Delivering therapeutic agents in a sufficient concentration to a target site is a major challenge in the treatment of many diseases including cancer treatment. Conventional ways of drug administration are often characterized by a limited effectiveness, poor biodistribution and lack of selectivity. Fortunately, this limitation could be overcome by using drug nanocarriers. In this context, metal-organic frameworks (MOFs) - porous coordination polymers – seem to be very promising. MOFs comprise two building components – metal ions (or clusters) and polydentate organic ligands. They are known as materials featuring a high internal surface area, which can be used to load various molecules including drugs. In our research we focus on the delivery of arsenic drugs. Arsenic compounds such as arsenic trioxide have been shown as promising drugs in cancer treatment but due their high intrinsic toxicity, their delivery in therapeutic concentrations is challenging. Here nanomedicine and nanoparticles as drug carriers show great potential. Moreover, by combining MOF nanoparticles with agents suitable for magnetic resonance imaging such as superparamagnetic Fe3O4 nanoparticles, we could prepare theranostics, i.e. materials suitable not only for drug delivery but also for diagnosis. Such materials are expected to play a significant role in the dawning era of personalized nanomedicine.