Abstract 1846: Enhancing chemoimmunotherapy of metastatic TNBC through potent and immunomodulatory drug combination nanoparticles

Abstract Breast cancer is the most common cancer in women worldwide, and triple-negative breast cancer (TNBC) represents the most challenging subtype to treat due to the lack of hormone or HER2 receptors. Current guidelines for metastatic TNBC recommend a combination of pembrolizumab with chemotherapy for patients with PD-L1 expression (CPS ≥ 10). To enhance the efficacy of existing regimens, we previously reported that water soluble gemcitabine (G) and insoluble paclitaxel (T) can be stabilized with lipid excipients and a novel process into drug combination nanoparticle suspension (GT-in-DcNPs, ∼60 nm) capable of co-delivering both drugs to the tumor. We demonstrated that a single IV dosing of GT-in-DcNP, but not soluble form, inhibited lung metastasis of 4T1 in mouse by nearly 100%. In this study, we hypothesized that GT-in-DCNPs not only improve tumor inhibition compared to free GT, but also increase PD-L1 expression in the primary tumor and reduce myeloid-derived suppressor cells (MDSCs), thereby sensitizing tumors to anti-PD-1/PD-L1 immune checkpoint inhibitor therapy. Consequently, combining antibody therapy with GT-in-DcNPs could further improve therapeutic outcomes. Our in vitro studies showed that free G or T increased PD-L1 mRNA and protein expression in 4T1 TNBC cells in a dose- and time-dependent manner. When the drugs were combined in G-to-T molar ratios of 30:1 to 1:1, PD-L1 mRNA levels were upregulated by 4- to 5-fold after 24 hours. Under static cellular incubations, GT-in-DcNPs and free GT counterpart produced a similar impact on PD-L1 upregulation. We further investigated orthotopic 4T1 tumoral PD-L1 upregulation in BALB/c mice. Six days after a single IV dose of 35/3.5 mg/kg GT-in-DcNP (∼30:1 molar ratio) but not free GT elevated PD-L1 mRNA expression in tumor by ∼70%. In a preliminary efficacy study with the orthotopic 4T1 model (∼100 mm3), a single GT-in-DcNP (35/3.5 mg GT, IV, day 0) and anti-PD-L1 antibody (100 μg, IP, day 6) combination exhibited significant tumor growth reduction (168%) than the GT-in-DcNP or free drug treatment (248% and 265%, respectively; n = 8, p < 0.05), by day 12. Flow cytometry analysis of tumor and spleen cell suspensions revealed that MDSC populations (Gr-1+ CD11b+) in both tissues were reduced by GT-in-DcNPs on day 6 post-treatment, whereas no changes were observed in the free drug groups. Furthermore, the spleen CD8+ T cell population on day 12 was increased by 65% in DcNP-antibody combination group compared to free GT-antibody combination group (p < 0.05). Overall, these data suggest that GT-in-DcNP treatment elevates PD-L1 expression in 4T1 TNBC cells in vitro and in vivo and reduces MDSC populations in vivo, and its combination with PD-L1 antibody enhances tumor inhibition beyond that achievable by GT in conventional dosage form. Thus inclusion of GT-in-DcNP with immune checkpoint inhibitor against PD-L1 may provide additional benefits. Citation Format: Caroline B. Linde, Xiaolin Xu, Rodney J. Ho, Mary L. Disis, Qingxin Mu. Enhancing chemoimmunotherapy of metastatic TNBC through potent and immunomodulatory drug combination nanoparticles [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1846.