Tachykinin Receptor-Selectivity of the Potential Glioblastoma-Targeted Therapy, DOTA-[Thi⁸,Met(O₂)¹¹]-Substance P

Radiopharmaceutical development hinges on the affinity and selectivity of the biological component for the intended target. An analogue of the neuropeptide Substance P (SP), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[Thi⁸,Met(O₂)¹¹]-SP (DOTA-[Thi⁸,Met(O₂)¹¹]SP), in the theranostic pair [⁶⁸Ga]Ga-/ [²¹³Bi]Bi-DOTA-[Thi⁸,Met(O₂)¹¹]SP has shown promising clinical results in the treatment of inoperable glioblastoma. As the theranostic targeting component, modifications to SP that affect the selectivity of the resulting analogue for the intended target (neurokinin-1 receptor [NK1R]) could be detrimental to its therapeutic potential. In addition to other closely related tachykinin receptors (neurokinin-2 receptor [NK2R] and neurokinin-3 receptor [NK3R]), SP can activate a mast cell expressed receptor Mas-related G protein-coupled receptor subtype 2 (MRGPRX2), which has been implicated in allergic-type reactions. Therefore, activation of these receptors by SP analogues has severe implications for their therapeutic potential. Here, the receptor selectivity of DOTA-[Thi⁸,Met(O₂)¹¹]SP was examined using inositol phosphate accumulation assay in HEK293-T cells expressing NK1R, NK2R, NK3R or MRGPRX2. DOTA-[Thi⁸,Met(O₂)¹¹]SP had similar efficacy and potency as native SP at NK1R, but displayed greater NK1R selectivity. DOTA-[Thi⁸,Met(O₂)¹¹]SP was unable to elicit significant activation of the other tachykinin receptors nor MRGPRX2 at high concentrations nor did it display antagonistic behaviour at these receptors. DOTA-[Thi⁸,Met(O₂)¹¹]SP, therefore has high potency and selectivity for NK1R, supporting its potential for targeted theranostic use in glioblastoma multiforme and other conditions characterised by NK1R overexpression.