Stealth liposomes and tumor targeting: one step further in the quest for the magic bullet.

At the turn of the 20th century, the German bacteriologist Paul Ehrlich coined the expression “magic bullets” in his search for chemotherapeutic agents with specific affinity for diseased tissues. Understanding target structure and function, developing drug delivery strategies to achieve controlled release, and targeting of drugs to specific tissues of the body have been a major focus of research in the last decades in an attempt to improve selectivity in cancer treatment. Shortly after they were first characterized (1), liposomes were proposed as drug carriers in cancer chemotherapy by Gregoriadis et al. (2). Since then, interest in liposomes as devices for drug delivery gradually increased, and they have been one of the main players in the cancer drug delivery arena for the last 25 years. However, the early enthusiastic notes on liposomes and cancer drug delivery were strongly criticized (3, 4) when it became apparent that liposomes are rapidly recognized and removed from the circulation by the RES, thus nullifying any possible chance of substantial localization in tumors. Moreover, the initial steps of the liposome approach were tainted by a nebulous scientific rationale: why should liposomes home to tumors? Why should liposomes spare some healthy tissues? To deal with these issues, a rational approach, linking liposome formulation with liposome pharmacology and its implications on biodistribution and extravascular transport, was needed. Clearly, if liposomes are to be used for targeting to extraRES tissues, a key issue is to reduce the rate of uptake by the RES so as to enable them to remain in the circulation longer. The effect of particle size in favor of small vesicles was recognized early (5). Thereafter, during the 1980s, liposome composition was found to play an important role in circulation time, and the key factors involved were characterized. High-phasetransition temperature phospholipids, a high fraction of cholesterol, and a small fraction of some specific glycolipids (e.g., monosialoganglioside and hydrogenated phosphatidylinositol) imparting a weak surface negative charge were recognized as factors contributing to the longer circulation half-lives of liposomes (6–9). About a decade ago, this liposome engineering process culminated with the observation that coating of liposomes with PEG, a synthetic hydrophilic polymer, would improve their stability and lengthen their half-lives in circulation (10–13), rendering the use of glycolipids obsolete. PEG coating inhibits protein adsorption and opsonization of liposomes, thereby avoiding or retarding liposome recognition by the RES. These PEG-coated liposomes are also referred to as sterically stabilized, or Stealth liposomes. The PEG stabilizing effect results from local surface concentration of highly hydrated groups that sterically inhibit both hydrophobic and electrostatic interactions of a variety of blood components at the liposome surface (14, 15). Although PEG is the most common polymer used for liposome coating, other polymers have also been shown to protect liposomes from opsonization and prolong their circulation time (16). The rationale for long-circulating liposomes in cancer drug delivery was based on data revealing a strong correlation between liposome residence time in blood and their uptake by implanted tumors in mice (8). We hypothesized then that liposome extravasation in tumors is the result of passive convective transport through a leaky endothelium. A longer blood residence time will result in repeated passages through the tumor microvascular bed of high concentrations of vesicles and, consequently, in a greater efficiency of extravasation per unit volume of convective transport. The physiopathological changes underlying the high permeability of tumor microvessels to liposomes, other nanoparticles, and macromolecules include large interendothelial fenestrations, discontinuous basement membranes, and a high rate of trans-endothelial transport (17) and appear to be secondary to the neoangiogenic stimulus caused by factors secreted by tumors cells such as vascular endothelial growth factor, formerly referred to as vascular permeability factor (18). An additional factor contributing to liposome accumulation is the lack of a functional lymphatic drainage in tumors, thus creating a “dead-end” for extravasated liposomes. The enhanced permeability and retention model, which has been proposed to explain the preferential accumulation of macromolecules in tumors (19, 20), is also applicable to liposomes. Morphological studies with colloidal gold-labeled Stealth liposomes (21) and in vivo dynamic observations in the skin-fold chamber model with fluorescent labels (22) indicate that liposomes remain in the tumor interstitial fluid in close vicinity to tumor vessels. Drug molecules are released from the extravasated liposomes as a consequence of poorly understood processes that may include chemical disruption of the gradient retaining the drug, as in the case of doxorubicin (23), and enzymatic breakdown of the liposome membrane. In this issue of Clinical Cancer Research, Harrington et al. (24) present important observations on the pharmacokinetics, biodistribution, and imaging of radiolabeled, drug-free Stealth liposomes in cancer patients. In this regard, this is probably one of the most complete studies on Stealth liposomes in humans. One of the strengths of the study by Harrington et al. (24) is the labeling methodology, which is based on the formation of an intraliposomal In-DTPA chelation complex (25). This complex, as also shown for Ga-deferoxamine (26), is highly stable Received 11/28/00; accepted 12/4/00. 1 To whom requests for reprints should be addressed, at Department of Oncology, Hadassah Medical Center, Kiryat Hadassah, Jerusalem il91120, Israel. Fax: 972-2-643-0622; E-mail: [email protected]. 2 The abbreviations used are: RES, reticuloendothelial system; PEG, polyethyleneglycol; DTPA, diethylenetriaminepentaacetic acid. 223 Vol. 7, 223–225, February 2001 Clinical Cancer Research