Cancer chemotherapy is based on the use of drugs that are selectively toxic (cytotoxic) to cancer cells (also referred to herein as tumorigenic cells, transformed cells or neoplastic cells). Harrison's Principles of Internal Medicine, Part 11 Hematology and Oncology, Ch. 296, 297 and 300-308 (12th ed. 1991). A preferred class of chemotherapeutic drugs inflict damage on cellular DNA. Drugs of these classes generally are referred to as genotoxic. Two widely used genotoxic anticancer drugs are cisplatin [cis-diamminedichloroplatinum(II)] and carboplatin [diaminine(1,1-cyclobutane dicarboxylato)platinum(II)]. Bruhn et al. (1990), 38 Prog. Inorg. Chem. 477, Burnouf et al. (1987), 84 Proc. Natl. Acad. Sci. USA 3758, Sorenson and Eastman (1987), 48 Cancer Res. 4484 and 6703, Pinto and Lippard (1985), 82 Proc. Natl. Acad. Sci., USA 4616, Lim and Martini (1984), 38 J. Inorg. Nucl. Chem. 119, Lee and Martin (1976), 17 Inorg. Chim. Acta 105, Harder and Rosenberg (1970), 6 Int. J. Cancer 207, Howle and Gale (1970), 19 Biochem. Pharmacol 2757. Cisplatin and/or carboplatin currently are used in the treatment of selected, diverse neoplasms of epithelial and mesenchymal origin, including carcinomas and sarcomas of the respiratory, gastrointestinal and reproductive tracts, of the central nervous system, and of squamous origin in the head and neck. Harrison's Principles of Internal Medicine (12th ed. 1991) at Ch. 301. Cisplatin currently is preferred for the management of testicular carcinoma, and in many instances produces a lasting remission. Loehrer and Einhorn (1984), 100 Ann. Int. Med. 704. Susceptibility of an individual neoplasm to a desired chemotherapeutic drug or combination thereof often, however, can be accurately assessed only after a trial period of treatment. The time invested in an unsuccessful trial period poses a significant risk in the clinical management of aggressive malignancies.
U.S. Pat. No. 5,359,047 (the teachings of which are incorporated herein by reference) recounts the discovery that eukaryotic cells contain one or more intracellular structure specific recognition proteins, or SSRPs, that bind to 1,2-dinucleotide intrastrand adducts formed in cellular DNA by therapeutically useful genotoxic metal coordination compounds, such as platinum(II) and platinum(IV) compounds. The class of genotoxic noble metal coordination compounds that form SSRP-recognized genomic lesions includes cisplatin (cis-diamminedichloroplatinum(II) or cis-DDP), carboplatin (diammine(1,1-cyclobutane-dicarboxylato)platinum(II), cis-diamminetetrachloroplatinum(IV), iproplatin (CHIP), DACCP, malonatoplatin, cis-dichloro(ethylenediamine)platinum(II), cis-dichloro(1,2-diaminocyclohexyl)platinum(II), and the like. For convenience, SSRP recognized 1,2-intrastrand dinucleotide adducts formed by any member of this class are referred to herein as cisplatin-type lesions (or adducts). Such lesions are formed by substitution of the cis-configured labile moieties of the genotoxin by atoms of the nucleotide bases, usually adenosine (A) or guanosine (G) residues. This produces a crosslink, bridged by the noble metal atom (e.g., platinum) between two vicinal, adjacent or paired nucleotide bases. Platinum-bridged crosslinks between adjacent adenosine and guanosine residues within a single polynucleotide strand (1,2-intrastrand dinucleotide adducts or lesions) of double stranded DNA are abbreviated herein as 1,2-d(A G) and 1,2-d(G G) lesions. The structural distortion associated with cisplatin-type genomic lesions produces a characteristic three-dimensional DNA structural motif, comprising a bend in the direction of the major groove.
The rate of cellular repair of genotoxin-induced DNA damage, as well as the rate of cell growth via the cell division cycle, affects the effectiveness of genotoxin therapy. Unrepaired lesions in a cell's genome can impede DNA replication, impair the replication fidelity of newly synthesized DNA or hinder the expression of genes needed for cell survival. Thus, one determinant of a genotoxic agent's cytotoxicity (propensity for contributing to cell death) is the resistance of genomic lesions formed therefrom to cellular repair. Genotoxic agents that form persistent genomic lesions, e.g., lesions that remain in the genome at least until the cell commits to the cell cycle, generally are more effective cytotoxins than agents that form transient, easily repaired genomic lesions. Hence, genotoxic agents that form persistent genomic lesions are preferred for use as chemotherapeutic agents in the clinical management of cancer.
When SSRP binds to a cisplatin-type genomic lesion, a DNA:protein complex is formed, such that the sterically large SSRP becomes localized in the immediate vicinity of the genomic lesion. This complex can be detected visually using techniques described in U.S. Pat. No. 5,359,047, including modified Western (Southwestern) blotting and electrophoretic mobility shift analysis (EMSA, also known as bandshift analysis). The SSRP is large enough to sterically obscure (cover) a region of cellular DNA extending from the lesion site in both the 5' and 3' direction for at least about five base pairs, and to shield the genomic lesion from repair by the cell's enzymatic DNA repair machinery. SSRP-shielded lesions persist in the genome longer than unshielded lesions. SSRP-shielded lesions accordingly are more effective for prejudicing the fidelity of DNA replication, hindering the expression of genes relevant to cell survival, and otherwise contributing to disarray of the cell's nuclear architecture. One or more of the foregoing can contribute to cell death, e.g., by triggering apoptosis. Indeed, there have been several reports in the literature that actively dividing cells that are unable to carry out DNA repair are extremely sensitive to cisplatin. Fraval et al. (1978), 51 Mutat. Res. 121, Beck and Brubaker (1973), 116 J. Bateriol 1247.
Most SSRPs reported thus far comprise at least one high mobility group, or HMG, structural domain. As described in U.S. Pat. No. 5,359,047, the HMG domain is involved in recognition of, and complex formation with, cisplatin-type DNA lesions. Thus, it is believed that all known and currently unknown cellular HMG domain proteins have SSRP activity for cisplatin-type lesions. Certain HMG domain proteins useful as SSRPs have been characterized in the literature as transcription factors that control or modulate the expression of one or more cellular genes, including genes that are relevant to cell metabolism or cell secretory function. Binding of such transcription factors to cisplatin-type lesions enhances cisplatin cytotoxicity, by disrupting cell metabolism and/or vital functions.
However, the DNA:protein complex formed by SSRP binding at the lesion site is non-covalent in nature. Thus, the SSRP-mediated repair shielding effect is dependent on non-covalent binding and is subject to dissociation of the DNA:protein complex.
Needs remain for increasing the incidence of selective cell death by increasing persistence of genomic lesions in tumorigenic cells. Needs remain also for enhancing effectiveness of chemotherapeutic drugs, such that satisfactory cell killing can be achieved with lower doses than are currently needed. Poignant needs remain for chemotherapeutic drugs with improved selectivity for destroying tumorigenic cells. Particularly poignant needs remain for ways to render tumorigenic cells selectively more vulnerable to killing through chemotherapy.