Nicotinamide Attenuates Focal Ischemic Brain Injury: Meta-Analysis or Mechanism of Protection
To the Editor:
We read with great interest the recent article by Macleod et al.1 They have shown a highly significant neuroprotective effect of nicotinamide on experimental stroke using the technique of systematic review or meta-analysis. The message is clear, and we believe this approach is useful and fairly robust in terms of statistical power. However, I would like to point out a possible pitfall of their overview. Nicotinamide, vitamin B3, is the precursor of nicotinamide adenine dinucleotide (NAD+), and it may act as a poly(ADP-ribose) polymerase (PARP) inhibitor. When ischemia-induced DNA strand breakage is too extensive, the overactivation of PARP may lead to intracellular NAD+ depletion and subsequent secondary energy failure (ie, ATP depletion).2 ATP levels can be dramatically increased following ischemia-reperfusion by nicotinamide.3 This scenario is generally accepted to explain the neuroprotective action of nicotinamide. However, it is also well known that high doses of nicotinamide (250 mg/kg in our case4) increase regional cerebral blood flow, and many studies have ignored cerebral blood flow. Therefore, we cannot jump into a conclusion that nicotinamide is a pure neuroprotectant independent of vasodilative action. If one does not consider divergent mechanisms of the beneficial effects of a candidate drug, such a promising drug based on animal experiments may fail to work under clinical settings, even though pooling of animal data reveals undoubtedly obvious neuroprotection.
Macleod MR, O’Collins T, Howells DW, Donnan GA. Pooling of animal experimental data reveals influence of study design and publication bias. Stroke. 2004; 35: 1203–1208.
Sasanaga-Akiyoshi F, Yao H, Tanuma S, Nakahara T, Hong JS, Ibayashi S, Uchimura H, Fujishima M. Nicotinamide attenuates focal ischemic brain injury in rats: with special reference to changes in nicotinamide and NAD+ levels in ischemic core and penumbra. Neurochem Res. 2003; 28: 1227–1234.
The comments of Yao and colleagues serve to further illustrate the potential uses of systematic review and meta-analysis in the context of the interpretation of data from animal studies in stroke.
Studies in focal cerebral ischemia may report measurement of cerebral blood flow (CBF). In most cases, they find no significant differences between control and treated groups, and use this to infer that any protective effect seen is not a consequence of changes in CBF. However, power calculations for such comparisons are seldom if ever reported; it may be that a true difference in CBF is missed because studies are underpowered for this comparison (ie, there is a type II or “false negative” error).
Analyses such as ours can illuminate the impact of study quality and study design on the estimate of efficacy. Equally, where data are available the same technique could be used to give a more precise estimate of the effect of a drug on CBF. This would reduce the probability of a type II error, and thereby reduce the risk that protection due to an effect on CBF might be incorrectly attributed to some other drug property.
It is likely that systematic review and meta-analysis can provide further insights both into the limits to the efficacy of individual drugs and into more generic determinants of outcome in experimental stroke. We are currently developing an international collaborative approach to establish priorities for future research and to develop a standard set of methodologies which might be used.