Role of radiolabeled octreotide therapy in patients with metastatic neuroendocrine neoplasms

Review: update on selection of optimal radiopharmaceuticals for clinical trials

Multiple formulations of radiopharmaceuticals (RPs) are possible because of engineering at the nanometer scale. Yet, numbers of patients are limited, and the cost of each clinical trial is high. Thus, there is the need of preclinical evaluation of one agent versus another for the selection of an optimal choice. In the application of RPs to cancer, this selection involves both visualization and treatment aspects. In this paper, we propose the use of imaging and therapeutic figures of merit (IFOM and TFOM, respectively) to select the optimal structure and radiolabel for subsequent clinical trials given animal biodistribution results. Limiting cases and Monte Carlo simulation were used to demonstrate that these modern figures of merit are superior to traditional ratio functions that have been employed in these two contexts. Finally, there is the question of how animal and human results resemble each other kinetically. We considered allometry and compared mouse and human results for several of the cognate cT84.66 antibodies (anti-CEA; carcinoembryonic antigen). While kinetics of intact and 120-kDa engineered proteins are similar across the two species, the 80-kDa cognate shows a manifest difference in the RP first moment in the blood. In particular, human blood clearance is slower than that seen in the nude mouse. We suggest that such allometric comparisons become standard in the reporting of clinical trials.

Targeted radionuclide therapy

Targeted radionuclide therapy (TRT) seeks molecular and functional targets within patient tumor sites. A number of agents have been constructed and labeled with beta, alpha, and Auger emitters. Radionuclide carriers spanning a broad range of sizes; e.g., antibodies, liposomes, and constructs such as nanoparticles have been used in these studies. Uptake, in percent-injected dose per gram of malignant tissue, is used to evaluate the specificity of the targeting vehicle. Lymphoma (B-cell) has been the primary clinical application. Extension to solid tumors will require raising the macroscopic absorbed dose by several-fold over values found in present technology. Methods that may effect such changes include multistep targeting, simultaneous chemotherapy, and external sequestration of the agent. Toxicity has primarily involved red marrow so that marrow replacement can also be used to enhance future TRT treatments. Correlation of toxicities and treatment efficiency has been limited by relatively poor absorbed dose estimates partly because of using standard (phantom) organ sizes. These associations will be improved in the future by obtaining patient-specific organ size and activity data with hybrid SPECT/CT and PET/CT scanners.