sdz-psc-833 and laniquidar

Modulation of P glycoprotein (Pgp) in clinical oncology has had limited success. Contributing factors have included the limitation in our understanding of the tumours in which Pgp overexpression is mechanistically important in clinical drug resistance; the failure to prove that concentrations of modulators achieved in patients were sufficient to inhibit Pgp; and the inability to conclusively prove that Pgp modulation was occurring in tumours in patients. New approaches are needed to determine the clinical settings in which Pgp overexpression plays a major role in resistance. (Clinical trials with third generation modulators are ongoing, including trials with the compounds LY335979, R101933 and XR9576. Using the Pgp substrate Tc-99m Sestamibi as an imaging agent, increased uptake has been seen in normal liver and kidney after administration of PSC 833, VX710 and XR9576. These studies confirm that the concentrations of modulator achieved in patients are able to increase uptake of a Pgp substrate. Furthermore, CD56+ cells obtained from patients treated with PSC 833 demonstrate enhanced rhodamine retention in an ex vivo assay after administration of the antagonist. Finally, a subset of patients treated with Pgp antagonists show enhanced Sestamibi retention in imaged tumours. These results suggest that Pgp modulators can increase drug accumulation in Pgp-expressing tumours and normal tissues in patients. Using third generation Pgp antagonists and properly designed clinical trials, it should be possible to determine the contribution of modulators to the reversal of clinical drug resistance.

Topics: Animals; Antineoplastic Agents; ATP Binding Cassette Transporter, Subfamily B, Member 1; Benzazepines; Clinical Trials as Topic; Cyclosporins; Dibenzocycloheptenes; Drug Interactions; Drug Resistance, Multiple; Drug Resistance, Neoplasm; Enzyme Inhibitors; Fluorescent Dyes; Gene Expression Regulation, Neoplastic; Genes, MDR; Humans; Mice; Mice, Knockout; Neoplasm Proteins; Neoplasms; Piperidines; Pyridines; Quinolines; Radionuclide Imaging; Radiopharmaceuticals; Rhodamines; Technetium Tc 99m Sestamibi; Tissue Distribution; Tumor Cells, Cultured

At present, P-glycoprotein (P-gp) function can be studied using positron emission tomography (PET) together with a labelled P-gp substrate such as R-[11C]verapamil. Such a tracer is, however, less suitable for investigating P-gp (over)expression. Laniquidar is a third-generation P-gp inhibitor, which has been used in clinic trials for modulating multidrug resistance transporters. The purpose of the present study was to develop the radiosynthesis of [11C]laniquidar and to assess its suitability as a tracer of P-gp expression. The radiosynthesis of [11C]laniquidar was performed by methylation of the carboxylic acid precursor with [11C]CH3I. The product was purified by HPLC and reformulated over a tC18 Seppak, yielding a sterile solution of [11C]laniquidar in saline. For evaluating [11C]laniquidar, rats were injected with 20 MBq [11C]laniquidar via a tail vein and sacrificed at 5, 15, 30 and 60 min after injection. Several tissues and distinct brain regions were dissected and counted for radioactivity. In addition, uptake of [11C]laniquidar in rats pretreated with cyclosporine A and valspodar (PSC 833) was determined at 30 min after injection. Finally, the metabolic profile of [11C]laniquidar in plasma was determined. [11C]Laniquidar could be synthesized in moderate yields with high specific activity. Uptake in brain was low, but significantly increased after administration of cyclosporine A. Valspodar did not have any effect on cerebral uptake of [11C]laniquidar. In vivo rate of metabolism was relatively low. Further kinetic studies are needed to investigate the antagonistic behaviour of [11C]laniquidar at tracer level.

Topics: Animals; ATP Binding Cassette Transporter, Subfamily B, Member 1; Benzazepines; Binding, Competitive; Carbon Radioisotopes; Cyclosporine; Cyclosporins; Quinolines; Radioactive Tracers; Rats; Rats, Wistar; Tissue Distribution

P-glycoprotein inhibitors can increase the oral bioavailability of paclitaxel. We have now explored the mechanisms that determine the efficacy of several novel P-glycoprotein inhibitors to increase the absorption of paclitaxel from the gut lumen of mice in both in vivo and in vitro experiments. The inhibitors studied were cyclosporin A, PSC 833, GF120918, LY335979 and R101933. Mass balance studies showed that GF120918 was the most effective inhibitor, resulting in almost complete uptake of paclitaxel. PSC 833 was slightly less effective, whereas cyclosporin A and LY335979 were moderately effective. R101933 had only marginal effects. These findings were in line with in vitro transport experiments using LLC-mdr1a cells. By studying the intra-intestinal kinetics of the agents we found that cyclosporin A, PSC 833 and GF120918 rapidly passed the stomach and traveled concurrently with paclitaxel through the intestines, whereas LY335979 and R101933 delayed stomach emptying. Moreover, these latter compounds appear to be more readily absorbed when released into the intestines thus reducing local intestinal concentrations. Due to their combined effects on absorption and metabolic elimination of paclitaxel, cyclosporin A and PSC 833 resulted in the highest paclitaxel levels in plasma. In conclusion, our models provide insight into the factors that determine the suitability of P-glycoprotein inhibitors to enable oral paclitaxel therapy and will be useful in selecting candidate inhibitors for clinical testing.

Topics: Acridines; Administration, Oral; Animals; Antineoplastic Agents, Phytogenic; ATP Binding Cassette Transporter, Subfamily B, Member 1; Benzazepines; Biological Availability; Biological Transport, Active; Cyclosporine; Cyclosporins; Dibenzocycloheptenes; Drug Interactions; Female; Gastric Mucosa; In Vitro Techniques; Intestinal Mucosa; Intestines; Mice; Models, Biological; Paclitaxel; Quinolines; Stomach; Tetrahydroisoquinolines; Time Factors