragments to introduce cellular specificity. 

Besides parenteral application of microspheres and nanoparticles for cell selective delivery of drugs, they have more recently been studied for their application in oral delivery of peptides and peptidomimetics [30]. Immunological tolerance induction against beta-lac-toglobulin could be achieved by application of this protein in a poly-lactic-glycolide microsphere formulation [31]. 

Polymeric micelles are mostly small (10-100 nm) in size and dmgs can be incorporated by chemical conjugation or physical entrapment. For efficient delivery activity, they shonld maintain their integrity for a sufficient amount of time after injection into the body. Most of the experience with polymeric micelles has been obtained in the field of passive targeting of anticancer drugs to tumours [33]. Attachment of antibodies or sugars, or introduction of a polymer sensitive to variation in temperature or pH has also been stndied [32]. 

Targeting of therapeutics, whether they are chemical entities, peptides, proteins or nucleic acid polymeric substances, relies on the release of the drug from the carrier and subsequent access to the molecular target. Advances in the understanding of membrane structure, functions and properties of the various cellular organelles is the basis for directing the pharmacologically active components to the correct cellular compartments [39]. 

In most other cases, the lysosomes are a transit compartment en route to the cytoplasm. In case the targeted agent is lysosomally unstable (e.g. DNA) this compartment should be avoided. 

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Cellular Carriers. Erythrocytes, leukocytes, platelets, islets, hepatocytes, and fibroblasts have all been suggested as potential carriers for drugs and biological substances. They can be used to provide slow release of entrapped drugs in the circulatory system, to deliver drugs to a specific site in the body, as cellular transplants to provide missing enzymes and hormones (in... 

LEUKOCYTES. Leukocytes are white blood cells involved in the cellular and humoral defense of the organism against foreign materials. They are grouped into two classes polymorphonuclear leukocytes, which comprise neutrophils, eosinophils, and basophils, and mononuclear leukocytes that include lymphocytes and monocytes. Of these, neutrophils and lymphocytes have been suggested as potential cellular carriers. 

Additionally, polycationic substitutions and various groups designed to bind to cellular carrier systems have been synthesized. Although many compounds have been synthesized, the data reported to date are insufficient to draw firm conclusions about the value of such approaches or structure-activity relationships (SARs) (332). 

Progress and Road Blocks in the Development of Carbon Nanotubes as Cellular Carriers for Nucleic Acids... 

A cellular licensee may serve all or just a portion of a designated MSA or RSA. Usually most of a MSA is served, although large portions of a RSA may be left unserved. The actual service area is defined by the cellular carrier and is called the cellular geographic service area (CGSA). Debate continues as to whether the FGG s effort to promote competition has been successful because there is often little difference in the prices charged by the wireline and nonwireline licensees or in the quality of service. Nonetheless, the overall price of cellular service has continued to drop and the number of subscribers has increased dramatically since service began in 1983. As of January 1996, there were approximately 34 million cellular subscribers in the US. and it is expected that there will be almost 65 million by the end of 2005. 

Initially, the development of digital cellular was the result of major concerns by the cellular carriers about their ability to meet a rapidly growing subscriber base. With growth rates of over 40%, it did not appear that existing analog technology would be able to cope with the projected numbers of subscribers within a few years. 

A cell site is a transmitter/receiver location, operated by the cellular carrier (or wireless carrier) through which radio links are established between the cellular system and the mobile units. The area reliably served by a given cell site is referred to as a cell. 

Cellular switches may be connected to the PSTN using several different configurations. The initial connection offered to cellular carriers was a trunk-side connection to an end office and is known as a Type 1 connection. Later, as a result of an FCC order, cellular carriers were able to obtain connections from a tandem office, which are known as Type 2A. There are also other variants available, such as high-usage trunks to specific end offices (Type 2B), trunks to 911 tandems (Type 2C), trunks to operator services tandems (Type 2D), and 56 kb/s circuits used for signaling system 7 (SS7) appUcations (Type S). 

Cellular carriers are entitled to telephone number assignments just like any LEC. They often have entire NXX codes (blocks of 10,000 numbers) dedicated to them for cellular service. This is particularly true when the cellular carrier has a tandem (Type 2) interconnection with the LEC. However, entire codes can also be used with other types of interconnection as well. 

Because of heavy fraud activity, cellular carriers sometime restrict mobile users to placing long distance calls by using either a calling card, collect, or third-number billing options. This is possible because of a service called selective class of call screening (SCCS) that is offered to the cellular carriers by the LECs. 

Wireless carrier (WC) A generic term used to describe entities providing wireless service. Such entities include, but are not limited to, cellular carriers, radio common carriers, and private carriers. 

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