AN EXAMPLE OF APPLICATION OF THE POPULATION APPROACH TO TOXICOLOGICAL STUDIES

1. AN EXAMPLE OF APPLICATION OF THE POPULATION APPROACH TO TOXICOLOGICAL STUDIES F. Fiorentini1, M. Simeoni2, I. Poggesi1, G. Westerberg1, M. Rocchetti1 1 Nerviano Medical Science, Nerviano, Italy, 2University of Pavia, Pavia, Italy Abstract Introduction Usually, pharmacokinetic information is obtained from satellite groups of animals during toxicological studies. This approach provides a characterization of the toxicological findings only in relation to the average estimates of the systemic exposure (typically Cmax, tmax, AUCs) in the satellite groups. In this way, the individual toxicological observations remain difficult to interpret in absence of a direct comparison with the exposure of the main study animals. A sparse sampling design together with a population analysis could however allow merging the individual toxicological observations with the corresponding estimate of the systemic exposure in the same animal. This allows a considerable expansion of the degree of information obtainable from the toxico-pharmacokinetic profile of the compound and, in addition, could provide reduction of costs in the subsequent phases of development. An example of a real application of this kind of approach is provided here. Three typical studies that constitutes part of a toxicological program of a new intravenous anticancer drug were considered: an acute study after single administration, a short term study (7 days repeated administration) and a cyclic dose study. The data were analysed by using a 2-compartment model with elimination from the central compartment and a proportional residual error. The results compared with those obtained with more traditional approaches confirmed the added value of the population analysis. During the pharmacotoxicological evaluation of a drug, the information on the exposure of a drug is often obtained from ‘satellite groups’ of animals. Due to this, the characterization of the toxicological findings is provided only in relation to the average estimates of the systemic exposure (typically Cmax, t max, AUCs). In this way, however, the individual toxicological observations remain difficult to be interpreteddue to the absence of a direct estimation of the exposure achieved in the individual animals. Sparse sampling techniques together with population pharmacokinetic analysis is able to merge the individual toxicological observations with the relevant estimates of the systemic exposure in the same animal, thereby allowing a smarter use of the information obtained from the toxico-pharmacokinetic profile of the compound1. In addition, using this approach, costs can be reduced in the subsequent phases of drug development when long term toxicity studies are needed. The data • Anticancer drug, IV bolus administration • Tox program in rats • Study 1 • 7-day repeated dose at 2.5, 5.0 and 10 mg/kg/day. Extensive sampling: 3 mouse/dose. Blood sampling at: pre-dose, 5 min, 0.5,1,3, 6 and 24h after 1st and 7th administration. Limited sampling performed on the main toxicological groups of animal. 8rats/dose, 1 sampling time per animal distributed between the following: 5 min, 0.5, 1, 3, 6 and 24h after 1st and 7th administration. • Study 2 • Three cycles of 3-day treatment separated by 5-day interval. Doses: 2.5, 5 and 10 mg/kg/day. Limited sampling on main toxicological groups. 12rats/dose, 1 sampling time/animal/cycle of treatment distributed at the following time points: 5 min, 0.5, 1, 3, 6 and 24h. The approach and the model • Working scheme: • 1. Analysis of study 1 (extensive sampling) for establishing the PK model. • 2. Two-stages approach to obtain initial estimates of the subject-specific parameters. • 3. Simultaneous analysis of studies 1 and 2 using a ‘population approach’ (analyeses carried out with WinNonMix version 2.0.1 and NONMEM version V). • 4. Comparison with the parameters obtained using the non-compartmantal analysis (NCA) from the average plasma concentration-time data • Method of analysis: • - 2-compartment open model with firts order elimination. • - Exponential terms for describing subject-specific random effects. • - Proportional residual error. • - First-order linearization. Results Study 1-limited sampling Study1-extensive sampling Study 2 Conclusions • Winnonmix provided results in good agreement with NONMEM analysis • Overall, there was an excellent agreement between the pharmacokinetic parameters obtained using the population approach and the non-compartmental analysis. The population approach allowed analysing studies with a repeated schedule of treatment with only 2 or 3 sampling times per animal and consequent reduction of disturbance. • In addition, it was made possible the correlation of the individual PK profiles with the corresponding toxicological findings elicited by the different schedule of treatment. • Finally, the possibility of using a limited sampling strategy allowed to plan additional long term studies with a substantial reduction of the cost/time of bioanalysis. • Overall, the use of population approaches confirmed to be useful also in preclinical development. • 1Poggesi I, Simeoni M. Population pharmacokinetics in preclinical development. An application to toxicokinetics. First Italian symposium on population Pharmacokinetics. Turin, March 19, 2002. PAGE Uppsala,17-18 June 2004