Combination Therapy with Immune Checkpoint Inhibitors and Histone Deacetylase Inhibitors or Alkylating Agents

Immune checkpoint inhibitors (CPIs) have been widely adopted in a number of early and advanced malignancies. Histone deacetylase inhibitors (HDACis) and alkylating agents (AAs) have been suggested to potentiate the actions of CPIs on tumor cells. We conducted a comprehensive literature review to explore the potential synergistic activity between CPIs, AAs, and HDACis.

Clinical and non-clinical studies describing outcomes in patients with cancer receiving CPIs and either concomitant or sequential (pre- or post-CPI) AAs or HDACis were identified in PubMed using pre-defined search strings. Manual searches of key oncology congresses were similarly performed. All relevant articles and abstracts were manually screened for relevance, classified according to the specific anticancer agents used (CPIs, AAs, or HDACis), tumor entity, and whether treatment was concomitant or sequential.

Overall, 227 unique clinical studies across a range of tumor types, both solid tumors and hematological malignancies, were identified. One hundred and fifty-nine publications on Phase I and II clinical studies together with 41 publications on Phase III studies were examined. The most commonly investigated tumor types were melanoma, triple-negative breast cancer, non-small cell lung cancer, and Hodgkin lymphoma. The randomized clinical studies identified, all of which reported on the combination of a CPI with an AA, demonstrated superior outcomes in the combination arm compared with CPI or AA monotherapy. Similarly, combination therapy with CPIs and HDACis demonstrated promising activity.

Sequential or concomitant administration of a CPI with an AA or an HDACi may improve outcomes for patients with a range of tumor types. There is a rationale to support further investigation into the potential for synergy between CPIs, alkylating agents and/or HDACis in both the non-clinical and clinical settings.