The most sensitive cell structure - a DNA molecule, is the common target of cancer therapy. DNA damage response (controlled by enzymes from the phosphatidylinositol 3-kinase-related kinases family - PIKK) presents many encouraging targets for improving both conventional cytotoxic anticancer therapy and individualized monotherapy. DNA-dependent protein kinase (DNA-PK) is a member of the PIKK superfamily and plays an important role in the detection and repair of DNA double-strand breaks via the non-homologous end-joining pathway. The ability of cancer cells to repair DNA damage is an important element determining their sensitivity to radio- or chemo-therapy. The overactivation of DNA-PK in cancers can result in resistance to anticancer therapy. The inhibition of DNA-PK is a very promising target in anticancer research. However, the specific DNA-PK inhibitors currently known are limited by poor solubility and high metabolic lability in vivo, leading to a short serum half-life. Construction of new compounds based on existing drugs is the most important strategy to improve drug efficacy, pharmacokinetic parameters and to reduce toxicity. This review will describe small molecule inhibitors and summarize their efficacy in synergizing radio- and chemotherapy in vitro.