This makes the overall monitoring and analysis of these populations quite challenging

This makes the overall monitoring and analysis of these populations quite challenging. == Parameters for analysis of leukemic stem cell in clinical trials == Current standard clinical endpoints may not be informative to evaluate the tumor in patients with relapsed or refractory disease. cells, dimethylaminoparthenolide, DMAPT, target, functional assay, differentiation == INTRODUCTION == It has been 14 years since John Dicks seminal paper inNatureprovided the first phenotypic handle on primary human leukemia stem cells.1A few years ago, the concept of targeting leukemia stem cells was just an interesting biological concept. However, there is now a certain degree of impatience in the research community surrounding this concept and how it can be put into practice. One challenge is to determine whether the agents that can target leukemic stem cells are effectively working in patients. The second challenge is to determine how to assess whether the leukemic stem cell is being targeted in vivo. == SELECTIVELY TARGETING LEUKEMIC STEM CELLS == == Small molecules == A variety of small-molecule regimens selectively target the leukemic stem cell. MG-132, a reagent-grade proteasome inhibitor that has similar activities to bortezomib, has significant selective activity for leukemia stem cells, both alone2and IPI-3063 in combination with idarubicin,3in both in vitro and xenograft model systems. ABT-737, a BCL-2 inhibitor and BH3 mimetic, has been shown to have activity against phenotypically described stem cells.4 TDZD-8 (4-benzyl, 2-methyl, 1,2,4-thiadiazolidine, 3,5 dione) is a very different class of molecule.5It has selective activity for leukemia stem cells, but the molecular mechanism behind it is different from some of the other small molecule compounds. TDZD-8 appears to selectively impair membrane integrity, specifically in leukemia cells. Parthenolide, a naturally occurring molecule found in the medicinal plant feverfew, induces apoptosis in acute myeloid leukemia (AML) stem cells.6Celastrol and 4-hydroxy-2-nonenal, which eradicate AML cells at the bulk, progenitor, and stem cell level, were discovered using high through-put screens that utilized the gene expression pattern evoked by parthenolide as a template.7 == Parthenolide analogs == DMAPT (dimethylaminoparthenolide), a parthenolide derivative (Figure 1), is a compound that has a fairly significant preclinical rationale for going forward.8DMAPT is readily water soluble and is 70% orally bioavailable. Pharmacologic studies in rodents and dogs have shown the drug to be tolerable well beyond the level at which in vitro activity is observed, without any known associated acute toxicity. However, the drug has a short half-life in rodents, making it challenging to set up xenograft models to test biological efficacy. == Figure 1. == Chemical structure of dimethylaminoparthenolide fumarate (DMAPT), a parthenolide analog Preclinically, DMAPT shows the preferred selective activity in an agent of this nature: it is selectively cytotoxic to CD34-selected bulk and primitive AML populations, and it does not cause death in the normal cell population. The IC50is approximately 10 times higher for normal cells. In immune-deficient mice, cells treated with DMAPT were significantly impaired in their engraftment ability, while the control specimens did not show IPI-3063 a significant effect. Although it is not known which IPI-3063 patient populations will benefit most from this drug, it is clear that there is some broad activity across the AML patient population. When canines with severe acute leukemia were treated with 50 mg/kg daily DMAPT, the levels of CD34-positive cells were largely reduced. The reduction was not likely due to overt cytotoxicity, since the overall white blood cell counts YWHAS were elevated at the beginning and remained elevated. DMAPT probably induces a biological change in the tumor population that is consistent with differentiation, as evidenced histologically by increased levels of maturing myeloid cells. A differentiation agent is an ideal way to treat a self-renewing stem cell because it inhibits the cells self-renewal and pushes differentiation. Furthermore, research has shown that NF-B signaling is a very common characteristic of leukemias, and therefore, inhibiting that pathway is potentially important. In vivo, DMAPT treated cells show increased cytoplasmic relocalization of the p65 subunit, which is IPI-3063 consistent with.