Increased brain P-glycoprotein in morphine tolerant rats.

The objective of this study was to determine whether chronic morphine exposure increased P-glycoprotein in rat brain. Male Sprague-Dawley rats were treated with morphine, saline, or dexamethasone for 5 days. On day 6, antinociceptive effect was measured to evaluate the extent of functional tolerance to morphine. Brain P-glycoprotein was detected by Western blot analysis of whole brain homogenate. Morphine- and dexamethasone-treated rats exhibited decreased antinociceptive response when compared to saline-treated controls. Brain P-glycoprotein was approximately 2-fold higher in morphine-treated rats compared to saline controls based on Western blot analysis. Chronic morphine exposure appears to increase P-glycoprotein in rat brain. P-glycoprotein induction may enhance morphine efflux from the brain, thus reducing morphine's pharmacologic activity. Induction of P-glycoprotein may be one mechanism involved in the development of morphine tolerance.

P-glycoprotein-mediated transport of morphine in brain capillary endothelial cells.

Cell accumulation, transendothelial permeability, and efflux studies were conducted in bovine brain capillary endothelial cells (BBCECs) to assess the role of P-glycoprotein (P-gp) in the blood-brain barrier (BBB) transport of morphine in the presence and absence of P-gp inhibitors. Cellular accumulation of morphine and rhodamine 123 was enhanced by the addition of the P-gp inhibitors N-{4-[2-(1,2,3,4-tetrahydro-6,7dimethoxy-2-isoquinolinyl)-ethyl]-phenyl}-9,10-dihydro-5-methoxy-9- carboxamide (GF120918), verapamil, and cyclosporin A. Positive (rhodamine 123) and negative (sucrose and propranolol) controls for P-gp transport also were assessed. Morphine glucuronidation was not detected, and no alterations in the accumulation of propranolol or sucrose were observed. Transendothelial permeability studies of morphine and rhodamine 123 demonstrated vectorial transport. The basolateral to apical (B:A) fluxes of morphine (50 microM) and rhodamine (1 microM) were approximately 50 and 100% higher than the fluxes from the apical to the basolateral direction (A:B), respectively. Decreasing the extracellular concentration of morphine to 0.1 microM resulted in a 120% difference between the B:A and A:B permeabilities. The addition of GF120918 abolished any significant directionality in transport rates across the endothelial cells. Efflux studies showed that the loss of morphine from BBCECs was temperature- and energy-dependent and was reduced in the presence of P-gp inhibitors. These observations indicate that morphine is transported by P-gp out of the brain capillary endothelium and that the BBB permeability of morphine may be altered in the presence of P-gp inhibitors.

Effects of a potent and specific P-glycoprotein inhibitor on the blood-brain barrier distribution and antinociceptive effect of morphine in the rat.

Previous data suggest that the analgesic effect of morphine may be modulated by P-glycoprotein (P-gp) inhibition. The effects of the P-gp inhibitor GF120918 on brain distribution and antinociceptive effects of morphine were examined in a rat cerebral microdialysis model. Pretreatment with GF120918 increased both the area under the concentration-time curve of unbound morphine in brain extracellular fluid (ECF) and morphine-associated antinociception. The area under the concentration-time curve ratio for unbound morphine in brain ECF versus unbound morphine in blood was significantly higher in GF120918-treated rats compared with control rats (1.21 +/- 0.34 versus 0.47 +/- 0.05, respectively; p <.05). Modulation of morphine brain-blood distribution was confirmed by quantitating brain tissue morphine in a separate group of rats; GF120918 increased the brain tissue:serum concentration ratio approximately 3-fold. The half-life of unbound morphine in brain ECF was approximately 3-fold longer in GF120918-treated rats compared with controls (p <.05). The fraction unbound of morphine in whole blood was not altered significantly in the presence of GF120918 (0.651 +/- 0.039) as compared with controls (0.662 +/- 0.035). Concentrations of unbound morphine-3-glucuronide in blood and brain ECF were increased in GF120918-treated rats versus controls. An integrated pharmacokinetic/pharmacodynamic model was developed to characterize the unbound blood and brain ECF morphine concentration profiles and concentration-effect relationships. The results of this study indicate that alteration of morphine antinociception by a potent P-gp inhibitor appears to be mediated at the level of the blood-brain barrier.

Phase I trial of a 96 h paclitaxel infusion with filgrastim support in refractory solid tumor patients.

A phase I study of a 96 h paclitaxel infusion with filgrastim support was performed to determine the toxicity, maximum-tolerated dose (MTD) and pharmacokinetics in patients with refractory solid tumors. In this phase I trial, the initial paclitaxel dose was 140 mg/m2/96 h followed by filgrastim (5 microg/kg/day s.c.) beginning 24 h after the paclitaxel and continued until granulocyte recovery. Cycles were repeated every 21 days. Patients with refractory solid tumors were eligible; however, only one previous chemotherapy regimen was allowed. The dose of paclitaxel was escalated by 20 mg/m2/96 h in subsequent cohorts until dose-limiting toxicity (DLT) occurred. Pharmacokinetic analysis was performed by quantitating paclitaxel concentrations at baseline, 24, 48, 72 and 96 h after the start of the paclitaxel infusion. Twenty-one patients were entered into this trial of which 19 were evaluable. A total of 52 treatment cycles were administered. DLT was seen in two of four patients at 200 mg/m2/96 h, and consisted of diarrhea, mucositis and granulocytopenic infection. The MTD of the 96 h paclitaxel infusion was 180 mg/m2 with filgrastim support. Mucosal and granulocyte toxicity were correlated with steady-state paclitaxel concentrations (Css) greater than 0.100 micromol/l. In the presence of liver function test 1.5 times or lower than normal, metastatic liver disease did not alter paclitaxel Css. Objective responses were observed in non-small cell lung cancer, small cell lung cancer and melanoma. The recommended phase II dose of paclitaxel infused over 96 h with filgrastim support is 180 mg/m2. Paclitaxel Css correlate with mucosal and granulocyte toxicity. In the presence of normal enzymatic function, metastatic liver disease does not affect paclitaxel clearance.

Effect of GF120918, a potent P-glycoprotein inhibitor, on morphine pharmacokinetics and pharmacodynamics in the rat.

PURPOSE: The objective of this study was to evaluate the effect of a potent P-gp inhibitor, GF120918, on the systemic pharmacokinetics and antinociceptive pharmacodynamics of a single intravenous dose of morphine in rats. METHODS: Male Sprague-Dawley rats received either 500 mg base/kg/d GF120918 or vehicle for 4 days by gavage, or no pretreatment. On day 4, morphine was administered as a 1- or 2-mg/kg i.v. bolus. Antinociception, expressed as percent of maximum possible response (%MPR), was evaluated over 300 min after morphine administration. Serial blood samples were collected and analyzed for morphine and morphine-3-glucuronide (M3G) by HPLC. RESULTS: Morphine clearance and distribution volume were not altered significantly by GF120918. M3G AUC in the GF120918-treated rats was approximately 2-fold higher than in vehicle-treated rats. For both morphine doses, %MPR and the area under the effect-time curve at 300 min were significantly higher in the GF120918-treated rats. A pharmacokinetic/pharmacodynamic effect model accurately described the effect-concentration data for the rats that received 1-mg/kg morphine; ke0 was significantly smaller for GF120918- vs. vehicle-treated and control rats (0.060 +/- 0.028 vs. 0.228 +/- 0.101 vs. 0.274 +/- 0.026 min-1, p = 0.0023). EC50 and gamma were similar between treatment groups. CONCLUSIONS: Pretreatment with GF120918 enhanced morphine antinociception, as assessed by the hot-lamp tail-flick assay, and elevated systemic M3G concentrations in rats. The differential pharmacologic response to morphine in the GF120918-treated animals could not be attributed to alterations in systemic morphine pharmacokinetics.