Species differences in troxacitabine pharmacokinetics and pharmacodynamics: implications for clinical development.
OBJECTIVE
Troxacitabine is the first unnatural L-nucleoside analog to show potent preclinical antitumor activity and is currently under clinical investigation. Significant differences in troxacitabine toxicity between mice, rats, monkeys, and humans were observed during preclinical and clinical evaluations. To better understand the different toxicity and efficacy results observed between the human xenograft mouse tumor models used for preclinical assessment and the clinical study results, the pharmacodynamics and pharmacokinetics of troxacitabine were reassessed in murine and human models.


METHODS
Clonal and thymidine incorporation assays were used to investigate the in vitro antiproliferative activity of troxacitabine on a selected panel of mouse and human tumor cell lines and normal hemapoietic cells. Analysis of the intracellular metabolites of [14C]troxacitabine was determined in mouse and human T-lymphocytes obtained from peripheral blood. The antitumor efficacy of troxacitabine administered either as single or repeated high-dose bolus administrations or as low-dose continuous infusions was evaluated in the human colon HT-29 xenograft model. We also determined plasma concentrations of troxacitabine using the different administration schedules.


RESULTS
Five to nine hundred-fold lower concentrations of troxacitabine were required to inhibit cell growth in human compared with murine tumor and normal hemapoietic cell lines. Furthermore, the sensitivity of cells of both species to troxacitabine was strongly time dependent, requiring >24 hours exposure for maximum activity. Analysis of the intracellular metabolites of [14C]troxacitabine in T-lymphocytes obtained from peripheral blood revealed subsequently higher levels of mono-, di-, and triphosphates in human compared with mouse. Antitumor efficacy studies revealed that prolonged exposure schedules (up to 6 days) showed equivalent efficacy to repeated high-dose bolus administrations. Five-day continuous infusion of 20 mg/mL troxacitabine via subcutaneous implanted mini-osmotic pump maintained systemic concentrations of 262 ng/mL (1.2 micromol/L) for the duration of administration, which are clinically achievable plasma concentrations, and led to significant antitumor activity [treated versus control (T/C) of 27% and tumor regression during treatment].


CONCLUSIONS
These studies support the hypothesis that troxacitabine infusions might be the administration regimen with the greatest likelihood of fully exploiting clinically the potent preclinical antitumor activity of troxacitabine.
Gourdeau
Henriette
H
Shire BioChem Inc., Laval, Quebec, Canada.
Leblond
Lorraine
L
Hamelin
Bettina
B
Dong
Kelly
K
Ouellet
France
F
Boudreau
Chantal
C
Custeau
Dominique
D
Richard
Annie
A
Gilbert
Marie-Josée
MJ
Jolivet
Jacques
J
D000818:Animals; D000970:Antineoplastic Agents / Q000493:pharmacokinetics* / Q000494:pharmacology*; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D003114:Colony-Forming Units Assay; D003596:Cytosine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics* / Q000494:pharmacology*; D004148:Dioxolanes / Q000493:pharmacokinetics* / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006412:Hematopoietic Stem Cells; D006801:Humans; D020128:Inhibitory Concentration 50; D008297:Male; D051379:Mice; D009368:Neoplasm Transplantation; D009369:Neoplasms / Q000188:drug therapy*; D013045:Species Specificity; D013997:Time Factors
D000970:Antineoplastic Agents; D004148:Dioxolanes; C074908:troxacitabine; D003596:Cytosine