quantitative structure activity relationships for substituted naloxone benzoylhydrazones....

Quantitative Structure Activity Relationships for SubstitutedNaloxone Benzoylhydrazones. 40-substituted NaloxoneBenzoylhydrazones

Marvin Charton*1, Grazyna R. Ciszewska2, James Ginos2, Kelly M. Standifer2, Andrew I. Brooks2, George P. Brown2, Jennifer P.

Ryan-Moro2 and Gavril W. Pasternak*2,3

1Department of Chemistry, Pratt Institut, Brooklyn, NY; 2The Cotzias Laboratory of Neuro-Oncology, Memorial Sloan-Kettering Cancer

Center; 3Departments of Neurology and of Neuroscience and Pharmacology, Cornell U. Medical College, New York, NY

Abstract

Quantitative structure activity relationships (QSAR) were

determined for the Ki values measured for the interaction of

40-substituted naloxone benzoylhydrazones with k1, k3, m1,

m2, and d receptors. Based on a comparison of the estimated

and observed values of Ki for two groups which were not in

the data sets used to obtain the initial QSAR the latter give

very satisfactory estimates for all but the d receptor. The

validity of the QSAR are further supported by a comparison

of the observed and calculated orders of Ki values for these

two groups. The effects of the 40 substituent on Ki are

similar for the m1 and m2 receptors. No other similarities

were observed. The results suggest that substituents of the

type CH2Z and NHZ where Z has large values of the

localized electrical effect parameter, s1, should exhibit a

ratio of Ki(k3) to Ki(k1) greater than 30. This is supported by

the result for the acetylamino group. For all receptors but dand possibly k3 the 40 substituent seems to be binding to a

hydrophobic region of the molecular framework through

Van der Waals interactions. The segmental parameteriza-

tion seems to be a better choice for the representation of

steric effects than a monoparametric parameterization. The

intermolecular force model is a useful alternative to the

Hansch-Fujita model. The QSAR are reliable for all but the

d receptor where the results are very uncertain.

Quant. Struct.-Act. Relat., 17 (1998) # WILEY-VCH Verlag GmbH, D-69469 Weinheim 0931-8771/98/0204±0109 $17.50+.50/0 109

QSAR for 40-substituted Naloxone Benzoylhydrazones QSAR

1 Introduction

The synthesis and characterization of substituted benzoyl-

hydrazones 2 of naloxone, 1, and their binding af®nities for

the m1, m2, d, k1, and k3 receptors were reported elsewhere

[1]. QSAR developed for these binding af®nities (Ki values)

are reported here.

The Hansch model [2±4] for QSAR is:

log ba � T 1tX � T 2t2X � rsX � AaX � SuX � Bo �1�

* To receive all correspondence

Key words: QSAR, 40-substituted naloxone benzoylhydrazones,

receptor site, receptor molecule, substituent binding effect,

substituent-receptor interaction, steric effects, segmental method,

intermolecular force model.

Abbreviations: VdW, Van der Waals.

where t is a transport parameter, s a Hammett type

electrical effect parameter, u is a steric effect parameter, and

a a polarizability parameter. Typical transport parameters

include log P (P is the 1-octanol=water partition coef®-

cient), p (de®ned as the difference between log PX and log

PH), and log k 0 (k 0 is the capacity factor determined from

high performance liquid chromatography). All of these are

composite parameters that include polarizability, electrical

effects, dipole moment, hydrogen bonding, ionic charge if

any, and steric effects. The Hammett s constant is a

composite electrical effect parameter [5]:

sX � lslX � dsdX � rseX � h �2�

where sl represents the localized (®eld) electrical effect, sd

the intrinsic delocalized (resonance) electrical effect, and se

the sensitivity to electronic demand. The polarizability

parameter a is de®ned as the difference between the group

molar refractivities of X and H divided by 100. The steric

parameter u is de®ned as the difference between the Van der

Waals radii of X and H [6]. The equation used to correlate

the log Ki values was obtained by substituting Eq. 2 in Eq.

1, using p as the transport parameter, and introducing a term

in the bond moment of the X-C(sp2) bond. The parameter mused is actually the dipole moment of PhX. The hydrogen

bonding parameters are nH, the number of OH or NH bonds

in X; and nn, the number of lone pairs on N or O atoms in X

[7]. The resulting correlation equation is:

log KiX � LslX � DsdX � RseX �MmX

� H1nHX � H2nnX � AaX � SyX � Bo �3�

2 Method and Results

The data were correlated with Eqs. 1 and 3 or relationships

derived from them by means of multiple linear regression

analysis. The substituent constants used in this work are

reported in Table 1.

The correlation matrix (values of the zeroth order partial

correlation coef®cients) for Eq. 3 is given in Table 2, that

for Eq. 8 is set forth in Table 3. The regression equations

reported are those which gave the best statistics. The

statistics reported are: 100R2, percent of the data accounted

for by the regression equation; A100R2, percent of the data

accounted for by the regression equation adjusted for the

number of independent variables, F, a test for the

signi®cance of the regression coef®cients and a measure

of the goodness of ®t; Sai, standard errors of the regression

coef®cients; Sest, standard error of the estimate; So, standard

error of the estimate divided by the root mean square of the

data. The Sai are given in parentheses after the regression

coef®cients in the regression equation, the other statistics

are reported beneath the regression equation.

Correlation of the log K i values for the k1 receptor with Eq.

3 gave:

Ki�k1�X � 5:36��1:22�slX � 1:91��0:631�sdX 12:9��3:73�seX

� 2:62��1:13�aX ÿ 0:967��0:276�mX ÿ 0:146��0:261��4�

n � 20 100R2 � 66:39 A100R2 � 57:42

Sest � 0:366 So � 0:693 F � 5:530�99:0�

Eq. 4 is signi®cant.

The correlation of log K i for the k3 receptor with Eq. 1

gave:

log Ki�k3�X � ÿ0:378��0:0744�pX � 5:75��1:12�aX

� 0:292��0:0901� �5�n � 20 100R2 � 62:01 A100R2 � 59:90

Sest � 0:209 So � 0:669 F � 13:87�99:9�

Again, the equation is signi®cant.

Correlation of the log K i values for the m1 receptor with Eq.

3 gave:

log Ki�m1�X � ÿ 0:562��0:372�slX � 1:33��0:542�aX

� 0:126��0:0711�mX ÿ 0:381��0:100� �6�n � 20 100R2 � 44:97 A100R2 � 38:50

Sest � 0:185 So � 0:829 F � 4:359�95:0�

Though Eq. 6 is signi®cant the ®t is only fair.

Correlation of the log Ki values for the m2 receptor with Eq.

1 gave:

Ki�m2�X � ÿ 0:205��0:0630�pX � 3:28��0:997�aX

� 0:555��0:221�yX ÿ 0:248��0:108� �7�n � 20 100R2 � 65:61 A100R2 � 61:57

Sest � 0:175 So � 0:657 F � 10:18�99:9�

Eq. 7 is signi®cant but dif®cult to interpret due to

collinearities among the variables. In order to obtain further

insight into the nature of the steric effect the data were

correlated with the equation:

log Ki � LslX � DsdX � RseX �MmX � AaX � H1nHX

� H2nnX � S1y1X � S2y2X � S3y3X � Bo �8�

in which the monoparametric steric term has been replaced

by the segmental method of steric effect parameterization

[8, 9]. In this method each atom of the longest chain in the

Marvin Charton et al.QSAR

110 Quant. Struct.-Act. Relat., 17 (1998)

Table 1. Parameter values used in the correlations.

X p p2 sl sd se m a

H 0 0 0 0 0 0 0

Br 0.81 0.74 0.47 7 0.27 7 0.018 1.70 0.079

Cl 0.71 0.50 0.47 7 0.28 7 0.011 1.70 0.050

NO2 7 0.28 0.078 0.67 0.18 7 0.077 4.28 0.063

OH 7 0.67 0.45 0.35 7 0.57 7 0.044 1.40 0.018

NH2 7 1.23 1.51 0.17 7 0.68 7 0.13 1.49 0.044

CN 7 0.57 0.32 0.57 0.12 7 0.055 4.14 0.053

I 1.12 1.15 0.40 7 0.20 7 0.057 1.71 0.129

F 0.14 0.02 0.54 7 0.48 0.041 1.66 7 0.001

OMe 7 0.02 0 0.30 7 0.55 7 0.064 1.36 0.068

Me 0.56 0.31 7 0.01 7 0.14 7 0.030 0.37 0.046

Et 1.02 1.04 7 0.01 7 0.12 7 0.036 0.37 0.093

iPr 1.53 2.34 0.01 7 0.15 7 0.040 0.40 0.140

Bu 2.13 4.54 7 0.01 7 0.15 7 0.036 0.37 0.186

tBu 1.98 3.92 7 0.01 7 0.15 7 0.036 0.52 0.186

Hx 3.14 9.86 7 0.01 7 0.14 7 0.036 0.37 0.276

Hp 3.65 13.3 7 0.01 7 0.14 7 0.036 0.37 0.322

cHx 2.51 6.30 0.00 7 0.14 7 0.036 0.37 0.257

CH2NH2 7 1.04 1.08 0.06 7 0.11 7 0.018 1.38 0.081

NHMe 7 0.47 0.22 0.13 7 0.67 7 0.18 1.77 0.093

X nH nn u u1 u2 u3

H 0 0 0 0 0 0

Br 0 0 0.65 0.65 0 0

Cl 0 0 0.55 0.55 0 0

NO2 0 4 0.35 0.35 0.32 0

OH 1 2 0.32 0.32 0 0

NH2 2 1 0.35 0.40 0 0

CN 0 1 0.40 0.40 0.40 0

I 0 0 0.78 0.78 0 0

F 0 0 0.27 0.27 0 0

OMe 0 2 0.36 0.32 0.52 0

Me 0 0 0.52 0.52 0 0

Et 0 0 0.56 0.52 0.52 0

iPr 0 0 0.76 0.76 0.52 0

Bu 0 0 0.68 0.52 0.52 0.52

tBu 0 0 1.24 1.24 0.52 0

Hx 0 0 0.73 0.52 0.52 0.52

Hp 0 0 0.73 0.52 0.52 0.52

cHx 0 0 0.87 0.76 0.52 0

CH2NH2 2 2 0.55 0.52 0.35 0

NHMe 1 1 0.39 0.35 0.52 0

Values of parameters are from: p, Hansch, C., Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biology, Wiley, New York 1979; sl,

sd, se, ref. 2; a, Charton, M. In The Chemistry of Sulfenic Acids, Esters and Derivatives, S. Patai (Ed.), Wiley, New York 1990, pp. 657±700; u, u1, u2, u3,

ref. 3; m, McClellan, A.L. Tables of Experimental Dipole Moments, W.H. Freeman, San Francisco 1963, McClellan, A.L. Tables of Experimental Dipole

Moments, Vol. 2, Rahara Enterprises, El Cerrito, Cal. 1974.

Table 2. Correlation matrix for Eq. 3.

sl sd se a nH nn u p p2 m

1 0.026 0.032 0.522 0.095 0.515 0.363 0.478 0.499 0.859 sl

1 0.393 0.238 0.475 0.016 0.205 0.310 0.185 0.153 sd

1 0.032 0.438 0.404 0.064 0.285 0.081 0.272 se

1 0.255 0.335 0.727 0.885 0.926 0.405 a1 0.260 0.246 0.589 0.187 0.081 nH

1 0.411 0.538 0.337 0.685 nn

1 0.691 0.525 0.315 u1 0.845 0.542 p

1 0.431 p2

1 m


Quant. Struct.-Act. Relat., 17 (1998) 111

substituent together with the atoms attached to it constitutes

a segment for which a separate parameter of same type as uis used. Eq. 8 is an form of the intermolecular force (IMF)

equation. The correlation matrix for Eq. 8 is given in Table

3. The equation obtained is:

log Ki�m2�X � ÿ 0:885��0:337�slX � 0:166��0:0645�mX

� 2:69��0:647�aX ÿ 0:898��0:288�u3X

� 0:00550��0:0928� �9�

n � 20 100R2 � 70:65 A100R2 � 65:15

Sest � 0:167 So � 0:626 F � 9:027�99:9�

Again Eq. 9 is signi®cant.

Correlation of the K i values for the m1 receptor with Eq. 8

gave best results on exclusion of the value for naloxone 4-

cyclohexylbenzoylhydrazone.

Ki�m1�X � ÿ0:655��0:248�slX � 0:500��0:172�y3X

� 0:152��0:0496�mX ÿ 0:325��0:0530� �10�

n � 19 100R2 � 56:73 A100R2 � 51:32

Sest � 0:130 So � 0:740 F � 6:556�99:5�

Correlation of the log K i values for the d receptor with Eq. 1

did not give a signi®cant result until the K i value for

X�OH was excluded. The regression equation is:

log Ki�d�X � ÿ 0:242��0:0907�pX ÿ 0:0940��0:0340�p2X

� 1:03��0:0792� �11�

n � 19 100R2 � 33:40 A100R2 � 29:49

Sest � 0:283 So � 0:888 F � 4:03�95:0�

The results are very poor and barely signi®cant. K i for this

receptor seems to be independent of 40-substitution.

The percent contribution, Ci, of each independent variable

in the regression equation is useful in discussing the results

[7]. Ci is written as:

Ci �100aixiPmiÿ1

aixi

�12�

where ai is the regression coef®cient of the i-th independent

variable and xi is its value for some reference group. A

hypothetical reference group was de®ned for which:

sl � sd � m � nH � nn � p � y � y1 � y2 � y3 � 1;

se � 0:1; a � 0:2:

Values of Ci for each receptor are set forth in Table 4.

Inspection of these values shows that substituent effect

composition on the Ki values is similar for the m1 and m2

receptors. The values of the coef®cients of sl and m are not

signi®cantly different. In view of the collinearity of a and u3

the lack of dependence of the m1 set on a and the difference

in sign of the coef®cient of u3 in the two sets is not

unreasonable.

Values of log Kio and log Kic, the observed and calculated

values of log Ki respectively, and of D log Ki, the difference

between the observed and calculated values of log Ki, are

given in Table 5. The D log Ki values show that there is

generally reasonable agreement between calculated and

observed values of log Ki. Also given in Table 5 is the range

of the data set which is de®ned as the difference between

the largest and the smallest data points in the set. The range

provides a measure of the sensitivity of the log Kio values to

structural effects.

The validity of the QSAR was tested by the classical

method of predicting bioactivities for compounds that have

not been studied, then determining the experimental values,

and comparing predicted and observed values. This is a very

good test of predictability, particularly if the substituents

chosen for study are very different from those used to obtain

the original QSAR. In Table 6 calculated values are given

Table 3. Correlation matrix for Eq. 8.

sl sd se a nH nn u1 u2 u3 m

1 0.026 0.032 0.522 0.095 0.515 0.247 0.448 0.384 0.859 sl

1 0.393 0.238 0.475 0.016 0.183 0.222 0.159 0.153 sd

1 0.032 0.438 0.404 0.063 0.253 0.084 0.272 se

1 0.255 0.335 0.531 0.657 0.711 0.405 a1 0.260 0.206 0.163 0.197 0.081 nH

1 0.362 0.028 0.247 0.685 nn

1 0.318 0.015 0.218 u1

1 0.405 0.149 u2

1 0.337 u3

1 m



for a number of substituents for which no experimental

determinations were carried out when the ®rst QSAR

models were developed. The parameter values used for

these calculations are reported in Table 7. The results in

Table 6 suggested that substituents of the type CH2Z and

NHZ where Z has a large value of the localized electrical

effect parameter, sl, should exhibit a ratio of Ki(k3) to

Ki(k1) greater than 30. To test these predictions Ki values

were determined for the SO2NH2 and the NHAc groups for

all receptors. These values are also reported in Table 6. It is

important to note that the SO2NH2 and NHAc groups are

very different structurally from the groups used to obtain

the original QSAR. There is no group comparable to the

SO2NH2 group in the original data set. Though NHR groups

(R�H or Me) are present. They differ greatly from the

NHAc group. The latter has a much more acidic NH bond, a

much less basic N atom, and a carbonyl group. Clearly these

two substituents should provide a good test of QSAR

validity. The quality of our QSAR as estimators of Ki can be

judged from the magnitude of f the absolute value of the

ratio of D log Ki to Sest, the standard error of the estimate for

the QSAR equation. When f< 2 the estimate is good, when

2<f< 3 the estimate is acceptable, and when f> 3 the

estimate is unacceptable. Values of D log Ki and f for the

SO2NH2 and NHAc groups are given in Table 8. The results

for the k3, m1, m2, and d receptors are very satisfactory.

Those for the k1 receptor are poor.

A second test of QSAR reliability is whether they can

predict the order of the Ki values for the receptors. The

predicted and observed orders are for SO2NH2

OBSERVED d > k3 > m2 > m1 > k1

CALCULATED d > k3 > m2 > m1 � k1

and for NHAc

OBSERVED k3 > d > m2 > m1 > k1

CALCULATED k3 > d� m2 > m1 �> k1

The observed and calculated orders are in good agreement,

supporting the validity of the QSAR.

The inclusion of the NHAc and SO2NH2 data points in the

correlations was examined. For the k1 receptor the best

regression equation (obtained on exclusion of the Ki value

for NHAc) is:

Ki�k1�X � 4:18��0:989�slX � 1:57��0:617�sdX

ÿ 10:1��2:95�seX � 2:83��1:18�aX

ÿ 0:665��0:203�mX ÿ 0:258��0:262� �40�

n � 21 100R2 � 60:80 A100R2 � 50:99

Sest � 0:383 So � 0:741 F � 4:652�99:0�

The coef®cients in Eq. 40 are not signi®cantly different from

those in Eq. 4 though the goodness of ®t is poorer. A plot of

log Ki calculated from both Eq. 4 and Eq. 40 versus log Ki

observed is given in Figure 1. A comparison of the

substituent effect composition for these equations is given

in Figure 2.

For the k3 receptor the best equation is:

log Ki�k3�X � ÿ 0:328��0:0559�pX � 6:02��0:952�aX

ÿ 0:440��0:116�y� 0:466��0:0913� �50�

n � 22 100R2 � 72:20 A100R2 � 69:27

Sest � 0:211 So � 0:583 F � 15:58�99:9�

The coef®cients of p and a in Eq. 50 are essentially

unchanged from those of Eq. 5. There is a new term in the

steric parameter u and the goodness of ®t is improved. A

plot of log Ki calculated from both Eq. 5 and Eq. 50 versus

log Ki observed is given in Figure 3. A comparison of the

substituent effect composition for these equations is given

in Figure 4.

Table 4. Values of Ci.

Set Eq. sl sd se nH nH a u u1 u2 u3 p p2 m

k1 4 53.3 19.0 12.8 0 0 5.21 0 nd nd nd 0 0 9.63

40 52.3 19.7 12.8 0 0 7.09 0 nd nd nd 0 0 8.32

k3 5 0 0 0 0 0 75.3 0 nd nd nd 24.7 0 0

50 0 0 0 0 0 61.1 16.6 nd nd nd 22.3 0 0

m1 10 50.1 0 0 0 0 0 nd 0 0 38.3 nd nd 11.6

100 47.5 0 0 0 0 0 nd 0 0 41.6 nd nd 11.0

m2 9 35.7 0 0 0 0 21.7 nd 0 0 36.1 nd nd 6.66

90 34.6 0 0 0 0 22.1 nd 0 0 36.9 nd nd 6.33

d 11 0 0 0 0 0 0 0 nd nd nd 71.9 28.1 0

110 0 0 0 52.9 21.5 0 0 nd nd nd 25.6 0 0

Sets k1, k3, and d were correlated with Eq. 3 (regression Eqs. 4, 40, 5, 50, 11, and 110). Sets m1 and m2 were correlated with Eq. 8 (regression Eqs. 9, 90, 10,

and 100). nd� not determined. Values in boldface are for nonpolar parameters, values in italics are for the composite parameters p and p2.



Table 5. Values of log Kio, log Kic, D log Ki, and the range of the data set.

Equation, Receptor 4,40(k1) 5,50(k3) 10,100(m1)

X log Kio log Kic D log Ki log Kio log Kic D log Ki log Kio log Kic D log Ki

H 7 0.222 7 0.146 7 0.076 7 0.046 0.292 7 0.338 7 0.523 7 0.325 7 0.198

7 0.258 0.036 0.466 7 0.511 7 0.319 7 0.204

Br 1.097 0.650 0.447 0.380 0.440 7 0.060 7 0.301 7 0.374 0.073

0.557 0.540 0.391 7 0.010 7 0.363 0.062

Cl 0.987 0.465 0.522 0.447 0.311 0.136 7 0.301 7 0.374 0.073

0.388 0.595 0.293 0.154 7 0.363 0.062

NO2 0.863 0.803 0.061 0.663 0.760 7 0.097 7 0.155 7 0.113 7 0.042

0.937 7 0.073 0.783 7 0.121 7 0.134 7 0.021

OH 7 0.523 7 0.102 7 0.421 1.076 0.649 0.427 7 0.155 7 0.341 0.186

7 0.126 7 0.397 0.653 0.422 7 0.334 0.179

NH2 7 0.046 7 0.191 0.145 1.041 1.010 0.031 7 0.097 7 0.209 0.113

0.166 0.119 0.980 0.061 7 0.218 0.121

CN 7 0.222 7 0.021 7 0.200 0.914 0.812 0.101 7 0.046 7 0.069 0.023

0.266 7 0.488 0.796 0.118 7 0.095 0.049

I 0.580 1.031 7 0.451 0.276 0.610 7 0.334 7 0.398 7 0.327 7 0.071

0.905 7 0.325 0.533 7 0.256 7 0.322 7 0.076

F 7 0.444 7 0.306 7 0.138 0.199 0.234 7 0.035 7 0.538 7 0.426 7 0.112

7 0.279 7 0.165 0.296 7 0.097 7 0.409 7 0.129

OMe 7 0.046 0.095 7 0.140 0.695 0.691 0.004 7 0.432 7 0.314 7 0.117

0.068 7 0.114 0.724 7 0.029 7 0.311 7 0.121

Me 0.134 7 0.319 0.452 0.576 0.345 0.231 7 0.097 7 0.262 0.165

7 0.332 0.466 0.331 0.245 7 0.264 0.167

Et 7 0.237 7 0.081 7 0.156 0.574 0.441 0.133 7 0.060 7 0.262 0.201

7 0.106 7 0.130 0.446 0.128 7 0.264 0.204

iPr 7 0.013 0.115 7 0.128 0.540 0.519 0.022 7 0.310 7 0.270 7 0.040

0.084 7 0.097 0.474 0.067 7 0.272 7 0.038

Bu 7 0.301 0.105 7 0.406 0.595 0.556 0.039 7 0.114 7 0.002 7 0.112

0.110 7 0.411 0.589 0.006 7 0.002 7 0.112

tBu 7 0.523 7 0.040 7 0.481 0.301 0.613 7 0.312 7 0.398 7 0.239 7 0.159

0.010 7 0.533 0.392 7 0.091 7 0.244 7 0.154

Hx 0.356 0.360 0.004 0.568 0.692 7 0.124 0.076 7 0.002 0.077

0.381 7 0.025 0.778 7 0.210 7 0.002 0.077

Hp 0.662 0.480 0.181 0.909 0.764 0.145 0.033 7 0.002 0.035

0.511 0.151 0.888 0.021 7 0.002 0.035

cHx 0.749 0.364 0.385 1.067 0.821 0.246

0.369 0.380 0.809 0.259

CH2NH2 7 0.658 7 0.926 0.269 1.046 1.151 7 0.105 7 0.167 7 0.154 7 0.013

7 0.686 0.029 1.057 7 0.011 7 0.170 7 0.002

NHMe 0.255 0.114 0.141 0.894 1.005 7 0.111 7 0.222 7 0.141 7 0.081

0.143 0.112 1.009 7 0.115 7 0.158 7 0.064

SO2NH2 0.009 7 0.319 0.328 1.166 1.268 7 0.102 0.117 0.112 0.005

NHAc 1.494 1.421 0.073 0.093 0.140 0.005

Range 1.755 1.122 0.613

Equation, Receptor 9,90(m2) 11,110(d)

X log Kio log Kic D log d Ki log Kio log Kic D log d Ki

H 7 0.097 0.006 7 0.102 0.857 1.030 7 0.172

7 0.011 7 0.108 0.851 0.006

Br 7 0.097 0.084 7 0.181 1.061 0.904 0.157

0.089 7 0.186 0.942 0.118

Cl 0.079 0.006 0.073 1.267 0.905 0.362

0.011 0.068 0.931 0.336

NO2 0.204 0.292 7 0.088 1.352 1.105 0.247

0.275 7 0.071 1.197 0.155

OH 0.079 7 0.024 0.103

7 0.020 0.099

NH2 0.398 0.221 0.177 1.294 1.470 7 0.175

0.216 0.182 1.271 0.024

(continued )



Table 6. Values of Ki for groups not included in the correlations.

Equation (Receptor) 4(k1) 5(k3) 10(m1) 9(m2) 11(d)

X Ki

CH2Cl 0.0119 5.94 1.14 4.45 9.81

CO2Et 17.2 16.3 0.575 3.08 10.4

OCF3 0.574 1.95 0.676 0.761 7.59

SO2CF3 7.49 5.79 0.789 2.79 8.42

SOMe 0.491 41.6 0.844 3.39 44.5

N3 83.9 4.44 0.427 0.656 8.68

CH2CN 0.00939 10.7 1.84 1.92 15.8

CH2Br 0.0156 5.09 1.13 5.23 7.89

CF3 0.833 1.55 0.704 1.71 7.76

NHAc 0.0363 28.7 1.67 2.93 22.6

0.90 31.20 1.24 6.96 13.60

NHCO2Me 0.117 21.0 1.63 3.16 13.6

NHCONH2 0.0116 31.8 2.19 3.68 31.9

NHSO2Me 1.01 46.7 1.82 3.51 28.0

SO2NH2 0.111 42.6 1.47 5.91 60.7

1.02 14.65 1.31 5.11 32.62

SCN 0.812 7.08 1.14 1.22 8.85

SF5 3.28 2.18 0.648 1.97 7.50

OSO2Me 0.852 28.0 1.12 1.75 20.7

Values in bold face were experimentally determined.

Table 5. (Continued )

Equation, Receptor 4,40(k1) 5,50(k3) 10,100(m1)

X log Kio log Kic D log Ki log Kio log Kic D log Ki log Kio log Kic D log Ki

CN 0.398 0.330 0.068 1.076 1.198 0.122

0.311 0.087 0.882 0.194

I 0.377 0.282 0.094 0.530 0.868 7 0.337

0.284 0.011 0.977 7 0.447

F 7 0.180 7 0.200 0.020 0.638 0.998 7 0.359

7 0.191 0.011 0.867 7 0.229

OMe 0.097 0.149 7 0.052 1.157 1.035 0.122

0.151 7 0.054 1.038 0.119

Me 0.255 0.200 0.056 1.117 0.924 0.193

0.200 0.055 0.914 0.202

Et 0.274 0.326 7 0.052 0.415 0.881 7 0.466

0.326 7 0.052 0.737 7 0.322

iPr 0.182 0.440 7 0.258 1.193 0.881 0.311

0.441 7 0.259 1.023 0.170

Bu 0.158 0.110 0.049 0.662 0.943 7 0.282

0.110 0.048 1.093 7 0.429

tBu 0.672 0.602 0.070 1.079 0.921 0.158

0.600 0.072 1.074 0.006

Hx 0.551 0.352 0.199 1.389 1.201 0.187

0.352 0.199 1.204 0.185

Hp 0.228 0.476 7 0.248 1.210 1.403 7 0.193

0.476 7 0.248 1.261 7 0.051

cHx 0.945 0.759 0.186 1.189 1.018 0.171

0.759 0.186 1.132 0.056

CH2NH2 0.544 0.399 0.145 1.763 1.383 0.380

0.391 0.153 1.292 0.471

NHMe 0.176 0.434 7 0.258 0.981 1.164 7 0.183

0.424 7 0.248 1.125 7 0.143

SO2NH2 0.708 0.734 7 0.026 1.513 1.582 7 0.068

NHAc 0.843 0.845 7 0.002 1.133 1.488 7 0.354

Range 1.125 1.348

Values in boldface are for Eqs. 40, 50, 90, 100, and 110.



Table 7. Parameter values used to estimate the Ki values in Table 6.

X p p2 a sl sd se m u3

N3 0.46 0.21 0.092 0.43 7 0.27 7 0.12 1.56 0.35

CH2CN 7 0.57 0.32 0.091 0.20 7 0.01 7 0.011 3.43 0.40

CH2Br 0.79 0.62 0.124 0.20 7 0.08 7 0.026 3.35 0

CH2Cl 0.17 0.03 0.095 0.17 7 0.06 7 0.024 3.24 0

CF3 0.88 0.77 0.040 0.40 0.13 7 0.026 2.86 0

CO2Et 0.06 0 0.164 0.30 0.18 7 0.064 1.849 0

OCF3 1.04 1.08 0.068 0.51 7 0.30 0.014 2.33 0.27

OSO2Me 7 0.88 0.77 0.143 0.55 7 0.23 7 0.065 3.77 0.32

NHAc 7 0.97 0.94 0.139 0.28 7 0.35 7 0.088 3.75 0.32

NHCO2Me 7 0.37 0.14 0.155 0.28 7 0.42 7 0.13 3.69 0.32

NHCONH2 7 1.30 1.69 0.125 0.23 7 0.45 7 0.13 4.31 0.32

NHSO2Me 7 1.18 1.39 0.162 0.42 7 0.21 7 0.18 4.60 0.32

SCN 0.41 0.17 0.124 0.56 7 0.15 7 0.040 3.62 0.40

SOMe 7 1.58 2.50 0.127 0.54 7 0.01 7 0.037 3.98 0

SO2NH2 7 1.82 3.31 0.113 0.44 0.23 7 0.082 5.13 0

S02CF3 0.55 0.30 0.118 0.71 0.29 7 0.056 4.52 0

SF5 1.23 1.51 0.089 0.59 0.04 7 0.040 3.44 0

Table 8. Values of DKi and f for SO2NH2 and NHAc.

Receptors

X k1 k3 m1 m2 d

SO2NH2 D log Ki 0.947 7 0.463 7 0.0493 7 0.0632 7 0.269

f 2.59 2.22 0.356 0.378 0.951

NHAc D log Ki 1.395 0.0362 7 0.128 0.376 7 0.220

f 3.81 0.173 0.985 2.25 0.777

Figure 1. Plot of log Ki, calculated from Eqs. 4 and 40against log Ki, observed.



The best equation for the m1 receptor was again obtained on

the exclusion of the Ki value for 40-cyclohexyl NBH, it is:

logKi�m1�X � ÿ 0:577��0:177�slX � 0:505��0:163�y3X

� 0:134��0:0262�mX ÿ 0:319��0:0480� �100�

n � 21 100R2 � 66:92 A100R2 � 63:24

Sest � 0:123 So � 0:639 F � 11:46�99:9�

Again, there is no signi®cant difference between the

coef®cients of Eqs. 10 and 100 though the goodness of ®t

is better in the latter equation. A plot of log Ki calculated

from both Eq. 10 and Eq. 100 versus log Ki observed is

given in Figure 5. A comparison of the substituent effect

composition for these equations is given in Figure 6. We are

unable at the present time to account for the peculiar

behavior of the cyclohexyl group. As it was not an outlier in

the case of the other receptors it may be characteristic of the

Figure 2. Substituent effect composition forthe k1 receptor.

Figure 3. Plot of log K, calculated from Eqs.5 and 50 against log Ki, observed.



m1 receptor or due to an error in the determination of Ki that

we have not been able to correct.

For the m2 receptor the equation is:

log Ki�m2�X � ÿ0:842��0:255�slX � 0:154��0:0358�mX

� 2:69��0:609�aX ÿ 0:897��0:270�y3X

� 0:0109��0:0847� �90�n � 22 100R2 � 78:05 A100R2 � 74:39

Sest � 0:157 So � 0:533 F � 15:11�99:9�

The coef®cients of Eqs. 9 and 90 are essentially the same,

the goodness of ®t is much better for Eq. 90. A plot of log Ki

calculated from both Eq. 9 and Eq. 90 versus log Ki observed

is given in Figure 7. Comparison of the substituent effect

composition for these equations is given in Figure 8.

For the d receptor the best equation (obtained on exclusion

of the data point for 40-HO-NBH) is:

Figure 5. Plot of log Ki, calculated from Eqs. 10 and 100 againstlog Ki, observed.

Figure 7. Plot of log Ki, calculated from Eqs. 9 and 90 against logKi, observed.

Figure 4. Substituent effect composition for the k3 receptor.

Figure 6. Substituent effect composition for the m1 receptor. Figure 8. Substituent effect composition for the m2 receptor.



log Ki�d�X � 0:112��0:0542�pX � 0:232��0:0995�nHX

� 0:0942��0:0534�nn � 0:851��0:0942� �110�n � 21 100R2 � 42:43 A100R2 � 36:04

Sest � 0:269 So � 0:843 F � 4:177�97:5�

The results for Eq. 110 show a better ®t to the data than

those obtained for Eq. 11. The two equations differ

dramatically however. The coef®cient of p has changed

sign in Eq. 110, and Eq. 11 shows a dependence on p2 while

Eq. 110 shows a dependence on the hydrogen bonding

parameters nH and nn. In view of the dramatic differences

between Eqs. 11 and 110 it is unlikely that either is a reliable

QSAR. A plot of log Ki calculated from both Eq. 11 and Eq.

110 versus log Ki observed is given in Figure 9. A

comparison of the substituent effect composition for these

equations is given in Figure 10.

3 Discussion

Although the QSAR reported here are signi®cant the

explained variance is low. We believe this is due to two

factors: dif®culty in measuring Ki, and the narrow range of

log Ki in the compounds available for study.

3.1 The Effect of 40-Substituents on Binding

3.1.1 Receptor Structure

For the purpose of discussion we de®ne the receptor site as

a combination of:

1. The cluster of active amino acid side chains involved in the

recognition of and=or the bioactivity resulting from interaction

with a bioactive substance.

2. The nearest neighbor residues of amino acids with active side

chains.

The molecular framework consists of the rest of the amino

acid residues to which these active side chains are attached.

Thus, the receptor molecule consists of a receptor site

embedded in a molecular framework. If that part of the

bioactive substance to which the substituent is attached is

large enough the substituent will bind to the molecular

framework rather than the receptor site. Recognition

requires directed interactions such as hydrogen bonding

and salt bridges. If we consider some typical compounds

that are known to bind strongly to opiate receptors and that

differ widely in structure (Table 9) we see that the common

features encountered in all of them are:

1. A tertiary alkylamino group (an N atom bonded to three sp3

hybridized carbon atoms).

2. At least one O atom that is part of a OR or CO-group (R�H or

alkyl).

3. At least one aromatic ring.

Features 1 and 2 are separated by from ®ve to seven bonds.

It seems reasonable that the tertiary alkylamino N atom, the

O atom, and possibly the aromatic ring as well, are

components of the pharmacophore. The receptor then

consists of those amino acid residues whose side chains

interact with the groups which comprise the pharmacophore.

The effect of the 40 substituent on binding should result

from a combination of two distinct contributions:

Figure 9. Plot of log Ki, calculated from Eqs. 11 and 110 againstlog Ki, observed.



1. The substituent could exert an electrical effect on the strength

of hydrogen bonding between the receptor site and hydrogen

acceptor atoms (N, O) of the naloxone moiety, and its two OH

groups, though this effect is likely to be very small due to the

large distance involved [10]. It may also have a small effect

on H acceptor bonding by the acylhydrazone group

(�NÿNH(C�O)ÿC6H4Xÿ40) at O or N, and H donor bonding

at NH, again due to distance. Substituent effects on hydrogen

bonding are inherently small. Finally, it may effect H acceptor

bonding to the benzene ring of the acylhydrazone group as

well.

2. The substituent may bind directly to the receptor site or more

probably the molecular framework by intermolecular forces

including hydrogen bonding and Van der Waals (VdW)

interactions. The latter require a dependence on a and m.

We now consider the results for the individual receptors.

3.2 The k1 Receptor

The ®nal QSAR for the k1 receptor (Eq. 90) shows a large

dependence on the electrical effect parameters sl, sd, and se

as is shown by the Ci values in Table 4. This is in accord

with an effect of the 40 substituent on the hydrogen bonding

of the naloxone acylhydrazone moiety. The small but

signi®cant dependence on m and a suggests binding of the 40

substituent to the receptor or the molecular framework due

to VdW interactions. A number of the substituents studied

in our data sets are capable of hydrogen bonding, including

the NH2, CH2NH2, NHAc, SO2NH2, OH, NHMe, NO2, and

OMe groups. If hydrogen bonding were involved in the

binding of the 40 substituents we should observe a

dependence on the nH and nn parameters. As no such

dependence is found we may conclude that hydrogen

bonding by the substituent is not involved and therefore the

40 substituents are binding to a hydrophobic region of either

receptor site or molecular framework.

3.3 The k3 Receptor

The ®nal QSAR for the k3 receptor (Eq. 50) shows a large

dependence on a, some dependence on p, and a small

dependence on u. The p parameter is itself a composite

parameter and is a function of electrical effects, polariz-

ability, dipole moment, and the hydrogen bonding para-

meters. It seems likely that there is little or no effect of the

40 substituent on hydrogen bonding in the rest of the

substrate but in view of the composite nature of the pparameter this is not certain. The effect of the 40 substituent

on binding is probably the result of VdW interactions and

possibly hydrogen bonding as well with the receptor site

and=or the molecular framework.

3.4 The m1 Receptor

The ®nal QSAR obtained for the m1 receptor (Eq. 100) is

shown by the Ci values of Table 4 to have a large

dependence on sl, it depends on u3 and m as well. Due to the

strong collinearity between u3 and a it is uncertain whether

this is due to a steric effect, to polarizability, or both. The

dependence on sl suggests that the 40 substituent may be

Figure 10. Substituent effect composition for the d receptor.

Table 9. Typical compounds that bind strongly to opiate receptors.

Structural Features

Compound Tertiary alkylamino group Oxygen atom functional groups nba Benzenoid ring

Levorphanol, 3 Yes OH 7 Yes

Pentazocine, 4 Yes OH 7 Yes

Meperidine, 5 Yes C�O 5 Yes

Methadone, 6 Yes C�O 5 Yes

Naloxoneb, 1 Yes OH, C�O, CÿOÿC 7, 6, 5 Yes

Naloxone benzoylhydrazoneb, 2 Yes OH, CÿOÿC 7, 5 Yes

All of the OH groups listed in the Table are phenolic. a The number of bonds between the oxygen and the nitrogen atoms. b These compounds have an

alcoholic OH group as well as a phenolic OH group.



exerting an effect on hydrogen bonding to the acylhydra-

zone group. The dependence on the dipole moment and

possible dependence on polarizability suggest that the

substituent is probably involved in binding to the receptor

site and=or the molecular framework by VdW interactions.

The absence of any dependence on the hydrogen bonding

parameters suggests that the binding of the substituent is to

a hydrophobic region of the receptor or molecular frame-

work.

3.5 The m2 Receptor

The ®nal QSAR (Eq. 90) is similar to that for the m1

receptor, the major difference being that Eq. 90 shows a

dependence on a whereas Eq. 100 does not. Again, there is a

possible effect of the 40 substituent on the hydrogen bonding

to the naloxone moiety and=or the acylhydrazone group,

and the substituent is binding directly to the receptor site or

the molecular framework by VdW interactions without

hydrogen bonding, indicative of interaction with a hydro-

phobic region.

3.6 The d Receptor

The ®nal QSAR (Eq. 110) shows a dependence on the

hydrogen bonding parameters which suggests that the 40

substituent is binding directly to the receptor site or

molecular framework largely by hydrogen bonding with a

smaller contribution from VdW interactions. The 40

substituent should therefore be interacting with a hydro-

philic area of the receptor site or framework. The 40

substituent seems to have no discernible effect on hydrogen

bonding to the naloxone moiety or to the acylhydrazone

group. In view of the lack of reliability of Eq. 110 these

inferences are tentative at best.

3.7 The Binding Site of the 40-substituent

The comments below are speculative. As was noted above

recognition and subsequent binding of naloxone derivatives

to an opiate receptor most probably involves hydrogen

bonding to the O and N atoms of the pharmacaphore. It is

quite likely that the 40 substituent projects out past the

receptor site and that with the possible exception of the dreceptor the substituent itself is binding to a hydrophobic

part of the molecular framework in which the receptor sites

are embedded.

4 Conclusion

QSAR for binding af®nities of 40-substituted nalaxone

benzoylhydrazones with the m1, m2, k1, k3, and d opiate

receptors were obtained. Those for the m1 and m2 receptors

are very similar. No other close similarities are found

among the QSAR. For all of the receptors but d and possibly

k3 the 40 substituent seems to be binding to a hydrophobic

region through VdW interactions. There is an effect of the

40 substituent on hydrogen bonding to the acylhydrazone

group in the case of the k1 receptor, and probably in the

case of the m1, and m2 as well. The QSAR are considered

reliable for all but the d receptor where the results are

considered uncertain.

Acknowledgment

The work was supported, in part, by a research grant

(DA06241) and a Research Scientist Award (DA00220) to

GWP from the National Institute on Drug Abuse and a core

grant from the National Cancer Institute (CA08748) to

MSKCC.

5 References

[1] Ciszewska, G.R., Ginos, J.A., Charton, M., Standifer, K.M.,Brooks, A.I., Brown, G.P., Ryan-Moro, J.P., Berzetei-Gurske, I., Toll, L. and Pasternak, G.W., Synapse 24, 193±201 (1996).

[2] Martin, Y.C., Quantitative Drug Design, Dekker, New York1978.

[3] Franke, R., Theoretical Drug Design Methods, Akademie-Verlag, Berlin 1984.

[4] Hansch C. and Leo, A., Exploring QSAR. Fundamentals andApplications in Chemistry and Biology, Vol. 1, Amer. Chem.Soc. Washington, D. C. 1995.

[5] Charton, M., Prog. Phys. Org. Chem. 16, 287±315 (1987).Note that the coef®cients of the pure parameters sl, sd, andse are lower case in equations relating them to compositeelectrical effect parameters and upper case in all otherrelationships.

[6] Charton, M., Topics in Current Chem. 114, 57±91 (1983).[7] Charton, M., Prog. Phys. Org. Chem. 18, 163±284 (1990).[8] Charton, M., Stud. Org. Chem. 42, 629±687 (1992).[9] Charton, M., Proceedings of the 5th International Symposium

on Pharmaceutical Sciences, J. Fac. Pharm. Ankara Univ.,OÈ zden, S., CË os;kun, M., BuÈyuÈkbingoÈl, E., Onur, F. andTarimji, N. (Eds.) Ankara, 1997 (1997), pp. 146±163.

[10] Charton, M. unpublished results on the variation of theelectrical effect with distance in the hydrogen bonding ofXGCN, data from Berthelot, M., Helbert, M., Laurence, C.and Le Questel, J.-Y. J. Phys. Org. Chem. 6, 302±306 (1993).

Received on October 6, 1997; accepted on December 5, 1997



quantitative structure activity relationships for substituted naloxone benzoylhydrazones....

Documents