332 Journal ofMedicinal Chemistry. 197.5, Vol. 18, N o . 4
Quinn, Uriscoll, Hansch
Structure-Activity Correlations among Rifamycin B Amides and Hydrazides Frank R. Quinn, John S. Driscoll,* I h g l)c,i.i-lopmentBranch. Drug Research and Ueuelopmt~nt,I)ir>i.\ionof ('nnwr Treatment, .Vational Cancer l n z t i t u f c ,Nationnl , hcsdn, Mar.yland 20111.1 lristrtu tcs o/ H ~ a i t hHcit
and Corwin Hansch 1)i~partmenti)f('hemi.\tr?,. Pomona C'olicge, ('lnrnmont, ('alifornia 91 71 1 Recc,ii,Pd August 26, 197-/
Structure-antibacterial activity corrrlation equations have been developed for a series of 44 amides and 25 hydrazides of rifamycin B in five bacterial systems. The best amide equations show that activity is a parabolic function of lo:! P. A wide variation in log P I was found for the various bacterial systems. The most important correlation parameter in the hydrazide equations is mi. 'l'he significance of'this finding is somewhat obscured by the high degree of collinearity among the parameters evaluated (a*,E.. log P ) . Two rifamycin B amides were prepared and evaluated as a result of' this study. The correlation equations quantitatively predicted their activity in five of six tests. The rifamycins, a group of metabolites of Streptomyces rnediterranei, inhibit gram-positive bacteria and have been used in the treatment of bacterial infections. One member of this family, rifamycin B, contains a free carboxyl group. In an attempt to increase the activity in this series, Sensi, et al , I prepared and tested a number of amide (I) and hydrazide (11) derivatives of rifamycin B. These 75 derivatives were tested against nine bacterial systems. A wide variation in activity was observed in five of the bacterial systems and the results appeared to constitute a good opportunity for a study of quantitative structure-activity relationships (QSAR) using substituent constants' ' and regression analysis.
'
I
correlations could be demonstrated for this large series (50 amides and 25 hydrazides) in bacterial systems, then there was good promise that quantitative correlations could be developed among rifamycin derivatives as reverse transcriptase inhibitors" when the appropriate test data become available. Methods. The biological activity, expressed as log ( l / C ) , is given for the rifamycin amides in Table I and for the hydrazides in Table 11. These data are derived from t h a t reported by Sensi, p t a l . , ] and the compound numbers used here are consistent with those used in that study. The molar concentration, C, is defined as the lowest concentration of antibiotic that prevents visible growth after 18-hr incubation. T h e physiocochemical parameters used are given in Tables 111 and IV for 70 of the 75 compounds in the original investigation.' Because of the uncertainty in the estimation of steric and electronic constants for compounds 7, 36, 37, 41, and 42, these were omitted from this study. The equations were obtained using the method of least squares and an IBM 370/165 computer. All equations designated as best for each bacterial system were determined to be statistically significant by a stepwise F test. Derivation of Log P Values. T h e log P of the parent compound, rifamycin SV, was determined experimentally in the octanol-water system.; T h e log P values of the congeners in this series were calculated from this base value. Partition coefficients for the rifamycin B amide derivatives (1-49) were calculated using the measured values for the partition coefficient of rifamycin SV and acetamide.H log P (rifamycin
SV) + log P (acetamide)
=
--1.46
+O .78
log P (compound 1) (1) -0.68 From the above relationship, the calculated log P value for the unsubstituted amide (compound 1) in the amide series (1-49) is -0.68. The log P values for the N-substituted derivatives were then calculated using eq 2.
log P = -0.68
A recent treatment of 16 N-disubstituted rifamycin B amides indicated a parabolic dependence of activity on lipophilicity in one of the five bacterial systems treated in the analysis described here.- I t was felt t h a t if quantitative
+
TR,
+
liR,
(2)
As a test of the accuracy of this method for calculating rifamycin log P values, a small sample of rifamycin B monoethylamide (3) was obtained and its log P value was determined. T h e found value of 0.13 was in satisfactory agreement with the 0.32 value calculated. T h e log P values for the hydrazides were obtained in a similar manner. The calculated log P value for unsubstituted rifamycin B hydrazide was obtained by adding the previously calculated base value for rifamycin B amide
Journal of Medicinal Chemistry, 1975, Vol. 18, No. 4
Rifamycin B Amides and Hydrazides
333
Table I. Antibacterial Activity of Rifamycin Amides [Log (l/C)] .I[.
Compd 1 2' 3
4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 38 39 40 43 44 45 46 47 48
49
S.f n ccnlis
ntli'rlls
Obsd
Preda
5.70 6.68 6.99 6.50 6.70 6.73 6.92 6.54 6.48 6.90 7.03 7.12 7.91 8.15 8.46 8.86 8.65 8.22 8.49 7.95 7.91 8.61 8.44 8.62 8.01 8.62 8.44 8.14 8.15 8.46 8.24 6.93 7.52 6.97 7.24 7.26 8.22 8.01 8.22 8.22 8.32 6.44 7.92 7.93
5.81 6.10 6.35 6.57 6.48 6.58 6.80 6.99 7.03 7.08 5.72",' 7.65 8.06 8.33 8.48 8.44 8.49 8.18 8.47 7.87 8.06 7.99 8.21 8.07 8.21 8.33 8.21' 8.25 8.34 8.36" 8.37 7.32 7.58 7.49 8.28' 7.91 8.12 8.06 8.15 8.15 8.18 7.54e.J 7.69 7.82
s. I l c l l l o l y t i C l I S
Obsd
Predb
Obsd
4.91 5.89 6.20 5.82 5.90 5.83 6.04 6.39 6.56 6.78 6.12 6.02 7.01 7.15 7.46 7.86 7.65 7.22 7.49 7.06 7.31 7.61 7.22 7.62 7.22 7.62 7.14 7.22 7.53 7.33 7.09 5.85 6.18 6.05 5.77 6.34 7.45 6.92 7.27 7.45 7.45 5.84 6.22 6.98
4.88 5.29 5.66 5.97 5.85 5. 99" 6.29 6.56 6.60 6.67 4.74"s' 6.45 7.02 7.40 7.57 7.53 7.54 7.20 7.48 6.76 7.02 6.92 7.24 7.04 7.24 7.40 7.23 7.29 7.40 7.43 7.45 5.96 6.34 6.21 7.33"3' 6.82 7.11 7.02 7.16 7.16 7.20 6.29 6.49 6.68
6.62 5.09 7.50 6.42 7.12 6.43 6.84 6.86 6.66 6.98 7.25 7.89 8.01 8.15 7.86 8.16 7.55 8.44 7.89 7.95 8.13 8.13 8.62 8.31 8.22 8.15 8.62 8.22 7.98 8.16 8.16 7.15 6.96 6.97 7.47 8.21 8.22 8.22 7.92 7.92 7.92 7.26 8.36 7.76
Pred' 6.02 6.31eif 6.54f 6.72" 6.65 6.73" 6.88 6.96 6.96 6.96 5.92' 7.72 8.03 8.14 8.05 8.11 7.76 8. 10" 7.60 7.90 8.03 7.99 8.11" 8.04 8.11 8.14 8.11" 8.13 8.14 8.14e 8.14 7.40 7.65 7.57 8.14 7.93 8.07 8.03 8.09 8.09 8.10 7.62 7.75e 7.86
B. subtilis Obsd
Predd
4.78 7.11 5.40 5.41 5.41 5.42 5.43 5.64 5.78 5.80 5.11 5.72 6.03 6.53 7.16 7.28 7.05 6.67 6.89 6.03 6.65 6.96 6.35 6.66 6.35 7.27 6.97 6.36 6.68 6.37 6.37 5.23 5.73 5.74 6.38 5.73 6.67 6.04 6.36 6.67 6.45 5.74 6.67 6.15
4.74 4.96e*f 5.17 5.36 5.28 5.37 5.57 5.79 5.83 5.90 4.67 5.92 6.29 6.59 6.84 6.75 7.03 6.42 7.08 6.11 6.29 6.22e*f 6.45 6.30 6.45 6. 59" 6.44e 6.49 6.60 6.63 6.65 5.65 5.86 5.79 6.53 6.15 6.35 6.29 6.39 6.39 6.42 5.83 5.95e.f 6.06
"Predicted by eq 18. bPredicted by eq 20. 'Predicted by eq 22. dPredicted by eq 24. eCompounds omitted in the development of eq 5-16. [Compounds omitted in the development of eq 17-24.
(-0.68) to an average of the experimental values8 of log P for dimethylamine (-0.25) and then subtracting 1.0 for the removal of two methyl groups (eq 3). Log P values for substituted hydrazides were calculated using eq 4.
log P ( r i f a m y c i n B hydrazide) = -0.68
- 1.0
log P (Me2")
log P = -1.93
+
7iR1
+
TR,
+
+ = -1.93
aR3
(3)
(4)
T h e value for the tert-butyl group (6) was obtained from the experimental valueYfor methyl tert-butyl ether by subtracting -0.97 for an ether oxygen and 0.5 for one methyl group. T h e value for the diallyl derivative 19 was obtained by adding -0.3 (for a double bond) to the value of a propyl radical. T h e value for the 1-propynyl group (28) was ob-
tained by subtracting 1.0 (for ethyl) from the experimentally obtained value of log P for 1-pentyne." T h e value for the -CH&H&N derivative 35 was obtained using a value of -0.84 for the CN group. Compounds 39-46 represent cyclized amides in which pyrrolidine, piperidine, or azepine rings have been formed. In the case of the unsubstituted pyrrolidine 39, the log P was calculated by adding 4 X 0.4 (ring CH?) to the base value of -0.68. T h e log P values for compounds 40-46 were calculated in a similar manner, adding 0.5 for each methyl substituent and subtracting 0.2 for each chain branch. The log P values of the morpholine derivatives of rifamycin B amide (47-49) were obtained by adding the log P values for diethyl ether (0.77) to the base amide value. The values for the cyclized hydrazides (68-75) were calculated
334 Journal of Medicinal Chemistry, 2975, Vol. 28, No. 4
Quinn, Driscoll. Hansch
Table 11. Antibacterial Activity of Rifamycin Hydrazides [Log (l/C)]
.\I.
(111 l ' ~ ' l l ~ \
__
s.
s.
fnccolis -___--
i/c~ilrolyticrrs __
-.
~
H . s 1 1 bt il is __
~...
.\I. Iuhercitlosis -_
Cumpd
Obsd
Prcd"
Obsd
Predb
Obsd
Prcdc
Obsd
Predd
Obsd
Prede
50
6.98 6.93 8.13 8.62 8.94 8.87 8.62 8.93 8.87 8.80 8.43 8.94 8.48 8.49 8.93 8.64 8.66 8.64 8.63 8.64 8.64 8.63 8.23 7.64 8.10 7.65
7.00 6.90 8.41 8.61 8.64 8.66 8.51 8.70 8.73 8.76 8.53 8.72 8.74 8.78 8.54 8.73 8.76 8.54 8.63 8.72 8.74 8.75 8.24 8.34' 8.35 8.37j
6.04 5.93 7.21 7.62 7.76 7.95 7.44 7.63 7.77 7.96 7.60 7.63 7.65 7.66 7.68 7.94 7.66 7.94 7.63 7.64 7.94 7.95 7.68 6.68 7 17 6.70
6.06 5.95 7.47 7.66 7.69 7.72 7.56 7.76 7.78 7.81 7.58 7.77 7.79 7.83 7.59 7.78 7.81 7.59 7.68 7.77 7.79 7.80 7.29 7.39' 7.41 7.42'
7.11 7.23 8.61 8.62 8.64 8.65 8.62 8.63 8.87 8.96 8.60 8.64 8.48 8.67 8.33 7.94 8.66 8.09 7.93 7.94 7.94 8.11 7.76 8.86 8.2'5 7.65
7.15 7.06 8.37 8.54 8.57 8.59 8.46 8.62 8.65 8.67 8.47 8.64 8.66 8.69 8.48 8.65' 8.67 8.48 7.90 7.39 8.00 8.01 8.22 8.31' 8.32 8.33'
5.40 5.44 6.65 6.96 6.98 7.30 6.62 7.63 7.29 7.31 6.65 7.64 7.17 7.49 7.28 7.64 7.31 7.28 6.98 6.98 7.29 7.30 6.36 6.06 6.38 5.78
5.27 5.48 6.68 6.98 7.14 7.31 6.83 7.13 7.29 7.46 6.91 7.22 7.37 7.55 6.99 7.29 7.46 7.06 7.01 7.16 7.25 7.33 6.41 6.56' 6.65 6 73f
6.71 7.36 6.65 6.35 6.37 7.00 6.62 6.76 6.69 7.00 6.73 6.76 7.00 7.01 6.76 6.69 7.00 6.68 6.98 7.28 7.29 7.00 6.68 6.37 6.38 5.78
6.84 7.25 6.51 6.60 6.75f 6.91 6.55 6.71 6.80 6.96 6.63 6.72 6.88 7.03 6.71 6.80 6.96 6.79 7.04 7.09 7.16 7.24 6.44 6 48 6 56 6.64'
51
52 53
54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
"Predicted by eq 26. bPredicted by eq 25. Predicted by eq 27. dPredicted by eq 28. 'Predicted by eq 29.{Compounds omitted in the development of eq 25-29. using the experimental values"' of log P for morpholine (-1.08) and piperidine (0.85). These were added to t h e base value for the amides (-0.68) to yield the hydrazide values. Calculation of E,. Steric constant values, except where noted, were those of Taft." T h e E , values for phenyl and p-halophenyl substituents (8-11) were assumed to be t h e same ( E . = -2.58), since t h e major contribution can be attributed to the ring. T h e value for 1-hydroxyethyl (12) is taken from a series of recently measured values by Talvik and Palm." T h e steric contribution of the allyl group (19) was considered to be equivalent to t h e average of t h e values for Et and Pr. T h e same value (-0.36) was used for the 2-propynyl group (28). T h e E , value for the -CH?CHyOH derivatives (33, 34) was estimated to be t h a t of n-propyl. T h e E , values for the cyclic amides and hydrazides (39-49, 68-75) were estimated by using t h e approximation that t h e total steric contribution for these rings could be deduced by summing the contribution of those moieties obtained by a scission giving t h e largest acyclic segments. For example, E . for pyrrolidine (39) was assumed to be roughly equivalent to t h e sum of t h e contribution of two ethyl groups. Similarly, t h e steric contribution of 2,5-dimethylpyrrolidine was assumed to be t h e equivalent of 2-isopropyl groups ( 2 X -0.4'7). Table V summarizes these values and the methods of calculation. Calculation of u * . T h e parameter chosen in this study t o account for polar effects was the Taft aliphatic substitue n t constant, u*. These summed values are given in Table 111. While t h e number of experimental u* values is limited," an analysis of t h e u* values of similar groups permits some reasonable approximations t o be made. For example,
t h e contribution of a n tu-methyl group appears to be a p proximately -0.1. This can be deduced from observing that the u* value for a benzyl substituent is 0.215 while t h a t for tu-methyl benzyl is 0.11.Again, t h e value for E t is -0.1, while t h a t for i-Pr is -0.19. Similarly, t h e values for i - P r and sec-butyl differ by -0.085. T h i s approximation was used t o calculate u* for the 1-hydroxyethyl group (12). In this instance, -0.1 was added to t h e value for -CHIOH (0.55) to yield a value of 0.45. T h e inductive effect of terminal alkyl groups becomes constant with increasing chain length. Hence t h e u* for the pentyl group (18) was assumed to be approximately the same as t h a t of t h e butyl group. T h e value for t h e allyl derivative (19) was obtained from t h e measured value for the vinyl group (0.65) by assuming a 50% decrease in u* for t h e effect of a n intervening methylene. Comparison of other groups in t h e T a f t series seems t o justify this approximation. T h e value for t h e 1-propynyl derivative 28 was taken from work of T h e value for t h e 2-hydroxyethyl substituent (33, 34) was obtained from t h e value for CH,iOH (0.555) using t h e factor of 0.5 as in t h e case of the allyl group. Similarly, t h e u* for the 2-cyanoethyl compound 35 was obtained by taking l/l of t h e value for -CH?CN. T h e u* values for t h e N-containing rings (39-46, 68-70) were calculated in a manner analogous to t h e calculation of the steric constants (Table V). T h e u* values for the morpholine derivatives (47-49, 72-75) were derived by considering t h e u* for morpholine t o be t h e sum of CH&HzO and Et values. T h e u* value for -CH&HZO (0.28) was obtained from t h e value for -CH20H by using t h e 50% factor for t h e intervening methylene. The uncertainty in this approximation is reflected in t h e correlation difficulties with some of the ring analogs.
Rifamycin B Amides and Hydrazides
Results and Discussion Rifamycin B Amides. Table VI shows some of the equations generated for the rifamycin B amides in the Micrococcus aureus system. Equations were derived for all linear u*, ( u * ) ? , and E,. Only the combinations of log P, (log P)?, most significant equations are shown. In general, the equations in Table VI are typical of those determined for Streptococcus faecalis, Streptococcus hemolyticus, and Bacillus subtilis. Forty-one of the 44 compounds in Table I11 were used to generate the data for the M. aureus system. The four compounds omitted from the derivation of the best equation (Table VI, eq 13) are designated in Table I. In the M. aureus system, the electronic contribution ( u * ) appears to be the most important single factor (eq 7). Equation 13, which was statistically significant (F test), best correlated the data in the M. aureus bacterial system. All three parameters are necessary with u* predominant. T h e best equations for the amides in the four bacterial test systems evaluated are shown in Table VII. M. aureus, B. subtilis, and S. faecalis (eq 13-15) all show a similar dependence on E,, log P, and U* with correlation coefficients >0.9 and standard deviations of 0.24-0.33. Only 36 of 45 amides (see Table I) were used t o derive the best equation for S. hemolyticus. T h e best amide equation in this system showed a dependence only on u*. No adequate quantitative correlation could be obtained for the rifamycin amides in the Mycobacterium tuberculosis system. T h e correlations obtained (eq 13-16) appear quite reasonable in that they explained about 80% of the variance in the data ( r 2 ) and are statistically significant relative to the next best equations containing fewer variables. T h e following points, however, caused some concern about the utility of these correlation equations. (a) Log P, an important term in all the equations except that for S. hemolyticus, had a positive coefficient and no equation which included a (log P)' term was significant. This indicated that unlimited increases in the lipophilicity of amide derivatives should result in unlimited increases in antibacterial activity. This is inconsistent with past experience. (b) As many as eight of the 4 4 derivatives had to be omitted (Table I ) in order to obtain the best correlations. (c) There was some collinearity between log P and E , for the compounds studied in several of the bacterial systems. In an attempt to improve the usefulness of the correlation equations, further studies were carried out. Molar refractivity'' ( M R ) was added as a dependent variable, but it did not prove useful due to its high degree of collinearity ( r l = 0.86) with the log P term for the substituents in the study. Inspection of the data shows that, in general, activity increases with increasing substitution of the amide nitrogen (di > mono > unsubstituted).' Additionally, it is known that amide partition coefficients are often not calculable as strictly linear combinations of x values for N-substituents."' For these reasons, a study of the effect of an additional parameter, D, was undertaken. A value of D = 1.0 was assigned to all N,N-disubstituted amides and D = 0.0 to the mono- and unsubstituted compounds. T h e single parameter equations in D and the best equations using log P, u*,E., and D as variables are given in Table VIII. The addition of the dummy parameter significantly altered the form of the optimum correlation equations (Tables VI1 and VIII) and effectively negated, or a t least reduced, many of the concerns stated above relative to the equations in Table VII. Inclusion of this parameter gave best equations which were parabolic rather than linear in log P. T h e greatest number of compounds which had to be omitted from any series in order to obtain good correlations
Journal of Medicinal Chemistry, 1975, Vol. 18, No. 4 335
Table 111. Rifamycin Amides. Physicochemical Parameters
1 2
3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 38 39 40 43 44 45 46 47 48 49
H H H H H H H H H H H Me Et )I - P r n -Bu i -Bu iz -Phenyl
H Me Et n -Pr i -Pr
p -C1 -Ph p -Br -Ph p -I -Ph
Allyl
Allyl
t -Bu Phenyl
C H (OH )Me Me Et
n -Pr -Bu i -Bu 12 -Pentyl !Z
Benzyl Benzyl Me Et Me Pr Me i -Pr Me n -Bu Me t -Bu Et n-Pr Et n -Bu n -Pr Propynyl Me CyclopentyT Me Cyclohexyl Et Phenyl Me Benzyl Me -CHzCHzOH Et -CHzCH20H Me XHZCH2CN -CHzCHz- -CH,CH2Cl c1 +CH2)4- C H (CH3)( CH2)zCH(CH3)-
2.48 1.24 1.17 0.88 0.77 -0.30 -1.34 -1.34 -1.34 -1.34 1.34 0.00 -0.14 -0.72 -0.78 -1.86 -0.80 -0.42 -0.76 -0.07 -0.36 -0.47 -0.39 -1.54 -0.43 -0.45 -0.57 -0.51 -0.79 -2.65 -0.38 -0.36 -0.43 -0.99 -1.80
-0.68 -0.18 0.32 0.82 0.62 0.85 1.45 2.15 2.31 2.57 -0.84 0.32 1.32 2.32 3.32 2.92 4.32 1.72 4.70 0.82 1.32 1.12 1.82 1.35 1.82 2.32 1.80 1.96 2.33 2.45 2.51 -0.34 0.16 -0.02 2.10
0.98 0.49 0.39 0.38 0.30 0.19 1.09 1.09 1.09 1.09 0.94 0.00 -0.20 -0.23 -0.26 -0.25 -0.26 0.32 0.43 -0.10 -0.11 -0.19 -0.13 -0.30 -0.22 -0.23 0.65 -0.20 -0.15 0.50 0.21 0.28 0.18 0.65 0.77
-0.14 -0.94
0.92 1.52
-0.20 -0.38
1.32 1.62 1.62 1.72 0.09 0.39 0.69
-0.22 -0.23 -0.30 -0.23 0.18 0.09 0.00
4.43 -0.46 -0.83 4-72 -0.43 --(cH,),ocH~cH(cH~)-0.83 -CH(CH,)CHZOCH~CH. -0.94 (CH3)-
-(CH2)5+CH2)2CH(CH,)(CH2)24 C H z )dCH(CHs)-(CH2 16
