Partition Coefficients of Indoles and Betacarbolines

+

+

Partition Coefficients of Indoles and Betacarbolines PILAR GUARDADOX, MANUEL BALON, CARMEN CARMONA, MARIA A. MUN˜ OZ,

AND

CARMEN DOMENE

Received March 4, 1996, from the Departamento de Quı´mica Fı´sica, Facultad de Farmacia, Universidad de Sevilla, 41012 Sevilla, Spain. Final revised manuscript received July 17, 1996. Accepted for publication October 1, 1996X. Abstract 0 Partition coefficients for substituted indoles and betacarbolines were determined in octan-1-ol/water and cyclohexane/water. A comparative study of the results in both systems allows us to discuss the effects played by the different molecular structures, substituents, and aromaticity on the distribution properties of these compounds. In particular, the hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) properties of these solutes were characterized and compared with those of structurally related compounds. The Abraham solute descriptors were estimated and partition coefficients (log P) calculated and compared with the experimental values. The results show that the HBD properties are similar for indoles and betacarbolines, and the HBA capacity, as expected, is significantly enhanced by the contribution of the extra pyridinic or piperidinic ring in betacarbolines. The effects of the substituent groups are presented in relation to their contribution to the distribution properties of the compounds studied.

Introduction Indole and its derivatives have attracted considerable interest because of their varied and potent biological activities such as tumor promoter (teleocidin), CNS stimulant (harmaline), and hypotensive (reserpine), antiinflamatory (indomethacin), antimicrobial (gliotoxin), and plant growth regulatory (indole acetic acid) properties. They are also building blocks of natural alkaloids and pharmacologically active compounds.1 A large number of indole alkaloids possess, as an integral part of their structures, a betacarboline (BC) nucleus (9Hpyrido[3,4-b]indole) in some of its different oxidation degrees, (Scheme 1). BC alkaloids are found in many plants,2 some of which have been used as hallucinogens and drugs, and they also occur as minor constituents in tobacco smoke. Several tetrahydrobetacarbolines (THBC) are endogenous, albeit trace, constituents of the mammalian brain. These so-called mammalian THBC alkaloids probably arise endogenously from the condensation of central nervous system (CNS) indolamines with an aldehyde or R-keto acid. THBCs may furtherly be oxidized to the corresponding dihydrobetacarbolines (DBC) and fully aromatic BCs. Despite increasing evidence that some BCs are normal constituents in the human body,3,4 very little information is known about the fate of those chemicals in humans. However, the central effect of these drugs shows that they are absorbed and can be transferred into the CNS. The pharmacokinetic of different BCs is highly dependent on their lipophilicity, and it is known that those derivatives without hydrophilic groups easily pass through the blood brain barrier (BBB).3,5 The lipophilicity of solutes can be related to their partition coefficients, (log P), which is of great significance from both physicochemical and biological viewpoints. Indeed, many studies have shown that log P is useful for correlating the transport processes of a drug and its interactions with receptor molecules. In fact, the maximum biological response (drug activity, membrane diffusion processes, etc.) for a series of related aromatic compounds has been defined in terms of molecular partitioning across nonpolar-polar phases and chemical reactivity.6 Partition coefficients, as well as other properties depending on solute-solvent interactions, have X

Abstract published in Advance ACS Abstracts, November 15, 1996.

106 / Journal of Pharmaceutical Sciences Vol. 86, No. 1, January 1997

Scheme 1

been correlated by linear solvation energy relationships (LSER) that are linear combinations of dependences on different solute parameters, such as molar volume, dipolarity/ polarizability, and hydrogen-bond acceptor (HBA) or donor (HBD) abilities.7-11 Because of our current interest in indole and BC chemistry,12-14 and with the aim to seek from a different route the importance of the various forces involved in the possible interactions of these compounds with biological receptors, we report in this paper results of the study of partition coefficients (log P) in octan-1-ol/water and cyclohexane/water for the aforementioned compounds together with some structurally related compounds. The choice of the systems was dictated by the difference in properties between them. The octan-1ol/water system mimics the lipophilicity of the membrane cell and the greatest number of biological interesting solute types has been measured with it; the partition coefficients of these solute types have been used in quantitative structure relationships (QSAR). On the other hand, due to the greater lipophilicity of the cyclohexane and its lack of hydrogen bonding,15 a system like cyclohexane/water provides complementary information of the role played by the different forces involved and, more specifically, by the HBD and HBA properties of the solutes.

Experimental Section The indoles and BCs were commercial products of the best available quality (> 98%; Aldrich Quimica, Sigma, or Lancaster) and used as received or prepared as described elsewhere.16 Solvents were saturated as described earlier.17 For the BC derivatives, 0.01 M sodium hydroxide aqueous solutions saturated with the organic solvent were used instead of water to have only the neutral form in both phases.16 The analysis of the concentrations of the partitioned substances was carried out in a Perkin Elmer Lambda 5 spectrophotometer. For the partitioning, the solute was usually dissolved in the organic phase. The volume ratio of the two phases and the amount of sample were selected to get absorbances between 0.1 and 0.9 in a 1 cm cell after equilibrium distribution was obtained. The flasks were kept in a thermostatted bath at 298 K and gently agitated for a period of time. After being centrifuged, the organic layers were removed with a syringe, and the water phases were analyzed. Each determination was carried out in at least duplicate at different volume ratios and/or initial concentrations of the solute. Care was taken to check that equilibrium had been attained by measuring absorbances of the sample at the maximum wavelength of the spectra until the values remained constant. The log P values were referred to the molar scale because it is usual for partition coefficients and the uncertainty in the reported values are <( 0.04 in both systems,

S0022-3549(96)00111-6 CCC: $14.00

© 1997, American Chemical Society and American Pharmaceutical Association

+

+

Table 1sLogarithms of Partition Coefficients in Octan-1-ol/Water (log PO-W) and Cyclohexane/Water (log PCH-W) of Indole and Betacarboline Derivatives Compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a

Solute Indole(I) 1 CH3−I 3 CH3−I 5 CH3−I 7 CH3−I 1,2 (CH3)2−I 2,3 (CH3)2−I 2,5 (CH3)2−I 5 OH−I 5 NH2−I 5 CH3O−I 5 CN−I 5 NO2−I 7 NO2−I 5 Cl−I

log PO-W 2.14a

2.14, 2.55 2.60a 2.67, 2.68a 2.56 2.82a 3.01 3.02 1.29 0.85 2.08 2.40 2.61 2.64 2.93

log PCH-W

∆(log P)

0.77 2.47

1.37 0.08

1.36 1.35

1.31 1.21

1.83

1.18

−0.65

1.94

0.56 −1.16 −0.28

1.52 3.56 2.89

1.40

1.53

Compound 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Solute Pyrrole THBC 1 CH3−THBC 6 NO2,1 CH3−THBC 6 Cl,1 CH3−THBC 6 CH3O,1CH3−THBC 7 CH3O,1CH3−THBC BC 1 CH3−BC 1,9 (CH3)2−BC THC CZ 9 CH3−CZ Tryptamine(T)

log PO-W 0.75a 1.98 2.06 2.07 2.30 1.15 1.37 2.65, 3.17b 2.89 3.01 3.00 3.48, 3.29a 3.99 1.35

log PCH-W −0.5 −0.8 −0.4 −0.5 −0.3 −0.6 0.2 1.70 1.50 2.24 3.73 <−1.4

∆(log P) 2.48 2.86 2.70 1.65 1.67 3.25 2.69 1.31 1.50 1.24 0.26 e2.75

Taken from ref 18. b Taken from ref 19.

Figure 1sRelationship between the substituent constant π (π ) log PX − log PH) for substituted indoles and benzenes in octan-1-ol/water. except for some THBCs and BCs in cyclohexane/water where bigger uncertainty exists (e( 0.5) because of the very small solubilities of these compounds in both phases.

Results and Discussion HBD and HBA Abilities of Indoles and BCs.sOur results on log Pobs in both solvent systems for a series of indoles and BCs, together with some structurally related compounds whose partition coefficients have been determined by us or found in the literature,18,19 are presented in Table 1. Where our results can be compared with the published values, a good agreement can be observed (see 1, 4, 23, and 27 in Table 1). Table 1 also includes the Seiler parameter20 [∆(log P)], which is defined as the difference between the log Pobs values in octan-1-ol/water and cyclohexane/water systems. This parameter has general utility in establishing correlations

Figure 2sRelationship between the substituent constant π (π ) log PX − log PH) for substituted indoles and phenols in octan-1-ol/water.

of distribution of solutes between blood and brain and in the design of potential drugs where brain penetration needs to be either improved or minimized. Although, it has been demonstrated that molar volume, dipolarity/polarizability, and hydrogen-bond basicity of the solutes also contribute to ∆(log P),21,22 the Seiler parameter may be considered as an indicator accounting mainly for the solute HBD ability. Thus, the larger the ∆(log P) is, the larger will be the hydrophilicity and the capability of hydrogen bonding of the molecule. Following this rationale, the HBD ability of indoles stands out by comparison of ∆(log P) for 4 or 5 and 2 with 1 in Table 1. If the HBD center is blocked by replacing the nitrogenbound proton with a methyl group to form 1 methylindol 2; the action of the site usually assumed to be the point of attachment for hydrogen bonding23 is cancelled out and consequently a greater decrease in ∆(log P) is found. Similar results are observed in Table 1 for the pairs 24 and

Journal of Pharmaceutical Sciences / 107 Vol. 86, No. 1, January 1997

+

+

Table 2sSolute Descriptors and Observed and Calculated Partition Coefficients log PO-W H

H

H

0

Compound

Solute

R2

π2

R2

β2

β2

VX

Obs

Calc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Indole (I) 1 CH3-I 3 CH3-I 5 CH3-I 7 CH3-I 1,2 (CH3)-I 2,3 (CH3)2-I 2,5 (CH3)2-I 5 OH-I 5 NH2-I 5 CH3O-I 5 CN-I 5 NO2-I 7 NO2-I 5 Cl-I Pyrrole THBC 1 CH3-THBC 6 NO2,1 CH3-THBC 6 Cl,1 CH3-THBC 6 CH3O,1 CH3-THBC 7 CH3O,1 CH3-THBC BC 1 CH3-BC 1,9 (CH3)2-BC THC CZ 9 CH3-CZ Tryptamine(T)

1.200 1.206 1.200 1.200 1.200 1.206 1.200 1.200 1.395 1.545 1.295 1.335 1.465 1.410 1.310 0.613 1.620 1.587 1.852 1.697 1.682 1.682 1.831 1.798 1.804 1.200 1.787 1.793 1.436

1.12 0.92 1.06 1.08 1.08 0.85 1.06 1.06 1.49 1.43 1.40 1.86 1.95 1.28 1.31 0.73 1.52 1.55 2.38 1.74 1.83 1.83 1.90 1.81 1.61 1.06 1.42 1.22 1.41

0.44 0.00 0.44 0.44 0.44 0.00 0.44 0.44 1.04 0.49 0.41 0.63 0.66 0.00 0.51 0.41 0.54 0.54 0.76 0.61 0.53 0.53 0.44 0.44 0.00 0.44 0.47 0.00 0.60

0.22 0.31 0.22 0.22 0.22 0.31 0.22 0.22 0.38 0.75 0.40 0.21 0.18 0.29 0.12 0.29 0.69 0.75 0.71 0.65 0.93 0.93 0.52 0.58 0.69 0.22 0.26 0.35 0.83

0.31 0.36 0.35 0.32 0.32 0.42 0.36 0.36 0.47 0.84 0.49 0.30 0.27 0.38 0.21

0.946 1.087 1.087 1.087 1.087 1.228 1.228 1.228 1.005 1.046 1.146 1.101 1.121 1.121 1.069 0.577 1.360 1.501 1.676 1.633 1.701 1.701 1.274 1.415 1.556 1.401 1.315 1.456 1.307

2.14 2.55 2.60a 2.67 2.56 2.82a 3.01 3.02 1.29 0.85 2.08 2.40 2.61 2.64 2.93 0.75a 1.98 2.06 2.07 2.30 1.15 1.37 2.65 2.89 3.01 3.00 3.48 3.99 1.35

2.13 2.70 2.61 2.68 2.68 3.09 3.10 3.10 1.54 0.55 2.03 2.06 2.22 2.80 2.79 0.88 2.21 2.49 2.58 3.20 2.39 2.39 2.36 3.08 3.51 3.76 3.20 4.12 1.09

a

0.69 0.75 0.71 0.65 0.93 0.93 0.47 0.48 0.53 0.36 0.31 0.36 0.96

log PCH-W Obs

Calc

0.77 2.47

0.82 3.04

1.36 1.35

1.45 1.45

1.83

2.24

−0.65

−2.44

0.56 −1.16 −0.28

0.57 −0.30 −0.22

1.40

1.38

−0.5 −0.8

−0.29 −0.01

−0.4 −0.5 −0.3 −0.6 0.2 1.70 1.50 2.24 3.73 〈−1.4

0.34 −0.34 −0.34 0.04 0.53 2.65 3.04 2.19 4.53 −1.41

Taken from ref 18.

25, and 27 and 28. Again, a significant decrease in the ∆(log P) is observed upon blocking the acidic NH pyrrolic center. The values of ∆(log P) for 1 and 27 indicate that the HBD ability of this center is not significantly affected by annellation of an extra benzene ring to the indole nucleus. However, the annellation of a piperidinic or pyridinic ring, as in 17 or 23, presents a very different situation. In our opinion, the marked increase of ∆(log P) for these compounds, and also for 29, should not be related with a significant increase of the HBD ability but, as mentioned before, other contributions should be taken into account. In fact, as data in Table 1 show, methylation of the pyrrolic nitrogen produces the same decrease in ∆(log P) for 1 and 24. With regard to the effects of the substituents in the benzene ring, the values of ∆(log P) might suggest that the substituents primarily modify the hydrogen-bonding ability of the pyrrolic nitrogen center of the indole and THBC derivatives. In fact, this explanation has been offered for substituted anilines and phenols, which have electron-withdrawing substituent groups.24,25 However, several clues direct us to think that this is not the predominant effect. Firstly, no linear correlation was found between our observed substituent constant π values (π ) log Px - log PH)18 and the Hammett parameter σ. Thus, substituents such as OH and CH3O with similar σp values present different effects on log Pobs. Furthermore, if we compare our results of substituted indoles with those published for substituted benzenes, where no modification of hydrogen bonding ability is expected, a plot of πind versus πbenz (Figure 1) gives a good linear correlation, with NO2 and CN substituents deviating from the plot. Therefore, the effects exerted by the substituents in our solutes have to be related not only to the modification of the HBD ability of the NH pyrrolic center, but also to the interaction of the groups themselves with the solvent systems, probably in relation with their acceptor capacities. Interestingly, an excellent correlation was found between the substituent constants π of indole and p-phenol derivatives in octan-1-ol/water (Figure 2). The 108 / Journal of Pharmaceutical Sciences Vol. 86, No. 1, January 1997

similarity in the π-excessive character of the indole ring and the molecule of phenol is probably responsible for the similar effects exerted by the substituents in both molecules. In the case of the THBC derivatives, the nonavailability of many substituted THBCs prevented us from doing a more systematic study, as in the indole series. However, as stated earlier, differences exist because of the extra ring with a group of marked HBA properties. The possibility of conjugation with the new ring could extend the influence of the substituents to the piperidinic ring. Analysis of Partition Coefficients: Solute DescriptorssIt is well known that the determination of solute descriptors and their correlations through LSER allows the estimation of distribution properties of compounds whose partition coefficients are difficult to obtain experimentally. Recently, Abraham et al.11,22 used a general LSER approach as previously described by Kamlet et al.8,9 but with a new set of solute descriptors. The application of this LSER to partition coefficients in a wide variety of solvents and to a greater number of solutes has resulted in the following equations for octan-1-ol/water and cyclohexanewater systems: H log PO-W ) 0.088 + 0.562R2 - 1.054 πH 2 + 0.034 R2 -

3.460 βH 2 + 3.814Vx

(1)

n ) 613 F ) 0.997 SD ) 0.12 F ) 23161.6 log PCH-W ) 0.127 + 0.816R2 - 1.731 π2H - 3.778 RH 2 4.905 βH 2 + 4.646Vx n ) 170 F ) 0.997 SD ) 0.13 F ) 5122.5

(2)

+

In eqs 1 and 2, R2 is an excess molar refraction,26 π2H is the solute dipolarity/polarizability,11,27-29 R2H is the effective or summation hydrogen-bond acidity,30 β2H is the effective or summation hydrogen-bond basicity,11,31 and VX is the McGowan's characteristic volume32 in units of (cm3 mol-1)/ 100. We carried out the estimation of the solute descriptors for our compounds from the published values of some model compounds and calculated the log P by the LSER described in eqs 1 and 2. The results of the log Pobs and log Pcalc, together with the aforementioned descriptors, are collected in Table 2. McGowan characteristic volumes, VX, were obtained according to earlier calculations.32 The methodology and approach used in the construction of the descriptor sets are similar to the determination by the fragment method proposed earlier.33 For the indole derivatives, the solute descriptors were estimated from the published values of some indoles31 and the contribution of substituents in p-phenol derivatives, because of the correlation mentioned before. Indoles are known to belong to the class of solutes for which the HBA ability appears to vary with the solvent system,31 so we calculated log P based on eqs 1 and 2, with β20 and β2H for octan-1-ol/water and cyclohexane/water systems, respectively. The solute descriptors R2, π2H, and R2H for THBC and BC were obtained by adding the values of 2,3 dimethylindole and piperidine (THBC) or pyridine (BC), respectively (the β2H values are those corresponding to the latter rings). This assumption is supported by published results on annellation of a ring to pyridine to give quinoline or isoquinoline.31 Also, the fairly good adherence of our experimental results to the calculated ones validates this approach. For tryptamine, summation of the descriptors of 2,3-dimethylindole and ethylamine were used to calculate the descriptors. For THBC derivatives, we used the same contribution of the substituents as in the indole series, although we are aware that some differences might exist. The possibility of conjugation with the new ring could extend the influence of the substituents to the piperidinic ring. Comparison of the observed and calculated values of log P in Table 2 indicates that, in general, there is good agreement between experimental and calculated values for octan-1-ol/ water and cyclohexane/water systems, although the calculated values in the latter system show greater deviations for some compounds. These deviations can possibly be related to the fact that eq 2 was obtained with a smaller number of compounds than eq 1, and this will introduce bigger uncertainties in the coefficients affecting the descriptors in eq 2. Furthermore, because of the greater magnitude of these coefficients, small errors in our estimated descriptors would lead to greater deviations in the calculated values in cyclohexane/water. In spite of these uncertainties, the good adherence between log Pcalc and log Pobs for most of the solutes studied allows us to have an estimation through the solute descriptors of the factors influencing the distribution of indole and BC derivatives in both systems. Moreover, as the data in Table 2 show, although hydrogen-bond acidity has an important contribution to ∆(log P), solute dipolarity/polarizability and solute hydrogenbond basicity also make contributions and lead to more positive values of this parameter. On the contrary, solute volume contributes in the opposite sense; that is, the larger the solute, the smaller the ∆(log P) will be. In conclusion, indole and its derivatives present significant HBD ability through the NH pyrrolic center. The BC deriva-

+

tives possess similar HBD capabilities and also strong HBA properties because of the presence of the piperidinic or pyridinic ring, the latter ability being greater for the less aromatic THBCs than for the fully aromatic BC derivatives.

References and Notes 1. Chadwick, D. J. In Comprehensive Heterocyclic Chemistry, Vol. 3; Katritzky, A. R.; Rees, C. W.; Bird, C. W.; Cheeseman, G. W. H. Eds.; Pergamon-Press: Oxford, 1987. 2. Allen, J. R. F.; Holmstedt, B. R. Phytochemistry 1980, 19, 15731582. 3. Airaksinen, M. M.; Kari, I. Med. Biol. 1981, 59, 21-34. 4. Bloom, F.; Barchas, J.; Sandler, M.; Usdin, E. In Progress in Clinical and Biological Research, Vol. 90; Alan R. Liss. New York, 1982. 5. Ho, B. T.; Fritchie, G. E.; Kralik, P. M.; Tansey, L. W.; Walker, K. E.; McIsaac, W. M. J. Pharm. Sci. 1969, 58, 1423-1425. 6. Hansch, C. Accounts Chem. Res. 1969, 2, 232-239. 7. Taft, R. W.; Abraham, M. H.; Famini, G. R.; Doherty, R. M.; Abboud, J. L. M.; Kamlet, M. J. J. Pharm. Sci. 1985, 74, 807814. 8. Kamlet, M. J.; Doherty, R. M.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Pharm. Sci. 1986, 75, 338-349. 9. Kamlet, M. J.; Doherty, R. M.; Abraham, M. H.; Marcus, Y.; Taft R. W. J. Phys. Chem. 1988, 92, 5244-5255. 10. Marcus, Y. J. Phys. Chem. 1991, 95, 8886-8891. 11. Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73-83. 12. Balo´n, M.; Mun˜oz, M. A.; Guardado, P.; Hidalgo, J.; Carmona, C. Trends in Photochem. Photobiol. 1994, 3, 117-138. 13. Mun˜oz, M. A.; Carmona, C.; Hidalgo, J.; Guardado, P.; Balo´n, M. Bioorg. Med. Chem. 1995, 3, 41-47. 14. Mun˜oz, M. A.; Guardado, P.; Carmona, C.; Balo´n, M. J. Photochem. Photobiol. A: Chem. 1996, 94, 139-143. 15. Leo, A. J. Adv. Chem. Ser. 1972, 114, 51-60. 16. Balo´n, M.; Hidalgo, J.; Guardado, P.; Mun˜oz, M. A.; Carmona, C. J. Chem. Soc., Perkin Trans. 2 1993, 91-97 and 99-104. 17. Yalkowsky, S. H.; Valvani, S. C.; Roseman, T. J. J. Pharm. Sci. 1983, 72, 866-870. 18. Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-616. 19. Biagi, G. L.; Pietrogrande, M. C.; Barbaro, A. M.; Guerra, A. M.; Borea, P. A.; Cantelli Forti, G. J. Chromatogr. 1989, 469, 121-126. 20. Seiler, P. Eur. J. Med. Chem. 1974, 9, 473-479. 21. El Tayar, N.; Tsai, R. S.; Testa, B.; Carrupt, P. A.; Leo, A. J. Pharm. Sci. 1991, 80, 590-598. 22. Abraham, M. H.; Chadha, H.; Whiting, G. S.; Mitchell, R. C. J. Pharm. Sci. 1994, 83, 1085-1100. 23. Tubergen, M. J.; Levy, D. H. J. Phys. Chem. 1991, 95, 21752181. 24. Fujita, T.; Iwasa, J.; Hansch, C. J. Am. Chem. Soc. 1964, 86, 5175-5180. 25. Sotomatsu, T.; Shigemura, M.; Murata, Y.; Fujita, T. J. Pharm. Sci. 1993, 82, 776-781. 26. Abraham, M. H.; Withing, G. S.; Doherty, R. M.; Shuely, W. J. J. Chem. Soc., Perkin Trans. 2 1990, 1451-1460. 27. Abraham, M. H.; Whiting, G. S.; Doherty, R. M.; Shuely, W. J. J. Chromatogr. 1991, 587, 213-228. 28. Abraham, M. H.; Whiting, G. S. J. Chromatogr. 1992, 594, 229241. 29. Abraham, M. H. J. Cromatogr. 1993, 644, 95-139. 30. Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem. Soc., Perkin Trans. 2 1989, 699711. 31. Abraham, M. H. J. Phys. Org. Chem. 1993, 6, 660-684. 32. Abraham, M. H.; McGowan, J. C. Chromatographia 1987, 23, 243-246. 33. Abraham, M. H.; Chadha, H. S.; Mitchell, R. C. J. Pharm. Sci. 1994, 83, 1257-1268.

Acknowledgments The financial assistance of the Direccio´n General de Investigacio´n Cientı´fica y Te´cnica (PB95-0530) and Junta de Andalucı´a is appreciated.

JS960111P

Journal of Pharmaceutical Sciences / 109 Vol. 86, No. 1, January 1997