2006 Background: QT interval prolongation associated with “torsades de pointes” (TdP) has been a common cause of withdrawal from the market for several promising drugs. We determined the degree of QT prolongation in patients treated within a phase I study with a thioxanthone derivative known to have arrhythmogenic potential. Methods: Clinical data and serial ECGs from 31 patients with advanced tumors who received 86 courses of investigational drug were prospectively reviewed. The drug has been administered intravenously over 24 hours every 3 weeks. Patients have been on a 24 hour Holter monitor until 24 hours after infusion was complete. Three baseline ECGs were done and compared to those every 6 hours during therapy and once 6 hours after the infusion was complete. ECGs were read at a central lab according to a standard protocol. All QT measurements were then corrected for heart rate (QTc) using Bazett’s formula (QTc = QT interval divided by the square root of the R-R interval). Results: Overall,843 ECG tracings were obtained, all evaluable for analysis. No basal ECG showed significant abnormalities. Prolonged QT intervals developed in 2 patients without clinical symptoms (1 patient had intervals 500 milliseconds).In both cases it was associated with the maximum concentration of the drug. Compared with baseline, the QTc interval was prolonged by 30 to 60 milliseconds in 20% of total tracings, and by more than 60 milliseconds in 2% of ECGs. In patients receiving multiple courses, QTc intervals returned to pretreatment levels before the second course. Conclusions: The assessment of QTc prolongation was a major effort in this study but produced an accurate information about such event. In phase I study such an effort is justified when arrhythmogenicity is suspected. The timing of collection of ECGs should be guided by the available preclinical information about the pharmacokinetic profile of the drug. Nevertheless uncertainty remains regarding the specific relationship between the degree of QT prolongation and the risk of life-threatening arrhythmias. The decision to use the drug ultimately has to be based on an estimation of the perceived risk relative to expected benefits for patients. No significant financial relationships to disclose.
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