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Imino derivatives of anthracyclines: Electronic properties, relative stabilities relationship to activity
Journal of Molecular Structure (Theochem), Elsevier Science Publishers B.V., Amsterdam
152 (1987) 351-355 -Printed in The Netherlands
Short communication
IMINO DERIVATIVES OF ANTHRACYCLINES: ELECTRONIC PROPERTIES, RELATIVE STABILITIES AND RELATIONSHIP TO ACTIVITY
VINCENZO Farmitalia
MALATESTA Carlo Erba, Via dei Gmcchi 35, Z-20146 Milan0 (Italy)
GRAZIELLA Zstituto
RANGHINO
Guido Donegani,
(Received
and CAMILLO TOSI Via G. Fauser 4, Z-281 00 Novara (Italy)
30 January 1987)
The anthracyclines comprise a large number of hydroxyquinones differing by the substitution pattern and the spectrum of action on neoplasies ranging from leukemias to solid tumors. A serious drawback with this class of drugs is the dose-dependent cardiac toxicity which results in severe damage to mitochondria and sarcoplasmic reticulum [ 11. The anthracycline structure has been modified with the aim of increasing potency and lowering cardiotoxicity. Particularly important has been the substitution of the C=N-H group for the C5=0 quinone carbonyl [2] : the 5-imino compound is indeed endowed with low cardiotoxicity, albeit reduced activity. In two recent papers [3, 41 Davies and Doroshow have emphasized the importance of knowing the redox properties of these drugs in order to assess the relevance of their involvement in mitochondrial electron transfer processes. They have for instance shown that the 5-imino derivative, in marked contrast with doxorubicin and daunorubicin, exhibits little or no tendency to undergo reduction by mitochondrial NADH dehydrogenase. It would be interesting to assess the properties of the other possible imino derivative of daunorubicin, viz. the 12-iminodaunorubicin. Up till now, however, this compound has not been reported, nor is there any indication of its formation during the synthesis of the 5-imino derivative [2]. We were puzzled by such a result and, having recently carried out extensive ab initio calculations on anthracyclines [ 51, looked for peculiar differences in the electronic structure of the two imino derivatives of daunorubicin, as they result from all-electron SCF-HF ab initio calculations with a minimal basis set of Gaussian orbitals [6] . Figure 1 shows the structure of the daunorubicm aglycone, i.e., daunomycinone (1). In the two possible imino derivatives (2) and (3), either the C5=05 or the C12==012 carbonyl group is replaced by C=N-H. The first row of Table 1 gives the total energies for the three compounds: as seen, 0166-1280/87/$03.50
0 1987 Elsevier Science Publishers B.V.
352
,CH3
CH30
R
OH
6H
Fig. 1. Molecular structure of daunomycinone (1: R = 0, Rl = 0), 5Gminodaunomycinone (2: R = NH, Rl = 0) and 12-iminodaunomycinone (3: R = 0, Rl = NH).
TABLE 1 Outcome of ab initio computations
on neutral molecules
Daunomycinone Energy (a.u.)
(1)
-1399.64379
Overlap population 05.** W08) 012*** H(Ol1) N5 * * * H(06) N12.m. H(Ol1)
0.099 0.049
Coefficients c5 05 Cl2 012
-1379.87012
12-@ino -1379.86323 0.098
0.048 0.153 0.064
Bond energy analysis, kcal mol-’ 05*** H(06) -84.1 012*.* H(Ol1) -52.0 N5 * * . NO61 N12 * * - H(Ol1) Net charges c5 05 Cl2 012 N5 N12 C6 06 Cl1 011
5-imino (2)
0.32 -0.40 0.30 -0.38
0.21 -0.53 0.22 -0.52 of the 2py and 2pz AOs in the LUMO -0.15, 0.24 -0.18,0.32 -0.19,0.25 -0.26,0.34
-84.1 -52.2 -104.0 -57.4 0.15 0.30 -0.38 -0.47 0.20 -0.54 0.22 -0.53
0.32 -0.40 0.11
-0.40 0.20 -0.53 0.21 -0.53
(3)
353
5-iminodaunomycinone is 4.3 kcal mol-’ more stable than 12-iminodaunomycinone. A comparison of the overlap between atomic orbitals of N5 - - * H(06) and N12 - * * H(Ol1) indicates that part of the extra stabilization energy of (2), as compared to (3), might be derived from a stronger hydrogen bond in the first molecule. Table 1 also gives the values of the overlap population for the hydrogen bonds involving 05 and 012. The orbital superposition in the pairs 05 * - - H(06) and 012.. - H(Ol1) in unsubstituted daunomycinone is quite different, and does not change on passing to the two imino derivatives. The N * - * H(0) overlaps are stronger than the corresponding 0 * - * H(0) overlaps, the increment amounting to 0.054 in (2) and 0.015 in (3). It has been reported [2] that the chemical shift of the H(06) proton decreases on going from daunorubicin to its 5-imino derivative and this is indicative of an increased, shielding of H(06) in the latter compound, in keeping with the calculated overlap increase. The interaction energy is proportional to the orbital superposition and in fact the pairwise partition of the total energy, carried out with Clementi’s Bond Energy Analysis formalism [7] , gives an interaction energy of -104 kcal mol-’ for the N5 - - * H(06) bond vs -84 kcal mol-’ for 05 *-a H(06) and -57 kcal mol-’ for N12 - * - H(Ol1) vs. -52 kcal mol-’ for 012 +- * H(Ol1). Hence the different stability of the two products could favour the formation of the 5-imino derivative in a thermodynamically controlled reaction. However, the different stability is not the only difference between these two molecules. If we turn our attention to the net charges and to the A0 distribution in the two frontier orbitals, HOMO and LUMO, of the parent compound daunomycinone, we observe that the main contributions to the LUMO (coefficients > 0.2) are from C3, C4A, C5, 05, C6A, C12, C12A and 012. In the reaction leading from daunorubicin to the imino derivative, the most probable site of nucleophilic attack will be the atom, among those that contribute to the LUMO, which bears the highest positive charge; the negative charge of the nucleophile will be accommodated in the LUMO, most probably on the atom(s) with the highest coefficients in the LCAO. Now, C5 has slightly higher net charge than Cl2 and the (X2=012 carbonyl has the largest contribution to the LUMO (see Table 1). Therefore, C5 should be the favoured site for the nucleophilic attack and the negative charge should be born mainly by the carbonyl12. The lower propensity of (2) to undergo reduction (Ep = -625 mV vs. Ag/AgCl, phosphate buffer, pH = 7.4) [8] might also be related to the lower stability of the partially or fully reduced species, i.e., semiquinone or hydroquinone. The interest in these species stems from the results of Davies and Doroshow [3, 41 and is related to the totally different behaviour of iminodaunorubicin and daunorubicin in the redox processes in the heart cell mitochondria. We have regarded the electronic distribution and the stability of negatively charged daunomycinone derivatives as models of the negatively charged
354
species that are formed in reductive processes. For this purpose, we have added a pair of electrons to the neutral molecules, and recalculated the Hartree-Fock energy and other electronic properties. It is known [5] that the reduction potential is higher for the 5-imino derivative than for daunorubicin, the difference being 3 kcal mol-’ if we refer to the half gap LUMOHOMO and 8 kcal mol-’ if we just consider the LUMO energy. Table 2 shows the energetic parameters for the dianionic species: the total energies are of course higher than those of the neutral molecules and the HOMOs have positive energy. The energy difference between the fully oxidized and the fully reduced molecule is 35.0 kcal mol-’ in (l), 46.4 kcal mol-’ in (2) and 44.9 kcal mol-’ in (3). As expected, the added electron pair is distributed between the two carbonyl groups, but 012 is slightly more negative and the HOMO has higher coefficients on the carbonyl 12 than on the carbonyl 5. The same remarks on net charges and HOMO gross atomic population hold for the imino derivatives. It is noticeable that the HOMO in the charged species has roughly the same composition as the LUMO in the neutral molecule. Hence, not only the 5-imino derivative of daunorubicin is less easily reduced than daunorubicin itself, but also the corresponding dianion, postulated in a redox cycle, is much less stable than daunorubicin dianion; as a consequence, the gap of 11.4 kcal mol-’ in stability, plus the larger energy needed in the reduction reaction, could lead to different cellular metabolism for the imino derivative and daunorubicin. In conclusion, it is also clear from these remarks on the reactivity that the mode of action of the antitumor agents should involve electron transfer reactions in vivo which, however, have an energy requirement compatible with biological mechanisms with which they, in this way, are able to interfere. TABLE 2 Outcome of ab initio computations on dianions Daunomycinone Energy (a.u.) E (red - ox), kcal mol-’ HOMO, kcal mol-*
-1399.58849 35.0 62.4
Gross atomic population 05 N5 012 N12
on the HOMO 0.24
Net charges 05 N5 012 N12
0.32
(1)
5imino (2) -1379.79628 46.4 66.8
12-imino -1379.79172 44.9 65.4 0.23
0.27 0.32 0.39
-0.58 -0.60
-0.62 -0.60
-0.58
-0.59
(3)
355 ACKNOWLEDGEMENT
One of us (V.M.) acknowledges partial financial support by the National Research Council of Italy, Special Project “Oncology”, contract No. 86. 00495.44. REFERENCES 1 2 3 4 5
L. Gianni, B. J. Corden and C. E. Myers, Rev. Biochem. Toxicol., 5 (1983) 1. G. L. Tong, D. W. Henry and E. M. Acton, J. Med. Chem., 24 (1981) 669. K. J. A. Davies and J. H. Doroshow, J. Biol. Chem., 261 (1986) 3060. J. H. Doroshow and K. J. A. Davies, J. Biol. Chem., 261(1986) 3068. G. Ranghino, C. Tosi, V. Malatesta and N. Sacchi, J. Mol. Struct. (Theochem), (1987) 287. 6 L. Gianolio, R. Pavani and E. Clementi, Gazz. Chim. Ital., 108 (1978) 181. 7 G. Corongiu and E. Clementi, Gazz. Chim. Ital., 108 (1978) 273. 8 V. Malatesta and N. Sacchi, unpublished results.
151
152 (1987) 351-355 -Printed in The Netherlands
Short communication
IMINO DERIVATIVES OF ANTHRACYCLINES: ELECTRONIC PROPERTIES, RELATIVE STABILITIES AND RELATIONSHIP TO ACTIVITY
VINCENZO Farmitalia
MALATESTA Carlo Erba, Via dei Gmcchi 35, Z-20146 Milan0 (Italy)
GRAZIELLA Zstituto
RANGHINO
Guido Donegani,
(Received
and CAMILLO TOSI Via G. Fauser 4, Z-281 00 Novara (Italy)
30 January 1987)
The anthracyclines comprise a large number of hydroxyquinones differing by the substitution pattern and the spectrum of action on neoplasies ranging from leukemias to solid tumors. A serious drawback with this class of drugs is the dose-dependent cardiac toxicity which results in severe damage to mitochondria and sarcoplasmic reticulum [ 11. The anthracycline structure has been modified with the aim of increasing potency and lowering cardiotoxicity. Particularly important has been the substitution of the C=N-H group for the C5=0 quinone carbonyl [2] : the 5-imino compound is indeed endowed with low cardiotoxicity, albeit reduced activity. In two recent papers [3, 41 Davies and Doroshow have emphasized the importance of knowing the redox properties of these drugs in order to assess the relevance of their involvement in mitochondrial electron transfer processes. They have for instance shown that the 5-imino derivative, in marked contrast with doxorubicin and daunorubicin, exhibits little or no tendency to undergo reduction by mitochondrial NADH dehydrogenase. It would be interesting to assess the properties of the other possible imino derivative of daunorubicin, viz. the 12-iminodaunorubicin. Up till now, however, this compound has not been reported, nor is there any indication of its formation during the synthesis of the 5-imino derivative [2]. We were puzzled by such a result and, having recently carried out extensive ab initio calculations on anthracyclines [ 51, looked for peculiar differences in the electronic structure of the two imino derivatives of daunorubicin, as they result from all-electron SCF-HF ab initio calculations with a minimal basis set of Gaussian orbitals [6] . Figure 1 shows the structure of the daunorubicm aglycone, i.e., daunomycinone (1). In the two possible imino derivatives (2) and (3), either the C5=05 or the C12==012 carbonyl group is replaced by C=N-H. The first row of Table 1 gives the total energies for the three compounds: as seen, 0166-1280/87/$03.50
0 1987 Elsevier Science Publishers B.V.
352
,CH3
CH30
R
OH
6H
Fig. 1. Molecular structure of daunomycinone (1: R = 0, Rl = 0), 5Gminodaunomycinone (2: R = NH, Rl = 0) and 12-iminodaunomycinone (3: R = 0, Rl = NH).
TABLE 1 Outcome of ab initio computations
on neutral molecules
Daunomycinone Energy (a.u.)
(1)
-1399.64379
Overlap population 05.** W08) 012*** H(Ol1) N5 * * * H(06) N12.m. H(Ol1)
0.099 0.049
Coefficients c5 05 Cl2 012
-1379.87012
12-@ino -1379.86323 0.098
0.048 0.153 0.064
Bond energy analysis, kcal mol-’ 05*** H(06) -84.1 012*.* H(Ol1) -52.0 N5 * * . NO61 N12 * * - H(Ol1) Net charges c5 05 Cl2 012 N5 N12 C6 06 Cl1 011
5-imino (2)
0.32 -0.40 0.30 -0.38
0.21 -0.53 0.22 -0.52 of the 2py and 2pz AOs in the LUMO -0.15, 0.24 -0.18,0.32 -0.19,0.25 -0.26,0.34
-84.1 -52.2 -104.0 -57.4 0.15 0.30 -0.38 -0.47 0.20 -0.54 0.22 -0.53
0.32 -0.40 0.11
-0.40 0.20 -0.53 0.21 -0.53
(3)
353
5-iminodaunomycinone is 4.3 kcal mol-’ more stable than 12-iminodaunomycinone. A comparison of the overlap between atomic orbitals of N5 - - * H(06) and N12 - * * H(Ol1) indicates that part of the extra stabilization energy of (2), as compared to (3), might be derived from a stronger hydrogen bond in the first molecule. Table 1 also gives the values of the overlap population for the hydrogen bonds involving 05 and 012. The orbital superposition in the pairs 05 * - - H(06) and 012.. - H(Ol1) in unsubstituted daunomycinone is quite different, and does not change on passing to the two imino derivatives. The N * - * H(0) overlaps are stronger than the corresponding 0 * - * H(0) overlaps, the increment amounting to 0.054 in (2) and 0.015 in (3). It has been reported [2] that the chemical shift of the H(06) proton decreases on going from daunorubicin to its 5-imino derivative and this is indicative of an increased, shielding of H(06) in the latter compound, in keeping with the calculated overlap increase. The interaction energy is proportional to the orbital superposition and in fact the pairwise partition of the total energy, carried out with Clementi’s Bond Energy Analysis formalism [7] , gives an interaction energy of -104 kcal mol-’ for the N5 - - * H(06) bond vs -84 kcal mol-’ for 05 *-a H(06) and -57 kcal mol-’ for N12 - * - H(Ol1) vs. -52 kcal mol-’ for 012 +- * H(Ol1). Hence the different stability of the two products could favour the formation of the 5-imino derivative in a thermodynamically controlled reaction. However, the different stability is not the only difference between these two molecules. If we turn our attention to the net charges and to the A0 distribution in the two frontier orbitals, HOMO and LUMO, of the parent compound daunomycinone, we observe that the main contributions to the LUMO (coefficients > 0.2) are from C3, C4A, C5, 05, C6A, C12, C12A and 012. In the reaction leading from daunorubicin to the imino derivative, the most probable site of nucleophilic attack will be the atom, among those that contribute to the LUMO, which bears the highest positive charge; the negative charge of the nucleophile will be accommodated in the LUMO, most probably on the atom(s) with the highest coefficients in the LCAO. Now, C5 has slightly higher net charge than Cl2 and the (X2=012 carbonyl has the largest contribution to the LUMO (see Table 1). Therefore, C5 should be the favoured site for the nucleophilic attack and the negative charge should be born mainly by the carbonyl12. The lower propensity of (2) to undergo reduction (Ep = -625 mV vs. Ag/AgCl, phosphate buffer, pH = 7.4) [8] might also be related to the lower stability of the partially or fully reduced species, i.e., semiquinone or hydroquinone. The interest in these species stems from the results of Davies and Doroshow [3, 41 and is related to the totally different behaviour of iminodaunorubicin and daunorubicin in the redox processes in the heart cell mitochondria. We have regarded the electronic distribution and the stability of negatively charged daunomycinone derivatives as models of the negatively charged
354
species that are formed in reductive processes. For this purpose, we have added a pair of electrons to the neutral molecules, and recalculated the Hartree-Fock energy and other electronic properties. It is known [5] that the reduction potential is higher for the 5-imino derivative than for daunorubicin, the difference being 3 kcal mol-’ if we refer to the half gap LUMOHOMO and 8 kcal mol-’ if we just consider the LUMO energy. Table 2 shows the energetic parameters for the dianionic species: the total energies are of course higher than those of the neutral molecules and the HOMOs have positive energy. The energy difference between the fully oxidized and the fully reduced molecule is 35.0 kcal mol-’ in (l), 46.4 kcal mol-’ in (2) and 44.9 kcal mol-’ in (3). As expected, the added electron pair is distributed between the two carbonyl groups, but 012 is slightly more negative and the HOMO has higher coefficients on the carbonyl 12 than on the carbonyl 5. The same remarks on net charges and HOMO gross atomic population hold for the imino derivatives. It is noticeable that the HOMO in the charged species has roughly the same composition as the LUMO in the neutral molecule. Hence, not only the 5-imino derivative of daunorubicin is less easily reduced than daunorubicin itself, but also the corresponding dianion, postulated in a redox cycle, is much less stable than daunorubicin dianion; as a consequence, the gap of 11.4 kcal mol-’ in stability, plus the larger energy needed in the reduction reaction, could lead to different cellular metabolism for the imino derivative and daunorubicin. In conclusion, it is also clear from these remarks on the reactivity that the mode of action of the antitumor agents should involve electron transfer reactions in vivo which, however, have an energy requirement compatible with biological mechanisms with which they, in this way, are able to interfere. TABLE 2 Outcome of ab initio computations on dianions Daunomycinone Energy (a.u.) E (red - ox), kcal mol-’ HOMO, kcal mol-*
-1399.58849 35.0 62.4
Gross atomic population 05 N5 012 N12
on the HOMO 0.24
Net charges 05 N5 012 N12
0.32
(1)
5imino (2) -1379.79628 46.4 66.8
12-imino -1379.79172 44.9 65.4 0.23
0.27 0.32 0.39
-0.58 -0.60
-0.62 -0.60
-0.58
-0.59
(3)
355 ACKNOWLEDGEMENT
One of us (V.M.) acknowledges partial financial support by the National Research Council of Italy, Special Project “Oncology”, contract No. 86. 00495.44. REFERENCES 1 2 3 4 5
L. Gianni, B. J. Corden and C. E. Myers, Rev. Biochem. Toxicol., 5 (1983) 1. G. L. Tong, D. W. Henry and E. M. Acton, J. Med. Chem., 24 (1981) 669. K. J. A. Davies and J. H. Doroshow, J. Biol. Chem., 261 (1986) 3060. J. H. Doroshow and K. J. A. Davies, J. Biol. Chem., 261(1986) 3068. G. Ranghino, C. Tosi, V. Malatesta and N. Sacchi, J. Mol. Struct. (Theochem), (1987) 287. 6 L. Gianolio, R. Pavani and E. Clementi, Gazz. Chim. Ital., 108 (1978) 181. 7 G. Corongiu and E. Clementi, Gazz. Chim. Ital., 108 (1978) 273. 8 V. Malatesta and N. Sacchi, unpublished results.
151