Comparison of the prognostic value of dipyridamole and dobutamine myocardial perfusion scintigraphy in hemodialysis patients.

Screening for coronary artery disease (CAD) in hemodialysis patients is hampered by contraindications and/or limitations of the available techniques in this population. Myocardial perfusion scintigraphy (MPS) using dipyridamole has been considered inaccurate due to abnormally high basal levels of adenosine in uremia that could blunt the vasodilatory response. Since dobutamine may be more reliable, we directly compared the two in patients on hemodialysis. We performed MPS at rest and after separate dipyridamole or dobutamine stress in 121 chronic hemodialysis patients. More numerous, larger, and more intense reversible lesions were induced with dobutamine than with dipyridamole, mainly in the anteroseptal segments. Reversibility with dipyridamole but not dobutamine MPS was independently and strongly related with mortality associated with CAD and with fatal and non-fatal CAD. We hypothesize that the chronotropic action of dobutamine induced alterations of wall motion, leading to spurious perfusion defects, not unlike artifacts seen with left bundle branch block. Our study shows that even though dobutamine induced more pronounced myocardial ischemia than dipyridamole in chronic hemodialysis patients, dipyridamole MPS more accurately identifies patients at high risk for subsequent cardiac death or non-fatal CAD than dobutamine.

[Characteristics of dipyridamol isolation from biological material].

Acetone is proposed as an isolating agent for extraction of dipyridamolum from biological material. Optimal conditions of isolating dipyridamolum from human cadaveric liver tissue with acetone are determined and quantitative estimation of isolation results is provided.

[Detection of dipiridamol in biological fluids].

Acetone is proposed as an isolating agent for dipiridamol isolation from biological fluids. Purification of the isolates was performed with liquid-liquid extraction and colon chromatography with silasorb C-18 sorbent. The technique of dipiridamol detection in the blood and urine is described. The assays results are presented.

5'-aminoimidazole-4-carboxamide riboside induces apoptosis in human neuroblastoma cells.

5'-Aminoimidazole-4-carboxamide riboside (AICA riboside) has been previously shown to be toxic to two neuronal cell models [Neuroreport 11 (2000) 1827]. In this paper we demonstrate that AICA riboside promotes apoptosis in undifferentiated human neuroblastoma cells (SH-SY5Y), inducing a raise in caspase-3 activity. In order to exert its effect on viability, AICA riboside must enter the cells and be phosphorylated to the ribotide, since both a nucleoside transport inhibitor, and an inhibitor of adenosine kinase produce an enhancement of the viability of AICA riboside-treated cells. Short-term incubations (2 h) with AICA riboside result in five-fold increase in the activity of AMP-dependent protein kinase (AMPK). However, the activity of AMPK is not significantly affected at prolonged incubations (48 h), when the apoptotic effect of AICA riboside is evident. The results demonstrate that when the cell line is induced to differentiate both toward a cholinergic phenotype (with retinoic acid) or a noradrenergic phenotype (with phorbol esters), the toxic effect is significantly reduced, and in the case of the noradrenergic phenotype differentiation, the riboside is completely ineffective in promoting apoptosis. This reduction of effect correlates with an overexpression of Bcl-2 during differentiation. AICA riboside, derived from the hydrolysis of the ribotide, an intermediate of purine de novo synthesis, is absent in normal healthy cells; however it may accumulate in those individuals in which an inborn error of purine metabolism causes an increase in the rate of de novo synthesis and/or an overexpression of cytosolic 5'-nucleotidase, that appears to be the enzyme responsible for AICA ribotide hydrolysis. In fact, 5'-nucleotidase activity has been shown to increase in patients affected by Lesch-Nyhan syndrome in which both acceleration of de novo synthesis and accumulation of AICA ribotide has been described, and also in other neurological disorders of unknown etiology. Our results raise the intriguing clue that the neurotoxic effect of AICA riboside on the developing brain might contribute to the neurological manifestations of syndromes related to purine dismetabolisms.

In vitro antioxidant and photo-oxidant properties of dipyridamole.

The in vitro antioxidant and photo-oxidant activity of dipyridamole was studied by its effect on superoxide- and singlet oxygen-mediated photohemolysis and viability of neutrophils. Dipyridamole was found to be phototoxic when examined by the photohemolysis on human erythrocytes and on linoleic acid as lipid peroxidation model at concentrations above 3.0 x 10(-5) M. On the contrary, when lower concentrations (1.0 x 10(-5) to 1.0 x 10(-6) M) were used, dipyridamole showed a protector action against singlet oxygen-mediated photohemolysis by other phototoxic compounds like triamterene. This antioxidant property is proposed to result from quenching of triamterene mediated by fluorescence energy transfer. Auto-oxidation and fluorescence-energy transfer is clearly an important mechanism for protection for this drug.

Potentiation of the cytotoxicity of thymidylate synthase (TS) inhibitors by dipyridamole analogues with reduced alpha1-acid glycoprotein binding.

Dipyridamole has been shown to enhance the in vitro activity of antimetabolite anticancer drugs through the inhibition of nucleoside transport. However, the clinical potential of dipyridamole has not been realized because of the avid binding of the drug to the plasma protein alpha1-acid glycoprotein (AGP). Dipyridamole analogues that retain potent nucleoside transport inhibitory activity in the presence of AGP are described and their ability to enhance the growth inhibitory and cytotoxic effects of thymidylate synthase (TS) inhibitors has been evaluated. Three dipyridamole analogues (NU3026, NU3059 and NU3060) were shown to enhance the growth inhibitory activity of the TS inhibitor CB3717 and block thymidine rescue in L1210 cells. The extent of potentiation at a fixed analogue concentration (10 microM) was related to the potency of inhibition of thymidine uptake. A further analogue, NU3076, was identified, which was more potent than dipyridamole with a Ki value for inhibition of thymidine uptake of 0.1 microM compared to 0.28 microM for dipyridamole. In marked contrast to dipyridamole, inhibition of thymidine uptake by NU3076 was not significantly affected by the presence of AGP (5 mg ml(-1)). NU3076 and dipyridamole produced equivalent potentiation of the cytotoxicity of the non-classical antifolate TS inhibitor, nolatrexed, in L1210 cells with both compounds significantly reducing the LC90, by > threefold in the absence of salvageable thymidine. Thymidine rescue of L1210 cells from nolatrexed cytotoxicity was partially blocked by both 1 microM NU3076 and 1 microM dipyridamole. NU3076 also caused a significant potentiation of FU cytotoxicity in L1210 cells. These studies demonstrate that nucleoside transport inhibition can be maintained in the absence of AGP binding with the dipyridamole pharmacophore and that such analogues can enhance the cytotoxicity of TS inhibitors.

Inhibition of herpes simplex virus reactivation by dipyridamole in a mouse model.

Herpes simplex virus (HSV) thymidine kinase (TK) has been demonstrated to be important for reactivation from latency. Specifically, HSV latency-associated transcripts (LAT) are expressed during latent infection established by TK-negative (TK-) HSV mutants, but reactivation is minimal. TK- HSV, however, readily reactivated in the presence of exogenous thymidine (TdR) in explant medium [Tenser et al. (1996): Journal of Virology 70:1271-1276]. In the present report this was further studied by evaluating the effect of dipyridamole (DPM) on HSV reactivation. DPM is known to interfere with nucleoside transport. Inhibition of TdR-enhanced reactivation of TK- HSV and inhibition of reactivation of wild-type TK+ HSV were evaluated in an experimental mouse model of latency. Without DPM, TK- HSV reactivation was increased from 0% to 88% with TdR in explant medium, demonstrating TdR-enhanced reactivation of TK- HSV (as seen previously), TdR-enhanced reactivation of TK- HSV was decreased when DPM (25 or 50 microM) was also present, to 30%-60% and to 0%, respectively. Secondly, DPM also decreased reactivation of wild-type TK+ HSV. The reactivation frequency of latently infected dorsal root ganglia was 90% in standard medium (no added TdR), and this was decreased by DPM to 9% and 0%, respectively. Reactivation of trigeminal ganglia in standard medium was 100%, and this decreased to 59% and 23%, respectively. The possibility of a direct toxic effect of DPM on ganglion neurons to explain the results was unlikely. DPM had a modest antiviral effect on HSV replication in cell culture, and its efficacy in blocking reactivation may be related to this activity, probably by inhibition of nucleoside transport.

Radioprotection of mouse hemopoiesis by dipyridamole and adenosine monophosphate in fractionated treatment.

The purpose of the studies reported here was to investigate the ability of the combined administration of dipyridamole and adenosine monophosphate, drugs known to elevate extracellular adenosine, to protect mice undergoing treatment with fractionated irradiation (five doses of 2 or 3 Gy each) given at 24-h intervals. Based on observations of hemopoietic recovery (endogenous hemopoietic spleen colony formation, marrow granulocyte-macrophage colony-forming cells, peripheral blood cells) after the completion of fractionated irradiation and on survival studies, it was demonstrated that the repeated administration of the drugs 60 min before each of the radiation fractions mitigates the hemopoietic injury and enhances the survival of mice irradiated with an additional "top-up" dose. It could be deduced that the single protective actions of the drugs retain their efficacy in repeated treatment and enhance the sparing effect of dose fractionation on hemopoiesis. Interestingly, the toxic side effects of the drugs tend to decrease when they are administered repeatedly, probably due to the development of tolerance to their cardiovascular action. This reduction in toxicity offers benefit with respect to the potential use of these hemopoiesis-protecting drugs in clinical radiotherapy.

Potentiation of cytotoxicity of mitoxantrone toward CHO-K1 cells in vitro by dipyridamole.

Dipyridamole (DP), a clinically used vasodilator and an antiplatelet compound, augmented the activity of the anticancer drug mitoxantrone (MXN) toward chinese hamster ovary (CHO-K1) cells in culture. Clonogenic assays indicated that DP (1.0, 2.5, and 5.0 microM) decreased the survival of cells treated with MXN (5 to 25 nM) in a dose-dependent manner. Further, DP (1 and 5 microM) decreased the MXN concentration required for 50% inhibition of cell growth from 3.2 to 1.8 and from 3.0 to 0.5 nM, respectively, over a period of 3 days. DP (10 microM) increased the accumulation of MXN by 1.8-fold in exponentially growing cells exposed to MXN. The enhanced levels of MXN in CHO-K1 cells in the presence of the chemosensitizer may account for the potentiation of MXN-cytotoxicity by DP.

5-Fluorouracil's cytotoxicity is enhanced both in vitro and in vivo by concomitant treatment with hyperthermia and dipyridamole.

We obtained evidence that the cytotoxic effect of 5-fluorouracil (5-FU) is augmented when the drug is given in combination with hyperthermia (HYP) and dipyridamole (DP). Nontoxic levels of DP enhanced the combined cytotoxicity of 5-FU and HYP against B16 melanoma and human tumor cells in vitro as measured by the succinate dehydrogenase inhibition (SDI) test. Growth of B16 melanoma that had been subcutaneously implanted into the feet of C57 BL mice was inhibited by treatment with the combinations of 5-FU and HYP, of 5-FU and DP, and of 5-FU, HYP and DP as compared with the administration of 5-FU alone. Treatment with HYP plus DP did not alter the body weight of mice that received 5-FU. The administration of DP plus HYP seemed to render the tumor cells more sensitive to 5-FU. The combination of 5-FU, HYP and DP shows promise for the treatment of patients suffering from malignant disease.

Effect of MK-801 and its interaction with adenosinergic agents and carbamazepine against hypoxic stress-induced convulsions and death in mice.

Effects of MK-801, adenosinergic agents and carbamazepine were investigated against hypoxic stress-induced convulsions and death in mice. MK-801, a N-methyl D-aspartate antagonist, offered a significant protective effect against hypoxic convulsions. Adenosinergic agents such as adenosine, 2-chloroadenosine, N6-cyclohexyladenosine and dipyridamole produced theophylline-sensitive protective effect as they dose-dependently prolonged the latencies for onset of convulsions and death. Pretreatment with adenosinergic agents and MK-801 markedly enhanced the protective effect of the stable analogs of adenosine namely, 2-chloroadenosine and N6-cyclohexyladenosine. However, pretreatment with combination of MK-801 and adenosine or dipyridamole did not show enhanced protective effect. Carbamazepine also exhibited dose-dependent theophylline-sensitive protective effect. The pretreatment with combination of carbamazepine and MK-801 showed significantly enhanced protective effect. The observed enhanced protective effect of MK-801 and adenosinergic agents was reversed by pretreatment with theophylline. These results indicate the possible involvement of adenosinergic mechanisms in preventing hypoxic stress-induced convulsions and the role of NMDA receptors in hypoxia-induced convulsions and death as MK-801 not only showed effect per se but also potentiated the response due to adenosinergic agents.

Potent convulsant actions of the adenosine receptor antagonist, xanthine amine congener (XAC).

The convulsant properties of xanthine amine congener (XAC, 8-(4-(2-aminoethyl)-aminocarboxylmethyloxy)phenyl-1,3-dipropylxant hine) are compared to those of caffeine. Male Swiss albino mice were infused with convulsants through a lateral tail vein. Convulsion thresholds (i.e. the amount of convulsants required to elicit convulsions) of 39.8 +/- 2.0 mg/kg (n = 10) and 109.8 +/- 2.3 mg/kg (n = 10) were calculated for XAC and caffeine respectively. Pretreatment of animals with the adenosine receptor agonists 2-chloroadenosine, N6-cyclohexyladenosine or 5'-N-ethylcarboxamido-adenosine (1 mg/kg, i.p., 20 minutes prior to infusion) significantly decreased the seizure threshold of both XAC and caffeine. The adenosine uptake blockers, 6-nitrobenzylthioinosine or dipyridamole (0.25 mg/kg, i.p., 20 minutes prior to infusion) did not significantly affect the seizure threshold to either XAC or caffeine. The benzodiazepine agonist diazepam (5 mg/kg, i.p., 20 minutes prior to infusion) significantly increased the seizure threshold to both XAC (p less than 0.05) and caffeine (p less than 0.01), whereas the benzodiazepine antagonist Ro 15-1788 (10 mg/kg, i.p., 20 minutes prior to infusion) significantly increased the seizure threshold to caffeine (p less than 0.01), but not XAC. The results suggest that actions at benzodiazepine receptors may be a tenable hypothesis to explain the convulsant actions of caffeine, but not those of XAC.

Potentiation of antimetabolite antitumor activity in vivo by dipyridamole and amphotericin B.

Previous studies have shown that dipyridamole (DP), a potent nucleoside transport inhibitor blocking the rescue effect of exogenous nucleosides, markedly potentiates the cytotoxicity of antimetabolites. However, no enhancement of the chemotherapeutic effect of antimetabolites by DP in vivo has yet been reported. This study provided evidence that the combination of DP and amphotericin B (AmB) significantly potentiated the inhibitory effect of 5-fluorouracil (FU) or methotrexate (MTX) against a panel of transplantable tumors including sarcoma 180, cervical carcinoma U14, and Lewis lung carcinoma in mice. No significant increase in toxicity was induced by this combination in treated mice. Our results indicate that the combination of DP and AmB with antimetabolites is potentially useful in cancer chemotherapy.

Cardiac imaging and myocardial kinetics of technetium-tertiary butyl-isonitrile during dipyridamole-induced hyperemia.

UNLABELLED: To determine the myocardial kinetics of technetium-tertiary-butyl-isonitrile (Tc-TBI) during dipyridamole-induced hyperemia, the circumflex coronary arteries (LCX) of 15 dogs were partially occluded. Dipyridamole was then infused intravenously over 4 minutes, creating hyperemic flows in the anterior descending (LAD) coronary system. Tc-TBI was administered, then LAD and LCX regional myocardial Tc-TBI activities were continuously monitored with miniature detectors and gamma camera imaging over 3 hours. Microsphere-determined regional myocardial blood flows demonstrated an LCX/LAD flow ratio of 0.81 +/- 0.21 at rest and 0.45 +/- 0.24 (SD) during dipyridamole infusion. Three-hour fractional Tc-TBI clearance rates were minimal and were equal in the LAD (0.14 +/- 0.11) and LCX (0.13 +/- 0.12) zones (p = ns). Excellent gamma camera images, demonstrating the LCX defect, were obtained in all dogs. The correlation coefficient was 0.98 for regional myocardial blood flow vs initial Tc-TBI distribution. IN CONCLUSION: (1) Dipyridamole vasodilation unmasked coronary stenoses despite no flow disparities at rest. (2) The initial distribution of Tc-TBI is proportional to regional myocardial blood flow. (3) There is minimal washout and no redistribution into the initial defect over time, and thus image quality is stable over time. (4) Tc-TBI myocardial kinetics may be applicable to closely related agents currently being developed.

Enhancement of the sensitivity of human colon cancer cells to growth inhibition by acivicin achieved through inhibition of nucleic acid precursor salvage by dipyridamole.

This study was undertaken to determine if salvage of nucleic acid precursors might constitute a mechanism of resistance to acivicin in human colon cancer cells and, if so, to establish whether dipyridamole, an inhibitor of nucleoside and nucleobase transport, can block the salvage process and restore sensitivity to acivicin. Acivicin inhibited the replication of human colon cancer cells (VACO 5) in vitro in a dose- and time-dependent fashion. In addition, marked cell lysis was evident after a 24-hr exposure to acivicin at concentrations greater than 1 microgram/ml. The primary metabolic effect of acivicin was depletion of the cytidine triphosphate and guanosine triphosphate pools. Adenosine triphosphate levels were also reduced, but apparently as a consequence of the guanosine triphosphate depletion. VACO 5 cells exposed to acivicin (3 micrograms/ml) efficiently salvaged low levels (1 micron) of cytidine, guanosine, and guanine and could, therefore, restore the depleted nucleotide pools. The combination of cytidine and guanosine, but not either nucleoside alone, provided significant protection against the growth-inhibitory properties of acivicin. Dipyridamole, at a noncytotoxic concentration (5 microM), blocked repletion of the cytidine triphosphate and guanosine triphosphate pools in cells exposed to acivicin and the nucleic acid precursors. As a result, the growth-inhibitory effects of acivicin were maintained. The salvage of cytidine was particularly sensitive to inhibition by dipyridamole, and no restoration of cytidine triphosphate pools was evident. The cellular uptake of a variety of nucleic acid precursors was differentially sensitive to inhibition by dipyridamole. The 50% inhibitory dose values ranged from 0.01 to 2.5 microM for cytidine and uridine, respectively. The results of this study indicate that, although the replication of VACO 5 cells was inhibited by acivicin, low levels of nucleosides and nucleobases can circumvent the cytotoxicity. Dipyridamole effectively blocked the salvage pathways and restored the sensitivity of the cancer cells to the antiproliferative actions of acivicin.

Potentiation of methotrexate toxicity by dipyridamole.

Dipyridamole, an inhibitor of facilitated transport systems for purines and pyrimidines, was shown to enhance the toxicity of methotrexate (MTX) against cells in culture and in mice. Under certain incubation conditions, the availability of performed purines and pyrimidines in undialyzed serum appeared to render Chinese hamster ovary cells insensitive to MTX. Addition to the culture of nontoxic levels of dipyridamole conferred sensitivity to MTX. Inhibition of [3H]thymidine uptake by dipyridamole paralleled the enhanced MTX toxicity in a comparison of the dose-effect relationships. Inhibition of [3H]hypoxanthine uptake also occurred, although approximately 10-fold higher levels of dipyridamole were required. In vivo dipyridamole enhanced MTX toxicity in mice; however, the antitumor activity of MTX toward Ridgway osteogenic sarcoma and L1210 leukemia was not dramatically improved.

Deleterious effects of vasodilating agents. Induction of subendocardial ischemia in dogs with proximal stenosis of coronary artery.

This study was designed to determine how distal vasodilatory stimuli can alter coronary hemodynamics and the appearance of myocardial ischemia in the presence of severe coronary stenosis. In anesthetized open chest dogs, left circumflex coronary artery (LCx) flow, distal LCx perfusion pressure (DCPP), heart rate, and aortic pressure were monitored and regional myocardial ischemia was estimated from ST segment deviation in the epicardial and intramyocardial electrograms obtained from the electrodes located in the area supplied by LCx. In the presence of severe LCx constriction, intracoronary administration of adenosine (0.01 mg/Kg/min) and dipyridamole (0.05 mg/Kg) failed to augment LCx flow and induced further reduction in DCPP without changing systemic hemodynamics. In parallel with these hemodynamic changes, significant ST elevation was observed in the inner layer of the area supplied by the LCx. These results show that subepicardial vasodilation following vasodilatory stimuli can induce transmural flow redistribution and lead to subendocardial ischemia in the presence of severe coronary stenosis.