Metastasis is known to be the prime cause of cancer associated fatality. It is therefore imperative to treat and prevent metastasis in order to curb cancer-associated morbidity and mortality. Metastatic lymph nodes (MLNs) are known to be crucial diagnostic and prognostic indicators. Furthermore, as per the lymph node mediated hematogenous theory, due to extant communication between the lymphatic network and the venous system, the lymphatic network facilitates the passage and subsequent colonization of tumor cells to distant sites, thus, serving as a starting point for distant metastasis even during the initial stages. Treatment of MLNs, therefore, is imperative. Proposed in 2015, the Lymphatic Drug Delivery System (LDDS) is a novel and promising drug delivery system for the treatment of MLNs that overcomes the limitations of conventional chemotherapy. Unlike systemic chemotherapy, LDDS entails a direct intranodal drug administration to sentinel lymph node, post its identification using radioisotopes and fluorescent molecules, in order to treat it and its downstream lymph nodes. Previous studies have documented the superior efficacy of LDDS as a MLN management strategy as compared to conventional therapy. Higher drug delivery to target sites and prolonged retention with lower systemic toxicity, and thereby stronger anti-tumor effect have been reported to have been achieved upon drug administration via LDDS. In the present study, MXH10/Mo/lpr mice and FM3A-Luc, mouse mammary carcinoma cells expressing the firefly luciferase gene, have been utilized for the establishment of a mouse model of Lymph node metastasis (LNM) to investigate the effect of drug osmotic pressure and viscosity on therapeutic efficacy of the LDDS. Two lymph nodes, exhibiting systemic lymphadenopathy, in the subiliac and axillary region, the subiliac lymph node (SiLN) and proper axillary lymph node (PALN) have been used to model a network of interconnected human lymph nodes. Drug solution of varying osmotic pressure and viscosity were administered via LDDS to the murine model of LNM and subsequently therapeutic efficacy was monitored using the high frequency ultrasound system, VEVO and bioluminescent imaging system, IVIS, and confirmed through histology. The study revealed an optimized window of drug osmotic pressure and viscosity (π = 1000 - 2000 kPa; µ = 0.9 - 11.5 mPa·s.) where a significantly stronger anti-tumor response was observed. Venturing below or beyond the optimized range results in decreased therapeutic efficacy. The study therefore concluded that drug osmotic pressure and viscosity are important fluid parameters that require careful consideration prior to preparation of drug cocktails as they can severely impact the prognosis. The findings of this study have the potential for translation to clinics and we anticipate that these results can be replicated in clinical trials, positively altering the fate of many cancer patients. Citation Format: Radhika Mishra, Ariunbuyan Sukhbaatar, Sora Shouta, Maya Sakamoto, Shiro Mori, Tetsuya Kodama. Importance of drug osmotic pressure and viscosity for efficient drug delivery using lymphatic drug delivery system [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr LBA051.