Abstract

Perifosine is an alkylphosphocholine with structural similarity to phospholipids that are the main constituents of cellular membranes. It is the first orally available compound inducing AKT-inhibition in clinical trials. In myeloma tumor cells, Perifosine can inhibit Akt and induce apoptosis. Perifosine inhibits the growth of myeloma tumor cells in vivo and prolongs survival in a murine myeloma model. Triple immune deficient female beige-nude-xid (BNX) mice were inoculated with subcutaneous myeloma tumors derived from MM.1S cells. When measurable growing tumors appeared, mice were randomized to receive Perifosine via oral gavage at either 250mg/kg/week in two divided doses or 36mg/kg/day, or vehicle control (PBS). Animals were sacrificed when tumors reached 2cm or if they appeared moribund. Tumor growth was evaluated using caliper measurements from the first day of treatment until day of first sacrifice, which was day 15 for controls, day 32 for daily treated and day 30 for weekly treated. The tumors of control mice all reached 2cm by day 60, whereas 71% of daily treated mice and 57% of weekly treated mice remained alive at this time, a significantly improved survival amongst treated mice (p=0.01). Surviving treated mice were sacrificed on day 81 for further histological and molecular analysis. Terminal blood was obtained from 4 control mice, 4 daily-treated mice, and 4 weekly-treated mice at the time of sacrifice. Peripheral blood was analyzed with an automated Coulter counter and differential counts were confirmed with a blood smear. In control mice, the mean hemoglobin was 12g/dl, white cell count 7x109/l, and mean platelet count was 292x109/l. In contrast, the respective values for weekly-treated mice were 11, 9 and 944; and for daily treated mice were 10, 11 and 752. The increase in white cell count was due predominantly to a neutrophilia, except in one weekly treated mouse a monocytosis was observed. Blood smears of treated mice confirmed neutrophilia, including a left shift and band donut cells, with thrombocytosis. In placebo-treated animals, splenic architecture consisted of a normal distribution of red pulp and white pulp. The marrow consisted of a normal distribution of hematopoietic cells, with red cell precursors, lymphocytes, maturing myeloid cells and megakaryocytes. There was a normal amount of fat throughout the marrow. In contrast, the splenic architecture of treated mice was partially effaced by a marked expansion of white pulp comprised of maturing myeloid cells and scattered admixed megakaryocytes. The marrow of treated mice displayed marked hypercellularity, consisting predominantly of myeloid hyperplasia, donut cells and band neutrophils. Histological examination of tumors revealed large areas with distinct histological features, characterized by large abnormal foamy cells not seen in control mice, giving a “popcorn lacunae” characteristic. Neutrophils could be observed within tumor blood vessels and band forms and donut cells were found infiltrating the tumors. These findings have clinical relevance because myeloid suppression is a dose-limiting toxicity of many cytotoxic agents, and myeloid hyperplasia is usually only observed in the setting of growth factor stimulation. Coupled with its remarkable in vitro myeloma cytotoxicity, these results strongly support the use of Perifosine in clinical trials for patients with multiple myeloma.

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