Modulation of gemcitabine (2',2'-difluoro-2'-deoxycytidine) pharmacokinetics, metabolism, and bioavailability in mice by 3,4,5,6-tetrahydrouridine.
OBJECTIVE
In vivo, 2',2'-difluoro-2'-deoxycytidine (dFdC) is rapidly inactivated by gut and liver cytidine deaminase (CD) to 2',2'-difluoro-2'-deoxyuridine (dFdU). Consequently, dFdC has poor oral bioavailability and is administered i.v., with associated costs and limitations in administration schedules. 3,4,5,6-Tetrahydrouridine (THU) is a potent CD inhibitor with a 20% oral bioavailability. We investigated the ability of THU to decrease elimination and first-pass effect by CD, thereby enabling oral dosing of dFdC.


METHODS
A liquid chromatography-tandem mass spectrometry assay was developed for plasma dFdC and dFdU. Mice were dosed with 100 mg/kg dFdC i.v. or orally with or without 100 mg/kg THU i.v. or orally. At specified times between 5 and 1,440 min, mice (n = 3) were euthanized. dFdC, dFdU, and THU concentrations were quantitated in plasma and urine.


RESULTS
THU i.v. and orally produced concentrations >4 microg/mL for 3 and 2 h, respectively, whereas concentrations of >1 microg/mL have been associated with near-complete inhibition of CD in vitro. THU i.v. decreased plasma dFdU concentrations but had no effect on dFdC plasma area under the plasma concentration versus time curve after i.v. dFdC dosing. Both THU i.v. and orally substantially increased oral bioavailability of dFdC. Absorption of dFdC orally was 59%, but only 10% passed liver and gut CD and eventually reached the systemic circulation. Coadministration of THU orally increased dFdC oral bioavailability from 10% to 40%.


CONCLUSIONS
Coadministration of THU enables oral dosing of dFdC and warrants clinical testing. Oral dFdC treatment would be easier and cheaper, potentially prolong dFdC exposure, and enable exploration of administration schedules considered impractical by the i.v. route.
Beumer
Jan H
JH
Molecular Therapeutics/Drug Discovery Program, University of Pittsburgh Cancer Institute, Room G.27d, Hillman Research Pavilion, 5117 Centre Avenue, Pittsburgh, PA 15213-1863, USA. beumerjh@upmc.edu
Eiseman
Julie L
JL
Parise
Robert A
RA
Joseph
Erin
E
Covey
Joseph M
JM
Egorin
Merrill J
MJ
D000284:Administration, Oral; D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000008:administration & dosage / Q000493:pharmacokinetics*; D019540:Area Under Curve; D001682:Biological Availability; D003841:Deoxycytidine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D004347:Drug Interactions; D008297:Male; D051379:Mice; D013767:Tetrahydrouridine / Q000008:administration & dosage / Q000493:pharmacokinetics*; D000093542:Gemcitabine
D000964:Antimetabolites, Antineoplastic; D003841:Deoxycytidine; D013767:Tetrahydrouridine; D000093542:Gemcitabine