Prospective, randomized, double-blind trial of BQ-123 and bosentan for prevention of vasospasm following subarachnoid hemorrhage in monkeys

An improved canine model of subarachnoid hemorrhage using intrathecal indwelling catheters

In the present study, the feasibility of intrathecal indwelling catheters in the preparation of a repeated subarachnoid hemorrhage (SAH) model in dogs, as well as chronic intrathecal administration of therapeutic agents against the ensuing cerebral vasospasm was examined. Briefly, through a small suboccipital incision, two catheters were introduced into the subarachnoid space so that their tips were positioned in the prepontine cistern. One was used to induce SAH by infusing autologous blood, and the other to administer pharmacological agents (saline and/or saline containing a dye in this study) by means of an osmotic pump. The occurrence of cerebral vasospasm was followed by angiography via the catheter placed in the vertebral artery. The obtained results show: i) the injected blood effectively formed a subarachnoid clot in the prepontine cistern, invariably leading to the occurrence of severe cerebral vasospasm of the basilar artery; ii) the fluid injected by the osmotic pump was evenly distributed in the cisterns around the brain stem; iii) on post mortem pathological examination, no injury of the brain or the major arteries ascribable to the placement of catheters was found. Therefore, the present model is considered to be useful for both the investigation of pathophysiology and therapy of cerebral vasospasm following SAH, to be more favorable from the standpoint of animal protection, and more convenient and reliable than those used until now.

Prevention and reversal of cerebral vasospasm by an endothelin-converting enzyme inhibitor, CGS 26303, in an experimental model of subarachnoid hemorrhage

Delayed cerebral ischemia due to cerebral vasospasm is a major cause of morbidity and mortality in patients with aneurysmal subarachnoid hemorrhage (SAH). Increasing evidence implicates the potent vasoconstrictor peptide endothelin (ET) in the pathophysiology of cerebral vasospasm. In the present study the authors examined the therapeutic value of blocking the production of ET-1 by inhibiting the conversion of its relatively inactive precursor, Big ET-1, to a physiologically active form. An inhibitor of ET-converting enzyme (ECE), CGS 26303, was injected intravenously after inducing SAH in New Zealand white rabbits. Injections of CGS 26303 were initiated either 1 hour after SAH (prevention protocol) or 24 hours after SAH (reversal protocol). One of three concentrations (3, 10, or 30 mg/kg) of CGS 26303 was injected twice daily, and all animals were killed by perfusion fixation 48 hours after SAH occurred. Basilar arteries were removed and sectioned, and their cross-sectional areas were measured in a blind manner by using computer-assisted videomicroscopy. Treatment with CGS 26303 attenuated arterial narrowing after SAH in both the prevention and reversal protocols. The protective effect of CGS 26303 achieved statistical significance at all dosages in the prevention protocol and at 30 mg/kg in the reversal protocol. These findings demonstrate that inhibiting the conversion of Big ET-1 to ET-1 via intravenous administration of an ECE inhibitor can be an effective strategy for limiting angiographic vasospasm after SAH. Moreover, the results demonstrate that treatment with the ECE inhibitor is capable of reducing vasospasm even when initiated after the process of arterial narrowing has begun. Finally, the results provide further support for the role of ET in the establishment of cerebral vasospasm. The ECE inhibitor CGS 26303 thus represents a promising therapeutic agent for the treatment of cerebral vasospasm following aneurysmal SAH.

Endothelin production links superoxide generation to altered opioid-induced pial artery vasodilation after brain injury in pigs

BACKGROUND AND PURPOSE

Traumatic brain injury conveys significant morbidity and mortality to infants and children. In the newborn pig, opioids contribute to pial artery vasconstriction after fluid percussion injury (FPI). FPI attenuates vasodilation and cGMP production by methionine enkephalin (Met) and leucine enkephalin (Leu) and reverse dynorphin (Dyn) from a dilator to a constrictor. Superoxide anion (O2-) production contributes to altered cerebral hemodynamics after FPI, and O2- scavengers partially restore decreased dilator responses after FPI. Endothelin-1 (ET-1), a purported mediator of cerebral vasospasm, has been suggested to alter nitric oxide function and cGMP concentration. The present study was designed to determine the contribution of ET-1 to altered opioid-induced dilation after FPI and the role of O2- in such altered responses.

METHODS

Injury of moderate severity (1.9 to 2.3 atm) was produced by the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation.

RESULTS

FPI increased cerebrospinal fluid ET-1 from 20 +/- 2 to 93 +/- 6 pg/mL (approximately 10(-10) mol/L). Topical ET-1 (10(-10) mol/L) increased SOD-inhibitable NBT reduction from 1 +/- 1 to 16 +/- 3 pmol/mm2, similar to previously reported NBT reduction after FPI (14 +/- 2 pmol/mm2). BQ123 (10(-6) mol/L), an ET-1 antagonist, blunted the NBT reduction observed after FPI (4 +/- 1 pmol/mm2). Met produced pial vasodilation that was attenuated by FPI and partially restored by BQ123 pretreatment (7 +/- 1%, 11 +/- 1%, and 17 +/- 1% versus 3 +/- 1%, 6 +/- 1%, and 9 +/- 2% versus 5 +/- 1%, 9 +/- 1%, and 14 +/- 2% for 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Met-induced dilation was associated with increased cerebrospinal fluid cGMP, and these biochemical changes were likewise blunted by FPI and partially restored by BQ123 (357 +/- 12, 455 +/- 15, 500 +/- 19, and 632 +/- 11 versus 264 +/- 4, 267 +/- 4, 295 +/- 12, and 305 +/- 15 versus 309 +/- 19, 432 +/- 11, 529 +/- 10, and 593 +/- 4 pg/mL for resting conditions, 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Similar partial restoration of vascular and biochemical parameters was observed for Leu and Dyn.

CONCLUSIONS

These data show that ET-1, in concentrations similar to that present in cerebrospinal fluid after FPI, increases O2- production. These data also indicate the opioid-induced vasodilation and cGMP production are partially restored after FPI by ET-1 receptor blockade. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI, at least in part, through elevated O2- production.

Interactions between vasoconstrictors in isolated human cerebral arteries

This study investigates whether different endogeneous vasoconstrictors exert synergistic effects in isolated human cerebral arteries, because potentiation of contractile effects may play a role in the pathogenesis of cerebral vasospasm. Isolated human pial arteries obtained from macroscopically intact tissue during brain tumour operations were mounted onto a wire myograph. Concentration-response curves of 5-hydroxytryptamine (5-HT) were constructed in the absence and presence of threshold concentrations of the thromboxane A2 (TXA)-analog U46619, and endothelin-1 (ET-1). Threshold concentrations of U46619 markedly enhanced the maximum contractile effect of 5-HT. The response to 5-HT remained markedly increased even after washout of U46619. Threshold concentrations of ET-1 increased the maximum response to 5-HT, and markedly shifted the dose-response curve to the left. Even after washout of ET-1, the dose-response curve of 5-HT remained shifted to the left. The increase of the contractile effect of 5-HT in the presence of U46619 did not correlate with the relaxant action of the endothelium-dependent vasodilator carbachol. Thus, synergism between contractile substances such as 5-HT, U46619, or ET-1 is seen in human cerebral arteries, and responses to 5-HT are potentiated even after washout of ET-1 and U46619. The potentiation does not depend on the endothelial function. We conclude that synergistic responses between endogeneous vasoconstrictors such as 5-HT, TXA and ET-1 may be involved in the pathogenesis of cerebral vasospasm after subarachnoid haemorrhage.

Role of endothelin in pial artery vasoconstriction and altered responses to vasopressin after brain injury

Pial artery constriction following fluid-percussion injury to the brain is associated with elevated cerebrospinal fluid (CSF) vasopressin concentration in newborn pigs. It has also been observed that fluid-percussion injury reverses the function of vasopressin from that of a dilator to a constrictor. Endothelin-1 (ET-1), a purported mediator of cerebral vasospasm, can be released by several stimuli, including vasopressin. The present study was designed to investigate the role of ET-1 in pial artery constriction and in the reversal of vasopressin from a dilator to a constrictor, which is observed after fluid-percussion injury. Brain injury of moderate severity (1.9-2.3 atm) was produced in anesthetized newborn pigs that had been equipped with a closed cranial window. Endothelin-1 elicited pial dilation at low concentrations and vasoconstriction at higher concentrations. Fluid-percussion injury reversed the process of dilation to that of constriction at the low ET-1 concentration and potentiated this constriction at high ET-1 concentrations (10% +/- 1%, -8% +/- 1%, and -15% +/- 1% vs. -6% +/- 1%, -17% +/- 1%, and -26% +/- 2% for 10(-12), 10(10),10(-8) M ET-1 before and after fluid-percussion injury, respectively). Vasopressin modestly increased CSF ET-1 concentration before fluid-percussion injury. Fluid-percussion injury markedly increased CSF ET-1 concentration and the ability of vasopressin to release ET-1 (20 +/- 2, 26 +/- 3, and 40 +/- 4 pg/ml vs. 93 +/- 6, 141 +/- 9, and 247 +/- 31 pg/ml for control, 40 pg/ml vasopressin, and 400 pg/ml vasopressin before and after fluid-percussion injury, respectively). An ET-1 antagonist, BQ 123 (10(-6) M) blunted pial artery constriction following fluid-percussion injury (146 +/- 5 microns -127 +/- 6 microns vs.144 +/- 5 microns-136 +/- 4 microns). The BQ 123 also blocked the reversal of vasopressin's function from that of a dilator to a constrictor after fluid-percussion injury (8% +/- 1%, 21% +/- 3%, and -5% +/- 1%, -14% +/- 2% vs. 8% +/- 1%, 21% +/- 2% and 4% +/- 1%, 2% +/- 1% for 40 and 4000 pg/ml vasopressin before and after fluid-percussion injury in the absence and presence of BQ 123, respectively). The BQ 123 blocked the constrictor component to ET-1, whereas it had no effect on the dilator component. These data show that ET-1 contributes to pial constriction after fluid-percussion injury. These data also indicate that vasopressin-induced release of ET-1 contributes to the reversal of vasopressin from a dilator to a constrictor following fluid-percussion injury. Furthermore, these data indicate that elevated CSF vasopressin and ET-1 interact in a positive feedback manner to promote pial artery constriction following fluid-percussion injury.

Systemic administration of an inhibitor of endothelin-converting enzyme for attenuation of cerebral vasospasm following experimental subarachnoid hemorrhage

The potent vasoconstrictor peptide, endothelin-1 (ET-1), has been implicated in the pathophysiology of cerebral vasospasm that occurs after subarachnoid hemorrhage (SAH). This peptide is synthesized as a large prepropeptide that requires a series of modifying steps for its activation. The last of these steps involves the proteolytic conversion of a relatively inactive propeptide, Big ET-1, to its active, 21-amino acid peptide form. The enzyme responsible for converting Big ET-1 to ET-1 is a metalloprotease called endothelin-converting enzyme (ECE). In the present study the authors examined the effects of a newly developed inhibitor of ECE on responses to ET peptides in the normal basilar artery and on pathophysiological constriction in the spastic basilar artery after SAH. In the first series of experiments the authors examined normal basilar arteries in the rabbit, which were exposed transclivally and measured on-line using videomicroscopy. Intravenous administration or topical application of an active inhibitor of ECE, CGS 26303, blocked vasoconstrictor responses to topically applied Big ET-1 but not to ET-1. In contrast, topical application of a structurally related compound that does not inhibit ECE, CGS 24592, was ineffective in blocking vasoconstriction that was elicited by a topical application of Big ET-1. These findings indicate that CGS 26303 when administered systemically is capable of blocking the conversion of Big ET-1 to ET-1 in the basilar artery without affecting the ability of the vessel to respond to ET-1. In the second series of experiments the authors examined the effects of the ECE inhibitor on cerebral vasospasm after experimental SAH. Intraperitoneal administration of CGS 26303 via osmotic minipumps significantly attenuated the delayed spastic response of the basilar artery to an intracisternal injection of autologous blood. This study provides the first evidence that systemic administration of an inhibitor of ECE is capable of preventing cerebral vasospasm after SAH. The results reinforce a growing body of evidence that ETs play a critical role in the development of spastic constriction after SAH. Moreover, the findings indicate that blocking the conversion of Big ET-1 to its active ET-1 form using CGS 26303 may represent a feasible strategy for ameliorating cerebral vasospasm.

Effect of BQ-123 and tissue plasminogen activator on vasospasm after subarachnoid hemorrhage in monkeys

BACKGROUND AND PURPOSE

We aimed to determine the effect of intracisternal administration of an endothelin-A receptor antagonist (BQ-123) against vasospasm in a monkey model and to determine whether this drug would have adverse interactions with intracisternal tissue plasminogen activator (TPA).

METHODS

Thirty-three monkeys were randomly allocated to undergo baseline cerebral angiography, creation of right subarachnoid hemorrhage (SAH), and intracisternal delivery of (1) placebo (n = 10); (2) low-dose BQ-123 (5 mg/kg per day, n = 7); (3) high-dose BQ-123 (10 mg/kg per day, n = 9); or (4) BQ-123 10 mg/kg per day plus TPA 1 mg every 12 hours for three doses (n = 7). Angiography was repeated after 7 days, and animals were killed. Vasospasm was assessed by comparisons of angiograms within groups across time by paired t test and by comparisons across groups at each time by ANOVA.

RESULTS

Significant clot remained in the basal cisterns in all groups except those receiving TPA, in whom complete clot clearance was noted. Comparisons of angiograms at baseline and after 7 days showed significant vasospasm of the right middle cerebral artery in animals receiving placebo (mean +/- SEM reduction in diameter, 36 +/- 7%; P < .05) and low- and high-dose BQ-123 (16 +/- 4% and 18 +/- 7%, respectively). Animals that received TPA did not develop significant right cerebral artery vasospasm. Comparisons of arterial diameters at day 7 revealed significant variance in right middle cerebral artery diameter, with animals in the placebo group having significantly more and animals in the TPA group having significantly less vasospasm than the BQ-123 groups. Histopathological examination of the brains did not show inflammation or pathological change in animals that received BQ-123 or BQ-123 plus TPA.

CONCLUSIONS

Intracisternal TPA was efficacious against vasospasm in monkeys. Combination therapy with TPA and BQ-123 was not associated with reduction in efficacy of either drug or with evidence of toxicity.

Changes in endothelial nitric oxide synthase mRNA during vasospasm after subarachnoid hemorrhage in monkeys

OBJECTIVE

We attempt to determine whether changes in messenger ribonucleic acid (mRNA) for nitric oxide synthase (NOS) and soluble guanylate cyclase, enzymes that mediate endothelium-dependent vasodilation in cerebral arteries, occur after subarachnoid hemorrhage (SAH) in monkeys.

METHODS

Baseline cerebral angiograms were obtained, and right-sided SAH was induced by microsurgically placing autologous blood clot against the right anterior circle of Willis in seven monkeys. Seven days later, angiographic studies were repeated and the animals were killed. Right (vasospastic) and left (control) middle cerebral arteries and underlying cortex were removed. The competitive reverse transcriptase polymerase chain reaction was used to quantify mRNA for soluble guanylate cyclase and two isoforms of constitutive NOS in these tissues.

RESULTS

Comparison of angiograms at baseline and after 7 days showed a 41 +/- 7% (mean +/- standard error of the mean, P < 0.05, Wilcoxon test) decrease in diameter of the right middle cerebral artery. After the animals were killed, comparison of right and left middle cerebral arteries showed a 56 +/- 11% decrease (P < 0.005, paired t test) in endothelial NOS mRNA. There was a 142 +/- 39% (P < 0.05) increase in right cortex endothelial NOS mRNA compared to the left cortex. There were no significant differences between right and left sides in mRNAs for soluble guanylate cyclase or brain NOS.

CONCLUSION

Decreased endothelial NOS mRNA in cerebral arteries 7 days after SAH may be caused by endothelial cell damage and could contribute to vasospasm after SAH. Increased endothelial NOS in brain tissue may reflect a compensatory vasodilator mechanism of the brain against the cerebral ischemia associated with vasospasm and SAH.

Increased expression of endothelin B receptor mRNA following subarachnoid hemorrhage in monkeys

These studies tested the hypothesis that the cerebral vasospasm that follows subarachnoid hemorrhage (SAH) is due to alterations in endothelin (ET) and ET receptor expression. Eight monkeys underwent cerebral angiography and induction of SAH. Angiography was repeated 7 days later to confirm the presence of cerebral vasospasm, and animals were killed. RNA was isolated from right (vasospastic) and left (control) side middle cerebral arteries and surrounding cerebral cortex. The levels of prepro (PP) ET-1 (ppET-1) and ppET-3 and ETA and ETB receptor MRNAs were determined using a quantitative reverse transcriptase polymerase chain reaction-based assay. ET-1 peptide was also measured in CSF at baseline and after 7 days. Specific agonist binding to ETA and ETB receptors in both middle cerebral arteries and in surrounding brain cortex was measured in three animals by autoradiographic binding assays. Levels of ETB receptor mRNA were 3.4 +/- 2.2-fold higher in the right than in the left cerebral arteries (p < 0.01). There were no significant differences in the levels of ppET-1, ppET-3, or ETA receptor mRNA in cerebral arteries. ET-1 peptide was not elevated in CSF. Levels of ETA and ETB receptor mRNAs were 2.6 +/ 1.1- and 2.1 +/ 1.3-fold higher, respectively, in the right than in the left cerebral cortex, while the level of ppET-3 mRNA was 2.1 +/- 1.0-fold lower. There were no differences in ppET-1 mRNA levels between right and left cerebral cortex. Binding to ETA and ETB receptors in cerebral arteries and cortex did not differ significantly between right and left sides. These results do not support the hypothesis that overexpression of ET-1 is principal cause of vasospasm, but rather they suggest that SAH causes complex changes in the ET system that together are responsible for the cellular response to SAH.