Pharmacokinetics of dipyridamole

Nucleotide metabolism: a pan-cancer metabolic dependency

Metabolic alterations are a key hallmark of cancer cells, and the augmented synthesis and use of nucleotide triphosphates is a critical and universal metabolic dependency of cancer cells across different cancer types and genetic backgrounds. Many of the aggressive behaviours of cancer cells, including uncontrolled proliferation, chemotherapy resistance, immune evasion and metastasis, rely heavily on augmented nucleotide metabolism. Furthermore, most of the known oncogenic drivers upregulate nucleotide biosynthetic capacity, suggesting that this phenotype is a prerequisite for cancer initiation and progression. Despite the wealth of data demonstrating the efficacy of nucleotide synthesis inhibitors in preclinical cancer models and the well-established clinical use of these drugs in certain cancer settings, the full potential of these agents remains unrealized. In this Review, we discuss recent studies that have generated mechanistic insights into the diverse biological roles of hyperactive cancer cell nucleotide metabolism. We explore opportunities for combination therapies that are highlighted by these recent advances and detail key questions that remain to be answered, with the goal of informing urgently warranted future studies.

An Integrative in silico Drug Repurposing Approach for Identification of Potential Inhibitors of SARS-CoV-2 Main Protease

Considering the urgent need for novel therapeutics in ongoing COVID-19 pandemic, drug repurposing approach might offer rapid solutions comparing to de novo drug design. In this study, we designed an integrative in silico drug repurposing approach for rapid selection of potential candidates against SARS-CoV-2 Main Protease (Mpro ). To screen FDA-approved drugs, we implemented structure-based molecular modelling techniques, physiologically-based pharmacokinetic (PBPK) modelling of drugs disposition and data mining analysis of drug-gene-COVID-19 association. Through presented approach, we selected the most promising FDA approved drugs for further COVID-19 drug development campaigns and analysed them in context of available experimental data. To the best of our knowledge, this is unique in silico study which integrates structure-based molecular modeling of Mpro inhibitors with predictions of their tissue disposition, drug-gene-COVID-19 associations and prediction of pleiotropic effects of selected candidates.

Brief Report: Dipyridamole Decreases Gut Mucosal Regulatory T-Cell Frequencies Among People With HIV on Antiretroviral Therapy

BACKGROUND

We had previously conducted a double-blind, randomized placebo-controlled, partial cross-over trial showing that 12 weeks of dipyridamole decreased CD8 T-cell activation among treated HIV(+) individuals by increasing extracellular adenosine levels.

METHODS

In this substudy, rectosigmoid biopsies were obtained from 18 participants (9 per arm), to determine whether 12 weeks of dipyridamole affects mucosal immune cells. Participants randomized to placebo were then switched to dipyridamole for 12 weeks while the treatment arm continued dipyridamole for another 12 weeks. We evaluated T-cell frequencies and plasma markers of microbial translocation and intestinal epithelial integrity. Linear regression models on log-transformed outcomes were used for the primary 12-week analysis.

RESULTS

Participants receiving dipyridamole had a median 70.2% decrease from baseline in regulatory T cells (P = 0.007) and an 11.3% increase in CD8 T cells (P = 0.05). There was a nonsignificant 10.80% decrease in plasma intestinal fatty acid binding protein levels in the dipyridamole arm compared with a 9.51% increase in the placebo arm. There were no significant differences in plasma levels of β-D-glucan. In pooled analyses, there continued to be a significant decrease in regulatory T cells (-44%; P = 0.004). There was also a trend for decreased CD4 and CD8 T-cell activation.

CONCLUSION

Increasing extracellular adenosine levels using dipyridamole in virally suppressed HIV (+) individuals on antiretroviral therapy can affect regulation of gut mucosal immunity.

On the Usefulness of Two Small-Scale In Vitro Setups in the Evaluation of Luminal Precipitation of Lipophilic Weak Bases in Early Formulation Development

A small-scale biphasic dissolution setup and a small-scale dissolution-permeation (D-P) setup were evaluated for their usefulness in simulating the luminal precipitation of three lipophilic weak bases—dipyridamole, ketoconazole and itraconazole. The transition from the gastric to intestinal environment was incorporated into both experimental procedures. Emulsification during the biphasic dissolution experiments had a minimal impact on the data, when appropriate risk mitigation steps were incorporated. Precipitation parameters estimated from the in vitro data were inputted into the Simcyp^® physiologically based pharmacokinetic (PBPK) modelling software and simulated human plasma profiles were compared with previously published pharmacokinetic data. Average C_max and AUC values estimated using experimentally derived precipitation parameters from the biphasic experiments deviated from corresponding published actual values less than values estimated using the default simulator parameters for precipitation. The slow rate of transport through the biomimetic membrane in the D-P setup limited its usefulness in forecasting the rates of in vivo precipitation used in the modelling of average plasma profiles.

Biopharmaceutic IVIVE-Mechanistic Modeling of Single- and Two-Phase In Vitro Experiments to Obtain Drug-Specific Parameters for Incorporation Into PBPK Models

The physiological relevance of single-phase (aqueous only) and 2-phase (aqueous and organic phase) in vitro dissolution experiments was compared by mechanistic modeling. For orally dosed dipyridamole, stepwise, sequential estimation/confirmation of biopharmaceutical parameters from in vitro solubility-dissolution data was followed, before applying them within a physiologically based pharmacokinetic (PBPK) model. The PBPK model predicted clinical dipyridamole luminal and plasma concentration profiles reasonably well for a range of doses only where the precipitation rate constant was derived from the 2-phase experiment. The population model predicted a distribution of maximal precipitated fractions from 0% to 45% of the 90 mg dose (mean 7.6%). Such population information cannot be obtained directly from a few in vitro experiments; however well they may represent an "average" and several extreme subjects (those with low-high luminal fluid volumes, pH, etc.) because there is no indication of outcome likelihood. For this purpose, direct input of in vitro dissolution/precipitation profiles to a PBPK model is insufficient-mechanistic modeling is required. Biopharmaceutical in vitro-in vivo extrapolation tools can also simulate the effect of key experimental parameters (dissolution volumes, pH, paddle speed, etc.) on dissolution/precipitation behavior, thereby helping to identify critical variables, which may impact the number or design of in vitro experiments.

A semi-quantitative translational pharmacology analysis to understand the relationship between in vitro ENT1 inhibition and the clinical incidence of dyspnoea and bronchospasm

Adenosine contributes to the pathophysiology of respiratory disease, and adenosine challenge leads to bronchospasm and dyspnoea in patients. The equilibrative nucleoside transporter 1 (ENT1) terminates the action of adenosine by removal from the extracellular environment. Therefore, it is proposed that inhibition of ENT1 in respiratory disease patients leads to increased adenosine concentrations, triggering bronchospasm and dyspnoea. This study aims to assess the translation of in vitro ENT1 inhibition to the clinical incidence of bronchospasm and dyspnoea in respiratory disease, cardiovascular disease and healthy volunteer populations. Four marketed drugs with ENT1 activity were assessed; dipyridamole, ticagrelor, draflazine, cilostazol. For each patient population, the relationship between in vitro ENT1 [³H]-NBTI binding affinity (K_i) and [³H]-adenosine uptake (IC₅₀) to the incidence of: (1) bronchospasm/severe dyspnoea; (2) tolerated dyspnoea and; (3) no adverse effects, was evaluated. A high degree of ENT1 inhibition (≥13.3x K_i, ≥4x IC₅₀) associated with increased incidence of bronchospasm/severe dyspnoea for patients with respiratory disease only, whereas a lower degree of ENT1 inhibition (≥0.1x K_i, ≥0.05x IC₅₀) associated with a tolerable level of dyspnoea in both respiratory and cardiovascular disease patients. ENT1 inhibition had no effect in healthy volunteers. Furthermore, physicochemical properties correlative with ENT1 binding were assessed using a set of 1625 diverse molecules. Binding to ENT1 was relatively promiscuous (22% compounds K_i<1μM) especially for neutral or basic molecules, and greater incidence tracked with higher lipophilicity (clogP >5). This study rationalises inclusion of an assessment of ENT1 activity during early safety profiling for programs targeting respiratory disorders.

Experience of low-dose aminophylline use to relieve minor adverse effects of dipyridamole in patients undergoing stress myocardial perfusion imaging

BACKGROUND

Intravenous administration of aminophylline is widely adopted to reverse dipyridamole-related adverse effects (AEs) during stress myocardial perfusion imaging (MPI). The study aimed to investigate the efficacy of lower-dose aminophylline to relieve minor AEs.

METHODS

2,250 consecutive patients undergoing dipyridamole-stressed MPI were enrolled. Information concerning AE occurrence and dosages of aminophylline was collected to evaluate the efficacy of lower-dose aminophylline. A logistic regression was used to determine independent predictors of dipyridamole-related AE occurrence.

RESULTS

No severe AE was noted. Overall mild AE incidence was 37.0% (833/2,250 patients). Initial low-dose (25 mg) aminophylline relieved symptoms in 98.8% of patients with mild AEs (823/833 patients). An extra 25 mg aminophylline sufficed to reverse all such AEs. Mean body mass index (BMI) differed significantly between patients with and without any AE [25.6 vs 25.1 (P = .009)]. There was no significant difference between two subgroups in mean age, male gender prevalence, body height and weight, dipyridamole dose/BMI, or prevalence of significant perfusion defect(s) on MPI. Multivariable logistic regression demonstrated BMI remained the independent predictor of dipyridamole-related AE occurrence (odds ratio 1.028, 95% confidence interval 1.007-1.049, P = .01).

CONCLUSION

Low-dose (≦50 mg, and usually 25 mg) aminophylline seems sufficient to relieve mild dipyridamole-related AEs during stress MPI.

Anti-platelet agents in pediatric cardiac practice

Pediatric patients with a variety of congenital and acquired cardiac conditions receive antithrombotic therapy. Many of the indications are empirical, and have either not been proven in controlled studies or are extrapolated from adult studies. This article reviews the current available literature regarding the use of anti-platelet drugs in the pediatric cardiac population.

Biotransformation and in vitro assessment of metabolism-associated drug-drug interaction for CRx-102, a novel combination drug candidate

CRx-102 is an oral synergistic combination drug which contains the cardiovascular agent, dipyridamole (DP) and a very low dose of the glucocorticoid, prednisolone (PRED). CRx-102 works through a novel mechanism of action in which DP selectively amplifies the anti-inflammatory activity of PRED without replicating its side effects. CRx-102 is in clinical trials for the treatment of osteoarthritis. Here we delineate the in vitro metabolism and explore the potential for a drug-drug interaction between the active agents in CRx-102. Our study using human hepatocyte suspensions showed that both DP and PRED were metabolized by CYP3A4 isozymes, resulting in the formation of diverse arrays of both oxidative and oxidative-reduced metabolites. Within phase 1 biotransformation, CYP3A4 was one of the pathways responsible for the metabolism of PRED, while phase 2 biotransformation played a significant role in the metabolism of DP. Glucuronidation of DP was substantial and was catalyzed by many UGT members, specifically those in the UGT1A subfamily. Based on the tandem mass (MS/MS) product ion spectra (PIS) acquired, the major metabolites of both agents, namely, monooxygenated, mono-N-deethanolaminated, dehydrogenated and O-glucuronidated metabolites of DP and the monooxygenated (e.g., 6-hydroxyl), dehydrogenated (prednisone) and reduced (20-hydroxyl) metabolites of PRED, were identified and elucidated. The affinities for DP biotransformation, including CYP3A4-mediated oxidative pathways and UGT-mediated O-glucuronidation, appeared high (K(m)<10 microM), as compared with the modest affinities of PRED biotransformation catalyzed by CYP3A4 (K(m) approximately 40-170 microM). DP, but not PRED, exerted a minimal inhibitory effect on the drug-metabolizing CYP isoforms, including CYP3A4, which was determined using a panel of CYP isoform-preferred substrate activities in pooled human liver microsomal (HLM) preparations and microsomal preparations containing the recombinant enzymes (K(i) approximately 2-12 microM). Using the DP maximal plasma concentration (C(max)) observed in the clinic and a predictive mathematical model for metabolism-associated drug-drug interaction (DDI), we have demonstrated that there is little likelihood of a pharmacokinetic interaction between the two active agents in CRx-102.

Clinical pharmacokinetics of antiplatelet agents used in the secondary prevention of stroke

Stroke is one of the leading causes of death and debilitation. Several million stroke survivors are alive throughout the world today. Prevention of recurrent stroke is of major importance to stroke survivors. Several pharmacological agents are currently available for use in secondary stroke prevention.Clopidogrel, the combination of immediate-release aspirin and extended-release dipyridamole and aspirin alone are the most widely recommended agents for use in the secondary prevention of strokes. Clopidogrel has shown superiority over aspirin in the combined endpoints of stroke, death and myocardial infarction. The immediate-release aspirin/extended-release dipyridamole combination has shown superiority to aspirin alone in the secondary prevention of stroke. Dipyridamole has been studied as an antiplatelet agent for several decades. Early trials to prove its efficacy compared with aspirin were not favourable, and patients often experienced many adverse effects. Researchers began developing an extended-release formulation in an effort to maintain therapeutic blood concentrations with less frequent daily administration and better adverse effect profile. Pharmacokinetic analysis of this new product showed it to have a more consistent and reproducible absorption compared with immediate-release dipyridamole. The rate of absorption of extended-release dipyridamole is considerably slower than that of immediate-release dipyridamole, while similar plasma concentrations are maintained to optimise antiplatelet efficacy. This allows extended-release dipyridamole to be administered twice daily rather than four times daily.A large-scale randomised trial was conducted with extended-release dipyridamole 200mg in combination with immediate-release aspirin 25mg given twice daily. The combination product showed a greater efficacy at preventing a recurring stroke then either agent administered alone. Indirect comparisons with clopidogrel show that the combination of immediate-release aspirin/extended-release dipyridamole may be more effective than clopidogrel at preventing a recurring stroke.

MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs

The multidrug transporter, MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs was examined in vitro by using the MDR1-overexpressing LLC-GA5-COL150 cells, which were established by transfection with human MDR1 cDNA into porcine kidney epithelial LLC-PK(1) cells. Amiodarone, its active metabolite monodesethyl-amiodarone (DEA), and quinidine markedly inhibited the basal-to-apical transport (renal secretion) of [(3)H]digoxin and increased the apical-to-basal transport (reabsorption), but cibenzoline and lidocaine showed slight inhibition of the transport, and disopyramide and mexiletin had no such effects. The IC(50) values for amiodarone, DEA and quinidine on [(3)H]digoxin transport in LLC-GA5-COL150 cells were 5.48 microM, 1.27 microM and 9.52 microM, respectively. These were comparable to, or only several times the achievable concentration in clinical use, suggesting that MDR1 could be responsible for the drug interaction between digoxin and amiodarone found in clinical reports and that DEA contributes the elevation of digoxin serum concentration. Similarly, dipyridamole altered the transport, but isosorbide showed only slight modification of the transport. The IC(50) value for dipyridamole was 40.0 microM, also only several times the achievable concentration in clinical use, indicating a risk of interaction.

Aggrenox: a fixed-dose combination of aspirin and dipyridamole

OBJECTIVE

To describe the pharmacology, pharmacokinetics, efficacy, and safety of a fixed-dose combination of aspirin and extended-release (ER) dipyridamole indicated for the secondary prevention of stroke.

DATA SOURCES

Published articles and abstracts were identified from a MEDLINE search (1966-December 1999) using the search terms dipyridamole, aspirin, antiplatelet, antiaggregation, and stroke prevention. Pertinent articles written in English were considered for review. Additional articles were identified from the references of retrieved literature.

STUDY SELECTION AND DATA EXTRACTION

Studies including a combination of aspirin/dipyridamole in human subjects were evaluated. Emphasis was placed on randomized, controlled trials.

DATA SYNTHESIS

Aspirin is a platelet inhibitor that works by inhibiting platelet cyclooxygenase, which reduces the production of thromboxane A2. Dipyridamole is a platelet inhibitor that is thought to work in part by inhibiting platelet cyclic-3',5'-adenosine monophosphate and cyclic-3',5'-guanosine monophosphate phosphodiesterase. The active metabolite of aspirin, salicylic acid, is highly bound to plasma protein and has a plasma half-life of two to three hours. Dipyridamole is also highly bound to plasma proteins, and the ER formulation has a plasma half-life of 13 hours. The first European Stroke Prevention Study (ESPS-1) found the combination of aspirin/dipyridamole to be superior to placebo in the prevention of stroke and transient ischemic attack (TIA). The ESPS-1, however, did not include an aspirin-only treatment arm. Therefore, it was unclear whether the combination of aspirin/dipyridamole was superior to aspirin alone. As a result, a second trial was conducted that included treatment arms of aspirin alone, ER dipyridamole alone, combination therapy, and placebo. The combination of aspirin 25 mg plus ER dipyridamole 200 mg twice daily was shown in the ESPS-2 to be significantly better than either agent given individually in preventing stroke and TIAs (p < 0.001).

CONCLUSIONS

The American College of Chest Physicians (ACCP) recommends aspirin 50-325 mg/d to be the initial antiplatelet of choice for the prevention of atherothrombotic cerebral ischemic events. However, with the favorable results of the ESPS-2, it may be appropriate to substitute aspirin/ER dipyridamole for aspirin alone as the drug of choice. This combination appears to have a favorable adverse effect profile. The relative effectiveness of aspirin/ER dipyridamole compared with clopidogrel and ticlopidine has yet to be determined. If alternative antiplatelet therapy is needed, the ACCP recommends clopidogrel rather than ticlopidine because of its lower incidence of adverse effects. The ACCP further states that the combination of aspirin plus dipyridamole may be more effective than clopidogrel; these agents have a similarly favorable adverse effect profile.

Pulmonary and systemic effects of the phosphodiesterase inhibitor dipyridamole in newborn lambs with persistent pulmonary hypertension

Nitric oxide (NO) relaxes vascular smooth muscle by increasing the intracellular concentration of cGMP. In the pulmonary circulation, cGMP is inactivated by specific phosphodiesterases (PDE5). Dipyridamole, a clinically approved drug, has inhibitory activity against PDE5 and has been reported to augment the response to inhaled NO in persistent pulmonary hypertension of the newborn (PPHN). We wished to determine whether dipyridamole alone, or in combination with NO, can be used to treat a newborn lamb model of PPHN. In newborn lambs with PPHN, dipyridamole infused at 0.02 mg/kg/min for 45 min alone, or in combination with 5 ppm of inhaled NO for the final 15 min, significantly decreased pulmonary and systemic blood pressure, decreased pulmonary vascular resistance, and increased pulmonary blood flow. There was no significant difference between the pulmonary vascular effects of 5 ppm NO alone compared with the effects of NO combined with dipyridamole. In control lambs, the 45-min infusion of dipyridamole did not change pulmonary pressure whereas systemic pressure decreased by 28 +/- 3%. These systemic effects in control lambs persisted 90 min after discontinuing the dipyridamole infusion. Systemic arteries isolated from both control and PPHN lambs were significantly more sensitive to dipyridamole than pulmonary arteries. We conclude that dipyridamole has significant hemodynamic effects in both the pulmonary and systemic circulations of newborn lambs with pulmonary hypertension as well as in the systemic circulation of newborn control lambs. The pronounced effects of dipyridamole on the systemic circulation limits its utility as an adjunct to inhaled NO in the treatment of PPHN.

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.

Combined dipyridamole and aspirin pellet formulation for improved oral drug delivery. Part 1: Development pharmaceutics

The dissolution profile of various weight fractions of dipyridamole: hydropropylmethylcellulose acetate succinate (HPMC-AS) and dipyridamole: hydroxypropylmethylcellulose phthalate co-precipitates lead to the choice of 1:2 dipyridamole: HPMC-AS as the controlled-release component. It was deposited to form two-third of the total dose as an inner layer on inert sucrose cores by air suspension coating for release mainly in the small intestine. Further examination of this material by IR spectroscopy, differential scanning calorimetry and X-ray diffraction indicated some free drug, preferentially soluble under gastric pH conditions. One-third of the total dose was applied by pan coating as an outer layer of micronized dipyridamole around the inner enteric co-precipitate layer. Aspirin-loaded cores were prepared also by pan coating for use in the final product, which contained both anti-platelet drugs.

Effect of dipyridamole on zidovudine pharmacokinetics and short-term tolerance in asymptomatic human immunodeficiency virus-infected subjects

Zidovudine delays the progression of infection and prolongs the survival of human immunodeficiency virus (HIV)-infected patients, but these benefits are limited by dose-related toxicity and the cost of the drug. Dipyridamole, in micromolar concentrations, acts synergistically with zidovudine, reducing the anti-HIV 95% inhibitory concentration of zidovudine 5- to 10-fold in vitro. We sought to establish a well-tolerated dose of dipyridamole for use in combination with zidovudine and to detect clinically significant pharmacokinetic interactions. Both objectives are essential for planning studies of the efficacy of the zidovudine-dipyridamole combination. Eleven asymptomatic HIV-infected subjects (median CD4+ cell count, 311 cells per mm3), 10 of whom had been on zidovudine at 500 mg/day for at least 6 months, were admitted to the study. Zidovudine pharmacokinetics were measured on day 1. Dipyridamole was then begun at 600 mg/day (subjects 1 to 3) or 450 mg/day (subjects 4 to 11), and zidovudine and dipyridamole pharmacokinetics were measured on day 5. All subjects given 600 mg of dipyridamole per day developed headache or nausea, or both. Six of eight subjects given dipyridamole at 450 mg/day developed headache or mild nausea that resolved after a median of 2 days. The area under the zidovudine concentration-time curve was not significantly different on day 1 in comparison with that on day 5 (P = 0.11). Symptoms were significantly correlated with the maximum zidovudine concentrations, which were achieved when dipyridamole was dosed concomitantly (p = 0.03). Total (free and protein-bound) dipyridamole trough concentrations were near those demonstrating synergy with zidovudine against HIV in vitro. Dipyridamole was highly protein bound, with a median free/total dipyridamole ratio of 0.7%; the percent free/total dipyridamole ratio was inversely correlated with alpha 1 acid glycoprotein concentrations (r2 =0.66). Results of the study indicate that adjustment of the zidovudine dose was not required to achieve equivalent zidovudine concentrations when zidovudine was administered in combination with dipyridamole at the doses studied. In the short study described here, the zidovudine-dipyridamole combinations was well tolerated in asymptomatic HIV-infected subjects after the occurrence of mild transient symptoms.

Comparison of accuracy for detecting coronary artery disease and side-effect profile of dipyridamole thallium-201 myocardial perfusion imaging in women versus men

Intravenous dipyridamole planar thallium-201 imaging is a safe and effective test for detection and prognosis of coronary artery disease (CAD) in the general population. The relative diagnostic accuracy and side-effect profile of dipyridamole thallium-201 stress imaging in women is not defined. Forty-three consecutive female and 71 male patients who underwent dipyridamole thallium-201 imaging (0.56 mg/kg) within 3 months of cardiac catheterization were studied. Scans were considered abnormal if fixed or reversible perfusion defects were detected. Stenosis severity of greater than or equal to 50% luminal diameter reduction of any artery defined CAD. Overall sensitivity for detection of CAD was 0.87 in women and 0.94 in men; specificity was 0.58 in women and 0.63 in men (p = not significant). Sensitivity for detection of 1-vessel CAD was 0.60 in women and 0.94 in men (p = 0.001). The sensitivity for detection of multivessel CAD (with or without surgical revascularization) was 1.0 and 0.94 in women and men, respectively. Adverse effects were reported in 62% of women and in 38% of men (p = 0.01). There was no significant difference in the incidences of chest pain, headache, nausea, flushing or electrocardiographic changes. The incidences of severe ischemia and dizziness were higher in women. Possible explanations for this difference in adverse effects include gender differences in the volume of distribution of dipyridamole due to varied fat-to-muscle ratios and different subjective nocioceptive sensitivities to the effects of dipyridamole. Overall sensitivity and specificity are comparable between the sexes.

A comparison of tablets with oral suspension formulation of dipyridamole in thallium myocardial imaging

Dipyridamole stress thallium imaging has been widely employed to diagnose and assess the extent of coronary heart disease in patients who cannot exercise. When oral dipyridamole administration was used, a wide range of results for sensitivity, specificity, hemodynamic response and side effect profile has been reported. The authors hypothesized that the formulation used for oral administration of dipyridamole plays a major factor in this variability, and that the pulverized form of dipyridamole will achieve faster and more consistent response than the standard tablet form. The authors studied 13 consecutive patients who underwent thallium scintigraphy. Eight patients received dipyridamole pulverized and dissolved in a glycol/aqueous base diluent (group A), and five patients received the standard form of dipyridamole (group B). In group A, mean peak systolic blood pressure decreased from 142 +/- 31 (mean +/- standard deviation) to 109 +/- 30 (P = .05), and mean diastolic blood pressure decreased from 76 +/- 14 to 51 +/- 5. The mean heart rate changed from 78 +/- 26 to 80 +/- 10. In group B, baseline systolic blood pressure was 165 +/- 12 and decreased to 156 +/- 7 at 45 minutes and to 155 +/- 14 at 90 minutes. Heart rate increased from baseline of 69 +/- 9 to 75 +/- 8 at 45 minutes and to 76 +/- 11 at 90 minutes. At 45 minutes, the systolic blood pressure of the 8 group A patients dropped by 33 +/- 19 mm Hg, whereas group B's changed by 9 +/- 6 mm Hg (P less than .005).(ABSTRACT TRUNCATED AT 250 WORDS)

Intravenous dipyridamole: use in thallous chloride TL 201 stress imaging

An intravenous form of dipyridamole has been approved for cardiac stress testing with thallous chloride Tl 201 (201Tl) imaging. The procedure provides an alternative method for stress testing of patients who cannot perform strenuous physical exercise because of physical limitations or those in the immediate postinfarction period. Dipyridamole simulates the physiologic effects of physical exercise by increasing adenosine concentrations, which in turn increases coronary blood flow. Dipyridamole maximizes the differences in uptake of radioactive thallium in myocardial regions supplied by normal coronary arteries and those distal to stenosed arteries where blood flow is reduced. The sensitivity and specificity of dipyridamole-201Tl stress imaging for the detection of coronary disease are comparable to those of traditional exercise-201Tl stress testing. The intravenous dipyridamole usually is well tolerated, and the majority of adverse effects can be reversed with the administration of aminophylline.

Pharmacokinetics of dipyridamole-beta-cyclodextrin complex in healthy volunteers after single and multiple doses

Dipyridamole is a well known anti-aggregating agent characterized by poor water solubility as well as scant and variable bioavailability. Recently, the compound was complexed with beta-cyclodextrin forming a molecular encapsulation resulting in better oral absorption and stronger biological activities in animals. In the present study, a randomized double blind cross-over comparison between dipyridamole-beta-cyclodextrin complex (dip-beta-CD) and dipyridamole was performed in 12 healthy subjects after single (75mg) and multiple oral treatments (75mg TID). Dip-beta-CD showed better bioavailability and less interindividual variability than dipyridamole either after single or multiple doses. In particular, dip-beta-CD had a greater AUC and Cmax, and a smaller Tmax even at the steady state. In addition, 100% of the subjects receiving a single dose of dip-beta-CD, as compared to 66.7% of those treated with dipyridamole, had plasma levels superior to 1 microgram/ml (which is the supposed anti-aggregating threshold level). In contrast, 0 and 33.03% of the subjects showed plasma levels superior to 2.5 micrograms/ml (which might cause the appearance of side-effects) on the 7th day of the multiple treatment with dip-beta-CD and dipyridamole, respectively. In fact, the subjects presenting higher levels after uncomplexed dipyridamole also complained of headache and/or dizziness on occasion. No adverse side effects were reported for dip-beta-CD.

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Dipyridamole: pharmacokinetics and effects on aspects of platelet function in man

1. The effect of dipyridamole on platelet function was measured in twelve normal subjects given 150 or 200 mg tablets as single and multiple doses, and in six subjects given single doses of 25, 50 and 100 mg and multiple doses of 50 mg 8 hourly. 2. Platelet aggregation was measured in response to ADP and collagen. In the subjects given 150/200 mg, the platelets were assayed for content of cyclic AMP and for formation of thromboxane after addition of collagen. The responses to ADP and collagen and the cyclic AMP content were assessed in both the presence and absence of added PGE1. The pharmacokinetics of dipyridamole were studied in all subjects. 3. One hour after 150/200 mg single doses of dipyridamole there was significant inhibition of platelet aggregation in response to both collagen and ADP. There was no detectable effect on aggregation at other time points or with lower doses of dipyridamole. The addition of PGE1 to platelets prior to testing did not enhance the effect of dipyridamole on platelet aggregation. 4. In multiple doses, dipyridamole (150/200 mg twice daily for 11 days) had no detectable effect on platelet aggregation. 5. Dipyridamole did not have any effect on platelet cyclic AMP content, whether or not PGE1 was added prior to assay. 6. Dipyridamole did not affect platelet thromboxane formation. 7. Plasma dipyridamole concentrations were maximal 1-2 h after ingestion, at the same time that inhibition of platelet aggregation was detected. The concentrations declined in a biexponential fashion, with a terminal half life of 24.1 +/- 1.9 h (mean +/- s.e. mean). In six of the 17 subjects, the mean steady state plasma concentration was less than 75% of the value predicted from the single dose data.

The effect of the nucleoside transport inhibitor dipyridamole on the incorporation of [3H]thymidine in the rat

Dipyridamole is a non-specific inhibitor of nucleoside transport into mammalian cells. It is currently undergoing clinical evaluation in combination with various antimetabolites in an attempt to enhance the activity of these anticancer drugs by blocking the salvage of extracellular nucleosides, an important determinant of their cytotoxicity. In the present study, the effect of i.v. infusions of dipyridamole on [3H]thymidine incorporation into DNA has been examined in the anaesthetized rat. The tissues studied were bone marrow, gastrointestinal tract epithelium and the ascitic form of the Walker carcinosarcoma. Dipyridamole at 10 mg/kg, given over 3 hr, led to plasma levels of less than 5 microM and did not reduce [3H]thymidine incorporation into any of the tissues studied. At 40 mg/kg dipyridamole (plasma levels 10-15 microM) [3H]thymidine incorporation into the DNA of bone marrow and gastrointestinal tract epithelium was reduced to 20-30% of control values. Increasing the dose to 100 mg/kg did not lead to a further suppression of incorporation. Measurement of [3H]thymidine plasma pharmacokinetics and the intracellular distribution of tritium suggested that the inhibition of [3H]thymidine incorporation was due to reduced cellular uptake. In contrast to the effects on normal tissues, even at a lethal dose (200 mg/kg) dipyridamole did not significantly inhibit [3H]thymidine incorporation into Walker tumour cells. The levels of dipyridamole found in the ascitic fluid, at 100 mg/kg approximately half those in plasma, argue against a pharmacokinetic basis for this difference. Dipyridamole was found to bind extensively (97%) to rat plasma proteins, which may explain the discrepancy between the concentrations of dipyridamole required to inhibit nucleoside incorporation in vitro, in serum-free media, and those needed in vivo. From a comparison of the plasma levels of dipyridamole which cause an inhibition of [3H]thymidine incorporation in the rat with those which can be achieved safely in patients, it is concluded that dipyridamole is unlikely to markedly reduce nucleoside salvage in man.

Thallium-201 myocardial imaging during pharmacologic coronary vasodilation: comparison of oral and intravenous administration of dipyridamole

Although the diagnostic utility of thallium-201 myocardial imaging after dipyridamole infusion is well established, the intravenous form of the drug is not yet commercially available in North America. Fifty patients referred for coronary angiography were prospectively studied. Within a 2 week period, each patient underwent cardiac catheterization and thallium-201 myocardial imaging after both oral and intravenous dipyridamole administration. For the oral protocol, patients were randomly assigned to treatment with either 200 or 400 mg of dipyridamole in tablet form. Coronary artery stenoses of 70% or greater were considered significant. For the 25 patients who received a 200 mg oral dose of dipyridamole, the scintigraphic study showed perfusion defects in 65% of patients with significant coronary artery disease after the oral dose and in 85% of patients after the intravenous dose. For the 25 patients who received a 400 mg oral dose, the sensitivity of the scintigram was 84% after the oral dose and 79% after the intravenous dose. Except for headache and nausea, side effects were less severe and less frequent with oral (either 200 or 400 mg) than with intravenous dipyridamole. Because of the delayed and variable absorption of dipyridamole tablets, the oral studies required a longer period of medical supervision (45 to 60 minutes), and aminophylline was empirically administered after completion of the first set of thallium-201 images. It is concluded from this study that thallium-201 myocardial imaging after coronary vasodilation with a 400 mg oral dose of dipyridamole is a safe, widely available and reliable alternative for the evaluation of coronary artery disease in patients unable to achieve an adequate exercise level on stress testing.

Inhibition of platelet aggregation in whole blood by dipyridamole and aspirin

We have examined the effects of dipyridamole on platelet aggregation in whole blood both in vitro and after administration to man. The effects of dipyridamole ex vivo were compared with those of aspirin and a combination of dipyridamole and aspirin. In vitro dipyridamole was most effective as an inhibitor of platelet aggregation induced by platelet activating factor (PAF) and low concentrations of arachidonic acid (AA). Its inhibitory effect was always potentiated by adenosine suggesting that its effect on aggregation may be via inhibition of adenosine uptake into blood cells. Ex vivo, dipyridamole, aspirin and the combination of these drugs inhibited the platelet aggregation induced by PAF and AA. Again, adenosine increased the degree of inhibition. These results stress the importance of measuring platelet aggregation in the natural whole blood environment for detection of the inhibitory effects of dipyridamole and suggest a mode of action for the drug.

Usefulness of oral dipyridamole suspension for stress thallium imaging without exercise in the detection of coronary artery disease

Stress thallium imaging with intravenous dipyridamole permits assessment of coronary artery disease (CAD) without the need for exercise. However, intravenous dipyridamole is available in the United States only on an experimental basis. To study the use of oral dipyridamole as a clinically available alternative to intravenous dipyridamole for this purpose, 100 patients underwent thallium imaging with oral dipyridamole. Each patient received 300 mg of pulverized tablets in a 30-ml suspension. Maximal increase in mean heart rate and decrease in mean blood pressure occurred 30 minutes after ingestion. At 45 minutes, 2 mCi of thallium was given intravenously and serial imaging was begun within 7 minutes. The serum dipyridamole level (mean +/- standard deviation) 45 minutes after 300 mg was administered orally (3.7 +/- 2.2 micrograms/ml) was similar to that 5 minutes after 0.56 mg/kg was given intravenously (4.6 +/- 1.3 micrograms/ml). Fifty-five patients had some adverse effects between 15 and 75 minutes after oral ingestion, including nausea, headache, dizziness, chest pain (25 patients) and electrocardiographic changes (14 patients). Intravenous aminophylline was used to resolve these adverse effects in 21 patients. There were no severe arrhythmias, myocardial infarctions or deaths. Of the 43 patients with angiographically documented CAD, 39 had an initial perfusion defect that redistributed on the delayed images. When the results in patients who had undergone catheterization were analyzed by individual segment, the presence of thallium redistribution was associated with normal or hypokinetic contrast left ventriculographic wall motion of that segment, whereas the presence of a persistent defect was associated with akinesia or dyskinesia (Fisher's standardized Z = 9.14).(ABSTRACT TRUNCATED AT 250 WORDS)

Dipyridamole: a critical evaluation

Dipyridamol is a vasodilator that is used primarily in clinical practice as an antiplatelet agent. It increases coronary blood flow and was originally introduced as an antianginal agent. An ability to prolong a shortened platelet survival has been used to justify its value in preventing thromboembolic complications. Conditions characterized by a reduction in platelet survival and where dipyridamole has been used include heart valve replacement, arterial grafting, cerebrovascular disorders, and disorders of peripheral circulation. The in vivo effect of dipyridamole on platelet aggregation has not been well defined and may depend on additional factors. Prostaglandins appear to have important roles in platelet homeostasis; their relationships to the action of dipyridamole are discussed. Dipyridamole usually is combined with aspirin for synergistic anti-aggregatory purposes. However, the nature of the interaction has not been elucidated and benefit from the addition of dipyridamole has not been demonstrated in clinical studies. A review of clinical studies using dipyridamole indicates that it currently has limited value.

Dipyridamole inhibition of adenosine metabolism in human blood

The effects of dipyridamole on the metabolism of adenosine were studied in human whole blood. The half-life for adenosine disappearance and the formation of deamination and phosphorylation products were determined by adding [3H]adenosine to blood incubating at 37 degrees C. The initial adenosine concentration was 10 nmol/ml blood. Aliquots of blood were removed at specific times following the addition of labelled adenosine. The aliquots were later assayed for adenosine and its products by reverse-phase high pressure liquid chromatography. The half-life for adenosine in undiluted blood was less than 10 s; therefore, to accurately evaluate the effects of dipyridamole, blood was diluted 1 : 12 in isotonic saline. At this dilution, the half-life for adenosine was 1.3 min. Dipyridamole concentrations of 1 nmol/ml blood caused 90% inhibition of adenosine metabolism. Inhibition was virtually complete, except for plasma deamination, at concentrations greater than 10 nmol/ml blood. Since these inhibitory concentrations of dipyridamole are on the same order as those achieved therapeutically in man, these data indicate that dipyridamole at therapeutic concentrations causes significant inhibition of adenosine metabolism in whole blood.

Plasma dipyridamole concentrations after two different dosage regimens in patients

Twenty patients received dipyridamole by two different dosage regimens yielding a total daily dose of 150 mg, either as 50 mg three times a day or 75 mg twice a day. The rationale for comparing these two regimens is that dipyridamole is usually given three times a day, but recent studies have revealed a final elimination half-life of the drug of about half a day. Based on drug cumulation during chronic dosing, the final half-life of dipyridamole observed in this study also averaged about half a day. The 75 mg b.i.d. regimen did not result in lower trough concentrations than the 50 mg t.i.d. regimen. There was wide interpatient variability in observed plasma dipyridamole concentrations for both regimens, averaging about 10-fold. These results suggest that dipyridamole could be administered twice a day and that dipyridamole levels should be monitored in clinical studies on the antithrombotic effect of the drug.

Determinants of the pharmacokinetics of antithrombotic drugs

This review describes a variety of factors which determine the pharmacokinetics of antithrombotic drugs, viz., elimination by hepatic and/or renal clearance, plasma protein binding and drug disposition in the geriatric population. An understanding of biotransformation of acetylsalicylic acid and sulfinpyrazone is critical to understanding their usefulness as antithrombotic agents. Acetylsalicylic acid is rapidly hydrolysed to salicylic acid in the body and the latter may be eliminated by five different routes. Two of the biotransformation processes involving salicylic acid are readily saturable. For this reason, salicylate concentration increases more than proportionately with an increase in dose. Acetylsalicylic acid, but not salicylic acid, acetylates cyclo-oxygenase resulting in its inactivation. It is important in pharmacokinetic studies to measure plasma levels of both acetylsalicylic acid and salicylic acid. Biotransformation of sulfinpyrazone involves oxidation to the sulfone and reduction to the sulfide. The sulfide has marked antiplatelet activity. Dipyridamole undergoes hepatic biotransformation to a monoglucuronide which is eliminated by biliary and fecal excretion. Sulfinpyrazone, dipyridamole and salicylic acid are highly bound to plasma proteins. Since binding diminishes with age, one must expect therapeutic and toxic effects at lower plasma drug concentrations in the geriatric population. Other factors to be considered in the geriatric group are altered distribution, diminished glomerular filtration rate, and diminished oxidative biotransformation.

Clinical pharmacokinetics of dipyridamole

The pharmacokinetics off dipyridamole were studied in six normal subjects and 20 patients. The normal subjects received 20 mg IV each and five also took a 50 mg oral dose. Concentrations after the intravenous dose showed a tri-exponential decline with a terminal half-life of 11.6 +/- 2.2 hr (mean +/- S.D.). Total plasma clearance was 138 +/- 30 ml/min and the apparent volume of distribution was 141 +/- 51 l. Peak concentrations after oral dipyridamole occurred 2--2.5 hr after the dose. Systemic availability of the oral dose was 52 +/- 23%. Plasma protein binding was 99.13 +/- 0.24%. Twenty patients, admitted for coronary artery bypass grafting, received total daily doses of 150 mg, either as 50 mg tid or 75 mg bid. Based on drug cumulation during chronic dosing, the terminal half-life averaged about half a day. There was wide interpatient variability, averaging about 10-fold, in observed plasma concentrations for both dosage regimens. The bid regimen was not associated with lower trough concentrations of the drug than the tid regimen. These results indicate that dipyridamole concentrations vary widely in patients receiving the drug, and suggest that it could be administered twice a day, and that dipyridamole levels should be monitored for the antithrombotic effect in clinical studies.

Lack of effect of ischemia and dipyridamole on prostacyclin production in arteriosclerosis obliterans

Six patients with advanced arteriosclerosis obliterans in the lower extremities were subjected to an exercise test on a tread mill with and without dipyridamole treatment. Prostacyclin (PGI2) release was measured by the concentration of its stable metabolite, 6-keto-prostaglandin F1 alpha in plasma. All the patients suffered from ischemic pain during both tests, but no changes were seen in plasma 6-keto-PGF1 alpha. Dipyridamole did not affect the physical performance. Our results suggest that atherosclerotic vessels do not increase PGI2 production in response to ischemia and that a single dose of dipyridamole does not change PGI2 production.

Myocardial pharmacokinetics and pharmacodynamics of dipyridamole in the isolated rabbit heart

The myocardial accumulation and disposition pharmacokinetics of the antithrombotic drug dipyridamole were investigated in isolated perfused and spontaneously beating rabbit hearts. The rabbit myocardium behaved pharmacokinetically as a two-compartment system with regard to the drug. The half-lives of the alpha-phase of distribution and of the beta-phase of disposition were about 1.4 and 6.3 min., respectively. Perfusion with a modified Krebs-Henseleit solution containing 5.1 microgram ml-1 of dipyridamole caused an accumulation of about 140 micrograms g-1 of myocardial tissue at steady state. The initial measured pharmacokinetic parameters were unchanged after perfusion with the drug for 60 min. The accumulating dipyridamole in the rabbit heart caused a progressive decrease in myocardial contractility to about 65%. This was accompanied by a decrease in the ratio of contraction rate to oxygen consumption to about 0.7 as an expression of reduced myocardial efficiency. The coronary flow rate was not significantly increased. The heart beating frequency decreased only slightly and no dromotropic effects were observed.

High-performance liquid chromatographic analysis of dipyridamole in plasma and whole blood

A rapid, sensitive, and specific high-performance liquid chromatographic method is described for the quantitative analysis of dipyridamole in plasma and whole blood. The method involves a single extraction of an alkalinized sample with diethyl ether followed by evaporation of the organic solvent and ion-pair chromatography using fluorescence detection. The lower limit of sensitivity for dipyridamole is 1 ng/ml. Concentrations of dipyridamole between 1 and 500 ng per sample are measured with an average coefficient of variation of 4.5% in plasma and 7.4% in whole blood.

Bioavailability and pharmacokinetics in man of orally administered theophylline

The pharmacokinetics of theophylline after both intravenous and oral administration was investigated in six hospitalized patients with normal renal, hepatic and pulmonary functions. A rather wide range of biological half-lives from about 3-16 hours and plasma clearance values of about 1.5-115 ml kg-1 hr-1 were found in the investigated patients, who were from 31 to 73 years of age. The apparent volumes of distribution during the eliminatory beta-phase (Vdbeta) were within the range 0.394-0.6161 kg-1 with a mean value of 0.484 1 kg-1 +/- 0.082 S.D., as determined from the intravenous data, and in excellent agreement with the value obtained from the peroral data. Except in one case theophylline exhibited two compartment characteristics after intravenous administration, while the oral data in only one patient showed this pharmacokinetic configuration and had to be analysed according to one-compartment characteristics in the other five subjects. In the oral experiments absorption rate constants of from about 0.57 to 2.17 hr-1 were found for the administered microparticulate theophylline tablet preparation, Nuelin from Riker Laboratories. A wide range of lag-times from 0 to 1.32 hours were also demonstrated in the experiments. The systemic availability of theophylline in this preparation varied from 82.8 to 103% as determined on basis of the ratios of areas under the oral and intravenous serum concentration curves. It is conclusively stated that therapeutic plasma concentrations of theophylline probably may be maintained and controlled efficiently with the investigated oral theophylline preparation. Because of the interindividual variability in the biological half-life of the compound monitoring of the serum theophylline concentration is generally advised in order to avoid toxic side effects, in particular in relation to the initial establishment of a therapeutic serum concentration level in the individual subjects to be treated.