Evaluation of dose-related effects of aspirin on platelet function: results from the Aspirin-Induced Platelet Effect (ASPECT) study.

BACKGROUND: The antiplatelet effect of aspirin is attributed to platelet cyclooxygenase-1 inhibition. Controversy exists on the prevalence of platelet resistance to aspirin in patients with coronary artery disease and effects of aspirin dose on inhibition. Our primary aim was to determine the degree of platelet aspirin responsiveness in patients, as measured by commonly used methods, and to study the relation of aspirin dose to platelet inhibition. METHODS AND RESULTS: We prospectively studied the effect of aspirin dosing on platelet function in 125 stable outpatients with coronary artery disease randomized in a double-blind, double-crossover investigation (81, 162, and 325 mg/d for 4 weeks each over a 12-week period). At all doses of aspirin, platelet function was low as indicated by arachidonic acid (AA)-induced light transmittance aggregation, thrombelastography, and VerifyNow. At any 1 dose, resistance to aspirin was 0% to 6% in the overall group when AA was used as the agonist, whereas it was 1% to 27% by other methods [collagen and ADP-induced light transmittance aggregation, platelet function analyzer (PFA-100)]. Platelet response to aspirin as measured by collagen-induced light transmittance aggregation, ADP-induced light transmittance aggregation, PFA-100 (81 mg versus 162 mg, P < or = 0.05), and urinary 11-dehydrothromboxane B2 was dose-related (81 mg versus 325 mg, P = 0.003). No carryover effects were observed. CONCLUSIONS: The assessment of aspirin resistance is highly assay-dependent; aspirin is an effective blocker of AA-induced platelet function at all doses, whereas higher estimates of resistance were observed with methods that do not use AA as the stimulus. The observation of dose-dependent effects despite nearly complete inhibition of AA-induced aggregation suggests that aspirin may exert antiplatelet properties through non-cyclooxygenase-1 pathways and deserves further investigation.

Assessment of clopidogrel responsiveness: measurements of maximum platelet aggregation, final platelet aggregation and their correlation with vasodilator-stimulated phosphoprotein in resistant patients.

BACKGROUND: Controversy surrounds the optimal platelet aggregation measurement to assess clopidogrel non-responsiveness. The P2Y12 reactivity ratio (PRR) determined by vasodilator-stimulated phosphoprotein phosphorylation levels has been used to indicate the extent of P2Y(12) blockade. OBJECTIVES: We sought to compare the prevalence of non-responsiveness measured by maximum (MA) and 6 min aggregation (FA) and correlate these measurements with PRR in patients with non-responsiveness. METHODS: MA and FA were measured in stented patients (n=100) before and after clopidogrel treatment. The PRR was determined in 22 non-responsive patients. Responsiveness was defined as pre-treatment minus post-treatment aggregation; and non-responsiveness was defined as <10% change in platelet aggregation. RESULTS: Responsiveness was greater as determined by FA, p=0.006 (5 microM ADP) and p=0.003 (20 microM ADP)). There was a strong correlation between MA and FA stimulated by 5 microM (r=0.84, p<0.0001) and 20 microM ADP (r=0.90, p<0.001). The prevalence of non-responsiveness rose with agonist concentration but did not differ significantly between methods: 5 microM ADP [22% (MA) vs. 17% (FA), p=0.186] and 20 microM ADP [33% (MA) vs. 29% (FA), p=0.270]. PRR correlated with both MA (r=0.66, p<0.001) and FA (r=0.74, p<0.001) in non-responsive patients indicating incomplete receptor blockade. CONCLUSION: Clopidogrel responsiveness is higher when measured by FA as compared to MA. However, these measurements are equivalent in determining the prevalence of non-responsiveness: FA and MA are affected to the same degree in patients with non-responsiveness. These findings are relevant to ongoing studies assessing platelet inhibition by P2Y(12) inhibitors and support previous studies that employed MA to assess non-responsiveness.

Delivery of liposomal doxorubicin (Doxil) in a breast cancer tumor model: investigation of potential enhancement by pulsed-high intensity focused ultrasound exposure.

RATIONALE AND OBJECTIVES: To investigate the potential of using pulsed high-intensity focused ultrasound (HIFU) exposures to enhance the delivery, and hence therapeutic effect of liposomal doxorubicin (Doxil) in a murine breast cancer tumor model. MATERIALS AND METHODS: Tumors were grown in the bilateral flanks of mice using a mammary adenocarcinoma cell line. Experiments consisted of exposing one of two tumors to pulsed-HIFU, followed by tail vein injections of Doxil. Tumor growth rates were monitored, and assays carried out for doxorubicin concentration in these tumors as well as in a second (squamous cell carcinoma) tumor model and in muscle. Laser scanning confocal microscopy was used with fluorescent probes to observe both the uptake of polystyrene nanoparticles and dilation of exposed blood vessels. Additional experiments involving histologic analysis and real-time temperature measurements were performed to determine the safety of the exposures. RESULTS: Pulsed-HIFU exposures were shown to be safe, producing no apparent deleterious effects in the tumors. The exposures, however, were not found to enhance the delivery of Doxil, and consequently did not allow for lower doses for obtaining tumor regression. Imaging with a fluorescent dextran showed blood vessels to be dilated as a result of the exposures. Experiments with polystyrene nanoparticles of similar size to the liposomes showed a greater abundance to be present in the treated tumors. CONCLUSION: Although past studies have shown the advantages of pulsed-HIFU exposures for enhancing delivery, this was not observed with the liposomes, apparently because of their inherent ability to preferentially accumulate into tumors on their own. Potential mechanisms for enhanced uptake of non-liposomal nanoparticles are discussed.

Antiplatelet therapy: current strategies and future trends.

Pharmacological management of thrombotic complications is strongly influenced by antiplatelet treatment strategies. Recent clinical trials have clearly indicated that current antiplatelet strategies may not inhibit recurrent thrombotic events in selected patients and improvement is necessary. Recently, there has been a gradual modification in the guidelines for clopidogrel dosing. In addition, newly developed P2Y(12) receptor inhibitors and thrombin inhibitors are undergoing Phase II and III clinical trials. Moreover, research related to novel agents that interfere with other steps in coagulation and platelet adhesion, and platelet thromboxane and thrombin receptor blockers, show promise. An important future step will probably be the development of personalized therapy based on defining the individual patient's propensity for thrombosis through investigation of platelet-thrombin-fibrin interactions. Such an approach will enhance the targeting of specific therapy based on the pathophysiology of the individual patient.