Population pharmacokinetic modelling of febuxostat in healthy subjects and people with gout.

AIMS: To investigate and characterise the pharmacokinetics of febuxostat and the effect of the covariates of renal function and body size descriptors on the pharmacokinetics of the drug. METHODS: Blood samples (n = 239) were collected using sparse and rich sampling strategies from healthy (n = 9) and gouty (n = 29) subjects. Febuxostat plasma concentrations were measured by a validated high-performance liquid chromatography method. Population pharmacokinetic analysis was performed using NONMEM. A common variability on bioavailability (FVAR) approach was used to test the effect of fed status on absorption parameters. Covariates were modelled using a power model. RESULTS: The time course of the plasma concentrations of febuxostat is best described by a two-compartment model. In the final model, the population mean for apparent clearance (CL/F), apparent central volume of distribution (Vc/F), apparent peripheral volume of distribution (Vp/F), absorption rate constant (ka) and apparent intercompartmental clearance (Q/F) were 6.91 l h-1 , 32.8 l, 19.4 l, 3.6 h-1 and 1.25 l h-1 , respectively. The population parmater variability (coefficient of variation) for CL/F, Vc/F and Vp/F were 13.6, 22 and 19.5%, respectively. Food reduced the relative biovailability and ka by 67% and 87%, respectively. Renal function, as assessed by creatinine clearance, was a significant covariate for CL/F while body mass index was a significant covariate for Vc/F. CONCLUSIONS: Renal function and body mass index were significant covariates. Further work is warranted to investigate the clinical relevance of these results, notably as renal impairment and obesity are common occurrences in people with gout.

Human Group IIA Phospholipase A2-Three Decades on from Its Discovery.

Phospholipase A2 (PLA2) enzymes were first recognized as an enzyme activity class in 1961. The secreted (sPLA2) enzymes were the first of the five major classes of human PLA2s to be identified and now number nine catalytically-active structurally homologous proteins. The best-studied of these, group IIA sPLA2, has a clear role in the physiological response to infection and minor injury and acts as an amplifier of pathological inflammation. The enzyme has been a target for anti-inflammatory drug development in multiple disorders where chronic inflammation is a driver of pathology since its cloning in 1989. Despite intensive effort, no clinically approved medicines targeting the enzyme activity have yet been developed. This review catalogues the major discoveries in the human group IIA sPLA2 field, focusing on features of enzyme function that may explain this lack of success and discusses future research that may assist in realizing the potential benefit of targeting this enzyme. Functionally-selective inhibitors together with isoform-selective inhibitors are necessary to limit the apparent toxicity of previous drugs. There is also a need to define the relevance of the catalytic function of hGIIA to human inflammatory pathology relative to its recently-discovered catalysis-independent function.

Limitations of drug concentrations used in cell culture studies for understanding clinical responses of NSAIDs.

In this review, the in vitro cellular effects of six nonsteroidal anti-inflammatory drugs (NSAIDs), salicylate, ibuprofen, naproxen, indomethacin, celecoxib and diclofenac, are examined. Inhibition of prostanoid synthesis in vitro generally occurs within the therapeutic range of plasma concentrations that are observed in vivo, consistent with the major action of NSAIDs being inhibition of prostanoid production. An additional probable cellular action of NSAIDs has been discovered recently, viz. decreased oxidation of the endocannabinoids, 2-arachidonoyl glycerol and arachidonyl ethanolamide. Many effects of NSAIDs, other than decreased oxidation of arachidonic acid and endocannabinoids, have been put forward but almost all of these additional processes are observed at supratherapeutic concentrations when the concentration of albumin, the major protein that binds NSAIDs, is taken into account. However, one exception is salicylate, a very potent inhibitor of the neutrophilic enzyme, myeloperoxidase, the inhibition of which leads to reduced production of the inflammatory mediator, hypochlorous acid, and inhibition of the inflammation associated with rheumatoid arthritis.

Assessing the accuracy of two Bayesian forecasting programs in estimating vancomycin drug exposure.

BACKGROUND: Current guidelines for intravenous vancomycin identify drug exposure (as indicated by the AUC) as the best pharmacokinetic (PK) indicator of therapeutic outcome. OBJECTIVES: To assess the accuracy of two Bayesian forecasting programs in estimating vancomycin AUC0-∞ in adults with limited blood concentration sampling. METHODS: The application of seven vancomycin population PK models in two Bayesian forecasting programs was examined in non-obese adults (n = 22) with stable renal function. Patients were intensively sampled following a single (1000 mg or 15 mg/kg) dose. For each patient, AUC was calculated by fitting all vancomycin concentrations to a two-compartment model (defined as AUCTRUE). AUCTRUE was then compared with the Bayesian-estimated AUC0-∞ values using a single vancomycin concentration sampled at various times post-infusion. RESULTS: Optimal sampling times varied across different models. AUCTRUE was generally overestimated at earlier sampling times and underestimated at sampling times after 4 h post-infusion. The models by Goti et al. (Ther Drug Monit 2018. 40: 212-21) and Thomson et al. (J Antimicrob Chemother 2009. 63: 1050-7) had precise and unbiased sampling times (defined as mean imprecision <25% and <38 mg·h/L, with 95% CI for mean bias containing zero) between 1.5 and 6 h and between 0.75 and 2 h post-infusion, respectively. Precise but biased sampling times for Thomson et al. were between 4 and 6 h post-infusion. CONCLUSIONS: When using a single vancomycin concentration for Bayesian estimation of vancomycin drug exposure (AUC), the predictive performance was generally most accurate with sample collection between 1.5 and 6 h after infusion, though optimal sampling times varied across different population PK models.

A pharmacokinetic-pharmacodynamic study of a single dose of febuxostat in healthy subjects.

AIMS: To examine the pharmacokinetic-phamacodynamic (PK-PD) relationships of plasma febuxostat and serum urate and the effect of a single dose of the drug on renal excretion and fractional clearance of urate (FCU). METHODS: Blood and urine samples were collected at baseline and up to 145 hours following administration of febuxostat (80 mg) to healthy subjects (n = 9). Plasma febuxostat and serum and urinary urate and creatinine concentrations were determined. Febuxostat pharmacokinetics were estimated using a two-compartment model with first-order absorption. An Emax PK-PD model was fitted to mean febuxostat and urate concentrations. Urinary urate excretion and FCU were calculated pre- and post-dose. RESULTS: Maximum mean plasma concentration of febuxostat (2.7 mg L-1 ) was observed 1.2 hours after dosage. Febuxostat initial and terminal half-lives were 2.0 ± 1.0 and 14.0 ± 4.7 hours (mean ± SD), respectively. The majority (81%) of the drug was eliminated in the 9 hours after dosing. Serum urate declined slowly achieving mean nadir (0.20 mmol L-1 ) at 24 hours. The IC50 (plasma febuxostat concentration that inhibits urate production by 50%) was 0.11 ± 0.09 mg L-1 (mean ± SD). Urinary urate excretion changed in parallel with serum urate. There was no systematic or significant change in FCU from baseline. CONCLUSION: The PK-PD model could potentially be used to individualise febuxostat treatment and improve clinical outcomes. A single dose of febuxostat does not affect the efficiency of the kidney to excrete urate. Further investigations are required to confirm the present results following multiple dosing with febuxostat.

The safety and pharmacokinetics of metformin in patients with chronic liver disease.

BACKGROUND: The FDA approved 'label' for metformin lists hepatic insufficiency as a risk for lactic acidosis. Little evidence supports this warning. AIMS: To investigate the safety and pharmacokinetics of metformin in patients with chronic liver disease (CLD). METHODS: Chronic liver disease patients with and without type 2 diabetes mellitus (T2DM) were studied by a cross-sectional survey of patients already prescribed metformin (n = 34), and by a prospective study where metformin (500 mg, immediate release, twice daily) for up to 6 weeks was prescribed (n = 24). Plasma metformin and lactate concentrations were monitored. Individual pharmacokinetics were obtained and compared to previously published values from healthy and T2DM populations without CLD. RESULTS: All plasma metformin and lactate concentrations remained below the putative safety thresholds (metformin, 5 mg/L; lactate, 5 mmol/L). Lactate concentrations were unrelated to average steady-state metformin concentrations. In patients with CLD, T2DM was associated with higher plasma lactate concentrations (48% higher than those without T2DM, P < 0.0001). CLD patients with cirrhosis had 23% higher lactate concentrations than those without cirrhosis (P = 0.01). The pharmacokinetics of metformin in CLD patients were similar to patients with T2DM and no liver disease. The ratio of apparent metformin clearance (CLMet /F) to creatinine clearance was marginally lower in CLD patients compared to healthy subjects (median, interquartile range; 12.6, 9.5-15.9 vs 14.9, 13.4-16.4; P = 0.03). CONCLUSIONS: The pharmacokinetics of metformin are not altered sufficiently in CLD patients to raise concerns regarding unsafe concentrations of metformin. There were no unsafe plasma lactate concentrations observed in CLD patients receiving metformin (ACTRN12619001292167; ACTRN12619001348145).

Comparison of the Area Under the Curve for Vancomycin Estimated Using Compartmental and Noncompartmental Methods in Adult Patients With Normal Renal Function.

BACKGROUND: Vancomycin pharmacokinetics are best described using a 2-compartment model. However, 1-compartment population models are commonly used as the basis for dose prediction software. Therefore, the validity of using a 1-compartment model to guide vancomycin drug dosing was examined. METHODS: Published plasma concentration-time data from adult subjects (n = 30) with stable renal function administered a single intravenous infusion of vancomycin were extracted from previous studies. The vancomycin area under the curve (AUC0-∞) was calculated for each subject using noncompartmental methods (AUCNCA) and by fitting 1- (AUC1CMT), 2- (AUC2CMT), and 3- (AUC3CMT) compartment infusion models. The optimal model fit was determined using the Akaike information criterion and visual inspection of the residual plots. The individual compartmental AUC0-∞ values from the 1- and 2-compartment models were compared with AUCNCA values using one-way repeated measures analysis of variance. RESULTS: The mean (±SD) AUC estimates were similar for the different methods: AUCNCA 180 ± 86 mg·h/L, AUC1CMT 167 ± 79 mg·h/L, and AUC2CMT 183 ± 88 mg·h/L. Despite the overlapping AUC values, AUC2CMT and AUCNCA were significantly greater than AUC1CMT (P < 0.05). The 3-compartment model was excluded from the analysis because of the failure to converge in some instances. CONCLUSIONS: Dose prediction software using a 1-compartment model as the basis for Bayesian forecasting underestimates drug exposure (estimated as the AUC) by less than 10%. This is unlikely to be clinically significant with respect to dose adjustment. Therefore, a 1-compartment model may be sufficient to guide vancomycin dosing in adult patients with stable renal function.

Is the use of metformin in patients undergoing dialysis hazardous for life? A systematic review of the safety of metformin in patients undergoing dialysis.

AIMS: Metformin may have clinical benefits in dialysis patients; however, its safety in this population is unknown. This systematic review evaluated the safety of metformin in dialysis patients. METHODS: MEDLINE, Embase, CENTRAL, PsycINFO and the Cochrane Library were searched for randomised controlled trials and observational studies evaluating metformin use in dialysis patients. Three authors reviewed the studies and extracted data. The primary outcomes were mortality, occurrence of lactic acidosis and myocardial infarction (MI) in patients taking metformin during dialysis treatment for ≥12 months (long term). Risk of bias was assessed using Risk Of Bias In Nonrandomised Studies of Interventions (ROBINS-1). Overall quality of evidence was assessed using Grading of Recommendations Assessment, Development and Evaluation (GRADE). RESULTS: Fifteen observational studies were eligible; 7 were prospective observational studies and 8 were case reports/case series. No randomised controlled trials were identified. The 7 prospective observational studies (n = 194) reported on cautious metformin use in patients undergoing maintenance dialysis. Only 3 provided long-term follow-up data. In 2 long-term studies of metformin therapy (≤1000 mg/d) in patients undergoing peritoneal dialysis (PD), 1 reported 6 deaths (6/83; 7%) due to major cardiovascular events (3 MI) and the other reported no deaths (0/35). One long-term study of metformin therapy (250 mg to 500 mg thrice weekly) in patients undergoing haemodialysis reported 4 deaths (4/61; 7%) due to major cardiovascular events (2 MI). These findings provide very low-quality evidence as they come from small observational studies. CONCLUSION: The evidence regarding the safety of metformin in people undergoing dialysis is inconclusive. Appropriately designed randomised controlled trials are needed to resolve this uncertainty.

Determination of febuxostat in human plasma by high performance liquid chromatography (HPLC) with fluorescence-detection.

Febuxostat prevents gout attacks by lowering serum urate. Aspects of the pharmacokinetic-pharmacodynamic relationship of febuxostat concentrations to urate in gout patients need further elucidation. In order to undertake these studies, the assay methodology for febuxostat has been enhanced and validated to meet FDA standards. An HPLC method with fluorescence-detection has been modified to increase sensitivity, reduce complexity, shorten the sample preparation process and improve the inter-day coefficient of variation of the lowest quality control sample (0.03 µg/L). Protein in plasma samples (200 µL) is now precipitated with acetonitrile (400 µL) containing the internal standard (2-naphthoic acid). The supernatant is analysed at excitation and emission wavelengths of 320 and 380 nm, respectively as in the previous method. A Luna C18 column (Phenomenex, Australia) at 40 °C with mobile phase of glacial acetic acid (0.032%) in acetonitrile:water (60:40, v:v), an injection volume of 10 µL and a flow rate of 1.5 mL/min is employed. Analysis time is 8 min. Calibration curves in drug-free plasma range from 0.005 to 10.00 µg/mL. Data points are fitted using linear regression with a weighting factor of 1/concentration. The inter-day accuracy and imprecision of the quality control samples (0.0075, 0.015, 3.00 and 9.80 µg/mL) is 90-115% and ≤ 14.5%, respectively.

Restarting antidepressant and antipsychotic medication after intentional overdoses: need for evidence-based guidance.

Intentional drug overdoses with antidepressant and antipsychotic medications are an increasingly common problem. Currently, there is little guidance with regard to reintroduction of these medications after intentional overdoses. We have used published toxicological and pharmacokinetic data to obtain factors which control the recovery from overdoses. From such data, we have proposed guidance regarding their reintroduction, provided there are no adverse effects or contraindications. Tentatively, we suggest that when adverse effects from the overdose are lost, treatment could recommence after a further mean half-life of elimination. Most antidepressant and antipsychotic drugs are metabolized by cytochrome P450 enzymes and, where cytochrome P450 inhibitors are co-ingested, serial plasma concentrations should optimally be obtained in order to assess a suitable time for reintroduction of the psychoactive drugs. We hope the proposals presented will stimulate research and discussion that lead to better guidance for clinicians concerning reintroduction of psychoactive medication after intentional overdose.

Predicting Response or Non-response to Urate-Lowering Therapy in Patients with Gout.

PURPOSE OF REVIEW: To review the extent of treatment success or failure with the xanthine oxidoreductase inhibitors allopurinol and febuxostat and indicate how the dosage of urate-lowering therapy (ULT) may be modified to increase the response in the majority of patients with gout. RECENT FINDINGS: Gout flares are associated with serum concentrations of urate above 0.42 mmol/L (7 mg/dL). Achieving and maintaining serum urate below 0.36 mmol/L is considered an effective response to ULT. On an intention to treat basis, clinical trials indicate that allopurinol at daily doses of 100 to 300 mg decreases serum urate adequately in only about 40% of gout patients while febuxostat 80 mg daily reduces serum urate adequately in approximately 70% of gout patients. Higher doses of ULT may be required in patients receiving concomitant diuretics. The addition of a uricosuric agent to allopurinol and febuxostat therapy significantly increases the proportion of patients achieving adequate lowering of serum urate. Finally, carriers of a genetic variant of the transporter, ABCG2 (BCRP), have a decreased response to allopurinol. Careful examination of medication adherence, titration of doses, and the addition of uricosuric agents increase the percentage of patients responding to allopurinol and febuxostat.

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 .

Allopurinol: insights from studies of dose-response relationships.

INTRODUCTION: Gout is the most common inflammatory arthritis in men and is increasingly prevalent. Allopurinol is very effective at reducing plasma urate concentrations to a level sufficient to dissolve monosodium urate crystals. However, many patients fail to achieve a sufficient therapeutic response to allopurinol. Areas covered: This review covers the metabolism and pharmacokinetics of allopurinol and its active metabolite, oxypurinol and how these factors affect the plasma concentrations of urate at initiation and during long-term therapy with allopurinol. Significant aspects discussed are the importance of adherence to allopurinol therapy, allopurinol hypersensitivity reactions and insights into hyperuricemia. Expert opinion: The initial dosage of allopurinol should be low, particularly in patients with renal impairment. The dose should then be increased slowly until plasma concentrations of urate are sufficient to dissolve monosodium urate crystals (≤ 0.36 mmol/L). For this target, the maintenance dose of allopurinol can be estimated from the equation: Dose = 1413*(Up-0.36) where Up is the pre-treatment concentration of urate. Poor adherence is a major factor limiting successful therapy with allopurinol; however, its use can be improved considerably by education of patients and clinicians. Allopurinol is generally well tolerated and screening for genetic factors predictive of allopurinol hypersensitivity reactions can now be undertaken.

Clinical Pharmacokinetics and Pharmacodynamics of Febuxostat.

Febuxostat is a xanthine oxidoreductase inhibitor that has been developed to treat chronic gout. In healthy subjects, the pharmacokinetic parameters of febuxostat after multiple oral dose administration include an oral availability of about 85 %, an apparent oral clearance (CL/F) of 10.5 ± 3.4 L/h and an apparent volume of distribution at steady state (V ss/F) of 48 ± 23 L. The time course of plasma concentrations follows a two-compartment model. The initial half-life (t ½) is approximately 2 h and the terminal t ½ determined at daily doses of 40 mg or more is 9.4 ± 4.9 h. Febuxostat is administered once daily. The maximum (peak) plasma concentrations are approximately 100-fold greater than the trough concentrations. Consequently, there is no significant accumulation of the drug during multiple dose administration. There are few data on the pharmacokinetics of febuxostat in patients with gout. While the pharmacokinetic parameters are not affected by mild to moderate hepatic impairment, there is no consensus on whether renal impairment has any effect on the pharmacokinetics of febuxostat. Febuxostat is extensively metabolised by oxidation (approximately 35 %) and acyl glucuronidation (up to 40 %); febuxostat acyl glucuronides are cleared by the kidney. In healthy subjects treated with multiple doses of febuxostat 10-240 mg, the concentrations of serum urate are reduced by a maximum of about 80 %. The percentage reduction in the concentrations of serum urate is slightly less in gouty patients than in healthy subjects.

Xanthine oxidoreductase and its inhibitors: relevance for gout.

Xanthine oxidoreductase (XOR) is the rate-limiting enzyme in purine catabolism and converts hypoxanthine to xanthine, and xanthine into uric acid. When concentrations of uric acid exceed its biochemical saturation point, crystals of uric acid, in the form of monosodium urate, emerge and can predispose an individual to gout, the commonest form of inflammatory arthritis in men aged over 40 years. XOR inhibitors are primarily used in the treatment of gout, reducing the formation of uric acid and thereby, preventing the formation of monosodium urate crystals. Allopurinol is established as first-line therapy for gout; a newer alternative, febuxostat, is used in patients unable to tolerate allopurinol. This review provides an overview of gout, a detailed analysis of the structure and function of XOR, discussion on the pharmacokinetics and pharmacodynamics of XOR inhibitors-allopurinol and febuxostat, and the relevance of XOR in common comorbidities of gout.

Hyperuricaemia: contributions of urate transporter ABCG2 and the fractional renal clearance of urate.

OBJECTIVE: To investigate the contributions towards hyperuricaemia of known risk factors, focusing on fractional (renal) clearance of urate (FCU) and variation in the ATP-binding cassette transporter, sub-family G 2 (ABCG2) gene. METHODS: The contributions of age, sex, ancestry, Q141K genotype for ABCG2, FCU, sugar-sweetened beverage and alcohol consumption, metabolic syndrome disorders and measures of renal function to the risk of hyperuricaemia were evaluated by comparing hyperuricaemic (serum urate≥0.42 mmol/L, n=448) with normouricaemic (serum urate<0.42 mmol/L, n=344) participants using stepwise logistic regression. Model performance was evaluated using the area under the receiver operator characteristic curve (AUROC). RESULTS: ABCG2 genotype, FCU, male sex, body mass index, serum triglyceride concentrations, estimated glomerular filtration rate and consumption of alcohol were the best predictors of hyperuricaemia (AUROC 0.90, 81% accuracy). Homozygosity in the 141K variant for ABCG2 conferred an adjusted OR of 10.5 for hyperuricaemia (95% CI 2.4 to 46.2). For each 1% decrease of FCU, the adjusted OR increased by 51% (OR 1.51, 95% CI 1.37 to 1.66). There was no association between ABCG2 genotype and FCU (r=0.02, p=0.83). CONCLUSIONS: The ABCG2 141K variant and the FCU contribute strongly but independently to hyperuricaemia. These findings provide further evidence for a significant contribution of ABCG2 to extra-renal (gut) clearance of urate.