Plasma levels of sulfinpyrazone and of two of its metabolites after a single dose and during the steady state

Intestinal UDP-glucuronosyltransferase as a potential target for the treatment and prevention of lymphatic filariasis

Lymphatic filariasis (LF), a morbid disease caused by the tissue-invasive nematodes Wuchereria bancrofti, Brugia malayi, and Brugia timori, affects millions of people worldwide. Global eradication efforts have significantly reduced worldwide prevalence, but complete elimination has been hampered by limitations of current anti-filarial drugs and the lack of a vaccine. The goal of this study was to evaluate B. malayi intestinal UDP-glucuronosyltransferase (Bm-UGT) as a potential therapeutic target. To evaluate whether Bm-UGT is essential for adult filarial worms, we inhibited its expression using siRNA. This resulted in a 75% knockdown of Bm-ugt mRNA for 6 days and almost complete suppression of detectable Bm-UGT by immunoblot. Reduction in Bm-UGT expression resulted in decreased worm motility for 6 days, 70% reduction in microfilaria release from adult worms, and significant reduction in adult worm metabolism as detected by MTT assays. Because prior allergic-sensitization to a filarial antigen would be a contraindication for its use as a vaccine candidate, we tested plasma from infected and endemic normal populations for Bm-UGT-specific IgE using a luciferase immunoprecipitation assay. All samples (n = 35) tested negative. We then tested two commercially available medicines known to be broad inhibitors of UGTs, sulfinpyrazone and probenecid, for in vitro activity against B. malayi. There were marked macrofilaricidal effects at concentrations achievable in humans and very little effect on microfilariae. In addition, we observed that probenecid and sulfinpyrazone exhibit a synergistic macrofilaricidal effect when used in combination with albendazole. The results of this study demonstrate that Bm-UGT is an essential protein for adult worm survival. Lack of prior IgE sensitization in infected and endemic populations suggest it may be a feasible vaccine candidate. The finding that sulfinpyrazone and probenecid have in vitro effects against adult B. malayi worms suggests that these medications have promise as potential macrofilaricides in humans.

Rationalization and prediction of in vivo metabolite exposures: the role of metabolite kinetics, clearance predictions and in vitro parameters

IMPORTANCE OF THE FIELD

Due to growing concerns over toxic or active metabolites, significant efforts have been focused on qualitative identification of potential in vivo metabolites from in vitro data. However, limited tools are available to quantitatively predict their human exposures.

AREAS COVERED IN THIS REVIEW

Theory of clearance predictions and metabolite kinetics is reviewed together with supporting experimental data. In vitro and in vivo data of known circulating metabolites and their parent drugs were collected and the predictions of in vivo exposures of the metabolites were evaluated.

WHAT THE READER WILL GAIN

The theory and data reviewed will be useful in early identification of human metabolites that will circulate at significant levels in vivo and help in designing in vivo studies that focus on characterization of metabolites. It will also assist in rationalization of metabolite-to-parent ratios used as markers of specific enzyme activity.

TAKE HOME MESSAGE

The relative importance of a metabolite in comparison to the parent compound as well as other metabolites in vivo can only be predicted using the metabolite's in vitro formation and elimination clearances, and the in vivo disposition of a metabolite can only be rationalized when the elimination pathways of that metabolite are known.

Are circulating metabolites important in drug-drug interactions?: Quantitative analysis of risk prediction and inhibitory potency

The potential of metabolites to contribute to drug-drug interactions (DDIs) is not well defined. The aim of this study was to determine the quantitative role of circulating metabolites in inhibitory DDIs in vivo. The area under the plasma concentration-time curve (AUC) data related to at least one circulating metabolite was available for 71% of the 102 inhibitor drugs identified. Of the 80 metabolites characterized at steady state, 78% had AUCs >10% of that of the parent drug. A comparison of the inhibitor concentration/inhibition constant ([I]/K(i)) ratios of metabolites and the respective parent drugs showed that 17 of the 21 (80%) reversible inhibitors studied had metabolites that were likely to contribute to in vivo DDIs, with some metabolites predicted to have inhibitory effects greater than those of the parent drug. The in vivo drug interaction risks associated with amiodarone, bupropion, and sertraline could be identified from in vitro data only, when data pertaining to metabolites were included in the predictions. In conclusion, cytochrome P450 (CYP) inhibitors often have circulating metabolites that contribute to clinically observed CYP inhibition.

Effective antiplatelet drug concentrations in experimental arterial thromboembolism

Although drugs that modify platelet function have been widely studied as antithrombotic agents in experimental and clinical studies, there is limited information regarding the relationship between in vivo drug blood concentrations and antithrombotic efficacy. This study compared the pharmacokinetics of three antiplatelet agents with their antithrombotic effects in an experimental model of arterial thromboembolism in baboons. Thrombus formation was measured as steady-state platelet utilization induced by thrombogenic arteriovenous cannulae. The drugs studied were aspirin, dipyridamole and sulfinpyrazone. Aspirin was administered in daily doses of 20 mg/kg, dipyridamole in daily doses of 2.5 and 10 mg/kg, and sulfinpyrazone in daily doses of 20 and 100 mg/kg; each drug was given in two equal doses per day. Multiple blood samples were collected for drug analysis after steady-state had been reached. The average concentrations of dipyridamole at steady-state were 26 +/- 15 and 79 +/- 69 ng/ml after 2.5 and 10 mg/kg/day. These concentrations were associated with 28 and 87% inhibition of cannula platelet consumption, respectively. The average steady-state concentrations of acetylsalicylic and salicylic acids were 0.67 +/- 0.80 and 3.76 +/- 2.60 micrograms/ml, respectively, after 20 mg/kg/day. Aspirin had no effect on platelet consumption. Average concentrations of sulfinpyrazone were 1.05 +/- 0.45 and 12.25 +/- 5.73 micrograms/ml after 20 and 100 mg/kg/day, with significant concentrations of the sulfide metabolite. These concentrations were associated with 23 and 85% inhibition of platelet consumption, respectively. No significant pharmacokinetic interactions were observed after concurrent administration of aspirin and dipyridamole or sulfinpyrazone. As the experimental model used involves thrombus formation on an artificial surface, it is likely that these results are most relevant to patients with arterial prosthetic devices.

Correlation between inhibitory effect on platelet aggregation and disposition of sulfinpyrazone and its metabolites in rabbits. Part II: Multiple dose study

In a crossover study rabbits were given perorally sulfinpyrazone (SO) and the sulfide metabolite (S) every 24 h for 5 days on separate occasions and inhibition of aggregation was measured. The results showed: the dosage regimen is effective if the minimum effective concentration of S is defined to be between 0.5-1.0 microgram ml-1, and the repeated dosing did not cause changes in disposition kinetics except that the terminal half-life of S was reduced after dosing with S. No significant accumulations in trough concentration and inhibition of aggregation were observed. The results obtained in this study could provide some useful information for design of dosage regimen and blood level monitoring for humans.

Electrical changes produced by injury to the rat myocardium in vitro and the protective effects of certain antiarrhythmic drugs

Glass microelectrodes were used to record intracellular electrical activity from rat isolated and superfused atrial myocardium during external electrical stimulation. After 2 h in normal oxygenated physiological salt solution the muscle was exposed for 30 min to a superfusate simulating the composition of extracellular fluid during myocardial ischaemia (SI). This fluid contained lactate (20 mM), a raised potassium concentration (7 mM), no glucose and a pH lowered to 6.4, and was gassed with N2 in place of O2 (hypoxia). During SI the diastolic threshold voltage for stimulation increased, the speed of action potential conduction between the right and left atria slowed, and both the effective and functional refractory periods of the right atrium shortened, as did the duration of the right atrial action potential. The only component of SI which separately caused electrical changes similar to those of the full simulation was hypoxia. Addition to the superfusate of verapamil (0.5 micrograms ml-1), sulphinpyrazone (1-20 micrograms ml-1) or indomethacin (10-20 micrograms ml-1) attenuated many of the SI-induced electrical changes, although indomethacin was much less effective than the other two drugs. Lowering the calcium concentration of the superfusate from 2 mM to 0.5 mM protected against the SI-induced electrical changes that were inhibitable with sulphinpyrazone and verapamil.

Pharmacokinetics of sulphinpyrazone and its major metabolites after a single dose and during chronic treatment

The pharmacokinetics of sulphinpyrazone and its major metabolites (sulfide, sulfone, p-hydroxysulfone and p-hydroxy-sulphinpyrazone) were investigated in 9 volunteers after a single oral dose as well as after chronic treatment for 23 days. Chronic administration of sulphinpyrazone, in comparison with a single oral dose, led to significant changes in plasma AUC (115.86 to 42.90 mg/l . h), in renal clearance (1.06 to 1.80 l/h), in hepatic intrinsic clearance (319.0 to 598.0 l/h), and in the unbound fraction in plasma 1.15 to 1.69%) and in tissue (2.73 to 1.31%). The volume of distribution changed from 20.24 to 52.04 l. The steady state concentrations predicted from the single dose were significantly higher than the values found after chronic treatment. The results suggest that sulphinpyrazone induces its own metabolism. The metabolism of the sulfone, p-hydroxysulfone and the p-hydroxy-sulphinpyrazone to further degradation products was also induced. Chronic treatment with sulphinpyrazone reduced the plasma AUC of the sulfide and caused a decrease in its elimination half-life (20.9 to 14.3 h). Since considerable amounts of the sulfide are formed in the G.I. tract, it is suggested that besides the induction of metabolism, bacteria which reduce sulphinpyrazone to the sulfide may also be responsible for the observed pharmacokinetic changes.

The site of reduction of sulphinpyrazone in the rabbit

Comparison of oral and i.v. administration of sulphinpyrazone (10 mg/kg) to rabbits showed that the oral route was associated with an incomplete bioavailability and a six-fold greater formation of the active sulphide metabolite. The bile was an important route of elimination of unchanged sulphinpyrazone in rabbits (18% of an i.v. dose in four hours). Only small amounts of the sulphide appeared in the bile after i.v. administration. Pretreatment with oral antibiotics decreased the area under the plasma concentration-time curve (AUC) for the sulphide but increased that of the parent drug. Excretion of the p-hydroxysulphide metabolite in urine was decreased 30-fold by antibiotic treatment. The contents of the caecum showed the greatest capacity for sulphinpyrazone reduction in vitro. The liver possessed a slight ability to reduce sulphinpyrazone in vitro under anaerobic, but not aerobic, conditions. The gut bacteria are the main site of reduction of sulphinpyrazone to the active sulphide metabolite in the rabbit. These findings contrast with those obtained for sulindac which was reduced extensively under both aerobic and anaerobic conditions by rabbit-liver soluble fraction in vitro. The sulphide metabolites of both sulphinpyrazone and sulindac were oxidized to the parent drug by rabbit-liver microsomes.

Warfarin-sulfinpyrazone interaction on binding to human serum albumin

Sulfinpyrazone displacement of warfarin from human serum albumin was studied in-vitro. At low sulfinpyrazone concentrations one molecule of warfarin is displaced on binding by one molecule of sulfinpyrazone. Clinical plasma concentrations of sulfinpyrazone are, however, too low to cause significant displacement.

Inhibition of human platelet cyclo-oxygenase activity by sulfinpyrazone and three of its metabolites

Sulfinpyrazone and three of its major metabolites were compared in vitro for their inhibitory effect on human platelet cyclo-oxygenase activity. Sulfinpyrazone appeared to be about 15-20 times less potent than its sulfide metabolite (G25671) and 6-7 times less potent than the other two compounds, the sulfone metabolite (G31442) and p-hydroxysulfide (G33378). All four compounds were apparently competitive inhibitors of platelet cyclo-oxygenase activity. Comparison of the potency of sulfinpyrazone and its metabolites, as determined in the present study and the plasma levels previously measured in man, indicates that sulfinpyrazone and G33378 were not potent enough to be effective in man. G31442 showed inhibitory potency slightly lower than its corresponding plasma levels, whereas G25671 was effective at concentrations well below those found in human plasma. This study supports the hypothesis that sulfinpyrazone metabolites (in particular the sulfide) rather than the drug itself affect platelet function when administered therapeutically.