Relationship Between Adalimumab Concentrations, Antidrug Antibodies, and Disease Activity in Rheumatoid Arthritis: A Cross-Sectional Observational Study.

OBJECTIVES: To determine the influence of patient characteristics and disease activity on adalimumab (ADA) concentrations; to assess the relationships between ADA concentrations, the presence of antidrug antibodies (ADAb), and disease activity in rheumatoid arthritis (RA); and to determine the association between cytokine concentrations and ADA concentrations. METHODS: A cross-sectional study of people with RA receiving ADA for at least 4 weeks was undertaken. Disease activity was assessed by the Disease Activity Score in 28 joints (DAS28), with responders defined as DAS28 ≤ 3.2. Serum and plasma were obtained for ADA concentrations and ADAb, and a panel of cytokines were obtained for a subgroup. ADA concentrations were compared between demographic and clinical subgroups using ANOVA. The independent associations between clinical and demographic features were analyzed using a general linear model. Variables significantly associated with ADA concentrations from the univariate analyses were entered into multivariate analyses. RESULTS: Of the 156 participants, 69.2% were female and the mean age was 57.4 (SD 12.7) years. Multivariate analysis revealed that higher C-reactive protein (P < 0.001) and higher weight (P < 0.004) were independently associated with lower ADA concentrations. ADA concentrations were higher in those with DAS28 ≤ 3.2 compared to those with DAS28 > 3.2 (median 10.8 [IQR 6.4-20.8] mg/L vs 7.1 [IQR 1.5-12.6] mg/L, P < 0.001). There was a significant negative correlation between interleukin 6 (IL-6) and ADA concentrations (r = -0.04, P < 0.01). CONCLUSION: ADA concentration correlates negatively with markers of inflammatory disease activity in RA, including IL-6. ADA concentration in the range 5 to 7 mg/L over the dose interval are associated with better disease control.

Immediate effect of caffeine on sympathetic nerve activity: why coffee is safe? A single-centre crossover study.

PURPOSES: Habitual coffee drinking is ubiquitous and generally considered to be safe despite its transient hypertensive effect. Our purpose was to determine the role of the sympathetic nervous system in the hypertensive response. METHODS: In a single-centre crossover study, medical caregivers were studied after consumption of standard coffee (espresso), water and decaffeinated coffee (decaff) given in random order at least 1 month apart. Plasma caffeine levels, mean arterial pressure, heart rate, total peripheral resistance and muscle sympathetic activity were recorded. Baroreflex activity was assessed using burst incidence and RR interval changes to spontaneous blood pressure fluctuations. RESULTS: A total of 16 subjects (mean [± standard error] age 34.4 ± 2 years; 44% female) were recruited to the study. Three agents were studied in ten subjects, and two agents were studied in six subjects. Over a 120-min period following the consumption of standard coffee, mean (± SE) plasma caffeine levels increased from 2.4 ± 0.8 to 21.0 ± 4 µmol/L and arterial pressure increased to 103 ± 1 mmHg compared to water (101 ± 1 mmHg; p = 0.066) and decaff (100 ± 1 mmHg; p = 0.016). Peripheral resistance in the same period following coffee increased to 120 ± 4% of the baseline level compared to water (107 ± 4; p = 0.01) and decaff (109 ± 4; p = 0.02). Heart rate was lower after both coffee and decaff consumption: 62 ± 1 bpm compared to water (64 bpm; p = 0.01 and p = 0.02, respectively). Cardio-vagal baroreflex activity remained stable after coffee, but sympathetic activity decreased, with burst frequency of 96 ± 3% versus water (106 ± 3%; p = 0.04) and decaff (112 ± 3%; p = 0.001) despite a fall in baroreflex activity from - 2.2 ± 0.1 to - 1.8 ± 0.1 bursts/100 beats/mmHg, compared to water (p = 0.009) and decaff (p = 0.004). CONCLUSION: The hypertensive response to coffee is secondary to peripheral vasoconstriction but this is not mediated by increased sympathetic nerve activity. These results may explain why habitual coffee drinking is safe.

Changes in inpatient medicine prescribing during COVID-19 lockdown.

BACKGROUND: New Zealand went into lockdown March 2020 successfully eliminating the circulation of the coronavirus disease 2019 (COVID-19) virus. During lockdown there were reduced rates of respiratory infections and hospital admission numbers were low. At the time, rumours of benefit and harm of medicines for COVID-19 were widespread in the lay and medical media. AIM: To describe changes in inpatient prescribing in an acute general medicine service during the New Zealand COVID-19 lockdown in 2020. METHODS: Rates of prescribing of medicines during the 33 days of lockdown were compared with a 33-day control period before lockdown. Prescriptions, patients and bed days were calculated from the hospital patient administration and electronic prescribing and administration systems. RESULTS: In the general medicine service, acute admissions were 20% lower during lockdown (from 1216 pre-lockdown to 974). There was a small decrease in the rate of prescriptions per patient (10.1 vs 10.4, P = 0.01) during lockdown, and the average length of stay was shorter (3.2 vs 3.6 days). Nebulised administration decreased by 75% (1.3% vs 5.3% of admissions) but unexpectedly there was no change in the prescribing rates of antibacterial medicines, e.g. amoxicillin (26% vs 26%). There were no changes in rates of prescribing of medicines being rumoured to potentially improve (e.g. hydroxychloroquine) or worsen (e.g. angiotensin-converting enzyme inhibitors) COVID-19 outcomes. CONCLUSIONS: Acute medical admissions decreased 20% during lockdown for COVID-19, with a proportional decrease in prescriptions. Reduced rates of respiratory tract infections did not lead to decreased prescribing of antibacterial medicines. Rumour-based prescribing did not eventuate.

Four-year review of New Zealand laboratory infliximab and adalimumab concentration results indicating potential for improved dosing.

A review of laboratory results across New Zealand for therapeutic drug monitoring (TDM) of infliximab and adalimumab concentrations and antidrug antibodies (ADAs) over 4 years was completed. Of 6591 results, the median serum concentration for infliximab was 5.7 mg/L and for adalimumab was 5.5 mg/L. Subtherapeutic drug concentrations (<7 mg/L) were measured in 54% of samples. Drug concentrations <2 mg/L were measured in 23% of samples, with ADAs detected in 51% of these. The high number of samples with subtherapeutic drug concentrations and common ADA detection is consistent with failing therapy but could also suggest that standard dosing is frequently too low for patients. These results reinforce the value of antitumour necrosis factor drug TDM in making decisions to adjust dosing or switch agents in patients taking infliximab and adalimumab.

A nationwide survey on therapeutic drug monitoring of anti-tumour necrosis factor agents for inflammatory bowel disease.

BACKGROUND: Routine therapeutic drug monitoring (TDM) during treatment with anti-tumour necrosis factor (anti-TNF) agents in inflammatory bowel disease may increase treatment efficacy and cost-effectiveness, and reduce the risk of loss of response. AIMS: To assess the current use of anti-TNF agent TDM, including trough concentration and anti-drug antibodies, among gastroenterology practitioners in New Zealand. METHODS: A web-based survey was delivered to gastroenterologists and advanced trainees in New Zealand, identified by the New Zealand Society of Gastroenterology. RESULTS: The response rate was 36% (48/134). Adalimumab was the most common initial anti-TNF agent used (78%, infliximab 22%). Ninety-three percent of those who completed the survey used TDM, mainly in cases of non-response or loss or response. Most respondents (93% and 83% for adalimumab and infliximab, respectively) measured trough concentrations within 24 h prior to the next administration. In patients in clinical remission but with endoscopic inflammation on anti-TNF agents, 72% would measure drug concentrations. In the presence of anti-drug antibodies, 45% would add an immunomodulator in patients with active disease and 47% would add an immunomodulator in patients in remission. With low trough concentrations, 77% would make no changes if the patient was in remission, and 75% would increase the dose in case of active disease. CONCLUSION: TDM was routinely used among inflammatory bowel disease gastroenterology clinicians who responded to this survey. However, interpretation of results and decision-making is variable, suggesting more guidance is required.

A 1 g dose of intravenous iron is sufficient to treat iron deficiency anaemia.

One hundred and ninety-four patient episodes were audited for response to a standardised 1 g intravenous iron infusion for medical outpatients with iron deficiency anaemia. Patients received either ferric carboxymaltose or iron polymaltose. At 5-7 weeks after infusion, mean increase in Hb was 26.7 g/L and ferritin was 161 mcg/L, and only one patient had Hb <100 g/L. This reassures that 1 g dose of intravenous iron is sufficient for most patients, with benefits for treatment costs and patient convenience.

Clinical decision support in a hospital electronic prescribing system informed by local data: experience at a tertiary New Zealand centre.

BACKGROUND: An electronic prescribing and administration (ePA) system has been progressively rolled out to Canterbury District Health Board (CDHB, Christchurch, New Zealand) public hospitals since 2014, and is currently used for around 1300 tertiary beds. ePA data can be used to monitor user behaviour, and to evaluate and inform the local customisation of clinical decision support (CDS) tools within the ePA system. AIMS: To describe retrospectively illustrative vignettes of CDHB ePA analyses that have been used for CDS. METHODS: Alerts were developed according to a set of common principles agreed upon by the CDHB CDS Working Group. Alerts were informed and evaluated by extracting and parsing data for various time periods during 2016 to 2018 from the CDHB ePA database. RESULTS: There was a median of 74 000 prescriptions a month. After examining 525 spironolactone prescriptions, the high dose alert threshold was set at 100 mg with an expected alert burden of 3%. The presence of a ceftriaxone shortage prescribing alert for 1 week was associated with a prescribing rate that was lower than 95% of the preceding 52 weeks. Following review of 367 fentanyl patch alerts, revision of the alert led to false positives falling from 43% to 3% (P < 0.0001). At the point of firing, 6% of antithrombotic drug interactions alerts led to immediate changes in prescriptions (94% overridden), and a further 22% were changed within 30 min after the alert. CONCLUSIONS: Local data extracts from ePA systems can inform iterative configuration of the software and monitor user behaviour.

Infliximab and adalimumab concentrations and anti-drug antibodies in inflammatory bowel disease control using New Zealand assays.

BACKGROUND: Therapeutic drug monitoring of anti-tumour necrosis factor (TNF) drugs and anti-drug antibodies (ADA) is now recommended in the treatment of inflammatory bowel disease. However, assay types and drug concentration thresholds are still debated. AIM: To correlate inflammatory bowel disease activity in a New Zealand cohort with trough concentrations of infliximab and adalimumab, and ADA using locally developed competitive-binding enzyme-linked immunosorbent assays (ELISA) to establish threshold concentrations. METHODS: Patients with ulcerative colitis (UC) and Crohn disease (CD) from Christchurch and Dunedin on anti-TNF drugs >12 weeks were enrolled. Trough blood samples were assayed for drug and ADA concentrations. Other data included quality of life, blood count, C-reactive protein, albumin, renal function and disease activity indices. RESULTS: Of 103 patients, 53 were on infliximab (36 CD, 15 UC and 2 unclassified) and 50 adalimumab (48 CD and 2 UC). Median (range) infliximab and adalimumab concentrations were 10.5 (0-41) and 9.61 mg/L (0-30). CD remission, Crohn Disease Activity Index <150, correlated with infliximab and adalimumab concentration in CD (infliximab, P = 0.03; adalimumab, P = 0.04), with too few UC patients for analysis. Receiver operator curve analysis suggested a threshold value of 5.1 mg/L for distinguishing active disease from remission for infliximab and 7.3 mg/L for adalimumab in CD. Of 13 patients with infliximab <2 mg/L, 10 were ADA positive by homogeneous mobility shift assay (HMSA), including five with neutralising antibodies using ELISA. Of six with adalimumab <2 mg/L, three were ADA positive using HMSA, including one with neutralising antibodies. CONCLUSION: Using the New Zealand ELISA assay, threshold concentrations of 5 mg/L for infliximab and 7 mg/L for adalimumab are suggested to aid dosing decisions, consistent with results internationally. Both neutralising (ELISA) and non-neutralising ADA (HMSA) are associated with low drug concentrations.

How to prevent allopurinol hypersensitivity reactions?

Allopurinol hypersensitivity syndrome (AHS) is a severe and sometimes life-threatening adverse drug reaction. Although AHS is rare, the number of patients with gout requiring allopurinol is high, and there are sufficient overall cases of AHS to warrant consideration of preventive measures. Most cases occur within 8-9 weeks of commencing allopurinol, and good patient education at initiation may lead to rapid drug cessation at onset of symptoms. Pretreatment testing for HLA-B*5801 and avoidance of allopurinol when positive reduces the risk of AHS and is cost-effective in some ethnic groups. A low starting allopurinol dose may reduce AHS risk, but the relationship between maintenance dose and AHS is more controversial. Chronic kidney disease increases AHS risk, but slowly increasing the allopurinol dose in chronic kidney disease has not been associated with AHS. Alternative newer treatments are available for patients at risk of AHS, but similar adverse reactions can also occur with these.

The impact of diuretic use and ABCG2 genotype on the predictive performance of a published allopurinol dosing tool.

AIM: This research aims to evaluate the predictive performance of a published allopurinol dosing tool. METHODS: Allopurinol dose predictions were compared to the actual dose required to achieve serum urate (SU) <0.36 mmol l-1 using mean prediction error. The influence of patient factors on dose predictions was explored using multilinear regression. RESULTS: Allopurinol doses were overpredicted by the dosing tool; however, this was minimal in patients without diuretic therapy (MPE 63 mg day-1 , 95% CI 40-87) compared to those receiving diuretics (MPE 295 mg day-1 , 95% CI 260-330, P < 0.0001). ABCG2 genotype (rs2231142, G>T) had an important impact on the dose predictions (MPE 201, 107, 15 mg day-1 for GG, GT and TT, respectively, P < 0.0001). Diuretic use and ABCG2 genotype explained 53% of the variability in prediction error (R2  = 0.53, P = 0.0004). CONCLUSIONS: The dosing tool produced acceptable maintenance dose predictions for patients not taking diuretics. Inclusion of ABCG2 genotype and a revised adjustment for diuretics would further improve the performance of the dosing tool.

A randomised controlled trial of the efficacy and safety of allopurinol dose escalation to achieve target serum urate in people with gout.

OBJECTIVES: To determine the efficacy and safety of allopurinol dose escalation using a treat-to-target serum urate (SU) approach. METHODS: A randomised, controlled, parallel-group, comparative clinical trial was undertaken. People with gout receiving at least creatinine clearance (CrCL)-based allopurinol dose for ≥1 month and SU ≥6 mg/dL were recruited. Participants were randomised to continue current dose (control) or allopurinol dose escalation for 12 months. In the dose escalation group, allopurinol was increased monthly until SU was <6 mg/dL. The primary endpoints were reduction in SU and adverse events (AEs). RESULTS: 183 participants (93 control, 90 dose escalation) were recruited. At baseline, mean (SD) urate was 7.15 (1.6) mg/dL and allopurinol dose 269 mg/day. 52% had CrCL<60 mL/min. Mean changes in SU at the final visit were -0.34 mg/dL in the control group and -1.5 mg/dL in the dose escalation group (p<0.001) with a mean difference of 1.2 mg/dL (95% CI 0.67 to 1.5, p<0.001). At month 12, 32% of controls and 69% in the dose escalation had SU <6 mg/dL. There were 43 serious AEs in 25 controls and 35 events in 22 dose escalation participants. Only one was considered probably related to allopurinol. Five control and five dose escalation participants died; none was considered allopurinol related. Mild elevations in LFTs were common in both groups, a few moderate increases in gamma glutamyl transferase (GGT) were noted. There was no difference in renal function changes between randomised groups. CONCLUSIONS: Higher than CrCL-based doses of allopurinol can effectively lower SU to treatment target in most people with gout. Allopurinol dose escalation is well tolerated. TRIAL REGISTRATION NUMBER: ANZCTR12611000845932; Results.

Individualising the dose of allopurinol in patients with gout.

AIMS: The aims of the study were to: 1) determine if a plasma oxypurinol concentration-response relationship or an allopurinol dose-response relationship best predicts the dose requirements of allopurinol in the treatment of gout; and 2) to construct a nomogram for calculating the optimum maintenance dose of allopurinol to achieve target serum urate (SU) concentrations. METHODS: A nonlinear regression analysis was used to examine the plasma oxypurinol concentration- and allopurinol dose-response relationships with serum urate. In 81 patients (205 samples), creatinine clearance (CLCR ), concomitant diuretic use and SU concentrations before (UP ) and during (UT ) treatment were monitored across a range of allopurinol doses (D, 50-700 mg daily). Plasma concentrations of oxypurinol (C) were measured in 47 patients (98 samples). Models (n = 47 patients) and predictions from each relationship were compared using F-tests, r2 values and paired t-tests. The best model was used to construct a nomogram. RESULTS: The final plasma oxypurinol concentration-response relationship (UT = UP - C*(UP - UR )/(ID50 + C), r2  = 0.64) and allopurinol dose-response relationship (UT = UP - D* (UP - UR )/(ID50 + D), r2  = 0.60) did not include CLCR or diuretic use as covariates. There was no difference (P = 0.87) between the predicted SU concentrations derived from the oxypurinol concentration- and allopurinol dose-response relationships. The nomogram constructed using the allopurinol dose-response relationship for all recruited patients (n = 81 patients) required pretreatment SU as the predictor of allopurinol maintenance dose. CONCLUSIONS: Plasma oxypurinol concentrations, CLCR and diuretic status are not required to predict the maintenance dose of allopurinol. Using the nomogram, the maintenance dose of allopurinol estimated to reach target concentrations can be predicted from UP .

A population pharmacokinetic model to predict oxypurinol exposure in patients on haemodialysis.

PURPOSE: The aims of this study were to characterise the population pharmacokinetics of oxypurinol in patients receiving haemodialysis and to compare oxypurinol exposure in dialysis and non-dialysis patients. METHODS: Oxypurinol plasma concentrations from 6 gout people receiving haemodialysis and 19 people with gout not receiving dialysis were used to develop a population pharmacokinetic model in NONMEM. Deterministic simulations were used to predict the steady-state area under the oxypurinol plasma concentration time curve over 1 week (AUC7days). RESULTS: The pharmacokinetics of oxypurinol were best described by a one-compartment model with a separate parameter for dialytic clearance. Allopurinol 100 mg daily produced an AUC7days of 279 µmol/L h in dialysis patients, a value 50-75 % lower than the AUC7days predicted for patients with normal renal function taking 200 to 400 mg daily (427-855 µmol/L h). Dosing pre-dialysis resulted in about a 25-35 % reduction in exposure compared to post-dialysis. CONCLUSIONS: Oxypurinol is efficiently removed by dialysis. The population dialytic and total (non-dialytic) clearance of oxypurinol were found to be 8.23 and 1.23 L/h, standardised to a fat-free mass of 70 kg and creatinine clearance of 6 L/h, respectively. Our results suggest that if the combination of low-dose allopurinol and haemodialysis does not result in sustained urate lowering below treatment targets (serum urate ≤0.36 mmol/L), then allopurinol doses may be increased to optimise oxypurinol exposure.

Deterministic identifiability of population pharmacokinetic and pharmacokinetic-pharmacodynamic models.

Identifiability is an important component of pharmacokinetic-pharmacodynamic (PKPD) model development. Structural identifiability is concerned with the uniqueness of the model parameters for a set of perfect input-output data and deterministic identifiability with the precision of parameter estimation given imperfect input-output data. We introduce two subcategories of deterministic identifiability, external and internal, and consider factors that distinguish between these forms. We define external deterministic identifiability as a function of externally controllable variables, i.e., the design, and internal deterministic identifiability as a function of the model and its parameter values. The concepts are explored using three common PK and PKPD models, and verified for their precision for the selected set of parameter values under optimal design.

Predicting allopurinol response in patients with gout.

AIMS: The primary aim of this research was to predict the allopurinol maintenance doses required to achieve the target plasma urate of ≤0.36 mmol l(-1) . METHODS: A population analysis was conducted in nonmem using oxypurinol and urate plasma concentrations from 133 gout patients. Maintenance dose predictions to achieve the recommended plasma urate target were generated. RESULTS: The urate response was best described by a direct effects model. Renal function, diuretic use and body size were found to be significant covariates. Dose requirements increased approximately 2-fold over a 3-fold range of total body weight and were 1.25-2 fold higher in those taking diuretics. Renal function had only a modest impact on dose requirements. CONCLUSIONS: Contrary to current guidelines, the model predicted that allopurinol dose requirements were determined primarily by differences in body size and diuretic use. A revised guide to the likely allopurinol doses to achieve the target plasma urate concentration is proposed.

Development of an ELISA-Based Competitive Binding Assay for the Analysis of Drug Concentration and Antidrug Antibody Levels in Patients Receiving Adalimumab or Infliximab.

BACKGROUND: There is increasing interest in measuring both drug and antidrug antibody (ADA) levels in patients receiving anti-tumor necrosis factor treatment as part of algorithms for guiding therapeutic strategies. Many of the current assays for ADA detection have limitations with respect to specificity, sensitivity, and/or laboratory requirements. Specific identification of ADA based on their inhibitory activity in a simple competitive binding assay remains problematic. The development of an enzyme-linked immunosorbent assay (ELISA)-based method for detection of both drug and ADA in patients receiving either adalimumab or infliximab would widen availability of monitoring for these patients. METHODS: An ELISA for the specific detection of adalimumab and infliximab using widely available reagents was developed. A simple modification for the detection of ADA capable of competitively inhibiting the in vitro binding of drug to solid phase tumor necrosis factor was also developed. Drug and ADA levels were analyzed in patients with rheumatoid arthritis and inflammatory bowel disease. RESULTS: The ELISA specifically detected drug concentrations in patient sera with no evidence of positive or negative interference by rheumatoid factor positive control sera. A subset of those patients with low drug concentrations had detectable levels of ADA with inhibitory activity in a competitive binding assay. Spiking with both drugs confirmed the specificity of the ADA detected. CONCLUSIONS: A modified ELISA protocol can be used to for the detection of both drug concentrations and ADA in patients receiving either adalimumab or infliximab. The ELISA incorporates those features identified in the literature as important for the accurate analysis of these antibodies and uses laboratory facilities and reagents that are widely available. It therefore provides a relatively simple and low cost assay for therapeutic drug monitoring of inpatients receiving adalimumab or infliximab.

Smoking behaviour modifies IL23r-associated disease risk in patients with Crohn's disease.

BACKGROUND AND AIM: The etiology of Crohn's disease (CD) implicates both genetic and environmental factors. Smoking behavior is one environmental risk factor to play a role in the development of CD. The study aimed to assess the contribution of the interleukin 23 receptor (IL23R) in determining disease susceptibility in two independent cohorts of CD, and to investigate the interactions between IL23R variants, smoking behavior, and CD-associated genes, NOD2 and ATG16L1. METHODS: Ten IL23R single-nucleotide polymorphisms (SNPs) were genotyped in 675 CD cases, and 1255 controls from Brisbane, Australia (dataset 1). Six of these SNPs were genotyped in 318 CD cases and 533 controls from Canterbury, New Zealand (dataset 2). Case-control analysis of genotype and allele frequencies, and haplotype analysis for all SNPs was conducted. RESULTS: We demonstrate a strong increased CD risk for smokers in both datasets (odds ratio 3.77, 95% confidence interval 2.88-4.94), and an additive interaction between IL23R SNPs and cigarette smoking. Ileal involvement was a consistent marker of strong SNP-CD association (P ≤ 0.001), while the lowest minor allele frequencies for location were found in those with colonic CD (L2). Three haplotype blocks were identified across the 10 IL23R SNPs conferring different risk of CD. Haplotypes conferred no further risk of CD when compared with single SNP analyses. CONCLUSION: IL23R gene variants determine CD susceptibility in the Australian and New Zealand population, particularly ileal CD. A strong additive interaction exists between IL23R SNPs and smoking behavior resulting in a dramatic increase in disease risk depending upon specific genetic background.

Comparison of intracellular methotrexate kinetics in red blood cells with the kinetics in other cell types.

AIM: To assess the similarities in intracellular pharmacokinetics (PK) of methotrexate (MTX) in red blood cells (RBCs) and other cell lines. METHODS: Three previously published PK models for intracellular MTX and MTX polyglutamate (MTXGlu2-5 ) concentrations were used: (i) a model for the kinetics in RBCs, (ii) a model for the kinetics in human breast cancer cells (HBCCs) and (iii) a model for the kinetics in various white blood cell (WBC) lines. All three models were used to simulate the response in a typical individual receiving 10 mg oral MTX once weekly and the predicted steady-state concentrations (Css ) and time to Css (tss ) were compared. RESULTS: The HBCC model showed a lower Css for MTXGlu2 and 3 and higher Css for MTXGlu4 and 5 compared with the RBC PK model, while tss and overall intracellular MTX exposure appeared similar. The WBC PK model showed much lower Css for the parent MTXGlu1 and of tss for all MTXGlun , as well as a much lower cumulative Css for MTXGlu2-7 for the majority of the WBC cell lines. CONCLUSION: RBC kinetics of MTX differ from the kinetics in other cell types such as WBCs and HBCCs to a variable degree. It is possible that similarly diverse profiles may exist across other cell lines, including those on the causal path in rheumatoid arthritis. Hence, there may not necessarily be a clear link between RBC MTX concentrations and disease control in rheumatoid arthritis.

Coagulation assays and plasma fibrinogen concentrations in real-world patients with atrial fibrillation treated with dabigatran.

AIMS: In patients with atrial fibrillation prescribed dabigatran, the aim was to examine the correlation between plasma dabigatran concentrations and the three screening coagulation assays [international normalized ratio (INR), activated partial thromboplastin time (aPTT) and thrombin time (TT)] as well as the dilute thrombin time (dTT) and to examine the contribution of plasma fibrinogen concentrations to the variability in TT results. METHODS: Plasma from patients with atrial fibrillation on dabigatran were analysed for clotting times and concentrations of fibrinogen and dabigatran. Correlation plots (and associated r(2) values) were generated using these data. The variability in TT results explained by fibrinogen concentrations was quantified using linear regression. RESULTS: Fifty-two patients (38-94 years old) contributed 120 samples, with plasma dabigatran concentrations ranging from 9 to 408 µg l(-1) . The r(2) values of INR, aPTT, TT and dTT against plasma dabigatran concentrations were 0.49, 0.54, 0.70 and 0.95, respectively. Plasma fibrinogen concentrations explained some of the residual variability in TT values after taking plasma dabigatran concentrations into account (r(2) = 0.12, P = 0.02). CONCLUSIONS: Of the screening coagulation assays, the TT correlated best with plasma dabigatran concentrations. Variability in fibrinogen concentrations accounts for some of the variability in the TT.

Thirteen years' experience of pharmacokinetic monitoring and dosing of busulfan: can the strategy be improved?

BACKGROUND: A busulfan concentration monitoring and dosing service has been provided by Christchurch Hospital since 1998. This study aimed to see (1) the percentage of patients with an area under the concentration time curve (AUC) outside the target range and had dose adjustment, (2) how busulfan clearance (CL) relates to body weight, and (3) if fewer samples could be used to predict doses. METHODS: Blood samples were taken from patients after oral administration, usually at 0.5, 1, 1.5, and 6 hours, and after the start of a 2-hour intravenous (IV) infusion of busulfan, at 1, 2, 2.5, 3, 6, and 8 hours. Dose adjustment was made based on the AUC compared with the target range. The relationship of CL and body weight for the IV group was used to develop a revised IV dosing schedule. The bias and imprecision of AUCs estimated using fewer sampling points were examined to see if sampling could be economized. RESULTS: Data were available for 150 patients but for 6 patients, data were incomplete and excluded. Of the remaining 144 patients (256 sample sets, 209 oral, 47 IV, 62% with repeats), 38% (IV) and 35% (oral) of patients had AUCs within the target range after the first dose. Dose adjustment was made in 47% and 34% of patients dosed IV and orally, respectively, after which there was a trend to more patients achieving the target AUC. A nonlinear relationship was found between CL and body weight. The initial IV dosing schedule was revised to take this into account. Sampling for busulfan concentration measurement at 3 points (2.5, 4, 8 hours) or 2 points (2.5, 8 hours) after the start of the infusion enabled accurate and precise estimates of AUC0₋24. CONCLUSIONS: Around two thirds of patients treated with busulfan were outside the target AUC range after the first dose. Dose adjustment was made in 37% of patients. The relationship between CL and body weight was used to revise the initial IV dosing schedule. Sampling for AUC estimation could be reduced to 2 time points after IV dosing.