Heterogeneity of [3H]dipyridamole binding to CNS membranes: correlation with [3H]nitrobenzylthioinosine binding and [3H]uridine influx studies

Design, synthesis, and evaluation of 2-diethanolamino-4,8-diheptamethyleneimino-2-(N-aminoethyl-N-ethanolamino)-6-(N,N-diethanolamino)pyrimido[5,4-d]pyrimidine-fluorescein conjugate (8MDP-fluor), as a novel equilibrative nucleoside transporter probe

Nucleoside transporters are integral membrane glycoproteins that play critical roles in physiological nucleoside and nucleobase fluxes, and influence the efficacy of many nucleoside chemotherapy drugs. Fluorescent reporter ligands/substrates have been shown to be useful in the analysis of nucleoside transporter (NT) protein expression and discovery of new NT inhibitors. In this study, we have developed a novel dipyridamole (DP)-based equilibrative nucleoside transporter 1 (ENT1) fluorescent probe. The potent ENT1 and ENT2 inhibitor analogue of dipyridamole, 2,6-bis(diethanolamino)-4,8-diheptamethyleneiminopyrimido[5,4-d]pyrimidine (4, 8MDP), was modified to replace one β-hydroxyethyl group of the amino substituent at the 2-position with a β-aminoethyl group and then conjugated through the amino group to 6-(fluorescein-5-carboxamido)hexanoyl moiety to obtain a new fluorescent molecule, 2-diethanolamino-4,8-diheptamethyleneimino-2-(N-aminoethyl-N-ethanolamino)-6-(N,N-diethanolamino)pyrimido[5,4-d]pyrimidine-fluorescein conjugate, designated 8MDP-fluorescein (8MDP-fluor, 6). The binding affinities of 8MDP-fluor at ENT1 and ENT2 are reflected by the uridine uptake inhibitory K(i) values of 52.1 nM and 285 nM, respectively. 8MDP-fluor was successfully demonstrated to be a flow cytometric probe for ENT1 comparable to the nitrobenzylmercaptopurine riboside (NBMPR) analogue ENT1 fluorescent probe SAENTA-X8-fluorescein (SAENTA-fluor, 1). This is the first reported dipyridamole-based ENT1 fluorescent probe, which adds a novel tool for probing ENT1, and possibly ENT2.

Synthesis, flow cytometric evaluation, and identification of highly potent dipyridamole analogues as equilibrative nucleoside transporter 1 inhibitors

Dipyridamole (Persantine) is a clinically used vasodilator with equilibrative nucleoside transporters 1 and 2 (ENT1 and ENT2) inhibitory activity albeit less potent than the prototype ENT1 inhibitor nitrobenzylmercaptopurine riboside (NBMPR). Dipyridamole is a good candidate for further exploration because it is a non-nucleoside and has a proven record of safe use in humans. A series of dipyridamole analogues were synthesized with systematic modification and evaluated as ENT1 inhibitors by flow cytometry. Compounds with much higher potency were identified, the best being 2,6-bis(diethanolamino)-4,8-diheptamethyleneiminopyrimido[5,4-d]pyrimidine (13) with a K(i) of 0.49 nM compared to a K(i) of 308 nM for dipyridamole. Compound 13 is similar in potency to the prototype potent ENT1 inhibitor NBMPR (0.43 nM). For the first time, a dipyridamole analogue has been identified that is equipotent with NBMPR. The SAR indicated that diethanolamine substituted analogues were more active than monoethanolamine compounds. Also, free hydroxyl groups are not essential for activity.

Contribution of the nucleoside transport system to doxorubicin transport in HL60 cells but not in mononuclear cells

Previously, we reported that pirarubicin (THP), an anthracycline, was transported, at least in part, via a nucleoside transport system in human leukemic HL60 cells, but not in mononuclear cells (MNCs). In this study, the contribution of the nucleoside transport system to the transport of other anthracyclines, doxorubicin (DOX), daunorubicin (DNR) and idarubicin (IDA), in HL60 cells and MNCs was investigated. The experiments were performed after both types of cells had been pretreated with a metabolic inhibitor, 2,4-dinitrophenol, to deplete cellular ATP. The DOX uptake by HL60 cells was partially inhibited by inhibitors of equilibrative nucleoside transporters. In HL60 cells, moreover, the uptake of DOX depended on an inwardly directed Na(+)-gradient, and was inhibited by concentrative nucleoside transporters, but there was no change in the DNR or IDA uptake under any of these conditions. On the other hand, the uptake of the three drugs by MNCs was not affected by any inhibitors of the nucleoside transporters, and there was no dependence of the uptake on an Na(+)-gradient. These results suggested that DOX, but not DNR or IDA, was partially transported in HL60 cells via the nucleoside transport system, whereas in MNCs the system did not contribute to the uptake of any of these three drugs. Thus, nucleoside transport systems contributing to the transport of anthracyclines may be different among different derivatives and cell types.

Possibility of contribution of nucleoside transport systems to pirarubicin uptake by HL60 cells but not mononuclear cells

Previously, we reported that pirarubicin (THP), an anthracycline, was taken up, at least in part, by both human leukemic HL60 cells and mononuclear cells (MNCs) via a carrier-mediated system. In this study, the possibility of a contribution of nucleoside transport systems to the uptake of THP by HL60 cells and MNCs was investigated. The experiments were performed after both types of cells had been pretreated with a metabolic inhibitor, 2,4-dinitrophenol, to deplete cellular ATP. In HL60 cells, THP uptake was increased and decreased significantly by treatment with equilibrative nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine and dilazep, in the presence and absence, respectively, of an inwardly directed Na+-gradient. THP uptake by HL60 cells showed an overshoot in the presence of the gradient, and was decreased by treatment of the cells with monensin, indicating that the uptake partially depended on the Na+-gradient. In HL60 cells in which equilibrative nucleoside transport was inhibited by NBMPR, THP uptake in the presence of the gradient was inhibited by Na+-dependent concentrative nucleoside transport inhibitors, but no inhibition was observed in the absence of the gradient. In MNCs, conversely, there was no effect of any equilibrative nucleoside transport inhibitor or the Na+-gradient on THP uptake. These results suggested that THP was taken up, at least in part, via both equilibrative and concentrative nucleoside transport systems in HL60 cells, but not in MNCs.

Flow cytometric studies of nucleoside transport regulation in single chromaffin cells

The present paper reveals that a fluorescent derivative of nitrobenzylthioinosine, 5-(SAENTA-x8)-fluorescein, is a highly specific inhibitor of the neural NBTI-sensitive nucleoside transporter. 5-(SAENTA-x8)-fluorescein inhibited adenosine transport and [3H]NBTI binding with a Ki of 4 nM in cultured chromaffin cells. Flow cytometry demonstrated that 5-(SAENTA-x8)-fluorescein specifically interacted with the NBTI-sensitive nucleoside transporters with high affinity (K[D] = 6 nM). Activation of protein kinases A and C with forskolin or nicotinic receptor agonists, respectively, resulted in 50% inhibition of the fluorescence bound to the cells. Flow cytometry will allow studying nucleoside transport in single cells from heterogeneous neural cell populations.

Modulation of excitatory synaptic transmission by adenosine released from single hippocampal pyramidal neurons

Adenosine is a potent neuromodulator in the CNS, but the mechanisms that regulate adenosine concentrations in the extracellular space remain unclear. The present study demonstrates that increasing the intracellular concentration of adenosine in a single hippocampal CA1 pyramidal neuron selectively inhibits the excitatory postsynaptic potentials in that cell. Loading neurons with high concentrations of adenosine via the whole-cell patch-clamp technique did not affect the GABAA-mediated inhibitory postsynaptic potentials, the membrane resistance, or the holding current, whereas it significantly increased the adenosine receptor-mediated depression of excitatory postsynaptic currents. The effects of adenosine could not be mimicked by an agonist at the intracellular adenosine P-site, but the effects could be antagonized by a charged adenosine receptor antagonist and by adenosine deaminase, demonstrating that the effect was mediated via adenosine acting at extracellular adenosine receptors. The effect of adenosine loading was not blocked by BaCl2 and therefore was not caused by an adenosine-activated postsynaptic potassium conductance. Adenosine loading increased the paired-pulse facilitation ratio, demonstrating that the effect was mediated by presynaptic adenosine receptors. Finally, simultaneous extracellular field recordings demonstrated that the increase in extracellular adenosine was confined to excitatory synaptic inputs to the loaded cell. These data demonstrate that elevating the intracellular concentration of adenosine in a single CA1 pyramidal neuron induces the release of adenosine into the extracellular space in such a way that it selectively inhibits the excitatory inputs to that cell, and the data support the general conclusion that adenosine is a retrograde messenger used by pyramidal neurons to regulate their excitatory input.

Chronic dipyridamole administration downregulates [3H]nitrobenzylthioinosine binding site affinity in guinea pig kidney but not heart and brain

Specific binding of the nucleoside transporter probe, [3H]nitrobenzylthioinosine, ([3H]NBMPR) was measured in washed guinea pig cardiac, renal and forebrain membranes after 14 days of treatment with dipyridamole (37.5 mg/kg, s.c., b.i.d.) or vehicle. When compared to values in vehicle-treated animals, a 100 percent increase in equilibrium dissociation constant (Kd) was observed in the kidney of dipyridamole-treated animals (0.51 +/- 0.04 to 1.0 +/- 0.06, p < 0.01). The maximal binding capacity (Bmax) was unaltered. No changes were observed in the heart or forebrain. The increase in Kd suggests that chronic dipyridamole treatment decreases the apparent binding affinity of NBMPR for kidney nucleoside transporters. Cardiac and brain nucleoside transporters may be either less susceptible to chronic dipyridamole administration or have a different adaptive mechanism.

Adenosine transporters

1. In mammals, nucleoside transport is an important determinant of the pharmacokinetics, plasma and tissue concentration, disposition and in vivo biological activity of adenosine as well as nucleoside analogues used in antiviral and anticancer therapies. 2. Two broad types of adenosine transporter exist, facilitated-diffusion carriers and active processes driven by the transmembrane sodium gradient. 3. Facilitated-diffusion adenosine carriers may be sensitive (es) or insensitive (ei) to nanomolar concentrations of the transport inhibitor nitrobenzylthioinosine (NBMPR). Dipyridamole, dilazep and lidoflazine analogues are also more potent inhibitors of the es carrier than the ei transporter in cells other than those derived from rat tissues. 4. The es transporter has a broad substrate specificity (apparent Km for adenosine approximately 25 microM in many cells at 25 degrees C), is a glycoprotein with an average apparent Mr of 57,000 in human erythrocytes that has been purified to near homogeneity and may exist in situ as a dimer. However, there is increasing evidence to suggest the presence of isoforms of the es transporter in different cells and species, based on kinetic and molecular properties. 5. The ei transporter also has a broad substrate specificity with a lower affinity for some nucleoside permeants than the es carrier, is genetically distinct from es but little information exists as to the molecular properties of the protein. 6. Sodium-dependent adenosine transport is present in many cell types and catalysed by four distinct systems, N1-N4, distinguished by substrate specificity, sodium coupling and tissue distribution. 7. Two genes have been identified which encode sodium-dependent adenosine transport proteins, SNST1 from the sodium/glucose cotransporter (SGLT1) gene family and the rat intestinal N2 transporter (cNT1) from a novel gene family including a bacterial nucleoside carrier (NupC). Transcripts of cNT1, which encodes a 648-residue protein, are found in intestine and kidney only. 8. Success in cloning the remaining adenosine transporter genes will improve our understanding of the diversity of nucleoside transport processes, with a view to better targeting of therapeutic nucleoside analogues and protective use of transport inhibitors.

Localisation of the adenosine uptake site in the human brain: a comparison with the distribution of adenosine A1 receptors

Using quantitative receptor autoradiography we investigated the distribution of the adenosine uptake site labelled with [3H]NBTI in post-mortem human brain and compared its distribution with that of the A1 adenosine receptor labelled with [3H]CHA. The highest levels of [3H]NBTI binding were found in the cortex and striatum, with moderate levels in the hippocampus, globus pallidus, cerebellum and some midbrain and spinal cord nuclei. The distribution of A1 receptors and this adenosine uptake site differed in the hippocampus where A1 receptors were highest in CA1 but the uptake site was low in CA1 and higher in the molecular layer of the dentate gyrus. These results define the anatomical distribution of the high affinity NBTI sensitive adenosine uptake site in the normal human brain.

Characterization of nucleoside transport activity in rabbit cortical synaptosomes

Rabbit central nervous system (CNS) preparations have been used to study the central effects of adenosine, but little is known about the specific uptake mechanisms in rabbit brain involved in the regulation of extracellular adenosine concentrations. The present study assessed the kinetic and pharmacological characteristics of the uptake of [3H]uridine (a poorly metabolized substrate for adenosine transporters) by rabbit cortical synaptosomes, to define the transporter subtypes involved and to evaluate species variability in transporter characteristics. [3H]Uridine transport into rabbit cortical synaptosomes was mediated by two saturable, facilitated diffusion systems with characteristics compatible with the es and ei transporter subtypes identified in other mammalian species. About 65% of the total transport was mediated by the es system, and Km estimates of 320 and 94 microM were determined for [3H]uridine uptake by the es and ei transporter, respectively. These results differ significantly from the subtype ratio and kinetic characteristics reported for rat and guinea pig cortical synaptosomes, where most of the transport was mediated by an ei subtype. Dipyridamole, dilazep, nitrobenzylthioinosine, R75231, soluflazine, and mioflazine were relatively more effective as inhibitors of es-mediated uptake (compared with ei), while the substrates adenosine, cytidine, and guanosine did not distinguish between the es and ei transporters in rabbit cortical synaptosomes. These results highlight the significant species-tissue variability in nucleoside transporter characteristics and subtype expression, and emphasize the need to characterize the transporters in human CNS tissue to allow the rational development of CNS-active therapeutics based on inhibition of nucleoside transport.

Effect of cellular differentiation on nucleoside transport in human neuroblastoma cells

The nucleoside transport characteristics of undifferentiated and differentiated LA-N-2 human neuroblastoma cells were compared through measurement of the cellular accumulation of [3H]formycin B in the absence and presence of specific nucleoside transport blockers such as dipyridamole and nitrobenzylthioinosine (NBMPR). [3H]NBMPR was also used as a high affinity probe to obtain an estimate of the number of NBMPR-sensitive nucleoside transport proteins. Undifferentiated LA-N-2 cells accumulated [3H]formycin B (25 microM) via a NBMPR/dipyridamole sensitive, Na(+)-independent, nucleoside transport system (Vi = 1.52 pmol/microliters/s; maximum intracellular concentration = 45 pmol/microliters cell water). The undifferentiated cells also had a high density of site-specific [3H]NBMPR binding sites (135,000 sites/cell; KD = 0.4 nM). When cell differentiation was induced by exposure to a serum-free defined medium, the initial rate of transporter-mediated [3H]formycin B uptake increased to 1.92 pmol/microliters/s, and the steady-state intracellular concentration of [3H]formycin B also increased significantly to 73 pmol/microliters. However, there was no concomitant change in the number of [3H]NBMPR binding sites, and the additional uptake was not Na(+)-dependent. This enhanced uptake in the differentiated cells appeared to be due, in part, to an increased functional expression of a NBMPR-resistant form of facilitated nucleoside transporter. Approximately 18% of the transporter-mediated uptake in the differentiated cells was resistant to inhibition by NBMPR at concentrations that blocked transport completely in the undifferentiated cells. This cell model may prove useful for basic studies on regulation of nucleoside transporter subtype expression in neural tissues, and for evaluation of the efficacy and potential host toxicity of cytotoxic nucleoside analogues (+/- specific transport blockers) in the treatment of neuroblastoma.

(3H)dipyridamole and (3H)nitrobenzylthioinosine binding sites at the human parietal cortex and erythrocyte adenosine transporter: a comparison

We compared the binding sites of the adenosine transport inhibitors (3H)dipyridamole (DPR) and (3H)nitrobenzylthioinosine (NBI) in human parietal cortex and erythrocytes. In comparison with guinea pig (3H)DPR marked only slightly more binding sites than (3H)NBI with a Bmax of 1080 +/- 29 and 780 +/- 7 fmol/mg protein respectively in parietal cortex and 24288 +/- 2725 and 20875 +/- 1905 fmol/mg protein respectively in erythrocytes. NBI displaced (3H)DPR binding completely from its binding sites at about KD/2 concentrations in parietal cortex as well as erythrocytes with inhibition constants comparable to its dissociation constants. Lineweaver-Burke analysis in erythrocytes indicated a competitive inhibition of (3H)DPR binding by NBI. Pharmacological characterization of (3H)DPR binding sites in human erythrocytes is consistent with their localization on adenosine transporters. These findings provide evidence that as opposed to guinea pig (3H)DPR and (3H)NBI largely label binding sites to the same adenosine transporter in human erythrocytes and parietal cortex.

Adenosine 5'-triphosphate modulation of nitrobenzylthioinosine binding sites in plasma membranes of bovine chromaffin cells

Nitrobenzylthioinosine (NBTI) is a high affinity probe for facilitated diffusion nucleoside transporters. Kinetic analysis of the binding of [3H]NBTI to plasma membranes of chromaffin cells was conducted in the presence or absence of adenosine 5'-triphosphate (ATP). Similar curvilinear plots with a Hill number of 1.32 were obtained in both conditions. ATP significantly increased the number of NBTI binding sites in these preparations showing Bmax values of 1.62 +/- 0.20 pmol/mg protein for controls and 3.22 +/- 0.31 pmol/mg protein in the presence of ATP. However, the affinity constant (KD) was not significantly modified. The non-metabolizable ATP analogue, 5'-adenylyl imidodiphosphate (AMP-PNP) and diadenosine tetraphosphate (Ap4A) can mimic the stimulatory ATP effect, but adenosine monophosphate (AMP) has no effect on the NBTI binding to plasma membranes. These results indicate a modulatory role for ATP, non-hydrolysis dependent, on nucleoside transport in chromaffin cells. Therefore, a nucleotide binding site on the nucleoside transporter similar to that described for glucose transporter could be suggested.

Interaction of the mioflazine derivative R75231 with the nucleoside transporter: evidence for positive cooperativity

This study investigated the interaction of the mioflazine derivative R75231 with the nucleoside transport system of rabbit cortical synaptosomes, and assessed the binding of [3H]R75231 to human erythrocyte ghost membranes. R75231 was a potent inhibitor of [3H]nitrobenzylthioinosine binding and [3H]uridine uptake in synaptosomes (Ki < 10 nM). This inhibition was evident even after extensive washing of the synaptosomes, subsequent to exposure to R75231. In addition to its tight binding characteristics, R75231 was shown to be a 'mixed' type inhibitor of [3H]nitrobenzylthioinosine binding (increased KD, decreased Bmax). [3H]R75231 bound with high affinity (KD = 0.4 nM) to erythrocyte membranes with a Bmax of 44 pmol/mg protein, which is comparable to the number of [3H]nitrobenzylthioinosine binding sites in this preparation. Binding of [3H]R75231 to these membranes was reversible, but the rate of dissociation was dependent upon the displacer used. Nitrobenzylthioinosine and dipyridamole each induced a complete dissociation of site-bound [3H]R75231 at rates not significantly different from those observed using a protocol involving a 100-fold dilution with buffer (no displacer). However, R75231 and mioflazine slowed the rate of dissociation of [3H]R75231 and actually caused an initial increase in the amount of site-bound [3H]R75231. Adenosine, on the other hand, enhanced the rate of [3H]R75231 dissociation. These results indicate that R75231 binding to the nucleoside transporter is a complex reaction, which may involve multiple interacting sites demonstrating positive cooperativity.

Nimodipine inhibits [3H]nitrobenzylthioinosine binding to the adenosine transporter in human brain

The inhibition of [3H]nitrobenzylthioinosine ([3H]NBI) binding to human parietal cortex membranes by adenosine transport inhibitors, adenosine receptor agonists and antagonists and dihydropyridines was investigated. While the adenosine transport inhibitors inhibited [3H]NBI binding with Ki values in the low nanomolar range and the adenosine A1 receptor agonist, cyclopentyladenosine, with a Ki in the low micromolar range, no IC50 values could be obtained for the adenosine receptor antagonists at concentrations up to 100,000 nM. Among the dihydropyridines (+)-nimodipine was the most potent with a Ki of 201 +/- 55 nM. Inhibition of adenosine transport thus may contribute to the clinical effects of nimodipine in the central nervous system.