Exploitation of CD133 for the Targeted Imaging of Lethal Prostate Cancer

PURPOSE

Aggressive variant prostate cancer (AVPC) is a nonandrogen receptor-driven form of disease that arises in men in whom standard-of-care therapies have failed. Therapeutic options for AVPC are limited, and the development of novel therapeutics is significantly hindered by the inability to accurately quantify patient response to therapy by imaging. Imaging modalities that accurately and sensitively detect the bone and visceral metastases associated with AVPC do not exist.

EXPERIMENTAL DESIGN

This study investigated the transmembrane protein CD133 as a targetable cell surface antigen in AVPC. We evaluated the expression of CD133 by microarray and IHC analysis. The imaging potential of the CD133-targeted IgG (HA10 IgG) was evaluated in preclinical prostate cancer models using two different imaging modalities: near-infrared and PET imaging.

RESULTS

Evaluation of the patient data demonstrated that CD133 is overexpressed in a specific phenotype of AVPC that is androgen receptor indifferent and neuroendocrine differentiated. In addition, HA10 IgG was selective for CD133-expressing tumors in all preclinical imaging studies. PET imaging with [⁸⁹Zr]Zr-HA10 IgG revealed a mean %ID/g of 24.30 ± 3.19 in CD133-positive metastatic lesions as compared with 11.82 ± 0.57 in CD133-negative lesions after 72 hours (P = 0.0069). Ex vivo biodistribution showed similar trends as signals were increased by nearly 3-fold in CD133-positive tumors (P < 0.0001).

CONCLUSIONS

To our knowledge, this is the first study to define CD133 as a targetable marker of AVPC. Similarly, we have developed a novel imaging agent, which is selective for CD133-expressing tumors, resulting in a noninvasive PET imaging approach to more effectively detect and monitor AVPC.

PEG10 Promoter-Driven Expression of Reporter Genes Enables Molecular Imaging of Lethal Prostate Cancer

The retrotransposon-derived paternally expressed gene 10 (PEG10) protein is ordinarily expressed at high levels in the placenta. Recently, it was discovered that PEG10 isoforms promote the progression of prostate cancer to a highly lethal androgen receptor (AR)-negative phenotype. The presence of PEG10 in other subtypes of prostate cancer has not been explored and a utility for PEG10 overexpression has not been developed. Here, we found that in addition to AR-null disease, PEG10 was also expressed in prostate cancer with constitutively active AR-splice variants. A molecular genetic imaging strategy for noninvasive imaging of AR-splice variant prostate cancer was developed by utilizing the cancer specificity of the PEG10 promoter to drive the expression of reporter genes. Plasmid insertion of a PEG10 promoter sequence optimized for enhanced output upstream of a reporter gene allowed detection of prostate cancer by near-infrared and positron emission tomography imaging after systemic administration of the plasmid in vivo. PEG10 expressing subcutaneous xenograft and intratibial tumor models were imaged by both modalities using this molecular genetic imaging strategy. This study demonstrates a preclinical proof-of-concept that the PEG10 promoter is a powerful and specific tool that can be utilized for noninvasive detection of aggressive prostate cancer subtypes. SIGNIFICANCE: PEG10 is expressed by prostate cancer with constitutively active AR-splice variants that can be exploited for noninvasive molecular imaging of this aggressive prostate cancer subytpe.

Abstract 1145: Biomarker-driven molecular imaging of prostate cancer using the PEG10 promoter

The retrotransposon-derived paternally expressed gene 10 (PEG10) protein is ordinarily expressed at high levels in the placenta. The over-expression of PEG10 has been documented in a number of malignancies including hepatocellular carcinoma and leukemia. Recently, it was discovered that PEG10 isoforms promoted the progression of prostate adenocarcinoma to a highly lethal non-androgen receptor (AR) driven subtype called aggressive variant prostate cancer (AVPC).

In this study, we found that PEG10 was also expressed in castration-resistant PCa possessing constitutively active AR-splice variants in addition to AR-negative AVPC. We subsequently developed a molecular genetic imaging strategy for the non-invasive imaging of PCa by utilizing the cancer specificity of the PEG10 promoter to drive the expression of reporter genes. The transcriptional output of the PEG10 promoter was enhanced to enable high expression of reporter genes by multiple imaging modalities and immunoassay. By using this PEG10 promoter upstream of a reporter gene in a plasmid, we were able to detect PCa by fluorescence and positron emission tomography (PET) imaging after systemic administration of the plasmid in mice. This method also allowed for the detection of PCa by an immunoassay of mouse urine.

Our study demonstrates a pre-clinical proof-of-concept that the PEG10 promoter is a powerful and specific tool that can be utilized for non-invasive detection of highly lethal PCa subtypes. This biomarker imaging technology has the potential to improve detection and can be further modified for therapeutic applications.

Citation Format: Mariya Shapovalova. Biomarker-driven molecular imaging of prostate cancer using the PEG10 promoter [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1145.

Development of a Cross-Reactive Monoclonal Antibody for Detecting the Tumor Stroma

Here, we document the discovery of a monoclonal antibody that selectively binds to both human and murine fibroblast activation protein alpha (FAP), a serine protease that is overexpressed on cancer-associated fibroblasts (CAFs), making it an attractive therapeutic target for the aiding and abetting tumor microenvironment. The lead antibody, B12, was identified from a naïve murine single-chain variable fragment antibody phage display library screened against recombinant human FAP on magnetic beads. The heavy and light chains of B12 were cloned into full-length human immunoglobulin 1 (IgG) vectors and expressed as a chimeric monoclonal antibody (B12 IgG). We engineered a drug-resistant prostate cancer cell line, CWR-R1-EnzR, to express human FAP for antibody characterization and validation (R1-EnzR^FAP). B12 IgG selectively bound to the R1-EnzR^FAP cells by flow cytometry and was internalized in vitro by confocal microscopy. B12 IgG was further evaluated as a near-infrared (NIR) optical imaging probe in R1-EnzR^FAP and parental xenograft models. High tumor uptake and retention of the NIR probe was observed in the R1-EnzR^FAP xenografts, and endogenous expression of murine stromal origin FAP was detected in the parental xenografts. Ex vivo evaluation of these models by immunohistochemistry documented B12 IgG localization to both human and murine FAP-expressing cells.

The Molecular Imaging of Natural Killer Cells

The recent success of autologous T cell-based therapies in hematological malignancies has spurred interest in applying similar immunotherapy strategies to the treatment of solid tumors. Identified nearly 4 decades ago, natural killer (NK) cells represent an arguably better cell type for immunotherapy development. Natural killer cells are cytotoxic lymphocytes that mediate the direct killing of transformed cells with reduced or absent major histocompatibility complex (MHC) and are the effector cells in antibody-dependent cell-mediated cytotoxicity. Unlike T cells, they do not require human leukocyte antigen (HLA) matching allowing for the adoptive transfer of allogeneic NK cells in the clinic. The development of NK cell-based therapies for solid tumors is complicated by the presence of an immunosuppressive tumor microenvironment that can potentially disarm NK cells rendering them inactive. The molecular imaging of NK cells in vivo will be crucial for the development of new therapies allowing for the immediate assessment of therapeutic response and off-target effects. A number of groups have investigated methods for detecting NK cells by optical, nuclear, and magnetic resonance imaging. In this review, we will provide an overview of the advances made in imaging NK cells in both preclinical and clinical studies.

The Rational Design of Therapeutic Peptides for Aminopeptidase N using a Substrate-Based Approach

The M1 family of metalloproteases represents a large number of exopeptidases that cleave single amino acid residues from the N-terminus of peptide substrates. One member of this family that has been well studied is aminopeptidase N (APN), a multifunctional protease known to cleave biologically active peptides and aide in coronavirus entry. The proteolytic activity of APN promotes cancer angiogenesis and metastasis making it an important target for cancer therapy. To understand the substrate specificity of APN for the development of targeted inhibitors, we used a global substrate profiling method to determine the P1-P4' amino acid preferences. The key structural features of the APN pharmacophore required for substrate recognition were elucidated by x-ray crystallography. By combining these substrate profiling and structural data, we were able to design a selective peptide inhibitor of APN that was an effective therapeutic both in vitro and in vivo against APN-expressing prostate cancer models.