Radiation sensitizers

The present invention relates to a method of potentiating radiation therapy using Co(III) complexes of water soluble, synthetic meso-substituted porphyrins. The method results in the sensitization of both well oxygenated (oxic) and oxygen deficient (hypoxic) tumor cells.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates, in general, to radiosensitization and, in 
particular, to a method of potentiating radiation therapy using Co(III) 
complexes of water soluble, synthetic porphyrins. 
2. Background Information 
The use of porphyrin derivatives in combination with visible light as a 
treatment for cancer is well established. The rationale behind this 
treatment is that certain porphyrins accumulate in neoplastic tissue; when 
these porphyrins are exposed to visible light, highly reactive chemical 
species are formed that destroy the tumor. 
One drawback to this "photodynamic therapy" is that deep seated tumors in 
internal organs are difficult to irradiate with visible light. This 
problem can be overcome, however, by the use of ionizing radiation, which 
penetrates tissues more readily than visible light, in conjunction with 
porphyrins that potentiate the effects of ionizing radiation on tumors. 
Several attempts have been made to use porphyrins and metalloporphyrins as 
radiosensitizers. Hematoporphyrin has been shown to potentiate the 
effectiveness of X-rays in treating rhabdomyosarcoma in mice. While little 
or no beneficial effect has been seen in human patients administered 
hematoporphyrin, metal complexes of meso-tetra(p-carboxyphenyl)porphine 
(particularly the Zn and Ni complexes) have been found to be 
radiosensitizers of human tumor cells in vitro 
Most of the radiosensitizers described in the literature sensitize only 
hypoxic (oxygen deficient) cells. Since some tumors are thought to have 
regions of hypoxic tissue which are insensitive to the effects of ionizing 
radiation, these radiosensitizers are therapeutically beneficial because 
they potentiate the radiation killing of hypoxic cells but do not 
sensitize normal tissue which is well oxygenated. These compounds, 
however, are not useful in sensitizing oxic (well oxygenated) tumor cells. 
SUMMARY OF THE INVENTION 
It is a general object of the invention to obviate or minimize the 
objections to prior art radiosensitizers. 
It is a particular object of the invention to provide a method for 
sensitizing both oxic and hypoxic cancer cells to ionizing radiation. 
Further objects and advantages of the present invention will be apparent 
from the following detailed description of the species thereof. 
The foregoing objects of the invention are achieved by the administration, 
together with a pharmaceutically acceptable carrier, of Co (III) complexes 
of water soluble, synthetic meso-substituted porphyrins of the general 
formula (I): 
##STR1## 
where X= 
##STR2## 
or the corresponding compound with the N in the 2 or 3 position; 
##STR3## 
or the corresponding compound with the N in the 2 or 3 position, where R=a 
C.sub.2 -C.sub.6 n-alkyl, advantageously, R=n-C.sub.4 H.sub.9 ; 
##STR4## 
or the corresponding compound with the N in the 2 or 3 position, where 
n=1-4 and R=methyl, ethyl, or hydrogen; 
##STR5## 
where R=C.sub.3 -C.sub.6 n-alkyl; 
##STR6## 
where Y=H, Cl, Br, or NO.sub.2 ; or 
##STR7## 
or the corresponding compound with the N in the 2 or 3 position. These 
compounds accumulate in cancer cells and interact with ionizing radiation 
administered to those cells, thereby increasing cell kill during cancer 
treatment. These cobalt complexes of porphyrins sensitize both oxic and 
hypoxic cells.

EXAMPLE 1 
Preparation of [Co(tetra-(trimethylaminophenyl)porphine)] [BF.sub.4 ].sub.5 
or Co(TMAP) (W. Szulbinski, M. Lapkowski, Inorg. Chim. Acta., 123, 
127-133, 1986). 
Two hundred and fifty milligrams of [tetra(trimethylaminophenyl)porphine] 
[tosylate].sub.4 (Strem Chemicals) were dissolved in water (50 ml) and 24 
g of CoCl.sub.2.6H.sub.2 O were added. The mixture was refluxed overnight. 
The mixture was cooled to room temperature and excess of NH.sub.4 PF.sub.6 
was added to precipitate a red-violet solid. This material was treated 
with anion exchange resin (Dowex 2-X8) in the chloride form and the 
resulting solution of Co(TMAP)(Cl.sub.5) was freeze-dried to yield 0.39 g 
of solid. This material was dissolved in water and treated with a 20% 
Zn(BF.sub.4).sub.2 solution to precipitate 0.3 g of [Co(TMAP)][BF.sub.4 
].sub.5.6H.sub.2 O. 
______________________________________ 
C H N 
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Analysis % Calc. 46.47 5.02 7.75 
% Found 46.41 4.99 8.44 
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EXAMPLE 2 
The product of Example 1 and metal complexes of 
tetra(4-N-methylpyridyl)porphine (TMPyP) and tetraphenylporphinesulfonate 
(TPPS), were examined for radiosensitizing activity in vitro. 
[Co(tetra(4-N-methylpyridyl)porphine) [tosylate].sub.5 (Co(TMPyP)) and 
Na.sub.3 [Co(tetraphenylporphinesulfonate)] (Co(TPPS)) were purchased from 
Porphyrin Products, Logan, Utah. 
The results are shown in Table I. Cobalt complexes display activity 
superior to other metal complexes and thus may be effectively used as 
radiosensitizers in combination with irradiation. 
TABLE I 
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SER 
(hypoxic SER 
Compound Conc. (mm) V79 cells) (oxic V79 cells) 
______________________________________ 
Co(TMPyP) 100 1.4 1.3 
TMPyP 100 1.1 1.0 
Fe(TMPyP) 100 1.3 1.0 
Cu(TMPyP) 100 1.3 1.3 
Zn(TMPyP) 100 1.3 1.2 
Co(TPPS) 25 2.1 1.3 
TPPS 100 1.2 1.0 
Fe(TPPS) 100 1.2 1.2 
Co(TMAP) 100 1.5 1.1 
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EXAMPLE 3 
Preparation of Co(tetra(4-N-n-butylpyridyl)porphine (metal free version 
described by V. N. Madakyan, et al, Arm. Khim. Zh. 38(6): 391-396 (1985)). 
An aqueous solution of 2.38 g CoCl.sub.2.2H.sub.2 O and 0.57 g 
tetra(4-N-n-butylpyridyl)porphine was refluxed in air for 21 h. A filtered 
solution of 4g NaBF.sub.4 in 30ml H.sub.2 O was added while the above 
solution was still hot. This solution was cooled to 5.degree. C. for 24 
hours during which time the product precipitated. The product was 
collected by vacuum filtration. The precipitate was redissolved in 
acetone/H.sub.2 O containing a small amount of NaBF.sub.4 and placed in an 
open beaker. As the acetone evaporated, dark purple needles slowly 
deposited and were collected by vacuum filtration. 
Yield: 0.5 g of [Co(tetra(4-N-nbutylpyridyl)porphine)] 
Elemental analysis for C.sub.56 H.sub.66 B.sub.5 CoF.sub.20 N.sub.8 O.sub.3 
: 
Calculation: C 48.32, H 4.78, N 8.05 
Found: C 48.11, H 4.87, N 8.08 
It will be recognized that various modifications may be made in the 
invention as described above. Accordingly, the scope of the invention is 
defined in the following claims wherein: