Evaluation of ABD-Linked\nRM26 Conjugates for GRPR-Targeted\nDrug Delivery

Targeting the gastrin-releasing peptide receptor (GRPR)\nwith the\nbombesin analogue RM26, a 9 aa peptide, has been a promising strategy\nfor cancer theranostics, with recent success in radionuclide imaging\nof prostate cancer. However, therapeutic application of the short\npeptide RM26 would require a longer half-life to prevent fast clearance\nfrom the circulation. Conjugation to an albumin-binding domain (ABD)\nis a viable strategy to extend the <i>in vivo</i> half-life\nof peptides and proteins. We previously reported an ABD-fused RM26\npeptide targeting GRPR (ABD-RM26 Gen 1) that showed prolonged and\nstable tumor uptake over 144 h; however, the observed high kidney\nuptake indicated that the conjugate’s binding to albumin was\nreduced and that this could be an obstacle for its use as a delivery\nsystem for targeted therapy, especially for radiotherapy. Here, we\nhave designed, produced, and preclinically evaluated a series of novel\nABD-RM26 conjugates with the aim of improving the conjugate’s\nbinding to albumin and decreasing the kidney uptake. We developed\nthree second-generation constructs with varying formats, differing\nin the relative positions of the targeting moieties and the radionuclide\nchelator. The produced conjugates were radiolabeled with indium-111\nand evaluated <i>in vitro</i> and <i>in vivo</i>. All constructs displayed improved biophysical characteristics,\nbiodistribution, and lower kidney uptake compared to previously reported\nfirst-generation molecules. The ABD-RM26 Gen 2A conjugate showed the\nbest biodistribution profile with a nearly 6-fold reduction in kidney\nuptake. However, the ABD-RM26 Gen 2A conjugate’s binding to\nGRPR was compromised. This conjugate’s assembly of albumin-\nand GRPR-binding moieties might be used for further development of\ndrug conjugates for targeted therapy/radiotherapy of GRPR-expressing\ncancers.