Thiopyranol[2,3,4-c,d]indoles as inhibitors of 5-lipoxygenase, 5-lipoxygenase-activating protein, and leukotriene C4 synthase.

The attachment of an arylacetic or benzoic acid moiety to the thiopyrano[2,3,4-c,d]indole nucleus results in compounds which are highly potent and selective 5-lipoxygenase (5-LO) inhibitors. These compounds are structurally simpler than previous compounds of similar potency in that they contain a single chiral center. From the data presented, 2-[[1-(3-chlorobenzyl)-4-methyl-6-[(5-phenylpyridin-2-yl)methoxy]-4,5-dihydro-1H-thiopyrano[2,3,4-c,d]indol-2-yl]methoxy] -phenylacetic acid, 14b, was shown to inhibit 5-hydroperoxyeicosatetraenoic acid (5-HPETE) production by human 5-LO (IC 50 of 18 nM). The acid 14b is highly selective as an inhibitor of 5-LO activity when compared to the inhibition of ram seminal vesicle cyclooxygenase (IC 50 > 5 μM) or human leukocyte leukotriene A 4 (LTA 4 ) hydrolase (IC 50 > 20 μM). In addition, 14b was inactive in a 5-lipoxygenase-activating protein (FLAP) binding assay at 10 μM. In vivo studies showed that 14b is bioavailable in rat and functionally active in the hyperreactive rat model of antigen-induced dyspnea (74% inhibition at 0.5 mk/kg po ; 2 h pretreatment). In the conscious squirrel monkey model of asthma, 14b showed excellent functional activity at 0.1 mg/kg against antigen-induced bronchoconstriction (94% inhibition of the increase in R L and 100% inhibition in the decrease in C dyn ; n = 4). Resolution of this compound gave (-)-14b, the most potent enantiomer (IC 50 = 10 nM in the human 5-LO assay), which was shown to possess the S configuration at the chiral center by X-ray crystallographic analysis of an intermediate. Subsequent studies on the aryl thiopyrano[2,3,4-c,d]indole series of inhibitors led to the discovery of potent dual inhibitors of both FLAP and 5-LO, the most potent of which is 2-[[1-(4-chlorobenzyl)-4-methyl-6-(quinolin-2-ylmethoxy)-4,5-dihydro-1H-thiopyrano[2,3,4-c,d]indol -2-yl]methoxy]phenylacetic acid, 19. Acid 19 has an IC 50 of 100 nM for the inhibition of 5-HPETE production by human 5-LO and is active in a FLAP binding assay with an IC 50 of 32 nM. Furthermore, thiopyrano[2,3,4-c,d]indoles such as 1 and 14b are capable of inhibiting the LTC 4 synthase reaction in a dose dependent manner (IC 50 s of 11 and 16 μM, respectively, compared to that of LTC 2 at 1.2 μM) in contrast to other, structurally distinct 5-LO inhibitors. It has also been observed that the thiopyrano[2,3,4-c,d]indole class of compounds strongly promotes the translocation of 5-LO from the cytosol to a membrane fraction in the presence or absence of the ionophore A23187. The membrane-bound 5-LO retains its activity, and this association with the membrane is not reversed with the FLAP inhibitor MK-886. These observations, in part, support the hypothesis that these compounds act by binding at the arachidonic acid-binding site on FLAP, 5-LO, and LTC 4 synthase and that the thiopyrano[2,3,4-c,d]indole is capable of mimicking an active conformation of arachidonic acid.