Plasma pharmacokinetics and oral bioavailability of the 3,4,5,6-tetrahydrouridine (THU) prodrug, triacetyl-THU (taTHU), in mice

Abstract

Purpose: Cytidine drugs, such as gemcitabine, undergo rapid catabolism and inactivation by cytidine deaminase (CD). 3,4,5,6-tetrahydrouridine (THU), a potent CD inhibitor, has been applied preclinically and clinically as a modulator of cytidine analogue metabolism. However, THU is only 20% orally bioavailable, which limits its preclinical evaluation and clinical use. Therefore, we characterized THU pharmacokinetics after the administration to mice of the more lipophilic pro-drug triacetyl-THU (taTHU).

Methods: Mice were dosed with 150 mg/kg taTHU i.v. or p.o. Plasma and urine THU concentrations were quantitated with a validated LC-MS/MS assay. Plasma and urine pharmacokinetic parameters were calculated non-compartmentally and compartmentally.

Results: taTHU did not inhibit CD. THU, after 150 mg/kg taTHU i.v., had a 235-min terminal half-life and produced plasma THU concentrations >1 μg/mL, the concentration shown to inhibit CD, for 10 h. Renal excretion accounted for 40-55% of the i.v. taTHU dose, 6-12% of the p.o. taTHU dose. A two-compartment model of taTHU generating THU fitted the i.v. taTHU data best. taTHU, at 150 mg/kg p.o., produced a concentration versus time profile with a plateau of approximately 10 μg/mL from 0.5-2 h, followed by a decline with a 122-min half-life. Approximately 68% of i.v. taTHU is converted to THU. Approximately 30% of p.o. taTHU reaches the systemic circulation as THU.

Conclusions: The availability of THU after p.o. taTHU is 30%, when compared to the 20% achieved with p.o. THU. These data will support the clinical studies of taTHU.

Fig. 1

Chemical structure of 3,4,5,6-tetrahydrouridine (THU), its prodrug 2′,3′,5′-triacetyl-3,4,5,6-tetrahydrouridine (taTHU), and all the possible metabolic intermediates: 2′,3′-diacetyl-THU, 2′,5′-diacetyl-THU, 3′,5′-diacetyl-THU, 2′-monoacetyl-THU, 3′-monoacetyl-THU, and 5′-monoacetyl-THU

Fig. 2

Production of dFdU after incubation of 500 µM (131 µg/mL) dFdC at 37°C with: recombinant human CD (white square, 191 ng/mL/min, R² 0.989); CD and 500 µM taTHU (white circle, 152 ng/mL/min, R² 0.965); and CD and 500 µM THU (white triangle)

Fig. 3

Predictions (solid lines) of the final model and actual data (white square) after administration of 150 mg/kg taTHU i.v. (a) or p.o. (b) or 100 mg/kg THU i.v (c) or p.o. (d) to mice, using a compartmental model (e), constructed to be fitted to THU mouse plasma concentration data obtained after administration of 150 mg/kg taTHU i.v or p.o., in combination with previously published data [3] of THU administered to mice at 100 mg/kg i.v., 300 mg/kg p.o., 100 mg/kg p.o. or 30 mg/kg p.o. The entire p.o. THU dose is delivered to compartment 8 and transferred to compartment 7 (k₈₇), from which it is either eliminated (k₇₀, never absorbed) or transferred into the central compartment V₁ via a non-linear rate (V_max, K_m). The i.v. THU dose is delivered to V₁ as a bolus. Distribution takes place to and from compartments 2 and 3 by their respective rate constants (k₁₂, k₂₁, k₁₃, k₃₁), and elimination from V₁ is determined by k₁₀. The i.v. taTHU dose is delivered as a bolus to compartment 4, from which it can distribute into compartment 5 according to the respective rate constants (k₄₅, k₅₄). The p.o. taTHU dose is delivered as a bolus into compartment 6, from which only a fraction (F_abs) reaches compartment 1 at a specific rate (F_abs*k₆₁), while the rest is lost (unabsorbed). A fraction of taTHU from compartment 4 (F_conv) is converted to THU at a specific rate (F_conv*k₄₁; with correction for difference in molecular weights), while the remainder is lost