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Phase I and Correlative Study of Combination Bryostatin 1 and Vincristine in Relapsed B-Cell Malignancies

¹ Developmental Therapeutics Program and the Division of Hematology/Oncology, Department of Medicine, Case Western Reserve University School of Medicine and University Hospitals of Cleveland, Cleveland, Ohio, and
² National Cancer Institute, Bethesda, Maryland

Purpose: Bryostatin 1 activates protein kinase C (PKC) with short-term exposure and results in depletion of PKC with prolonged exposure. Preclinical in vitro and in vivo studies demonstrate synergistic activity and increased tumor apoptosis in B-cell malignancies when a prolonged infusion of bryostatin 1 is followed by vincristine.

Experimental Design: We embarked on a Phase I trial of bryostatin 1 as a 24-h continuous infusion followed by bolus vincristine in patients with refractory B-cell malignancies other than acute leukemias. Twenty-four evaluable patients were enrolled.

Results: The dose-limiting toxicity was myalgia. The MTD and recommended Phase II dose of bryostatin 1 was 50 µg/m²/24 h followed by vincristine 1.4 mg/m² (maximum total dose of 2 mg) repeated every 2 weeks. Significant antitumor activity was observed in this relapsed population, including patients who had failed high-dose chemotherapy. This included 5 durable complete and partial responses and 5 patients with stable disease lasting 6 months (range, 6–48+ months). Median time to response was 8 months. Correlative studies demonstrated a progressive increase in serum interleukin-6 with bryostatin 1 infusion followed by an additional increase after vincristine. Flow cytometry for detection of apoptosis in B and T cells showed an initial decrease in apoptotic frequency in CD5+ cells within 6 h of bryostatin 1 infusion compatible with its known increase in PKC activity in the majority of patients followed by a return to baseline or overall increase in apoptotic frequency after completion of infusion. All (5 of 5) patients who had an overall increase in apoptotic frequency in CD5+ cells achieved either a clinical response or prolonged stable disease. Four of these 5 patients did not have the initial decrement in apoptosis at 6 h.

Conclusions: Given the lack of myelosuppression and early evidence of clinical efficacy, additional exploration of this regimen in non-Hodgkin’s lymphoma and multiple myeloma is warranted.

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