Design of Programmable Antitumor Drugs: Fatal Engineering

It is a goal of cancer chemotherapy to achieve the selective killing of tumor cells while minimizing toxicity to normal tissues. Our studies on cisplatin suggested a new approach toward the design of anticancer drugs. Specifically, we decided to synthesize a DNA binding molecule that will form adducts that, if not repaired, would inhibit DNA replication and transcription. To accomplish this objective, we would exploit the abundance of known tumor specific proteins to bias repair so that it would occur in normal cells and fail in tumor cells, thus making the latter preferentially succumb to the drug.

o test this new concept we designed a series of selective toxins that form DNA adducts attractive to the estrogen receptor (ER), a protein that is overly expressed in many human breast and ovarian tumors; this is the "tumor specific protein." The compounds consist of 4-(3-aminopropyl)-N,N-(2-chloroethyl)-aniline linked to 2-(4'-hydroxyphenyl)-3-methyl-5-hydroxy-indole. The former moiety is a DNA damaging nitrogen mustard and the latter is a ligand for the ER. The connection between these groups was refined to permit efficient covalent modification of DNA while preserving the ability of the ligand to interact with the ER, even when the mustard was bound to DNA. By using oligonucleotides containing specific DNA adducts, it was determined that monoadducts and putative intrastrand crosslinks were preferred targets for the ER over interstrand crosslinks. A series of structurally related 2-phenylindole-mustards was prepared, some of which were selectively toxic to MCF-7 breast cancer cells, which express the ER, as compared to MDA-MB231 cells, which do not. It was found that the ability both to bind to DNA and to interact significantly with the ER was essential in order to achieve selective lethality toward ER positive human cells. Compounds forming DNA adducts without the ability to attract the receptor showed equal toxicities in the two cell lines. Several models are consistent with the selective toxicity of the mustard-phenylindole compounds toward ER positive cells. The first model suggests that a mustard-DNA adduct is shielded by the ER from DNA repair enzymes and hence cells possessing an abundance of the ER selectively retain the adduct and are killed. A second model proposes that the DNA adducts formed act as decoy binding sites for the ER and thus prevent its aberrant functioning as a transcription factor in ER positive cells. Evidence was obtained to support the first proposed model in that DNA crosslinks of the phenylindole-mustard were retained longer in ER positive cells than in ER negative cells.

One of the attractive features of this approach toward therapeutics is the fact that the toxin can be re-programmed with ligands that attract other tumor specific proteins. We have begun to prepare molecules that form DNA adducts attractive to the androgen receptor, which is over expressed in prostate cancer cells. This overall strategy could be generalized to combat a wide range of cancers and possibly even viral diseases.