3α-OL-5-β-pregnan-20-one hemisuccinate, a steroidal low-affinity NMDA receptor antagonist improves clinical rating scores in a rabbit multiple infarct ischemia model: Synergism with tissue plasminogen activator

We previously showed that the neuroactive steroid 3α-ol-5-β-pregnan-20-one hemisuccinate (ABHS), a low-affinity NMDA receptor antagonist, improves behavior in rabbits following spinal cord ischemia. The present study assessed whether this also occurs following cerebral ischemia. We used the rabbit small clot embolic stroke model (RSCEM), a model of multiple infarct ischemia, which has a well-defined behavioral endpoint. Quantal analysis for each treatment determines microclots (mg) that produce neurologic dysfunction in 50% of a group of animals (P50), with treatment considered beneficial if it increases the P50 compared to controls. ABHS (25 mg/kg) injected 5 min post-embolization significantly (P = 0.046) increased the P50 to 2.60 ± 0.69 mg compared to 1.15 ± 0.19 mg in controls but was ineffective at longer intervals. For combination studies with tissue plasminogen activator (tPA), a cumulative control curve (P50 = 1.18 ± 0.25 mg) was used for statistical comparisons. Low-dose tPA (0.9 mg/kg) and ABHS, given 60 min post-embolization, improved behavior synergistically (P50 = 2.44 ± 0.44 mg), compared to tPA (P50 = 1.25 ± 0.25 mg) or ABHS (P50 = 1.05 ± 0.24 mg) alone. A standard tPA dose (3.3 mg/kg) significantly increased the P50 at 60 (2.69 ± 0.19 mg) but not 180 min (1.28 ± 0.31 mg) post-embolization. However, when combined with ABHS, an effect was evident even 180 min post-embolization (P50 = 2.31 ± 0.48 mg). Although the therapeutic window for ABHS is limited, it significantly enhanced neuroprotection by low-dose tPA and increased the therapeutic window for tPA, suggesting that it may be most useful as a co-therapy with thrombolytics for stroke.