Date of Award
Doctor of Philosophy (PhD)
Multiple myeloma (MM) is a debilitating hematologic malignancy of terminally differentiated plasma cells in the bone marrow (BM). Advances in therapeutic regimens and the use of autologous stem cell transplantation have significantly improved survival rates and quality of life in patients. However, the disease remains incurable, with shorter, successive remission cycles following relapse. To reduce systemic, off-target toxicity and improve quality of life, there is a need for improved stratification of responding patients. Identification of specific, noninvasive, imaging biomarkers that correlate to therapeutic efficacy is an attractive strategy for stratifying responding patients, since the use of positron emission tomography (PET), computed tomography (CT), and magnetic resonance imaging (MRI) is clinically established. Here, we have developed a strategy for imaging MM disease pathogenesis and response to clinically relevant therapeutics by studying the bidirectional interactions between the BM microenvironment and myeloma cells at the cellular, environmental, and anatomical levels. Specifically, we have validated imaging markers that identify BM and myeloma-specific behaviors through three specific aims:
The first aim validated the use of the phenylalanine analog 18F-FDOPA for monitoring the uptake and efficacy of the DNA alkylating agent melphalan, which is used extensively in elderly, non-transplant eligible patients and in relapsed, refractory disease. 18F-FDOPA uptake was significantly reduced in melphalan-treated mice with orthotopic myeloma tumors, and was concordant with the established 18F-FDG-PET imaging. Immunohistochemistry was used to validate 18F-FDOPA uptake results. Importantly, expression of LAT1, which is known to mediate 18F-FDOPA and melphalan uptake, was visibly increased, although this may be a result of increased tumor vascularity. Our results suggest that 18F-FDOPA-PET can provide complementary imaging to 18F-FDG-PET for monitoring response to melphalan therapy and overall LAT1 expression in MM.
The second aim assessed the specificity and sensitivity of the peptidomimetic near-infrared fluorophore LLP2A-Cy5 for imaging the expression of the activated conformation of the VLA-4 integrin on the surface of myeloma cells. LLP2A-Cy5 imaging was also used to study response to treatment with the proteasome inhibitor bortezomib, which forms the backbone of several front-line MM therapy strategies. Uptake of LLP2A-Cy5 was significantly reduced in bortezomib-treated mice bearing intramedullary tumors, indicating a reduction in the expression of activated VLA-4. These observations are concordant with the known downregulation of adhesion-mediated drug resistance and VLA-4 by bortezomib. Our results indicate the viability of using LLP2A-Cy5 near-infrared imaging for sensitive, longitudinal assessment of VLA-4 expression for monitoring bortezomib treatment response.
Finally, the third aim validated the use of preclinical, multi-parametric MRI for studying changes in the BM in a diffuse infiltrative intramedullary tumor model. Longitudinal imaging of the BM in the femur and tibia demonstrated significant regional differences in T1-weighted contrast uptake and parametric T2 that correlated to changes in viable tumor burden following treatment with bortezomib. Hematoxylin and eosin staining (H&E) was used to validate the MRI observations. H&E showed complete diffuse infiltration of the BM in untreated animals, while bortezomib therapy caused the concentration of tumor burden near the epiphyseal plate of the distal femur and proximal tibia. These observations, in combination with MRI results, establish the use of preclinical MRI for studying effects of disease progression and therapy response on the BM in a longitudinal, noninvasive manner.
In summary, these studies established a combination of qualitative observations and quantitative results in PET, optical, and MRI based strategies. Thus, this project has integrated a structured, multi-modal approach for assessing changes in tumor burden and monitoring therapy response at varying granular levels within the myeloma/BM interaction spectrum. Future studies would adapt this approach into different cell lines and tumor models.
Samuel Achilefu, Kooresh Shoghi, Jonathan Silva, Mark Anastasio,