Influence of St. John's Wort on Intravenous Fentanyl Pharmacokinetics, Pharmacodynamics, and Clinical Effects: A Randomized Clinical Trial.

BACKGROUND: Patients often use complementary and alternative herbal medicines, hence, potential exists for adverse herb-drug interactions. Fentanyl is metabolized by hepatic CYP3A4 and considered transported by blood-brain barrier P-glycoprotein. Both disposition processes could be upregulated by the herbal St. John's wort. This investigation evaluated effects of St. John's wort on fixed-dose and apparent steady-state IV fentanyl pharmacokinetics, pharmacodynamics, and clinical effects. METHODS: Healthy volunteers received a fentanyl fixed-dose infusion and an individually tailored target controlled infusion on separate days, before and after 30-day St. John's wort (300 mg thrice daily; n = 8) or placebo control (n = 8) in a randomized parallel-group design. Fentanyl plasma concentrations, pupil diameter, analgesic response to experimental pain (cold pressor), subjective side effects, and cognitive effects were measured. Plasma fentanyl concentrations and changes in pupil diameter were subjected to pharmacokinetic-pharmacodynamic modeling. RESULTS: St. John's wort did not alter fentanyl pharmacokinetics. Clearance (l/min) before and after St. John's wort (1.13 ± 0.29 and 1.24 ± 0.26, respectively) or placebo (0.96 ± 0.28 and 1.12 ± 0.27, respectively) were not different. St. John's wort also did not affect fentanyl pharmacodynamics as measured by pupil constriction after fixed-dose and tailored fentanyl infusions. EC50 (ng/ml) was 1.1 ± 0.7 and 1.4 ± 0.9 before and after St. John's wort versus 1.2 ± 0.8 and 1.4 ± 1.7 before and after placebo. Effect site equilibration time, T½,ke0 (min), was 12.8 ± 5.3 and 11.3 ± 6.4 before and after St. John's wort versus 11.4 ± 6.4 and 11.1 ± 5.6 before and after placebo. St. John's wort had no influence on analgesia, cognitive performance, or somatic cognitive-affective effects of fentanyl. CONCLUSIONS: St. John's wort did not alter fentanyl pharmacokinetics, pharmacodynamics or clinical effects, suggesting no effect on hepatic clearance or blood-brain barrier efflux. Patients taking St. John's wort will likely not respond differently to IV fentanyl for anesthesia or analgesia.

Impact of chronic migraine attacks and their severity on the endogenous µ-opioid neurotransmission in the limbic system.

OBJECTIVE: To evaluate, in vivo, the impact of ongoing chronic migraine (CM) attacks on the endogenous µ-opioid neurotransmission. BACKGROUND: CM is associated with cognitive-emotional dysfunction. CM is commonly associated with frequent acute medication use, including opioids. METHODS: We scanned 15 migraine patients during the spontaneous headache attack (ictal phase): 7 individuals with CM and 8 with episodic migraine (EM), as well as 7 healthy controls (HC), using positron emission tomography (PET) with the selective µ-opioid receptor (µOR) radiotracer [11C]carfentanil. Migraineurs were scanned in two paradigms, one with thermal pain threshold challenge applied to the site of the headache, and one without thermal challenge. Multivariable analysis was performed between the µ-opioid receptor availability and the clinical data. RESULTS: µOR availability, measured with [11C]carfentanil nondisplaceable binding potential (BPND), in the left thalamus (P-value = 0.005) and left caudate (P-value = 0.003) were decreased in CM patients with thermal pain threshold during the ictal phase relative to HC. Lower µOR BPND in the right parahippocampal region (P-value = 0.001) and right amygdala (P-value = 0.002) were seen in CM relative to EM patients. Lower µOR BPND values indicate either a decrease in µOR concentration or an increase in endogenous µ-opioid release in CM patients. In the right amygdala, 71% of the overall variance in µOR BPND levels was explained by the type of migraine (CM vs. EM: partial-R2 = 0.47, P-value<0.001, Cohen's effect size d = 2.6SD), the severity of the attack (pain area and intensity number summation [P.A.I.N.S.]: partial-R2 = 0.16, P-value = 0.031), and the thermal pain threshold (allodynia: partial-R2 = 0.08). CONCLUSIONS: Increased endogenous µ-opioid receptor-mediated neurotransmission is seen in the limbic system of CM patients, especially in right amygdala, which is highly modulated by the attack frequency, pain severity, and sensitivity. This study demonstrates for the first time the negative impact of chronification and exacerbation of headache attacks on the endogenous µ-opioid mechanisms of migraine patients. ClinicalTrials.gov identifier: NCT03004313.

µ-opioid receptor system mediates reward processing in humans.

The endogenous µ-opioid receptor (MOR) system regulates motivational and hedonic processing. We tested directly whether individual differences in MOR are associated with neural reward responses to food pictures in humans. We scanned 33 non-obese individuals with positron emission tomography (PET) using the MOR-specific radioligand [11C]carfentanil. During a functional magnetic resonance imaging (fMRI) scan, the subjects viewed pictures of appetizing versus bland foods to elicit reward responses. MOR availability was measured in key components of the reward and emotion circuits and used to predict BOLD-fMRI responses to foods. Viewing palatable versus bland foods activates regions involved in homeostatic and reward processing, such as amygdala, ventral striatum, and hypothalamus. MOR availability in the reward and emotion circuit is negatively associated with the fMRI reward responses. Variation in MOR availability may explain why some people feel an urge to eat when encountering food cues, increasing risk for weight gain and obesity.

Effect of Modulated Electrohyperthermia on the Pharmacokinetics of Oral Transmucosal Fentanyl Citrate in Healthy Volunteers.

PURPOSE: This study aimed to determine whether changes occur in fentanyl absorption and disposition when administered in conjunction with modulated electrohyperthermia (mEHT) treatment. METHODS: A randomized, single-dose, crossover, open-label study was used to investigate the effect of mEHT on the pharmacokinetic properties of fentanyl in 12 healthy volunteers. The 12 healthy volunteers were each administered a single dose of oral transmucosal fentanyl citrate (OTFC) or a single dose of OTFC with mEHT. mEHT was performed on the abdomen for 1 hour. Blood samples were collected for 24 hours after dosing. The temperature of the abdominal skin surface was assessed before dosing and at 10, 20, and 60 minutes after dosing. FINDINGS: Geometric mean ratios (ratio of fentanyl with mEHT to fentanyl alone) for the Cmax and AUC0-last were 1.20 (90% CI, 1.09-1.32) and 1.15 (90% CI, 0.99-1.33), respectively. The mean temperature of the abdominal skin surface increased by approximately 4°C. IMPLICATIONS: There was an increase in the overall exposure to the drug without implications of any clinical significance. OTFC can be administered without limitations in combination with mEHT, and it is not necessary to modify the dosing regimen. cris.nih.go,kr Identifier: KCT0001286.

Effect of dosing interval on pharmacokinetics of fentanyl pectin nasal spray from a crossover study.

OBJECTIVE: To evaluate the effects of different dosing intervals on multiple-dose pharmacokinetics, and safety and tolerability of fentanyl pectin nasal spray (FPNS). METHODS: This was an open-label study in healthy volunteers. Five FPNS treatments (1 × 100 µg; 2 × 100 µg at 4-, 2-, and 1-hour intervals, and 8 × 100 µg consecutively) were administered to the right nostril, with a ≥ 3-day washout period. Blood samples were collected at predose and up to 1,440 minutes postdose. Plasma fentanyl concentrations were determined. Pharmacokinetic parameters-peak concentration (C(max)), time to C(max) (t(max)), and area under the concentration-time curve (AUC)-were derived using noncompartmental method. For the two-dose regimens, pharmacokinetic parameters were compared between doses using a paired t-test with p < 0.05 as statistically significant. RESULTS: Thirteen subjects were enrolled and 10 completed the study. Median tmax was 10-15 minutes across five regimens. Cmax post the second dose significantly increased for 1-hour (p < 0.0001) and 2-hour (p < 0.001) but not 4-hour intervals (p = 0.462). C(max) and AUC(0-24) following 8 × 100 µg were approximately fivefold of those following 1 × 100 µg. Dizziness (11.9 percent) and somnolence (4.9 percent) were most common adverse events (AEs). 12.9 percent of patients discontinued due to AEs. CONCLUSIONS: FPNS exhibited consistently rapid t(max). When intervals between two doses were shorter, the difference in C(max) between the first and second dose was larger. All regimens of FPNS were well tolerated. Exposure reached a plateau after eight consecutive doses, suggesting potential limited absorption through the nasal mucosa.

Low- and high-frequency transcutaneous electrical acupoint stimulation induces different effects on cerebral µ-opioid receptor availability in rhesus monkeys.

Although systematic studies have demonstrated that acupuncture or electroacupuncture (EA) analgesia is based on their accelerating endogenous opioid release to activate opioid receptors and that EA of different frequencies is mediated by different opioid receptors in specific areas of the central nervous system, there is little direct, real-time evidence to confirm this in vivo. The present study was designed to investigate the effects of transcutaneous electrical acupoint stimulation (TEAS), an analogue of EA, at low and high frequencies on µ-opioid receptor (MOR) availability in the brain of rhesus monkeys. Monkeys underwent 95-min positron emission tomography (PET) with (11) C-carfentanil three times randomly while receiving 0, 2, or 100 Hz TEAS, respectively. Each TEAS was administered in the middle 30 min during the 95-min PET scan, and each session of PET and TEAS was separated by at least 2 weeks. The results revealed that 2 Hz but not 100 Hz TEAS evoked a significant increase in MOR binding potential in the anterior cingulate cortex, the caudate nucleus, the putamen, the temporal lobe, the somatosensory cortex, and the amygdala compared with 0 Hz TEAS. The effect remained after the end of TEAS in the anterior cingulate cortex and the temporal lobe. The selective increase in MOR availability in multiple brain regions related to pain and sensory processes may play a role in mediating low-frequency TEAS efficacy.

A pharmacokinetic assessment of an alternate titration strategy for fentanyl pectin nasal spray.

OBJECTIVE: Fentanyl pectin nasal spray (FPNS) is approved for management of breakthrough pain in cancer. It is available in 100 and 400 µg strength products which allow for doses of 100 - 800 µg (1 or 2 sprays). Existing titration strategies require a transition from the 100 µg product to the 400 µg product when increasing the dose from 200 to 400 µg. This study assessed the pharmacokinetic (PK) profile of FPNS administered as 4 sprays of 100 µg as an alternate titration strategy. METHODS: In this 3-way, crossover study, healthy subjects aged 18 - 65 years were randomized to receive each of 3 dosages of FPNS (4 × 100 µg, 2 × 100 µg, and 1 × 400 µg). PK samples were collected over 24 hours. RESULTS: Of 22 subjects randomized, 20 were included in the PK analysis. Administration of both 400 µg regimens (4 × 100 µg and 1 × 400 µg) provided greater systemic fentanyl exposure compared with the 200 µg dose (C(max): 1,748 and 1,485 pg/ml vs. 1,051 pg/ml; AUC(0-1h): 1,012 and 944 pg×h/ml vs. 665 pg×h/ml; and tmax: 0.25 hours and 0.50 hours vs. 0.25 hours); fentanyl exposure after 4 × 100 µg and 1 × 400 µg regimens was similar. Adverse events (AEs) were all mild or moderate in intensity; most common AEs were nausea (50%) and headache (23%). AE frequency was similar across treatments without reports of nasal effects. CONCLUSIONS: Given that systemic fentanyl exposure from FPNS administered as 4 × 100 µg is similar to that from FPNS as 1 × 400 µg, the 4 × 100 µg regimen provides an alternate titration strategy for patients needing more than 200 µg. This alternate strategy will facilitate a patient's ability to achieve an optimized FPNS regimen and reduce opioid wastage.

Long-term tolerability, efficacy and acceptability of fentanyl pectin nasal spray for breakthrough cancer pain.

PURPOSE: Previous studies show that fentanyl pectin nasal spray (FPNS) rapidly provides clinically meaningful pain relief in the treatment of breakthrough cancer pain (BTCP). This study assessed the long-term tolerability, acceptability and consistency of effect of FPNS in patients with BTCP. METHODS: Patients (new and rolled over from earlier controlled studies) with cancer experiencing one to four episodes per day of BTCP whilst taking ≥ 60 mg/day of morphine (or equivalent) given orally for cancer pain entered an open-label 16-week safety study. Safety and tolerability were assessed by adverse events (AEs), adverse drug reactions (ADRs), withdrawal due to AEs and by nasal assessments. Acceptability assessments included ratings of overall satisfaction with each treated episode and ease of use and convenience of FPNS. Additional rescue medication and dose stability were used to evaluate the consistency of effect. RESULTS: Four hundred three patients were included in the safety and intent-to-treat analysis (42,227 episodes), 356 entered the treatment phase and 110 completed 16 weeks. Overall, 24.6% of 403 patients reported treatment-related treatment-emergent AEs that were generally mild/moderate and typical of opioids; 20 patients discontinued treatment due to an AE (9 were ADRs). Nasal assessments revealed no clinically significant effects; 94% of FPNS-treated episodes required no additional rescue medication. More than 90% of patients did not have to increase their dose during the study. Patients reported overall satisfaction with FPNS for 90.1% of episodes. At week 12, 96.9% of patients were satisfied with the ease of use and 97.9% with the convenience of FPNS. CONCLUSIONS: FPNS was generally well tolerated and well accepted for the treatment of BTCP, and doses remained stable over the 4-month study period.

Randomized 5-treatment crossover study to assess the effects of external heat on serum fentanyl concentrations during treatment with transdermal fentanyl systems.

This randomized, open-label, 5-treatment, 5-sequence crossover study was designed to evaluate the effects of a heating pad on serum fentanyl concentrations with reservoir and matrix transdermal fentanyl systems. Subjects were randomized to 1 of 5 treatment sequences, receiving 5 fentanyl treatments (1 per period) for 36 hours: 25 µg/h reservoir without heat, 25 µg/h reservoir with heat, 25 µg/h matrix without heat, 25 µg/h matrix with heat, and a 50 µg/h reservoir without heat. The 25 µg/h systems with heat had a heating pad applied from 0 to 10 and 26 to 36 hours post application. Washout periods between treatments were 5 to 14 days. Naltrexone was given to block the opioid effects of fentanyl. Study results indicate that external heat had a similar effect on both matrix and reservoir systems, with heat applied during the first 10 hours of treatment increasing fentanyl exposure by approximately 61% to 81% at 10 hours (observed serum concentration at 10 hours) and overall exposure (area under the curve from 0 to 10 hours) by approximately 120% to 184%, but had minimal effect from 26 to 36 hours. The increased exposure observed with heat in both 25 µg/h systems, between 0 and 10 hours, was higher than that obtained with the 50 µg/h reservoir system applied without heat.

Fentanyl nasal spray for the treatment of cancer pain.

INTRODUCTION: Breakthrough pain, a transitory flare of pain in patients with otherwise controlled chronic pain, has been well characterized in cancer patients but despite medical awareness, sometimes remains underdiagnosed and therefore undertreated. AREAS COVERED: Oral transmucosal fentanyl citrate (OTFC) and fentanyl buccal tablets are the first medications developed specifically for the treatment of breakthrough pain in opioid-tolerant patients. Since oral administration of fentanyl is not an option for many cancer patients, the development of intranasal fentanyl spray (INFS) emerged as a more effective method of administration. Intranasal administration of fentanyl has several advantages over the oral/gastrointestinal route and clinical trials have shown that it is superior to OTFC while being well tolerated and more acceptable by the majority of patients. EXPERT OPINION: The aim of this review is to summarize the pharmacological characteristics and data obtained from clinical studies of INFS in the past few years, and present Fentanyl Pectin Nasal Spray (PecFent), which uses an innovative delivery system and is now approved in the EU. Finally, we discuss the impact that it may have in the future management of breakthrough pain in cancer patients, because an accurate diagnosis followed by the best treatment is crucial for effective pain alleviation.

Case report: delirium due to a diltiazem-fentanyl CYP3A4 drug interaction.

INTRODUCTION: Fentanyl and diltiazem are frequently used medications. Diltiazem inhibits cytochrome P450 3A4 isoenzymes. This can suppress fentanyl metabolism. METHOD: We present a case of delirium after coadministration of fentanyl and diltiazem. DISCUSSION: Cautious use is warranted while concomitantly administering fentanyl and diltiazem as this can potentiate fentanyl toxicity. Other 3A4 inhibitors include ketoconazole, erythromycin, nefazodone, ritonavir, delavirdine, aprepitant and imatinib. Psychosomatic medicine psychiatrists, pain and palliative care physicians and cardiologists in particular should be aware of this interaction.

Pharmacokinetics and relative bioavailability of fentanyl pectin nasal spray 100 - 800 µg in healthy volunteers.

OBJECTIVES: Fentanyl pectin nasal spray (FPNS) is formulated as a solution utilizing PecSys®; pectin based enabling technology (Archimedes). On contact with the nasal mucosa the formulation will gel and modulate fentanyl absorption while limiting nasal drip or runoff. This single-dose volunteer study compared the pharmacokinetics of FPNS 100, 200, 400, and 800 µg doses and assessed bioavailability relative to oral transmucosal fentanyl (OTFC) 200 µg. Safety and dose proportionality were also examined. SUBJECTS AND METHODS: 16, opioid-naïve subjects were dosed on five separate visits under naltrexone block. FPNS doses were administered using a Pfeiffer device delivering 100 µl. Devices were filled with either 1.57 mg/ml (100 and 200 µg dosing) or 6.28 mg/ml fentanyl citrate (400 and 800 µg). Venous blood samples were collected up to 48 h after dosing and plasma fentanyl concentrations measured. RESULTS: Median tmax values for FPNS ranged from 15 to 21 min post-dose and were dose-independent. At 200 µg Cmax values were 2.3-fold higher for FPNS compared with OTFC. Mean relative bioavailability of FPNS to OTFC ranged from 103% to 163%. Dose proportionality for Cmax and AUC0-1 across the FPNS range was statistically confirmed. Drug absorption also increased in a close to dose-proportional manner for AUC0-inf. CONCLUSIONS: FPNS has a shorter tmax, higher Cmax and greater bioavailability than OTFC and is well tolerated. The dose proportionality of Cmax and AUC0-1 was demonstrated. It is concluded that the pharmacokinetic profile of FPNS suggests this product is suitable for clinical investigation in breakthrough pain in cancer patients.

Assessment of canine sensory function by using sine-wave electrical stimuli paradigm.

The paradigm of sine-wave electrical stimuli has been used for sensory neurological assessment in humans. In the present study, we applied the paradigm to the dog for the quantitative assessment of sensory function. Sine-wave electrical current stimuli at frequencies of 2000, 250, and 5Hz were delivered to bipolar electrodes attached to the skin surface of the hind paws. The stimulation intensity was gradually increased, and the minimum intensity required to elicit the lifting behavior in the stimulated paw was determined as current threshold (CT) for each of the three frequencies. Dogs consistently showed the lifting behavior at CTs without showing aversive behaviors such as vocalization and wriggling. The baseline CTs (mean+/-SEM, n=12) were 4430+/-110microA for CT2000, 2215+/-173microA for CT250, and 2305+/-152microA for CT5. The CTs immediately increased after bolus intravenous injection of fentanyl at 10microg/kg, although the significant increase disappeared within 1h. The time course for the CTs was parallel to that of plasma fentanyl concentration. In conclusion, the present study applied the paradigm of transcutaneous sine-wave electrical stimuli to the dog, and used the hind paw lifting as endpoint behavior. This paradigm is simple, non-invasive, useful in the assessment of sensory function, and can be adapted to investigate the pharmacokinetics/pharmacodynamics relation of drugs. Further studies are needed to give the conclusive interpretation of the endpoint behavior.

Pharmacokinetic comparisons of three nasal fentanyl formulations; pectin, chitosan and chitosan-poloxamer 188.

OBJECTIVES: To optimize the absorption profile and reduce C(max), three new fentanyl nasal spray formulations have been developed: fentanyl pectin (FPNS), fentanyl chitosan (FChNS) and fentanyl in chitosan-poloxamer 188 (FChPNS). The venous pharmacokinetic profiles and tolerability of these formulations were assessed and compared with oral transmucosal fentanyl citrate (OTFC) lozenge. SUBJECTS AND METHODS: This randomized, open-label, crossover study was conducted in opioid-naïve, healthy adult volunteers. Subjects were dosed under naltrexone blockade on four occasions with three nasal sprays (100 microg in 100 microl) and OTFC 200 microg. Fentanyl venous plasma concentrations were measured up to 24 h post-dose. Tolerability was assessed by clinical nasal assessments and a nasal reactogenicity questionnaire. RESULTS: 18 subjects were enrolled and completed the study. The mean dose-normalized AUC(0-infinity) for each nasal formulation was significantly higher (p < 0.05) compared with OTFC. Bioavailability compared with OTFC was significantly greater for all nasal fentanyl formulations (FPNS 132.4%, FChNS 154.1%, FChPNS 122.3%). Median tmax (FPNS 0.33 h, FChNS 0.17 h, FChPNS 0.26 h) were significantly (p < 0.001) reduced (OTFC 1.5 h) and mean C(max) significantly increased with all nasal formulations compared with OTFC. Nasal reactogenicity symptom incidence was lowest for the FPNS formulation (FPNS 2, FChNS 28 and FChPNS 45). CONCLUSIONS: All nasal formulations demonstrated significantly increased systemic exposure and reduced times to peak plasma values compared with OTFC. The FPNS formulation exhibited the most favorable nasal and general tolerability profiles. It appears suitable for further investigation in breakthrough cancer pain management.

PecSys: in situ gelling system for optimised nasal drug delivery.

PecSys (PS) is a proprietary pectin-based drug delivery system designed to gel when applied to mucosal surfaces and with potential areas of application for drugs used in local and systemic disease therapy. The current area of focus is intranasal drug delivery where PS is being used to optimise absorption of lipophilic drugs into the systemic circulation. Pectin is described as GRAS (generally regarded as safe) with an excellent regulatory position through its long history of pharmaceutical and food usage. Tests to measure the functional gelling properties of pectin raw material and PS have been devised and validated. The PS-based products at the most advanced stages of development are intranasal formulations containing opioid analgesics intended to provide rapid pain relief with simple and convenient dosing and minimal side effects. The profile of such drugs may not be optimal through current routes of delivery and the ability of PS to modulate their pharmacokinetic profiles, such as attenuation of the peak plasma concentration (Cmax), has been demonstrated in clinical testing. The lead product using PS is a fentanyl nasal spray formulation (NasalFent), which has successfully met the primary objective in a pivotal Phase III clinical study and is scheduled for regulatory filings in the first half of 2009.

Altered central micro-opioid receptor binding after psychological trauma.

BACKGROUND: Functional neuroimaging studies have detected abnormal limbic and paralimbic activation to emotional probes in posttraumatic stress disorder (PTSD), but few studies have examined neurochemical mechanisms that underlie functional alterations in regional cerebral blood flow. The mu-opioid neurotransmitter system, implicated in responses to stress and suppression of pain, is distributed in and is thought to regulate the function of brain regions that are implicated in affective processing. METHODS: Here we examined the micro-opioid system with positron emission tomography and the micro-opioid receptor-selective radiotracer [11C] carfentanil in 16 male patients with PTSD and two non-PTSD male control groups, with (n = 14) and without combat exposure (n = 15). Differences in micro-opioid receptor binding potential (BP2) were detected within discrete limbic and paralimbic regions. RESULTS: Relative to healthy controls, both trauma-exposed groups had lower micro-opioid receptor BP2 in extended amygdala, nucleus accumbens, and dorsal frontal and insular cortex but had higher BP2 in the orbitofrontal cortex. PTSD patients exhibited reduced BP2 in anterior cingulate cortex compared with both control groups. Micro-opioid receptor BP2 in combat-exposed subjects without PTSD was lower in the amygdala but higher in the orbitofrontal cortex compared with both PTSD patients and healthy controls. CONCLUSIONS: These findings differentiate the general response of the micro-opioid system to trauma from more specific changes associated with PTSD.

Prolonged central mu-opioid receptor occupancy after single and repeated nalmefene dosing.

The opioid antagonist nalmefene offers an alternative to traditional pharmacological treatments for alcoholism. The present study was designed to investigate the relationship between nalmefene plasma concentration and central mu-opioid receptor occupancy after a clinically effective dose (20 mg, orally). Pharmacokinetics and mu-opioid receptor occupancy of nalmefene after single and repeated dosing over 7 days was studied in 12 healthy subjects. Serial blood samples were obtained after both dosings, and pharmacokinetic parameters for nalmefene and main metabolites were determined. Central mu-opioid receptor occupancy of nalmefene was measured with positron emission tomography (PET) and [(11)C]carfentanil at four time points (3, 26, 50, 74 h) after both dosings. Nalmefene was rapidly absorbed in all subjects. The mean t(1/2) of nalmefene was 13.4 h after single and repeated dosing. The accumulation of nalmefene and its main metabolites in plasma during the repeated dosing period was as expected for a drug with linear pharmacokinetics, and steady-state was reached for all analytes. Both nalmefene dosings resulted in a very high occupancy at mu-opioid receptors (87-100%), and the decline in the occupancy was similar after both dosings but clearly slower than the decline in the plasma concentration of nalmefene or metabolites. High nalmefene occupancy (83-100%) persisted at 26 h after the dosings. The prolonged mu-opioid receptor occupancy by nalmefene indicates slow dissociation of the drug from mu-opioid receptors. These results support the rational of administering nalmefene when needed before alcohol drinking, and they additionally suggest that a high mu-opioid receptor occupancy can be maintained when nalmefene is taken once daily.

In vivo evaluation of new carfentanil-based radioligands for the mu opiate receptor.

Eight derivatives of [(11)C]carfentanil were evaluated as alternative mu opioid receptor radioligands with the potential for lower pharmacological activity, faster pharmacokinetics, and/or lower non-specific binding. Derivatives with aryl ring substituents or alkyl group substitutions were prepared in carbon-11 labeled form and examined for initial brain uptake and regional brain tissue pharmacokinetics in mouse brain. Promising derivatives with chloro, methoxy and methyl substituents on one aryl ring were then evaluated for specific binding in an equilibrium infusion rat model of regional brain distributions. Although no derivatives were identified with improved pharmacokinetics or lower non-specific binding, several derivatives show acceptable in vivo specific binding properties and may deserve further evaluation as less potent and thus safer compounds for in vivo imaging studies.

Quantitative analysis of fentanyl in rat plasma by gas chromatography with nitrogen-phosphorus detection.

A sensitive assay method was developed to determine fentanyl, an opiate agonist, in rat plasma by gas chromatography with nitrogen-phosphorus detection. For the pretreatment of plasma samples, sodium hydroxide was added to denature protein and n-butyl chloride was used to extract fentanyl. The calibration curve was linear within the concentration range 0.5 to 50 ng/ml (r=0.9997). The limit of detection was 0.1 ng/ml, and 0.5 ng/ml could be quantified with acceptable precision. Furthermore, fentanyl could be determined in only 200 microl of rat plasma. The method has been successfully applied to an intramuscular pharmacokinetic study at a dose of 10 microg/kg. Therefore, the current method is a valuable analytical tool for investigating the pharmacokinetics of fentanyl at low clinical doses.