Genetically engineered Newcastle disease virus for prostate cancer: a magic bullet or a misfit

Keywords::

• Newcastle disease virus
• oncolytic virotherapy
• oncolytic virus
• prostate cancer
• prostate-specific antigen retargeting

Prostate cancer (CaP) is the second leading cause of cancer-related deaths in the USA [1]. The US Institute of Medicine has identified CaP as among the top 25 priorities for future comparative effectiveness research in the USA [2]. Radical prostatectomy is the most common treatment for organ confined or localized CaP in the USA. Radical prostatectomy and radiation therapy result in an overall rate of 5–10 years disease-free survival (DFS). Hormonal treatment, referred to as androgen deprivation therapy (ADT) is also used to treat patients with localized CaP. However, the persistence of castration-resistant cells within prostate tumors and their ability to proliferate in the absence of androgen causes the lethal hormone-refractory CaP [3]. Most of the patients that received ADT show an initial response, but eventually develop androgen-independent CaP within 12–18 months with rising prostate-specific antigen (PSA). Chemotherapeutic agents and anticancer drugs such as docetaxel are used for treating hormone-refractory CaP and are palliative. Clinical experience with the US FDA approved CaP vaccine, sipuleucel-T (Provenge^®) is limited but it has also been shown to only improve survival. Unfortunately, second-line treatment options are limited and most patients presented with metastases perish within 5 years of diagnosis [4]. There is currently no effective cure for advanced and metastatic CaP and novel therapeutic strategies are essential considering the fact that one man in 36 will die of CaP.

Viral therapy of cancer

The use of viruses to treat cancer is by no means new and Newcastle disease virus (NDV) treatment for cancer is no exception. Early clinical trials prior to the 1960s such as those for metastatic melanoma with live-attenuated rabies virus, although induced remissions in some, resulted in releapse in most patients, which lead to the abandonment of virus therapy for cancer [5]. Wheelock and Dingle provided the first evidence in 1964 that with a combination of influenza A and B, Newcastle disease and arboviruses resulted in partial remissions of acute myelogenous leukemia in a patient who subsequently died [6]. Cassel and Garrett, injected 2.4 × 10¹² NDV particles directly into the cervical tumor of a single patient and demonstrated intratumoral regression of the local cancer and also a distant malignant lymph node [7]. Subsequently several different NDV strains including 73-T, Ulster, MTH-68, Italien, Hickman, PV701, HUJ and LaSota were used in various forms and in surprisingly large doses by repeated injections in human clinical trials against a variety of neoplasms (reviewed in [8]). Overall, direct administration of NDV to patients has been shown to result in minimal toxicity with a suggestion of clinical benefit in some patients [8].

Limitations of viral therapy

It was evident from many of the Phase I and II clinical trials that despite repeated administrations of large doses of NDV, complete remissions of metastatic disease seldom occurred, the disease stabilized but relapsed in most cases. It is possible that a nonuniform intravenous delivery and transport to tumor interstitium and inefficient transport through the interstitium to target cells may limit viral therapy [9]. It is also plausible that repeated large doses of the virus, induced tolerance to NDV or there was no effect on self-renewing cancer stem cells leading to relapse. Therefore, it is logical to develop an oncolytic NDV with a better therapeutic index that would lead to complete tumor eradication and DFS.

Viruses against CaP

Respiratory syncytial virus selectively induced apoptosis in PC-3 CaP cells in vitro and in in vivo xenografts mediated through downregulation of NF-κB activity [10]. Similarly, inactivated Sendai virus (hemagglutinating virus of Japan envelope) directly induced cytotoxicity in PC-3 and DU145 cells and completely regressed PC3 xenografts in 85% of the Severe combined immunodeficient mice [11]. Prostate specific membrane antigen receptor-induced tumor regression of LNCaP and PC3 tumor xenografts by a live-attenuated strain of measles virus has been reported [12]. The efficacy of oncolytic vesicular stomatitis virus depends on the type of infected cells. While LNCaP cells were sensitive, PC-3 cells were resistant to vesicular stomatitis virus-mediated cell death [13].

NDV for CaP

With the advent of reverse genetics, it became possible to genetically manipulate NDV [14], Since then, NDV has been modified to enhance its antitumor activity with approaches that increase cytolytic properties, fusogenicity, apoptosis and inhibition or enhancement of innate immune responses; as well as increasing antitumor immunity by expressing tumor-associated antigens, tumor-directed antibodies for activation of antibody-dependent cellular cytotoxicity and expression of immunostimulatory cytokines for the recruitment of lymphocytes and dendritic cells to the tumor bed (reviewed in [8]).

NDV strain 73-T is the most well-characterized oncolytic strain [15]. Several solid tumors including prostate (PC-3), epidermoid (KB8-5-11), colon (SW620 and HT29), large cell lung (NCIH460), breast (SKBR3) and low passage colon (MM17387) carcinoma xenografts treated intratumorally with the strain 73-T regressed effectively [16]. None of the NDV Phase I/II clinical trials included patients with CaP.

NDV kills cancer cells of ecto, endo and mesodermal origin by the activation of both extrinsic and intrinsic pathways of apoptosis in cell type and virus-specific manner [17]. Cancer cell type-specific innate immune responses against NDV determine tumor selectivity [18]. Mutations of the V protein of NDV enhance the safety but retain the oncolytic efficacy [19]. Conventional methods of growing NDV in developing chicken embryos would mark the virus for complement-mediated neutralization in cancer patients and NDV meant for human clinical trials should be grown in human cells where NDV acquires complement regulatory proteins on the envelope to resist human complement [20].

Very recently, PSA retargeted NDV is purported to be a potential oncolytic agent against CaP [21]. Onolcytic virus (OV) therapy with NDV offers many advantages. The mainstay for localized, locally advanced and metastatic CaP is ADT. The underlying principle of ADT is to deprive androgen from CaP cells. ADT with estrogen agonists, gonadotropin hormone-releasing agonists and antagonists lead to adverse effects and affect the quality of life (QoL) of patients. The timing of and proper use of these agents are also controversial. While ADT proved to be beneficial in locally advanced CaP, adjuvant ADT results in substantial side effects besides being noncurative [22]. As many as 94% of the CaP patients respond to ADT [23] but the effects are short lived and the tumors become androgen hypersensitive leading to increased PSA production and tumor regrowth. The PSA retargeted NDV is inherently tumor selective and also tailored for these situations. First, let us consider locally advanced CaP. The management of locally advanced CaP remains controversial due to side effects and QoL issues despite neoadjuvant ADT combined with irradiation is considered standard therapeutic choice [24]. Overuse of ADT might partly explain the notably high rates of cancer-specific mortality among men diagnosed with high-risk CaP [25]. PSA retargeted NDV is free from side effects and QoL issues. PSA-retargeted NDV will be beneficial to treat CaP from the initial organ-confined stage to the advanced metastatic stages. Besides, it will also seek out PSA producing metastatic foci of CaP cells in lymphnodes and elsewhere. In these instances, there would be sufficient active PSA in cancer cells. The reported mean PSA in the serum of patients treated with ADT and radiation therapy ranges from 14.1 to 51.3 ng/ml [26]. For the replication of PSA-retargeted NDV in prostate cancer cells in in vitro cell cultures, 100 ng/ml of PSA is sufficient [21]. This suggests that the amount of PSA available in the tumor microenvironment and CaP cells would be more than sufficient for NDV to effect oncolysis.

With increasing numbers of men undergoing PSA testing, the disease is being diagnosed earlier and the costs of ADT, with uncertain survival benefits and associated risks, have risen dramatically. Clinical studies of potent novel agents have shown survival benefits in advanced disease, but timing, risks and cost–effectiveness of treatment remain controversial [27]. Besides, observational evidence associates the use of ADT with an increased risk of diabetes, stroke, fatal and nonfatal myocardial infarction in men with CaP [28]. As stated by Bourke et al., it is inevitable, that should a man survive long enough on ADT, castrate-resistant CaP will develop and until recently, few treatment options had been shown to be effective in improving overall survival in men with such disease. Taxane-based chemotherapy has been shown to be effective in this group of men, but is associated with significant toxicity [27]. Considering all these risks and the increased cost of ADT and QoL issues, OV therapy with PSA-retargeted NDV appears promising.

Future perspective

The complex, cellular, molecular and physical features in CaP present major challenges to effective treatment, as well as vulnerabilities that could be exploited. Increased vascular permeability contributes to elevated interstitial fluid pressure, within solid tumors. The major limitation for NDV-based therapy to CaP would be the lack of ability to achieve effective intratumoral penetration. Androgen receptor engagement with synthetic androgen analogs such as R1881 increased PSA levels in invasive WPE-int cells and enhanced virus spread of PSA retargeted NDV[21]. The situation is analogous to ADT in locally advanced and advanced CaP. Furthermore, other options are available for extracellular matrix degradation [29] including engineering extracellular matrix degrading proteins or antiangiogenic factors in NDV. The second major concern would be the need for identifying an effective dose for long-term DFS that could be answered only from clinical trails. There is a concern that repeated administrations of NDV would lead to antiviral immunity. The fact that there are nine established antigenically distinct serotypes of avian paramyxoviruses (APMV1-9), offers envelope exchange, which would greatly alleviate this concern. Additionally, whether metastatic foci would be effectively targeted by NDV is unknown. The precise nature of the genetic modification in PSA-retargeted NDV would allow only targeting PSA producing cells, whether in circulation or in solid tumors or metastatic sites. A recent study with recombinant NDV attested to this approach by showing efficacy in the treatment of peritoneally disseminated gastric cancer in a xenograft model [30]. Our recent unpublished studies indicate that PSA-retargeted NDV also efficiently kills CaP stem cells. Mutually, synergistic therapies with NDV and other oncolytic viruses or radiation, chemotherapy and ADT are also possible, in which the relative doses of toxic therapies could be moderated to avoid side effects and QoL issues. These arguments provide further rationale for clinical trials with PSA retargeted NDV against CaP.

Financial & competing interests disclosure

The prostate-specific antigen-retargeted Newcastle disease virus discussed in the manuscript was developed through fundnig from US Department of Defense under a ‘congressionally directed Medical Research Program for prostate cancer’ concept award to S Elankumaran. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.