Antibodies that can inhibit negative regulators of T-cell activation have been the subject of great interest recently, with blockade of CTLA-4 and PD-1 or of PD-L1 showing impressive results in clinical trials for a number of solid tumors, as well as Hodgkin lymphoma.1 There is also emerging evidence that small molecules with activity in lymphoma, such as inhibitors of phosphoinositide-3-OH kinase (PI-3K) p110 delta, may have effects upon T-cell responses,2 raising the possibility of additive or even synergistic effects between these different classes of drugs. This study from the laboratory of Dr. Ronald Levy at Stanford University extends this idea to the Bruton tyrosine kinase inhibitor ibrutinib, which has shown activity against chronic lymphocytic leukemia, mantle cell lymphoma, activated diffuse large B-cell lymphoma and lymphoplasmacytic lymphoma, by inhibition of the signaling pathways downstream of the B-cell receptor. Ibrutinib is also an inhibitor of interleukin-2 inducible T-cell kinase (ITK), which is an important mediator of survival for Th2 cells. These investigators therefore hypothesized that ibrutinib might act as an immunomodulator by altering the balance of Th1/Th2 response to tumors.
Initial experiments established that the syngeneic lymphoma cell line A20 is insensitive to ibrutinib, both in vitro and in vivo, with no evidence of cell killing or reduced tumor growth at doses up to 10 μM. Similarly, A20 is not killed in vitro by anti−PD-L1 antibodies, despite expressing high levels of the antigen, although there was evidence of slight growth retardation in vivo. However, the combination of ibrutinib and anti-PD-L1 demonstrated much more substantial therapeutic effects in vivo, with around half the mice cured after treatment of established lymphoma. This effect was lost if the mice were depleted of T cells, suggesting an active immune response. This was confirmed by the demonstration of intracellular interferon-γ in splenic T cells co-cultured with irradiated A20 cells, which was only detectable when the T cells came from animals treated with the ibrutinib/anti−PD-L1 combination.
Further experiments confirmed similar findings with the ibrutinib-insensitive J558 myeloma cell line and with two solid tumor cell lines: 4T1, a triple-negative breast cancer; and CT26, a colon cancer, neither of which express Bruton tyrosine kinase. The in vivo therapeutic effect was more impressive with the CT26 model, which is known to express a murine leukemia virus antigen AH1. In mice treated with the ibrutinib/anti−PD-L1 combination, AH1-specific CD8 cells were detectable in the peripheral blood by MHC tetramer staining. Re-challenge of cured animals with further doses of 106 CT26 cells demonstrated the presence of immune memory, with resistance to their engraftment.
This study adds further evidence to the view that molecules that can affect signaling pathways in malignant lymphocytes may also have potent effects upon the normal cells of the immune system. That this could be shown in a tumor model where the malignant cells lack any Btk expression is particularly interesting, and it suggests that ibrutinib may be able to augment weak T-cell responses in tumors that respond less well to immunomodulating antibodies. The translation of this idea to clinical investigation is appealing, though previous experience of severe liver toxicity when vemurafinib was combined with ipilimumab highlights the risks of uncontrolled autoimmunity and the need for caution in trial design with this class of agents.
Dr. Johnson indicated no relevant conflicts of interest.