the influence of the aforementioned amidino compounds ( table 1 ) on rs virus was revealed by their effect on various aspects of virus growth in cell cultures . for an inhibitor to be judged acceptable , cell toxicity had to be excluded as the basis of the observed anti - rs virus activity . the specificity of the compounds was demonstrated by their inability to interfere with the growth of other viruses studied . the examples below are offered to more fully describe the present invention , but are not to be construed as limiting the scope thereof . hep - 2 , hela , and cv - 1 cells were propagated in eagle minimal essential medium ( mem ) supplemented with 10 % fetal calf serum ( fcs ). in experiments involving protease inhibitors , hep - 2 cells were used between passages 371 and 377 . an a 2 strain of rs virus was grown in hep - 2 cells . stock cultures of virus were prepared by inoculating suspensions of hep - 2 cells . following virus adsorption , the infected cells were seeded in 250 ml plastic culture flasks containing mem plus 10 % fcs . when viral cytopathic effects were maximal ( 72 - 96 hr .) the flasks were frozen at - 70 ° c . typical lysates contained 10 7 - 10 8 tcid 50 / ml . parainfluenza type 3 ( p - 3 ) was obtained from the research resources branch , niaid ( cat # v - 323 - 002 - 020 ). stocks of p - 3 were produced in hela cells by infecting confluent monolayers and harvesting culture fluid after 48 hrs . stock cultures containing 10 8 tcid 50 / ml were stored at - 70 ° c . a polykaryoctye - forming mutant ( mp ) of herpes simplex type 1 ( hsv - 1 ) was also employed . all virus stocks and cell lines were determined to be free of mycoplasma contamination by a modification of the fluorochrome technique of chen . ( in situ determination of mycoplasma contamination in cell cultures by fluorescent hoeshst 33258 stain . experimental cell research , 104 , 255 - 262 ( 1977 )). virus yields were quantitated by calculating 50 percent end points according to the method of reed and muench ( a simple method for estimating fifty percent end - points . am . j . hyg . b 27 , 493 - 497 ( 1938 )). briefly , 0 . 05 ml of virus diluted in mem plus 10 % fcs was added to micro - titer plates . this was followed by the addition of 40 , 000 - 60 , 000 hep - 2 cells in a volume of 0 . 05 ml . plates were incubated at 36 ° c . for 5 - 7 days . six or eight wells were used for each dilution . standard error analysis was done according to the method of pizzi ( sampling variation of the fifty percent endpoint determined by the reed - muench ( behrens ) method . human biol . 22 , 151 - 180 ( 1950 )). thirteen competitive , reversible inhibitors of trypsin - like enzymes selected for this study are listed with their structures in table 1 . the first compound , benzamidine , represents the prototype of inhibitors with an amidine nature , and since the time it was first described by mares - guia and shaw ( studies on the active center of trypsin . the binding of amidines and guanidines as models of the substrate side chain . j . biol . chem . 140 , 1579 - 1585 ( 1965 )) it has served as a key building block for other , more potent agents , such as compounds no . 2 - 6 . pentamidine , which is active against a wide variety of trypsin - like enzymes , is generally an order of magnitude more effective than benzamidine ( geratz , specific low - molecular weight inhibitors of trypsin . their structure - activity relationships and possible clinical uses . in pulmonary emphysema and proteolysis . ( c . mittman , ed .) pp . 325 - 340 . academic press , new york , 1972 ; geratz et al ., diamidino - α , ω - diphenoxylalkanes . structure - activity relationships for the inhibition of thrombin , pancreatic kallikrein , and trypsin . j . med . chem ., 16 , 970 - 975 , 1973 ; geratz and tidwell , the development of competitive reversible thrombin inhibitors . in chemistry and biology of thrombin . ( r . l . lundblad , j . w . fenton , ii , and k . g . mann , eds . ), pp . 179 - 196 , ann arbor science , 1977 ). compounds 3 and 4 are similar in structure to pentamidine , but they possess only a single amidino group and are endowed with increased hydrophobicity as a consequence of the lengthened hydrocarbon chain . compound 5 represents a strong inhibitor of the plasminogen activator urokinase with a k i value of 3 . 2 × 10 - 6 m ( geratz and cheng , the inhibition of urokinase by aromatic diamidines . thromb . diath . haemorrh . 33 , 230 - 243 , 1975 ), while no . 6 is one of the most powerful inhibitors of pancreatic kallikrein with a k i value of 3 . 1 × 10 - 8 m ( geratz et al ., novel bis ( benzamidino ) compounds with an aromatic central link . inhibitors of thrombin , pancreatic kallikrein , trypsin , and complement . j . med . chem ., 19 , 634 - 639 , 1976 ). in no . 7 we selected the amidino compound with the greatest antitrypsin activity so far reported ( k i = 1 . 8 × 10 - 8 m ) ( tidwell et al ., diarylamidine derivatives with one or both of the aryl moieties consisting of an indole or indole - like ring . inhibitors of arginine - specific esteroproteases . j . med . chem . 21 , 613 - 623 , 1978 ), and in no . 8 its closest homolog was included , differing from the parent compound only by bearing an additional hydrocarbon in the central molecular chain . compounds 9 - 11 are isosteric amidino - substituted heterocycles of small molecular size which can be accommodated solely in the specificity pockets of their target enzymes and achieves much tighter binding than benzamidine ( geratz et al ., amidino - substituted aromatic heterocycles as probes of the specificity pocket of trypsin - like proteases . arch . biochem . biophys ., 197 , 551 - 560 , 1979 ). compounds 12 and 13 , finally , are derivatives of 5 - amidinoindole bearing a substitution on the indole nitrogen ( geratz et al ., 1979 , ibid .). compounds 3 and 4 were synthesized in our laboratory and have not been previously reported . the two novel derivatives were prepared and purified according to a general synthetic procedure for the synthesis of unsymmetrical α , ω - diphenoxyalkane derivatives ( geratz et al ., diamidino - α , ω - diphenoxylalkanes . structure - activity relationships for the inhibition of thrombin , pancreatic kallikrein , and trypsin . j . med . chem ., 16 , 970 - 975 , 1973 ). the melting points ( m . p .) and elemental analyses of compounds 3 and 4 are as follows : __________________________________________________________________________compound 3 : c . sub . 19 h . sub . 24 n . sub . 2 o . sub . 2 . hcl molecular weight = 348 . 88m . p . = 290 - 292 ° c . analysis : c h n calculated 65 . 42 7 . 22 8 . 03 found 65 . 32 7 . 02 8 . 29compound 4 : c . sub . 21 h . sub . 28 n . sub . 2 o . sub . 2 hcl . o . 5h . sub . 2 molecular weight = 385 . 93m . p . = 100 - 102 ° c . analysis : c h n calculated 65 . 35 7 . 83 7 . 25 found 65 . 58 8 . 07 7 . 34__________________________________________________________________________ the melting points were determined on a thomas - hoover capillary melting point apparatus and are uncorrected . hep - 2 cells or cv - 1 cells were seeded in well trays to contain approximately 10 6 cells per well at 72 hrs . rs virus or p - 3 virus added at various input multiplicities of infection in 0 . 2 ml was adsorbed for 2 hrs . at 36 ° c . following this period , the wells were rinsed twice with 1 ml of mem and received either inhibitor - containing or control media consisting of mem with 2 % fcs and 1 % methyl sulfoxide ( dmso ). cell viability was measured by trypan blue exclusion and cytopathology were scored at 24 and 40 hr . following addition of the inhibitors . total virus yields from cultures harvested at 40 hr . post infection were determined following one freeze - thaw cycle at - 70 ° c . the effect of the inhibitors on viral penetration ( table 6 ) was determined as follows : rs virus grown in the presence or absence of compound 7 was serially diluted in mem containing 2 % fcs and 1 % dmso with or without 50 μm of compound 7 . aliquots of 0 . 05 ml were added to wells of microtiter plates containing monolayers of hep - 2 cells . virus was adsorbed for 2 hrs . at 36 ° c . rs virus and then antiserum diluted 1 : 8 in mem , was added to the appropriate wells . following an additional one hour of incubation , all monolayers were then rinsed and overlayed with 0 . 1 ml of mem plus 5 % fcs . titers were determined at 5 - 7 days . in order to separate the cytotoxic properties of the inhibitors from specific antiviral activities , hep - 2 cell cultures incubated with each of the compounds for 24 and 40 hrs . were examined for viability by the trypan blue exclusion test . table 2 lists as toxic concentrations of inhibitors which resulted in the uptake of dye by more than 10 % of the cells harvested . among the 13 compounds investigated , only no . 2 ( pentamidine ) and no . 1 ( benzamidine ) had previously been tested for their cytotoxic potential . the former reduced cell growth rates at concentrations of 5 - 40 μm ( goldberg et al ., plasminogen activators of transformed and normal cells . in proteases and biological control ( e . reich , d . b . rifkin , and e . shaw , eds . ), pp . 857 - 868 , cold spring harbor laboratory , 1975 ) and the latter at concentrations greater than 500 μm ( taylor and lembach , reversible inhibition of cell multiplication in vitro by inhibitors of arginine esteroproteases . biochim . biophys . acta 329 , 58 - 71 , 1973 ). those concentrations of the two agents are in the same general range in which clear - cut toxicity was observed in the present assay system ( table 2 ). from the data in table 2 it can also be seen that only one other compound , no . 6 , was as toxic as pentamidine . five inhibitors , nos . 3 - 5 , 12 and 13 were intermediate in their cytotoxicity , and an additional five , nos . 7 - 11 , possessed the same or even less toxic activity than benzamidine . it was also established that hep - 2 cells are not unique in their response to amidines . an identical pattern of toxicity was observed with a representative group of the compounds ( nos . 2 , 3 , 6 and 7 ) when tested on three other cell lines , i . e ., a549 ( human lung ), cv - 1 ( monkey kidney ), and bhk - 21 ( hamster kidney ) ( data not shown ). table 2______________________________________cytotoxic effect of amidines on hep - 2 cell viabilitycompound concentration ( μm ) number 5000 1000 500 100 50 10 1 . 0 0 . 1______________________________________1 t . sup . 1 nd . sup . 2 t nd c . sup . 3 nd nd nd2 t nd t nd t t c c3 nd t nd t nd c nd nd4 nd t nd t nd c nd nd5 nd t nd t nd c c c6 nd t nd t nd t c c7 t nd c nd c c c c8 nd t nd c nd c c c9 nd t nd c nd c c c10 nd t nd c nd c nd nd11 nd nd nd c nd c c c12 nd nd nd t nd c c c13 nd nd nd t nd c c c______________________________________ . sup . 1 toxic . sup . 2 not done . sup . 3 control viability ( no cytotoxicity ) influence of amidino compounds on the cytopathic effects of rs virus and p - 3 virus the role of proteolytic modification of viral glycoproteins in the activation of cell fusing activity is well established for paramyxoviruses . with this in mind , the presence or absence of syncytium formation was initially used to score the impact of the protease inhibitors on the replication of rs virus and p - 3 virus . the amidino compounds were added to the hep - 2 cell cultures after viral adsorption and penetration ( 2 hour post infection ). therefore , any inhibitor - related modification of the cytopathic effects must have had its origin in the alteration of the later stages of virus replication . all compounds were tested in serial dilutions and syncytium formation was evaluated by inspection of infected cultures at low microscopic power . in the case of p - 3 virus , none of the compounds exhibited any effect on the cytopathic effect and inhibitor - treated cultures were morphologically identical to the control . this was in striking contrast to the rs virus - infected cultures in which seven compounds ( nos . 5 , 7 - 9 , 11 , 12 and 13 ) suppressed the appearance of the characteristic syncytia ( table 3 ). one of the amidines ( no . 7 ) was active at concentrations at least 100 times lower than their cytotoxic level . the structural specificity of the inhibition is shown by the ability of compound 7 to inhibit syncytium formation at a concentration as low as 1 . 0 μm and by the lower activity of compound 8 which is a structural homolog of compound 7 . morphologically , the changes in the rs virus cytopathic effect produced by the inhibitors were easily recognized . uninfected control cultures showed some single rounded cells of the monolayer while virus - infected hep - 2 cells ( multiplicities of infection 1 - 2 , 40 hrs . post infection ) had formed syncytia of varying sizes . treatment of rs virus - infected cells with a 1 . 0 μm concentration of compound 7 resulted in abolition of fusion to the extent that infected monolayers did not appear different than uninfected controls . the modification of the cytopathic effect at the level of individual foci of infection ( multiplicity of infection 0 . 01 , 96 hrs . post infection ) was also striking . the characteristic pattern of rs virus cpe in hep - 2 cells involves the fusion of cells adjacent to the initially infected cell . this results in the progressive enlargement of the syncytium until the edges of the syncytium begin to contract . this contraction results in a thinning of the mono - layer around the syncytium or the development of an area devoid of cells . in the presence of the inhibitors , no fusion of cells was seen at the focus of infection . instead , the infected cells aggregate , forming clusters of non - fused cells with no apparent thinning of the adjacent monolayer . this reduction of cytopathology was even more obvious in rs virus infected cv - 1 cells examined at 8 days post infection . it was observed that in untreated infected cultures over 50 % of the monolayer was destroyed during the multiple cycles of rs virus growth . however , treatment of similarly infected cv - 1 cells with a 50 μm concentration of compound 7 resulted in the preservation of the monolayer with individual foci of infection appearing to be inhibited in their spread to uninfected areas of the culture . this inhibition of the rs virus cytopathic effect was not due to toxicity of compound 7 over a prolonged period of cell exposure . cv - 1 cells maintained in a medium containing a 50 μm concentration of compound 7 for 6 days produced normal yields of p - 3 and hsv - 1 when challenged on day 6 . it should also be noted that compound 7 had no effect on the ability of the mp mutant of hsv - 1 to fuse cv - 1 cells . table 3______________________________________effect of amidines on rs virus cytopathologycompound concentration ( μm ) number 1000 500 100 50 10 1 . 0 0 . 1______________________________________1 nd . sup . 1 t . sup . 2 nd s . sup . 3 nd nd nd2 nd t nd t t s s3 t nd t nd s nd nd4 t nd t nd s nd nd5 t nd t nd ns . sup . 4 s s6 t nd t nd t s s7 nd ns nd ns ns ns s8 t nd ns ns ns s s9 t nd ns ns ns s s10 t nd s nd s nd nd11 nd nd ns nd s s nd12 nd nd nd nd ns ns ns13 nd nd nd nd ns s s______________________________________ . sup . 1 not done . sup . 2 toxic . sup . 3 syncytia present . sup . 4 absence of characteristic syncytial development with paramyxoviruses , the coordinate expression of infectivity and the ability to induce cell fusion strongly suggests that these two activities are controlled by the same viral protein . the relationship between infectivity and cell fusion has not been established for rs virus . the availability of the fusion inhibitors permitted us to examine this question for rs virus . with a large inoculum ( multiplicities of infection 1 - 2 ) added prior to the addition of the inhibitors , the suppression of virus - induced cell fusion had no detrimental effect on essentially single - cycle yields of rs virus as shown in table 4 . lower than control values were obtained only in those instances where toxicity had been demonstrated ( table 2 ). in fact , the yield data seem to duplicate the findings with the trypan blue exclusion test indicating that the outcome of either assay reflects the integrity and competence of the inhibitor - treated cells . in addition , the amidino compounds did not alter the yields of p - 3 virus at high multiplicities of infection . table 4______________________________________effect of amidines on single cycle yields of rs viruscompound concentrations ( μm ) number 1000 500 100 50 10______________________________________1 nd . sup . a 2 . 8 × 10 . sup . 4c nd 1 . 1 × 10 . sup . 7 nd2 nd t . sup . b nd 6 . 3 × 10 . sup . 4 nd3 t nd t nd 3 . 5 × 10 . sup . 74 t nd t nd 3 . 5 × 10 . sup . 75 t nd 5 . 0 × 10 . sup . 5 nd 0 . 5 × 10 . sup . 76 t nd t nd 6 . 3 × 10 . sup . 67 nd 2 . 0 × 10 . sup . 7 nd 6 . 3 × 10 . sup . 7 nd8 t nd 7 . 9 × 10 . sup . 7 nd 6 . 3 × 10 . sup . 79 t nd 6 . 3 × 10 . sup . 7 nd 2 . 9 × 10 . sup . 710 t nd 6 . 3 × 10 . sup . 7 nd 1 . 1 × 10 . sup . 711 nd nd 8 . 7 × 10 . sup . 7 nd nd12 nd nd nd nd nd13 nd nd nd nd nd______________________________________ . sup . 8 not done . sup . b toxic . sup . c rs virus titers in tcid . sub . 50 / ml control rs virus titers 1 - 5 × 10 . sup . 7 at low multiplicities of infection , however , the findings were different from those just discussed . for these experiments , hep - 2 cells were infected with either rs virus or p - 3 virus at multiplicities of infection of 0 . 1 , 0 . 01 , or 0 . 001 . following a 2 hour adsorption period , the cultures received medium with or without a 50 μm concentration of compound 7 . the monolayers were harvested after multiple cycles of replication and the yields were determined in inhibitor - free microtiter assays . as shown in table 5 , considerably less rs virus was obtained from the inhibitor - treated monolayers than from the controls . this confirmed the impressions from the earlier visual observations that presence of inhibitor greatly reduced the spreading of rs virus . even at high multiplicities of infection , pretreatment of hep - 2 cells with a 50 μm concentration of compound 7 resulted in a 1 - 2 log reduction of rs yields . the results in table 5 also reveal that the inhibitor produced a slight , but consistent decrease in the yield of p - 3 virus . table 5______________________________________multiple cycle yields of rs virus and p - 3 virus yields ( tcid . sub . 50 / ml ) virus moi control treated . sup . d______________________________________rs 0 . 1 . sup . a 5 . 0 × 10 . sup . 7 6 . 3 × 10 . sup . 6rs 0 . 01 . sup . b 3 . 2 × 10 . sup . 7 1 . 2 × 10 . sup . 5rs 0 . 001 . sup . b 2 . 8 × 10 . sup . 6 1 . 1 × 10 . sup . 4p - 3 0 . 1 . sup . c 3 . 5 × 10 . sup . 8 7 . 9 × 10 . sup . 7p - 3 0 . 01 . sup . c 7 . 9 × 10 . sup . 7 4 . 6 × 10 . sup . 7p - 3 0 . 001 . sup . a 2 . 0 × 10 . sup . 8 6 . 3 × 10 . sup . 7______________________________________ . sup . a harvested 72 hr post infection . sup . b harvested 120 hr post infection . sup . c harvested 48 hr post infection . sup . d 50 μm compound 7 in the following experiments , the ability of compound 7 to interfere with adsorption and / or penetration of rs virus was studied since restrictions in these areas of the infection cycle could explain the data in table 5 . interference with viral penetration can be assessed by determining whether adsorbed virus is accessible to virus specific antiserum . this approach was used employing a modification of the standard tcid 50 assay of rs virus . briefly , rs virus was grown on hep - 2 cell monolayers in the absence or presence of a 50 μm concentration of compound 7 . after 40 hours , the supernatant was harvested and serially diluted in mem with or without 50 μm of compound 7 . a 0 . 05 ml aliquot of each dilution was added to eight preformed hep - 2 cell monolayers in microtiter plates . following a 2 hour adsorption period at 36 ° c ., 0 . 05 ml of either mem or mem containing a 1 : 8 dilution of rs virus antiserum was added to the wells and the incubation continued for an additional hour . the monolayers were then rinsed and overlayed with 0 . 1 ml of mem plus 5 % fcs . titers were determined at 5 - 7 days . from the data in table 6 , it is evident that attachment of the virus was not blocked by the inhibitor because the final titers in group 2 versus 4 and group 6 versus 8 were identical . since the inocula in these groups were all removed at the same time , any interference with adsorption should have expressed itself in a reduction in titer in the inhibitor - containing assays as compared to the inhibitor - free controls . a comparison of groups 3 versus 4 and groups 6 versus 7 shows that the addition of antiserum to the inhibitor - free virus cultures did not significantly affect the final titer , i . e ., most of the adsorbed virus had already penetrated the cells by 2 hours and was beyond neutralization by antibodies . however , if inhibitor was present during the adsorption period and the cultures were then exposed to antiserum , a significant reduction in titer was observed as compared to the inhibitor - free , antiserum - treated controls ( group 1 versus 3 ; group 5 versus 7 ). these findings indicate that a delay in rs virsus penetration occurred under the influence of inhibitor so that the varions remained susceptible to the inactivating effect of the antiserum . it should also be noted that the rs virus which had replicated in the presence of inhibitor ( rsi series in table 6 ) did not behave differently from the control ( c series ) which had been grown in the absence of inhibitor . with virus from both sources , penetration was delayed to a similar degree and , as already pointed out , there was not interference with adsorption . this result argues against the idea that inhibition by amidines of post - translational proteolytic modification of newly synthesized viral proteins was responsible for the delay in viral penetration . there was recorded , however , throughout all control groups a higher titer than in the rsi series . since only released virus and not total virus was harvested , no significance can be attached to the differences between the rsi series and the control series . table 6______________________________________effect of fusion inhibitor on thepenetration of rs virustiter titer tcid . sub . 50 standard # conditions ( log 10 ) error______________________________________1 rsi + i + a 4 . 11 . 332 rsi + i + a 5 . 97 . 263 rsi - i + a 5 . 50 . 304 rsi - i + a 5 . 97 . 265 c + i + a 5 . 59 . 316 c + i - a 6 . 92 . 377 c - i + a 6 . 73 . 248 c - i + a 6 . 97 . 26______________________________________ rsi = rs virus grown in presence of cpd 7 ( 50 μm ) c = rs virus grown in absence of cpd 7 + i = presence of cpd 7 during adsorption of rs virus - i = absence of cpd 7 during adsorption of rs virus + a = addition of antirs serum 2 hrs . after addition of virus to hep2 cell monolayers - a = no antirs serum treatment the invention having thus described , it will be apparent that various departures can be made therefrom without departing from the scope or spirit thereof .
a number of aromatic mono - and bis - amidines are capable of blocking cell fusion induced by respiratory syncytial virus . suitable amidino compounds include those selected from the group consisting of 1 - 4 - di - 2 - butanol ; bismethane ; 1 , 2 - bisethane ; 5 - amidino - indole ; 5 - amidinobenzimidazole , 5 - amidino - 1 - methylindole and 5 - amidino - 1 - indole . the most powerful of the compounds , bismethane , is able to achieve complete suppression of syncytium formation at a concentration of 1 μm . inhibition occurs in rs virus - infected hep - 2 cells as well as cv - 1 cells . babim also causes a significant retardation of rs virus penetration , but does not interfere with adsorption . addition of the amidines after the penetration of rs virus does not affect single cycle yields . the compounds may be used in the prophylactic control of rs virus in man .