Cytokines
are small proteins that modulate the activity of the
immune system. Because of their potent immunomodulatory properties,
some recombinant cytokines have undergone clinical development and
have gained marketing authorization for the therapy of certain forms
of cancer. Recombinant cytokines are typically administered at ultralow
doses, as many of them can cause substantial toxicity even at submilligram
quantities. In an attempt to increase the therapeutic index, fusion
proteins based on tumor-homing antibodies (also called “immunocytokines”)
have been considered, and some products in this class have reached
late-stage clinical trials. While antibody-cytokine fusions, which
preferentially localize in the neoplastic mass, can activate tumor-resident
leukocytes and may be more efficacious than their nontargeted counterparts,
such products typically conserve an intact cytokine activity, which
may prevent escalation to curative doses. To further improve tolerability,
several strategies have been conceived for the development of antibody-cytokine
fusions with “activity-on-demand”, acting on tumors
but helping spare normal tissues from undesired toxicity. In this
article, we have reviewed some of the most promising strategies, outlining
their potential as well as possible limitations.