Neuroprotective Activity of 3‐Aminopyridine‐2‐Carboxaldehyde Thiosemicarbazone (PAN‐811), a Cancer Therapeutic Agent

3‐aminopyridine‐2‐carboxaldehyde thiosemicarbazone (3‐AP) is a highly‐hydrophobic small molecule that was originally developed for cancer therapy (Triapine®, Vion Pharmaceuticals) due to its ability to inhibit ribonucleotide reductase, a key enzyme required for DNA synthesis. 3‐AP has a high affinity for divalent cations, chelating the Fe 2+ at the R2 subunit of the enzyme and inhibiting formation of a tyrosyl radical essential for ribonucleotide reduction. We have demonstrated that 3‐AP is also a potent neuroprotectant (as such, it is referred to as “PAN‐811”). In vitro it completely blocks ischemic neurotoxicity at a concentration of 0.5 μM (EC 50  ≊ 0.35 μM) and hypoxic toxicity at 1.2 μM (EC 50  ≊ 0.75 μM). Full protection of primary cortical and striatal neurons can be achieved with 3‐AP when it is added to the medium at up to six hours after an ischemic insult. 3‐AP also suppresses cell death induced by neurotoxic agents, including staurosporine, veratridine and glutamate, indicating activity against a central target(s) in the neurodegenerative process. 3‐AP acts via neutralization of two important intracellular effectors of excitatory neurotoxicity; calcium and free radicals. Its reported ability to elevate anti‐apoptotic proteins is likely to be a consequence of the suppression of excessive intracellular free calcium. In a rat model of transient ischemia, a single bolus delivery of 3‐AP 1 h after the initiation of ischemic attack reduced infarct volume by 59% when administered i.c.v. (50 μg per rat) and by 35% when administered i.v. (1 mg/kg). In Phase I clinical trials in cancer therapy 3‐AP had no cardiovascular, CNS or other major adverse effects. Thus, 3‐AP has a high potential for development as a novel, potent neuroprotectant for the treatment of neurodegenerative diseases.