Original article
New amido derivatives as potential BKCa potassium channel activators. XI

https://doi.org/10.1016/j.ejmech.2007.06.005Get rights and content

Abstract

The vasorelaxing effects of exogenous activators of large-conductance calcium-activated potassium channels (BK channels) can furnish the pharmacological rational basis for the treatment of hypertension and/or other diseases related with an impaired contractility of vessels. Since in previous works some benzanilide derivatives showed BK channel-induced vasorelaxing activity, in this paper we have taken into consideration the introduction of methylene spacer(s) between the amide linker and one or both the aromatic substituents, to evaluate the pharmacological effect caused by these lengthenings and to obtain possible useful information about structure–activity relationships. Overall, the main findings of this work suggest that the introduction of one or two methylene group(s) in the amide linker exerts a negative influence on the BK-opening properties, which can be due to an excessive lengthening of the spacer between the two aromatic rings and/or to further degrees of conformational freedom.

Introduction

The large-conductance, calcium-activated potassium channels (BK, also termed BKCa, Slo, or MaxiK) are distributed in both excitable and non-excitable cells. They are involved in many cellular functions such as action potential repolarization, neuronal excitability, neurotransmitter release, hormone secretion, tuning of cochlear hair cells, innate immunity, and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues [1], [2], [3].

BKCa channels characteristically respond to two distinct physiological stimuli, changes in membrane voltage and in cytosolic Ca2+ concentration. The BK channels gate-open in response to an increase in cytosolic Ca2+ concentration and membrane depolarization, resulting in an increase of K+ efflux, which leads to rapid hyperpolarization of the excitatory membrane and reduces Ca2+ influx through voltage-dependent Ca2+ channels.

Then, the availability of exogenous compounds able to activate BK channels can guarantee an innovative pharmacological tool for the clinical management of many pathological states, due to a cell hyperexcitability, such as asthma, urge incontinence and bladder spasm, gastric hypermotility, neurological and psychiatric disorders [1], [2].

As concerns the cardiovascular system, it is now widely accepted that BK channels ensure the predominant component of the outward K+ current in vascular smooth muscle cells, accounting for fundamental function of such ion channels in the modulation of the muscular tone of vessels [4], [5]. Consequently, the vasorelaxing effects of exogenous BK-openers can furnish the pharmacological rational basis for the treatment of hypertension and/or other diseases related with an impaired contractility of vessels (for example, coronary vasospasm) [1], [2].

In a previous work [6], we could observe that the synthesised 1-(2′-hydroxybenzoyl)-5-methyl-benzotriazole, showing structural analogies with the reference BK-openers NS 004 and NS 1619 and exhibiting vasorelaxing effects probably due to the activation of vascular BK channels, was able to confer a significant protection of the myocardial function, in isolated rat hearts submitted to ischemia/reperfusion cycles. This result (originally unexpected) can be now explained; thanks to more recent experimental evidence showing that the activation of cardiac calcium-activated potassium channels could be involved in the cardioprotective mechanisms of “ischemic preconditioning” and that the administration of BK-openers, such as NS 1619, could reduce the cardiac injury following an ischemic event [7], [8], [9]. Of course, these reports let us foresee a further potential use of BK-activators in cardiovascular pharmacotherapy, as anti-ischemic drugs.

Our research program about potential activators of BK channels, in the beginning considered heterocyclic compounds [6], [10], [11], [12] which referred to the benzimidazolone derivatives NS 004 and NS 1619 reported in the literature [13] as BK-openers. After regarding to an hypothetic simple pharmacophore (Fig. 1A) which consisted of two appropriately substituted phenyl rings connected by a linker, 1,2,3-triazole derivatives [14], [15], [16] and benzanilides [17], [18], [19] were synthesised and tested, in which the 1,2,3-triazole ring and the amide function, respectively, represented the linker.

In particular, after a first paper [17] which had shown a BK channel-induced vasorelaxing activity of the N-(2-hydroxy-5-chlorophenyl)-2-methoxy-5-chlorobenzamide (Fig. 1B) higher than NS 1619, the research about new compounds bearing the amide linker was advanced changing the benzanilide structure [18] by the introduction of a five or six membered aromatic heterocyclic substituent in the acid moiety of the amide, keeping unaltered the basic moiety as 2-hydroxy-5-chloro-anilide (Fig. 1C). In addition an aromatic or aliphatic six or five membered heterocyclic substituent was introduced in the basic moiety of the amide, keeping unaltered the acid moiety as 2-methoxy-5-chlorobenzoyl, which was then converted to the corresponding 2-hydroxy derivative (Fig. 1D). The pharmacological results indicated that the presence of nitrogen heterocycles on the acid side of the amide linker seems to be a negative requirement, while furan and thiophene rings were well tolerated. On the contrary, the introduction of unsaturated heterocyclic rings (pyridine and thiazole) on the basic side of the amide linker, led to satisfactory biological activity, but the presence of aliphatic heterocycles lowered the pharmacological effect. The presence of a phenolic function as a certain H-bond donor was confirmed.

Another structural change of the benzanilide pharmacophore concerned a further deepening of the structure–activity relationships of benzanilide derivatives previously studied as BK channel activators [19]. In this paper were reported several substitutions on the phenyl rings of the reference benzanilides, which possessed particular and specific properties from a mesomeric and/or steric point of view. The pharmacological results indicated that several compounds exhibited vasorelaxing effects which could not be attributed to the activation of BK channels, but two derivatives showed a clear profile of BK-activators with a vasodilator activity comparable to or slightly lower than that recorded for the reference benzimidazolone NS 1619.

In this paper we have taken into consideration the introduction of methylenic spacers between the amide linker and the two aromatic substituents which form the benzanilide pharmacophore to evaluate the pharmacological effect caused by these lengthenings.

Section snippets

Chemistry

At first a methylenic bridge was introduced in the acid moiety of the benzanilide, by the preparation of a new series of phenylacetanilides (Scheme 1). Compounds 3ag were prepared by reaction of 2-methoxy-phenylacetic acid chloride (1) with the suitable substituted aniline (2ag) in toluene in the presence of triethylamine. The employed anilines had differentiated substituents as 2-hydroxy-5-chloro- (2a), 2-hydroxy-5-methyl- (2b), 2-methoxy-5-nitro- (2c), 2-methyl-4-nitro- (2d), 4-nitro- (2e),

Pharmacology

As a preliminary indication about a possible BK-activating mechanism of action, the vasodilating effect of the new compounds was studied in vitro on isolated rat aortic rings precontracted with KCl (20 mM) (see later for the pharmacological details).

Results and discussion

The vasorelaxing efficacy and potency of the tested compounds and NS 1619 are summarised in Table 3. Many of the synthesised compounds resulted ineffective or exhibited low levels of vasorelaxing efficacy (<50%).

The first evidence emerging from the pharmacological results seems to give a clear indication about the negative impact exerted by the structural features showed by compounds 8, 9, 11 and 12.

In particular, these compounds, all characterised by very low levels of vasorelaxing efficacy,

Chemistry

Melting points were determined on a Kofler hot-stage and are uncorrected. IR spectra in nujol mulls were recorded on a Mattson Genesis series FTIR spectrometer. 1H NMR spectra were recorded with a Varian Gemini 200 spectrometer in DMSO-d6 or CDCl3, in δ units, using TMS as internal standard. Mass spectra were performed with a Trace GC Q plus, thermo quest Finnigan. Elemental analyses (C, H, N) were within ±0.4% of the theoretical values and were performed on a Carlo Erba Elemental Analyzer Mod.

References (24)

  • S. Ghatta et al.

    Pharmacol. Ther.

    (2006)
  • G. Biagi et al.

    Farmaco

    (2001)
  • Y. Shintani et al.

    J. Mol. Cell. Cardiol.

    (2004)
  • G. Biagi et al.

    Eur. J. Med. Chem.

    (2000)
  • B. Baragatti et al.

    Eur. J. Med. Chem.

    (2000)
  • G. Biagi et al.

    Farmaco

    (2001)
  • S.P. Olesen et al.

    Eur. J. Pharmacol.

    (1994)
  • G. Biagi et al.

    Farmaco

    (2004)
  • V. Calderone et al.

    Farmaco

    (2005)
  • V. Calderone et al.

    Eur. J. Med. Chem.

    (2005)
  • G. Biagi et al.

    Eur. J. Med. Chem.

    (2004)
  • V. Calderone et al.

    Eur. J. Med. Chem.

    (2006)
  • View full text