Genome-Wide Association Study of Atorvastatin Pharmacokinetics: Associations With SLCO1B1, UGT1A3, and LPP

In a genome-wide association study of atorvastatin pharmacokinetics in 158 healthy volunteers, the SLCO1B1 c.521T>C (rs4149056) variant associated with increased area under the plasma concentration-time curve from time zero to infinity (AUC0-∞) of atorvastatin (P = 1.2 × 10-10), 2-hydroxy atorvastatin (P = 4.0 × 10-8), and 4-hydroxy atorvastatin (P = 2.9 × 10-8). An intronic LPP variant, rs1975991, associated with reduced atorvastatin lactone AUC0-∞ (P = 3.8 × 10-8). Three UGT1A variants linked with UGT1A3*2 associated with increased 2-hydroxy atorvastatin lactone AUC0-∞ (P = 3.9 × 10-8). Furthermore, a candidate gene analysis including 243 participants suggested that increased function SLCO1B1 variants and decreased activity CYP3A4 variants affect atorvastatin pharmacokinetics. Compared with individuals with normal function SLCO1B1 genotype, atorvastatin AUC0-∞ was 145% (90% confidence interval: 98-203%; P = 5.6 × 10-11) larger in individuals with poor function, 24% (9-41%; P = 0.0053) larger in those with decreased function, and 41% (16-59%; P = 0.016) smaller in those with highly increased function SLCO1B1 genotype. Individuals with intermediate metabolizer CYP3A4 genotype (CYP3A4*2 or CYP3A4*22 heterozygotes) had 33% (14-55%; P = 0.022) larger atorvastatin AUC0-∞ than those with normal metabolizer genotype. UGT1A3*2 heterozygotes had 16% (5-25%; P = 0.017) smaller and LPP rs1975991 homozygotes had 34% (22-44%; P = 4.8 × 10-5) smaller atorvastatin AUC0-∞ than noncarriers. These data demonstrate that genetic variation in SLCO1B1, UGT1A3, LPP, and CYP3A4 affects atorvastatin pharmacokinetics. This is the first study to suggest that LPP rs1975991 may reduce atorvastatin exposure. [Correction added on 6 April, after first online publication: An incomplete sentence ("= 0.017) smaller in heterozygotes for UGT1A3*2 and 34% (22%, 44%; P × 10-5) smaller in homozygotes for LPP noncarriers.") has been corrected in this version.].

Influence of Intravenous S-Ketamine on the Pharmacokinetics of Oral Morphine in Healthy Volunteers

BACKGROUND

Subanesthetic ketamine may reduce perioperative consumption of opioids. We studied whether intravenous S-ketamine alters the pharmacokinetics of oral morphine in healthy volunteers.

METHODS

In this paired, randomized, double-blind, crossover trial, 12 participants under a 2-hour intravenous S-ketamine (0.57 mg/kg/h) or placebo infusion received oral morphine (0.2 mg/kg) at 30 minutes. Plasma concentrations of ketamine, morphine, and their major metabolites were quantified for 24 hours. The primary end point was area under the curve (AUC) 0-24 of morphine. Other pharmacokinetic variables for morphine and its metabolites were studied as secondary end points. The data were analyzed as between-phase comparisons for each participant using Wilcoxon matched-pairs signed-rank tests ( tmax ) or paired t -tests on log-transformed variables (other variables).

RESULTS

While the AUC 0-24 was similar between the 2 phases, S-ketamine reduced the AUC 0-1.5 of oral morphine by 69% (ratio to control, 0.31; 90% confidence interval [CI], 0.15-0.65; P = .0171) and increased its tmax from 0.5 (range, 0.50-1.5) to 1.0 hour (range, 0.50-4.0; P = .010). The AUC 0-1.5 of morphine-6-glucuronide (M6G) was reduced by 84% (0.16; 90% CI, 0.07-0.37; P = .0025) and maximum plasma concentration ( Cmax ) by 43% (0.57; 90% CI, 0.40-0.81; P = .0155), while its tmax was increased from 1.5 (range, 1.0-2.0) to 4.0 (range, 1.0-8.0; P = .0094) hours by S-ketamine. Similarly, the AUC 0-1.5 of morphine-3-glucuronide (M3G) was reduced by 85% (0.15; 90% CI, 0.05-0.43; P = .0083), and tmax increased from 1.0 (range, 0.5-1.5) to 4.0 hours (range, 1.0-8.0; P = .0063). In addition, the M6G-to-morphine and M3G-to-morphine metabolic AUC ratios were decreased by 47% (0.53; 90% CI, 0.39-0.71; P = .0033) and 52% (0.48; 90% CI, 0.27-0.85; P = .0043) during 0 to 1.5 hours and by 15% (0.85; 90% CI, 0.78-0.92; P = .0057) and 10% (0.90; 90% CI, 0.83-0.98; P = .0468) during 0 to 24 hours, respectively. One participant was excluded from the analyses due to vomiting in the S-ketamine phase.

CONCLUSIONS

Intravenous S-ketamine inhibited the metabolism of oral morphine and delayed its absorption, resulting in a net reduction in the exposure to morphine during the first 1.5 hours. Intravenous S-ketamine may delay the absorption and impair the efficacy of orally administered analgesics and other drugs.

Introduction of an electrochemical point-of-care assay for quantitative determination of paracetamol in finger-prick capillary whole blood samples

AIMS

Measuring venous plasma paracetamol concentrations is time- and resource-consuming. We aimed to validate a novel electrochemical point-of-care (POC) assay for rapid paracetamol concentration determinations.

METHODS

Twelve healthy volunteers received 1 g oral paracetamol, and its concentrations were analysed 10 times over 12 h for capillary whole blood (POC), venous plasma (high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS)), and dried capillary blood (HPLC-MS/MS).

RESULTS

At concentrations >30 μM, POC showed upward biases of 20% (95% limits of agreement [LOA] -22 to 62) and 7% (95% LOA -23 to 38) compared with venous plasma and capillary blood HPLC-MS/MS, respectively. There were no significant differences between mean concentrations for the paracetamol elimination phase.

CONCLUSIONS

Upward biases in POC compared with venous plasma HPLC-MS/MS were likely due to higher paracetamol concentrations in capillary blood than in venous plasma and to faulty individual sensors. The novel POC method is a promising tool for paracetamol concentration analysis.

A SPECT-based method for dynamic imaging of the glymphatic system in rats

The glymphatic system is a brain-wide waste drainage system that promotes cerebrospinal fluid circulation through the brain to remove waste metabolites. Currently, the most common methods for assessing glymphatic function are ex vivo fluorescence microscopy of brain slices, macroscopic cortical imaging, and MRI. While all these methods have been crucial for expanding our understanding of the glymphatic system, new techniques are required to overcome their specific drawbacks. Here, we evaluate SPECT/CT imaging as a tool to assess glymphatic function in different anesthesia-induced brain states using two radiolabeled tracers, [111In]-DTPA and [99mTc]-NanoScan. Using SPECT, we confirmed the existence of brain state-dependent differences in glymphatic flow and we show brain state-dependent differences of CSF flow kinetics and CSF egress to the lymph nodes. We compare SPECT and MRI for imaging glymphatic flow and find that the two imaging modalities show the same overall pattern of CSF flow, but that SPECT was specific across a greater range of tracer concentrations than MRI. Overall, we find that SPECT imaging is a promising tool for imaging the glymphatic system, and that qualities such as high sensitivity and the variety of available tracers make SPECT imaging a good alternative for glymphatic research.

Glymphatic-assisted perivascular brain delivery of intrathecal small gold nanoparticles

Nanoparticles are ultrafine particulate matter having considerable potential for treatment of central nervous system (CNS) disorders. Despite their tiny size, the blood-brain barrier (BBB) restricts their access to the CNS. Their direct cerebrospinal fluid (CSF) administration bypasses the BBB endothelium, but still fails to give adequate brain uptake. We present a novel approach for efficient CNS delivery of ¹¹¹In-radiolabelled gold nanoparticles (AuNPs; 10-15 nm) via intra-cisterna magna administration, with tracking by SPECT imaging. To accelerate CSF brain influx, we administered AuNPs intracisternally in conjunction with systemic hypertonic saline, which dramatically increased the parenchymal AuNP uptake, especially in deep brain regions. AuNPs entered the CNS along periarterial spaces as visualized by MRI of gadolinium-labelled AuNPs and were cleared from brain within 24 h and excreted through the kidneys. Thus, the glymphatic-assisted perivascular network augment by systemic hypertonic saline is a pathway for highly efficient brain-wide distribution of small AuNPs.

A comprehensive pharmacogenomic study indicates roles for SLCO1B1, ABCG2 and SLCO2B1 in rosuvastatin pharmacokinetics

AIMS

The aim was to comprehensively investigate the effects of genetic variability on the pharmacokinetics of rosuvastatin.

METHODS

We conducted a genome-wide association study and candidate gene analyses of single dose rosuvastatin pharmacokinetics in a prospective study (n = 159) and a cohort of previously published studies (n = 88).

RESULTS

In a genome-wide association meta-analysis of the prospective study and the cohort of previously published studies, the SLCO1B1 c.521 T > C (rs4149056) single nucleotide variation (SNV) associated with increased area under the plasma concentration-time curve (AUC) and peak plasma concentration of rosuvastatin (P = 1.8 × 10^-12 and P = 3.2 × 10^-15 ). The candidate gene analysis suggested that the ABCG2 c.421C > A (rs2231142) SNV associates with increased rosuvastatin AUC (P = .0079), while the SLCO1B1 c.388A > G (rs2306283) and SLCO2B1 c.1457C > T (rs2306168) SNVs associate with decreased rosuvastatin AUC (P = .0041 and P = .0076). Based on SLCO1B1 genotypes, we stratified the participants into poor, decreased, normal, increased and highly increased organic anion transporting polypeptide (OATP) 1B1 function groups. The OATP1B1 poor function phenotype associated with 2.1-fold (90% confidence interval 1.6-2.8, P = 4.69 × 10^-5 ) increased AUC of rosuvastatin, whereas the OATP1B1 highly increased function phenotype associated with a 44% (16-62%; P = .019) decreased rosuvastatin AUC. The ABCG2 c.421A/A genotype associated with 2.2-fold (1.5-3.0; P = 2.6 × 10^-4 ) increased AUC of rosuvastatin. The SLCO2B1 c.1457C/T genotype associated with 28% decreased rosuvastatin AUC (11-42%; P = .01).

CONCLUSION

These data suggest roles for SLCO1B1, ABCG2 and SLCO2B1 in rosuvastatin pharmacokinetics. Poor SLCO1B1 or ABCG2 function genotypes may increase the risk of rosuvastatin-induced myotoxicity. Reduced doses of rosuvastatin are advisable for patients with these genotypes.

SPECT/CT imaging reveals CNS-wide modulation of glymphatic cerebrospinal fluid flow by systemic hypertonic saline

Intrathecal administration enables central nervous system delivery of drugs that do not bypass the blood-brain barrier. Systemic administration of hypertonic saline (HTS) enhances delivery of intrathecal therapeutics into the neuropil, but its effect on solute clearance from the brain remains unknown. Here, we developed a dynamic in vivo single-photon emission computed tomography (SPECT)/computed tomography (CT) imaging platform to study the effects of HTS on whole-body distribution of the radiolabeled tracer ^99mTc-diethylenetriaminepentaacetic acid (DTPA) administered through intracisternal, intrastriatal, or intravenous route in anesthetized rats. Co-administration of systemic HTS increased intracranial exposure to intracisternal ^99mTc-DTPA by ∼80% during imaging. In contrast, HTS had minimal effects on brain clearance of intrastriatal ^99mTc-DTPA. In sum, SPECT/CT imaging presents a valuable approach to study glymphatic drug delivery. Using this methodology, we show that systemic HTS increases intracranial availability of cerebrospinal fluid-administered tracer, but has marginal effects on brain clearance, thus substantiating a simple, yet effective strategy for enhancing intrathecal drug delivery to the brain.

•

We established a SPECT/CT platform for imaging glymphatic drug delivery in vivo

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Hypertonic saline (HTS) elevates intracranial availability of CSF-infused ^99mTc-DTPA

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HTS triples the availability of ^99mTc-DTPA in several deep brain regions

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HTS has negligible effects on clearance of intrastriatally infused DTPA

Systemic hypertonic saline enhances glymphatic spinal cord delivery of lumbar intrathecal morphine

The blood-brain barrier significantly limits effective drug delivery to central nervous system (CNS) targets. The recently characterized glymphatic system offers a perivascular highway for intrathecally (i.t.) administered drugs to reach deep brain structures. Although periarterial cerebrospinal fluid (CSF) influx and concomitant brain drug delivery can be enhanced by pharmacological or hyperosmotic interventions, their effects on drug delivery to the spinal cord, an important target for many drugs, have not been addressed. Hence, we studied in rats whether enhancement of periarterial flow by systemic hypertonic solution might be utilized to enhance spinal delivery and efficacy of i.t. morphine. We also studied whether the hyperosmolar intervention affects brain or cerebrospinal fluid drug concentrations after systemic administration. Periarterial CSF influx was enhanced by intraperitoneal injection of hypertonic saline (HTS, 5.8%, 20 ml/kg, 40 mOsm/kg). The antinociceptive effects of morphine were characterized, using tail flick, hot plate and paw pressure tests. Drug concentrations in serum, tissue and microdialysis samples were determined by liquid chromatography-tandem mass spectrometry. Compared with isotonic solution, HTS increased concentrations of spinal i.t. administered morphine by 240% at the administration level (T13-L1) at 60 min and increased the antinociceptive effect of morphine in tail flick, hot plate, and paw pressure tests. HTS also independently increased hot plate and paw pressure latencies but had no effect in the tail flick test. HTS transiently increased the penetration of intravenous morphine into the lateral ventricle, but not into the hippocampus. In conclusion, acute systemic hyperosmolality is a promising intervention for enhanced spinal delivery of i.t. administered morphine. The relevance of this intervention should be expanded to other i.t. drugs and brought to clinical trials.

Rifampin Reduces the Plasma Concentrations of Oral and Intravenous Hydromorphone in Healthy Volunteers

Several opioids are metabolized by the inducible cytochrome P450 (CYP) 3A isozymes. Coadministration with strong inducers of drug metabolism, such as rifampin, can dramatically reduce systemic exposure to these opioids. As the CYP metabolism of hydromorphone is of minor importance, we studied in healthy volunteers whether hydromorphone would be an effective analgesic for patients who concomitantly receive the prototypical enzyme inducer rifampin.

Neurophysiological response properties of medullary pain-control neurons following chronic treatment with morphine or oxycodone: modulation by acute ketamine

Morphine and oxycodone are two clinically used strong opioids. Chronic treatment with oxycodone as well as morphine can lead to analgesic tolerance and paradoxical hyperalgesia. Here we show that an N-methyl-d-aspartate receptor-dependent pronociceptive change in discharge properties of rostroventromedial medullary neurons controlling spinal nociception has an important role in antinociceptive tolerance to morphine but not oxycodone. Interestingly, chronic oxycodone did not induce pronociceptive changes in the rostroventromedial medulla.

Dexmedetomidine enhances glymphatic brain delivery of intrathecally administered drugs

Drug delivery to the central nervous system remains a major problem due to biological barriers. The blood-brain-barrier can be bypassed by administering drugs intrathecally directly to the cerebrospinal fluid (CSF). The glymphatic system, a network of perivascular spaces promoting fluid exchange between CSF and interstitial space, could be utilized to enhance convective drug delivery from the CSF to the parenchyma. Glymphatic flow is highest during sleep and anesthesia regimens that induce a slow-wave sleep-like state. Here, using mass spectrometry and fluorescent imaging techniques, we show that the clinically used α₂-adrenergic agonist dexmedetomidine that enhances EEG slow-wave activity, increases brain and spinal cord drug exposure of intrathecally administered drugs in mice and rats. Using oxycodone, naloxone, and an IgG-sized antibody as relevant model drugs we demonstrate that modulation of glymphatic flow has a distinct impact on the distribution of intrathecally administered therapeutics. These findings can be exploited in the clinic to improve the efficacy and safety of intrathecally administered therapeutics.

Increased glymphatic influx is correlated with high EEG delta power and low heart rate in mice under anesthesia

The glymphatic system is responsible for brain-wide delivery of nutrients and clearance of waste via influx of cerebrospinal fluid (CSF) alongside perivascular spaces and through the brain. Glymphatic system activity increases during sleep or ketamine/xylazine (K/X) anesthesia, yet the mechanism(s) facilitating CSF influx are poorly understood. Here, we correlated influx of a CSF tracer into the brain with electroencephalogram (EEG) power, heart rate, blood pressure, and respiratory rate in wild-type mice under six different anesthesia regimens. We found that glymphatic CSF tracer influx was highest under K/X followed by isoflurane (ISO) supplemented with dexmedetomidine and pentobarbital. Mice anesthetized with α-chloralose, Avertin, or ISO exhibited low CSF tracer influx. This is the first study to show that glymphatic influx correlates positively with cortical delta power in EEG recordings and negatively with beta power and heart rate.

Interactions of (2S,6S;2R,6R)-Hydroxynorketamine, a Secondary Metabolite of (R,S)-Ketamine, with Morphine

Ketamine and its primary metabolite norketamine attenuate morphine tolerance by antagonising N-methyl-d-aspartate (NMDA) receptors. Ketamine is extensively metabolized to several other metabolites. The major secondary metabolite (2S,6S;2R,6R)-hydroxynorketamine (6-hydroxynorketamine) is not an NMDA antagonist. However, it may modulate nociception through negative allosteric modulation of α7 nicotinic acetylcholine receptors. We studied whether 6-hydroxynorketamine could affect nociception or the effects of morphine in acute or chronic administration settings. Male Sprague Dawley rats received subcutaneous 6-hydroxynorketamine or ketamine alone or in combination with morphine, as a cotreatment during induction of morphine tolerance, and after the development of tolerance induced by subcutaneous minipumps administering 9.6 mg morphine daily. Tail flick, hot plate, paw pressure and rotarod tests were used. Brain and serum drug concentrations were quantified with high-performance liquid chromatography-tandem mass spectrometry. Ketamine (10 mg/kg), but not 6-hydroxynorketamine (10 and 30 mg/kg), enhanced antinociception and decreased rotarod performance following acute administration either alone or combined with morphine. Ketamine efficiently attenuated morphine tolerance. Acutely administered 6-hydroxynorketamine increased the brain concentration of morphine (by 60%), and brain and serum concentrations of 6-hydroxynorketamine were doubled by morphine pre-treatment. This pharmacokinetic interaction did not, however, lead to altered morphine tolerance. Co-administration of 6-hydroxynorketamine 20 mg/kg twice daily did not influence development of morphine tolerance. Even though morphine and 6-hydroxynorketamine brain concentrations were increased after co-administration, the pharmacokinetic interaction had no effect on acute morphine nociception or tolerance. These results indicate that 6-hydroxynorketamine does not have antinociceptive properties or attenuate opioid tolerance in a similar way as ketamine.

A Novel Small Molecule GDNF Receptor RET Agonist, BT13, Promotes Neurite Growth from Sensory Neurons in Vitro and Attenuates Experimental Neuropathy in the Rat

Neuropathic pain caused by nerve damage is a common and severe class of chronic pain. Disease-modifying clinical therapies are needed as current treatments typically provide only symptomatic relief; show varying clinical efficacy; and most have significant adverse effects. One approach is targeting either neurotrophic factors or their receptors that normalize sensory neuron function and stimulate regeneration after nerve damage. Two candidate targets are glial cell line-derived neurotrophic factor (GDNF) and artemin (ARTN), as these GDNF family ligands (GFLs) show efficacy in animal models of neuropathic pain (Boucher et al., 2000; Gardell et al., 2003; Wang et al., 2008, 2014). As these protein ligands have poor drug-like properties and are expensive to produce for clinical use, we screened 18,400 drug-like compounds to develop small molecules that act similarly to GFLs (GDNF mimetics). This screening identified BT13 as a compound that selectively targeted GFL receptor RET to activate downstream signaling cascades. BT13 was similar to NGF and ARTN in selectively promoting neurite outgrowth from the peptidergic class of adult sensory neurons in culture, but was opposite to ARTN in causing neurite elongation without affecting initiation. When administered after spinal nerve ligation in a rat model of neuropathic pain, 20 and 25 mg/kg of BT13 decreased mechanical hypersensitivity and normalized expression of sensory neuron markers in dorsal root ganglia. In control rats, BT13 had no effect on baseline mechanical or thermal sensitivity, motor coordination, or weight gain. Thus, small molecule BT13 selectively activates RET and offers opportunities for developing novel disease-modifying medications to treat neuropathic pain.

Ketamine coadministration attenuates morphine tolerance and leads to increased brain concentrations of both drugs in the rat

Background and Purpose

The effects of ketamine in attenuating morphine tolerance have been suggested to result from a pharmacodynamic interaction. We studied whether ketamine might increase brain morphine concentrations in acute coadministration, in morphine tolerance and morphine withdrawal.

Experimental Approach

Morphine minipumps (6 mg·day^–1) induced tolerance during 5 days in Sprague–Dawley rats, after which s.c. ketamine (10 mg·kg^–1) was administered. Tail flick, hot plate and rotarod tests were used for behavioural testing. Serum levels and whole tissue brain and liver concentrations of morphine, morphine-3-glucuronide, ketamine and norketamine were measured using HPLC-tandem mass spectrometry.

Key Results

In morphine-naïve rats, ketamine caused no antinociception whereas in morphine-tolerant rats there was significant antinociception (57% maximum possible effect in the tail flick test 90 min after administration) lasting up to 150 min. In the brain of morphine-tolerant ketamine-treated rats, the morphine, ketamine and norketamine concentrations were 2.1-, 1.4- and 3.4-fold, respectively, compared with the rats treated with morphine or ketamine only. In the liver of morphine-tolerant ketamine-treated rats, ketamine concentration was sixfold compared with morphine-naïve rats. After a 2 day morphine withdrawal period, smaller but parallel concentration changes were observed. In acute coadministration, ketamine increased the brain morphine concentration by 20%, but no increase in ketamine concentrations or increased antinociception was observed.

Conclusions and Implications

The ability of ketamine to induce antinociception in rats made tolerant to morphine may also be due to increased brain concentrations of morphine, ketamine and norketamine. The relevance of these findings needs to be assessed in humans.

The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception

Aims

Spironolactone, an antimineralocorticoid, has been reported to potentiate the cataleptic effect of morphine in the rat. Since no previous research exists on the matter and the interaction might be clinically significant, the effects of spironolactone on morphine antinociception and pharmacokinetics in the rat were investigated.

Methods

Male SD rats were used to assess the effects of spironolactone on acute morphine-induced antinociception, development of morphine tolerance, and established morphine tolerance in the tail-flick and hot plate tests. Spironolactone was also administered with loperamide to assess whether spironolactone enhances the brain distribution of the acknowledged P-glycoprotein substrate across the blood-brain barrier.

Results

Spironolactone had no antinociceptive effects of its own but when co-administrated with morphine the antinociceptive effect of morphine was greatly enhanced. Morphine concentrations in the brain were increased fourfold in the spironolactone co-administrated group. Spironolactone did not inhibit the formation of pro-nociceptive morphine-3-glucuronide, nor did inhibit the development of tolerance. The peripherally restricted opioid, loperamide, had no antinociceptive effects by itself, but co-administration with spironolactone produced a clear change in the hot plate test.

Conclusions

Although mineralocorticoids have been proposed to take part in pain signaling, in our setting spironolactone did not have antinociceptive properties of its own. The increased antinociceptive effect of morphine is apparently caused by the increased morphine brain concentrations. We suggest this to be due to P-glycoprotein inhibition, as indicated by the loperamide assay. The clinical relevance of P-glycoprotein inhibition by spironolactone should be studied.

The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception

BACKGROUND

Spironolactone, a commonly used mineralocorticoid receptor antagonist, has been reported to potentiate the effect of morphine in the rat. The aim of this study was to investigate the effects of spironolactone on morphine antinociception and tissue distribution.

METHODS

The effects of spironolactone on acute morphine-induced antinociception, induction of morphine tolerance and established morphine tolerance were studied with tail-flick and hot plate tests in male Sprague-Dawley rats. Serum, brain, and liver morphine and its metabolite concentrations were quantified using high-pressure liquid chromatography-tandem mass spectrometry. Spironolactone was also administered with the peripherally acting, P-glycoprotein (P-gp) substrate loperamide to test whether spironolactone allows loperamide to pass the blood-brain barrier.

RESULTS

Spironolactone (50 mg/kg, i.p.) had no antinociceptive effects of its own, but it enhanced the antinociceptive effect of morphine in both thermal tests. Two doses of spironolactone enhanced the maximum possible effect (MPE) from 19.5% to 100% in the hot plate test 90 min after administration of 4 mg/kg morphine. Morphine concentrations in the brain were increased fourfold at 90 min by spironolactone. Spironolactone did not inhibit the formation of morphine-3-glucuronide. Acute spironolactone restored morphine antinociception in morphine-tolerant rats but did not inhibit the development of tolerance. The peripherally restricted opioid, loperamide (10 mg/kg), had no antinociceptive effects when administered alone, but co-administration with spironolactone produced a 40% MPE in the hot plate test.

CONCLUSIONS

Spironolactone has no antinociceptive effects in thermal models of pain, but it enhances the antinociceptive effects of morphine mainly by increasing morphine central nervous system concentrations, probably by inhibiting P-gp.