The
vitamin D(3) catabolizing
enzyme, CYP24, is frequently over-expressed in
tumors, where it may support proliferation by eliminating the growth suppressive effects of
1,25-dihydroxyvitamin D(3) (
1,25(OH)(2)D(3)). However, the impact of CYP24 expression in
tumors or consequence of CYP24 inhibition on
tumor levels of
1,25(OH)(2)D(3)in vivo has not been studied due to the lack of a suitable quantitative method. To address this need, an LC-MS/MS assay that permits absolute quantitation of
1,25(OH)(2)D(3) in plasma and
tumor was developed. We applied this assay to the H292 lung
tumor xenograft model: H292 cells eliminate
1,25(OH)(2)D(3) by a CYP24-dependent process in vitro, and
1,25(OH)(2)D(3) rapidly induces CYP24 expression in H292 cells in vivo. In
tumor-bearing mice, plasma and
tumor concentrations of
1,25(OH)(2)D(3) reached a maximum of 21.6 and 1.70ng/mL, respectively, following intraperitoneal dosing (20μg/kg
1,25(OH)(2)D(3)). When co-administered with the CYP24 selective inhibitor CTA091 (250μg/kg),
1,25(OH)(2)D(3) plasma levels increased 1.6-fold, and
tumor levels increased 2.6-fold. The
tumor/plasma ratio of
1,25(OH)(2)D(3) AUC was increased 1.7-fold by CTA091, suggesting that the inhibitor increased the
tumor concentrations of
1,25(OH)(2)D(3) independent of its effects on plasma disposition. Compartmental modeling of
1,25(OH)(2)D(3) concentration versus time data confirmed that:
1,25(OH)(2)D(3) was eliminated from plasma and
tumor; CTA091 reduced the elimination from both compartments; and that the effect of CTA091 on
tumor exposure was greater than its effect on plasma. These results provide evidence that CYP24-expressing lung
tumors eliminate
1,25(OH)(2)D(3) by a CYP24-dependent process in vivo and that CTA091 administration represents a feasible approach to increase
tumor exposure to
1,25(OH)(2)D(3).