Understanding the dose-response relationship of allopurinol: predicting the optimal dosage

A Genome-Wide Association Study of Oxypurinol Concentrations in Patients Treated with Allopurinol

Cohort studies have identified several genetic determinants that could predict the clinical response to allopurinol. However, they have not been commonly used for genome-wide investigations to identify genetic determinants on allopurinol metabolism and concentrations. We conducted a genome-wide association study of a prior cross-sectional investigation of patients from the Montreal Heart Institute Biobank undergoing allopurinol therapy. Four endpoints were investigated, namely plasma concentrations of oxypurinol, the active metabolite of allopurinol, allopurinol, and allopurinol-riboside, as well as allopurinol daily dosing. A total of 439 participants (mean age 69.4 years; 86.4% male) taking allopurinol (mean daily dose 194.5 mg) and who had quantifiable oxypurinol concentrations were included in the genome-wide analyses. Participants presented with multiple comorbidities and received concomitant cardiovascular medications. No association achieved the predefined genome-wide threshold values for any of the endpoints (all p > 5 × 10-8). Our results are consistent with prior findings regarding the difficulty in identifying genetic determinants of drug concentrations or pharmacokinetics of allopurinol and its metabolites, as well as allopurinol daily dosing. Given the size of this genome-wide study, collaborative investigations involving larger and diverse cohorts may be required to further identify pharmacogenomic determinants of allopurinol and measure their clinical relevance to personalize allopurinol therapy.

The development and evaluation of dose-prediction tools for allopurinol therapy (Easy-Allo tools)

AIMS

Dose escalation at the initiation of allopurinol therapy can be protracted and resource intensive. Tools to predict the allopurinol doses required to achieve target serum urate concentrations would facilitate the implementation of more efficient dose-escalation strategies. The aim of this research was to develop and externally evaluate allopurinol dosing tools, one for use when the pre-urate-lowering therapy serum urate is known (Easy-Allo1) and one for when it is not known (Easy-Allo2).

METHODS

A revised population pharmacokinetic-pharmacodynamic model was developed using data from 653 people with gout. Maintenance doses to achieve the serum urate target of <0.36 mmol L-1 in >80% of individuals were simulated and evaluated against external data. The predicted and observed allopurinol doses were compared using the mean prediction error (MPE) and root mean square error (RMSE). The proportion of Easy-Allo predicted doses within 100 mg of the observed was quantified.

RESULTS

Allopurinol doses were predicted by total body weight, baseline urate, ethnicity and creatinine clearance. Easy-Allo1 produced unbiased and suitably precise dose predictions (MPE 2 mg day-1 95% confidence interval [CI] -13-17, RMSE 91%, 90% within 100 mg of the observed dose). Easy-Allo2 was positively biased by about 70 mg day-1 and slightly less precise (MPE 70 mg day-1 95% CI 52-88, RMSE 131%, 71% within 100 mg of the observed dose).

CONCLUSIONS

The Easy-Allo tools provide a guide to the allopurinol maintenance dose requirement to achieve the serum urate target of <0.36 mmol L-1 and will aid in the development of novel dose-escalation strategies for allopurinol therapy.

Predictors of Inadequate Serum Urate Response to Low-Dose Febuxostat in Male Patients with Gout

Objective

This study aimed to understand predictors of inadequate response (IR) to low-dose febuxostat treatment based on clinical variables.

Methods

We pooled data from 340 patients of an observational cohort and two clinical trials who received febuxostat 20 mg/day for at least 3 months. IR was defined as failure to reach the target serum urate level (sUA<6 mg/dL) at any time point during 3 months treatment. The potential predictors associated with short- or mid-term febuxostat IR after pooling the three cohorts were explored using mixed-effect logistic analysis. Machine learning models were performed to evaluate the predictors for IR using the pooled data as the discovery set and validated in an external test set.

Results

Of the 340 patients, 68.9% and 51.8% were non-responders to low-dose febuxostat during short- and mid-term follow-up, respectively. Serum urate and triglyceride (TG) levels were significantly associated with febuxostat IR, but were also selected as significant features by LASSO analysis combined with age, BMI, and C-reactive protein (CRP). These five features in combination, using the best-performing stochastic gradient descent classifier, achieved an area under the receiver operating characteristic curve of 0.873 (95% CI [0.763, 0.942]) and 0.706 (95% CI [0.636, 0.727]) in the internal and external test sets, respectively, to predict febuxostat IR.

Conclusion

Response to low-dose febuxostat is associated with early sUA improvement in individual patients, as well as patient age, BMI, and levels of TG and CRP.

Moving urate-lowering therapy in gout beyond guideline recommendations

The 'treat-to target serum urate strategy' when using urate-lowering therapy has been recommended by most specialist rheumatology societies for many years. An alternative "treat-to-avoid-symptoms" in gout has been suggested, albeit without a clear definition of what this means and how it might be implemented in clinical trials or clinical practice. This has hampered efforts to design clinical trials that compare the "treat-to-target [urate]" and "treat-to-avoid-symptoms" strategies in the long-term management of gout. In this review we consider the rationale for the treat-to-target urate strategy when using urate-lowering therapy, potential definitions of a "treat-to-avoid-symptoms" strategy, or perhaps what is not "treat-to-avoid-symptoms", and approaches that might address this uncertainty.

Population pharmacokinetics, pharmacodynamics and pharmacogenetics modelling of oxypurinol in Hmong adults with gout and/or hyperuricemia

AIMS

The aim of this study was to quantify identifiable sources of variability, including key pharmacogenetic variants in oxypurinol pharmacokinetics and their pharmacodynamic effect on serum urate (SU).

METHODS

Hmong participants (n = 34) received 100 mg allopurinol twice daily for 7 days followed by 150 mg allopurinol twice daily for 7 days. A sequential population pharmacokinetic pharmacodynamics (PKPD) analysis with non-linear mixed effects modelling was performed. Allopurinol maintenance dose to achieve target SU was simulated based on the final PKPD model.

RESULTS

A one-compartment model with first-order absorption and elimination best described the oxypurinol concentration-time data. Inhibition of SU by oxypurinol was described with a direct inhibitory Emax model using steady-state oxypurinol concentrations. Fat-free body mass, estimated creatinine clearance and SLC22A12 rs505802 genotype (0.32 per T allele, 95% CI 0.13, 0.55) were found to predict differences in oxypurinol clearance. Oxypurinol concentration required to inhibit 50% of xanthine dehydrogenase activity was affected by PDZK1 rs12129861 genotype (-0.27 per A allele, 95% CI -0.38, -0.13). Most individuals with both PDZK1 rs12129861 AA and SLC22A12 rs505802 CC genotypes achieve target SU (with at least 75% success rate) with allopurinol below the maximum dose, regardless of renal function and body mass. In contrast, individuals with both PDZK1 rs12129861 GG and SLC22A12 rs505802 TT genotypes would require more than the maximum dose, thus requiring selection of alternative medications.

CONCLUSIONS

The proposed allopurinol dosing guide uses individuals' fat-free mass, renal function and SLC22A12 rs505802 and PDZK1 rs12129861 genotypes to achieve target SU.

Persistence with urate-lowering therapy in Australia: A longitudinal analysis of allopurinol prescriptions

AIM

Gout is the most common form of inflammatory arthritis in men. Despite the availability of effective urate-lowering therapies (ULT), the management of gout is suboptimal due to poor persistence with ULT. This study examined national prescribing patterns of ULT to determine persistence with allopurinol in Australia.

METHODS

A 10% sample of the Australian Pharmaceutical Benefits Scheme dispensing claims database was used to identify individuals initiated on allopurinol between April 2014 and December 2019. The number of allopurinol scripts dispensed was used to estimate persistence with allopurinol. Persistence was defined as the number of months from initiation until discontinuation (last prescription with no further scripts acquired for a period thereafter). Kaplan-Meier curves were used to examine persistence, while Cox regression analysis was used to examine the influence of gender, concomitant colchicine and age.

RESULTS

The largest drop in persistence occurred immediately after initiation, with 34% of patients discontinuing allopurinol 300-mg therapy in the first month. Median persistence with allopurinol 300 mg was 5 months (95% confidence interval 4.76-5.24), with around 63% of individuals not persisting with this therapy for more than 12 months. Concomitant prescription of colchicine on the day of allopurinol initiation only occurred in 7% of allopurinol initiations. No increase in persistence was observed for those co-prescribed colchicine.

CONCLUSION

Persistence with allopurinol was poor. More effective methods targeting prescribers, patients and systems are required to promote persistence with allopurinol. Improving persistence to allopurinol is an important public health goal given the proven potential of this medication to eliminate gout.

Critical appraisal of serum urate targets in the management of gout

Gout management involves two broad aspects: treatment of gout flares to provide rapid symptomatic relief and long-term urate-lowering therapy to lower serum urate sufficiently to prevent gout flares from occurring. All of the major rheumatology societies recommend a target serum urate of <5 mg/dl (<0.30 mmol/l) or <6 mg/dl (<0.36 mmol/l), both of which are below the point of saturation for urate and therefore lead to monosodium urate crystal dissolution. In this Review, we describe the rationale for treat-to-target urate approach in the long-term management of gout and the current evidence and controversy around the appropriate serum urate targets.

Honorary Professor Garry Graham

An appreciation of the contribution of Professor Gary Graham to anti-inflammatory and antirheumatic pharmacology and clinical pharmacology.

Better outcomes for patients with gout

Gout is increasing in prevalence despite effective pharmacotherapies. Barriers to effective management are largely educational deficiencies. Sufferers, usually men, need to understand more about gout, especially that maintaining serum urate below 0.36 mmol/L will eliminate recurrent attacks. Also, of great importance is appreciating that sub-optimal adherence to urate-lowering therapy (ULT) will result in a return of attacks. Prescribers also need to understand that acute attacks are likely to occur in the first few months of urate-lowering therapy (ULT), but these can be mitigated by commencing with a dose of ULT reflective of renal function and escalating the dose slowly, every 2-5 weeks until target serum urate is achieved. Prophylaxis against acute attacks over the initial 6 months period of ULT can be enhanced further with concomitant colchicine or nonsteroidal anti-inflammatory drugs (NSAIDs).Gout is largely managed in primary care. Rates of adherence to ULT are 50% or less, worse than most other chronic illnesses. Efforts at educating primary care physicians to, firstly, manage gout effectively and, secondly, to educate their gout patients sufficiently have not been successful. Allied health practitioners, such as nurses, working with prescribers in primary care settings and given the mandate to educate and manage patients with gout, have been spectacularly effective. However, this approach is resource intensive. 'Personalised' eHealth interventions show promise as an alternative strategy, notably in improving adherence to ULT.Numerous applications for smart phones (apps) are now available to assist people with chronic health conditions. Their design needs to accommodate the barriers and enablers perceived by patients to maintaining adherence to prescribed therapies. Personalised feedback of serum urate may represent an important enabler of adherence to ULT in the case of gout.Harnessing mobile apps to support patients managing their chronic illnesses represents an important opportunity to enhance health outcomes. Rigorous, patient-centred and driven development is critical. These tools also require careful evaluation for effectiveness.

Relationships Between Allopurinol Dose, Oxypurinol Concentration and Urate-Lowering Response-In Search of a Minimum Effective Oxypurinol Concentration

The aims of this study were to determine factors that predict serum urate (SU) lowering response to allopurinol and the conversion of allopurinol to oxypurinol, and to determine a minimum therapeutic oxypurinol concentration. Data from 129 participants in a 24-month open, randomized, controlled, parallel-group, comparative clinical trial were analyzed. Allopurinol dose, SU, and plasma oxypurinol concentrations were available at multiple time points. The slope for the association between allopurinol dose and SU was calculated as a measure of sensitivity to allopurinol. The slope for the association between allopurinol dose and oxypurinol was calculated as a measure of allopurinol metabolism. Receiver operating characteristic (ROC) curves were used to identify a minimum oxypurinol concentration predictive of SU < 6 mg/dL. There was a wide range of SU concentrations for each allopurinol dose. The relationship between sensitivity to allopurinol and allopurinol metabolism for each 100 mg allopurinol dose increase varied between individuals. Body mass index (P = 0.023), creatinine clearance (CrCL; P = 0.037), ABCG2 Q141K (P = 0.019), and SU (P = 0.004) were associated with sensitivity to allopurinol. The minimum oxypurinol concentration for achieving the urate target was found to be about 104 μmol/L, but predictive accuracy was poor (ROC curve area under the curve (AUC) 0.65). The minimum therapeutic oxypurinol concentration was found to increase with decreasing renal function. Although there is a positive relationship between change in oxypurinol and change in SU concentration, a minimum therapeutic oxypurinol is dependent on CrCL and cannot reliably predict SU target. Other variables, including ABCG2 Q141K genotype, impact on sensitivity to allopurinol (ACTRN12611000845932).

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.

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.

Current and future therapies for gout

INTRODUCTION

Gout is a common disease responsible for recurrent flares triggered by the deposition of monosodium urate crystals secondary to longstanding hyperuricaemia. The management of gout implies both the treatment of flares and the treatment of hyperuricaemia itself. Recent improvement in the understanding of the disease led to the development of new drugs. Areas covered: This review covers data related to 'old' treatments of flares and hyperuricaemia, evidence on the recently approved drugs and emerging therapies in development. Expert opinion: Recent data provide a good grasp of the optimal use of colchicine, corticosteroids and NSAIDs for the treatment of flares. Interleukin-1 blocking therapies have an increasing role in the management of difficult-to-treat gout. Sub-optimal use of allopurinol is common and its potency to reduce serum uric acid (SUA) levels is underestimated. Febuxostat effectively reduces SUA levels. New uricosurics, notably lesinurad and arhalofenate, in combination with xanthine oxidase inhibitors, offer promising perspectives to help a greater number of patients achieve sufficient SUA reduction.

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 (CL_CR ), concomitant diuretic use and SU concentrations before (U_P ) and during (U_T ) 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 (U_T  = U_P  - C*(U_P  - U_R )/(ID₅₀  + C), r²  = 0.64) and allopurinol dose-response relationship (U_T  = U_P  - D^* (U_P  - U_R )/(ID₅₀  + D), r²  = 0.60) did not include CL_CR 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, CL_CR 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 U_P .

The management of gout

Gout is a common inflammatory arthritis that is increasing in prevalence. It is caused by the deposition of urate crystals. Non-steroidal anti-inflammatory drugs, colchicine and corticosteroids are options for the management of acute gout. They are equally efficacious and comorbidities guide the best choice. Allopurinol is an effective treatment for reducing concentrations of uric acid. Renal function guides the starting dose of allopurinol and the baseline serum uric acid concentration guides the maintenance dose. Febuxostat is another xanthine oxidase inhibitor. It is clinically equivalent to allopurinol. Uricosuric drugs, such as probenecid, increase uric acid excretion. New drugs in this class will soon become available and are likely to have a role in the treatment of patients who do not respond to other drugs.

The Budget Impact of Increased Use of Febuxostat in the Management of Gout: A US Health Plan Managed Care Pharmacy and Medical Costs Perspective

PURPOSE

Gout is a chronic disease characterized by the deposition of urate crystals in the joints and throughout the body, caused by an excess burden of serum uric acid (sUA). The study estimates pharmacy and medical cost budgetary impacts of wider adoption by US payers of febuxostat, a urate-lowering therapy (ULT) for the treatment of gout.

METHODS

A US payer-perspective budget impact model followed ULT patients from a 1,000,000-member plan over 3 years. The current market share scenario, febuxostat (6%) and ULT allopurinol (94%), was compared with an 18% febuxostat market share. Data were implemented from randomized controlled trials, census and epidemiologic studies, and real-world database analyses. An innovation was the inclusion of gout-related chronic kidney disease costs. Cost results were estimated as annual and cumulative incremental costs, expressed as total costs, cost per member per month, and cost per treated member per month. Clinical results were also estimated.

FINDINGS

Increasing the febuxostat market share resulted in a 6.3% increase in patients achieving the sUA target level of <6.0 mg/dL and a 1.4% reduction in gout flares during the 3-year period. Total cost increased 1.4%, with a 49.9% increase in ULT costs, a 1.4% reduction in flare costs, a 1.2% reduction in chronic kidney disease costs, and a 2.8% reduction in gout care costs. The cumulative incremental costs were $1,307,425 in the first year, $1,939,016 through the second year, and $2,092,744 through the third year. By the third year, savings in medical costs offset most of the increase in treatment costs. Impacts on cumulative cost per member per month and cumulative cost per treated member per month followed the same pattern, with the highest impact in the first year and cumulative impacts declining during the 3-year period. The cumulative cost per member per month impact was estimated as $0.109, $0.081, and $0.058 and the cumulative cost per treated member per month impact was estimated as $12.416, $9.207, and $6.625 in the first year, through the second year, and through the third year, respectively.

IMPLICATIONS

Expanding the febuxostat market share would result in improved clinical outcomes, but with an overall increase in costs over 3 years due to higher costs of treatment. By the third year, savings in medical costs, primarily in chronic kidney disease costs, would offset most of the increase in treatment costs. Expanded use of febuxostat in the treatment of all gout patients, independent of renal impairment status, should be considered based on improved clinical outcomes and longer-term medical cost savings associated with these improved outcomes.

ABCG2 loss-of-function polymorphism predicts poor response to allopurinol in patients with gout

Many patients fail to achieve the recommended serum urate (SU) target (<6 mgdl^-1) with allopurinol. The aim of our study was to examine the association of ABCG2 with SU target in response to standard doses of allopurinol using a cohort with confirmed adherence. Good response was defined as SU<6 mgdl^-1 on allopurinol ⩽300 mgd^-1 and poor response as SU⩾6 mgdl^-1 despite allopurinol >300 mgd^-1. Adherence was confirmed by oxypurinol concentrations. ABCG2 genotyping was performed using pre-designed single nucleotide polymorphism (SNP) TaqMan assays. Of 264 patients, 120 were good responders, 68 were poor responders and 76 were either non-adherent or could not be classified. The minor allele of ABCG2 SNP rs2231142 conferred a significantly increased risk of poor response to allopurinol (odds ratio=2.71 (1.70-4.48), P=6.0 × 10^-5). This association remained significant after adjustment for age, sex, body mass index, ethnicity, estimated glomerular filtration rate, diuretic use and SU off urate-lowering therapy. ABCG2 rs2231142 predicts poor response to allopurinol, as defined by SU⩾6 mgdl^-1 despite allopurinol >300 mgd^-1.

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.

Impaired response or insufficient dosage? Examining the potential causes of "inadequate response" to allopurinol in the treatment of gout

OBJECTIVES

Gout is one of the most common forms of arthritis. It is well established that urate-lowering therapy that aims for a serum urate less than at least 0.36 mmol/l (6 mg/dl) is required for the successful management of gout. Allopurinol, a xanthine oxidase (XO) inhibitor, is the most commonly used urate-lowering therapy. However, many patients fail to achieve the target serum urate on allopurinol; these patients can be considered to have "inadequate response" to allopurinol. Herein, we examine the potential mechanisms and implications of inadequate response to allopurinol.

METHODS

The literature was reviewed for potential causes for failure to reach target serum urate in patients receiving allopurinol.

RESULTS

The two most common causes of inadequate response to allopurinol are poor adherence and under-dosing of allopurinol. Adherent patients who fail to achieve target serum urate on standard doses of allopurinol form a group that could be considered to be "partially resistant" to allopurinol. There are four potential mechanisms for partial allopurinol resistance: decreased conversion of allopurinol to oxypurinol; increased renal excretion of oxypurinol; abnormality in XO structure and/or function such that oxypurinol is rendered less effective and/or drug interactions.

CONCLUSIONS

It is important to determine the reasons for failure to achieve treatment targets with allopurinol, particularly as newer agents become available. The knowledge of the mechanisms for inadequate response may help guide the clinician towards making a therapeutic choice that is more likely to result in achieving the serum urate target.

Pharmacokinetics considerations for gout treatments

INTRODUCTION

Patients with gout often have comorbid conditions such as renal failure, cardiovascular disease and metabolic syndrome. The presence and required treatment of these conditions can make the treatment of gout challenging. Knowledge of the pharmacokinetics of the available drugs for the management of gout is mandatory.

AREAS COVERED

A MEDLINE PubMed search for articles published in English from January 1990 to January 2014 was completed using the terms: pharmacokinetics, colchicine, canakinumab, allopurinol, febuxostat, pegloticase, gout, toxicity, drug interaction.

EXPERT OPINION

Colchicine is a drug with a narrow therapeutic-toxicity window. Co-prescription with strong CYP3A4 or P-glycoprotein inhibitors can greatly modify its pharmacokinetics and is to be avoided. Elimination of canakinumab mainly occurs via intracellular catabolism, following receptor mediator endocytosis. Canakinumab appears to be a good alternative for patients with contraindications to colchicine, NSAIDs and corticosteroids. For patients with renal impairment, some authors recommend that the allopurinol maximum dosage should be adjusted to creatinine clearance. If the urate target cannot be achieved, the therapy should be switched to febuxostat, which is appropriate with mild-to-moderate renal failure. Anti-pegloticase antibodies affect the pharmacokinetics of the drug because they increase its clearance, with loss of pegloticase activity.

Safety and efficacy of allopurinol in chronic kidney disease

OBJECTIVE

To review the evidence surrounding the use of allopurinol in chronic kidney disease (CKD) and discuss safety and efficacy considerations of such use.

DATA SOURCES

A literature search was conducted through MEDLINE (1950-July 2013), PubMed (1965-July 2013), and International Pharmaceutical Abstracts (1970-July 2013) using the search terms allopurinol and kidney or renal. In addition, reference citations from publications identified were reviewed.

STUDY SELECTION AND DATA EXTRACTION

All articles in English identified from the data sources were evaluated for inclusion.

DATA SYNTHESIS

Gout management with allopurinol in patients with CKD can be challenging because of the risk of adverse events and uncertain efficacy. Not all gout treatment guidelines provide recommendations for allopurinol use specifically in patients with CKD. Literature regarding the safety and efficacy of dosing allopurinol in CKD has shown inconsistent results and is based primarily on retrospective, case cohort or observational data. Some trials have demonstrated an increased risk of allopurinol-induced adverse reactions in patients with CKD, whereas others have not confirmed renal insufficiency as a risk factor. More CKD patients achieved a target uric acid level in studies where the allopurinol dose was titrated to effect as compared with those studies in which patients were given renally adjusted or untitrated allopurinol doses.

CONCLUSIONS

Studies evaluating allopurinol use in patients with CKD have reported inconsistent findings relative to safety and efficacy. Providers should be aware of the potential risk of allopurinol hypersensitivity syndrome as well as the need for reducing the initiation dose and gradual titration of allopurinol to safely achieve a target serum urate level in this population.