Using HIV drugs to target human papilloma virus

Abstract

Over the past decade it has been demonstrated that HIV protease inhibitors have various off-target activities that has enabled them to be repositioned as treatments for a range of other pathologies. Human papilloma virus and related malignancies have been shown to be susceptible to these agents and current progress with this indication is summarized here together with a discussion of the rationale for the off-target effects of these compounds.

Keywords::

• cervical cancer
• HIV
• HPV
• protease inhibitors
• therapy

High-risk forms of the human papilloma virus (HPV) are well established as the main causative agent for anogenital cancers, particularly cancer of the cervix. A major concern is that vulval, penile and anal cancers are increasing in incidence worldwide. Added to the fact that the incidence of oropharyngeal malignancies associated with HPV is also rising dramatically, this highlights the virus as a major player in human cancer and a key target for therapy.

Globally, with >400,000 new cases per annum, cervical cancer remains the most common HPV-linked malignancy resulting in >275,000 deaths from this disease every year. Indeed, it is in this cancer that most studies of the virus have been undertaken. In the developed countries that have cervical screening programs, HPV-related cervical lesions can be detected at an early stage, and yet, despite this, there is currently no treatment other than surgery. Significantly, surgical intervention does not always eliminate the virus and can lead to treatment-associated complications [1]. Furthermore, while cryotherapy and loop electrosurgical excision have been shown by some to be effective in elimination of HPV, in many low-resource settings (e.g. sub-Saharan Africa), cryotherapy machines and the equipment and expertise required to perform the loop electrosurgical excision procedure are scarce [2]. Paradoxically, these areas have the greatest burden of this disease (>80%).

A variety of non-surgical approaches have been explored as potential complementary therapies to surgery for cervical cancer or alternative (reviewed [3]). For pre-malignant HPV-related dysplasia, these incorporate topical IFN-α, imiquimod, 5-Fluorouracil and cidofovir, although, in 2004, Bernard concluded that none of these were recommended due to side effects, limited efficacy and high recurrence rates [4]. Moreover, many of these treatments are still physician applied and are costly, which would severely limit their use in resource-limited countries.

In 2006, the authors made the first observation that HIV protease inhibitors (PIs) might have activity against HPV-related cervical dysplasia [5] and the authors subsequently published additional pre-clinical work that supported this indication [6,7]. These studies identified that the drug lopinavir had the greatest activity against HPV-immortalized/transformed cells and also defined, in part, its mode-of-action, that is, prevention of HPV E6-mediated degradation of the tumor suppressor p53 via inhibition of the proteasome. In addition, the authors also showed that approximately 10-times the concentration of lopinavir was needed for this effect than could be obtained from normal oral dosing for HIV therapy. Although their results indicated that lopinavir was most effective in vitro against HPV-related disease, the authors also showed that saquinavir, nelfinavir and ritonavir had substantial activity in this respect. Work by other groups has since confirmed the anti-HPV effects of both saquinavir and ritonavir [8]. Inhibition of the 20S proteasome was one of the first reported off-target antineoplastic effects of HIV PIs. Subsequent studies have suggested that inhibition of the PI3K/phospho-Akt pathway, induction of endoplasmic reticulum stress, stimulation of apoptosis and inhibition of matrix metalloproteases can also be added to the anti-cancer repertoire of many HIV PIs [8,9]. In support of these findings, lopinavir is normally co-administered with ritonavir (4:1) but early studies in vitro by the authors suggested this combination was less effective against HPV than lopinavir alone, despite the observation that both HIV PIs could inhibit the proteasome [6,10]. This suggested that other additional off-target effects of these compounds were also having activity against HPV and indeed the authors went on to show that lopinavir treatment leads to an induction of numerous pro-apoptotic proteins and the anti-viral protein RNase L [6]. Recent unpublished work by the author’s group has observed that a further effect of lopinavir is its ability to inhibit activation of the mammalian target of rapamycin – a current key cancer target.

Clearly, there is now a large body of pre-clinical evidence accumulating for the alternative use of HIV PIs to treat a range of different pathologies, including HPV-related cancers and pre-cancerous lesions, but what about clinical trial data for this use? It is fair to say that evidence of efficacy here is limited. For non-HPV-related malignancies, ritonavir has been co-administered with lopinavir (4:1) for the treatment of progressive high-grade glioma and yet did not achieve the desired efficacy endpoint [11]. Furthermore, in spite of good preclinical evidence for the use of nelfinavir as an anti-tumor agent [12], this had little activity when given as a monotherapy for refractory adenoid cystic carcinoma [13]. This may be more effective when used alongside other treatment modalities, such as in combination with cisplatin and radiation to treat advanced cervical cancer [14]. When considering the use of HIV PIs for the treatment of HPV cervical disease, the question most often asked is, does highly active antiretroviral therapy in HIV-positive women have an effect on the progression of HPV-related lesions? However, despite numerous studies, the picture remains unclear. While some report an association of highly active antiretroviral therapy with a reduction in the rate of lesion incidence and progression (e.g., [15]), others do not (e.g., [16]). This may of course, in part, be due to the levels of PI that need to be delivered to the cervix as discussed in the authors’ previous publication [5].

What is clear from the lack of efficacy of HIV PI monotherapy seen thus far in a clinical setting's that it exemplifies the huge problems encountered when moving anticancer therapeutics from the pre-clinical in vitro studies into the clinic, and it is most likely that this relates to the fundamental nature of treating malignant disease when this is at an advanced stage. Cancer cells evolve built in redundancy with the capacity to undergo further evolutionary changes, which enable them to escape treatments that are too focused. This is precisely why non-specific treatments, such as radiotherapy and cytotoxic agents, work but with the caveat that they also cause genetic damage, resulting in the introduction of more mutations which in turn enhances the ability of any surviving tumor cells to evolve therapy resistance.

Thus, for the aforementioned reasons, it is better to treat cancers at an early stage before they have had time to acquire numerous genetic lesions. The authors propose that HIV PIs are an extremely good candidate for this indication since they do not cause DNA damage like other agents trialed for this purpose and have the added advantage of being FDA approved, readily available and possess a proven track record of minimal toxicity. Indeed, a clinical trial of orally administered nelfinavir for the treatment of high-grade precancerous cervical lesions [17] is currently underway and the authors have recently completed a clinical trial of topically applied lopinavir/ritonavir (4:1) for the same indication. As of June 2014, the nelfinavir study has not yet reported, while findings by the authors (manuscript submitted) have demonstrated no adverse reactions to treatment and good positive efficacy (∼80% improvement over 3 months as measured by resolution of lesion and viral clearance). The authors used HPV testing plus liquid-based cytology to identify and subsequently evaluate women with both high- and low-grade cervical disease with a histological diagnosis at the end of treatment. While initially the trial eligibility criteria of the authors was not based on a histological analysis (since biopsy-related damage could enhance spontaneous regression), future larger studies would look to incorporate this as the acknowledged gold standard for diagnostic accuracy. One limitation of the study by the authors was the lack of a placebo control arm, but spontaneous regression alone cannot account for the improvements seen (See the LOTT TrialISRCTN48776874 [15]). Moving forwards, it will be important to delineate which group of women might benefit most from this treatment. This will require consideration of the time to progression and regression rates seen in HPV infections when factors including age, HPV subtype, grade of presenting dysplasia and possibly co-infections and viral load are factored in (reviewed [18]).

The fundamental question remains of why should a drug designed to target the HIV encoded aspartyl protease also have activity against HPV-related disease? The answer is not simple and the authors speculated on the rationale for this in their editorial ‘Lessons from Repurposing HIV Drugs?’ [19]. It most likely relates to the way viruses from disparate families have evolved to deregulate important cellular pathways at multiple intervention points – an effect which is also seen in cancer. Consequently, any therapy that targets a single specific action of a viral protein on a cellular pathway will usually be rendered ineffective as the virus will just ‘re-route’ and produce the same outcome by a different mechanism. Again, this is also observed with deregulated pathways in cancer. Since many viruses have limited genome size and thus a restricted ability to carry the information needed to evolve new strategies for completion of the virus life cycle, they often, by necessity, use similar tactics to one another to produce targeted effects in the host. So how does all this relate to the function of the HIV PIs? For so-called small molecule therapeutics, these molecules are necessarily quite large complex structures and, as previously discussed, it is abundantly clear that they have numerous off-target effects that also contribute to their general activity against HIV [20]. Therefore, the authors speculate that the ‘off-target’ effects of HIV PIs are affecting the common mechanisms that are used by other viruses. If this were accepted as a general paradigm, it would make intuitive sense to explore the activity of this class of compounds, and combinations of these, against many other viral-related disease pathologies.

Financial & competing interests disclosure

The authors have no 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.