A tethered ascorbate-norepinephrine compound, 4-UT, displays long-acting adrenergic activity on rabbit aortic smooth muscle

We previously demonstrated that adrenergic and histaminergic receptors have an ascorbic acid (vitamin C) binding site on the first extracellular loop, immediately adjacent to the aminergic binding site. Binding of ascorbate to this site strongly potentiates any sub-maximal dose of an adrenergic or histaminergic compound, significantly increasing its duration of activity. We report here the successful synthesis of a tethered compound that mimics the combined effects of a mixture of ascorbate with norepinephrine. The tethered compound uses a four-unit polyethylene linker to tether ascorbate to norepinephrine. The tethered compound is about tenfold less effective than norepinephrine in stimulating rabbit aortic smooth muscle, but has a very significantly enhanced duration of activity compared with norepinephrine alone and comparable to a mixture of norepinephrine and ascorbate. Additional ascorbate does not enhance the tethered compound's effects and we demonstrate that the compound binds to a synthetic peptide spanning the ascorbate binding site of the receptor. These experiments strongly suggest that the compound binds to both the adrenergic binding site and the ascorbate binding site simultaneously. Tethered compounds with linkers of other lengths did not have these properties. We believe that the synthesis of enhanced adrenergic and histaminergic drugs by tethering them to potentiators such as ascorbate will permit a new class of potential drugs to be created with high specificity and long duration of activity. Drug Dev Res 69:242–250, 2008. © 2008 Wiley-Liss, Inc.