Currently, the limited penetration of nanoparticles remains a major challenge for antitumor nanomedicine to penetrate into the tumor tissues. Herein, we propose a size-shrinkable drug delivery system based on a polysaccharide-modified dendrimer with tumor microenvironment responsiveness for the first time to our knowledge, which was formed by conjugating the terminal glucose of hyaluronic acid (HA) to the superficial amidogen of poly(amidoamine) (PAMAM), using a matrix metalloproteinase-2 (MMP-2)-cleavable peptide (PLGLAG) via click reaction. These nanoparticles had an initial size of ∼200 nm, but once deposited in the presence of MMP-2, they experienced a dramatic and fast size change and dissociated into their dendrimer building blocks (∼10 nm in diameter) because of cleavage of PLGLAG. This rapid size-shrinking characteristic not only promoted nanoparticle extravasation and accumulation in tumors benefited from the enhanced permeability and retention effect but also achieved faster nanoparticle diffusion and penetration. We have further conducted comparative studies of MMP-2-sensitive macromolecules (HA-pep-PAMAM) and MMP-2-insensitive macromolecules (HA-PAMAM) synthesized with a similar particle size, surface charge, and chemical composition and evaluated in both monolayer cells and multicellular spheroids. The results confirmed that the enzyme-responsive size shrink is an implementable strategy to enhance drug penetration and to improve therapeutic efficacy. Meanwhile, macromolecule-based nanoparticles with size-variable characteristics not only promote drug penetration, but they can also be used as gene delivery systems, suggesting great potential as nano-delivery systems.

Immune checkpoint blockade (ICB) is a promising novel therapy for multiple cancer types; however, most patients show limited or no clinical response. Accumulating evidence indicates that myeloid-derived suppressor cells (MDSCs) are a major factor responsible for immunosuppression in patients with cancer. Therefore, identifying effective therapies that deplete or modulate MDSCs is essential. In this study, we focus on the anticonvulsant drug valproic acid (VPA), which has additional activities including anticancer and immunoregulation by inhibition of histone deacetylases. We showed that VPA decreased the proportion of polymorphonuclear (PMN)-MDSCs in vitro and showed for the first time that VPA greatly attenuated the immunosuppressive function of MDSCs in a dose-dependent manner. Moreover, we demonstrated that in vitro differentiated VPA-conditioned MDSCs exhibited impaired ability to stimulate tumor progression in vivo. We also showed the possible involvement of several mechanisms in the VPA-induced attenuation of the immunosuppressive function of MDSCs, including the interleukin-4 receptor-α (IL-4Rα)/arginase axis, programmed cell death 1 ligand 1 (PD-L1) and toll-like receptor 4 (TLR4) signaling pathways, and retinoblastoma 1 (Rb1) derepression. This research highlights the potential of combining VPA with ICB in cancer treatment.

Myeloid-derived suppressor cells (MDSCs) are immunosuppressive cells that promote tumor progression by inhibiting anti-tumor immunity and may be the cause of patient resistance to immune checkpoint inhibitors (ICIs). Therefore, MDSCs are a promising target for cancer immunotherapy, especially in combination with ICIs. Previous studies have shown that the anticonvulsant drug valproic acid (VPA) has additional anti-cancer and immunoregulatory activities due to its inhibition of histone deacetylases. We have previously shown that VPA can attenuate the immunosuppressive function of differentiated MDSCs in vitro. In the present study, we utilized anti-PD-1-sensitive EL4 and anti-PD-1-resistant B16-F10 tumor-bearing mouse models and investigated the effects of VPA on MDSCs with the aim of enhancing the anti-cancer activity of an anti-PD-1 antibody. We showed that VPA could inhibit EL4 and B16-F10 tumor progression, which was dependent on the immune system. We further demonstrated that VPA down-regulated the expression of CCR2 on monocytic (M)-MDSCs, leading to the reduced infiltration of M-MDSCs into tumors. Importantly, we demonstrated that VPA could relieve the immunosuppressive action of MDSCs on CD8 + T-cell and NK cell proliferation and enhance their activation in tumors. We also observed that the combination of VPA plus an anti-PD-1 antibody was more effective than either agent alone in both the EL4 and B16-F10 tumor models. These results suggest that VPA can effectively relieve the immunosuppressive tumor microenvironment by reducing tumor infiltration of M-MDSCs, resulting in tumor regression. Our findings also show that VPA in combination with an immunotherapeutic agent could be a potential new anti-cancer therapy.

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Myeloid-derived suppressor cells (MDSCs) are major immunosuppressive cells that accumulate in tumor-bearing hosts. Since MDSCs suppress anti-tumor immunity and promote tumor progression, they are promising targets for cancer immunotherapy. Granulocyte colony-stimulating factor (G-CSF) is an agent used for the treatment of chemotherapy-induced febrile neutropenia (FN) in patients with cancer. However, several reports have revealed that G-CSF plays crucial immune-related adverse roles in tumor progression through MDSCs. In this study, we showed that MDSCs differentiated in the presence of G-CSF in vitro exhibited enhanced proliferation and immunosuppressive activity compared to those differentiated without G-CSF. RNA sequencing analysis demonstrated that G-CSF enhanced the immunosuppressive function of MDSCs by upregulating gamma-glutamyltransferase (GGT) 1. Moreover, in the EL4 lymphoma-bearing neutropenic mouse model, administration of recombinant G-CSF increased the number of MDSCs and attenuated the anti-cancer effect of chemotherapy. We showed that the combination of GGsTop, a GGT inhibitor, could prevent G-CSF-induced tumor growth, without affecting the promotion of myelopoiesis by G-CSF. These results suggest that targeting GGT1 can mitigate G-CSF-induced enhanced immunosuppressive functions of MDSCs and can eliminate the tumor-promoting effect of G-CSF. Furthermore, GGsTop could be an attractive combination agent during G-CSF treatment for FN in patients with cancer.