Druggable Mitochondrial Targets for Treatment of Cerebral Ischemia

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Quality survival after brain injury is currently the greatest challenge for critically ill or injured infants and children. A universal contributor limiting quality survivorship is the devastating impact of hypoxic-ischemic encephalopathy (HIE), either as a primary consequence in cases of cardiac arrest, stroke, or intracranial hemorrhage or as secondary sequelae in cases of status epilepticus, circulatory or septic shock, neuroinflammation, or traumatic brain injury (TBI); with the principal cause of HIE spanning from infancy through adolescence a consequence of cardiac arrest. As to-date a cure for HIE has not been discovered, a paradigm-shifting strategy is likely necessary to improve neurological outcome for victims of HIE. Accordingly, we have developed a new class of therapeutics to treat HIE via preservation of critical cellular energy stores by selectively targeting poly(ADP-ribose) polymerase (PARP) in mitochondria (mtPARP), linking the mitochondria-targeting moieties hemi-gramicidin S (XJB) or triphenylphosphonium (TPP) to PARP inhibitors used clinically. Ischemia-induced PARP overactivation triggered by DNA damage consumes NAD+, generating branch chain poly(ADP-ribose) polymers (PARylation) resulting in ATP depletion, energy failure, and cell death by necrosis and/or apoptosis-inducing factor (AIF)-mediated parthanatos. As mitochondria are the major source of ATP and NAD+ in aerobic organisms, preservation of mitochondrial energy stores represents a logical “druggable” target for mitigation of HIE. We recently reported that the mitochondria- targeting PARP1 inhibitor XJB-veliparib preserves NAD+ stores and prevents neuronal death after oxygen- glucose deprivation (OGD) in vitro at nanomolar concentrations. Importantly, XJB-veliparib selectively targets mitochondria and thereby does not impede nuclear DNA repair in vitro. We present provocative pilot data suggesting that XJB-veliparib and the readily translatable mitochondria-targeting compound TPP-veliparib may be efficacious after cardiac arrest in post-natal day (PND) 17 rats, a developmental age equivalent to a young child and a time associated with peak cerebral metabolism. This new class of therapeutics has the advantage of preventing PARP-mediated energy failure and cell death by selectively targeting mtPARP while sparing PARP1-facilitated nuclear DNA repair and provide a tool to definitively establish (or refute) a role for mtPARP in the pathogenesis of HIE. If proven effective, mtPARP1 inhibitors would represent novel, safe (in terms of nuclear DNA repair), and translatable therapies to mitigate HIE, with special potential in the highly vulnerable, developing brain where metabolic rate is at its peak.