In vitro binding evaluation of ¹⁷⁷Lu-AMBA, a novel ¹⁷⁷Lu-labeled GRP-R agonist for systemic radiotherapy in human tissues

Members of the gastrin-releasing peptide (GRP) family and its analogs bombesin (BBN) have been implicated in the biology of several human cancers including prostate, breast, colon and lung. To date, three mammalian GRP/BBN receptor subtypes have been cloned and characterized: the neuromedin B receptor (NMBR), the GRP receptor (GRPR) and the BBN-receptor subtype 3 (BB₃). The fourth BBN receptor subtype, BB₄, has only been identified in amphibian and at present no mammalian equivalent of this receptor has been described. GRPR analogs have been used as carriers to deliver drugs, radionuclides and cytotoxins to target various cancer types that are GRPR positive. We investigated the in vitro binding properties of ¹⁷⁷Lu-AMBA, a novel radiolabelled BBN analog currently undergoing clinical trial as systemic radiotherapy for hormone refractory prostate cancer (HRPC) patients. Pharmacological analyses of the ¹⁷⁷Lu-AMBA was determined using in vitro binding studies using membrane target system containing specific receptor subtypes. We investigated the distribution of binding sites for ¹⁷⁷Lu-AMBA by receptor autoradiography on human neoplastic and non-neoplastic tissues. Pharmacological characterizations of ¹⁷⁷Lu-AMBA shows, high affinity towards NMB and GRP receptors, while little or no affinity towards BB₃ receptor. Among the 40 different types of non-neoplastic tissues tested seven of them showed limited but specific binding of ¹⁷⁷Lu-AMBA. Fourteen of 17 primary prostate cancers, six of 13 primary breast cancers expressed binding sites for ¹⁷⁷Lu-AMBA. Furthermore, no apparent differences in ¹⁷⁷Lu-AMBA-binding sites expression were observed between matched pairs (primary vs. secondary) of prostate and breast cancer tissues. These data represent the molecular basis for clinical applications of ¹⁷⁷Lu-AMBA for diagnosis and treatment of GRP-R and NMB-R positive tumors.