Background: Metastatic prostate cancer (mPrC), with a median survival of under 2 years, represents an important unmet medical need which may benefit from the development of more effective targeted drug delivery systems. Several cell surface receptors have been identified as candidates for targeted drug delivery to mPrC cells; however, these receptors were selected for their overabundance on PrC cells rather than for their suitability for targeted delivery and uptake of cytotoxic drug payloads. Methods: We describe a novel, unbiased strategy to isolate peptides that fulfill functional criteria required for effective intracellular drug delivery and the specific cytotoxicity of PrC cells without prior knowledge of the targeted receptor. Phage clones displaying 7-mer cyclic peptides were negatively selected in vivo and then positively biopanned through a series of parent and drug-resistant mPrC cells. Peptides from the internalized clones were then subjected to a panel of biochemical and functional tests that led to the selection of several peptide candidates. Results: The selected peptides do not bind PSMA. Peptide-drug conjugates (PDCs) incorporating one of the peptides selectively killed wild-type and drug-resistant PrC cell lines and patient PrC cells but not normal prostate tissue cells in vitro. The PDC also halted the growth of PC3 tumors in a xenograft model. Conclusions: Our study demonstrates that adding unbiased, functional criteria into drug carrier selection protocols can lead to the discovery of novel peptides with appropriate properties required for effective targeted drug delivery into target cancer cells.

Multiple Myeloma (MM) progresses over 2-3 decades through two pre-malignant stages (MGUS and SMM), culminating in clinically active disease. Given the limitations in acquiring sequential bone marrow (BM) samples from patients over this time frame, the mechanisms that compromise immunosurveillance and promote the development of MM remain Balb/c mice inoculated with MOPC315.BM myeloma cells were followed over the next 220 days. Blood and bone marrow samples were collected on days 80, 150, and 220 post cell inoculation. Blood samples were used to monitor levels of paraprotein and whole blood cell counts. BM aspirates were used for deep immune profiling by flow cytometry and for T cell function assays. Blood analyses validated that the model reflects serological features of human MM. Analysis of BM samples revealed a biphasic behavior of T regulatory cells, Th17 cells, CD8+ cytotoxic T cells and NK cells, as well as skewing of CD4+ and CD8+ T memory cell subset distributionss, suggesting failure of an early anti-myeloma response, which is replaced by progressive immunosuppression, and dysfunction and exhaustion of CD8+ T cell tumor cytotoxicity. Our new model is a flexible tool to investigate the early cellular interactions that initiate immunosuppression and MM disease progression. The model can also be used to test the efficacy of new therapeutic strategies.

Background: Malignant prostate cancer (PrC) remains challenging to treat due to tumor heterogeneity and the limited availability of ligands that target disease-associated surface markers with properties appropriate for targeted drug delivery systems. To overcome this hurdle, we used an unbiased but stringent selection strategy to discover a series of phage-displayed peptides that internalize specifically into PrC tumors. Methods: Here we report the characteristics, properties and function of one of these peptides, Pr10, and validate its ability to specifically deliver cytotoxic drugs into PrC cells and kill them, both in vitro and in xenograft models. Results: Biochemical and proteomic studies identified the receptor for Pr10 as Desmocollin-3 (DSC3). This finding was confirmed by demonstrating the expression of the DSC-3 protein on PrC cells; by siRNA knockdown of DSC3 expression, which abrogated Pr10 function; and by in silico docking experiments. Conclusions: Together, these findings identify DSC3 as a novel, functional receptor on malignant prostate cancer cells and establish Pr10 as an effective and PrC-selective ligand for drug delivery PrC cells. More broadly, this work highlights the ability of unbiased screening approaches to identify and isolate novel target receptors with properties appropriate for use in effective target drug delivery systems for cancer therapy.