Provenge ®

The term cancer vaccine refers to a vaccine that either prevents infections with cancer-causing viruses, treats existing cancer or prevents the development of cancer in certain high risk individuals. (The ones that treat existing cancer are known as therapeutic cancer vaccines.)

Some cancers, such as cervical cancer and some liver cancers, are caused by viruses, and traditional vaccines against those viruses, such as HPV vaccine and Hepatitis B vaccine, will prevent those cancers. (These anti-viral vaccines are not further discussed in the rest of this article)

Scientists have also been trying to develop vaccines against existing cancers. But there is no such vaccine that can cure a person having cancer. Some researchers believe that cancer cells routinely arise and are destroyed by the healthy immune system^[1]; cancer forms when the immune system fails to destroy them^[2]. One approach to cancer vaccination is to separate proteins from cancer cells and immunize cancer patients against those proteins, in the hope of stimulating an immune reaction that would kill the cancer cells. Therapeutic cancer vaccines are being developed for the treatment of breast, lung, colon, skin, kidney, prostate, and other cancers.^[3].

In a phase III trial of follicular lymphoma (a type of non-Hodgkin’s lymphoma), investigators reported that the BiovaxID prolonged remission by 44.2 mo, versus 30.6 mo for the control, at the June 2009 meeting of the American Society of Clinical Oncology.^[4]

On April 14 2009 Dendreon Corporation announced that their Phase III clinical trial of Provenge, a cancer vaccine designed to treat prostate cancer, had demonstrated an increase in survival. Dendreon expects Provenge to be approved for use in the United States by the U.S. Food and Drug Administration (FDA) by 2010 ^[5]

Another approach to therapeutic anti-cancer vaccination is to generate the immune response in situ in the patient. This approach has been successful used by BioVex Inc, Woburn, MA with their product OncoVEX GM-CSF. OncoVEX GM-CSF is a version of herpes simplex virus which has been engineered to replicate selectively in tumor tissue and also to express the immune stimulatory protein GM-CSF. This enhances the anti-tumor immune response to tumor antigens released following lytic virus replication providing an in situ, patient specific anti-tumor vaccine as a result. This is currently in Phase 3 trials in melanoma following a very high degree of efficacy having been observed in Phase 2 trials.

Desired characteristics

Effective cancer vaccines must resolve several challenges. Cancer vaccines seek to target an antigen specific to the tumor and distinct from self-proteins. Selection of the appropriate adjuvant, molecules that activate antigen-presenting cells to stimulate immune responses,is required; at the present time, only BCG, aluminum-based salts and a squalene-oil-water emulsion are approved worldwide for clinical use. The effective vaccine also should seek to provide longterm memory to prevent tumor recurrence. Some scientists believe that for total tumor elimination, both the innate and adaptive immune systems should be activated.^[6]

Antigen candidates

Tumor antigens have been divided into two broad categories: shared tumor antigens; and unique tumor antigens. Shared antigens are expressed by many tumors. Unique tumor antigens result from mutations induced through physical or chemical carcinogens; they are therefore expressed only by individual tumors.

In one approach, vaccines contain whole tumor cells, though these vaccines have been less effective in eliciting immune responses in spontaneous cancer models. Defined tumor antigens decrease the risk of autoimmunity but because the immune response is directed to a single epitope, tumors can evade destruction through antigen loss variance. A process called “epitope spreading” or “provoked immunity” may mitigate this weakness, as sometimes an immune response to a single antigen will lead to development of immunity against other antigens on the same tumor.^[6]

Problems

A vaccine against a particular virus is relatively easy to create. The virus is foreign to the body, and therefore will express antigens the immune system can recognize. Furthermore, there are usually only a few viable variants of the virus in question. It is very hard to develop vaccines for viruses that mutate constantly such as influenza or HIV.

A tumour can have many different types of cells in it, each with different cell-surface antigens. Furthermore, those cells are derived from the individual with cancer, and therefore display few if any antigens that are foreign to that individual. This makes it difficult for the immune system to distinguish the cancer cells from normal cells. Some scientists believe that Renal cancer and melanoma are the two cancers with most evidence of causing spontaneous and effective immune responses, possibly because they often display antigens that are recognized as foreign. Therefore, many attempts at developing cancer vaccines are directed against these tumors. However, given Dendreon’s success in prostate cancer, a disease that never spontaneously regresses, cancers other than melanoma and renal cancer may be equally amenable to immune attack.

However, most clinical trials investigating a cancer vaccine have failed. The precise reasons are unknown, but possible explanations include:

1) disease stage being treated was too advanced: it is difficult to get the immune system to fight bulky tumor deposits, because tumors actively suppress the immune system using a variety of mechanisms (e.g. secretion of cytokines that inhibit immune activity). The most suitable stage for a cancer vaccine is likely to be early disease, when the tumor volume is low, but the problem there is that clinical trials take upwards of five years and require high numbers of patients to reach measurable end points. The alternative is to target patients with minimal residual disease after de-bulking of the tumor by surgery, radiotherapy or (providing it does not in itself harm the immune system) chemotherapy.

2) escape loss variants (cancer vaccines that target just one tumor antigen are likely to be less effective. Tumors are highly heterogeneous and antigen expression differs markedly between tumors (even within deposits in the same patient). The most effective cancer vaccine is likely to raise an immune response against a broad range of tumor antigens to minimise the chance of the tumor being able to mutate and become resistant to the therapy.)

3) prior treatments (numerous clinical trials in the past have treated patients who have received numerous cycles of chemotherapy. Chemotherapy is often myelosuppressive and destroys the immune system. There is little point giving a cancer vaccine to a patient who is immune suppressed).

4) some tumors progress very rapidly and/or unpredictably, and they can literally outpace the immune system. It may take two to three months for an immune response to a vaccine to mature, but some cancers (e.g. advanced pancreatic) can produce marked clinical deterioration, or even death, within this timeframe.

5) many cancer vaccine clinical trials examine immune responses by patients as their primary goal. Correlations are then made, typically showing that the patients who made the strongest immune responses were the ones who lived the longest, and this is taken as evidence that the vaccine is working. The alternative explanation, however, is that the patients who made the best immune responses were the healthier patients with the better prognosis, and they would have survived longest in any event, even without the vaccine. In other words, the immune responses may simply be a simple reflection of a better health status, not an indication that the vaccine had any beneficial effects. As such, these immune ‘false friends’ may have tempted some to embark on expensive phase III trials without a solid rationale.

Drug development

Most of the cancer vaccines in development are addressing specific cancer types and are therapeutic vaccines. Several cancer vaccines are currently in development by companies such as Antigenics Inc. (Oncophage), Celldex Therapeutics (CDX110, CDX1307 and CDX1401), Geron Corporation (GRNVAC1), Dendreon Corp (Provenge), BN ImmunoTherapeutics[1] (PROSTVAC), GlobeImmune[2] (Tarmogens, GI-4000^[7][8]), Advaxis, Inc (Lovaxin C), Accentia Biopharmaceuticals majority owned subsidiary Biovest International [BiovaxID], GeneMax Corp (GMXX), (Apthera, Inc.[3] (NeuVax), and Avax Technologies[4] [AC Vaccine].

Cell Genesys (GVAX), CancerVax (Canvaxin), Genitope Corp (MyVax personalized immunotherapy), and Favrille Inc (FavId) are examples of cancer vaccine projects that have been terminated due to poor phase III results (despite promising phase II data and strong immune responses).

On April 8 2008, New York-based company Antigenics announced that it had received approval for the first therapeutic cancer vaccine in Russia. It is the first approval by a regulatory body of a cancer immunotherapy. The treatment, Oncophage, increased recurrence-free survival by a little more than a year according to the results of a phase III clinical trial. The approval is for a subset of kidney cancer patients who are at intermediate risk for disease recurrence. It awaits approval in the US and EU.^[9] but will need a new trial for FDA approval.

In January 2009, a review article was published in Expert Reviews in Anticancer Therapy (Vol 9, #1, pages 67–74) which highlighted past program failures and made recommendations for success as follows:

1. Target settings with a low or very low burden of disease; it is clear that vaccines will not work in patients with advanced metastatic disease.

2. Conduct randomized Phase II trials so that the Phase III program is sufficiently powered – resist the temptation to leap into Phase III prematurely.

3. Do not randomize antigen plus adjuvant versus adjuvant alone. The goal is to establish clinical benefit of the immunotherapy (i.e.,adjuvanted vaccine) over the standard of care, not over standard of care plus adjuvant. The adjuvant may have a low-level clinical effect that would skew the statistical powering of the trial, increasing the chances of a false negative.

4. Base development decisions on clinical data, not just immune responses. Time-to-event end points are more valuable and clinically relevant. To date, immune responses have not been predictive of clinical benefit. It is possible that the ability to mount an immune response is merely a prognostic factor that identifies patients with pretreatment characteristics that favor longer survival.

5. Regulatory compliance needs to be designed into the program from inception; invest in the manufacturing process and product assays early. It is much more difficult to retro-fit.

2009 Merck are starting phase III trials of Stimuvax for breast cancer.^[10] It had promising results from a phase IIB trial for inoperable lung cancer.