There is widespread interest in macrophages as a therapeutic target in cancer. Here, we demonstrate that trabectedin, a recently approved chemotherapeutic agent, induces rapid apoptosis exclusively in mononuclear phagocytes. In four mouse tumor models, trabectedin caused selective depletion of monocytes/macrophages in blood, spleens, and tumors, with an associated reduction of angiogenesis. By using trabectedin-resistant tumor cells and myeloid cell transfer or depletion experiments, we demonstrate that cytotoxicity on mononuclear phagocytes is a key component of its antitumor activity. Monocyte depletion, including tumor-associated macrophages, was observed in treated tumor patients. Trabectedin activates caspase-8-dependent apoptosis; selectivity for monocytes versus neutrophils and lymphocytes is due to differential expression of signaling and decoy TRAIL receptors. This unexpected property may be exploited in different therapeutic strategies.

Molecular alterations in genes involved in DNA mismatch repair (MMR) promote cancer initiation and foster tumour progression. Cancers deficient in MMR frequently show favourable prognosis and indolent progression. The functional basis of the clinical outcome of patients with tumours that are deficient in MMR is not clear. Here we genetically inactivate MutL homologue 1 (MLH1) in colorectal, breast and pancreatic mouse cancer cells. The growth of MMR-deficient cells was comparable to their proficient counterparts in vitro and on transplantation in immunocompromised mice. By contrast, MMR-deficient cancer cells grew poorly when transplanted in syngeneic mice. The inactivation of MMR increased the mutational burden and led to dynamic mutational profiles, which resulted in the persistent renewal of neoantigens in vitro and in vivo, whereas MMR-proficient cells exhibited stable mutational load and neoantigen profiles over time. Immune surveillance improved when cancer cells, in which MLH1 had been inactivated, accumulated neoantigens for several generations. When restricted to a clonal population, the dynamic generation of neoantigens driven by MMR further increased immune surveillance. Inactivation of MMR, driven by acquired resistance to the clinical agent temozolomide, increased mutational load, promoted continuous renewal of neoantigens in human colorectal cancers and triggered immune surveillance in mouse models. These results suggest that targeting DNA repair processes can increase the burden of neoantigens in tumour cells; this has the potential to be exploited in therapeutic approaches.

Trabectedin (ET-743) is a marine alkaloid isolated from the Caribbean tunicate Ecteinascidia turbinata, with a chemical structure characterized by three fused tetrahydroisoquinoline rings. Two of these rings (subunits A and B) provide the framework for covalent interaction with the minor groove of the DNA double helix, whereas the third ring (subunit C) protrudes from the DNA duplex, apparently allowing interactions with adjacent nuclear proteins. The compound's chemical interactions trigger a cascade of events that interfere with several transcription factors, DNA binding proteins, and DNA repair pathways, likely to be different from other DNA-interacting agents. Trabectedin also causes modulation of the production of cytokines and chemokines by tumor and normal cells, suggesting that the antitumor activity could also be ascribed to changes in the tumor microenvironment. The promising data on the combination of trabectedin with other anticancer agents, observed in preclinical systems, have prompted several clinical studies that are currently ongoing. One of these combinations (trabectedin-pegylated liposomal doxorubicin) was recently authorized by the European Commission for the treatment of patients with relapsed platinum-sensitive ovarian cancer.

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