Thresholds of O6-Alkylguanine-DNA Alkyltransferase which Confer Significant Resistance of Human Glial Tumor Xenografts to Treatment with 1,3-Bis(2-chloroethyl)-1-nitrosourea or Temozolomide1
- Demetrius M. Kokkinakis2,
- Dora B. Bocangel,
- S. Clifford Schold,
- Robert C. Moschel and
- Anthony E. Pegg
- Department of Neurosurgery, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235-8855 [D. M. K.]; University of Texas Medical Branch, Sealy Center for Molecular Science, Galveston, Texas 77555 [D. B. B.]; National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, Maryland 21702-1201 [R. C. M.]; Departments of Cellular and Molecular Physiology and Pharmacology, Pennsylvania State University, College of Medicine, The Milton Hershey Medical Center, Hershey, Pennsylvania 17033 [A. E. P.]; and University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213 [S. C. S.]
Abstract
Bis-2-chloroethylnitrosourea (BCNU) or temozolomide (TMZ) were tested alone or in combination with the AGT inhibitors O6-benzyl-2′-deoxyguanosine (dBG) or O6-benzylguanine (BG) against human glial tumor xenografts growing s.c. in athymic mice. Four glioblastoma (SWB77, SWB40, SWB39, and D-54) and one anaplastic oligodendroglioma (SWB61) xenografts having O6-alkylguanine-DNA alkyltransferase (AGT) activities of 75, 45, 10, <10, and 16 fmol/mg protein, respectively, were used. BCNU at 35 mg/m2 was ineffective against these tumors, although 70 mg/m2 (LD10, 75 mg/m2) produced a marked tumor growth delay (T-C) in D54 but had no effect against SWB40 or SWB77. Coadministration of BG or dBG and BCNU necessitated reduction of the BCNU dose to a maximum of 30 and 35 mg/m2, respectively, because of increased toxicity. Optimized treatment with dBG (250 mg/m2) and BCNU (35 mg/m2) resulted in T-Cs of 30, 29, 11, 16, and 14 days for SWB77, SWB40, SWB39, D-54 and SWB61, respectively. These delays were more pronounced than those induced with optimized, isotoxic treatments with BG (180 mg/m2) and BCNU (30 mg/m2). In comparison to BCNU, TMZ was less toxic, with an LD 10 of 400 mg/m2. TMZ (300 mg/m2) was more effective than BCNU against SWB77, SWB40, and SWB61, inducing T-Cs of 23, 53, and 56 days, respectively. BG and dBG enhanced the toxicity of TMZ in athymic mice by decreasing the LD10 from 400 to 200 mg/m2. TMZ (180 mg/m2) with either BG (180 mg/m2) or dBG (250 mg/m2) resulted in T-Cs of 31 and 49 days in SWB77, respectively, as compared with 16 days for TMZ (180 mg/m2) alone. In SWB40, the combination of TMZ with dBG, but not with BG, was significantly more effective than the maximum tolerated dose of TMZ (300 mg/m2) alone. The combination of TMZ with AGT inactivators had no benefit, as compared with TMZ alone, against xenografts with marginal AGT activity. In conclusion, at equimolar doses dBG was less toxic than BG in athymic mice when combined with either BCNU or TMZ. In this regard, BCNU or TMZ can be used at higher doses in combination with dBG than with BG. This study further demonstrates that there is a significant benefit of depleting AGT with nonspecific AGT inhibitors prior to treatment with either BCNU or TMZ in tumors having AGT activity >45 fmol/mg protein.
Footnotes
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵1 Supported by National Cancer Institute Grants CA 57725 and CA 78561 and the National Cancer Institute, Department of Health and Human Services, under contract with ABL and The Children’s Brain Tumor Foundation of the Southwest.
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↵2 To whom requests for reprints should be addressed, at Department of Neurological Surgery, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-8855. Phone: (214) 648-6314; Fax: (214) 648-2265; E-mail: dkokki{at}mednet.swmed.edu
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↵3 The abbreviations used are: BCNU, 1,3-bis(2-chloroethyl)-1-nitrosourea; AGT, O6-alkylguanine-DNA alkyltransferase; BG, O6-benzylguanine; dBG, O6-benzyl-2′-deoxyguanosine; MMR, mismatch repair; TMZ, temozolomide; PEG, polyethyleneglycol.
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↵4 D. B. Bocangel, S. Mitra, and D. M. Kokkinakis, unpublished observations.
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- Accepted November 9, 1900.
- Received July 21, 1900.
- Revision received November 9, 1900.
