Targeted drug delivery to tumors: myths, reality and possibility.

The ultimate goal of drug delivery research is to help patients by developing clinically useful formulations. During the last several decades controlled drug delivery technology has advanced significantly, leading to the development of various clinical formulations improving patient compliance and convenience [1]. Current technologies allow delivery of drugs at desired release kinetics for extended periods of time ranging from days to years. Oral and transdermal drug delivery systems routinely deliver drugs for 24 h, substantially improving drug efficacy and minimizing side effects. Implantable systems can locally deliver drugs for months, even years. While significant advances have been made, there are still areas where substantial improvements need to be made to reach the next level of clinical relevance. One such area is targeted drug delivery to solid tumors. The clinically significant impact of targeted drug delivery lies in the ability to specifically target a drug or drug carrier to minimize drug-originated systemic toxic effects. Successful translation (from bench to bedside) of potential cancer and gene therapies, particularly small interfering RNA (siRNA) delivery, will largely depend on targeted drug delivery strategies. Overcoming the many challenges of identifying a successful targeted drug delivery strategy requires an understanding of events involving transport of drug or drug carrier to a target site after intravenous (i.v.) administration as well as issues relevant for specific target diseases and the body's response toward a drug delivery system. The current lack of clear recognition of problems facing the drug delivery field can be anticipated to result in only marginal advances in targeted drug delivery technologies in the coming years. The current unmet needs and challenges in this area were summarized by Professor Alexander T. Florence who is one of the few who raised awareness on the exaggerated claims of the nanoparticle-based drug targeting [2,3]. They need to be better appreciated and understood for achieving greater success in drug targeting to tumors. Thus, it would be profitable to address a variety of issues and factors that could affect the development of improved targeted drug delivery systems. Many terms have been used to describe nano-sized drug delivery systems, and here the term “nanoparticle” is used to represent a spectrum of systems, including nanocarrier, nanovehicle, nanosystem, nanostructure, and other terms used in the literature.