Capecitabine in the treatment of brain metastases of breast cancer: Critical role of enzymatic targeting of key metabolic species

Breast cancer brain metastases (BCBM) are a grave health problem with limited therapeutic options because of poor drug delivery. Capecitabine, an enzymatically targeted prodrug of 5-fluorouracil (5-FU), shows promise in combination therapy of BCBM. However, limited information exists regarding its distribution and efficacy in BCBM. We investigated distribution of capecitabine and its metabolites in mouse models of BCBM with varying enzymatic profiles. Capecitabine was administered orally to athymic NuNu mice bearing intracranial MDA-MB-231-GFP/LUC, MCF-7/GFP or ZR-75-1 human breast cancer tumors or to matching healthy tumor-free animals at doses that either mimic human plasma levels (150 mg/kg) or are at the maximal tolerable level (750 mg/kg). Tissue and plasma samples were collected from 0.5-8 h and compound levels were measured by LC-MS/MS. Enzymatic biotransformation of capecitabine and three of its metabolites in brain and tumor samples was assessed in vitro. In tumor free brain, capecitabine delivered 1/50th the brain exposure to neuro-toxic compounds (e.g., 5-FU, F-uridine, F-deoxyuridine) than 5-FU did at matching molar dose. Capecitabine also demonstrated enzyme selective targeting to brain metastasis which varied among the tumor models by >9 fold and matched that predicted by the ratio of thymidine phosphorylase (TP)/dihydropyrimidine dehydrogenase (DPD) (>6 fold; P<0.05). In brain metastases with the higher TP/DPD ratio, tumor 5-FU concentration Cmax approached IC50 values in vitro. The TP/DPD ratio, an indicator of drug efficacy, was elevated in ZR-75-1 (~6-fold) and MDA-MB-231-GFP/LUC BCBM (~2-fold), but ~ equal in MCF-7/GFP compared to brain. Efficacy of capecitabine in intra-cranial tumor was assessed by measuring inhibition of thymidylate synthase enzyme and induction of apoptosis. Administration of maximum tolerated dose of capecitabine led to 39% inhibition of TS activity and induction of early apoptosis in 40% cells in intra-cranial tumor of MDA-MB-231-GFP/LUC model. In a capecitabine clinical study conducted in parallel to this preclinical study, brain metastasis 5-FU concentrations varied widely among tumors, with only 20% approaching the values associated with significant TS inhibition or apoptosis in preclinical studies. These are very exciting findings, show robust translational impact and warrant further study to modulate enzymatic conversion to improve delivery and efficacy of capecitabine in BCBM.