Passive, active and physical. Passive targeting relies on the natural distribution pattern of the drug-carrier system. For example, particles 5 micrometer or smaller are readily removed from the blood by macrophages of the Reticulo Endothelial System (RES) when administered systemically. This neutral defense mechanism of the RES thus provides an opportunity to target drug, encapsulated in or conjugated to an appropriate carrier system, to macrophages.
Mechanical filtration of large carriers by capillary blockage can also be exploited to target drugs to the lungs by the venous supply and to the organs through the appropriate arterial supply. By controlling the rate of drug release, once can achieve the desired therapeutic action in the targeted organ.
Passive targeting also includes delivery of drug-carrier systems directly to a discrete compartment in the body (e.g. different regions of the GI tract, eye, nose, knee joints, lungs, vagina, rectum, respiratory tract, or other). This offers the opportunity for the treatment of diseases that require a persistent and sustained presentation of drug at that site.
Active targeting employs a deliberately modified drug-drug-carrier molecule capable of recognizing and interacting with a specific cell, tissue, or organ in the body. Modifications of the carrier system may include a change in the molecular size, alteration of the surface properties, incorporation of antigen-specific antibodies, or attachment of cell receptor-specific ligands.
Physical targeting refers to delivery system that releases a drug only when exposed to a specific macro-environment, such as change in pH or temperature or the use of an external magnetic field.
Particulate Drug Delivery Systems:
The concept of using particles to deliver drugs to selected sites in the body originated from their use as radio diagnostic agents in medicine in the investigation of the RES (liver, spleen, bone marrow and lymph nodes), gastrointestinal examination, and so on. Particles ranging in sizes from 20 up to 300 micrometer or lower have been proposed for drug targeting.
Because of the small size of the particles, particulate drug delivery systems can be introduced directly into the central circulation by intra-articular or intravenous injection, or delivered to a given body compartment, for example, by injection into a joint or by administration by an aerosol to the lungs and nose. Subcutaneous and intraperitoneal administration route have also been used to deliver drugs to the lymphatic system and regional lymph nodes.
Particulate drug delivery system can be monolithic (i.e. containing an intimate mixture of drug and the core material), capsular (in which the drug is surrounded by the carrier material), or emulsion (in which the drug is dispersed in a suspension of the carrier material) types. The biofate (passive targeting of particulate drug delivery systems depend on the size and shape, charge and surface hydrophobicity of the particles.