This invention relates to pharmaceutical compositions comprising plant deoxyribonucleoside kinase enzymes (dNK) capable of phosphorylating nucleoside analogs and to medical use of said dNKs. More specifically the invention relates to the medical use of deoxyribonucleoside kinase enzymes derived from (Arabidopsis thaliana), from loblolly pine (Pinus taeda), from tomato (Lycopersicum esculentum), from maize (Zea mays) or from rice (Oryza sativa). The invention also relates to methods of sensitising cells to prodrugs, and to methods of inhibiting pathogenic agents in warm-blooded animals using said plant dNKs. In another aspect the invention relates to plant derived deoxyribonucleoside kinase enzymes provided in isolated form from loblolly pine (Pinus taeda), from tomato (Lycopersicum esculentum), from maize (Zea mays) or from rice (Oryza sativa). In further aspects the invention provides polynucleotides encoding the plant dNKs, vector constructs comprising the polynucleotide, packaging cell lines capable of producing said vector, and genetically modified isolated host cells transduced/transfected/-transformed with the vector.