Peptide-based cancer vaccines activate cancer specific T - cell immune-responses

Mutant RAS proteins are neo-antigens and drivers for development of cancer. RAS mutations are exclusively found in cancer cells and are therefore cancer specific targets for attacking cancer immunologically.

Targovax’s mutant RAS targeting immunotherapy is designed to activate T cells of the patient’s own immune system to attack the cancer cells. The immunotherapy is designed with mutant RAS mimicking peptides (antigens) that are long enough to be complexed with MHC class II molecules for stimulation of CD4+ T helper cells, and to allow antigen processing to shorter peptides that can be complexed with MHC class I molecules for stimulation of CD8+ cytotoxic T cells. The therapy is therefore able to activate both types of T cells necessary for achieving efficient anti-cancer immune activity.

Peptides are not immunogenic by themselves and need an adjuvant that can trigger the peptide immunization process resulting in activation of the desired anti-cancer specific T cells. The quality of the immune reaction to peptides is completely dependent on the adjuvant.

Targovax has selected the immune stimulator GM-CSF (granulocyte-macrophage colony-stimulating factor) as adjuvant - one of the most potent adjuvants used for peptide vaccinations since the early 1990s, producing a very convincing track record of potency and effectiveness. Targovax avoids the side effects associated with systemic administration of GM-CSF by only injecting minute quantities intra-dermally for local activation of the immune cells.

By targeting the central cancer neo-antigen mutant RAS and using the right type of adjuvant Targovax hope to succeed in developing a clinically efficient immunotherapy - which will benefit all patients with RAS mutated cancers.

Targovax’s RAS peptides are small proteins which can be produced chemically in quantities of many kilograms. They are also very stable and can be stored for several years.

Berit Iversen
VP, Head of CMC

T-cells are cells of the immune system that defend the host against intracellular changes and infections. By using peptides mimicking special intracellular changes, like RAS mutations, a subset of T-cells can be induced that recognizes cells with such mutations and triggers immunological eradication of these cells. T-cells also provide immunological memory and are rapidly retriggered upon reappearance of cells with the specific intracellular changes.

T-Cells, the Human Immune System and the Role of RAS Targeted Immunotherapies

The human immune system is the body’s scanner and defense against infectious organisms and other invaders. It works on two levels: through the innate immune system and the adaptive immune system.

The innate immune system is always active and responds to general threats such as invading microbes or debris from dead cells after injury. The advantage of the innate immune system is that it is always active and responsive. The disadvantage is that it is largely non-specific.

The immune system has its “front line soldiers” – cells which are non-specific, such as natural killer cells and macrophages. These will attack a threat from, for example, cancer cells without knowing anything about them. This is the innate immune system.

The adaptive immune system mounts targeted responses to specific threats. It is slower than the innate immune system to be activated, sometimes taking weeks or months for an adaptive immune response to be fully active. Once activated, however, it is extremely effective at eliminating a specific threat such as a virus or, in this case, cancer.

Except for a very few exceptions, all human cells showcase on their surface a sample of the proteins of which they are made. The proteins are broken up into short peptides (i.e. protein fragments) and displayed on the cellular surface by MHC (Major Histocompatibility Complex) class I protein complexes.

Normal cells display a range of normal peptides on their MHC class I complexes, which do not trigger a reaction by CD8+ cytotoxic T-cells (cells of the adaptive immune system that can kill other cells if needed).

Cancer cells carry mutations in some genes and thus produce mutated proteins. They display on their surface mostly normal peptides, but also some abnormal peptides. If the CD8+ cytotoxic T-cells recognize an MHC class I complex when it presents an abnormal peptide, the T-cells will kill the cancer cell. This is how, it is believed, the immune system eliminates most cancer cells. In fact, it is likely we all experience cancer every now and then but are not aware of it because these emergent cancer cells are quickly being cleared out by the immune system.

Sometimes the mutations of the cancer cell do not drastically change the shape of the proteins and the cancer cell displays thus a range of only slightly abnormal peptides on its surface. Cancers with RAS mutations are one such example. CD8+ cytotoxic T-cells do not easily recognize these slightly abnormal RAS peptides and therefore are not able to eliminate RAS mutated cancer cells, as for example in pancreatic cancer. Here, the CD8+ cytotoxic T-cells need help to learn to recognize RAS mutated peptides. This is where Targovax comes in.

Targovax’s lead peptide-based targeted immunotherapy TG01 is a collection of 7 mutated RAS peptides, which are injected into the skin just like most targeted immunotherapies. TG01 is administered together with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF), a substance that boosts immune reactions.

T-cells attacking a cancer cell. Targovax is developing immuno-therapies aimed at stimulating the immune system to produce cancer specific T-cells. We expect that their effect will be enhanced further by Checkpoint Inhibitors (CPIs).

GM-CSF is injected in the skin to activate the local antigen presenting cells (APC) like dendritic cells (DC). After injection of RAS peptides at the same site as GM-CSF the peptides are taken up by the activated DCs, which carry the TG01 peptides to the lymph nodes - the immune system power hubs. There, the DCs present the RAS peptides, eventually after processing to shorter peptides, on MHC class I and II protein complexes to both CD4+ and CD8+ T cells. T cells that are able to recognize the mutant RAS peptides are activated, proliferate to increase in number and enter systemic circulation in the body.

As their name implies, CD4+ helper T-cells are helpful indeed: they secrete growth factors to provide the right environment for CD8+ cytotoxic T-cells to proliferate. They live a long time, some a lifetime, and form a lasting memory of the immune system: when the immune system is challenged a second time with the same specific threat, the adaptive immune system will be ready to intervene at short notice. And the CD4+ helper T-cells down regulate excessive immune responses in order to prevent auto-immunity.

In short, CD4+ helper T-cells are an essential part of a well-calibrated immune response. An important twist to this story is that the MHC class II complexes require peptides of about 17-25 amino acids (a peptide is a short string of amino acids), while MHC class I complexes can recognize peptides of 8-10 amino acids long. MHC class II complexes are needed to generate CD4+ helper T-cells and specific CD8+ cytotoxic T-cells. Importantly, the RAS mutated peptides in TG01 are sufficiently long as to be recognized by both MHC Class II complexes as well as the MHC class I complexes. The peptides are degraded by the cells and the shorter degradation products can then be recognized by MHC class I complexes. This way, Targovax’s RAS peptide-based targeted immunotherapies activate both CD4+ helper T-cells and CD8+ cytotoxic T-cells.

Tumors can avoid the attack from the immune system by putting the brakes on it.

Activated CD4+ helper T-cells and CD8+ cytotoxic T-cells are often not enough. Tumors are very clever at putting the brakes on the immune system (immune suppression). Activated CD8+ cytotoxic T-cells are prevented by the tumor from taking action against the cancer cells by various means but there appears to be a solution to this problem. Recently discovered immunomodulatory drugs called Immune Checkpoint Inhibitors (CPIs) remove the brakes from the immune system. It would therefore be advantageous to explore combinations of Targovax’s RAS targeted immunotherapies with CPIs in future clinical trials in additional indications. For TG01’s initial target indication - patients with resected (i.e. surgically removed) pancreatic cancer - there is no tumor left to suppress the immune system, so a monotherapy approach is a logical one. The purpose of targeted immunotherapy in this patient population is therefore to prevent the cancer from coming back. 

RAS mutations and Cancer

Targovax is developing a highly targeted immunotherapy for patients with RAS-mutated tumors - the most frequently mutated oncogene in cancer, with no approved therapies. Mutation of RAS disrupts normal cell division signaling and occurs early in the transformation from a normal to a cancer cell.

RAS mutations are a key driver of cancer progression and treatment resistance. They are found in 20 - 30% of all cancers1, over 90% of pancreatic cancers2, 50% of colorectal cancers3, between 20 and 30% of non-small cell lung cancers4 and between 20 and 30% of malignant melanomas1.

Few treatment options are available for patients with RAS mutations and of limited efficacy, highlighting the significant unmet medical need for these patients.

References:

  1. Fernandez-Medarde, A. and Santos, E.; RAS in Cancer and Developmental Diseases; Genes & Cancer. 2011;2(3):344–358
  2. Raphael B.J. et al; Integrated genomic characterization of pancreatic ductal adenocarcinoma; Cancer Cell 32, 185-203, August 14, 2017
  3. Van Cutsem, E. et al; Fluorouracil, Leucovorin, and Irinotecan Plus Cetuximab Treatment and RAS Mutations in Colorectal Cancer; J Clin Oncol. 2015; 33(7):692-700
  4. D’Arcangelo, M. and Cappuzzo, F.; K-RAS Mutations in Non-Small-Cell Lung Cancer: Prognostic and Predictive Value; International Scholarly Research Network, ISRN Molecular Biology, Volume 2012, Article ID 837306, 8p.