smooth muscle contraction plays a fundamental role in regulating the functions of the hollow organs in the body such as the blood vessels , intestines , bladder , airways , and uterus .
dysfunctional contraction of the smooth muscles is a key pathological feature of many diseases including hypertension , bladder overactivity ( oab ) , irritable bowel syndrome , erectile dysfunction , and asthma .
abnormal uterine contractions may also lead to preterm labor , the latter being responsible for prenatal mortality , neonatal morbidity , and childhood developmental disorders . by and large
, smooth muscle hyperactivity disorders involve immense social cost and financial burden to the health services , thus , a considerable effort has been made to understand their etiologies and also to develop drugs with potent smooth muscle relaxant activities .  
contractions of all smooth muscles absolutely depend on the presence of ca which activates the contractile machineries [ 2 , 3 ] .
elevating intracellular ca , which can be achieved by agonists ( e.g. , acetylcholine , oxytocin , and prostaglandin f2 ) , causes smooth muscle contractions . acetylcholine ( ach )
, the main contractile transmitter in many smooth muscle tissues ( e.g. , the urinary bladder and gastrointestinal tract ) , activates muscarinic ( m3 ) receptors and raises intracellular ca levels by activating the gq - phospholipase c- ( plc- ) inositol triphosphate ( ip3 ) pathway .
it is also believed that   extracellular ca facilitates an increase in intracellular ca via opening of the voltage - gated ca channels .
m2 receptor binding of ach also elevates intracellular ca through a number of controversial mechanisms including inhibition of the production of cyclic amp ( camp ) . in the bladder smooth muscles , where muscarinic receptor is the primary efferent receptor ,
agonist - induced contraction is largely dependent on ca entry through nifedipine - sensitive channels and activation of the rho - kinase pathway .
on the other hand , oxytocin and prostaglandin f2 act on oxytocin and prostaglandin receptors , respectively , and release ca from intracellular stores through stimulation of the gq - plc - ip3 system [ 810 ] .
they may also propagate the influx of extracellular ca through voltage - gated l - channels .
in addition , oxytocin may activate   nonselective cation channels and ca - activated cl channels leading to depolarization of myometrial cells and , eventually , the opening of voltage - dependent ca channels [ 2 , 11 ] .   in light of these observations , drugs that could block the effects of these agonists induce smooth muscle relaxation through some mechanisms that could block or interfere with ca entry .
antimuscarinic agents , those that oppose the effects of ach , are effective bladder and intestinal smooth muscle relaxants and are well - known standard therapies for oab and in some forms of gastrointestinal motility disorders .
in addition , ca channel blockers ( ccbs ) are effective oab interventions although they are more commonly used for hypertension and other cardiovascular diseases .
ccbs block ca entry by binding to the l - type ca channels in the heart and smooth muscles of the peripheral vasculature , thereby generating vasodilation and eventually lowering blood pressure .  
oxytocin antagonists , ccbs , prostaglandin synthase inhibitors , and -adrenergic agonists are used as tocolytics ( medications to suppress premature labor ) by virtue of their influence on lowering intracellular ca levels .
-adrenergic agonists , alike some nonsteroidal anti - inflammatory drugs , increase the level of camp which results in the decrease in intracellular ca by stimulating efflux of ca from the cell and also uptake by the sarcoplasmic reticulum .  
however impressive the above - named agents are in managing abnormal smooth muscle contractions , their efficacy and application are limited due to some reported drug - induced side effects .
in fact , the application of these compounds may exacerbate the diseases albeit only in extreme cases .  
antimuscarinic drugs , although effective in inhibiting bladder and intestinal contractility , also influence normal contractility thus affecting normal voiding and excretion functions .
moreover , muscarinic m3 receptors are found in the salivary glands thus severe dry mouth is expected with the use of antimuscarinic agents .
the standard tocolytics , although efficacious in arresting preterm labor , also produce serious maternal and cardiovascular or adverse fetal side effects [ 15 , 16 ] .
altogether , these findings indicate that there is a necessity to develop other smooth muscle relaxants , preferably those that act on a different mechanism .    
another way to counteract defective smooth muscle contractility is to enhance repolarizing ( potassium [ k ] ) currents .  
k channels are abundantly expressed in smooth muscles where they play an important role in determining and regulating the excitability of the cell by acting as an excitability  brake . 
a number of k channel openers have been developed , and they showed promise in preclinical and clinical studies for a variety of smooth muscle hyperactivity disorders . among them
are the openers of the atp sensitive k ( katp ) channels and ca - activated k ( kca ) channels .
however , as k channels are ubiquitously expressed in virtually all cell types , it has been thought that k channel openers may not show tissue selectivity .
indeed , the most investigated katp   channel openers pinacidil and cromakalim effectively abolished unwanted bladder contractions without affecting normal voiding .   however , they also exhibited limited bladder selectivity and influenced cardiovascular functions [ 1923 ] .   
at least two noncollaborating laboratories have reported the synthesis of novel k channel openers which they thought as effective interventions for smooth muscle hyperactivity disorders , particularly in oab .
butera and colleagues   first reported the production of benzofuroindole analogues in their continued effort to develop potent bladder relaxants with minimal hemodynamic effects .
these benzofuroindole compounds were produced by manipulating the structure of the benzopyran - based antihypertensive and prototype katp channel opener celikalim .
initial structural modifications of celikalim accidentally led to the production of the fisher - indole product 10h - benzofuro[3,2-b]indole ( compound 7 ) ( figure 1 ) , a derivative which displayed not only potent bladder relaxant effects in in vitro screenings , but also high bladder ( versus aorta ) selectivity .
on the other hand , another group produced benzofuroindole compounds by overlaying compound 7 ( see above ) , with bms-204352 , a prototypical opener of one type of kca channels , the large conductance ca - activated k ( bkca ) channel .
one of the derivatives , compound 22 , ( 4-chloro-7-trifluoromethyl-10h - benzofuro[3,2-b]indole-1-carboxylic acid ) ( figure 1 ) demonstrated in vitro inhibition of bladder contractions without influencing contractility of the blood vessels .  
multiple analyses showed that the above - mentioned benzofuroindole compounds were potent activators of the bkca channels [ 2426 ] .
bkca channels , relative to other k channel types , have more superior biophysical , molecular , and pharmacological properties making them more appealing targets to achieve smooth muscle relaxation ( see below ) .   in the succeeding sections ,
the attractive features of benzofuroindole compounds as smooth muscle relaxants are described , as well as some of the concerns that need to be addressed if they were used clinically as antispasmodics or tocolytics .
as stated above , the first ever synthesized benzofuroindole analogue ( compound 7 ) was derived from a katp   channel opener .
thus , it came as a surprise when the compound potently activated the bkca channels .
accordingly , the effects of compound 7 were readily reversed by the specific bkca channel blocker iberiotoxin , but not by glyburide ( a selective katp channel blocker ) .
furthermore , voltage clamp studies on isolated rat bladder myocytes showed that the compound caused an iberiotoxin - sensitive increase in hyperpolarizing current , further intensifying its bkca channel - potentiating properties . on the other hand , gormemis et al .  
compound 22 , along with other novel benzofuroindole derivatives , was shown to effectively potentiate cloned bkca channels expressed in xenopus laevis oocytes .
the ionic currents caused by compound 22 were blocked by the peptide bkca   channel blocker charybdotoxin indicating selective activation of the bkca   channels .
further electrophysiological characterizations of one the potent derivatives , compound 8 ( 7-trifluoromethyl-10h - benzofuro[3,2-b]indole-1-carboxylic acid ) , showed that it highly activated cloned bkca channels from the extracellular side independent of  subunits and regardless of the presence of intracellular ca ( for a review on bkca   channel structure , see figure 2 ) .  
in addition , it activated native bkca channels from rat hippocampus pyramidal neurons , a finding which might have important clinical roles .
but just how remarkable is it when a compound is an opener of the bkca channels ?
some excellent reviews on the structure , pharmacology , functions ( figure 2 ) , and the potentiality of bkca channels as novel therapeutic targets have been made [ 29 , 30 ] .
structurally , bkca channels are composed of two different subunits : the pore - forming  subunit and the auxiliary  subunits .
although channels formed only by four  subunits can be functional ,  subunits alter the biophysical and pharmacological properties of homomeric channels , including ca and voltage sensitivity and gating kinetics [ 28 , 3134 ] .
these characteristics of bkca channels make them appealing targets , and their activators potent therapies for many diseases : ( 1 ) abundant distribution like other k channel types , ( 2 ) high conductance ( ~200 ps ) even at low probability of opening , thus facilitating more efficient k efflux and membrane hyperpolarization ( relaxation ) , ( 3 ) high sensitivity to both intracellular ca concentrations and voltage , ( 4 ) ca independence , that is , bkca channels can open even in the absence of ca and the ca and membrane potential dependence of the channels are independent of each other [ 29 , 30 ] .  
a number of bkca channel openers , derived from natural products and from synthetic chemistry , have been developed and reported ( e.g. , dehydrosoyasaponin - i , maxikdiol , ns1619 , bms-204352 , 17-estradiol , ethylbromidetamoxifen , pimaric acid , and epoxyeicosatrienoic acids [ 3538 ] .   
these substances , however , differ in properties and in some respects , mechanisms of action .
for instance , dehydrosoyasaponin - i and 17-estradiol may require  subunits for optimum channel potentiation [ 31 , 39 ] , while some compounds ( e.g. , dehydrosoyasaponin - i and 17-estradiol ) may act only on the intracellular side of the channel by being highly impermeable .
unexpectedly ,   given the potentiality of bkca channel openers as future interventions in many disease states , it is surprising that only four bkca channel openers have entered clinical development ( ns-8 , ta-1702 , bms-223131 , and bms-204352 ) . to the best of our knowledge , clinical trials for the bkca channel openers ns8 , bms204352 , and ta-1702 have been discontinued , while only one drug candidate , andolast ( for the treatment of asthma ) , remains in the early phase of clinical development .  
in the view of garcia et al . , there is still much validation required for bkca channel openers to progress as future smooth muscle relaxants .  
novel bkca channel openers must show appropriate potency and selectivity , efficacy in preclinical disease models , and , most of all , lesser toxicity .
aside from remarkably potentiating bkca channels from rat bladder myocytes , benzofuroindole compounds developed by butera and colleagues were also shown to be highly bladder selective with aorta / bladder ic50 ratios ranging from 8- to 46-fold .
this was ascertained through organ bath studies with isolated rat bladder and aortic rings . in their studies ,
compound 15 showed to be the most bladder selective ( ic50 ratio aorta / bladder = 46 ) .
the structure - activity relationships for these compounds have been reported and reviewed [ 14 , 24 ] .   by looking at the structures of compounds 7 , 14 , 15 and those of the other highly bladder selective derivatives ( compounds 22 , 23 , and 24 ) ,
bladder specificity could be attributed to the imbedded 5,5 ring system that is fairly tolerant of structural modifications   ( figure 3 ) . meanwhile , dela pea et al .  
also disclosed the bladder ( versus aorta ) selectivity profile of compound 22 , the benzofuroindole analogue synthesized by gormemis and colleagues .   in their multiple screenings ,
compound 22 or ldd175 displayed 20-fold selectivity for the rat bladder compared with the aorta ( when emax values are compared ) .  
what is more , compound 22 did not have any significant vasorelaxant activity . in vivo screenings in the spontaneously hypertensive rat ( shr ) ,
an animal model of hypertension also showed that compound 22 did not alter the rat 's hemodynamic activities .
in addition , the same group demonstrated that oral administration of compound 22 reduced voiding frequency and lengthened void intervals in shr , a putative animal model of oab .
it is noteworthy that these effects were seen only in the shr and not in the normotensive strain , the wistar kyoto rats , a finding that might have significant clinical implications .   in certain disease states such as oab
, a major drawback of current pharmacotherapies as well as those drugs in development , is their ability to affect cardiovascular activities .  
katp channel openers , compounds first developed for oab , also activated katp channels in the heart and peripheral blood vessels and brought hemodynamic side effects .  
for this reason , the development of katp   drugs for oab has been abandoned in recent years .  
the focus has been shifted to other k channel openers , such as bkca and the recently identified kcnq channels openers .
compared with katp channels , bkca channels are less expressed in the heart tissue [ 29 , 30 ] but are abundant in the bladder smooth muscles and also in neuronal tissues . with regard to their expression in neuronal tissues ,
bkca activators could then impact oab whether the underlying etiology is either neurogenic or myogenic in nature .  
in fact , it is proposed that targeting the neuronal channels could minimize cardiovascular side effects , although it might also lead to the emergence of unwanted neuronal side effects .
bkca channels are abundantly distributed in many smooth muscle types such as   gastrointestinal smooth muscles   and the uterus [ 4547 ] .  
as not much is known about the effects of benzofuroindole compounds in other tissues , an ongoing effort has been made to investigate this matter .
compound 22 also displayed potent inhibition of both spontaneous and agonist - induced contractions of the ileum   and uterus .
ec50 values of the relaxant effects of compound 22 are shown in table 1 . while compound 22 did not have any intrinsic relaxant activity in the bladder ( suggesting its lack of effect on myogenic contractions ) , it significantly inhibited spontaneous contractions of the ileum and the uterus .   however , the effect of compound 22 was more demonstrated in ach - induced contractions of the bladder versus ileum and uterus .
the detrusor was not responsive to the effects of compound 22 in electric field stimulation- ( efs- ) induced contractions .
conversely , the relaxative effect of compound 22 in efs - induced ileal contraction was comparable to that induced by ach . in bladder and ileum strips , however , compound 22 was not as potent as atropine , an antimuscarinic drug , in inhibiting ach - induced contractions [ 26 , 43 ] . in the uterus , although compound 22 relaxed oxytocin , prostaglandin f2 , and ach - induced contractions , its effects were not as potent as those of standard tocolytics ( note , however , the similarity in the degree of potency between diclofenac and compound 22 in inhibiting pgf2-induced contractions ) .   nevertheless , the effects of compound 22 were all consistent with the activation of bkca channels , as evidenced by the counter effects of bkca channel blockers iberiotoxin or penitrem a [ 26 , 43 , 44 ] on the relaxative effects of compound 22 in high k- ( 20  mm kcl- ) induced contractions ( table 1 ) .
other ion channels or contractile machineries may also be at play considering the effects of compound 22 in high - k- ( 6080  mm kcl- ) induced contractions [ 26 , 43 , 44 , 48 , 49 ] .  
taken together , these findings indicate that in addition to bladder - relaxant activities , compound 22 also influences intestinal and uterine contractions .
although k channel openers hold promise as effective and safer alternatives to many smooth muscle relaxants that are used today , most k channel openers in development have not lived up to our expectation . a majority of katp channel openers lacked selectivity and brought unwanted side effects ( e.g. , cardiovascular ) considered worse than those wrought by standard antispasmodics or tocolytics .
thus , the development of katp channel openers has been discontinued , and the interest was shifted to developing other k channel openers that are as efficacious as standard smooth muscle relaxants , but with significantly better side effect profile .  
recent years have seen great advances in our understanding of the structure and function of existing k channels . due to unmet expectations with katp channel openers , newer channels or associated channel proteins
have been characterized as potential drug targets . as stated above , among those   explored
were the bkca channels . while the role of bkca channels in the cns is complex and still an area of active academic research , there appears to be a consensus on the contribution of these channels to the regulation of smooth muscle tone .   in this paper , we reviewed the contribution of benzofuroindole derivatives in the field that searches for ideal compounds for oab ( or for other pathophysiologic conditions ) .  
we have stated the remarkable profiles of these compounds if considered as future oab drug treatment .  
some investigators , however , showed the potency of a certain benzofuroindole analogue ( compound 22 ) to relax ileum and uterine contractions , indicating that like other k channel openers , selectivity is still an area of concern with benzofuroindole compounds . however , it is still too early to conclude that benzofuroindole analogues have no place in the roll of alternative or safer smooth muscle relaxants .
more investigations are required to better understand their mechanics and characteristics and ultimately to address their lack of selectivity .
the potency of various benzofuroindole compounds in smooth muscle types can be compared   and from there we may discover the drugs ' most appropriate clinical application .   finally , the worth of bkca channel openers as  better  smooth muscle relaxants is just proven theoretically but not in clinical practice .
not much is known about the physiology of bkca channels and their activators in the disease state , thus , whether or not a bkca channel opener will be found to have therapeutic utility will depend on the appropriate counterbalance of bkca channel activation versus other excitatory inputs . moreover , some reported bkca channel openers do not satisfy some of the criteria set in clinical tests to prove their worth as effective smooth muscle relaxants ( for review see ) .
this explains , in part , the slow pace in the development of bkca channel openers as smooth muscle hyperactivity interventions .
however , as molecular biology and drug development techniques are getting more and more advanced , it is plausible that , in the next few years , concerns that limit the potential use of bkca channel openers ( especially those that are recently characterized ) will be resolved
. therefore , the road ahead may be tedious for benzofuroindole compounds , but there is still optimism with regard to their potential use as effective smooth muscle relaxants .
 at least two laboratories have independently reported the synthesis of benzofuroindole compounds having potential therapeutic implications in many disease states including those that involve smooth muscle hyperactivity . through a series of in vitro screenings 
 , they demonstrated the efficacy ( and selectivity ) of these compounds to potentiate large conductance calcium- ( ca2 + - ) activated k+ ( bkca ) channels , by far , the most characterized of all ca2 + -dependent k+ channels . 
 interestingly , promising benzofuroindole derivatives such as compound 7 ( 10h - benzo[4,5]furo[3,2-b]indole ) and compound 22 ( 4-chloro-7-trifluoromethyl-10h - benzo[4,5]furo[3,2-b]indole-1-carboxylic acid ) both exhibited high bladder ( versus aorta ) selectivity , making them attractive alternative treatments for bladder overactivity . in recent reports , 
 compound 22 ( ldd175 or tbic ) also showed inhibition of ileum and uterine contractions , indicating multiple target tissues , which is not surprising as bkca channels are ubiquitously expressed in the animal and human tissues . in this paper , the authors discuss the value of benzofuroindole compounds and the challenges that need to be overcome if they were considered as smooth muscle relaxants . 
1. Introduction
2. Benzofuroindole Compounds, BK
3. Benzofuroindole Compounds, BK
4. Compound 22 and Its Effects in Other Smooth Muscles
5. Concluding Remarks
 Conflict of Interests