Design principles of drug combinations for chemotherapy

Combination chemotherapy is the leading clinical option for cancer treatment. The current approach to designing drug combinations includes in vitro optimization to maximize drug cytotoxicity and/or synergistic drug interactions. However, in vivo translatability of drug combinations is complicated by the disparities in drug pharmacokinetics and activity. In vitro cellular assays also fail to represent the immune response that can be amplified by chemotherapy when dosed appropriately. Using three common chemotherapeutic drugs, gemcitabine (GEM), irinotecan (IRIN), and a prodrug form of 5-flurouracil (5FURW), paired with another common drug and immunogenic cell death inducing agent, doxorubicin (DOX), we sought to determine the in vitro parameters that predict the in vivo outcomes of drug combinations in the highly aggressive orthotopic 4T1 murine breast cancer model. With liposomal encapsulation of each drug pair, we enabled uniform drug pharmacokinetics across the drug combinations, thus allowing us to study the inherent benefits of the drug pairs and compare them to DOX liposomes representative of DOXIL®. Surprisingly, the Hill coefficient (HC) of the in vitro dose-response Hill equation provided a better prediction of in vivo efficacy than drug IC₅₀ or combination index. GEM/DOX liposomes exhibited a high HC in vitro and an increase in M1/M2 macrophage ratio in vivo. Hence, GEM/DOX liposomes were further investigated in a long-term survival study and compared against doxorubicin liposomes and gemcitabine liposomes. The GEM/DOX liposome-treated group had the longest median survival time, double that of the DOX liposome-treated group and 3.4-fold greater than that of the untreated controls. Our studies outline the development of a more efficacious formulation than clinically representative liposomal doxorubicin for breast cancer treatment and presents a novel strategy for designing cancer drug combinations.