Monthly Archives: January 2021

Fidgeting with TIGIT – Part 2 – pathway complexity

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 ( 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 (

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 (  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.

Stay tuned.



Fidgeting about TIGIT

Part 1 of 2

Pathways and targets covered: TGF-beta, PD-L1, PD-1, TIGIT

Companies mentioned: Merck KgaA, GSK, Roche, Merck, Mereo, iTeos, BMS, Arcus/Gilead, Compugen, Seagen, Beigene, Innovent, Agenus

Last week we had the bad news that Merck KGaA and GSK had thrown in the towel on bintrafusp alfa therapy for first-line advanced NSCLC.  Bintrafusp alfa is an anti-PD-L1/TGFbR2 TRAP therapeutic designed to selectively antagonize TGF-beta isoforms 1 and 3 while also blocking PD-L1, thereby delivering two-for-one anti-immunosuppression.  Bintrafusp alfa was being tested in a head-to-head trial vs. pembrolizumab and showed no added benefit in a patient population selected for PD-L1-high tumor expression (50%+ of cells in the tumor biopsy sample positive for expression).

This stirred up a fair amount of discussion, as TGF-beta blocking therapies are in vogue for immuno-oncology (IO), with small molecules, biologics, RNA-antagonists and genetic knockouts (in CAR T cells) all in the pipeline. I have high hopes for this space, despite the news out of Darmstadt. And to be fair, the press release stressed the ongoing bintrafusp alfa trials in bladder cancer, cervical cancer, and NSCLC using various drug combinations, and noted new trials in urothelial cancer and TNBC (  Still, the failure stung, due mainly to the promise of the early (open label) Phase 1 expansion cohort data that had suggested significant benefit from the therapy.

This got me thinking about TIGIT, another hot IO target.  The last time I wrote about TIGIT I ended with this question: “How to select patients who should respond to anti-TIGIT co-therapy (or anti-TIM-3 or anti-LAG-3)…?” ( This is a question we should ask about any pathway – including TGF-beta of course – particularly as we are now in the post-immune-checkpoint era, that is, in a setting where many patients in the most IO-responsive indications like melanoma and NSCLC will have already been treated with an anti-PD-1 or anti-PD-L1.  So, is there anything known about TIGIT expression that can guide us in patient (or indication) selection?

Roche leads the field with tiragolumab an anti-TIGIT Fc-competent IgG1 that has shown activity in combination with the anti-PD-L1 antibody atezolizumab in first-line NSCLC, and only in patients with PD-L1- expressing tumors (> 1% of cells in the tumor biopsy sample positive for expression).  We can pause here to recall that this is about where we started the discussion above regarding the TGF-beta TRAP/anti-PD-L1 asset from Merck KGaA, being trialed in the PD-L1-high (>50%) setting in NSCLC.

In front-line NSCLC (EGFR and ALK wildtype), Roche reported responses higher than with atezolizumab alone. Data were shown at AACR and then updated at ASCO.  Here are some of the ASCO data:


The response rate with dual therapy looks rather better than atezo alone, especially in the PD-L1 high cohort (middle panel).  Atezo alone appears to have underperformed, with an ORR = 21% (left panel, all patient data (ITT)).  In the comparable phase 3 trial of atezo vs chemotherapy in front-line NSCLC (also EGFR and ALK wildtype) the ORR = 38.3% in the atezo arm (n=285) and 28.6% in the chemotherapy arm (n=287), see Regardless the 66% response rate in the PD-L1-high cohort (middle panel) attracted attention.

The PFS data were also striking when compared to the prior trial.  This is tiragolumab plus atezolizumab / PD-L1 high cohort:


We can go back and compare this to the atezo alone Phase 3 interim data shown at ESMO in 2019 (I was stuck in the overflow “room” which was a curtained space on the floor of the Barcelona convention center).  This is the PD-L1-high cohort:


Here the median PFS is 8 months, certainly shorter than what is shown for tiragolumab plus atezolizumab, but again, note the disparity with the atezo alone arm of the study (medPFS for = 4 months).

Just to be clear, here are the PD-L1-high patient data compared:


We’re left with the always troubling question of variability between trials and the possibility that the tiragolumab plus atezolizumab results are a fluke.  Unfortunately, we will have to wait and see.

There are two features here worth noting.  One is that TIGIT, the target, is expressed on T cells, along with PD-1.  So far this makes sense – they might very well synergize, particularly given the function of DNAM-1 in the context of T cell signaling (see part 2).  But the anti-TIGIT antibody is an IgG1 isotype, thought to trigger ADCC and CDC-mediated target cell (ie. the T cell) death.  But we want the T cells, that’s the whole point of blocking PD-L1 with atezo.  So what the heck is going on here?

Merck seems to have an answer, but first, some more data.  Merck’s anti-TIGIT antibody, vibostolimab, like Roche’s tiragolumab, is a wildtype IgG1.  Early data on the combination of vibostolimab and pembrolizumab (anti-PD-1), presented at ESMO2020, looked promising in immune checkpoint naïve patients (75% had prior chemotherapy, the rest were treatment naïve):



We can benchmark these results to monotherapy, just as we did with the Roche data, focusing on the PD-L1-positive subset (here we can see data using a cutoff of >1% or >50% of cells positive in the tumor biopsy):



The results compare favorably with pembro-alone using the >1% PD-L1 cutoff and are similar to pembro-alone using the >50% PD-L1 cutoff.  As usual it is difficult to compare between trials, but the signal is encouraging.

Preclinically, Merck has addressed the MOA, stressing the requirement for the intact Fc functionality imparted by the IgG1 antibody isotype.  As mentioned earlier, the mechanistic puzzle is that canonical IgG1 activity includes the triggering of target cell killing via ADCC and CDC mediated cytotoxicity.  Of course, TIGIT is expressed on the very T cells we want to preserve and activate, not kill.  Given this reality we need alternate hypotheses for the action of the IgG1 antibodies.  The predominant hypothesis is that anti-TIGIT antibodies are selectively depleting T-regulatory cells that are TIGIT-bright and immunosuppressive.  This is reminiscent of the now-T-regulatory cells that are TIGIT-bright and immunosuppressive.  It’s an easy hypothesis to advance, similar to the now-debunked arguments made on behalf of anti-CTLA4 and anti-GITR antibodies, and very likely incorrect.

Merck has demonstrated in preclinical models that antagonistic anti-TIGIT antibodies having a  FcgR-engaging isotype induce strong anti-tumor efficacy whereas anti-tumor activity is drastically reduced when using the same anti-TIGIT antibodies that are null for FcgR-engagement (doi: 10.3389/fimmu.2020.573405). These results are consistent with data presented by multiple groups, eg. Mereo and iTeos.  The Merck team further showed shown that FcgR engagement persistently activated myeloid lineage antigen-representing cells APCs, including the induction of proinflammatory cytokines and chemokines while TIGIT blockade simultaneously enhanced T cell activation including elevated secretion of granzyme B and perforin, which synergizes with anti-PD-1 antagonism.  I favor this hypothesis.  Nb. This suggests we’ve a lot to learn still about the best way to engage Fcg receptors, a theme I introduced in the last post (link).

Where does this hypothesis leave everyone else in the TIGIT space?  Let’s line them up:


A few quick notes: EMD Serono/Merck KGaA and Innovent have anti-TIGIT programs without disclosed isotype information; Arcus has disclosed a second, Fc-competent, anti-TIGIT program (AB308); Agenus is developing both IgG1 and IgG4 anti-TIGIT antibodies.

A question: is Seagen’s hyper-killing IgG1 a step too far?

In summary, we have preliminary data in NSCLC that suggest that anti-TIGIT may synergize with anti-PD-1 or anti-PD-L1 therapies, consistent with the expression of TIGIT on PD-1 positive (ie. activated) T cells.  We have several hypotheses addressing the Fc-end of the therapeutics, and some information on why blocking TIGIT may enhance T cell responses.

Other than selecting patients with PD-L1-positive tumors, can we gate on TIGIT expression?  Apparently not, at least not in NSCLC, as just reported at the World Conference on Lung Cancer (abstract P77.02 – Efficacy of Tiragolumab + Atezolizumab in PD-L1 IHC and TIGIT Subgroups in the Phase II CITYSCAPE Study in First-Line NSCLC).

Here’s their text:

“Among the 135 enrolled patients with PD-L1-positive NSCLC (intent-to-treat [ITT] population), 113 had results from the SP263 assay and 105 had results from the TIGIT assay. The biomarker-evaluable populations (BEP) for both of these assays were similar to the ITT population. Comparable PFS improvement with tira + atezo relative to atezo monotherapy was seen in PD-L1–high (≥50% TC) subgroups defined by SP263 (PFS HR 0.23, 95% CI: 0.10–0.53) when compared with PD-L1-high subgroups defined by 22C3. However, for patients whose tumors were defined as TIGIT-high (≥5% IC), no strong association with PFS improvement was observed.

Biomarker subgroup Subgroup, n (BEP, N) PFS HR (CI) relative to atezo monotherapy arm
ITT (PD-L1 IHC 22C3 >1% TPS) 135 (135) 0.58 (0.39–0.88)
PD-L1 IHC 22C3 (≥50% TPS) 58 (135) 0.30* (0.15–0.61)
PD-L1 IHC SP263 (≥50% TC) 45 (113) 0.23* (0.10–0.53)
TIGIT IHC (≥5% IC) 49 (105) 0.62* (0.30–1.32)
*Unstratified HR

Prevalence of PD-L1 subgroups in the BEP was comparable with previous reports for both IHC assays. The PFS benefit observed with tira + atezo in patients with tumors defined as PD-L1-high by 22C3 was also observed using the SP263 IHC assay, but not in tumors classified as TIGIT-high using an exploratory TIGIT IHC assay. Our results suggest that PD-L1 expression, assessed by 22C3 or SP263, may be a biomarker for tira + atezo combination therapy in metastatic PD-L1-positive untreated NSCLC.”

So that the answer to the question we started with, can we pick patients, is ‘no’ for TIGIT expression, at least in this indication.

Regardless, to actually understand what blocking TIGIT does, we need to better understand the pathway.

That will be discussed in Part 2, coming soon.

Stay tuned.