Circumvention of adriamycin resistance by dipyridamole analogues: a structure-activity relationship study

Proton Transfer in Bacterial Reaction Centers and Bacteriorhodopsin in the Presence of Dipyridamole

Dipyridamole, 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d]pyrimidine (DIP), a well known vasodilator and inhibitor of membrane peroxidation has recently been shown a potential co-activator (modulator) in the MDR phenomenon in cancer therapy. It inhibits P-glycoprotein (Pgp) which is a efflux pump of anticancer drugs in tumor cells. For the first time it is shown that dipyridamole, markedly slows down the kinetics of the electrogenic phase of the photoelectric response in Rb. sphaeroides chromatophores which is due to the proton transfer from the external medium to the secondary quinone acceptor in the reaction center. In purple membranes from H. salinarium containing bacteriorhodopsin (bR) dipyridamole (in its charged state) significantly slows down the kinetics of the proton transfer to the Schiff base from the primary donor Asp-96 (in wild type bacteria) or from the surrounding (in D96N mutant). Dipyridamole is supposed to affect the proton-transfer via changes in structural dynamics of membrane proteins including modification of their system of hydrogen-bonds.

Dipyridamole prevents triple-negative breast-cancer progression

Dipyridamole is a widely prescribed drug in ischemic disorders, and it is here investigated for potential clinical use as a new treatment for breast cancer. Xenograft mice bearing triple-negative breast cancer 4T1-Luc or MDA-MB-231T cells were generated. In these in vivo models, dipyridamole effects were investigated for primary tumor growth, metastasis formation, cell cycle, apoptosis, signaling pathways, immune cell infiltration, and serum inflammatory cytokines levels. Dipyridamole significantly reduced primary tumor growth and metastasis formation by intraperitoneal administration. Treatment with 15 mg/kg/day dipyridamole reduced mean primary tumor size by 67.5 % (p = 0.0433), while treatment with 30 mg/kg/day dipyridamole resulted in an almost a total reduction in primary tumors (p = 0.0182). Experimental metastasis assays show dipyridamole reduces metastasis formation by 47.5 % in the MDA-MB-231T xenograft model (p = 0.0122), and by 50.26 % in the 4T1-Luc xenograft model (p = 0.0292). In vivo dipyridamole decreased activated β-catenin by 38.64 % (p < 0.0001), phospho-ERK1/2 by 25.05 % (p = 0.0129), phospho-p65 by 67.82 % (p < 0.0001) and doubled the expression of IkBα (p = 0.0019), thus revealing significant effects on Wnt, ERK1/2-MAPK and NF-kB pathways in both animal models. Moreover dipyridamole significantly decreased the infiltration of tumor-associated macrophages and myeloid-derived suppressor cells in primary tumors (p < 0.005), and the inflammatory cytokines levels in the sera of the treated mice. We suggest that when used at appropriate doses and with the correct mode of administration, dipyridamole is a promising agent for breast-cancer treatment, thus also implying its potential use in other cancers that show those highly activated pathways.

Semiempirical quantum mechanical calculations of dipolar interaction between dipyridamole and dipalmitoyl phosphatidyl choline in Langmuir monolayers

Recent studies have shown that dipalmitoyl phosphatidyl choline (DPPC) monolayers respond cooperatively to the presence of dipyridamole (DIP) guest molecules even at small concentrations, which is a signature of molecular recognition. Using semiempirical quantum mechanical calculations for the DIP-DPPC system, we show that the incorporation of DIP causes large changes in the vertical dipole moment of the DIP-DPPC system, which can explain why measurable changes in surface potential are observed experimentally even at very low DIP concentrations. The calculations are also consistent with the anomalous concentration dependence of the surface pressure and surface potential isotherms for DIP-DPPC monolayers. Rather than saturation or a continuous increase in the effects caused by the incorporation of increasing amounts of DIP, the experimentally observed inversion in the behavior of the surface potential as the DIP concentration reaches 0.5 mol % would be caused by a change in DIP conformation, from a vertical arrangement for the DIP rings to a horizontal or intermediate arrangement. The strong dipolar interactions indicated in the calculations may also be the origin of the drastic changes in monolayer morphology seen in fluorescence microscopy images, with triskellion-shaped domains being formed for condensed DIP-DPPC monolayers.

Fragmentation of dipyridamole and related dipyrimidines by electrospray ionization collisional activated decomposition mass spectrometry

The coronary vasodilator, co-activator of antitumor compounds and antioxidant drug dipyridamole and several of its derivatives were studied by electrospray ionization (ESI) combined with collisional activated decomposition (CAD) in both positive and negative modes. These compounds produce abundant monocharged ions ([M + H](+)) under ESI. Interpretation of the CAD spectra showed that fragmentation occurs preferentially in the ethanolamine groups attached at C-2, C-4, C-6 and C-8. 2-Methoxyethanol is eliminated when ethanolamines are in positions C-2/C-6 and 2-aziridinethanol is eliminated from C-4/C-8 ethanolamines. The proposed fragmentation schemes were supported by deuterium labeling experiments and tandem mass spectrometry.

Among substituted 9,10-dihydro-9,10-[1,2]benzenoanthracene-1,4,5,8-tetraones, the lead antitumor triptycene bisquinone TT24 blocks nucleoside transport, induces apoptotic DNA fragmentation and decreases the viability of L1210 leukemic cells in the nanomolar range of daunorubicin in vitro

In contrast to their inactive parent compound triptycene (code name TT0), several new synthetic analogs (TT code number) have antileukemic activities and remain effective in daunorubicin (DAU)-resistant tumor sublines in vitro. Among variously substituted 9,10-dihydro-9,10-[1,2]benzenoanthracene-1,4,5,8-tetraones, a total of six lead antitumor compounds have been identified, and their code names are TT2, TT13, TT16, TT19, TT21 and TT24. These active antitumor triptych structures have bisquinone functionality, and various bromo, methoxy, methylamino and/or dimethylamino substitutions with or without longer alkyl chains on the amino function. Like the anthracycline quinone antibiotic DAU, these triptycene (TT) bisquinones also inhibit DNA synthesis and induce DNA cleavage in relation with their cytotoxic activities, but have the additional advantage of blocking the cellular transport of purine and pyrimidine nucleosides, an effect which DAU cannot do. As demonstrated by intact chromatin precipitation and agarose gel electrophoresis, the ability of TT bisquinones and DAU to induce DNA fragmentation is biphasic with a peak that shifts to lower concentrations with increasing times of drug exposure. The most effective lead antitumor compound, TT24, induces DNA cleavage in the same concentration-dependent manner as DAU at 24 h (similar peak in response to 1.6 microM) and is nearly equipotent to DAU against L1210 tumor cell viability at day 4 (IC50 values of TT24 and DAU: 48 and 25 nM, respectively). The mechanism by which TT24 induces DNA fragmentation is inhibited by actinomycin D, cycloheximide, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, benzyloxycarbonyl-Ile-Glu-Thr-Asp-fluoromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone and ZnSO4, suggesting that TT bisquinones trigger apoptosis by caspase and endonuclease activation. Since TT24 is cytotoxic in the nanomolar range of DAU, but might have a more versatile mechanism of action than DAU in wild-type and multidrug-resistant tumor cells, this new class of DNA-damaging quinone antitumor drugs inhibiting nucleoside transport might be valuable to develop new means of polychemotherapy.

Nucleoside transport inhibitors, dipyridamole and p-nitrobenzylthioinosine, selectively potentiate the antitumor activity of NB1011

NB1011, a novel anticancer agent, targets tumor cells expressing high levels of thymidylate synthase (TS). NB1011 is converted intracellularly to bromovinyldeoxyuridine monophosphate (BVdUMP) which competes with the natural substrate, deoxyuridine monophosphate, for binding to TS. Unlike inhibitors, NB1011 becomes a reversible substrate for TS catalysis. Thus, TS retains activity and converts BVdUMP into cytotoxic product(s). In vitro cytotoxicity studies demonstrate NB1011's preferential activity against tumor cells expressing elevated TS protein levels. Additionally, NB1011 has antitumor activity in vivo. To identify drugs which interact synergistically with NB1011, we screened 13 combinations of chemotherapeutic agents with NB1011 in human tumor and normal cells. Dipyridamole and p-nitrobenzylthioinosine (NBMPR), potent inhibitors of equilibrative nucleoside transport, synergized with NB1011 selectively against 5-fluorouracil (5-FU)-resistant H630R10 colon carcinoma cells [combination index (CI)=0.75 and 0.35] and Tomudex-resistant MCF7TDX breast carcinoma cells (CI=0.51 and 0.57), both TS overexpressing cell lines. These agents produced no synergy with NB1011 in Det551 and CCD18co normal cells (CI > 1.1) lacking TS overexpression. Dipyridamole potentiated NB1011's cytotoxicity in medium lacking nucleosides and bases, suggesting a non-salvage-dependent mechanism. We demonstrate that nucleoside transport inhibitors, dipyridamole and NBMPR, show promise for clinically efficacious combination with NB1011.

Synthetic 1,4-anthracenediones, which block nucleoside transport and induce DNA fragmentation, retain their cytotoxic efficacy in daunorubicin-resistant HL-60 cell lines

Anthracene-1,4-dione and 6,7-dichloro-1,4-anthracenedione (code names AQ1 and AQ4, respectively) are cytostatic (IC50: 53 and 110 nM, respectively) and cytotoxic (IC50: 100 and 175 nM, respectively) in wild-type drug-sensitive HL-60-S tumor cells at day 4 in vitro. Therefore, the antitumor effects of these drugs were assessed and compared to those of daunorubicin (DAU) in HL-60-RV and HL-60-R8 tumor cells, which are, respectively, P-glycoprotein-positive and -negative multidrug-resistant (MDR) sublines. In contrast to DAU, which loses its cytostatic [resistance factors (RFs): 30.3-31.8] and cytotoxic (RFs: 48.8-58.1) activities in MDR sublines, AQ1 inhibits cell proliferation (RFs: 0.9-1.3) and cell viability (RFs: 1.4-1.6) as effectively in HL-60-RV and HL-60-R8 as in HL-60-S cells. Similarly, DAU decreases the rate of DNA synthesis less effectively in MDR sublines (RFs: 8.0-13.3) but AQ1 inhibits the incorporation of [3H]thymidine into DNA to the same degree in HL-60-S as in HL-60-RV and HL-60-R8 cells (RFs: 0.9-1.1). In contrast to DAU, which is ineffective, the advantage of AQ1 is its ability to block the cellular transport of purine and pyrimidine nucleosides in HL-60-S cells, an effect which persists in the MDR sublines (RFs: 1.1). AQ4, which mimics to a lesser degree all the antitumor effects of AQ1, except the inhibition of adenosine transport, also retains its effectiveness in MDR sublines (RFs: 1.1-3.1). The peaks of DNA cleavage caused by DAU and AQ1 in HL-60-S cells shift to lower concentrations with increasing times of drug exposure but DAU loses most of its ability to induce DNA fragmentation in MDR sublines, whereas the levels of AQ1-induced DNA cleavage at 16 and 24 h are nearly equivalent in HL-60-S, HL-60-RV and HL-60-R8 cells. Because they not only mimic the antitumor effects of DAU in the nM range but also block nucleoside transport and remain effective in tumor cells that have developed different mechanisms of MDR, AQ1 and AQ4 analogs might be valuable to develop new means of polychemotherapy.

Cooperativity of phospholipid reorganization upon interaction of dipyridamole with surface monolayers on water

Results from various surface sensitive characterization techniques suggest a model for the interaction of the piperidinopyrimidine dipyridamole (DIP)--known as a vasodilator and inhibitor of P-glycoprotein associated multidrug resistance of tumor cells--with phospholipid monolayers in which the drug is peripherally associated with the membrane, binding (up to) five phospholipids at a time. These multiple interactions are responsible for a very strong association of the drug with the lipid monolayer even at exceedingly low concentrations (approximately 0.2 mol%). Electrostatic interactions and hydrogen bonding are likely involved in the binding of DIP to DPPC. Cooperative effects among the lipids are invoked to explain the macroscopically measurable changes of lipid monolayer properties even when only one out of 100 DPPC molecules is directly associated with a DIP molecule. A reversal of the observed changes upon drug association with the membrane as the DIP concentration surpasses a threshold concentration (c(crit)approximately 0.5 mol%) may be explained by cooperativity in a different context, the self-aggregation of drug molecules. With its implications for the interaction of DIP with phospholipid films, this work provides a first approach to the explanation of the high sensitivity of cell membranes to piperidinopyrimidine drugs on a molecular level.

Quinone isomers of the WS-5995 antibiotics: synthetic antitumor agents that inhibit macromolecule synthesis, block nucleoside transport, induce DNA fragmentation, and decrease the growth and viability of L1210 leukemic cells more effectively than ellagic acid and genistein in vitro

Antibiotic WS-5995A (code name J4) and two of its synthetic analogs, o-quinone J1 and model p-quinone J7, which show some structural similarity with both ellagic acid (EA) and genistein (GEN), were compared for their antileukemic activity in L1210 cells in vitro. Overall, J4 is more cytostatic and cytotoxic than J1 and J7, suggesting that methyl and methoxy substitutions, a p-quinone moiety, and a hydrogen bonding phenolic group may enhance the antitumor potential of these naphthoquinone lactones, which are all more potent than EA and GEN. For instance, the lead compound J4 inhibits tumor cell proliferation and viability at day 4 (IC(50): 0.24--0.65 microM) more effectively than EA (IC(50): 5--6 microM) and GEN (IC(50): 7 microM). Since J4 does not increase but rather decreases the mitotic index of L1210 cells at 24 h, it is not an antitubulin drug but might arrest early stages of cell cycle progression like EA and GEN. A 1.5- to 3-h pretreatment with J4 is sufficient to inhibit the rates of DNA, RNA and protein syntheses (IC(50): 2.0--2.5 microM) determined over 30- to 60-min periods of pulse-labeling in L1210 cells in vitro, whereas EA (IC(50): 20-130 microM) and GEN (IC(50): 40--115 microM) are less effective against macromolecule synthesis. In contrast to 156 microM EA, which is inactive, a 15-min pretreatment with 10--25 microM J4 has the advantage of also inhibiting the cellular transport of both purine and pyrimidine nucleosides over a 30 s period in vitro, an effect which can be mimicked by 156 microM GEN. Hence, the WS-5995 analogs and GEN may prevent the incorporation of [(3)H]adenosine and [(3)H]thymidine into DNA because they rapidly block the uptake of these nucleosides by the tumor cells. After 24 h, the concentration-dependent induction of DNA cleavage by J4 peaks at 10 microM and declines at 25 microM, whereas EA and GEN are ineffective at 10 microM but maximally stimulate DNA cleavage at 62.5 microM. Like EA and GEN, the mechanism by which J4 induces DNA fragmentation is inhibited by actinomycin D, cycloheximide, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone and ZnSO(4), suggesting that J4 triggers apoptosis by caspase and endonuclease activation. Because they are more potent than EA and GEN, and affect both nucleoside transport and DNA cleavage, the WS-5995 antitumor antibiotics might be valuable in polychemotherapy to potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.

Inhibition of human multidrug resistance P-glycoprotein 1 by analogues of a potent δ-opioid antagonist

Analogues Dmt-Tic (2',6'-dimethyl-L-tyrosine-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) pharmacophore, a potent delta-opioid receptor antagonist, inhibited hMDR1 P-GP expressed in a G-185 fibroblast cell line in a manner similar to verapamil. N,N(Me)2-Dmt-Tic-NH-1-adamantane, H-Dmt-Tic-NH-1-adamantane, H-Dmt-Tic-Ala-NH-1-adamantane and N,N(Me)2-Dmt-Tic-NH-tBut were highly effective inhibitors. Weaker inhibition was observed with N,N(Et)2-Dmt-Tic-OH, H-Dmt-Tic-Ala-NH-tert-butyl amide and cyclo(Dmt-Tic). Results demonstrate that N- and C-terminal hydrophobic/lipophilic analogues of the Dmt-Tic pharmacophore inhibit hMDR1 and point to a potential role as chemosensitizing agents in chemotherapy for cancers containing hMDR1.

Classification of multidrug-resistance reversal agents using structure-based descriptors and linear discriminant analysis

Linear discriminant analysis is used to generate models to classify multidrug-resistance reversal agents based on activity. Models are generated and evaluated using multidrug-resistance reversal activity values for 609 compounds measured using adriamycin-resistant P388 murine leukemia cells. Structure-based descriptors numerically encode molecular features which are used in model formation. Two types of models are generated: one type to classify compounds as inactive, moderately active, and active (three-class problem) and one type to classify compounds as inactive or active without considering the moderately active class (two-class problem). Two activity distributions are considered, where the separation between inactive and active compounds is different. When the separation between inactive and active classes is small, a model based on nine topological descriptors is developed that produces a classification rate of 83.1% correct for an external prediction set. Larger separation between active and inactive classes raises the prediction set classification rate to 92.0% correct using a model with six topological descriptors. Models are further validated through Monte Carlo experiments in which models are generated after class labels have been scrambled. The classification rates achieved demonstrate that the models developed could serve as a screening mechanism to identify potentially useful MDRR agents from large libraries of compounds.

1,4-Anthraquinone: an anticancer drug that blocks nucleoside transport, inhibits macromolecule synthesis, induces DNA fragmentation, and decreases the growth and viability of L1210 leukemic cells in the same nanomolar range as daunorubicin in vitro

1,4-Anthraquinone (AQ) was synthesized and shown to prevent L1210 leukemic cells from synthesizing macromolecules and growing in vitro. In contrast, its dihydroxy-9,10anthraquinone precursor, quinizarin, was inactive. The antitumor activity of AQ was compared to that of daunorubicin (DAU), which is structurally different from AQ but also contains a quinone moiety. AQ is equipotent to DAU against L1210 tumor cell proliferation (IC50: 25 nM at day 2 and 9 nM at day 4) and viability (IC50: 100 nM at day 2 and 25 nM at day 4), suggesting that its cytostatic and cytotoxic activities are a combination of drug concentration and duration of drug exposure. Since AQ does not increase but rather decreases the mitotic index of L1210 cells at 24 h, it is not an antitubulin drug but might arrest early stages of cell cycle progression. Like DAU, a 1.5-3 h pretreatment with AQ is sufficient to inhibit the rates of DNA, RNA and protein syntheses (IC50: 2 microM) determined over 30-60 min periods of pulse-labeling in L1210 cells in vitro. In contrast to DAU, which is inactive, a 15 min pretreatment with AQ has the advantage of also inhibiting the cellular transport of both purine and pyrimidine nucleosides (IC50: 2.5 microM) over a 30 s period in vitro. Hence, AQ may prevent the incorporation [3H]thymidine into DNA because it rapidly blocks the uptake of these nucleosides by the tumor cells. After 24 h, AQ induces as much DNA cleavage as camptothecin and DAU, two anticancer drugs producing DNA strand breaks and known to, respectively, inhibit topoisomerase I and II activities. However, the concentration-dependent induction of DNA cleavage by AQ, which peaks at 1.6-4 microM and disappears at 10-25 microM, resembles that of DAU. The mechanism by which AQ induces DNA cleavage is inhibited by actinomycin D, cycloheximide and aurintricarboxylic acid, suggesting that AQ activates endonucleases and triggers apoptosis. The abilities of AQ to block nucleoside transport, inhibit DNA synthesis and induce DNA fragmentation are irreversible upon drug removal, suggesting that this compound may rapidly interact with various molecular targets in cell membranes and nuclei to disrupt the functions of nucleoside transporters and nucleic acids, and trigger long-lasting antitumor effects which persist after cessation of drug treatment. Because of its potency and dual effects on nucleoside transport and DNA cleavage, the use of bifunctional AQ with antileukemic activity in the nM range in vitro might provide a considerable advantage in polychemotherapy to potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.

Dipyridamole and its derivatives modify the kinetics of the electron transport in reaction centers from Rhodobacter sphaeroides

A well known vasodilator dipyridamole (DIP), 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d]pyrim idine, and its derivatives have recently been shown as potential co-activators (modulators) in the phenomenon of multidrug resistance (MDR) in cancer therapy. They inhibit the specific function of a transmembrane P-glycoprotein responsible for the ex-flux of anti-cancer drugs from tumor cells. To clarify molecular mechanisms of the anti-MDR activity of DIP and its two derivatives, RA25 and RA47, we have studied their effects on electron transport in reaction centers (RC) from purple photosynthetic bacteria Rb. sphaeroides, using RC as a model system. Increasing concentrations of DIP and RA47 progressively accelerate the back electron transfer from the primary quinone acceptor QA to the bacteriochlorophyll dimer Bchl2 (Bchl2+ -QA- recombination). In the absence of o-phenantroline, when both quinone acceptors QA and QB are involved in the electron transport, RA47 is more effective than DIP. DIP stabilizes the electron on the secondary quinone acceptor QB, the effect manifested as the retardation of Bchl2+ -QB- recombination. Effects of RA25 are negligible in all cases. The drugs are proposed to change the electron transport affecting the RC structural dynamics and the stabilization of the electron on quinone acceptors through modification of H-bonds in the system.

Binding of dipyridamole to phospholipid vesicles: a fluorescence study

Binding and localization of the vasodilator and antitumor drug coactivator dipyridamole (DIP) and one of its derivatives, RA25, to phospholipid vesicles of DMPC (dimyristoylphosphatidylcholine) and DPPC (dipalmitoylphosphatidylcholine) was studied using fluorescence spectroscopy as well as quenching of fluorescence. The analysis of fluorescence data indicates that neutral dipyridamole binds to the phospholipids in their liquid crystalline phase with an association constant of 950 M(-1) and 1150 M(-1) to DMPC and DPPC, respectively. Protonation of DIP leads to a 3-fold reduction of the association constant. For the gel phospholipid phase, the binding is smaller (a factor of 2), independently of pH, suggesting that the more flexible lipid packing in the liquid crystalline phase facilitates the binding of the drug. The association constant of RA25 neutral form is considerably lower than for DIP, being around 295 M(-1). Fluorescence quenching with nitroxides TEMPO and stearic acid doxyl derivatives suggests the localization of DIP to be closer to the 5th carbon of alkyl chain. The quenching effect of 5-DSA below the lipid phase transition suggests that a strong static quenching may be operative. The quenching effect of 16-DSA is almost as great as that for 5-DSA below the phase transition, being even higher above the phase transition. This effect is probably due to the trans-gauche isomerization of the stearic acid nitroxide, making the encounter of its paramagnetic fragment with the DIP chromophore possible. Our data are consistent with DIP location close to the bilayer surface in the border of hydrophobic-polar heads interface which is similar to the data in micellar systems. In the case of RA25, the drug is in the outer part of the head group interface being much exposed to the aqueous phase and being significantly less accessible to the membrane nitroxide quenchers.

Dipyridamole Interacts with the Polar Part of Cationic Reversed Micelles in Chloroform: 1H NMR and ESR Evidence

The interaction of dipyridamole (DIP) with reversed micelles (RM) of cetyltrimethylammonium chloride (CTAC) in CDCl3 at different water contents was studied. The position and T1 relaxation of the water peak upon addition of extra water revealed three concentration ranges of CTAC: <10 mM (impurity water is mainly dispersed in CDCl3), >50-100 mM (water mainly inside the RM), and intermediate range. The resonances of CTAC protons in the polar layer broadened and displaced by up to 0.07 ppm as a function of CTAC concentration and extra water. At 10 mM CTAC, the addition of 40 mM DIP shifted the head group signals to high field by about 0.1 ppm. At high and intermediate CTAC concentrations, four nitroxide spin probes, hydrophobic 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), hydrophilic 4-amine-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPAMINE), and lipophilic 5- and 16-doxyl stearic acids (5- and 16-DSA), underwent partial immobilization. The rotational correlation time of TEMPAMINE (rather than TEMPO, 5-, and 16-DSA) in RM moderately increased upon addition of 1.5-2.0 mM DIP. At an excess of CTAC, only one DIP peak at 3.88 ppm remained measurable, and its selective T1 fell from 0.34 to 0.12 s. The association constant for DIP and CTAC was between 10 and 35 M-1. Thus, DIP incorporates into the polar region of RM influencing packing and dynamics of surfactant head groups. In contrast, in aqueous CTAC micelles, the preferential localization of DIP substituents is inside the nonpolar micelle core, and the binding constant is two orders of magnitude above that for RM.

Interaction of dipyridamole with lipids in mixed Langmuir monolayers

Dipyridamole (DIP), a well known coronary vasodilator and coactivator of anti-tumor activity of a number of drugs, forms stable Langmuir monolayers with the zwitterionic lipid dipalmitoylphosphatidylcholine (DPPC) and the negatively charged dipalmitoylphosphatidylglycerol (DPPG) at an air/aqueous solution interface. The drug binds to the lipid molecules and change their packing density in the monolayer in the process of compression, the effect depending on the drug location in the monolayer, protonation of the drug and also on the charge state of the lipid. The incorporation of dipyridamole (DIP) into neutral DPPC monolayers causes them to be more expanded at low DIP concentrations but more condensed at high concentrations, resembling the effect of cholesterol. Maximum expansion occurs for a DIP concentration of 2 mol%. For slightly charged DPPG monolayers spread on ultra pure water, the monolayers become increasingly more expanded with increasing DIP concentrations. For the negatively charged DPPG monolayers spread on buffer solutions, the incorporation of DIP has similar effects to that observed for DPPC monolayers. This is probably due to the interaction between the charged DPPG molecules and the protonated DIP molecules. Also, introduction of protonated DIP brings an increase in surface potential of DPPG monolayers because the negative contribution from the double layer is decreased. The results indicated that DIP molecules are located deeper in the hydrophobic region of DPPC monolayers, whereas in DPPG ones they appear to be located very close to the polar head region. Due to the electrostatic interaction of protonated DIP with the charges on the polar heads of lipids it is inclined with respect to the plane of the monolayer.

Antifolates: current developments

In summary, the problem of MTX resistance has been approached in a mechanistic fashion, based on the wealth of information generated over the years. To date, these strategies have produced several new classes of anticancer drugs, with a variety of anticipated and unanticipated mechanisms of action. Several of these have shown promising preclinical activity, and these are moving into more stringent testing in the clinic.

Dipyridamole mediated enhanced antiproliferative activity of 10-ethyl-10-deazaaminopterin (10-EDAM) against human lung cancer cell lines

10-ethyl-10-deazaaminopterin (10-EDAM) is a rationally designed derivative of the antifolate, methotrexate (MTX). In a number of tumor models these design features have resulted in an improved spectrum of antiproliferative activity as compared with the parent compound. Using an MTT growth assay, we compared in vitro antiproliferative activity of 10-EDAM with MTX in eight lung cancer cell lines. Growth was inhibited in all lines tested by clinically achievable concentrations of 10-EDAM (0.1-1,000 nM). 10-EDAM was more cytotoxic than MTX at the same concentrations in all eight lung cancer cell lines. In an effort to enhance the antiproliferative effect, we evaluated the addition of dipyridamole (DPM), an inhibitor of nucleoside transport, to 10-EDAM (0.1-10 microns). DPM decreased the concentration of 10-EDAM required to cause 50% growth inhibition (IC50) in all eight cell lines tested. This suppression was statistically significant by 2-sided sign test (P = .0078). By contrast, the IC50 of MTX was decreased in only two of the eight cell lines when DPM was added (0.1-10 microM). In defined thymidine depleted media, cell kill by the combination of 10-EDAM and DPM was no greater than 10-EDAM alone, consistent with the possibility that DPM exerts some of its effect by inhibition of extrinsic nucleoside salvage. In consideration of the published activity of 10-EDAM in lung cancer and the modest clinical toxicity of DPM based biochemical modulation, we conclude the current in vitro data provide justification for clinical evaluation of this combination in patients with lung cancer.

Localization of dipyridamole molecules in ionic micelles: effect of micelle and drug charges

The localization of the coronary vasodilator dipyridamole (DIP) in cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS) and zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and lysophosphatidylcholine (HPS and LPC) micelles was investigated using fluorescence quenching by quenchers with known localization in the micelle (TEMPO and 5-doxyl and 12-doxyl stearic acids). The use of fluorescence quenching jointly with fluorescence and 1H-NMR spectral measurements shows that DIP molecules in both protonated and nonprotonated forms are localized in micelles near the region which separates their polar and nonpolar parts, the polarizable heteroaromatic cycle of DIP being close to the polar part and the nonpolar substituents penetrating the hydrophobic interior of the micelle. The electrostatic interaction between the protonated DIP molecules and micelle charges either moves DIP into the micelle interior (for cationic and zwitterionic micelles) or draws it closer to the micelle surface (for anionic ones). Our results could be relevant to the mechanism of DIP action since many data indicate the interaction of the drug with cell membranes. The ability of DIP to localize near the membrane surface with the substituents immersed into a hydrophobic moiety could be essential for the drug interaction with P-glycoprotein, which is responsible for mediation of the effects of several antitumour drugs.

Biochemical modulation of 'classical' multidrug resistance by BIBW22BS, a potent derivative of dipyridamole

BACKGROUND

Modulators of the 'classical' multidrug resistance (mdr) phenotype have low efficacy in patients with solid tumors. We analyzed BIBW22BS, 4-[N-(2-hydroxy-2-met- hyl-propyl)-ethanolamino]-2,7-bis(cis-2,6-dimethyl-morpho- lino)-6-phenylpteridine, a derivative of dipyridamole, for its higher potential to modulate mdr.

MATERIALS AND METHODS

Four human malignant cell lines: BRO, A2780, GLC4, SW1573, the Pgp-positive sublines: BRO/mdr1.1, 2780AD and the non-Pgp sublines: GLC4/ADR, SW1573/2R120 were used in vitro to investigate BIBW22BS as a modulator of the antiproliferative effects of vincristine and doxorubicin and to compare the potency of BIBW22BS with that of dipyridamole, verapamil, bepridil and flunarizine. BRO/mdr1.1 s.c. well-established xenografts in nude mice were used to study the modulating properties of BIBW22BS 50 mg/kg i.v. followed after one h by vincristine 1 mg/kg i.p. or doxorubicin 8 mg/kg i.p. weekly x 2.

RESULTS

BIBW22BS was 20- to 100-fold more potent than dipyridamole in the reversal of resistance in the Pgp-positive sublines. Reversal of resistance was obtained in a dose-dependent manner and was complete at concentrations of 0.5-2.5 microM. At non-toxic, equimolar concentrations of 1.0 microM BIBW22BS showed higher modulating potency than the calcium-channel blockers. BIBW22BS did not affect resistance in the non-Pgp sublines. BRO/mdr1.1 s.c. xenografts have stable multidrug-resistance characteristics upon serial transplantation. BIBW22BS, vincristine, or doxorubicin as single agents were not effective in vivo, while the addition of BIBW22BS could significantly reduce the tumor growth expressed as the T/C% of vincristine from 109% to 48% and that of doxorubicin from 55% to 32%. However, reversal of vincristine resistance in BRO/mdr1.1 xenografts was not complete when compared to the efficacy of vincristine in BRO xenografts.

CONCLUSION

The results encourage the further preclinical development of BIBW22BS as a modulator of 'classical' multidrug resistance in cancer patients.

Dipyridamole modulates multidrug resistance and intracellular as well as nuclear levels of doxorubicin in B16 melanoma cells

Simultaneous occurrence of resistance to many chemotherapeutic agents, termed multidrug resistance (MDR), is a complex phenotype. MDR occurs due to several reasons, including over-expression of a 170-kDa membrane-bound protein, called P-glycoprotein (P-gp), which apparently participates in active drug efflux. Multidrug-resistant cells also frequently exhibit an altered pattern of intracellular drug distribution, resulting in a reduction in the nuclear level of drugs such as doxorubicin (DOX). In this study, the effect of dipyridamole (DP) on drug resistance and on intracellular as well as nuclear levels of DOX in multidrug-resistant melanoma cells has been examined. For this purpose, drug-sensitive murine melanoma cells (B16V) and their multidrug-resistant variant cells, (B16VDXR; selected for resistance to DOX) which over-produce P-gp, were employed. B16VDXR cells were cross-resistant to several anti-cancer agents including etoposide (VP-16) and mitoxantrone (Mitox). DP (10 microM) significantly potentiated the cytotoxicity of DOX, VP-16 and Mitox towards multidrug-resistant B16VDXR cells but not in parental drug-sensitive B16V cells. The presence of DP resulted in a 3.7-fold increase in the total cellular level and a 4.2-fold increase in the nuclear content of DOX in the resistant cells. Isobologram analysis indicates that DP at several pharmacologically relevant concentrations synergistically potentiates the activity of DOX in B16VDXR cells.

Reversal of multidrug resistance by bis(phenylalkyl)amines and structurally related compounds

We have previously reported that multidrug (MDR)-reversal activity can be exerted by compounds in which two ring structures of certain types are connected by one alkyl bridge to a secondary or tertiary amine group. In the present investigation we studied the MDR-reversal activity of compounds in which the two ring structures were connected by separate alkyl bridges to the amine group. The structure-activity relationship of these compounds verified previous findings on the structural features that support MDR-reversal activity as well as the features that reduce such activity. In addition, the present study reveals additional chemical groups and ring structures that support MDR-reversal activity as well as those that reduce it.

Pharmacologic circumvention of multidrug resistance

The ability of malignant cells to develop resistance to chemotherapeutic drugs is a major obstacle to the successful treatment of clinical tumors. The phenomenon multidrug resistance (MDR) in cancer cells results in cross-resistance to a broad range of structurally diverse antineoplastic agents, due to outward efflux of cytotoxic substrates by the mdr1 gene product, P-glycoprotein (P-gp). Numerous pharmacologic agents have been identified which inhibit the efflux pump and modulate MDR. The biochemical, cellular and clinical pharmacology of agents used to circumvent MDR is analyzed in terms of their mechanism of action and potential clinical utility. MDR antagonists, termed chemosensitizers, may be grouped into several classes, and include calcium channel blockers, calmodulin antagonists, anthracycline and Vinca alkaloid analogs, cyclosporines, dipyridamole, and other hydrophobic, cationic compounds. Structural features important for chemosensitizer activity have been identified, and a model for the interaction of these drugs with P-gp is proposed. Other possible cellular targets for the reversal of MDR are also discussed, such as protein kinase C. Strategies for the clinical modulation of MDR and trials combining chemosensitizers with chemotherapeutic drugs in humans are reviewed. Several novel approaches for the modulation of MDR are examined.

Radiosensitizing and repair-inhibiting properties of dipyridamole

Radioresistance and postirradiation repair of potentially lethal damage (PLD repair) are important factors underlying failure to control local disease in cancer. Dipyridamole (DP) is known as a modifier of the action of cytotoxic drugs. We therefore investigated DP as a potential radiosensitizer and inhibitor of PLD repair in X-irradiated Chinese hamster ovary (CHO) cells in vitro. Exposure to the drug alone resulted in a slight reduction of the clonogenic capacity of the cells. Preincubation for 18 h with 10 and 20 microM DP in cells subcultured at low density, led to a significant radiosensitization. In confluent density-inhibited cultures, preincubation alone as well as pre- and postincubation with 20 microM DP resulted in a significant inhibition of PLD repair. Dipyridamole and related compounds may thus be useful tools for modifying and investigating the response of mammalian cells to radiation.

Dipyridamole enhances an anti-proliferative effect of interferon in various types of human tumor cells

The anti-proliferative activity of human interferon (HuIFN) was enhanced by dipyridamole, 2,6-bis-(diethanolamino)-4,8-dipiperidinopyrimido-[5,4-d]-py rimidine, when tested against various human tumor cell lines, including KT (breast carcinoma), PLC/PRF/5 (hepatoma), MGC-I, U251-SP and T98 (glioma), HAC-2 and SHIN-3 (ovarian carcinoma), and MM-ICB (melanoma). The enhancement occurred irrespective of the kind of HuIFN used (alpha, beta or gamma) and the original degree of susceptibility of the cells to HuIFN. Even low doses down to 0.01 microM of dipyridamole that had no intrinsic anti-proliferative activity could enhance the effect of HuIFN. The enhancement of HuIFN effects seems not to be caused by induction of HuIFN production, because neither anti-viral activity nor HuIFN antigens were detected in culture medium in cells treated with dipyridamole. Mopidamole, a derivative of dipyridamole lacking one piperidine residue, produced little enhancement of the effects of HuIFN. Among ovarian cancer cell lines tested, the enhancement of the activity of HuIFN by dipyridamole for HAC-2 and SHIN-3 cells was equivalent to or greater than that for 3 chemotherapy agents (adriamycin, vincristine, and a camptothecin derivative). However, neither HOC-21 ovarian cancer cells nor HEC-1 endometrial adenocarcinoma cells were susceptible to any combinations. When MGC-1, U251-SP, and HAC-2 cells were injected into nude mice, the growth of tumors was more markedly inhibited by the subcutaneous administration of HuIFN in combination with oral administration of dipyridamole than by the HuIFN alone. Thus, this combination therapy seems to be worth trying for human cancer, although the enhancement of the effects of HuIFN by dipyridamole varied among the cell lines examined.

Reversal of multidrug resistance by phenothiazines and structurally related compounds

The multidrug-resistance (MDR)-reversal activity of 232 phenothiazines and structurally related compounds was tested in MDR P388 cells. Such activity was found among compounds exhibiting two ring structures (phenyl, cyclopentyl, cyclohexyl, thienyl or 5-norbornen-2-yl but not pyridinyl) linked by a variety of bridge types and possessing a secondary or tertiary amine group. Among 192 such compounds, 31.8% displayed good activity (MDR-reversal ratio, greater than or equal to 10) and 8.3%, outstanding activity (MDR-reversal ratio, greater than or equal to 30). In a subgroup comprising 56 compounds with a carbonyl residue, 4 with sulfuryl residue and 1 with thienyl residue, 42.7% showed good activity and 18%, outstanding activity. The contribution of these residues to the MDR-reversal activity was particularly evident among compounds containing a cyclic tertiary amine. Among 49 such compounds, 51% displayed good activity and 20.4%, outstanding activity, whereas among the 85 compounds lacking such groups, only 31.8% showed good activity and 4.7%, outstanding activity. Enhancement of this activity by the carbonyl group is also obtained when the latter is part of an amide bond of a tertiary amine. As compounds with a carbonyl group located on the rings, on the bridge to the amine group or beyond the amine are efficient MDR reversers, it seems that the exact molecular location of the carbonyl group is not critical for the elicitation of this activity.

Photoaffinity substrates for P-glycoprotein

A variety of compounds can inhibit the function of P-glycoprotein (Pgp) by binding to it and preventing the efflux of anticancer drug substrates. While the molecular architecture of the drug binding site(s) in Pgp is not known, it is clear that modulators in general appear to conform to some general physical-chemical rules. In this paper, we discuss the basic concepts of drug recognition by Pgp as currently understood. We also examine the compounds used to photoaffinity label this protein and discuss their utility in identifying drug binding sites. Finally, we show that a photoaffinity analog of daunorubicin, [3H]azidobenzoyl-daunorubicin ([3H]AB-DNR), is a good affinity labeling reagent for Pgp. A finding of interest is that vinblastine and verapamil compete more effectively than daunorubicin for [3H]AB-DNR binding to Pgp, suggesting that vinblastine and verapamil have similar structural features not shared by daunorubicin.

Modulation of vinblastine sensitivity by dipyridamole in multidrug resistant fibrosarcoma cells lacking mdr1 expression

We examined the ability of dipyridamole (DPM) to act synergistically with vinblastine (VBL) in HT1080 fibrosarcoma cells and a drug-resistant variant, HT1080/DR4, which lacks mdr1 expression, in order to determine whether DPM requires P-glycoprotein to modulate drug sensitivity. Median effect analysis of clonogenic assay was used to produce the combination index (CI50, values less than 1 indicate synergy). DPM was mildly synergistic with VBL producing a CI50 of 0.83 +/- 0.13 for HT1080 cells and 0.80 +/- 0.16 for HT1080/DR4 cells. HT1080 and HT1080/DR4 cells accumulated 6.7 +/- 0.7 and 5.6 +/- 0.9 pmol 3H-VBL mg cells-1 at steady state (Css) and 20 microM DPM elevated the Css by 1.8 and 2.9-fold, respectively. In comparison, the CI50 was 1.1 +/- 0.2 in parental KB-3-1 cells and 0.1 +/- 0.1 in mdr1-expressing KB-GRC1 cells. The KB-3-1 and KB-GRC1 cells had a Css of 3.8 +/- 0.8 and 0.7 +/- 0.2 pmol 3H-VBL mg cells-1, respectively, and DPM elevated the Css by 9.2-fold in KB-GRC1 cells. These studies demonstrate that DPM can produce synergy independently of mdr1 expression but that much greater levels of synergy are achievable in mdr1-expressing tumour cells.

Activity of various amphiphilic agents in reversing multidrug resistance of L 1210 cells

Several compounds (bamipine, chlorphenoxamine, estracyt, hycanthone, quinidine, quinine, tamoxifen, trifluoperazine and verapamil) have a common basic structure with the following features: lipophilic aromatic ring system; linked chain hydrophilic N-alkyl group. They are used medically for varying diseases. Their activity in reversing multidrug-resistance (MDR) with other compounds (diethylstilbestrol, beta-estradiol, methylbiguanide, methylpiperazine, testosterone) lacking one of these chemical features is compared. The in vitro test system we used was the nucleoside incorporation assay using parental L 1210 ascites tumor cells and a doxorubicin resistant subline, which expresses the MDR phenotype. The substances lacking one of these features were not effective in reversing the MDR whereas all other tested substances demonstrated modulating potential in the MDR resistant L 1210 cells.

Glucose transport in Acanthocheilonema viteae

The uptake of glucose by Acanthocheilonema viteae was studied in vitro. The process was selective for the D-isomer and saturatable with a Km of 2 mM. The rate of glucose transport/utilization was inhibited by 2-deoxyglucose, mannose, 5-thioglucose and dipyridamole but, unlike mammalian systems, was not impaired by cytochalasin B, phloretin, phloridzin, 3-O-methylglucose and 4,6-ethylideneglucose. A potential chemotherapeutic advantage of selectively inhibiting filarial glucose transport exists for the following reasons. (1) The glucose transporter present in A. viteae was shown to be different from the one present in some mammalian systems. (2) Incubation under glucose-free conditions led to glycogen depletion, loss of motility and worm death. (3) Worms maintained in vitro for more than 18 h without glucose did not survive when implanted into gerbils.

Cyclosporin A and verapamil have different effects on energy metabolism in multidrug-resistant tumour cells

Cyclosporin A (Sandimmune) rapidly induced an increase in daunorubicin accumulation in multidrug-resistant human ovarian carcinoma cells (2780AD) and was more potent than verapamil. Steady-state 3H-cyclosporin A accumulation at 37 degrees C in 2780AD cells was 60-70% of that in the sensitive A2780 cells. A rapid increase of ATP consumption and lactate production was induced in 2780AD cells by verapamil, but not by cyclosporin A. These results suggest that the interactions of cyclosporin A and verapamil with P-glycoprotein, which leads to inhibition of drug transport, have a different mechanistic basis.