Pharmacokinetics of enoxacin and its oxometabolite after multiple oral dosing and penetration into prostatic tissue

B. Hamela, N. Mottetb, M. Audranc, P. Costab and F. Bressollea,*

a Laboratoire de Pharmacocinétique Clinique, Faculté de Pharmacie, Université Montpellier I; b Service d'Urologie Andrologie, Hôpital G. Doumergue, Nîmes; c Laboratoire de Biophysique, Faculté de Pharmacie, Université Montpellier I, France


    Abstract
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The objective of this study was to determine the concentrations of enoxacin and its oxo-metabolite in human prostatic tissue after multiple oral doses (400 mg bd) in 13 patients. On the first day of treatment, elimination half-lives were 6.8 h for enoxacin and 7.1 h for its metabolite; they were increased on day 4 (10.3 and 13.2 h, respectively). The ratios of drug concentration in prostatic tissue and plasma averaged 2.2 for enoxacin and 1.4 for its metabolite. In conclusion, concentrations of enoxacin achieved within the prostatic tissue were higher than plasma concentrations suggesting that there was an active transport mechanism.


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Enoxacin is a synthetic antibacterial drug of the fluoroquinolone class, and is rapidly bactericidal against Gram-positive and -negative organisms including Pseudomonas aeruginosa and Enterobacteriaceae.1 This drug has a bioavailability of c. 77–90%, a large volume of distribution (c. 200 L), an elimination half-life of 3.3–5 h and a total clearance averaging 480 mL/min.2,3 However, enoxacin kinetics were dose-dependent at doses between 200 and 800 mg.2,3 The main metabolite, oxoenoxacin, had antibacterial activity about one-tenth that of the parent drug.1

The distribution of enoxacin into prostatic tissue has been widely studied but no data are available on the distribution of the oxometabolite.

The aim of this study was to assess the pharmacokinetic profile of enoxacin and oxoenoxacin after multiple oral dosing, and to investigate their penetration into the prostatic tissue.


    Materials and methods
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Patients

This study was carried out in 13 male patients, ranging from 48 to 75 years old (mean ± S.D., 62.8 ± 7.33 years), and from 62 to 93 kg in weight (mean ± S.D., 74.7 ± 8.65 kg). These patients were hospitalized for benign prostatic adenoma (suprapubic or transurethral prostatectomy). They had no history of allergy to antimicrobial agents, and the results of physical examination, routine haematology, blood chemistry and urinalysis studies performed on each patient before the study were normal. The patients presented no evidence of acute progressive disease or renal (creatinine clearance, calculated according to Cockcroft and Gault formula, ranging from 56.2 to 88.7 mL/min) or hepatocellular failure. The patients were enrolled in the study after giving written informed consent. The study protocol was reviewed and approved by the institutional review board.

Study design

Before surgery, each patient was premedicated with enoxacin 400 mg bd orally for 3 days. A final (seventh) dose was given the day of surgery (day 4), c. 6 h before surgical incision, with a 12 h interval between the sixth and the seventh doses. Blood samples (6 mL) were collected in heparinized glass tubes: (i) immediately pre-dose; (ii) 0.5, 1, 2, 3, 4, 8 and 12 h post-dose on days 1 and 4; (iii) just before the first doses on days 2 and 3; (iv) at 16, 24 and 36 h post-dose on day 4; and (v) the day of surgery at the time of tissue sample. Plasma was separated from blood by centrifugation (3000g for 10 min), then immediately put in to two polypropylene tubes.

To prevent urinary contamination of prostatic chips, the bladder was emptied of all urine before their removal. Prostatic tissue samples (c. 300 mg) were washed for 30 s in physiological saline to remove traces of blood or urine, dried on gauze and then put in a polypropylene tube.

Plasma and tissue samples were immediately frozen and stored at –80°C until assayed. Concentrations of enoxacin and oxoenoxacin were determined by high-performance liquid chromatography.4

Pharmacokinetic analysis

Cmin and Cmax are trough and peak observed concentrations, respectively. The time of the Cmax was designated tmax. Elimination half-life was determined from the slope of the log-linear part curves.

In order to take into account the residual plasma concentration before the seventh dose, the plasma concentrations (Ccorrected) at each sampling time were corrected as follows: Ccorrected = CobservedCresidual and Cresidual = C0 e{lambda}2t, where C0 is the plasma concentration before the seventh dose, and {lambda}2 is the elimination rate constant and t is the time post-dose.

The areas under the curve (AUCs) were obtained by log–linear trapezoidal approximation: (i) on days 1 and 4 from zero (pre-dose) to 12 h (AUC0–12); (ii) on day 1, from 0 to infinity by dividing the last observed data point by the elimination rate constant (AUC0–{infty}); and (iii) on day 4, using corrected concentrations, from 0 to 12 h, and from 0 to infinity.

The total body clearance of the parent drug was calculated from the relationship: CL/F = dose/AUC.

The accumulation ratios for the parent drug and its metabolite were calculated from: (i) 1/[1 – e{lambda}2{tau}], where {tau} is the dosing interval; (ii) AUC (0–12 h)day 4/AUC (0–12 h)day 1; and (iii) from Cmax day 4/Cmax day 1.

Statistical analysis

Pharmacokinetic parameters (tmax, Cmax, AUC0–12, AUC0–{infty}, CL/F and elimination half-life) evaluated after the first oral administration and after the last dose (computed using corrected concentrations) were compared using the Friedman test. In order to avoid a possible bias caused by different sampling schedules (12 versus 36 h, respectively), the elimination half-life and AUC were computed, on day 4, from drug concentrations versus time curve between 0 and 12 h.

A two-way ANOVA was performed to compare measured trough values. In case of a significant result, a simple contrast test was used to compare 2-by-2 each group. Before analyses, AUCs, Cmax and Cmin were previously transformed into their logarithms.

Unweighted least-squares regression analyses of AUC versus age, AUC versus creatinine clearance and plasma concentrations versus tissue concentrations were carried out. The significance of the regression was confirmed by the F-test.

A P value <0.05 was taken as the threshold of probability.


    Results
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Table IGo summarizes mean observed and calculated pharmacokinetic parameters; trough plasma concentrations are reported in Table IIGo.


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Table I. Mean (± S.D.) pharmacokinetic parameters of enoxacin and oxoenoxacin
 

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Table II. Mean (± S.D.) trough concentration values
 
For both the parent drug and its metabolite, significant differences occurred between the following parameters: AUC0–12, AUC0–{infty}, CL/F and elimination half-life, determined on day 4 and those determined on the first day of treatment. The accumulation ratios are presented in Table IGo. Methods B and C gave nearly identical results, but they were significantly different from those computed from the equation 1/[1 – e{lambda}2{tau}].

Mean tissue/plasma ratios averaged 2.2 for enoxacin and 1.4 for oxoenoxacin. When tissue concentrations of enoxacin were plotted against corresponding plasma concentrations, a statistically significant straight line could be fitted to the data (r = 0.57, P = 0.0386). For the metabolite there was no significant relationship with corresponding plasma concentrations.

Strong correlations were found between creatinine clearance and AUC0–12 (r = –0.815, P < 0.0008 on day 1, and r = –0.707, P < 0.007 on day 4 for enoxacin; r = –0.741, P < 0.004 on day 1 for oxoenoxacin). No significant relationship was found between AUC and age.


    Discussion
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Enoxacin showed a 37% reduction in CL/F and a 51.4% increase in elimination half-life after 4 days of treatment compared with day 1. Similar dose-dependent changes were reported after single2 and repeated3 doses and could be due to a saturable metabolism. The elimination half-life of the oxometabolite is similar to that of enoxacin and increased from day 1 to day 4.

Enoxacin is minimally bound to serum proteins (c. 35%); thus, when enoxacin concentrations are compared with the in vitro activity of the drug, it might be expected that repeated administration of enoxacin (400 mg bd) would lead to plasma enoxacin concentrations exceeding the MIC90 of many Gram-negative bacteria (Enterobacteriaceae MIC90 < 1 mg/L, Haemophilus influenzae MIC90 = 0.12 mg/L) and methicillin-suceptible Staphylococcus (MIC90 = 1 mg/L).5 The MIC90 of Pseudomonas spp. (2 mg/L) is also covered.5

The prostatic tissue concentrations of enoxacin, 35–320% higher than plasma concentrations, suggested an active transport mechanism. The low protein binding and the pKa values (6 and 8) of enoxacin may favour entrapment in the human prostatic environment which has a lower pH than plasma (pH 7.28 in normal individual).6 The tissue concentrations reported in this study were all above the MIC of enoxacin for usual pathogens causing infections at this site (Enterobacteriaceae, methicillin-suceptible Staphylococcus aureus, Neisseria gonorrhoeae MIC90 = 0.08–0.16 mg/L) and enoxacin can be considered as a useful drug in the treatment of prostatitis and in prophylaxis of prostatic surgery.

As previously reported, enoxacin is tolerated well by patients, many of them elderly, with few side effects.7


    Acknowledgments
 
The authors thank Melle B. Marion and Mr G. Bougard for their excellent technical assistance, and gratefully acknowledge support of this work by the Pierre Fabre Research Institute, Castres, France.


    Notes
 
* Correspondence address. Laboratoire de Pharmacocinétique Clinique, Faculté de Pharmacie, 34060 Montpellier Cedex 2, France. Tel: +33-4-67-54-80-75; Fax: +33-4-67-79-32-92; E-mail: FBressolle{at}aol.com Back


    References
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 . Siporin, C. & Towse, G. (1984). Enoxacin worldwide in-vitro activity against 22,451 clinical isolates. Journal of Antimicrobial Chemotherapy 14, Suppl. C, 47–55.[ISI][Medline]

2 . Chang, T., Black, A., Dunky, A., Wolf, R., Sedman, A., Latts, J. et al. (1988). Pharmacokinetics of intravenous and oral enoxacin in healthy volunteers. Journal of Antimicrobial Chemotherapy 21, Suppl. B, 49–56.[Abstract]

3 . Wolf, R., Eberl, R., Dunky, A., Mertz, N., Chang, T., Goulet, J. R. et al. (1984). The clinical pharmacokinetics and tolerance of enoxacin in healthy volunteers. Journal of Antimicrobial Chemotherapy 14, Suppl. C, 63–9.[ISI][Medline]

4 . Hamel, B., Audran, M., Costa, P. & Bressolle, F. (1998). Reversed-phase high-performance liquid chromatographic determination of enoxacin and 4-oxo enoxacin in human plasma and prostatic tissue. Application to a pharmacokinetic study. Journal of Chromatography A 812, 369–79.[ISI][Medline]

5 . Heifetz, C. L., Bien, P. A., Cohen, M. A., Dombrowski, M. E., Griffin, T. J., Malta, T. E. et al. (1988). Enoxacin: in-vitro and animal evaluation as a parenteral and oral agent against hospital bacterial isolates. Journal of Antimicrobial Chemotherapy 21, Suppl. B, 29–42.

6 . Fair, W. R., Crane, D. B., Schiller, N. & Heston, W. D. (1979). A reappraisal of treatment in chronic bacterial prostatitis. Journal of Urology 121, 437–41.[ISI][Medline]

7 . Foot, M., Williams, G., Want, S., Roe, M., Quaghebeur, G. & Bates, S. (1988). An open study of the safety and efficacy of enoxacin in complicated urinary tract infections. Journal of Antimicrobial Chemotherapy 21, Suppl. B, 97–103.[ISI][Medline]

Received 22 May 2000; returned 2 August 2000; revised 21 August 2000; accepted 30 August 2000