Aspirin kinetics and platelet aggregation in man

Aspirin augments IgE-mediated histamine release from human peripheral basophils via Syk kinase activation

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs), especially aspirin, and food additives (FAs) may exacerbate allergic symptoms in patients with chronic idiopathic urticaria and food-dependent exercise-induced anaphylaxis (FDEIA). Augmentation of histamine release from human mast cells and basophils by those substances is speculated to be the cause of exacerbated allergic symptoms. We sought to investigate the mechanism of action of aspirin on IgE-mediated histamine release.

METHODS

The effects of NSAIDs, FAs or cyclooxygenase (COX) inhibitors on histamine release from human basophils concentrated by gravity separation were evaluated.

RESULTS

Benzoate and tartrazine, which have no COX inhibitory activity, augmented histamine release from basophils similar to aspirin. In contrast, ibuprofen, meloxicam, FR122047 and NS-398, which have COX inhibitory activity, did not affect histamine release. These results indicate that the augmentation of histamine release by aspirin is not due to COX inhibition. It was observed that aspirin augmented histamine release from human basophils only when specifically activated by anti-IgE antibodies, but not by A23187 or formyl-methionyl-leucyl-phenylalanine. When the IgE receptor signaling pathway was activated, aspirin increased the phosphorylation of Syk. Moreover, patients with chronic urticaria and FDEIA tended to be more sensitive to aspirin as regards the augmentation of histamine release, compared with healthy controls.

CONCLUSIONS

Aspirin enhanced histamine release from basophils via increased Syk kinase activation, and that the augmentation of histamine release by NSAIDs or FAs may be one possible cause of worsening symptoms in patients with chronic urticaria and FDEIA.

Pharmacokinetic profile of atenolol aspirinate

We report microwave-assisted synthetic routes, the pharmacokinetic profile along with results from ulcerogenicity and mutagenicity studies of atenolol aspirinate, and an already described derivative, in which acetyl salicylic acid (aspirin) was connected to atenolol by an ester linkage. Atenolol aspirinate was stable towards aqueous hydrolysis but rapidly hydrolyzed in plasma (t(1/2) = 7.6 min). The results showed that the rapid and complete hydrolysis generates atenolol salicylate, which assumes a conformation stabilized by two intramolecular H-bonds, avoiding its further hydrolysis to salicylic acid and atenolol.

Effects of different aspirin formulations on platelet aggregation times and on plasma salicylate concentrations

BACKGROUND

Early aspirin treatment is widely used to inhibit platelet activity and to reduce morbidity and mortality in patients presenting with an acute myocardial infarction or a stroke. A number of different aspirin formulations have been used for this purpose; however, a comparison of their effectiveness in inhibiting early platelet aggregation has not been determined.

METHODS

In this study, we determined plasma salicylate concentrations and platelet inhibitory activities at various times after ingestion of three commonly used aspirin formulations: soluble aspirin (Alka-Seltzer), 325 mg, chewed baby aspirin, 324 mg, and whole compressed non-enteric coated aspirin, 324 mg. Twenty-four healthy volunteers, 18-39 years of age, participated in the prospective single-blinded triple-crossover study. Plasma salicylate concentrations and inhibition of arachidonic acid-induced platelet aggregation were determined on post-dose blood samples collected at 2.5, 5.0, 7.5, 10, 15, 20, 25, 30, and 40 min. All subjects crossed over to the other two formulations with at least 2 weeks between ingestions.

RESULTS

The median platelet inhibition times for the chewed, soluble, and whole aspirin formulations were 7.5, 7.5, and 10.0 min, respectively. Soluble and chewed aspirin were found to inhibit platelet aggregation faster than whole aspirin (p<0.001); however, there were no significant differences in platelet aggregation times between the soluble and chewed formulations (p<0.163). Inhibition of platelet aggregation was found to occur at an average plasma salicylate concentration of 2.46 microg/mL, regardless of method of ingestion.

CONCLUSION

The results indicate that soluble and chewed aspirin inhibit platelet aggregation in a shorter period of time than does whole aspirin. The results suggest that chewing baby aspirin or taking soluble buffered aspirin may be the preferred route of administration for early platelet inhibition.

Bioequivalence Assessment of Two Enteric-Coated Aspirin Brands, Nu-seals® and Loprin®, after a Single Oral Dose of 150mg in Healthy Male Adults

AIM

The bioequivalence of aspirin from two enteric-coated brands, Nu-seals and Loprin, identified as the reference (R) and test (T) products, respectively, was assessed.

METHODS

A two-period randomised crossover design with a washout interval of 15 days was used in this study. The study results were determined in 16 healthy volunteers, all males with ages ranging from 19-28 (23.33 +/- 3.74) years and bodyweights of 52-92 (65.89 +/- 11.39) kg. After oral ingestion of 150mg of the either brand with 200 mL of water, serial blood samples were obtained over a period of 24 hours. Plasma, harvested from blood was analysed for the concentration of salicylic acid, a deacetylated metabolite of aspirin, by a validated high performance liquid chromatography (HPLC) method. Pharmacokinetic parameters were determined for both formulations by an interactive computer-assisted PK II procedure. A general linear model for repeated measures and 90% confidence intervals (CI) was employed to assess the sequence of treatment effects and to exclude differences between the parameters due to the product and period of administration, respectively.

RESULTS

The observed 90% CI ratios (Loprin/Nu-seals) for peak concentration, time to reach the peak and area under the plasma-concentration time curve from zero to infinity of 1.03,1.08; 1.04,1.05 and 1.01,1.15, respectively, were within the bioequivalence range (0.80,1.25) stipulated by the US Food and Drug Administration.

CONCLUSION

On the basis of the findings, the test (Loprin) and reference drug (Nu-seals) were deemed bioequivalent.

Antiplatelet and antithrombotic effects of a novel selective phosphodiesterase 3 inhibitor, NSP-513, in mice and rats

We investigated the effects of NSP-513, (R)-4,5-dihydro-5-methyl-6-[4-(2-propyl-3-oxo-1-cyclohexenyl)amino] phenyl-3(2H)-pyridazinone, on phosphodiesterase (PDE) isozyme activities, in vitro platelet aggregation and in vivo thrombus formation. NSP-513 selectively inhibited human platelet PDE 3 isozyme with an IC50 value of 0.039 microM. In an in vitro human platelet aggregation assay, the IC50 values (microM) of NSP-513 for platelet aggregation induced by collagen, U-46619, arachidonic acid, adenosine diphosphate (ADP), epinephrine and thrombin were 0.31, 0.25, 0.082, 0.66, 0.23 and 0.73, respectively. In a mouse pulmonary thromboembolism model, orally administered NSP-513 showed in vivo antithrombotic effects that were 320 to 470 times more potent than those of cilostazol. In a rat carotid arterial thrombosis model, intraduodenally administered NSP-513 (0.1 mg/kg), cilostazol (30 mg/kg) and aspirin (30 mg/kg) reduced thrombus formation by 75%, 66% and 48%, respectively. However, intravenously administered dipyridamole (10 mg/kg) did not significantly prevent thrombus formation. These results demonstrate that NSP-513 has the potential to prevent not only in vitro platelet aggregation but also in vivo thrombus formation and indicate that the highly selective PDE 3 inhibitory effect of NSP-513 may make this compound useful for assessing the physiological role of PDE 3.

Focused antithrombotic therapy: novel anti-platelet salicylates with reduced ulcerogenic potential and higher first-pass detoxification than aspirin in rats

The use of aspirin as an anti-platelet drug is limited by its propensity to induce gastric injury and by its adverse effect on vascular prostacyclin formation. Two phenolic non-steroidal anti-inflammatory drugs (salicylic acid and diflunisal) were modified by esterification with a series of O-acyl moieties. The short-term ulcerogenic in vitro and in vivo anti-platelet properties, pharmacodynamic profiles, and extent of hepatic extraction of these phenolic esters were compared with aspirin (acetylsalicylic acid). The more lipophilic esters (longer carbon chain length in O-acyl group) show significantly less gastrotoxicity in stressed rats than does aspirin after a single oral dose. The in vitro and in vivo anti-platelet studies show that these phenolic esters inhibited (1) arachidonate-triggered human platelet aggregation and (2) thrombin-stimulated rat serum thromboxane A2 production by platelets in the clotting process almost as effectively as aspirin. The hepatic extractions of these O-acyl derivatives are significantly higher than those of aspirin. The pharmacodynamic studies show that these O-acyl derivatives of salicylic acid and diflunisal probably bind to, or combine with, the same site on the platelet cyclooxygenase as aspirin. Replacing the O-acetyl group with longer chain O-acyl moiety in this series of phenolic esters markedly reduced the potential of these agents to induce short-term gastric injury but did not lessen their activity as inhibitors of platelet aggregation. These non-acetyl salicylates may therefore represent a novel class of anti-platelet drugs with less ulcerogenic potential.

Hepatic structure-pharmacokinetic relationships: the hepatic disposition and metabolite kinetics of a homologous series of O-acyl derivatives of salicylic acid

1. The hepatic disposition and metabolite kinetics of a homologous series of O-acyl (acetyl, propionyl, butanoyl, pentanoyl, hexanoyl and octanoyl) esters of salicylic acid (C2SA, C3SA, C4SA, C5SA, C6SA and C8SA, respectively) was determined using a single-pass, in-situ rat liver preparation. 2. The hepatic venous outflow profiles for the parent esters and the generated metabolite, salicylic acid (SA) were analysed by HPLC. Non-parametric moments analysis was used to determine the area under the curve (AUC'), mean transit time (MTT) and normalized variance (CV2) for the parent esters and generated SA. 3. Pregenerated SA ([14C]-salicylic acid) was injected into each liver with the parent ester to determine its distribution characteristics. 4. The overall recovery of ester plus metabolite was 89% of the ester dose injected and independent of the ester carbon number, suggesting that ester extraction was due to hepatic metabolism to salicylic acid. 5. The metabolite AUC' value increased directly with the lipophilicity of the parent ester (from 0.12 for C2SA to 0.95 for C8SA). By contrast, the parent AUC' decreased with the lipophilicity (from 0.85 for C2SA to zero for C8SA). The metabolite MTT value also showed a trend to increase with the lipophilicity of the parent ester (from 15.72 s for C3SA to 61.97 s for C8SA). However, the parent MTT value shows no significant change across the series. 6. The two-compartment dispersion model was used to derive the kinetic parameters for parent ester, pregenerated SA and generated SA. Consequently, these parameters were used to estimate the values of AUC', MTT and CV2 for the parent ester and metabolite. The moments values obtained using the two-compartment dispersion model show similar trends to the corresponding moments values obtained from the outflow profiles using a non-parametric approach. 7. The more lipophilic aspirin analogues are more confined to the portal circulation after oral administration than aspirin due to their more extensive hepatic elimination avoiding systemic prostacyclin inhibition. Given that aspirin's selectivity as an anti-thrombotic agent has been postulated to be due to selective anti-platelet effects in the portal circulation, the more lipophilic and highly extracted analogues are potentially more selective anti-thrombotic agents than aspirin.

Hepatic disposition and metabolite kinetics of a homologous series of diflunisal esters

The hepatic disposition and metabolite kinetics of a homologous series of diflunisal O-acyl esters (acetyl, butanoyl, pentanoyl, and hexanoyl) were determined using a single-pass perfused in situ rat liver preparation. The experiments were conducted using 2% BSA Krebs-Henseleit buffer (pH 7.4), and perfusions were performed at 30 mL/min in each liver. O-Acyl esters of diflunisal and pregenerated diflunisal were injected separately into the portal vein. The venous outflow samples containing the esters and metabolite diflunisal were analyzed by high performance liquid chromatography (HPLC). The normalized outflow concentration-time profiles for each parent ester and the formed metabolite, diflunisal, were analyzed using statistical moments analysis and the two-compartment dispersion model. Data (presented as mean +/- standard error for triplicate experiments) was compared using ANOVA repeated measures, significance level P < 0.05. The hepatic availability (AUC'), the fraction of the injected dose recovered in the outflowing perfusate, for O-acetyldiflunisal (C2D = 0.21 +/- 0.03) was significantly lower than the other esters (0.34-0.38). However, RN/fu, the removal efficiency number RN divided by the unbound fraction in perfusate fu, which represents the removal efficiency of unbound ester by the liver, was significantly higher for the most lipophilic ester (O-hexanoyldiflunisal, C6D = 16.50 +/- 0.22) compared to the other members of the series (9.57 to 11.17). The most lipophilic ester, C6D, had the largest permeability surface area (PS) product (94.52 +/- 38.20 mL min-1 g-1 liver) and tissue distribution value VT (35. 62 +/- 11.33 mL g-1 liver) in this series. The MTT of these O-acyl esters of diflunisal were not significantly different from one another. However, the metabolite diflunisal MTTs tended to increase with the increase in the parent ester lipophilicity (11.41 +/- 2.19 s for C2D to 38.63 +/- 9.81 s for C6D). The two-compartment dispersion model equations adequately described the outflow profiles for the parent esters and the metabolite diflunisal formed from the O-acyl esters of diflunisal in the liver.

Pharmacokinetics of a new nitroderivative of acetylsalicylic acid after a single dose in rats

The pharmacokinetics of a new ASA-nitroderivative compound, NCX 4016 (ASA-NO2), was evaluated using an HPLC method. After single equimolar doses of ASA (35 mg kg(-1)) or ASA-NO2 (65 mg kg(-1)), no detectable levels of these compounds have been observed in rat plasma samples. SA peak levels were obtained at 3 h and 6 h after ASA and ASA-NO2 administration respectively. The elimination rate constants of SA were similar after ASA and ASA-NO2, suggesting a similar elimination phase of this metabolite in rats. From these data it is evident that ASA-NO2 is slowly metabolized in ASA, which is rapidly converted to SA.

A comparison of aspirin with placebo in patients treated with warfarin after heart-valve replacement

BACKGROUND

Despite the use of warfarin, major systemic embolism remains an important complication in patients with heart-valve replacement. Although the addition of antiplatelet agents has the potential to reduce this complication, their efficacy and safety when given in combination with warfarin are uncertain.

METHODS

In a randomized, double-blind, placebo-controlled trial, we assessed the efficacy and safety of adding aspirin (100 mg per day) to warfarin treatment (target international normalized ratio, 3.0 to 4.5) in 370 patients with mechanical heart valves or with tissue valves plus atrial fibrillation or a history of thromboembolism.

RESULTS

A total of 186 patients were randomly assigned to aspirin and 184 to placebo, and they were followed for up to 4 years (average, 2.5). Major systemic embolism or death from vascular causes occurred in 6 aspirin-treated patients (1.9 percent per year) and 24 placebo-treated patients (8.5 percent per year) (risk reduction with aspirin, 77 percent; 95 percent confidence interval, 44 to 91 percent; P < 0.001). Major systemic embolism, nonfatal intracranial hemorrhage, or death from hemorrhage or vascular causes occurred in 12 patients assigned to aspirin (3.9 percent per year) and 28 patients assigned to placebo (9.9 percent per year) (risk reduction, 61 per cent; 95 percent confidence interval, 24 to 80 percent; P = 0.005); major systemic embolism or death from any cause occurred in 13 patients (4.2 percent) and 33 patients (11.7 percent), respectively (risk reduction, 65 percent; 95 percent confidence interval, 33 to 82 percent; P < 0.001); and death from all causes occurred in 9 patients (2.8 percent) and 22 patients (7.4 percent), respectively (risk reduction, 63 percent; 95 percent confidence interval, 19 to 83 percent; P = 0.01). Bleeding occurred in 71 patients in the aspirin group (35.0 percent), as compared with 49 patients in the placebo group (22.0 percent) (increase in risk, 55 percent; 95 percent confidence interval, 8 to 124 percent; P = 0.02); major bleeding occurred in 24 and 19 patients, respectively (increase in risk, 27 percent; 95 percent confidence interval, -30 to 132 percent; P = 0.43).

CONCLUSIONS

In patients with mechanical heart valves and high-risk patients with prosthetic tissue valves, the addition of aspirin to warfarin therapy reduced mortality, particularly mortality from vascular causes, together with major systemic embolism. Although there was some increase in bleeding, the risk of the combined treatment was more than offset by the considerable benefit.

Single-dose effect of enteric-coated aspirin on platelet function and thromboxane generation in middle-aged men

OBJECTIVE

To evaluate the effect of a single dose of enteric-coated aspirin (ECA) in three different dosages on platelet function and thromboxane generation in middle-aged men.

DESIGN AND METHODS

In a nonblind, nonplacebo-controlled, crossover study, a single dose of ECA (50, 250, or 1000 mg) was given in a tablet form to a group of healthy, middle-aged men. Ten subjects, aged 50-67 years, volunteered to participate in this study. Platelet functions including bleeding time, platelet aggregation, adenine nucleotides, beta-thromboglobulin, platelet factor 4, thromboxane generation, and aspirin measurement were determined.

RESULTS

Before ECA ingestion, the intracellular adenine nucleotides (adenosine triphosphate, adenosine diphosphate) were decreased, and both beta-thromboglobulin and platelet factor 4 were increased. These observations suggested that platelets were activated in vivo in middle-aged men. These findings returned to normal within 8 hours after the ingestion of ECA, and maintained normal for at least two days. Bleeding time was significantly prolonged at 8 and 24 hours compared with that before ingestion of ECA 1000 mg (p < 0.05). The generation of platelet thromboxane was maximally inhibited by approximately 40 percent in the samples 8 hours after ECA ingestion. Abnormal values of adenine nucleotides, beta-thromboglobulin, and platelet factor 4 returned to normal within 8 hours. Arachidonic acid-induced platelet aggregation was inhibited compared with that before treatment (p < 0.01) and the inhibitory effect was maintained for at least three days. Adenosine diphosphate- and epinephrine-induced aggregations were less inhibited than those induced by arachidonic acid. Inhibitory effects of ECA on platelet aggregation were dose dependent.

CONCLUSIONS

Our study indicates that platelets are activated in middle-aged men and that a single dose of ECA 50 mg is safe and can inhibit thromboxane synthesis and platelet aggregation. These results suggest that a daily dose of ECA 50 mg may be useful for blocking platelet activation and preventing thrombosis.

The effects of varying doses of aspirin on human platelet activation induced by PAF, collagen and arachidonic acid

1. The effect of increasing doses of orally administered aspirin (30-900 mg) on platelet aggregation and ATP release induced by arachidonic acid (AA), collagen and platelet activating factor (PAF) was assessed in 12 normal volunteers. 2. Aspirin ingestion was associated with a significant increase in EC50 for AA (P less than 0.0001) and collagen (P less than 0.0001) but not for PAF (P greater than 0.495) although the normal biphasic aggregation response for the latter was abolished. Maximum ATP release was reduced by aspirin for all three agonists. 3. The mean maximum degrees of inhibition of platelet aggregation induced by aspirin for AA, collagen and PAF were 100%, 48% and 21% of baseline, respectively. The corresponding mean maximum inhibition of ATP release was 100%, 63% and 57%. The minimum cumulative doses of aspirin producing these effects were 240, 240 and 90 mg for AA, collagen and PAF respectively. For collagen alone, there was a significant decrease in the degree of inhibition of aggregation between the last dose on day 1 (150 mg) and the baseline measurement on day 2. 4. Platelets from female subjects were more sensitive to collagen (P less than 0.05) and AA (P less than 0.01) stimulation compared with males. However, prior to aspirin ingestion, PAF produced a greater maximum response in platelets from females (P less than 0.02) while following aspirin ingestion PAF-induced activation was inhibited to a greater degree in females (P less than 0.02). 5. These results indicate that collagen- and PAF-induced platelet activation are only partially dependent on cyclo-oxygenase and for PAF this seems related only to the second phase of aggregation.(ABSTRACT TRUNCATED AT 250 WORDS)

Two year Cottbus reinfarction study with 30 mg aspirin per day

All 701 heart infarction patients admitted to 15 hospitals in the district of Cottbus between 1981 and 1983 were randomly administered 30, 60 or 1000 mg aspirin daily according to the territorial affiliation of their local hospitals. The physical and drug therapy during the 2 years follow-up was highly standardized; deviations--as far as they occurred--were documented. Lower all-cause mortality was statistically demonstrated in patients over 60 and a lower fatal reinfarction rate in patients over 50 as well as in men. Deaths and fatal reinfarctions were significantly lower among patients with a history of angina pectoris, marked ST-depression, with an infarction location except for the posterior wall and among hypercholesterolemic patients. The preventive effect of 60 mg aspirin daily was less than that of 30 mg in comparison to the 1000 mg group. Side effects were seen in 4 and 8% (first and second year), respectively, of the patients administered 30 mg aspirin as opposed to 22 and 17% in patients allocated 1000 mg. We conclude that the optimum dose of aspirin for preventing reinfarctions could be as low as 30 mg daily.

Suppression of thromboxane A2 but not of systemic prostacyclin by controlled-release aspirin

BACKGROUND

The antithrombotic efficacy of aspirin is attributed to its inhibition of the enzyme prostaglandin G/H synthase, which is necessary for the formation of thromboxane A2 in platelets. Thromboxane A2 is a potent vasoconstrictor and platelet agonist. However, the formation of prostacyclin by vascular endothelium also requires prostaglandin G/H synthase, and prostacyclin exerts opposite effects on platelet function and vascular tone. We wanted to see whether controlled-release aspirin would affect the formation of thromboxane A2 but not prostacyclin by reducing the aspirin concentration that reaches the posthepatic circulation.

METHODS

A controlled-release formulation containing 75 mg of aspirin, designed to release 10 mg per hour, was developed to inhibit prostaglandin G/H synthase in platelets in the prehepatic circulation. The effects of the controlled-release preparation on plasma levels of aspirin and salicylate, serum levels of thromboxane B2, and urinary dinor metabolites of prostacyclin and thromboxane B2 (measured by gas chromatography-mass spectrometry) were compared with those of conventional immediate-release aspirin in normal volunteers. Prostacyclin release was stimulated by intravenous bradykinin.

RESULTS

Steady-state inhibition of serum thromboxane B2 required two to four days and appeared slower with 75 mg of controlled-release than with the same amount of immediate-release aspirin. Maximal inhibition was achieved rapidly by adding a single loading dose of 162.5 mg of immediate-release aspirin to the regimen. Over a 28-day period, suppression of thromboxane A2 with this regimen was comparable to that with immediate-release aspirin taken either as 162.5 mg daily or as 325 mg on alternate days, despite the minimal systemic bioavailability of controlled-release aspirin. Bleeding time was prolonged to a similar degree with each of the three regimens. The five- to sixfold increase in the prostacyclin metabolite induced by bradykinin was depressed by pretreatment for four days with 75 mg of immediate-release aspirin, but not by 75 mg of controlled-release aspirin.

CONCLUSIONS

Maximal inhibition of platelet thromboxane A2 production was sustained during long-term dosing with controlled-release aspirin, whereas basal prostacyclin biosynthesis fell only slightly and systemic synthesis of prostacyclin stimulated by bradykinin was preserved. Controlled-release aspirin may facilitate determination of the clinical importance of preserving prostacyclin during platelet inhibition in humans.

Selective inhibition of platelet cyclooxygenase with controlled release, low-dose aspirin

The hypothesis that slow administration of low doses of aspirin may selectively inhibit platelet cyclooxygenase and thromboxane A2 formation was evaluated using controlled release aspirin formulations. In the first study, doses of either 50, 100, 325 and 1,300 mg of these formulations and 300 mg soluble aspirin were ingested daily by healthy volunteers for one week. In the second study, doses of 5, 10, 25 and 50 mg controlled release aspirin, 50 mg soluble aspirin and 100 mg aspirin and glycine formulation were ingested daily for ten days. Platelet function and urinary prostaglandin production were assessed immediately before and on the seventh day of dosing in both studies and in the second study, repeated on the tenth day of dosing. Platelet function and serum thromboxane B2 production were fully inhibited by all formulations of 50 mg aspirin and above, but not by doses of controlled release aspirin below 50 mg doses. The excretion of urinary 6-keto-PGF1 alpha (a major metabolite of prostacyclin) was significantly reduced at controlled release aspirin doses above 100 mg and at all doses of rapidly absorbed aspirin tested. As no significant reduction in the urinary 6-keto-PGF1 alpha production was observed at doses of controlled release aspirin of 50 and 100 mg and below, it appeared that these doses did not inhibit the systemic vascular cyclooxygenase. These data are consistent with a selective inhibition of platelet function by daily doses of 50 and 100 mg of the controlled release formulation of aspirin.

The antiplatelet effect of daily low dose enteric-coated aspirin in man: a time course of onset and recovery

We have studied the onset and recovery of inhibition of platelet function by low dose aspirin. Enteric-coated aspirin 50mg daily was administered to five human volunteers for five weeks and then 100mg daily was given for a further five weeks. We studied platelet aggregation and thromboxane formation in response to a range of stimuli: ADP, adrenaline, arachidonate and collagen, and also measured thromboxane formation after coagulation of whole blood (serum thromboxane). The onset of inhibition of platelet aggregation was progressive over several days for each of the four platelet stimuli, and was synchronous with the inhibition of thromboxane formation. Maximum inhibition occurred by day three for the weak stimuli ADP and adrenaline, by day five for the stronger stimuli arachidonate and collagen, but did not occur until day eight for serum thromboxane. Further inhibitory effects on both aggregation and thromboxane generation were observed after 100mg daily. Two weeks after the cessation of aspirin the responses to collagen and arachidonate and serum thromboxane had returned to normal. Platelet aggregation in response to the weaker stimuli, ADP and adrenaline, still showed detectable inhibition two weeks after cessation of aspirin, but had returned to normal by four weeks. These experiments provided no evidence for an effect of aspirin on platelets separate to its effect on cyclooxygenase. The onset and recovery of inhibition of platelet function by low dose aspirin was dependent on the strength of the stimulus studied.

Effects of aspirin and prostaglandin E1 on in vitro thrombolysis with urokinase. Evidence for a possible role of inhibiting platelet activity in thrombolysis

The formation of thrombi in vivo includes the activation of both platelets and the coagulation cascade. Conventional thrombolytic therapy is primarily directed toward the dissolution of fibrin. To evaluate the possibility that platelet activity impairs the lysis of thrombi, we studied the effects of aspirin and platelet-deaggregating prostaglandin E1 on thrombolysis with urokinase. Combined platelet and fibrin thrombi were produced in vitro by adding CaCl2 and collagen (1 microgram/ml) to citrated platelet-rich plasma (250,000 platelets per microliters). Urokinase (500-10,000 units/ml) caused a dose-dependent weight loss of the thrombi that was maximal at 2,000 units/ml. The addition of aspirin (10-200 micrograms/ml) to platelet-rich plasma before thrombus formation markedly enhanced thrombolysis with urokinase. This effect was most pronounced at 20 micrograms/ml aspirin. However, when aspirin was added after completion of thrombus formation, no significant effect on thrombolysis was noted. Prostaglandin E1 (1-100 mumol/l) improved the lysis with urokinase of the combined platelet and fibrin thrombi. This effect was maximal at 20 mumol/l prostaglandin E1. When pure fibrin thrombi were produced in platelet-free plasma, prostaglandin E1 was without effect on lysis. Thus, in vitro lysis with urokinase of combined platelet and fibrin thrombi was enhanced by the addition of platelet-deaggregating prostaglandin E1 and by pretreatment with aspirin.

Inhibition of platelet function by low-dose plain and micro-encapsulated acetylsalicylic acid

The effect of two acetylsalicylic acid (ASA) formulations, plain (Magnyl) and micro-encapsulated (Globentyl), on platelet aggregation, thromboxane formation, and bleeding time was studied in 12 healthy volunteers in a randomized double-blind cross-over study. All subjects were treated with Magnyl and Globentyl (75 mg daily) in periods of 2 weeks, separated by a wash-out period of 2 weeks. Both drugs significantly depressed platelet aggregation and thromboxane formation and prolonged bleeding time without difference in mode of action of the drugs. It is concluded that significant inhibition of platelet activity may be achieved by low-dose ASA treatment with micro-encapsulated as well as with plain formulations.

Pharmacokinetics of low-dose oral modified release, soluble and intravenous aspirin in man, and effects on platelet function

The pharmacokinetics of low-dose aspirin and the resulting salicylic acid were studied in 6 healthy volunteers. Each received a single 50-mg dose of (1) oral modified release capsules, (2) oral solution and (3) intravenous solution. The volunteers also received 50 mg modified release capsules daily for 6 days to determine the effect on collagen, ADP and arachidonate induced platelet aggregation and thromboxane production, and to compare the pharmacokinetics after repeated dosing with the parameters obtained after the single dose. The formulation and route of administration profoundly influenced several pharmacokinetic parameters for aspirin: the maximum concentration (Cmax, ng.ml-1) was 221 and 191 after modified release for single and chronic dosing respectively, 1323 after the oral solution and 6000 after intravenous injection; the time to achieve this maximum concentration (tmax, h) was 3.42 and 3.02 after modified release for single and chronic dosing respectively, and 0.29 after the oral solution; the area under the plasma drug concentration versus time curve (AUC, microgram.h.ml-1) was 0.38 and 0.27 after modified release single and chronic dosing respectively, 0.68 after the oral solution and 1.57 after intravenous injection. The elimination of aspirin after the two solutions was at least biphasic. The terminal phase rate constant ranged from 1.52 h-1 after intravenous injection to 1.88 h-1 after the oral modified release form. The absorption of the oral forms of aspirin was complete as reflected by the total recovery of the doses as salicylic acid in urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma drug and antiplatelet profiles of the original acetylsalicylic acid preparations used in the AMIS, PARIS and German-Austrian trials for secondary prevention of myocardial infarction

In a cross-over study 6 healthy male subjects were given for 9 days the acetylsalicylic acid (ASA) preparations used in the Aspirin Myocardial Infarction Study (AMIS), Persantine-Aspirin Reinfarction Study (PARIS) and German-Austrian secondary heart attack prevention trials, exactly according to the original study protocols. Plasma concentrations of ASA and its main metabolites salicylic acid (SA) and salicyluric acid (SUA), as well as platelet function (collagen-induced platelet aggregation; tissue extract-induced change in platelet shape) were studied repeatedly on the first day of each medication period and were again examined on the sixth and ninth days. Differences in the plasma concentrations of ASA and its metabolites were found only on the first day, probably as a result of different absorption rates. Collagen-induced platelet aggregation was more rapidly inhibited the faster the preparation was absorbed. Each ASA preparation inhibited tissue extract-induced platelet shape change from the first dose, although statistically significant inhibition was seen only with the AMIS preparation. It is concluded that differences in the antithrombotic efficiency of ASA cannot be explained by differences in the pharmacokinetic and antiplatelet profiles of the various ASA preparations tested.

Effective antiplatelet drug concentrations in experimental arterial thromboembolism

Although drugs that modify platelet function have been widely studied as antithrombotic agents in experimental and clinical studies, there is limited information regarding the relationship between in vivo drug blood concentrations and antithrombotic efficacy. This study compared the pharmacokinetics of three antiplatelet agents with their antithrombotic effects in an experimental model of arterial thromboembolism in baboons. Thrombus formation was measured as steady-state platelet utilization induced by thrombogenic arteriovenous cannulae. The drugs studied were aspirin, dipyridamole and sulfinpyrazone. Aspirin was administered in daily doses of 20 mg/kg, dipyridamole in daily doses of 2.5 and 10 mg/kg, and sulfinpyrazone in daily doses of 20 and 100 mg/kg; each drug was given in two equal doses per day. Multiple blood samples were collected for drug analysis after steady-state had been reached. The average concentrations of dipyridamole at steady-state were 26 +/- 15 and 79 +/- 69 ng/ml after 2.5 and 10 mg/kg/day. These concentrations were associated with 28 and 87% inhibition of cannula platelet consumption, respectively. The average steady-state concentrations of acetylsalicylic and salicylic acids were 0.67 +/- 0.80 and 3.76 +/- 2.60 micrograms/ml, respectively, after 20 mg/kg/day. Aspirin had no effect on platelet consumption. Average concentrations of sulfinpyrazone were 1.05 +/- 0.45 and 12.25 +/- 5.73 micrograms/ml after 20 and 100 mg/kg/day, with significant concentrations of the sulfide metabolite. These concentrations were associated with 23 and 85% inhibition of platelet consumption, respectively. No significant pharmacokinetic interactions were observed after concurrent administration of aspirin and dipyridamole or sulfinpyrazone. As the experimental model used involves thrombus formation on an artificial surface, it is likely that these results are most relevant to patients with arterial prosthetic devices.

Effect of 50 mg enteric-coated aspirin (Astrix) on thromboxane and prostacyclin synthesis

Although low-dose soluble aspirin can be recommended as a useful anti-thrombotic drug regimen in patients with vascular disease, enteric-coated preparations have a theoretical advantage for aspirin preparations which are to be ingested daily for many years. We have demonstrated that a 50 mg enteric-coated aspirin formulation (Astrix) which has an absorption rate much lower than soluble aspirin, is sufficient to inhibit platelet thromboxane synthesis while causing no major decrease in vascular prostacyclin synthesis.

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.

A dose-ranging study of the antiplatelet effect of enteric coated aspirin in man

Enteric coated aspirin was given to eight human volunteers in escalating doses (20, 40, 60, 80, 100 mg daily), each dose being given over two weeks. In addition, to measure the maximum effect of aspirin, each volunteer was given two single doses of 600 mg of soluble aspirin. At the end of each dosing interval we measured platelet aggregation and thromboxane formation in response to four aggregating agents and to whole blood coagulation. The doses of aspirin required to inhibit platelet aggregation in response to various stimuli were: for collagen 60-80 mg, for adenosine diphosphate and adrenaline 60 mg, and for arachidonate 40 mg. For maximum inhibition of thromboxane formation the doses were: for collagen greater than 100 mg, for adenosine diphosphate and adrenaline 60 mg, for arachidonate 80 mg, and for whole blood coagulation 100 mg. Different aspirin doses are required to inhibit the responses to different stimuli. Furthermore, for some stimuli, inhibition of thromboxane generation may require more aspirin than is required for inhibition of aggregation. The clinical implications of these findings are uncertain since we do not know which stimuli are important in arterial thrombosis in man.

Effect of alternate-day regular and enteric-coated aspirin on platelet aggregation, bleeding time, and thromboxane A2 levels in bleeding-time blood

The effectiveness of low-dose aspirin for primary prevention of cardiovascular mortality is being assessed among the nearly 22,000 United States physicians currently participating in the Physicians' Health Study. Because of occasional reports of gastric irritation among study participants, two enteric-coated aspirin preparations were tested as possible alternatives to regular compressed aspirin for platelet inhibition. Thirty-three volunteers were assigned randomly to one of four treatment groups: regular aspirin (325 mg), placebo, and two enteric-coated aspirin preparations (325 mg). Pills were administered every other day, duplicating the regimen used in the Physicians' Health Study. Bleeding times, platelet aggregation, and thromboxane A2 levels produced by aggregating platelets in vitro, as well as in collected bleeding-time blood, were determined. Measurements were taken before and after a single dose as well as after seven alternate-day doses. Regular and enteric-coated aspirin preparations were equally efficacious in prolonging the bleeding time, inhibiting platelet aggregation, and suppressing thromboxane A2 production. There was virtually complete suppression of thromboxane A2 production (over 99 percent), by platelets in vitro and in collected bleeding-time blood. The levels were still profoundly reduced (89 percent) 48 hours after the last dose. Enteric-coated aspirin may provide an alternative to regular aspirin in a low-dose regimen designed to inhibit platelet activity.

Extrahepatic metabolism and distribution of aspirin in vascular beds of sheep

The dispositions of aspirin, its metabolite, salicylic acid, and its subsequent metabolite, salicyluric acid, were studied in eight anesthetized sheep infused with aspirin (61 and 485 microgram X min-1 X kg-1) for 75 min. Plasma samples were withdrawn from the portal vein, hepatic vein, pulmonary artery, left ventricle, left femoral vein, and left femoral artery. Significant extraction of aspirin occurred across the liver and hind leg, with mean availabilities of 0.75, 0.98, and 0.82 observed across the liver, lung, and hind leg, respectively. The extraction of aspirin was not affected by coadministration of sodium salicylate (30-1200 mg equiv salicylic acid). This extraction reflected hydrolysis of aspirin to salicylic acid; the metabolism of aspirin in the hind leg, lung, and liver being confirmed with tissue homogenate studies. The metabolism of aspirin in the extrahepatic tissues is significant in relation to the proposed selective presystemic acetylation of platelet cyclooxygenase by aspirin and the use of low-dose aspirin for thrombotic indications.

Biochemical selectivity of oral versus intravenous aspirin in rats. Inhibition by oral aspirin of cyclooxygenase activity in platelets and presystemic but not systemic vessels

In rats intravenous aspirin was only slightly more effective an inhibitor of platelet thromboxane B2 (TxB2) than of aorta 6-keto-prostaglandin (PGF)1 alpha generation (1.9 versus 2.1 mg/kg). In contrast, oral aspirin was about five times more effective on platelet than on aorta cyclooxygenase activity. The "biochemical selectivity" of aspirin as an inhibitor of platelet and vascular cyclooxygenase thus was not apparent after intravenous administration of the drug. However, this could be achieved by relatively low doses of oral (or intraduodenal) aspirin, on account of "presystemic" acetylation of platelet cyclooxygenase. Even in this condition, though, aspirin selectivity was relative to "systemic" peripheral vessels but not to the vessels of the enterohepatic circulation. Indeed after an oral or intraduodenal dose of 5 mg/kg aspirin, generation of portal vein 6-keto-PGF1 alpha was inhibited to much the same extent as platelet TxB2, while inferior vena cava 6-keto-PGF1 alpha formation was spared.

The effects of different doses of some acetylsalicylic acid formulations on platelet function and bleeding times in healthy subjects

In an attempt to find the most appropriate dose and formulation(s) of acetylsalicylic acid (ASA) for thrombosis prophylaxis, healthy volunteers were given doses of ASA for 1 wk daily ranging from 50 to 3900 mg as either Aspro-Clear (soluble), Astrix (enteric coated pellets) or Ecotrin (enteric coated tablets). Platelet function and bleeding times were monitored. All doses of ASA significantly inhibited platelet function (p less than 0.05) and increased bleeding times (p less than 0.05) relative to control values. Irrespective of the formulation, maximum increases in bleeding time and platelet dysfunction were obtained with daily doses of about 100 mg, and no further changes were observed with higher doses.

Hypotensive and antiplatelet actions of motapizone depend on dose and time after ingestion

Single doses of motapizone 1 to 10 mg were given to 12 healthy subjects. Before and up to 8 h after each dose the blood pressure and heart rate were measured, as well as thrombocyte aggregation "ex vivo" with collagen, ADP and adrenaline. Motapizone produced a dose-dependent reduction in diastolic blood pressure and an increase in heart rate. These effects were demonstrated with individual variations after 1 to 3 mg and as a rule they were very marked after more than 6 mg. With the highest dose (mean 7.7 +/- 2.3 mg) the diastolic pressure fell by an average of 23% 1 h after medication as compared to with the baseline values. At the same time there was marked inhibition of thrombocyte aggregation, which also became apparent after about 3 mg and increased in proportion to the dose. The inhibition of aggregation peaked after 2 h and had disappeared within 8 h. The inhibition of ADP-induced aggregation was particularly marked.

Inhibition of thrombocyte aggregation by oral motapizone and other drugs

Ten healthy subjects took single oral doses of placebo, 8.8 +/- 1.8 mg motapizone, 40 +/- 13 mg captopril, 25 mg dihydralazine, 20 mg nifedipine and 4.5 +/- 1.1 mg prazosin in random order, and, as the last preparation 500 mg acetylsalicylic acid. Thrombocyte aggregation induced "ex-vivo" with collagen, ADP and adrenaline was measured before and after 60 min. Immediately before each dose, the "threshold concentration" of each agent was determined in each subject, i.e. the concentration producing about 90% of maximal aggregation. After the preparation had been taken, aggregation was induced with 1-, 2- and 4-times the threshold concentration. Both motapizone and also acetylsalicylic acid caused marked inhibition of aggregation at up to 4-times the threshold concentration; the dose ratio was about 1:50. Motapizone produced greater inhibition of the aggregation induced by ADP and acetylsalicylic acid than of that due to collagen. The inhibitory actions after captopril, dihydralazine, nifedipine and prazosin were weak and did not significantly differ from placebo.

Pharmacology of platelet inhibition in humans: implications of the salicylate-aspirin interaction

The current dispute over the effects of "low" vs "high" doses of aspirin should take into consideration the pharmacokinetics of this drug. In fact, different pharmaceutical formulations of aspirin may deliver little or no aspirin to the systemic blood. This was the case, for instance, in healthy volunteers taking 320 mg of compressed aspirin or 800 mg of enteric-coated aspirin. In all instances thromboxane B2 generation in serum was fully inhibited. Platelet cyclooxygenase might therefore be effectively acetylated by exposure to aspirin in the portal circulation, whereas vascular cyclooxygenase could be spared. Thus aspirin formulations ensuring complete first-pass deacetylation should be sought rather than "low" or "high" doses of unspecified aspirin formulations. Regardless of the type and dose of aspirin administered, salicylate is formed and accumulates in the circulation. It may antagonize the effects of aspirin on cyclooxygenase, at least in acute conditions. As an example, after administration of 1 g of salicylate to healthy volunteers, when plasma levels of the drug were about 75 micrograms/ml, the effect of 40 mg iv aspirin (given 40 min later) on platelet cyclooxygenase and aggregation was significantly diminished. In contrast, in patients undergoing saphenectomy, the same dose of salicylate (1 g) gave plasma drug levels of about 25 micrograms/ml; salicylate was unable to prevent the inhibitory effect on platelets of 40 mg iv aspirin (given 1 hr later) but did act on vascular prostacyclin. Thus the combination of salicylate with aspirin at an appropriate dose and blood level ratio may result in almost complete dissociation of the drug's effect on platelets and vessels in man.(ABSTRACT TRUNCATED AT 250 WORDS)

The biochemical pharmacology of thromboxane synthase inhibition in man

Selective inhibitors of thromboxane synthase have two theoretical advantages over inhibitors of the cyclooxygenase enzyme as potential antithrombotic compounds. First, they do not prevent formation of prostacyclin, a platelet-inhibitory, vasodilator compound, coincident with inhibiting thromboxane biosynthesis. Second, the prostaglandin endoperoxide substrate that accumulates in the platelet in the presence of thromboxane synthase inhibition may be donated to endothelial prostacyclin synthase at the site of platelet-vascular interactions (endoperoxide "steal"). Selective inhibition of thromboxane biosynthesis coincident with enhanced prostacyclin formation in vivo has been observed after administration of these compounds to man. Despite these attractive features and the efficacy of these compounds in diverse short-term animal preparations of thrombosis, investigations of their efficacy in human disease have proven disappointing. This may reflect on the importance of thromboxane A2 in the diseases that have been investigated. Alternatively, the lack of drug efficacy may have resulted from either incomplete suppression of thromboxane biosynthesis and/or substitution for the biological effects of thromboxane A2 by prostaglandin endoperoxides during long-term dosing studies. Given that selective inhibition of thromboxane formation can be approached with aspirin, the particular value of these compounds is dependent on enhancing prostacyclin formation. Aspirin inhibits thromboxane-dependent platelet activation, but many platelet agonists are likely to act in concert in vivo and prostacyclin inhibits platelet aggregation induced by both thromboxane-dependent and thromboxane-independent mechanisms. To test the hypothesis that thromboxane synthase inhibitors are efficacious in human disease, compounds of longer duration of action are required. Combination with antagonists of the prostaglandin/thromboxane A2 receptor may be necessary to reveal their full beneficial action.

Plasma concentrations and anti-platelet effects after low dose acetylsalicylic acid

The present study has investigated whether low-dose acetylsalicylic acid (ASA) can inhibit platelet aggregation locally at its site of gastrointestinal absorption without concentrations in the systemic circulation high enough for inhibition of cyclooxygenase. For this purpose platelet aggregation, thromboxane formation as well as ASA plasma concentrations were measured in 8 volunteers before oral intake of 100 mg ASA as well as 20 to 300 minutes thereafter. At each time 5 ml of blood were mixed with 5 ml of blood obtained from a second, untreated volunteer. Aggregation and thromboxane formation were also determined in these mixed blood samples. The same protocol was performed with 4 volunteers after administration of 1500 mg ASA as well as after no drug intake. In a separate experiment the concentration-effect-relationship of ASA was assessed in vitro. One hundred and forty minutes after administration of 100 mg ASA aggregation and thromboxane formation were significantly decreased to 49.4 and 4.5% of the initial values, respectively, whereas in the mixed blood sample aggregation was not impaired. Inhibition of thromboxane formation was constantly 73% of the inhibition observed in the unmixed sample throughout the study period and thus most probably was caused by dilution of the platelets of the untreated volunteer by the inactivated platelets of the ASA-treated volunteer. These data suggest the absence of pharmacologically active drug concentrations in the peripheral blood. ASA plasma concentration was highest after 40 minutes (2.2 +/- 1.6 microgram/ml; n = 5). After the 1500 mg ASA dose platelet function and thromboxane formation decreased to 29.8 and 2.0% of the initial values, respectively. Furthermore, aggregation and thromboxane formation in the mixed blood sample were markedly reduced. Thus, after the high dose of ASA effective plasma concentrations were present in the peripheral circulation. Highest ASA plasma concentrations were 21.1 +/- 8.9 micrograms/ml. IC50 values were 1.00 +/- 0.36 and 0.30 +/- 0.05 microgram/ml for aggregation and thromboxane formation in vitro, respectively. It is concluded that low dose ASA can effectively inhibit platelet function without producing pharmacologically active concentrations in the peripheral circulation.

Cumulative antiplatelet effect of low-dose enteric coated aspirin

Enteric coated aspirin (ECA) at doses of 325-1300 mg is an effective alternative to regular aspirin for inhibition of platelet activity while avoiding gastric irritation. The objectives of this study were to determine: (1) the lowest chronic dose of ECA providing effective inhibition of platelet activities, (2) the time course of the inhibition, and (3) the reappearance of platelet cyclo-oxygenase activity. Seven subjects were studied before and after seven daily doses of 40-325 mg ECA. Serum thromboxane (TX) B2 levels indicated that the lowest dose of ECA resulting in greater than 90% inhibition of platelet cyclo-oxygenase was 80 mg/d. Platelet aggregation and ATP release in response to collagen (1 microgram/ml) and arachidonic acid (1 mM) were abolished and bleeding times were prolonged from 6.1 +/- 1.5 min to 9.7 +/- 2.8 min (mean +/- SD, P less than 0.01). Examination of platelet cyclo-oxygenase activity on a daily basis revealed that 24 h after the first 80 mg dose serum TXB2 had decreased by approximately 60% and was suppressed by more than 90% after four doses. Recovery of platelet cyclo-oxygenase activity after a single 80 mg dose of ECA was delayed for 48-72 h indicating that aspirin reached the systemic circulation. We conclude that chronic inhibition of platelet activity may be achieved in a cumulative manner with 80 mg ECA/d.

Plasma salicylate levels and platelet function after acute and chronic administration of slow-release acetylsalicylic acid (Monobeltin)

The relationships between the antiplatelet effects and the pharmacokinetics of a slow release formulation of acetylsalicylic acid (ASA) have been investigated. After acute intake of 750 mg ASA in a slow-release formulation (Monobeltin), a slow increase in plasma ASA was paralleled by a gradual decrease in certain platelet functions. During chronic medication (750 mg twice daily), ASA was present in plasma at all times accompanied by full inhibition of platelet aggregation. For chronic antiplatelet therapy, this slow release formulation of ASA appears to be very effective, unless rapid inhibition of platelet function must be achieved.

Human pharmacokinetics of analgesics and methods for their determination in biological fluids

The main pharmacokinetic data of analgesics--biological half-lives, apparent volumes of distribution, total body clearances--obtained in humans, and their clinical relevance are summarized. Special emphasis has been given to the analytical methods used for the quantitative determination of these drugs in biological fluids.