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*(U_p-0.36) where U_p 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.     

Shape and steepness of toxicological dose–response relationships of continuous endpoints

A re-analysis of a large number of historical dose–response data for continuous endpoints indicates that an exponential or a Hill model with four parameters adequately describes toxicological dose–responses. No exceptions were found for the datasets considered, which related to a wide variety of endpoints and to both in vivo and in vitro studies of various types. For a given endpoint/study type dose–response shapes were found to be homogenous among chemicals in the in vitro studies considered, while a mild among-chemical variation in the steepness parameter seemed to be present in the in vivo studies. Our findings have various practical consequences. For continuous endpoints, model selection in the BMD approach is not a crucial issue. The often applied approach of using constraints on the model parameters to prevent “infinite” slopes at dose zero in fitting a model is not in line with our findings, and appears to be unjustified. Instead, more realistic ranges of parameter values could be derived from re-analyses of large numbers of historical dose–response datasets in the same endpoint and study type, which could be used as parameter constraints in future individual datasets. This approach will be particularly useful for weak datasets (e.g. few doses, much scatter). In addition, this approach may open the way to use fewer animals in future studies. In the discussion, we argue that distinctions between linear, sub/supralinear or thresholded dose–response shapes, based on visual inspection of plots, are not biologically meaningful nor useful for risk assessment.     

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

Aims

The aim of the study was to identify and quantify factors that control the plasma concentrations of urate during allopurinol treatment and to predict optimal doses of allopurinol.

Methods

Plasma concentrations of urate and creatinine (112 samples, 46 patients) before and during treatment with various doses of allopurinol (50–600 mg daily) were monitored. Non-linear and multiple linear regression equations were used to examine the relationships between allopurinol dose (D), creatinine clearance (CL_cr) and plasma concentrations of urate before (U_P) and during treatment with allopurinol (U_T).

Results

Plasma concentrations of urate achieved during allopurinol therapy were dependent on the daily dose of allopurinol and the plasma concentration of urate pre-treatment. The non-linear equation: U_T = (1 – D/(ID₅₀ + D)) × (U_P – U_R) + U_R, fitted the data well (r² = 0.74, P < 0.0001). The parameters and their best fit values were: daily dose of allopurinol reducing the inhibitable plasma urate by 50% (ID₅₀ = 226 mg, 95% CI 167, 303 mg), apparent resistant plasma urate (U_R = 0.20 mmol l⁻¹, 95 % CI 0.14, 0.25 mmol l⁻¹). Incorporation of CL_cr did not significantly improve the fit (P = 0.09).

Conclusions

A high baseline plasma urate concentration requires a high dose of allopurinol to reduce plasma urate below recommended concentrations. This dose is dependent on only the pre-treatment plasma urate concentration and is not influenced by CL_cr.     

Enzyme therapy for the treatment of type 1 Gaucher disease: clinical outcomes and dose – response relationships

Background: Enzyme therapy for Gaucher disease has improved the lives of many patients, by reducing the burden of their disease. Several studies have sought to determine to what extent optimal clinical outcomes are a function of the prescribed enzyme dose and its resultant costs. Objective: Issues concerning dose – response relationships during initial and maintenance treatment phases and currently applied treatment goals have been addressed. Methods: All studies that aimed to describe the efficacy of different doses of treatment and approaches for maintenance, such as lowering the dose or changing to less frequent infusions and the effects of drug interruptions, were reviewed. Results/conclusion: Dose – response relationships do exist, but doses between 30 and 60 U/kg per month may be sufficient in a large majority of patients. Goals of treatment should include important clinical end points, such as enhanced quality of life and decreased risk for malignancies and other morbidities. The relationship between these important end points and treatment schedules deserve further study.     

Muller’s Nobel lecture on dose–response for ionizing radiation: ideology or science?

In his Nobel Prize Lecture of December 12, 1946, Hermann J. Muller argued that the dose–response for radiation-induced germ cell mutations was linear and that there was “no escape from the conclusion that there is no threshold”. However, assessment of correspondence between Muller and Curt Stern 1 month prior to his Nobel Prize Lecture reveals that Muller knew the results and implications of a recently completed study at the University of Rochester under the direction of Stern, which directly contradicted his Nobel Prize Lecture. This finding is of historical importance since Muller’s Nobel Lecture gained considerable international attention and is a turning point in the acceptance of the linearity model in risk assessment for germ cell mutations and carcinogens.     

Processing of facial affect in social drinkers: a dose–response study of alcohol using dynamic emotion expressions

Rationale

Attention dysfunction is the hallmark of cognitive deficits associated with major psychiatric illnesses including schizophrenia. Cognitive deficits of schizophrenia have been attributed to reduced function of the N-methyl-d-aspartate (NMDA) receptor or reduced expression of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase-67, which presumably leads to attenuated neurotransmission at GABA_A receptors.

Objective

The present study used a rodent model to compare the inhibition of NMDA and GABA_A receptors, and GAD activity on attention. We tested the impact of inhibiting these proteins brain wide or in the anterior cingulate cortex (ACC), a prefrontal cortex region critical for attentional processing.

Methods

Rats were trained on the three choice serial reaction time task (3-CSRT), an attention test. The impact of systemic or intra-ACC injection of drugs on performance was measured in well-trained rats.

Results

Reducing GABA_A receptor function within the ACC with the direct antagonist SR95531 (1 or 3 ng/side) or brain wide using systemic injection of the benzodiazepine inverse agonist FG7142 (5 mg/kg) impaired accuracy and increased omissions. Systemic or intra-ACC inhibition of NMDA receptors using MK-801 (at 3 mg/kg or 3 μg, respectively) also impaired performance. Inhibition of GAD with 3-mercaptopropionic acid, even at high doses, had no effect on 3-CSRT accuracy or omissions when administered systemically or within the ACC.

Conclusions

These data demonstrate that, while tonic stimulation of NMDA and GABA_A receptors within the ACC are critical for attentional performance, reduction in GAD activity may have little functional significance and is not indicative of reduced GABA neurotransmission.     

Isobolographic analysis of interaction between drugs with nonparallel dose–response relationship curves: a practical application

The objective of this study was to characterize the anticonvulsant and acute adverse-effect potentials of topiramate (TPM) and gabapentin (GBP)—two second-generation antiepileptic drugs administered alone and in combination in the maximal electroshock (MES)-induced seizures and chimney test in mice. The anticonvulsant and acute adverse effects of the combination of TPM with GBP at the fixed ratio of 1:1 were determined using the type I isobolographic analysis for nonparallel dose–response relationship curves (DRRCs). To ascertain any pharmacokinetic contribution to the observed interaction between TPM and GBP, total brain concentrations of both drugs were determined. The isobolographic analysis of interaction for TPM and GBP, whose DRRCs were not parallel in both MES and chimney tests, was accompanied with a presentation of all required calculations allowing the determination of lower and upper lines of additivity. The isobolographic analysis revealed that TPM combined with GBP at the fixed-ratio combination of 1:1 interacted supraadditively (synergistically) in terms of suppression of MES-induced seizures, and simultaneously, the combination produced additive interaction with respect to motor coordination impairment (adverse effects) in the chimney test. The evaluation of pharmacokinetic characteristics of interaction for the combination of TPM with GBP revealed that neither TPM nor GBP affected their total brain concentrations in experimental animals, and thus, the observed interaction in the MES test was pharmacodynamic in nature. In conclusion, the combination of TPM with GBP, because of supraadditivity in the MES test and additivity in terms of motor coordination impairment in the chimney test as well as lack of pharmacokinetic interactions between drugs, fulfilled the criterion of a favorable combination, worthy of recommendation in further clinical practice.     

Limitations in dose–response and surrogate species methodologies for risk assessment of Cry toxins on arthropod natural enemies

Dose–response assays and surrogate species are standard methods for risk analysis for environmental chemicals. These assume that individuals within a species have unimodal responses and that a surrogate species can predict responses of other related taxa. We exposed immature individuals of closely related aphidophagous coccinellid predators, Cycloneda sanguinea and Harmonia axyridis, to Cry1Ac and Cry1F toxins through uniform and constant artificial tritrophic exposure through Myzus persicae aphids. Both toxins were detected in coccinellid pupae, with individual and interspecific variation. Uptake was significantly higher in H. axyridis than in C. sanguinea, both in the proportion of individuals and the concentrations per individual. We also observed bimodal uptake of the Cry toxins by H. axyridis, which indicated that some individuals had low bioaccumulation and some had high bioaccumulation. This suggests that standard dose–response assays need to be interpreted with caution and future assays should examine the modality of the responses. In addition, the similarity in the biological effects of the Cry toxins in the two predators was due to different biological exposure mechanisms. The majority of H. axyridis were exposed both internally and in the gut, while C. sanguinea was exposed primarily in the gut. Thus, despite their close phylogenetic relatedness, these species would not be good surrogates for each other and the surrogate species methodology should be tested more rigorously.     

Metabolomics of silver nanoparticles toxicity in HaCaT cells: structure–activity relationships and role of ionic silver and oxidative stress

The widespread use of silver nanoparticles (AgNPs) is accompanied by a growing concern regarding their potential risks to human health, thus calling for an increased understanding of their biological effects. The aim of this work was to systematically study the extent to which changes in cellular metabolism were dependent on the properties of AgNPs, using NMR metabolomics. Human skin keratinocytes (HaCaT cells) were exposed to citrate-coated AgNPs of 10, 30 or 60?nm diameter and to 30?nm AgNPs coated either with citrate (CIT), polyethylene glycol (PEG) or bovine serum albumin (BSA), to assess the influence of NP size and surface chemistry. Overall, CIT-coated 60?nm and PEG-coated 30?nm AgNPs had the least impact on cell viability and metabolism. The role of ionic silver and reactive oxygen species (ROS)-mediated effects was also studied, in comparison to CIT-coated 30?nm particles. At concentrations causing an equivalent decrease in cell viability, Ag^+?ions produced a change in the metabolic profile that was remarkably similar to that seen for AgNPs, the main difference being the lesser impact on the Krebs cycle and energy metabolism. Finally, this study newly reported that while down-regulated glycolysis and disruption of energy production were common to AgNPs and H₂O₂, the impact on some metabolic pathways (GSH synthesis, glutaminolysis and the Krebs cycle) was independent of ROS-mediated mechanisms. In conclusion, this study shows the ability of NMR metabolomics to define subtle biochemical changes induced by AgNPs and demonstrates the potential of this approach for rapid, untargeted screening of pre-clinical toxicity of nanomaterials in general.     

Critical review of dose–response options for F344 rat mammary tumors for acrylamide – Additional insights based on mode of action

Previous risk assessment reviews analyzed the potential for dietary acrylamide to increase breast cancer risk. Here, we critically review acrylamide animal bioassay data on mammary tumors for human relevance. We applied a systematic evaluation using reasonable standards of scientific certainty and a systematic weight of evidence (WOE) approach to evaluate several hypothesized modes of action (MOA), including (1) genotoxicity related to glycidamide formation and oxidative stress, (2) endocrine effects due to age-related hyperprolactinemia or secondary to neurotoxicity, and (3) epigenetic effects. We conclude that the appropriate approach for low-dose extrapolation of the rat mammary tumors can be narrowed to two options: (1) linear low-dose extrapolation (i.e., based on a MOA of mutagenicity from direct DNA interaction) from a point of departure (POD) for the combined incidence of adenomas and adenocarcinomas, since these tumor types are related; or (2) non-linear extrapolation, using uncertainty factors to estimate a Reference Dose (RfD) from a POD for tumor promotion derived using the combined fibroadenoma, adenoma and adenocarcinoma data. Non-linear extrapolation is used in the latter approach because these combined tumor types are unlikely to be exclusively caused by mutagenicity. Comparison of the WOE for each alternative MOA indicates that a non-linear approach (option 2) is more appropriate for evaluation of acrylamide-induced mammary tumors; a linear approach (option 1) is shown for comparison.     

Information contributed by meta-analysis in exposure–response modeling: application to phase 2 dose selection of guselkumab in patients with moderate-to-severe psoriasis

Ustekinumab, a human immunoglobulin G1 kappa (IgG1κ) monoclonal antibody that binds with high affinity to human interleukin (IL)-12 and IL-23, has been approved to treat patients with psoriasis. Guselkumab is a related human IgG1 monoclonal antibody in clinical development which specifically blocks IL-23. The objective of this study was to study the exposure–response relationship of guselkumab to guide dose selection for a Phase 2 study in patients with moderate-to-severe psoriasis. Data were available from a Phase 1 study of 47 healthy subjects and 24 patients with psoriasis who received various doses of guselkumab. Disease severity was assessed using Psoriasis Area and Severity Index (PASI) scores in all studies. Individual pharmacokinetic parameters were derived from population pharmacokinetics modeling for the purpose of exposure–response modeling to guide dosing regimen selection. A population mechanism-based exposure–response model of guselkumab was developed to evaluate the association of guselkumab dosing with PASI scores using a Type I indirect response model, with placebo effect empirically modeled. The model was subsequently updated, first by incorporating data from psoriasis patients who received placebo (n = 765) and from patients actively treated with ustekinumab 45 or 90 mg (n = 1,230) in two ustekinumab Phase 3 trials. Inclusion of this additional ustekinumab data and the consequent contributions to specific model components substantially reduced uncertainties in all model components except for one parameter. Additional sensitivity analyses showed that the dose selection decision was robust to this remaining uncertainty. The described approach underscores the importance of utilizing all available sources of information in dose selection decisions, along with the importance of effective development team interaction.     

Acquisition of cocaine self-administration in male Sprague–Dawley rats: effects of cocaine dose but not initial locomotor response to cocaine

Rationale  

We have previously described a model in which adult outbred male Sprague–Dawley rats are classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced open-field activation. This model revealed important individual differences in cocaine's effects, including that LCRs exhibited greater responding than HCRs on a progressive ratio schedule of cocaine reinforcement. However, no LCR/HCR differences in acquisition of cocaine self-administration (0.25 mg/kg/12 s infusion) were observed under these conditions.     

Effect of grapefruit juice dose on grapefruit juice–triazolam interaction: repeated consumption prolongs triazolam half-life

Objective: Grapefruit juice inhibits CYP3A4-mediated metabolism of several drugs during first pass. In this study, the effect of grapefruit juice dose on the extent of grapefruit juice–triazolam interaction was investigated. Methods: In a randomised, four-phase, crossover study, 12 healthy volunteers received 0.25 mg triazolam with water, with 200 ml normal-strength or double-strength grapefruit juice or, on the third day of multiple-dose [three times daily (t.i.d.)] administration of double-strength grapefruit juice. Timed blood samples were collected up to 23 h after dosing, and the effects of triazolam were measured with four psychomotor tests up to 10 h after dosing. Results: The area under the plasma triazolam concentration–time curve (AUC_0–∞) was increased by 53% (P < 0.01), 49% (P < 0.01) and 143% (P < 0.001) by a single dose of normal-strength, a single dose of double-strength and multiple-dose administration of double-strength grapefruit juice, respectively. The peak plasma concentration (C_max) of triazolam was increased by about 40% by a single dose of normal-strength grapefruit juice (P < 0.01) and multiple-dose grapefruit juice (P < 0.01) and by 25% by a single dose of double-strength grapefruit juice (P < 0.05). The elimination half-life (t _1/2) of triazolam was prolonged by 54% during the multiple-dose grapefruit juice phase (P < 0.001). A significant increase in the pharmacodynamic effects of triazolam was seen during the multiple-dose grapefruit juice phase in the digit symbol substitution test (DSST, P < 0.05), in subjective overall drug effect (P < 0.05) and in subjective drowsiness (P < 0.05). Conclusions: Even one glass of grapefruit juice increases plasma triazolam concentrations, but repeated consumption of grapefruit juice produces a significantly greater increase in triazolam concentrations than one glass of juice. Thet _1/2 of triazolam is prolonged by repeated consumption of grapefruit juice, probably due to inhibition of hepatic CYP3A4 activity. Received: 30 December 1999 / Accepted in revised form: 11 April 2000     

A triazolam/amphetamine dose–effect interaction study: dissociation of effects on memory versus arousal

Rationale In addition to producing robust memory impairment, benzodiazepines also induce marked sedation. Thus, it is possible that the observed amnestic effects are secondary to more global sedative effects and do not reflect a specific primary benzodiazepine effect on memory mechanisms. Objective The objective was to use the nonspecific stimulant d-amphetamine to dissociate the sedative and memory-impairing effects of the benzodiazepine triazolam. Materials and methods Single oral doses of placebo, triazolam alone (0.25, 0.50 mg/70 kg), d-amphetamine sulfate alone (20, 30 mg/70 kg), and triazolam (0.25, 0.50 mg/70 kg) and d-amphetamine sulfate (20, 30 mg/70 kg) conjointly (at all dose combinations) were administered to 18 healthy adult participants across nine sessions in a double-blind, staggered-dosing, crossover design. In addition to standard data analyses, analyses were also conducted on z-score standardized data, enabling effects to be directly compared across measures. Results Relative to the sedative measures, the memory measures generally exhibited a pattern of less reversal of triazolam’s effects by d-amphetamine. The memory measures ranged in degree of reversal such that the most reversal was observed for reaction time on the n-back working memory task, and the least reversal was observed for accuracy on the Sternberg working memory task, with most measures showing an overall pattern of partial reversal. Conclusions Benzodiazepines have specific effects on memory that are not merely a by-product of the drugs’ sedative effects, and the degree to which sedative effects contribute to the amnestic effects varies as a function of the particular memory process being assessed.     

Different behavioral effect dose–response profiles in mice exposed to two-carbon chlorinated hydrocarbons: Influence of structural and physical properties

The present study aimed to clarify whether dose–response profiles of acute behavioral effects of 1,2-dichloroethane (DCE), 1,1,1-trichloroethane (TCE), trichloroethylene (TRIC), and tetrachloroethylene (PERC) differ. A test battery involving 6 behavioral endpoints was applied to evaluate the effects of DCE, TCE, TRIC, and PERC in male ICR strain mice under the same experimental conditions. The behavioral effect dose–response profiles of these compounds differed. Regression analysis was used to evaluate the relationship between the dose–response profiles and structural and physical properties of the compounds. Dose–response profile differences correlated significantly with differences in specific structural and physical properties. These results suggest that differences in specific structural and physical properties of DCE, TCE, TRIC, and PERC are responsible for differences in behavioral effects that lead to a variety of dose–response profiles.