Substitution of l-Tryptophan by α-Methyl-l-Tryptophan in ¹⁷⁷Lu-RM2 Results in ¹⁷⁷Lu-AMTG, a High-Affinity Gastrin-Releasing Peptide Receptor Ligand with Improved In Vivo Stability

Chemical structure of RM2 and its α-Me-l-Trp (α-Me-l-Trp⁸) modified derivatives AMTG and AMTG2, as well as reference ligand NeoBOMB1. Structural differences from RM2 are highlighted in red.

Preclinical data of ^nat/177Lu-RM2 (red), ^nat/177Lu-NeoBOMB1 (gray), ^nat/177Lu-AMTG (green), and ^nat/177Lu-AMTG2 (blue). (A) Affinity data on PC-3 and T-47D cells (1.5 × 10⁵ cells/mL/well) using 3-¹²⁵I-d-Tyr⁶-MJ9 (0.2 nM/well) as radiolabeled reference (2 h, room temperature, Hanks’ balanced salt solution plus 1% bovine serum albumin [v/v]). (B) GRPR-mediated internalization (0.25 pmol/well) on PC-3 cells as percentage of applied activity (incubation at 37°C for 1 h, Dulbecco modified Eagle medium/F-12 plus 5% bovine serum albumin [v/v], 1.5 × 10⁵ cells/mL/well). Data were corrected for nonspecific binding (10⁻³ M ^natLu-RM2). (C) Lipophilicity at pH 7.4 (logD_7.4). (D) Metabolic stability in vitro in human plasma (left) (37°C, 72 ± 2 h; n = 4), and metabolic stability in vivo in murine plasma (middle) and murine urine (right) at 30 min after injection (n = 3). Data are expressed as mean ± SD. Metabolic stability of ¹⁷⁷Lu-RM2 derivatives was determined in vitro and in vivo.

Maximum-intensity projection of PC-3 tumor–bearing CB17-SCID mice injected with ¹⁷⁷Lu-RM2 and ¹⁷⁷Lu-AMTG (100 pmol each). Images were acquired at 1, 4, 8, 24, and 28 h after injection into PC-3 tumors (arrows). p.i. = after injection.

(A) Biodistribution of ¹⁷⁷Lu-RM2 (red) and ¹⁷⁷Lu-AMTG (green) in selected organs at 24 h after injection in PC-3 tumor–bearing CB17-SCID mice (100 pmol each). Data are %ID/g, mean ± SD (n = 4). (B) Tumor-to-background ratios for selected organs for ¹⁷⁷Lu-RM2 (red) and ¹⁷⁷Lu-AMTG (green) at 24 h after injection in PC-3 tumor–bearing CB17-SCID mice. Data are mean ± SD (n = 4).