Abstract
PEG-Asparaginase is an important part of many treatment protocols for ALL. In many centres Asparaginase activity is measured after administration of PEG-asparaginase. However, a predictive pharmacokinetic model is lacking. Such a model would be helpful for dose adjustment and decision making when to switch to another preparation due to the development of inactivating antibodies. Previously described models like linear one-compartment [V.I. Avramis et al., Blood 2002, 99: 1986–1994] or a one-compartment Michaelis-Menten model [H.J. Mueller et al., Cancer Chemother. Pharmacol. 2002, 49, 149–154] describe the data sufficiently for one dose alone, but cannot account for the phenomenom that the time to reach a lower activity limit after administration is not increasing with increasing the dose. Therefore, we analysed 1189 serum activity measurements in 185 children from the ALL-BFM 95 study. Patients received 500, 1000 or 2500 U/m2 PEG-Asp on up to 9 occasions. Serum asparaginase activity was measured using a semi-automatic enzymatic assay with a limit of quantification of 2 U/l [C. Lanvers et al. Anal. Biochem. 2002, 309, 117–126]. Data analysis was done using nonlinear mixed effects modelling (NONMEM Vers. V). Different models like Michaelis-Menten, linear first-order, Weibull and gamma models were tested. The best model applicable to all dosing groups was a modified first-order one-compartmental model with clearance increasing with time according to the formula: Cl = Cli*exp(0.0853*t) with Cli=initial clearance, and t=time. Addition of a second compartment did not improve the model. A typical activity-time course of a patient receiving 1000 U/m2 is shown below displaying the typical shape observable in all patients and in all doing groups. The population parameters found were: Volume of distribution (V) 1.05 ± 27.3% l/m2, Cli 60.3 ± 70.8% ml/day/m2 (mean ± interindividual variability). Interoccasion variability was substantial with 0.223 l/m2 for V and 37.7 ml/day/m2 for Cl, respectively. A subgroup of one third of the patients is identifiable showing a high clearance probably due to the development of inactivating antibodies. Drug monitoring of serum PEG-Asparaginase activity is required to identify these patients who do not benefit from PEG-Asp therapy. The pharmacokinetic model presented here should help to reduce the number of required serum samples per patient.
Figure Figure
Pharmacokinetics of Ethambutol under Fasting Conditions, with Food, and with Antacids
