Malignant pleural effusion (MPE) is a buildup of fluid in the cavity between the lungs and chest wall, occurring secondary to metastatic cancer. The appearance of MPE is indicative of the late stage of cancer and poor prognosis with an average survival of 4-9 months. Moreover, accumulation of pleural effusion commonly causes breathlessness, pain, extreme bodyweight loss and reduced physical activity, which severely compromises patient’s quality of life. Current standard of care for MPE includes catheter drainage of the fluid or chemical/surgical procedure to seal up the cavity, but is largely palliative. The emergence of cancer immunotherapy is providing tremendous promise in cancer treatment. Recent clinical trials with the immune checkpoint inhibitors (ICI), i.e., anti-PD-1/PD-L1 and anti-CTLA antibodies, have shown some encouraging data in MPE patients. However, only a fraction of MPE patients benefit from the ICI immunotherapy and some patients suffer from serious immunotoxicity. As such, there is an unmet need for developing novel therapeutics that can enhance the ICI immunotherapy while minimizing unwanted side effects.
Clinical studies have shown that MPE comprises abundant tumor associated immune cells that promote rather than inhibit tumor growth as well as high levels of immunosuppressive cytokines, indicating the immunologically ‘cold’ MPE. Growing body of evidence suggests that the immune cold tumor microenvironment (TME) is a major contributor to the failure of ICI. We thus sought to develop a new therapeutic strategy to mitigate the immune cold MPE. In this study, we developed a liposomal nanoparticle loading a potent immunostimulant, cyclic dinucleotide (LNP-CDN). LNP-CDN provides excellent protection from enzymatic degradation of CDN by ENPP1, a phosphodiesterase, that is found at high concentrations in MPE. Given the prevalent population of tumor associated myeloid cells in MPE, which contribute significantly to the immune cold MPE, LNP-CDN is designed to enable targeted delivery of CDN to activate these immune cells. Loading of CDN complexed with calcium phosphate enables pH-responsive release of CDN from endosome to cytosol, where it ligates STING to initiate STING signaling and type I interferon (IFN) production.
Accompanying MPE, disseminated tumors (carcinomatosis) often grow on the cavity surface. These two distinct TME compartments, the effusion and solid tumors, co-exist in the cavity, presenting a major challenge for therapeutic interventions and drug delivery. Here, we proposed to administer LNP-CDNs directly into the pleural cavity to achieve close proximity to tumor and immune cells in MPE while possibly minimizing systemic toxicities. We found that intrapleurally administered LNP-CDNs dispersed well not only in the fluid but also in the pleural solid tumors; importantly, LNP-CDNs were also located in tumor draining lymph nodes (TDLNs), which are critical sites for initiation of tumor immunosurveillance and antitumor immune responses.
Through unbiased single-cell RNA sequencing (scRNA-seq) analysis, intraplerual LNP-CDN was found to promote innate and adaptive immune responses by repolarizing tumor associated myeloid cells toward the proinflammatory phenotype, activating dendritic cells for effective immune sensing and cross-presentation of tumor antigens, and expanding the populations of polyfunctional effector NK cells and CD8+T cells in a MPE mouse model. We further hypothesized that LNP-CDN-induced inflammatory MPE set a stage for response to anti-PD-L1 immunotherapy. Indeed, combination immunotherapy with blockade of PD-L1 potently reduced MPE volume and inhibited tumor growth, conferring significantly prolonged survival of MPE-bearing mice. Furthermore, the LNP-CDN-induced immunological effects were also observed with clinical MPE samples, suggesting the potential of intrapleural LNP-CDN for clinical MPE immunotherapy. Importantly, intrapleural LNP-CDN alone or in combination with anti-PD-L1 Ab is safe without causing immunotoxicity.
In the context of clinical practice, LNP-CDN can be administered serially via indwelling pleural catheters. Clinically, the presence of MPE often precludes surgical intervention, and many patients with MPE are not fit for chemotherapy due to the extremely poor condition of these patients. Thus, successful management of MPE may renew the opportunities for combining with other treatment options to maximize therapeutic efficacy.