Neurochemical and morphological correlates of protection against hypoxic/ischemic injury by adenosine A1
receptors have been described using in vitro
Several recent reviews provide details of experimental in vivo
Both focal and global ischemia have been studied, with the duration of insults varying between 5 and 30 min. Drugs were administered either prior to or shortly after cerebrocirculatory arrest, and several measures were employed in determining the outcome, e.g.
, mortality, neuronal loss in selectively vulnerable regions, or the extent of neurological dysfunction. The reports indicated a significant amelioration of all studied parameters,1,2,3,8
Ultimately, the effectiveness of acute adenosine A1
receptor stimulation in preventing ischemic brain damage has been stressed by several experiments in which both selective26
receptor antagonists have been used. Invariably, treatment with these agents resulted in a significant worsening of postischemic necrosis27
Despite unequivocal, albeit experimental, demonstration that both pre- and postischemically administered adenosine A1
receptor agonists reduce postischemic neuronal loss,1–4
cardiovascular side effects (bradycardia and hypotension), which accompanied administration of even low doses of the drugs available in the recent past, mitigated against their clinical implementation. In addition, the criticism based upon the assertion that the neuroprotective effects of A1
receptor agonists were primarily based upon their hyperthermia-inducing properties provided another stumbling block.3
Subsequent experiments in which body and brain temperatures were carefully controlled1,2,4
clearly showed that hypothermia was not the principal mechanism of A1
receptor-mediated neuroprotection. Moreover, the results of these studies were also confirmed by the in vitro
Thus, although the contributory aspect of A1
receptor agonist-induced hypothermia to neuroprotection cannot be readily dismissed, it must be viewed in the context of the overall effect of these drugs, i.e.
, as a direct consequence of diminished electrical activity/metabolism resulting from A1
receptor stimulation. Hence, the depression of body/brain temperature resulting from administration of A1
receptor agonists is the consequence, and not a very direct one at that, of other actions elicited by these drugs, constituting a part of the neuroprotective complex rather than a side effect.
Recently, the issue of the presence of cardiovascular complications induced by A1
agonists has been experimentally addressed. In their recent publication29
Novo Nordisk disclosed a series of potent and selective A1
agonists including NNC 21-0136, whose early postischemic administration resulted in neuroprotection, yet was remarkably free of both bradycardia and hypotension.
Virtually all early studies in which neuroprotective properties of A1
agonists have been demonstrated employed acute pre- or postischemic administration. In retrospect, such limitation of the treatment regimen is surprising, since A1
receptor therapies have been advocated not only for the treatment of acute pathologies (i.e.
, stroke) but also chronic, such as seizures. Studies aimed at correcting this deficiency provided a puzzling result indicating that the outcome of the treatment with A1
receptor agonists was highly dependent on the drug administration regimen.9
Thus, acute administration of the potent and selective A1
-cyclopentyladenosine (CPA) at 1 mg/kg either 15 min prior to or after 10 min of forebrain ischemia led to increased survival and vastly improved neuronal preservation. Chronic treatment (1 mg/kg daily for 15 days) with the same drug, however, resulted in postischemic survival that was significantly worse than in the controls (60% vs 14%) and a correspondingly poor neuronal preservation in the brains of the treated animals. The results of chronic treatment with the selective A1
receptor antagonist 1,3-dipropyl-8-cyclopentylxanthihe (CPX) were exactly opposite.26
Similar data were obtained when experimental treatment of seizures with CPA was attempted.29
Although receptor downregulation/desensitization may be involved, the precise nature of this “regimen-dependent effect reversal”9
is still unclear. Interesting in terms of the understanding of basic phenomena involved, the dependence of the therapeutic outcome on the treatment regimen indicated that caution was needed when contemplating adenosine-based drugs for practical use.
It is unknown whether the therapeutic properties of novel A1
agonists recently disclosed by Novo Nordisk28
will be sustained also in the chronic administration. While synthesis of the latter drugs is based on the “classical” medicinal chemistry of adenosine receptor-active agents, the functionalized congener concept, i.e.
, addition of a long side chain to the phenyl substituent of N6
-phenyladenosine introduced a series of highly selective and potent A1
receptor agonists and antagonists.30
One of these drugs, adenosine amine congener (ADAC, ) has been studied extensively as a potential candidate for the treatment of ischemic brain damage.31–32
The therapeutic dose of ADAC is much lower (micrograms) than that of the hitherto studied agents: either pre- or postischemic administration of ADAC at 75–100 µg/kg substantially reduces postischemic mortality, and leads to a high degree of neuroprotection. Furthermore, preischemic treatment with the drug given at 100 µg/kg results in a virtually complete elimination of postischemic spatial memory and learning deficits ( and ). However, in contrast to most other A1
agonists, ADAC does not induce cardiovascular side effects ().
Chemical structure of the A1 agonists CPA and ADAC and the A1 antagonist CPX.
Effect of ADAC administration (single dose of 100 µg/kg) at 6 hr and 12 hr postischemia on neuronal survival following 5 min ischemia in gerbils. ap < 0.05. Bars: SEM.
10 day survival of gerbils following 10 min ischemia after chronic administration of ADAC (60 days) at different doses.
Cardiovascular Effects of ADAC Administration at a Dose of 100 µg/kg to Nonischemic Gerbils
ADAC has been shown to reduce glutamate release from gerbil cortical slices in a dose dependent manner.33
However, involvement of other mechanisms is quite likely in view of the fact that postischemic treatment with ADAC protects against neuronal damage and memory loss even when instituted as late as 12 hr postischemia,32
, the time when ischemia-elevated glutamate release is fully normalized. Immunocytochemical studies emphasize the extensive character of ADAC-mediated neuroprotection as evidenced by a significant reduction in the degeneration of microtubule associated protein 2 (MAP-2).
The finding that acute treatment with ADAC effectively prevents brain damage even when instituted as late as 12 hr after the insult () conflicts with the belief that the “time window” for A1
receptor therapy is rather brief.8
It does, however, underline the emerging notion that the interplay between the effects elicited by stimulation of A1
receptors located on different cell types (i.e.
, neurons, glia) may be exceedingly complex and far from fully elucidated.4,34
ADAC proved equally effective in chronic administration. shows the effect of different doses of the drug given once daily for 60 days. As can be seen, prolonged treatment with ADAC at doses of 10–100 µg/kg resulted in a significant increase of postischemic survival and neuronal preservation (). Most importantly, however, the results of chronic treatment with ADAC are in stark contrast to those obtained with CPA (see above), i.e., long-term administration of ADAC is not associated with the “regimen-dependent effect reversal.”
In summary, the currently available data indicate that ADAC, and possibly other compounds based upon the functionalized congener concept,30
may belong among the leading candidates for practical treatment of neurological diseases. The newly emerging compounds are free of cardiovascular side effects and appear to be equally effective in acute as well as chronic administration. The latter aspect is particularly important in view of the fact that adenosine A1
-based therapies have been suggested not only for stroke but also for other disorders of the central nervous system (e.g.
, seizures) in which long-term treatment is mandatory. Unquestionably, several aspects of adenosine receptor involvement in neurodegenerative processes that accompany ischemia and other neuron-destroying diseases need further studies. Nonetheless, it also appears that, despite the initial setbacks, the notion of clinical applications of A1
receptor-acting drugs is now more real than ever.