The native transportation protein serum albumin represents an attractive nano-sized transporter for drug delivery applications due to its beneficial safety profile. Existing albumin-based drug delivery systems are often limited by their low drug loading capacity as well as noticeable drug leakage into the blood circulation. Therefore, a unique albumin-derived core-shell doxorubicin (DOX) delivery system based on the protein denaturing-backfolding strategy was developed. 28 DOX molecules were covalently conjugated to the albumin polypeptide backbone via an acid sensitive hydrazone linker. Polycationic and pegylated human serum albumin formed two non-toxic and enzymatically degradable protection shells around the encapsulated DOX molecules. This core-shell delivery system possesses notable advantages, including a high drug loading capacity critical for low administration doses, a two-step drug release mechanism based on pH and the presence of proteases, an attractive biocompatibility and narrow size distribution inherited from the albumin backbone, as well as fast cellular uptake and masking of epitopes due to a high degree of pegylation. The IC50 of these nanoscopic onion-type micelles was found in the low nanomolar range for Hela cells as well as leukemia cell lines. In vivo data indicate its attractive potential as anti-leukemia treatment suggesting its promising profile as nanomedicine drug delivery system.
The G protein-coupled receptor 56 (GPR56) was identified as part of the molecular signature of functionally validated leukemic stem cells isolated from patients with acute myeloid leukemia (AML). This report now demonstrates particularly high expression of GPR56 in patients with mutant NPM1 and FLT3-length mutation and association of high GPR56 expression with inferior prognosis in a large patient cohort treated in two independent multicenter phase III trials. Functional relevance of GPR56 expression was validated in mice, in which co-expression of Gpr56 significantly accelerated HOXA9-induced leukemogenesis and vice versa knockdown of Gpr56 delayed onset of HOXA9/MEIS1-induced AML. Overexpression of Gpr56 grossly changed the molecular phenotype of Hoxa9-transduced cells affecting pathways involved in G protein-coupled receptors (GPRCs) and associated intracellular signaling. Blockage of surface GPR56 by an anti-GPR56 antibody successfully impaired engraftment of primary human AML cells. In summary, these data demonstrate that high expression of GPR56 is able to contribute to AML development and characterize the GPR56 as a potential novel target for antibody-mediated antileukemic strategies.
Leukemic stem cells (LSC) might be the source for leukemic disease self-renewal and account for disease relapse after treatment, which makes them a critical target for further therapeutic options. We investigated the role of cytotoxic T-lymphocytes (CTL) counteracting and recognizing LSC. Leukemia-associated antigens (LAA) represent immunogenic structures to target LSC. We enriched the LSC-containing fraction of 20 AML patients and hematopoietic stem cells (HSC) of healthy volunteers. Using microarray analysis and qRT-PCR we detected high expression of several LAA in AML cells but also in LSC. PRAME (p 5 0.0085), RHAMM (p 5 0.03), WT1 (p 5 0.04) and Proteinase 3 (p 5 0.04) showed significant differential expression in LSC compared with HSC. PRAME, RHAMM and WT1 are furthermore also lower expressed on leukemic bulk. In contrast, Proteinase 3 indicates a higher expression on leukemic bulk than on LSC. In colony forming unit (CFU) immunoassays, T cells stimulated against various LAA indicated a significant inhibition of CFUs in AML patient samples. The LAA PRAME, RHAMM and WT1 showed highest immunogenic responses with a range up to 58-83%. In a proof of principle xenotransplant mouse model, PRAME-stimulated CTL targeted AML stem cells, reflected by a delayed engraftment of leukemia (p 5 0.0159). Taken together, we demonstrated the expression of several LAA in LSC. LAA-specific T cells are able to hamper LSC in immunoassays and in a mouse model, which suggests that immunotherapeutic approaches have the potential to target malignant stem cells.
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