Background: Despite substantial in vitro activity, the demonstration of mass tumor response to N-(4-Hydroxyphenyl) retinamide (fenretinide; 4-HPR, a synthetic retinoid) in murine xenografts has been challenging despite activity being observed in early phase clinical trials of more bioavailable formulations. 4-HPR systemic exposures achieved in mice are generally lower than levels predicted to be active in vivo and are lower than exposures seen in recent human clinical trials. We hypothesized that: 1) Differences in the in vivo metabolism of 4-HPR exist between humans and mice; 2) Co-administration of an inhibitor of retinoid metabolism (ketoconazole, KETO) could increase 4-HPR plasma and tissue levels in mice. Methods: Cohorts of NOD/SCID mice were treated with 4-HPR by gavage at 90 mg/kg/dose twice daily with or without KETO (25 mg/kg/dose twice a day). Blood was collected four hours after the fifth dose of 4-HPR in heparinized tubes. Plasma was separated by centrifugation, and metabolites were identified using HPLC-ABI 4000QTRAP tandem mass spectrometer and LightSight® metabolite identification software. After identifying major metabolites, their relative ratio to 4-HPR were determined employing an Agilent 1200 HPLC with UV detector, Waters® Symmetry C18 column (4X150 mm), and gradient mobile phase of water/acetonitrile/formic acid at 1ml/min flow rate. Human plasma samples were obtained from patients enrolled in a Phase I clinical trial in hematologic malignancies (Ph I-42) and received 905 mg/m2/day intravenous 4-HPR emulsion for 5 days as a continuous infusion. Results: Common 4-HPR metabolites (retention time in parenthesis) identified in mouse and human were hydroxy-4-oxo-4-HPR type I (7.6 min), hydroxy-4-oxo-4-HPR type II (10.9 min), dehydro-4-HPR (15.9), 4-oxo-4-HPR (16.6), 4-HPR-glucuronide (22.3),and 4-(methoxyphenyl) retinamide (4-MPR) (28.0 min). In humans, 4-MPR appeared to be the most abundant metabolite and accumulated in plasma to the same level as 4-HPR. Other metabolites were present at relatively low levels (0.33% to 10% relative to 4-HPR) in human plasma. By contrast, in mice the 4-oxo-4-HPR, dehydro-4-HPR and hydroxyl-4-oxo-4-HPR and 4-MPR were all equally abundant. In mice co-treated with KETO, 4-HPR, 4-MPR, and 4-oxo-4-HPR levels were increased by 59%, 49%, and 41%, respectively; hydroxyl-4-oxo-4-HPR and dehydro-4-HPR levels were unaffected. These data indicate that KETO acted to decrease the secondary metabolism of 4-HPR, 4-MPR and 4-oxo-4-HPR. Conclusions: Humans and mice metabolize 4-HPR differently: humans metabolized 4-HPR mainly to 4-MPR and 4-oxo-4-HPR whereas additional metabolites were present in mice. Modulation of 4-HPR metabolism with KETO achieves higher 4-HPR levels in mice, suggesting KETO might also increase 4-HPR levels in humans. Further 4-HPR metabolism and xenograft response studies in mice co-treated with 4-HPR + KETO are ongoing.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2686.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO