16 T u m o r I n h i b i t i o n b y M e t a l l o c e n e D i h a l i d e s of Early Transition Metals Chemical and Biological Aspects H. KÖPF Technische Universität Berlin, Institut für Anorganische und Analytische Chemie, 1 Berlin 12, Federal Republic of Germany
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P. KÖPF-MAIER Freie Universität Berlin, Institut für Anatomie, 1 Berlin 33, Federal Republic of Germany 5
Metallocene dihalides, (η -C H ) MX , represent the first organometallic antitumor agents. Their anti tumor activity has been established in vitro and in vivo, e.g. against the Ehrlich ascites tumor, the lymphoid leukemia L1210, or the lymphocytic leuke mia P388. Considering the structure-activity rela tion three statements can be made: (i) There ex ists a strong dependence of the cytostatic activity of the metallocene dichlorides (C H ) MCl upon the central metal atom M. ( i i ) Within the titanocene dihalide system (C H ) TiX the Cl ligands can be replaced by other halide or pseudohalide ligands without reduction of the tumor-inhibiting potency, (iii) Chemical modification of the cyclopentadienyl rings, e.g. mono- or 1,1'-disubstitution with orga nic residues, leads to a diminution of the cytosta tic activity in dependence on the degree of substi tution. With regard to the mechanism of action of the metallocene dihalides, precursor incorporation studies indicate an attack within the nucleic acid metabolism. This is confirmed by cytokinetic and by microanalytical (electron energy loss-spectrosco pic) investigations as well as by the light-micro scopic and ultrastructural alteration patterns of treated tumor cells. 5
5 2
2
5
5
5 2
5 2
2
2
The development of the inorganic compound cis-diamminedichloroplatinum(II) (DDP) to a potent antitumor drug during the last twelve years, leading to a valuable enrichment of the thera peutic arsenal of cytostatic drugs (J^, 2), has strongly stimula ted the search for other metal-containing tumor-inhibiting spe cies. Thus, the antitumor activity of certain representatives of the metallocene dihalides was detected three years ago (3). Since then, the cytostatic behavior of the metallocene dihalides has 0097-6156/83/0209-0315$06.00/0 © 1983 American Chemical Society Lippard; Platinum, Gold, and Other Metal Chemotherapeutic Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
316
METAL CHEMOTHERAPEUTIC AGENTS
been demonstrated at the experimental stage a g a i n s t v a r i o u s a n i mal tumor systems as w e l l as a g a i n s t i n v i t r o c u l t u r e d c e l l s . The metallocene d i h a l i d e s represent the f i r s t example of a group of o r g a n o m e t a l l i c compounds which e x h i b i t canceros t a t i c p r o p e r t i e s by t h e i r own. Both c o n t a i n i n g a c i s - d i h a l o m e t a l moiety, the metallocene d i h a l i d e s a r e , i n c o n t r a s t to the tumori n h i b i t i n g i n o r g a n i c noble metal complexes of the p l a n a r L2MX2 type, t e t r a h e d r a l l y coordinated and are c h a r a c t e r i z e d by e a r l y t r a n s i t i o n metals f e a t u r i n g as the c e n t r a l atoms M and r r - c y c l o p e n t a d i e n y l r i n g s as the l i g a n d s L.
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Structure-Activity Relation To study the s t r u c t u r e - a c t i v i t y r e l a t i o n of the metallocene d i h a l i d e s , the i n f l u e n c e of chemical v a r i a t i o n upon the tumor-inh i b i t i n g a c t i v i t y was i n v e s t i g a t e d . P r i n c i p a l l y , a metallocene d i h a l i d e molecule o f f e r s t h f e e d i f f e r e n t p o s i t i o n s to be m o d i f i e d (Figure 1): ( i ) At the p o s i t i o n of the c e n t r a l metal atom M, one of e i g h t d i f f e r e n t e a r l y t r a n s i t i o n metals of the t i t a n i u m , vanadium, and chromium subgroups of the P e r i o d i c Table can be i n t r o duced, ( i i ) The p o s i t i o n s of the acido l i g a n d s X can be occupied by v a r i o u s h a l i d e as w e l l as pseudohalide l i g a n d s . ( i i i ) S i n g l e hydrogen atoms of the two c y c l o p e n t a d i e n y l r i n g s may be r e p l a c e d by other groups i n d i f f e r e n t s u b s t i t u t i o n modes such as monosubs t i t u t i o n , 1 , 1 ' - d i s u b s t i t u t i o n , or b r i d g i n g of the two r i n g s by a b i f u n c t i o n a l group. Studies on the s t r u c t u r e - a c t i v i t y r e l a t i o n of the m e t a l l o c e ne d i h a l i d e s were g e n e r a l l y performed u s i n g as experimental system the E h r l i c h a s c i t e s tumor (EAT) growing i n female CF1 mice. The t h e r a p e u t i c e f f e c t was judged by determining the mean s u r v i v a l time (MST) and by c a l c u l a t i n g the i n c r e a s e i n l i f e span (ILS) compared to the untreated c o n t r o l s . The key-date f o r the s u r v i v a l s t u d i e s was day 180 a f t e r tumor t r a n s p l a n t a t i o n (Table I , I I I ) or day 60 (Table V I I I ) . V a r i a t i o n of the Metal Atom. C o n s i d e r i n g the i n f l u e n c e of the c e n t r a l metal M w i t h i n the metallocene d i c h l o r i d e system ^ I ^ ^ M C ^ upon the t u m o r - i n h i b i t i n g a c t i v i t y , there i s a s t r o n g dependence of the antitumor a c t i o n upon the metal atom present C 3 - 7 ) . Whereas the complexes c o n t a i n i n g the f i r s t and second row t r a n s i t i o n metals T i , V, Nb, or Mo e f f e c t marked tumor i n h i b i t i o n w i t h d i s t i n c t d o s e - a c t i v i t y r e l a t i o n s and w i t h cure r a t e s of 100 % at optimum dose (Table I ) , the metallocene d i c h l o r i d e s of the h i g h e r homologues Ta or W only s p o r a d i c l y induce the s u r v i v a l of t r e a t e d tumor-bearing mice. The extreme case of t h i s tendency i s reached at the metallocene d i c h l o r i d e s of Zr and Hf, f o r which no t u m o r - i n h i b i t i n g p r o p e r t i e s could be observed a g a i n s t EAT. Regarding the p o s i t i o n of the c e n t r a l metal atoms i n the Per i o d i c Table, a d i a g o n a l r e l a t i o n T i - Nb and V - Mo i s obvious of those metals which d i s p l a y as c e n t r a l atoms i n metallocene d i c h l o r i d e s marked c a n c e r o s t a t i c a c t i v i t y ( 6 ) . This corresponds to
Lippard; Platinum, Gold, and Other Metal Chemotherapeutic Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
16.
KOPF AND KÔPF-MAIER
317
Metallocene Dihalides
Figure 1. Molecular structure of the metallocene dihalides, fa -C H ) MX . The arrows indicate the three positions that can be chemically modified. 5
s
s
2
t
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Table I Pharmacological data of metallocene d i c h l o r i d e s
M = Ti
b)
Réf.
Optimum dose
Cure r a t e at o p t i mum dose
ILS at o p t i - ν mum dose
(mg/kg)
(%)
(%)
(mg/kg)
30-60
80-90
842-951
100
3.3
75
->
(4)
220
J*>
(4)
c)
T I L D
50
d
M - Zr .d)
M = Hf
(3,4)
M =V
80-90
100
984
110
1.4
M = Nb
20-25
1Q0
1076
35
3.5
(6)
M - Ta
80-220
220
.e)
-
M - Mo
75-100
175
2.9
M =W
100-525
e)
570 100 e)
f )
1039 360
f )
500
(6)
a) I L S (increase i n l i f e span) values are defined as p e r c e n t u a l increase of the mean s u r v i v a l time (MST) of a dose group r e l a t e d to the untreated tumor-bearing c o n t r o l group (key-date: day 180). With T i , V, Nb, Mo i n optimum doses, the MST values are s i g n i f i c a n t l y d i f f e r e n t from c o n t r o l values (Wilcoxon-Mann-Whitney Us t a t i s t i c s ) . b) Τ I (therapeutic i n d e x ) , defined as the r e l a t i o n LD50/ED75. c) 100 % a f t e r a p p l i c a t i o n i n buffered s o l u t i o n (17). d) No cures o r any therapeutic e f f e c t observed, e) Only sporadic cures w i t h i n the i n d i c a t e d dose range, f ) Maximum v a l u e .
Lippard; Platinum, Gold, and Other Metal Chemotherapeutic Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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METAL CHEMOTHERAPEUTIC AGENTS
the s i m i l a r i t y of atomic r a d i i w i t h i n a diagonal p a i r of e l e ments, e.g. T i (1.32 Â) and Nb (1.34 Â ) , and p o i n t s to the s i g n i f i c a n c e of e f f e c t s of s i z e f o r the mechanism of a c t i o n . Thus, the s i z e of M i n f l u e n c e s the C l - M - C l bonding angle, the M-CI bonding d i s t a n c e and the non-bonding Cl...CI d i s t a n c e ( b i t e ) w i t h i n the d i c h l o r o m e t a l moiety of the complexes. These s t r u c t u r a l parameters are given i n Table I I f o r metallocene d i c h l o r i d e s (8 - 11) and f o r the i n o r g a n i c c y t o s t a t i c drug DDP (12). For DDP the C l . . . C I b i t e amounts to 3.35 Â and corresponds to the d i s t a n c e between two appropriate DNA-base donor atoms of a DNA h e l i x (13) i n a manner that c r o s s - l i n k s can be b u i l t up a f t e r d i s s o c i a t i o n of the CI atoms (13 - 16). I f the s i m p l i f i c a t i o n i s admitted that the b i t e between the CI atoms as the l e a v i n g groups i s d i r e c t l y c o r r e l a t e d to the d i s t a n c e of adjacent c o o r d i n a t i o n s i t e s at the metal atom against another given p a i r of entrant l i g a n d s , the p o s s i b i l i t y of a s i m i l a r molecular a t t a c k f o r complexes e x h i b i t i n g s i m i l a r b i t e values has to be taken i n account. I t i s remarkable that w i t h a l l t u m o r - i n h i b i t i n g m e t a l l o cene d i c h l o r i d e s t h i s b i t e i s of a s i m i l a r s i z e as w i t h DDP, i . e . the value i s at the most by 0.12 Â higher or 0.11 Â lower than the b i t e of DDP. In c o n t r a s t , the zirconocene and hafnocene d i c h l o r i d e s , which do not induce a n t i p r o l i f e r a t i v e a c t i v i t y , are c h a r a c t e r i z e d by a b i t e which i s markedly (by 0.25 or 0.31 Â) l a r g e r than that of DDP. These two C l . . . C I d i s t a n c e s exceed a c r i t i c a l value l y i n g between 3.50 and 3.60 Â which seems to be an upper l i m i t f o r the formation of DNA-metal c r o s s - l i n k s , and are t h e r e f o r e p o s s i b l y r e s p o n s i b l e f o r the t h e r a p e u t i c i n e f f e c t i v i t y of zirconocene and hafnocene d i c h l o r i d e s . From these c o n s i d e r a t i o n s i t can be deduced that the c i s - d i chlorometal moiety w i t h i n the metallocene d i h a l i d e s i s apparently of c r i t i c a l s i g n i f i c a n c e f o r the mechanism of a c t i o n . The format i o n of adjacent, perhaps c h e l a t i n g bonds to b i o l o g i c a l macromol e c u l e s a f t e r d i s s o c i a t i o n of the h a l i d e l i g a n d s seems to be an important step. V a r i a t i o n of the Acido Ligands. In c o n t r a s t to the v a r i a t i o n of the c e n t r a l metal atoms, the exchange of the CI l i g a n d s i n the titanocene model system (05115)2^X2 by the h a l i d e or pseudohalide l i g a n d s X • F, Br, I , NCS, N3 does not a f f e c t the a n t i tumor a c t i v i t y (17). Table I I I shows that a l l i n v e s t i g a t e d compounds l e a d to cure r a t e s of 100 % when a p p l i e d i n optimum doses. This means that the antitumor a c t i v i t y of the metallocene species i s independent of the h a l i d e l i g a n d X; provided that a c e n t r a l metal i s present which causes marked antitumor a c t i v i t y , the a c i do l i g a n d s can apparently be widely v a r i e d without l o s s or d i m i n u t i o n of the c a n c e r o s t a t i c e f f e c t i v i t y . In some cases, e s p e c i a l l y i n the case of X - Br, the t h e r a p e u t i c range i s even enlarged l e a d i n g to an increase of the t h e r a p e u t i c index (Τ I) to 4.5, compared to 3.3 a f t e r treatment w i t h titanocene d i c h l o r i d e . Moreover, the a p p l i c a t i o n of the titanocene d i h a l i d e s i n so-
Lippard; Platinum, Gold, and Other Metal Chemotherapeutic Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
16.
KÔPF A N D KÔPF-MAIER
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319
Metallocene Dihalides
II
S t r u c t u r a l parameters of DDP and some metallocene d i c h l o r i d e s
(C H ) MoCl 5
5
2
2
(C H CH ) VC1 5
4
3
2
2
cis-(NH ) PtCl 3
2
2
b )
Réf.
Cl-M-Cl
Cl...Cl
(Â)
(°)
(Â)
Mo
2.471(5)
82.0(2)
3.242
(8)
V
2.398(2)
87.06(9)
3.303
(9)
Pt
2.330(1)
91.9(3)
3.349
(12)
M
Compound
M-Cl
a )
(C H ) NbCl
2
Nb
2.470(4)
85.6(1)
3.356
(8)
(C H ) TiCl
2
Ti
2.364(3)
94.43(6)
3.470
(10)
Hf
2.423(3)
95.87(8)
3.598
