Modulation of chemotherapy resistance in regional therapy:: a novel therapeutic approach to advanced extremity melanoma using intra-arterial temozolomide in combination with systemic O6-benzylguanine

This study investigated whether the therapeutic index of regional melanoma therapy using parenteral temozolomide could be improved by chemomodulation with O-6- benzylguanine (O(6)BG), an inhibitor of the DNA repair enzyme O-6-alkylguanine-DNA alkyltransferase (AGT). Using a nude rat s.c. human melanoma xenograft model of the extremity, tumors were analyzed for AGT level 2 to 3 hours after the i.p. injection of 3.5 to 70.0 mg/kg O(6)BG to inhibit AGT activity. Survival studies were conducted using animals that were treated with a 15-minute isolated limb infusion with 10% DMSO in PBS (control), temozolomide alone, or temozolomide in conjunction with single or multiple doses of i.p. O(6)BG. Tumor volume and toxicity level were monitored every other day. Administration of 3.5 mg/kg O(6)BG depleted tumor AGT activity by 93.5% (P < 0.01). Groups treated with regional temozolomide alone (350 mg/kg), systemic temozolomide with O(6)BG, or vehicle combined with O(6)BG showed no significant tumor responses compared with controls. Whereas use of regional temozolomide alone at a higher dose (750 mg/kg) showed some degree of tumor response, regional temozolomide given in conjunction with multiple dosages of O(6)BG showed a marked (P < 0.01) reduction in tumor growth with minimal toxicity. Our findings suggest that AGT modulation by the administration of O(6)BG in combination with temozolomide regional chemotherapy leads to a significant improvement in melanoma antitumor responses. Clinical trials using chemotherapy modulation may improve response rates in future regional infusion and perfusion drug trials.