Carmustine and Streptozocin in Refractory Melanoma: An Attempt at Modulation of 06-Alkylguanine-DNA-alkyltransferase1

The activity of the enzyme 06-alkylguanine-DNAalkytransferase (AGAT) protects cells from the cytotoxic effects of alkylating agents. This Phase II trial was designed to assess the efficacy of a strategy designed to modulate the resistance to carmustine (BCNU) mediated by AGAT using streptozocin (STZ) in patients with advanced refractory melanoma. Seventeen patients who had failed prior chemotherapy were treated with STZ at 500 mg/rn2 daily for 4 days with BCNU at 150 mg/rn2 on day 3. Peripheral blood lymphocytes for assay of AGAT activity levels were collected prior to therapy and following the third dose of STZ. There were two partial responses in the 15 patients evaluable for response (13%). Most patients received only a single cycle of therapy due to rapidly progressive disease. Two patients developed fatal pulmonary toxicity, and one developed myelodysplasia. Other toxicities included transient rises in liver function tests. AGAT levels decreased by a mean of 53 % in 9 patients but actually increased over baseline in 3 patients while on therapy. Based on these data, BCNU and STZ are not an effective combination for the therapy of advanced refractory melanoma, and pulmonary toxicity due to this combination appears to be increased compared with BCNU alone. STZ is not an effective modulator of AGAT activity when given on this schedule. New strategies designed to deplete AGAT activity using 06-benzylguanine or ternozolornide should be explored with careful attention to the possibility that this approach may potentiate both the toxicity and efficacy of BCNU. INTRODUCTION Alkylating agents are among the most potent antineoplastic drugs in experimental systems and clinical practice. The nitroReceived 1/3 1/96; revised 3/29/96: accepted 4/5/96. I This work was supported in part by Grants 5P30-CA-479()4-07. UOICA75525, and P01-43703 from the NIH and the National Cancer Institute. 2 To whom requests for reprints should be addressed. at University of Michigan Medical Center. Division of Hematology/Oncology. 31 19 Taubrnan Center, 0374, 1500 East Medical Center Drive. Ann Arbor, MI 48109-0374. Phone: (313) 936-5281; Fax: (313) 936-7376. soureas are classic alkylating agents, which have shown substantial promise in preclinical evaluations ( 1 ). They have been incorporated into a variety of chemotherapeutic regimens used in the treatment of lymphoma, myelorna, lung and colon cancer. melanoma, and brain neoplasms (2). Unfortunately. most tumors eventually develop resistance to the cytotoxic effects of the nitrosoureas. One mechanism of this resistance is mediated by the enzyme AGAT3 (3, 4). The 06 position of guanine is a preferred site of activity for the chloroethylnitrosoureas such as BCNU, which initially attaches an alkyl group at the (/‘ position, followed several hours later by the formation of a crosslink with the cytidine of the complementary DNA strand (S. 6). During this period, the alkyl group is vulnerable to the activity of AGAT. This enzyme acts as a suicide enzyme, covalently binding alkyl and methyl groups attached to the O position and removing them (7). In doing so, the enzyme is depleted. resulting in increased vulnerability of cells to the cytotoxic activity of alkylating agents. Levels of AGAT are variable in tumor tissues, and expression can be induced by exposure to alkylating agents (3, 8). One strategy to improve the efficacy of alkylating agents is based on this observation. Agents such as STZ efficiently methylate the 06 position of guanine (9). This methyl group can be removed from DNA by AGAT, depleting enzyme activity. In vitro, BCNU-resistant HT-29 cells are sensitized by pretreatment or concurrent exposure to STZ via this mechanism (10. 1 1 ). STZ at noncytotoxic levels decreases AGAT activity by 75%, with subsequent exposure to BCNU resulting in a 3-4-log increase in cell killing compared with BCNU alone. The AGAT activity can be reduced by up to 90% following a single large dose of STZ in this system, with >50% depletion maintained for 24 h. Peripheral blood lymphocyte levels of AGAT have been used as intermediate markers of drug effect. These levels are reduced by 40% by a single 500-mg/m2 dose of STZ and by 75% by three doses ( 12). Based on these data. Phase I trials of the combination of BCNU and STZ have been conducted by Micetich et a!. (13), who gave STZ at 2 g/m2 as a single dose, followed 20 mm later by BCNU, and Panella ci a!. ( 14), using STZ at 500 mg/m2/day for 4 days with BCNU after the third dose of STZ. The maximum tolerated doses of BCNU in these trials were 125 and ISO mg/m2. respectively. A third Phase I study using a single 2-g/m2 dose of STZ followed 6 h later by escalating doses of BCNU showed a maximum tolerated dose of 130 mg/rn2 (15). DTIC and BCNU are among the most active drugs in the therapy of melanoma ( I 6). These agents share a common site of activity at the 06 position of guanine. Thus. a possible strategy 3 The abbreviations used are: AGAT. 06-alkylguanine-DNA-alkyltransferase: BCNU, carmustine: STZ. streptozocin: DTIC. dacarbazine: ECOG, Eastern Cooperative Oncology Group. Research. on December 30, 2017. © 1996 American Association for Cancer clincancerres.aacrjournals.org Downloaded from Table 1 Patient characteristics ii (evaluable) Male/female Age: median (range) Prior therapy Chemotherapy Radiation Surgery Immunotherapy 17(15) I 1/6 55 (27-70)