PET Imaging of Nucleoside Metabolism for Individualized Therapy

Molecular imaging diagnostics such as Positron Emission Tomography (PET) have significantly improved cancer patient management. PET allows the non-invasive, whole body imaging of metabolic processes and has been extensively used to detect lesions and monitor patient response to therapeutic intervention. Despite the development of a large number of PET probes, only a limited few find utility for imaging cancer. Furthermore, complex probe metabolism may render image interpretation difficult. With these factors in mind, my work focuses on improving PET imaging of nucleoside metabolism for cancer detection and prediction of treatment response.The thymidine analog, 3‘-deoxy-3‘-18F-fluorothymidine (18F-FLT), is extensively used as a measurement of tumor proliferation. However, its use is limited in tissues of murine origin, which do not avidly take up 18F-FLT. Here, we describe a pharmaceutical intervention using dipyridamole (DPA) to extend the utility of 18F-FLT PET for imaging mice tissues and tumors. Administration of DPA ten minutes prior to injection of probe allowed visualization of thymus, bone marrow and spleen tissues in C57/BL6 mice and murine tumor models of melanoma, lung carcinoma, and leukemia.The recent development of 1-(2‘-deoxy-2‘-18F-fluoro-β-D-arabinofuranosyl)cytosine (18F-FAC), a PET probe for deoxyctidine kinase (dCK), provides a new tool to measure the other arm of salvage metabolism. We show that, in addition to phosphorylation by dCK, 18F-FAC is highly susceptible to deamination and limits the utility of the probe for measuring dCK activity. We describe the development of a panel of L-analog probes that maintain affinity for dCK, low susceptibility to deamination, high uptake in dCK-expressing cell lines, and biodistribution in mice reflective of the tissue-expression pattern of dCK. Two lead compounds, 1-(2‘-deoxy-2‘-18F-fluoro-β-L-arabinofuranosyl)cytosine (L-18F-FAC) and 1-(2‘-deoxy-2‘-18F-fluoro-β-L-arabinofuranosyl)-5-methylcytosine (L-18F-FMAC), were validated in mice models of leukemia and autoimmunity.We show that (i) 18F-FAC tumor uptake is influenced by cytidine deaminase (CDA), a determinant of gemcitabine resistance; (ii) PET imaging with 18F-FAC and L-18F-FMAC can stratify tumors for dCK and CDA activities; and (iii) application of this PET assay can predict differential tumor responses to dCK-dependent nucleoside analog prodrugs, gemcitabine and clofarabine, the latter being resistant to deamination. These results support PET in patient stratification and predictive medicine.Findings here demonstrate the complexities of nucleoside analog probe metabolism and their effect on image interpretation. Understanding the biological relevance of these measurements, combined with complementary modalities such as magnetic resonance imaging (MRI), can provide new opportunities for personalized medicine