| Abstract: | Despite progress in systemic small interfering RNA (siRNA) delivery to the liver and to solid tumors, systemic siRNA delivery to leukocytes remains challenging. The ability to silence gene expression in leukocytes has great potential for identifying drug targets and for RNAi-based therapy for leukocyte diseases. However, both normal and malignant leukocytes are among the most difficult targets for siRNA delivery as they are resistant to conventional transfection reagents and are dispersed in the body. We used mantle cell lymphoma (MCL) as a prototypic blood cancer for validating a novel siRNA delivery strategy. MCL is an aggressive B-cell lymphoma that overexpresses cyclin D1 with relatively poor prognosis. Down-regulation of cyclin D1 using RNA interference (RNAi) is a potential therapeutic approach to this malignancy. Here, we designed lipid-based nanoparticles (LNPs) coated with anti-CD38 monoclonal antibodies that are specifically taken up by human MCL cells in the bone marrow of xenografted mice. When loaded with siRNAs against cyclin D1, CD38-targeted LNPs induced gene silencing in MCL cells and prolonged survival of tumor-bearing mice with no observed adverse effects. These results highlight the therapeutic potential of cyclin D1 therapy in MCL and present a novel RNAi delivery system that opens new therapeutic opportunities for treating MCL and other B-cell malignancies.RNA interference (RNAi) can be activated by introducing synthetic short double-stranded RNA fragments, termed small interfering RNAs (siRNAs), into cells to silence genes bearing complementary sequences. RNAi holds great promise as a powerful tool for evaluating the role of specific genes in cellular and disease processes and for therapeutic applications (1, 2). siRNAs that manipulate gene expression in leukocytes could be used to understand hematologic cell biology and to develop novel therapeutic approaches to dampen inflammation and the harmful immune responses that occur during autoimmunity; to suppress lymphotropic viral infections, such as HIV; or to treat blood cancers (3). However, the lack of systemic delivery startegies to target leukocytes foils these applications.Here we devised a new strategy to target B-cell malignancies using mantle cell lymphoma (MCL) as a prototypic blood cancer to validate this siRNA delivery approach. MCL is an aggressive B-cell malignancy characterized by a t(11:14) chromosomal translocation that juxtaposes the proto-oncogene encoding cyclin D1 (cycD1) to the Ig heavy chain gene promotor (4). This leads to constitutive overexpression of cycD1, a protein that is not expressed in healthy B-lymphocytes. Current MCL therapy relies mainly on conventional chemotherapy; anti-CD20 cytotoxic monoclonal antibodies; autologous stem cell transplantation; and, more recently, small molecule inhibitors of critical molecular pathways, such as the BTK inhibitor ibrutinib (5). Unfortunately, relapse and progressive resistance to treatment lead to short median survival. MCL has one of the worst prognoses among lymphomas (6–8). Thus, there is a need for new therapeutic approaches.We previously showed that cycD1 down-regulation in MCL cell lines using RNAi inhibits proliferation and causes cell cycle arrest and apoptosis (9). However, the clinical application of this approach is hindered by the lack of appropriate systems that could deliver RNAi payloads to MCL cells in an efficient and safe manner (10, 11). RNAi therapeutics for B-cell malignancies is especially challenging because these cells are dispersed and are intrinsically resistant to transfection with nucleic acids (3, 12, 13). Therefore, to test the potential therapeutic effect of cycD1 inhibition in vivo and to demonstrate the feasibility of RNAi therapeutics in MCL, we needed to develop a suitable RNAi-delivery platform for potent gene silencing.Lipid-based nanoparticles (LNPs), composed of ionizable lipids that incorporate siRNAs, can induce potent gene silencing in the liver (14, 15). These are currently being evaluated in a phase III clinical study to knock down the TTR gene expressed in the liver to treat familial amyloidosis (16). We recently demonstrated that LNPs could be surface-modified with a natural ligand or a monoclonal antibody to improve in vivo delivery of siRNA payloads (17, 18). Here we investigate the use of antibody-targeted LNPs to deliver siRNAs to MCL cells. The blood supply in the hematological tissues where MCL cells mostly reside, including spleen and bone marrow, is made up of sinusoids that allow small nanoparticles tissue access. Selective targeting of lymphoma cells by antibody-targeted delivery should be clinically beneficial because it could reduce the total amount of drug required for therapeutic benefit and reduce toxicity to bystander cells (2, 12).CD38 is expressed on the surface of immature hematopoietic cells, including immature B cells. Its expression is tightly regulated during B-cell ontogeny; it is expressed on bone marrow precursors, but not mature B cells. CD38 is expressed on most MCLs (19). In the present study, we show that CD38 is a suitable target for antibody-mediated delivery of therapeutic siRNAs to MCL. LNPs–siRNA coated with an anti-CD38 monoclonal antibody (αCD38 mAb) showed specific MCL binding in vitro (in MCL cell lines and MCL primary lymphomas) and in vivo (in mice xenografted with a human MCL cell line). CD38-targeted LNPs (αCD38-LNPs) entrapping siRNA against cycD1 (siCycD1) were specifically taken up by MCL xenografts. αCD38-LNPs-siCycD1 induced gene silencing, suppressed tumor cell growth in vitro, and prolonged the survival of MCL-bearing mice. Our data demonstrate the effectiveness of inhibiting cycD1 in MCL in vivo and highlight αCD38–LNPs–siRNA as part of a strategy that could ultimately become a novel therapeutic modality for treating MCL and other CD38-expressing hematological malignancies. |
