Biomedical subjects

B K Sinha

Publications and source records attributed to B K Sinha.

At least 19 recordsLinked to original sources

PubMed1992

Synergistic antiproliferative effects of the combination of interleukin-1 alpha and doxorubicin against human melanoma cells.

We have investigated the antiproliferative effects of recombinant human interleukin-1 alpha (IL-1) combined with the cytotoxic antitumor drug doxorubicin against A375 human melanoma IL-1-sensitive (C6) and IL-1-resistant (C5) clonal cell lines. Growth inhibition was assessed by the MTT assay, and C5 cells were 10-fold less sensitive to IL-1 than the C6 cells, but both cell lines were equally sensitive to doxorubicin. Synergistic antitumor activity between the two agents was evaluated by median effects/combination index analysis, and IL-1 and doxorubicin were strongly synergistic over a broad range of drug concentrations. The strongest synergism occurred when C6 cells were exposed to IL-1 prior to doxorubicin, and when C5 cells were pretreated with doxorubicin for 6 hr prior to IL-1 additions. An examination of various ratios of the two agents revealed a maximum 20-fold potentiation of doxorubicin, and a 30-fold potentiation of IL-1 median dose values in the combination compared to the median dose values obtained with doxorubicin or IL-1 alone. Doxorubicin treatment enhanced the binding and internalization of [125I]IL-1 after 24 and 48 hr at 37 degrees C, but IL-1 binding to cells incubated on ice was increased only marginally by doxorubicin pretreatment. Treatment of C6 cells with IL-1 for 24 hr did not alter the cellular accumulation of doxorubicin. A recombinant protein IL-1 receptor antagonist that binds to both the 80 kDa type I and the 65 kDa type II IL-1 receptors, blocked the cytostatic effects of IL-1 and abrogated the synergism with doxorubicin. In cells synchronized following release from aphidicolin block, doxorubicin caused a G2 + M accumulation, IL-1 alone had no effect, and the combination of both agents resulted in a G2 + M block similar in magnitude to that caused by doxorubicin alone. These results provide preclinical evidence that doxorubicin combined with IL-1 may be beneficial in the clinical treatment of malignant melanoma and possibly other types of solid tumors.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed1992

Selective formation of tumor necrosis factor-alpha (TNF) degradation products contributes to TNF mediated cytotoxicity.

We compared tumor necrosis factor (TNF) metabolism by wild-type MCF-7 (WT) cells, by 40-fold doxorubicin resistant (40F) breast cancer cells and by PC3 and LNCaP prostate cancer cell lines. MCF-7 WT and LNCaP cell lines were sensitive to TNF cytotoxicity and both lines produced two major intracellular TNF degradation products of 15 kDa and 5.5 kDa. The MCF-7 40F and the PC3 cell lines were resistant to TNF and produced multiple TNF degradation products with molecular weights lower than 15 kDa. Both the breast and prostate lines showed TNF receptor crosslinking patterns consistent with a molecular weight of 55 kDa. The breast and LNCaP lines expressed TNF receptors with an apparent dissociation constant (Kd) of 0.4 to 0.6 nM, while the TNF resistant line had a Kd of 2 nM. Similar receptor numbers per cell were found for all cell types (4,000 to 8,000/cell), and comparable levels of TNF internalization were noted. TNF-conditioned medium from the TNF-sensitive cell types was cytotoxic toward both the TNF-sensitive and TNF-resistant lines, and the toxicity was significantly blocked by an anti-TNF monoclonal antibody. Hydrophobic interaction column HPLC fractionation of the TNF-degradation products produced by MCF-7 WT and LNCaP cells revealed that the trimeric, monomeric, and 5.5 kDa fractions possessed the greatest in vitro antitumor activity. These findings suggest that a TNF degradation product, produced selectively by TNF-sensitive cells, may contribute to the antitumor action of TNF.

Antibodies, Monoclonal↗

PubMed1991

Etoposide-induced DNA damage in human tumor cells: requirement for cellular activating factors.

Previous studies with the multidrug-resistant human HL60 cell line have shown a 3-4-fold decrease in VP-16 accumulation compared to the sensitive cell line, while the degree of resistance to VP-16 was 300-fold, indicating that other mechanisms of resistance are also operative. Since VP-16 has been shown to interfere with topoisomerase II activity, we have evaluated VP-16-dependent DNA strand break formation in the drug-sensitive and -resistant HL60 cells. Studies reported here show that the drug-resistant HL60 cells are extremely resistant to VP-16-dependent DNA cleavage compared to the sensitive cells. This decrease in DNA cleavage activity in the presence of VP-16 was, in part, related to a 2-3-fold decrease in both the amount and activity of topoisomerase II in the resistant cell line compared to the sensitive cells. Nuclei from the resistant cell line were markedly more resistant to VP-16-dependent DNA cleavage than the WT cell nuclei. Interestingly, WT nuclei were found to be relatively more resistant to VP-16-induced DNA cleavage than the intact WT cells. Addition of WT cytosolic proteins to WT nuclei, however, significantly stimulated VP-16-dependent DNA cleavage and slightly increased DNA cleavage in resistant cell nuclei. In contrast, cytosolic proteins from the resistant cells had no effect on DNA cleavage in nuclei isolated from either cell line. These observations indicate that a decrease in the amount and activity of topoisomerase II in resistant HL60 cells translates into a decrease in VP-16-dependent DNA breakage and contributes to the resistance to VP-16. Furthermore, the cytosolic fraction from WT cells contains some factor, not present in the resistant cells, which is necessary for the maximal drug-induced DNA cleavage.

Blotting, Western↗

PubMed1991

Biochemical and pharmacological characterization of MCF-7 drug-sensitive and AdrR multidrug-resistant human breast tumor xenografts in athymic nude mice.

The phenotypic expression of multidrug resistance by the doxorubicin-selected AdrR human breast tumor cell line is associated with overexpression of plasma membrane P-170 glycoprotein and increased cytosolic selenium-dependent GSH-peroxidase activity relative to the parental MCF-7 wild-type line (WT). To determine whether doxorubicin resistance by AdrR cells persists in vivo, and to further investigate the possibility of biochemical differences between WT and AdrR solid tumors, both tumor cell lines were grown as subcutaneous xenografts in athymic nude mice. Tumorigenicity depended upon cell inoculation burden, and tumor incidence was similar for both cell lines (greater than 80% tumor takes at 10(7) cells/mouse) at 14 days, provided 17 beta-estradiol was supplied to the animals bearing the WT tumors. However, the growth rate for the AdrR xenografts was only about half that of WT xenografts. Doxorubicin (2-8 mg/kg, i.p., injected weekly) significantly diminished the growth of the WT tumors, but AdrR solid tumors failed to respond to doxorubicin. The accumulation of 14C-labeled doxorubicin was 2-fold greater in WT xenografts that in AdrR, although there were no differences in host organ drug levels in mice bearing either type of tumors. Membrane P-170 glycoprotein mRNA was detected by slot-blot analysis in the AdrR tumors, but not in WT. Electron spin resonance 5,5-dimethylpyrroline-N-oxide-spin-trapping experiments with microsomes and mitochondria from WT and AdrR xenographs demonstrated a 2-fold greater oxygen radical (superoxide and hydroxyl) formation from activated doxorubicin with WT xenographs compared to AdrR. Selenium-dependent glutathione (GSH)-peroxidase, superoxide dismutase and GSH-S-aryltransferase activities in AdrR xenografts were elevated relative to WT. Although the activities of the latter two enzymes were similar to those measured in both tumor cell lines, GSH-peroxidase activities were elevated 70-fold (WT) and 10-fold (AdrR) in xenografts compared to tumor cells. In contrast, in both WT and AdrR solid tumors in vivo, catalase, NAD(P)H-oxidoreductases, and glutathione disulfide (GSSG)-reductase activities, and GSH and GSSG levels were not markedly different, and were essentially the same as in cells in vitro. Like the MDR cells in culture, AdrR tumor xenografts were extremely resistant to doxorubicin and retained most of the characteristics of the altered phenotype. These results suggest that WT and AdrR breast tumor xenografts provide a useful model for the study of biochemical and pharmacological mechanisms of drug resistance by solid tumors in vivo.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1991

A role for the interleukin 1 receptor in the synergistic antitumor effects of human interleukin 1 alpha and etoposide against human melanoma cells.

To investigate the possibility that anticancer drugs combined with cytokines may show increased activity, human tumor cells were treated with combinations of human recombinant interleukin 1 alpha (rIL-1 alpha) and etoposide (VP-16). The cytotoxicity of these combinations was evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay using rIL-1 alpha-sensitive A375-C6 melanoma cells and A375-C5 cells, a clonal variant line that is resistant to IL-1 alpha. Data were analyzed for synergism by the median effect principle of T-C. Chou and P. Talalay (J. Biol. Chem., 252: 6438-6442, 1977). At a dose ratio of VP-16 to rIL-1 alpha of 12 nM:1 unit/ml in either simultaneous or sequential exposure (VP-16 first), the calculated combination index values indicated synergistic cytotoxicity toward both A375-C6 cells and A375-C5 cells. IL-1 alpha treatment 24 h prior to VP-16 exposure had no advantage over simultaneous treatment. Surface IL-1 alpha receptors on both A375-C6 and A375-C5 cells were measured using 125I-radiolabeled rIL-1 alpha binding; A375-C6 cells had 701 +/- 128 (SD) receptor molecules/cell and A375-C5 cells only had 58 +/- 33 receptor molecules/cell. The dissociation constants for IL-1 alpha were similar in both cell types (19 +/- 6 pM for A375-C6 and 17 +/- 2 pM for A375-C5). The specific binding of rIL-1 alpha to the surface IL-1 alpha receptors of both sensitive and resistant cells was significantly increased in a dose-dependent fashion by the prior treatment with VP-16 (1.75-fold on A375-C6 cells and 3.5-fold on A375-C5 cells). VP-16 also enhanced the internalization of receptor-bound rIL-1 alpha, suggesting that a possible mechanism of the synergistic cytotoxicity of rIL-1 alpha and VP-16 might be related to the modulation of rIL-1 alpha receptors by VP-16, resulting in increased internalization of rIL-1 alpha.

Drug Administration Schedule↗

PubMed1991

Increased glutathione peroxidase activity in a human sarcoma cell line with inherent doxorubicin resistance.

Several mechanisms of drug resistance have been defined using cell lines selected for resistance in vitro. However, the relevance of these to tumor cell resistance in vivo remains unclear. We established tumor cell lines from biopsies of human sarcomas before and after doxorubicin therapy. One pretreatment sarcoma line, STSAR90, was 6-fold less sensitive to doxorubicin than was a normal fibroblast line, AG1522. The sensitivities of six other sarcoma lines were similar to that of AG1522. STSAR90 cells did not overexpress P-glycoprotein mRNA, by Northern analysis with the pCHP1 complementary DNA fragment. Photoaffinity labeling with the vinblastine analogue N-(p-azido-3-125I-salicyl)-N'-beta-aminoethylvindesine did not show increased P-glycoprotein concentrations. Accumulation of [3H]daunomycin was not decreased in STSAR90 compared with a less resistant sarcoma line, STSAR11, nor was the doxorubicin sensitivity of STSAR90 increased by coincubation with verapamil. Glutathione levels were twice as high in STSAR90 as in STSAR11, and glutathione peroxidase activity was 3.5- to 6-fold higher. This was due mostly to an increase in selenium-dependent peroxidase activity. After exposure to doxorubicin, STSAR90 cells formed only half as much measurable hydroxyl radical as STSAR11, as detected by electron spin resonance spectrometry. Doxorubicin sensitivity was increased in STSAR90 cells when intracellular glutathione levels were reduced by buthionine sulfoximine. These results indicate that multidrug resistance due to P-glycoprotein-mediated drug efflux is not the only mechanism of doxorubicin resistance that occurs in sarcomas and that glutathione peroxidase-dependent detoxification of doxorubicin-induced oxygen radicals may contribute to clinical doxorubicin resistance.

Adolescent↗

PubMed1991

Doxorubicin-induced cross-resistance to tumor necrosis factor (TNF) related to differential TNF processing.

To evaluate whether cells selected for doxorubicin resistance were cross-resistant to tumor necrosis factor, the effects of doxorubicin and recombinant human tumor necrosis factor-alpha (TNF) on doxorubicin-sensitive (WT) and 40-fold doxorubicin-resistant (40F) MCF-7 cell proliferation were assessed. The median dose (MD) for doxorubicin was 14.5 nM for WT cells and 474 nM for 40F cells. The MD for TNF was 0.18 nM for WT cells, while 40F cells were highly resistant to TNF concentrations up to 60 nM. Doxorubicin and TNF in combination were synergistic against WT cells, but not 40F cells. Glutathione depletion by buthionine sulfoxamine sensitized WT cells threefold to TNF, with no change in their response to doxorubicin, while 40F cells showed a twofold increase in doxorubicin sensitivity, with no apparent change in their resistance to TNF. No significant differences in TNF receptor number, Kd, or capacity for TNF internalization were noted between the two cell types. WT cells produced a single 15 kDa TNF degradation product, while the 40F cells produced three lower molecular weight degradation products. We conclude that cross-resistance to TNF in doxorubicin-resistant MCF-7 cells may be explained in part by altered TNF degradation.

Buthionine Sulfoximine↗

PubMed1991

Cytochrome P-450- and peroxidase-dependent activation of procarbazine and iproniazid in mammalian cells.

Metabolism of hydrazine derivatives, procarbazine and iproniazid, to reactive free radical intermediates has been studied using spin-trapping techniques in intact human promyelocytic leukemia (HL60) and mouse hepatic cell lines. While HL60 cells have been shown to contain both myeloperoxidase and cytochrome P-450 enzymes, the hepatic cell line shows only cytochrome P-450 activity. Both peroxidases and cytochrome P-450 have been reported to catalyze biotransformation of hydrazines. Procarbazine and iproniazid were rapidly metabolized in these cell lines to methyl and isopropyl radicals, respectively. However, in HL60 cells, procarbazine was metabolized by myeloperoxidase while iproniazid was metabolized mostly by the cytochrome P-450 system. In the hepatic cells, both of these compounds were metabolized by the P-450 system.

Biotransformation↗

PubMed1991

Selenium-dependent glutathione peroxidase expression is inversely related to estrogen receptor content of human breast cancer cells.

The absence of estrogen receptors (ER) in human breast tumors has been associated with a poorer prognosis compared to patients with ER positive breast cancer. Previous studies from our laboratory have shown that a multidrug resistant human breast cancer cell line selected for resistance to Adriamycin (ADR) exhibited markedly increased expression of both the pi class glutathione S-transferase (GST-pi) and the selenium-dependent glutathione peroxidase. These studies also revealed that the ER status was inversely related to the expression of GST-pi in six human breast cancer cell lines and primary tumor specimens. In the present study, we have examined the relationship between ER status and several biological properties of these cells, including their levels of glutathione peroxidase (GSH-Px) and catalase expression, their capacity to generate toxic hydroxyl radicals (degrees OH) by redox cycling of ADR, and their sensitivities to the cytotoxic effects of ADR and the oxidant, H2O2. Our results show that expression of GSH-Px, but not catalase, is inversely related to the ER status in these cell lines. Formation of the degree OH induced by treatment of cells with ADR was inversely proportional to the GSH-Px activity in these cell lines, and thus directly related to the ER status. Sensitivity of these cells to ADR or to H2O2, however, was not consistently related to ER status, GSH-Px, or catalase activity, or to ADR induced degree OH radical formation. These results indicate that these parameters are not predictive of cellular susceptibility to oxidative damage in these cell lines under the conditions studied.

Blotting, Northern↗

PubMed1990

Copper ion-dependent oxy-radical mediated DNA damage from dihydroxy derivative of etoposide.

The dihydroxy etoposide, a metabolite of the clinically active anticancer drug, VP-16, induced extensive DNA damage in the presence of copper ions. While superoxide dismutase was without any effect on the DNA damage, catalase and inhibitors of free hydroxyl radicals inhibited the DNA degradation, indicating that hydroxyl radicals were responsible for this drug-Cu-dependent DNA damage.

Animals↗

PubMed1990

Synergistic activity of suramin with tumor necrosis factor alpha and doxorubicin on human prostate cancer cell lines.

We evaluated the action of suramin, doxorubicin, and tumor necrosis factor alpha (TNF-alpha) on the testosterone-responsive human prostate cell line LNCaP and on the testosterone-independent human prostate cell line PC-3. The synergistic action of these agents in combination was tested by the Chou and Talalay method (quantitative analysis of dose-effect relationships) to determine whether in vitro doses were active at levels safely achieved in vivo. The action of suramin was potentiated threefold by doxorubicin for the PC-3 line and seven-fold by doxorubicin for the LNCaP line. Both the suramin-TNF-alpha and the doxorubicin-TNF-alpha combinations showed synergistic action against the LNCaP line. Synergistic activity was noted at drug concentrations routinely achieved clinically. This study demonstrates that suramin, doxorubicin, and TNF-alpha are active agents against prostate cancer cell lines and that their activity can be enhanced when they are used in combination.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed1990

DNA interstrand cross-link and free radical formation in a human multidrug-resistant cell line from mitomycin C and its analogues.

A subline of the human breast tumor cell line (MCF-7), selected for resistance to Adriamycin and having the multidrug resistance phenotype, also developed significant cross-resistance to mitomycin C and its two analogues, BMY 25282 and BMY 25067. Because mitomycin C and the analogues contain both quinone and aziridine moieties, the mechanism of tumor cell kill is thought to involve alkylation and cross-linking of DNA molecules, hence they are not expected to show cross-resistance to cells selected for resistance to a DNA intercalator. Studies to understand this novel observation show that the resistant MCF-7 cells form significantly less hydroxyl radical and DNA cross-linking in the presence of mitomycin C and BMY 25282 than the sensitive cells. Although BMY 25067 formed less free radicals in the resistant cells, similar to the other two drugs, the formation of DNA cross-links was identical in both cell lines, indicating a somewhat different mechanism of tumor cell kill by this analogue. DNA cross-link formation increased slightly with time in the sensitive cells while there was a small decrease in the resistant cells. This difference in the formation of toxic intermediates appeared to result from enhanced detoxification of reactive species (hydrogen peroxide and alkylating intermediates) as a result of significantly higher glutathione peroxidase (14-fold) and glutathione S-transferase (44-fold) activities in the resistant cell line. These events, i.e., free radical formation and DNA alkylation, showed a good correlation with the cytotoxicity in drug-sensitive cells, indicating that both mechanisms contribute to cell killing of human breast tumor cells.

Alkylating Agents↗

PubMed1990

Role of oxygen free radical formation in the mechanism of menogaril resistance in multidrug resistant tumor cells.

The mechanisms of action and resistance to menogaril, a clinically active anthracycline antitumor drug, were evaluated in sensitive and doxorubicin-selected multidrug resistant human breast tumor (MCF-7) cell lines. While MCF-7/ADRR cells were highly resistant (250-500-fold) to doxorubicin, they displayed only marginal resistance (10-fold) to menogaril. In contrast to doxorubicin, the mechanism of resistance to menogaril in these cells does not involve differential inhibition of DNA synthesis as measured by thymidine incorporation. P-170-glycoprotein-dependent drug transport did not contribute to resistance as there was no difference in the accumulation and retention of menogaril by sensitive and resistant cell lines. However, there was a 2-fold decrease in oxygen free radical formation in the resistant cells, compared to sensitive cells, in the presence of menogaril. Since resistant cells contain 12-fold higher glutathione peroxidase activity than the parental sensitive cells, the detoxification of hydrogen peroxide may be responsible for the decreased free radical formation and thus, may play a role in the resistance to menogaril.

Antineoplastic Agents↗

PubMed1990

Structure-activity relations, cytotoxicity and topoisomerase II dependent cleavage induced by pendulum ring analogues of etoposide.

The cytotoxicity of etoposide and its analogues, dihydroxy (DHVP), o-quinone (VP-Q) and o-methyl (VP-OMe), was evaluated in human breast (MCF-7) and HL60 tumour cells. Although less potent than etoposide, both DHVP and VP-Q were cytotoxic to these cells. However, VP-OMe was inactive. Studies with purified topoisomerase II showed that the intensity of DNA cleavage and the pattern of cleavage were similar for DHVP, VP-Q and etoposide. In contrast, the VP-OMe failed to induce DNA cleavage, indicating that the presence of 4'-OH is essential for metabolism, induction of topoisomerase II-mediated DNA cleavage and cytotoxicity of etoposide and its analogues.

Breast Neoplasms↗

PubMed1990

Free radicals and anticancer drug resistance: oxygen free radicals in the mechanisms of drug cytotoxicity and resistance by certain tumors.

Certain anticancer agents form free radical intermediates during enzymatic activation. Recent studies have indicated that free radicals generated from adriamycin and mitomycin C may play a critical role in their toxicity to human tumor cells. Furthermore, it is becoming increasingly apparent that reduced drug activation and or enhanced detoxification of reactive oxygen species may be related to the resistance to these anticancer agents by certain tumor cell lines. The purposes of this review are to summarize the evidence pointing toward the significance of free radicals formation in drug toxicity and to evaluate the role of decreased free radical formation and enhanced free radical scavenging and detoxification in the development of anticancer drug resistance by a spectrum of tumor cell types. Studies failing to support the participation of oxyradicals in the cytotoxicity and resistance of adriamycin are also discussed.

Animals↗

PubMed1990

Tyrosinase-induced free radical formation from VP-16,213: relationship to cytotoxicity.

Tyrosinase-dependent activation of hydroxybenzenes forms reactive compounds, including catechols and o-quinones, and some of which show antitumor activity against pigmented melanomas. Since VP-16 is a phenoxy-containing antitumor drug, forms free radicals and reactive o-quinones during peroxidative activation, we evaluated the cytotoxicity of VP-16 to both tyrosinase-containing and non-tyrosinase-containing tumor cells. Our results show that VP-16 is significantly more cytotoxic to B-16/F-10 melanoma cells than human MCF-7 breast tumor cells. Phenylthiocarbamide, an inhibitor of tyrosinase activity, selectively decreased VP-16 toxicity only in melanoma cells. Furthermore, VP-16 was readily activated to its phenoxy free radical intermediate by purified tyrosinase, indicating tyrosinase may play a role in VP-16 toxicity in pigmented melanomas.

Animals↗

PubMed1990

Potentiation of doxorubicin cytotoxicity by (+)-1,2-bis-(3,5-dioxopiperazinyl-1-yl) propane (ICRF-187) in human leukemic HL-60 cells.

The bisdioxopiperazine propane, ICRF-187, has been reported to potentiate doxorubicin cytotoxicity in certain tumor cell lines; however, the mechanism of this interaction is not known. In order to define the mechanism of this interaction, we examined the effects of ICRF-187 on doxorubicin cytotoxicity, free radical formation, and drug accumulation in human leukemia HL-60 cells. Studies show that ICRF-187 synergistically potentiated doxorubicin cytotoxicity in HL-60 cells. This potentiation of doxorubicin cytotoxicity by ICRF-187 appeared to result from enhanced drug dependent free radical formation without effecting doxorubicin uptake in HL-60 cells.

Biological Transport↗

PubMed1990

P-glycoprotein-independent mechanism of resistance to VP-16 in multidrug-resistant tumor cell lines: pharmacokinetic and photoaffinity labeling studies.

The interaction of etoposide (VP-16), Vinca alkaloids, and verapamil with the P-glycoprotein (P-gp) was studied in human breast (MCF-7) and Chinese hamster lung (DC3F) cell lines and the corresponding multidrug-resistant MCF-7/ADR and DC3F/ADX tumor cell lines, selected for resistance to Adriamycin and actinomycin D, respectively, and overexpressing P-gp. Verapamil (10 microM) markedly reversed resistance to vincristine (11-fold in DC3F/ADX and 125-fold in MCF-7/ADR; 1-hr exposure), but it had a very modest effect on resistance to VP-16 (3- to 4-fold; 1-hr exposure). Resistant cells accumulated 2- to 4-fold less VP-16 and vincristine than the parental cell lines. Verapamil (10 microM) significantly increased accumulation and retention of vincristine, but not of VP-16, in resistant cell lines. Photoaffinity labeling of resistant cell lines with radioactive analogs of verapamil [N(p-azido-3-125I-salicyl)-N'-beta-aminoethylverapamil (NASVP)] and vinblastine[N-(p-azido-3-125I-salicyl)-N'-beta-aminoethylvindesine (NASV)] showed distinctly labeled P-gp bands in both resistant cell lines, compared with wild-type cells. Excess nonradioactive vinblastine or verapamil effectively competed with the P-gp photolabeling by either NASVP or NASV, with IC50 levels of 0.6 and 10 microM, respectively. In contrast, nonradioactive VP-16 was 100- to 500-fold less potent than vinblastine in competing with P-gp photolabeling, suggesting that VP-16 has significantly lower affinity for P-gp than Vinca alkaloids have. Taken together, our data indicate that P-gp glycoprotein by itself may not be important in the transport/efflux of VP-16 and, thus, in the mechanism of resistance to VP-16 in these cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗