Indian Journal of Chemistry Vol. 45A, January 2006, pp. 34-44

QSAR studies on cis-hexa and tetra-hydrophthalazinones: A new dass of selective PDE-4 inhibitors

M Lakshmi, Anshuman Dixit & Anil K Saxena*

Medicinal and Process Chemistry Division , Central Drug Research Institute. Lucknow 226 OOL India

Email: anilsak@hotmail.com

Received 21 January 2005; revised 8 November 2005

Quantitative structure activity relationship studies have been carried out on cis-hexa and tetra-hydrophthalazinones, a new class of selective PDE-4 inhibitors. The 78 compounds analyzed were divided into a training set of 62 and test set of 15 molecules (excluding one compound) each describing a simi lar range of biological activity. Among several 20 and 3D parameters. the molar refractivity, dielectric energy and structural parameters as indicator variables (l J, 12, I]) best describe the variation in PDE-4 inhibitory activity in the training set with high correlation(r=>0.9) of >99% significance. These equations also show a good. test set prediction (r>0.8) and thus may be useful in desi gning new potential PDE4 inhibitors.

The PDE-4 is an important isozyme among the family of eleven phosphodiesterase isozymes . It is present in airway smooth muscle, immune and inflammatory cells and its selective inhibitors form the largest group of inhibitors for any PDE family. These PDE-4 inhibitors have shown great potential as anti-inflam­matory drugs targeting asthma, chronic obstructive pulmonary disease (COPD), rheumatoid arthritis, multiple sclerosis, type II diabetes , atopic dermatitis, septic shock and other autoimmune diseases 1-4. The selective inhibitors are considered as novel target for the treatment of asthma and COPD 5. 6 and Rolipram is the first generation selective inhibitor of PDE-4. However, its clinical utility is limited due to adverse side effect like emesis, etc. The continuing research efforts in this field have led to the development of second-generation PDE-4 inhibitors like Roflumilast, with improved efficacy and safet/. Several PDE-4 inhibitors are under clinical development and the molecules belonging to various chemical classes, viz., benzodiazepines, benzimidazoles, oxindoles, substi­tuted furans, arylsulphonylhydroxamic acids and phathalazines have been investigated for their activity towards PDE-48

. Tetra- and hexa-hydrophthalazi­nones are the new class of selective PDE-4 inhibitors reported recently9-

J

2. The studies reported by Mey (:( af. 12 indicate [hat the cis form of phthalazinones is more potent than the trans form. As very limited structure activity relationship studies have been reported in these molecules, in the present study,

systematic QSAR studies have been carried out in order to identify the essential structural features and physicochemical properties for PDE-4 inhibitory activity in these molecules.

Materials and Methods Two series, series-A II (Table 1) conSIStll1g of 55

racemic N-substituted cis tetra- and hexa-hydro­phthalazinones, and, series B 12 (Tables 2 & 3) consis­ting of 23 racemates and enantiomers of cis-tetra and hexahydrophthalazinones, were selected for the QSAR analysis. The PDE-4 inhibitory activity (pICsQ)

of both the series has been determined in cytosol of the human neutrophils under similar conditions 11.12;

hence the compounds of both series were considered together and were divided into the training and test set (Tables 1, 2 & 3). The criterion of selection of molecules for the training and the test set is based on the variation in the observed pICsQ values. So all the molecules were first arranged in increasing order of prCSQ values and every 5 th molecule was selected for test set and the remaining ones for the training set. Thus, both the training and test set represented the full range of prCSQ values .

Computational approach BiomedCache l 3 software from Fujitsu was used for

sketching the molecules and for calculation of various physico-chemical parameters. In addition to these parameters, different physico-chemical parameters

LAKSHMI e( at .: QSAR STUDIES ON HYDROPHTHALAZINONES

Table I- PDE-4 inhibitory activities of N-substituted hexahydrophthalazinones (Series A)

OMe OMe OMe

MeO MeO R1

R2

NY NY NY X I I I

I R/N

aNo

y R/ N

0 0

Compo No. 1-42 Compo No. 43-52 Comp: No. 53-55

Compo R pICso" Compo R pICso " 1 H 6 .4 33 CHz-3-HOC6H4 8.0

2 CH3 6.8 34 CHz-4-HOC6H4 83.0

3 CzHs 7.4 35 CHz-3-HOOCC6H4 7.9

4 /1- C3H7 7.8 36 CHz-4-HOOCC6H4 7.3

5 i-C3H7 8.6 37b CHz-4-MeOOCC6H4 7.9

6 l-Bu 7.9 38 CHz-2-CsH4N 7.6

7 CCSH9 8.4 39b CHz-3-CsH4N 7.6

8 CC7H13 8.6 40b CHz-4-CsH4N 7.8

9 CHzCHCH 7.6 41 (CHz)-2-imidazol-l-yl 6.3

lOb CHzCCH 7.4 42 (CHz)-4-imidazol-l-yl 8.0

11 CHzCN 7.2 43 CHz-c-Pr 8.1

12 (CHz)zOH 6.8 44 i-Pr 8.9

13 (CH2)30H 7.5 45 Cyclo-CsH9 9.3

l4b CH2COOEt 7.2 46 Cyclo-C6H II 9.2

15 CH2COOH 5.7 47 Tetrahydropyran-4-yl 8.9

16# (CH2)sCOONa 7.5 48b Tetrahydrothiopyran-4-yl 8.9

17b (CH2)7COOH 7.6 49 Cyclo-octyl 9.3

18b (CH2)zN(Me)z 6.5 50 Adamantan-2-yl 9.4

19 (CH2)4N(Me)z 7.0 51 Indan-2-yl 9.1

20 (CH2)6N(Me)2 7.0 52b Cyclo-heptyl 9 .1

21 (CH2)6NHz 7. 1

22 Ph 7.6 Compo RI Rz X-V plCso a

23 Bn 8.1

24 (CH2)zPh 7.6

C5 25 CHzCOPh 8.0 53 HzC-CH2 92

26 (CH2)30Ph 7.9

27 CH2CH=CHPh 7. 8

X 28 CH2-2-MeOC6H4 7.7 54 HC=CH 9

29 CH 2-3-MeOC6H4 8.5

30b CHz-4-MeOC6H4 8.2 .. 31 CH2-3,5-(MeO)zC6H3 7.6 55 (5 HC=CH 92 32 CH2-2-HOC6H4 8.1

apDE_4 inhibitory ac ti vity is in log (I/ICso) form, "Compounds of test set, #the compound was omitted because of non­availability of parameters.

35

36 INDI AN J CHEM. S EC A, JAN UARY 2006

Table 2-PDE-4 inhibitory activity of racematcs and enantiomers of aryl-substituted hexa and tetrahydrophthal azinones (Series B)

OMe

R1

R2

N Y X I I

/N Y R

0

Compo No. 56-72

Comp. RI R2 X-Y piCso a Isomer Compo RI R2 X-Y pICso

, Iso mer

56 OMe H H1C-CHz 6.9 (+) 65 OCCSH9 H HC=CH 7.4 (+)

57 OMe H H1C-CH2 5. 1 (-) 66 OCCSH9 H HC=CH 6.7 ( -)

58b OMe H HC=C H 7 (±) 67 C5 HzC-CHz 7.3 (±)

59 OMe H HC=CH 7.3 (+) 68 as 67 H1C-CH2 7.8 (+)

60 OMe H HC=C H 6. 1 (- ) 69b as 67 H1C-CHl 5.6 (-)

61h OCCSH9 H HzC-C Hz 6.8 (±) 70b as 67 HC=CH 8 (±)

62 OCCSH9 H HzC-C H1 7. 1 (+) nb as 67 HC=CH 8.5 (+)

63 OCCSH9 H H1C-C H1 5. 8 (-) 72 as 67 HC=CH 6.6 (-)

64 OCCSH9 H HC=C H 7 .1 (±)

apDE_4 inhibitory activi ty is in log ( 1IICso) fo rm .

Table 3--PDE-4 inhibi tory acti vity of race mates and enanti omers of N-subustituted tetrahydrophthalaz inones

Comp.

73 74 75

R,

Me

Me

Et

Rz

Me

Me

Et

R3

Adamantan-2-yl

Adamantan-2-yl

4-HOOCC6H4

apDE_4 inhibitory activi ty is in log (1IICso) form .

p ICso

93

6.9

7.7

o

Comp. No. 73-78

, Isomer Compo R , Rz R3 p rc so" Iso mer

(+) 76 Et Et 4-HOOCC6H4 8.3 (+)

(- ) 77 Et Et 4-EtOOCC6H4 8 (± )

(± ) 78 El Et 4-EtOOCC6H4 8 2 (+ )

LAKSHMI (' I al. QSAR STUD IES ON HYDROPHTH ALAZ INON ES 37

such as Ii pophi I icity CFR), molar refracti vity (MRs), hydroge ri bond donor (HD) and hydrogen bond acceptor (HA) properties were al so computed from the lite rature va lues

l4. The multiparamete r regress ion

analysis was carried o ut using SYSTAT 7.01.) software.

Results and Discussion Various parameters li ke connecti vity index, dipo le

moment, e lectron affinity , conformation minimum energy, di e lec tric energy, steric energy, heat of format ion, HOMO, LUMO, ionizati on potential , log P, molar refract ivity, shape index, solvent access ibl e surface area and va lency connecti vity index ha~,e been calcu lated for the compounds of both seri es (A and B) by BiomedCache software. These parameters were used as an independent and PDE-4 inhibitory activity as dependent variable for deriving the QSARs using multiple linear regress ion ana lysis. In order to consider different prototypes, stereo-isomers and enan tiomers together for the QSAR ana lys is, diffe re nt indicator variables viz., indicator variable 11 (11 = 1 fo r the presence and 11=0 for the absence of the branched substitution at N-2) , h (h= 1 and 0 for the presence and the absence of double bond in the cis fused hydrocarbon ring of phtha lazinone group respec­tively) and I, (1, being 0, +1 and - 1 for raccmates, dex tro and levo isome rs, respectivel y) have been used (Table 4). The stepwise MLR anal ys is fo r the training set resul ted in Eq. 1, which shows the positi ve

contri bution of dielectric energy (DE) and mo lar refractivity (MR) parameters a long with the indicator va ri ab les I I, h and I, for the PDE4 inhibiti on. Though the Eq. L shows a reasonable corre lation(r=0.795) of hi gh statistica l significance (>99.9%) the regress ion coefficients with DE shows sli ghtly lesser stati stical significance as compared with other parameters (s ignificance -98.5%) and a lso had some inter­correlation ( r=O. 73) with I, (Tab le 5). Hence, Eq. 2 was derived after re moval of DE parameter, which al so sho wed a linear correlatio n (r=0.769) of hi gh stati st ica l significance (>99.9%) as expected. Both the equations explained the va ri ance in activity reaso­nably well as shown by the comparison between the observed and calculated activity values (Tabie 6) . Thi s is a lso apparent from the plots between the observed and estimated activity by Eq . L (Fig. 1) and Eq. 2(Fig. 2). The confidence e ll ipses are shown as graphical indicator of corre lation where the min or axi s of the e ll ipse centered on the sample means of the va ri ables describes the standard deviati on.from the regress ion line. When two vari ables are unc"orrelated , the confidence e llipse is circu lar in shape, while it beco mes more e lo ngated with the in ti'ease In corre lat ion be tween two variab les.

-log p1Cso= L .224(0.49)DE+0 .030(±0.006 )MR +0.803 (±O.217) I 1+0.609(±O. j 64)h +5.00 I (±0.644)

n=62, r=0.795 , / =0.632, Adj . ,) =0.606,

r2 CY( Loo)=0.46, s= 0.607, F=24.49 . .. (1)

Tab le 4--Physico-chemical parameters and biological acti vity for training set molecu les

Compo DE MR" MR,h I I 12 I, BA(obs)

-0.6 80.6 15 1 03 0 0 0 7.6

2 -0.55 85.5 12 5.65 0 0 0 8. 1

3 -0.54 90.26 103 0 0 0 7.6

4 -0.46 94.784 14.96 0 0 0 8

5 -0.52 94.678 14.96 0 0 0 7.9

6 -0.44 99.315 19.62 0 0 0 7.8

7 -0.47 101924 22.02 0 0 7.7

8 -0.49 1111 26 3 1.34 I 0 0 85

9 -0.55 94.674 14.4 1 0 0 0 7.6

11 -0 .67 90.576 10. 11 0 0 0 8. 1

12 -0.58 91804 11 .8 1 0 0 0 8

13 -0.56 96.669 16.48 0 0 0 8

15 -0.73 91.596 28.38 0 0 0 7.9

19 -0.64 11 304 32 .72 0 0 0 7.3

20 -0.65 122.242 4199 0 0 0 7.6

(Collld. )

38

Compo

21 22 23 24 25 26

27

28

29

31 32 33 34 35 36 38 41 42 43 44 45 46 47

49

50

51 53 54 55 56 57

59

60 62 63 64 65 66 67

68 72 73 74 75

76 77 7!i

INDlA N J CHEM, SEC A, JA NU ARY 2006

Tablc 4(Collld.)---Physico-chcmical parameters and biological acti vity for training se l molecules

DE

-0.66

-0.63

-0.5

-0.56

-0.75

-0.61

-0.54

-0 .6

-0.55

-0.62

-0.59

-0.65

-0.64

-0.72

-0.95

-0.66

-0.89

-0. 89

-0.46

-0.52

-0.46

-0.49

-0.59

-0.49

-0.51

-0.57

-0.47

-0.52

-0.49

-0 .6 -0.53

-0.62

-0.62 -0.5 1

-0 .56

-0.58

-0.55

-0.6 -0.6 -0.6 1

-0.61 -051

-0.49

-0.8 I

-0.82 -0 .7)

-0.76

MR"

11 2.173

105.289

110.1 24

114.879

115.41 3

120.929

120.443

11 6.587

116.587

123051

I 11. 8 18

II 1. 8 18

I I 1. 8 18

11 6.883

11 6.883

107.595

107.484

11 6.993

98.57

95 .794

103041

107.642

105 .104

11 6.844

122.244

119.287

130.662

124.532

131.779

80.615

80.6 15

8 1.732

81.732

97027 97027 98. 144

98. 144

98 .144

100. 15 1

100.15 1

101.268

122.078

!22078 122.66

122 66

132. 177 132 . 177

32.22

25.36

30.0 1

34.63

34.58

40.95

37.59

36 82

3682

43.66

3 1.83

3 1.83

31.83

35.88

35.88

2303

29. 1

38.41

18. 15

14.96

2202 26.69

23.7

3598

36.63

38.26

3 1.34

3 1.34

31.34

1.03

1.03

1.03

1. 03

1.03

1.03

1 03

1.03

103 1.03

1.03

1.03

36.63 36.6:1

30. 36 3036 4 1.09 41.09

o o o o o o o o o o o o o o o o o o o o

I

I

o o o o o o o o o o o I

I

o o o o

o o o o o o o o o o o o o o o o o o

I

o o

I

o o

I

o o

o o o o o o o o o o o o o o o o o o o o )

o o o o o o o o

-I

- I

-I

o I

- I

o

- I

-I

o I

o

"MR calcul ated using Bi oM edCachc6.0 for whole molecule ; hCalcu laled us ing litera tu re values fo r slI hstituelll R.

BA(obs)

6.3

7.6

8. 1

7.6

8 7.9

7.8

7.7

8.5

7.6

8. 1

8 8

7.9

7.3

7.6

63

8

8. 1

8.9

9.3

9 .2

8.9

9.3

9.4

9. 1

9.2

9

9.2

6.9

5. 1

7.3

6. 1

7.1

5.8 7.1

7.4

6.7

7.3

7.8

6.6

9.3

6.9

7.7

1::.3 8

8 2

LAKSHMI ef al.: QSAR STUDIES ON HYDROPHTHALAZ INONES 39

-log pICso= 0.028(±0.006)MR+ 1.057(±O.200)IJ +0.572(±O.170) h+4.498(±0.638)

n=62, r=0.769, /=0.592, Adj. r~=0.571, r2CV(LOO)=0.502, s= 0.633 , F=28.04 . . . (2)

In view of the major contribution by the bulk parameter MR given by BioMedCache software where it is taken as an overall shape parameter, it is of interest to focus our attention to study the contribution of this parameter MR for the substituent rather than the whole molecule. So, the MR values of the substituent 'R ' (MRs) have been computed from the literature (Table 4) and along with the indicator variables as an independent variable PDE-4 inhibitory activity (pIC50) have been correlated as dependent variable by multiparameter regression analysis, which led to the derivation of the QSAR Eq. 3. This equation shows a moderate correlation (r=0 .868) of PDE-4 inhibitory activity with the MR and indicator variables. The Eq . 3 has higher correlation (r=0.868) as compared to Eqs 1 and 2 with almost non-inter­correlated (r<0.5) variables . (Table 7) .

Table 5--The correlation matri x for physi cochemical parameters used in equations

DE MR II 12 13 BA

DE LOO

MR -0.03 LOO

II 0.44 0 27 LOO

12 0.21 025 0.46 LOO

13 -0.73 0.096 0 .05 0.02 LOO

BA 0.35 0 .56 059 0.40 0.34 LOO

11

10 0

a 9 0

UJ 0 0"

I 0

f-(fJ

8 UJ

7

0

6 5 6 7 8 9 10

BA

Fig . I- The plot between observed (SA) and es tim ated (EST _ EQ I) acti vity. (Eq. I)

N a

9

WI 8 f-(fJ W

7 o o

00 o

BA

o

9 10

Fig. 2- The plot between observed (BA) and estimated (EST_ EQ2) acti vity. (Eq . 2).

Table 6--Calculated and residual activiti es or compounds or training set using derived equations (from BioMedCache software)

Comp. Obs." Cal b Eq I Res ." Eq I CaL b Eq 2 Res." Eq 2

1 6.4 6.69 -0 .29 6.76 -0.36 2 6 8 6 89 -009 6.89 -0.09 3 7.4 7.05 0.35 7.03 0 .37 4 7 .8 7 .29 0.5 1 7. 15 0 .65 5 8.6 7.2 1.4 7. 15 1.45 6 7.9 7.45 0.45 7.28 0.62 7 8.4 8.29 0.11 8.4 1 -0 .01 I; 8.6 8.54 0.06 8.67 -007 9 7.6 7. 17 0 .43 7 .1 5 0.45 11 7. 2 6.89 0.3 1 7.03 0.17 12 6.8 7.04 -0.24 7.07 -0.27 13 7.5 7.22 0 .28 7. 2 0. 3 15 5.7 6.86 -1.16 7 06 - 1.36 19 7 7.61 -0 .62 7.66 -0 .66 2() 7 7. 88 -0. 88 7.92 -0.9:2

(CoJller )

40 INDIA I J C1IEM. SEC A, JA UA RY 2006

Table 6(ColI/(l. )---Calcul ated and residual act iviti es o r compounds or training: sctusing derived cqual ions (from BioMedCachc soflware)

Compo Obs." Cal h Eq I Res .' Eq I Cal. h Eq 2 Res.' Eq 2

21 7. 1 7.56 -0.46 7.64 -0.54

22 7. 6 7.39 0.21 7.45 0.1 5

23 8.1 7.69 0.4 1 7. 58 0.52

24 7. 6 7.76 -0. 16 7.7 1 -0. 11

25 8 7.54 0.46 7.73 0.27

26 7.9 789 00 1 7.88 0.02

27 7.8 7.95 -0. 15 7.87 -0 .07

28 7. 7 7. 76 -0.06 7. 76 -006

29 8.5 783 067 7.76 0.74

3 1 7.6 793 -0 .33 7. 9'+ -0.34

32 8. 1 7.63 0.47 7.63 0.47

33 8 7.56 0.44 7.63 0.37

34 8. 3 757 0.43 7.63 0.37

35 7 .9 7.63 0.27 7.77 0. 13

36 7.3 7.34 -004 7.77 -0.47

38 7.6 7.42 0. 18 7.5 1 0.09

41 6.3 7.14 -0.84 7.5 1 - 1.2 1

42 8 7.43 0.57 7.77 0.23

43 8 I 7.4 0.7 7.26 0.84 44 8 9 7.24 1.66 7. 18 1. 72 45 9.3 8 33 0.97 8.44 0 86 46 9.2 8.43 0.77 8.57 0.63 47 8.9 824 0.66 8.5 0.4 49 9.3 8. 7 1 0.59 8.83 0.47 50 9.4 10. 1 -0.7 8.98 0.42 51 9. 1 8.68 0.42 8.9 0.2 53 9.2 9. 14 0.06 9.2 1 -0 .0 1 54 9 8.9 1 0.09 904 -0.04 55 . 92 9. 15 0.05 9. 24 -0 .04 56 6.9 8. 1 - \.2 838 -1.48 57 5.1 6. 16 - 1.06 6. 18 - 1. 08 59 7.3 7. 31 -0.01 736 -006 6() 6. 1 6.08 002 6.2 1 -0. 11 62 7. 1 7.9 -0 8 7.79 -0.69 63 5.8 6.62 -0 .82 6.64 -0.84 64 7. 1 7. 24 -0. 14 7. 25 -0. 15 65 7.4 7. 88 -0.48 7.82 -0.42 66 6 7 6.6 0.1 6.67 0.03 67 7.3 7.27 0.Q3 7.3 0 68 7.8 7. 87 -007 7. 87 -0.07 72 6.6 6.68 -0.08 676 -0. 16 73 9.3 9.45 -0. 15 9 5') -0:2:" 74 (). 9 8.26 - 1.36 8 . .+ -1. 5 75 7.7 769 0.01 7.93 -0,23 76 8.3 8. 29 001 8.5 -0.2 77 8 8.05 -0.05 8.2 ,9·2 78 8.2 8. 65 -0.45 8. 77 -0 .57

"Obse rved ac tivity: hC tl culatcd aClivity hy I'cs[lcc tive equa tions; ' Rc:ii dual activity bc twcc llthc observcd and calculatcd acti vit y.

LAKSH[VII ('{ af.: QSAR STUDIES ON HYDROPHTH ALAZ INONES 41

Table 7--Correlati on malrix rar the pal'amctcrs calculatcd by literature (training sell

MRs II 12 I, BA

MRs 1.00 II 0.22 1.00 12 0.01 0.463 1.00 I, 0.061 0.04 0.02 1.00

BA 0.529 0.68 0.39 0.35 1.00

-log pICso= 0.025(±O.004) MRs+ 1.065(±0.162) II +0.237(±0.143) h+0.682(±0.128) 1} +6 . 770(±0.1)

n=62, r =0.868, /=0.753, Adj. /=0.736, ""cV(l .00)=0.70, s= 0.477, F=43 .52 . . . (3)

However, thi s equati on with four parameters includes the indicator variable h instead of DE as fourth parameter in Eq. I. Though thi s equati on has high stati sti cal significance (FJ.58uUUO I=6 .8 1, FJ .58(,OOO I=55.41 ), the contribution of the parameter h is less significant as indicated by its hi gh standard error. Hence, the multiparameter regress ion analysis was carried out by removing thi s parameter which resulted in a three parameter equation (Eq . 4) with the linear correlation (r=0.861 ) of high stati stical signi ­fi cance CF4.57 uOOIII =5.81, F4.S7 u.o()o l=43.52) between molar refracti vity (MRJ , indicator va riabl es (I I and IJ) and the bio logica l acti vity. These eq uations (Eqs 3 and 4) well describe the variation in activity with the change in structural and MR parameters as shown by correlation between observed and calculated activity. The confidence e llipses for these equations are given in Figs 3 & 4. The positive coefficient with the MR parameter in Eqs 3 & 4 suggest that the bulk at N-2 position is also important for activity and its increase enhances the activ ity. The positi ve coeffici ents of the i nd icator va riabl e I I suggest the presence of the branched substituti on at N-2 is fa vourable for activity. The positi ve coefficient with indicator va riable 1, indicates that the dextrorotatory compo unds have hi ghest acti vi ty as compared to racemic and levoro­tatory compounds, whil e racemic mixture is less acti ve than dextro isomer and more active than levo isomer. The calculated and residual acti viti es for the compounds of the training set using the Eqs 3 & 4 are given in Ta ble 8.

-log pICso=OJ)24(±O.004)MRs + 1.292(±O.144)II +0.692 (±0. 130) 10 + 6. 844(±O. 1 18)

n=62, 1'=0.861 , ,)=O.74L Adj. r2=0.728, r2('\'( 1.00)=0.695. s= 0.484, F=55.410 (4)

10 ,---,----,----,----,--~

C')

a w

9

I 8 f-(f) W

7

o

6 ~O~~ ____ ~ __ ~ ____ L_ __ ~

5 6 7 8 9 10 SA

Fi g. 3~The plot between observed (BA) and estimated (EST _EQ3 ) activity. (Eq . 3)

1 0,---,----,----,---,---~

9

..;­a w

I 8 f-(f) W

7

G 0 co 6LL __ ~ __ ~ __ _L __ -L __ ~

5 6 7 8 9 10 SA

Fig. 4----Thc plot between obse rved (BA) and estimatcd (EST_EQ4) act ivity. (Eq. 4)

External validation

Though the equations described above also show good / cv(LOO ) value particularly for Eq. 3 (0.736), but in order to va lidate the relevance of the mode ls, ex ternal test set val idation has been carried out. It consists of making predictions for an independent set of compounds not used in mode l training. In the present study, the predictivity of generated QSAR eq uations was tested against a test set consisting of fi fteen compounds (Table 9). The predicted activities for the fifteen compounds by all the four equations are described in Table 10, alongwith the correlation of the predicted vs. observed activities (predictive / for £qs 1,2,3 and 4 is 0.67, 0.70, 0.71 and 0.67, respectively). The pred icti ve / Was based onl y on mol ecules not included in the training set and is defined as r "prccl

=(SD-PRESS)/SD W:lere SD is the sum of the squared dev iations between the biological acti vity of

42 INDI AN J CH EryL SEC A, JANUA RY 2006

Table 8---Calcul ared and res idual activit ies o r training sct compounds using Eqs 3 and 4

Compo 1 2 3 4

5 6

7

8

9

11

12 13

15 19

20

21 22 23 24 25

26 27

28

29

31

32 33

34

35

36 38

Cal. Eq. 3 6.80

6.9 1

7.03

7. 14

8.2 1

8.33

8.39

8.62

7 . 13

7.02

7.07

7. 18

7.48

7.59

7 .82

7.58

7.40

7.52

7.64

7.63

7.79

7.7 1

7.69

7.69

7.86

7.57

7.57

7.57

7 .67

7 67

735

Res . Eq. 3 -0.40

-0 . 11

0.37

0 .66

0.39

-0.43

0.01

-0 .02

0.47

01 8

-0.27

0.32

0 .02

-0.59

-0.82

-OA8 0.20

0.58

-0.04

0.37

0.11

0.09

001

0.81

-0 .26

0 .53

0.43

0 .43

023

-0.37

0.25

Cal. Eq. 4 6.87

6.98

7.09

720 8.50

8.61

8.66

8.89

7 .19

7.09

7. 13

7.24

7.53

7 .63

7.85

7.62

7.45

7.56

7.68

7.67

7.83

7.75

7 .73

7.73

7.89

7 .61

7.61

7 .61

7.71

7.7 1

7. 40

9,---.----,----,---.---~

8

7

6 L---~---L--~----~--~

5 6 7 8 9 10 SA

Res . Eq. 4 -0.47

-0. 18

0 3 1

0.60

0 .10

-0 .71

-0.26

-0.29

0.41

0 . 11

-0 33

0.26

-0.03

-0.63

-0.85

-0 .52

0 .15

0 .54

-0 .08

0 .33

0 .07

0 .05

-0.03

0 .77

-0.29

OA9 0.39

0 39

01 9

-OAI

0.20

Fig. 5--Thc plot between observcd (BA) and estimatcd (EST ~EQ I) acti vity fo r the test se t by Eq . I.

Compo 41

42 43

44 45

46

47

49

50

51 53 54

55

56

57

59

60

62 63 64

65

66

67

68

72

73

74

75

76

77

78

Cal. Eq. 3 7.50

N a w f-I (j)

W

7.73

7.46

8 45

8.62

8.74

8.66

8.97

8.99

9.03

8.86

8.86

8.86

7.48

6. 11

7.7 1

6. 35

7A8 6. 11

7.03

7.7 1

6.35

6.80

7.48

6. 35

9 .67

8.3 1

7.77

8. 45

8.03

8.72

7

o

Res. Eq. 3 Cal. Eq. 4 -1 .20 7.54

0 27 7 .77

064 7.28

04') 8.50

0 .68 8.66

0.46 8.78

0 24 8.70

0 .33 9 .00

0.41 902

0.07 9.05

0 .14 8.89

0. 34 8.89

0 .24 8.89

-0 58 7.56

-1.01 6.1 8

-0.41 7.56

-0 .25 6. 18

-0.38 7.56

-0 .3 1 6. 18

0 .07 6.87

-0.3 1 7.56

0 .35 6. 18

0.50 6.87

0 .32 7.56

0 .25 6. 18

-0.37 9. 71

-IAI 8.32

-007 7 .57

-0 .15 8.26

-0.03 7. 83

-0 .')2 8.52

o

o

Res. Eg. 4 -1 .24

0.23

0 .82

0.40

0 .64

OA2 0.20

0 .30

0. 38

005

0. 11

0.3 1

0.2 1

-0 .66

-1.08

-0.26

-0 .08

-0.46

-0.38

0.23

-0. 16

0 .52

0.43

0 .24

OA2 -0 .41

-1.42

0 .13

0 .04

0. 17

-0 .32

6 L---~--~----L---~--~

5 6 7 8 9 10 SA

Fig. 6--The pl ot between observed (BA) and esti matcd (EST~EQ2) acti vity fo r the tes t set by Eq. 2.

LAKSHMI e/ al.: QSAR STUDI ES ON HYDROPHTH ALAZINONES 43

Table 9--{:alculatcd parameters ror test set co mpou nds

Compo DE" MR" MR (Litll 11

10 -0.549 93.092

14 -0. 742 101.1 13

17 -0. 857 119.454

18 -0.558 103.53 I

30 -0.586 116.587

37 -0.778 12 1.652

39 -0.643 107.967

40 -0.626 107.967

48 -0 553 II 1.278

52 -0. 5 17 112.243

58 -0.62 8 1.732

61 -0.533 97.027

69 -0.595 100.15 1

70 -0.603 101.268

71 -0.594 101.268

12.87

21.38

33.88

23.4 1

36.82

41.82

23 .03

23.03

29.7

3 1.34

1.03

1.03

1.03

1.03

1.03

0

0

0

0

0

0

0

0

1

I

0

0

0

0

0

o o o o o o o o

I

o o

o o o o o o o o o o o o -I

o

"parameters calculated using BioMedCache, h parameters calculated using literature values.

Tab le I O--Calculated and re sidual act ivities of compou nds of test set using derived equations

Compo

10 14

. 17 18

30 37 39 40 48 52 58 61 69 70 71

pred r2

BA (obs )

7.40

7.20

7.60

6.50

8.20

7. 90

7.60

7.80

8.90

9.10

7. 00

6.80

560

8.00

8.50

Prcd. Eq.

7. 12

7.1 3

7.54

7.42

7.78

7.70

7.45

7.47

8.47

8.54

6.69

7.26

6.67

730 7.92

067

Res i. Eq .

0.28

0.07

0.06

-0.92

0.42

0.20

0.15

033

0.43

0.56

0.31

-0 .46

- 1.07

070

0.58

Prcd . Eg . 2

7.10

7.33

7.84

7.40

7.76

7.90

7.52

7.52

8.67

870

6.79

7. 21

6.73

7.33

7. 91

0.70

molecules In the test set and the mean biol ogica l activity and PRESS IS the sum of the squared deviations be tween predicted and actual ac ti vity va lues for eac h molec ule in the test set. Like ,) c\' . the predictive / can ass ume a negative va lue reflecting a comp lete lack of predicti ve ab ility of the training set for the molecules inc luded in the test set.

T he good rredict ive ab ility of the generated eq uations I. e. predict i ve / =0.7 1 for the mode l generated uSin g litera ture va lues and predictive

Rcs i. Eq. 2

0.30

-0.13

-0.24

-0.90

0.44

000

0.08

0.28

0.23

0.40

0.21

-0.41

- 1.1 3

0.67

0.59

Pred. Eq. 3

7.09

730 7.62

7.36

7.69

7.82

7.35

7.35

8.8 1

8.62

7 03

6.80

6.11

7.03

7.7 1

0. 7 1

Rc si. Eg. 3

0.31

-0.10

-0.02

-0.86

0.5 1

0.08

0.25

0.45

0.09

0.58

-0 .03

0.00

-0.51

0.97

0.79

Pred. Eq. 4

7. 15

7.36

7.66

7 .41

7.73

7.85

7.40

7.40

8.85

8.89

6.87

6.87

6.18

6.87

7.56

0.67

Resi. Eq. 4

0.25

-0.1 6

-006

-0.91

0.47

0.05

0.20

0.40

0.05

0.3 1

0.13

-0 .07

-058

1.1 3

0.94

/ =0.70 for the mode l generated by BioMedCache parameters shows the ro bustness of the mode l. Thi s is depicted I n Fi gs 5-8 111 the fo rm of confidence e llipses. which become more e longated with the increasing corre lat ion between two variables.

Conclusions Among several parameters used for the 2D QSAR

studi es on cis-hexa dnd tetra-hydrophthalaz inones. a new c lass of se lec ti ve PDE-4 inhibitors, only mo lar

44 IND IA N JCHEM, SEC A. JANUARY 2006

(")

a w

I f­(j)

W

o

o

8

7

6~~~--~----~--~--~

5 6 7 8 9 10 BA

Fig. 7-The plot bet ween observed (BA) and estimated (EST _EQ3) activity for the test set by Eq. 3.

refractivity (MR) and dielectric energy (DE) showed positive correlation with PDE-4 inhibitory actIvity along with the structural parameters as indicator variables II (11=1 and 0 for the presence and absence of the branched substitution at N-2, respectively) , h (h=l and 0 for the presence and the absence of doubl e bond in the cis fused hydrocarbon ring of phthal­azinone group respectively) and I, (I, being 0, +1 and -1 for racemates , dextro and levo i somers respec­tive ly) . As the models explain well the observed variance in PDE-4 inhibitory activity both in the training and test set, these may be useful for the design and synthesis of nove l molecules .

Acknowledgements The authors are thankful to Mr. Philip Prathipati

for critical suggestions and Mr. A.S. Kushwaha , Technical Officer, for the technical support. They also acknowledge the CSIR, New Delhi for financial support.

References I Geimbycz M A, Drugs, 59 (2000) 193.

-.;t

a w

I f­(j) W

BA

Fi g. 8-The plot between observed (BA) and estimated (EST_EQ4) activity for th e test set by Eq. 4.

2 Geimbycz M A. MOlluld Arch Ches! Dis, 57 (2000) 48. 3 Piaz V 0 & Giovannoni P, ["111' J Med ChOIl, 35 (2000) 463. 4 Barnette M S & Underwood 0 C. Cll rr Opin Pll tlll Med, 6

(2000) 164. 5 Turner C R, Esser K M & Whee ldon E B. Cire Shock, 39

(1993) 237. 6 Ouplantier A J & Cheng J B. AIlIl Rep Med C/WIl, 29 ( 1994)

73. 7 Hay 0 W P & Barnette M S. AIlI! Rep .'vled Chelll, 34 (1999)

III. 8 Montana J G & Dyke H J, AIlII Rep Met! ChOIl, 36 (200 I) 41. 9 Mey M V, Hatze lmann A, Laan I J V. Sterk G K. Thibaut V

& Ti mmcrmann H, J Med C/1('I1I , 44 (200 I) 251 I. 10 Mey M V, Hatze lmann A, Vanklink G P M, Laan I J V.

Sterk G K. Thibaut V. Ulrich W R & Timmermann H . .1 Med Chell!, 44 (200 I) 2523.

II Mey M V. Boss H, Hatzelmann A .. Laan I J V, Sterk G K & Timmermann H . .1 Met! Chelll, 45 (2002) 2520.

12 Mey M V, Boss H, Couvenberg 0 , Hatzc lmann A, Sterk G K, Goubitz K. , Schenk H. & Timmermann H, .I Med CiWIl,

45 (2002) 2526. 13 BIOMEOCACHE Vcr 60. (FUjitsu Limited . Oxi'ord

Molecular Ltd). 14 Hansch C & Leo A .I , in Substitl/cllt COllstClllls/or Allalysis ill

Chelllistry alld Biology (John Wiley & Sons. Inc, New York) 1979. pp 48-63.

15 SYSTAT Ver7.0. (S PSS. Inc, Chicago, USA).