Adenovirus targeting to prostate-specific membrane antigen through virus-displayed, semirandom peptide library screening

Oncolytic Adenoviruses: Strategies for Improved Targeting and Specificity

Cancer is a major health problem. Most of the treatments exhibit systemic toxicity, as they are not targeted or specific to cancerous cells and tumors. Adenoviruses are very promising gene delivery vectors and have immense potential to deliver targeted therapy. Here, we review a wide range of strategies that have been tried, tested, and demonstrated to enhance the specificity of oncolytic viruses towards specific cancer cells. A combination of these strategies and other conventional therapies may be more effective than any of those strategies alone.

Retargeting adenoviruses for therapeutic applications and vaccines

Adenoviruses (Ads) are robust vectors for therapeutic applications and vaccines, but their use can be limited by differences in their in vitro and in vivo pharmacologies. This review emphasizes that there is not just one Ad, but a whole virome of diverse viruses that can be used as therapeutics. It discusses that true vector targeting involves not only retargeting viruses, but importantly also detargeting the viruses from off-target cells.

Gene Therapy Leaves a Vicious Cycle

The human genetic code encrypted in thousands of genes holds the secret for synthesis of proteins that drive all biological processes necessary for normal life and death. Though the genetic ciphering remains unchanged through generations, some genes get disrupted, deleted and or mutated, manifesting diseases, and or disorders. Current treatment options—chemotherapy, protein therapy, radiotherapy, and surgery available for no more than 500 diseases—neither cure nor prevent genetic errors but often cause many side effects. However, gene therapy, colloquially called “living drug,” provides a one-time treatment option by rewriting or fixing errors in the natural genetic ciphering. Since gene therapy is predominantly a viral vector-based medicine, it has met with a fair bit of skepticism from both the science fraternity and patients. Now, thanks to advancements in gene editing and recombinant viral vector development, the interest of clinicians and pharmaceutical industries has been rekindled. With the advent of more than 12 different gene therapy drugs for curing cancer, blindness, immune, and neuronal disorders, this emerging experimental medicine has yet again come in the limelight. The present review article delves into the popular viral vectors used in gene therapy, advances, challenges, and perspectives.

Tropism and transduction of oncolytic adenovirus 5 vectors in cancer therapy: Focus on fiber chimerism and mosaicism, hexon and pIX

The cellular internalization (infection of cells) of adenovirus 5 (Ad5) is mediated by the initial attachment of the globular knob domain of the capsid fiber protein to the cell surface coxsackievirus and adenovirus receptor (CAR), then followed by the interaction of the virus penton base proteins with cellular integrins. In tumors, there is a substantial intra- and intertumoral variability in CAR expression. The CAR-negative cells generally exhibit very low infectability. Since the fiber knob is a primary mediator of Ad5 binding to the cell surface, improved infectivity of Ad5-based vectors as oncolytic agents may be achieved via genetic modifications of this domain. The strategies to modify or broaden tropism and increase transduction efficiency of Ad5-based vectors include: 1) an incorporation of a targeting peptide into the fiber knob domain (the HI loop and/or C-terminus); 2) fiber knob serotype switching, or pseudotyping, by constructing chimeric fibers consisting of the knob domain derived from an alternate serotype (e.g., Ad5/3 or Ad5/35 chimeras), which binds to receptor(s) other than CAR (e.g., desmoglein 2/DSG2 and/or CD46); 3) "fiber complex mosaicism", an approach of combining serotype chimerism with peptide ligand(s) incorporation (e.g., Ad5/3-RGD); 4) "dual fiber mosaicism" by expressing two separate fibers with distinct receptor-binding capabilities on the same viral particle (e.g., Ad5-5/3 or Ad5-5/σ1); 5) fiber xenotyping by replacing the knob and shaft domains of wild-type Ad5 fiber protein with fibritin trimerization domain of T4 bacteriophage or σ1 attachment protein of reovirus. Other genetic approaches to increase the CAR-independent transduction efficiency include insertion of a targeting peptide into the hypervariable region of the capsid protein hexon or fusion to the C-terminus of pIX. Finally, we consider a yet unsolved molecular mechanism of liver targeting by Ad5-based vectors (CAR-, integrin-, fiber shaft KKTK motif-, and hepatic heparan sulfate glycosaminoglycans-independent, but fiber-, hexon- and blood factor X-dependent).

Retargeted and detargeted adenovirus for gene delivery to the muscle

We previously selected muscle binding peptides 12.51 and 12.52 from "context-specific" phage display libraries for introduction into adenovirus (Ad) vectors. In this work, these peptides were inserted into the hypervariable region (HVR) 5 loop of the Ad5 hexon protein to display 720 peptides per virions. HVR-12.51 and 12.52 increased transduction of C2C12 cells up to 20-fold when compared to unmodified Ad5. 12.51 increased in vivo muscle transduction 2 to 7-fold over unmodified Ad after intramuscular injection in mice and hamsters. 12.52 did not increase muscle transduction. Notably, insertion of 12.51 into the hexon reduced liver transduction 80-fold when compared to unmodified Ad5 after intravenous injection. Increased muscle transduction in mice translated into increased immune responses after gene-based vaccination. These data suggest there are merits to retargeting and detargeting benefits to modifying the hexons of Ads with peptide ligands.

Recent advances in genetic modification of adenovirus vectors for cancer treatment

Adenoviruses are widely used to deliver genes to a variety of cell types and have been used in a number of clinical trials for gene therapy and oncolytic virotherapy. However, several concerns must be addressed for the clinical use of adenovirus vectors. Selective delivery of a therapeutic gene by adenovirus vectors to target cancer is precluded by the widespread distribution of the primary cellular receptors. The systemic administration of adenoviruses results in hepatic tropism independent of the primary receptors. Adenoviruses induce strong innate and acquired immunity in vivo. Furthermore, several modifications to these vectors are necessary to enhance their oncolytic activity and ensure patient safety. As such, the adenovirus genome has been engineered to overcome these problems. The first part of the present review outlines recent progress in the genetic modification of adenovirus vectors for cancer treatment. In addition, several groups have recently developed cancer-targeting adenovirus vectors by using libraries that display random peptides on a fiber knob. Pancreatic cancer-targeting sequences have been isolated, and these oncolytic vectors have been shown by our group to be associated with a higher gene transduction efficiency and more potent oncolytic activity in cell lines, murine models, and surgical specimens of pancreatic cancer. In the second part of this review, we explain that combining cancer-targeting strategies can be a promising approach to increase the clinical usefulness of oncolytic adenovirus vectors.

Prostate-specific membrane antigen targeted protein contrast agents for molecular imaging of prostate cancer by MRI

Prostate-specific membrane antigen (PSMA) is one of the most specific cell surface markers for prostate cancer diagnosis and targeted treatment. However, achieving molecular imaging using non-invasive MRI with high resolution has yet to be achieved due to the lack of contrast agents with significantly improved relaxivity for sensitivity, targeting capabilities and metal selectivity. We have previously reported our creation of a novel class of protein Gd(3+) contrast agents, ProCA32, which displayed significantly improved relaxivity while exhibiting strong Gd(3+) binding selectivity over physiological metal ions. In this study, we report our effort in further developing biomarker-targeted protein MRI contrast agents for molecular imaging of PSMA. Among three PSMA targeted contrast agents engineered with addition of different molecular recognition sequences, ProCA32.PSMA exhibits a binding affinity of 1.1 ± 0.1 μM for PSMA while the metal binding affinity is maintained at 0.9 ± 0.1 × 10(-22) M. In addition, ProCA32.PSMA exhibits r1 of 27.6 mM(-1) s(-1) and r2 of 37.9 mM(-1) s(-1) per Gd (55.2 and 75.8 mM(-1) s(-1) per molecule r1 and r2, respectively) at 1.4 T. At 7 T, ProCA32.PSMA also has r2 of 94.0 mM(-1) s(-1) per Gd (188.0 mM(-1) s(-1) per molecule) and r1 of 18.6 mM(-1) s(-1) per Gd (37.2 mM(-1) s(-1) per molecule). This contrast capability enables the first MRI enhancement dependent on PSMA expression levels in tumor bearing mice using both T1 and T2-weighted MRI at 7 T. Further development of these PSMA-targeted contrast agents are expected to be used for the precision imaging of prostate cancer at an early stage and to monitor disease progression and staging, as well as determine the effect of therapeutic treatment by non-invasive evaluation of the PSMA level using MRI.

A novel approach for detecting viable and tissue-specific circulating tumor cells through an adenovirus-based reporter vector

BACKGROUND

Circulating tumor cells (CTCs) hold great promise as biomarkers and are a direct source of tumor cells through a simple blood draw. However, CTCs are rare and their detection requires sensitive and specific methods to overcome the overwhelming hematocyte population. Therefore, CTC detection remains technically challenging.

METHODS

An assay was developed for detecting viable and tissue-specific CTCs using a tropism-enhanced and conditionally replicating reporter adenovirus (CTC-RV). Adenoviral replication was made prostate-specific by placing the E1A gene under the control of the probasin promoter and prostate-specific antigen enhancer (PSE-PBN). Viral tropism was expanded through capsid-displayed integrin targeting peptides. A secreted reporter, humanized Metridia Luciferase (hMLuc), was engineered for expression during the major late phase of viral replication. The assay involves red blood cell lysis, cell collection, viral infection, and subsequent quantification of reporter activity from cellular media. Assay and reporter stability, cell specificity and sensitivity were evaluated in cell dilution models in human blood.

RESULTS

A conditionally replicating prostate-selective adenovirus reporter and CTC assay system were generated. The secreted reporter, MLuc, was found to be stable for at least 3 days under assay conditions. CTC detection, modeled by cell dilution in blood, was selective for androgen receptor positive prostate cancer (PCa) cells. Serial dilution demonstrated assay linearity and sensitivity to as few as three cells. Prostate cancer cell viability declined after several hours in anticoagulated blood at ambient temperatures.

CONCLUSIONS

Conditionally replicative adenoviral vectors and secreted reporters offer a functional method to detect viable CTCs with cell specificity and high sensitivity.

Adenoviruses using the cancer marker EphA2 as a receptor in vitro and in vivo by genetic ligand insertion into different capsid scaffolds

Adenoviral gene therapy and oncolysis would critically benefit from targeted cell entry by genetically modified capsids. This requires both the ablation of native adenovirus tropism and the identification of ligands that remain functional in virus context. Here, we establish cell type-specific entry of HAdV-5-based vectors by genetic ligand insertion into a chimeric fiber with shaft and knob domains of the short HAdV-41 fiber (Ad5T/41sSK). This fiber format was reported to ablate transduction in vitro and biodistribution to the liver in vivo. We show that the YSA peptide, binding to the pan-cancer marker EphA2, can be inserted into three positions of the chimeric fiber, resulting in strong transduction of EphA2-positive but not EphA2-negative cells of human melanoma biopsies and of tumor xenografts after intratumoral injection. Transduction was blocked by soluble YSA peptide and restored for EphA2-negative cells after recombinant EphA2 expression. The YSA peptide could also be inserted into three positions of a CAR binding-ablated HAdV-5 fiber enabling specific transduction; however, the Ad5T/41sSK format was superior in vivo. In conclusion, we establish an adenovirus capsid facilitating functional insertion of targeting peptides and a novel adenovirus using the tumor marker EphA2 as receptor with high potential for cancer gene therapy and viral oncolysis.

Development of peritoneal tumor-targeting vector by in vivo screening with a random peptide-displaying adenovirus library

The targeting of gene transfer at the cell-entry level is one of the most attractive challenges in vector development. However, attempts to redirect adenovirus vectors to alternative receptors by engineering the capsid-coding region have shown limited success, because the proper targeting ligands on the cells of interest are generally unknown. To overcome this limitation, we have constructed a random peptide library displayed on the adenoviral fiber knob, and have successfully selected targeted vectors by screening the library on cancer cell lines in vitro. The infection of targeted vectors was considered to be mediated by specific receptors on target cells. However, the expression levels and kinds of cell surface receptors may be substantially different between in vitro culture and in vivo tumor tissue. Here, we screened the peptide display-adenovirus library in the peritoneal dissemination model of AsPC-1 pancreatic cancer cells. The vector displaying a selected peptide (PFWSGAV) showed higher infectivity in the AsPC-1 peritoneal tumors but not in organs and other peritoneal tumors as compared with a non-targeted vector. Furthermore, the infectivity of the PFWSGAV-displaying vector for AsPC-1 peritoneal tumors was significantly higher than that of a vector displaying a peptide selected by in vitro screening, indicating the usefulness of in vivo screening in exploring the targeting vectors. This vector-screening system can facilitate the development of targeted adenovirus vectors for a variety of applications in medicine.

High expression of prostate-specific membrane antigen in the tumor-associated neo-vasculature is associated with worse prognosis in squamous cell carcinoma of the oral cavity

Prostate-specific membrane antigen (PSMA) is a transmembrane protein expressed in prostate cancer as well as in the neo-vasculature of nonprostatic solid tumors. Here, we determined the expression pattern of PSMA in the vasculature of oral squamous cell carcinoma. Using a previously validated antibody, PSMA staining distribution and cyclooxygenase 2 (COX2) expression status was evaluated in a cohort of patients with squamous cell carcinoma of the oral cavity (n=96) using immunohistochemistry and was correlated with clinicopathological features as well as outcome. Twenty-four (25%) cases showed no detectable PSMA staining, 48 (50%) demonstrated positive immunoreactivity for PSMA in less than 50% of microvessels and 24 (25%) cases showed strong endothelial PSMA expression in more than 50% of tumor-associated microvessels. High endothelial PSMA expression was associated with greatly reduced survival (18.2 vs 77.3 months; P=0.0001) and maintained prognostic significance after adjusting for grade and stage in multivariate analysis (hazard ratio=2.19, P=0.007). Furthermore, we observed a strong association between endothelial PSMA and cancer cell-specific COX2 expression. In conclusion, we provide the first evidence for the prognostic significance of endothelial PSMA expression in oral squamous cell carcinoma and, suggest a potential interaction between arachidonic acid metabolites and endothelial PSMA expression in the tumor neo-vasculature.

2-(3-{1-Carboxy-5-[(6-[18F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid, [18F]DCFPyL, a PSMA-based PET imaging agent for prostate cancer

PURPOSE

We have synthesized and evaluated in vivo 2-(3-{1-carboxy-5-[(6-[(18)F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid, [(18)F]DCFPyL, as a potential imaging agent for the prostate-specific membrane antigen (PSMA). PSMA is upregulated in prostate cancer epithelia and in the neovasculature of most solid tumors.

EXPERIMENTAL DESIGN

[(18)F]DCFPyL was synthesized in two steps from the p-methoxybenzyl (PMB) protected lys-C(O)-glu urea precursor using 6-[(18)F]fluoronicotinic acid tetrafluorophenyl ester ([(18)F]F-Py-TFP) for introduction of (18)F. Radiochemical synthesis was followed by biodistribution and imaging with PET in immunocompromised mice using isogenic PSMA PC3 PIP and PSMA- PC3 flu xenograft models. Human radiation dosimetry estimates were calculated using OLINDA/EXM 1.0.

RESULTS

DCFPyL displays a K(i) value of 1.1 ± 0.1 nmol/L for PSMA. [(18)F]DCFPyL was produced in radiochemical yields of 36%-53% (decay corrected) and specific radioactivities of 340-480 Ci/mmol (12.6-17.8 GBq/μmol, n = 3). In an immunocompromised mouse model [(18)F]DCFPyL clearly delineated PSMA+ PC3 PIP prostate tumor xenografts on imaging with PET. At 2 hours postinjection, 39.4 ± 5.4 percent injected dose per gram of tissue (%ID/g) was evident within the PSMA+ PC3 PIP tumor, with a ratio of 358:1 of uptake within PSMA+ PC3 PIP to PSMA- PC3 flu tumor placed in the opposite flank. At or after 1 hour postinjection, minimal nontarget tissue uptake of [(18)F]DCFPyL was observed. The bladder wall is the dose-limiting organ.

CONCLUSIONS

These data suggest [(18)F]DCFPyL as a viable, new positron-emitting imaging agent for PSMA-expressing tissues.

Advances and future challenges in adenoviral vector pharmacology and targeting

Adenovirus is a robust vector for therapeutic applications, but its use is limited by our understanding of its complex in vivo pharmacology. In this review we describe the necessity of identifying its natural, widespread, and multifaceted interactions with the host since this information will be crucial for efficiently redirecting virus into target cells. In the rational design of vectors, the notion of overcoming a sequence of viral "sinks" must be combined with re-targeting to target populations with capsid as well as shielding the vectors from pre-existing or toxic immune responses. It must also be noted that most known adenoviral pharmacology is deduced from the most commonly used serotypes, Ad5 and Ad2. However, these serotypes may not represent all adenoviruses, and may not even represent the most useful vectors for all purposes. Chimeras between Ad serotypes may become useful in engineering vectors that can selectively evade substantial viral traps, such as Kupffer cells, while retaining the robust qualities of Ad5. Similarly, vectorizing other Ad serotypes may become useful in avoiding immunity against Ad5 altogether. Taken together, this research on basic adenovirus biology will be necessary in developing vectors that interact more strategically with the host for the most optimal therapeutic effect.