Targeting Chronic Myeloid Leukemia Stem/Progenitor Cells Using Immunolipsome Loaded Venetoclax
Authors: Houshmand Mohammad , Circosta Paola , Garello Francesca , Gaidano Valentina , Cignetti Alessandro , Stefania Rachele , Aime Silvio , Saglio Giuseppe 
Published: November 2020

The introduction of different generations of tyrosine kinase inhibitors (TKIs) significantly improved outcome and survival rate in chronic myeloid leukemia (CML) patients. However, long-term use of TKIs is concomitant with many side effects that affect the quality of life in patients. Approximately half of CML patients achieve deep molecular response (DMR), this makes them suitable candidates to discontinue the TKI therapy in a controlled condition, and about half of them will remain in treatment free remission (TFR) after discontinuation. It has been shown that a small population of leukemia stem cells (LSCs) as the residual disease burden is present at diagnosis, during the treatment, and in patients who are in TFR. While CML LSCs have many features in common with HSCs, they express specific markers such as CD25, CD26, IL1-RAP, etc., which can be used for the diagnosis and targeting. Protection by the bone marrow microenvironment and activity of signaling pathways such as WNT/β catenin, Hedgehog, PI3K, JAK/STAT in CML LSCs in a BCR-ABL dependent and independent manner guarantee their survival and elimination of these cells solely using TKIs seems ineffective.
Herein we designed a pegylated liposomal nanocarrier conjugated with a specific antibody against CD26 (Begelomab, ADIENNE, Lugano, Switzerland). Then we loaded this immunoliposome with venetoclax, a BCL2 inhibitor, to eliminate CML LSCs selectively and to spare normal HSCs.
First, we measured the expression of CD26 in the bone marrow and peripheral blood samples of newly diagnosed patients. We had a high expression of CD26 in CD34+/CD38- of both PB and BM, and a low expression on CD34+/CD38+ (progenitors) cells. Also, the expression of this marker in resistant patients to TKIs was visible while it was absent in normal stem cells.
After the synthesis of the liposome, we conjugated Begelomab to the liposome. Then, we tested the selectivity of the designed system in different positive and negative cells. Our designed immunoliposome showed a strong selectivity toward CD26 positive cells. We also tested the selectivity on CML primary cells; in particular, we sorted newly diagnosed CML samples based on CD34+/CD38-/CD26- for HSCs and CD34+/CD38-/CD26+ for LSCs. Based on the confocal and flow cytometry analysis, our designed immunoliposome selectively targets LSCs and spares HSCs. Then we loaded this immunoliposome with venetoclax, and we treated CD26 positive and negative cells with this system. Based on our preliminary results, this immunoliposome loaded venetoclax specifically induced apoptosis in CD26+ cells, with higher activity compared to free venetoclax at the same dose. However, more analysis will be performed to confirm the selectivity of this system.
Based on the obtained results, CD26 in newly diagnosed CML patients is expressed by CML LSCs and is a suitable option for diagnosis and targeting. Our preliminary data strongly suggest that we can selectively target CML LSCs. The main advantage of this system is its precision to hit the target. So we expect that after the drug release, the LSCs will be eliminated without any side effects on normal cells. Liposomes are suitable carriers because of their biocompatibility, self-assembly, large drug payload, and minimal toxicity. This strategy may help us to increase the number of patients attaining and maintaining TFR without relapsing.

Disclosures
Saglio: Ariad: Research Funding; Pfizer: Research Funding; Incyte: Research Funding; Roche: Research Funding; Bristol-Myers Squibb: Research Funding; Novartis: Research Funding.