New inhibitors of poly(ADP-ribose) polymerase and their potential therapeutic targets

Poly(ADP-ribose) polymerase (PARP) is a DNA-binding protein that is activated by nicks in the DNA molecule. It regulates the activity of various enzymes, including itself, that are involved in the control of DNA metabolism. Evidence obtained with both benzamide and isoquinolinone PARP inhibitors and the PARP-1(^-/-) phenotype, clearly indicate that PARP plays an important role in NO/ROS-induced cell damage during inflammation, ischaemia and neurodegeneration. PARP is involved in the maintenance of genomic stability and PARP inhibition may also potentiate the cytotoxic action of agents used in cancer therapy. Benzamides, although not very potent (IC₅₀ ~ 20 – 50 μM) PARP inhibitors, have been widely used to probe PARP functions, because of their lack of toxicity both in vitro and in vivo, even at high doses. In the early 1990s, a new class of very potent PARP inhibitors (i.e., at least 100-fold more potent thatn benzamide), the dihydroisoquinolinones, benzamide derivatives with the carbamoyl group constrained into the antiorientation, was discovered. At the same time, a large structure–activity surevey identified over 13 chemical classes of PARP inhibitors, the most potent calss sharing a common structural feature, the presence of a carbonyl group built into a polyaromatic heterocyclic skeleton or a carbamoyl group attached to an aromatic ring. Recently, a better knowledge of the PARP catalytic domain and the use of its crystal structure have led to the design and synthesis of the tricyclic lactam indoles, active at low nanomolar concentrations, and with favourable physical properties and in vivo characteristics. In the last few years the interest in PARP as a therapeutic target has been rapidly growing. This article reviews the patents filed for new PARP inhibitors over the last three years, up to February 2002, and their development status.