The pharmacokinetics and protein-binding of fusidic acid in patients with severe renal failure requiring either haemodialysis or continuous ambulatory peritoneal dialysis

Population pharmacokinetics of rifampicin in adult patients with osteoarticular infections: interaction with fusidic acid

AIMS

Rifampicin represents the key antibiotic for the management of osteoarticular infections. An important pharmacokinetic variability has already been described, particularly for absorption and metabolism. All previous pharmacokinetic studies have been focused only on patients treated for tuberculosis. The objective of the present study was to describe a population pharmacokinetic model of rifampicin in patients with staphylococcal osteoarticular infections, which has not been investigated to date.

METHOD

Rifampicin concentrations were collected retrospectively from 62 patients treated with oral rifampicin 300 mg three times daily. Plasma concentration-time data were analysed using NONMEM to estimate population pharmacokinetic parameters. Demographic data, infection characteristics and antibiotics taken in addition to rifampicin antibiotics were investigated as covariates.

RESULTS

A one-compartment model, coupled to a transit absorption model, best described the rifampicin data. Fusidic acid coadministration was identified as a covariate in rifampicin pharmacokinetic parameters. The apparent clearance and apparent central volume of distribution mean values [95% confidence interval (CI)] were 5.1 1 h^-1 (1.2, 8.2 1 h^-1 )/23.8 l (8.9, 38.7 l) and 13.7 1 h^-1 (10.6, 18.0 1 h^-1 )/61.1 1 (40.8, 129.0 1) for patients with and without administration of fusidic acid, respectively. Interindividual variability (95% CI) in the apparent clearance and apparent central volume of distribution were 72.9% (49.5, 86.0%) and 59.1% (5.5, 105.4%), respectively. Residual variability was 2.3 mg l^-1 (1.6, 2.6 mg l^-1 ).

CONCLUSION

We developed the first population pharmacokinetic model of rifampicin in patients with osteoarticular infections. Our model demonstrated that fusidic acid affects rifampicin pharmacokinetics, leading to potential high drug exposure. This finding suggests that fusidic acid dosing regimens should be reconsidered.

Screening of drug metabolizing enzymes for fusidic acid and its interactions with isoform-selective substrates in vitro

1. Fusidic acid (FA) is widely used for the treatment of infections of sensitive osteomyelitis or skin and soft tissue caused by bacteria. However, the role of cytochrome P450s (CYPs) in the metabolism of FA is unclear. In the present study, we screened the main CYPs for the metabolism of FA and studied its interactions with isoform-selective substrates in vitro. 2. The main CYP450s were screened according to the inhibitory effect of specific inhibitors on the metabolism of FA in human liver microsomes (HLMs) or recombinant CYP isoforms. Enzyme kinetic parameters including K_i, K_i', V_max, and IC₅₀ were calculated to determine the potential of FA to affect CYP-mediated metabolism of isoform-selective substrates. 3. FA metabolism rate was inhibited by 49.8% and 83.1% under CYP2D6, CYP3A4 selective inhibitors in HLMs. In recombinant experiment, the inhibitory effects on FA metabolism were 83.3% for CYP2D6 and 58.9% for CYP3A4, respectively. FA showed inhibition on CYP2D6 and CYP3A4 with K_is of 13.9 and 38.6 μM, respectively. Other CYP isoforms including CYP1A2, CYP2A6, CYP2C9, CYP2E1, and CYP2C19 showed minimal or no effect on the metabolism of FA. 4. FA was primarily metabolized by CYP2D6 and CYP3A4 and showed a noncompetitive inhibition on CYP2D6 and a mixed competitive inhibition on CYP3A4. Drug-drug interactions between FA and other chemicals, especially with substrates of CYP2D6 and CYP3A4, are phenomena that clinicians need to be aware of and cautious about.

Fusidic Acid Inhibits Hepatic Transporters and Metabolic Enzymes: Potential Cause of Clinical Drug-Drug Interaction Observed with Statin Coadministration

Fusidic acid (FA), which was approved in the 1960s in many European and Asian countries, has gained renewed interest due to its continued effectiveness against methicillin-resistant Staphylococcus aureus As rhabdomyolysis has been reported upon coadministration of FA with statins, we aimed to elucidate the underlying molecular mechanisms that contribute to FA-statin drug-drug interactions. Because of the association between rhabdomyolysis and increased exposure to statins, we investigated if cytochrome P450 (CYP) enzymes and transporters involved in the disposition of various statins are inhibited by FA. FA was found to inhibit BCRP and OATP1B1 but not P-gp. In overexpressing cell systems, FA inhibited BCRP-mediated efflux (50% inhibitory concentration [IC50], ∼50 to 110 μM) and OATP1B1-mediated uptake (IC50, ∼4 to 35 μM) of statins at clinically relevant concentrations achievable in the intestine and liver (based on a 550-mg oral dose of FA, the expected maximum theoretical gastrointestinal concentration is ∼4 mM, and the maximum total or unbound concentration in the inlet to the liver was reported to be up to 223 μM or 11 μM, respectively, upon multiple dosing). Similarly, FA inhibited metabolism of statins in human liver microsomes (IC50, ∼17 to 195 μM). These data suggest that FA inhibits at least 3 major dispositional pathways (BCRP, OATP1B1, and CYP3A) and thus affects the clearance of several statins. We confirmed that FA is eliminated via phase 1 metabolism (primarily via CYP3A); however, there is also some phase 2 metabolism (mediated primarily by UGT1A1). Taken together, these data provide evidence for molecular mechanisms that may explain the occurrence of rhabdomyolysis when FA is administered with statins.

'Old' antibiotics for emerging multidrug-resistant bacteria

PURPOSE OF REVIEW

Increased emergence of bacterial resistance and the decline in newly developed antibiotics have necessitated the reintroduction of previously abandoned antimicrobial agents active against multidrug-resistant bacteria. Having never been subjected to contemporary drug development procedures, these 'old' antibiotics require redevelopment in order to optimize therapy. This review focuses on colistin as an exemplar of a successful redevelopment process and briefly discusses two other old antibiotics, fusidic acid and fosfomycin.

RECENT FINDINGS

Redevelopment of colistin led to an improved understanding of its chemistry, pharmacokinetics and pharmacodynamics, enabling important steps towards optimizing its clinical use in different patient populations. A scientifically based dosing algorithm was developed for critically ill patients, including those with renal impairment. As nephrotoxicity is a dose-limiting adverse event of colistin, rational combination therapy with other antibiotics needs to be investigated.

SUMMARY

The example of colistin demonstrated that state-of-the-art analytical, microbiological and pharmacokinetic/pharmacodynamic methods can facilitate optimized use of 'old' antibiotics in the clinic. Similar methods are now being applied to fosfomycin and fusidic acid in order to optimize therapy. To improve and preserve the usefulness of these antibiotics rational approaches for redevelopment need to be followed.

Population pharmacokinetics of fusidic acid: rationale for front-loaded dosing regimens due to autoinhibition of clearance

The objectives of this analysis were to develop a population pharmacokinetic (PK) model to describe the absorption and disposition of fusidic acid after single and multiple doses and to determine the effect of food on the rate and extent of bioavailability. Plasma PK data from three phase 1 studies (n = 75; n = 14 with and without food) in which healthy subjects received sodium fusidate (500 to 2,200 mg) as single or multiple oral doses every 8 h (q8h) or q12h for up to 7 days were modeled using S-ADAPT (MCPEM algorithm). Accumulation of fusidic acid after multiple doses was more than that predicted based on single-dose data. The PK of fusidic acid was best described using a time-dependent mixed-order absorption process, two disposition compartments, and a turnover process to describe the autoinhibition of clearance. The mean total clearance (% coefficient of variation) was 1.28 liters/h (33%) and the maximum extent of autoinhibition was 71.0%, with a 50% inhibitory concentration (IC(50)) of 46.3 mg/liter (36%). Food decreased the extent of bioavailability by 18%. As a result of the autoinhibition of clearance, steady state can be achieved earlier with dosing regimens that contain higher doses (after 8 days for 750 mg q12h and 1 day for 1,500 mg q12h on day 1 followed by 600 mg q12h versus 3 weeks for 500 mg q12h). Given that large initial doses autoinhibit the clearance of fusidic acid, this characteristic provides a basis for the administration of front-loaded dosing regimens of sodium fusidate which would allow for effective concentrations to be achieved early in therapy.

In vitro elution of moxifloxacin and fusidic acid by a synthetic crystallic semihydrate form of calcium sulphate (Stimulan)

Stimulan was evaluated in vitro as a biodegradable carrier for local delivery of moxifloxacin and fusidic acid. Moxifloxacin or fusidic acid was mixed with calcium sulphate at a ratio of 95:5 to prepare five replicas per antibiotic. In vitro elution was estimated daily using a high-performance liquid chromatography (HPLC) system. Elution of moxifloxacin lasted for 31 days. Eluted concentrations reached their peak on Day 13 (mean level 745 microg/mL); the lowest eluted concentration was detected on Day 30 (mean level 367 microg/mL). Elution of fusidic acid lasted for 14 days. Eluted concentrations reached their peak on Day 6 (mean value 249.5 microg/mL); the lowest eluted concentration was detected on Day 13 (mean value 10.9 microg/mL). The presented results revealed that Stimulan may allow adequate in vitro elution of moxifloxacin and fusidic acid. The latter results support the application of this system in experimental models of osteomyelitis.

Fusidic acid pharmacology, pharmacokinetics and pharmacodynamics

Fusidic acid comes in a variety of formulations for oral, intravenous and topical use. After oral administration of 500 mg Cmax values range from 14.5-3.3 mg/l and an elimination half-life of 8.9-11.0 h. Similar values are obtained with intravenous administration of the sodium salt, although peaks tend to be higher. Bioavailability for the new film-coated tablet is approximately 91% while that of the suspension formulation appears to be much lower. Repeated dosing results in substantial drug accumulation when given 8-hourly, and to a variable extent depending on dose when administered 12-hourly. One study has demonstrated a modest dose-dependency for pharmacokinetics, with decreased clearance at higher doses. Fusidic acid is primarily eliminated by non-renal mechanisms, and a proportion of the drug is metabolised to seven or more breakdown products that can be detected in bile. Hypoalbuminaemia increases fusidic acid clearance, while clearance is decreased in the presence of severe cholestasis, and essentially unchanged in renal failure. Fusidic acid is highly protein-bound (91-98S), but has good penetration to a number of tissues including skin blisters, burns, infected bone and joints. Topical application of fusidic acid results in poor penetration through skin but good penetration into aqueous and vitreous humour. Little is known about the pharmacodynamics of fusidic acid, apart from the fact that it is slowly bactericidal against Staphylococcus aureus, and produces moderate post-antibiotic effects in vitro.