A 4-h posttreatment with 4 µm β-lapachone was previously shown to enhance the lethality of X-rays against human laryngeal epidermoid carcinoma (HEp-2) cells (D. A. Boothman et al., Cancer Res., 47: 5361–5366, 1988). We now show that β-lapachone (a) activates the DNA-unwinding activity of topoisomerase I, (b) inhibits the fast component of potentially lethal damage repair (PLDR) carried out by HEp-2 cells when present during or immediately following X-irradiation, (c) specifically and synergistically enhances the cytotoxic effects of DNA-damaging agents which induce DNA strand incisions, such as neocarzinostatin or X-rays, against a radioresistant human malignant melanoma (U1-Mel) cell line, (d) does not synergistically potentiate melphalan-induced lethality against U1-Mel cells but inhibits survival recovery and increases sister chromatid exchanges, and (e) does not further enhance the lethal effects of X-rays following prolonged drug exposures, indicating that β-lapachone modifies initially created DNA lesions or inhibits lesion repair but does not create lethal lesions by itself.

β-Lapachone accelerated the DNA-unwinding activity of topoisomerase I derived from avian erythrocytes, calf thymus, or HEp-2 cells. β-Lapachone did not intercalate into DNA, nor did it inhibit topoisomerase II or ligation carried out by mammalian or T4 DNA ligases. Structurally similar analogues, α-lapachone, lapachol, and dichloroallyl lawsone, did not enhance X-ray-induced cytotoxicity nor did they activate topoisomerase I. Camptothecin, a specific inhibitor of topoisomerase I, significantly radiosensitized HEp-2 cells, in a manner similar to β-lapachone. These results suggest a role of topoisomerase I in DNA repair.

The PLDR capacity of confluent-arrested HEp-2 cells was inhibited when β-lapachone was given immediately following or during X-irradiation. The effect decreased when the drug was added at later times. β-Lapachone may enhance lethality by converting single- into double-stranded DNA breaks during PLDR or through DNA conformational changes which inhibit PLDR. We propose that either mechanism of enhanced lethality may result from the ability of β-lapachone to activate topoisomerase I.

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Supported by Grant CA 22427 to A. B. P. from the National Cancer Institute and by a Biomedical Research Support Grant and Training Grant 5T32 CA 09361 to D. A. B.

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