Through optimization of the linker between our DO3A-chelate and the biologically active Bombesin (BBN) [7-14], we have developed a radiolabeled agonist with an improved pharmacokinetic profile, 177Lu-AMBA. This compound targets two bombesin receptor subtypes, the Gastrin Releasing Peptide receptor (GRP-R), and the Neuromedin-B receptor (NMB-R) which are upregulated in several human cancers. In vitro studies (PC-3, human prostate) included direct binding, competition binding, and downregulation/regeneration of receptor. 177Lu-AMBA has an IC50 of 3 nM, relative to 125I-Tyr4-BBN. Following incubation with 177Lu-AMBA for 40 min at 37oC, 74% of cell-associated counts were internalized with only 7% efflux in 2h. The BBN-R was rapidly down regulated in PC3 cells (50% max in 3 min), and the receptor was almost completely regenerated in 24 h. Biodistribution of 177Lu-AMBA (formulated/non-purified vs. HPLC-purified) in male PC-3 tumor-bearing mice showed favorable uptake in tumors (4-6% ID/g), rapid clearance from blood (<1% ID/g at 1 h); renal excretion (44-49% ID at 1 h); and low kidney retention (3.5-6.6% ID/g at 1 h; 2% ID/g at 24 h). Multidose radiotherapy efficacy studies (1 or 2 doses spaced 14 days apart) were performed in the PC-3 model using 30 mCi/kg of 177Lu-AMBA s.c. (n=32-36), or vehicle control s.c. (n=16), and followed for up to 120 days. The 2-dose treatment demonstrated an increase in tumor growth delay and in overall survival over the single dose regimen (47% vs. 38%). The following comparison of 1 vs. 2 dose groups using modified WHO criteria demonstrates that there is a favorable shift toward complete and partial response with multidose treatment: CR= 31% vs. 39%, PR=44% vs. 61%, SD=3% vs. 0%, PD=22% vs. 0%. A seven day dosimetry study performed in normal rats was used to estimate the absorbed radiation dose to humans as calculated using the Adult Male Phantom for 177Lu. Due to favorable PK, we estimate that in humans as much as 1.6 Ci of 177Lu-AMBA could be delivered without exceeding the 23 Gy limit for kidney or the 2.5 Gy limit for bone marrow. 177Lu-AMBA demonstrates a very favorable risk-benefit ratio and is now in full development.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]