Radiotherapy and dosimetry of ⁶⁴Cu-TETA-Tyr³-octreotate in a somatostatin receptor-positive, tumor-bearing rat model

⁶⁴Cu [T( 1/2 ) = 12.8 h; β⁺ = 0.655 MeV (19%); β^- = 0.573 MeV (40%)] has shown promise as a radioisotope for targeted radiotherapy. It has been demonstrated previously that the somatostatin analogue ⁶⁴Cu-TETA-octreotide (⁶⁴Cu-TETA-OC, where TETA is 1,4,8,11-tetraazacyclotetradecane-N,N',N_,N'_- tetraacetic acid) significantly inhibited the growth of somatostatin receptor-positive CA20948 rat pancreatic tumors in Lewis rats (C. J. Anderson et al., J. Nucl. Med., 39: 1944-1951, 1998). In this study, we evaluated the radiotherapeutic efficacy of a new ⁶⁴Cu-labeled somatostatin analogue, ⁶⁴Cu-TETA-Tyr³-octreotate (⁶⁴CuTETA-Y3-TATE), in CA20948 tumor-bearing rats. A single dose of 15 mCi (555 MBq) of ⁶⁴Cu-TETA-Y3-TATE was shown to be more effective in reducing tumor burden than the same dose of ⁶⁴Cu-TETA- OC. In multiple dose experiments, tumor-bearing rats were administered three doses of either 10 or 20 mCi (370 or 740 MBq) of ⁶⁴Cu-TETA-Y3TATE at 48-h intervals. Rats given 3 x 10 mCi (3 x 370 MBq) showed extended mean survival times compared with rats given a single dose; however, no complete regressions occurred. Complete regression of tumors was observed for all rats treated with 3 x 20 mCi (3 x 740 MBq), with no palpable tumors for ~10 days; moreover, the mean survival time of these rats was nearly twice that of controls. Toxicity was determined by physical appearance and hematological and enzyme analysis, which revealed no overt toxicity and only transient changes in blood and liver chemistry. Absorbed dose estimates showed the dose-limiting organ to be the kidneys. The radiotherapy results, along with absorbed dose estimates to target and clearance organs, confirm that ⁶⁴Cu- labeled somatostatin analogues warrant continued consideration as agents for targeted radiotherapy.