Radiolabeled liposome imaging determines an indication for liposomal anticancer agent in ovarian cancer mouse xenograft models

Currently, several liposomal anticancer agents are approved by the FDA to treat the following cancers: doxorubicin‐encapsulated liposomes (Doxil®) for ovarian cancer, breast cancer, and Kaposi's sarcoma; daunorubicin‐encapsulated liposomes (DaunoXome®) for Kaposi's sarcoma; vincristine‐encapsulated liposomes (Marqibo®) for acute lymphoblastic leukemia; and cytarabine‐encapsulated liposomes (DepoCyt®) for lymphomatous meningitis.1, 2 Additional liposomal agents are under investigation towards their clinical application. It is expected that advantages of liposomal anticancer agents include tumor‐selective targeting of macromolecules based on the concept of the enhanced permeability and retention (EPR) effect, which consequently leads to higher accumulation of free drugs in the tumor, and reduction of side‐effects that are related to their free drugs.3, 4, 5, 6 For example, liposomal formulations of doxorubicin and daunorubicin, as well as vincristine and cytarabine, reduced side‐effects such as cardiotoxicity and hematological toxicity, respectively, leading to improved overall compliance and quality of life for cancer patients.7, 8, 9, 10 Due to progressive development of technology related to improvement of drug delivery systems, as well as the expansion of the aging population worldwide with cancer, the number of liposomal as well as lipid‐based anticancer agents is expected to increase. Despite the expected advantages, liposomal agents provide clinical benefits for only a limited number of cancer patients. Indeed, the overall response rate (complete response or partial response [PR]) was 19.7% for Doxil® in recurrent epithelial ovarian carcinoma, 25% for DaunoXome® in AIDS‐related Kaposi's sarcoma, 25% for Marqibo® in refractory aggressive non‐Hodgkin's lymphoma, and 26% for DepoCyt® in a clinical trial with solid tumor patients.11, 12, 13 These records indicate that there is a necessity to evaluate tumor characteristics that are favorable for liposomal anticancer agents and, based on these findings, to develop biomarkers that are capable of predicting efficacies of liposomal agents. In recent years, importance of diagnostic imaging has been emphasized for treatment planning of many diseases, especially cancer. Within nanomedicine, there are particularly interesting possibilities to generate nanocarriers that can be engineered to transport diagnostic and therapeutic agents, a concept termed theranostics.14, 15 Several theranostic procedures have already been used in routine clinical practice. For example, when octreotide (a synthetic analog of somatostatin that binds to somatostatin receptors) is labeled with positron‐emitting 68Ga, expression of somatostatin receptors on tumors can be visualized, and positive scintigraphic findings allow us to proceed to radionuclide therapy to treat gastroenteropancreatic neuroendocrine tumors with radiolabeling of octreotide with Auger electron‐ and β‐emitting 90Y or 177Lu.16 Applying this concept to the liposomal agents, we listed the current set of FDA‐approved liposomal anticancer agents, specifically focusing on their lipid compositions and sizes, and examined if we could generate diagnostic versions of the therapeutic liposomes by altering the interior contents from encapsulation of anticancer agents to radionuclides. Although a few radiolabeled liposomes have already been proposed as nanocarriers for theranostic applications,17, 18, 19 their usefulness has not been clearly determined and no effective biomarkers for these liposomal agents have been established yet. As a strategy to develop the biomarkers with capacities to predict the effectiveness of liposomal anticancer agents, we initially compared the pharmacological effects of Doxil® and doxorubicin among mouse xenograft models bearing four different human ovarian cancers (Caov‐3, SK‐OV‐3, KURAMOCHI, and TOV‐112D), and carried out histopathological analyses to correlate the therapeutic effects of Doxil® and histological characteristics linked to EPR. We clearly showed a close correlation between them. We next generated 111In‐encapsulated liposomes with lipid composition and size identical to Doxil®, and applied single‐photon emission computed tomography (SPECT)/CT imaging of the aforementioned mouse xenograft models. Differing accumulation levels of the radiolabeled liposomes offered a unique opportunity to investigate the feasibility of the radiolabeled liposomes to individually diagnose and predict the efficacy of Doxil® treatment. Hence, we examined each xenografted mouse to determine tumor accumulation of the radiolabeled liposomes by SPECT/CT imaging and, remarkably, we confirmed a clear correlation between the accumulation and the effects of liposomal formulation. Our integrated preclinical findings showed the merit of the radiolabeled liposomes, designed for SPECT/CT imaging, as imaging biomarkers to decide the indications for Doxil® treatment.