Poly(ADP-ribose) polymerase-1 (PARP-1), when activated by DNA damage, promotes both cell death and inflammation. Here we report that PARP-1 enzymatic activity is directly inhibited by minocycline and other tetracycline derivatives that have previously been shown to have neuroprotective and anti-inflammatory actions. These agents were evaluated by using cortical neuron cultures in which PARP-1 activation was induced by the genotoxic agents N-methyl-N -nitro-N-nitrosoguanidine (MNNG) or 3-morpholinosydnonimine (SIN-1). In both conditions, neuronal death was reduced by >80% either by 10 M 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone, an established PARP inhibitor, or by 100 nM minocycline. Neuronal NAD ؉ depletion and poly(ADP-ribose) formation, which are biochemical markers of PARP-1 activation, were also blocked by 100 nM minocycline. A direct, competitive inhibition of PARP-1 by minocycline (K i ‫؍‬ 13.8 ؎ 1.5 nM) was confirmed by using recombinant PARP-1 in a cell-free assay. Comparison of several tetracycline derivatives showed a strong correlation (r 2 ‫؍‬ 0.87) between potency as a PARP-1 inhibitor and potency as a neuroprotective agent during MNNG incubations, with the rank order of potency being minocycline > doxycycline > demeclocycline > chlortetracycline. These compounds are known to have other actions that could contribute their neuroprotective effects, but at far higher concentrations than shown here to inhibit PARP-1. The neuroprotective and antiinflammatory effects of minocycline and other tetracycline derivatives may be attributable to PARP-1 inhibition in some settings.death ͉ doxycycline ͉ inflammation ͉ neuron M inocycline and other tetracycline derivatives have neuroprotective effects unrelated to their antimicrobial properties (1). Minocycline can reduce neuronal death after excitotoxicity and ionizing radiation in culture (2, 3), and in animal models of stroke (1, 3-5), Parkinson's disease (6, 7), Huntington's disease (8), and amyotrophic lateral sclerosis (9). The neuroprotective effects of minocycline have been attributed both to reduced inflammation and a direct effect on neuronal survival (2, 3, 5, 9, 10).Poly(ADP-ribose) polymerase-1 (PARP-1) plays a key role in neuronal death and survival under stress conditions (11). PARP-1 is the most abundant of several PARP family members, accounting for Ͼ85% of nuclear PARP activity (11). When activated by DNA damage, PARP-1 consumes NAD ϩ to form branched poly(ADP-ribose) on target proteins. Poly(ADPribose) formation on histones and enzymes involved in DNA repair appears to facilitate DNA repair by preventing chromatid exchange and by loosening histone wrapping (12, 13). Poly(ADPribose) formation also has effects on gene transcription through interactions with transcription factors, notably NF-B (14-16), and PARP-1 inhibition or gene deletion attenuates the brain microglial response to cytokines and other triggers (17-19). Extensive PARP-1 activation can, in addition, lead to neuronal death through mechanisms linked to NAD ϩ deplet...