Part 2 of 2
Pathways and targets covered: TIGIT, PVR, PVRL2, PVRIG, DNAM-1
Companies mentioned: Compugen, Surface Oncology, Eli Lilly, Merck, Roche
In Part 1 (http://www.sugarconebiotech.com/?page_id=37) I did a drive-by on TIGIT targeting, more or less in isolation. But TIGIT exists in a complex web of ligands and receptors, expressed on diverse cell types. Here is a simplified view:
In the context of anti-tumor immunity we want to know how ligands are expressed on myeloid cells, dendritic cells and the tumor cells in the tumor microenvironment (TME) and how the receptors are expressed on infiltrating T and NK cells. Expression of both ligands, PVRL2 and PVR, is upregulated in many cancers. Also, shed PVR is thought to prevent productive signaling through DNAM-1. TIGIT and PVRIG expression is bright on activated T cells, including PD-1-positive T cells. DNAM-1 expression is variable, in part due to internalization and degradation induced by PVR binding. DNAM-1 activity is also negatively regulated in cis by TIGIT and is reportedly downregulated in the TME of many cancers.
So, it’s complicated.
As noted above we can break this into two main pathways. Compugen has argued that these pathways represent parallel means of regulating DNAM-1 interactions by coopting ligand binding, ie. that the negative signaling receptors TIGIT and PVRIG both compete with DNAM-1 for ligand engagement. These data below are from a Compugen paper (DOI: 10.1158/2326-6066.CIR-18-0442). The cytokine data are from a 2-week antigen-dependent activation assay that yields CD8+ T cell responses that can be measured in the presence of blocking antibodies.
The take home message is that one only sees synergistic activation of IFNy when the parallel pathways are both blocked, thus blocking both TIGIT/PVRL2 or TIGIT/PVRIG or PVR/PVRl2 or PVR/PVRIG, but not both sides of the same pathway, eg. TIGIT/PVR. Here again are the two pathways:
The effect can be duplicated by blocking either side plus blocking PD-1. Compugen also showed that blocking PVRIG induced upregulation of TIGIT, suggesting a compensatory effect that may be overcome by blocking both sides..
These data are the basis for Compugen’s “triple” combination study in collaboration with BMS (NCT04570839). The trial will evaluate the simultaneous blockade of three immune checkpoint pathways, PVRIG (COM701), TIGIT (BMS-986207) and PD-1 (nivolumab), in patients with advanced solid tumors including those that have failed anti-PD-1 or anti-PD-L1 as prior therapy. A quick note here: Compugen’s anti-PVRIG is an IgG4 isotype antibody (like nivolumab) and the BMS anti-TIGIT is a FcγR-null IgG1. The idea here (as with anti-PD-1) is that you don’t want to deplete the T cells, just block the pathways. The role of FcR-engagement in this space is controversial as was discussed in Part 1 (http://www.sugarconebiotech.com/?page_id=37).
Compared to the wealth of programs targeting TIGIT only a few efforts to tackle the other pathways have been disclosed. Surface Oncology presented anti-CD112R (PVRIG) data in an AACR 2020 poster. This included in vivo data using syngeneic mouse models, including in the rechallenge setting.
Note the focus on comparing antibody isotypes (mouse IgG2a > mouse IgG1), which is reminiscent of the findings surrounding anti-TIGIT antibody isotypes, where Fc-effector function may be important for efficacy (note that mouse IgG2a is equivalent to human IgG1). We’ve not touched on the complexity of target expression on both T cells and NK cells, but here Surface shows a role for each, as depletion of either cell type prevented activity in their in vivo model:
Similar to Merck’s publication, Surface identified a key role for FcgR-engagement in mediating activity. Finally, they demonstrated synergy with anti-PD-1 treatment:
Compugen’s anti-PVRIG data were generated with an IgG4, showing additive activity with anti-PD-L1. These data are compared to gene KO results across three different models (from SITC 2019 poster):
The poster also showed robust biophysical characterization of the anti-PVRIG antibodies where the isotype perhaps mattered less. The in vivo modeling data are somewhat modest – note the short duration of modeling. Regardless, the combination KO data suggest that blocking both arms (TIGIT and PVRIG) is more efficacious than blocking either single arm, which is in line with their in vitro analyses.
A proposed mechanism of action for blocking two pathways
An interesting model is that blocking either pathway (PVRL2/PVRIG or PVR/TIGIT) will “free” DNAM-1 to engage ligands to transduce a productive immune signal. Of note TIGIT blockade can also release DNAM-1 (from disruptive interaction in cis) as shown by the Roche group:
Therefore, anti-TIGIT and anti-PVRIG (CD112R) blockade may both increase surface DNAM-1 expression or availability. This is important since several papers have directly examined the role of DNAM-1 expression in immune responses. DNAM-1 is degraded upon activation, in a phosphorylation followed by ubiquitination-dependent manner. Mark Smythe’s lab has examined DNAM-1 degradation in the context of anti-tumor immunity. In vivo modeling data showed an improvement in tumor control when DNAM-1 degradation is blocked, and synergy with immune checkpoint blockade is also demonstrated - nb. this paper is full of very untraditional gating of flow cytometry data (doi.org/10.1016/j.immuni.2020.09.010). A follow-on paper from the same lab uses DNAM-1 expression to examine the functional state of TIL, suggesting that down-regulation of DNAM-1 in the tumor microenvironment contributes to T cell dysfunction – this echoes Compugen’s analyses. There is little work directed to DNAM-1 itself although apparently Eli Lilly has a agonist antibody in the clinic (LY3435151).
TIGIT-related pathways in IO resistance
TIGIT is reproducibly identified on PD-1-positive T cells and appears as a signal of resistance to anti-PD-1 therapy. DNAM-1 downregulation is consistently seen in TIL subsets linked to exhaustion. Of course, whether these two observations are linked is not known. In contrast, PVR upregulation has not been identified in any of the many unbiased profiling studies on mechanisms of resistance to or relapse from anti-PD-1 or anti-PD-L1 therapy. We should recognize that these relationships are complicated – recall that CGEN showed that PVRIG blockade increased TIGIT expression and Roche showed that TIGIT binding in cis can disrupt DNAM-1 activity. Some of these features are likely to be seen in the context of anti-PVR blockade – we do not have enough data to know.
Of note we do have enough (clinical) data that shows that single agent anti-TIGIT antibody treatment is ineffective, and co-administration of anti-PD-L1 or anti-PD-1 is needed. It is reasonable to further hypothesize that antagonism of both sides of this complex inhibitory network – eg. anti-TIGIT plus anti-PVRIG – may produce optimal synergy with anti-PD-1 or anti-PD-L1 therapies.
Compugen has presented anti-PVRIG monotherapy clinical data that suggests some activity:
Several partial responses were reported. With respect to their conclusions we will want to assess the biomarker data alluded to below:
Compugen, in collaboration with BMS, is running a “triple” study: anti-TIGIT, anti-PVRIG, anti-PD-1 (nivolumab). This sounds promising and will yield useful information one way or the other, since, as noted earlier, the anti-TIGIT antibody being used is an IgG4, as is the anti-PVRIG antibody – activity with this combination would further complicate our understanding of the mechanisms of action.
Updates will post as we get more clinical data from these interesting targets.