worldwide , open - angle glaucoma ( oag ) is estimated to affect almost 45 million adults over the age of 40 years , and the number is expected to reach 59 million by 2020.1 since the loss of vision associated with glaucoma is irreversible,2 early diagnosis and treatment are key to preserving visual function . because intraocular pressure ( iop ) is a major risk factor in glaucoma and glaucoma suspects , all currently marketed treatments for glaucoma aim to lower iop . accordingly , the administration of topical agents that reduce the production of aqueous humor and/or increase outflow is the mainstay of therapy.26 multidose formulations of topical antiglaucoma medications contain preservatives , of which the most commonly used is benzalkonium chloride ( bak ) , a potent bactericidal and fungicidal agent.7,8 the majority of patients tolerate bak , but it has been associated with ocular sensitivity in some cases.912 patients who require multiple medications to reach / maintain target iop may be at higher risk of bak sensitivity , as are those with severe dry eye disease because of reduced dilution in the tear film.9 consequently , preservatives causing less irritation , as well as many single - dose , preservative - free ( pf ) formulations , have been developed . in clinical trials and clinical practice , pf formulations have demonstrated noninferior or equivalent efficacy to their respective preserved formulations , with potential for a reduced incidence of ocular adverse events.1322 bimatoprost / timolol ( bim / tim ) ophthalmic solution ( ganfort ; allergan plc , irvine , ca , usa ) is a fixed - combination formulation of bimatoprost 0.03% ( a synthetic prostamide ) and timolol 0.5% ( a nonselective -adrenergic receptor antagonist ) preserved with bak.23 clinical trials have demonstrated that when dosed once daily , the fixed combination produces greater iop reduction than each of the active components dosed as monotherapy.24,25 accordingly , bim / tim is used in a number of countries worldwide to treat patients with primary oag ( poag ) or ocular hypertension ( oht ) who do not reach target iop with monotherapy . however , because of the increasing awareness of patient sensitivity to preservatives , bim / tim pf ( allergan plc , irvine , ca , usa ) was developed ; this pf formulation is identical to the original bim / tim formulation , except for the removal of 50 ppm of bak . in a 12-week , multicenter , randomized , phase iii study of 561 patients with oag or oht , bim / tim pf demonstrated noninferiority and equivalence to bim / tim in terms of iop - lowering efficacy ( without significant differences in safety / tolerability),26 which led to its marketing approval in the european union in september 2013.27 despite these findings , when the between - treatment difference in average eye iop was compared , a consistent trend toward lower iop favoring bim / tim pf over bim / tim was observed in seven of the nine time points measured.26 this observation suggested that bak removal might have enhanced the efficacy of bim / tim . given that bak is a well - known permeability enhancer that has been reported to increase drug penetration into ocular tissues,2831 it was surprising that bim / tim pf produced a greater iop - lowering effect than the bim / tim formulation containing bak.26 a preliminary literature search exploring possible explanations for this clinical observation found data suggesting that timolol in preserved ophthalmic solutions may display an inverted , u - shaped dose response curve for iop lowering , with an optimal concentration between 0.25% and 0.5%.32 removal of bak from 0.5% timolol ophthalmic solution formulations could reduce timolol bioavailability enough to achieve a concentration that optimizes intraocular efficacy.32 this literature review aims to explain the findings of the phase iii study demonstrating enhanced efficacy of bim / tim pf over preserved bim / tim by exploring the relationship between the concentration of timolol in preserved ophthalmic solutions and its iop - lowering efficacy . an initial search of the literature for timolol dose  response in humans was performed in scopus ( elsevier , philadelphia , pa , usa ) . articles that were published in english between 1960 and july 2014 were identified using the keywords  timolol and ( intraocular pressure or iop ) and human ( or derivatives ) and ( 1% or 0.5% or 0.25% )  , and screened for relevance to iop lowering after ocular instillation of topical timolol 0.25% , 0.5% , or 1.0% . similarly , a literature search spanning the years 1960 to july 2014 was conducted in pubmed ( http://www.ncbi.nlm.nih.gov/pubmed ) using the keywords  timolol and ( intraocular pressure or iop ) and ( 1% or 0.5% or 0.25% )  , with filters set to display articles involving human patients in clinical trials published in english . articles that reported direct comparisons of the effect of at least two concentrations of timolol on iop in adult patients were included . studies in which patient subgroups received a higher concentration of timolol after responding poorly to an initial lower concentration were excluded . an initial search of the literature for timolol dose  response in humans was performed in scopus ( elsevier , philadelphia , pa , usa ) . articles that were published in english between 1960 and july 2014 were identified using the keywords  timolol and ( intraocular pressure or iop ) and human ( or derivatives ) and ( 1% or 0.5% or 0.25% )  , and screened for relevance to iop lowering after ocular instillation of topical timolol 0.25% , 0.5% , or 1.0% . similarly , a literature search spanning the years 1960 to july 2014 was conducted in pubmed ( http://www.ncbi.nlm.nih.gov/pubmed ) using the keywords  timolol and ( intraocular pressure or iop ) and ( 1% or 0.5% or 0.25% )  , with filters set to display articles involving human patients in clinical trials published in english . articles that reported direct comparisons of the effect of at least two concentrations of timolol on iop in adult patients were included . studies in which patient subgroups received a higher concentration of timolol after responding poorly to an initial lower concentration were excluded . given its long history in the management of glaucoma , many reports of the iop - lowering effects of timolol have been published , but only a few provide data related to dose response . a comparable search in pubmed yielded 607 references , including ten relevant clinical trials . accounting for overlap , the scopus and pubmed searches together found a total of 17 relevant studies ( figure 1 ) . all trials were randomized except one,33 and 14 were double - masked.3447 eight studies included parallel groups,34,37,41,43,44,4648 six involved dose escalation,33,35,38,42,45,49 and three had one or two crossover(s).36,39,40 five studies enrolled healthy volunteers,34,38,42,43,49 whereas the other 12 enrolled patients with oag or oht . timolol is commonly prescribed as a 0.25% or 0.5% preservative - containing ( usually bak ) ophthalmic solution . however , the selected studies covered a broader range of concentrations : 0.008% , 0.025% , 0.0625% , 0.08% , 0.1% , 0.125% , 0.25% , 0.5% , 1.0% , and 1.5% ( table 1 ) . nine studies measured iop after weeks to months of timolol administration once40,48 or twice33,3537,41,44,45 daily . the remaining eight studies assessed the short - term ( ie , within 28 hours ) iop - lowering effect following a single application.34,38,39,42,43,46,47,49 all studies that evaluated the efficacy of timolol over weeks or months found that the concentrations tested ( ie , 0.1% , 0.25% , 0.5% , and 1% ) provided statistically significant iop reductions compared with baseline and/or controls ( untreated or placebo - treated eyes ) , regardless of the instillation schedule ( ie , once versus twice daily).33,3537,40,41,44,45,48 however , results of short - term studies varied , depending on the study population . in patients with elevated iop , concentrations of 0.1% , 0.25% , 0.5% , 1.0% , and 1.5% provided statistically significant iop reductions compared with baseline and/or controls ( untreated or placebo - treated eyes).34,38,43,46,47 even a concentration as low as 0.008% ( administered as gel or solution ) was effective in lowering iop in patients with oht and a baseline iop > 22 mmhg , compared with placebo controls.39 in contrast , studies of healthy volunteers with baseline iop of approximately 1314 mmhg reported no significant iop - lowering effect when treated with timolol 0.008% or 0.025%,42 or 0.0625% , 0.125% , or 0.25%.49 four studies compared the iop - lowering effect of timolol ophthalmic solution at concentrations of 0.5% or greater and found that timolol 0.5% was as effective as or more effective than higher concentrations . three of these studies involved a single application of drug.38,46,47 in a parallel - group , single - dose study in 30 patients with oag , timolol 0.5% was as effective as or numerically more effective than timolol 1.5% at six of eight time points measured ( figure 2a).46 in a similarly designed study in 20 patients with oag , timolol 0.5% was as effective as or numerically more effective than timolol 1.0% at the 4- , 12- , and 24-hour time points ( figure 2b).47 also , in a dose escalation study in 30 healthy volunteers , katz et al38 found that iop reduction from baseline was numerically greater at 3 , 5 , and 7 hours after a single application of timolol 0.5% ( 25% , 26% , and 23% ) , compared with timolol 1.0% ( 23% , 19% , and 22% ) or 1.5% ( 24% , 17% , and 17% ) , respectively . timolol 0.5% was also numerically more effective than timolol 1.0% at 2 hours postinstillation ( figure 2c ) . the fourth and most clinically relevant study involved administration of timolol 0.5% and 1.0% twice daily for 1 week in a dose escalation design,45 and found that patients with oht or oag had a numerically lower mean iop at four of six postinstillation time points during treatment with timolol 0.5% than during treatment with timolol 1.0% ( figure 2d).45 the consistent findings suggest that on a concentration basis , timolol 0.5% is more effective than higher concentrations . such an inverted u - shaped dose  response relationship , in conjunction with the systemic cardiovascular risk associated with timolol as a nonselective -adrenoceptor antagonist,5056 explains why timolol is not prescribed at a dose strength higher than 0.5% . of the 17 studies , 13 compared timolol 0.25% and 0.5% , currently the most commonly used concentrations . among these , two studies showed that timolol 0.25% was either significantly or numerically more effective than timolol 0.5% in iop lowering . the first , a parallel - group study of timolol administered twice daily over 12 months , indicated that when statistically significant differences were found between doses , they always favored timolol 0.25% ( figure 3a).41 the second , a double - masked , three - phase study of the effects of timolol 0.25% and 0.5% administered once daily for 10 days , reported that timolol 0.25% was numerically ( but not statistically significantly ) better than the 0.5% concentration in lowering iop over the majority of time points measured ( figure 3b).40 three other studies provided data supporting a dose - dependent difference in iop - lowering favoring the 0.5% concentration . results from a 1-week , dose escalation study of twice - daily administration indicated that iop lowering from baseline was numerically greater with timolol 0.5% ( 21% and 23% ) than with timolol 0.25% ( 17% and 20% ) at 1 and 6 hours postinstillation , respectively , but similar or greater with timolol 0.25% ( 21% and 24% ) than with timolol 0.5% ( 21% and 15% ) at 3 and 8 hours postinstillation , respectively.45 however , the iop - lowering effect appeared to be better sustained at 12 hours postinstillation ( trough ) with timolol 0.5% ( 19% ) than timolol 0.25% ( 11%).45 similarly , two short - term studies with assessments that spanned 90 minutes43 and 28 hours47 postinstillation of timolol 0.25% or 0.5% found a dose - dependent , numerical difference in iop lowering at later time points that favored the 0.5% concentration ( figure 4 ) , although statistical analysis of the difference between concentrations was not provided . the remaining eight studies concluded that there was no difference in iop lowering between timolol 0.25% and 0.5% . in a double - masked , three - period crossover study of timolol administered twice daily for 4 weeks , the mean iop reduction at study end was 11.313.18 mmhg ( 34.67% ) with timolol 0.25% versus 12.033.72 mmhg with timolol 0.5% ( 35.95% ; p>0.5).36 findings were comparable when timolol 0.25% and 0.5% were dosed once daily for 8 weeks ( figure 5a),48 or twice daily for 6 months ( figure 5b).44 in a randomized , double - masked , dose escalation study , timolol 0.25% induced a 26% reduction in iop from baseline when administered twice daily for 1 week . an increase in concentration to timolol 0.5% only produced a modest , additional iop reduction of approximately 4% , but a statistical analysis of the between - concentration difference in iop lowering was not provided.35 in a similar dose escalation study of timolol 0.25% and 0.5% twice daily , increasing the concentration from 0.25% to 0.5% did not result in significant additional iop lowering from baseline ; iop reduction reached 25% and 27% after 34 weeks of treatment with timolol 0.25% and 0.5% , respectively.33 a short - term study that assessed iop levels at 1 , 2 , 4 , 6 , 8 , 12 , and 24 hours postinstillation and included statistical analysis of the between - concentration difference in iop lowering from baseline also concluded that the effect of timolol 0.5% was not superior to that of timolol 0.25% ( figure 5c).34 an area under the curve analysis of iop versus time ( ie , 12 and 24 hours ) yielded similar values for both concentrations : 43.66.2 and 74.911.0 for timolol 0.25% versus 38.19.4 and 69.116.4 for timolol 0.5% , respectively.34 importantly , the largest study identified in this systematic review compared the iop - lowering efficacy of timolol 0.25% and 0.5% administered twice daily as a maleate or hemihydrate solution over 8 weeks in 371 patients with poag or oht.37 the authors concluded that when either formulation was administered , the 0.25% and 0.5% dose strengths were equally effective from week 1 to week 12 ( figure 5d ) ; equivalence between the two concentrations at the final visit was established by statistical analysis . taken together , the aforementioned results suggest that an inverted u - shaped dose  response curve may exist for timolol in terms of iop lowering , and that the concentration that elicits maximum iop lowering likely lies between 0.25% and 0.5% , as illustrated in figure 6 . a possible explanation for the inverted u - shaped dose the association between repeated ocular timolol use and tachyphylaxis has long been described,5765 and although some patients exhibit relatively stable iop reductions for years in response to timolol ophthalmic solution , some demonstrate an upward iop drift after days or months , reflecting a partial or complete loss of response to timolol.59,62 timolol concentrations of 0.5% and higher may be associated with an increased occurrence of receptor upregulation,62,66,67 which leads to tachyphylaxis . in contrast , there has been no report of an association between bimatoprost and tachyphylaxis in the literature , and preclinical studies of bimatoprost in dogs and monkeys have revealed a flat dose response curve between 0.001% and 0.1% , suggesting that preservative removal is unlikely to affect the effective concentration of bimatoprost.68 given its long history in the management of glaucoma , many reports of the iop - lowering effects of timolol have been published , but only a few provide data related to dose response . a comparable search in pubmed yielded 607 references , including ten relevant clinical trials . accounting for overlap , the scopus and pubmed searches together found a total of 17 relevant studies ( figure 1 ) . all trials were randomized except one,33 and 14 were double - masked.3447 eight studies included parallel groups,34,37,41,43,44,4648 six involved dose escalation,33,35,38,42,45,49 and three had one or two crossover(s).36,39,40 five studies enrolled healthy volunteers,34,38,42,43,49 whereas the other 12 enrolled patients with oag or oht . timolol is commonly prescribed as a 0.25% or 0.5% preservative - containing ( usually bak ) ophthalmic solution . however , the selected studies covered a broader range of concentrations : 0.008% , 0.025% , 0.0625% , 0.08% , 0.1% , 0.125% , 0.25% , 0.5% , 1.0% , and 1.5% ( table 1 ) . nine studies measured iop after weeks to months of timolol administration once40,48 or twice33,3537,41,44,45 daily . the remaining eight studies assessed the short - term ( ie , within 28 hours ) iop - lowering effect following a single application.34,38,39,42,43,46,47,49 all studies that evaluated the efficacy of timolol over weeks or months found that the concentrations tested ( ie , 0.1% , 0.25% , 0.5% , and 1% ) provided statistically significant iop reductions compared with baseline and/or controls ( untreated or placebo - treated eyes ) , regardless of the instillation schedule ( ie , once versus twice daily).33,3537,40,41,44,45,48 however , results of short - term studies varied , depending on the study population . in patients with elevated iop , concentrations of 0.1% , 0.25% , 0.5% , 1.0% , and 1.5% provided statistically significant iop reductions compared with baseline and/or controls ( untreated or placebo - treated eyes).34,38,43,46,47 even a concentration as low as 0.008% ( administered as gel or solution ) was effective in lowering iop in patients with oht and a baseline iop > 22 mmhg , compared with placebo controls.39 in contrast , studies of healthy volunteers with baseline iop of approximately 1314 mmhg reported no significant iop - lowering effect when treated with timolol 0.008% or 0.025%,42 or 0.0625% , 0.125% , or 0.25%.49 four studies compared the iop - lowering effect of timolol ophthalmic solution at concentrations of 0.5% or greater and found that timolol 0.5% was as effective as or more effective than higher concentrations . three of these studies involved a single application of drug.38,46,47 in a parallel - group , single - dose study in 30 patients with oag , timolol 0.5% was as effective as or numerically more effective than timolol 1.5% at six of eight time points measured ( figure 2a).46 in a similarly designed study in 20 patients with oag , timolol 0.5% was as effective as or numerically more effective than timolol 1.0% at the 4- , 12- , and 24-hour time points ( figure 2b).47 also , in a dose escalation study in 30 healthy volunteers , katz et al38 found that iop reduction from baseline was numerically greater at 3 , 5 , and 7 hours after a single application of timolol 0.5% ( 25% , 26% , and 23% ) , compared with timolol 1.0% ( 23% , 19% , and 22% ) or 1.5% ( 24% , 17% , and 17% ) , respectively . timolol 0.5% was also numerically more effective than timolol 1.0% at 2 hours postinstillation ( figure 2c ) . the fourth and most clinically relevant study involved administration of timolol 0.5% and 1.0% twice daily for 1 week in a dose escalation design,45 and found that patients with oht or oag had a numerically lower mean iop at four of six postinstillation time points during treatment with timolol 0.5% than during treatment with timolol 1.0% ( figure 2d).45 the consistent findings suggest that on a concentration basis , timolol 0.5% is more effective than higher concentrations . such an inverted u - shaped dose  response relationship , in conjunction with the systemic cardiovascular risk associated with timolol as a nonselective -adrenoceptor antagonist,5056 explains why timolol is not prescribed at a dose strength higher than 0.5% . of the 17 studies , 13 compared timolol 0.25% and 0.5% , currently the most commonly used concentrations . among these , two studies showed that timolol 0.25% was either significantly or numerically more effective than timolol 0.5% in iop lowering . the first , a parallel - group study of timolol administered twice daily over 12 months , indicated that when statistically significant differences were found between doses , they always favored timolol 0.25% ( figure 3a).41 the second , a double - masked , three - phase study of the effects of timolol 0.25% and 0.5% administered once daily for 10 days , reported that timolol 0.25% was numerically ( but not statistically significantly ) better than the 0.5% concentration in lowering iop over the majority of time points measured ( figure 3b).40 three other studies provided data supporting a dose - dependent difference in iop - lowering favoring the 0.5% concentration . results from a 1-week , dose escalation study of twice - daily administration indicated that iop lowering from baseline was numerically greater with timolol 0.5% ( 21% and 23% ) than with timolol 0.25% ( 17% and 20% ) at 1 and 6 hours postinstillation , respectively , but similar or greater with timolol 0.25% ( 21% and 24% ) than with timolol 0.5% ( 21% and 15% ) at 3 and 8 hours postinstillation , respectively.45 however , the iop - lowering effect appeared to be better sustained at 12 hours postinstillation ( trough ) with timolol 0.5% ( 19% ) than timolol 0.25% ( 11%).45 similarly , two short - term studies with assessments that spanned 90 minutes43 and 28 hours47 postinstillation of timolol 0.25% or 0.5% found a dose - dependent , numerical difference in iop lowering at later time points that favored the 0.5% concentration ( figure 4 ) , although statistical analysis of the difference between concentrations was not provided . the remaining eight studies concluded that there was no difference in iop lowering between timolol 0.25% and 0.5% . in a double - masked , three - period crossover study of timolol administered twice daily for 4 weeks , the mean iop reduction at study end was 11.313.18 mmhg ( 34.67% ) with timolol 0.25% versus 12.033.72 mmhg with timolol 0.5% ( 35.95% ; p>0.5).36 findings were comparable when timolol 0.25% and 0.5% were dosed once daily for 8 weeks ( figure 5a),48 or twice daily for 6 months ( figure 5b).44 in a randomized , double - masked , dose escalation study , timolol 0.25% induced a 26% reduction in iop from baseline when administered twice daily for 1 week . an increase in concentration to timolol 0.5% only produced a modest , additional iop reduction of approximately 4% , but a statistical analysis of the between - concentration difference in iop lowering was not provided.35 in a similar dose escalation study of timolol 0.25% and 0.5% twice daily , increasing the concentration from 0.25% to 0.5% did not result in significant additional iop lowering from baseline ; iop reduction reached 25% and 27% after 34 weeks of treatment with timolol 0.25% and 0.5% , respectively.33 a short - term study that assessed iop levels at 1 , 2 , 4 , 6 , 8 , 12 , and 24 hours postinstillation and included statistical analysis of the between - concentration difference in iop lowering from baseline also concluded that the effect of timolol 0.5% was not superior to that of timolol 0.25% ( figure 5c).34 an area under the curve analysis of iop versus time ( ie , 12 and 24 hours ) yielded similar values for both concentrations : 43.66.2 and 74.911.0 for timolol 0.25% versus 38.19.4 and 69.116.4 for timolol 0.5% , respectively.34 importantly , the largest study identified in this systematic review compared the iop - lowering efficacy of timolol 0.25% and 0.5% administered twice daily as a maleate or hemihydrate solution over 8 weeks in 371 patients with poag or oht.37 the authors concluded that when either formulation was administered , the 0.25% and 0.5% dose strengths were equally effective from week 1 to week 12 ( figure 5d ) ; equivalence between the two concentrations at the final visit was established by statistical analysis . taken together , the aforementioned results suggest that an inverted u - shaped dose response curve may exist for timolol in terms of iop lowering , and that the concentration that elicits maximum iop lowering likely lies between 0.25% and 0.5% , as illustrated in figure 6 . a possible explanation for the inverted u - shaped dose  response curve might involve receptor upregulation and tachyphylaxis . the association between repeated ocular timolol use and tachyphylaxis has long been described,5765 and although some patients exhibit relatively stable iop reductions for years in response to timolol ophthalmic solution , some demonstrate an upward iop drift after days or months , reflecting a partial or complete loss of response to timolol.59,62 timolol concentrations of 0.5% and higher may be associated with an increased occurrence of receptor upregulation,62,66,67 which leads to tachyphylaxis . in contrast , there has been no report of an association between bimatoprost and tachyphylaxis in the literature , and preclinical studies of bimatoprost in dogs and monkeys have revealed a flat dose response curve between 0.001% and 0.1% , suggesting that preservative removal is unlikely to affect the effective concentration of bimatoprost.68 the evidence gathered appears to support our hypothesis that the optimal iop - lowering concentration of timolol lies between 0.25% and 0.5% when administered as a bak - preserved formulation . the majority of studies found no significant differences in iop lowering between the 0.25% and 0.5% concentrations , and some studies showed statistically significantly increased efficacy with the 0.25% concentration . therefore , if the removal of bak would result in lower ocular concentrations of timolol in patients treated with bim / tim pf ( containing 0.5% timolol ) than in those treated with preserved bim / tim ( also containing 0.5% timolol ) , it is reasonable to believe that the reduced exposure to timolol 0.5% in the pf formulation at the target site may bring the effective concentration within the 0.25%0.5% range , thus maximizing the iop - lowering effect of timolol . this supposition , however , should be considered in light of the limitations of the available data : the variability of the study designs , the small sample sizes of some studies , and the lack of studies that compared the efficacy of various concentrations of timolol in preserved versus pf formulations . early pharmacokinetic and pharmacodynamic studies showed that timolol is well absorbed through the cornea and rapidly distributes into ocular tissues following topical ocular administration ; it can be measured in the human aqueous humor for up to 12 hours . timolol significantly lowered iop in healthy volunteers and in patients with oag , and in dose ranging studies , the maximum effect appeared to occur with the 0.5% concentration.69 the current knowledge of the dose response relationship between iop lowering and timolol concentration in preserved topical ophthalmic solutions is highlighted in our systematic literature review . studies that evaluated the efficacy of timolol over weeks or months in patients with oag or oht are likely more relevant to our hypothesis as they are indicative of the relationship between efficacy and steady - state concentrations of timolol in the eye ( compared with studies assessing the iop - lowering effects within minutes or hours of treatment with a single dose ) . these studies concluded that timolol ophthalmic solutions at 0.1% , 0.25% , 0.5% , and/or 1% provide statistically significant iop reductions from baseline and/or compared with untreated or placebo - treated eyes , regardless of the instillation schedule . nevertheless , all but two short - term studies ( including one study in eleven patients that could not detect significant iop reduction at every time point with timolol 0.5% ) , reached the same conclusion , even for timolol concentrations 0.1% . overall , these data suggest that on a concentration basis , timolol 0.5% is more effective at lowering iop than higher concentrations , and that the optimal iop - lowering concentration of timolol is likely between 0.25% and 0.5% . furthermore , these findings support the clinical observation that removal of bak from the bim / tim formulation provided more optimal iop reduction .
purposemany patients with glaucoma require combination therapies to achieve target intraocular pressure ( iop ) and preserve visual function . 
 ocular hypotensives often contain a preservative ( eg , benzalkonium chloride [ bak ] ) , but preservative - free ( pf ) formulations have been developed for patients with sensitivity . 
 a phase iii study found the efficacy of bimatoprost 0.03%/timolol 0.5% ( bim / tim , ganfort ) pf to be equivalent to that of preserved bim / tim , although a trend favoring bim / tim pf was observed . 
 as bak is a corneal penetration enhancer , this literature review aims to explain these findings by exploring the relationship between timolol concentration and its iop - lowering effect.methodssystematic searches were performed in scopus and pubmed for clinical trials published in english between 1960 and july 2014 using the keywords  timolol  ,  intraocular pressure  , and the concentrations  1% , 0.5% , or 0.25%  . 
 articles that directly compared iop - lowering effects of 2 concentrations of timolol were identified by manual screening , and cross - checked for duplication.resultsseventeen studies that included 10371 patients were evaluated ; the majority were randomized ( 16/17 ) , double - masked ( 14/17 ) , and enrolled patients with open - angle glaucoma or ocular hypertension ( 12/17 ) . 
 all studies investigated timolol in preserved formulations . 
 timolol concentrations tested ranged from 0.008% to 1.5% . 
 of 13 studies comparing timolol 0.25% versus 0.5% , two found the 0.25% dose to have greater iop - lowering effects , and three reported the opposite ; eight reported similar iop lowering . 
 results also indicate that timolol 0.5% may be more effective than higher concentrations.conclusionthe evidence suggests that timolol may have an inverted u - shaped dose 
 response curve , and that its optimal iop - lowering concentration is between 0.25% and 0.5% . compared with bim / tim , removal of the permeability enhancer bak in bim / tim pf could have resulted in a lower timolol concentration at the target site , bringing the effective concentration within the 0.25%0.5% range and enhancing the efficacy of bim / tim pf .