Musculoskeletal side effects were common in AI-treated patients, resulting in therapy discontinuation in more than 10% of patients. There are no identifiable pre-therapy indicators of risk, and the etiology remains elusive.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT • Anastrozole is primarily cleared by hepatic metabolism via oxidative and conjugating enzymes. WHAT THIS STUDY ADDS • Anastrozole is oxidized to hydroxyanastrozole mainly by CYP3A4/5 and glucuronidated to anastrozole glucuronide predominantly by UGT1A4 in vitro. • Hydroxyanastrozole glucuronide and hydroxyanastrozole were quantified as the major metabolites of anastrozole in plasma of breast cancer patients. • This study describes for the first time anastrozole metabolic pathways and the enzymes involved, which may serve as the scientific basis for pharmacogenetic and drug‐interaction assessments. AIMS Little information is available regarding the metabolic routes of anastrozole and the specific enzymes involved. We characterized anastrozole oxidative and conjugation metabolism in vitro and in vivo. METHODS A sensitive LC‐MS/MS method was developed to measure anastrozole and its metabolites in vitro and in vivo. Anastrozole metabolism was characterized using human liver microsomes (HLMs), expressed cytochrome P450s (CYPs) and UDP‐glucuronosyltransferases (UGTs). RESULTS Hydroxyanastrozole and anastrozole glucuronide were identified as the main oxidative and conjugated metabolites of anastrozole in vitro, respectively. Formation of hydroxyanastrozole from anastrozole was markedly inhibited by CYP3A selective chemical inhibitors (by >90%) and significantly correlated with CYP3A activity in a panel of HLMs (r= 0.96, P= 0.0005) and mainly catalyzed by expressed CYP3A4 and CYP3A5. The Km values obtained from HLMs were also close to those from CYP3A4 and CYP3A5. Formation of anastrozole glucuronide in a bank of HLMs was correlated strongly with imipramine N‐glucuronide, a marker of UGT1A4 (r= 0.72, P < 0.0001), while expressed UGT1A4 catalyzed its formation at the highest rate. Hydroxyanastrozole (mainly as a glucuronide) and anastrozole were quantified in plasma of breast cancer patients taking anastrozole (1 mg day−1); anastrozole glucuronide was less apparent. CONCLUSION Anastrozole is oxidized to hydroxyanastrozole mainly by CYP3A4 (and to some extent by CYP3A5 and CYP2C8). Once formed, this metabolite undergoes glucuronidation. Variable activity of CYP3A4 (and probably UGT1A4), possibly due to genetic polymorphisms and drug interactions, may alter anastrozole disposition and its effects in vivo.

Treatment failure in breast cancer is largely the failure to control metastatic dissemination. In this study, we investigated the efficacy of an antibody against the rat variant of HER-2/neu, labeled with the A-particle emitter 213 Bi to treat widespread metastases in a rat/neu transgenic mouse model of metastatic mammary carcinoma. The model manifests wide-spread dissemination of tumor cells leading to osteolytic bone lesions and liver metastases, common sites of clinical metastases. The maximum tolerated dose was 120 MCi of 213 Bi-7.16.4. The kinetics of marrow suppression and subsequent recovery were determined. Three days after left cardiac ventricular injection of 10 5 rat HER-2/neu-expressing syngeneic tumor cells, neu-N mice were treated with (a) 120 MCi 213 (b) .4 increased median survival time to 41 days compared with 28 days for the untreated controls (P < 0.0001); corresponding median survival times for groups b, c, and d were 36 (P < 0.001), 31 (P < 0.01), and 33 (P = 0.05) days, respectively. Median survival relative to controls was not significantly improved in mice injected with 10-fold less cells or with multiple courses of treatment. We concluded that A-emitter 213 Bi-labeled monoclonal antibody targeting the HER-2/neu antigen was effective in treating early-stage HER-2/neu-expressing micrometastases. Analysis of the results suggests that further gains in efficacy may require higher specific activity constructs or target antigens that are more highly expressed on tumor cells. [Cancer Res 2008;68(10):3873-80]

Purpose: Animal models of breast cancer metastases that recapitulate the pattern of metastatic progression seen in patients are lacking; metastatic breast cancer models do not currently exist for evaluation of immune-mediated therapies. We have developed and characterized a preclinical model for the evaluation of immune-mediated metastatic breast cancer therapies. Experimental Design: The NT2.5 mammary tumor cell line was injected into the left cardiac ventricle of immunotolerant transgenic neu-N mice and athymic nu/nu mice. Metastatic progression was monitored by bioluminescent, small-animal magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography/computed tomography imaging, and also by histopathology. Antigen expression in normal organs and tumor metastases was evaluated by Western blot analysis and flow cytometry. Results: Left cardiac ventricle injection of NT2.5 cells yielded widespread metastases in bones, liver, and spleen. Three to four weeks after injection, mice exhibited hind limb paralysis and occasional abdominal enlargement. Bioluminescence imaging of metastatic progression was successful in nude mice but the bioluminescent cells were rejected in immunocompetent mice. Other imaging modalities allowed successful imaging of nonbioluminescent cells. Small-animal positron emission tomography imaging allowed visualization of disease, in vivo, in the bones and liver. Magnetic resonance imaging revealed initial dissemination of the tumor cells to the bone marrow. Small-animal single-photon emission computed tomography/computed tomography imaging identified metastatic bone lesions targeted by a radiolabeled antibody. Conclusion: The model closely recapitulates the pattern of metastatic spread in breast cancer. This immunotolerant metastatic model is a novel addition to existing breast cancer models and coupling the model with in vivo imaging greatly facilitates the evaluation of targeted immunotherapies of metastasis.