4-Phenoxybutoxy-substituted heterocycles – A structure–activity relationship study of blockers of the lymphocyte potassium channel Kv1.3

Abstract

The voltage-gated potassium channel Kv1.3 constitutes an attractive pharmacological target for the treatment of effector memory T cell-mediated autoimmune diseases such as multiple sclerosis and psoriasis. Using 5-methoxypsoralen (5-MOP, 1), a compound isolated from Ruta graveolens, as a template we previously synthesized 5-(4-phenoxybutoxy)psoralen (PAP-1, 2) which inhibits Kv1.3 with an IC50 of 2 nM. Since PAP-1 is more than 1000-fold more potent than 5-MOP, we here investigated whether attaching a 4-phenoxybutoxy side-chain to other heterocyclic systems would also produce potent Kv1.3 blockers. While 4-phenoxybutoxy substituted quinolines, quinazolines and phenanthrenes were inactive, 4-phenoxybutoxy substituted quinolinones, furoquinolines, coumarins or furochromones inhibited Kv1.3 with IC50s of 150 nM to 10 µM in whole-cell patch-clamp experiments. Our most potent new compound is 4-(4-phenoxybutoxy)-7H-furo[3,2-g]chromene-7-thione (73, IC50 17 nM), in which the carbonyl oxygen of PAP-1 is replaced by sulfur. Taken together, our results demonstrate that the psoralen system is a crucial part of the pharmacophore of phenoxyalkoxypsoralen-type Kv1.3 blockers.

Potassium Channel Block by a Tripartite Complex of Two Cationophilic Ligands and a Potassium Ion

“…When attaching the 4-phenoxybutoxy side chain of PAP-1 to other planar heterocyclic systems, we had observed that phenoxyalkoxypsoralen-type Kv1.3 blockers have a very steep structure-activity relationship and that the coumarin system cannot be replaced (Bodendiek et al, 2009). Our model explains these structure-activity findings and the Hill coefficient of 2 and shows that the coumarin rings of two PAP-1 molecules are located in the inner pore but do not block ion permeation per se.…”

“…These results suggest that the ether oxygens are very important for PAP-1 activity and explain why compound SB9 (Fig. 3) is ϳ4.5 times more active than its analog 1,2-dihydro-N-methyl-4-(4-phenoxybutoxy)quinolin-2-one (SB4) (Bodendiek et al, 2009), in which the ether oxygen is replaced by an N-CH 3 group (Fig. 3).…”

“…Our study proposes a K ϩ ion to be an indispensable component of the PAP-1 receptor and explains our previous failure to design a potent Kv1.3 blocker that lacks the coumarin ring system (Bodendiek et al, 2009), which we had wanted to eliminate because this structural element can potentially act as a Michael acceptor and cause liver toxicity by reacting with glutathione. When attaching the 4-phenoxybutoxy side chain of PAP-1 to other planar heterocyclic systems, we had observed that phenoxyalkoxypsoralen-type Kv1.3 blockers have a very steep structure-activity relationship and that the coumarin system cannot be replaced (Bodendiek et al, 2009).…”

“…The Hill coefficient of 2 determined for PAP-1 with a 10-concentration dose-concentration curve (Schmitz et al, 2005) for approximately 50 very closely related psoralens (Vennekamp et al, 2004;Schmitz et al, 2005;Bodendiek et al, 2009) and approximately 70 structurally related khellinones (Baell et al, 2004;Harvey et al, 2006) by whole-cell patch-clamp experiments unambiguously demonstrates that more than one cationophilic molecule binds simultaneously to one Kv1.3 channel. However, this Hill coefficient does not rule out the possibility that more than two PAP-1 ligands could bind to the channel.…”

“…3), each of which can attract a K ϩ ion. To define the oxygen atoms involved in K ϩ coordination, we synthesized 27 PAP-1 analogs in which the oxygens were either removed or substituted with other heteroatoms (Bodendiek et al, 2009). A selection of these compounds is shown in Fig.…”