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Paclitaxel is an alkaloid that inhibits endothelial cell proliferation, motility, and tube formation at nanomolar concentrations. Cationic liposome preparations have been shown to target blood vessels. We wished to explore the possibility that paclitaxel encapsulated in cationic liposomes carries paclitaxel to blood vessels and thereby provides an antiangiogenic effect. We used a humanized SCID mouse melanoma model, which allowed us to analyze tumor growth and tumor angiogenesis in an orthotopic tumor model. Here, human melanoma cells grow on human dermis and are in part nourished by human vessels. We show that paclitaxel encapsulated in liposomes prevents melanoma growth and invasiveness and improves survival of mice. Moreover, liposome-encapsulated paclitaxel reduces vessel density at the interface between the tumor and the human dermis and reduces endothelial cell mitosis to background levels. In contrast, equimolar concentrations of paclitaxel solubilized in Cremophor EL(R) had only insignificant effects on tumor growth and did not reduce the mitotic index of endothelium in vivo, although the antiproliferative effect of solubilized paclitaxel in Cremophor EL(R)in vitro was identical to that seen with liposome-coupled paclitaxel. In conclusion, we present a model of how to exploit cytotoxic effects of compounds to prevent tumor growth by using cationic liposomes for targeting an antiproliferative drug to blood vessels.

Primary cultures of bovine adrenal chromaffin cells provide large quantities of a homogeneous population of target cells for nerve growth factor (NGF) and, thus, are a suitable system for studying the molecular mechanism of action of NGF. In this study, we have shown that NGF mediates the specific induction of the key enzymes in catecholamine biosynthesis, tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). Acetylcholinesterase (AChE), an enzyme which catalyzes the breakdown of acetylcholine, is also induced by NGF. We have compared NGF-mediated TH and AChE induction and have provided pharmacological evidence that TH induction involves a post-transcriptional, polyadenylation-dependent event (blockable by 9-beta-arabinofuranosyladenine but not by alpha- amanitin), whereas AChE induction requires transcription (blockable by alpha-amanitin). DBH and PNMT appear to be regulated via the same mechanism as TH. The time course of TH induction is such that NGF must be continuously present for at least the first 36 hr (during which time TH levels remain unchanged), and then the entire increase takes place during the subsequent 12 hr. In contrast, AChE induction proceeds linearly with time of NGF exposure. These data suggest that there may be multiple mechanisms by which NGF regulates enzyme induction. We have also compared the effects of cAMP with those of NGF. As compared to NGF, cAMP produces a different pattern of enzyme induction (in addition to TH, DBH, PNMT, and AChE, dopa decarboxylase (DDC) is also induced), it acts rapidly (a 12-hr exposure produces the full effect), and it acts only at the transcriptional level (its effects are blocked by alpha-amanitin). These data provide evidence that cAMP does not act as a second messenger for NGF with regard to enzyme induction.