Doxycycline-Induced Changes in Circulating MMP or TIMP2 Levels Are Not Associated with Skeletal-Related Event-Free or Overall Survival in Patients with Bone Metastases from Breast Cancer

SIMPLE SUMMARY: The current study was a retrospective exploratory study evaluating whether circulating levels of known targets of doxycycline were altered following administration of doxycycline to bone metastatic breast cancer patients as an indicator of its expected on-target efficacy. Although we saw predicted changes in these proteins, namely MMP2, MMP9 and TIMP2 pre- and post-doxycycline, they were not prognostic for skeletal-related event-free or overall survival in this cohort. This is likely due in part to confounding effects of doxycycline administration on other cell types in the bone and effects of concurrent treatment regimens on these same target proteins. Although use of doxycycline in cancer patients remains an attractive modality, our findings suggest that studies assessing biomarkers of doxycycline efficacy should carefully consider putative confounding factors and account for this prospectively in the study design. ABSTRACT: Doxycycline is often used as a promoter of inducible gene expression in preclinical models; however, it can also have direct effects on tumor growth and survival. This is due in part to its ability to inhibit cell invasion and regulate matrix metalloproteinase (MMP) expression. Given that doxycycline is also osteotropic, a clinical study to assess its effects on modulation of tumor progression or prevention of skeletal-related events (SRE) in patients with bone metastases from breast cancer (the Achilles trial) was undertaken. Patients received 100 mg of oral doxycycline twice daily for 12 weeks, with serum obtained at baseline and 4, 8 and 12 weeks post-initiation of doxycycline treatment. Exploratory analysis of the effects of doxycycline on circulating levels of MMP or tissue inhibitor of matrix metalloproteinase 2 (TIMP2) was performed in enrolled patients. Statistically significant associations were observed between MMP2, MMP9 and TIMP2 at baseline with significant associations maintained between absolute levels and changes in levels of MMP2 and TIMP2 at weeks 4–12 post initiation of doxycycline. Treatment with doxycycline generally resulted in decreases in MMP2 and MMP9 levels with concurrent upregulation of TIMP2 at 12 weeks post-initiation of doxycycline treatment. Despite this, we observed no association with the levels of any of these factors with either SRE-free or overall survival in this patient cohort. In summary, despite observing hypothesized effects of doxycycline administration on surrogate markers of its anti-tumor activity, measures of circulating levels of these biomarkers were not prognostic in this patient population.