Factors associated with persistent opioid use 6-12 months after primary total knee arthroplasty.

Persistent pain following knee arthroplasty occurs in up to 20% of patients and may require ongoing analgesia, including extended opioid administration. A comprehensive secondary analysis was performed from results of a study that considered persistent postoperative pain in 242 patients who underwent unilateral knee arthroplasty using a standardised enhanced recovery programme. Opioid prescribing for 12 months before and 12 months after surgery was evaluated and converted to oral morphine equivalents. Demographic, functional, psychological and pain questionnaires were completed along with quantitative sensory testing and genetic analysis. Forty-nine percent of patients had at least one opioid prescription in the 12 months before surgery. Opioid prescriptions were filled in 93% of patients from discharge to 3 months and in 27% of patients ≥6 months after surgery. Persistent opioid use ≥6 months after surgery was strongly associated with pre-operative opioid use (RR 3.2, p < 0.001 (95%CI 1.9-5.4)). The median (IQR [range]) oral morphine equivalent daily dose was 3.6 (0.9-10.5 [0-100.0]) mg pre-operatively, 35.0 (22.5-52.5 [4.6-180.0]) mg in hospital, 12.8 (5.1-24.8 [0-57.9]) mg from discharge to 3 months and 5.9 (4.5-12.0 [0-44.5]) mg at ≥6 months following surgery. Predictors of increased daily oral morphine equivalent ≥6 months after surgery included increased average daily oral morphine equivalent dose compared with previous values (lag), increased body mass index and three or more comorbid pain sites. Persistent opioid use was not associated with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain (RR 1.003, p = 0.655, 95%CI 0.65-1.002) or WOMAC function (RR 1.001, p = 0.99, 95%CI 0.99-1.03) outcomes 6 months after surgery. There was no association between persistent opioid use and pre-operative quantitative sensory testing results or psychological distress. Pre-operatively, patients with a higher body mass index, more comorbid pain sites and those who had filled an opioid prescription in the last 12 months, were at increased risk of persistent opioid use and a higher oral morphine equivalent daily dose ≥ 6 months after surgery. Strategies need to be developed to limit dose and duration of persistent opioid use in patients following knee arthroplasty surgery.

Persistent postoperative pain after total knee arthroplasty: a prospective cohort study of potential risk factors.

BACKGROUND: Persistent postoperative pain (PPP) is common after total knee arthroplasty (TKA). The primary aim of this prospective cohort study was to identify important predictors of moderate to severe PPP 6 and 12 months after TKA. METHODS: Consenting patients (n=300) undergoing primary unilateral TKA attended a preoperative session to collect clinical information (age, gender, BMI, preoperative knee pain, comorbid pain, likely neuropathic pain) and psychological variables (depression, anxiety, catastrophising, expected pain). Quantitative sensory testing (pressure pain thresholds, temporal summation, conditioned pain modulation) was performed, and blood samples were obtained for subsequent genotyping of OPRM1 and COMT. Acute postoperative pain was measured at rest and during movement. Surgical factors (surgery time, patella resurfacing, anaesthetic type) were collected after operation. Follow-up questionnaires were sent 6 and 12 months after surgery. Multivariate logistic regression was used to identify predictors of PPP. RESULTS: The prevalence of moderate to severe PPP was 21% (n=60) and 16% (n=45) 6 and 12 months after surgery, with 55% (n=33) and 60% (n=31) of PPP likely neuropathic in nature. At 6 months, a combination of preoperative pain intensity, expected pain, trait anxiety, and temporal summation (Akaike information criterion, 309.9; area under receiver operating characteristic (ROC) curve, 0.70) was able to correctly classify 66% of patients into moderate to severe PPP and no to mild PPP groups. At 12 months, preoperative pain intensity, expected pain, and trait anxiety (Akaike information criterion, 286.8; area under ROC curve, 0.66) correctly classified 66% of patients. CONCLUSIONS: Findings from this study highlight several factors that may be targeted in future intervention studies to reduce the development of PPP. TRIAL REGISTRY NUMBER: ACTRN12612001089820.

Placebo-controlled pilot trial testing dose titration and intravenous, intramuscular and subcutaneous routes for ketamine in depression.

OBJECTIVE: This pilot study assessed the feasibility, efficacy and safety of an individual dose-titration approach, and of the intravenous (IV), intramuscular (IM) and subcutaneous (SC) routes for treating depression with ketamine. METHOD: Fifteen treatment-refractory depressed participants received ketamine or midazolam (control treatment) in a multiple crossover, double-blind study. Ketamine was administered by IV (n = 4), IM (n = 5) or SC (n = 6) injection. Dose titration commenced at 0.1 mg/kg, increasing by 0.1 mg/kg up to 0.5 mg/kg, given in separate treatment sessions separated by ≥1 week, with one placebo control treatment randomly inserted. Mood, psychotomimetic and hemodynamic effects were assessed and plasma ketamine concentrations assayed. RESULTS: Twelve participants achieved response and remission criteria, achieved at doses as low as 0.1 mg/kg. All three routes of administration resulted in comparable antidepressant effects. Fewest adverse effects were noted with the SC route. Antidepressant response, adverse effects and ketamine concentrations were dose-related. CONCLUSION: Antidepressant response occurred at a range of doses and at <0.5 mg/kg. The dose-titration approach is a practical method for optimizing the efficacy - side-effects trade-off on an individual patient basis. This pilot study provides preliminary evidence for SC injection as a practical, feasible and efficacious treatment approach.

Pharmacogenetics of opioid response.

For opioids requiring CYP2D6 O-demethylation to active metabolites, poor metabolizers have reduced metabolite formation and minimal pain reduction. Clinically, this has only reliably been shown for tramadol. Ultra-rapid metabolizers have an increased risk of toxicity especially for codeine. ABCB1 genetics show no consistent findings. In Asian populations, the high OPRM1 118A>G frequency associates with higher opioid dosage requirements. Clinical translation of opioid genetics is premature because many important pain and addiction phenotype factors contribute.

Opioid activation of toll-like receptor 4 contributes to drug reinforcement.

Opioid action was thought to exert reinforcing effects solely via the initial agonism of opioid receptors. Here, we present evidence for an additional novel contributor to opioid reward: the innate immune pattern-recognition receptor, toll-like receptor 4 (TLR4), and its MyD88-dependent signaling. Blockade of TLR4/MD2 by administration of the nonopioid, unnatural isomer of naloxone, (+)-naloxone (rats), or two independent genetic knock-outs of MyD88-TLR4-dependent signaling (mice), suppressed opioid-induced conditioned place preference. (+)-Naloxone also reduced opioid (remifentanil) self-administration (rats), another commonly used behavioral measure of drug reward. Moreover, pharmacological blockade of morphine-TLR4/MD2 activity potently reduced morphine-induced elevations of extracellular dopamine in rat nucleus accumbens, a region critical for opioid reinforcement. Importantly, opioid-TLR4 actions are not a unidirectional influence on opioid pharmacodynamics, since TLR4(-/-) mice had reduced oxycodone-induced p38 and JNK phosphorylation, while displaying potentiated analgesia. Similar to our recent reports of morphine-TLR4/MD2 binding, here we provide a combination of in silico and biophysical data to support (+)-naloxone and remifentanil binding to TLR4/MD2. Collectively, these data indicate that the actions of opioids at classical opioid receptors, together with their newly identified TLR4/MD2 actions, affect the mesolimbic dopamine system that amplifies opioid-induced elevations in extracellular dopamine levels, therefore possibly explaining altered opioid reward behaviors. Thus, the discovery of TLR4/MD2 recognition of opioids as foreign xenobiotic substances adds to the existing hypothesized neuronal reinforcement mechanisms, identifies a new drug target in TLR4/MD2 for the treatment of addictions, and provides further evidence supporting a role for central proinflammatory immune signaling in drug reward.

Contrasting effects of diclofenac and ibuprofen on active imatinib uptake into leukaemic cells.

BACKGROUND: The human organic cation transporter-1 (OCT-1) is the primary active protein for imatinib uptake into target BCR-ABL-positive cells. Non-steroidal anti-inflammatory drugs (NSAIDs) are frequently used by chronic myeloid leukaemia (CML) patients on imatinib to manage musculoskeletal complaints. METHODS: Here we investigated the impact of NSAIDs on functional activity of the OCT-1 (OCT-1 activity; OA) in CML cells. RESULTS: Although ten of twelve NSAIDs tested had no significant impact on OA (P>0.05), we observed increased OA (27% increase in K562; 22% increase in KU812 cells, P<0.05) and reduced IC50(imatinib) when treated with diclofenac. Co-incubation with imatinib and diclofenac resulted in a significantly lower viable cell number compared with imatinib alone. In contrast, ibuprofen led to a significant decrease in OA, an increase in IC50(imatinib) and thus reduced the cytotoxicity of imatinib. In primary CML samples, diclofenac significantly increased OA, particularly in patients with low OA (<4 ng per 200 000 cells), and significantly decreased IC50(imatinib). Ibuprofen induced significant decreases in OA in CML samples and healthy donors. CONCLUSION: On the basis of the expected impact of these two drugs on OA, ibuprofen should be avoided in combination with imatinib. Further studies are warranted regarding the potential benefit of diclofenac to improve OA in a clinical setting.

Acute pain management in opioid-tolerant patients: a growing challenge.

In Australia and New Zealand, in parallel with other developed countries, the number of patients prescribed opioids on a long-term basis has grown rapidly over the last decade. The burden of chronic pain is more widely recognised and there has been an increase in the use of opioids for both cancer and non-cancer indications. While the prevalence of illicit opioid use has remained relatively stable, the diversion and abuse of prescription opioids has escalated, as has the number of individuals receiving methadone or buprenorphine pharmacotherapy for opioid addiction. As a result, the proportion of opioid-tolerant patients requiring acute pain management has increased, often presenting clinicians with greater challenges than those faced when treating the opioid-naïve. Treatment aims include effective relief of acute pain, prevention of drug withdrawal, assistance with any related social, psychiatric and behavioural issues, and ensuring continuity of long-term care. Pharmacological approaches incorporate the continuation of usual medications (or equivalent), short-term use of sometimes much higher than average doses of additional opioid, and prescription of non-opioid and adjuvant drugs, aiming to improve pain relief and attenuate opioid tolerance and/or opioid-induced hyperalgesia. Discharge planning should commence at an early stage and may involve the use of a 'Reverse Pain Ladder' aiming to limit duration of additional opioid use. Legislative requirements may restrict which drugs can be prescribed at the time of hospital discharge. At all stages, there should be appropriate and regular consultation and liaison with the patient, other treating teams and specialist services.

Potentiation of buprenorphine antinociception with ultra-low dose naltrexone in healthy subjects.

Previous reports have demonstrated greater antinociception following administration of a buprenorphine/naloxone combination compared to buprenorphine alone among healthy volunteers. The aim of the current investigation was to determine whether buprenorphine antinociception could be enhanced with the addition of ultra-low dose naltrexone, using a range of dose ratios. A repeated-measures, double-blind, cross-over trial was undertaken with 10 healthy participants. The effects of each buprenorphine:naltrexone ratio (100:1, 133:1, 166:1, and 200:1) on cold pressor tolerance time and respiration were compared to the effects of buprenorphine only. The 166:1 ratio was associated with significantly greater tolerance time to cold pressor pain than buprenorphine alone. Minimal respiratory depression and few adverse events were observed in all conditions. These findings suggest that, as previously described with naloxone, the addition of ultra-low dose naltrexone can enhance the antinociceptive effect of buprenorphine in humans. This potentiation is dose-ratio dependent and occurs without a concomitant increase in adverse effects.

Pharmacokinetics and pharmacodynamics of the short-acting sedative CNS 7056 in sheep.

BACKGROUND: CNS 7056 is a new short-acting esterase-metabolized benzodiazepine. We report the first pharmacokinetic (PK) and pharmacodynamic (PD) study of CNS 7056 and its inactive metabolite CNS 7054 in sheep. METHODS: The stability of CNS 7056 in blood samples was examined ex vivo. Six sheep were prepared with physiological instrumentation, and were given doses of 0.37, 0.74, and 1.47 mg kg(-1) (2 min infusion) of CNS 7056 in alternating order on separate days. RESULTS: CNS 7056 was degraded in warm whole sheep blood (23% over 2 h), but not in plasma or blood stored on ice. Using non-compartmental analysis (NCA), CNS 7056 had a mean (sd) clearance of 4.52 (0.96) litre min(-1) and a terminal half-life of 21.3 (10.9) min. There was a rapid conversion of CNS 7056 to its metabolite CNS 7054, which had a terminal half-life of 22.5 (3.4) min. The arterial kinetics of CNS 7056 could be described by a three-compartment model, with volumes of 1.9, 3.9, and 79 litre, a clearance of 4.2 litre min(-1), and inter-compartmental clearances of 2.85 and 1.44 litre min(-1), while the metabolite could be described by a two-compartment model. Cardiac output was an important covariate. Sedation as measured by the alpha power band of the EEG showed rapid onset and offset. The t(1/2,)(k)(e0) for sedation was 1.78 min, and the EC(50) was 0.10 µg ml(-1). CONCLUSIONS: CNS 7056 has PK-PD properties compatible with its potential human use as a short-acting i.v. sedative.

Possible involvement of toll-like receptor 4/myeloid differentiation factor-2 activity of opioid inactive isomers causes spinal proinflammation and related behavioral consequences.

Opioid-induced glial activation and its proinflammatory consequences have been associated with both reduced acute opioid analgesia and the enhanced development of tolerance, hyperalgesia and allodynia following chronic opioid administration. Intriguingly, recent evidence demonstrates that these effects can result independently from the activation of classical, stereoselective opioid receptors. Here, a structurally disparate range of opioids cause activation of signaling by the innate immune receptor toll like receptor 4 (TLR4), resulting in proinflammatory glial activation. In the present series of studies, we demonstrate that the (+)-isomers of methadone and morphine, which bind with negligible affinity to classical opioid receptors, induced upregulation of proinflammatory cytokine and chemokine production in rat isolated dorsal spinal cord. Chronic intrathecal (+)-methadone produced hyperalgesia and allodynia, which were associated with significantly increased spinal glial activation (TLR4 mRNA and protein) and the expression of multiple chemokines and cytokines. Statistical analysis suggests that a cluster of cytokines and chemokines may contribute to these nociceptive behavioral changes. Acute intrathecal (+)-methadone and (+)-morphine were also found to induce microglial, interleukin-1 and TLR4/myeloid differentiation factor-2 (MD-2) dependent enhancement of pain responsivity. In silico docking analysis demonstrated (+)-naloxone sensitive docking of (+)-methadone and (+)-morphine to human MD-2. Collectively, these data provide the first evidence of the pro-nociceptive consequences of small molecule xenobiotic activation of spinal TLR4 signaling independent of classical opioid receptor involvement.

Benzodiazepine dependence: focus on withdrawal syndrome.

Benzodiazepines are potentially addictive drugs: psychological and physical dependence can develop within a few weeks or years of regular or repeated use. The socioeconomic costs of the present high level of long-term benzodiazepine use are considerable. These consequences could be minimised if prescriptions for long-term benzodiazepines were decreased. However, many physicians continue to prescribe benzodiazepines and patients wishing to withdraw receive little advice or support. Particular care should be taken in prescribing benzodiazepines for vulnerable patients such as elderly persons, pregnant women, children, alcohol- or drug-dependent patients and patients with comorbid psychiatric disorders. The following update gives recent research results on the withdrawal pathophysiology and practical information in order to treat or prevent benzodiazepine withdrawal syndrome.

Enhanced buprenorphine analgesia with the addition of ultra-low-dose naloxone in healthy subjects.

Animal studies have demonstrated that co-administration of an ultra-low-dose opioid antagonist with an opioid agonist may result in enhanced analgesia. Investigation of this effect in humans has been limited and produced inconsistent findings, with previous reports suggesting that dose ratio may be critical to analgesic potentiation. The aim of the current investigation was to determine whether buprenorphine analgesia could be enhanced with the addition of ultra-low-dose naloxone among healthy volunteers, using a range of dose ratios. Tolerance to cold pressor pain was significantly greater with the combination of buprenorphine and naloxone compared to buprenorphine alone, and this effect was dose ratio dependent. Importantly, this enhanced analgesia occurred without an increase in adverse effects; indeed at some ratios, respiratory depression was attenuated. These findings demonstrate that the addition of ultra-low-dose naloxone can enhance the analgesic effect of buprenorphine in humans without a concurrent increase in side effects.

Lack of influence of CYP2D6 genotype on the clearance of (R)-, (S)- and racemic-methadone.

OBJECTIVE: To investigate the influence of CYP2D6 genotype on the oral clearance of (R)-, (S)- and rac-methadone. METHODS: In this retrospective study, CYP2D6 genotypes were identified in 56 methadone maintained subjects. Plasma concentrations of (R)-, (S)- and rac-methadone were determined by stereoselective HPLC and sufficient data were available to estimate the apparent oral clearances of (R)-, (S)- and rac-methadone using a population kinetic model in 37 of the genotyped subjects. RESULTS: The CYP2D6 allele frequencies were similar to those previously reported in Caucasians, the most common being: CYP2D6*1 (35.2%), CYP2D6*2 (12.0%) and CYP2D6*4 (22.2%). Three unknown SNPs were found in four subjects: 1811G > A (n = 1), 1834C > T (n = 1) and 2720G > C (n = 2). The oral clearances of (R)-, (S)- and rac-methadone varied 5.4-, 6.8- and 6.1-fold, respectively. No significant differences in methadone oral clearance were found between CYP2D6 genotypic PM, IM and EM (p = 0.57, 0.40 and 0.43 for (R)-, (S)- and rac-methadone, respectively). Only 1 subject had duplication of functional CYP2D6 alleles and the oral clearance of the three analytes was not markedly altered. CONCLUSIONS: CYP2D6 poor, intermediate and extensive metabolizer genotypes did not appear to impact on the oral clearance of (R)-, (S)- or rac-methadone. In addition, methadone dosage requirements were not influenced by CYP2D6 genotypes in these subjects. However, the impact of duplication of functional CYP2D6 alleles on oral clearance and dosage requirements requires further investigation.

Blood-brain distribution of morphine-6-glucuronide in sheep.

BACKGROUND AND PURPOSE: At present there are few data regarding the rate and extent of brain-blood partitioning of the opioid active metabolite of morphine, morphine-6-glucuronide (M6G). In this study the cerebral kinetics of M6G were determined, after a short-term intravenous infusion, in chronically instrumented conscious sheep. EXPERIMENTAL APPROACH: Five sheep received an intravenous infusion of M6G 2.2 mg kg(-1) over a four-minute period. Non-linear mixed-effects analysis, with hybrid physiologically based kinetic models, was used to estimate cerebral kinetics from the arterio-sagittal sinus concentration gradients and cerebral blood flow measurements. KEY RESULTS: A membrane limited model was selected as the final model. The blood-brain equilibration of M6G was relatively slow (time to reach 50% equilibration of the deep compartment 5.8 min), with low membrane permeability (PS, population mean, 2.5 ml min(-1)) from the initial compartment (V1, 13.7 ml) to a small deep distribution volume (V2) of 18.4 ml. There was some between-animal variability (%CV) in the initial distribution volume (29%), but this was not identified for PS or V2. CONCLUSION AND IMPLICATIONS: Pharmacokinetic modelling of M6G showed a delayed equilibration between brain and blood of a nature that is primarily limited by permeability across the blood-brain-barrier, in accordance with its physico-chemical properties.

A new simple diagnostic assay for the identification of the major CYP2D6 genotypes by DNA sequencing analysis.

AIM: To establish a method suitable for diagnostic genotyping of CYP2D6 alleles by DNA sequencing. METHODS: Initial PCR reactions were performed to specifically amplify exons 3, 4, 5 and 6 of the CYP2D6 gene using primers previously published. New primers were used to identify *2, *3, *4, *6, *7, *8, *9 and *41 in 2 sequencing reactions. Additional primers were designed for reverse sequencing in samples with 1 or 3 b.p. deletions. Previously published assays were used to detect *5, *10 and *16 alleles to complete genotype assignment. RESULTS: We reliably detected the nonfunctional alleles, *3, *4, *6, *7 and *8, which are associated with the poor metabolizer phenotype, and 2 important alleles associated with decreased enzyme activity, *9 and *41. Observed allele frequencies were comparable to those found previously in Caucasian populations. CONCLUSION: CYP2D6 genotype has been shown in previous clinical studies to be a good predictor of CYP2D6 phenotype and, therefore, related to therapeutic response and the risk of drug toxicity. This genotyping method is simple and reliable, and, therefore, can be routinely performed on an isolated patient sample, providing a relatively quick turnaround time needed for clinical practice. In addition, the simultaneous drawing of blood with the commencement of drug therapy will allow dosage adjustment on the basis of the CYP2D6 genotype to reduce the risk of adverse drug reactions.

CYP3A4 mediates dextropropoxyphene N-demethylation to nordextropropoxyphene: human in vitro and in vivo studies and lack of CYP2D6 involvement.

The individual cytochrome P450 isoforms in dextropropoxyphene N-demethylation to nordextropropoxyphene were determined and the pharmacokinetics of dextropropoxyphene and nordextropropoxyphene in cytochrome P4502D6 (CYP2D6) extensive (EM) and poor (PM) subjects were characterized. Microsomes from six CYP2D6 extensive metabolizers and one CYP2D6 poor metabolizer were used with isoform specific chemical and antibody inhibitors and expressed recombinant CYP enzymes. Groups of three CYP2D6 EM and PM subjects received a single 65-mg oral dose of dextropropoxyphene, and blood and urine were collected for 168 and 96 h, respectively. Nordextropropoxyphene formation in vitro was not different between the CYP2D6 extensive metabolizers (Km = 179 +/- 74 microM, Cl(int) = 0.41 +/- 0.26 ml mg(-1)h(-1)) and the PM subject (K = 225 microM, Cl(int) = 0.19 ml mg(-1) h(-1)) and was catalysed predominantly by CYP3A4. There was no apparent difference in the pharmacokinetics of dextropropoxyphene and nordextropropoxyphene in CYP2D6 EM and PM subjects. CYP3A4 is the major CYP enzyme catalysing the major metabolic pathway of dextropropoxyphene metabolism. Hence variability in the pharmacodynamic effects of dextropropoxyphene are likely due to intersubject variability in hepatic CYP3A4 expression and/or drug-drug interactions. Reported CYP2D6 phenocopying is not due to dextropropoxyphene being a CYP2D6 substrate.

Polymorphic hydroxylation of perhexiline in vitro.

AIMS: The aims of this study were to examine the in vitro enzyme kinetics and CYP isoform selectivity of perhexiline monohydroxylation using human liver microsomes. METHODS: Conversion of rac-perhexiline to monohydroxyperhexiline by human liver microsomes was assessed using a high-performance liquid chromatography assay with precolumn derivatization to measure the formation rate of the product. Isoform selective inhibitors were used to define the CYP isoform profile of perhexiline monohydroxylation. RESULTS: The rate of perhexiline monohydroxylation with microsomes from 20 livers varied 50-fold. The activity in 18 phenotypic perhexiline extensive metabolizer (PEM) livers varied about five-fold. The apparent Km was 3.3 +/- 1.5 micro m, the Vmax was 9.1 +/- 3.1 pmol min-1 mg-1 microsomal protein and the in vitro intrinsic clearance (Vmax/Km) was 2.9 +/- 0.5 micro l min-1 mg-1 microsomal protein in the extensive metabolizer livers. The corresponding values in the poor metabolizer livers were: apparent Km 124 +/- 141 micro m; Vmax 1.4 +/- 0.6 pmol min-1 mg-1 microsomal protein; and intrinsic clearance 0.026 micro l min-1 mg-1 microsomal protein. Quinidine almost completely inhibited perhexiline monohydroxylation activity, but inhibitors selective for other CYP isoforms had little effect. CONCLUSIONS: Perhexiline monohydroxylation is almost exclusively catalysed by CYP2D6 with activities being about 100-fold lower in CYP2D6 poor metabolizers than in extensive metabolizers. The in vitro data predict the in vivo saturable metabolism and pharmacogenetics of perhexiline.

Classification of the kinetics of factor VIII inhibitors in haemophilia A: plasma dilution studies are more discriminatory than time-course studies.

Factor VIII inhibitors have previously been classified as type I or type II using complex experiments that study the time course of inactivation of factor VIII and the effect of varying the antibody concentration. Classification may be important to better understand inhibitor behaviour in vivo. To determine the most reliable method of classifying the kinetics of factor VIII inactivation, we studied 11 patients with haemophilia A, comprising five severe, three mild and three acquired cases, and compared the classification obtained from plasma dilution studies and time-course studies. The plasma dilution studies showed two distinctly different patterns: a steep slope with complete FVIII:C inactivation at high antibody concentrations for type I inhibitors and a FVIII:C plateau with incomplete inactivation for type II inhibitors. Six type I (four severe, one mild and one acquired) and two type II (one mild and one acquired) inhibitors were classified using either plasma samples or purified and concentrated IgG, while the remaining were undetermined owing to insufficient available plasma. In contrast, the time-course studies could not discriminate between these groups. We recommend that plasma dilution studies be used for the classification of in vitro kinetics of factor VIII inhibitors.

Determination of in vivo absorption, metabolism, and transport of drugs by the human intestinal wall and liver with a novel perfusion technique.

BACKGROUND AND AIMS: The contribution of the gastrointestinal tract in comparison with the liver for the low and variable bioavailability of orally administered drugs is still poorly understood. Here we report on a new intestinal perfusion technique for the direct assessment of absorption, metabolism, and transport of drugs by the intestinal wall. METHODS: In 6 healthy volunteers a multilumen perfusion catheter was used to generate a 20-cm isolated jejunal segment that was perfused with 80 mg verapamil. Simultaneously, 5 mg [(2)H(7)]verapamil was given intravenously. Blood, perfusate, and bile samples were analyzed for parent verapamil and its major metabolites. RESULTS: The mean fraction of the verapamil dose absorbed from the 20-cm segment was 0.76 but substantial interindividual variability (0.51-0.96) was shown. Bioavailability was low (19.3%). The intestinal wall contributed to the same extent as the liver to extensive first-pass metabolism (mean extraction ratio, 0.49 versus 0.48). Substantial transport of verapamil metabolites from the systemic circulation via the enterocytes into the intestinal lumen was observed. Compared with biliary excretion, intestinal secretion into a 20-cm jejunal segment contributed to drug elimination to a similar extent. CONCLUSION: First-pass metabolism by the intestinal wall is extensive and contributes to the same extent as the liver to low bioavailability of some drugs such as verapamil. Moreover, intestinal secretion is as important as biliary excretion for the elimination of metabolites.

Precipitated withdrawal following codeine administration is dependent on CYP genotype.

The role of metabolic polymorphism in the development of physical dependence to codeine was assessed in cytochrome P450 2D2 (CYP2D2) deficient Dark Agouti and CYP2D2 intact Sprague-Dawley rats by assessment of the severity of naloxone precipitated withdrawal after codeine and morphine administration. Plasma morphine concentrations after codeine were significantly higher (P<0.01) in Sprague-Dawley than in Dark Agouti rats with metabolic ratios of 0.71 +/- 0.27 and 0.07 +/- 0.04, respectively. Withdrawal after codeine resulted in significantly greater hypothermia (3.5-4 degrees C, P<0.0001) in Sprague-Dawley animals compared to the other groups. Body weight loss was similar for all groups ranging from 6.2 +/- 0.4 to 8.2 +/- 0.6 g. When strain and treatment data were combined, a relationship between body temperature and plasma morphine concentration could be described by the inverse Hill equation (r(2)=0.76, EC(50)=556 +/- 121 ng/ml, n=2.9 +/- 1.5). These data indicate that dependence and withdrawal after codeine administration are dependent on its bioconversion to morphine.