Pharmacokinetics and metabolism of allopurinol riboside

Allopurinol Disrupts Purine Metabolism to Increase Damage in Experimental Colitis

Inflammatory bowel disease (IBD) is marked by a state of chronic energy deficiency that limits gut tissue wound healing. This energy shortfall is partially due to microbiota dysbiosis, resulting in the loss of microbiota-derived metabolites, which the epithelium relies on for energy procurement. The role of microbiota-sourced purines, such as hypoxanthine, as substrates salvaged by the colonic epithelium for nucleotide biogenesis and energy balance, has recently been appreciated for homeostasis and wound healing. Allopurinol, a synthetic hypoxanthine isomer commonly prescribed to treat excess uric acid in the blood, inhibits the degradation of hypoxanthine by xanthine oxidase, but also inhibits purine salvage. Although the use of allopurinol is common, studies regarding how allopurinol influences the gastrointestinal tract during colitis are largely nonexistent. In this work, a series of in vitro and in vivo experiments were performed to dissect the relationship between allopurinol, allopurinol metabolites, and colonic epithelial metabolism and function in health and during disease. Of particular significance, the in vivo investigation identified that a therapeutically relevant allopurinol dose shifts adenylate and creatine metabolism, leading to AMPK dysregulation and disrupted proliferation to attenuate wound healing and increased tissue damage in murine experimental colitis. Collectively, these findings underscore the importance of purine salvage on cellular metabolism and gut health in the context of IBD and provide insight regarding the use of allopurinol in patients with IBD.

From Benznidazole to New Drugs: Nanotechnology Contribution in Chagas Disease

Chagas disease is a neglected tropical disease caused by the protozoan Trypanosoma cruzi. Benznidazole and nifurtimox are the two approved drugs for their treatment, but both drugs present side effects and efficacy problems, especially in the chronic phase of this disease. Therefore, new molecules have been tested with promising results aiming for strategic targeting action against T. cruzi. Several studies involve in vitro screening, but a considerable number of in vivo studies describe drug bioavailability increment, drug stability, toxicity assessment, and mainly the efficacy of new drugs and formulations. In this context, new drug delivery systems, such as nanotechnology systems, have been developed for these purposes. Some nanocarriers are able to interact with the immune system of the vertebrate host, modulating the immune response to the elimination of pathogenic microorganisms. In this overview of nanotechnology-based delivery strategies for established and new antichagasic agents, different strategies, and limitations of a wide class of nanocarriers are explored, as new perspectives in the treatment and monitoring of Chagas disease.

Lack of Clinical Pharmacokinetic Studies to Optimize the Treatment of Neglected Tropical Diseases: A Systematic Review

INTRODUCTION

Neglected tropical diseases (NTDs) affect more than one billion people, mainly living in developing countries. For most of these NTDs, treatment is suboptimal. To optimize treatment regimens, clinical pharmacokinetic studies are required where they have not been previously conducted to enable the use of pharmacometric modeling and simulation techniques in their application, which can provide substantial advantages.

OBJECTIVES

Our aim was to provide a systematic overview and summary of all clinical pharmacokinetic studies in NTDs and to assess the use of pharmacometrics in these studies, as well as to identify which of the NTDs or which treatments have not been sufficiently studied.

METHODS

PubMed was systematically searched for all clinical trials and case reports until the end of 2015 that described the pharmacokinetics of a drug in the context of treating any of the NTDs in patients or healthy volunteers.

RESULTS

Eighty-two pharmacokinetic studies were identified. Most studies included small patient numbers (only five studies included >50 subjects) and only nine (11 %) studies included pediatric patients. A large part of the studies was not very recent; 56 % of studies were published before 2000. Most studies applied non-compartmental analysis methods for pharmacokinetic analysis (62 %). Twelve studies used population-based compartmental analysis (15 %) and eight (10 %) additionally performed simulations or extrapolation. For ten out of the 17 NTDs, none or only very few pharmacokinetic studies could be identified.

CONCLUSIONS

For most NTDs, adequate pharmacokinetic studies are lacking and population-based modeling and simulation techniques have not generally been applied. Pharmacokinetic clinical trials that enable population pharmacokinetic modeling are needed to make better use of the available data. Simulation-based studies should be employed to enable the design of improved dosing regimens and more optimally use the limited resources to effectively provide therapy in this neglected area.

Addition of Allopurinol for Altering Thiopurine Metabolism to Optimize Therapy in Patients with Inflammatory Bowel Disease

Thiopurine drugs, including azathioprine and 6-mercaptopurine, are used commonly in patients with inflammatory bowel disease for maintenance of remission. Although generally well tolerated, adverse effects lead to discontinuation in a significant minority of patients. Pharmacogenomic studies have suggested that metabolic breakdown of azathioprine in an individual is genetically determined. Coupled with the fact that certain thiopurine metabolites, notably 6-thioguanine nucleotide and 6-methylmercaptopurine, are associated with antiinflammatory effects and adverse effects, respectively, some investigators have examined intentionally shunting the metabolism of azathioprine toward increasing 6-thioguanine nucleotide levels by using low doses of the xanthine oxidoreductase inhibitor allopurinol to improve efficacy and decrease toxicity of azathioprine in patients with inflammatory bowel disease. We performed a search of the MEDLINE and Embase databases for basic and clinical research reports of this modality. Pertinent articles were retrieved, reviewed, and assessed by the authors. Case series, cohort studies, and one randomized trial have investigated adding allopurinol to azathioprine therapy in patients with inflammatory bowel disease. Most reports primarily examined metabolite levels in these patients. In general, the literature suggests that this modality was successful at significantly increasing 6-thioguanine nucleotide levels while decreasing 6-methylmercaptopurine levels. Several small reports have suggested that patients with increased 6-thioguanine nucleotide levels had improved symptoms or symptom remission. Adverse effects and discontinuation rates remained similar or were improved in patients who were taking a thiopurine and started allopurinol. In conclusion, the addition of allopurinol may be an option for optimizing thiopurine metabolite production in select patients with low 6-thioguanine nucleotide levels. Appropriate care and monitoring of these patients are mandatory to prevent neutropenia or other adverse effects.

Trypanosoma brucei adenine-phosphoribosyltransferases mediate adenine salvage and aminopurinol susceptibility but not adenine toxicity

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African trypanosomes possess two distinct adenine phosphoribosyltransferases.

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Trypanosoma brucei TbAPRT1 is cytosolic, TbAPRT2 localizes to the glycosome.

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Aprt1,2 null mutants are viable but do not incorporate adenine into nucleotides.

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Aprt1,2 null mutants are resistant to aminopurinol but still sensitive to adenine.

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Aminopurinol is a trypanocide with submicromolar activity against T. brucei.

African trypanosomes, like all obligate parasitic protozoa, cannot synthesize purines de novo and import purines from their hosts to build nucleic acids. The purine salvage pathways of Trypanosoma brucei being redundant, none of the involved enzymes is likely to be essential. Nevertheless they can be of pharmacological interest due to their role in activation of purine nucleobase or nucleoside analogues, which only become toxic when converted to nucleotides. Aminopurine antimetabolites, in particular, are potent trypanocides and even adenine itself is toxic to trypanosomes at elevated concentrations. Here we report on the T. brucei adenine phosphoribosyltransferases TbAPRT1 and TbAPRT2, encoded by the two genes Tb927.7.1780 and Tb927.7.1790, located in tandem on chromosome seven. The duplication is syntenic in all available Trypanosoma genomes but not in Leishmania. While TbAPRT1 is cytosolic, TbAPRT2 possesses a glycosomal targeting signal and co-localizes with the glycosomal marker aldolase. Interestingly, the distribution of glycosomal targeting signals among trypanosomatid adenine phosphoribosyltransferases is not consistent with their phylogeny, indicating that the acquisition of adenine salvage to the glycosome happened after the radiation of Trypanosoma. Double null mutant T. brucei Δtbaprt1,2 exhibited no growth phenotype but no longer incorporated exogenous adenine into the nucleotide pool. This, however, did not reduce their sensitivity to adenine. The Δtbaprt1,2 trypanosomes were resistant to the adenine isomer aminopurinol, indicating that it is activated by phosphoribosyl transfer. Aminopurinol was about 1000-fold more toxic to bloodstream-form T. brucei than the corresponding hypoxanthine isomer allopurinol. Aminopurinol uptake was not dependent on the aminopurine permease P2 that has been implicated in drug resistance.

Drug resistance in leishmaniasis

Leishmaniasis is a complex disease, with visceral and cutaneous manifestations, and is caused by over 15 different species of the protozoan parasite genus Leishmania. There are significant differences in the sensitivity of these species both to the standard drugs, for example, pentavalent antimonials and miltefosine, and those on clinical trial, for example, paromomycin. Over 60% of patients with visceral leishmaniasis in Bihar State, India, do not respond to treatment with pentavalent antimonials. This is now considered to be due to acquired resistance. Although this class of drugs has been used for over 60 years for leishmaniasis treatment, it is only in the past 2 years that the mechanisms of action and resistance have been identified, related to drug metabolism, thiol metabolism, and drug efflux. With the introduction of new therapies, including miltefosine in 2002 and paromomycin in 2005-2006, it is essential that there be a strategy to prevent the emergence of resistance to new drugs; combination therapy, monitoring of therapy, and improved diagnostics could play an essential role in this strategy.

Serum concentrations of acute-phase proteins in dogs with leishmaniosis during short-term treatment

OBJECTIVE

To evaluate changes in serum concentrations of acute-phase proteins in dogs with leishmaniosis during short-term therapy in accordance with 2 treatment protocols and determine whether concentrations of acute-phase proteins could be used to monitor the initial response of dogs to treatment.

ANIMALS

12 dogs naturally infected with Leishmania infantum.

PROCEDURE

Dogs were allocated into 2 groups. Dogs of group 1 were treated by use of meglumine antimonate (100 mg/kg, SC, q 24 h) administered concurrently with allopurinol (15 mg/kg, PO, q 12 h) for 20 days and then with allopurinol alone at the same dosage for the subsequent 30 days. Dogs of group 2 were treated by administration of allopurinol alone (15 mg/kg, PO, q 12 h) for 60 days). Blood samples were obtained before and during treatment for measurement of serum concentrations of acute-phase proteins and determination of CBC counts, serum biochemical analyses, and electropherograms.

RESULTS

All dogs evaluated in the study had increased concentrations of C-reactive protein, haptoglobin, and ceruloplasmin at the time of diagnosis of leishmaniosis. Mean concentration of serum amyloid A before treatment was also increased, but some of the dogs had concentrations of serum amyloid A that were within the reference range. Concentrations of C-reactive protein and ceruloplasmin decreased significantly in all dogs at the end of the study period.

CONCLUSIONS AND CLINICAL RELEVANCE

Measurement of concentrations of selected acute-phase proteins, such as C-reactive protein or ceruloplasmin, could be used to evaluate the initial response of dogs with leishmaniosis to treatment.

Use of allopurinol for maintenance of remission in dogs with leishmaniasis

Current treatments for infected dogs with leishmaniasis do not always provide long-term control of the disease and clinical relapses are common. In this study, the usefulness of long-term allopurinol administration in the maintenance of clinical remission in canine leishmaniasis was evaluated. Fifteen dogs with natural leishmania infection were subjected to an initial treatment based on the simultaneous administration of meglumine antimoniate (100 mg/kg/day) and allopurinol (30 mg/kg/day). Once clinical remission was achieved, a maintenance treatment with allopurinol (20 mg/kg/day) administered for one week a month was instituted. Results were compared with those of a retrospective control group comprising 15 infected dogs which only followed the induction treatment. Relapses occurred in 86 per cent of control dogs within 14 months of discontinuing treatment. In contrast, those dogs on intermittent oral allopurinol administration were successfully maintained in clinical remission for a follow-up period of 10 to 44 months. In this latter group, specific antibody titres decreased or were unchanged, no side effects directly attributable to allopurinol were seen and treatment was well accepted by the owners. It is concluded that long-term intermittent administration of allopurinol is an effective way of maintaining clinical remission in dogs with leishmaniasis.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Effects of probenecid on the pharmacokinetics of allopurinol riboside

Allopurinol riboside is an experimental agent for the treatment of leishmaniasis and American trypanosomiasis. Previous studies showed that after oral administration, unexpectedly low levels of allopurinol riboside in plasma are attributable to incomplete absorption and rapid renal clearance. In this randomized, crossover evaluation in healthy volunteers, probenecid reduces the renal clearance of allopurinol riboside, extends the half-life of allopurinol riboside in plasma, and triples the levels of allopurinol riboside in plasma.