QSAR of Adenosine Receptor Antagonists II:

QSAR of Adenosine Receptor Antagonists II:Exploring Physicochemical Requirements for Selective Binding of2-Arylpyrazolo[3,4-c]quinoline Derivatives with Adenosine A1 andA3 Receptor SubtypesKunal Roy

Drug Theoretics and Cheminformatics Lab, Division of Medicinal and Pharmaceutical Chemistry, Department of PharmaceuticalTechnology, Jadavpur University, Calcutta 700 032, India; e-mail: [email protected]; URL: http://www.geocities.com/kunalroy_in

Abstract

Considering potential of selective adenosine receptorsubtype ligands in the development of prospective drugcandidates, A1 and A3 receptor binding affinity data of 2-arylpyrazolo[3,4-c]quinoline derivatives have been sub-jected to QSAR analyses to explore the physicochemicalrequirements for selective binding. The study has beencarried out with Wang-Ford charges of the common atomsof the molecules calculated from their energy minimizedconformations using AM1 technique. Apart from the

charge parameters, physicochemical variables like parti-tion coefficient and molar refractivity of the wholemolecules have been used along with suitable indicatorvariables. The study shows that substituents on theappended 2-phenyl ring and 4-amino or 4-keto substitutionon the pyrazolo[3,4-c]quinoline nucleus modulate theselectivity pattern. Further, negative charge on the quino-line nitrogen and volume and lipophilicity of the wholemolecules are important contributors to the selectivity.

1 Introduction

Adenosine receptors (AR) represent one of the promisingdrug targets of contemporary interest. As nearly all cellsexpress specific adenosine receptors, adenosine serves asimportant physiological regulator and acts as cardioprotec-tor, neuroprotector, chemoprotector and immunomodula-tor [1 ± 4]. There exist a large number of ligands that havebeen generated by introducing several modifications in thestructure of the lead compounds. Based on pharmacologicalstudies and molecular cloning, four different subtypes (A1,A2A, A2B and A3) of G-protein coupled adenosine receptorshave been identified [2]. New genetic and epigenetic tools,such as antisense and gene −knockin× and −knockout×techniques, have been used for elucidation of the functionsof these receptors [5].It has been reported that somnogenic effect of adenosine

in the basal forebrain areamay bemediated byA1 receptorsand its expressionmight be regulated by induction ofNF-�Bprotein as transcription factor [6]. SelectiveA1 agonists havehigh neuroprotective actions [7].

A2A receptor is thought to play a role in a number ofphysiological responses and pathological conditions. An-tagonistic interactions between A2A receptors and dopa-mine D2 receptors have been described. A2A receptorantagonists may be useful for treatment of neurodegener-ative disorders such as Parkinson×s disease while A2A

agonists may treat certain types of sleep disorders [8 ± 13].A2A receptor desensitizes A1 receptor and reduces A1

mediated effects [14]. A2A receptor has been identified asnovel target in renal diseases [12].The knowledge of A2B receptors lags behind that of other

receptor types. A2B receptors have been implicated in theregulation of vascular smooth muscle tone, cell growth,intestinal function and neurosecretion. The role of A2B

receptors in mast cell activation and the potential relevanceof this action on asthma have also been reviewed [15].Activation of A3 receptors has been shown to stimulate

phospholipase C and D and to inhibit adenylate cyclase. Italso causes release of inflammatory mediators like hista-mine from mast cells leading to inflammation and hypo-tension.Highly selectiveA3 antagonists have been indicatedas potential drug for the treatment of asthma and inflam-mation while agonists have been shown to possess cardio-protective action [1, 2].Potent and selective A1 and A3 ligands have been

developed. However, potent and selective A2A and A2B

614 QSAR Comb. Sci. 22 (2003) DOI: 10.1002/qsar.200330821 ¹ WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1611-020X/03/0607-0614

Key words: QSAR, AM1 calculations, adenosine receptor ligands,2-arylpyrazolo[3,4-c]quinoline derivates

Kunal Roy

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receptor ligands are still lacking. Since the treatment withAR agonists may lead to fast desensitization of thereceptors, partial agonists, and indirect AR agonists, suchas adenosine kinase inhibitors, or allosteric enhancers ofadenosine binding, are being developed as site- and event-specific agents [16]. Research is going on to synthesize andevaluate selective adenosine receptor ligands as possiblerenal protective, anti-Parkinson, anti-inflammatory, anti-asthmatic and anti-ischemic agents [17].

Recently, the author has explored QSAR of adenosinereceptor antagonist 1,2-dihydro-2-phenyl-1,2,4-triazolo[4,3-a]quinoxaline-1-one derivatives [18]. In the present com-munication, A1 and A3 receptor binding affinity of 2-arylpyrazolo[3,4-c]quinoline derivatives, recently reportedby Colotta et al. [19], have been modeled with Wang-Fordcharges of different common atoms alongwith some indica-tor parameters and physicochemical variables in order toexplore selectivity pattern for the adenosine receptor

Table 1. Structural features, physicochemical parameters and important charge parameters of 2-arylpyrazolo[3,4-c]quinoline derivatives

1 ± 10 11 ± 35

Sl.no.

R R1 R2 logP MR* q5 q7 q10 q13 q15 q18

1 H H H 2.90 7.643 �0.022 � 0.503 �0.178 �0.405 �0.102 �0.0792 H Cl H 3.46 8.103 �0.101 �0.523 �0.162 �0.410 �0.108 �0.0763 2-Me H H 3.39 8.232 �0.004 �0.500 �0.216 �0.405 0.102 �0.0694 3-Me H H 3.39 8.232 �0.019 �0.506 �0.178 �0.404 �0.134 �0.0545 4-Me H H 3.39 8.232 �0.017 �0.505 �0.181 �0.408 �0.100 �0.1266 3-F H H 3.06 7.683 �0.025 �0.050 �0.168 �0.408 �0.208 �0.0567 4-OMe H H 2.78 8.367 �0.017 �0.508 �0.174 �0.408 0.016 �0.2088 4-Cl H H 3.46 8.103 �0.028 �0.502 �0.170 �0.407 �0.160 0.0799 H H Me 3.14 8.143 0.041 �0.310 �0.223 �0.407 �0.099 �0.081

10 H H n-Pr 3.96 9.083 �0.008 �0.439 �0.195 �0.403 �0.122 �0.07611 H H H 3.45 7.939 �0.271 �0.732 0.074 �0.464 �0.099 �0.11412 H Cl H 4.01 8.399 �0.321 �0.741 0.037 �0.475 �0.102 �0.08213 2-Me H H 3.94 8.528 �0.231 �0.730 �0.010 �0.478 0.103 �0.06914 3-Me H H 3.94 8.528 �0.244 �0.730 0.028 �0.471 �0.128 �0.05315 4-Me H H 3.94 8.528 �0.243 �0.728 0.027 �0.474 �0.088 �0.12316 3-F H H 3.61 7.979 �0.246 �0.728 0.031 �0.476 �0.199 �0.05717 4-OMe H H 3.32 8.663 �0.241 �0.731 0.031 �0.474 0.023 �0.19618 4-Cl H H 4.01 8.399 �0.256 �0.727 0.034 �0.471 �0.149 0.07719 H H c-Hx 5.30 10.476 �0.185 �0.055 �0.668 �0.424 �0.110 �0.08320 H Cl c-Hx 5.86 10.936 �0.265 �0.042 �0.689 �0.426 �0.110 �0.08421 3-Me H c-Hx 5.79 11.066 �0.180 �0.057 �0.668 �0.427 �0.165 �0.06822 4-Me H c-Hx 5.79 11.066 �0.193 �0.051 �0.671 �0.421 �0.154 0.07023 3-F H c-Hx 5.46 10.516 �0.181 �0.056 �0.669 �0.432 �0.209 �0.04824 4-Cl H c-Hx 5.86 10.936 �0.193 �0.050 �0.670 �0.422 �0.155 0.07225 H H c-C5H9 4.88 10.016 �0.206 �0.022 �0.678 �0.444 �0.110 �0.08426 2-Me H c-C5H9 5.37 10.606 �0.188 �0.049 �0.674 �0.460 0.087 �0.08227 3-Me H c-C5H9 5.37 10.606 �0.201 �0.024 �0.678 �0.445 �0.166 �0.06828 3-F H c-C5H9 5.04 10.057 �0.202 �0.022 �0.679 �0.451 �0.208 �0.05029 H H Bn 5.48 10.825 �0.199 �0.645 0.017 �0.444 �0.102 �0.09030 H H CH2Bn 5.76 11.285 �0.225 �0.697 0.013 �0.432 �0.104 �0.08331 H H Ac 3.16 8.772 �0.195 �0.588 �0.012 �0.401 �0.098 �0.08432 H H Bz 5.06 10.785 �0.194 �0.584 �0.019 �0.405 �0.102 �0.08633 H H COBn 5.00 11.216 �0.172 �0.577 �0.051 �0.411 �0.100 �0.09734 H H CONHPh 4.73 11.073 �0.236 �0.606 0.028 �0.459 �0.112 �0.07735 H H CONHBn 4.80 11.614 �0.216 �0.618 �0.011 �0.423 �0.110 �0.075

Me�Methyl, OMe�Methoxy, n-Pr� normal-Propyl, c-Hx� cyclo-Hexyl, c-C5H9� cyclo-Pentyl, Bn�Benzyl, Ac�Acetyl, Bz�Benzoyl, Ph�Phenyl.* Scaled to a factor of 0.1 as usual

QSAR Comb. Sci. 22 (2003) 615

Physicochemical Requirements for Selective Adenosine A1 and A3 Receptor Subtypes

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binding affinity in terms of structural requirements. It maybe mentioned here that Colotta et al. [19] tested the bindingaffinities of the compounds with adenosine A1 receptor ofbovine cerebral cortical membranes and human clonedadenosine A3 receptor. Adenosine receptors from differentspecies show amino acid sequence homology (82 ± 93%)with the only exception being adenosineA3 subtype [19, 20].Though bovine data may not be well suitable for drugdevelopment, a preliminary attempt ismade here to explorethe selectivity requirements for receptor binding affinity.

2 Materials and Methods

Adenosine receptor binding affinity data [19] of 2-arylpyr-azolo[3,4-c]quinoline derivatives (Tables 1 and 2) have beenused for the present QSAR study. Though the originalauthors [19] reported binding affinity for A2A receptor also,these could not be considered due to insufficient quantita-tive data points. The biological activity data [Ki(nM)] wereconverted to logarithmic scale [pKi(�M)] and then used forsubsequent QSAR analyses as the response variable. Thebiological activity values and structural features of thecompounds are presented in Table 1. Quantum mechanicalcalculations were done according to AM1 (AustinModel 1)[21 ± 23] method using Chem 3D Pro [24] package. Thegeneral structure of the compounds (Figure 1) was drawn inChem Draw Ultra ver 5.0 [24] and it was saved as thetemplate structure. For every compound, the templatestructure was suitably changed considering its structuralfeatures, copied to Chem 3D ver 5.0 [24] to create the 3-Dmodel and finally the model was ×cleaned up×. The non-

hydrogen common atoms of the compounds were given aserial number so that these maintain same serials in all themodels (Figure 1). Energy minimization was done underMOPAC module using RHF (restricted Hartree-Fock:closed shell) wave function. The energy minimized geom-etry was used for calculation of Wang-Ford charges (ob-tained from molecular electrostatic potential surface) ofdifferent atoms. The charges (qx) of different atoms (x) weresubjected to intercorrelation study. The biological activitydata of the compounds [pK(�M)] were subjected toregression with the charges of different common atomsand also different combinations of them [25] to obtain thebest relations using the programAUTOREG [26] developedby the author. The relations were improved further usingindicator variables and/or physicochemical variables [hy-drophobicity (partition coefficient, log P) and steric param-eter (molar refractivity, MR)]. The software Chem DrawUltra ver 5.0 was used for the calculation of log P and MRvalues (Ghose and Crippen×s fragmentation method [27]).The regression analyses were carried out using a GW-

BASIC program RRR98 [26]. The statistical quality of theequations [28] was judged by the parameters like explainedvariance (Ra

2, i.e., adjusted R2), correlation coefficient (r orR), standard error of estimate (s), average of absolute valuesof the residuals (AVRES), variance ratio (F) at specifieddegrees of freedom (df), 95% confidence intervals of theregression coefficients, cross-validation R2 (Q2) [29], pre-dicted residual sum of squares (PRESS) [29], standarddeviation based on PRESS (SPRESS) [30], standard deviationof error of prediction (SDEP) [30] and average absolutepredicted residual (Presav). PRESS (leave-one-out) statistics[29, 30] were calculated using the programs KRPRES1 andKRPRES2 [26]. Use of more than one variable in amultivariate equation was justified by intercorrelationstudy. All the accepted equations have regression constantsand F ratios significant at 95% and 99% levels respectively,if not stated otherwise. A compound was considered as anoutlier if the residual ismore than twice the standard error ofestimate for a particular equation.

3 Results and Discussion

TheWang-Ford charges of selected non-hydrogen commonatoms of 2-arylpyrazolo[3,4-c]quinoline derivatives alongwith physicochemical parameter values (log P andMR) aregiven in Table 1. Definitions of different indicator variablesare given in Table 3.

3.1 QSAR of A1 Receptor Binding Affinity

The A1 receptor binding affinity of thirty-four 2-arylpyr-azolo[3,4-c]quinoline derivativeswere subjected tomultipleregression with Wang-Ford charges of different combina-tions of atoms that are notmuch autocorrelated and the bestrelation obtained was the following:

616 QSAR Comb. Sci. 22 (2003)

Figure 1. General structure of 2-arylpyrazolo[3,4-c]quinolinederivatives : the common atoms have been numbered 1 through19 (it has no relation to chemical nomenclature system) andimportant fragments responsible for selective adenosine A1/A3

binding have been marked.

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QSAR Comb. Sci. 22 (2003) 617

Table 2. Observed, calculated and predicted adenosine A1 and A3 receptor binding affinity and selectivity data of 2-arylpyrazolo[3,4-c]quinoline derivatives

Compd.no.

A1 binding affinity(pKA1)

A3 binding affinity(pKA3)

Selectivity(�pK�)

Obs.a Calc.b Pred.b Obs.a Calc.c Pred.c Obs.a Calc.d Pred.d

1 �0.322 0.008 0.045 1.511 1.482 1.477 1.833 1.075 0.9552 0.547 0.013 �0.047 1.356 1.450 1.461 0.809 1.296 1.3643 �0.505 �0.533 �0.547 1.101 1.561 1.616 1.606 1.358 1.3234 0.081 0.007 �0.002 2.301 2.357 2.368 2.220 2.413 2.4475 �0.591 0.154 0.267 2.495 2.243 2.201 3.086 2.413 2.2946 0.234 0.199 0.191 1.344 0.469* 0.123 1.110 1.095 1.0927 �0.033 0.128 0.185 2.495 2.269 2.233 2.528 2.477 2.4688 0.333 �0.355 �0.659 2.538 2.247 2.196 2.205 2.351 2.3769 �0.009 0.007 0.018 0.928 1.100 1.124 1.018 1.315 1.357

10 0.022 0.048 0.051 1.165 1.614 1.656 1.143 1.766 1.87011 1.160 1.020 1.003 0.259 0.347 0.361 �0.901 �0.780 �0.75512 1.065 0.925 0.908 0.510 0.125 0.057 �0.555 �0.559 �0.56013 0.284 �0.033 �0.193 �0.556 0.037 0.148 �0.840 �0.497 �0.45214 0.889 0.492 0.428 1.003 0.989 0.986 0.114 0.557 0.63915 0.668 0.617 0.607 0.726 0.900 0.933 0.058 0.557 0.64916 0.959 0.682 0.611 0.103 0.009 �0.010 �0.856 �0.761 �0.74117 0.623 0.575 0.555 1.045 0.926 0.904 0.422 0.622 0.65718 �0.360 0.127 0.332 0.824 0.964 0.989 1.184 0.496 0.36319 2.066 1.839 1.785 0.151 0.434 0.489 �1.915 �1.113 �0.98020 1.567 1.843 1.908 �0.328 0.418 0.580 �1.895 �0.892 �0.73221 1.654 1.524 1.502 1.356 1.196 1.154 �0.298 0.225 0.32722 1.143 1.053 1.029 0.939 1.351 1.456 �0.204 0.225 0.30923 1.262 1.569 1.631 0.810 0.218 0.088 �0.452 �1.093 �1.19924 0.321 1.049 1.250 1.248 1.303 1.316 0.927 0.163 0.01825 2.495 1.843 1.687 1.218 1.059 0.972 �1.277 �1.333 �1.34426 0.830 0.940 0.999 0.818 0.752 0.716 �0.012 �1.050 �1.21927 1.390 1.526 1.550 1.652 1.877 2.025 0.262 0.005 �0.04228 2.032 1.573 1.481 ± ± ± ± ± ±29 0.879 0.950 0.959 1.446 1.145 1.104 0.567 0.604 0.61030 1.221 0.933 0.899 1.483 1.654 1.693 0.262 0.824 0.96331 1.120 0.921 0.898 1.317 1.933 2.019 0.197 �0.380 �0.44632 ± ± ± 2.678 2.104 1.990 ± ± ±33 0.971 0.968 0.968 2.004 1.984 1.980 1.033 0.791 0.73534 �0.176 0.934* 1.068 0.967 0.680 0.632 1.143 0.723 0.63535 0.730 0.922 0.946 2.081 1.791 1.719 1.351 0.982 0.863

��pK�pKA3�pKA1a Ref. [19]; b according to eq. (2); c according to eq. (5); d according to eq. (7)* Outlier

Table 3. Definitions of the indicator parameters

Parameters Definition

I Presence (value 1) or absence (value 0) of cyclopentylamino or cyclohexylamino substituent at the 4 position of thepyrazolo[3,4-c]quinoline nucleus

ICO Presence or absence of nuclear keto (� C�O) group at the 4 position of the pyrazolo[3,4-c]quinoline nucleus

I/CO Presence or absence of nuclear or extranuclear keto (� C�O) group at the 4 position of the pyrazolo[3,4-c]quinolinenucleus

IR Presence or absence of R substituent (non-hydrogen) on appended 2-phenyl moiety when 4-amino group is present onthe pyrazolo[3,4-c]quinoline nucleus

I/R Presence or absence of meta or para substituents having size higher than that of hydrogen on the appended 2-phenylmoiety.

I2Me Presence or absence of 2-methyl group on the appended 2-phenyl moiety.Ipent Presence or absence of cyclopentylamino substituent at the 4 position of the pyrazolo[3,4-c]quinoline nucleus


� ��

pKA1��4.468 (�1.601) q5� 1.422 (�0.559) q10� 2.800(�2.197) q15� 4.040 (�2.845) q18� 0.910 (�0.482) (1)n� 34, Q2� 0.582, Ra

2� 0.663, R2� 0.704, R� 0.839,s� 0.444, F� 17.2 (df 4, 29), AVRES� 0.315,PRESS� 8.053, SDEP� 0.487, SPRESS� 0.527,Presav� 0.372

All regression coefficients of Eq. 1 are significant at 95%level. The 95% confidence intervals of the regressioncoefficients are shown within parentheses. This Eq. canexplain 66.3% variance and predict 58.2% variance of A1

receptor binding affinity. Attempt was made to obtainsuperior relation using physicochemical (log P andMR) andsuitable indicator variables. However, in the final relation,there is no log P or MR term.

pKA1��2.495 (�2.104) q15� 3.214 (�2.788) q18� 0.891(�0.380) I � 0.907 (�0.393) ICO� 0.404 (�0.379)IR� 0.407 (�0.405) (2)n� 34, Q2� 0.618, Ra


Eq. 2 can explain 69.9% variance vis-a-vis predict 61.8%variance of A1 binding affinity. Compound 34 behaves as anoutlier. The calculated and predicted A1 binding affinity ofdifferent compounds are shown in Table 2, which showslarger residuals for less active compounds. Intercorrelation(r) among different variables of Eqs. 1 and 2 is given inTable 4.Eq. 1 shows the importance of Wang-Ford charges of

atom numbers 5, 10, 15 and 18. From the sign of theregression coefficients, it is evident that higher negativecharges on these atoms are conducive to A1 binding affinity.Eq. 2 suggests that presence of cyclohexylamino or cyclo-pentylamino group at 4 position of the pyrazolo[3,4-c]quinoline nucleus favours binding affinity while a nuclearcarbonyl group at the same position decreases bindingaffinity. Eq. 2 further suggests that in presence of a 4-aminosubstituent, any substituent on the appended 2-phenyl ringis detrimental for A1 binding affinity.

3.2 QSAR of A3 Receptor Binding Affinity

When A3 receptor binding affinity data of thirty-four 2-arylpyrazolo[3,4-c]quinoline derivatives were subjected tomultiple regression with Wang-Ford charges of differentcombinations of common atoms, no statistically acceptablerelation could be generated. However, from among therelations obtained, atoms 7 and 13 emerged as importantcontributors (R� 0.661) amongst the all atoms. In anattempt to improve the relations by incorporating phys-icochemical (log P and MR) and/or indicator parameters,the following Eqs. were obtained:

pKA3��2.065 (�0.684) q7� 27.374 (�5.920) q13� 0.690(�0.294) I/R � 0.433 (�0.512)* I2Me � 1.438 (�0.601)Ipent� 0.130 (�0.112) MR� 10.597 (�2.795) (3)n� 34, Q2� 0.688, Ra


pKA3��0.621 (�0.663)* q7� 1.300 (�0.329) I/CO� 0.756(�0.333) I/R� 0.543 (�0.572)* I2Me � 0.888 (�0.637)Ipent� 0.193 (�0.129) MR� 1.753 (�1.276) (4)n� 34, Q2� 0.635, Ra


However, regression coefficient of the variable I2Me in Eq. 3and those of I2Me and q7 in Eq. 4 are significant at 90% level.Omitting the variables I2Me and I/CO, the following relationcontaining all significant regression coefficients was ob-tained:

pKA3��2.067 (�0.706) q7� 27.979 (�6.072) q13� 0.756(�0.293) I/R� 1.319 (�0.604) Ipent� 0.145 (�0.115)MR� 10.668 (�2.866) (5)n� 34, Q2� 0.683, Ra


Eq. 5 can explain 75.5% variance vis-a-vis predict 68.3%variance of A3 binding affinity. All regression coefficientsare significant at 95% level. The calculated and predictedA3

binding affinity of different compounds according to Eq. 5are shown in Table 2. The intercorrelation (r) amongdifferent variables of Eqs. 3 to 5 are given in Table 5. Thelog P values are found to be highly correlated with MRvalues (r� 0.921); however, MR serves here as betterdescriptor in comparison to log P as evident from qualityof the equations.Eqs. 3 and 4 suggest that increase in negative charge on

atom 7 (quinoline N) increases A3 receptors binding affinitywhile increase in negative charge on atom 13 (pyrazole N)

618 QSAR Comb. Sci. 22 (2003)

Table 4. Intercorrelation (r) among the important variables(eqs. 1 and 2, n� 34)

pC1 q5 q10 q15 q18 I ICO IR

pC1 1.000 0.567 0.499 0.302 0.040 0.648 0.645 0.188q5 1.000 0.038 0.076 0.041 0.251 0.935 0.434q10 1.000 0.198 0.255 0.955 0.136 0.287q15 1.000 0.437 0.225 0.088 0.059q18 1.000 0.260 0.074 0.275I 1.000 0.417 0.422ICO 1.000 0.508IR 1.000

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decreases binding affinity. Further, presence of a nuclear orextranuclear carbonyl group at the 4 position of pyrazo-lo[3,4-c]quinoline nucleus andmeta or para substituent withhigher bulk (size) thanHon the appended2-phernyl ring areconducive to theA3 binding affinity while presence of ortho-methyl group on the appended 2-phenyl ring decreasesactivity. Again, a 4-cyclopentyamino substituent increasesA3 binding affinity. Molar refractivity of the molecules alsoshows positive impact on the binding affinity. However,molar refractivity is found to be highly correlated with logP(r� 0.921) and hence the impact ofMR is certainly not dueto only steric effect and a major lipophilicity componentexists.

3.3 Exploring Selectivity

Selective structural requirements for specific binding affin-ity for either receptor subtype were attempted to exploreconsidering the variables which emerged as importantcontributors in explaining the variance of binding affinitiesin QSAR of individual series. Compounds 28 and 32 had tobe omitted while considering selectivity requirements. Thedifference (�pK� pKA3� pKA 1) between binding affinityvalues was taken as the response variable. Considering allstatistical parameters, the best two selectivity relations arenoted below:

�pK� 2.598 (�0.615) ICO� 2.112 (�0.913) q7� 0.902(�0.444) I/R� 0.472 (�0.237) MR �5.985 (�2.295) (6)n� 33, Q2� 0.710, Ra


�pK��1.549 (�0.556) I� 1.997 (�0.527) ICO� 1.055(�0.412) I/R� 0.479 (�0.212) MR� 4.585 (�2.039) (7)n� 33, Q2� 0.759, R2

a � 0.800 , R2� 0.825, R� 0.909,s� 0.542, F� 33.1 (df 4, 28), AVRES� 0.412,PRESS� 11.4, SDEP� 0.587, SPRESS� 0.637,Presav� 0.486

The explained variance and predicted variance of Eq. 7 are80.0% and 75.9% respectively. This Eq. has a high variance

ratio (significant at 99.9% level) which implies the stabilityof the regression coefficients. There is no outlier for Eq. 7.Statistical quality of Eq. 7 shows that the relation issufficiently robust one. The intercorrelation among differ-ent variables of Eqs. 6 and 7 is given in Table 6. Thecalculated and predicted selectivity values according toEq. 7 are given in Table 2.Eqs. 6 and 7 suggest that presence of cyclopentylamino or

cyclohexylamino substituent at the 4 position of thepyrazolo[3,4-c]quinoline nucleus favours binding with A1

receptor over A3 receptor type while presence of nuclear 4-keto group favours A3 receptor binding. The importance ofthe carbonyl group for A3 binding affinity has beensuggested to be due to its interaction with proton donorpresent on N6 region of the A3 receptor subtype [19]. Anyincrease in negative charge on atom 7 (quinoline N)

QSAR Comb. Sci. 22 (2003) 619

Table 6. Intercorrelation (r) among the important variables(Eqs. 6 and 7, n� 33)

�pK I ICO q7 I/R Ipent MR

�pK 1.000 0.523 0.703 0.255 0.355 0.218 0.313I 1.000 0.404 0.868 0.103 0.516 0.601ICO 1.000 0.001 0.050 0.209 0.599q7 1.000 0.021 0.464 0.426I/R 1.000 0.020 0.091Ipent 1.000 0.243MR 1.000

Figure 2. Energy minimized geometries of the most selective (a)A1 receptor antagonist (compound 19) and (b) A3 receptorantagonist (compound 5) of the series.

Table 5. Intercorrelation (r) among the important variables(Eqs. 3 ± 5, n� 34)

pC2 q7 q13 I/CO I/R I2Me Ipent MR

pC2 1.000 0.056 0.601 0.625 0.329 0.301 0.010 0.051q7 1.000 0.339 0.161 0.033 0.014 0.466 0.401q13 1.000 0.732 0.081 0.169 0.192 0.036I/CO 1.000 0.160 0.068 0.276 0.274I/R 1.000 0.230 0.013 0.112I2Me 1.000 0.269 0.072Ipent 1.000 0.230MR 1.000


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increasesA3binding affinity. The importance of thenegativecharge on the quinoline nitrogen may be due to its ability tomodulate the property of 4-substitution (amino or keto).Further, presence ofmeta and para substituents substituentbulkier than hydrogen atom on the appended 2-phenyl ringis conducive to the A3 binding affinity. Any increase in themolar refractivity value (a major component being lip-ophilicity, intercorrelation r� 0.920) of the whole moleculeincreases A3 binding affinity. The major fragments in themolecules responsible for the selective receptor binding areshown in Figure 1. The energy minimized geometries of themost selective A1 receptor antagonist (compound 19) andthe most selective A3 receptor antagonist (compound 2) areshown in Figure 2.

4 Conclusion

The quantitative observations of this QSAR study are inaccordance with the qualitative conclusions drawn byColotta et al [19]. The substituents on the 2-phenyl ring arefound to modulate the selectivity pattern. Presence of anuclear 4-keto group favours A3 binding affinity while an 4-amino substitution is essential for A1 binding affinity. Anyincrease in negative charge on atom 7 (quinoline N)increases A3 binding affinity. Volume and lipophilicity ofthe whole molecules are also important for the selectivity.

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Kunal Roy

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Received on January 6, 2003; Accepted on February 4, 2003

QSAR Comb. Sci. 22 (2003) 621


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