Polymer Based Nuclear Imaging and Radiotherapy

With the appropriate delivery system, radioisotopes have a significant advantage over other therapy agents, namely, the emission of energy that can kill at a distance from the point of radioisotope localization. This diameter of effectiveness helps to overcome the problem of tumor heterogeneity because, unlike other molecular therapy (cell toxins, chemotherapy, etc.), not all tumor cells need to take up the radioisotope to eradicate a tumor. There are also physical characteristics (type of particle emission, emission energy, half-life) of different radioisotopes that may be selected to enhance therapeutic effectiveness.174 For example, different isotopes deliver beta particulate ionization over millimeters (131I) to centimeters (90Y). Long-lived isotopes such as 131I that remain within the tumor target may provide extended radiation exposure and high radiation dose, especially if there is progressive renal clearance and high target to non-target ratios.

RGD peptides labeled with therapeutically relevant isotopes such as b-particle emitters have been investigated as potential angiogenesis targeted radiotherapy (Table 9.5). The chelation conditions for 90Y and lutetium-177 (177Lu) labeled RGD have revealed that time, temperature, pH, presence of trace metal contaminants, and stoichiometric ratio of chelator to isotope all have significant effects on the rate of chelation and radiolabeling efficiency.175 A major challenge in development of therapeutic radiopharmaceuticals is radiolytic degradation of radiolabeled products because of production of free radicals in the presence of a large amount of high energy b-particles.176 A study on the stability of 90Y labeled, dimeric RGD peptide showed that presence

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