Differential effects of estrogen, tamoxifen and the pure antiestrogen ICI 182,780 in human drug-resistant leukemia cell lines

Synthesis and evaluation of 5-Aryl-3-(4-hydroxyphenyl)-1,3,4-oxadiazole-2-(3H)-thiones as P-glycoprotein inhibitors

5-Aryl-3-(4-hydroxyphenyl)-1,3,4-oxadiazole-2(3H)-thiones 3 were prepared by cyclocondensation of 1-(4-hydroxyphenyl)-2-aroylhydrazines with thiophosgene. All compounds exhibited antiproliferation activity in K562, IC(50) ranging from 24 to 94 micro M comparable efficacy with apigenin and genistein and showed more potent antiproliferation of K562/adr cells, highly expressing P-glycoprotein. Compounds 3g, 3e and 3a inhibited the function of P-glycoprotein with the alpha(0.5) equal to 10+/-3 micro M, 21+/-5 micro M and 34+/-7 micro M, respectively.

Pharmacokinetic and pharmacodynamic implications of P-glycoprotein modulation

P-glycoprotein (P-gp) is a cell membrane-associated protein that transports a variety of drug substrates. Although P-gp has been studied extensively as a mediator of multidrug resistance in cancer, only recently has the role of P-gp expressed in normal tissues as a determinant of drug pharmacokinetics and pharmacodynamics been examined. P-glycoprotein is present in organ systems that influence drug absorption (intestine), distribution to site of action (central nervous system and leukocytes), and elimination (liver and kidney), as well as several other tissues. Many marketed drugs inhibit P-gp function, and several compounds are under development as P-gp inhibitors. Similarly, numerous drugs can induce P-gp expression. While P-gp induction does not have a therapeutic role, P-gp inhibition is an attractive therapeutic approach to reverse multidrug resistance. Clinicians should recognize that P-gp induction or inhibition may have a substantial effect on the pharmacokinetics and pharmacodynamics of concomitantly administered drugs that are substrates for this transporter.

Tamoxifen inhibits the release of arachidonic acid stimulated by thapsigargin in estrogen receptor-negative A549 cells

In pre-labelled A549 cells the tumour promoter thapsigargin (50 nM) stimulates the release of [5,6,8,9,11,12,14,15-3H(N)]-arachidonic acid (3H-AA) by ca. 300% above basal levels. A549 cells are estrogen receptor negative (ER-), yet this stimulation by thapsigargin is inhibited in a dose-dependent manner by a 3 h pre-treatment with the anti-estrogen tamoxifen (1-20 microM). Moreover, the presence of excess (100 microM) estradiol does not reverse this effect of tamoxifen. Thapsigargin stimulated 3H-AA release is not inhibited over the same concentration range by 4 hydroxy-tamoxifen nor by the steroidal anti-estrogen ICI 164384. However, the steroidal anti-estrogen ICI 182780 inhibits thapsigargin stimulated 3H-AA release in a similar manner to tamoxifen and this effect is also not reversed by the presence of excess estradiol. Stimulation of 3H-AA release by EGF (10 nM), IL-1beta (1 ng ml-1) and bradykinin (100 nM) was unaffected by these concentrations of tamoxifen. Ionomycin (10 microM) stimulates 3H-AA release by ca. 700% and A23187 (10 microM) by ca. 300% above basal levels. Pre-treatment with tamoxifen (1-20 microM) inhibits 3H-AA release stimulated by both these agents and again the presence of excess estradiol does not reverse this effect. Unlike the effects of glucocorticoids on 3H-AA release in A549 cells the effects of tamoxifen are not reversed by neutralizing anti-bodies to lipocortin 1. Arachidonic acid release is central to cell proliferation in A549 cells and we propose that this action of tamoxifen could explain the anti-proliferative effect seen in these cells and could have important implications for control of cell proliferation of ER- cells in general.

Modulation of the function of human MDR1 P-glycoprotein by the antimalarial drug mefloquine

MDR1 P-glycoprotein in membranes of human tumor cells of the CEM/VBL100 line was selectively labelled using photoreactive analogs of verapamil, N-(p-azido-3-[125I]salicyl)amino-verapamil ([125I]ASA-V) and prazosin, 2-[4-(4-azido-3-[125I]iodobenzoyl)piperazin-1-yl]4 -amino-6,7-dimethoxyyquinazoline ([125I]ASA-P). Mefloquine, a quinolinemethanol antimalarial drug, was shown to inhibit the labelling of P-glycoprotein with an efficiency similar to that for verapamil, a known chemosensitizer. By contrast, chloroquine competed poorly for the binding site on P-glycoprotein. Mefloquine also inhibited the functional activity of P-glycoprotein. It decreased the rates of extrusion of [3H]vinblastine and the fluorescent dyes, fluo-3 acetomethoxy ester and rhodamine 123, from drug-resistant cells and decreased the level of resistance of these cells to vinblastine. The ability of mefloquine to inhibit P-glycoprotein function may be involved in the neurotoxic side-effects occasionally associated with the use of mefloquine as an antimalarial drug.

Modulatory effect of tamoxifen and ICI 182,780 on adriamycin resistance in MCF-7 human breast-cancer cells

In this study the ability of the new pure anti-estrogen ICI 182,780 to modulate the cytotoxic action of adriamycin (ADR) on parental and ADR-resistant MCF-7 (MCF-7 ADRr) human breast-cancer cells was investigated and compared with that of tamoxifen (TAM). TAM enhanced ADR cytotoxicity in MCF-7 ADRr cells in a dose-related manner, but this effect was slight or absent in MCF-7 WT. In contrast, ICI 182,780 was able to enhance ADR toxicity both in MCF-7 ADRr and in the parental cell line. ICI 182,780 was up to 2.5-fold more effective than TAM in reducing the IC50 of ADR in MCF-7 ADRr cells. Analysis of the data by the isobole method showed that the combination ADR/TAM and ADR/ICI 182,780 produced synergistic anti-proliferative activity in MCF-7 ADRr cells. Because ADR resistance in these cells is associated with the expression of high levels of P-glycoprotein (Pgp), we evaluated the effect of anti-estrogens on Pgp expression and activity. Both ICI 182,780 and TAM failed to modulate Pgp expression as assessed by flow cytometry and Western-blot analysis, performed using the monoclonal antibodies MM4.17 and C 219, which are specific for an external or an internal determinant respectively. Pgp activity was investigated by flow cytometry measuring the extrusion of ADR and the cationic dye Rhodamine 123 (Rh 123). ICI 182,780, but not TAM, reduced the activity of Pgp in MCF-7 ADRr cells. Flow cytometry was also used to investigate cell-cycle modifications induced by ADR in MCF-7 ADRr cells, both in the presence and in the absence of anti-estrogens. After 72 hr, higher doses induced an arrest of cells at the G2/M phase. The same effect was visible when lower doses of ADR were combined with ICI 182,780 or TAM. In terms of cell-cycle-blocking activity ICI 182,780 was largely more effective than TAM.

The pure antiestrogen ICI 182,780 binds to a high-affinity site distinct from the estrogen receptor

Both estrogen receptor-positive (ER+), tamoxifen-sensitive (5-21) and tamoxifen-resistant (5-23) subclones of the parental MCF-7 breast cancer cell line were used in competitive ligand binding studies involving either [3H]ICI 182,780 or 4-OH-[3H]tamoxifen (4OHT) displacement by unlabelled estradiol (E2) or the antiestrogens (AE) 4OHT and ICI 182,780. Neither radioligand was displaced significantly by E2 over a range of concentrations; binding was predominantly inhibited by the corresponding radio-inert ligand. Scatchard analysis of the data revealed that the binding capacities of both cell lines for ICI 182,780 were approximately 7-fold greater than the previously determined number of ER sites per cell, with the affinity being an order of magnitude less than that of E2 for ER. No difference was found between the TAM-sensitive and -resistant cells in their binding of either AE. When cells were preincubated with either E2, TAM or 4OHT at a high, fixed concentration to block the ER or AE binding sites (AEBS), respectively, displaceable binding of [3H]ICI 182,780 was still observed, indicating binding at a site other than the classical ER or previously described AEBS. Our results suggest that there is a specific, saturable and relatively high-affinity binding site for ICI 182,780 in MCF 5-21 and MCF 5-23 breast cancer cells. However, the physiological relevance of this binding site requires further clarification because in cell growth assays, E2 (at 1/10 the dose of ICI 182,780) overcame the inhibitory effect of the antiestrogen in both of the cell lines.