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The quassinoids bruceantin, brucein D, brucein E, bruceoside A, and brusatol significantly inhibited P-388 lymphocytic leukemic cell RNA and protein synthesis in tissue culture. However, DNA synthesis inhibition seemed to correlate more directly with the anti-neoplastic activity of these compounds in the in vivo P-338 survival system. In vitro, brusatol and bruceoside A marginally inhibited 10-day P-388 lymphocytic leukemia DNA polymerase, RNA polymerase, thymidylate synthetase, dihydrofolate reductase, phosphoribosyl pyrophosphate aminotransferase, and cathepsin protease activities. In vivo studies demonstrated similar inhibition and elevated cyclic AMP levels, correlating positively with the antineoplastic activity of individual compounds. Purine synthesis was inhibited drastically by brusatol in vivo, and one key inhibition site in purine synthesis was at phosphoribosyl pyrophosphate aminotransferase, the regulatory enzyme. Histone phosphorylation and ribonucleotide reductase activity also were inhibited marginally by brusatol.
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A phase I clinical study of bruceantin was conducted in 66 patients with various types of advanced solid tumors to evaluate its toxicity and efficacy. The initial dose of 0.2 mg/m2/day x 5 days repeated at 2-week intervals was progressively increased to a maximum dose of 4.5 mg/m2/day. Hypotension was the dose-limiting toxic effect; it was delayed, cumulative, and occurred more often in patients with abnormal pretreatment liver function. Nausea, vomiting, and fever were common at higher doses, and diarrhea, stomatitis, alopecia, paresthesia, and rash were observed in some patients. The hematologic toxicity of bruceantin was moderate at high doses and was manifested mainly as thrombocytopenia; it was more severe in patients with abnormal hepatic and renal functions. No objective tumor regressions were observed. The recommended dose of bruceantin is 3.5 mg/m2/day x 5 days for phase II studies.
Bruceantin was administered to 33 patients with advanced solid tumors or hematologic neoplasms. The schedule consisted of four weekly iv injections at one dose level followed by a 2-week "no-treatment" period; this program was repeated every 6 weeks. A total of 156 weekly injections were given over a dose range of 0.8-8.5 mg/m2. Systolic and diastolic hypotension was dose-related and dose-limiting at doses of greater than or equal to 6.0 mg/m2. Transient febrile reactions were dose-related. Gastrointestinal toxicity was commonly observed but was not dose-limiting. There was no hematologic or renal toxicity; hepatic toxicity was subclinical and reversible. Of 18 patients evaluable for antitumor response one patient with adenocarcinoma of the cervix experienced a less than 50% disease regression lasting 10 months and one patient with pleural mesothelioma experienced disease stabilization lasting 4 months. A phase II study of bruceantin is recommended at a starting dose of 5.5 mg/m2 weekly for 4 weeks as a 4-hour iv infusion repeated at 6-week intervals.