Clinical application of cytokine therapy is limited by two main challenges: (1) dose-limiting toxicity and (2) a short half-life in the bloodstream. Hence, an unmet need exists for strategies that reduce systemic toxicity and confine the therapeutic activity of cytokines to the tumor site. Fusion proteins comprising an antibody fragment, targeting surface antigens expressed on cancer cells, have been developed. However, these cannot be applied to tumors that lack a known antigenic signature. Utilizing universal physiological features of solid tumors could circumvent this limitation. Recently near-infrared heptamethine cyanine (HMC) fluorophores featuring a rigid chlorine-substituted cyclohexenyl ring were reported to strongly accumulate in many preclinical solid tumor models. This was partially ascribed to a strong affinity of HMC dyes for albumin. Here we will investigate whether HMC dyes can serve as a molecular anchor to deliver cytokines to solid tumors. Hereto we will synthesize multiple HMC analogues and conjugate these to IL-2 and IL-12 as cytokines of interest. We will investigate how molecular architecture and length of a poly(ethylene glycol) (PEG) linker affects albumin binding in vitro and tumor-targeting in vivo, an whether the therapeutic window of cytokine treatment can be improved.