Lack of synergistic interaction between the two mechanisms of action of tapentadol in gastrointestinal transit.

BACKGROUND: A multi-mechanistic approach offers potential enhancement of analgesic efficacy, but therapeutic gain could be offset by an increase in adverse events. The centrally acting analgesic tapentadol [(-)-(1R,2R)-3-(3-dimethylamino-1-ethyl-2-methyl-propyl)-phenol hydrochloride] combines µ-opioid receptor (MOR) agonism and neuronal noradrenaline reuptake inhibition (NRI), both of which contribute to its analgesic effects. Previously, isobolographic analysis of occupation-effect data and a theoretically equivalent methodology determining interactions from the effect scale demonstrated pronounced synergistic interaction between the two mechanisms of action of tapentadol in two models of antinociception (low-intensity tail-flick and spinal nerve ligation). The present study investigated the nature of interaction of the two mechanisms on a surrogate measure for gastrointestinal adverse effect (inhibition of gastrointestinal transit). METHODS: Dose-response curves were generated in rats for tapentadol alone or in combination with the opioid receptor antagonist, naloxone, or the α2 -adrenoceptor antagonist, yohimbine, to reveal the effect of tapentadol based upon MOR agonism, NRI, and combined mechanisms. RESULTS: The dose-effect curve of tapentadol was shifted to the right by both antagonists, thereby providing data to distinguish between MOR agonism and NRI. Analysis revealed a simple additive interaction between the two mechanisms on this endpoint, in contrast to the synergistic interaction previously demonstrated for antinociception. CONCLUSIONS: We believe this is the first published evaluation of mechanistic interaction for a surrogate measure of adverse effect of a single compound with two mechanisms of action, and the results suggest that there is a greater separation between the analgesic and gastrointestinal effects of tapentadol than expected based upon its analgesic efficacy.

A search for interaction among combinations of drugs of abuse and the use of isobolographic analysis.

WHAT IS KNOWN AND OBJECTIVE: Individuals who abuse drugs usually use more than one substance. Toxic consequences of single and multi-drug use are well documented in the Treatment Episodes Data Set that lists drug combinations that result in hospital admissions. Using this list as a guide, we focused our attention on combinations that result in the most hospital admissions and searched the PubMed database with the objective of determining the number of such publications and, in particular, those that used the term synergism in their titles or abstracts. COMMENT: Using the search criteria produced an extensive list of published articles. However, a further intersection of the search terms with the term isobole revealed a surprisingly small number of literature reports. WHAT IS NEW AND CONCLUSION: Because the method of isoboles is the most common quantitative method for distinguishing between drug synergism and simple additivity, the small number of investigations that actually employed this quantification suggests that the term synergism is not properly documented in describing the toxicity among abused substances. The possible reasons for this lack of quantification may be related to a misunderstanding of the modelling equations. To help rectify this possible hurdle to understanding and clinical utility, the theory and modelling are discussed here.

Mephedrone ("bath salt") pharmacology: insights from invertebrates.

Psychoactive bath salts (also called meph, drone, meow meow, m-CAT, bounce, bubbles, mad cow, etc.) contain a substance called mephedrone (4-methylcathinone) that may share psychostimulant properties with amphetamine and cocaine. However, there are only limited studies of the neuropharmacological profile of mephedrone. The present study used an established invertebrate (planarian) assay to test the hypothesis that acute and repeated mephedrone exposure produces psychostimulant-like behavioral effects. Acute mephedrone administration (50-1000 µM) produced stereotyped movements that were attenuated by a dopamine receptor antagonist (SCH 23390) (0.3 µM). Spontaneous discontinuation of mephedrone exposure (1, 10 µM) (60 min) resulted in an abstinence-induced withdrawal response (i.e. reduced motility). In place conditioning experiments, planarians in which mephedrone (100, 500 µM) was paired with the non-preferred environment during conditioning displayed a shift in preference upon subsequent testing. These results suggest that mephedrone produces three behavioral effects associated with psychostimulant drugs, namely dopamine-sensitive stereotyped movements, abstinence-induced withdrawal, and environmental place conditioning.

Synergistic interaction between the two mechanisms of action of tapentadol in analgesia.

The novel centrally acting analgesic tapentadol [(-)-(1R,2R)-3-(3-dimethylamino-1-ethyl-2-methyl-propyl)-phenol hydrochloride] combines two mechanisms of action, µ-opioid receptor (MOR) agonism and noradrenaline reuptake inhibition (NRI), in a single molecule. Pharmacological antagonism studies have demonstrated that both mechanisms of action contribute to the analgesic effects of tapentadol. This study was designed to investigate the nature of the interaction of the two mechanisms. Dose-response curves were generated in rats for tapentadol alone or in combination with the opioid antagonist naloxone or the α(2)-adrenoceptor antagonist yohimbine. Two different pain models were used: 1) low-intensity tail-flick and 2) spinal nerve ligation. In each model, we obtained dose-effect relations to reveal the effect of tapentadol based on MOR agonism, NRI, and unblocked tapentadol. Receptor fractional occupation was determined from tapentadol's brain concentration and its dissociation constant for each binding site. Tapentadol produced dose-dependent analgesic effects in both pain models, and its dose-effect curves were shifted to the right by both antagonists, thereby providing data to distinguish between MOR agonism and NRI. Both isobolographic analysis of occupation-effect data and a theoretically equivalent methodology determining interactions from the effect scale demonstrated very pronounced synergistic interaction between the two mechanisms of action of tapentadol. This may explain why tapentadol is only 2- to 3-fold less potent than morphine across a variety of preclinical pain models despite its 50-fold lower affinity for the MOR. This is probably the first demonstration of a synergistic interaction between the occupied receptors for a single compound with two mechanisms of action.

Effects on the visual system might contribute to some of the cognitive deficits of cancer chemotherapy-induced 'chemo-fog'.

The diminution in certain aspects of cognitive function that is reported to occur in some patients during or after adjuvant cancer chemotherapy is variously known as 'chemo-fog', 'chemo-brain' or other such term. In addition to reported deficits in attention, concentration and other functions, most, if not all, of the studies report deficits involving visual-spatial function or visual memory. Since the visual system is part of the nervous system, it seems reasonable to ask if it is susceptible to some of the deleterious effects produced by adjuvant chemotherapeutic drugs. We propose here the possibility that some portion of the vision-related aspects of the 'chemo-fog' spectrum of cognitive deficits results from a direct action of the adjuvant drugs on the visual system or from drug/drug or site/site interaction between effects on the visual system and other critical brain regions.

Oxycodone combinations for pain relief.

No single analgesic drug provides the perfect therapeutic/adverse effect profile for every pain condition. In addition to convenience and possibly improved compliance, a combination of analgesic drugs offers the potential, requiring verification, of providing greater pain relief and/or reduced adverse effects than the constituent drugs when used individually. We review here analgesic combinations containing oxycodone. We found surprisingly little preclinical information about the analgesic or adverse effect profiles of the combinations (with acetaminophen, paracetamol, nonsteroidal anti-inflammatory drugs, morphine, gabapentin or pregabalin). Clinical experience and studies suggest that the combinations are safe and effective and may offer certain advantages. As with all combinations, the profile of adverse effects must also be determined in order to provide the clinician with the overall benefit/risk assessment.

GABAA receptors modulate cannabinoid-evoked hypothermia.

Cannabinoids evoke hypothermia by stimulating central CB(1) receptors. GABA induces hypothermia via GABA(A) or GABA(B) receptor activation. CB(1) receptor activation increases GABA release in the hypothalamus, a central locus for thermoregulation, suggesting that cannabinoid and GABA systems may be functionally linked in body temperature regulation. We investigated whether GABA receptors modulate the hypothermic actions of [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one] (WIN 55212-2), a selective cannabinoid agonist, in male Sprague-Dawley rats. WIN 55212-2 (2.5 mg/kg im) produced a rapid hypothermia that peaked 45-90 min postinjection. The hypothermia was attenuated by bicuculline (2 mg/kg ip), a GABA(A) antagonist. However, SCH 50911 (1-10 mg/kg ip), a GABA(B) blocker, did not antagonize the hypothermia. Neither bicuculline (2 mg/kg) nor SCH 50911 (10 mg/kg) by itself altered body temperature. We also investigated a possible role for CB(1) receptors in GABA-generated hypothermia. Muscimol (2.5 mg/kg ip), a GABA(A) agonist, or baclofen (5 mg/kg ip), a GABA(B) agonist, evoked a significant hypothermia. Blockade of CB(1) receptors with SR141716A (2.5 mg/kg im) did not antagonize muscimol- or baclofen-induced hypothermia, indicating that GABA-evoked hypothermia does not contain a CB(1)-sensitive component. Our results implicate GABA(A) receptors in the hypothermic actions of cannabinoids and provide further evidence of a functional link between cannabinoid and GABA systems.

L-NAME (N omega-nitro-L-arginine methyl ester), a nitric-oxide synthase inhibitor, and WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], a cannabinoid agonist, interact to evoke synergistic hypothermia.

Cannabinoids evoke profound hypothermia in rats by activating central CB(1) receptors. Nitric oxide (NO), a prominent second messenger in central and peripheral neurons, also plays a crucial role in thermoregulation, with previous studies suggesting pyretic and antipyretic functions. Dense nitric-oxide synthase (NOS) staining and CB(1) receptor immunoreactivity have been detected in regions of the hypothalamus that regulate body temperature, suggesting that intimate NO-cannabinoid associations may exist in the central nervous system. The present study investigated the effect of N(omega)-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, on the hypothermic response to WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], a selective cannabinoid agonist, in rats. WIN 55212-2 (1-5 mg/kg, i.m.) produced dose-dependent hypothermia that peaked 45 to 90 min post-injection. L-NAME (10-100 mg/kg, i.m.) by itself did not significantly alter body temperature. However, a nonhypothermic dose of L-NAME (50 mg/kg) potentiated the hypothermia caused by WIN 55212-2 (0.5-5 mg/kg). The augmentation was strongly synergistic, indicated by a 2.5-fold increase in the relative potency of WIN 55212-2. The inactive enantiomer of WIN 55212-2, WIN 55212-3 [S-(-)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-napthanlenyl) methanone mesylate] (5 mg/kg, i.m.), did not produce hypothermia in the absence or presence of L-NAME (50 mg/kg), confirming that cannabinoid receptors mediated the synergy. The present data are the first evidence that drug combinations of NOS blockers and cannabinoid agonists produce synergistic hypothermia. Thus, NO and cannabinoid systems may interact to induce superadditive hypothermia.

N-methyl-D-aspartate antagonists and WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-i,j]quinolin-6-one], a cannabinoid agonist, interact to produce synergistic hypothermia.

CB(1) cannabinoid receptors mediate profound hypothermia when cannabinoid agonists are administered to rats. Glutamate, the principal excitatory neurotransmitter in the central nervous system (CNS), is thought to tonically increase body temperature by activating N-methyl-D-aspartate (NMDA) receptors. Because NMDA antagonists block cannabinoid-induced antinociception and catalepsy, intimate glutamatergic-cannabinoid interactions may exist in the CNS. The present study investigated the effect of two NMDA antagonists on the hypothermic response to WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-i,j]quinolin-6-one], a selective cannabinoid agonist, in rats. WIN 55212-2 (1-10 mg/kg i.m.) produced dose-dependent hypothermia that peaked 60 to 180 min postinjection. Dextromethorphan (5-75 mg/kg i.m.), a noncompetitive NMDA antagonist, or LY 235959 [(-)-6-[phosphonomethyl-1,2,3,4,4a,5,6,7,8,8a-decahydro-isoquinoline-2-carboxylate]](1-4 mg/kg i.m.), a competitive and highly selective NMDA antagonist, evoked hypothermia in a dose-sensitive manner, suggesting that endogenous glutamate exerts a hyperthermic tone on body temperature. A dose of dextromethorphan (10 mg/kg) that did not affect body temperature by itself potentiated the hypothermic response to WIN 55212-2 (1, 2.5, or 5 mg/kg). The enhancement was strongly synergistic, indicated by a 2.7-fold increase in the relative potency of WIN 55212-2. Similarly, a dose of LY 235959 (1 mg/kg) that did not affect body temperature augmented the hypothermia associated with a single dose of WIN 55212-2 (2.5 mg/kg), thus confirming that NMDA receptors mediated the synergy. We have demonstrated previously that CB(1) receptors mediate WIN 55212-2-evoked hypothermia in rats. The present data are the first evidence that NMDA antagonists exert a potentiating effect on cannabinoid-induced hypothermia. Taken together, these data suggest that interactions between NMDA and CB(1) receptors produce synergistic hypothermia.

Drug synergism: its detection and applications.

Two drugs that produce overtly similar effects will sometimes produce exaggerated or diminished effects when used concurrently. A quantitative assessment is necessary to distinguish these cases from simply additive action. This distinction is based on the classic pharmacologic definition of additivity that, briefly stated, means that each constituent contributes to the effect in accord with its own potency. Accordingly, the relative potency of the agents, not necessarily constant at all effect levels, allows a calculation using dose pairs to determine the equivalent of either agent and the effect by using the equivalent in the dose-response relation of the reference compound. The calculation is aided by a popular graph (isobologram) that provides a visual assessment of the interaction but also requires independent statistical analysis. The latter can be accomplished from calculations that use the total dose in a fixed-ratio combination along with the calculated additive total dose for the same effect. Different methods may be used, and each is applicable to experiments in which a single drug is given at two different sites. When departures from additivity are found, whether in "two-drug" or "two-site" experiments, the information is useful in designing new experiments for illuminating mechanisms. Several examples, mainly from analgesic drug studies, illustrate this application. Even when a single drug (or site) is used, its introduction places it in potential contact with a myriad of chemicals already in the system, a fact that underscores the importance of this topic in other areas of biological investigation.

Possible mechanism of hypothermia induced by intracerebroventricular injection of orphanin FQ/nociceptin.

Orphanin/nociceptin (OFQ/N), a 17-amino-acid peptide, is an endogenous peptide, the receptor for which is similar to mu-, delta- and kappa-opioid receptors ( approximately 65% homology). Reports indicate that OFQ/N can block the antinociception induced by mu-, delta- and kappa-opioid agonists in the rat and in the mouse, indicating that there is a functional interaction between opioid receptors and OFQ/N receptors in the nervous system. It is well known that activation of the mu- and kappa-opioid receptors results in hyperthermia and hypothermia, respectively, in Sprague-Dawley rats. The present studies were designed to examine effects of OFQ/N on body temperature (Tb) and explore whether the mechanism of T(b) change induced by OFQ/N involved the opioid system. The results show that (1) i.c.v. injection of a high dose of OFQ/N (9-18 micro g) produces hypothermia in adult rats; (2) OFQ/N (1.8 micro g, i.c.v., t=+30 s after morphine) can decrease morphine-induced hyperthermia; (3) neither the opioid receptor antagonist, naloxone (10 mg/kg, s.c., t=-15 s before OFQ/N) nor the kappa-opioid receptor antagonist nor-BNI (1 micro g/5 microl, i.c.v., t=-30 s before OFQ/N) reduces the hypothermia induced by i.c.v. injection of OFQ/N at dose of 18 micro g (P>0.05); (4) 60 micro g/5 microl AS oligo (i.c.v. treatment on days 1, 3 and 5) against OFQ/N receptors significantly reduces the hypothermia induced by i.c.v. injection of 9 micro g OFQ/N (P<0.01). These results suggest that the hypothermia induced by i.c.v. injection of a high dose of OFQ/N (9 or 18 micro g) is mediated, at least partially, by its own receptor, independent or downstream of opioid receptors in the rat brain and that OFQ/N probably acts as a physiological antagonist to reduce morphine-induced hyperthermia.

Unexpected and pronounced antinociceptive synergy between spinal acetaminophen (paracetamol) and phentolamine.

Acetaminophen was administered to mice by spinal (intrathecal, i.t.) injection alone or with phentolamine (11.3 microg = 0.03 micromol). Acetaminophen produced dose-related antinociception in the abdominal irritant test with an ED(50) value of 137.2 microg (0.9 micromol) Phentolamine had no effect. For combined administration, the potency of acetaminophen was significantly increased (ED50=24.4 vs. 137.2 microg), indicative of multiplicative interaction and strong synergism. These results reveal the significant and surprising interaction of spinal cord adrenoceptors or ion channel subtypes with acetaminophen-induced antinociception.

Discovery of "self-synergistic" spinal/supraspinal antinociception produced by acetaminophen (paracetamol).

The mechanism of the analgesic action of one of the world's most widely used drugs-acetaminophen (paracetamol)-remains largely unknown more than 100 years after its original synthesis. Based on the present findings, this elusiveness appears to have resulted from experimental strategies that concentrated on a single target site or mechanism. Here we report on the use of analyses that we previously developed to investigate possible brain/spinal-cord site-site interaction in acetaminophen-induced antinociception. Spinal (intrathecal) administration of acetaminophen to mice produced dose-related, naloxone-insensitive antinociception with an ED(50) value of 137 (S.E. = 23) microgram = 907 (S.E. =153) nmol. In contrast, supraspinal (i.c.v.) acetaminophen administration had no effect. However, combined administration of acetaminophen in fixed ratios to brain and spinal cord produced synergistic antinociception, ED(50) = 57 (S.E. = 9) microgram, that reverted toward additivity, ED(50) = 129 (S.E. = 23) microgram, when the opioid antagonist naloxone was given spinally (3.6 microgram = 10 nmol) or s.c. (3.6 mg/kg). These findings demonstrate for the first time that acetaminophen-induced antinociception involves a "self-synergistic" interaction between spinal and supraspinal sites and, furthermore, that the self-synergy involves an endogenous opioid pathway.

Loss of antiallodynic and antinociceptive spinal/supraspinal morphine synergy in nerve-injured rats: restoration by MK-801 or dynorphin antiserum.

The co-administration of morphine at spinal (i.th.) and supraspinal (i.c.v.) sites to the same rat produces antinociceptive synergy, a phenomenon which may underlie the clinical analgesic utility of this drug. In animals with peripheral nerve injury, however, the antinociceptive potency and efficacy of i.th. morphine is significantly decreased. Here, the possible loss of spinal/supraspinal morphine antinociceptive synergy and relationship to elevation of spinal dynorphin content was studied. Ligation of lumbar spinal nerves resulted in elevated dynorphin in the ipsilateral lumbar and sacral spinal cord. In sham-operated rats supraspinal/spinal co-administration of morphine produced synergistic antinociception which was unaffected by i.th. MK-801 or dynorphin A((1-17)) antiserum. In nerve-injured rats, i.th. morphine was inactive against tactile allodynia and showed diminished in potency against acute nociception without supraspinal/spinal antinociceptive synergy. Antiserum to dynorphin A((1-17)) or the non-competitive NMDA antagonist MK-801 increased the antinociceptive potency of i.th. morphine, restored supraspinal/spinal morphine antinociceptive synergy and elicited a dose-related i.th. morphine antiallodynic action. These agents did not demonstrate antinociceptive or antiallodynic activity alone and did not alter morphine actions in sham-operated animals. The loss of spinal/supraspinal antinociceptive synergy and lack of antiallodynic activity of spinal morphine appear to be due to the elevation across multiple spinal segments of dynorphin following nerve injury. Pathological actions of elevated dynorphin may directly or indirectly modulate the NMDA receptor, result in a loss of supraspinal/spinal morphine synergy and may thus account for the decreased clinical analgesic efficacy of morphine in peripheral neuropathies.

Interaction between medullary and spinal delta1 and delta2 opioid receptors in the production of antinociception in the rat.

Previous work supports the existence of two types of delta opioid receptor (delta1 and delta2) and a role of both subtypes in the spinal cord and the ventromedial medulla (VMM) in the production of antinociception. Although it is well established that spinal and supraspinal mu opioid receptors interact in a synergistic manner to produce antinociception, little is known about the interaction of delta opioid receptors. This study used isobolographic analysis to determine how delta1 and delta2 opioid receptors in the VMM interact with their respective receptors in the spinal cord to produce antinociception. Concurrent administration of the delta1 opioid receptor agonist [D-Pen2,D-Pen5]enkephalin at spinal and supraspinal sites in a fixed-dose ratio produced antinociception in an additive manner in the tail-flick test. In contrast, concurrent administration of very low doses of the delta2 opioid receptor agonist [D-Ala2,Glu4]deltorphin at spinal and medullary sites produced antinociception in a synergistic manner. However, as the total dose of [D-Ala2,Glu4]deltorphin increased, this interaction converted to additivity. These observations suggest that different mechanisms mediate the antinociceptive effects of different doses of delta2 opioid receptor agonists. The difference in the nature of the interaction produced by delta1 and delta2 opioid receptor agonists provides additional evidence for the existence of different subtypes of the delta opioid receptor. These results also suggest that delta2 opioid receptor agonists capable of crossing the blood-brain barrier will be more potent or efficacious analgesics than delta1 opioid receptor agonists after systemic administration.

Response surface analysis of synergism between morphine and clonidine.

Graded doses of morphine sulfate and clonidine hydrochloride were administered intrathecally to mice that were then tested for antinociception in the 55 degrees C tail immersion test. The dose-effect relations of each compound were used in calculations that permitted the construction of a three-dimensional plot of the expected additive effect (vertical scale) against the planar domain of dose pairs representing combinations administered simultaneously. This additive response surface became the reference surface for viewing the actual effects produced by three different fixed-ratio combinations of the drugs that were used in our tests. Each combination produced effects significantly greater than indicated by the additive surface, thereby illustrating marked synergism and a method for quantifying the synergism. This quantification, measured by the value of the interaction index (alpha), was found to be dependent on the fixed-ratio combination; accordingly, the actual response surface could not be described by a single value of the index alpha. Furthermore, we found that application of the common method of isoboles gave estimates of the index that agreed well with those obtained from the more extensive surface analysis. These results confirm earlier studies, which found synergism for these drugs while also providing surface views of additivity and synergism that form the basis of isobolographic analysis.

Efficient designs for studying synergistic drug combinations.

Distinguishing between pharmacologically additive and synergistic drug combinations requires experimental designs and statistical analyses that often require appreciable numbers of animals and much experimenter time. The current study employed a design in which individual dose-effect data from each drug were translated into theoretically additive total dose combinations, in a fixed drug proportion, in order to produce a composite additive dose-effect relation that could be compared with that of an actual mixture having the same proportion. Results from this approach, using a combination of intrathecal doses of morphine and clonidine, were virtually identical to those using isobolographic analysis of the same data set. Both analyses showed significant synergism for this combination and, in each method, it was not necessary to constrain the drug regression lines to parallelism. In contrast to the isobole approach, the use of the composite additive dose-effect relation also allows observation of the interaction over a range of effects while reducing the size of the data sets needed.

Synergism between buprenorphine and cocaine on the rotational behavior of the nigrally-lesioned rat.

Buprenorphine, a partial mu-opioid receptor agonist, has been proposed as a treatment for cocaine abuse. However, studies in animals have produced conflicting results on the nature of the interaction between buprenorphine and cocaine. In some studies, buprenorphine attenuated the effects of cocaine and in others it enhanced them. The purpose of the present study was to evaluate the interaction of buprenorphine and cocaine on the rotational behavior of the nigrally-lesioned rat. Both buprenorphine (0.003-0.1 mg/kg) and cocaine (1.0-30 mg/kg) alone produced dose-dependent increases in rotational behavior. Buprenorphine produced long-lasting turning with a peak at 60 min after administration, while cocaine produced turning that peaked immediately after administration and lasted for about 2 h. To distinguish simple additivity from other possible outcomes, we determined the relative potency of each drug alone, using a defined level of effect: 150 turns above the saline control in 4 h. This effect was produced by 10.0 mg/kg cocaine alone and by 0.0175 mg/kg buprenorphine alone. Based on these results, fixed ratio combinations were tested and the experimentally derived effects were compared to the theoretically additive values, using an isobolographic analysis. The fixed ratio combinations of the two drugs tested produced turning greater than predicted from simple additivity. This finding provides statistically-supported evidence for synergism between the actions of buprenorphine and cocaine.

Theory and statistics of detecting synergism between two active drugs: cocaine and buprenorphine.

This article discusses the theory and statistical aspects in the design and analysis of experiments to detect synergism between two drugs that produce overtly similar effects. The current analysis extended and simplifies previously published work in this area. Application is made to a study by Kimmel et al. in this issue that examined the combined action of buprenorphine and cocaine in producing turning in rats having unilateral nigrostriatal lesions produced by 6-hydroxydopamine. The use of turning as an endpoint is unusual in quantitative studies of synergism in that no clear maximum effect (turning), could be elicited. Data from the turning study are analyzed statistically and reveal that the combination of buprenorphine and cocaine in each of two fixed ratio mixtures tested is synergistic for this effect.

Antivasoconstrictor and antiaggregatory activities of picotamide unrelated to thromboxane A2 antagonism.

Picotamide is a dual thromboxane (Tx) A2 receptor antagonist/Tx synthase inhibitor although some observations suggest an anti-vasoconstrictor effect independent of TxA2 inhibition/antagonism. The aim of our study was to assess whether picotamide antagonises vascular contractions induced by different vasoactive substances in vitro. Picotamide inhibited competitively the contraction of rabbit aortic rings induced by the TxA2 mimetic U46619 (pA2 = 3.59) but also the contractions induced by phenylephrine (pA2 = 3.93) and serotonin (5-HT) (pA2 = 5.81) although in a not competitive way. Picotamide did not inhibit potassium-induced contractions, thus excluding aspecific effects on vascular smooth muscle. Picotamide inhibited 5-HT-induced platelet aggregation in vitro with an IC50 (212 microM) similar to that found when other aggregating stimuli are used, but it did not affect shape change (IC50 > 1 mM) suggesting that the effects of picotamide can not be ascribed to 5-HT2-receptor antagonism; in the same experimental conditions neither a Tx-receptor antagonist (BM13.177) nor a dual Tx-receptor antagonist/synthase inhibitor (ridogrel) affected 5-HT-induced platelet responses. Our studies demonstrate that picotamide exerts antivasoconstrictor and platelet inhibitory effects unrelated to TxA2 antagonism. This activity may contribute to the anti-thrombotic/anti-ischaemic effects of the drug in vivo.