Influence of blood-brain barrier efflux pumps on the distribution of vincristine in brain and brain tumors

Vincristine (VCR) is efficacious in some but not all brain cancers and an established substrate of Pgp and Mrp1. However, the extent to which such transporters affect the VCR penetration through the blood-brain barrier (BBB) is poorly understood. To evaluate the role of Pgp and Mrp1 in VCR CNS distribution, VCR concentrations were analyzed under steady-state conditions in normal brain, brain tumor, and bone marrow in wild-type (WT), Mrp1 ko (mrp1−/−), Pgp ko (mdr1a−/−:mdr1b−/−), and TKO (mdr1a−/−:mdr1b−/−:mrp1−/−) mice. VCR normal brain partition coefficients (i.e. tissue/plasma VCR concentrations) in TKO mice were greater than those in WT mice at both targeted 10 and 50 ng/mL plasma VCR concentrations, and ranged from 1.3- to 3.6-fold. VCR brain tumor partition coefficients in Mrp1 mice were greater than WT mice at both doses, being 1.5- and 2.4-fold higher at low and high doses, respectively. TKO mice also showed elevated VCR brain tumor penetration with a brain tumor partition coefficient of 1.9-fold greater than that in WT mice at the high-dose level. The bone marrow partition coefficient in Mrp1 ko mice was 1.65-fold greater than that in WT mice. Within strain comparisons revealed that VCR brain tumor concentrations were significantly greater than normal brain in all strains, ranging from 9- to 40-fold. These findings indicate that disruption of the BBB caused the largest enhancement in VCR tumor concentrations, yet the absence of Mrp1 on the brain tumor vasculature could enhance the penetration compared with that in normal brain.     

Increased penetration of paclitaxel into the brain by inhibition of P-Glycoprotein.

P-Glycoprotein (Pgp) in the blood-brain barrier limits the uptake of substrate drugs into the brain. We have determined the efficacy of several (putative) inhibitors of Pgp (cyclosporin A, PSC833, GF120918, and Cremophor EL) on the penetration of paclitaxel into the mouse brain. Pgp inhibitors were administered p.o. before i.v. paclitaxel. Plasma and tissues were collected at 1, 4, 8, and 24 h and analyzed for paclitaxel by high-performance liquid chromatography. Pgp knockout mice were used as reference for complete blockade of Pgp and to determine the selectivity of Pgp inhibitors on the pharmacokinetics of paclitaxel. Cremophor EL had no effect at all. Increased brain uptake was observed with cyclosporin A (3-fold), PSC833 (6.5-fold), and GF120918 (5-fold), although the levels were lower than that observed in Pgp knockout mice (11-fold increase). Both cyclosporin A and PSC833 also markedly increased the plasma levels of paclitaxel in contrast to GF120918. Obviously, cyclosporin A and PSC833 markedly inhibited several elimination pathways of paclitaxel, whereas the reduced clearance of paclitaxel by GF120918 was most probably related to the inhibition of Pgp alone. After further optimization of the dose and schedule of GF120918, we could achieve paclitaxel brain levels of about 80-90% of those reached in Pgp knockout mice. It is warranted to test paclitaxel in combination with GF120918 in experimental brain tumor models and in clinical trials.     

Improved penetration of docetaxel into the brain by co-administration of inhibitors of P-glycoprotein

P-glycoprotein (Pgp) in the blood-brain barrier limits the brain's uptake of many anticancer drugs. We have investigated whether the Pgp inhibitors cyclosporin A, valspodar (PSC833) and elacridar (GF120918) increase the accumulation of docetaxel in the brain. Pgp knockout mice served as a reference model for the complete absence or complete inhibition of Pgp. Plasma and tissues were analysed by high-performance liquid chromatography. Cyclosporin A, valspodar and elacridar significantly increased the brain concentrations of docetaxel in wild-type mice to 38%, 56% and 59%, respectively, of those achieved in Pgp knockout mice. Valspodar and cyclosporin A also increased the docetaxel concentration in plasma and other tissues by 2- and 3-fold, whereas elacridar did not change the clearance. All three inhibitors therefore inhibit Pgp in the blood-brain barrier. Elacridar increases the accumulation of docetaxel in the brain without significant effects on systemic exposure. Further clinical tests with this latter combination are warranted.     

Correlation between functional and molecular analysis of mdr1 P-glycoprotein in human solid-tumor xenografts

The contribution of P-glycoprotein (Pgp) to multidrug resistance in human solid tumors is generally estimated from bulk mRNA measurements or immunohistochemistry, while direct measurement of the effect of Pgp on intracellular drug concentrations has not been pursued. We investigated the feasibility and sensitivity of a method for probing Pgp-mediated drug transport in cells isolated from solid tumors, using xenograft models. Human tumor xenografts (XG) were grown by s.c. injection of Pgp-expressing cell lines 2780^AD, BRO/mdrl and KB8-5. Tumor uptake of doxorubicin (DOX) after administration of DOX to the mice was determined. XG from untreated mice were enzymatically dissociated. The effect of the Pgp modulator bepridil on steady-state cellular daunorubicin (DNR) and vincristine (VCR) accumulation and chemosensitivity of these XG cells was compared with its effects in the cell lines (CL). mdrl mRNA and Pgp (by flow cytometry) were measured. Also, the dependence on intraceilular ATP concentration, [ATP]_i, of the modulator effect was determined in intact KB8-5 cells. The results showed that i.v. administration of DOX to the mice led to lower DOX levels in the Pgp-expressing XG than in the “sensitive” XG, suggesting the presence of an in vivo functional Pgp in these XG tumor models. Dissociated, viable XG cells appeared to have ATP levels sufficient to sustain Pgp-ATPase-coupled drug transport. This was inferred from experiments using KB8- 5 CL, which showed half-maximal inhibition of DNR transport at an [ATP]_i of 1 to 2 mM. The effect of bepridii on DNR and VCR accumulation and chemosensitivity in the XG cells was in accordance with the XG expression of mdr1/Pgp, In KB8-5 XG cells, Pgp function was hardly detectable, in accordance with decreased mdrl/Pgp expression in vivo. In conclusion, Pgp activity can be determined in freshly dissociated XG human tumor cells. The results obtained with the more necrotic KB8-5 XG may represent some of the interpretation problems arising when low levels of Pgp expression occur within a heterogeneous cell population, such as may be expected in clinical human tumors. Also our results indicate that Pgp activity may be impaired in vivo at [ATP]_i below 2 mM, which are realistic values for human solid tumors. © 1995 Wiley-Liss, Inc.     

Increased accumulation of doxorubicin and doxorubicinol in cardiac tissue of mice lacking mdr1a P-glycoprotein.

To gain more insight into the pharmacological role of endogenous P-glycoprotein in the metabolism of the widely used substrate drug doxorubicin, we have studied the plasma pharmacokinetics, tissue distribution and excretion of this compound in mdr1a(-/-) and wild-type mice. Doxorubicin was administered as an i.v. bolus injection at a dose level of 5 mg kg(-1). Drug and metabolite concentrations were determined in plasma, tissues, urine and faeces by high-performance liquid chromatography. In comparison with wild-type mice, the terminal half-life and the area under the plasma concentration-time curve of doxorubicin in mdr1a(-/-) mice were 1.6- and 1.2-fold higher respectively. The retention of both doxorubicin and its metabolite doxorubicinol in the hearts of mdr1a(-/-) mice was substantially prolonged. In addition, a significantly increased drug accumulation was observed in the brain and the liver of mdr1a(-/-) mice. The relative accumulation in most other tissues was not or only slightly increased. The differences in cumulative faecal and urinary excretion of doxorubicin and metabolites between both types of mice were small. These experiments demonstrate that the absence of mdr1a P-glycoprotein only slightly alters the plasma pharmacokinetics of doxorubicin. Furthermore, the substantially prolonged presence of both doxorubicin and doxorubicinol in cardiac tissue of mdr1a(-/-) mice suggests that a blockade of endogenous P-glycoprotein in patients, for example by a reversal agent, may enhance the risk of cardiotoxicity upon administration of doxorubicin.     

Targeted delivery of a peripheral benzodiazepine receptor ligand-gemcitabine conjugate to brain tumors in a xenograft model

PURPOSE: Peripheral benzodiazepine receptors (PBRs) are overexpressed in brain tumors compared to normal brain, and could serve as a target to selectively increase anticancer drug delivery through a PBR ligand-drug conjugate system. We have previously synthesized PBR ligand-gemcitabine conjugates based on the model PBR ligand, PK11195. The goal of the current study was to examine this new drug delivery strategy in an intracerebral xenograft model by measurement of steady-state drug distribution following administration of gemcitabine (GEM) and PK11195-GEM. METHODS: In vitro PBR receptor binding and cytotoxicity assays were used to screen three different PK11195-GEM conjugates (GG01, GG02, GG03) in human SF126 glioma cells. Based on these findings and the favorable chemical stability of GG01, here referred as PK11195-GEM, pharmacokinetic investigations of PK11195-GEM and GEM were conducted in male rats. These studies consisted of single-dose and steady-state dosing regimen studies, the latter to assess drug distribution in normal brain and brain tumors. PK11195-GEM and GEM were measured in blood and tissue samples by HPLC. RESULTS: All PBR-GEM conjugates demonstrated appreciable receptor binding affinity and cytotoxicity with mean IC50 values ranging from 248 to 376 nM and 5.6 to 29.1 nM, respectively. The cytotoxicity of GEM was comparable with a mean IC50 value of 5.9 nM. Following administration of single 8 mg/kg doses of PK11195-GEM to rats (n=4), PK11195-GEM had a mean total clearance of 126.3 +/- 29.6 ml/min per kg, and a volume of distribution at steady-state of 1,261.9 +/- 31.05 ml/kg that resulted in a very short elimination half-life of 16.1 +/- 5.8 min. In comparison GEM had a similar volume of distribution (993.8 +/- 131.6 ml/kg), reduced clearance (3.4 +/- 0.8 ml/ min per kg), and longer half-life (235.6 +/- 26.7 min). In nude rats bearing intracerebral tumors, mean steady-state tumor/plasma, tumor/right brain, and tumor/left brain PK 11195-GEM concentration ratios were 1.75 +/- 0.46, 5.49 +/- 5.2, and 9.96 +/- 3.2, respectively. The analogous values following GEM administrations were 0.81 0.5, 3.67 1.57, and 5.21 +/- 1.95, respectively. These values indicate a minimum twofold increase in tumor target selectivity for the conjugate delivery system compared to GEM treatment. CONCLUSION: Targeting intracellular PBRs is a new drug delivery strategy based on the use of low molecular weight drug conjugates that can be administered systemically. It was demonstrated under steady-state conditions that PK11195-GEM possessed a twofold enhancement in brain tumor selectivity compared to GEM alone. This type of target selectivity would allow higher tumor concentrations to be achieved in conjunction with lower drug concentrations in normal or non-target tissues.     

The influence of the P-glycoprotein inhibitor zosuquidar trihydrochloride (LY335979) on the brain penetration of paclitaxel in mice

We determined the effect of zosuquidar·3HCl, an inhibitor of P-gp, on the penetration of the anticancer drug paclitaxel into the brain. Zosuquidar·3HCl was administered orally at 25 and 80 mg/kg 1 h before i.v. paclitaxel and i.v. at 20 mg/kg 10 min and 1 h before paclitaxel. The concentrations of paclitaxel in plasma and tissues and of zosuquidar·3HCl in plasma were quantified by high-performance liquid chromatography. The results revealed 3.5-fold and 5-fold higher paclitaxel levels in the brain of wild-type mice treated orally with 25 and 80 mg/kg zosuquidar·3HCl, respectively. However, complete inhibition as in P-gp knockout mice (11-fold increase) was not achieved. Zosuquidar·3HCl also increased the paclitaxel concentrations in plasma and tissues to levels similar to those observed in P-gp knockout mice, suggesting selective P-gp inhibition of zosuquidar·3HCl. When zosuquidar·3HCl was administered i.v. 10 min before paclitaxel, the paclitaxel levels in the brain of wild-type mice increased by 5.6-fold, whereas the increase was only 2.1-fold when zosuquidar·3HCl was administered 1 h before paclitaxel. This suggests that the inhibition of P-gp at the blood-brain barrier by zosuquidar·3HCl is rapidly reversible and that the concentrations of zosuquidar·3HCl in the plasma have already declined to levels insufficient to inhibit P-gp at the blood-brain barrier. In conclusion, zosuquidar·3HCl is only moderately active as an inhibitor of P-gp at the blood-brain barrier.     

Effects of orally active taxanes on P-glycoprotein modulation and colon and breast carcinoma drug resistance

BACKGROUND: The taxane paclitaxel (Taxol) is often of limited efficacy in chemotherapeutic regimens because some cancer cells express high levels of the efflux pump, P-glycoprotein (Pgp), which removes the drug from the cells. The orally active paclitaxel analog IDN-5109 has been reported to overcome Pgp-mediated drug resistance. We tested whether IDN-5109 acts by modulating Pgp activity. METHODS: Human MDA435/LCC6mdr1 and MDA435/LCC6 breast carcinoma cells, which express and do not express Pgp, respectively, were incubated with [3H]IDN-5109 and paclitaxel to determine intracellular drug accumulation. Flow cytometry was used to analyze intracellular retention of two Pgp substrates, rhodamine 123 (Rh-123) and doxorubicin, in both breast carcinoma cell lines and in human colon carcinoma cells (SW-620, DLD1, and HCT-15, whose Pgp levels vary) treated with different taxanes. The effects of IDN-5109 and paclitaxel on tumor growth in vivo were studied with the use of tumors established through xenografts of Pgp-expressing SW-620 and DLD1 cells in severe combined immunodeficiency mice. All statistical tests were two-sided. RESULTS: Pgp-expressing cells treated with IDN-5109 or with the taxane-based drug resistance reversal agent tRA96023, which blocks Pgp activity, retained 8.1- and 9.4-fold more Rh-123 (P =.0001), respectively, and 1.7- and 1.9-fold more doxorubicin (P =.001), respectively, than cells treated with paclitaxel. Non-Pgp-expressing cells treated similarly demonstrated no increased retention of either substrate. MDA435/LCC6mdr1 cells retained 5.3-fold more [3H]IDN-5109 than [3H]paclitaxel after 2 hours (P =.01). IDN-5109 showed statistically significantly higher tumor growth inhibition than paclitaxel against the SW-620 xenograft (P =.003). CONCLUSIONS: IDN-5109 modulates Pgp activity, resulting in superior tumor growth inhibition against Pgp-expressing tumors as compared with paclitaxel. IDN-5109 may broaden the spectrum of taxane use to include colon tumors.     

Adenosine 5'-triphosphate-sensitive potassium channel-mediated blood-brain tumor barrier permeability increase in a rat brain tumor model

Brain tumor microvessels/capillaries limit drug delivery to tumors by forming a blood-brain tumor barrier (BTB). The BTB overexpresses ATP-sensitive potassium (K(ATP)) channels that are barely detectable in normal brain capillaries, and which were targeted for BTB permeability modulation. In a rat brain tumor model, we infused minoxidil sulfate (MS), a selective K(ATP) channel activator, to obtain sustained, enhanced, and selective drug delivery, including various sized molecules, across the BTB to brain tumors. Glibenclamide, a selective K(ATP) channel inhibitor, significantly attenuated the MS-induced BTB permeability increase. Immunocytochemistry and glibenclamide binding studies showed increased K(ATP) channel density distribution on tumor cells and tumor capillary endothelium, which was confirmed by K(ATP) channel potentiometric assay in tumor cells and brain endothelial cells cocultured with brain tumor cells. MS infusion in rats with brain tumors significantly increased transport vesicle density in tumor capillary endothelial and tumor cells. MS facilitated increased delivery of macromolecules, including Her-2 antibody, adenoviral-green fluorescent protein, and carboplatin, to brain tumors, with carboplatin significantly increasing survival in brain tumor-bearing rats. K(ATP) channel-mediated BTB permeability increase was also demonstrated in a human, brain tumor xenograft model. We conclude that K(ATP) channels are a potential target for biochemical modulation of BTB permeability to increase antineoplastic drug delivery selectively to brain tumors.     

P-glycoprotein and breast cancer resistance protein: two dominant transporters working together in limiting the brain penetration of topotecan.

PURPOSE: The brain is a pharmacologic sanctuary site, due to the presence of the blood-brain barrier (BBB). Whereas the effect of P-glycoprotein (P-gp) at the BBB is well established, the role of breast cancer resistance protein (BCRP) that is also expressed at the BBB is not. EXPERIMENTAL DESIGN: We have studied the effect of BCRP by administering topotecan to wild-type (WT), single Mdr1a/b(-/-) and Bcrp1(-/-), and compound Mdr1a/b(-/-)Bcrp1(-/-) knockout mice. Drug levels in plasma and tissues were determined by high-performance liquid chromatography. RESULTS: The area under the plasma and tissue concentration-time curve (AUC) of topotecan in brains of Mdr1a/b(-/-) and Bcrp1(-/-) mice was only 1.5-fold higher compared with WT mice, but in Mdr1a/b(-/-)Bcrp1(-/-) mice, where both transporters are absent, the AUC increased by 12-fold. The AUC in plasma was approximately 0.75-, 2.4-, and 3.7-fold higher in Mdr1a/b(-/-), Bcrp1(-/-), and Mdr1a/b(-/-)Bcrp1(-/-) mice, respectively, resulting in 2.0-fold (P < 0.01), 0.65-fold (P, not significant), and 3.2-fold (P < 0.01), respectively, higher brain-to-plasma AUC ratios. Results using Mrp4(-/-) mice showed that this transporter had no effect on the brain penetration of topotecan. The P-gp/BCRP inhibitor elacridar fully inhibited P-gp-mediated transport of topotecan, whereas inhibition of Bcrp1-mediated transport by elacridar was minimal. CONCLUSIONS: Our results using Mdr1a/b(-/-)Bcrp1(-/-) mice clearly show the effect of Bcrp1 at the BBB and also show how two drug transporters act in concert to limit the brain penetration of topotecan. We expect that this finding will also apply to other drugs that are substrates of both P-gp and BCRP. Consequently, to improve the brain penetration of such compounds for targeting intracranial malignancies in patients, it will be essential to use potent inhibitors of both drug transporters.     

Predicting human tumor drug concentrations from a preclinical pharmacokinetic model of temozolomide brain disposition.

PURPOSE: Knowledge of drug concentrations in tumors is critical for understanding the determinants of drug accumulation in tumors. Because significant obstacles prevent making these measurements in humans, development of a predictive pharmacokinetic model would be of great value to the translation of preclinical data to the clinic. Our goal was to show how the latter could be achieved for temozolomide, an agent used in the treatment of brain tumors, using an orthotopic brain tumor model in rats. EXPERIMENTAL DESIGN: Rats bearing i.c. tumors received 20 mg/kg i.v. of temozolomide followed by the subsequent measurement of serial plasma, cerebrospinal fluid (CSF), normal brain, and brain tumor temozolomide concentrations. The resultant data provided the framework to develop a hybrid physiologically based pharmacokinetic model for temozolomide in brain. The preclinical pharmacokinetic model was scaled to predict temozolomide concentrations in human CSF, normal brain, and brain tumor, and through a series of Monte Carlo simulations, the accumulation of temozolomide in brain tumors under conditions of altered blood-brain barrier permeability, fractional blood volume, and clinical dosing schedules was evaluated. RESULTS: The developed physiologically based pharmacokinetic model afforded a mechanistic and accurate prediction of temozolomide brain disposition in rats, which through model scale-up procedures accurately predicted the CSF/plasma area under the drug concentration-time curve ratios of 0.2 reported in patients. Through a series of model simulations, it was shown that the brain tumor accumulation of temozolomide varied substantially based on changes in blood-brain barrier permeability and fractional tumor blood volume but minimally based on clinical dosing regimens. CONCLUSIONS: A physiologically based pharmacokinetic modeling approach offers a means to translate preclinical to clinical characteristics of drug disposition in target tissues and, thus, a means to select appropriate drug dosing regimens for achieving optimal target tissue drug concentrations.     

Randomized study of paclitaxel and tamoxifen deposition into human brain tumors: implications for the treatment of metastatic brain tumors.

PURPOSE: Drug resistance in brain tumors is partially mediated by the blood-brain barrier of which a key component is P-glycoprotein, which is highly expressed in cerebral capillaries. Tamoxifen is a nontoxic inhibitor of P-glycoprotein. This trial assessed, in primary and metastatic brain tumors, the differential deposition of paclitaxel and whether tamoxifen could increase paclitaxel deposition. EXPERIMENTAL DESIGN: Patients for surgical resection of their primary or metastatic brain tumors were prospectively randomized to prior paclitaxel alone (175 mg/m(2)/i.v.) or tamoxifen for 5 days followed by paclitaxel. Central and peripheral tumor, surrounding normal brain and plasma, were analyzed for paclitaxel and tamoxifen. RESULTS: Twenty-seven patients completed the study. Based on a multivariate linear regression model, no significant differences in paclitaxel concentrations between the two study arms were found after adjusting for treatment group (tamoxifen versus control). However, in analysis for tumor type, metastatic brain tumors had higher paclitaxel concentrations in the tumor center (1.93-fold, P = 0.10) and in the tumor periphery (2.46-fold, P = 0.039) compared with primary brain tumors. Pharmacokinetic analyses showed comparable paclitaxel areas under the serum concentration between treatment arms. CONCLUSIONS: Paclitaxel deposition was not increased with this tamoxifen schedule as the low plasma concentrations were likely secondary to concurrent use of P-450-inducing medications. However, the statistically higher paclitaxel deposition in the periphery of metastatic brain tumors provides functional evidence corroborating reports of decreased P-glycoprotein expression in metastatic versus primary brain tumors. This suggests that metastatic brain tumors may respond to paclitaxel if it has proven clinical efficacy for the primary tumor's histopathology.     

The pharmacological phenotype of combined multidrug-resistance mdr1a/1b- and mrp1-deficient mice

Two major classes of plasma membrane proteins that actively extrude a wide range of structurally diverse hydrophobic amphipathic antineoplastic agents from cells, with different mechanisms of action, lead to multidrug resistance. To study the importance of these ATP-binding cassette transporters to the toxicity of cancer chemotherapy agents, we have used mice genetically deficient in both the mdr1a and mdr1b genes [mdr1a/1b(-/-) mice], the mrp1 gene [mrp1(-/-) mice], and the combined genes mdr1a/1b and mrp1 [mdr1a/1b(-/-), mrp1(-/-) mice] and embryonic fibroblasts derived from wild-type mice and from the three gene knockout animals. The consequences of export pump deficiencies were evaluated primarily using vincristine and etoposide. Mice deficient in the three genes, mdr1a/1b and mrp1, exhibited a 128-fold increase in toxicity to vincristine and a 3-5-fold increase in toxicity to etoposide; increased toxicity to embryonic fibroblast cells from triple knockout mice also occurred with vincristine and etoposide. Vincristine, which normally does not express toxicity to the bone marrow and to the gastrointestinal mucosa when used at therapeutic doses, caused extensive damage to these tissues in mdr1a/1b(-/-), mrp1(-/-) mice. The findings indicate that the P-glycoprotein and mrpl are compensatory transporters for vincristine and etoposide in the bone marrow and the gastrointestinal mucosa and emphasize the potential for increased toxicities by the combined inhibition of these efflux pumps.     

Increased oral bioavailability of paclitaxel by GF120918 in mice through selective modulation of P-glycoprotein.

Previous studies in mice with disrupted mdr1a P-glycoprotein genes have shown that the oral bioavailability of paclitaxel is very low because of the presence of this drug-transporting protein in the intestinal wall. Additional studies with cyclosporin A have shown that this P-glycoprotein-inhibiting agent is able to increase the bioavailability of paclitaxel in mouse models and in patients. However, the potential immune-suppressive side effects of cyclosporin A renders this compound less suitable for chronic use in cancer patients. In this paper we present the results obtained with GF120918, an experimental P-glycoprotein inhibitor, on the oral bioavailability of paclitaxel in both wild-type and mdrlab knockout mice. GF120918 (25 mg/kg) was administered p.o. by gavage 15 min or 2 h before oral or i.v. dosing of paclitaxel, respectively. Paclitaxel plasma levels were quantified by high-performance liquid chromatography. GF120918 increased the plasma values for areas under the concentration-time curve of oral paclitaxel in wild-type mice by 6.6-fold from 408 to 2701 ng x ml(-1) h. Calculated relative to their respective values for area under the concentration-time curve after i.v. administration, GF120918 increased the oral bioavailability of paclitaxel in wild-type mice from 8.5 to 40.2%. The plasma pharmacokinetics of paclitaxel in mdr1ab knockout mice was not altered by GF120918, whereas the pharmacokinetics of paclitaxel in wild-type mice receiving GF120918 became comparable with mdr1ab knockout mice. This result indicates that GF120918 at this dose-level selectively and completely blocks P-glycoprotein in the intestines and does not notably interfere in the elimination of paclitaxel by metabolism or other transporters. On the basis of this result, GF120918 has been selected for additional study in humans.     

Peptide-mediated targeting to tumor blood vessels of lung cancer for drug delivery

Antiangiogenesis therapies for the treatment of cancers hold the promise of high efficacy and low toxicity. In vivo phage display was used to identify peptides specifically targeting tumor blood vessels. The peptide SP5-52 recognized tumor neovasculature but not normal blood vessels in severe combined immunodeficiency mice bearing human tumors. Synthetic peptide was shown to inhibit the binding of PC5-52 phage particles to the tumor mass in the competitive inhibition assay. Several selected phage clones displayed the consensus motif, proline-serine-proline, and this motif was crucial for peptide binding to the tumor neovasculature. SP5-52 peptides also bound vascular endothelial growth factor-stimulated human umbilical vein endothelial cells and blood vessels of human lung cancer surgical specimens. Furthermore, this targeting phage was shown to home to tumor tissues from eight different types of human tumor xenografts following in vivo phage display experiments. An SP5-52 peptide-linked liposome carrying doxorubicin enhanced the therapeutic efficacy of the drug, markedly decreased tumor blood vessels, and resulted in higher survival rates of human lung and oral cancer-bearing xenograft mice. The current study indicates that ligand-targeted therapy offers improved therapeutic effects over conventional anticancer drug therapy, and that the peptide SP5-52 specifically targets tumor neovasculature and is a good candidate for targeted drug delivery to solid tumors.     

Concentrations of VP16 and VM26 in human brain tumors

VP16 and VM26 were determined by high pressure liquid chromatography in intracerebral tumors, adjacent normal brain tissue, plasma and CSF samples from 24 patients given the two drugs before surgical resection of the tumor. The drugs were administered at doses of 100-150 mg/m2 as a 1-hour i.v. infusion, between 1.5 and 12 hours before surgery. Concentrations of VP16 ranged between 1.05 and 3.28 micrograms/g in tumors in the series of patients who received the drug 1.5-3 hours before surgery and between less than 0.05 and 1.12 micrograms/g in four patients who received the drug 9-13 hours before surgery. Tumor concentrations of VM26 also varied, ranging from less than 0.05 to 1.68 micrograms/g between 1.5 and 12 hours before surgery. VP16 and VM26 in the apparently normal brain tissue surrounding the tumor were low or undetectable except in one patient who had received radiotherapy, in whom we found 3.1 micrograms/g.     

Lipid association improves the therapeutic index of lomustine [1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea] to suppress 36B-10 tumor growth in rats

The therapeutic efficacy and tumor accumulation of a liposome formulation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), an effective agent used in the treatment of malignant brain tumors, was examined in an animal tumor model. Pharmacokinetic studies in normal and tumor-bearing rats indicated that a 2-fold greater plasma exposure was achieved with liposome-formulated CCNU compared with the free drug. In Fisher rats bearing s.c. tumors 36B-10, tumor growth was delayed substantially when liposomal CCNU was delivered compared with free-drug treatment. In single-dose treatments of 20, 35, and 50 mg/kg, tumor progression after each dose was reduced approximately 2-fold with liposomal compared with free CCNU (four animals in each treatment group). Multiple-dose treatments (given as three weekly doses with eight animals in each treatment group) with cumulative doses of 80 and 100 mg/kg of free and liposomal CCNU also resulted in a 2-fold reduction in tumor progression when compared with free-drug treatment. When drug levels in tumors relative to plasma were examined, it was observed that tumor drug concentrations did not exceed those found in plasma after administration of free CCNU; after administration of liposomal CCNU, however, tumor concentrations exceeded those in plasma by nearly 10-fold. These results suggest that the increased efficacy of liposome-formulated CCNU may be attributable to enhanced drug accumulation in tumor tissues.     

Expression of bisecting GlcNAc in pediatric brain tumors and its association with tumor cell response to vinblastine.

Increased expression of the bisecting GlcNAc has been correlated with tumor progression in several experimental tumor models. Its expression and function in brain tumors are, however, not yet known. In this study, we investigated expression of the bisecting GlcNAc structure in a series of pediatric brain tumors and its relationship to tumor response to vinblastine. A plant lectin (E-PHA) that recognizes the bisecting GlcNAc structure was used for detection of this molecule in a total of 90 pediatric brain tumors and normal brain tissue specimens. Our results showed that, whereas E-PHA staining was undetectable in the normal brain tissue, pediatric brain tumor specimens exhibited different levels of reactivity. Lectin staining was particularly prominent in high-grade astrocytomas (73%) and ependymomas (72%). In astrocytomas, there was a positive correlation with the tumor grade, which suggests that the bisecting GlcNAc may be of particular interest as a tumor marker for diagnosis and/or prognosis. By using a human glioma cell culture model, we have found that treatment of these cells with E-PHA lectin enhances their sensitivity to vinblastine. E-PHA interacted directly with the drug transporter P-glycoprotein and inhibited its drug efflux function. In a drug-resistant glioma cell line transfected with the mdr1 gene, drug resistance was reversed by E-PHA. Our findings indicate that: (a) expression of the bisecting GlcNAc in pediatric brain tumors may have a potential relevance as a tumor marker; and (b) glioma response to chemotherapy may be modulated through the bisecting GlcNAc.