Vaccines versus immunotherapy: Overview of approaches in deciding between options

Abstract

This review compares the optimal use of vaccines vs. other forms of immunotherapy, which includes cytokines, such as IL-2, monoclonal antibodies, such as the ‘checkpoint inhibitors’, against CTLA-4 and PD-1.

The review includes both prophylactic and therapeutic vaccines using a variety of technologies. It is already established that vaccines can be enhanced by other immunotherapies, such as cytokines (IL-2) and there is scope for combining both of these with the ‘checkpoint’ antibodies. Moreover, both can be enhanced with other modalities, such as radiotherapy, ablative therapy and both high and low dose chemotherapies.

Keywords:

• antibodies
• cytokines
• checkpoint inhibitors
• immunotherapy
• immune modulators
• therapeutic vaccines

Abbreviations

BCG	=	Bacillus Colmette Guerin
CpG	=	cytosine-phosphate-guanosine
TLR	=	Toll-like receptor
IL-2	=	Interleukin-2
GM-CSF	=	Granulocyte-macrophage colony-stimulating factor
PFS	=	progression free survival
PSA	=	Prostate-specific antigen
TGFβ	=	Tumour growth factor beta
HPV	=	Human papilloma virus
HBV	=	Human hepatitis virus

Introduction

This issue covers a number of vaccines and some immunotherapy non-vaccine approaches in great detail and therefore detailed review of vaccines and non-vaccine immunotherapeutics will not be covered unless they are not included elsewhere and it is relevant to the topic. Indeed, this will be an overview of approaches and deciding between options.

Semantically, therapeutic cancer vaccines are part of the modality of immunotherapy. Here, in this review, immunotherapy is referred to as non-vaccine based treatment that alters the immune response and this includes cytokines and growth factors, such as Interferons and cytokines, including GM-CSF. More recently, this includes the group of antibodies that are now referred to as ‘check-point’ inhibitors.¹ The first of these, an antibody to CTLA4, now known as Ipilimumab or Yervoy, was the first immunotherapy treatment approved for melanoma for many years, since Alpha-Interferon and Interleukin-2.² Although the response to Ipilimumab is not great and the side effect profile has been very problematical, it was approved on its significant increase in overall survival. There is real excitement in this area as there are many other ‘check-point’ targets to consider and the next one of these to be developed is against PD1, as well as its ligand.³ Although they have yet to be approved, the early studies suggest that in the case of PD1, in particular, that it induces a higher response rate with a low side effect profile. In addition, pre-clinical studies show clear synergy between PD1 antibodies and CTLA4 antibodies, leading to complete tumor control in murine models.⁴ The combination is already on trial in the clinic. With regards to the concept of vaccines and cancer, it is important to be aware that there are 2 prophylactic vaccine types already licensed that have been shown to have a major impact on the incidence of Hepatitis B virus induced primary liver cancer⁵ and Human Papiloma Virus induced cervical anal cancer.⁶ Indeed, the finding of HPV, associated with other cancers in the aerodigestive tract, means that there is a strong possibility that these vaccines may reduce the incidence of other cancer types, such as HPV associate oral cancer.⁷

Therapeutic cancer vaccines given after a cancer has appeared and been removed or used as a direct therapeutic to induce an immune response against an established non-resectable tumor, are usually aimed at tumor specific antigens or tumor associated antigens and maybe either aimed at a single antigen or multiple antigens, which is often the case with peptide, DNA and mRNA based vaccines.⁸ Needless to say that whole cell based vaccines, whether autologous or allogeneic, have multiple antigens. Work by Thierry Boon and collaborators has shown that targeting a single antigen in melanoma leads to it being down regulated with concomitant tumor escape from the induced immune response.⁹ In view of this, there is certainly a trend now for new vaccine approaches to incorporate a number of antigens. Nevertheless, the first vaccine licensed by the FDA was Provenge, now known as Sipuleucel-T, developed by Dendreon. It is essentially an autologous ex-vivo dendritic cell based vaccine, pulsed with a single antigen of prostatic acid phosphatase (PAP) and linked to GM-CSF. It was licensed as it was able to show a small but significant increase in overall survival in a randomized study.¹⁰ Interestingly, in common with some other vaccines aimed at this group of hormone resistant prostate cancer patients, it failed to achieve the progression free survival, which was its primary endpoint. Another such example was the allogeneic cell based vaccine from Onyvax which showed an increasing PFS, which is associated with a reduction in the rate of rise of PSA and a cell mediated based cytokine response in a non-randomized setting also did not show any improvement in PFS in a multi-center randomized study.¹¹

The earliest and non-specific vaccine immunotherapy grounded in a scientific background has to be the bacterial cell wall preparations developed by William Coley, the New York surgeon who noted that patients who survived sepsis at the turn of the last century sometimes had complete resolution of their cancer.¹² The cell wall preparations were developed to induce the acute inflammatory immune response without killing the patient with sepsis, as this was in the pre-antibiotic era. Patients, who made an acute inflammatory response, maintaining a high temperature for many hours, were the only ones that benefited from this regimen. The development of this approach was dropped following the advent of radiotherapy. The only modern day equivalent of an agent inducing an acute inflammatory response to irradicate cancer is that of BCG, which is instilled intravesicularly to treat superficial bladder cancer. It is the acute inflammatory response that intravesicular BCG induces that leads to the cancer control and it is not thought to be due to any vaccine effect.¹³

BCG,¹⁴ together with other Mycobacterium species,¹⁵ given either as cell wall preparations or heat inactivated preparations induce a non-specific immune response that has been shown to be very effective when injected intratumourally into melanoma, as well as boosting a cell mediated immune response associated with gamma interferon, IL-2 and Alpha-TNF cytokines, when given as a vaccine in low doses. This has been associated with good tumor control and, importantly, led to BCG being used as an adjuvant with antigen specific vaccine, such as those in tumor specific lysate or whole cell preparations.^16,17

There are many studies where a strong cell mediated responses with Th-1 cytokines has been shown to be associated with a good outcome in vaccination and a humoural based response with Th-2 cytokines, such as IL-4, 5 and 6, has been associated with a poor outcome.¹⁸ Unfortunately, if BCG is given at too high a dose and with multiple doses then it induces Th-2 response and all benefit is lost. Heat killed Mycobacteria, such as Mycobacterium vaccae and Mycobacterium obuense, do not induce this switch and can be given repeatedly to maintain a cell mediated response. In contrast with some single antigen specific vaccines this non-specific response has been associated with a marked increase in survival in stage IV patients in melanoma.¹⁹ Although a Mycobacterium based preparation will have many different antigens, some of which may cross-react with tumors, it is the non-specific activity, which includes inducing natural killer and gamma delta T-cell activity, that separates them from the tumor antigen specific vaccines.^20,21 This non-antigen specific activity is indeed a form of immune modulation (the ability to switch from a humoural based response to a cell mediated based response) and therefore fits as a non-antigen specific immune therapy. The mechanism of action must include its ability to stimulate toll-like receptors (TLRs)^22,23 especially those on the outside of the cell, including TLR-2 and −4. This is also the mechanism of action of some other non-specific immune stimulators/modulators, such as CpG 7909, an oligonucleotide into which CpG (2-deoxy-ribo-cytosine-phosphate-guanosine) motifs are engineered and which signals through the intracellular receptor TLR9. More recently the ability of Imiquimod, or Aldara, to stimulate through TLR7 and 8, has also been exploited as an antigen specific vaccine adjuvant.²⁴

Current Use of Vaccines and Other Forms of Immunotherapy

A very brief review suggests that following the demonstration that BCG could induce regression of melanoma lesions when injected intratumourally, this led to the development of BCG associated antigens, with or without autologous and allogeneic antigens, which were then used in optimal minimal disease scenarios after complete resection in stage III and stage IV melanoma.^16,25 In spite of the enormous number of different approaches to this scenario there is still no registered vaccine for disease free melanoma at the time of writing.

Immunotherapy is used to encompass both non-specific immune stimulation, such as BCG and cytokines, as well as immune modulators, which include check point inhibitor antibodies and drugs such as the Thalidomide derived IMiDs, e.g. Revlimid/Lenalidomide. Vaccines used to describe agents which will induce an active response to a tumor antigen, such as MAGE or NY-ESO, etc. As such, these vaccines are therapeutic cancer vaccines, as opposed to preventative anti-viral vaccines which reduce the incidence of liver and cervical cancer by targeting HBV and HPV.

The other major strategy for vaccine development has been to add it in more advanced disease when first line treatment has failed and this has led to the first approved vaccine, namely Sipuleucel-T, for advanced prostate cancer.¹⁰ A full review of cancer vaccine usage suggests that they have usually been used alone and/or in late stage disease.⁸ Over the last few years it has become increasingly obvious that the tumors learn to defend themselves from even a very appropriate vaccine-induced immune response by putting up various shields, such as FAS-L, CD55 and CD95, as well as deploying immune suppressive factors, such as TGF-β and IL-10, which can also recruit T-regulatory cells and the expansion of myeloid derived suppressor cells.^26-30 This would strongly suggest that the antigen induced immune response needs help, either in the form of enhanced immune response, which can be achieved by adding in cytokines, such as Interleukin-2 (as well as IL-7, 12, 15, 21), or that tumor activity has to be greatly reduced by tumor cell kill, either with radiotherapy or chemotherapy, and that specific treatments to downregulate T-regulatory myeloid derived suppressor cells be considered.³¹ It has been shown that Interleukin-2 even at low doses can boost the effects of even the non-specific vaccine³² and that some drugs are also able to enhance vaccine effectiveness, such as Cyclophosphamide³³ which is able to inhibit T-regulatory activity even at low doses and Gemcitabine³⁴ which is a strong inhibitor of myeloid derived suppressor cells.

The Inflammatory Paradox

It has been mentioned above that Coley's toxins and intravesicle BCG have marked anti-cancer activity by inducing an acute inflammatory response. This is paradoxical, given the fact that the majority of cancers are thought to evolve and be fed by chronic inflammatory states. The significance of this has been reviewed in much detail elsewhere and the practical implications have been confirmed by the reduction in colorectal cancer incidents in patients taking long term anti inflammatories, such as aspirin or Sulindac.^35-37 Chronic inflammation induces local immune suppression and marked angiogenesis and, as such, raises the question whether anti-inflammatories and even anti-angiogenics might enhance an immune response. Pre-clinical models have clearly shown evidence in favor of this for both COX-2 inhibitors and Avastin.^38-40

During the development of Thalidomide, which was shown to have a strong Th-2 down regulatory activity in leprosy and Leishmaniasis,³³ analogs were developed to enhance this activity while trying to avoid the teratogenicity of the parent compound.⁴¹ In a phase I/II study of the first of these analogs to be developed, namely Revlimid/Lenalidomide, it was noted that patients responded as if they had had some form of immune therapy with regards to cell subset changes and cytokine levels.⁴² A murine tumor vaccine model clearly showed that the pre-treatment of mice with these agents greatly enhanced the survival to a tumor specific vaccine when challenged with live tumor. This was dose dependent and yet the drugs alone had no effect in this system.⁴³ This adjuvant effect has now been reported to occur in the clinical state as patients on Revlimid respond much better to Prevnar,⁴⁴ the pneumococcal vaccine than those on other treatments. This has led to therapeutic clinical studies looking at pre-treating patients prior to therapeutic vaccines with Revlimid. The same properties are also shown by Pomalidomide/Pomalyst, which is now also available in the clinic.⁴⁵ The basic properties of Lenalidomide and Pomalidomide show that in addition to being anti-inflammatory agents they are both co-stimulants and immune modulators, as well as being anti-angiogenic and thus attack all 3 arms of the so-called inflammatory triangle of chronic inflammation, suppressed immune response and pro-angiogenesis.^41,46

From the above it would be reasonable to conclude that it would be rather naive to trial a therapeutic vaccine in the absence of integration into other modalities. Furthermore, it would be very logical to combine vaccines with other immunotherapies, particularly those that act through toll-like receptors, such as BCG, CpG, Imiquimod, etc., and of course there is evidence that they do boost vaccine activity by acting as adjuvants. In addition, induced immune responses can be enhanced with cytokines, such as IL-2. The vaccine response can also be enhanced by the addition of other chemotherapy.⁴⁷ This chemotherapy may act as an immune modulator, an anti-inflammatory agent, a co-stimulatory agent, as well as the effect on T-regs and suppressor cells. Moreover, the effect of vaccination can also be greatly enhanced with a marked reduction on tumor bulk and its suppressor activity, as well as the ability to shed antigens to the immune system through radiotherapy, chemotherapy and direct ablative techniques. This can also have a very positive effect on the immune response to a vaccine by inducing epitope spreading, which is a feature that would appear to have to occur if there is to be any benefit from a single antigen vaccine.⁴⁸

There are numerous reports on the ability of Interleukin-2 to enhance the immunogenicity of a variety of vaccines.³² Steve Rosenberg's group reported that the addition of gp100, a melanoma antigen in a peptide formulation, had significant improvement in clinical response, progression free survival, as well as overall survival, as opposed to those who just received Interleukin-2 alone.⁴⁹ However, the IL-2 dose was relatively high compared to other lower dose regimens, which can also enhance the effect of vaccines without inducing significant toxicity.

Immunotherapy/Cytokines

The comments about vaccines needing to be combined with other modalities can equally be applied to cytokines. One of the first immunotherapies, namely α-interferon, which can make a great impact on certain lesions, was applied to many studies with melanoma and any benefit seen, whether at high doses or low doses, was not significant compared to the toxicity profile and although licensed, it is rarely used outside of a clinical trial in the UK. Interleukin-2 at high doses has shown some remarkable long-term complete responses, albeit in a minority of people. Once again, it has guaranteed toxicity and is very expensive, not only involving more drug use but also in the support of care required to give this treatment. Lower dose treatments, however, can be used with other modalities with additional benefit besides those described with vaccines. It would appear to enhance the effect of radiotherapy and when given after chemotherapy, even at low doses, it can maintain expansion of the activated T-cells, leading to additional responses. It is also used for the same reason after ablation procedures.

Monoclonal Antibodies As Immunotherapeutic Agents

Ipilimumab the anti-CTLA4 agent has been given to stage IV melanoma patients on the grounds that blocking the co-stimulatory inhibitory response via CTLA4 would enhance existing T-cell responses and allow them to expand. It has been shown that in certain patients Ipilimumab induces CD8+ T-cell responses to several major melanoma antigens, including NY-ESO, MART-1 and gp100.⁵⁰ It would therefore make sense to combine this approach with vaccines. An initial randomized study had 2 arms of the same dose, one of which included the gp100 vaccine used in the IL-2 study. There was some surprise when there was no benefit with the vaccine and indeed there was a slight reduction in efficacy when the 2 were used together.² However, this may not necessarily be the case when other vaccine candidates are used in combinations and possibly different scheduling. The responses against Ipilimumab alone were not dramatic, with very few complete responses, but reduction in large scale metastases was seen and important unexpected finding was reported in that the disease could continue to increase over the first 2 or 3 courses before a reduction would occur and this late impact response has been flagged as a unique feature of this type of immunotherapy,⁵¹ although it was also noted in non-resectable melanoma treated with the CancerVax vaccine by Donald Morton many years earlier.¹⁷

A major problem with Ipilimumab has been the side effects caused by unfettered immune responses against normal antigens and major clinical problems with colitis and antibodies to endocrine organs have required special interventions when they occur. Although treatment of colitis has greatly improved, some of the other problems, such as pituitary, etc., can be permanent.

To enhance the effect of Ipilimumab it has been combined with chemotherapy and indeed the outcome is better in the combination arm. One reason for this is that the chemotherapy, even though it does not induce a significant immune response, may kill enough cells to shed tumor antigen, thus deflecting the enhanced immune response onto the tumor antigens, as opposed to the self-antigens. It is doubtful that Ipilimumab will rule the roost for very long as the new immunotherapy as antibodies targeted at other components of the checkpoint inhibition system, such as those against the program death PD1 molecule, as well as its ligand, have shown superior response rates with less side effects and it is inevitable that these agents will become drugs of choice. Although Ipilimumab has only been licensed for melanoma it has been tried in numerous other solid tumors, as will be the new antibodies against PD1, etc. The fact that there appears to be synergy in some cases with other modalities will certainly lead to studies in combination against numerous other solid tumors.

Vaccines or Immunotherapy, Combinations or Sequential Protocols

It has been much speculated that the ability of the only licensed vaccine, Sipuleucel-T, to give a survival advantage in prostate cancer, having failed to give a progression free survival benefit, maybe due to the fact these patients are likely to have been given other agents, such as chemotherapy, after the vaccine has finished. This enhancement effect of chemotherapy on the immune response has been noted in other vaccine programmes as well. Moreover, in the phase I/II study of Revlimid/Lenalidomide in solid tumors, previously mentioned, significant clinical responses were noted in this study and rarely seen in a randomized study.⁴² It was noted that these responses were all in pre-vaccinated patients from earlier studies. (Dalgleish, unpublished observation)

Vaccines are usually not effective when there is a large tumor mass or a tumor that is clearly growing too fast.^52-54 In this scenario, slowing the tumor growth and inhibiting its ability to deliver the immunosuppressive effect of the immune response, as well as shedding new antigens, may be the ideal scenario to add in vaccines and/or immunotherapy, which could include cytokines and the checkpoint inhibitors. Indeed, giving immunotherapy after significant tumor cell death, whether it be by ablation, radiotherapy or chemotherapy, does appear to be effective in inducing an immune response, which can lead to an abscopal effect, whereby non-treated lesions, in the case of ablation and radiotherapy, regress.⁵⁵

When it is not possible to kill all of the available tumor, it is then important to consider the use of low dose chemotherapies to suppress the T-regulatory cells⁵⁶ and the myeloid derived suppressor cell activity.^55,57 With regards T-cell activity, it would appear that the majority of vaccine based trials, on the available databases, now include using cyclophosphamide to reduce the T-cell regulatory activity. It should be stressed that this activity is not limited to cyclophosphamide alone and is seen with several chemotherapy agents at lower doses.^56,58

Conclusions

From the above it is fairly clear that it is not as simple a case of vaccines or immunotherapy and that a variety of combinations have already been shown to be more effective than either alone. In addition, the sequence of agents in different disease types and presentations may be very important.

In non-resectable disease there is overwhelming literature that these tumors are immune suppressive and this can be demonstrated very elegantly by showing the immune suppression disappears in those tumors that can be fully resected.^57,59 In disease that cannot be fully resected a treatment plan should look at boosting the immune response in non-resectable disease.^58,60 Both non-specific vaccine agents, such as Mycobacterium derived preparations, as well as immuno- modulatory drugs (IMiDs), such as the Thalidomide-derived analogs, can both do this and I would argue it should be the first therapeutic intervention in such patients. IMiDs are drugs which can attenuate an overactive autoimmune response, as well as enhancing suppressed cel mediated responses commonly present in cancer. For those with significant volume disease it would then be pertinent to attack this with appropriate therapy, whether systemic chemotherapy pertaining to the tumor type or, where possible, radiotherapy or ablation. At the same time, antigen specific vaccine may be appropriate to use to take advantage of epitope spreading. This would also be the time, especially when the therapy has induced some tumor cell killing reduction, to consider the use of the checkpoint inhibitors which when available are more likely to include the PD1 and its ligand agent. Given that complete responses in many tumor types are still rare it would then make sense to expand memory activated T-cells with cytokines.^59,61 Although clear benefit has been reported with Interleukin-2, which also theoretically can expand regulatory cells, related cytokines such as IL-12 and 15 may be more effective at doing this when available.

The majority of solid tumor types when no longer fully resectable clearly benefit from using more than one modality. Previously this could include radiotherapy and/or endocrine therapy, and/or radiotherapy/ablation. It is now becoming increasingly clear that there is a role for immunotherapy/vaccines in a great many cancers and the integration of this modality with the other established ones is the key to long term control.

Disclosure of Potential Conflicts of Interest

The author is Chief and Principal Investigator on trials and on the Sicentific Advisory Board for Immodulon Therapeutics, as well as the Scientific Advisory Boards of Curevac and Bionorpharma. The author is also on the Scientific Advisory Board and receives research support from Celgene.

Funding

The author is supported by the Cancer Vaccine Institute (registered charity: 3884777), he has research support from Celgene and GW Pharma.