Lets quickly set the stage. In part 1 we reviewed the CTLA4 and PD-1 pathways and therapeutics targeting these pathways. In part 2 we brought in a few more targets within the immunoglobulin superfamily: LAG-3, TIM-3, B7-H3, B7-H4, and very briefly TIGIT and VISTA. Then we reviewed therapeutics being developed to target proteins in the TNF receptor (OX40, CD40, 4-1BB, CD27, GITR) and ligand (CD70) superfamilies.
While some of these pathways play a role in the innate immune system, they are all more closely aligned with the adaptive immune system. The innate immune system is hard-wired, triggering a rapid immune response, while the adaptive immune system relies on the orchestrated interaction of antigen presenting cells (dendritic cells, macrophages, etc) with T cells and B cells, leading to a robust immune response and, importantly, immunologic memory, i.e. memory of “that which” induced the immune response in the first place. Memory underlies immunity, as in “I am immune to…”, and is the basis for vaccination. In the context of oncology, memory allows sustained immune response to cancer cells over time.
In most immune responses to pathogens, both the innate and adaptive arms of the immune system are critical for efficient and sustained protection. We are learning from work with innate immune checkpoint therapeutics that the same may hold true for anti-tumor immunity.
One of the critical cells in the innate immune response is the natural killer (NK) cell. The name tells their story, as these cells are primed to disgorge toxins onto pathogens and pathogen-infected cells or tissue. Recently, an adaptive immune role for NK cells has been described, a finding that only increases the importance of this cell type. The activity of NK cells is controlled to a great degree by the killer inhibitory receptors.
Killer inhibitory receptors come in 2 flavors: killer cell immunoglobulin-like receptors (KIRs) and C‑type lectin transmembrane receptors. There are many different proteins within these groups, with various functions. KIRs are normally kept quiescent through interaction with cell surface HLA proteins. Both HLA and KIR have variable genotypes, and not all are compatible. Further complicating the picture is the existence of multiple KIR family proteins. We are just beginning to understand the expression and regulation of these proteins in the context of tumor biology, and choosing which of the 20 or more receptors to target remains an open question. However, some progress has been made.
Innate Pharmaceuticals (OTC: IPHYF) is taking the first steps in exploring NK cell therapeutics. The company’s lead drug is lirilumab, a first-in-class anti-KIR antibody that specifically recognizes the KIR2DL1, -2, and -3 receptors, and prevents their inhibitory signaling. The antibody increases NK cell–mediated killing of HLA-C–expressing tumor cells. A phase II study of lirilumab in 150 patients with acute myeloid leukemia is in progress. Lirilumab has been licensed by Bristol-Myers Squibb. BMY is sponsoring Phase 1 trials of lirilumab in combination with ipilimumab (anti-CTLA4) and nivolumab (anti-PD-1) in patients with solid tumors. These early lirilumab trials will start to read out over the next 2 years and the data will generate considerable interest.
Innate Pharma’s expertise in NK cell biology has produced several other programs. KIR3DL2 is another KIR family protein that is highly expressed in aggressive forms of cutaneous T cell lymphoma. Innate has developed an anti-KIR3DL2 antibody that has cytotoxic activity against cutaneous T cell lymphoma in vivo (mouse models) and ex vivo (primary patient cells). They gave an update at the T cell lymphoma forum in January: (http://www.innate-pharma.com/sites/default/files/tclforum2014_iph41_1.pdf). The company will file an IND this year. Innate Pharma also has several interesting earlier stage programs.
One of the reasons this is an exciting pathway is reflected in the combination therapy approaches mentioned, in which a boost in T cell activity is combined with a boost in NK cell activity. Other combinations worth considering include KIR inhibition with 4-1BB agonist activation. Ron Levy (Stanford) has described the transient expression of 4-1BB on NK cells that are exposed to tumor cells coated with antibody (e.g. lymphoma cells coated with rituximab or breast cancer cells coated with herceptin). This suggests that the presence of activating antibody induces 4-1BB expression on NK cells. If so, and if one could get the timing right, very potent combinations can be considered. One might also consider such mechanisms in the development of bispecific therapeutics. Obviously there are critical considerations here – one is toxicity (will the combination be safe) and second is timing, if the antibodies are administered separately.
Another critical cell in the innate immune response is the macrophage, that has an ancient and fundamental role in the clearance of dead, dying and infected cells from the body. Galectin-3 is an anti-apoptotic protein that is widely expressed, and may regulate apoptosis of tumor cells and tumor-associated macrophages. There are also reports that galectin-3 can regulate macrophage/T cell interaction, although the mechanism of action is unclear.
I honestly don’t know what to make of therapeutics targeting galectin-3 as this is a very promiscuous protein. That does not mean such therapeutics won’t be useful, it is just a point of risk assessment. Galectins bind to sugar moieties that are hanging off of proteins or bound to extracellular matrix (ECM). In this sense galectins are “sticky”, capable of binding distinct targets. There are about 15 different human galectins, and to add to the fun, some of these can oligomerize with each other.
Specificity is imposed by the preference of galectins for sugars having a terminal galactose. Further specificity is imposed by the preference of different galectins for different sugars adjacent to the terminal galactose in the oligosaccharide chain. Oligomerization allows galectins to support cell-cell and cell-matrix interactions, either of which can induce cell signaling. Galectins are most highly expressed in macrophages but are pretty ubiquitous. Galectins, including galectin-3, are proposed to play a role in diverse diseases, including asthma, fibrosis, cardiovascular disease, inflammatory disease and oncology. Well, that gives me pause, as a lot of biology is involved here. Already some big bets on galectin-3 have failed, such as BG Medicine’s cardiovascular disease program.
Galectin Therapeutics Inc (NASDAQ: GALT) has several galectin-targeting programs in development for liver fibrosis (notably, non-alcoholic steatohepatitis aka NASH) and oncology. GR-MD-02 and is a polysaccharide polymer that binds to galectin-3 and galectin-1, with higher affinity for galectin-3.
Mouse models have demonstrated that GR-MD-02 plus ipilimumab enhances T-cell function and anti-tumor responses greater than either agent alone. The data suggest a role of GR-MD02 in promoting the CD8+ T cell response to tumor antigens. GR-MD-02 is in Phase 1 testing to establish dose and tolerability. Providence Portland Medical Center has filed an IND to test GR-MD-02 plus ipilimumab in a Phase 1B study, enrolling patients with metastatic melanoma. Galecto Biotech has also developed galectin-3 inhibitors, these are preclinical stage programs.
The mechanism of action remains unclear. This remains the biggest issue with galectin-3 drug development – we really have no idea how it the system works. Galectin Therapeutics has built mechanism of action studies into the oncology clinical trials, which is a good step forward. To be clear, this is not to suggest that galectin-3 is not a good target, but if it is it will be nice to know more about the mechanism of action.
Phosphatidylserine (PS) can be classified as a pattern recognition target, making its expression a component of innate immunity regulation. This may or may not have anything to do with the mechanism of action of Peregrine Pharmaceuticals (NASDAQ: PPHM) anti-PS antibody bavituximab. Bavituximab is being tested in multiple solid tumor settings. It’s an oversimplification, but let’s define PS as an immunosuppressive molecule. PS is an inner membrane protein that is “flipped” to the cell membrane surface in cells undergoing apoptosis. PS is also a component of ECM, and is found on the surface of activated cells, such as activated T cells, although at much lower levels than on apoptotic cells. Tumor cells can express a lot of PS on their cell surface, and this is thought to provide protection from immune cells because the immune system will often ignore cells undergoing normal (i.e. programmed) cell death, which always occurs by apoptosis. Bavituximab may act to block PS and therefore allow an immune response to cancer cells.
Somewhat amazingly, a Phase 2 trial of bavituximab in advanced NSCLC yielded positive results on PFS and more importantly, improved OS, from less than 6 months to 12 months. This was a second line study in patients who had failed chemotherapy. FDA granted fast track designation for bavituximab for second line NCSLC even though the drug had failed as a first-line therapy in Phase 2. In other words, there is some disconnect in the results. Peregrine is currently testing bavituximab in phase 3 trials of advanced NSCLC as second-line therapy.
Again what we have here is a drug with a poorly defined mechanism of action. This will not matter if the Phase 3 results are positive, but it will complicate rationale design of co-therapies. One wonders how this drug might be paired with another immunotherapeutic, or even with a targeted therapy like ramucirumab (anti-VEGFR2 from Eli Lilly), which just reported out positive Phase 3 data in a very similar patient population.
The SIRPalpha/CD47 system is another fundamental component of blocking innate immune responses, in this case by macrophages. There are a few companies trying to utilize antibodies to CD47 or to SIRPalpha (CD172a). Expression of CD47 on tumors provides a shutdown signal to macrophages via CD127a, that basically prevents phagocytosis. Of note, cancer stem cells also utilize CD47 to escape the attention of macrophages. The approach is effective in models of human tumors in mouse, and there is a report of synergy with rituximab in a non-hodgkin’s lymphoma model. Irving Weissman and colleagues at Stanford and Oxford Universities will initiate Phase 1 testing of an anti-CD47 antibody this year. Several very small biotechs have begun working on antibodies to CD47 and CD127a.
There are other targets in this class although most are even earlier in development. TGFbeta is a good example of a target around which there is a lot of early activity in oncology (among other things). There are also, scattered in the literature, hints as to the next wave of immune checkpoint targets emerging.
OK, on to IDO.
Indoleamine 2,3-dioxygenase (IDO1) is an IFN-inducible enzyme that catabolizes the essential amino acid tryptophan from the cellular microenvironment. IDO1 is induced by interferon gamma, and is therefore elevated in settings of innate or adaptive immune responses. Elevated tryptophan degradation stops T cell activation and induces T-cell apoptosis. Furthermore, generation of biologically active tryptophan metabolites are associated with the induction of immune tolerance. Therefore IDO expression in APCs or the tumor cells is a potential mechanism by which the immune tolerance to tumor antigens is induced.
Incyte Corporation (NASDAQ: INCY) has developed a clinical stage oral IDO1 inhibitor. INCB24360 is currently in Phase 1 and 2 for metastatic melanoma in combination with ipilimumab and as monotherapy for ovarian cancer. Incyte presented PK/PD and tolerability data from a phase 1 trial at ASCO in 2012, and monotherapy data in advanced disease was presented at ASCO in 2013. In general that data showed only modest efficacy, with some patients being able to maintain stable disease, and these were late-stage patients. Results from the ipilimumab combo trial should be available this year, probably at ASCO.
Earlier this month Incyte entered into a collaboration with Merck to evaluate INCB24360, in combination with Merck’s anti-PD-1 antibody MK-3475. The first trial is a Phase 1/2 study in advanced or metastatic cancers including melanoma and NSCLC. The trial is designed to set appropriate doses and then randomize to MK-3475 with or without INCB24360. NewLink Genetics (NASDAQ: NLNK) is also in phase 2 with indoximod, a tryptophan analogue inhibitor of IDO1. The mechanism of action of this drug is not well understood, as it does not appear to influence free tryptophan levels. NewLink presented Phase 1 data at ASCO last year, showing good tolerability and some early signals of clinical activity.
Several companies have preclinical inhibitors of IDO and related enzymes. Privately held iTeos Therapeutics has preclinical drug discovery programs on IDO1 and Tryptophan 2,3-dioxygenase (TDO2), a second key enzyme in tryptophan catabolism. ToleroTech is developing an siRNA approach to targeting IDO. Other programs are no doubt underway in pharma and biotech.
Recall that in Part 1 we identified five or six distinct areas of immunotherapeutic development, and immune checkpoint inhibition was only one of these. We’ve stretched the definition a little bit to allow coverage of some of the targets in this last bit, Part 3. We’ve also deliberately skipped over the Toll-Like Receptor (TLR) field as these agents may best be viewed in the context of tumor vaccines and adjuvants.
There are compelling questions to consider.
1- how the heck will “healthcare” fold all these therapeutics together in a way that makes the best sense for individual patients?
2- how are companies coping with the overload of targets and modalities? How do you build a credible pipeline?
3- what modality is best suited for which tumor types? plus we’ll need biomarkers to sort out responses to these new therapeutics and combinations- what are they?
4- can we identify gaps that can be filled by new targets, perhaps new companies?
5- can we find hidden drug development gems already out there waiting to be licensed or bought?
6- what are the pivotal data coming up that will move companies and their stock prices?
7- can we foresee changes in clinical practice that will support some therapeutic modalities, and doom others?
8- where are the transformative therapies that will change the clinical landscape
We have spent a lot of time working on this competitive landscape, and have arrived at some very interesting answers (and lots more questions).
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What will we cover next? Honestly, I don’t know yet.