1441 The treatment of glioblastoma with chemotherapeutic agents is hampered by the inability of most drugs to sufficiently penetrate the blood brain barrier and deliver adequate drug exposure. Taxanes have been an important class of agents in cancer treatment, but have contributed little to therapeutic advances in glioblastoma treatment. TPI 287 is a highly lipophylic third generation taxane which avoids the GP170 drug efflux pump, is orally bioavailable, and penetrates the blood brain barrier and accumulates in the brain of rats and mice. After a single 20mg/kg iv dose, TPI 287 brain/plasma ratio increases to 63.79 at 96h post dose, with 261 nM drug concentration in the brain of mice.The mean residence time for TPI 287 exposure was 42.5h and 17.6h for brain and plasma, respectively. In cell culture the U251 glioblastoma tumor cell line was shown to be very sensitive to TPI 287 with a 6 nM IC50 in standard MTS cytotoxicity assay. In addition, subcutaneous tumors were also sensitive to a single Q4Dx3 cycle of treatment by iv. and po. drug administration. In the U251 intracranial glioblastoma antitumor efficacy study TPI 287 as single agent or in combination with temozolomide increased survival of treated mice p

A192 Tumor resistance involves several factors including the extrusion of chemotherapeutic agents by constitutively expressed or upregulated ABC transporter pumps such as MDR1 (P-gp 170 kDa) which is capable of exporting a variety of hydrophobic compounds before they reach their drug targets. This pump is also found in the cells of the intestine, liver, kidney, and blood-brain barrier, sites which affect the ability of the drug to reach the tumor. TPI 287 is a taxane derivative with previously shown enhanced oral bioavailability and penetrance of the blood-brain barrier in rodents. Because of the molecular complexity of the P-gp pump, we explored several different assays to show that TPI 287 behaves differently from paclitaxel and ortataxel. In a 72 hr proliferation assay, mdr1(-) tumor cells from several different tissues (breast, colon, brain, lung, ovary) exhibit paclitaxel IC50s below 0.1nM and TPI 287 IC50s in the 0.5-10nM range. However, in mdr1(+) tumor cells (as determined by Western blot), the paclitaxel IC50 is 50-200 times higher than that for TPI 287. Further, TPI 287 blocks the efflux of Rhodamine 123 (an MDR1 substrate) from mdr1(+) cells, as does ortataxel, whereas paclitaxel and docetaxel do not. This fluorescence-based assay with the SK-N-FI(mdr1+) and MV522/mdr1+ cell lines generates a 30-40 fold higher Blocking Factor (BF) for TPI 287 versus docetaxel or paclitaxel. TPI 287 and ortataxel generate similar high BFs for R123 efflux in MV522/mdr1+ cells. In contrast, TPI 287 does not block efflux of an MRP1 substrate from an mrp1+ cell type any more than docetaxel. In a luciferase assay, with cell-free P-gp membranes, which measures the ATPase activity of the P-gp pump, TPI 287 behaves differently from paclitaxel and ortataxel. TPI 287 causes a 2-3 fold higher consumption of ATP than paclitaxel at any given concentration, and ortataxel stimulates a very low level of ATPase activity. When tested against the high verapamil stimulation of the P-gp ATPase, Cyclosporin-A and ortataxel cause a dose dependent decrease in the level of ATPase activity, but paclitaxel and TPI 287 have little effect. In addition, Caco-2 and MDCKII/mdr1+ permeability analyses show that paclitaxel is a good substrate of the P-gp pump in the basolateral to apical direction (5-fold greater activity than in the apical to basal direction in MDCKII/mdr1+), but TPI 287 shows no such polarization, suggesting that it is not a substrate of the active P-gp transport system in these cells. In xenograft studies of MDR1 tumors in nude mice, TPI 287 has shown efficacy where paclitaxel has none, suggesting avoidance or modulation of the tumor cell MDR1 pump. And in a pancreatic tumor xenograft, oral administration of TPI 287 had very similar efficacy to IV administration, suggesting avoidance of the MDR1 pump in brush border cells of the intestine. These data demonstrate that TPI 287 is distinct from other taxanes and functions in mdr1+ tumor cells by avoidance of the P-gp efflux mechanism and may act as a blocking agent in some circumstances in order to reduce drug efflux. This profile further supports the development of TPI 287 as an oral anticancer agent.