2-(4,5-Dihydroimidazol-2-yl)benzimidazoles as highly selective imidazoline I2/adrenergic α2 receptor ligands

N-(Imidazolidin-2-ylidene)-1-arylmethanamine oxides: synthesis, structure and pharmacological evaluation

A high yielding three-step procedure was applied for the synthesis of N-(imidazolidin-2-ylidene)-1-arylmethanamine oxides 3 (α-aminonitrones) starting from the easily accessible imidazolidin-2-one O-benzyl oxime 1. The α-aminonitrone-α-iminohydroxyloamine tautomerism of these products was studied theoretically and the structures of the synthesised compounds were confirmed by single crystal X-ray crystallographic analysis. The compounds were evaluated in vitro for their binding affinities to α(1) and α(2) adrenoceptors as well as imidazoline I(1) and I(2) receptors. The highest potencies at the α(2) adrenergic receptors were observed for compounds bearing biphenyl (4h, K(i)  = 9 nM) and naphthyl (4i, K(i)  = 92 nM) moieties. Compounds 4h and 4i were further tested in vivo for their cardiovascular and sedative-hypnotic effects in rats.

Imidazoline I2 receptors: target for new analgesics?

Pain remains a major clinical challenge because there are no effective analgesics for some pain conditions and the mainstay analgesics for severe pain, opioids, have serious unwanted effects. There is a dire need for novel analgesics in the clinic. Imidazoline receptors are a family of three receptors (I(1), I(2) and I(3)) that all can recognize compounds with an imidazoline structure. Accumulating evidence suggests that I(2) receptors are involved in pain modulation. Ligands acting at I(2) receptors are effective for tonic inflammatory and neuropathic pain but are much less effective for acute phasic pain. When studied in combination, I(2) receptor ligands enhance the analgesic effects of opioids in both acute phasic and chronic tonic pain. During chronic use, patients can develop tolerance to and dependence on opioids. Imidazoline I(2) receptor ligands can attenuate the development of tolerance to opioid analgesia and inhibit drug withdrawal or antagonist precipitation induced abstinence syndrome in animals. Taken together, drugs acting on I(2) receptors may be useful as a monotherapy or combined with opioids as an adjuvant for treating pain. Future studies should focus on understanding the relative efficacy of I(2) receptor ligands and developing new compounds to fill the gap in intrinsic efficacy continuum of I(2) receptors.