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On the Stereoselective Pharmacokinetics of Eflornithine and Prediction of Drug Tissue to Plasma Concentration Ratios

by Jansson Löfmark, Rasmus, PhD

Abstract (Summary)
Eflornithine is one of two registered drugs for the treatment of late-stage human African trypanosomiasis, a uniformly fatal neglected disease with sixty million people are at risk of being infected. Eflornithine is efficacious but the cumbersome intravenous administration leaves numerous patients untreated. A simplified mode of administration, preferably oral, would enable more patients having access to treatment. The trypanostatic agent eflornithine is administered as a racemate where the L – form has a several-fold greater in vitro potency compared to the D – enantiomer. Despite the difference in potency of the enantiomers, the stereoselective pharmacokinetics of eflornithine has not been considered. This thesis aimed to study L – and D – eflornithine pharmacokinetics in the rat, in Caco-2 cells and in late-stage human African Trypanosomiasis patients. A secondary aim was also to develop a general method for predicting drug tissue to plasma concentration ratios. In the rat, eflornithine displayed stereoselective absorption where the more potent L – form had an approximately 50% lower fraction absorbed compared to D – eflornithine. The stereoselective mechanism was not detected in the present Caco-2 cell assay. Late-stage HAT patients, treated with racemic oral eflornithine, had an approximate 50% lower exposure of L – compared to D – eflornithine, similar to those in rat. The findings suggested that previous attempts to develop an oral eflornithine dosage regimen have failed due to unfavorable stereoselective absorption. High plasma exposure for both L – and D – eflornithine were significantly correlated to the probability of being cured. For the secondary aim of this thesis, the novel method to predict drug tissue distribution, based on a measured volume of distribution in combination with drug lipophilicity performed reasonably well. Predicted drug tissue to plasma concentration ratios agreed reasonably well with experimentally determined values with 85% being within a factor of ±3 to experimental values (n=148). In conclusion, this thesis present the stereoselective pharmacokinetics of eflornithine that can give information on whether a much needed oral eflornithine can be developed or not. In addition, the thesis also presents a general method to predict drug tissue to plasma concentration ratios.
Bibliographical Information:

Advisor:

School:Göteborgs universitet

School Location:Sweden

Source Type:Doctoral Dissertation

Keywords:MEDICINE; Human African trypanosomiasis; HAT; Pharmacokinetics; NONMEM; Stereoselectivity; Eflornithine; Tissue distribution

ISBN:978-91-628-7818-4

Date of Publication:01/01/2009

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