Diagnostic assessment of osteosarcoma chemoresistance based on Virtual Clinical Trials

Mass Spectrometric-Based Proteomics for Biomarker Discovery in Osteosarcoma: Current Status and Future Direction

Due to a lack of novel therapies and biomarkers, the clinical outcomes of osteosarcoma patients have not significantly improved for decades. The advancement of mass spectrometry (MS), peptide quantification, and downstream pathway analysis enables the investigation of protein profiles across a wide range of input materials, from cell culture to long-term archived clinical specimens. This can provide insight into osteosarcoma biology and identify candidate biomarkers for diagnosis, prognosis, and stratification of chemotherapy response. In this review, we provide an overview of proteomics studies of osteosarcoma, indicate potential biomarkers that might be promising therapeutic targets, and discuss the challenges and opportunities of mass spectrometric-based proteomics in future osteosarcoma research.

A Correlation Between Intracellular Zinc Content and Osteosarcoma

Zinc is a trace element in human body involved in many biological processes. It is critical for cell growth and acts as a cofactor for the structure and function of a wide range of cellular proteins such as enzymes. Mounting evidence has shown the involvement of intracellular zinc in the bone-related biological processes such as bone growth, homeostasis, and regeneration; however, the molecular mechanism(s) whereby zinc impels tumorigenesis in bone remains largely unexplored. In this article, selective outline related to the content of intracellular zinc in osteosarcoma cells was provided, and its correlation with signaling molecules that are activated and consequently guide the cells toward tumorigenesis or osteogenesis was discussed. Based on preclinical and clinical evidence, dysregulation of zinc homeostasis, both at intracellular and tissue level, has the main role in the pathogenesis of osteosarcoma. Based on the intracellular zinc content, this element could have a direct role in the dynamics of bone cell transformation and tumor development and play an indirect role in the modulation of the inflammatory and pro/antitumorigenic responses in immune cells. In this context, zinc transporters and the proteins containing zinc domain are regulated by the availability of zinc, playing a crucial role in bone cell transformation and differentiation. According to recent studies, it seems that intracellular zinc levels could be considered as an early prognosis marker. Besides, identification and targeting of zinc-dependent signaling molecules could tilt the balance of life and death toward the latter in chemoresistant malignant cells and may pave a way for designing of the novel osteosarcoma treatment strategies.

Decrease in tumor content assessed in biopsies is associated with improved treatment outcome response to pembrolizumab in patients with rare tumors

BACKGROUND

Decreased tumor content (TC) in resection specimens after neoadjuvant therapy is used to predict prognosis. We investigated whether TC assessed in biopsy specimens or the shift in TC from baseline to on-treatment can be used accordingly to predict response in patients with rare tumors who were treated with pembrolizumab.

METHODS

A total of 57 tumors (represented by 173 baseline and 179 on-treatment biopsies) from 57 patients with rare tumors participating in an ongoing phase II clinical trial of pembrolizumab were evaluated. TC was estimated on H&E-stained slides and tumors were dichotomized into low and high TC according to a cut-off of 10%. Necrosis, proliferative fibrosis (PF) and normal tissue were assessed in on-treatment biopsies. TC at baseline and on-treatment, as well as the shift in TC from baseline to on-treatment, was correlated with clinical response defined according to Response Evaluation Criteria in Solid Tumors.

RESULTS

A decrease in TC was seen in 14% (n=8); no change in TC was seen in 75% (n=43); and an increase in TC from baseline to on-treatment was seen in 11% (n=6). Objective response was significantly associated with decrease in TC from baseline to on-treatment (38%, 3/8) compared with no change/increase in TC (6%, 3/49) (p=0.031). Patients with a decrease in TC had a significantly increased time to progression (TTP) (75% probability) compared with patients with an increase (20% probability) or no change in TC (19% probability) (p=0.0042). Low TC was seen in 23% (13/57) of the tumors at baseline and in 26% (15/57) on-treatment. High TC was seen in 77% (44/57) of tumors at baseline and in 74% (42/57) on-treatment. No significant associations with response were seen for necrosis, PF or normal tissue in on-treatment biopsies.

CONCLUSION

Patients with a decrease in TC from baseline to on-treatment had a significant improvement in objective response and a longer TTP. Our data suggest that the shift in TC might be used to predict response to pembrolizumab in rare tumors. However, further investigations in larger cohorts are needed to determine the clinical value of TC, the shift in TC and the cut-off of 10% assessed in biopsies.

TRIAL REGISTRATION NUMBER

NCT02721732.

Overexpression of miR-329-3p sensitizes osteosarcoma cells to cisplatin through suppression of glucose metabolism by targeting LDHA

Osteosarcoma (OS) is one of the most frequent malignant bone tumor types. Traditional treatments of OS involve standard chemotherapy or combination with radiation before and after surgery. Cisplatin is one of the most effective chemotherapeutic drugs used for treating osteosarcoma. However, patients with advanced tumor stages develop cisplatin resistance, leading to a major clinical challenge. In this study, we investigated the roles of miR-329-3p in cisplatin sensitivity of osteosarcoma cells. We found miR-329-3p was significantly downregulated in osteosarcoma tissues compared with normal bone tissues. Overexpression of miR-329-3p suppressed osteosarcoma cell proliferation. Moreover, we observed low-toxic cisplatin treatments suppressed miR-329-3p but higher concentrations of cisplatin-induced miR-329-3p expression. In addition, miR-329-3p was significantly downregulated in cisplatin-resistant Saos-2 cells which displayed elevated glucose metabolism. Overexpression of miR-329-3p significantly impaired glucose metabolism of Saos-2 cells. Bioinformatics analysis and luciferase assay consistently demonstrated the glycolysis enzyme, lactate dehydrogenase-A (LDHA) was a direct target of miR-329-3p in osteosarcoma cells. Rescue experiments revealed restoration of LDHA in miR-329-3p-overexpressed cisplatin-resistant cells effectively recovered glucose metabolism, resulting in increased cisplatin resistance. This study demonstrates a miR-329-3p-LDHA-glucose metabolism-cisplatin resistance axis in osteosarcoma cells, providing a miRNA-based therapeutic strategy against chemoresistant osteosarcoma.

Mechanisms of Resistance to Conventional Therapies for Osteosarcoma

Osteosarcoma (OS) is the most common primary bone tumor, mainly occurring in children and adolescents. Current standard therapy includes tumor resection associated with multidrug chemotherapy. However, patient survival has not evolved for the past decades. Since the 1970s, the 5-year survival rate is around 75% for patients with localized OS but dramatically drops to 20% for bad responders to chemotherapy or patients with metastases. Resistance is one of the biological processes at the origin of therapeutic failure. Therefore, it is necessary to better understand and decipher molecular mechanisms of resistance to conventional chemotherapy in order to develop new strategies and to adapt treatments for patients, thus improving the survival rate. This review will describe most of the molecular mechanisms involved in OS chemoresistance, such as a decrease in intracellular accumulation of drugs, inactivation of drugs, improved DNA repair, modulations of signaling pathways, resistance linked to autophagy, disruption in genes expression linked to the cell cycle, or even implication of the micro-environment. We will also give an overview of potential therapeutic strategies to circumvent resistance development.

Circular RNA hsa_circ_0003496 Contributes to Tumorigenesis and Chemoresistance in Osteosarcoma Through Targeting (microRNA) miR-370/Krüppel-Like Factor 12 Axis

Background

Osteosarcoma (OS) is the most common primary malignancy of bone with a high incidence in children. Circular RNAs (circRNAs) play crucial roles in the carcinogenesis and chemoresistance of OS. In the current work, we focused on the function and mechanism of hsa_circ_0003496 (circ_0003496) in OS progression and chemoresistance.

Materials and Methods

The expression levels of circ_0003496, miR-370 and Krüppel-like factor 12 (KLF12) mRNA were measured by quantitative real-time polymerase chain reaction (qRT-PCR). The Cell Counting Kit-8 (CCK-8) assay was used to assess the 50% inhibitory concentration (IC50) value and cell proliferation. Cell migration, invasion and apoptosis were detected by transwell assay and flow cytometry, respectively. Western blot analysis was performed to assess the protein level. Targeted relationships among circ_0003496, miR-370 and KLF12 were validated by dual-luciferase reporter, RNA immunoprecipitation (RIP) and RNA pull-down assays. Animal studies were carried out to observe the role of circ_0003496 in vivo.

Results

Our results indicated that circ_0003496 up-regulation was associated with doxorubicin (DXR) resistance of OS. Circ_0003496 knockdown repressed DXR-resistant OS cell proliferation, migration and invasion, and enhanced apoptosis and DXR sensitivity. Circ_0003496 functioned as a sponge of miR-370, and miR-370 mediated the regulatory effect of circ_0003496 depletion on DXR-resistant OS cell progression and DXR sensitivity. KLF12 was a direct target of miR-370, and miR-370 overexpression suppressed cell progression and enhanced DXR sensitivity by KLF12. Moreover, circ_0003496 protected against KLF12 repression through sponging miR-370. Additionally, circ_0003496 knockdown hampered tumor growth and promoted DXR sensitivity in vivo.

Conclusion

Our present work suggested that the knockdown of circ_0003496 suppressed OS progression and enhanced DXR sensitivity at least partially through modulating KLF12 expression via functioning as a miR-370 sponge, highlighting new opportunities for OS management.

Blackboard to Bedside: A Mathematical Modeling Bottom-Up Approach Toward Personalized Cancer Treatments

Cancers present with high variability across patients and tumors; thus, cancer care, in terms of disease prevention, detection, and control, can highly benefit from a personalized approach. For a comprehensive personalized oncology practice, this personalization should ideally consider data gathered from various information levels, which range from the macroscale population level down to the microscale tumor level, without omission of the central patient level. Appropriate data mined from each of these levels can significantly contribute in devising personalized treatment plans tailored to the individual patient and tumor. Mathematical models of solid tumors, combined with patient-specific tumor profiles, present a unique opportunity to personalize cancer treatments after detection using a bottom-up approach. Here, we discuss how information harvested from mathematical models and from corresponding in silico experiments can be implemented in preclinical and clinical applications. To conceptually illustrate the power of these models, one such model is presented, and various pertinent tumor and treatment scenarios are demonstrated in silico. The presented model, specifically a multiscale, hybrid cellular automaton, has been fully validated in vitro using multiple cell-line-specific data. We discuss various insights provided by this model and other models like it and their role in designing predictive tools that are both patient, and tumor specific. After refinement and parametrization with appropriate data, such in silico tools have the potential to be used in a clinical setting to aid in treatment protocols and decision making.

SLIC-supervoxels-based response evaluation of osteosarcoma treated with neoadjuvant chemotherapy using multi-parametric MR imaging

OBJECTIVE

Histopathological examination (HPE) is the current gold standard for assessing chemotherapy response to tumor, but it is possible only after surgery. The purpose of the study was to develop a noninvasive, imaging-based robust method to delineate, visualize, and quantify the proportions of necrosis and viable tissue present within the tumor along with peritumoral edema before and after neoadjuvant chemotherapy (NACT) and to evaluate treatment response with correlation to HPE necrosis after surgery.

METHODS

The MRI dataset of 30 patients (N = 30; male:female = 24:6; age = 17.6 ± 2.7 years) with osteosarcoma was acquired using 1.5 T Philips Achieva MRI scanner before (baseline) and after 3 cycles of NACT (follow-up). After NACT, all patients underwent surgical resection followed by HPE. Simple linear iterative clustering supervoxels and Otsu multithresholding were combined to develop the proposed method-SLICs+MTh-to subsegment and quantify viable and nonviable regions within tumor using multiparametric MRI. Manually drawn ground-truth ROIs and SLICs+MTh-based segmentation of tumor, edema, and necrosis were compared using Jacquard index (JI), Dice coefficient (DC), precision (P), and recall (R). Postcontrast T1W images (PC-T1W) were used to validate the SLICs+MTh-based necrosis. SLICs+MTh-based necrosis volume at follow-up was compared with HPE necrosis using paired t test (p ≤ 0.05).

RESULTS

Active tumor, necrosis, and edema were segmented with moderate to satisfactory accuracy (JI = 62-78%; DC = 72-87%; P = 67-87%; R = 63-88%). Qualitatively and quantitatively (DC = 74 ± 9%), the SLICs+MTh-based necrosis area correlated well with the hypointense necrosis areas in PC-T1W. No significant difference (paired t test, p = 0.26; Bland-Altman plot, bias = 2.47) between SLICs+MTh-based necrosis at follow-up and HPE necrosis was observed.

CONCLUSION

The proposed multiparametric MRI-based SLICs+MTh method performs noninvasive assessment of NACT response in osteosarcoma that may improve cancer treatment monitoring, planning, and overall prognosis.

KEY POINTS

• The simple linear iterative clustering supervoxels and Otsu multithresholding-based technique (SLICs+MTh) successfully estimates the proportion of necrosis, viable tumor, and edema in osteosarcoma in the course of chemotherapy. • The proposed technique is noninvasive and uses multiparametric MRI to measure necrosis as an indication of anticancer treatment response. • SLICs+MTh-based necrosis was in satisfactory agreement with histological necrosis after surgery.

Risk factors for lung metastasis at presentation with malignant primary osseous neoplasms: a population-based study

Background

Large population-based studies of risk factor for lung metastases at the presentation with primary osseous neoplasms are lacking and necessary. We aim to examine potential risk factors of lung metastases at presentation with primary osseous neoplasms using Surveillance, Epidemiology, and End Results (SEER) database tool.

Methods

We collected patients diagnosed with primary osseous neoplasms between 2010 and 2015 from the SEER database. Patients were divided into two groups: patients with lung metastases or patients without lung metastases. Patient characteristics such as age, sex, race, tumor size, histologic types, histologic grade, and lung metastasis were collected. Univariate and multivariate logistic regression analyses were applied to determine which characteristics were risk factors for lung metastasis at diagnosis.

Results

A total of 4459 patients were collected, and 507 patients had lung metastases at presentation. Data on age, race, gender, primary site, grade, tumor size, and histology types were enrolled into the multivariate logistic analysis. Higher grade (OR = 5.197, 95% CI 3.328 to 8.117), histology type (Ewing sarcoma: OR = 1.432, 95% CI 1.020 to 2.009; osteosarcoma: OR = 1.597, 95% CI, 1.073 to 2.377), and larger tumor size (≥ 5 cm: OR = 3.528, 95% CI 2.370 to 5.251) were associated with an increased risk of lung metastasis at presentation.

Conclusion

Histology types (osteosarcoma and Ewing sarcoma) were related to a higher risk of lung metastases in primary osseous neoplasms patients. Patients with osteosarcoma and lager tumors or higher tumor grade were associated with higher possibility of lung metastases. Patients with Ewing sarcoma and larger tumors have more tendency of lung metastases. These patients are supposed to receive chest CT scans at the presentation with primary osseous neoplasms.

Towards personalized computational oncology: from spatial models of tumour spheroids, to organoids, to tissues

A main goal of mathematical and computational oncology is to develop quantitative tools to determine the most effective therapies for each individual patient. This involves predicting the right drug to be administered at the right time and at the right dose. Such an approach is known as precision medicine. Mathematical modelling can play an invaluable role in the development of such therapeutic strategies, since it allows for relatively fast, efficient and inexpensive simulations of a large number of treatment schedules in order to find the most effective. This review is a survey of mathematical models that explicitly take into account the spatial architecture of three-dimensional tumours and address tumour development, progression and response to treatments. In particular, we discuss models of epithelial acini, multicellular spheroids, normal and tumour spheroids and organoids, and multi-component tissues. Our intent is to showcase how these in silico models can be applied to patient-specific data to assess which therapeutic strategies will be the most efficient. We also present the concept of virtual clinical trials that integrate standard-of-care patient data, medical imaging, organ-on-chip experiments and computational models to determine personalized medical treatment strategies.

miR-34 inhibits growth and promotes apoptosis of osteosarcoma in nude mice through targetly regulating TGIF2 expression

miR-34 was reported to be involved in multiple tumors occurrence and development. The aim of the present study was to explore the impact of miR-34 on osteosarcoma and related mechanisms. Tumor tissues and non-tumor tissues of 34 patients with osteosarcoma were collected. qRT-PCR detection revealed that miR-34 was significantly down-regulated in tumor tissues (P<0.05). hFOB 1.19 and MG-63 cells were cultured. qRT-PCR detection showed that miR-34 was also significantly down-regulated in MG-63 cells (P<0.05). After transfection by miR-34 mimics, MG-63 cells proliferation in nude mice was significantly impaired (P<0.05), and percentage of apoptosis as well as caspase-3 positive cells proportion of osteosarcoma tissue in nude mice was markly increased (P<0.05). Western blot and immunofluorescence results also demonstrated that TGIF2 relative expression and TGIF2 positive cells proportion were both dramatically decreased (P<0.05). By luciferase reporter assay, we found that TGIF2 was the target gene of miR-34. After transfected by TGIF2 overexpression vector or co-transfected by miR-34 mimics and TGIF2 overexpression vector, we observed that, compared with blank group, tumor volume was significantly increased and apoptotic cells as well as caspase-3 positive cells proportion was obviously decreased in TGIF2 group (P<0.05), while no significant difference was found in these indicators between blank group and TGIF2 + mimics group. We concluded that miR-34 inhibited growth and promoted apoptosis of osteosarcoma in nude mice through targetting regulated TGIF2 expression.

Targeting Ligand Specificity Linked to Tumor Tissue Topological Heterogeneity via Single-Cell Micro-Pharmacological Modeling

Targeted therapy has held promise to be a successful anticancer treatment due to its specificity towards tumor cells that express the target receptors. However, not all targeting drugs used in the clinic are equally effective in tumor eradication. To examine which biochemical and biophysical properties of targeted agents are pivotal for their effective distribution inside the tumor and their efficient cellular uptake, we combine mathematical micro-pharmacological modeling with in vivo imaging of targeted human xenograft tumors in SCID mice. The mathematical model calibrated to experimental data was used to explore properties of the targeting ligand (diffusion and affinity) and ligand release schemes (rates and concentrations) with a goal to identify the properties of cells and ligands that enable high receptor saturation. By accounting for heterogeneities typical of in vivo tumors, our model was able to identify cell- and tissue-level barriers to efficient drug uptake. This work provides a base for utilizing experimentally measurable properties of a ligand-targeted agent and patient-specific attributes of the tumor tissue to support the development of novel targeted imaging agents and for improvement in their delivery to individual tumor cells.

Micropharmacology: An In Silico Approach for Assessing Drug Efficacy Within a Tumor Tissue

Systemic chemotherapy is one of the main anticancer treatments used for most kinds of clinically diagnosed tumors. However, the efficacy of these drugs can be hampered by the physical attributes of the tumor tissue, such as tortuous vasculature, dense and fibrous extracellular matrix, irregular cellular architecture, tumor metabolic gradients, and non-uniform expression of the cell membrane receptors. This can impede the transport of therapeutic agents to tumor cells in sufficient quantities. In addition, tumor microenvironments undergo dynamic spatio-temporal changes during tumor progression and treatment, which can also obstruct drug efficacy. To examine ways to improve drug delivery on a cell-to-tissue scale (single-cell pharmacology), we developed the microscale pharmacokinetics/pharmacodynamics (microPKPD) modeling framework. Our model is modular and can be adjusted to include only the mathematical equations that are crucial for a biological problem under consideration. This modularity makes the model applicable to a broad range of pharmacological cases. As an illustration, we present two specific applications of the microPKPD methodology that help to identify optimal drug properties. The hypoxia-activated drugs example uses continuous drug concentrations, diffusive-advective transport through the tumor interstitium, and passive transmembrane drug uptake. The targeted therapy example represents drug molecules as discrete particles that move by diffusion and actively bind to cell receptors. The proposed modeling approach takes into account the explicit tumor tissue morphology, its metabolic landscape and/or specific receptor distribution. All these tumor attributes can be assessed from patients' diagnostic biopsies; thus, the proposed methodology can be developed into a tool suitable for personalized medicine, such as neoadjuvant chemotherapy.

The natural sweetener metabolite steviol inhibits the proliferation of human osteosarcoma U2OS cell line

Steviol is the colonic metabolite of the natural sweetener steviol glycosides. It does not diffuse to the blood and the half maximal inhibitory concentration of steviol is longer compared with that of current chemotherapy agents, including 5-fluorouracil and doxorubicin. The present study demonstrated that steviol inhibits the proliferation of the human osteosarcoma U2OS cell line in a dose- and time-dependent manner, and that the inhibition rate is comparative with that of doxorubicin and 5-fluorouracil. The mechanism of this anticancer activity is also investigated. The results indicated that steviol inhibits U2OS cells through inducing G1 phase cell cycle arrest, downregulating the ability of colony formation via a mitochondrial apoptotic pathway, which was indicated by an increase of the Bax/Bcl-2 ratio and activation of cyclin-dependent kinase inhibitor 1, tumor protein 53 and cyclin-dependent kinase; whereas a Survivin and Caspase 3-independent mechanism was involved. Considering that steviol appears minimally in the plasma during metabolism, and possesses a median lethal dose of 100-fold greater compared with that of 5-fluorouracil, it may become a potential chemotherapy agent.

Hypoxia promotes chemotherapy resistance by down-regulating SKA1 gene expression in human osteosarcoma

Drug resistance has always been the main problem in osteosarcoma treatment, and hypoxia seems to be one of the many causes for drug resistance. Therefore, in this study, we investigated how hypoxia triggers chemotherapy resistance in osteosarcoma. We first screened hypoxia- and normoxia- cultured osteosarcoma cells in silico to identify the differentially expressed genes specifically related to drug resistance. This led to the identification of spindle and kinetochore associated complex subunit 1 (SKA1) as a probable gene of interest. SKA1 was further overexpressed by a lentiviral vector into an osteosarcoma cell line to study its role in chemoresistance. Our data revealed that SKA1 overexpression reduced the expression of some multidrug resistance genes, and enhanced the sensitivity of two common chemotherapeutic drugs used in osteosarcoma patients, epirubicin (EPI) and ifosfamide (IFO). In addition, we also confirmed the role of SKA1 in EPI drug sensitivity in vivo. Taken together, our study indicated that hypoxia mediated downregulation of SKA1 expression increased the chemotherapy resistance in human osteosarcoma cells.

Prospective Preliminary In Vitro Investigation of a Magnetic Iron Oxide Nanoparticle Conjugated with Ligand CD80 and VEGF Antibody As a Targeted Drug Delivery System for the Induction of Cell Death in Rodent Osteosarcoma Cells

Target drug deliveries using nanotechnology are a novel consideration in the treatment of cancer. We present herein an in vitro mouse model for the preliminary investigation of the efficacy of an iron oxide nanoparticle complex conjugated to vascular endothelial growth factor (VEGF) antibody and ligand cluster of differentiation 80 (CD80) for the purpose of eventual translational applications in the treatment of human osteosarcoma (OSA). The 35 nm diameter iron oxide magnetic nanoparticles are functionalized with an n-hydroxysuccinimide biocompatible coating and are conjugated on the surface to proteins VEGF antibody and ligand CD80. Combined, these proteins have the ability to target OSA cells and induce apoptosis. The proposed system was tested on a cancerous rodent osteoblast cell line (ATCCTM^NPO CRL-2836) at four different concentrations (0.1, 1.0, 10.0, and 100.0 μg/mL) of ligand CD80 alone, VEGF antibody alone, and a combination thereof (CD80+VEGF). Systems were implemented every 24 h over different sequential treatment timelines: 24, 48, and 72 h, to find the optimal protein concentration required for a reduction in cell proliferation. Results demonstrated that a combination of ligand CD80 and VEGF antibody was consistently most effective at reducing aberrant osteoblastic proliferation for both the 24- and 72-h timelines. At 48 h, however, an increase in cell proliferation was documented for the 0.1 and 1 μg/mL groups. For the 24- and 72-h tests, concentrations of 1.0 μg/mL of CD80+VEGF and 0.1 μg/mL of VEGF antibody were most effective. Concentrations of 10.0 and 100.0 μg/mL of CD80+VEGF reduced cell proliferation, but not as remarkably as the 1.0 μg/mL concentration. In addition, cell proliferation data showed that multiple treatments (72-h test) induced cell death in the osteoblasts better than a single treatment. Future targeted drug delivery system research includes trials in OSA cell lines from greater phylum species having spontaneous OSA, such as the dog, and on a human OSA cell line model.

Microenvironmental Niches and Sanctuaries: A Route to Acquired Resistance

A tumor vasculature that is functionally abnormal results in irregular gradients of metabolites and drugs within the tumor tissue. Recently, significant efforts have been committed to experimentally examine how cellular response to anti-cancer treatments varies based on the environment in which the cells are grown. In vitro studies point to specific conditions in which tumor cells can remain dormant and survive the treatment. In vivo results suggest that cells can escape the effects of drug therapy in tissue regions that are poorly penetrated by the drugs. Better understanding how the tumor microenvironments influence the emergence of drug resistance in both primary and metastatic tumors may improve drug development and the design of more effective therapeutic protocols. This chapter presents a hybrid agent-based model of the growth of tumor micrometastases and explores how microenvironmental factors can contribute to the development of acquired resistance in response to a DNA damaging drug. The specific microenvironments of interest in this work are tumor hypoxic niches and tumor normoxic sanctuaries with poor drug penetration. We aim to quantify how spatial constraints of limited drug transport and quiescent cell survival contribute to the development of drug resistant tumors.