Affinity proteins constitute an important category of cancer therapeutics. Owing to properties such as high target affinity and selectivity, therapeutic proteins offer more targeted therapy than small molecule drugs. The target molecules are typically proteins that are overexpressed on the surface of tumour cells, such as membrane-bound receptors. However, these surface proteins are usually expressed in normal tissues as well, resulting in on-target off-tumour toxicity. Proteins with a higher tissue selectivity are thus needed. Here, this has been addressed by developing prodrug proteins dependent on cancer-specific proteases for activation. The prodrugs were composed of a target-binding affibody (active domain) connected to a masking affibody (masking domain) by a peptide linker including a protease substrate. The target of the prodrugs developed in this project was the HER2 receptor, which is overexpressed in several cancer types. Three prodrug candidates were developed, produced and characterised based on their ability to be activated by their respective protease. The hypothesis that the prodrugs could be activated and thus bind to HER2 in cancer cells was tested using biosensor assays, as well as preliminary cancer cell assays. One of the three candidates showed strong potential to be used as a targeted therapy for cancer treatment in the future. / Affinitetsproteiner utgör en viktig kategori av cancerläkemedel. Jämfört med småmolekylära läkemedel är affinitetsproteiner mer riktade, då de har högre affinitet och selektivitet än små molekyler. Oftast utgörs det molekylära målet av ett protein som överuttrycks på ytan av cancerceller, så som membranbundna receptorer. Dessvärre uttrycks de flesta cancerspecifika proteiner i mindre mängd även i normal vävnad. Detta leder till oönskade effekter som kan ge upphov till biverkningar. I syfte att utveckla mer vävnadsspecifika läkemedel har här affibody-baserade “prodrugs”, beroende av cancerspecifika proteaser för aktivering, tagits fram. Prodrug-proteinerna i detta projekt är riktade mot HER2-receptorn, som är överuttryckt i flera typer av cancer. Tre kandidater togs fram och utvärderades med avseende på deras förmåga att aktiveras av sina respektive proteaser. För att testa hypotesen att kandidaterna kunde binda till HER2 på cancerceller efter proteasaktivering användes biosensoranalys samt experiment med cancerceller. En av kandidaterna visade stark potential att kunna användas som ett riktat läkemedel mot cancer i framtiden.