Pre-clinical experience with Re-188-RC-160, a radiolabeled somatostatin analog for use in peptide-targeted radiotherapy.

The clinical potential of radiolabeled peptides such as octreotide and VIP has been widely established for tumor localization. Radiotherapy based on the tumor binding potential of the peptides and the radiotoxic effects of beta- or a-emitting radionuclides is an extension of such applications. Rhenium-188 (T1/2 16.9 hr, beta-max 2.1 MeV) coupled to the analogue RC-160 has been used to establish the feasibility of treating tumors with radiolabeled peptides, and our experience with this approach is summarized. In three different experimental tumor models (human prostate, mammary gland, and small cell lung carcinomas) in nude mice, treatment resulted in significant reduction or elimination of tumor burden. Two routes of administration were used: intra-lesional injection (prostate carcinoma) and intra-cavity injection (mammary and SCLC). Re-188-labeled negative control peptides bound to tumor cells to a low extent and did not exhibit therapeutic benefit. RC-160 by itself did not result in therapeutic benefit. Tumors which did not bind Re-188-RC-160 did not evidence a therapeutic benefit. Uncoupled Re-188 (control) was rapidly excreted via the urinary bladder and did not accumulate in either tumors or normal tissues even following direct injection. Instant radiolabeling kits containing 200 micrograms of RC-160 were labeled with < 3000 MBq of Re-188 in 30 minutes with no need for subsequent purification. These studies establish the conceptual feasibility of targeted radiotherapy based on the local or regional administration of radiolabeled peptides.

Experimental radiotherapy of receptor-positive human prostate adenocarcinoma with 188Re-RC-160, a directly-radiolabeled somatostatin analogue.

The therapeutic potential of the somatostatin analogue RC-160 radiolabeled with 188Re was evaluated in nude mice bearing xenografts of human prostate adenocarcinoma. 188Re-RC-160 was selectively retained in both DU-145 and PC-3 tumors following direct intra-tumor injection at all time points examined (2, 6 and 24 hr post-injection). Unbound 188Re-RC-160 was rapidly excreted via the hepatobiliary system and, with the exception of the gastrointestinal tract, very little normal organ uptake was found at any time point examined. Negative control compounds, 188Re-perrhenate and 188Re-mercaptoacetyl-triglycine (188Re-MAG3), were essentially washed out of the tumor by 6 hr post-injection and were rapidly excreted through the kidneys. 131I-RC-160, used as reference compound, had a biodistribution in tumor-bearing animals similar to that of 188Re-RC-160. In PC-3 xenografts, 188Re-RC-160 gave a dose-dependent therapeutic response (stasis or regression) even in animals with relatively large tumor masses (greater than 600 mm3), whereas the macro-aggregated form of 188Re-RC-160 did not. Long-term studies with 188Re-RC-160 demonstrated a protracted reduction of tumor volume and a positive effect on animal survival. Neither RC-160 by itself nor a 188Re-labeled peptide, unrelated to somatostatin (PA-22-2, a laminin peptide), demonstrated the reduction in tumor mass observed with 188Re-RC-160. 188Re-RC-160 shows potential as a new clinical agent for treatment of somatostatin-receptor-positive cancers.