Resistance Development: A Major Piece in the Jigsaw Puzzle of Tumor Size Modeling

Mathematical models of tumor size (TS) dynamics and tumor growth inhibition (TGI) need to place more emphasis on resistance development, given its relevant implications for clinical outcomes. A deeper understanding of the underlying processes, and effective data integration at different complexity levels, can foster the incorporation of new mechanistic aspects into modeling approaches, improving anticancer drug effect prediction. As such, we propose a general framework for developing future semi-mechanistic TS/TGI models of drug resistance.

FOLFOX4 plus cetuximab treatment and RAS mutations in colorectal cancer

BACKGROUND

The OPUS study demonstrated that addition of cetuximab to 5-fluorouracil, folinic acid and oxaliplatin (FOLFOX4) significantly improved objective response and progression-free survival (PFS) in the first-line treatment of patients with KRAS exon 2 wild-type metastatic colorectal cancer (mCRC). In patients with KRAS exon 2 mutations, a detrimental effect was seen upon addition of cetuximab to FOLFOX4. The current study reports outcomes in subgroups defined by extended RAS testing.

PATIENTS AND METHODS

Samples from OPUS study KRAS exon 2 wild-type tumours were reanalysed for other RAS mutations in four additional KRAS codons (exons 3-4) and six NRAS codons (exons 2-4) using BEAMing. A cutoff of ⩾5% mutant/wild-type sequences was selected to define RAS status; we also report an analysis using a cutoff based on the technical lower limit for mutation identification (0.1%).

RESULTS

Other RAS mutations were detected in 31/118 (26%) evaluable patients. In the extended analysis of RAS wild-type tumours (n=87), objective response was significantly improved by addition of cetuximab to FOLFOX4 (58% versus 29%; odds ratio 3.33 [95% confidence interval 1.36-8.17]; P=0.0084); although limited by population size, there also appeared to be trends favouring the cetuximab arm in terms of PFS and overall survival in the RAS wild-type group compared with the RAS evaluable group. There was no evidence that patients with other RAS mutations benefited from cetuximab, but small numbers precluded precise estimations of treatment effects. In the combined population of patients with any RAS mutation (KRAS exon 2 or other RAS), a clear detrimental effect was associated with addition of cetuximab to FOLFOX4.

CONCLUSION

Patients with RAS-mutant mCRC, as defined by mutations in KRAS and NRAS exons 2-4, derive no benefit and may be harmed by the addition of cetuximab to FOLFOX4. Restricting cetuximab administration to patients with RAS wild-type tumours will further tailor therapy to maximise benefit.

Effects of selective COX-2 inhibition on prostanoids and platelet physiology in young healthy volunteers

BACKGROUND

Selective inhibitors of cyclooxygenase-2 (COX-2) called coxibs, are effective anti-inflammatory and analgesic drugs. Recently, these drugs were associated with an increased risk for myocardial infarction and atherothrombotic events. The hypothesis of thromboxane-prostacyclin imbalance has been preferred to explain these unwanted effects.

METHODS

We studied the effects of 14 days intake of rofecoxib (25 mg q.d.), celecoxib (200 mg b.i.d.), naproxen (500 mg b.i.d.) and placebo in a randomized, blinded, placebo-controlled study in young healthy volunteers (median age 25-30 years, each group n = 10). We assessed prostanoid metabolite excretion (PGE-M, TXB(2), 6-keto-PGF(1alpha), 11-dehydro-TXB(2), 2,3-dinor-TXB(2), and dinor-6-keto-PGF(1alpha)), the expression of platelet activation markers (CD62P, PAC-1, fibrinogen), platelet-leukocyte formation, the endogenous thrombin potential, platelet cAMP content and plasma thrombomodulin level.

RESULTS

Naproxen suppressed biosynthesis of PGE-M, prostacyclin metabolites and thromboxane metabolites and thrombomodulin levels. In contrast, both coxibs had an inhibitory effect only on PGE-M, 6-keto-PGF(1alpha), and on dinor-6-keto-PGF(1alpha), whereas TXB(2), 2,3-dinor-TXB(2) and 11-dehydro-TXB(2) excretion were unaffected. None of the coxibs exerted significant effects on the expression of platelet activation markers, cAMP generation, platelet-leukocyte formation, or on thrombomodulin plasma levels. Interestingly, platelet TXB(2) release during aggregation was enhanced after coxib treatment following arachidonic acid or collagen stimulation.

CONCLUSION

In young healthy volunteers coxibs inhibit systemic PGE(2) and PGI(2) synthesis. Platelet function and expression of platelet aggregation markers are not affected; however, coxibs can stimulate TXB(2) release from activated platelets. Combined decrease in vasodilatory PGE(2) and PGI(2) together with increased TXA(2) in proaggregatory conditions may contribute to coxib side effects.

The Retention Index Test Homburg (RTH-II): Response to clopidogrel and comparison with aggregometry and CD62P-expression

The Retention Index Test Homburg (RTH-II) is quoted to detect effects of shear stress on platelets, which involve ADP receptor signaling. RTH-II might be a tool for monitoring antiplatelet therapy for compounds that interfere with ADP induced platelet activation and secretion. In a series of investigations, we used an ADP (2 microM) triggered RTH-II in parallel with light-transmittance aggregometry and flow cytometry in subjects before and after clopidogrel. A loading dose of 225 mg clopidogrel leads to a significant reduction (p < 0.01) in the ADP-stimulated retention index (RI) from 69 +/- 15 to 48 +/- 21%, in the aggregation response to 5 microM ADP (from 50 +/- 20 to 29 +/- 21%) and the expression of CD62P (from 64 +/- 11 to 41 +/- 17%). Correlation analysis showed that the RI corresponds significantly to CD62P-expression (p < 0.01) but not to aggregation. We also found a strong correlation (p < 0.01) between the ADP-stimulated RI and the expression of CD62P after stimulation with 2 microM ADP, whereas no correlation was seen for RI vs. binding of PAC-1 or aggregation. Platelets not retained in the filter had lower CD62P expression than measured in the sample before the filter passage (54 vs. 35%). A direct interaction of CD62P with platelet ligands might lead to enhanced retention in RTH and explain the correlation of RI with CD62P expression. The RTH-II might be a simple and easy to handle platelet function assay for monitoring effects on P2Y(12)-inhibitors on platelet degranulation, perhaps in addition to aggregometry.

P-selectin (CD62p) and P-selectin glycoprotein ligand-1 (PSGL-1) polymorphisms: minor phenotypic differences in the formation of platelet-leukocyte aggregates and response to clopidogrel

INTRODUCTION

Formation of platelet-leukocyte aggregates (PLA) via the CD62p-ligand PSGL-1 represents an important mechanism by which leukocytes contribute to thrombotic and inflammatory events. Deficient variants (namely the Thr715Pro-SNP for CD62p and a VNTR-polymorphism for PSGL-1) might affect PLA formation and probably the response to clopidogrel (which is known to reduce PLA-formation).

METHODS

CD62p-expression, PLA-formation and the up-regulation of CD11b before (V1) and 24 hours after (V2) a loading dose of clopidogrel 225 mg were investigated in ten wild-type controls, ten heterozygote carriers of the Thr715Pro-allele and five carriers of the rare PSGL-1 B-allele (2 A/B and 3 B/B).

RESULTS

CD62p-expression before application of clopidogrel and under clopidogrel treatment in Pro715-haplotype samples did not differ from that in wild-type subjects. The response to clopidogrel was similar in all subjects. Pro715-carriers exhibited a significantly lower percentage of monocytes with platelets attached prior to clopidogrel treatment (ADP: median 22 (1st-3rd quartile 20-23), TRAP: 27 (25 - 38)) compared to the wild-type (ADP: 37 (31-44), TRAP: 55 (37-63)). These differences were not present under clopidogrel, and CD11b-expression was significantly reduced in both groups (controls: median 150 (quartile range 121 - 230) to 113 (121 - 230), Pro715-carriers: 147 (139 - 221) to 126 (109 - 170); all values refer to mean fluorescence intensity). Statistical analysis was not done in the case of PSGL-1 B-allele carriers, but PLA-formation before and under clopidogrel was always at the bottom end of the range seen in the control group and the Pro715-carriers or even below this range.

CONCLUSION

Minor phenotypic differences in the CD62p-PSGL-1 axis could be demonstrated in this study. Carriers of these polymorphisms showed a full response to clopidogrel comparable to that in control subjects.

Effects of antiplatelet agents on platelet-induced thrombin generation

OBJECTIVES

The influence of antiplatelet agents on platelet-induced thrombin generation may increase the risk of bleeding. Assessment of the endogenous thrombin potential (ETP), is therefore a parameter deserving attention in early pharmacodynamic studies with antiplatelet drugs. The aim ofthis study was to assess whether an automated ETP-assay can be used to determine possible inhibitory effects of antiplatelet drugs on platelet-associated thrombin generation.

METHODS

We first characterized the in vitro dose-response relationship of several platelet agonists (ADP, collagen, U46619, TRAP (amino acid sequence: SFLLRNP) and tissue factor (TF) using the generation of ETP. One submaximal concentration of each agonist was then used to assess the influence of in vivo treatment with aspirin (single oral dose of 500 mg as inhibitor of thromboxane synthesis) and clopidogrel (given orally for 6 days, as an inhibitor of the purinergic P2Y12-receptor on platelets) and in vitro treatment with abciximab (which inhibits the platelet glycoprotein IIb/IIIa-receptor for fibrinogen), on the ETP.

RESULTS

The effect of TF and the other platelet inducers on thrombin generation was dose-dependent. Repeat measurements on samples from the same subject, with the same inducer concentration on 2 different occasions showed a variability of approx. 22% (absolute difference between 2 measurements as % of mean). The coefficient on variation of repeat measurements of one sample varied between 7% and 17%, depending on the inducer. After a single dose of aspirin, ETP was reduced by 25-40%, depending on the platelet activating agent used. The reduction in ETP with abciximab in vitro was more pronounced. In contrast, TF-induced ETP was not influenced by aspirin or abciximab. Clopidogrel, administered for 6 days, reduced the ETP by 60% when platelets were stimulated using 20 microM ADP, whereas collagen-induced ETP and TF-induced ETP remained unchanged.

CONCLUSIONS

The ETP-method is a sensitive and reproducible method for the detection of drug effects on platelet-induced thrombin generation of high throughput, and can be recommended for studies on the pharmacodynamic profile of drugs interfering with platelet function.

Angiotensin II infusion increases plasma erythropoietin levels via an angiotensin II type 1 receptor-dependent pathway

BACKGROUND

Angiotensin-converting enzyme inhibitors (ACEIs) have been shown to lower hematocrit and erythropoietin (EPO), but a direct link between angiotensin II (Ang II) and EPO in humans has not been shown.

METHODS

Placebo or Ang II was infused for six hours in nine healthy male volunteers with and without blockade of the Ang II subtype 1 receptor (AT1R). EPO concentrations were measured 3, 6, 12, and 24 hours after the start of the infusion.

RESULTS

Ang II raised the mean arterial pressure by about 20 mm Hg. Consistent with the known diurnal variation, EPO levels rose significantly (P < or = 0.02) during the day in all groups. During Ang II infusion, EPO levels rose to significantly higher levels after 6 and 12 hours compared with placebo [9.9 +/- 3.5 vs. 7.2 +/- 3.1 mU/mL (3 h, P = NS); 16.9 +/- 4.5 vs. 8.8 +/- 3.7 mU/mL (6 h, P = 0.01); 17.0 +/- 8.6 vs. 11.1 +/- 4.7 mU/mL (12 h, P = 0.01)] and returned to baseline after 24 hours (7.9 +/- 3.8 vs. 10.6 +/- 8.6 mU/mL, P = NS). With AT1R blockade, blood pressure remained normal during Ang II infusion, and EPO levels were never significantly different from placebo [6.8 +/- 4.8, 10.5 +/- 5.6, 13.1 +/- 9.0, and 12.4 +/- 10.1 mU/mL at 3, 6, 12, and 24 h after infusion, respectively, P = NS].

CONCLUSIONS

Ang II increases EPO levels in humans. This increase requires the participation of AT1R.

Platelet CD62 expression and PDGFAB secretion in patients undergoing PTCA and treatment with abciximab

AIMS

To investigate a correlation of the platelet activation marker CD62 and secretion of the growth factor PDGF from platelets in coronary patients under therapy with the GPIIb/IIIa-inhibitor abciximab.

METHODS

Flow cytometric assessment of fibrinogen binding (GPIIb/IIIa-binding site) and CD62 expression, as well as PDGF release of human platelets (immunoassay) and platelet aggregation with 20 microM ADP and 2 microg ml(-1) collagen were evaluated in nine patients with stable coronary artery disease. Patients were undergoing elective balloon angioplasty and were treated with aspirin (100 mg day(-1)), heparin (ACT < 220 s) and abciximab (bolus and infusion over 12 h). Blood samples were obtained before initiation of abciximab therapy (under aspirin and heparin) (I), 3 h after angioplasty under abciximab (II) and 12 h after termination of abciximab infusion (III).

RESULTS

Compared with sample I before abciximab therapy, fibrinogen binding was reduced to 37% (+/- 34 s.d., P < 0.05) (II) and 55% (+/- 40 s.d., P < 0.05) (III). Reduced fibrinogen binding also led to a significant reduction of the aggregation response to ADP (down to 37% +/- 20) and collagen (down to 0%). Mean fluorescence intensity of CD62-expression was 78 units (+/- 20 s.d.) (I), 72 units (+/- 14 s.d.) (II) and 64 units (+/- 12 s.d., P < 0.05) (III). PDGF release from isolated, washed platelets was 99 (+/- 33 s.d.) ng/10(9) platelets at (I), 82 (+/- 31 s.d.) ng/10(9) platelets and 96 (+/- 30 s.d.) ng/10(9) platelets.

CONCLUSIONS

The results indicate that despite a strong reduction of GPIIb/IIIa-binding and platelet aggregation, CD62 as a marker of platelet secretion and the secretion product PDGF were only slightly reduced under abciximab treatment. No direct correlation between CD62 expression and PDGF release could be demonstrated.

In vitro dose response to different GPIIb/IIIa-antagonists: inter-laboratory comparison of various platelet function tests

AIMS

The aim of this study was to assess the inter- and intra-laboratory variation of the concentration-response to the GPIIb/IIIa-antagonists abciximab and eptifibatide on platelet aggregometry and to compare results with flow cytometric tests as well as the rapid platelet function analyser (RPFA).

METHODS

In five different laboratory sites, blood from three to five healthy donors was spiked with abciximab or eptifibatide, followed by the assessment of: (1) aggregometry (anticoagulant: sodium citrate 3.18% or hirudin 5 microg/ml); (2) flow cytometry (fibrinogen binding or PAC1-expression), or (3) RPFA. Dose-response curves were established on the basis of a sigmoidal Imax)-model [I=(Imax)*Cg)/(IC50g + Cg)].

RESULTS

For citrated blood, aggregation induced by 20 microM ADP was blocked up to 100% by both GPIIb/IIIa-antagonists, IC50 values varied between 0.11-0.22 microg/ml for eptifibatide and 1.25-2.3 microg/ml for abciximab. I(max) of the response to 5 microg/ml collagen ranged from 46% to 100%, and IC50 values varied between 0.28-0.34 microg/ml for eptifibatide and 2.3-3.8 microg/ml for abciximab. In hirudinized blood, IC50 values for eptifibatide were 1.5- to 3-fold higher than those obtained with citrated plasma. Inhibition of PAC1-expression by abciximab (IC50) 0.84 microg/ml) showed results similar those of the RPFA (approx. 1.0 microg/ml); larger differences between PAC1 and RPFA results were observed for eptifibatide. Based on aggregometry, eptifibatide concentrations for 80% inhibition varied from 0.27 to 0.55 microg/ml, and were considerably less when the RPFA was taken as basis (0.15 or 0.22 microg/ml). A similar pattern was observed for abciximab.

CONCLUSIONS

We found quite a low inter- and intra-laboratory variation in the in vitro pharmacodynamic characterization of GPIIb/IIIa-antagonists by aggregometry, making results of these tests obtained from different laboratories during clinical trials at least comparable. The RPFA exhibits a higher sensitivity to inhibitory GPIIb/IIIa-effects, in keeping with the "real" inhibition of the activated receptor (PAC1) as assessed with more elaborate flow cytometry.

Effect of angiotensin II infusion with and without angiotensin II type 1 receptor blockade on nitric oxide metabolism and endothelin in human beings: a placebo-controlled study in healthy volunteers

BACKGROUND

Angiotensin II has been shown to induce the synthesis of endothelium-derived relaxing factor nitric oxide (NO) and endothelin in vitro. In human beings, to our knowledge, no data on NO release in response to angiotensin II and on the influence of angiotensin II type 1 receptor blockade have been published.

METHODS

In a placebo-controlled study in nine healthy volunteers, angiotensin II was administered intravenously for 6 hours with and without pretreatment with valsartan, a specific angiotensin II type 1 receptor antagonist. NO (NO2 + NO3) and endothelin plasma concentrations, clearance values for inulin and paraaminohippuric acid and NO (NO2 + NO3) excretion in urine were determined.

RESULTS

During angiotensin II infusion NO plasma concentrations remained unaltered compared with placebo after 3 hours: 6.66 +/- 5.49 versus 5.56 +/- 3.09 micromol/L (P = ns) but increased after 6 hours: 18.36 +/- 20.02 versus 7.13 +/- 3.87 micromol/L (P < .04). The same was noted after pretreatment with valsartan: 7.61 +/- 5.69 versus 5.56 +/- 3.09 micromol/L (P= ns) after 3 hours, and 21.70 +/- 11.51 versus 7.13 +/- 3.87 micromol/L (P = .02) after 6 hours. In urine fractional NO excretion decreased after angiotensin II infusion: 0.87 +/- 0.72 versus 0.95 +/- 0.71 (P = .5) during the first 3 hours, and 0.44 +/- 0.39 versus 0.78 +/- 0.43 (P = .01) during the following 3 hours. After valsartan pretreatment the decrease in fractional urinary NO excretion began earlier: 0.40 +/- 0.15 versus 0.95 +/- 0.71 (P = .04) during the first 3 hours, and 0.17 +/- 0.11 versus 0.78 +/- 0.43 (P = .01) during the following 3 hours. Endothelin plasma concentrations showed no difference after angiotensin II infusion with or without valsartan.

CONCLUSIONS

Our observations demonstrate for the first time that angiotensin II increases NO plasma concentrations in human beings and that this response is not mediated by angiotensin II type 1 receptor. In spite of increased NO plasma levels, urinary NO excretion decreased. Endothelin plasma levels remained unchanged during angiotensin II infusion.

Thrombolytics: drug interactions of clinical significance

Thrombolytic agents activate plasminogen and induce a systemic fibrinolytic and anticoagulant state. Interaction of fibrinolysis with coagulation and platelet aggregation might be important for synergistic interactions with other antiplatelet or anticoagulant drugs. Thrombolytic agents are most often used in patients with coexisting cardiovascular medication, including various antihypertensives, beta-blocking agents, nitrates and aspirin (acetylsalicylic acid). In acute coronary syndromes, anticoagulants and antiplatelet compounds such as clopidogrel or glycoprotein IIb/IIIa receptor antagonists might be given. Inducers or inhibitors of the cytochrome P450 system are not reported to affect the pharmacokinetics of any thrombolytic agent. Since the elimination of the recombinant plasminogen activators saruplase and alteplase is dependent on liver blood flow, drugs affecting hepatic blood flow could theoretically affect the hepatic clearance of these agents. In fact, a reduction in thrombolytic activity has only been demonstrated for alteplase with nitroglycerin (glyceryl trinitrate). Pharmacodynamic interactions occur more often. The additive and beneficial effect of aspirin as concomitant therapy to thrombolysis has been demonstrated without excessive bleeding rates. No data are available on the interaction between ticlopidine or clopidogrel and thrombolytic agents in humans. Anticoagulation by heparin concomitantly with thrombolysis improves the patency rate of the occluded coronary vessel, but bleeding complications are seen more frequently. Although there has been no controlled study on the interaction between oral anticoagulants and thrombolytic agents, patients with myocardial infarction who were taking an oral anticoagulant before admission seem to be at higher risk for intracranial haemorrhage during thrombolytic therapy. Currently, no recommendations can be given for possible dose adjustment of thrombolytic therapy in patients receiving antiplatelet comedication. For comedication with heparin, it has been advised to monitor activated partial thromboplastin time frequently and to avoid values >2.5-fold normal. Patients receiving thrombolytic treatment should be monitored frequently for bleeding and the physician should be aware of any comedication exerting antiplatelet (e.g. aspirin, clopidogrel and ticlopidine) or anticoagulant (e.g. warfarin) effects.

[Dose-response relationship: relevance for medical practice]

Dose-finding studies are performed routinely in patients and--if appropriate surrogate models exist--also in healthy volunteers. Such studies aim at establishing the optimal dose range for further clinical studies on the efficacy and the risk-benefit ratio of a new drug. The dose-response relationship of a drug is most often described by a sigmoidal curve. Its parameters include the mean effective dose, the maximal effect and the steepness. Interpretation of such curves should be done in the context of the intended clinical indications of the drug. The risk-benefit ratio of a drug can be assessed by overlapping the dose-response curve of wanted and unwanted clinical effects, again, any overlapping (which can be described e.g. by the therapeutic index) should be seen in the context of the indication and available therapeutic alternatives.

Ex vivo--in vitro interaction between aspirin, clopidogrel, and the glycoprotein IIb/IIIa inhibitors abciximab and SR121566A

OBJECTIVES

To assess the interaction between aspirin and clopidogrel in healthy male volunteers and the interaction of the glycoprotein IIb/IIIa (GPIIb/IIIa) inhibitors abciximab and SR121566A with blood from those pretreated subjects (ex vivo-in vitro).

METHODS

Aspirin (300 mg/day), clopidogrel (75 mg/day), or the combination of both drugs were administered orally for 8 days. Group 1 (n = 5) started with aspirin and group 2 (n = 5) with clopidogrel. From day 4 to day 8, subjects of both groups received the combined treatment. Blood from these subjects was spiked with abciximab (0.5 and 1.5 microg x mL(-1)) and SR121566A (31 and 62 ng x mL(-1)).

RESULTS

In vivo, average bleeding times were 6.8 minutes at baseline, 20.3 minutes for clopidogrel alone (P < .01), 10.9 minutes for aspirin alone (difference not significant), and 24.0 minutes (P < .01) for the combined treatment. Fibrinogen binding to the platelet GPIIb/IIIa receptor was reduced for aspirin to 69% (difference not significant), to 63% for clopidogrel (difference not significant), and to 63% for the clopidogrel plus aspirin combination (P < .01). CD62 expression as a marker of platelet granular secretion was reduced to 66% by clopidogrel (P < .01) and to 41% by the combination of clopidogrel and aspirin; aspirin alone had no effect. In vitro, with pretreatment with aspirin and clopidogrel, inhibitory effects of the GPIIb/IIIa inhibitors on fibrinogen binding were additive to changes observed with aspirin or clopidogrel alone. No effect on CD62 expression was observed with either GPIIb/IIIa inhibitor. Aspirin and clopidogrel reinforced effects of the GPIIb/IIIa inhibitors on adenosine diphosphate (5 micromol/L)-induced aggregation in an additive manner, a supra-additive effect was observed with collagen (2 microg x mL(-1))-induced aggregation.

CONCLUSION

The augmentation of the antiaggregatory effects of GPIIb/IIIa inhibitors by aspirin and clopidogrel and the lack of antisecretory effects of GPIIb/IIIa inhibitors may favor their combination with clopidogrel.

Differential in vitro effects of the platelet glycoprotein IIb/IIIa inhibitors abixicimab or SR121566A on platelet aggregation, fibrinogen binding and platelet secretory parameters

The aim of this study was to compare fibrinogen binding, inhibition of platelet aggregation and secretory potential of the MAb abciximab (0.5-5 microg/mL) and the peptidomimetic compound SR121566A (15-250 ng/mL) in vitro in whole blood. Fibrinogen binding was followed by flow cytometry; platelet function was evaluated by light transmittance and by impedance aggregometry. Secretory functions of platelets were evaluated using ATP as marker for early secretion by dense granulae and P-selectin (CD62) for alpha-granular secretion as well as CD63 for lysosomal degranulation. Results showed that fibrinogen binding induced by 5 microM TRAP was maximally inhibited greater than 80% at 3 microg/mL abciximab or at 250 ng/mL SR121566A. At these concentrations of antagonists, platelet aggregation induced by 5 microM ADP or 2 microg/mL collagen was inhibited completely. Expression of CD62 was reduced 34% with abciximab or 15% with SR121566A; CD63 expression was reduced 22% with both agents. With both agents, the EC50 for inhibition of CD62 and CD63 expressions was in similar magnitudes than the EC50 for fibrinogen binding inhibition. With 3 microg/mL abciximab, ATP secretion was maximally reduced to 50% of the control, whereas SR121566A at 250 ng/mL had no inhibitory effect on this parameter. A slight increase in ATP secretion was seen with 0.5 microg/mL abciximab and with SR121566A in concentrations of less than 45 ng/mL. The data suggest a discoupling between the anti-aggregatory and the antisecretory effects of IIb/IIIa antagonists. Because it is not established to what extend CD62 or CD63 expression can be reduced by any means, the reduction by 20-30% obtained by 3 microg/mL abciximab or 250 ng/mL SR121566A might already be the maximum possible inhibition by these agents.

[Dose-response relation: relevance for clinical practice]

Dose-finding studies are performed routinely in patients and--if appropriate surrogate models exist--also in healthy volunteers. Such studies aim at establishing the optimal dose range for further clinical studies on the efficacy and the risk-benefit ratio of a new drug. The dose-response relationship of a drug is most often described by a sigmoidal curve. Its parameters include the mean effective dose, the maximal effect and the steepness. Interpretation of such curves should be done in the context of the intended clinical indications of the drug. The risk-benefit ratio of a drug can be assessed by overlapping the dose-response curve of wanted and unwanted clinical effects, again, any overlapping (which can be described e.g. by the therapeutic index) should be seen in the context of the indication and available therapeutic alternatives.