The lack of validated molecular profiles clearly distinguishing aggressive phenotypes of prostate cancer, and the absence of suitable intermediate endpoints make this tumor difficult to manage rationally. We have pioneered and developed an in vivo screening system in which peptide-targeted particles capable of homing to tumors are recovered from a phage display random library following intravenous administration. In an unbiased screening of an established patient-derived xenograft (PDX) with several biological attributes reminiscent of human metastatic prostate cancer, we first isolated and validated tumor-homing peptides targeting the cell surface-associated glucose-regulated protein-78 kD (GRP78); we also screened a patient directly to isolate and validate ligands targeting the interleukin-11 receptor (IL11R). We next inserted cis- genetic elements from adeno-associated virus (AAV) within the phage genome and established that hybrid ligand-directed AAV/phage (AAVP) particles enable systemic tumor targeted delivery and molecular-genetic imaging of reporter transgene in preclinical models of prostate cancer. We hypothesize that promoters of upregulated transgenes in prostate cancer should enable transcription-driven suicide gene expression for tumor imaging with in tandem growth suppression. Work from our group (among others) shows that overexpression of GRP78 and IL11R is related to disease progression. As such, promoters for these genes will be evaluated in new AAVP constructs.
In Aim 1, we will validate GRP78 and IL11R, in addition to promoters of other genes identified by transcriptome analysis of prostate cancer patient-derived samples. These promoters will be cloned into reporter plasmids to initially assess their properties relative to a standard CMV promoter in prostate cancer cells and 3-dimensional patient-derived osteogenic prostate cancer cultures in vitro.
In Aim 2, the constructs will be evaluated in animal models. Our strategy relies on intravenously injected GRP78-targeting AAVP engineered to deliver a Herpes Simplex Virus thymidine kinase (HSVtk) transgene. Promoter-driven TK expression will be imaged with [18F]-FEAU by PET/CT and quantified by qPCR. Tumor growth inhibition after ganciclovir treatment will be assessed in vivo by imaging, and tumor sections will be evaluated by immunohistochemistry. The integration of transcriptional profiling, prostate cancer cell targeting and molecular-genetic imaging within a single platform will potentially allow for the monitoring of progression and response to therapy in pathologically indistinguishable, yet biologically diverse patient cohorts with different disease outcomes. Given that the previous body of work has established GRP78 and IL11R as bona fide molecular targets in human prostate cancer, our methodological approach and findings bear mechanistic and translational significance.
Monitoring of tumor progression and/or response to therapy in pathologically indistinguishable (yet biologically and clinically diverse) cohorts of prostate cancer patients remains a substantial unmet need, so here we seek to integrate an adaptive theranostic gene therapy-based approach using highly engineered phage-AAV chimeric vectors to specifically deliver both an imaging and therapeutic agent. Novel tumor targeting vectors will be available for potential evaluation in first-in-human studies. This work addresses an unmet need in prostate cancer and, if successful, it will ultimately lead to a blueprint towards translational applications in an accelerated framework.