An Engineering Approach to Understanding the Bone-Tumor Microenvironment: In Vitro Modeling and Drug Delivery Strategies to Improve Bone-Metastatic Patient Outcomes
Vanderburgh, Joseph Paul
Metastasis is the major prognostic factor for cancer patient survival, and breast tumors have a striking propensity for metastasis to bone. Once tumors establish in bone, they initiate what is colloquially known as the vicious cycle of tumor-induced bone disease (TIBD), where tumor cells interact with native bone cells and the bone microenvironment to drive bone destruction and subsequent tumor growth. The complexity of bone-tumor interplay precludes facile screening of drugs due to limitations in modeling the relevant cell types and the rigid bone matrix and structure in vitro. Further, the efficacy of TIBD drug candidates is often hampered by poor bioavailability and uptake in the bone microenvironment upon systemic administration. This work details engineering approaches to address these challenges with the goal of providing in vitro modeling and drug delivery platforms to enable study and treatment of TIBD, respectively. First, tissue engineered bone constructs (TEBCs) were fabricated via a new micro‐CT/3D inkjet printing process that reproducibly creates constructs with anatomic site-specific morphometric and mechanical properties of human trabecular bone. Incorporation of TEBCs into a perfusion bioreactor model with coculture of bone and tumor cells supported long-term coculture and evidence of TEBC resorption was observed. These findings suggest the model can replicate in vivo bone and tumor cell behavior. Next, a drug delivery platform was developed to enable the in vivo efficacy of GANT58, a small molecule inhibitor of the TIBD-associated protein Gli2. Polymeric micellar nanoparticles (GANT58-NPs) were fabricated to colloidally stabilize GANT58, providing a fully aqueous, intravenously injectable formulation. In an intratibial model of breast cancer bone metastasis, treatment with GANT58-NPs resulted in a 2.5-fold increase in trabecular bone volume. In vitro studies suggested GANT58-NP treatment would also reduce tumor burden in vivo, however no effect on tumor burden was observed. To improve on this delivery platform, a new bone-targeted nanoparticle chemistry (GANT58-BTNP) was synthesized utilizing the bisphosphonate alendronate as the bone-targeting agent. GANT58-BTNP treatment significantly improved bone outcomes over the non-targeted GANT58-NP and untreated controls in an intracardiac model of breast cancer bone metastasis. However, histological analysis revealed spatial heterogeneity in Gli2 expression in the bone-tumor microenvironment that restricted GANT58 effect to the bone surface. Therefore, Gli2 spatial heterogeneity limits GANT58-BTNP efficacy as an antitumor treatment, however GANT58-BTNP acts as a potent tumor-mediated antiresorptive therapy. The GANT58-BTNP findings demonstrate the important and nuanced bone-tumor interactions that drive TIBD progression, further compelling the need for in vitro models that recapitulate in vivo behavior. Taken together, the tools developed in this dissertation provide the platform to further investigate how spatial heterogeneity drives tumor progression in bone.