A Targeted Nanomedicine Prototype Against Enzalutamide-resistant Prostate Cancer

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ABSTRACT Eight percent of patients diagnosed with prostate cancer progress to lethal metastatic disease. Development of resistance to androgen-deprivation therapy and eventually, to last line chemotherapeutics such as enzalutamide (ENZ), contribute to lethal, metastatic prostate cancer. While interest to identify tumor-specific molecular signatures, termed precision medicine, is gaining popular favor, it requires identification of physiologically accessible targets. By diverting the function of a molecular tumor target by conventional anti- cancer drugs, rates of tumor growth are expected to decrease; however, this does not take into account acquired drug resistance mechanisms which are dependent on systemic drug stability, solubility or toxicity. One method to stabilize poorly soluble and/or highly toxic drugs, and potentially overcome resistance, is to encapsulate drugs in nanoparticles (NPs) to prevent their degradation and enhance their circulation time. Moreover, accumulation of loaded NPs at the tumor site can be improved by adding tumor-specific targeting moieties that induce NP endocytosis, thereby improving the therapeutic index while minimizing collateral damage to healthy cells. A prostate tumor-specific biomarker, the 78 kDa glucose-regulated protein (GRP78), was identified by the Pasqualini and Arap team by screening antibodies from prostate cancer patient sera. GRP78 is a biomarker of disease progression and, crucial to our proposed research, we recently identified human recombinant anti-GRP78 antibodies with optimal in vivo tumor targeting. In this proposal, our objective is to generate GRP78-targeted NPs against ENZ-resistant prostate cancer. We will employ the novel, modular “protocell” platform developed by the Brinker team. Protocells consist of a porous silica core, which can be engineered to accommodate varied and combination cargos, encapsulated within a supported lipid bilayer that protects and retains the cargo, and provides a biocompatible surface for conjugation to targeting and/or trafficking ligands. The Brinker team demonstrated exceptional stability of targeted, first-generation protocells in vivo with specific binding and cargo delivery to individual circulating leukemia cells. Instead of delivering chemotherapeutic drugs that work at the protein level, we propose to deliver small interfering RNAs (siRNAs) directed against the long non-coding RNA, PCA3. We showed that interfering with PCA3 inhibits growth of human prostate xenografts. Guided by predictive modeling conducted by the Cristini team, our modular GRP78-targeted protocells will be designed to package PCA3 siRNAs to selectively bind to GRP78-expressing prostate cancer cells, and deliver PCA3 siRNAs intracellularly to inhibit tumor growth. Our project is a first-in- field study that galvanizes our current combined expertise and technology. The dual prostate tumor “centric” feature of these next generation NP prototype platforms increases their specificity and efficacy, and overcomes the limitation of conventional standard-of-care drugs, particularly in the case of acquired drug resistance.