CELT-133 is a selective fluorescent antagonist for the hα1A adrenergic receptor, demonstrating complete selectivity towards α1A over α2A receptors, with a Ki of 5 nM and 15% displacement at 1 μM in radioligand binding assays, respectively. It displays equivalent affinity across all α1 adrenergic receptor subtypes. Notably, CELT-133 features excitation and emission maxima (λ) at 560 and 571 nm, respectively, making it suitable for use as an acceptor dye in time-resolved Förster resonance energy transfer (TR-FRET) assays when paired with the CoraFluor 1 TR-FRET donor.