Implantable drug delivery (IDD) devices offer more uniform drug release rates, lower required doses, and localized delivery. These devices are generally composed of the drug of interest distributed in a polymer matrix. Examples of marketed implantable drug-delivery formulations include Norplant for birth control (Wyeth Laboratories), Gliadel for localized delivery of a chemotherapeutic agent to the brain (Guilford Pharmaceuticals), and Viadur for slow release of hormones for prostate cancer treatment (Bayer). The sales of Gliadel in 2003 were nearly $20 million (an increase of 32 percent from 2002), and annual Viadur sales are projected to reach $150 million.
Based on these trends, the demand for implantable drug-delivery systems is expected to exceed $2 billion by 2012. There is still room for improvement in IDD technology. Two emerging nanotechnologies with applications in implantable drug delivery are nanoporous membranes and biochips.
Nanoporous membranes are microfabricated with well-defined pores (diameters in the tens of nanometers). The membranes can be used to deliver small-molecule, peptide, or protein drugs (Figure 16-5). One application under investigation involves encapsulation of pancreatic islet cells for insulin delivery. The reproducible and uniform pore size precisely controls the material exchange across nanoporous membranes: Nutrients for the cells and secreted insulin can pass through the pores, but proteins and cells from the immune system that may attack the implanted islet cells are restricted from entering the biocapsules due to their size.
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