ST 91

Synergistic interactions between two alpha(2)-adrenoceptor agonists, dexmedetomidine and ST-91, in two substrains of Sprague-Dawley rats.[Pubmed:10692612]

Pain. 2000 Mar;85(1-2):135-43.

Several lines of evidence indicate that the antinociception produced by intrathecal administration of the alpha(2)-adrenoceptor agonists dexmedetomidine or ST-91 is mediated by different subtypes of the alpha(2)-adrenoceptor. We recently provided additional pharmacologic evidence for this idea, as well as for differences in the function of these receptors between Harlan and Sasco rats, two widely-used outbred substrains of Sprague-Dawley rat. The present study used isobolographic analysis to further characterize the receptors at which intrathecally administered ST-91 and dexmedetomidine act in these two substrains. The rationale for these studies derives from the assumption that if dexmedetomidine and ST-91 act as agonists at the same receptor then they should interact in an additive manner. However, if they interact in a supra-additive manner, then they must act at different subtypes of the alpha(2)-adrenoceptor. In the tail-flick test, the dose-effect relationship for a 1:3 mixture of dexmedetomidine and ST-91 was shifted significantly to the left of the theoretical dose-additive line in both Harlan and Sasco Sprague-Dawley rats. A similar finding was made in the hot-plate test despite the fact that the dose-response characteristics of the agonists were different in this test. Thus, in Harlan rats, in which ST-91 is a full agonist and dexmedetomidine is essentially inactive, the dose-effect relationship for the mixture of dexmedetomidine and ST-91 was shifted far to the left of the dose-additive line. Similarly, in Sasco rats, in which ST-91 is a partial agonist and dexmedetomidine is inactive, co-administration of the two agonists also shifted the dose-response relationship to the left of the dose-additive line. The consistent finding that these two alpha(2)-adrenoceptor agonists interact in a supra-additive manner provides strong evidence that dexmedetomidine and ST-91 produce antinociception by acting at different alpha(2)-adrenoceptor subtypes in the spinal cord. This conclusion is consistent with the earlier proposal that dexmedetomidine acts predominantly at alpha(2A)-adrenoceptors whereas ST-91 acts predominantly at non-alpha(2A)-adrenoceptors. Recent anatomical evidence indicates that these non-alpha(2A) adrenoceptors may be of the alpha(2C) type. The synergistic combination of an alpha(2A)- and an alpha(2C)-adrenoceptor agonist may provide a unique and highly effective drug combination for the treatment of pain without the sedation produced by an equianalgesic dose of a single alpha(2)-adrenoceptor agonist.

Carboplatin/ifosfamide window therapy for osteosarcoma: results of the St Jude Children's Research Hospital OS-91 trial.[Pubmed:11134210]

J Clin Oncol. 2001 Jan 1;19(1):171-82.

PURPOSE: To determine the activity of carboplatin/ifosfamide in patients with previously untreated osteosarcoma and to estimate patient outcomes after a multiagent chemotherapy protocol that eliminated cisplatin. PATIENTS AND METHODS: Sixty-nine patients with newly diagnosed, previously untreated osteosarcoma received three cycles of carboplatin (560 mg/m(2) x 1) and ifosfamide (2.65 g/m(2)/d x 3). Assessment of response was evaluated after two (week 6) and three (week 9) chemotherapy cycles. At week 9, histologic response was assessed. Adjuvant therapy comprised two additional carboplatin/ifosfamide cycles, doxorubicin, and high-dose methotrexate. Patients were stratified at enrollment: stratum A, resectable primary tumor without metastases; stratum B, unresectable primary tumor; and stratum C, metastatic disease at diagnosis. Week 6 response was compared with that of a historic group that received only ifosfamide during the initial window evaluation. RESULTS: The clinical and radiographic response rate to three cycles of carboplatin/ifosfamide was 67.7% (95% confidence interval, 55.0% to 78.8%). Compared with the historic population who received only ifosfamide, the combination of carboplatin and ifosfamide reduced the progressive disease rate at week 6 (31.9% v 9%, P: = .003). For patients in stratum A, the 3-year event-free survival and survival were 72.3% +/- 6.7% and 76.4% +/- 6.4%, respectively. Patients who received carboplatin-based therapy had less long-term renal toxicity and ototoxicity. CONCLUSION: This pilot trial suggests that carboplatin/ifosfamide combination chemotherapy has substantial antitumor activity. In the context of a multiagent chemotherapy protocol comprising high-dose methotrexate and doxorubicin, we found that the addition of carboplatin/ifosfamide resulted in patient outcomes comparable to trials using cisplatin-based therapy with less long-term toxicity.

alpha2B-adrenoceptor agonist ST-91 antagonizes beta2-adrenoceptor-mediated relaxation in rat mesenteric artery rings.[Pubmed:18054907]

Eur J Pharmacol. 2008 Feb 12;580(3):361-5.

We sought an isolated vascular preparation and experimental setting where the function of alpha2B-adrenoceptors could be demonstrated by non-recombinant technique. ST-91 (2-[2,6-diethylphenylamino]-2-imidazoline), an alpha 2B-adrenoceptor agonist with a mixed alpha adrenergic receptor type/subtype selection profile antagonized the relaxant effect of isoproterenol in endothelium-denuded rat mesenteric artery rings precontracted with phenylephrine. At 10(-7) M of ST-91, the antagonism was characterized by a rightward shift of isoproterenol dose-response curve (A50=6.81+/-1.40 e-7 (n=4) vs the control 1.29+/-0.25 e-7 M (n=4)) with no E(max) depression. At 10(-6) M the Emax depression was prevalent (36.1+/-7.0% (n=4) vs the control 79.9+/-5.1% (n=4)); both actions could be antagonized by the alpha2-adrenoceptor antagonist yohimbine. The not subtype-selective alpha(2)-adrenoceptor agonist xylazine (10(-7) M) did not affect the relaxant action of isoproterenol. Present findings are discussed in the light of previously reported hemodynamic effects attributed to alpha2B-adrenoceptors in receptor subtype-knockout animals.

Dexmedetomidine and ST-91 analgesia in the formalin model is mediated by alpha2A-adrenoceptors: a mechanism of action distinct from morphine.[Pubmed:18846040]

Br J Pharmacol. 2008 Dec;155(7):1117-26.

BACKGROUND AND PURPOSE: Intrathecal administration of alpha(2)-adrenoceptor agonists produces potent analgesia. This study addressed the subtype of spinal alpha(2)-adrenoceptor responsible for the analgesic effects of i.t. dexmedetomidine and ST-91 in the formalin behavioural model and their effects on primary afferent substance P (SP) release and spinal Fos activation. EXPERIMENTAL APPROACH: The analgesic effects of i.t. dexmedetomidine and ST-91 (alpha(2) agonists) were tested on the formalin behavioural model. To determine the subtype of alpha(2)-adrenoceptor involved in the analgesia, i.t. BRL44408 (alpha(2A) antagonist) or ARC239 (alpha(2B/C) antagonist) were given before dexmedetomidine or ST-91. Moreover, the ability of dexmedetomidine and ST-91 to inhibit formalin-induced release of SP from primary afferent terminals was measured by the internalization of neurokinin(1) (NK(1)) receptors. Finally, the effects of dexmedetomidine on formalin-induced Fos expression were assessed in the dorsal horn. KEY RESULTS: Intrathecal administration of dexmedetomidine or ST-91 dose-dependently reduced the formalin-induced paw-flinching behaviour in rats. BRL44408 dose-dependently blocked, whereas ARC239 had no effect on the analgesic actions of dexmedetomidine and ST-91. Dexmedetomidine and ST-91 had no effect on the formalin-induced NK(1) receptor internalization, while morphine significantly reduced the NK(1) receptor internalization. On the other hand, both dexmedetomidine and morphine diminished the formalin-induced Fos activation. The effect of dexmedetomidine on formalin-induced Fos activation was reversed by BRL44408, but not ARC239. CONCLUSION AND IMPLICATIONS: These findings suggest that alpha(2A)-adrenoceptors mediate dexmedetomidine and ST-91 analgesia. This effect could be through a mechanism postsynaptic to primary afferent terminals, distinct from that of morphine.

Cardiopulmonary effects of the alpha2-adrenoceptor agonists medetomidine and ST-91 in anesthetized sheep.[Pubmed:10215644]

J Pharmacol Exp Ther. 1999 May;289(2):712-20.

To test the hypothesis that pulmonary alterations are more important than hemodynamic changes in alpha2-agonist-induced hypoxemia in ruminants, the cardiopulmonary effects of incremental doses of (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imadazole) hydrochloride (medetomidine; 0.5, 1.0, 2.0, and 4 micrograms/kg) and 2-(2, 6-diethylphenylamino)-2-imidazol (ST-91; 1.5, 3.0, 6.0, and 12 micrograms/kg) were compared in five halothane-anesthetized, ventilated sheep using a placebo-controlled randomized crossover design. Pulmonary resistance (RL), dynamic compliance, and tidal volume changes in transpulmonary pressure (DeltaPpl) were determined by pneumotachography, whereas cardiac index (CI), mean pulmonary artery pressure (Ppa), and pulmonary artery wedge pressure (Ppaw) were determined using thermodilution and a Swan-Ganz catheter. The most important finding was the fall in partial pressure of oxygen in arterial blood (PaO2) after administration of medetomidine at a dose (0.5 micrograms/kg) 20 times less than the sedative dose. The PaO2 levels decreased to 214 mm Hg as compared with 510 mm Hg in the placebo-treated group. This decrease in PaO2 was associated with a decrease in dynamic compliance and an increase in RL, DeltaPpl, and the intrapulmonary shunt fraction without changes in heart rate, CI, mean arterial pressure, pulmonary vascular resistance, Ppa, or Ppaw. On the other hand, ST-91 only produced significant changes in PaO2 at the highest dose. After this dose of ST-91, the decrease in PaO2 was accompanied by a 50% decrease in CI and an increase in mean arterial pressure, Ppa, Ppaw, and the intrapulmonary shunt fraction without significant alterations of RL and DeltaPpl. The study suggests that the mechanism(s) by which medetomidine and ST-91 produce lower PaO2 are different and that drug-induced alterations in the pulmonary system are mainly responsible for the oxygen-lowering effect of medetomidine.

Characterization of the pharmacology of intrathecally administered alpha-2 agonists and antagonists in rats.[Pubmed:1349648]

J Pharmacol Exp Ther. 1992 May;261(2):764-72.

To examine the pharmacology of the spinal alpha receptor which modulates nociceptive transmission, the antinociceptive effects (52.5 degrees C hot plate; HP) of three i.t. administered alpha-2-preferring agonists [dexmedetomidine (DMET); clonidine (CLON) and ST-91] were determined. The antagonist potency of atipamezole (ATI), idazoxan (IDAZ), yohimbine (YOH) and prazosin (PRA), adrenergic antagonists with differing alpha-2-preferring profiles, were then examined for each of the three agonists. The three agonists produced a dose-dependent block of the HP response with the ED50 and the dose which was just maximally effective being DMET (3.2 and 10 micrograms); CLON (27 and 100 micrograms) and ST-91 (6.1 and 20 micrograms). After determining the time of peak antagonist effect, studies were run in which the just maximally effective dose of each agonist was given in conjunction with one of several doses of the several antagonists. The rank order of potency (and ID50 in microgram) for the several antagonists against each of the three agonists was: DMET = [IDAZ (1.9); ATI (4.1); YOH (70); PRA (greater than 100)]; CLON = [ATI (2.7); IDAZ (23); YOH (52); PRA (greater than 100)]; ST-91 = [PRA (38); YOH (69); ATI (greater than 100); IDAZ (greater than 100)]; where antagonists joined by a common line display overlapping 95% confidence intervals and greater than (greater than) indicates failure to achieve a 50% reversal at the highest antagonist dose.(ABSTRACT TRUNCATED AT 250 WORDS)